NAT'L INST OF STANDARDS & TECH R.I.C.

A1 11 03066576

n^-"?RS^M2l°.Ik°A'i=^'"-^^«'' Induced damag QC100 .U57 NO.509, 1978 C.1 NBS-PUB-C fg

Q S SPECIAL PUBLICATION 509

U.S. DEPARTMENT OF COMMERCE / National Bureau of Standards

in Optical IVIaterials:

1977

iiiiiiilM iirtiwiiiiiBi^
NATIONAL BUREAU OF STANDARDS

The National Bureau of Standards^ was established by an act of Congress March 3, 1901. The Bureau's overall goal is strengthen and advance the Nation's science and technology and facilitate their effective application for public benefit. To t; end, the Bureau conducts research and provides: (1) a basis for the Nation's physical measurement system, (2) scientific £,l technological services for industry and government, (3) a technical basis for equity in trade, and (4) technical services to p- mote public safety. The Bureau consists of the Institute for Basic Standards, the Institute for Materials Research, the Instill! for Applied Technology, the Institute for Computer Sciences and Technology, the Office for Information Programs, and : Office of Experimental Technology Incentives Program.

THE INSTITUTE FOR BASIC STANDARDS provides the central basis within the United States of a complete and consi ent system of physical measurement; coordinates that system with measurement systems of other nations; and furnishes ess:-

tial services leading to accurate and uniform physical measurements throughout the Nation's scientific community, indusi,, and commerce. The Institute consists of the Office of Measurement Services, and the following center and divisions:

Applied Mathematics — Electricity — Mechanics — Heat — Optical Physics — Center for Radiation Research — L • oratory Astrophysics" — Cryogenics^ — Electromagnetics^ — Time and Frequency'.

THE INSTITUTE FOR MATERIALS RESEARCH conducts materials research leading to improved methods of measi>

ment, standards, and data on the properties of well-characterized materials needed by industry, commerce, educational iri- tutions, and Government; provides advisory and research services to other Government agencies; and develops, produces, li distributes standard reference materials. The Institute consists of the Office of Standard Reference Materials, the Office of r and Water Measurement, and the following divisions:

Analytical Chemistry — Polymers — Metallurgy — Inorganic Materials — Reactor Radiation — Physical ChemistrjTHE INSTITUTE FOR APPLIED TECHNOLOGY provides technical services developing and promoting the use of avl- able technology; cooperates with public and private organizations in developing technological standards, codes, and test m i- ods; and provides technical advice services, and information to Government agencies and the public. The Institute consist; )f the following divisions and centers:

Standards Application and Analysis — Electronic Technology — Center for Consumer Product Technology: Procct Systems Analysis; Product Engineering — Center for Building Technology: Structures, Materials, and Safety; Buikig

Environment; Technical Evaluation and Application — Center for Fire Research: Fire Science; Fire Safety Engineeri,'.

THE INSTITUTE FOR COMPUTER SCIENCES AND TECHNOLOGY conducts research and provides technical serves designed to aid Government agencies in improving cost effectiveness in the conduct of their programs through the selectn,

acquisition, and effective utilization of automatic data processing equipment; and serves as the principal focus wthin the ecutive branch for the development of Federal standards for automatic data processing equipment, techniques, and comper

languages. The Institute consist of the following divisions: Computer Services — Systems and Software — Computer Systems Engineering — Information Technology.

THE OFFICE OF EXPERIMENTAL TECHNOLOGY INCENTIVES PROGRAM seeks to affect public policy and pro*

to facilitate technological change in the private sector by examining and experimenting with Government policies and pctices in order to identify and remove Government-related barriers and to correct inherent market imperfections that im]de

the innovation process. THE OFFICE FOR INFORMATION PROGRAMS promotes optimum dissemination and accessibility of scientific infoiiation generated within NBS; promotes the development of the National Standard Reference Data System and a system oin- formation analysis centers dealing with the broader aspects of the National Measurement System; provides appropriate series

to ensure that the NBS staff has optimum accessibility to the scientific information of the world. The Office consists ofhe

following organizational units:

Office of Standard Reference Data — Office of Information Activities — Oflnice of Technical Publications — Librar;-

Office of International Standards — Office of International Relations.

ji

' Headquarters and Laboratories at Gaithersburg, Maryland, unless otherwise noted; mailing address Washington, D.C. 20234.

J ' Located at Boulder, Colorado 80302. j.
lal Bureau et Standans

2,1978

100 Laser Induced Damage

%c, In Optical Materials: 1977 5

Proceeding of a Symposium Sponsored by:

National Bureau of Standards

American Society for Testing and Materials

Office of Naval Research

Energy Research and Development Administration

Defense Advanced Research Project Agency

October 4-6, 1977

NBS Boulder, Colorado 80303

Edited by

Alexander J. Glass

Lawrence Livermore Laboratory

\ Livermore, California 94550

and

Arthur- H. Guenther

Air Force Weapons Laboratory

Kirtland AFB, New Mexico 87117

U.S. DEPARTMENT OF COMMERCE, Juanita M. Kreps, Secretary

Dr. Sidney Harman, Under Secretary

Jordan J. Baruch, Assistant Secretary for Science and Technology

u, NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director

Issued December 1977
Library of Congress Catalog Card Number: 78-600021

National Bureau of Standards Special Publication 509

Nat. Bur. Stand. (U.S.), Spec. Publ. 509, 561 pages (Dec. 1977)

CODEN: XNBSAV

U.S. GOVERNMENT PRINTING OFFICE

WASHINGTON: 1978

For sale by the Superintendent of Documents, U.S. Government Printing Office, Washington, D.C. 20434

(Order by SD Catalog No. C13. 10:509). Stock No. 003-003-01911-9 Price $6.15

(Add 25 percent additional for other than U.S. mailing).
Foreword

The Proceedings contain the papers presented at the 9th Annual Sjrmposium on Optical Materials for

High Power Lasers held at the National Bureau of Standards in Boulder, Colorado, on October 4-6, 1977.

The Symposium was jointly sponsored by the National Bureau of Standards, the American Society for

Testing and Materials, the Office of Naval Research, the Defense Advanced Research Projects Agency,

and the Energy Research and Development Administration. The Symposium was attended by about 185

scientists from the United States, the United Kingdom, France, Canada, South Africa, Australia and the

Soviet Union. It was divided into sessions devoted to the following topics: Infrared window

materials, mirrors and surfaces, thin films, and fundamental mechanisms. The Symposium Co-chairpersons

were Dr. Alexander J. Glass of the Lawrence Livermore Laboratory and Dr. Arthur H. Guenther of the Air

Force Weapons Laboratory, who also served as Editors of this report.

The Editors assume full responsibility for the summary, conclusions, and recommendations contained

in the report, and for the summaries of discussion found at the end of each paper. The manuscripts of

the papers presented at the Sjnnposium have been prepared by the designated authors, and questions pertaining to their content should be addressed to those authors. The interested reader is referred to the bibliography at the end of the summary article for general references to the literature of laser

damage studies.

The 10th Annual Symposium on this topic will be held in Boulder, Colorado, from September 12 to

14, 1978. It is anticipated that there will be more extensive foreign participation than in previous

years. Also, a concerted effort has been made to ensure closer liaison between the practitioners of

high peak power and the high average power community.

The principal topics to be considered as contributed papers in 1978 do not differ drastically

from those enumerated above. We expect to hear more about improved scaling relations as a function of

pulse duration, area, and wavelength, and see a continuing transfer of information from research

activities to industrial practice. New sources at shorter wavelengths continue to be developed, and

a corresponding shift in emphasis to short wavelength damage problems is anticipated. Fabrication

and test procedures will continue to advance, particularly in the micro-machined optics and thin film

areas.

At the 10th Annual Symposium we expect to pause and reflect upon the past 10 years' activities.

It is our intent to have distinguished speakers who are acknowledged principal contributors in the

many facets of the laser damage field. We will ask them to summarize and quantify as best as possible

the state-of-the-art and the state-of-the-understanding in their specialty. These review or tutorial

lectures will then be assembled and published as a separate volume on the subject of laser induced

damage in optical materials. We are also contemplating having round-table discussions on those specific subjects in which there is still some question as to the correctness of our understanding. The

purpose of these Sjrmposia is to exchange information about optical materials for high power lasers.

The Editors will welcome comment and criticism from all interested readers relevant to this purpose,

and particularly relative to our plans for the 10th Annual Symposium.

A. H. Guenther and A. J. Glass

Co-Chairmen

DISCLAIMER

Certain commercial equipment, instruments, or materials are identified in this publication in order to adequately specify the experimental procedure. In no case does such identification imply recommenmtl:°'^A°\^'^ /^"^^""^ National Bureau of Standards, nor does it imply that the material or equip- ment Identified is necessarily the best available for the purpose.

This publication is a contribution of the National Bureau of Standards and is not subject to copyright.

iii

CONTENTS

Section Page

Foreword iil

A. H. Guenther and A. J. Glass

Sununary and Conclusions viii

A. J. Glass and A. H. Guenther

Introduction and Welcome 1

A. H. Guenther

SI Conversion Units 4

LASER WINDOWS AND MATERIALS

Extrinsic Absorption in Infrared Laser-Window Materials 5

M. Flannery and M. Sparks

Possible Processes of Laser Window Degradation 24

W. J. Fredericks

Purification of Potassium Bromide and Alkaline 37

W. J. Fredericks

Frequency and Temperature Dependence of Residual Infrared Absorption in Mixed Fluoride Crystals. 56

W. G. Lipson, A. Hordvik, B. Bendow, S. S. Mitra, & J. J. Martin

Bulk Laser Absorption Homogeneity 62

T. J. Moravec and E. Bernal G.

Refractive Properties of Infrared Window Materials 74

A. Feldman, D. Horowitz, and R. M. Waxier

Refractive Properties of CVD Zinc Sulfide 83

M. J. Dodge

Critical Orientation for Eliminating Stress-Induced Depolarization in Crystalline

Windows and Rods 89

R. E. Joiner, J. H. Marburger, and W. H. Steier

Lattice Defect Equilibrium in KCl:Eu 96

J. B. Wolfenstine and T. G. Stoebe

Detection of Oxygen in Calcium Fluoride Laser Window Materials by XPS 105

T. N. Wittberg, J. R. Hoenigman, W. E. Moddeman, and C. L. Strecker

Internal-Ref lection-Spectroscopy Study of Water Adsorbed on CaF^ 112

J. W. Gibson, R. T. Holm, and E. D. Palik

Pulsed HF/DF Laser Damage in Window Materials 118

E. W. Van Stryland, M. Bass, M. J. Soileau and C. C. Tang

Optical Absorption in UV Laser Window Materials 127

J. A. Harrington, B. L. Bobbs, and M. Braunstein

UV Reflectance, Transmission, and Photoluminescence of LiYF , and the Bulk Loss

Coefficient in CaF 132

V. Rehn, D. L. Burdick, and V. 0. Jones

Fluence Dependence of the Absorptance of Some Alkaline Earth Fluorides at 0.36 pm 137

P. A. Temple and D. L. Decker

MIRRORS AND SURFACES

Surface Statistics of Selected Optical Materials 142

J. M. Bennett and J. Merle Elson

Surface Roughness Statistics of Fused Silica as a Function of Surface Preparation and Treatment. 157

R. A. House, II, A. H. Guenter, and J. M. Bennett

V
The Effects of Surface Roughness on 1064-nm, 150-ps Laser Damage

D. Milam, W. L. Smith, M. J. Weber, A. H. Guenther, R. A. House, and J. R. Bettis

166

Low Scatter Finishing of Aspheric Optics 174

W. P. Barnes, Jr., and R. R. McDonough

Measurement of Mirror and Window Characteristics For Use With 10.6 ym Lasers 183

S. Sharma, R. M. Wood, and R. C. C. Ward

Polishing Single Point Diamond Turned Mirrors 196

R. E. Parks, R. E. Sumner and R. E. Strittmatter

Pulsed-Laser Stress Phenomena on Highly Reflecting Metal and Alloy Surfaces 204

J. 0. Porteus, C. W. Fountain, J. L. Jernigan, W. N. Faith, and H. E. Bennett

Low-Expansion Laser Mirrors 215

P. Pirooz, G. Dube, and N. L. Boling

THIN FILMS

A New Chalcogenide-Glass Antiref lection Coating for KCl 222

A. D. McLachlan and W. E. K. Gibbs

Intrinsic and Thermal Stress Modeling for Thin-Film Multilayers 230

A. M. Ledger and R. C. Bastien

Hydroxyl Influence and Refractive Index Dependence in Picosecond Thin-Film Damage 244

W. L. Smith, D. Milam, M. J. Weber, A. H. Guenther, J. R. Bettis, and R. A. House

Further Studies of the Role of Electric Field Strength in Laser Damage of Dielectric Layers . . . 251

J. H. Apfel

j The Effects of Barrier Layers and Surface Smoothness on 150-ps, 1.064-iJm Laser Damage

of AR Coatings on Glass 255

J. H. Apfel, E. A. Enemark, D. Milam, W. L. Smith, and M. J. Weber

f Use of Non-Quarter-Wave Designs to Increase the Damage Resistance of Reflectors at 532 and 1064 Nanometers 260

D. H. Gill, B. E. Newnam, J. McLeod

Characteristics of R. F. Sputter-Deposited Zinc Selenide Films on Calcium Fluoride 271

D. A. Walsh and R. V. Bertke

A Study of Infrared Absorption in Zinc Selenide Thin Films 276

D. F. O'Brien

The Design and Operation of a Precise, High Sensitivity Adiabatic Laser Calorimeter for

Window and Mirror Material Evaluation 281

D. L. Decker and P. A. Temple

Modulated Light Ellipsometer Measurements of Strain-Induced Anisotropy in the Refractive

Index of ks^Se^ and As^S^ Films on KCl Substrates at 10.6 ym 286

M. E. Pedinoff, M. Braunstein, and 0. M. Stafsudd

Damage Resistance of AR-Coated Germanium Surfaces for Nanosecond CO Laser Pulses 298

B. E. Newman and D. H. Gill

10.6 ym Laser Damage in Coatings Containing As^S^ and As^Se^ 316

C. C. Tang, M. Bass, M. J. Soileau, and E. W. Van Stryland

Large-Spot DF Laser Damage of Dielectric-Enhanced Mirrors 325

D. B. Nichols, R. B. Hall, and R. A. House, II

Defects and Impurities in As^S^, As^Se^, and NaF Coatings 342

T. M. Donovan, A. D. Baer, J. H. Dancy, and J. 0. Porteus

Optical Constants of As S By a Wedged-Film Technique 352

D. L. Burdick

i1

vi
/

Dielectric Coatings for Deformable Mirrors 358

J. M. Rowe

Quantitative Characterization of Coatings: Part I: Adhesion 363

S. R. Scheele and J. W. Bergstrom

Quantitative Characterization of Coatings: Part II: Abrasion Resistance 378

R. A. West and C. W. Nichols

Dielectric Enhanced Mirrors for the 2-6 ym Region 385

S. J. Holmes

LASER GLASS AND GLASS LASERS

Temperature and Concentration Dependence of Laser Bulk Damage to Neodymium Glass 399

J. G. Sliney, Jr., and L. G. DeShazer

Direct Measurement of Inversion Density in Silicate and Phosphate Laser Glass AlO

J. A. Abate, D. C. Brown, C. Cromer, S. D. Jacobs, J. Kelly, J. Rinefierd

Figures of Merit and Correlations of Physical and Optical Properties in Laser Glasses 416

D. C. Brown, S. D. Jacobs, J. A. Abate, 0. Lewis, J. Rinefierd

Progress in Claddings for Laser Glasses 434

R. B. Bennett, K. R. Shillito, and G. J. Linford

Damage History of Argus, A 4TW Nd Laser System 440

I. F. Stowers and H. G. Patton

FUNDAMENTAL MECHANISMS

New Results on Avalanche Ionization as a Laser Damage Mechanism in Transparent Solids 455

A. A. Manenkov

Multiphoton Theory of Optical Breakdown in Alkali Halides 465

A. Schmid, P. Kelly, and P. Braunlich

The Role of Carrier Diffusion in Laser Damage of Semiconductor Materials 473

M. Kruer, L. Esterowitz, F. Bartoli, and R. Allen

Multiphoton-Induced Conductivity in Ultraviolet-Transmitting Materials

1. Free Carrier Lifetime 481

R. T. Williams, P. H. Klein, and C. L. Marquardt

Absolute Two-Photon Absorption Coefficients at 355 and 266 nm 489

P. Liu, W. L. Smith, H. Lotem, J. H. Bechtel, N. Bloembergen, and R. S. Adhav

Statistical Considerations in Laser Damage Measurements 496

J. A. Detrio and A. P. Berens

Multithreshold Measurement and Analysis of Pulsed Laser Damage on Optical Surfaces 507

J. 0. Porceus, J. L. Jernigan, and W. N. Faith

APPENDIX I

Participants 517

vii
»

Laser Induced Damage in Optical Materials

9th ASTM Symposium

October 4-6, 1977

The Ninth Annual Symposium on Optical Materials for High Power Lasers (Boulder Damage

Symposium) was held at the National Bureau of Standards in Boulder, Colorado, from

4 to 6 October 1977. The Symposium was held under the auspices of ASTM Committee F-1,

Subcommittee on Laser Standards, with the joint sponsorship of NBS, the Defense Advanced

Research Project Agency, the Department of Energy (formerly ERDA) , and the Office of

Naval Research. About 185 scientists attended the Symposium, including representatives

of the United Kingdom, France, Canada, Australia, Union of South Africa, and the Soviet

Union. The Symposium was divided into sessions concerning Laser Windows and Materials,

Mirrors and Surfaces, Thin Films, Laser Glass and Glass Lasers, and Fundamental Mechanisms.

As in previous years, the emphasis of the papers presented at the Symposium was directed

toward new frontiers and new developments. Particular emphasis was given to materials for use from 10.6 pm to the UV region. Highlights included surface characterization, thin

film-substrate boundaries, and advances in fundamental laser-matter threshold interactions

and mechanisms. The scaling of damage thresholds with pulse duration, focal area, and

wavelength were also discussed. Alexander J. Glass of Lawrence Livermore Laboratory and

Arthur H. Guenther of the Air Force Weapons Laboratory were co-chairpersons of the

Symposium. The Tenth Annual Symposium is scheduled for 12-14 September 1978 at the

National Bureau of Standards, Boulder, Colorado.

Key words: Laser damage; laser interaction; optical components; optical fabrication;

optical materials and properties; thin film coatings.

1. Introduction

The ninth annual Symposium on Optical Materials for High Power Lasers (Boulder Damage Symposium)

was held, as in previous years, at the National Bureau of Standards in Boulder, Colorado, from 4 October

to 6 October 1977. The Symposium was held under the auspices of the ASTM Committee F-1, Subcommittee

on Laser Standards, with the joint sponsorship of NBS, the Defense Advanced Research Projects Agency,

the Energy Research and Development Administration and the Office of Naval Research. This year's

meeting coordinated the Damage Symposium with the DARPA Laser Window Conference. The Window Conference

was held on Monday, 30 October, with the majority of the attendees remaining in Boulder for the entire

week. Working sessions of the Committee F-1 Subcommittee on Lasers were held on Thursday afternoon

and Friday 6-7 October. About 185 scientists attended the symposium, including representatives of the

United Kingdom, France, Canada, Australia, the Union of South Africa, and the Soviet Union. The

symposium was divided into sessions concerning infrared window materials, mirrors and surfaces, thin

films, laser glass and glass lasers, and fundamental mechanisms. As initiated last year at the eighth

symposium, two poster sessions were held. ' The general consensus of both those presenting poster papers

and those viewing them was highly favorable. In all, over fifty technical presentations were made.

Alexander J. Glass of Lawrence Livermore Laboratory and Arthur H. Guenther of the Air Force Weapons

Laboratory were co-chairpersons of the sjrmposium. The tenth symposium is scheduled for 12-14 September,

1978, at the National Bureau of Standards, Boulder, At the tenth Annual Sjrmposium we expect to pause

and reflect upon the past 10 years' activities. It is our intent to have distinguished speakers who are acknowledged principal contributors in the many facets of the laser damage field. We will ask them to summarize and quantify as well as possible the state of the art and the state of the understanding in

their specialty. These review or tutorial lectures will then be assembled and published as a separate

volume on the subject of laser induced damage in optical materials. We are also contemplating having

round-table discussions on those specific subjects in which there is still some question as to the

correctness of our understanding. The purpose of these Symposia is to exchange information about

optical materials for high power lasers. The authors will welcome comment and criticism from all , interested readers relevant to this purpose and particularly relative to our plans for the 10th Annual

Sjrmposium.

2. Principal Conclusions

The identification and development of good window materials for the infrared has been a major

activity of the optical materials community over the past few years. Under Defense ARPA sponsorship,

extensive research has been carried out to identify and eliminate residual impurities, which lead to deleterious absorption in the infrared. Certain conclusions can now be drawn from this activity. First,

it is very difficult to reduce bulk absorption much below 10 ^ cm anywhere between 10 ym and 1 pm. In the vicinity of 3.8 ym and 5.5 ym, regions occur where few impurities exhibit strong absorption, and ,

viii
at 2.7 um the presence of OH absorption makes low absorption less probable, but overall, impurity

-4 -1 absorption quite uniformly limits window performance. In short, bulk absorption of 10 cm is about

what systems designers will have to accept. Even this level of impurities is obtained only by an intensive effort at keeping starting materials pure, and maintaining a contamination-free environment during

the entire process of material manufacture and component fabrication. Surface contamination is especially

hard to prevent, since very thin layers of adsorbed or other materials, especially organics, are

easily formed, and can contribute significant absorption. The picture is further complicated by

synergistic effects, such as the accelerated diffusion of one impurity species into the material due

to the presence of a second species. This migration can occur from the surface, or from aggregates in

the bulk, or grain boundaries in polycrystalline materials.

Even if ultrapure materials are available, and ultra-clean fabrication techniques are developed

for components, this may not solve the problem. First of all, the cost of fabrication must be kept

at a reasonable level, and second, even if a component is clean and pure to start, it may not remain

so for long in the environment of the operating system. It is clearly unproductive to design a system around a level of impurities which cannot be maintained in practice.

A variety of diagnostic tools are available for identification of surface contaminants. Both

attenuated total reflection (ATR) and x-ray photoelectron spectroscopy (XPS, or ESCA) studies were

reported at the conference as means of identifying impurities adsorbed on window surfaces. Combined

with ion milling, and other techniques such as Auger spectroscopy, these afford powerful means for

surface diagnosis.

As interest turns to shorter wavelength operation, in the visible and uv, the picture, if anything,

grows bleaker. Impurity absorptions in the electronic spectrum are broader than in the IR, oscillator

strengths are larger, and combined effects can become overriding. Current measurements indicate that

for materials in which two-photon absorption is energetically allowed, i.e., for which the gap energy

-2 -3

is less than twice the photon energy, absorption coefficients of 10 to 10 cm/MW are expected. Thus,

2 -1 -1 even at 10-100 MW/cm , effective bulk absorption of ^10 cm is expected, far more than can be

tolerated. Note that some materials, oxides in particular, seem to exhibit two-photon absorption

coefficients down by an order of magnitude from these values, even when energetically allowed. Nevertheless, it is unlikely that high-power lasers can be operated with window materials in which two- photon absorption takes place.

An example of the unpleasant surprises which await the laser builder in the uv, is the observation

of a cumulative damage mechanism in CaF^ at 360 nm. Due possibly to color center formation, the

absorption in fluorite materials increases linearly with prolonged exposure, reaching a value of

-2 -1 2

2 X 10 cm after an exposure of 700 KJ/cm . Further elucidation of this effect is clearly needed.

Not all the conclusions drawn from this year's Symposium are negative, however. It is possible

to. eliminate stress-induced birefringence, by proper choice and orientation of the window material.

More and more low absorption materials are becoming available, most of them useful both in the IR and

uv. The development of uv window materials has clearly been accelerated by the infrared window program.

Many wide-gap materials are now available for uv windows, in ultrapure form, due to the infrared window

effort. These materials should be used as the point of departure for uv window studies. The NBS measurement program, along with measurements carried out in other laboratories, has led to the accumulation of a valuable data base on optical materials, and the development of precise measurement

techniques

.

The very complex nature of the thin film coating of optical elements has caused this area to be

the least understood as far as laser induced damage in optical materials is concerned. This is due

in large measure to the numerous variables that govern the optical properties and performance of thin

films. This area of investigation is characterized by the almost impossible task of attempting to isolate one factor while keeping the multitude of others constant, as well as by control of specific

laser parameters used in the damage testing. However, recent work has led to several useful design

tools, e.g., reduction of electric field strengths within film multilayers for pulsed laser damage

resistance, reduction of coating absorption for reducing thermal distortion in high average power

applications, etc. Numerous proposed figures of merit and rules of thumb have been proposed, e.g.,

jise of low index materials in short pulse applications and the use of low thermal coefficients

of expansion in mirror substrates for cw applications. However, there are undoubtedly synergistic

effects taking place of which we are unaware. The only solution to this problem would be a massive

undertaking that would entail the correlation of fabrication and environmental variables with

resultant film structures, and a further correlation of these structural characteristics with optical

properties prior to laser damage testing.

ix
It is noteworthy that the approach being followed today contains not only well thought out parametric analyses and component testing, but the beginnings of an understanding of the correlations

of optical material, fabrication variables, film structure, optical properties, and damage threshold,

and from this we are encouraged. Thus the principal conclusions in the thin film area still comprise

a potpourri of apparently disconnected findings. However, the great virtue of this Symposium is the

beneficial synergism derived from the mutual exchange among workers in a very narrow and highly

specialized field of endeavor.

One area which has received considerable attention is that of optical surfaces. Obviously it is

desirable to understand the response of bare optical surfaces prior to embarking on an assessment of

coated surfaces. At this year's meeting, tutorial presentations relating to the topographical characterization of surfaces were presented, particularly as relates to light scattering and absorption

mechanisms. One important area discussed was the potential role of dipole scattering from isolated

particles in the infrared, and its possible relation to absorption. Additional papers dealt with the

topography of optical surfaces treated by difficult procedures including ion polishing, acid etching,

bowl-feed and conventional polishing, flame polishing and overcoating (e.g., a film of Si02 deposited

on polished fused silica) . It was seen that those techniques which tended to eliminate defects or absorbing impurities such as ion or flame polishing and acid etching resulted in higher damage

thresholds. This increased damage resistance was less dependent, in these cases, on final surface

roughness. An additional finding was that material properties such as surface roughness or substrate

OH ion concentration, with certain types of deposited films, which strongly affect the damage

resistance at moderate pulse lengths of 40 nsec, are not very influencial at the short pulse lengths

of 150 psec. This probably relates to the fact that damage thresholds at ultra-short pulse lengths

are deterministic compared to the more probablistic processes governing long pulse length damage

thresholds. However, in many cases, the proposed t pulse length scaling of damage fluence (area/

energy density) was reported over pulse lengths from psec to nsec in the near and mid infrared region

of the spectrum.

Several papers concentrated on inter-film and film/substrate boundaries as potential regions of

importance for some coated elements. The OH ion concentration of fused silica substrates was seen

to have a relatively minor effect on the damage resistance of Zr02 , ThF^, KglP^^ and Si02 coatings on

fused silica substrates. The importance of maintaining a background environment rich in selenium to achieve proper stoichiometric films of ZnSe was identified. An additional area, somewhat related, concerns the adsorption of water layers on certain materials. The technique of attenuated total

reflection spectroscopy (ATR) has been shown to be extremely valuable in these studies. Conclusions

as to the nature of chemi-sorbed and physi-sorbed layers and their structure and optical properties

are readily drawn. The ATR technique will undoubtedly see a continued increase in utility as one moves from the relatively pristine environment of the laboratory to the vagaries of the real world.

As equally important a concern, as damage resistance, is environmental durability. Several papers

treated not only the environmental degradation of optical thin films but the development of quantitative

methods for the measurement of abrasion resistance and adhesion of thin film structures. Of course, environmental durability assessment must be accomplished with the intended application firmly in mind.

In some case, gross film modification can take place after exposure to the atmosphere. It was reported

that some optical films containing arsenic undergo a phase modification leading to the growth of

arsenic-rich absorbing crystallites after a period of atmospheric exposure. This is obviously a very

important observation, yet unknown in detail as to mechanism. The behavior and response of films

containing arsenic is quite perplexing. One sees crystallite growth, non-stochiometric behavior and

apparently dissimilar performance of the related materials ^s^S^ and As2Se2. However, it was stated

that As^S^ and As2Se2 films generally fail due to a thermal process, not directly related to the local

electric field strength, with As^Se^ being a much worse performer than As^S^. A somewhat similar

behavior was noted for coated and uncoated germanium saturable absorbers where the pulsed damage level was directly related to a thermal mechanism. Here the culprit was identified as the substrate surface,

in that the damage threshold was similar for both coated and uncoated elements. However, to further

compound the picture, no relation to the structure and form of germanium was noted. Acid etching

caused slight ('^ 10%) improvement in the damage resistance.

One rapidly developing field which will place especially stringent durability requirements on

thin film coatings is the area of adaptive optics. Preliminary research efforts indicate that this

may not be as serious a problem as one might expect, considering the relatively fragile nature of thin

films compared to other optical elements. Cycled deformable coatings did not undergo appreciable

absorption changes or mechanical failure. However, these films were not subjected to damage tests.

Several contributions treated important features of surface cleaning, film deposition conditions

and resulting film structure. It was seen that sputter-etch cleaning is an excellent preparatory

procedure for many materials, but bias etching is even better (the maintenance of a dc potential

between the sample and sputtering source). The homogeneity and structure of ZnSe films can be greatly

improved not only by employing a Se-rich-background during deposition, as has been mentioned, but

through very high substrate temperatures and very slow deposition rates, less than 5 A/sec.

X
Additional presentations treated the utility of novel film designs for improving pulsed laser

damage resistance. These included the use of barrier layers in anti-reflection coating to shield

the high index layer, in this case TiO , from the polished surface, suppressing possible chemical

reactions. Advantageous electric field distributions were obtained by the use of nonquarterwave

multilayers. Anticipated improvements and damage tests were, in general, in agreement.

New film materials were discussed including the chalcogenide glass GATS (Ge^QAs^^Te^QSe^^)

proffered as an excellent high index film material at 10.6 ym. To these materials were added new

polishing procedures for producing low scatter damage resistant aspheric surfaces, a welcome

revelation

.

We must end our review on surfaces, mirrors and thin films by noting that several accurate and

high sensitivity measurement techniques were unveiled. These instrumental procedures dealt with the

development of improved calorimetric methods especially addressing the surface and film absorption

problem, the use of modulated light ellipsometry to determine the strain- induced birefringence in

films, and the facility of using wedged film techniques on witness plates to evaluate the optical

constants of optical film as a function of thickness.

Turning our attention to Nd:glass lasers, certain conclusions can be drawn. First, bulk damage

in laser glass does not limit system performance. Instead, it is the failure of multilayer dielectric

coatings used in polarizers, beam splitters, and antiref lection coatings, which sets the performance

limits for large glass laser systems. In subnanosecond pulses, self-focusing limits the peak power

density, while in long pulses, saturation limits the available fluence (areal energy density), but in

both regimes, coating damage generally sets the absolute performance limit.

The effect of glass composition on optical and laser properties is becoming clearer. More compositions of laser glass are now available, silicates, phosphates, f luorophosphates , and, in limited

quantity, beryllates. Thus one can choose between high and low gain, and high and low dispersion

and nonlinear index values. Furthermore, these properties, within glass families, correlate with

other parameters of the glass, such as thermal conductivity, density, stimulated emission cross-section,

etc

.

As experience is accumulated on large laser systems, certain effects are becoming manifest. This

year, the experience on the Argus laser at the Lawrence Livermore Laboratory was reported, including

the appearance of a film, possibly a lithium-nitrogen compound, on surfaces of ED-2 disks. The need

for cleanliness in systems with hundreds of surfaces was emphasized, and the growth of beam perturbations from small particles on the optical surfaces was documented. It is now accepted that optimal

placement of spatial filters throughout such a system is essential to peak performance.

Finally, there is still much to be learned regarding the ultimate failure mechanism of bulk

dielectrics, breakdown and ionization. Two points of view have emerged, one emphasizing multiphoton

processes, while the other views the breakdown process strictly as an electron avalanche. New experiments, such as those reported by Alexander Manenkov of the Lebedev Institute, showing damage thresholds

as a function of wavelength, band gap, and temperature are very important. In both the avalanche

picture and the multiphoton picture, phonon processes play an important role. The temperature dependence

of the breakdown threshold is sensitive to phonon processes, and may help to discriminate among competing

theories. The pulse length dependence of the damage threshold should be measured as well, for the

materials currently under study, since competing theories differ in their predictions of pulse length

dependence.

It is essential to carry out these investigations on ultrapure materials. Manenkov has proposed a methodology for eliminating impurity dominated damage results, one which warrants further investigation.

If his view is correct, it may be nearly impossible to understand surface damage on a first principles

basis, since it is surely always dominated by impurities and defects.

Detailed models of the effects of conduction electrons are emerging. These include effects like

defocusing of the incident beam by the incipient plasma, diffusion of conduction electrons, and con- centration dependent electron lifetimes. It appears as if the models previously proposed for optical

dielectric breakdown were too naive, in ignoring these effects.

Information regarding the behavior of conduction electrons in dielectrics prior to breakdown is

available from measurements like photoconductivity. In wide gap materials, two-photon induced photoconductivity is a useful technique to provide this information. Two-photon absorption, overall, is

emerging as more complicated than anticipated. The relation of the measured coefficients to the

structure and composition of the material remains to be elucidated, as does the density of states

near the band edge, accessible to two-photon transitions.

The importance of defining what is meant by a damage theshold cannot be emphasized often enough,

because people still report numbers without stating their definition. In most materials, several

thresholds exist, for different competing phenomena. The characterization of the respective thresholds

for these competing effects (multi threshold analysis), and how they vary with material preparation,

xi
provides a very complete picture of the material's properties, and is especially useful in surface

damage studies. However, the complexity and expense of this kind of measurement may preclude its use

as a general means of surface characterization.

3. Summary of Papers

The 1977 Symposium was divided into five sessions at which thirty-five papers were formally presented. These sessions were augmented by two increasingly popular poster sessions, each of three hours

duration, at which nineteen additional presentations were made. The general topical areas included

(1) Laser Windows and Materials, (2) Mirrors and Surfaces, (3) Thin Films, (4) Laser Glass and Glass

Lasers, and (5) Fundamental Mechanisms. In the following summary a concise statement of the principal

results of each paper is presented. Related individual papers are grouped together within each topical

area to aid in appreciation of their common thrust. Readers are directed to the topics of their

particular areas of interest, and at the same time, presented with an overall view of the principal

consensus of the Symposium. We have tried to provide a concise summary of the main fields of current

interest and the level of understanding of the overall subject of laser induced damage in optical

materials

.

3.1 Laser Windows and Materials

In any laser system designed to run at high average power, thermal damage to the components, due

to residual absorption in the bulk materials, or at the surface, is the primary materials limitation.

Window materials are chosen to operate in the region of high transparency, so it is absorption due to impurities and contaminants, rather than intrinsic causes, which is the problem. The identification

of possible sources of residual absorption, and ways to avoid these impurities, is a major concern of

the optical materials community. The papers presented in this section fall in three categories, bulk

absorption in the IR, surface absorption due to adsorbed contaminants, and bulk absorption in the

visible and uv. In addition, several papers relating to other bulk properties, such as birefringence,

dn/dT, and damage resistance, are included.

It is well established that bulk absorption coefficients, in the region of transparency, are totally

determined by impurity absorption. M. Flannery and M. Sparks, of Xonics, Inc., have identified and

tabulated a large number of absorbing impurities, which can contribute to bulk absorption in the infrared.

In particular, they have examined the properties of various anions in KCl, and their respective contributions to the residual absorption at 2.7, 3.8, 5.25 and 10.6 \im. The resulting picture is not encour-

-4 -1

aging, from the standpoint of reducing absorption below 10 cm

The authors find that many impurities can contribute to absorption at 10.6 pm, even at concentrations well below one ppm. However, at 5.5 ym and 3.8 wm, one would expect a lower residual absorption

due to fewer coincidences with absorption lines in commonly occuring impurities. At 2.7 ym, however, near coincidence with the absorption of the ever-present OH radical suggests that very few materials

will be free from residual absorption. Although generally, molecular impurity cations are excluded from

alkali halides due to the small size of the alkali cation, the authors do consider the case of NH^ in

KCl, and find it especially deleterious at 3.8 ym.

In an extension of this work, the authors tabulated 144 common molecular impurities, mostly

organics, which could account for the high observed values of surface absorption. The location of

the principal absorption bands of these impurities are identified. There are essentially no spectral

regions between 1 ym and 14 ym in which impurity induced surface absorption cannot take place.

Continuing the discussion of impurity absorption in window materials, W. Fredericks of Oregon

State University discussed some of the processes whereby impurities can migrate into crystalline

materials, and subsequently aggregate. Of particular interest was his discussion of the diffusion of

impurities. He pointed out that in general, impurity ions migrate in association with either charge

compensating defects or ions of the opposite charge. Diffusion rates can be significantly enhanced

by the simultaneous presence of ionic impurities of opposite sign. Thus, while mercury diffuses very

little into pure KCl, the diffusion is greatly enhanced in KCl containing an hydroxide impurity. The

2+ mercuric ion bound with OH is much more mobile than Hg . Other cations show similar effects.

Fredericks concluded that in crystals containing divalent cation impurities, water diffusion will

shorten the life of the material as a laser window.

Clustering and precipitation of impurities were also discussed. Evidence was presented from

experiments on KC1:B02 , showing that the spectrum exhibits features not only of the metaborate ion,

but of aggregates as well. Thus, referring to the preceding paper, we see that Sparks' tabulation

represents, if anthing, an optimistic view of impurity absorption in the infrared. Furthermore,

Fredericks points out that if grain boundaries are present, as in polycrystalline or forged materials,

impurity phases can concentrate along the boundary. From the previous discussion, these regions can

xii
be susceptible to anomalous transport of H^O, and other pernicious impurities. We thus see the com- plicated and synergistic effects of impurites and defects in nominally pure and transparent materials.

Having established that impurities lead to unwanted absorption in laser window materials, and

further, that synergistic effects cause the presence of one impurity to enhance the influence of

another, we turn our attention to the problem of eliminating impurities from the materials to be used.

W. Fredericks discussed the problem of purification of the starting materials for growth of KBr and

alkaline earth fluorides for windows. He discussed the use of ion exchange resins and selective ion

filters. In the latter, which is the preferred method, a series of resins is used, such that only the

desired ion will be passed through them. Selective ion filters can be constructed for cations, but

anion resins with differing order of selectivity for several ions do not exist. Other means for

separating anions must be used.

The author discussed the prevalence of common impurities in the laboratory and presents a tabulation of impurity levels in various materials used in laboratory apparatus. As one would expect, iron

is especially prevalent, but so are Cr, Co, Zn and Sb. Frederick discussed the care which must be

exercised in designing a purification apparatus, to minimize further contamination of the material

being prepared. The preparation of ultrapure KBr was described, and the claim made that the impurity

level in the final KBr is one ppm for monovalent impurities, and much less for aliovalent impurities.

The levels achievable in the system for alkaline earth fluorides have not yet been documented, but are expected to be satisfactorily low.

Among the materials of interest for high power laser windows in the infrared, the cubic perovskites

have many attractive features. These are mixed fluoride crystals with the formula ABF^, where A is an alkali metal ion, and B an alkaline earth or other divalent cation. The residual infrared absorption

of these materials in the region of transparency has been studied in depth by H. Lipson, A. Hordvik,

and B. Bendow, of the RADC Deputy for Electronic Technology at Hanscom Field, working in collaboration

with S. Mitra of the University of Rhode Island and J. Martin of Oklahoma State University. At OSU,

crystals of KMgF and KZnF have been prepared from purified starting materials. Absorption measurements

-1 were carried out from 800 to 2000 cm using a Fourier transform spectrophotometer, and compared at several CW laser frequencies to photoacoustic calorimeter measurements. Good agreement was obtained.

Both the frequency and temperature dependance of the absorption coefficient were measured, from 80°C

to 550°C. For MgF^ and KMgF^, good agreement was found with a theory based on intrinsic multiphonon

absorption, but the KZnF showed evidence of extrinsic absorption. At low levels of absorption

-4 -1

10 cm ), evidence of extrinsic absorption was also seen in the KMgF^. The absorption properties of

the perovskite KMgF^ are determined primarily by the Mg-F bond, which has a higher frequency than the

K-F band. Since this crystal is strong, transparent, and chemically stable, it may prove useful for

infrared coatings or as a laser window material.

In order to ascertain the homogeneity of optical absorption across a window specimen, T. J. Moravec

and E. Bernal G. of the Honewell Corporate Materials Science Center have constructed a scanning Doppler

interferometer, which is used to measure thermally induced index changes in a heated sample. Heating

is provided by a 350 W CO2 laser. The sample is placed in the interferometer beam on a motor-driven

translation stage, controlled by an HP 9825A calculator. The sample is scanned, with the heating beam

and interferometer beam always remaining coincident. From the recorded phase shifts, a map of absorption homogeneity is produced. In high loss materials, deviations of the order of 50% from the average

absorption are seen across the sample. For low- loss samples, deviations of 100% from the average are

seen. The absolute deviation for a given material is essentially the same from sample to sample.

Although not as accurate as calorimetry, this method is less sensitive to surface absorption. In fact,

as these authors have previously shown, by passing the heating beam through the sample at an angle to the interferometer beam, surface contributions can be entirely eliminated. The automated scanning

interferometer provides a useful technique for measuring homogeneity and sample-to-sample variation,

rather than absolute absorption.

At the National Bureau of Standards in Gaithersburg, the Optical Materials Characterization Program

has been established, with the purpose of providing accurate values of important optical parameters for

materials of interest in laser optics. A. Feldman, D. Horowitz, and R. M. Waxier presented a review of

recent results from this program, especially measurements of change in refractive index with temperature

and stress. Two methods have been developed to measure refractive index, and thereby, dn/dT. One is

to measure the index over a continuous range of frequencies using a precision spectrometer and minimum

deviation prism. The disadvantages of this method are that it measures the index relative to that of

air, and that the measurement can only be made over a limited temperature range. To circumvent these

difficulties, NBS has developed a second method, based on measuring the fringe shift in a Fizeau

interferometer induced by temperature change in the sample. This provides absolute values of dn/dT,

but requires a coherent light source, and so is limited to a discrete set of wavelengths in the infrared,

and requires a separate measurement of the thermal expansion coefficient.

xiii
Data were presented on dn/dT at 0.63, 1.15, and 10.6 ym, obtained by the Fizeau method for BaF2,

CaF2, KBr, KCl, LiF, NaF, SrF2, ZnS, and ZnSe. In a companion paper, M. Dodge of MBS compared dn/dT

values for ZnS obtained using the precision spectrometer method to the Fizeau values obtained by

Feldman et al. The agreement was satisfactory, considering the sample to sample variation in refractive

index in any material. Feldman et al. compared their values to those obtained by other researchers,

and found good agreement, once certain discrepancies in the assumed values of the linear expansion

coefficients, used in the analysis, were taken into account.

The continuing measurements program at NBS has resulted in the evaluation of precise and accurate

measurement techniques for a number of parameters of interest for laser optics, and yielded a body of

data on these materials of high reliability and great utility. As the interest in laser materials

extends to the visible and uv, it is essential that the measurement program continue to provide these

data, in the wavelength region of interest. The maxim, "What you can't measure, you can't understand,"

applies with undiminished force to optical materials.

Stress-induced birefringence is a serious problem in high power laser windows and in laser rods.

It was pointed out by R. E. Joiner, J. H. Marburger, and W. H. Steier, of the University of Southern

California, that under certain conditions, it is possible to find an orientation of a window with

respect to the crystal axes of the material such that plane-polarized light will not undergo any

depolarization. This situation requires that the major and minor axes of the polarization ellipsoid

in the material be uniformly oriented over the entire window. Since the stress field (or strain, in a rod) is not uniform over the aperture, one requires that the relevant coefficients vanish, so that the

depolarizing terms vanish independent of the values of the stress.

The authors show that provided the material satifies certain symmetry requirements, there is a critical angle with respect to the crystal axes at which the depolarization vanishes. This angle is

determined by the relation tan^ = -Ilk, where A = "^44/ ('5i]^~'5]^2

^ ''"^ stress-optic anisotropy ratio.

Here the q's are elements of the stress-optic tensor. Thus, in addition to symmetry requirements on the

window material, the ratio A must be negative at the operating wavelength. The authors show that this

condition is satisfied in KCl, CaF2, BaF2 and SrF2 in the visible, and KCl at 10.6 pm. Their analysis

has been confirmed experimentally. They point out that not only is this effect useful in eliminating

stress-induced depolarization in windows and rods, it also provides a sensitive measurement of the

photoelastic constants.

Pure alkali halides are excellent optical materials, but exhibit poor mechanical properties. A

standard method of strengthening KCl is by the addition of europium. A technique for optical monitoring

of the europium concentration was advanced by J. Wolfenstine and T. Stoebe, of the University of

Washington. They monitored the optical absorption of the sample at 2A3 nm and 343 nm, regions of strong

2+ absorption of Eu . In order to be sure that this was the dominant charge state for the europium ion,

they separately measured the absorption in the known absorption bands of the trivalent ion, in the

3+ visible. No absorption was detectable which could be attributed to Eu . The authors find that

2+

optical monitoring is a reliable test of europium concentration, as long as the Eu ion is the

prevalent form of the impurity. They conclude that, usually, europium is present in KCl in the form

of a divalent ion associated with a cation vacancy.

A comparison was made with ionic conductivity measurements, and with polarographic measurements of

the europium concentration. A linear relationship was found between the ionic conductivity and the

optical absorption. In three of the four samples tested, the optical absorption correlated well with

the polarographic analysis. The reason for discrepancy in the fourth sample was not clear.

The role of surface contaminants in laser damage is not fully understood. The presence of contaminants at the interface between substrate and coating materials is known to affect the damage threshold,

but the mechanism whereby the contaminant influences the damage level has not been identified. There is

strong interest in identifying surface contaminants, measuring their concentration and distribution,

and finding out the origin of the contamination.

An analysis of surface contamination in CaF2 was presented by T. N. Wittberg, J. R. Hoenigman, and

W. E. Moddeman of the University of Dayton Research Institute, and C. Strecker of the Air Force Materials

Laboratory. They examined CaF2 window samples, both uncoated, and coated with ZnSe, using x-ray photoelectron spectroscopy (XPS, or ESCA) as a diagnostic tool. Samples were polished with alumina, washed,

rinsed with distilled water and alcohol, and cleaned with vapors. Coatings were prepared by electron

—8

beam deposition. ESCA diagnostics were measured at a vacuum of 10

xiv
All samples showed significant oxygen and carbon contamination. Ion beam etching revealed that a high degree of carbon contamination was present at the surface on all samples, coated or uncoated. The

authors ascribe this to absorption of or other carbon compounds from the air. The possibility of

contamination of the surface during the cleaning process cannot be completely ruled out. Oxygen was

seen at the surface, and to a lesser degree, even in the bulk, by an inspection of freshly cleaved

surfaces. The authors propose that either the surface was quickly contaminated by atmospheric water

or that oxygen occurs in the bulk of CaF^ at a level of a few atomic percent.

Following upon last year's exposition of the great utility of Attenuated Total Reflection (ATR)

spectroscopy for the characterization of coatings, J. W. Gibson, R. T. Holm, and E. D. Palik of the

Naval Research Laboratory, used ATR to study the properties and structure of water adsorbed on CaF2

crystals. From this analysis the growth of adsorbed water layers was followed, affording information

about the index of refraction and the product of the extinction coefficient and film thickness. This

information was derived from a detailed analysis of the unpolarized absorption and the S/P polarization

ratio of the absorptance. It was concluded that the optical constants of the adsorbed water were significantly less than those for bulk water, again calling attention to the modification of films by

substrate surfaces.

The principal conclusions are that adsorbed water films are less dense than bulk water (p '^^ 0. 7 g/cm^)

and that absorption bands shift to slightly higher frequencies with some band narrowing. Other specifics

o o include an adsorbed water layer 4 to 6 A in thickness, consisting of a 1.7 A chemisorbed layer and

a thicker 4.6 A physisorbed layer, with a resulting background index of 1.21 for these water films on

CaF2. Discussions as to the possible effects of impurities in the films were included. This paper

continued to show the many attributes of careful ATR analysis.

There has been a paucity of experimental data on damage thresholds in the mid-IR. The availability

of HF and DF lasers make it possible to probe that region of the spectrum. E. Van Stryland, M. Bass,

M. Soileau and C. Tang of USC contributed a study of pulsed HF/DF laser damage in a number of transparent dielectrics. Their laser source operated with either gas, radiating at 2.7 ym or 3.8 ym in a Gaussian beam with a 22 mm diameter (full width at 1/e^). The temporal pulse shape was complex, but was carefully monitored. The equivalent pulse duration was about 170 ns.

The materials examined included NaCl, KCl, KBr, MgF2, BaF2, SrF2, CaF2, sapphire, spinel, quartz,

ZnSe, and AS2S2. Damage tests were carried out with various spot sizes, 27 pm, 38 pm, and 59 ym.

Damage was identified as intrinsic or extrinsic, on the basis of morphological investigation.

Both bulk and surface damage were investigated. Sapphire and the divalent fluorides exhibited

high damage thresholds in the bulk, on the oreder of 10-15 KJ/cm^ at the 27 um spot size, approximately

half that value at 59 ym spot size, in keeping with the scaling law previously proposed by Bettis et al. For the lower threshold materials, this scaling is not as apparent, but this may be due to the presence

of impurities.

The authors attempt to correlate their results with damage data on the same materials taken at other

wavelengths by other investigators. Unfortunately, they fail to apply the scaling in spot size and

pulse duration which their own measurements tend to bear out. Thus it is difficult to draw meaningful

conclusions from these comparisons.

As interest shifts from IR lasers to shorter wavelength operation, in the visible and uv, the

problem of window absorption grows more severe. Impurity absorptions are not as well localized in

frequency as they are in the IR, so that the tables of contaminants generated by Sparks and co-workers

are not as instructive as at longer wavelengths. As was borne out in this year's discussion, cumulative

and synergistic effects of impurity absorption are more severe in the uv.

Using both an argon ion laser calorimeter and wavelength modulation spectroscopy, J. A. Harrington,

B. L. Bobbs, and M. Braunstein of the Hughes Research Laboratories, and R. Braunstein, R. Y. Kim, and

R. Stearns of UCLA, measured the residual absorption in 11 large-gap, uv window materials, including

KCl, NaCl, NaF, LiF, LaF^, CaF2, SrF2, MgF2, Si02, AI2O2 and MgO. The absorption decreased monotonically

with increasing wavelength, from 250 to 515 nm, in almost every case. Some evidence of structure in the

absorption spectrum is seen by wavelength modulation spectroscopy. The values obtained for the best

materials, essentially those developed for the infrared window program, to have low IR absorption, show

absorption coefficients near 10~'^cm~''' in the visible. Higher absorption values are seen for forged

materials than for single crystal samples. For the best materials, the values observed are significantly lower than extrapolations from previous measurements, attesting to the higher purity of the

samples

.

XV
i ;

Two photon absorption effectively excludes the use of any high-power window material for which

the band gap is less than twice the photon energy. At wavelengths shorter than 300 nm, the availability

of optical materials is limited. Thus there is great interest in new, wide-gap materials for potential

uv applications. V. Rehn, D. L. Burdick, V. 0. Jones of the Michelson Laboratories reported the first

studies of the vacuum ultraviolet transmission, reflectance, and photo luminescence of LiYF^, a tetragonal crystal with a 10.4 eV absorption edge. This material is biref ringent , with refractive indices

close to that of CaF^, about 1.45 in the visible.

The measurements were made over the range 4 eV to 25 eV, using synchrotron radiation from the

Stanford Synchrotron Radiation Project. The material was not especially pure, and significant impurity

effects were observed near the uv edge. As purer material is available, the intrinsic properties of

LiYF, will become more accessible from measurements of this kind.

4

In a parallel study, the uv absorption of CaF^ was measured over the range from 390 nm to 230 nm. A strong absorption band at 305 nm was observed in two samples of diverse origins. This absorption was 3+ attributed to the presence of Ce ionic impurities in both samples.

Infrared window materials suffer from absorption in the bulk and birefringence. The .bulk absorp-

-4 -1

tion is typically less than 10 cm in good window materials, almost entirely due to residual impurities.

With Increased interest in short wavelength lasers, however, new phenomena must be considered in bulk

absorption. One such new phenomenon, not previously reported, was the subject of a paper by P. A. Temple

and D. L. Decker of the Michelson Laboratories. They reported that when fluorite windows (CaF2, SrF2,

MgF2) were irradiated at 350 nm, using an argon ion laser, with a power density of 350 W/cm^, the loss

coefficient was seen to increase linearly with the cumulative fluence, /idt, until it had reached a -2-1 5 2 value of 2 X 10 cm after 7 x 10 J/cm exposure. Before exposure, the materials absorbed less than

-3 -1

10 cm . The loss was identified as absorption, since it was measured calorimetrically , but could

have arisen from one of two causes, the formation of organic polymers on the surface, due to pump oil

contamination, or the formation of absorbing defects such as color centers in the bulk material.

Further work is required to elucidate the exact mechanism. The implication for high average power

3 2 lasers is severe. At 10 W/cm , a reasonable operating level, the windows would have to be replaced

5 2 after. an exposure of about 10 J/cm , an operating time of 100 sec. This is clearly unacceptable. It is noteworthy that a similar darkening of CaF. windows was reported in uv lasers by Tachisto, Inc., a

2 laser manufacturer, after 100 hours of operation at a peak power density of 5 nW/cm , which corresponded

5 2 to a cumulative exposure of 10 J/cm .

3.2 Mirrors and Surfaces

Surfaces still appear to be sensitive areas in which damage will first appear. Therefore, a complete understanding of the response of surfaces is necessary before one can feel comfortable in

appreciating all the many details of threshold interactions. In some cases, experimental and theoretical research has indicated some properties which play a dominant role in minimizing the deleterious

aspects of intense laser illumination, such as the desirability of low expansion mirror substrates to reduce thermal distortion in high average power lasers. It is, as well, generally conceded that

surface characteristics dominate many facets of the laser damage scenario for exposed surfaces of

reflective and transparent optical elements. However, a comprehensive understanding of the exact

relationship is not at hand. Part of this problem stems from the difficulty of experimentally characterizing the surface and isolating one particular feature to determine its exact role. Thus, several

papers at this year's meeting dealt with surface characterization (primarily topographical statistics) ' and attempts to correlate certain features with the damage sensitivity. To this were added contribu- ^ tions on surface finishing procedures and the efficacy of various techniques for the improvement of '' damage resistivity. Diamond turned elements were highlighted again this year.

In a rather elegant exposition J. M. Bennett and J. M. Elson of the Naval Weapons Center discussed

the salient features of surface statistics and how they relate to scattering behavior. Besides being

a very complex subject, it is most important for characterizing optical surfaces prior to either damage

testing or overall system performance. Subjects covered included the traditional properties such as total intergrated scattering, rms roughness, rms slope and rms height as well as the height, slope and

auto-covariance functions. Specific materials evaluated included fused silica, molybdenum, potassium

chloride, copper and silicon carbide.

i

I

i

t xvi j

I
Characterization included correlation of scattering behavior with large defects, isolated defects

and micro-irregularities corresponding to the cases where the wavelength of light is smaller than,

comparable to, and less than the dimensions of the defect. To this analysis was added the importance

of surf ace-plasmon excitation which can lead to second order light scattering. By far the greatest

contribution to scattering for high quality optical surfaces arises from microirregularities.

After a discussion of the above points, tests of scattering for typical materials were reported.

It was concluded that, for many materials, the total scattering in the infrared is dominated by dipole

scattering from isolated particulates as opposed to surface microirregularities. Concerning the

correlation between theory and experimental observation, it is frequently possible to predict the

shape of angular scattering curves using measured surface statistics. The auto-covariance function

is by far the most important property in describing an optical surface.

Two years ago at the 1975 Damage Symposium, a systematic study of the effects of surface roughness

and finishing techniques on laser induced damage thresholds were described. At this year's meeting

R. A. House, II, and A. H. Guenther of the Air Force Weapons Laboratory and J. M. Bennett of the Naval

Weapons Center reported on the surface roughness statistics of fused silica substrates as a function

of surface preparation and treatment. The samples studied were a representative selection of those

previously employed in the earlier damage studies. They reported on a detailed statistical and topographical analysis to more fully characterize the surfaces and search for any correlation which might

elucidate the underlying factors affecting the previous results.

The analysis included a determination of the mean slope, slope distribution function, mean height,

height distribution function, autocovariance function and rms surface roughness as determined by total

integrated scattering, Talystep prof ilometry, and Nomarski microscopy. Conclusions as to surface

character were as follows. For flame polished and conventionally polished samples, the height and

slope distribution functions were not Gaussian, since surface statistics were dominated by strongly

bound particles with a very smooth background. On the contrary, bowl-feed polished samples were

relatively free of particles, surface scratches, etc., characterized by a Gaussian slope distribution

function and a slightly skewed height distribution function.

For Xe-ion polished surfaces, the surfaces were characterized by nearly Gaussian statistics,

while HNO^ acid etched surfaces were, relatively speaking, very rough and contained many holes, up

to 1.5 ym in diameter and 1-2 ym deep. These artifacts skew otherwise normal distribution functions.

The last set of samples evaluated were Si02 overcoated surfaces. Here the surfaces were

extremely smooth (much smoother than the polished substrate) and they exhibited the most Gaussian

distributions of all of the surfaces studied.

The only obvious conclusion to be made at the present time, in this continuing study, is that

techniques which have a tendency to eliminate impurities, i.e., flame polishing and acid or ion

etching, lead to improved damage resistance, while those other techniques (compared to conventional

polishing) such as bowl feed or overcoating lead to reduced damage thresholds. Much additional work

is necessary to more fully describe this most complex state of affairs and allow us to theoretically

predict appropriate surface character/damage threshold relationships.

In a paper assessing the effects of surface roughness on laser induced damage threshold of bare

silica surfaces and coated optical elements for 150 ps pulses at 1064 nm, D. Milam, W. L. Smith, and

M. J. Weber of the Lawrence Livermore Laboratory together with A. H. Guenther, R. A. House and

J. R. Bettis concluded that although there was a general trend in the correlation, the effect was much less pronounced than previously observed when 40 ns pulses were utilized. Bare substrate

o o roughnesses ranged from 15 A to 350 A rms. At roughnesses comparable to normal optically polished

o surfaces (< 50 A rms) no positive effect was suggested by the experimental data. This effect "surface

roughness vs. damage threshold" as a function of pulse length should be pursued over a greater range of

sample types and roughnesses in which the sample topography is more fully characterized and accurately

determined. It is in this latter situation that the greatest error is introduced in scaling and correlation experiments such as these, i.e., inaccuracy in adequately describing the surfaces. For the case

of coated elements no correlation was evident at all. However, pulse length scaling between the 150 ps

and 40 ns did follow the previously proposed square root of pulse length dependence put forth by Bettis,

House, and Guenther at the 1976 Damage Symposium.

Aspheric surfaces have always been difficult optical elements to polish effectively since they do not readily permit the close fitting of lap and work surfaces normally required. To overcome this

deficiency W. P. Barnes, Jr., and R. R. McDonough of the Itek Corporation discussed the utility of

short term polishing with colloidal silica and non-woven poromeric (flexible) laps for producing high

quality (low scatter) aspheric surfaces. Test results on three f/2 paraboloids polished in this manner indicated the achievement of low scatter surfaces whose quality is nearly equal to the best

bowl-feed finished elements as determined by scattered light and damage threshold measurements.

xvii
Last year at this symposium the group from the General Electric Co., Ltd., outlined their program

on laser induced damage thresholds at 10.6 ym. This year S. Sharma, R. M. Wood, and R. C. C. Ward of

that organization reported on the mirror and window characterization portion of that program. Their

report specifically dealt with the development of accurate reflectance and transmittance apparatuses,

and the correlation of these measurements with absorption, surface finish and laser induced damage

thresholds employing a TEA laser emitting a 60 ns FWHM pulse focused to approximately 100 ym at the 1/e intensity points.

For diamond turned OFHC copper mirrors, a SEM examination of uncoated elements indicated a generally

o high quality surface which contained numerous small shallow spherical holes (approximately 500 A in

o diameter and 250 A deep). There was a strong indication of reduced damage threshold at these pin holes

compared to clean surfaces. Similarly, correlation of damage resistance with absorptivity, reflectance,

and scatter were evident. Other samples analyzed included gold coated OFHC and ThF, overcoated,

diamond-turned copper mirrors as well as ZnSe window specimens.

In an attempt to remove the high frequency ripple from diamond turned surfaces, R. E. Parks, R. E. Sumner, and R. E. Strittmatter of the University of Arizona investigated the utility of hand polishing

techniques employing laps of sufficient rigidity to bridge the micro ripple yet soft enough not to scratch the metal mirror surface. This technique can require that substantial material be removed, in

which case careful testing must be done throughout the polishing. Phase contrast micrographs show that

the polished surface is "almost" as free of mechanical defects as the original machined surface. This

polishing technique can lead to large reductions in surface scatter and roughness. Unfortunately,

previous reports by other individuals indicate that this polishing, while improving surface cosmetics

will generally lead to a considerable decrease in the damage resistance of such diamond turned metal

mirrors. No laser damage tests were performed during the course of this program to date.

In addition to the papers dealing with surface statistics and finishing, presentations on the

mechanical response of metallic reflectors and the utility of low expansion glass ceramic substrates

for reflectors were heard.

In their continuing investigation of the threshold response of metal and alloys surfaces to intense short duration (100 ns) 10.6 ym laser pulses, J. 0. Porteus, C. W. Fountain, J. L. Jernigen,

W. N. Faith and H. E. Bennett of the Naval Weapons Center reported on the stress induced aspects of

laser interactions. This interaction can produce many material modifications. These many responses

include production of slip bands, intergranular slip melting, pit or crater production, ion emission,

light emission, and finally work function changes. Which one of these responses is the first to appear

as incident laser power is raised, is governed by many influences such as intrisic material properties,

impurities, processing variables in fabrication, environmental factors, handling, laser parameters,

etc. They reported that many of the metallurgical modifications such as slip were seen to arise

at laser powers less than those required for melting, and furthermore these types of responses may be

significant in evaluating total system performance. Structural alloys were seen to have comparatively

low damage thresholds.

Results of testing specially prepared surfaces Indicate that diamond turning and ion-implantation

are generally ineffective against slip, but sputtering, ion milling and forging may be effective.

However, these later treatments may lead to increased sensitivity to other types of damage. Most

noteworthy was a study of the damage threshold for copper tested in a 1 atm N2 environment and under

vacuum. A substantial increase in the threshold for melting and other catastrophic effects due to increased shielding was evidenced when tested in the N2 atmosphere. However at lower energy densities

where slip first occurs, no plasma was detected and damage thresholds were comparable in the two cases.

It has long been recognized that a most important characteristic of high power laser mirrors should

be low expansion. P. Pirooz, G. Dube, and N. L. Boling of Owens-Illinois evaluated a unique concept

for producing low expansion IR laser mirrors. The technique studied involved the self-generation of

thin copper films by diffusion from the bulk on "very" low expansion glass-ceramic (Cervit) substrates

previously doped with copper. Under these conditions, excellent adhesion was achieved for the

Li20-Al202-Si02 system but poor for the ZnO-Al202-Si02 system. If one assumes that all candidate

mirror materials can achieve approximately the same reflectivity, then a figure of merit for minimizing

thermal distortion is FM = pC/a where p = material density, C is the heat capacity, and o the thermal

expansion coefficient. On this basis, with copper normalized to 1; SiC = 2.5; Mo = 2.2 while

Cervit > 6.5. This advantage stems almost totally from the glass-ceramics low coefficient of thermal

expansion. There is unfortunately the question of figure stability during the various fabrication

steps remaining to be addressed.

xviii
3.3 Thin Films

As might be expected, the largest number of papers presented at this years damage symposium were

concerned with thin films, as they represent the weakest optical feature in any optimally designed high

power laser system. In point of fact, they can be considered the prime constraint to improved system

performance. While optical surfaces are weaker than within the bulk of a material, the addition of

coatings for anti-reflection, high reflection or environmental protection has in general caused a reduction in the laser induced damage resistance of the element. The papers dealing with thin films

at this years symposium can be loosely grouped into the following categories; new film materials and

designs, including analysis and experiments dealing with various parameters which might affect the

damage sensitivity; methods of film characterization and property determination; recent damage

investigations; and finally, the mechanical response and behavior of coatings as affects their

environmental durability.

The first group of papers are concerned with the development of new coating materials for use in

the infrared, thermal stress modeling of film structures, film/substrate chemistry, and the influence

of the electric field distribution on the pulsed laser induced damage threshold for specific and unique

multilayer designs.

A new chalcogenide-glass coating material, suitable as the high index material in two-layer antireflection (AR) coatings on KCl for use at 10.6 ym, was described by A. D. McLachlan and W. E. K. Gibbs

of the Materials Research Laboratories, Melbourne, Australia. The new material GATS (Ge^g As^^ ''"^30

5^22^ ^ refractive index of about 3.11 at 10.6 ym. In the course of their study, the compositional

homogeneity, structure, refractive index, optical dispersion, absorption and pulsed/cw damage threshold

of GATS films were determined. AR coatings employing GATS and AS2S2 (low index material) exhibited

absorptions ranging from 0.02% to 0.07% at 10.6 ym and reflectivities as low as 0.025%. These films

were electron beam deposited at a base pressure of 0.1 mPa (1x10 ^ torr).

Employing a TEA laser, operating in the TEM^^ mode, and characterized by a 0.2 ysec gain switched

spike followed by a 3 ysec decaying nitrogen tail, focused to a 1/e^ diameter of 280 ym, the pulsed

2 2 damage threshold was ^ 7J/cm . For the cw case, the laser was focused to an area 3 mm square (9 mm ) 2 for three seconds. Failure started to occur at 7 KW/cm . The obvious advantage of these coatings are

their very low absorptance, offering a distinct benefit when one is concerned with minimizing thermal

distortion.

It is well established that one of the many parameters that affect not only laser induced pulsed

damage resistance but the durability of thin films, as well, is the film's residual stress structure.

On this subject, A. M. Ledger and R. C. Bastien of the Perkin-Elmer Corporation examined several models

of both intrinsic and thermal stress in thin-film multilayer stacks. Such theoretical models are widely

employed for stress compensation in the design of multilayers. To perform these analyses several

important film properties must be evaluated. To this end, a system of -in i-Ltu cat's eye stress interferometers have been developed to measure the intrinsic stress, thermal expansion coefficient and

Youngs modulus, the thin film properties of prime importance. Material in thin film form exhibits

properties considerably different than those reported for the bulk material. In point of fact, thin

film stress levels often exceed the self-breaking strength of the material in bulk form. The resultant

stress is comprised of the intrinsic stress of the material, together with the stress contributed by

the difference in thermal expansion coefficients of the film and substrate. This latter factor is of

obvious importance considering the thermal cycling which occurs during deposition and cool-down. In many cases to obtain films of high quality they must be deposited at elevated substrate temperatures.

To more fully describe the stress scenario, one should include the effects of adhesive forces between

the individual layers and substrate and other factors.

After a discussion of the analytical models and experimental techniques, detailed measurements of

the stress levels in ThF^, ZnSe, and Til materials as a function of deposition conditions (rate, tem- perature, etc.) were given. The unit stress a(t) was found to fit to the following equation:

a(t) = Ae"^" (1 - e"")

where t is the film thickness.

Another paper addressing the differences between the short ("^ 150 ps) and long ('v 40 ns) pulse

duration response of coated elements was performed by W. L. Smith, D. Milam, and M. J. Weber of the

Lawrence Livermore Laboratory and A. H. Guenther, J. R. Bettis, and R. A. House, of the Air Force

Weapons Laboratory. This work attempted to quantify the role of the OH ion content of commercially

available fused silica substrates on the damage resistance of fluoride and oxide coatings to 1064 nm, 150 ps Irradiations. Four substrate materials were employed, being Suprasil-Wl, Optosil-2, Homosil,

and Suprasil-1 with OH concentrations of approximately 5, 130, 130, and 1200 ppm respectively.

xlx
Coatings Included A/2 (at 1.06 ym) layers of ZrO^, Si02, ThF^, and MgF2 . Bare uncoated substrates

were tested as well. Unlike the previous AO ns data by House, at al., there was no evident correlation between OH concentration and damage threshold, as might be expected from anomalous production of

SiO, in the case of Si02 films or MgO in the case of MgF^ films, etc., again pointing out the relative

insensitivity of short pulse experiments to parametric variations which are quite obvious at long pulse

durations. As discussed in the paper, this may indicate that at AO ns the presence of 0H~ (as well as surface roughness) may cause a weakening of the Si02 structure leading to an assistance in the liberation of charge carriers during the breakdown process. This could cause a bottlenecking effect during

the early phases of long duration breakdown studies. Converely, in the picosecond regime, the higher

field strengths required to provide faster ionization and sufficient incident flux for heating more effectively drive initial nonlinearities to provide numerous free carriers early in the pulse.

This indicates a highly deterministic behavior, due to the high fields required.

A second aspect of the study did corroborate the previously observed /F pulse length and refractive

index dependances observed at AO ns. A principal conclusion is that low index coatings (fluorides) are quite damage resistant at short pulse lengths, i.e., ThF^ and MgF2 were found to be twice as damage

resistant as ZrO_ with SiO„ 70% more damage resistant than ZrO_. For the substrates, Suprasil Wl

2 2 damaged at 12.7 ± 2.7 Joules/cm while the other fused silicas damaged between 7.2 and 9.8 J/cm . This

study also indicates that coating a surface reduces the damage threshold below the value observed for

the bare substrate.

In a companion paper J. H. Apfel of Optical Coating Laboratories investigated the role of electric

field strength in laser damage of dielectric layers. Films tested included a Si02/Ti02 quarter-wave

stack reflector, a four layer anti-reflection (AR) coating and a combination of the two. Tests were performed using a 180 ps, 106A nm pulsed laser. At damage threshold the field is highest for the V

reflector stack and lowest for the AR coating with the combination film in appropriate order compared I

to the reflector. Damage thresholds for the eleven layer quarter-wave reflector, four layer anti- I

reflection coating and combination AR and HR coating were 7.0 ± 0.5 J/cm^, 3.2 ± 0.3 J/cm^, and I

5.1 ± 0.5 J/cm^, respectively. The anomalously low threshold for the AR coating is attributed to ^ a vulnerability at the substrate-film interface. ^

In a continued effort to employ our increasing but still scant knowledge of those factors affecting

the damage sensitivity of thin film coatings, J. H. Apfel and E. A. Enemark of Optical Coating Laborator;

and 0. Milam, W. L. Smith, and M. J. Weber of the Lawrence Livermore Laboratory evaluated the utility

of barrier layers at the film substrate interface for both AR and HR coatings. The damage threshold

difference between these two cases is attributed in part to the coating substrate interface region

being illuminated for the AR case and protected by interference reflection when HR coated.

Experiments were performed using 150 ps, 106A nm laser on coatings of Si02 and Ti02. Conclusive

evidence was found that the introduction of a barrier layer of Si02 on BK-7 substrates improved the

damage resistivity, from zero to 90% depending on the materials used, film design, and thickness of

the barrier layer. As covered in another paper at this meeting, no conclusive evidence was found

for a surface roughness dependance on the damage threshold using 150 ps laser pulse lengths.

Through the use of non-quarter wave thin film designs D. H. Gill, B. E. Newnam, and J. McLeod of

the Los Alamos Scientific Laboratory studied the possible improvement in laser damage resistance of

reflectors at 532 and 106A nm. The non-quarter wave designs were employed to reduce the electric

field in the normally damage sensitive high index materials as well as move the field maxima from

film layer interfaces where possible. The results were quite positive for films supplied from

several vendors. Damage thresholds for Ti02/Si02 and Zr02/Si02 stacks tested with 21 ps, 532 nm,

and 30 ps, 1.06 nm exhibited improvements as high as 2.6 over their all quarter wave counterparts.

It was noted that these modified designs are quite sensitive to errors in coating thickness

but they were quick to point out that several vendors supplied non-quarter wave stacks with thickness errors of less than 1%. Appropriate equations governing the design and behavior of these novel films were presented.

As we have said before, if you can't measure it, you can't understand it. This applies doubly

to thin films, where methods to quantify the properties and behavior of thin films are needed, before

any real advancement can be made. In this regard, the next group of papers were concerned with the

characterization of ZnSe films from a structural and absorption standpoint, together with the

development of new measurement techniques such as modulated light ellipsometry and adiabatic

calorimetry

.

XX
A study of the optical, physical and chemical properties of R.F. bias-sputtered CVD ZnSe films on polycrystalline CaT^ as a function of sample preparation procedures and deposition parameters was reported

by D. A. Walsh and R. V. Bertke of the University of Dayton Research Institute. Optical measurements at 5.3 ym on coated polytran CaF^ subtrates indicate that the absorption of bias-sputtered films were con- sistently lower than when non-biasing techniques were employed. Rutherford backscatter analysis of the

ZnSe film indicates it was '^'11% rich in Zn. For the bias-sputtered samples, oxygen was found at the

f ilm/substrate interface, but its origin is not known. In {,-Cta sputter etching of the substrate surface

immediately before coating produces optimum film adhesion and lowest absorption. More detailed studies

of this type would be very beneficial.

It is well recognized that ZnSe is a most useful material in the infrared. Along this line

D. F. O'Brien of the Air Force Materials Laboratory reported on the infrared absorption of ZnSe thin

films as a function of deposition conditions. It had been previously reported that high absorption

in ZnSe film probably arose from a variation in stoichiometry . In this study ZnSe was deposited on

CaF under a variety of conditions. It was shown that the IR absorption decreased as the deposition

-1 ° -1 ° rate was lowered, from as high as 40 cm at 40 A/sec to < 5 cm at 1 A sec. Furthermore, deposition in a Se rich vapor environment increases the probability of producing a stoichiometric film

since high absorption films tend to exhibit a high Zn/Se ratio indicating an excess of Zn in the

film. Although not specifically demonstrated in this paper, it was pointed out that higher substrate

temperatures should also be beneficial in improving stoichiometry of ZnSe films. A final comment,

through the use of AES and ESCA profile measurements of the Zn and Se concentration, it was determined

that stoichiometry variations were found to exist as point to point differences in the Zn/Se ratio

and not as a uniform deviation throughout the film. Thus, ZnSe films tend to exhibit highly localized

absorption regions.

One area of continuing interest and utility is calorimetry as employed to evaluate new candidate

window and mirror materials or finished components. To this purpose, D. L. Decker and P. A. Temple

of the Naval Weapons Center reported on the design and operation of a precise, high resistivity

adiabatic laser calorimeter. Features of this vacuum calorimeter, normally employed for low-absorption

materials under low illumination levels, are given together with a detailed description of the theoretical equilibrium temperature response. The calorimeter exhibits a sensitivity of 10 ^ in

absorptance at laser illumination levels of 1 watt. The calorimeter is capable of absolute electrical

calibration. Major sources of systematic error arise from calibration of the external laser power

meter and uncontrolled sample scatter. Accuracy is thus limited to ± 5% for high quality low

scatter samples. By careful attention to the stray light problem this instrument should be suitable from

the near UV to the mid IR.

Another property of thin films, of which knowledge is important, concerns their anisotropy in

refractive index. Addressing the measurement of this feature, M. E. Pedenoff and M. Braunstein of

Hughes Research Laboratories and 0. H. Stafsudd of the University of California at Los Angeles presented

a most comprehensive discourse on the use of elasto-optical modulated light ellipsometer measurements

of strain induced refractive index anistropy in films of As2Se2 and As^S^ deposited on KCl substrates

at 10.6 pm. The instrument and computational produces developed by them allow one to estimate the

anisotropic change in refractive index due to a uniaxial stress applied to the substrate. Preliminary

measurements indicate anisotropics of 0.6% are generated in films of As2Se2 and ^s^S,^ deposited on KCl

by strains of 4 x 10 ^ induced in the substrate.

As they say, "the proof of the pudding is in the testing." One might add, it is a way to understanding and validate our theories, as well as assessing our ability to fabricate damage resistant

elements. Thus, several papers were concerned with the damage response of thin film coatings at several wavelengths in the infrared, with heavy emphasis on ks^S^ and As2Se2, and a characterization

of the optical constants of these materials.

An evaluation of the damage resistance of gallium-doped germanium saturable absorbers at 10.6 \m

was reported by B. E. Newnam and D. H. Gill of the Los Alamos Scientific Laboratory. Employing laser

pulses of 1.2 ns FWHM they found several interesting results. Damage was observed for bare Ge substrates

2 (reflectivity ^ 1% at 10.6 ym) at a threshold ranging from 0.55 to 0.70 J/cm and for anti-reflection

2 (AR) coated surfaces at 0.49 ± 0.3 J/cm employing fourteen coating designs using eight film materials

from nine coating manufacturers. In all cases, only front surface damage for these 6 mm thick samples

were observed, with the damage limited by the germanium surface properties. An HNO^ acid treatment

raised the damage threshold of bare Ge about 10%. Multi-shot conditioning (n on 1) produced the same threshold as single shot (1 on 1) tests. There was no appreciable difference in damage sensitivity for

samples of varied doping or crystallinity (single crystal, p-doped and undoped, and undoped polycrystalline) for three different conventionally polished substrates. In an additional experiment per2 1, formed at a pulse width of 70 nsec a pulse width dependence Ejj(J/cm ) , t was observed, in agreement

with previously proposed scaling relationships.

xxi
Pulsed laser damage testing of As^S^ and As^Se^ coatings at 1.6 pm was the subject of a paper by

C. C. Tang, M. Bass, M. J. Soileau and E. W. VanStryland of the University of Southern California. In particular the role of mode locked pulses on the damage threshold and process was assessed. For AS2S2 coatings, damage was noted only concurrently with a visible spark. However, for As2Se2 coatings,

two distinct thresholds were found. In this case the sparking threshold occurred for large diameter

damage sites while at an intensity level , 30% lower, small discreet damage sites were noted in the

absence of any spark or incandescence. For As^S^/KCl/As^S^ anti-reflection (AR) coatings on KCl,

substrate damage was , 430 MW/cm^ and was independent of the presence of a mode locked train of pulses.

2 By comparison three layer AR coatings containing As2Se2 damaged at ^ 20 MW/cm . The independence of

damage thresholds of these two materials on coating designs strongly indicates that damage is not related

to field strength nearly as much as the energy absorbed/unit mass i.e., a predominantly thermal failure

mechanism. An examination of the morphology of the damage sites indicates that damage is initiated at either microscopic defects or absorbing inclusions.

The growing interest in high power chemical lasers pointed the way for a study of dielectric

coated mirrors in the 2-5 urn region by D. B. Nichols and R. B. Hall of the Boeing Aerospace Company and

R. A. House II of the Air Force Weapons Laboratory. Using a photo-initiated HF/DF laser emitting a FWHM pulse of 4.0 ysec, they tested two substrate materials (molybderum and Cer-Vit) and five coating

3 1 designs deposited by Coherent Radiation and Northrop Corporation [Cr + (A1„0 /Si) ; Cr + Ag + (SiO/Si );

1 3 3 Cr + Au + (SiO/Si) ; Ag + (ThF /ZnS) ; and Ag + (ThF /ZnSe) ] which exhibited reflectivities between

0.9792 and 0.9978.

2 2 Irradiations began at exposures less than 4J/cm (1 MW/cm ) and proceeded at the same site with a nominal increase of 50 to 100% until small scale and then catastrophic damage was observed. Based on these tests the following conclusions were drawn: firstly, the two coatings having the lowest absorp3 3 tion, namely, Ag + (ThF^/ZnS) and Ag + (ThF^/ZnSe) were the most damage resistant, secondly, the

absolute value of damage thresholds for the best mirror coating are comparable to thresholds for formation of surface plasmas on bare metals, thirdly, there was no discernible threshold difference between

Cer-Vit or molybdenum substrates inferring that the thermal conductivity of the metal substrate gave no significant advantage for these single exposure experiments, which is not surprising considering these

are not cw experiments and finally, damage thresholds for these class of materials under these experimental conditions are not appreciably different for 1.0 and 0.1 cm beam spot diameters.

In a very revealing and important paper, T. M. Donovan, A. D. Baer, J. H. Dancy, and J. 0. Porteus

of the Naval Weapons Center presented results from a very detailed analysis of formation and characteristics of defects and impurities in As^S^, ks^Se^ and NaF coatings. It was noted that the damage

thresholds of chalcogenide glass thin films are less than for corresponding thin films of NaF even though

the absorption of NaF films is relatively high. Conclusions drawn from these two observations were explained by noticing that: first, chalcogenide glass films form vm size, arsenic rich crystallites

over a period of a few days after deposition (these are sulfides and selenides, and not oxides as previously reported), and secondly the high absorption of NaF films is probably due to a thin layer

of adsorbed water which rapidly forms upon exposure to the atmosphere. Attempts to eliminate these

problems by stabilizing the chalcogenide coating by depositing thin layers of Ag or NaF -in i'Ltu or employing water-free As^S^^ evaporant both failed. In the case of NaF, it was suggested that by

eliminating surface oxygen, film absorption approaching that of bulk NaF might be attained.

The disparity in absorption coefficients for materials in bulk and thin film form probably indicates

better than any other measure the vast difference between materials in these two forms. Both measurements can be difficult, e.g., very low absorption in bulk materials, versus very thin samples for films.

D. L. Burdick of the Naval Weapons Center has attacked the thin film side of this divergence in properties through the development of a novel wedged-film technique in which the film is deposited in a wedge

form (i.e., wedged in thickness). Through careful measurement of film properties as a function of

thickness on this one sample, he has been able to accurately measure optical constants of As„S. films

-1 4 -1

in the visible spectral region, including absorption coefficients ranging from 3 cm to 10 cm , and

refractive indices from 2.47 to 2.72 with precisions ranging from 0.3 to 1%. He pointed out a number

of advantages of this method over other methods of measuring optical constants of thin films.

The final group of papers dealing with thin films is best described as being related to the

environmental durability of coated elements. Included are papers on the performance of coatings on deformable mirrors, the degradation of coated elements subjected to real world environments of

humidity, dust and sand, temperature cycling and flexure.

xxii
Recognizing the impending advent of adaptive optic systems, J. M. Rowe of the Northrop Research

and Technology Center reported on dielectric coatings for deformable mirrors. In this study the

results of an experimental program to determine the optical and physical properties of high-reflectance

dielectric coatings subjected to as many as 10 deformation cycles was discussed. Films studied included

2 2 (SiO/Si) and (Al^O^/Si) designs for operation at 3.8 ym on top of silver coated mo substrates. Measurements included residual coating stress (taken immediately after deposition), spectral reflectance,

absorptance at 3.8 pm and Normarski interrogation. Investigations included so called "DC" cycling in

which deformation was of one sign only by use of a bias e.g., flat to convex and back to flat. Zero

to peak displacements of 10.9 pm at 10 Hz were employed leading to a peak stress of 9.9 x lO^Pa

comparable to the residual stress produced as a result of the deposition on these types of substrates.

No physical degradation (cracking, etc.) nor appreciable reflectance variation was observed. However,

in some cases, a slight variation in absorptance was noted. Additional work is underway on the study

of other candidate film systems, to evaluate adhesion and humidity properties as well.

It is only appropriate at an ASTM sponsored meeting that one discuss measurements and standards

along with the more fundamental interaction mechanisms. In this light S. R. Scheele and J. W. Bergstrom

of the Hughes Aircraft Company reported on the initial phase of a program dealing with the broad category

of quantitative characterization of the mechanical properties of coatings. The specific topic of their

presentation dealt with film adhesion testing. After a thorough literature search, survey and review

of existing test procedures, three specific mechanical pull-off tests were compared on a large number

of coated glass optical substrates as potential replacements for the current MIL-Spec tests. The tests included direct pull-off, a topple test and finally a peel test. Present tests are generally qualitative

and subjective, as well as not being well controlled, a generally undesirable situation.

A variety of coatings and substrates were utilized in comparing the various adhesion tests. An interesting generality was that it was difficult to remove coatings with good adherence to glass

substrates, but relatively easy to remove coatings from infrared substrates. Comparitively speaking,

the plug pull test was least desirable because of relatively poor repeatability and high forces

involved. The topple test gave better results and was less expensive; however, unexplained substrate

damage was frequently observed. While the peel test was relatively expensive to implement it required

the lowest force and had the advantage of affording highly localized sampling.

In a companion paper, R. A. West and C. W. Nichols also of the Hughes Aircraft Company treated

another aspect of coating characterization, abrasion resistance. In this instance the ASTM falling sand

test was modified to perform abrasion resistance tests in a controlled manner. Quantification was achieved by correlation with transmittance degradation. The final carefully evaluated test procedure

was correlated with other existing tests such as the military eraser and cheesecloth abrasion tests.

Abrasion resistance, like adhesion, is an important indicator of a film's durability when exposed

to a range of real world environments and a need for a quantitative test is readily identified. After

evaluation of many types of test specimens, the modified falling sand test was seen to be an easily

implemented, sensitive, repeatable and adequately quantitative test procedure. There was little

obvious correlation of the results of this test with those presently used, which are more subjective

and more poorly controlled.

Continuing their interest in the near-infrared, S. J. Homes reported on Northrop' s latest activities

in dielectric enhanced mirrors for use in the 2-6 ym region. Of major concern in this paper was a cataloging of performance characteristics such as reflectivity and absorption of a large variety of

dielectric multilayers for high reflectivity metallic mirrors (> 99.9% reflectivity at 2.8 \im, 3.8 pm

and 5.3 ym) together with a testing of their environmental durability. This study was performed to evaluate the reduced cooling requirements afforded by higher reflectivity mirrors, for high average

power applications, over uncoated thermally conducting substrates. At least ten different dielectric

enhanced mirror designs were evaluated at each of the three laser wavelengths. There were deposited

on conventionally and bowl feed polished (Al202)/Ag overcoated molybdenum substrates. Cleaning and

deposition parameters were covered in detail. The environmental evaluation included tests according to Mil-m-13508C for durability, adhesion, and humidity resistance. No laser damage testing was reported.

3.4 Laser Glass and Glass Lasers

With the continued emphasis on large, high power Nd:Glass lasers in the laser fusion laboratories

around the world, interest persists in the properties of Nd:Glass as a laser material. Obviously, the

damage threshold of laser glass is of importance, although in all large glass laser systems operating

today, the damage limit for the system is determined by failure of optical coatings, rather than bulk

failure in the glass itself. Other properties of laser glasses are also of importance, especially now

that several compositions with significantly different properties are available. The laser designer

must ask, which is the best glass for the system I want to design, given the system architecture and

desired operating parameters. These, in turn, must be related to composition.

xxiii
1

Adding another parameter to the growing list of relevant variables, J. G. Sliney, Jr., and

L. G. DeShazer of the University of Southern California have studied the bulk damage threshold of

various Nd-doped laser materials, both crystalline and glass. Samples were irradiated with a 10 ns pulse from a NdrYAG laser oscillator, amplified by a ND: glass rod amplifier. The beam was focused

with a 25.4 mm focal length lens which unfortunately was plano-convex. Accordingly, significant

spherical aberration was generated.

Nd-doped, ED-2 laser glass was tested at room temperature and at 110°K, at four concentrations of

Nd, 0, 1, 2, and 3%. At room temperature, some decrease in the damage threshold was noted with increasln;!

concentrations, while at reduced temperature, the threshold was 33% higher, and seemed less sensitive

to Nd concentration. It should be pointed out that due to uncertainties in this experiment, the reported

concentration dependence of damage is not well established. The increase in damage threshold with

lowered temperature is similar to that observed in NaCl by Manenkov.

Of the various host materials tested, LiYF^ and ED-2 glass showed the highest thresholds, 1.9 and

1.7 times that reported for YAG, while YVO and GGG showed the lowest, at about 0.6 times the value

2 for YAG. The computed power densities at which damage occured seem small, 7.6 GW/cm for NaCl, for

example, but the actual focal spot size and peak intensity are somewhat uncertain in these experiments.

Although not directly concerned with the damage properties of optical materials, two papers which

were presented by the University of Rochester Laboratory for Laser Energetics concerning the other

properties of laser glass were of great interest. J. A. Abate, D. C. Brown, C. Cromer, S. D. Jacobs,

J. Kelly, 0. Lewis, and J. Rinefierd contributed to the two papers.

The first concerned correlations of physical and optical properties of laser glasses and the statement and evaluation of figures of merit for laser materials. The authors pointed out that simply

considering the nonlinear index, n^, or even including the gain or cross-section for the glass did not

suffice to define the utility of a given glass in a laser system. They defined eight merit factors,

including the effects of nonlinear phase distortion, specific gain, heat dissipation, quantum efficiency, thermal diffusivity, heat capacity, thermo-optic distortion, and stress birefringence. In any system evaluation, these eight factors would have to be combined in some weighted sum, and cost

factors taken into account to obtain a true economic figure of merit. In the present work, these eight

merit factors were evaluated for five commercial laser glasses, ED-2 and four phosphates. The phosphatei

have higher cross-section and lower n^ values, as well as lower values of thermo-optic distortion and

stress birefringence, but poorer thermal diffusivity and heat capacity factors. Futhermore, phospate

glasses show a lower resistance to thermal shock. Other factors considered included the ease of manufacture and chemical stability, which can be improved by slight compositional changes.

The authors examined correlations among the properties of these glasses and f luorophosphates

to see if they can discover why some compositions show better thermal properties, and others better

optical properties. They observe that low index (and low index nonlinearity) glasses show higher

thermal expansion and less hardness. For phosphates and silicates, decreasing n^ correlates with a decreasing Young's modulus and decreasing thermal conductivity, however, this trend does not extend

to f luorophosphates

.

The difficulty in establishing these correlations seems to be the fact that differences in band

polarizabilities have been ignored. The optical properties of oxide glasses, whether silicate or phosphate, reside in the oxygen bonds. Non-bridging oxygens are more polarizable than bridging oxygens,

while certain network modifiers strengthen and densify the glass. Thus stronger, more compact glasses

tend to be more polarizable, with higher values of n and n2 . For fluorides, however, the fluoride

bond takes the place of the oxide, and a new set of correlations apply.

This paper does point up the necessity for examining all relevant factors in choosing a laser

glass for a system. Cost cannot be ignored, nor can surface damage in the system elements. The

isolation of the individual merit factors allows the glass user to emphasize those properties of

greatest importance for his application. Correlation of these desired properties with glass composition is then a task for the manufacturer.

The second paper by the same authors reported the measurement of inversion density in both silicate

and phosphate laser glass. The purpose of this measurement was to check the validity of assumptions

made in the computer code GENEFF, used at the Lawrence Livermore Laboratory to model the efficiency of

conversion of absorbed flashlamp light into population inversion.

The laser glass samples used were cut in the shape of a rectangular parallelepiped, with a

1 cm X 1 cm square base, and 16 cm length. Collimated flashlamp light was incident from one end. The

inversion density was monitored by recording the fluorescence as a function of distance along the side

of the sample. Nd concentration of the sample was determined by wet chemical and x-ray fluorescence

analyses

.

XXiv
The Argus laser at the Lawrence Livermore Laboratory has been in operation since late in 1975. The

system has been fired about 600 times, at power levels in excess of 4 TW in 0.1 ns pulses, and energy

levels up to 2 kJ in one ns. I. Stowers and H. Patton of the Lawrence Livermore Laboratory reported on

the experience accumulated on the Argus system in the identification and analysis of damage phenomena.

Three kinds of damage phenomena were noted. These can be described as surface damage due to foreign

particles, film growth on laser glass surfaces, and coating damage.

The surface damage arises from particles deposited on optical surfaces either by dislocation from

the surrounding surfaces or from the cooling nitrogen stream. Damage was seen to be more prevalent on upward facing surfaces, due to gravitational settling of particles, and on the first surface encountered

by the gas stream. No significant aging effect was seen. After ten to twenty firings, no subsequent

accumulation of particulate damage was seen, as long as the system was kept clean.

The phenomena of film growth on laser glass had not been extensively investigated previously. It was found that a cloudy film develops on ED-2 laser glass, even when it is not exposed to flashlamp or laser light. The film is found to consist of fibers of 2 to 5 ym diameter and up to 0.5 mm length,

containing Li and N in high concentration. Since ED-2 is rich in Li, this suggests that the film

evolves due to decomposition of the glass, perhaps under attack by a moisture-laden atmosphere. The

film can be removed by washing in ethanol.

Coating damage remains the major impediment to high power laser performance. Due to heavy use of

spatial filtering, the Argus system has not experienced severe coating damage, however. The spatial

filter lenses, especially on the input side, are most vulnerable to damage. Some evidence has been

seen of a greater tendency to damage on coatings in vacuum, possibly due to removal of absorbed

materials from the pores of the coating.

The most important measure for the prevention of damage in a large laser system is seen to be the

maintenance of cleanliness throughout the system. Coating damage can be held to an acceptable level

by maintenance of beam quality through spatial filtering.

3.5 Fundamental Mechanisms

The ultimate limit to the optical flux which an be transmitted through a transparent dielectric is

determined by the onset of optical breakdown. A plasma forms at the dielectric and quickly grows to a density sufficient to absorb the laser light strongly. The consequent heating of the lattice by the

plasma causes irreversible physical damage. This problem has been studied intensively for at least a decade. It is well known that at very short pulse durations, multiphoton ionization plays a major

role in promoting electrons to the conduction band, where they can be subsequently accelerated by the

optical field. It is also well known that at long pulse durations, where carrier lifetimes in the

conduction band are important, avalanche ionization can occur, with the electrons gaining energy by

successive collisions with the lattice, until they are sufficiently energetic to cause secondary

ionization. Two points of view have emerged, however, regarding the relative importance of multiphoton ionization and avalanche ionization in causing material breakdown. These two points of view

are represented by the first two papers in this section.

Alexander Manenkov of the Lebedev Physical Institute in Moscow reported on recent results obtained

in his laboratory on avalanche ionization in crystalline dielectrics. Based on a detailed investigation

of the quantum kinetic equation for the electric energy distribution, Manenkov derived the dependence of

the breakdown threshold on the various parameters of the experiment, including laser frequency, sample

temperature, ionization potential, and pulse duration. The predicted pulse duration dependence is very

weak, rather than scaling like x , but this is due to the absence of transport terms in the original

theoretical analysis.

Experiments were carried out for a large number of dielectric crystals with pulses of a few ns

in duration at 10.6 pm, 1.06 pm, 0.69 ym and 0.53 ym. The sample temperatures were varied from 100°K

to 800°K. The damage threshold was expected to decrease with increasing temperature at optical

frequencies well above the effective electron-phonon collision frequency, and to increase, but weakly,

with increasing temperature for optical frequencies below the collision frequency. For selected

samples of alkali halides, this dependence was observed from 1.06 ym to 0.69 ym, but no temperature

dependence was seen at 0.53 ym. This anomaly was attributed to multiphoton processes.

Manenkov tested hundreds of samples. He found that only those with high damage thresholds

exhibited consistent dependence on temperature and frequency. The low-threshold damage was attributed

to impurities. The threshold in these materials could be raised by heating the materials, which

Manenkov felt was due to dispersal of absorbing aggregates. The thresholds observed in the highthreshold materials were significantly higher than those reported elsewhere.

XXV
Although any theoretical analysis of damage is limited, due to the difficulty of treating all

possible effects simultaneously, this paper put forward a concrete model of avalanche ionization.

The experimental data are of particular interest, representing the first simultaneous and consistent

study of the temperature and frequency dependence of bulk damage by pulsed lasers.

The point of view that optical breakdown in alkali halides is solely due to multiphoton excitation

of polarons, followed by phonon heating of the lattice, was put forward by A. Schmid and P. Kelly of

the National Research Council of Canada, and P. Braunlich of Washington State University. They

calculated the rate at which conduction electrons are generated by multiphoton absorption, and the

consequent heating by polaron-phonon scattering. From this calculation, they obtain a predicted dependence of the breakdown field at threshold on pulse duration and laser wavelength. The results are significantly different from those of Manenkov. A strong dependence on pulse duration is predicted,

2/3 with the energy density at threshold increasing roughly as t^ at the ruby wavelength. Whereas the

avalanche theory predicts a monotonic increase of threshold with increasing optical frequency, the

multiphoton theory predicts a decrease for short wavelengths. It is of interest to note that Manenkov':

measurements deviate from the pure avalanche model at high frequency, raising the possibility that

multiphoton processes are contributing to the damage, as the model of Schmid et^ al would indicate.

With increased interest in short wavelength lasers, the consequence of promoting significant

numbers of electrons into the conduction band in dielectric solids is of increasing importance. This

phenomenon arises due to multiphoton absorption in window materials, but much can be learned about it

by looking at semiconductors, where interband absorption occurs via one-photon absorption. M. Kruer,

L. Esterowitz, F. Bartoli, and R. Allen of the Naval Research Laboratory have studied laser-induced

damage in semiconductors (Ge, Si, In, Sb) irradiated at 0.69 um, 1.06 ym, 5 ym and 10.6 pm. Pulse

durations ranged from a few ns to several seconds (CW) . Damage occured due to surface melting. It

was found that for very high absorption (lO^cm or more), the effective depth of heating is greater

than the absorption depth due to carrier diffusion. Diffusive distances of the order of 0.5 ym were

observed. These are much shorter than values obtained using room temperature material parameters.

The high concentration of conduction electrons generated in these experiments shorten the carrier

lifetime, due to Auger recombination, and so greatly reduce the diffusion length.

The question of the concentration dependence of free carrier lifetimes in insulators was explored

in depth by K. Williams, P. Klein, and C. Marquardt, of the Naval Research Laboratory. They considered

the effects of shallow traps, deep traps, and electron-hole recombination on the kinetics of conduction

electrons in insulators. Shallow traps reduce the electron mobility, but do not affect the lifetime.

They show that in both insulators and semiconductors, deep traps reduce the lifetime to an extrinsic

value which is essentially independent of carrier density, depending only on trap density, at high trap

concentrations. The intrinsic lifetime of conduction electrons is different in semiconductors and

insulators. In semiconductors, it is determined by the equilibrium concentration of carriers, and so, although temperature dependent, it is not sensitive to the initial concentration of photo-electrons

produced in a pulse of illumination, except through the Auger process indicated above. In insulators,

however, the equilibrium concentration of carriers is zero, so an inverse dependence of carrier lifetime on concentration is expected. The authors present arguments indicating that the Auger process is

17 -3

not important in alkali halides, even at carrier concentrations of 5 x 10 cm .

They present experimental data on measurements of multiphoton-induced photo-conductivity in alkali

halides, using a doubled ruby laser with a pulse duration of 20 ps. They observe a decrease of the

carrier lifetime from 20 ns, at low carrier concentration, to less than 10 ps at carrier concentrations

12 -3

which are high (> 10 cm ), but well below those expected at the level of optical damage. At high

concentrations, the authors expect their measurements to be uncertain, due to space charge effects.

They emphasize the importance of using concentration-dependent carrier lifetimes in theories of optical

damage

.

They further report a dependence of carrier lifetime on the method of material preparation in the

regime of extrinsic lifetime. In NaF, purification of the material shortened the carrier lifetime,

ostensibly due to an increase in carrier mobility, while in Al^O^, the lifetime was sensitive to small

deviations from stoichiometry in the crystal. The measurement technique is advanced as a sensitive,

non-destructive test for the evaluation of wide-gap materials.

XXVi
Two-photon absorption coefficients have been measured at Harvard University for a variety of widegap materials, again mostly fluorides and oxides, using third and fourth harmonic generation from a 30 ps, Nd: YAG laser. The authors included P. Liu, H. Lotem, and N. Bloembergen of Harvard University,

W. L. Smith, now at the Lawrence Livermore Laboratory, J. H. Bechtel, of the General Motors Research

Laboratory, and R. S. Adhav, of Quantum Technology. At 355 nm, the results were consistent with the

expectation, in that for materials with band gap greater than twice the photon energy, negligible

-2 absorption was seen, while for materials with band gap less than 2hu, coefficients from 10 to

10 cm/MW were observed. At 266 nm the same results were obtained for alkali halides and CaF2, but

for oxides and phosphate materials, the two-photon absorption coefficients were observed to be from

5 X 10~^ to 3 X 10 ^, even when the gap was significantly less than 2h(jj. The authors caution against

ignoring two-photon-absorption attenuation in measuring bulk damage at short wavelengths.

The final two papers in this section deal with a general methodology for identification of damage

mechanisms and definition of damage thresholds. It is essential that when a damage threshold is

quoted, everyone agree on the significance of the terminology. It is also important to know which

of several possible effects was responsible for the damage. The threshold is thus defined as the

illumination level at which a certain phenomenon will take place with a certain probability.

The problem of determining the damage threshold from a finite sample of observations was considered

by J. A. Detrio and A. P. Berens of the University of Dayton Research Institute. The objective of the

study is to evolve a stategy requiring the smallest number of observations to determine the damage

threshold with the greatest confidence level. The damage threshold is defined, in this work, as the

power or energy level at which the probability of damage (observed by some predetermined criterion) is

one-half. The objective of a damage test can also be to determine which of two specimens has the higher

threshold

.

The authors conclude that, to determine the threshold, the "up-and-down" method is the most efficient. In this method, if the sample does not damage on a given shot, the power is raised for the next

shot. If it does damage, the power is lowered. A requirement of this method is that the experimenter

is able to choose the incident power level within some range of values. The choice of increment and

sample size is discussed.

In determining the damage threshold of either a reflecting or transmitting surface at 10.6 \im, a number of competing effects can give rise to damage. J. Porteus, J. Jernigan, and W. Faith of the

Michelson Laboratories have developed a method of measurement and analysis, called the "multi-threshold"

method, for determining the effective threshold value for each of these competing effects. They have

constructed an ultra-high vacuum irradiation chamber, with the following diagnostics: a 20 power

optical microscope, Faraday cup for charge collection, and Auger analyzer with an electron imager,

which is sensitive to variations of the work function over the surface. Targets are exposed to a train

of pulses from a mode-locked CO2 laser. Up to 50 sites are irradiated with one pulse each, and the

observer notes the appearance of either visible plasma formation or pitting. The incident power density

varies from shot to shot over a substantial range of values. After irradiation, the type of damage, if any, at each site is characterized.

A computer algorithm has been devised to then determine the effective threshold value and standard ' deviation for each of the identified damage phenomena. For each process, it is assumed that the singleshot probability of damage is zero, for fluences less than <i> -/3a, (where 4 is the threshold

o o fluence, and a the standard deviation) increases to unity for ((>^ + /J a and is unity above that value.

The effective sample values of <t>^ and a are then determined from the principle of maximum likelihood.

The procedure has been calibrated using a gold surface electroplated on diamond-turned copper.

The results are highly reproducible. A number of materials have been examined by this process, and

the results are presented in a companion paper by the same authors.

The multi-threshold method is particularly valuable, because rather than identifying the damage

threshold of a surface as a single number, it provides a profile of the various damage thresholds,

which can be correlated with surface treatment, material purity, aging, or other parameters of the

surface preparation.

4. Recommendations

The ultimate objective of laser materials studies is better lasers, better in the sense of higher

power and energy, greater reliability, and reduced cost. Much of the research reported in the nine

volumes of these Symposia Proceedings has been motivated by the high power laser programs in the DoD

and DOE (formerly ERDA) . These programs center around the construction of very large, very expensive

lasers, so that considerations of performance, reliability and economy are of great importance.

XXV ii
Large systems with single-pulse energies of tens of kilojoules are in operation or under construction at several national laboratories around the country. Both lasers and Nd:Glass lasers are involved. Even larger systems, with single-pulse energies of hundreds of kilojoules, are being planned.

This raises problems for the optics industry and laser community, as well as affording new opportunities. The manufacturers and users of special optics can anticipate heavy pressure on the fabrication

capability of the U.S. in the area of high power laser components. This pressure will undoubtedly

divert work to manufacturers not hitherto involved in high power laser optics. It is essential that

the expansion of the special optics industry be carried out with due attention to the lessons of the

past, as recorded in these proceedings and elsewhere in the literature.

The flow of information back to these symposia is essential, also. As more experience is gained

on large laser systems, we would like to hear more reports like that of Patton and Stowers of LLL at this symposium, recounting the damage history of an operating system. We strongly encourage the high

power laser laboratories to keep their resident experts in laser damage in touch with the day-to-day

experience of the operating system. There is a tendency, once the laser is operating, to turn it over

to the applications group, and only to call in the damage experts when the system has failed. This

Symposium needs feedback from the laser laboratories regarding the operating experience of large laser

systems

.

We still lack a reliable rule of thumb, predicting the lifetime of a laser component as a function

of the fraction of the nominal damage level at which it is operated. What is a "safe" level of operation for thousand-shot lifetime? This question must be answered if laser systems are going to operate

reliably for reasonable periods of time in fusion power plants, for example. (At 1 pps, a laser fires

almost 10^ shots per day.) This safe operating level must reflect the actual operating environment of

the laser, obviously.

As higher yield fusion experiments are carried out, it becomes possible to conduct experiments on

the effect of target debris on optical elements. Metal mirrors, dielectric multilayers, and "blast

shields" are the elements most likely to be exposed to target debris. These experiments should be

carried out with due attention to characterization of the samples, both before and after exposure. Here

again, close cooperation between the laser damage specialist and the applications group is of great

benefit to all concerned.

At present, large laser system performance is severely limited by component damage, thin film

failure in glass systems, surface damage in single shot systems, and thermal damage in high average

power systems. We will consider each of these areas in detail as we proceed through our recommendations.

Let us turn our attention to surfaces and mirrors. While it is generally conceded that thin

films are presently the most damage sensitive element in optical systems, it should be obvious that one must understand surfaces and their characteristics before one can assess the role they play as substrates

for coated elements. In addition, much of the present interest in surfaces is concerned with surface

absorption, environmental contamination, and adsorbed impurities (both a physics and chemistry problem),

and the characterization and maintenance of the surface structure. These are all problems common to thin films. One obvious area of corollary interest is in fiber optics, where all of these topics can greatly influence the performance of fiber elements. Fiber optics will undoubtedly play a major role

as control and instrumentation components in laser fusion systems. To this one might add adaptive

optics. One must assess the long term modification of surfaces by mechanical fatigue of these deformable optic elements.

From a systems standpoint, there is a need to transfer laboratory experience to the real world

scene. Much of the material presented at these symposia deal with very basic research efforts, and

the development of improved processing procedures designed to approach the intrinsic qualities of

optical materials. However, one must back off from these "idealized" levels to safe operating levels

which can be utilized, taking into account potential environmental degradation. This concern is

certainly application dependent. Consider if you will the space scenario, the laser fusion target

chamber, or the high average power laser used in materials working such as welding in a machine shop.

Each of these arenas imposes uniquely different environmental constraints on safe operating levels.

We see a need for studies on fully characterized optical elements, in realistic environmental conditions,

along with subsequent careful laser damage testing.

xxviii
To return to our initial point, i.e., the careful characterization of surfaces in order to assess

surface quality and monitor gradual modifications, scattering techniques still appear to be the best

tool for quantifying surface topography. Unfortunately the theory is incomplete and measurments made

at different laboratories are usually far apart in the values obtained for such simple indicators as RMS surface roughness. About the best that can be said is that they usually agree in relative smoothness. This disagreement arises in part from the fact that different techniques give different answers.

To date there has been no standardization of measurement technique or even agreement on theory. This is

a very fruitful and important area for research and development. Scatter measurements will become of even greater utility as we move to the uv, where, due to the shorter wavelength of light, there is an increase in scattering which constitutes a loss in the optical system. The uv will also place a greater

burden on fabrication due to the more stringent tolerances required. Some accepted diagnostic method

is needed. We might add that long term or environmental degradation of optical tolerances must be

assessed as well to realize the full benefit to be obtained from ultraviolet lasers. We must demand

tighter tolerance specifications and improve our evaluation of system performance.

From the interaction standpoint, scaling laws have been enunciated relating surface damage

thresholds to laser parameters (frequency, pulse duration, spot size) and material parameters (surface

roughness, refractive index). Additional work is required to define the range of applicability of

these scaling laws and to refine their predictive value, both to improve their utility in arriving at realistic system specifications as well as an aid in defining theoretical bases. A word of caution

is appropriate. We should not greatly extend scaling laws beyond the range of experimental determination,

i.e., to grossly different conditions, for they may no longer be valid.

Thin films have been described as the Achilles Heel of high power laser systems. In large measure

this is because they themselves contain many Achilles Heels. We start by considering a substrate

surface. Prior to coating it is cleaned, ground, polished, and cleaned again. New and better methods

of cleaning are needed in order to remove surface impurities and improve film adhesion. This requires

the attention of both the surface physicist and surface chemist. The properties of the film/surface

interface greatly influence both the structure of the films and film adhesion. The latter continues

to be a problem in the durability of coated elements and requires continuing attention, especially for

machined optics where cutting oils, etc., must be scrupulously removed. Environmental degradation

occurring after the film is put into use has already been discussed.

Over the past several years we have seen many new coating materials introduced and novel design

concepts suggested. At the time they seemed most noteworthy. However, in many cases these improvements have not been put into use. We don't know if this is a result of overselling, a reluctance on

the part of manufacturers to deal with new materials or design, or the consequence of trying new ideas

and materials and finding them wanting. Unsuccessful attempts to implement new ideas are not usually

reported. One must transfer knowledge of failure as well as success. Without question more materials

and design concepts are needed, together with a careful and complete evaluation of their performance

and characteristics. It is hard to imagine that all possibilities have been explored.

An area which received some attention this year was the modeling of film stress. In the proposed

models, the stress arising from dissimilar coefficients of expansion of the substrate and film was

treated together with the intrinsic stress of the film materials. Work should be continued in this

area, including stress generated by entrapped gas or impurity atoms, stress introduced by the freezingin of lattice defects (which probably arise in good measure from the substrate surface properties) and purely

surface effects. The last arises from a variety of causes including surface tension effects due to small

thickness, the hindrance of dislocation motion, and the influence of oxide or other chemically bound

surface layers, similar to that arising from inter-layer forces. This analysis must as well be corro- borated by careful measurement and appropriate damage testing.

Of even greater concern to the burgeoning field of adaptive optics is film performance under

continued deformation cycling. Initial studies were reported this year, but only extended over a few

thousand cycles and did not include damage testing. If adaptive optics is to become practical, we will probably need "rubber" films.

There is still need for further parametric studies of films to develop appropriate scaling laws.

The subject of thin films in itself is so complex we cannot hope to understand the full picture prior

to a film's employment. What has been done to date has provided helpful guidelines for the selection

of candidate film materials, multilayer designs, and deposition procedures. The choice of film

materials and design is determined by the application. These studies have also uncovered new unex- pected results which need to be addressed further, such as the separation of thermal and electric

field induced damage in the arsenic sulfides and selanides and on the coatings of germanium saturable

absorbers where the exact crystalline form of the substrate was found to be unimportant. This is one

case in which a coating did not have an overwhelming deleterious effect on the damage threshold. We

might add it did not have a beneficial effect either.

xxix
A question we can ask is "Can you make the material in thin film fonn behave as it does in the

bulk?" Probably not. But how close we can come, can only be determined by a most comprehensive and

detailed research program. One must begin with starting materials of known purity and proceed from

known substrate character through a carefully controlled preparation procedure to a detailed evaluation

of film performance. From this we must correlate the physical structure with the optical properties

exhibited by the film. The studies must include damage testing and potential environmental or aging

effects. This a major undertaking and probably can only be realized by the intelligent infusion of

money, organization, and technology transfer by the government. This will require the development of

diagnostic instrumentation, consensus definitions, and standard evaluation techniques suitable for

reduction to widespread practice.

Although surface damage and thin film failure limit laser performance, especially in high peak

power systems, investigation of bulk properties remain a central concern. The efforts to relate the

optical and mechanical properties of laser glasses, and optical materials in general, to composition,

purity, and structure should continue, primarily because great progress is being made in this area. The technological payoff of these investigations extends beyond the high power laser field. Research

in fiber optics materials is proceeding along very similar lines. Low-loss optical fibers have been

developed with alternation as low as 0.7 dB/km, corresponding to an attenuation coefficient of 0.16

-4 -1 X 10 cm . From arguments presented at this symposium, the impurity content of these fibers, which

are made of a germanate glass, must be extremely low. Surface losses must be extremely low, as well.

Laser materials researchers may be able to learn much about low-loss materials from the fiber community.

Conversely, as fiber optics users try to propagate increasing amounts of energy through monomode

fibers, questions of laser damage will surely arise. A few milliwatts in a monomode fiber corresponds

2 to megawatts/cm . Similarly, manufacturers of junction diodes are already concerned with damage

processes. We want to emphasize that laser damage considerations are not restricted to the world of

high power lasers.

As far as infrared window materials are concerned, the problem has been identified as essentially

one of impurities. The solution, in so far as the problem can be solved, lies in the processes of manufacture of the material and fabrication of the component. To do this economically is a challenge to the industry. In future, emphasis should be placed on questions of process control, to ensure low

impurity levels, and on detection techniques, to monitor those levels. Reduction of surface contamination remains a serious problem.

The extensive research carried out on infrared window materials provides an excellent point of

departure for the development of ultraviolet materials. Wide gap materials of high purity have been

developed and should be used in uv studies. The methodology of the studies should also follow the

procedures developed in previous work. This is especially true for damage studies. Presently, there

is very sparse data on damage levels at short wavelengths. These data will not be useful unless

obtained with well-characterized lasers, on well-characterized materials, with a clear definition of

what constitutes damage. "Those who fail to study history are doomed to relive it" and perhaps even more painfully.

New phenomena occur in the uv which require investigation. The fact that electronic transactions

are involved and that electrons can easily be promoted to the conduction band, either by multiphoton or multi-step processes, leads to new and complicated effects. It is not clear that many of the scaling

laws previously observed apply at short wavelengths. Scaling with wavelength, bandgap, pulse duration,

and spot area must be re-examined. The cumulative loss mechanism reported at this conference clearly

warrants further investigation. There may be other cumulative or aging processes which degrade uv optics over prolonged exposure. Here again, feedback from the system builder is of great value.

Further development of new uv window materials should continue.

The fundamental mechanism of two-photon absorption is generally understood. Due to the importance

of this phenomenon in limiting uv laser performance, further study is clearly desired. The results

reported this year or the difference in behavior of halides and oxides is a challenging subject for

further investigation. The effects of impurities on two-photon absorption, especially near the band

edge, is poorly understood. The general question of the density of states accessible to two-photon

processes, at non-zero temperature, near the band edge is both interesting and important. Scaling

studies, showing the dependence of two-photon absorption on frequency, composition and structure, and

temperature, are required. Above all, as in any study, the principal requirement is for good data, on well-characterized materials.

Unresolved issues still remain in the subject of the optical breakdown of solids. This subject

may be essentially academic, since other damage mechanism occur before bulk breakdown. Furthermore,

the relevance of bulk breakdown to surface breakdown has been called into question, especially by

Manenkov, since it is unlikely that the surface can be made free of defects, inclusions, and absorbed

impurities. Nevertheless, it is important to obtain a complete understanding of the physics of this

process, as part of our fundamental understanding of materials.

XXX
The scaling of breakdown thresholds with temperautre and wavelength is of great importance in

discriminating between competing theories. Scaling in pulse duration is needed to complete the

picture. As Kelly and co-authors point out, careful attention must be paid to focusing conditions, and

to defocusing by the incipient plasma. Pre-breakdown studies, like those reported by the NRC group

using photoconductivity, provide valuable data on the behavior of electrons in the conduction band,

data which must be fed back into the models used to describe the breakdown process.

Finally, it is not clear why different investigators get such widely disparate values for damage

thresholds in alkali halides. The attribution of this disparity to the presence of impurities should

be investigated, and an exchange of samples, if feasible, would clearly be of great benefit.

As stated in previous years, the need exists for a documentation project, consolidating the data

which has been obtained over the past decade on laser materials, new coating materials, impurity effects,

and damage thresholds. Without such a project, valuable information on optical materials may be overlooked, with an unnecessary repetition of measurements as a consequence.

5. Acknowledgment

The editors would like to acknowledge the invaluable assistance of Dr. Harold S. Boyne, Ms. Marilee

Hood, Ms. Margaret Wooley, Ms. Sheila Aaker, and Ms. Florence Indorf and the other involved staff

members of the National Bureau of Standards in Boulder, Colorado, for their interest, support, and

untiring efforts in the professional operation of this Symposium and in the preparation and publication

of the Proceedings. To these we must add Dr. James Harris, Ms. Patsy Collins, and others from the

University of Dayton Research Institute. The continued success of the Damage Symposia would not have

been possible without the enthusiastic support of those named above.

6. Bibliography

[1] "Damage in Laser Glass," A. J. Glass and

A. H. Guenther, Editors, ASTM Special

Technical Publication 469, ASTM,

Philadelphia, PA (1969).

[2] "Damage in Laser Materials," A. J. Glass

and A. H. Guenther, Editors, NBS Special

Publication 341, U.S. Government Printing

Office, Washington, D.C. (1970).

[3] "Fundamentals of Damage in Laser Glass,"

N. Bloembergen, National Materials Advisory

Board Publication NMAB-271, National Academy

of Sciences, Washington, D.C. (1970).

[4] "High Power Infrared Laser Windows,"

N. Bloembergen, National Materials Advisory

Board Publication NMAB-292, National

Academy of Sciences, Washington, D.C. (1972).

[5] "Damage in Laser Materials: 1971,"

A. J. Glass and A. H. Guenther, Editors,

NBS Special Publication 356, U.S. Government

Printing Office, Washington, D.C. (1971).

[6] "Laser Induced Damage in Optical Materials:

1972, " A. J. Glass and A. H. Guenther,

Editors, NBS Special Publication 372, U.S. Government Printing Office, Washington, D.C.

(1972) .

[7] "Laser Induced Damage of Optical Elements, A Status Report," A. J. Glass and

A. H. Guenther, Applied Optics 12 , pp. 637-649 (1973).

[8] "Laser Induced Damage in Optical Materials:

1973, " A. J. Glass and A. H. Guenther,

Editors, NBS Special Publication 387, U.S. Government Printing Office, Washington, D.C. (1973) .

[9] "Laser Induced Damage in Optical Materials,

1973: A Conference Report," A. J. Glass and

A. H. Guenther, Applied Optics 14, pp. 74-88

(1974) .

[10] "Laser Induced Damage in Optical Materials:

1974, " A. J. Glass and A. H. Guenther,

Editors, NBS Special Publication 414, U.S. Government Printing Office, Washington,

D.C. (1974).

[11] "Laser Induced Damage in Optical Materials:

6th ASTM Symposium," A. J. Glass and

A. H. Guenther, Applied Optics 14 , pp. 698-715 (1975).

[12] "Laser Induced Damage in Optical Materials:

1975, " A. J. Glass and A. H. Guenther,

Editors, NBS Special Publication 435, U.S. Government Printing Office, Washington, D.C. (1975) .

[13] "Laser Induced Damage in Optical Materials:

7th ASTM Symposium," A. J. Glass and

A. H. Guenther, Applied Optics 15, No. 6, pp. 1510-1529 (1976).

[14] "Laser Induced Damage in Optical Materials:

1976, " A. J. Glass and A. H. Guenther,

Editors, NBS Special Publication 462, U.S. Government Printing Office, Washington,

D.C. (1976).

[15] "Laser Induced Damage in Optical Materials:

8th ASTM Symposium," A. J. Glass and

A. H. Guenther, Applied Optics 1^, No. 5, pp. 1214-1231 (1977).

XXX i

A. J. Glass

A. H. Guenther

INTRODUCTION AND WELCOME

Arthur H. Guenther

Air Force Weapons Laboratory/CA

Kirtland AFB, New Mexico 87117

Haynes Lee, Chairman of Subcommittee 2 on Lasers of the ASTM, has asked me to welcome you on behalf of the ASTM. Unfortunately he will not be able to join us this week.

The ASTM is pleased to again collaborate in cultivating the interchange of information vital to its primary aims. The ASTM looks to this conference as one of the most important inputs to the development of standards relating to lasers and related fields. Obviously one must develop standards from

a basis of knowledge soundly developed, freely discussed and finally tested. This is a prime goal of

the ASTM.

The Subcommittee on Lasers will be holding their working session this Thursday afternoon and

Friday at the conclusion of this meeting, including the working groups on optically pumped lasers,

light emitting diodes, energy and power measurement and finally infrared lasers and laser materials.

These groups have collectively produced several standards this past year with many more in various

stages of preparation. Don Liebenberg of the Los Alamos Scientific Laboratory, Chairman of the

Optically Pumped Group, has asked me to mention their plans for a mini-symposium early next year at the

next ASTM meeting in Florida on the manual and automatic reduction of interf erograms . You can contact

him for further details.

Haynes Lee has also asked me to invite anyone interested in the development of standards in the

laser or allied fields to participate in these working sessions, particularly the infrared laser and

laser materials group under the chairmanship of Herb Lipson. This group will be working on standard

specifications for light scattering as well as methods of testing optical absorption by calorimetry

for both bulk materials and coated surfaces. Initial discussions on loss or refractive index variations in fiber optics may also take place.

Finally I'd like to mention that the ASTM is a voluntary standards group consisting of members

from government, industrial and academic institutions. It is the only organization with a concentrated

effort underway in developing standards in the laser field. At the present time discussions are underway concerning possible joint ventures in the standards area between the Optical Society of America

and the ASTM, probably using the existing ASTM mechanism for the development of consensus standards.

We hope many of you will stay for the working sessions, become familiar with our operation, and participate in our activities.

Now to the subject at hand, speaking for Alex Glass and myself, I would like to welcome you to the

Ninth Annual Boulder Damage Sjmiposium. After eight years of operating this meeting one wonders how he

gets his adrenalin up to go through the pains of yet another symposium, now formally entitled

"Materials for High Power Lasers." But upon reflection, it is quite easy to understand. It is because

of the support that we have received not only from the National Bureau of Standards and the ASTM but

perhaps more important the energetic and cooperative attitude of those individuals who aid us in the

operation of this meeting, and this, of course, includes you the participants.

I have already talked of the ASTM under whose aegis this series of meetings was initiated. The

prime mover here at the National Bureau of Standards is Dr. Hal Boyne, Chief of the Electromagnetics

Division. To him and his staff we are grateful for the hospitality and the use of these excellent

facilities. Those NBS individuals to whom we are especially indebted include Florence Indorf, Margaret

Woolley, and Marilee Hood who are concerned with the details of the operation of the meeting, the

assembling of the proceedings, and financial management, respectively. Of course, there are others to whom Alex and I are also thankful for making the operation of this meeting a most rewarding experience.

They are those sponsors who have tangibly supported this meeting through underwriting the expense of

its operation. They include Joel Weiss at ERDA, Harry Winsor at ARPA, and Herschel Piloff of the

Office of Naval Research. We would be remiss in not acknowledging the active participation of Jim

Harris, Patsy Collins and others from the University of Dayton Research Institute who have aided not

only in the operation of this meeting, but certainly and most important in coordinating activities

during this week between ARPA, the ASTM, and the National Bureau of Standards.

We fully expect this to be our biggest and best symposium, i.e., until next year when we hold our tenth annual meeting. At that time we expect to pause and reflect upon the past ten years' activities.

It is our intent to have distinguished speakers who are acknowledged principal contributors in the

many facets of the laser damage field. We will ask them to summarize and quantify as best as possible

the state-of-the-art and the state-of-the-understanding in their specialty. These review or tutorial

-1-
lectures will then be assembled and published as a separate volume on the subject of laser induced

damage in optical materials. We are also thinking of the possibility of having round-table discussions on those specific subjects in which there is still some question as to the correctness of our understanding. Of course, we will have contributed papers as well. We solicit your thoughts on this proposed approach and comments or criticisms on the operation of this meeting and our intentions

for our tenth annual meeting next year are welcomed.

Returning to this year's meeting, I would like to mention some pertinent features. Because of

the economies and improved communications realized last year by holding several meetings on related

subjects consecutively at the same location, we again closely coordinated the activities of this meeting with ARPA and its contractors. After initial paper selections by a joint committee, those papers

most appropriate because of content were identified to be presented at the recently concluded ARPA

meeting. Greater than 60 papers remained as submitted to this, the ASTM meeting. Of these, roughly

32 were selected for oral presentation to the participants at this meeting, while 20 were selected for

presentation in a poster format. Other papers were selected by title only, with some rejected outright. The sum total of this effort was to hold the meeting to two days, allowing the ASTM to conduct their business meetings Thursday afternoon and Friday.

As a result of the highly successful initiation and popularity of poster sessions last year and

the very favorable comments received from both presenters and participants at the meetings, we selected!

those papers which contained detailed data, descriptions of complex instruments, and particularly those

^

papers dealing with comparative data which is best viewed side by side in close proximity for presentation in two poster sessions. These types of papers would be difficult to present effectively using

slides or vugraphs. There were, as well, many papers submitted specifically requesting presentation

at a poster session. In general, they were from people who had presented poster papers last year and

appreciated the personal interest, the close interaction and idea exchange that was evidenced at those sessions.

It has always been our intention to avoid concurrent sessions or unduly short presentations.

Furthermore, this meeting has had a trademark of remaining on schedule while leaving ample time for

discussion. The increase in poster sessions follows naturally from these objectives. Each poster

paper will be manned approximately 3 hours as indicated on your program. These sessions are interspersed to break up long periods which would be necessary for your attendance in the auditorium. My

one note of caution is to ask you to please return, however, to the presented papers session on time

at the conclusion of each poster session.

Alex and I would like to issue an especially warm welcome to our foreign participants from

Canada, England, South Africa, France, Australia, and the Soviet Union. We would particularly like to thank Prof. Manenkov for taking the time to travel this great distance to present a paper on his recent

work dealing with avalanche breakdown. His paper will be presented on Thursday morning.

We have seen a dynamic yet gradual change from one year to another in this meeting. We have

always left room to develop a detailed appreciation of factors and interactions of new features,

revealed by greater understanding, from the establishment of a solid foundation arrived at through an open and frank discussion of previous contributions. As can be anticipated, papers presented at this

meeting continue to concentrate on those areas of most importance to the high power, high energy

laser community, those being infrared laser window, surfaces and mirrors, thin films, and finally the

theoretical aspects of fundamental mechanisms describing the threshold laser interaction with optical

media. This year, as expected, we note a continued trend from the infrared to shorter wavelengths,

and this trend has been programmed into this meeting so that it will flow smoothly from yesterday's

meeting toward the shorter wavelengths at the end of the week. You will note additional scaling laws

presented this year and verification where possible of those surfaced last year. Several papers deal

with pulse length, surface roughness, area, and wavelength dependence of laser induced damage. These

contributions are principally covered in the poster sessions.

Last year in his opening remarks, Alex Glass speculated on why the same topics keep appearing.

It is obviously because we are dealing with real materials in the real world. In some cases, mechanisms are sufficiently understood that the salient material property has been identified; thus

detailed information concerning materials properties is now of prime importance, that is, the achievable material properties, not the ideal, perfect material properties. While in other areas we are searching for clues through theoretical analysis and through careful parametric studies and derivation

of scaling laws which will form the basic data to which future theories can be compared, still others

have become so complex that no theory is universally accepted. Here I speak of avalanche breakdown

mechanisms and particularly in the related case of breakdown statistics. There just appears to be too many variables and the identification of a new unexpected influence only uncovers a much more complex

situation. An appreciation of this complexity is particularly evident when dealing with real world

materials. You will find in this meeting that we have attempted to strike a balance between performance, properties and understanding of the laser damage process in materials. I don't wish to leave
you with the impression that we haven't made progress, for today little is said of the influence of

absorbing inclusions in the bulk as a pulsed breakdown mechanism or of self-focusing. Improved processing has solved the first and an adequate theory has allowed design around the second, pointing the

appropriate direction in which to go for developing improved materials.

I would like to dwell, however, a moment or two on one of Alex's and my favorite subjects, that is,

thin films and its place in the arena of laser induced damage. Laser interaction studies with thin

films have revealed a lack of basic understanding regarding thin films. These films represent effectively a new state of matter since in layers, from 100 to 1,000 nanometers thick, electronic and structural properties are significantly different than in the bulk material. This is in general the reason

why no material has been found to damage at the same level in the bulk and thin film form. One way of

assessing the reason why the state of knowledge of optical thin films is as It is would be by contrasting them with electronic materials In thin film forms. I would like to quote from a paper entitled

"The Importance of Being Defective" by Shulman and Schlifkin of the University of North Carolina. They

state "Because solids are rather susceptible to the occurrence of atomic scale defects in their structures, impurity atoms, vacant lattice sites, interstitial atoms and aggregations of these Imperfections,

they are actually much more interesting and versatile materials than they would otherwise be. Thus the

useful electronic behavior of insulating and seml-conductlng crystals comes about in the vast majority

of cases from imperfections built Into them in a controlled fashion. These defects exert and Influence

physical properties quite out of proportion to the low concentrations in which they are incorporated."

Thus, you see that it is viewed that the Important electronic properties of thin films depend in large measure on defects, that is, their departure from a perfect structure and pure material. Furthermore,

the properties of interest are generally those which describe some average behavior, such as conductivity. This is usually measured in the long dimension parallel to the surface. (In addition, these

materials are almost always employed in very small sizes, such as chips.) While electronic materials

may be made In large sizes, it is a simple matter to reject any mlcrosized element which does not exhibit the desired properties. There Is no question that great advances have been made in our understanding of electronic materials in thin film form because of their tremendous commercial value. Thus

there is an economic incentive for research and development in this field, that is, the electronic

characterization of materials in thin film form.

In contrast, optical materials when employed in thin film form at high levels of optical radiation

will fail at some Achilles heel, that is, at a defect or structure modification such as crystal disorder, vacancy. Impurity, dislocation, etc. This factor compounds the difficulty in producing these

coatings for large optical elements or components in which the cost of the substrate may exceed by

many times that of the deposited thin film. Generally one is concerned with the properties of the film

in the short dimension perpendicular to the surface, but spatially resolved. In addition, there is no commercially intensive market for large quantities of optics to be used under intense levels of radiation at the present time. For this reason there is insufficient payoff for large Industrial research

and development activities, particularly when one realizes that the majority of optical coatings are produced by a handful of generally small commercial organizations. The whole picture Is compounded

when one realizes that optical thin films may be multi-layers of many elements each having different

physical as well as optical properties and can be deposited on surfaces of other than plane geometry

where varying angles of incidence may produce undesirable or non-uniform characteristics. We could

list a multitude of other factors too long and probably incomplete that influence the ultimate

behavior of deposited thin films, that is, their optical behavior.

There appears to be only one sufficiently fundamental thread of knowledge which would help unravel

the above described state of affairs. That would be to understand the correlation between the detailed

structure and purity of the thin film and its resultant optical properties. This appears to be a very

appropriate area in which funding agencies could establish some measurable research effort since it

will probably not be sufficiently attacked by industry. Its solution could lead to real advances in

the utility of coated elements. The principal questions regarding thin films are the following: How

do the optical properties of the film, such as refractive index, absorption, scattering, breakdown

threshold relate to the physical and chemical parameters such as morphology, composition, phase,

porosity, impurity and defect composition? And secondly, how do these physical parameters depend on the preparation methods and on the basic materials and substrate utilized? To these two principal

questions I would add the following comment, that the cost of characterization and instrumentation

required may exceed that of the optical element itself or the research project as well. But this is

the only way that we can identify those parameters which are important in correlating structure and

real world optical behavior. With that thought I would like now to turn the meeting over to what was bound to happen as can be noted in your program - your chairman for the first session on Infrared

Laser Window Materials - Arthur Glass.

-3-
SI CONVERSION UNITS

In view of present accepted practice in this technological area, U. S. customary units of measurement have been used throughout this report. It should be noted that the U. S. is a signatory to the

General Conference on Weights and Measures which gave official status to the metric SI system of SI units in 1960. Readers interested in making use of the coherent system of SI units will find con- version factors in ASTM Standard Metric Practice Guide, ASTM Designation E 380-76 (available from

American Society for Testing and Materials, 1916 Race Street, Philadelphia, Pennsylvania 19103). Conversion factors for units used in this paper are:

Length

1 in = 0.0254* meter

1 ft = 0.3048* meter

—8

1 microinch = 2.5400* x 10 meter

Area

1 in^ = 6.4516* x

10~^ meter^

1 ft^ = 9.2903 X 10~^ meter^

Force

1 lb (Ibf) = 4.448 newton

1 kip = 4448 newton

Pressure Stress

1 psi = 6895 pascal

1 psf - 47.88 pascal

1 torr (mmHg.O°C) - 133.32 pascal

Energy

1 ft-lbf = 1.3558 joule

Moment

1 Ibf-ft = 1.3558 newtonmeter

Temperature

To^, = 5/9 (Top - 32)

Heat

Thermal conductivity, ic

1 cal (thermochemical) cm.s. °C = 418.40* watt/meter kelvin

specific heat, C

1 cal (thermochemical) /gm.°C = 4184.00* joule/kilogram-kelvin

*Exact Value

-4-
EXTRINSIC ABSORPTION IN INFRARED LASER-WINDOW MATERIALS

M. Flannery and M. Sparks

Xonics, Incorporated

Santa Monica, California 90401

Duthler's analysis and tabulation of molecular-ion-impurity lines is updated and extended to include infrared wavelengths other than 10.6 \im, to include other molecular ions, and to include other types of impurities. The

results support our previous suggestion that the most likely explanation of

typical lower absorptance at 5.25 ym than at 10.6, 3.8, or 2.8 pm is simply

the difference between the extrinsic absorptance at the different frequencies.

The results are displayed in correlation-type charts to allow either the identification of impurities to avoid at a given wavelength or the selection of a wavelength at which impurity absorption is likely to be small. Absorption at the low level of B

= 10"^ cm"^ can occur at frequencies many line widths from

an absorption peak.

Key words: Infrared absorption; laser windows; molecular impurities;

molecular ions; molecular spectra; optical materials; surface absorption.

Introduction

Deutsch [1]^ has studied the infrared absorptance of various impurities on the surfaces of window

samples, and Duthler [2] proposed that an important class of impurities limiting infrared transmission

in al kal i-hal ide laser window materials is molecular-ion impurities which substitute for the halide ion

in these crystals. In the present investigation, we extend Duthler's analysis and tabulation to include other materials and other infrared laser frequencies, in addition to 10.6 ym.

Recently there has been considerable interest in reducing the absorption coefficient B of transparent materials used in high-power infrared laser systems. Laser windows can fail catastrophical ly by

fracturing due to inhomogeneous internal stresses, or less dramatically due to thermal defocusing of

the beam which result from heating by absorbed radiation.

Among the promising materials for use with high-power 10.6 pm wavelength CO2 lasers are the alkali

halides, in particular KC2. and KBr. The mechanism for intrinsic infrared absorption in these materials

is multiphonon absorption, with the fundamental Reststrahl frequency being 6 to 8 times smaller than

the CO2 laser frequency of 943 cm"\ It has been established experimentally [1] and theoretically [3]

that for V > vj^, the intrinsic multiphonon absorption coefficient B-jp^ decreases nearly exponentially

with increasing frequency. Extrapolation of the measured low-frequency intrinsic absorption coefficients yields ^-^^ = 8 x lO"^ cm'^ for KC£ and 6^^^. = 5 x 10"^ cm'^ for KBr at 10.6 ym. However,

absorption in the best presently available KBr is impurity dominated with B > 10"^ cm"^ at the CO2 laser

frequency, while KC£ has been produced with intrinsic absorption at this frequency. At higher fre-

:|quencies the absorption in both materials is extrinsic.

The purposes of the present paper are to study the important class of infrared-absorbing polyatomic molecular-anion impurities which substitute for the halide ions in the crystal lattices and to identify those ionic impurities that absorb most strongly at the infrared laser frequencies: HF, 2.7 ym

(3704 cm'^); DF, 3.8 ym (2630 cm'^ ) ; CO, 5.25 ym (1905 cm"^ )

; and CO2, 10.6 ym (943 cm'^). Particular

[emphasis will be given to the HF and CO2 laser frequencies. The influence of absorbing inclusions on

the bulk absorption coefficient and the failure of materials containing inclusions have been considered in previous publications [4,5].

Nominally pure commercial crystals are commonly observed to contain molecular-anion impurities,

2-

such as the metaborate ion (BOg) [6,7] and the carbonate ion (CO^") [6] , using conventional absorption

spectroscopy. The hydroxyl ion (OH") is especially difficult to remove from alkali halides during

purification [8], and may be the ion limiting absorption at 2.7 ym. These anion impurities along with

divalent metal cation impurities [8,9] are most likely still present in small concentrations in

1. Figures in brackets indicate the literature references at the end of this paper.

-5-
specially purified laser window materials since less than one part per million of certain anions can result in the presently observed extrinsic absorption.

Molecular-cation impurities are less likely to be found in alkali-halide crystals because of the

small size of the alkali ions which they replace, although NHJ in KC£ has been studied [10]. Monatomic impurities can also absorb infrared radiation either through local modes of vibration or by

activating phonons other than the fundamental Reststrahl phonon [11]. However, absorption due to mon- atomic impurities from 943 cm"^ to 3704 cm"^ is expected to be negligible because phonon and local mode

frequencies are generally much smaller than the laser frequency: The local mode of H" in KCi (U center

at 500 cm"^ has the highest frequency of this group. Absorption due to electronic transitions of

impurities is not considered in this paper [12].

Impurity Spectra

The infrared absorption spectrum of a crystal containing substitutional molecular-anion impurities typically consists of a number of sharp lines corresponding to the infrared active internal

vibration modes of the polyatomic molecular ion. Generally the internal vibrations of the impurity

ions are only slightly affected by the host crystal: There are small shifts in frequency and in- creases in the line width which depend upon the particular host. As a result of a lowering of the sym- metry when the ion interacts with the host, infrared inactive modes may become weakly absorbing and de- generate modes may be split into several components. In fact, the technique of alkali-halide matrix

isolation has been used extensively to isolate and concentrate these ions for study by infrared

absorption spectroscopy.

The probability of the laser frequency being exactly coincident with the peak of a given impurity absorption line is extremely small, the laser line usually falling in the wings of the absorption

line. In order to estimate the absorption coefficient 6 at the laser frequency v due to an absorption

line centered at frequency Vq, a Lorentzian line shape

6(v„)(Av/2)2

0(v) = ^ p (1)

(v - v^r +

is assumed, where Av is the full width at half maximum. With 0.1 percent impurity concentration, the

2 -1 absorption coefficient &{vr.) at the center of an allowed transition typically ranges from 10 cm to 3-1 -1 -1 10 cm and Av ranges from 4 cm to 50 cm , depending on the particular molecular species and

host.

Although experimentally observed line-shapes near resonance are described quite accurately by

eq. (1) using a constant relaxation frequency Av [13-16], for extrapolation to the wings it is used

with certain reservations. Widths of the fundamental absorption lines of matrix-isolated molecular

impurities are in general a result of weak coupling between the internal molecular vibrations and the

lattice vibrations of the host crystal [17]. With |v-Vq| < v^, where v^ is the maximum phonon frequency (210 cm"^ for KCH), there will be structure in Av, hence in B(v), reflecting the one-phonon

density of states and phonon selection rules. This will be important for extrapolation of narrow

lines (Av < 10 cm ^), since the measured line width in this case samples only a small part of the

phonon spectrum.

For extrapolation far from the line center |v-Vq| > v^^, Av is expected to decrease with increasing |v-Vq| resulting in a more rapid decrease in 6 than is predicted by eq. (1). In this region it is

likely that 3 will decrease exponentially with increasing |v-Vg| in analogy with intrinsic multiphonon absorption [3]. Nevertheless, judicious use of eq. (1) together with measured impurity line

strengths and line widths from the experimental literature is felt to be reliable for order-ofmagnitude upper-bound estimates of 3 at any particular laser frequency.

The fundamental absorption lines of molecular-ion impurities are often accompanied by other

nearby lines, which may not be resolved at room temperature and may be confused with the fundamental

line width. In the case of small ions such as OH" and NO2 the additional lines arise from rotational

and local translational motions of the ions [17-23]. (Recall that for a freely rotating diatomic

molecule, the angular momentum selection rule requires that the vibrational transition be a doublet.,

The fundamental absorption lines of large polyatomic ions are often accompanied by nearby "hot" bands

from transitions originating from low-energy excited states, and by isotope lines [7,24].

-6-
A number of the molecular-ion impurities that have been isolated in KCl crystals are listed

together with their measured absorption frequencies in the first two columns of table 1 [25-51]. The

third through sixth columns present the estimated impurity concentrations in parts per million for each

-4 -1 ion that are required to produce B = 10 cm at 300 K and 2.7, 3.8, 5.25, and 10.6 pm, respectively.

These estimates were obtained by extrapolating the measured absorption lines using an assumed

Lorentzian line shape with measured line widths where they were available. The estimates that appear

in brackets are especially low because the laser line is more than 10 Av from any impurity line.

Details of the spectra can be found in the original literature references which are listed in the

last column.

Isolated diatomic ions typically have fundamental absorption frequencies that are far removed

from the C0„ laser frequency 943 cm"^. As a result, impurities such as OH" that are commonly observed

in alkali halides should have little effect on absorption at 943 cm in spite of the large line width

of typically 50 cm ^ at room temperature. Using 6(Vq) - 30 cm~^ and Av = 50 cm~^ in eq. (1) for

1000 ppm, OH" in KC«, yields 6(943 cm'^ ) ^ 10"'*cm"^ at 40 ppm [22].

The polyatomic impurities BO^", HCO^, SO^', CrO^", MnO^", and CJlO^ yield 6(943 cm"^ ) > 10"^cm"^

at concentrations less than 0.1 ppm. These impurities, if present in alkali-halide laser windows, will

result in a severe limitation in the laser transmission.

3_

The case of BO3 illustrates the effect of near coincidence of the laser frequency with an absorption line of the molecular ion. Here approximately 45 percent of the absorption at 943 cm"^ is

contributed by the strong absorption line at = 1222 cm"^ (with Av = 30 cm"^ ) while most of the

-1 -1 remaining absorption is from the weak absorption at v, = 949 cm (with Av = 5 cm ), which is only

_] ' 6 cm from the laser frequency.

With HCO" the principal absorption at 943 cm"^ is from the vc -13-1-1= 971 cm"^ fundamental. Estimating

from published absorption curves that 6(971 cm ) = 10 cm and Av = 12 cm with 400 ppm, eq. (1)

yields 6(943 cm"^ ) = 29 cm ^

. Including additional contributions from v-, = 1218 cm ^ and v. = 1346

-1 -1 -1 -1 -4 cm which have Av - 15 cm and Av = 50 cm , respectively, results in 6(943 cm ) = 10 with a concentration of 0.1 ppm.

The CAOj ion produced a 6(943 cm~^) = 10"^ cm~^ at a concentration of only 0.001 ppm, the lowest

concentration for any ion studied. This was produced by the two strong, broad absorptions just below

the laser frequency: v^ = 970 cm ^ with Av = 45 cm"^ and v^ = 940 cm ^ with Av = 15 cm ^.

The polyvalent ions listed in table 1 each have several absorption lines near 943 cm ^. In order

to preserve the charge neutrality of the crystal, divalent impurity anions must be accompanied by

compensating anion vacancies, or as is frequently observed, by a divalent cation impurity (such as 2+ 2+ 2+ 2+ 2+

Mg , Ca , Ba , Pb , or Sr ) which are usually located at neighboring lattice sites. In heavily

doped crystals, the solubility of the divalent anions is enhanced by simultaneous doping with divalent

cations. The exact absorption frequencies and relative line strengths of the anions are dependent on

2- the particular accompanying cation, especially in the case of CO, which is strongly perturbed with

different metal ions.

2- 2- 2- 2- 2- 2- The group of divalent ions SO^ , CrO^ , MnO^ , SeO^ , MoO^ , and WO^ have similar spectra with

the strong v^ fundamental of these ions falling very near to 943 cm~\ Due to their large size, MoO^'

and WO?" are reported to be insoluble in KCd and KBr [52]. Depending on the particular perturbing

2- cation impurity, the normally threefold degenerate v-, fundamental of SO. is split into three strongly

-1 -1 -1 absorbing components from 1050 cm to 1200 cm each with Av = 15 cm . The otherwise forbidden v,

-1 -1 fundamental becomes weakly absorbing in the crystal with frequencies from 975 cm to 980 cm and

with Av-1-1 - 15 cm"^ . Additional absorption lines due to the v. and v, fundamentals are observed near

'i c 2- 600 cm and 500 cm , respectively. The components of the v, fundamental of CrO. are even closer

2- -1 -1 to the laser line than SO^ with frequencies in this case varying from 880 cm to 950 cm .

As table 1 shows, the CO laser operating at 1905 cm~^ has the lowest absorption from molecular

3- -4 -1 ions. The BO3 ion is the only one predicted to give an absorption of 6 = 10 cm at a
concentration of 0.1 ppm. For this ion, 80 percent of the absorption at 1905 cm-1 comes from the

fundamental at = 1222 cm ^ -1-1 with Av = 30 cm"\ with an additional 15 percent from the v, + v- sumd s mation band at v = 1944 cm with Av = 5 cm . Since such a large portion of the absorption at

1905 cm ^ comes from the v., fundamental which is over twenty line widths away, the estimate of 0.1 ppm

3- BOj might reasonably be revised upward to 0.3 ppm.

-4 -1 At the CO laser frequency, only four other ions produce an absorption of 3 = 10 cm at con- centrations less than 1 ppm: BH^ and NH^ at 0.3 ppm, and 80^ and HCO^ at 0.5 ppm.

At the DF laser frequency, v = 2632 cm"\ only SH', NH^, and NCO' yield 6(2632 cm"^ ) = 10'^ cm'^

at concentrations less than 0.1 -1-1 ppm. For SH and NH. the absorption is from the fundamental S-H and

- -1 N-H stretching modes at 2590 cm and 3100 cm , respectively. The absorption of NCO at 2632 cm

is expected to be particularly sensitive to the host and its preparation because the absorption at

-1 -1 -1 2632 cm is dominated by a weak fourth harmonic at 2613 cm (Av = 9 cm ) of the fundamental.

The absorption coefficient of KCl at the HF laser frequency, v = 3704 cm~\ will certainly be

dominated by absorption from the hydroxyl radical. Using 6(vo = 3640 cm'^) = 30 cm'^ and Av = 50 cm'^

in eq. (1) for 1000 ppm, OH" in KCJ!, yields 6(3704 cm"^ ) = 10"^ cm'^ at 0.03 ppm. This ion will be

particularly difficult to exclude from the alkali halides because of the ubiquitous nature of water

and the similarity in size and electronegativity of the halide and hydroxyl ions.

At concentrations of 0.1 ppm the ammonium ion yields an absorption of 6(3704 cm~^ ) = lO"'^ cm'^

from a broad N-H stretching fundamental at 3100 cm'\ but this ion is less likely to be present than

the hydroxide ion.

Temperature Dependence

Absorption frequencies and line widths of matrix-isolated ions are observed to be temperature

dependent, which can result in temperature-dependent absorption at the laser frequency. In the case

of ions which have broad absorption lines resulting from coupling to the lattice phonons, and ionic

fundamentals within the approximate range of 50 cm"^ to 300 cm~^ from the laser frequency, the tem- perature dependence can be simply predicted. With |v-Vq| >> (Av/2), eq. (1) can be written

6(v) = 6(vq)(Av/2)^/(v- Vq)2 . (2)

Above 100 K, the line widths Av of the nitrite [13] (NO2), nitrate [13] (NO^), cyanate [14] (NCO"),

and hydroxyl [22] (OH") ions are observed to vary as T" with n - 1 .6, and will dominate the small

temperature-dependent changes in v„ in eq. (2). As an example of this result, 6(943 cm ^) will vary

16 - as T , especially in the important case of NOl, where the approximations are fairly well satisfied.

-1 For OH , the use of the Lorentzian line shape is well justified only at 3704 cm . The absorption

at the longer laser wavelengths due to this ion should have a stronger temperature dependence with

n > 1.6, in analogy with intrinsic multiphonon absorption [53].

With the ions SO^", CrO^", and HCOg at the CO2 laser frequency and NCO" and BH^ at the CO laser

frequency the laser is operating near many relatively narrow lines and the temperature dependence of

the absorption is more difficult to predict. Published spectra of these ions taken at different tem- peratures show small increases in line width and small shifts in frequency. In these cases changes

in Vq can be nearly as important as changes in Av, and it is not clear that the observed line widths

are entirely due to phonon coupling. That is, part of the absorption in the wings which contributes

at 943 cm"^ could be due to hot bands, and, in the case of divalent ions, to different perturbing

impurities accompanying the molecular ion. As a result, the 943 cm"^ absorption of crystals containing these ions is expected to be less temperature dependent than crystals containing smaller, mono- valent ions. It is interesting to note that figures 2 and 3 of Dem'yanenko, et al . [48], indicate

that with CrO^ in KCJ,, the 943 cm absorption could even decrease with increasing temperature.

-8-
Additional Ions and Crystal Structures

The list in table 1 of some of the absorbing impurities that have been isolated and studied in

KCl crystals illustrates the importance of substitutional molecular-anion impurities on infrared laser

-1 2- transmission. Other ions that are likely to limit transmission at 943 cm are BeF. , which has absorptions at approximately 909, 870 and 770 cm in KCl, the exact frequencies depending on the compensating

-1 3-

ion, and BF. , which absorbs at 1131 cm in KCil [54]. The molecular ion PO. has a strong absorption

-1 3- 4- near 1050 cm , while AsO^ probably absorbs at somewhat lower frequencies [55]. Also SiO^ has a

strong absorption near 1000 cm ^ in several glasses [56].

There may be significant absorption at the CO laser frequency, 1905 cm \ from SCN , which has a strong absorption at 2038 cm ^ in KI and is known to be at least sparingly soluble in all the alkali

halides despite its large size [57]. The SeH" ion has an absorption at 2294 cm"^ in KCi, which could

produce significant absorption at both CO' and OF laser frequencies [58]. Finally, there may be absorp-

-1 - 2- tion at the HF laser frequency, 3704 cm , from NH„ or NH ions which have bond-stretching frequencies

near 3400 cm [55].

These results for molecular ions isolated in KCH can be expected to hold for other alkali halides

if the laser frequency is not close to an impurity absorption frequency. As a general rule the absorption frequency of a molecular-ion impurity shifts by less than 5 percent as the host is changed from

NaC£ to Csl [53], while the solubility of the ion greatly increases. Since the solubility is controlled

by the relative sites of the impurity ion and the host ion it replaces, the solubility decreases precipitously when the host anion is smaller than the impurity ion. With the exception of OH" and possibly CN~, the molecular-ion impurities listed in table 1 have ionic radii larger than those of F',

2- 0.136 nm, or 0 , 0.140 nm, but smaller than C2. , 0.181 nm. Even in very thick samples only the

strongest impurity absorption lines are detected when the ions are isolated in alkali fluorides, while

very weak absorption lines are detected in KCl at concentrations of 1000 ppm. Since the ratio of

3 strong to very weak absorptions often exceeds 10 , we estimate that the solubility limit of the molecular ions in fluorides and oxides is in the range 0.1-1 ppm. Therefore, most molecular-ion impurities

are not expected to be a major source of absorption in alkaline-earth fluorides or oxides such as

AJI2O2 used in HF, DF, or CO laser components. However, considerable concentrations of OH" could be

present in the alkaline-earth fluorides or Al^'^^, which would account for the higher absorption

observed at 2.7 ym compared to that at 3.8 or 5.25 pm [59].

There has been little systematic experimental work done on the effects of molecular-ion impurities on absorption in materials used for high-power laser components. Further studies of impurityinduced absorption at all the laser frequencies are needed to more firmly establish the relationship

between the absorption in the wings and the fundamental absorption at the line centers. The temperature dependence of the absorption at laser frequencies found in intentionally doped crystals can possibly be used as a guide in identifying unknown impurities in other crystals.

A study of Pb-doped KCS- found little correlation between absorption at 943 cm"^ and the Pb con- centration [60]. Isolated Pb ions are not expected to absorb at this frequency, but complexes of

2+

Pb with the divalent anions listed in table 1 do absorb. The reason for the negative result of this

experiment could be that there were no uncompensated divalent anions in the samples studied. In 2+

crystals containing divalent anions without accompanying divalent cations, the introduction of Pb

2- could have a significant effect, particularly in the case of CO^ .

- 2- -1 Doping KCJi. crystals with OH and CO. was found to have little effect on 943 cm absorption

-1 when the ions were introduced separately, but a significant increase in 6(943 cm ) was observed when

both ions were present [51]. It is possible that in this case the-1-2- enhanced absorption was due to the

formation of HCO^ which is much more strongly absorbing at 943 cm than either OH or CO^ .

In internally stressed KCt crystals it was observed that 3(943 cm ^) was larger by a factor of

two than after the same crystals were annealed [8]. This is difficult to explain on the basis of

intrinsic absorption. Although there is little information available regarding the effects of inhomogeneous stresses on molecular-impurity absorption, we may speculate that the increased absorption

observed in stressed crystals could be due to increased molecular-ion line widths and shifts in

frequencies.

-9-
In addition to the substitutional ionic impurities, there are other possible molecular impurities

that will limit infrared laser transmission. Neutral molecules such as or can exist at interstitial sites or at anion vacancies accompanied by cation vacancies in the bulk of the crystal

[61-63]. Absorption from organic or inorganic molecules condensed on crystal surfaces is considered

in the next section [64].

Surface Contamination

Recent measurements of bulk and surface absorption, particularly at 2.8 or 3.8 ym, are dominated

by large surface absorptions as the total absorption approaches 6 = 10"^ cm~^[65-67]. Efforts have

been made to correlate this surface absorption with crystal -growth techniques, mechanical or chemical

polishes, and chemical surface cleaners [68-70]. Although specific culprits have been found and eliminated in some cases, there remains a high surface absorption which generally increases on prolonged

exposure to the atmosphere. We purpose that a large portion of this residual absorption is due to a conglomeration of impurities from the local environment deposited on the surface. This conglomeration

may vary widely in its specific chemical composition, but must be heterogeneous enough to produce a fairly uniform absorption throughout the infrared spectral region.

The expected absorption from impurity compounds deposited on infrared optical surfaces is a com- plicated function of the number of chemical structures that absorb at a given frequency, the strength

and width of their absorptions, the occurrence frequency of these chemical structures among all chemical compounds, the relative abundances of these compounds, and the dispersion characteristics of

chemicals in the environment, such as vapor pressure, reactivity with other chemicals, etc. Figure 2 shows the absorption peaks of many of the ions in table 1, a few simple compounds, and some common

chemical structures arranged in the order of their highest frequency absorption between 2 and 10 pm. The leading edge of the absorptions in this type of correlation chart tends to have steps, which occur

at frequencies where few types of chemical structures absorb, and plateaus, which occur at frequencies

where large numbers of chemical structures absorb. Figure 1 has plateaus at ~2.9 pm, ~4.6 ym, and

~6.8 urn which correspond quite nicely to distinct peaks in the total number of absorption bands at 2.8-3.0 pm, 4.4-5 ym, and 6.6-7.8 ym.

Unfortunately this simple picture becomes clouded as one assembles simple chemical structures

into complex chemicals. As the number of atoms increases, the number of strong fundamental vibration

modes increases, bringing an increase in the number of weak difference modes. Beyond a certain point,

adding atoms to large molecules tends not to increase the number of strong fundamental modes because

remote parts of the molecule become weakly coupled. However, this weak coupling between adjacent

structures increases the anharmonicity of the local fundamental modes which increases the strengths of

overtone and combination modes [71]. These interactions in concert with external hydrogen or ionic

bonding also tend to broaden the absorption bands. Therefore, to evaluate the absorption from a conglomeration of molecules, a correlation chart including some indications of the strength and width

of the absorptions is needed. Such a chart is shown in figure 2 for a large number of chemical bonds,

molecular ions, radicals and compounds between 0.5 and 15 ym. The approximate spectrum of a complicated molecule can be assembled by a superposition of the spectra of its component parts. Additional

correlation charts arranged by chemical structure can be found in references [55], [72], and [73].

Figure 2 tends to show that there is a rather even distribution of absorptions throughout the infrared

region except for gaps at 4 - 5 ym and 10- 11 ym. However, there do appear to be large numbers of

strong absorptions at 2.9-3.5 ym, around 6 ym, and again around 8 ym which could lead to increased

absorption in these regions.

In addition to the location and strength of infrared absorption bands, it is necessary to have

estimates of how frequently different chemical structures occur in compounds. Figure 3 shows the occurrence frequency of elements in 13 520 organic chemicals listed in The CRC Handbook of Chemistry

and Physics [72]. Of these compounds, 99 percent contained hydrogen, 74 percent oxygen, 42 percent

nitrogen, and 16 percent chlorine, while no other element was present in more than 8 percent of the

compounds. Therefore chemical bonds among this short list of elements should dominate the infrared

surface absorption from organic compounds. Unfortunately, single bonds often suffer shifts in frequency and changes in absorption strength when they are combined in complex structures, necessitating

the classification of the infrared absorptions of a much larger number of more complex structures

which have consistent group absorption frequencies. Figure 4 lists 8462 organic chemicals according

to their principal chemical group [73]. Generally compounds in a group may contain the chemical

structures listed above it, but not those listed below. Most of the groups of compounds listed in

this figure have well defined infrared frequencies commonly listed in infrared correlation charts. It

is interesting to note that most of the structures listed contain only the four most common elements

in figure 3: carbon, hydrogen, oxygen, and nitrogen.

Similar figures could be made for inorganic chemicals, but the results are less definitive.

Oxygen is certainly the most pervasive element, probably followed by nitrogen or sulfur and then the

halides [74]. But there is also a long list of metals which can cause rather large shifts in group

frequencies and thus relatively poor correlations with simple structures.

-10-
Acknowledgments

We are especially grateful to Dr. C. J. Duthler for discussion of previous work on this topic and

for supplying a large amount of literature. This work was supported by the Defense Advanced Research

Projects Agency.

T. F. Deutsch, J. Phys. Chem. Solids 34,

2091 (1973).

C. J. Duthler, J. Appl. Phys. 45, 2668

(1974).

For theories of intrinsic multiphonon absorption see M. Sparks and L. J. Sham,

Phys. Rev. B 8, 3037 (1973) and references

therein.

M. Sparks and C. J. Duthler, J. Appl. Phys.

44, 3038 (1973).

C. J. Duthler, Appl. Phys. Lett. 24. 5 (1974).

References

[18

H. W. Morgan and P. A. Phys. 33, 364 (1962).

Staats , J . Appl

.

J. C. Decius, J. L. Jacobson, W. F. Sherman,

and G. R. Wilkinson, J. Chem. Phys. 43,

2180 (1965).

R. C. Pastor and M. Braunstein, Hughes

Research Laboratories Technical Report

No. AFWL-TR-72-152, Vol. II, July (1973)

(unpublished)

.

T. I. Maksimova, Phys. Stat. Sol. 33, 547

(1969).

W. Vedder and D. F. Hornig, J. Chem. Phys.

35, 1560 (1961)

A. A. Maradudin in Solid State Physics , Vol. 18, p. 274 and Vol. 19, p. 1, ed. by

F. Seitz and D. Turnbull ; M. A. Ivanov,

M. A. Krivoglas, D. N. Mirline, and I. I. Reshina, Sov. Phys. -Solid State 8, 150

(1966).

See for example J. T.

S. Roberts, and A. E. 2, 4313 (1970).

Vallin, G. A. Slack,

Hughes, Phys. Rev. B

V. P. DemS'anenko and Yu. P. Tsyashchenko,

Sov. Phys. -Solid State H, 3043 (1970).

I. I. Kondilenko, Yu. P. Tsyashchenko, and

V. A. Pasechnyi, Sov. Phys. -Solid State 12,

2990 (1971)

V. Schettino and I. C. Hisatsune, J. Chem.

Phys. 52, 9 (1970).

M. Tsuboi and I. C. Hisatsune, J. Phys. 57, 2087 (1972).

Chem.

R. Metselaar and J. van der Elsken, Phys.

Rev. 165, 359 (1968).

[19

[20

[21

[22

[23

[24

[25

[26

[27

[28

[29

[30

[31

[32

[33

[34

[35

[36

[37

[38

R. Bonn, R. Metselaar, and J. van der Elsken,

J. Chem. Phys. 46, 1988 (1967).

V. Narayanamurti , W. D. Seward, and R. 0. Pohl, Phys. Rev. 148, 481 (1966).

W. D. Seward and V. Narayanamurti, Phys. Rev. 148, 463 (1966).

G. R. Field and W. 47, 2378 (1967).

F. Sherman, J. Chem. Phys.

B. Wedding and M. V. Klein, Phys. Rev. 177 , 1274 (1969).

M. V. Klein, B. Wedding, and M. A. Levine,

Phys. Rev. 180, 902 (1969).

D. F. Smith, J. Overend, J. C. Decius, and

D. J. Gordon, J. Chem. Phys. 58, 1636 (1973).

G. K. Pandey and D. K. Shukla, Phys. Rev. B 4, 4598 (1971).

A. I. Stekhanov and T. I. Maksimova, Sov. Phys. -Sol id State 9, 2900 (1968).

C. K. Chau, M. V. Klein, and B. Wedding,

Phys. Rev. Lett. ]7_, 521 (1966).

C. K. Chi and E. R. Nixon, J. Phys. Chem.

Solids 33, 2101 (1972).

I. C. Hisatsune and N. H. Suarez, Inorg.

Chem. 3, 168 (1964).

J. C. Decius, C. R. Becker, and W. J. Fredericks, J. Chem. Phys. 56, 5189 (1972).

K. 0. Hartman and I. C. Hisatsune, J. Chem.

Phys. 44' 1913 (1966).

J. I. Bryant and G. C. Turrell, J. Phys. 37, 1069 (1962).

Chem.

A. Maki and J. 772 (1959).C. Decius, J. Chem. Phys. 31_,

R. Kato and J. Rolfe, J. Chem. Phys. 47,

1901 (1967).

A. R. Evans and D. B. Fitchen, Phys. Rev. B 2, 1074 (1970).

A. Strasheim and K. Buijs, J. Chem. Phys.

34, 691 (1961).

J. van der Elsken and S. G. Phys. 41, 3451 (1964).

Kroon, J. Chem.

V. P. Demyanenko, Yu. P. Tsyashchenko, and

E. M. Verlan, Sov. Phys. -Solid State U,

767 (1971).

-11-
N. G. Politov and B. M. Trakhbrot, Sov. Phys. -Solid State ]±, 483 (1972).

S. C. Jain, A.V.R. Warrier, and H. K. Sehgal , J. Phys. C 6, 189 (1973).

V. P. Demyanenko, Yu. P. Tsyashchenko , and

E. M. Verlan, Phys. Stat. Sol. 48, 737

(1971).

D. L. Bernitt, K. 0. Hartman, and I. C. Hisatsune, J. Chem. Phys. 42, 3553 (1965).

E. H. Coker and D. E. Hofer, J. Chem. Phys.

48, 2713 (1968).

J. C. Decius, E. H. Cqker, and G. L. Brenna, Spectrochimica Acta 1_9, 1281

(1963).

E. H. Coker, J. C. Decius, and A. B. Scott,

J. Chem. Phys. 35, 745 (1961).

D. N. Mirlin and I. I. Reshina, Sov. Phys.-

Solid State 10, 895 (1968).

V. P. Demyanenko, Yu. P. Tsyashchenko, and

E. M. Verlan, Sov. Phys. -Solid State 12^, 2527 (1971).

V. P. Demyanenko, Yu. P. Tsyashchenko, and

E. M. Verlan, Sov. Phys. -Solid State 12,

417 (1970).

S. C. Jain, A.V.R. Warrier, and S. K. Agarwal, J. Phys. Chem. Solids 34, 209

(1973).

P. Manzelli and G. Taddei , J. Chem. Phys.

51_, 1484 (1969).

H. G. Lipson, J. J. Larkin, and B. Bendow,

J. Electronic Mat. 4, 1 (1975).

I. I. Kondilenko, V. A. Pasechny, and Yu.

P. Tsyashchenko, Phys. Stat. Sol. 54,

783 (1972).

M. Sparks and L. J. Sham, Phys. Rev. Lett.

31, 714 (1973).

S. C. Jain, A.V.R. Warrier, and S. K. Agarwal, National Bureau of Standards,

NSRDS-NBS 52, U.S. Government Printing

Office, Wash. D.C. , 1974.

N. B. Colthup, L. H. Daly, and S. E. Wiberley, Introduction to Infrared and

Raman Spectroscopy , Academic Press, N.Y.,

1975.

F. Matossi and 0. Bronder, Z. Physik 111 ,

1 (1938).

M. A. Cundill and W. F. Sherman, Phys. Rev. 168 , 1008 (1968).

F. Z. Fischer, Z. Phys. 187, 262 (1965).

[59] C. Willingham, D. Bua, H. Statz, and F. Horrigan, Raytheon Research Division Final

Technical Report, Contract DAAH01-74-C0719, August 1975.

[60] F. W. Patten, R. M. Garvey, and M. Hass,

Mat. Res. Bull. 6, 1321 (1971).

[61] T. J. Neubert and D. L. Roskelley, J. Phys.

Chem. Solids 34' ^93 (1973).

[62] H. Grlindig and C. Ruhenbeck, Z. Physik 183 , 274 (1965).

[63] C. Ruhenbeck, Z. Physik 207, 446 (1967).

[64] T. F. Deutsch and R. I. Rudko, Raytheon

Research Division Final Technical Report,

Contract No. DAAHOl -72-C-Ol 94 (1973)

(unpubl ished)

.

[65] A. Hordvik, B. Bendow, H. G. Lipson, L. H. Skolnik, and R. N. Brown in Laser Induced

Damage in Optical Materials: 1976 , 13-15

July 1976, Boulder, Colo., ed. by A. J. Glass and A. H. Guenther, NBS Special

Publication 462, U.S. Government Printing

Office, Wash. D.C. , 1976, p. 50.

[66] J. A. Harrington in Laser Induced Damage in Optical Materials: 1976 , 13-15 July 1976,

Boulder, Colo., ed. by A. J. Glass and A. H. Guenther, NBS Special Publication 462, U.S. Government Printing Office, Wash. D.C,

1976, p. 45.

[67] J. Harrington, D. Gregory, and W. Otto in Fifth Annual Conference on Infrared Laser

Window Materials , 1-4 Dec. 1975, Las Vegas,

Nev., DARPA, Arlington, VA, Feb. 1976,

p. 872.

[68] M. Soileau, P. Temple, J. Bethke, and J. Shaffer in Fifth Annual Conference on Infrared Laser Window Materials , 1-4 Dec. 1975,

Las Vegas, Nev., DARPA, Arlington, VA,

Feb. 1976, p. 91

[69] R. Turk, R. Pastor, A. Timper, M. Braunstein

and G. Heussner in Fifth Annual Conference

on Infrared Laser Window Materials , 1-4

Dec. 1975, Las Vegas, Nev., DARPA,

Arlington, VA, Feb. 1976, p. 103.

[70] J. Davisson in Fifth Annual Converence on Infrared Laser Window Materials , 1-4 Dec. 1975, Las Vegas, Nev., DARPA, Arlington,

VA, Feb. 1976, p. 113.

[71] N. B. Colthup, L. H. Daly, and S. E. Wiberley, Introduction to Infrared and

Raman Spectroscopy , Academic Press, N.Y.,

1975, Chapter 1.

[72] Robert C. Weast, ed. for Handbook of Chemistry and Physics , 55th ed.. The Chemical

Rubber Co., Cleveland, Ohio, 1973,

pp. C-609 - C-638. .

-12-
Handbook of Tables for Organic

Compound Identification , 3rd ed; com- piled by Zvi Rappoport, The Chemical

Rubber Co., Cleveland, Ohio, 1967.

[74] Robert C. Weast, ed. for Handbook of

Chemistry and Physics , 55th ed.. The Chemical Rubber Co., Cleveland, Ohio, 1973,

pp. C-609 - C-638.

Table and Figures

1 TJ

•1- OJ

4-> 4->

(/I (X3 <U E

O) +J .c in

4-> LU

+J

•1- i>i oCO O 1— ro

ro (U

4-> 4-> c

t/J n3 O OJ c

o

o< U

C_) c 4-' ^ OJ Q.

S_ c cr o •1- Ol OO

S- -a 4- ro

CLI +J s- >.

(T3 <U

1— uo o ro s- in 1

•1

a.E

00 O •r— c o

o >,

•I- -oE ro >> ro +J E

o C o s- Q-E - t/1 •1- Ll_ Q<U o

E ' O lo- 3

co 3=

O) M- tt) O -C

OJ 4-> I—

•r- ro r—

Ol I = E c

a- u 0)

Ol +J

S_ J- M- I c o ro C I— .c o +J

•r- II +-> O) CLCQ S_

S- o

0 O) E m u

ro "Do ro o s01 Q. CO

Ol

S- O QJ 0) +J ^ CO ^ c ro 0 o s-

•r- J3

I— ro o) <— s- s- ro 4-> ro a Q) cr

01 U lO E =

r- O C

S- O -r- Q- >, CO X +J c

UJ •- o

S_ .r- 3 +J Q. ro

• E sO) -o <u I— Q) (J -O +-> c

ro ro o

I— E o

I— fo

no 1

^ r- ^ 00

I— CvJ OO I

I—

«— If)

o in I

CO I

I— in

I— ir> r— 1^

.3- .—

>— r— CO

CO r—

If) t— O r— o r-~ CD

If) I

OJ I—

.— 00

CO rm CM cn ro CO

CM CVJ C\J ^

-13-
2 \im 4 ym 6 pm 8 \im 10 ym

Impuri ty

SH"

-CH3

-CH2-

Si-OH

Amides

H2O liquid

HC03

KN03

N02

CN'

Si-H

NCO"

Aroma tics

Ge-O-Ge

Esters

C-Cl

B-N, B-0

C F

2 > ^ ^

3

Impurity

CO2

CO

COF2

OH'

BH4

NH,NH2

KHCO3

U^O

KHCO2

P-OH

NO-3

XEY,X=Y=Z'

Si-O-Si

BO2

BOCarbonyl

CO^

C02-

P-O-C

S02

2- SO^

•'X.Y = C.N; Z = C,N,0,S: XeY means C=C, ChN, etc.

Figure 1. Correlation chart showing the absorption lines between 2 and

10 ym of some chemical bonds, molecular ions, radicals, and compounds.

Key: • peak positions; range of peak positions in various

materials.

-14-
Figure 2. Correlation chart showing the absorption lines between 0.5 and

15 ym of some chemical bonds, molecular ions, radicals, and compounds.

• peak positions

range of peak positions in various materials

Absorption strength:

S strong

M medium

W weak

V variable

Absorption line width:

B broad

Sh sharp

Numbers associated with an absorption band indicate the number of

peaks occurring within that band.

If several designations apply to one peak, they are separated by

commas

.

If a designation applies to two adjacent peaks, it may be written

between them.

aliph. = aliphatic

arom. = aromatic

Me = metal atom.

Impuri ty Wavelength (pm)

1 2 3 4 5 6 7 8 9 10 11 12 13 14

1 1 1 1 1 1 1 1 1 1 1 1 1 1

Phenols S,l-2-^Bonded (B) M S

Alcohol

s

Monomer(Sh) MS W S •• - — ^Dimer(B) — Unbonded

Tertiary alcohols MS,B M S

Secondary alcohols MS,B M M

-CH3

_S S_ _M

-CH2 i S W

u " — CH^ chains — —

Aromatic C-H W-H

Allene: >C=C=CH2

S,l-2

;;c=cH2 M,l-2 MVS S S

Amines: NH, NH^ 1-2, 1-2 M S M M,B

-ChC-H Sh M M,B S,B

Epoxides, oxiranes S M ......

-15-
Impuri ty 12 3 4

1 1 1 1

5

1 61

Wavelength (vim)

7 8 9

1 1 1 10

1 11 12

1 1 13 14 — W

-Cil-H

H 0 f 1 iQuidI

S,B

• • • M,B S,Ei S.B

Inndes: -CO-NH-CO- • • —

S_

-HC=CH- M

• • —

M S M

>P-H

>C=0 S

Aromatic aldehydes • • — • S M M

_S_ M M M

Jn W w

w s s W W

• • w

• • M

CO w S

Impurity 12 3 4

1 1 1 I

5

1

6

Wavelength (pm)

7 8 9

1 1 1 101 11 12 13 14

I

-C = N S_

-F-H

COFj

S S

Benzene W S MW S W M W

• • • • M

• • S

• •

S°2 • • •

Si-OH w s

SD"

w M

OH"

CO 0^2 • • • • S w S

Acetone

W MW s S S M

• • • • •

w W

•

BH4

U

• • • S.B

Amides: -CO-NH2 M S M

-16-
Impuri ty Wavelength (ym)

1 2 3 4 5 6 7 8 9 10 11 12 13 14

1 1 1 1 1 1 1 1 1 1 1 1 1 1

KHCO^

• •

M M

Amides: -uu-nh- M W S M

NCO"

w s s

• • • •

n-butyl alcohol S,B S M W M S M W M,B

NH^

Fonnami de SRMW SB SB M MB SF

••••• • , ,

W M S S

• • • ••

HCO3 B B B

• •••••• • «

HH^ salt S W M

K2CO3 • •

P-NH2 M M M

Impurity

Wavelength (pm)

1 2 3 4 5 6 7 8 9 10 11 12 13 14

1 1 1 1 1 1 L 1 1 1 1 1 1 1

Lactams: -CO-NH- M W S

Isopropyl : (CH3)2CHM "- S - M — ^ - w —•- w —

Carboxylic acid dimer S M SMS M,B

Ethyl : CH3CH2- S M M W M

Ethyl ether S WW MSMSSM W M

••M • •••• • • • • • • * •

Toluol SW MSMMS M S

(-0-Si(CH3)2-0-Si(CH3)2-) MW W S S,B M S W

• • ••••• •

KHCO2 • •

Tetrahydrofuran SW W MWMSM S

HCCij

M W W S W W S.B M •••• • 1

NH2'^ salt _S_ _M_

NO3- W S M

-17-
Impuri ty 1 2 3 4 5 6 7 8 9 10 1 1 12 13 14

I 1 1 1 1 1 1 1 1 1 t 1 1 1

KNO3

S ,B M W

• • • • •

P-OH

NH"^ salt s

N02' s

KNO2 S,B W

» • • •

C-D, CD^ M S MM

M n

-C=CW

XsY, X=Y=Z

X,Y = C,N; Z = C,N,0,S

s

Si-H V ,s

Si-O-Si w s

B.-H..B bridge :i i 2-4

Impuri ty Wavelength (ym)

1 2 3 4 5 6 7 8 9 10 n 12 13 14

1 1 1 1 1 1 J 1 1 1 J 1 1 1

• H

• Diborane: B !

_ ^

_ ; B M W M S

Ge-H

O3 w s w

• • • •

BO'

Mo(CO),

b S.B

• • WWW ••••• ,

Aromatic rings W,2-7 M W,2-6 S-W,2-3

BO3' • • • •

Sn-H

Thiol esters

s s s

Cyclic anhydrides -1

^

Anhydrides: -CO-O-CO- -i s

-18-
Impuri ty Wavelength (ym)

1 2 3 4 5 6 7 8 9 10 1 1 12 13 14

1 1 1 1 1 1 1 1 1 1 1 1 1 1

Ge-O-Ge

• —

Anil n n L t

>C=CF2

c

Esters: -CO-0- S S

Diene esters: -CO-0- ^

s

Aromatic esters: -CO-0- s s

Ketones S arnm. H — M aliph.

Nitrites: -0-NO

c c

C=C i,V

C=N, N=N

V

FHF S ,B

Nitrates: -O-NO^

S S

Impurity

Wavelength (ym)

1 2 3 4 5 6 7 8 9 10 11 12 13 14

1 1 1 1 1 1 1 1 1 1 I 1 1 1

Carboxyl salt: -CO" _S_ _M_

CO" S

Furans : [f

'

Triazine ring: (i*'*^ S,l-2 M-W

Nitro-: NO^ _S_ M_

Nitroso-: -N-N=0

Ail-0

•

.3. Thiophenes : [['

' ^

2-

S 1-2

Azoxy: -N=NOC-Ci S

P-O-C M,l-4 M,l-3 S M,V

-19-
Impuri ty Wavelength (pm)

1 2 3 4 5 6 7 8 9 10 11 12 13 14

1 1 1 1 J L 1 1 1 1 1 1 1 , B-N, B-0

SO2 S

Aromatic /~^\_^^

amines: \ / ^

S

s

Cyclic P=N s

Aromatic ethers S M

Inorganic PO^ salts s

P=0 s

s

s s

s=o s

N Pyridine ring: S S,2

Impurity

Wavelength (ym)

1 2 3 4 5 6 7 8 9 10 11 12 13 14

1 1 L 1 1 1 1 r 1 1 1 1 1 1

C=S v,s

SO4" S V.S W S M

Organic PO^ salts S

Aliphatic ethers s

(^Me

PO4" S,B

Me=0

PO^" salts

S M

W S W S.B

czo- S.B S S.B• • • •

Pyrimidine ring: ||° ,'l _s_ s

P-F S

-20-
Impuri ty Wavelength (ym)

1 2 3 4 S fi 7 H Q 1 n n 1 9 1 T 1 /I ' ^ J 0 / o y lu il Ic Ij 14

1 1 1 1 1 1 1 J 1 1 1 1 1 1

CroJ- • • • •

MnO^-

ml S S SS

* * * *

Ti-O-Ti

s s

• • • • •

Si-C

SeO^

3

BrO:

J

s

B

^°3

S W

• •

C-S w

C-Br s

-21-
Figure 3. Occurrence frequency of elements in 13,520 organic compounds.

A small number of compounds also contained Na, K, Mg, Sb, Cr, and D [72].

-22-
Alkanes, cycloalkanes

Alkenes, cycloalkenes

Al kynes

Aromati cs Fluorides

Chlorides

Bromi des

Iodides

Al cohol

s

Phenol

s

Ethers

Al dehydes

Ketones

Qu i nones Carboxyl i c acids

Acid anhydrides

Amides, i mi des

Esters

Ami no aci ds

Ami nes Carbohydrates

Nitro compounds

N i t r i 1 e s Sulfonic acids

Sulfonyl chlorides

Sulfonamides, sulfonanil ides

Thiol

s

Thio ethers

1 1 1 1 r

2 3 4 5 6 7 8

Hundreds of Compounds

Figure 4. Occurrence frequency of the principal chemical groups in 8462

organic compounds and their derivatives. A compound may include functional

groups listed above it, but not those listed below [73].

-23-
POSSIBLE PROCESSES OF LASER WINDOW DEGRADATION

W.J. Fredericks

Chemistry Department

Oregon State University

Corvallis, Oregon 97331

When judged by gross characteristics, many crystals appear to be immutable

under ambient conditions. However, most crystals are continuously undergoing

changes in their properties through interactions among the impurities they

contain and with reactive components in their environment. These changes can cause a decrease in the transparency of the window and lead to its ultimate

failure. The purpose of this paper is to consider the processes by which this

may occur. A brief discussion of impurities, their interactions, and of

various mass transport mechanisms [vacancy, vacancy pair, grain boundary,

anomalous) in ionic crystals will be given. But most of the discussion will concern experimental evidence for such changes and their effect on optical

properties. Particular attention will be given to the interactions and

reactions of OH" and H^O and to a variety of impurity clustering effects at

various stages in their development. Some of these effects occur in

remarkably short times.

Key words: Clusters; deterioration; diffusion; enhanced diffusion; impurity

reactions; laser window; precipitate.

Introduction

Degradation or loss of transparency of a laser window occurs through spontaneous changes in the

impurity system contained in the window. If a crystal contains very few impurities, its optical

properties are stable for an extremely long time. The results given in table 1 are measurements of

laser colorimetry [1] on a crystal grown in 1967. Measurements were made in 1976 or 1977.

Table 1. Additional round robin absorption in high purity KCl,

Oregon State University, Sample 5-M20-3-6.

-4 -1 Laboratory 6(10 cm ) Etching and drying procedure

1 2.0 ±.3 10 min. cone. HCl etch, acetone rinse,

heated during rinse.

5 1.75 ± .05 <1 min. cone. HCl etch, filtered

isopropanol spray rinse, dried with

freon vapor.

3 4.4 Not etched (as received).

3 6.8 10 sec. cone. HCl etch, air dried

without heating.

3 3.8 10 sec. 50/50 formic/acetic etch,

5 sec. acetic acid etch, dried 10

sec. with freon.

3 2.2 10 min. cone. HCl etch, acetone rinse,

dried with hot air.

When a crystal is grown, the impurities present in it are not necessarily in equilibrium with the \

storage temperature of the finished crystal or necessarily in equilibrium with its external environment.

As the impurity system shifts toward equilibrium, the window may change its optical properties by forming impurity compounds or the scattering centers. The fundamental process concerned with these changing properties is diffusion. Here we must consider some of the unusual mechanisms involved in diffusion

in ionic crystals. The traditional diffusion mechanism associated with ionic crystals is the vacancy

mechanism in which the impurity on one sublattice jumps to an adjacent vacancy in the same sublattice.

While this mechanism applies to alkali halides and many other ionic crystals in some crystals such as | the silver halides, the mechanism of cation migration is the interstitialcy mechanism in which a cationj

in an interstitial position jumps to a cation site knocking the cation in that site to an interstitial % position. The jump distance in this mechanism is not limited to adjacent lattice sites. For certain

small ions, an interstitial mechanism can occur in which the impurity moves through the crystal without

occupying normal lattice sites. In this paper, we will discuss only the alkali halides as they are important to laser window technology. In figure 1, we show the results of a literature survey of the

diffusion of divalent ions in sodium chloride [2]. The only obvious feature of this figure is that

there is little agreement between the migration energies, and little agreement between various experi

ments. If we illustrated the n'igration of monovalent impurities in a similar plot, we would find the

same disagreement. Even in self -diffusion , there is little agreement between the migration energies in

alkali halides. Some of this scatter may be due to experimental error but most is due to differences

-24-
in the impurity content of the crystals used in the experiments. The line shown represents careful

measurements of migration energies on very pure crystals for the case of diffusion by the vacancy

mechanism. To understand why reasonable experiments produce such variation in migration energies we need to consider in detail the migration processes that occur in alkali halides. It is primarily

impurity interactions that distort the diffusion process.

Impurity Reactions

When an alkali halide contains an impurity, the equilibrium condition for that impurity may not be

as a randomly distributed isolated ion. In the case of aliovalent impurities, there must always be a charge compensating vacancy introduced in the same sublattice. The vacancy and the impurity bear

opposite charges and so undergo a mutual attraction forming impurity-vacancy complexes when in adjacent

lattice sites. In tlie case of monovalent impurities, the impurity may be of such different size than a normal ion of the host lattice that the elastic distortion of the lattice will cause the thermally

generated vacancies to preferentially associate with the monovalent ion. This effect is generally

neglected in interpretation of experiments involving monovalent impurities. Most experiments have

concerned aliovalent ions, in particular divalent ions, and their diffusion and association into

impurity-vacancy complexes in the lattice. In table II, we show various kinds of impurity reactions that occur in the alkali halides that will distort diffusion processes. Simple association is relatively

well understood, but if the crystal contains more than one divalent ion then common ion effects must

be Considered as shown in example 2. When a divalent ion is very much smaller than the host ion it

replaces, a complex with the vacancy in the next nearest neighbor position may be more stable than the

Table 2. Impurity reactions in alkali halides.

Type

1. Simple Association

General Reaction

V A c

Examples*

NaClrPb*'*' or

NaCl:Cd'^"^, etc.

3.

Common Ion M

A

V

V

M V

A c M„V

A c B c

Nearest Neighbor i Next Nearest Neighbor

NaCl:Pb :Cd or

+ + + + KC 1 : Pb : Cd

M + V A c A c M + V ^ M,V (n) or

A c Ac

NaC 1 : Mn or

KCl rMn"^"^

M.V -> M,V (n) A c -<- A c ^

Cation-Anion (1) M„

A c B a V V

A c

c a ]

(2) . Xg-M^Xg(i) . or )

' Va I

KCl :HgO

KCl:Hg :0H

(3) M, + X„ ^ M, (i) X„(i) + V V

^ ^ A B ^ A^ B c a etc

.

5. Polymerization aX, ^ (XJ

A 1- A a KCl :3B0, KCl: (BOpj

V + KCliMn*"^ J KCl: (MnV ) c c'S

6. Phase Separation

(a) Cluster aM.V + 2aX -> aM,X^ + 2aV V. Ac -<- A 2 c d

(d) Ordered Phase Phases

nearest neighbor configuration. This is well established in the case of manganese and zinc and potassium chloride [3,4]. If the host crystal contains not only cationic but anionic impurities, they have

a tendency to form compounds within the lattice. A good example is mercury and the hydroxide ion or mercury and the oxide ion [5]. If the crystal is allowed to reach equilibrium, these often cluster

into large groups which we represent here as polarimization of the BO2 ion [6] . The final stage occurs

when a local region of the crystal becomes saturated and larger clusters or ordered phases begin to appear. Ail of these processes have been well established and can lead to deterioration of the optical

properties of the crystal. Here we will give some examples of this type of behavior. But for purposes

-25-
of this paper the question is can these reactions occur at normal temperatures. In order to discuss

this, we first need to look in detail at diffusion mechanisms in ionic crystals.

Diffusion

The diffusion coefficient obtained from experimental observation consists of diffusion by a vacancy

mechanism, ; diffusion by vacancy pairs, diffusion along dislocations, ; and diffusion along

grain boundaries, ^g^> ^'^Y other possible mechanism that may occur in the crystal. Thus the

observed diffusion coefficient is given by

D,=D+D +D_, + D obs V vp d ^ (1)

The classic case of vacancy diffusion of monovalent cations in alkali halides is represented by equation 2.

= 4a fo) exp(-//kT) = 4aY<^ exp (sVk) exp(-h^/kT) = exp[-h^/kT) , (2)

where a is a cation-anion distance, f the correlation factor, to the cation attempt frequency, x the

c

^ c c c

^ concentration of cation vacancies, g is the Gibbs free energy of migration with g = h - Ts . If "m b/ 6 ''m m m the only source of cation vacancies are Schottky defects , then eq. (2) can be written as shown in eq. (3) in which the concentration of cation vacancies is expressed as a Schottky product with hg being the

enthalpy of formation of a Schottky pair and s^ the entropy of formation of a Schottky pair.

D*^ = 4aVi^ exp { [s'^ + (s /2]/k} exp{ [h'^ + h /2]/kT}

V c ^ m s ' ^ ' m s (3)

If the crystal contains both anion and cation vacancies, these experience an electrostatic

attractive force and form a certain number of pairs in thermal equilibrium with the crystal at the time

it was grown. The mass action expression for this is given by

s exp[-{g = a )/kT]

^ p

(4)

where z is the configuration enthropy, g and g are the free energies of formation for Schottky defects

s p and pairs, respectively. The pairs can contribute to both anion and cation diffusion in eq. (5) the

given expression for the diffusion of cations by the vacancy pair mechanism. The usual experimental

d'^ = 8aV exp{[(s /2) + s + s'^P]/k} exp{ [-h /2) + h + h'^Pj/kT}

vp -^pr c ^ ^ s p m ' s p m

(5)

practice for separating diffusion by vacancies and diffusion by vacancy pair is to perform an electromigration experiment. As the vacancies are charged they will migrate in the electric field; the

vacancy pairs will not, as they are not charged. And by analysis of diffusion profiles obtained with

and without an electric field, the two contributions can be separated [71.

Diffusion along dislocations is not well developed. Often investigators use single crystals to separate the vacancy and vacancy pair contribution assuming that any difference between the observed

diffusion coefficient and the sum of D and D to constitute that portion of the overall diffusion

V vp

process caused by diffusion along dislocations. There have been very few systematic studies of dislocation diffusion in the alkali halides.

On the other hand, there have been several excellent studies of diffusion along grain boundaries.

The grain boundary diffusion coefficient as developed by Whipple [8] is given by

D', 6 = gb

ainx^' -2 4D

V 1/2 ainx^

[ J t 1/2

where (6)

x/(D^t)^''^ and e = 8(D t) V

1/2

The terms n and 6 are defined below the equation. D^^ is the grain boundary diffusion coefficient, 6

is the grain boundary width, t is the time, the concentrations x^, are given in mole fractions, and x is

the distance. L.B. Harris [9,10] has carried out extensive studies of diffusion along single grain

boundaries. He finds that the ratio of the grain boundary diffusion coefficient to that for vacancy

22 -9 2 diffusion for Na in sodium chloride is 450. The magnitude of D^^(Na] in NaCl is 1.2 x 10 cm at

633°K.

The diffusion of aliovalent ions in alkali halides is slightly more complex than that of mono- valent ions in that the impurity and the vacancy on the corresponding sublattice associate and it is

-26-

I
these vacancy-impurity complexes that constitute the diffusion flux. Since the impurity has an excess positive charge and the cation vacancy has an excess negative charge, the two attract with an energy of formation for the complex g^. The concentration of complexes is given by eq. (7a,b ,c ,d ,e , )

.

+ V' M V (7b.) A C ^ A C

at concentrations y , X^> Xi then electro neutrality requires x. = C. - k, C ^/c / M ^A A /c (7b)

k^(T) = \_ = exp(-<?^/kT) ; = 12 (7c)

= p , the degree of. association (7d)

^A

k^(T) = P

3 = 12C^ exp(-^? /kT) (7e)

(1-p)

Since the diffusion occurs by a vacancy process the impurity flux is given by eq. (8) and note that the

flux is proportional to the concentration gradient of the complexes. Since that gradient is concentration dependent the diffusion coefficients will be concentration dependent. It's convenient to consider

J„ = -ND, Vx, (8)

two limiting cases of divalent ion diffusion. The low concentration limit given by eq. (9) in which the

= |aVa)2P when « k^(T)^/^ (9)

concentration of the impurity is very much less than the concentration of thermal vacancies produced

from Schottky defects and that given by eq. (10) in which the concentration of the impurity exceeds that

= ^^f<j,^[p/(Up)] when >> k^(T)^/^ (10)

of the concentration of vacancies produced by Schottky defects . In the former case the diffusion co- efficient is proportional to the degree of association, while eq. (10) describes the situation in which

the degree of association is becoming larger and the diffusion coefficient is no longer directly proportional to the degree of association. As p approaches 1 the diffusion becomes constant, ie a saturation diffusion coefficient D^, given by eq. (11). This describes the behavior of a simple system

= a^fu>j3 = Dg (11)

containing a single aliovalent ion and represents the basic case of aliovalent ion diffusion. If other

diffusion processes are occurring such as grain boundary diffusion, then as used in eq. (5) is given

by the diffusion coefficients described by eq. (8) and (9). If the crystal contains more than a single

aliovalent ion on the same sublattice then the vacancy is a common ion between two reacting impurity

systems and one obtains coupled diffusion coefficients, the diffusion coefficient of one ion depending

on that of the second ion present.

Figure 2 shows the behavior of the diffusion coefficient of cadmium in NaCl as a function of

concentration [11]. If in the interpretation of a diffusion experiment the diffusion coefficient does

not reach saturation it can lead to a downward curvature in the Arrhenius plot which causes a low migration energy as illustrated in figure 3 [11]. An illustration of the dependence of the diffusion

coefficient of one ion on the presence of a second ion on the same sublattice is shown in figure 4. In that particular experiment, both cadmium and lead were simultaneously diffused into purified potassium

Chloride. Note the initial portion of the cadmium diffusion profile and the build-up of cadmium at the termination of the lead penetration into the crystal. Under these experimental conditions the lead

contributed most of the charge compensating vacancies it introduced into KCl to the cadmium and the

cadmium diffused at its saturation diffusion coefficient value. Even though its concentration was well

below that which would cause saturation to occur in a single ion experiment and the lead was retarded

in its diffusion in KCl by the lack of vacancies with which to associate. If this experiment had been

performed in a crystal containing an unsuspected quantity of lead uniformly distributed throughout the

crystal, the entire profile would have been distorted and interpreted as extremely rapid diffusion with

a low migration energy and a large preexponential factor.

The foregoing examples related to diffusion at high temperatures, but they indeed illustrate the

peculiar behavior that can occur in diffusion studies. It's essential to understand the effect of one impurity on another in understanding diffusion processes that can occur rapidly at low temperatures.

One additional high temperature example is shown in figure 5. This is the diffusion of mercury into

potassium chloride [13]. Notice the extremely low concentrations of mercury that will diffuse into the

purified potassium chloride, while under exactly the same conditions that 5.2 times more mercury is

diffused into potassium chloride containing hydroxide. Mercury forms an anion-cation impurity compound

-27-
1

[5] within the lattice which is quite immoble. Notice the rapid decrease of the Hg concentration

below that in the purified crystal at a penetration distance beyond the HgOH maximum. The Hg^*:OH':

2- " 0 system is complex [5], but this figure illustrates that when the mecuric ion is bound with hydroxide

in a potassium chloride lattice the energy of the impurity lattice compound is much lower than that of

an isolated mecuric ion or a mercuric ion vacancy complex. This enhances the diffusion such that at a specific temperature and in a specific time much more mercury diffuses into the crystal than Hg^^ in a purified crystal. Thus we have impurity enhanced diffusion. It is this impurity enhancement of diffusion that greatly speeds low temperature diffusion into the lattice.

| In keeping with its reputation for damaging the optical properties of alkali halides, the

ubiquitous impurity, hydroxide, exhibits anomalous diffusion characteristics. The only values of the

diffusion coefficient of hydroxide in KCl arise from effusion measurements [14] of hydroxide effusing

from a KCl. The diffusion coefficient is given by equation 12. The migration energy 2.0 eV is I D(OH") = 1.2 x 10^ exp(-2. OeV/kT) cm^/sec (12)

characteristic of a vacaricy mechanism for the diffusion process but the preexponential factor is m extremely large for a simple vacancy mechanism and makes hydroxide the fastest moving ionic impurity ^

observed in potassium chloride. These values have been confirmed by Ikeda in an independent experiment

[15]. However, if the atmosphere external to the crystal is reactive, such as a chlorine atmosphere,

the rate of migration of hydroxide becomes unbelievably rapid. If it is assumed that the concentration

profile within the crystal is an error function complement, then the diffusion coefficient given by

eq . (13) is obtained. Note that the migration energy does not change significantly but the preexponential factor increases by 100 due solely to the presence of the external reactive atmosphere. In similar

D(OH') (Cl^atm) = 3 x 10^ exp(-2.0eV/kT) cm^/sec (13)

effusion measurements, Ikeda has shown that it is not the hydroxide that diffuses but that the hydroxide

is destroyed more rapidly than it diffuses by forming an intermediate 0^ impurity. The resulting

diffusion profile resembles an error function complement with a displaced threshold (fig. 6). The

increased magnitude of the preexponential factor in the diffusion coefficient is not real. The absorption at 204 nm is destroyed by decomposition of the hydroxide and an intermediate formation of a band

ascribed to 0^ at 240 nm. The process by which this occurs is not well understood. It may be by a hole migration in the crystal and subsequent reaction with the hydroxide. Ikeda only reported on a single experimental measurement; much more work must be done before the exact process is understood.

However, this work presents a mechanism hither to unsuspected which clearly demonstrates the effect of

the external atmosphere on impurities far removed from the crystal surface. If the diffusion coefficient obtained in the vacuum experiments applies at temperatures well below those at which they were

-33 2 -1 measured, the diffusion coefficient of hydroxide at room temperature is of the order 10 cm sec This is far too small to cause degradation of optical properties in a reasonable window lifetime. If

accelerated processes occur, such as those in a chlorine atmosphere, substantially larger diffusion

coefficients could occur at room temperature but still not large enough to markedly affect the properties within a year lifetime of a laser window, but in the region of the crystal through which the laser

beam passes substantial changes in the impurity distribution could occur.

Grtlndig and Rtlhenbeck [16,17,18] developed a method of studying the diffusion of water into

potassium halides by observing the bleaching of the color centers in crystals to which or K had been

added. Diffusion under these conditions is enhanced diffusion similar to that of mercury into KC1:0H

discussed previously. Results of their experiments are summarized in table 3.

The temperature dependence of the solubility of water in these crystals is unusual. Initially the

solubility is high then decreases as the temperature increases to a minimum at 350°C in potassium

chloride, 324°C in potassium bromide, and 290°C in potassium iodide then again increases as the temperature increases. In each case the increase in solubility in the high temperature region is much greater

than its decrease in the low temperature region. Note the magnitudes of W in table 3, and that water

is extremely soluble in these crystals under enhanced diffusion conditions as was mercury in KC1:0H".

They also observed that in K doped zone refined crystals the temperature solubility transition is

well defined, but when Ca^* is added to the crystal a temperature independent region occurs that ex- tends well below and well above the transition temperature found in purer crystals. The solubility was enhanced in this region over that of zone refined crystals. The formation of a rather stable compound

between calcium and the products of water diffused into the crystal would explain this behavior.

Evidence for such a calcium hydroxide impurity compound was reported from conductivity studies by Fritz,

Laty , and Anger [19]

.

The diffusion coefficients reported are given in the lower part of table 3. Note the rather low

migration energies for water in KCl, KBr, and KI and the very large preexponential factors. Water

diffuses rapidly in these crystals when the diffusion is enhanced by the formation of a lattice com- pound within the crystal. Grtldig and ROhenbeck suggest that the diffusion may be by an interstitial

-28-
Table 3. Solubility and diffusion of H^O and its products in K^O- and K-doped crystals

Solubility

Crystal

KCl

KBr

KI

pC (cm J o

8.9 X 10"

9.1 X 10

6.2 X 10

7.6 X 10

1.5 X 10

1.8 X 10

16

10

16

13

16

W

CeV)

-0.20

0.67

-0.15

0.53

-0.06

0.33

Temp.

T < -350

T > -350

T < -325

T > -325

T < -290

T > -290

Reference

RUhenbeck (1967)

Diffusion

Crystal

KCl

KBr

KI

Dif fusant

"2°

0

"2°

Ho

"2°

T(°C)

700-180

420-180

600-180

400-180

600-180

420-180

r 2 -1, (cm sec J 62

630

1.5 X 10

7.7

50

0.48

5.6

U(eV) Reference

0.80 RUhenbeck (1967

0.97 GrUndig and

Riihenbeck (1972)

1.25 Stasiw (1935)''

0.69 RUhenbeck (1967)

0.81 GrUndig and

Rtlhenbeck (1972)

0.56 RUhenbeck (1967)

0.64 GrUndig and

RUhenbeck (197 2)

For doped zone refined crystals.

Given by RUhenbeck (1967),

c -3-1

p(Torr)C^(cm Torr ).

For OH diffusion, see section V and table V.

process. The low migration energies for such a large molecule do indeed suggest this as do the large

preexponential factors, although no other evidence is currently available to support that supposition.

If these diffusion coefficients hold to lower temperatures, water can certainly damage a crystal con- taining an impurity that will enhance its diffusion. In figure 7 is shown the ratio C/C^, assuming an extended source of concentration C^. as a function of distance into the crystal at 25"C for 180 days,

and 360 days for KCl, and 180 for KBr. For comparison the diffusion profile for OH under a chlorine

atmosphere is also shown. In order for the concentration of OH" to be comparable with that of water

There is significant penetration of water into the crystals to This would produce a significant amount of absorption in a

it must diffuse at 327°C for 180 days,

a depth of 0.6 mm at room temperature,

crystal used for a laser window.

There have been no direct experiments on the measurement of the diffusion of water into a divalent

cation doped alkali halide. However, evidence does exist that it occurs at room temperature to a significant degree in a period of a year. In figure 8 the relaxation time t versus the reciprocal

temperature is shown for a cadmium doped KCl crystal that had been exposed to atmospheric water for a period of 12 months [20] . This measurement occurred inadvertantly in that a crystal that had been

stored in a closed container for the period of a year was measured in a double crystal dipole relaxation measurement and exhibited a new relaxation process shown near A of figure 8 with a relaxation

energy of 0.57 eV . The line near B is the normal cadmium-vacancy complex relaxation. To confirm that

this relaxation was due to water, an additional experiment was conducted using a freshly grown KCL:Cd

crystal and exposing to water vapor for 1.5 hr at 400°C. The same relaxation at A appeared in this

crystal which previously had only the relaxation B. Further experiments showed that exactly the same

type of behavior occurred when a KCl:Pb^* crystal was exposed to water vapor for a similar period of

time. Since most divalent ions form stable impurity-hydroxide compounds when they are incorporated

in alkali halides as impurities this experiment suggests that a crystal containing a divalent ion will

have a much shorter lifetime as a laser window than a purer crystal. From these measurements, it

appears that a period of a year in a reasonably humid atmosphere would be sufficient to destroy the

optical quality of a single crystal laser window.

-29-
Clusters and Precipitates

In addition to the degradation of the optical properties crystal by the absorption of water,

another process, clustering, can reduce the optical quality of a laser window. It is convenient to divide this process into two stages, microclusters and macroclusters or precipitates. It is often

assumed that when an impurity is incorporated in an ionic crystal it is rather homogeneously distributed

throughout the crystal. This is not the case [21] and the discussion of the microclustering behavior of

impurities in crystals will clearly demonstrate this.

The classic case of the formation of microclusters concerns manganese. For years work showed that

manganese clusters by a third order process [22,23] and only recently has experimental evidence been

presented to show that there is an initial stage in which dimers are formed and then a second stage in

which trimers are formed [24,25,26]. However, manganese is not a common impurity in alkali halides and we will discuss in detail a more common impurity, strontium. The same processes occur for many other

aliovalent impurities. The evidence for clustering of strontium comes from ionic thermal current

measurements. These measurements determine the temperature at which a field oriented a dipole relaxes

within a crystal and the magnitude of the charge stored in that particular relaxation process. In figure 9 are shown the results of studies of strontium in KCl by Jacobs, et al [27]. In diagram (a)

is shown the current versus the temperature for depolarization of the crystal. Initially at (a) we havi one rather large relaxation peak. As time passes the peak shifts to higher temperature and decreases

in amplitude through (a), (b) , (c) , (d) . This change in relaxation properties occurs in 17 hours at 250°K, well below room temperature. The quantative analysis of this change in relaxation properties is

given in the table included in this figure. There not only is a shift in the energy at which this

relaxation occurs, but a decrease in the total amount of charge that can be stored in the crystal. At this temperature the change in relaxation properties cannot be due to the diffusion of an additional

impurity into the crystal from an external source. It could be due to the formation of an anion-cation

lattice compound such Sr(OH)^ only if the hydroxide were in close proximity to the strontium initially.

Crystals used in this experiment were not produced under a reactive atmosphere and could indeed contain

hydroxide. However, Jacobs et al . assigned this change in dipole relaxation to the formation of

impurity-vacancy complex dimers as shown at the bottom of figure 9. Recent measurements [28] on the

diffusion of strontium in KCl gave a diffusion coefficient represented by eq . (14) • From the magnitudeof the

DgCSr^"") = 1.20 X lO"'^ exp(-0.871eV/kT) cm^/sec (14)

diffusion coefficients its clear that this change cannot be due to a normal diffusion process occurring

at 250°K. Optical studies were not reported which could have eliminated the possibility of the hydroxide formation. Whatever the mechanism of this change in the dipole relaxation properties of KCl:Sr^

the significant point is that it occurs in such a short time at such low temperatures that it cannot occur by any normal diffusion process. One must conclude that either an unknown mechanism of diffusion

exists in the alkali halide or that the impurities are distributed within the crystal in such a way

that they are nearly adjacent to each other. While this type of clustering will only increase the

general background scattering of the crystals slightly, another type of clustering can significantly

alter the optical properties of the crystal.

It has been amply demonstrated that the vibrational spectrum of an anionic molecular impurity in an alkali halide depends on the cation associated with it as a charge compensating cation [29,30,31].

Another type of clustering occurs with molecular anionic impurities that has been less well discussed

in the literature. The only documented case of polymerization of an anion molecular impurity is a case

of metaborate in sodium chloride, potassium chloride, and potassium bromide [6]. The metaborate ion

introduces a spectrum with well known fermi doublet due to an isolated BO2 impurity [3]. However, that

spectrum is often accompanied by a very complex spectrum that extends over a wide region of the infrared. This is shown in figure 10. The complexities of this spectrum can be explained by the formation of a complex of BO^ ions. While the exact structure of the complex leading to this spectrum is

being debated by the investigators, the consequences of such polymerization are obvious in examining

the curves for the annealed crystal. As the crystal is annealed the spectral lines broaden until they

produce absorption over a large portion of the infrared. As the annealing continues actual

clusters of metaborate can be observed by an ultramicroscope as shown in figure 11. The clustering is

caused primarily by the misfit of the polymer in the lattice. Note that no clusters are shown for KCl

and that the effect of annealing on KC1:B02 is much less than on sodium chloride or potassium bromide.

At the onset of this precipitation stage in the crystal, not only broad absorption bands but signifiamounts of scattering from the forming precipitates was observed . This process is often called Ostwald

Ripening [33,34,35,36] and occurs in any local region of a crystal that is supersaturated with respect

to an impurity at the temperature of that region of the crystal. While such a process would not be

rapid in a crystal at room temperature, if such a crystal were exposed periodically to high intensity

electomagnetic radiation that heated local regions of the crystal in that region such polymerization

would initially occur finally reaching a point where precipitates could form. Thus, a stepwise, but

not necessarily linear, increase in absorption on repeated exposure to high intensity radiation in

local regions of a laser window may be observed. It appears probable that such a crystal would fail

-30-
before the stage of actual precipitaticT occurred. The size distribution of the precipitate depends on mechanism (bulk, grain boundary or surface diffusion] by which the precipitate is formed [37]. None

of the literature data is suitable for kinetic analysis of clustering or precipitating.

The final stages of precipitation in a crystal often results in the formation of an ordered second

phase. This generally is a phase composed of a divalent cationic impurity M, their associated

vacancies, and a sufficient number of host ions to produce a second phase with a composition MCl^ *

6KC1. These phases are called Suzuki phases [38] , and in figure 12 transmission electron micrograph [39]

of such a phase is shown. Such phases are only found in relatively impure crystals that have been well

annealed

.

The foregoing discussion has been primarily concerned with single crystals containing trace

impurities, both anionic and cationic. However, if the crystal contains a grain boundary as may occur

in poorly grown crystals or may be produced in forging a crystal, other rapid low temperature degradation processes may occur. In figure 13 is shown a scanning electron micrograph of an uncoated surface

of sodium chloride containing a single grain boundary [40]. The sodium chloride was nominally pure but

note the impurity phases clustered along the grain boundary. This crystal was pulled from a melt from

ultrapure Merk "pro analysi" sodium chloride. Analysis showed it to contain trace amounts of calcium

(Z=20) and potassium (Z=19). Due to the similarity and electron reflection characteristics of these

two ions, it is not possible to tell whether the impurity clusters contain calcium or potassium. The

foregoing discussion in this paper would suggest that they are more likely calcium than potassium. If

that is so, in a region of the crystal particularly penetrable by moisture, we have an ion which will

enhance diffusion. It is likely that the diffusion along the grain boundary for moisture is enhanced

at least as much as it is for sodium along a similar grain boundary; that is by a factor of around

450-500 times that in the bulk crystal. This would lead to more rapid deterioration of the optical

properties of such a crystal than a crystal containing a similar impurity concentration in its bulk.

Whether such clusters form on forged crystals is unknown, but in the forging process the effective

temperature of the crystal is increased and it is reasonable to expect that the concentration of

impurities would be enhanced along these grain boundaries. Measurements of the mechanical properties

of such a crystal indicate that divalent impurities will increase the hardness of a crystal by precipitating at the grain boundaries.

Conclusion

In this paper we have attempted to draw from the body of knowledge concerning the behavior of

impurities in alkali halides to discuss mechanisms which could possibly lead to deterioration of the

optical properties of a laser window. These processes conveniently divide into aging which occurs at storage temperature and clustering through use. Pure single crystals should be least susceptible to these degradation processes. The inclusion of aliovalent impurities in the lattice will increase the

rate of aging and eventually contribute to clustering. If the window contains grain boundaries or large numbers of dislocations as well as aliovalent impurities the useful lifetime of the window may be

markedly shortened.

References

[1] Lipson, H. and Ligor, P., "Round Robin on

, Colorimetric Measurement of 10.6pm Absorption in KCl." Air Force Cambridge Research

Laboratories

.

[2] Fredericks, W.J., in "Diffusion in Solids

Recent Developments." Academic Press,

New York (1975), data from p. 431.

[3] Watkins, CD., Phys. Rev. 113 , 79 and 81 (1959) .

[4] Dreyfus, R.W., Phys. Rev. 121 , 1675 (1961).

[5] Allen, C.A. and Fredericks, W.J., Phys.

Status Solidi 3a, 143 (1970).

[6] Becker, C.R., Thesis, Oregon State University, Chemistry Department, Corvallis,

Ore. (1969).

[7] Nelson, V.C. and Friauf, R.J., J. Phys.

Chem. Solids 3^, 825 (1970).

[8] Whipple, R.T.P., Phil. Mag. 45, 1225

(1954) .

[9] Harris, L.B. and Fiasson, J., Phys.

Status Solidi (a) 33^, 697 (1976).

[10] Harris, L.B., J. Phys. (Paris) 37, C7-

365 (1976).

[11] Allen, C.A., Ireland, D.T., and Fredericks,

W.J., J. Chem. Phys. 47^, 3068 (1967).

[12] Krause, J.L. and Fredericks, W.J., J. Phys. (Paris) 34_, C9-13 (1973).

[13] Allen, C.A. and Fredericks, W.J., Phys.

Status Solidi 55b, 615 (1973).

[14] Allen, C.A. and Fredericks, W.J., J. Solid

State Chem. 205 (1970).

[15] Ikeda, T., Japan J. Appl. Phys. !£,

1810 (1973).

[16] GrOndig, H. and RUhenbeck, C, Z. Phys.

183, 274 (1965).

[17] RUhenbeck, C, Z Phys. 207 , 446 (1967).

[18] GrUndig H. and RUhenbeck, C, Z. Phys.

249, 269 (1972).

[19] Fritz, B., LUty, F., and Anger, J., Z. Phys. 174 , 240 (1963).

[20] Chaney, R.E. and Fredericks, W.J., J. Solid State Chem. 6, 240 (1973).

[21] Allnatt, A.R. and Chadwick, A.V., Trans. Faraday Soc . 63^ 1929 (1967).

[22] Cook, J.S. and Dryden, J.S., Proc. Phys.

Soc. London 80, 479 (1962).

-31-
Dryden, J.S. and Harvey, G.G., Proc. Phys.

Soc. London (Solid State Phys.) 2, 6C3

(1969)

.

Crawford, J.H., Jr., J. Phys. Chem. Solids

3]_, 399 (1969).

Symmons, H.F., Proc. Phys. Soc. London

(Solid State Phys.) 3^, 1846 (1970).

ibid. 4, 1945 (1971) .

Hor, AH Mee, Jacobs, P.W.M., and Moodie,

K.S., Phys. Stat. Sol. (a) 30, 293

(1976) .

Machida, H. and Fredericks, W.J., to be

published

.

Coker, E.H., Decius, J.C., and Scott,

A.B., J. Chem. Phys. 35^, 745 (1961).

Decius, J.C., Coker, E.H., and Brenna,

G.L., Spectrochim. Acta 19^, 1281 (1963).

Decius, J.C., Spectrochim. Acta 21, 15 (1965).

Decius, J.C., Becker, C.R., and Fredericks,

W.J., J. Chem. Phys. 56, 5189 (1972).

[33]

[34]

[35]

[36]

[37]

[38]

[39]

[40]

Greenwood, G.W., in the "Mechanism of

Phase Transformations in Crystalline

Solids," Institute of Metals Monograph

No. 33. (1969) p. 105. Lifshitz, I.M. and Slezov, V.V., Zh. Exptl.

Theor. Phys. (USSR) 35^479 (1958); translation: Sov. Phys. JETP 38^331 (1959).

Lifshitz, I.M. and Slezov, V.V., J. Phys.

Chem. Solids 19_, 35 (1961).

Wagner, C, Z. Electrochem. 65_, 581 (1961).

Jain, S.C. and Hughes, A.E., J. Phys.

(Paris) 37, C7-463 (1976)

.

Suzuki

,

(1961)

.

K. , Japan J. Phys. Soc. 16^, 67

Yacaman, M.J., Goringe, M.J., and Hobbs,

L.W., UKAEA Research Report AERE-R846

(1976) Phys. Stat. Sol. (a) 39, K85 (1977).

Harris, L.B., Moncrieff, D.A. and Robinson,

V.N.E., Phys. Stat. Sol. 35, 371 (1976).

Figures

Figure 1. Migration energies of divalent

impurities in sodiumchloride as a function of

impurity radius. Solid line shows least

squares fit to D values.

Figure 2. Diffusion coefficient of Cd in

sodium chloride as a function of cadmium

concentration. The data were measured at

(A) f20''C, (B) 472°C, (C) 422*C, (D) 376''C,

and (E) 301°C. (ref. 11).

-32-
Figure 3. Log Dg vs 1/T for Cd

2+

in NaCl.

(A) Saturation diffusion data. (B) Literature

data with large D and bent Arrhenous plot

(Ikeda, T. , Japan J. Phys. Soc. 19, 858 (1964).

2+

(C) KCl:Cd (Keneshea, F.J. and Fredericks, W.

J. Phys. Chem. Solids 26, 501 (1965). (D)

Arrhenous plot of constant concentration D from figure 2. (ref. 11).

distanct (10- cm)

Figure 5. Comparison o pure KCl (circles) and

(KC1:2 X 10"^ mfOH~) (t

experiments the crystal

and the diffusion time source of HgCl^ at 180'

provided the diffusant.

was arbitrarily brought

concentration as found

Figure 4. Penetration profile for simultaneous

diffusion of cadmium and lead ions into purified

KCl at 455''C for 3.920 x lO^s. (ref. 12).

f mercury diffusion in

hydroxide doped KCl

riangles). In both

temperature was 473°C

7.58 X lO^S. A vapor

C in 1/3 atm Cl^

The KCl: OH profile

to the same surface

for pure KCl. (ref. 13)

.

r- 7

'EEZ 6

o

o

a.

<

OH

#*°204nm

oo

°0 Ql 0.2 0.3 0.4

Distance (mm)

0.6

Figure 6. Distribution of the 240 nm 0 band

relative to the 204 nm OH band after exposure

of KCl: OH to CI2 at 1180 torr for 20 hr at 600°C. Initial OH concentration not specified,

(ref. 15).

-33-
H :a.<a.z bth.>

Figure 7. Fractional diffusion profiles C/C

vs distance for and OH in KCl and KBr

°

when diffusion is enhanced by Impurity reactions. The lowest curve represents in

KCIiK"*" after 180d at 25°C. The next represents OH in KCl if enhanced by CI2 after

180d at 327°C. The third curve represents

H^O in KC1:K"^ after 360 d at 25° C. The

highest curve represents H2O in KBriK"^

after 180 days at 25°C. These profiles

were calculated assuming the values of D and U obtained by Griindig and Riihenbeck

(18) are valid at these temperatures. The

figure should represent the "worst case"

.condition.

4.0 4.2 lO'/T CK)

Figure 8. Dipole relaxation time t vs 1/T

for a KCl:Cd^''" (60 x lO"^ mol fraction)

showing the additional relaxation formed

by exposure to atmospheric HO for 12 mo.

(ref. 20). ^

Plotn of the thermal (lepolkriution current / agkinU T for a cr>-aUl of KChSr" annealed Ht 623 K Jor 10 h.

{a.) Immedintely after qu«nclung, (b) aller I h mt 200 K,

(t) «ftor a lurther hour ftt 250 K, (d) •fter 17 h at 250 K

Figure 9.

Thermol depolariinlion pornmclen tor KChSr"*

mncntoU nt (133 K (or 13 h, Ullcrs idenlLfying runs ore cxplainsl in the caption to Fig, 2

(10-" C) (eV)

I {

LTpeftk, T„ 210 K

1.44 ! 0.079

2.02 I 0,070

0.12 j 0.670 I HT pe»k. = 221 K

0,439 0.74 0.4S9 I 0.74 0.753 ! 0.74 0.795 I 0.74 Oei0©O©O

©O© O ©o©

Ponibic .©Q© dimv conflguratioiu in the NoCI©©©. atructurc villi a) no net ilipolc uid b] a net

dipo'e component along ^100>

Thermal depolarization of KCl: Sr. (ref. 27).

Figure 10. Spectra of boron containing species

in NaCl, KCl, and KBr taken at T - 100 °K (700-

1600 cm""*") and T = 300°K (1930-2080 cm"-^) which

demonstrate the effect of thermal history upon

the equilibria between species, a. (before

quenching), b. (after quenching from 605°C, and

c. (after annealing) denote the thermal history

of a particular crystal. The small numbers near

symbol refer to isotopic composition of the

BO^. (ref. 6).

-34-
Figure 11. Light scattering observed by means

of an ultramicroscope . Large clusters in (a)

KBr, (b) NaCl, (c) KBr, and (d) NaCl. All

crystals doped with metaborate ion. (ref. 6).

Figure 12. Suzuki phase precipitates (6NaCl • MnCl2)

showing interface dislocation structure in a TEII micrograph at 15°K. (ref. 39)

High-Resolution Examination of Uncoated Insulators by SEM

((in 1 1 In.TVstal siirf;ii>c sKowin)! sojircfjaliiiu 111 ]>f I'ci jiit^Ui'.'^ i » liUi-i to tiruin bouniliu-y I'vcrli'Ml. Iihi.'k), SiR-ciim-ii tilt 45'

{abiiui ii(in/iiii!.»! axis), accotpral iiij; vi)lUiirr 25 kV

of ( liliii mm jili'iloy ^ anil civ ilirs k spots irnlri:ilril by arrows) ;il ;;raiii lioLintlrtry, .Sjjcirinu'n tilt (tibimt lionwmtitl

axis), acpclt-rjiting v<)Ua;Lr<' l^V

Figure 13. Precipitation along a grain boundary

in NaCl. (ref. 40).

-35-
COMMENTS ON PAPER BV FREPERI CKS

In KupoYKit to a quutlon about poiiibZe, ctuitznlriQ calcxum hijdfLoyJ,de. bnpunJjtiu, in potaaium

chlonA.de. windowi the. ipeakeA nemoAked that wheAzoi iome. hydn.oxyt C-ompound^ ^om cZuu>teu in a veAy

ihont tAjmz, ha, ej>tAjnoitioni> indicate, that 6omeXhinQ tike, a yeoA mold be ncquAJied don. the {jOnmation o^

iignif^icank calcium hydAoxidc ctuAteM. He du/utheA commented that the e(j£ect of, uttnaviolet nadicution

in enhancing di{j(jUyiion and ctiUteAing o{j impuAitieA hai not been exptoned bat i,ince uttAavioZet excitation changeA the etectAonic itAuctoAe o^ the lattice and couue can lead to colon, centeting, one muZd expect the mignatton o{i ijmpuAittei through the lattice to be a{^{^ected by ultAaviolet Kadiiation.

-36-
PURIFICATION OF POTASSIUM BROMIDE AND ALKALINE

EARTH FLUORIDES FOR LASER COMPONENTS

W.J. Fredericks

Chemistry Department

Oregon State University

Corvallis, Oregon 97331

The use of selective ion filters and reactive gas treatment for purification of KBr SrF^ and BaF^ is considered. The principles of design of selective ion filters and the

choice of materials for such systems is discussed. Examples of the construction of such

systems are given. A system for removing monovalent impurities from alkaline earth fluorides

is shown. A greaseless reactive gas system and a three stage reactive gas seal were developed.

Key words: Potassium bromide; strontium fluoride; barium fluoride; ion exchange purification;

selective ion filters; reactive gas purification.

Introduction

In using ion exchange resins to separate a mixture of trace ionic impurities from a desired major

constituent one utilizes the property of the resin to bind these various ions to itself with varying

degrees of tenacity. The common measure of the property is called selectivity. It is measured as an equilibrium quantity of the reaction of the type given by

i nK"" + K^R- = i m"" R- . , (1) n n n

'

then the selectivity E is a mass action constant given by

E^!/" = ^Vn^R ^V^S . . (2) K

IVhere a is the activity of the various components in the resin phase R or in the solution phase S. The

selectivity is not constant for a particular ion on a particular resin but is a function of any of the

conditions of the resin or solution that affect the activity of the components in either phase. The

major factors that affect E are given in Table 1. Table 1. E is a function of any factor

that affects the various activities.

Examples:

1. Resin type and structure (energy of sorption)

2. Extent of resin cross linking

3. Resin swelling

4 . Ionic charge

5. Total ionic strength of passing solution

6. Relative ionic strength of exchanging ions

7. Extent of hydration of ions (size)

8. Strength of hydration bond

9. pH of the solution

10. Temperature

This property of differential absorption for various ions on the resin can be used in many ways to separate a mixture of such ions. In Table 2 we list the principal ways in which one employs these

resins to separate mixtures of ions.

Table 2. Application of resins.

1 . Ion exchange chromatography

2. Ion exclusion

3. Ion conversion

4. Frontal analysis

5. Selective ion filters (requires

several stages)

Selectivity coefficients are determined by rather tedious equilibrium measurements in which a small

quantity of resin is shaken for several hours with a small quantity of solution. Then the solution is

removed and the resin is washed to remove the ions it has absorbed, then both solutions are analyzed.

However, a more practical measurement is a quantity we call the separation coefficient which is given by

equation 3.

Si = ^iil- = f (Resin, C^ , T, Q, . .. .

, etc) (3)

'-oi.iJ t -37-
This quantity is not a thermodynamic quantity and simply the ratio of the concentration of an impurity

in the elutant from a column containing a measured quantity of the resin to that in the original

solution. It is not a equilibrium quantity.

Of the five methods given in table 2, the two most useful in preparation of materials for crystal

growth are ion exchange chromotography and selective ion filters. For ion exchange chromotography the

major component and the impurities contained in its solution are absorbed on a resin. Then the various

ions are eluted from the resin in a solvent in a sequential order. This process is not particularly

efficient when one of the components far exceeds the others in concentration. It is a process of last

resort and purification of salt systems for crystal growth simply because of the concentration problems

with overlapping elution bands and the size of the columns required. The preferred method is a selective ion filter. These operate with a combination of resins in which the orders of selectivity for

various ions differ between two or more resins. In table 3 we list the selectivities of four cation

exchange resins in which the order of the monovalent ions differ.

Table 3. Dilute solution selectivities.

Chelex 100 H >> Li > Na > K > Rb > Cs ; Cu*'^/Na* >> 100

AG50W Ag > Rb > Cs > K > NH^ > Na > Li

Bio Rex 40 Cs > Rb > K > Na > H > Li

Bio Rex 70 H > Ag > K > Na > Li

The Chelex 100 will absorb hydrogen, lithium, sodium more strongly than potassium while in the other

three, in particular AG50W, silver ribidium and caesium are absorbed more strongly than potassium and

potassium more strongly than ammonia, sodium, and lithium. Thus, if you pass a solution containing a mixture of these ions first through Chelex in potassium form the hydrogen, lithium, and sodium will be

removed. Then passing the same solution through AG50W in the potassium form ammonia, sodium and lithium

will be removed. Thus by passing a solution through the two resins we have a filter that passes only

potassium ion. I list only the copper-sodium ratio of selectivities which is far greater than a hundred

as it is typical of the much greater tenacity with which the divalent ions are bound to the resin than

the monovalent ions.

The selectivities given in table 3 are for very dilute solutions passed under ideal conditions. In order to actually construct a selective ion filter, one must measure a property that describes a separation of the ions under actual operating conditions. For this we use the separation coefficient and measure it as a function of concentration of the solution, pH of the solution, and any other parameters

that one may vary in the development of the separation process. These can be measured rather quickly on

a small column. However, one must measure them over a rather wide range of conditions in order to be

assured that the operating latitude of the columns will be sufficient to accommodate a large volume of

solution. In figure 1 we provide an example of the type of measurements one makes to measure the

separation coefficients for ion exchange filters.

Clearly for the separation of the divalent ions on an ion selective filter a chromatography column

must first be used to absorb the desired divalent ions and allow the monovalent ions to elute. Note

again the ratio of selectivities between divalent ions and monovalent ions are the order of a hundred

or more. However, the resin has a finite capacity for the amount of ion it can absorb and thus extremely large columns are required for separation of the alkalies from the desired divalent ion. An alternative is to use a chemical process which will remove the monovalent ions to the extent that they are no longer troublesome. This sort of a system will be described later in this paper.

The anion resins are not available with differing orders of selectivity. One can build a system

using several anion resins in which the initial resin absorbs most of the polyvalent ions and the anion

complexes that may be present while the second resin produces an extremely strong acid solution of the

salt which will have very minor amounts of other anions present. That type of system is used in both

the monovalent and aliovalent ion systems. The anion resins again possess the property that they absorb

the polyvalent ions much more strongly than the monovalent ions and that larger ions are much more strongly absorbed than smaller ions. Thus a solution passing through this double anion exchange resin

system will be free most anionic impurities with the exception of other ions very similar to it. For example, in our systems we expect no carbonate or hydroxide or sulfate or bisulfate or bisulfite ions

but we do expect other halide ions with the exception of iodine. These halogens can be removed by HBr

in the reactive gas treatment occurring later in the process of purification.

Selection of Materials of Construction

One of the best means of preventing contamination of the solutions is to allow them to contact only

materials which they do not wet and do not strongly absorb impurities. Many plastics possess these

desirable qualities and are satisfactory for the construction of these systems. However, condensation

polymers that contain amide or ester bonds, such as nylon or dalrin or any of the polyesters, must be

avoided as these bonds are hydrolized by the solutions used in these processes. The best polymers for use in these systems are the addition polymers such as polyethylene, polypropylene, and teflon. However

methylmethacrylate which contains an acid side change not involved in the polymer bond is also satisfactory. The latter has a lifetime of about 10 years in chloride systems and should be similar in

bromide systems. It fails by swelling and checkering and not by decomposition of the polymer bond.

Thus it does not release organics into the solution. It will not take materials such as gaseous

-38-
hydrogen bromide which cause it to crack rather quickly. Any substance used in construction contains

impurities. Table 4 shows a common impurities found on and in various materials commonly used in

purification and crystal growth. It contains both useful and useless materials to provide a comparison.

The impurities in highest concentration are mostly iron, zinc, cobalt on the plastics listed in this

table. They come from mold release compounds or extrusion greases used in forming the materials and the

great portion of them are on surfaces. They will not be removed by ordinary washing but can be removed

by chemical treatment. They are in the form of stearates or oleates and can be removed by first treating with a strong acid to hydrolize the insoluble soap bond forming a soluble metal nitrate or halide

then washing this portion free leaving the fatty acid behind. Next treat with a strong alkali such as potassium hydroxide that forms a soluble soap which can then be washed out of the system. Otherwise

they remain on the system and slowly contribute ions to the solutions being purified. We have found the

above treatments when associated with ultrasonic cleaning in the rinse cycle to be very effective in

reducing the contaminants on the plastics we used. Examination of the elastimers shows them to be even more contaminated and while a good portion of this contamination is on the surface a large amount is

incorported in the compound itself. We have not successfully been able to remove this by any cleaning

process to a low enough level to allow these materials to be in direct contact with the flow stream of

the material being purified. Thus in the portions of the system where elastimers must be used, they

should form the seal in a blind seal chamber. This is done in our systems by allowing tubing that is

sealed by an o-ring to extend beyond the o-ring a half inch into a tightly pressed seat. Then the

o-ring is driven against the external portion of the tubing by a compression nut. Not all elastimers

are suitable for use in our systems. We have found that polyethylene -polypropylene o-rings are the only

materials that hold up to our solutions without decomposition.

Design of the KBr Purification System

The system used for purification of the potassium bromide is shown in figure 2. It consists of seven columns, four of which are ion exchange columns and the other three are used to control the pH of

the solution passing through the systems. It contains five reservoirs, one for raw KBr, one for pure

KBr, one for KOH, one for water, and one for HBr. The latter four reservoirs are used in regeneration

of the columns or in changing the pH of the solution as it passes through the system for purification.

The first cation resin is Chelex 100 in the potassium for, the second cation resin is AG50W

in potassium form, the first anion resin is AG2 and the second anion resin is AGl both in the bromide

form. All the places marked F are filters, the first is a millipore filter with five micron pores, the

second is a polypropylene filter, the third is a polypropylene filter, and the fourth is a millipore

filter containing five micron pores. All the millipore filters are teflon-based mitex filters. The

only commerically available parts are the check valves that occur in the lines used to adjust pH or concentration. This is to prevent back flow and contamination of the reservoirs should the pressure of

the KBr solution exceed that in the reservoir. If an operator inadvertantly opened the wrong valve, he

could cause the system to flow backwards into an unpressurized reservoir. These valves are completely

made of teflon and are available from the Fluorocarljon Company. Pressurization is done by filtered

regulated nitrogen. The system is usually operated at 5-10 pounds guage pressure. All other components

are constructed of polyethylene, polypropylene, methyl methacrylate , or teflon. The on and off valves

are designated by a cross in a circle, the adjustable flow valves are denoted by a half-filled circle

and sampling valves denoted by a cross with an external dot. The boxes marked with an M and a subscript

are measuring stations. These consist of a methyl methacrylate body with a teflon plug containing a silver chloride/glass hydrogen electrode and are used to measure pH at that point. Note that all

measuring stations are in a line in which a portion of the solution being purified is passed to a waste

line and does not return to the main stream. This is to avoid contamination of the solution by the

electrode. Not shown in the diagram is a hydrostatic head device used to assure that the electrodes are always emersed in solution. The pH adjusting columns shown in the diagram contain a series of baffels

to assure that the solution which is brought in to the central portion of the bottom column cap is

thoroughly mixed with the adjusting solutions that come in through side of the bottom cap. These are

all constructed on the same module for efficiency in the machining of the parts. The on-off valves, the

adjustable flow valves, the sampling valves and the sampling chambers are all constructed on the same module which is shown in figure 3 with the various devices that fit into it. An additional device,

which converts any of the valves to a point at which resin or other materials can be inserted or removed

from the system at a direction perpendicular to the plane of the drawing. In the potassium bromide

system, these are used only when filling the columns with resin or removing resin from a column. Each

of the columns has a polypropylene filter built into its base to prevent resins from passing through

into the finer filters marked F in the system.

The hydrogen bromide is made from 15 Mfi water and electronic grade hydrogen

bromide gas. It is made in a polyethylene system and analysis shows the concentration of impurities to

be below the limit at which we can detect them (a fraction of a part per million). The potassium

hydroxide is purified within a column system of Chelex 100 in the potassium form and AGl in the

hydroxide form. It is not ultrapure but is low in aliovalent ions and in alkalies other than potassium.

The raw potassium bromide solution is 2 molar and has been passed through analytical grade filter to remove the insoluble materials always found in reagent grade chemicals. It is initially made acid by

the addition of hydrogen bromide but once in the reservoir potassium hydroxide is added to it to bring

its pH to 7.5 to 8. Table 5 gives an analysis the worst lot of potassium bromide obtained from

Mallinkrodt Chemical Company. The raw salt contains an amazing quantity of caesium and sizable amounts

-39-
of lithium and ribidium. The aliovalent ions iron and nickel are present in larger quantities than most

other impurities for which we analyzed. The analyses were done on a Perkin-Elmer 403 atomic absorption

spectrograph using an HGA 2100 graphite tube furnace as the evaporation source. The detection limit is the actual measured detection limit from the statistics from the various measurements.

To place the system in operation, it is first washed with hydrobromic acid, followed by deionized

water, followed by potassium hydroxide, followed by hydrobromic acid, followed by deionized water until

all the pH electrodes indicate that the system is neutral. Then the resins are washed into columns one, three, five, and six. The resins are washed in as the converted form and require only further rinsing

with deionized water. The potassium bromide is passed into column one and the entire amount is wasted

Out Ml until the pH of the effluent is greater than 9. Then it is allowed to enter the first pH column

and the effluent is wasted out M2 until the pH at M2 reaches about 7.5. Then acid is added to the

bottom of column two at a very small rate until the pH at M2 is less than 6 then it is allowed to enter

the third column. The effluent from column three is wasted through M3 until M3 indicates a solution has

reached that point then it is allowed to enter column 4 where the pH adjusted to about 6, as measured by

M4 then allowed to enter the first anion column. It emerges from the first anine column rather acid and

when M5 indicates it has reached that point it is allowed to pass to the second anion column. When M6

indicates an extremely acid solution of pH near 0.8 it is passed into the last column and wasted through

M7 until the solution reaches M7 at the appropriate pH then M7 is closed to a slight drip as were all

the previous metering stations. The solution is then collected in polypropylene container in the vacuum

evaporator. The output of the glass-AgCl electrode at each metering station passes to an operational

amplifier interface which converts the impedance from the 1000 MQ of the electrode to 4000fl to match

the John Fluke 2240A data logger. This device has separate high-low limits which operates a red (high)

and a green (low) LED limit indicator at each metering station so that any variance in the operating

conditions can be immediately detected by operator and corrected by adjustment of a suitable valve.

During the course of a successful run, about 3 to 4 gallons of purified potassium bromide are collected. There are two evaporators available so that the jars can be changed with only a momentary

interruption in the flow process. The vacuum evaporator containing a collection jar is shown in the

figure 4. After the jars of KBr solution have been collected the evaporator is placed in a nitrogen

tent where the jars are covered and transferred to a vacuum oven to reduce the solution to a reasonably

dry solid. The jars are closed, the vacuum oven opened, and the jar replaced in a nitrogen tent and the

KBr transferred to a polypropylene bottle for storage.

The purified KBr is placed in a clean quartz crucible while in a nitrogen tent and while in the

same tent the cruicible is transferred to a 3-inch diameter by 18-inch long quartz test tube. This tube

is sealed with a Rh-plated nickel seal which has a smaller three stage seal for a Rh-plated nickel cold

finger. for crystal pulling. The entire assembly is closed and placed in a crystal growing furnace and

the cold finger connected to the pulling mechanism. The furnace has been described elsewhere [2]. A photograph of the three stage seal is shewn in figure 5. The system is connected to the reactive gas

manifold shown in figure 6. The low pressure HBr is brought in through one of the ports at the side

of the cold finger seal. The first stage of cold finger seal is at the same pressure as the interior

of the crystal growing chamber, the second stage is at HBr at about 5 pounds guage pressure and the final

stage filled with argon at about 7 pounds guage pressure. This arrangement allows only HBr to leak past

the seal into the crystal growth chamber and prevents the HBr from leaking into the laboratory. The

other port in the seal connects to a separate vacuum system with a liquid nitrogen trap closed by

teflon valves. The crystal growth chamber is exhausted through this second vacuum system to prevent

contamination of the pure HBr. The gas manifold providing the pure HBr is arranged such that the HBr

can be distilled and other gasses, such as bromine or chlorine, can be added to the crystal growth

chamber if desired.

When these salts undergo reactive gas treatment there is always some sublimation from the salt in

the crucible to cold walls at the top and bottom of the crystal growth chamber. It is with this salt

that any volitile compounds that may remain in the system are carried away from the melt. Although the

purified salt should not contain such impurities this design provides an additional safeguard against

impurities such as Tl*.

The Alkaline Earth Fluoride Purification System

The program for purification of alkaline earth fluorides is not yet complete. However, the ion

exchange systems for their preparation have been designed and constructed and will be discussed here.

The materials of construction and components themselves are similar to those developed for the potassium

bromide system. However, as the alkaline earth fluorides are not soluble enough for use in ion exchange

purification, there are some changes. Further since the alkaline earths have a very high absorption

energies on most ion exchange resins, it's necessary to remove the much less tightly bound alkalies from

the solutions prior to attempting separation of the alkaline earths and other divalent ions on the

exchange resins.

Figure 7 shows the first stage in the purification of the alkaline earths. This stage functions by

precipitating an insoluble alkaline earth compound which can be washed free of the alkalies, then con- verted into an alkaline earth halide and transferred to the initial reservoir for the ion exchange

system. This must be done without exposing the alkaline earth solution to an outside environment,

figure 8 shows the construction of the system. The raw salt solution reservoirs are on the shelf above

the filter systems at the top of this picture. The column on the left is one used to purify potassium

-40-
hydroxide and is not part of this system.

Initially a solution of a soluble barium or strontium salt such as an acetate or nitrate is prepared. It is made acidic then filtered through course of filters to remove the major insoluble com- pounds, then placed in the initial reservoir and a quantity retained on top of a lu hydrophobic teflon

filter located in the vacuum funnels on top the tvvo-gallon intermediate storage vessel. The solution

remains on top of the IM filter until the vacuum is applied in the container below the filter.

Initially CO^ is allowed to pass through the filter into the region containing the soluble alkaline

earth salt. This causes the carbonates to precipitate and this process is carried out until no further

precipitation occurs in the solution as observed from the upper transparent port. Then additional

solution is allowed to enter the chamber, more precipitation carried out until a quantity of alkaline

earth carbonate is collected on the filter. We then apply a vacuum, wash, remove the solution from the

alkaline earth carbonate, and start washing with the ionized water. Washing is continued with the

effluent passing into our waste system. On the way to the waste system, samples are taken of the

effluent using a sampling valve. This is carried out until we no longer detect alkalies, particularly

sodium, in the effluent of the system. Then purified hydrobromic acid is allowed to enter the filter

chamber. This reacts with the carbonate producing the alkaline earth bromide and C02- A vacuum is

again applied to the system, and the solution passes into the intermediate storage chamber. During this

time the is pumped out through the vacuum and hydrobromic acid is added until the solution in the

intermediate storage chamber is slightly acidic as measured on the electrode in the waste line. The

alkaline earth bromide is then transferred to the lower 5 gallon container and when that has been filled,

nitrogen pressure is used to force the alkali free, alkaline earth bromide into the first container of

the ion exchange system.

From that first reservoir the solution is then passed through a cation exchange resin. The first

column in that figure is labeled "alkali pass" but is now used to separate the barium and other ions

from the strontium. The solution passes from that column into a pH adjust column thence into a second

exchange resin to another pH column to a third ion exchange resin to another pH adjust column, then

through two anion exchange resins and finally into the last column used to convert bromide to fluoride.

At each stage in the system the pH is measured on a calomal/glass electrode. The output of these

electrodes go into a separate set of channels on the Fluke data logger. There are again red and green

LED lamps provided at each measuring chamber to alert the operator to deviations from permissible

operating conditions for the system. The first eight columns are exactly similar to those used in the

potassium bromide system and require no further discussion here. The final column is unique in that it

is a countercurrent precipitation column. The bromide passes through the center of the top plate is

immediately surrounded by a left swirling hydrofluoric acid solution which meets an upward right

swirling hydrofluoric acid within the column and the precipitation occurs in the turbulent mixing

region. The center of the lower plate is funnel shaped and collects the precipitate as it leaves the

column. The precipitate then goes to a filter system similar to that used in the initial stage of this

process where it is collected under a vacuum filtration. The effluent is pumped to a lime reservoir

that will react with the excess hydrofluoric acid. The ion exchange system is shown schematically in

figure 9 and figure 10 shows the basis of the system as constructed prior to its installation in our

laboratory. Final filtrate collection chamber is shown in figure 11. In figure 11 in the upper right

hand corner the bottom of the precipitation column can be seen with its externally flowing HF and

internal funnel.

The anion exchange resins are exactly the same as those used in the potassium bromide system.

However the cation exchange resins are different and have not been completely characterized at this time.

Because of the acidity of the solution Chelex 100 may not be as useful as it is in the alkali

halide purification systems and that AG50W, Bio Rex 40 and Bio Rex 70 may be the best choices for resins

purification of the alkaline earths. The measurements for characterization are currently underway and

will be shortly completed.

The purification systems described here when operated correctly, should reduce impurities in

potassium bromide to the part per million level for alkalies and other halides and for the aliovalent

ions to a much lower level. The alkaline earth fluoride systems have not been operated sufficiently to make estimates on the quality of salt they produce. We expect the operating details on the systems will

be available shortly.

References

[1] Rearrangement of data from Bawyer, D.T. and [2] Fredericks, W.J., L.W. Schuerman, and L.C.

J.L. Roberts Jr. "Experimental Electro- Lewis, "An Investigation of Crystal Growth

chemistry for Chemists." Academic Press, Processes." Final Report, AFOSR-217-66

,

New York, 1974, p. 135. Chemistry Department, Oregon State University,

1967.

-41-
rH

cd Hu

<0

•t-> m6m3o

•H

oi O. > C C •H

•H

cn

to

co

c

H

<u 4-1 e OJ

(i> ^1

t-H 4-> 0) c

0)

O

o

c

nJ

o

Si

o

H

E- 1 o LO lO in 11 1 1 o 1-H t 1 CO o rH o o l-H ^ \/

^"

LO

d OJ 1 u Cs] at 1 m

O 1—1 » 1 o

o

o

V r-HV 1 1 vO

o

t-O rH o o r—( o o CO O to o o . -1 <c O o K) o o IT) KH CM o O hH rH .-H V V V V V V i-H rH t oV , o o o

.—4 LO LT) o o O o • - 1 o

o

o • OO rH CZi Cl t—* o o r-H o c 1—1

1-H

o I-H

VV

VVV

V f-H

Oo

o

o rj- CI

CO

Oo

o 1 CD 1 o

t

o

oo

o ID CO Cl

V

V 1^ ' C)

o

o -a- CTir I fH }—1 u t—(

lO ro

o

O I V OI n 1 Q o 1 o o Q - r

o

oo 1 o --(- \} - h oo

u • _ o o o

o ^ ^ J3 ^3 CO _

toQ CD (3) oo

orJ - ^." tnO o

O .-' H

O """^

—

K) 1 1 1/1 t 1

CO* c _

Ol CM 1 ( o

Si ~; to

CO r--( oo -H

o L» -H t/i r-H •H X) to

t/) tiO X) p. -H nJ

P

o

4-1 •H f-l fH Jh LO cy 'H

CS

X J_> r-H i—io 0)

o bo ^J

u

O

r-H rG idu +->u

£3 1—*

o rn QJ

P

oo

O a.

cc <1>

o

D (/) +J

H rH

M 10

i-H

o 0)

u Jh •H cn XI

H

o nJ 1—1

c—<

+->

o to rH f->

o

cn j3 CJ 4-1

o -H CO < u (/) CO CD o 1-H u to •H -p r-H -H O -rH <y f- H tsj (> c:i P- o i-H +-> CCJ r: o. •H ^ ^1 <H p. (/I fH 4-1 o +J <U »H

o

-t >.

H

o CJl ^ P.

p

OO fH P- 5 <4-l !-( .-H r-H > rH O o o Jh a rti -H

c

!-( 1)

Ooo

o :3

o

H

O CO a- 1 < Cu 1 a. o C/5 :2 ca s CIS 1

-A2-
Element

Cs

Na

Li

Rb

Ca

Mg

Cu

Table 5. Levels of impurities in KBr mink #WD6K

(Expressed as ppm or pg impurity/ g KBr)

Four solutions were made, on days 3-22-77, 2-38-77, 4-1-77, and

4-4-77. The values obtained from each solution are shown in

chronological order.

1200 ± 160

1350 ± 250

1280 ± 154

1210 ± 121

63.8 ± 12 68.8 ± 9.3

57.9 ± 3.3

64.3 ± 6.3

224 ± 24 206 ± 4.7

186 ± 24

181 ± 13

247 ± 35 246 ± 51 238 + 25

350 ± 46

24.3 ± 21 5.5 + 1.7

15.0 ± 14 21.7 ± 11

1.04 ± 1.34

.29 ± 4.7

.58 ± .62 3.30 ± .29

5.2 + .82 5.1 ± .81 5.0 ± .79 6.4 ± .79

Detection Limit'

242

Element

Cu

10.9

23

55

16.9

2.5

1.1

Fe

Mn

Zn

Co

Ni

Cr

££m

5.2 + .87 5.1 ± .85 5.0 ± .83 6.5 ± .83

91.1 ± 26 83.3 ± 10

75.9 ± 10

70.1 ± 17

2.00 ± .22 1.53 ± .21 1.92 ± .21 1.88 ± .75

2.30 ± .30

2.42 ± .25 2.21 ± .29

1.71 ± .75

12.1 + 2.7

13.1 ± 1.7

10.8 ± 0 10.8 ± 0

151 ± 50

185 ± 49

166 ± 25

94.4 ± 23

02 ± 0.0

0 ± 1.3

0 ± 1.2

0 ± 1.2

Detection Limit

1.2

22

.49

1.7

52.1

1.3

Four samples run on the M gave the same peak height.

2The peak height was very small, and came where the standard curve intercepted zero. dA

Detection limit calculated as D.L. /2 a (samples)dc

-43-
Figures

-44-
15 MEGOHM

WATER

FILTERED 81

REGULATED

' NITROGEN.

TD DRAIN a PUMP

POTASSIUM BROMIDE ION EXCHANGE SYSTEM

Figure 2(a). Flow diagram of KBr ion-exchange system.

-45-
-46-


Figure 5. Crystal pulling chamber closure and cold finger seal.

All metal is rhodium plated nickel. 0-rings are per fluoroelas timer

.

-49-
CO

_Jo

CD

>-m

0 Q ®

>

UI

liJ

UJ

CD

<.y

Si

o_i

(TO

OLlI OCT

ID

CO<

UJ

X

o

<q: UJ

UJ OO I- <

IDO

Iq:o

oh:! 2Z

< llJ

LU

_J IUJ

< <

liJ (D

O

O

O

cr. DC

o

o

CO

-IXIE-J

-50-

Figure 8. Apparatus for monovalent ion removal.

-52-
Ld

ILU

CD<XoX

LdQ

cro

_J

Li_ O

q:

I-co

to

in

(1) 00B

IDo

<uIco

>4

Iduo

<4-l a

(0u

so3

60

•H

-53-
-54-
Figure 11. Closed filter system for SrF . z

-55-
FREQUENCY AND TEMPERATURE DEPENDENCE OF

RESIDUAL INFRARED ABSORPTION IN MIXED FLUORIDE CRYSTALS

H. G. Lipson, A. Hordvik and B. Bendow

Rome Air Development Center

Deputy for Electronic Technology

Hanscom AFB, MA 01731

and

S. S. Mitra*

Dept. of Electrical Engineering

University of Rhode Island

Kingston, RI 02881

and

J. Dept. of Physics

Oklahoma State University

Stillwater, OK 74074

We have utilized Fourier spectroscopy and photoacoustic calorimetry to measure

infrared absorption of the mixed fluoride crystal KMgF3 in the 800 to 2400 cm"-'- frequency range at temperatures from 80 to 535°K and compared the results with those

obtained for binary fluorides such as MgF2 and CaF2. KMgFg exhibits an exponentiallike decrease in absorption as a function of frequency at room and elevated temperatures characteristic of intrinsic multiphonon absorption in most ionic materials.

At low temperatures, however, well-defined structural features are observed in the

absorption spectrum which may be attributed to phonon density-of-states effects.

The data indicate that the Mg-F interaction has the principal influence on the

KMgF3 spectra. We also observe a similar exponential-like absorption vs. frequency

for other mixed fluorides such as KZnF3 . However, in the 2500 to 4000 cm~ range our measurements suggest that residual absorption in both mixed crystals

is dominated by extrinsic effects.

Key words: Infrared absorption; Magnesium fluoride; Potassium magnesium fluoride;

Intrinsic multiphonon absorption; Absorption temperature dependence.

Introduction

Recent investigations have established that the residual absorption in the highly transparent

regime of pure non-metallic solids is due to higher order multiphonon processes.. In this regime the

spectrum is often structureless and the absorption coefficient usually has a nearly exponential dependence on frequency [ 1 ] . Extensive experimental investigations of the temperature dependence of

residual absorption have been carried out for a variety of ionic crystals, particularly the alkali

halides [2-3] . A number of theoretical models [4-8] based on multiphonon interactions have been advanced from which the observed data can be interpreted. The temperature dependence of the absorption

coefficient can be understood to a large extent in terms of phonon occupation numbers [9] and some additional considerations [6] related to crystal structure.

We have previously reported [10,11] nearly structureless exponential behavior for the frequency

dependence of the absorption coefficient of alkaline earth fluorides. The temperature dependence can

be understood in terms of nearly Bose-Einstein like multiphonon behavior. The purpose of the present

investigation is to extend these measurements to various fluoride crystals other than those of the

fluorite structure, namely, MgF2 which crystallizes in the rutile structure, and the perovskites KMgF3

and KZnF3.

The cubic perovskites are mixed fluoride crystals of the general formula ABF3 where A is an alkali

metal ion and B a divalent metal ion. Mixed crystals are known to have better mechanical properties,

e.g. hardness, than the end member pure crystals [12], and can be attractive candidate window materials

'Present address: Elab, 7034 Trondheim nth, Norway

Supported by Deputy for Electronic Tech/RADC , under Contract F19628-75-C-0163

**Supported by Deputy for Electronic Tech/RADC, under Contract F19628-77-C-0176

-56-
for high energy infrared lasers if prepared with sufficiently low absorption. Moreover, many of the

perovskites are water-insoluble, similar to the alkaline earth fluorides. The perovskites studied in

this investigation, viz., KMgF3 and KZnF3 were found to be transparent in the CO and chemical laser

regions, and also stronger than KCl or KCl-KBr alloys.

Infrared active lattice vibrations in the one-phonon region of MgF2, KMgF3 and KZnF3 as well as two-phonon infrared spectra have been reported in the literature [13-16]. First order Raman spectrum

[17], and multiphonon infrared absorption [2] of MgF2 have also been reported previously. This paper

presents the first measurements of temperature dependence of multiphonon absorption for MgF2 and

KMgF3 as well as an analysis in terms of a simplified model [11].

Experimental Procedure

The KMgF3 and KZnF3 crystals were prepared at Oklahoma State University from stoichiometric mixtures of KF and MgF2 and KF and ZnF2. Crystals were grown by the Bridgman method in graphite or vitreous carbon crucibles using extreme care to minimize water pick-up in the KF during loading, and

NH4HF2 to RAP the system. The crucible was evacuated overnight while held at 300°C, and then it was pressured with 10 psig of gettered argon and raised to the melting point, 1070'-'C for KMgF3 and 870^0

for KZnF3 . After a temperature cycle to freeze and remelt the material, the crystal was grown at 0.15 cm/hr. The KMgF3 crystals used for most of the optical measurements were pulled from the melt.

A pull rate of < 3 mm/hr was used with starting material prepared by the Bridgman technique, as described above. The MgF2 samples used for transmission measurements were commercial material obtained

from Optovac, while the one used for photoacoustic measurements was obtained in polished form from

A. Meller Inc.

Absorption values were determined from transmission measurements made with a Digilab Fourier

transform spectrophotometer operated in the double beam mode using a nitrogen gas atmosphere. Elevated

temperature measurements were made in a small furnace which was placed close to the focal point of the

spectrophotometer. Temperature measurements were recorded with Cu-constantan thermocouples placed on opposite sides of the sample periphery, and temperature was controlled with a d.c. power supply.

Liquid nitrogen temperature measurements were made in a Janis dewar positioned in the same way, with

the sample clamped against an indium ring in contact with the coolant reservoir.

Sample thicknesses used for room and elevated temperature measurements ranged from 0.18 to 2.46

cm. Low temperature measurements on KMgF3 and MgF2 were made on samples 0.549 and 0.660 cm thick

respectively. In the case of MgF2, all transmission measurements were made with the beam directed

along the c-axis.

Absorption calculations were made from transmission data stored on magnetic tape using the

expression:

^ = - m

(l-R)'

2T

2 \ 2 -i 1/2

(1)

where d = sample thickness, T = transmission and R = reflectivity.

Corrections for small variations of the 100% line with wave number were made from comparison to measurements with no sample in the beam using the on-line computer of the spectrophotometer. Reflectivities were determined either from the maximum transmission in an adjacent spectral region where

no variation was found, or from comparison with absorption values obtained by photoacoustic

calorimetry . More details of the absorption measurement method are given elsewhere [18].

For absorption coefficients above 5x10"'^ cm"-'-, little variation was found between absorption

measured by photoacoustic calorimetry and that calculated from corrected transmission data. Corrections

for the variation of reflectivity with wave number or the change of refractive index with temperature

also included in the computer program were found to be negligible.

The photoacoustic calorimetric measurements were carried out at several CW laser wavelengths.

The laser radiation incident on the sample was periodically interrupted by a chopper, and the resulting acoustic wave amplitude was measured by a piezoelectric ceramic transponder in conjunction with a lock-in amplifier. Experimental details of this method are given in previous publications [18,19].

Results

Figure 1 shows the room temperature absorption coefficient of MgF2, KMgF3 and KZnF3 from 800 to 2000 cm"-*- as obtained by Fourier spectroscopy. On the same figure are points representing absorption

at selected wave numbers measured by photoacoustic calorimetry which show good agreement with the

-57-
spectroscopic data. KMgF3 and MgF2 show exponentially decreasing slopes characteristic of intrinsic

multiphonon absorption with some structure evident, while KZnFg appears to display extrinsic absorption

over much of the frequency range investigated, certainly above 1400 cm""*"

.

Photoacoustic calorimetry results obtained at DF and HF wavelengths are given in Table 1.

Table 1 Total Absorption Determined by Photoacoustic Calorimetry

DF HF

3 .8 ;li m 2.7 ijim (2630 cm-1) (3700 cm-^)

KZnFg

-2 -1

5x10 cm

KMgFg

-4 -1

7x10 cm

-4 -1

7x10 cm

MgF^*

-4 -1 6.7x10 cm (bulk)

-4

2.5x10 cm

''" (bulk)

1.2x10""'^ (per surface

)

-4

8x10 (per surface)

*1.27 cm thick

The total absorption values for KMgFg prepared at Oklahoma State University are slightly lower than

those of commercial MgF2 at 2630 cm-I, but at 3700 cm"-*- the strong surface absorption of MgF2 becomes

dominant and makes comparison difficult. The absorption value of 7xl0~^ cm~l for KMgFg is considerably

higher than that expected from an extrapolation of the exponential slope of figure 1. This indicates

extrinsic behavior resulting at least partially from the surface or bulk scattering. The relatively

high value of 5x10"^ cm"-'- is again evidence of the extrinsic behavior demonstrated in figure 1.

The temperature and frequency dependence of the absorption coefficient of MgF2 and KMgF3 are shown in figures 2 and 3. The exponential dependence of absorption coefficient on frequency persists

at higher temperatures for both materials. Above 1800 cm"-'- additional absorption is evident which is

not present in the initial room temperature data of figure 1. This absorption is attributed to changes

in surface condition from temperature cycling during the measurements. KMgF3 shows more detailed

structure in the absorption spectra than MgF2, particularly at 80°K.

Discussion

The perovskitesABF3 can be thought of as mixed crystals of AF and BF2. Thus one might at first

expect to obtain their optical properties by simple addition of the properties of the two end members

In figure 4 the absorption of KMgF3 is compared with that of KF and MgF2. Actually, a close resemblance

is observed for the Mg compounds and a much lower absorption and a steeper slope found for KF. This

indicates that the Mg-F vibrations are dominant in the multiphonon spectrum of KMgF3. Quantitatively,

this can be understood from a comparison of the long wavelength longitudinal optical mode frequencies

corresponding to frequencies in the vicinity of the cut-off of the one-phonon density of states. These are 326 cm~^ (KF), 551 cm"! (KMgF3) and 617 cm~^ (MgF2). Thus at a particular frequency such as 1300 cm" , KF multiphonon absorption is a 5-phonon or higher order process whereas those of KMgF3 and

MgF2 are still in the 3-phonon regime.

The temperature and frequency dependence of MgF2 and KMgF3 is next analyzed in terms of a ^

simplified theory [11] which employs an average oscillator frequency to represent the phonon spectrum.

Following Bendow [21] the absorption coefficient is expressed as

I

w

[n(uj^) + 1] ^ ("'^^ = ^o n(oo) ^ 1 ^^'P Yw

"I o J

(2)

where n(aj) = [exp (kcj/kT) - 1] represents the phonon occupation number at temperature T, /3q a scaling

factor, the effective phonon frequency and Y the coefficient which governs the exponential decay.

From the above expression it is evident that increases at high T according to the usual T-^~ law for

a j ( = ^ ) - phonon process [9], provided that oj^ is independent of T. In general, 0)^ (T) decreases

o with increasing T, somewhat suppressing the T dependence of arising from the Bose-Enstein factors, an effect which becomes increasingly important at higher frequencies.

Before the above relation can be used to represent the T dependence of 0 it is necessary to specify u^. Because the contribution of acoustic phonons are suppressed due to energy conservation,

-58-
an average optical phonon frequency such as the Brout frequency [22,11], used in our previous analysis

of alkaline earth fluoride data, may be an appropriate choice. In the case of MgF2 where all of the

long wavelength optic mode frequencies are known from infrared [13] and Raman [23] measurements, the

Brout frequency turns out to be about 415 cm"''' . If this particular value is used for to^ in the above

equation, only fair agreement is achieved with experimental data. Using the Brout frequency instead as

a starting point for a least squares fit, the best agreement with experimental data is obtained for

Uq = 439 cm"-*- . The set of calculated absorption coefficients /3 versus temperature at various values of

frequency, w, are shown in figure 5. The experimental data designated with points are seen to agree

quite well over the frequency range 1300 to 1800 cm"-*^ and from 300 to 500°K. Deviations occurred at 80°K where structure was clearly evident in the spectra of figure 2 andthe simple expression (2) for (3 is no longer applicable, and also above 1800 cm""*" where extrinsic absorption begins to dominate. Furthermore, the agreement might well be Improved if the T dependence of uJq is included, but this has not been

attempted in the calculations displayed here. The long wavelength optic mode frequencies for KMgF3 are not as well known as those for MgF2. However, if the value Ug = 439 cm"-*- for MgF2 is scaled by the

ratio of the corresponding maximum longitudinal optic mode frequencies, (viz., 551 cm"-'' for KMgF3 and

617 cm"-'- for MgFg ) , a value of 392 cm"^ is obtained for the Uq for KMgF3. This roughly determined

value yields only fair agreement between the calculated and observed absorption coefficients. However,

the best fit is obtained for Wq = 402 cm""*- as indicated in figure 6, where good agreement is found between calculated and experimental /3 values, except at 80°K. Again the discrepancy at low temperature

may be attributed to the pronounced structure in the absorption spectrum of KMgF3 (figure 3).

A comparison of structure in the absorption spectrum of KF, MgF2 and KMgF3 at 80°K is shown in

figure 7. The KMgF3 and MgF2 data are our experimental data, and the KF curve is that obtained

theoretically by Boyer, et al [8]. It is evident that at 80°K KMgF3 has the most structure and KF the

least. The lack of structure in KF is characteristic of a rather smooth phonon density of states. In

the case of KMgF3 presumably the contribution of KF combined with that of MgF2 introduces a gap in its

phonon density of states and, as a result, persistent structure in the multiphonon spectrum as well.

With good transmission throughout the CO and chemical laser wavelength ranges and a lower absorption than MgF2, KMgF3 may be also a good possibility for a coating material, especially with the

additional strength inherent in the mixed crystal.

Acknowledgement

The authors would like to thank Mr. D. Bandes of Parke Mathematical Laboratories for his assistance

with the computer programming.

References

[1] See for example, "Optical Properties of Highly Transparent Solids" edited by S.S. Mitra

and B.Bendow, pp. 3-127 (Plenum Press, New

York, NY, 1975).

[2] T.F. Deutsch, J. Phys. Chem. Solids 34, 2091

(1973)

.

[3] J. A. Harrington and M. Hass, Phys. Rev. Lett.

11 , 710 (1973).

[4] B. Bendow, S.C. Ying and S.P. Yukon, Phys.

Rev. B8, 1679 (1973)

.

[5] T.C. McGill, R.W. Hellwarth, M. Mangir and

H.V. Winston, J. Phys. Chem. Solids, 34 2105

(1973) .

[6] K.V. Namjoshi and S.S. Mitra, Phys. Rev. B9,

815 (1974).

[7] L.J. Sham and M. Sparks, Phys. Rev. B9, 827

(1974) .

[8] L. Boyer, J. A. Harrington, M. Hass and H.B. Rosenstock, Phys. Rev. Bll , 1665 (1975).

[9] J.R. Hardy and B.S. Agrawal, Appl. Phys.

Lett. 22, 236 (1973),

[10] K.V. Namjoshi, S.S. Mitra, B. Bendow, J. A. Harrington and D.C. Stierwalt , Appl . Phys.

Lett. 26, 41 (1975).

[11] H.G. Lipson, B. Bendow, N.E. Massa and S.S. Mitra, Phys. Rev. B13, 2614 (1976).

[12] J.N. Plendl, Appl. Opt. 10, 1134 (1971).

[13] A.S. Barker, Jr., Phys. Rev. 136, A1290 (1964).

[14] G.R. Hunt, C.H. Perry and J. Ferguson, Phys.- Rev. 134, A688 (1964)

.

[15] C.H. Perry and E.F. Young, J. Appl. Phys. 38,

4616 (1967).

[16] E.F. Young and C.H. Perry, J. Appl. Phys. 38,

4624 (1967).

[17] S.P.S. Porto and J.F. Scott, Phys. Rev. 157 , 716 (1967).

[18] H.G. Lipson, Applied Optics, to be published,

[19] A. Hordvik, B, Bendow, H.G. Lipson, L.K. Skolnik

and R.N. Brown, "Laser Induced Damage in Optical Materials: 1976, A.J. Glass and A.H. Guenther, editors (NBS Special Publication 462,

1976 p. 50)

.

[20] A. Hordvik & H. Schlossberg, Appl. Opt. 16,

101 (1977).

[21] B. Bendow, Phys. Rev. B8, 5821 (1973).

[22] See, for example, S.S. Mitra, in Optical Properties of Solids, edited by S. Nudelman and

S.S. Mitra (Plenum, New York, 1969).

[23] S.P.S. Porto, P. A. Fleury and T.C. Damen,

Phys. Rev. 154, 522 (1967).
Figures

TRANSMISSION SPECTROSCOPY

.«..?..».. PHOTOACOUSTIC CALORIMETRY

1200 1600 WAVENUMBER ( cm"! ) 2000

Fig. 1. Absorption coefficient vs. wavenumber

for KZnF^, KMgF^ and MgF2 measured by

Fourier spectroscopy (solid and dashed

lines) and photoacoustic calorimetry

for KMgFj (dots) and KZnF^ (crosses).

1000 1200 1400 1600 1800 2000 2200

FREQUENCY (cm"')

Fig. 2. Measured absorption coefficient versus

frequency for MgF2 at various temperatures

Fig. 4. Comparison of absorption coefficient

o , , ^. i-ci J ^ versus frequency behavior of MgF„,

Fig. 3. Measured absorption coefficient versus

-ni v h kf

frequency for KMgF^ at various temper- KMgF^ and Ki

.

atures.

-59-
2.0 I

1 1 1 1 1 r 1300

TEMPERATURE (°K)

Figure 5 - Absorption coefficient versus temperature for MgF2 at various

representative frequencies. The solid lines are calculations

obtained from equation (2) utilizing = 439 cm~l, /Sq = 29,922,

7 = 3.496, while the individual points are experimental data.

0 ICQ 200 300 400 500 600

TEMPERATURE (°K)

Figure 6 - Absorption coefficient versus temperature for KMgFg at various

representative frequencies. The solid lines are calculations

obtained from equation (2) utilizing = 402, j3 q = 5328,

7 = 2.953, while the individual points are experimental data.

-60-
KM9F3 ISO" K EXPl

Figure 7 - Comparison of structure in low temperature spectra of KMgF3 and

MgF2 (experimental data) and KF (theoretical calculation from

reference 8). KF data plotted on a reduced scale relative to KMgF3 and MgF2.

COmENTS ON PAPER BV LIPSON, HOWVJK, BEmOW, MITRA, MV MARTIN

It MoA poi.YVttd oat tn the. cLLiciUi-ion that potcmZum magnejstum ltaonA.de, -c6 a inixe,d {jlaotide cAyitat

lA'kich appecULi ottAactivz {^ofi ait cu> a coating mateJujjUL. It MOi tnduicated that inve.itigaJU.on 0^ tiiLi maXeAA.at (^on. coaJxngi, aj> andeAway.

-61-
BULK LASER ABSORPTION HOMOGENEITY*

T. J. Moravec and E. Bernal G. Honeywell Corporate Materials Science Center

Bloomington, Minnesota 55A20

Doppler Interferometry has been used to measure the optical change in

path upon CO2 laser irradiation and thus the absorption. This absorption

has been measured at many points over the full aperture of several laser

windows and materials. The entire experiment is under control of an HP9825A calculator. Details are given of the experiment and its operation.

Key words: Automated calorimetry, calorimetry, Doppler interferometry

,

laser absorption uniformity.

1. Introduction

The objective of this experiment is to examine the spatial variation of absorption in infrared

laser window materials and coatings. Our approach is to measure by interferometric techniques the

localized change in optical path induced by irradiation of a window by a focused laser beam.

2. Experimental Description

Figure 1 shows the experimental arrangement. The idea is to bring the interferometric beam in

coincidence with the CO2 beam so the point of irradiation can be changed. The measurements are normalized to the time of irradiation and power transmitted, i.e. the energy. The experiment & X-Y table

used for mounting and positioning the sample are under computer control.

A sample can be scanned and the change in optical path length due to CO2 laser irradiation can

be measured at many points over thp clear anprture of the sample. The number of points is limited by

the size of the CO2 beam. The change in path length is proportional to the absorption coefficient

of the sample. Thus, a plot of the homogeneity of the absorption can be obtained by plotting the

change in optical path length. Because the interferometer measures optical path by h e t e r o dy ning

two light beams of different frequency, we will refer to this experimental technique as scanning

^ Doppler interferometry (SDI) . The experimental approach is similar to that of Skolnik et. al. Hill

3. Noise Limitations

An extensive program was undertaken during the first part of this year to determine the

accuracy and noise limitations of the instrument. The interfermoter has an instrument resolution of

0.016 pm determined by the wavelengths and electronics used.

To test the instrument, a program was written that does not move the X-Y table, but rather

samples the readings of the interferometer consecutively. Without any induced changes or motions,

the readings are essentially the background fluctuations or moise of the interferometer. Figure 2 shows a typical plot of 100 consecutive readings spaced 0.3 second apart that are the difference

(or change) from the zero reading. Large fluctuations are evident. These fluctuations are independent of the floating of the optical table and thus are not due to mechanical vibrations. However,

they appear to be due to air currents. Figure 3 shows a typical run with the room air circulating

system turned off. It was also determined that the same effect could be obtained with the interferometer beam enclosed in beam pipes and thus protected from air currents. The experiment then can be

run with the room air circulating.

Some drift can be noted in Figure 3 which we believe is due to thermal changes in the optical

table. We have eliminated this by sending the interferometer reference beam over the same path as the sample beam. With this configuration, data as shown in figure 4 is obtained. This shows sufficient stability to perform the homogeneity experiment. The absolute resolution and stability "now is

about 0.03 ijm with the present configuration.

* This work was supported by the Defense Advanced Research Projects Agency under Contract No. DAHC15-73-C-0464, ARPA Order No. A02416.

1. Figures in brackets indicate the literature references at the end of this paper.

-62-
4. Experimental Results

In order to examine the homogeneity of a sample, a program was written that moves the sample

and irradiates each point for a fixed length of time determined by the operator. Thus the change

in optical path length per transmitted laser power at each point is measured by the scanning

interferometer, . As derived in reference [[2] this is given by

ft ^ h

^"^'^^""^^ t(xi,yi) AT(xi,yi) (D

where n is the refractive index, t(x^,y-j^) is the thickness and AT(x^,yj^) is the change in temperature

at (x^,y^). This is directly proportional to the temperature rise at (xi,y/), and hence the local

absorption coefficient.

The present program will sample points in a 3 by 3 matrix up to a 9 by 9 matrix. The distance

between points is set by the operator and the size of the sample. The order of the data points for

a 5 by 5 and a 7 x 7 matrix is shown in table 1. The others are done in a similar fashion. The

procedure is very fast with a 7 x 7 array taking about 12 minutes for a 10 second irradiation at each

point. The data is plotted as an isographic projection of the x and y coordinates on the sample and

the value of AL-j^/Pi (time).

Figures 5 thru 9 are data taken on forged KCl samples and Figure 10 is that for a ZnSe sample.

In each case, a photograph of the sample through Crossed polarizers is shown in the upper right hand

corner. The area scanned is delineated by a white border whose dimensions are ^ 1" x 1". No

correlation with birefringence can be detected. The min. and max. are given in micrometers per

joule units.

Table 2 presents a summary of the results along with calorimetry measurements taken near

the center of each sample.

5. Conclusions

We conclude that there are no order of magnitude effects but variations of the order of three

are common in state-of-the-art materials. Change in path in alkali halides can be extremely small,

but fluctuations are then large. Materials that have larger changes have smaller variations. We note that values for average path change scale with the absorption calorimetry measurements. So

complete evaluation of potential laser windows should include these tests for checking the expected

laser absorption. It should be pointed out that while this technique is faster than adiabatic

techniques, it requires a large laser with an output of ~ 200 watts.

6. References

[1] L. H. Skolnik, A. Hordvik, and A. Kahan,

Appl. Phys. Lett. 23, 477 (1973)

[2] E. Bernal C, R.H. Anderson, B.G. Koepke,

T.J. Moravec, R.A. Skogman, D. Wertman,

R.J. Stokes, J.S. Loomis, Honeywell Semiannual Technical Report No. 6 on ARPA

Contract DAHC15-73-C-0464 , 31 December 1976.

-63-
LASER ABSORPTION SCANNING PATTERN

5X5

14 4 9 19

12 2 7 17

11 1 6 16

13 3 8 18

15 5 10 20

X-Axis

7X7

34 20 6 13 27 41

32 18 4 11 25 39

30 16 2 9 23 37

29 15 1 8 22 36

31 17 3 10 24 38

33 19 5 12 26 40

35 21 7 14 28 42

X-Axis
TABLE 2 SUMMARY OF SAMPLE HOMOGENEITY AT ICeym

-L -A -1 Sample Path Change (XIO urn) Calorimetry 6 (XIO cm )

Min Max Ave Std.Dev. Ave Std.Dev.

ZnSe #353 11.28 43.96 21.58 9.12 66.8 1.3

KCl #520 0.37 1.65 0.77 0.71 21.7 2.1

KCl #482 0.76 3.70 2.09 0.52 44.7 3.2

KCl #K-8 0.34 2.89 1.32 0.50 16.4 2.7

W/RbCl

KCl #476 2.75 5.18 3.50 0.52 55.7 2.3

-65-
DIAGRAM OF SCANNING DOPPLER

INTERFEROMETER

MOTOR-DRIVEN X-Y STAGE

RETROREFLECTOR

SHUTTER

INTERFEROMETER

CUBE

DUAL WAVELENGTH

STABILIZED LASER

He - Ne

Figure 1 Experimental arrangement of scanning Doppler Interferometer

B.H - H-P DOPPLER INTERFEROMETER

Figure 2 Background Noise Fluctuations of Optical Path Difference

during 30-Second Time Interval Represented by 100 Data

Points on x-axis

-66-
W/RDOM HIR OFF

Figure 3 Same as Figure 2 but with Room Air Circulating System Turned

Off

B.3lik B . I fS/M/77; tE; COVERED; X-RXIS

W/REFERENCE PRISM

iTmiii <j,MMHi Kaij iiiiiitwTTi i ri> ii m « iii

lL

-B . 3 i-

-la.M

Figure A Same as Figure 3 but with Reference Beam Sent over Same

Path as Sample Beam (but not through sample) and Interferometer Enclosed in Box

-67-





COMMEWTS ON PAPER BV MORAVEC ANV BERWAL G.

Iw A.e4pon4e to a quution abomC iCiotteA^ng, (kt ipmkoA mphcalztd that thu rmoMiAenmnt dztzatzd thz

theAmctllij induced path change and mcu> theAzf^ona not itn&itivt to !>cattzfU.nQ bat only to abiolptAve.

n\(LchanAj>m

.

-73-
REFRACTIVE PROPERTIES OF INFRARED WINDOW MATERIALS*

A. Feldman, D. Horowitz, and R. M. Waxier

Institute for Materials Research

National Bureau of Standards

Washington, D. C. 20234

The results of the Optical Materials Characterization Program at the National

Bureau of Standards are reviewed. The techniques for measuring refractive index

and the change of refractive index with stress and temperature are enumerated and

the materials on which these measurements have been made are listed. Data are presented for the change of refractive index with temperature of single crystal

specimens of BaF2, CaF2, reactive atmosphere processed (RAP) KCl and KBr, LiF, NaF and SrF2, and polycrystalline chemical vapor deposited (CVD) ZnSe and ZnS . The

measurements were done by the method of Fizeau interferometry over the temperature

range -180 to 200 °C at the wavelengths 0.6328 um, 1.15 pm, 3.39 pra and 10.6 ym. The results were compared to the results of other workers and found to be in

general agreement. We attempt to explain disagreements with other workers.

Key words: BaF2; CaF2; KBr; KCl; LiF; NaF; refractive index; SrF2; thermal coefficient of refractive index; ZnS; ZnSe.

1. Introduction

When high-power laser radiation propagates through a laser window, the residual absorption in the

window leads to a temperature rise. The temperature distribution is, in general, nonuniform; hence, it

will cause a nonuniform optic path variation across the window aperture resulting in a distortion of

the beam wavefront. The distortion arises from the change of refractive index with temperature, the

change of thickness with temperature, and the change of refractive index and thickness produced by

stress caused by thermal gradients. In order to predict the optical distortion due to thermal

gradients it is important to determine the refractive index, n, the change of refractive index with

temperature, dn/dT, the linear thermal expansion coefficient, a, the piezo-optic constants, q^^., and

the elastic constants, s.. or c.., of laser window materials. The Optical Materials Characterization

Program has been established at the National Bureau of Standards to measure these quantities.

In this paper are listed the methods we have used for measuring these parameters and materials on which measurements have been made. We do not discuss the details of the experimental procedures [1]^ or the experimental data that have appeared in the literature, but we do present our most recent data

of dn/dT on BaF2, CaF2, KBr, KCl, LiF, NaF, SrF2, ZnS, and ZnSe. A comparison is made with the data of

other workers. The results of refractive index measurement on ZnS are presented in an accompanying

paper by M. J. Dodge.

2. Experimental Methods

2.1. Refractive index

The refractive indices of prismatic specimens are measured by the method of minimum deviation on

the following instruments: Precision spectrometer with glass optics - The ultimate accuracy is several

parts in 10^; the wavelength range is 0.4 pm to 1.08 yra. Precision spectrometer with mirror optics - the ultimate accuracy is several parts in 10^; the wavelength range is 0.21 ym to SO ym.

As part of the infrared laser window program, we have measured the refractive indices of the

following materials: commercial KCl [2], reactive atmosphere processed (RAP) KCl [3], KCl nominally

doped with 1.5% KI (KC1:KI) [3], chemical vapor deposited (CVD) ZnSe [4], CVD ZnS (see paper by M. J. Dodge in these proceedings), hot forged CaF2 [5], fusion cast CaF2 [6], and eight specimens of CaF2

doped with from 0.001% to 3% Er [7]. The spectrometer with mirror optics was used. All of the refractive index values obtained were relative to the refractive index of air, n . , that is, we measured

, air

n/n . . air

* Research supported in part by the Defense Advanced Research Projects Agency.

1. Figures in brackets indicate the literature references at the end of this paper.

-74-
2.2. Temperature coefficient of refractive index

The temperature coefficient of refractive index, dn/dT, is measured by either of two methods. In the first method we measure the refractive index at 20 °C and at 30 °C on a precision spectrometer.

This method has the advantage of giving values for dn/dT over the entire wavelength range. It has sev- eral disadvantages: (1) Values can be obtained only near room temperature; (2) Because the values of

refractive index are obtained relative to the refractive index of air, in order to obtain dn/dT for a material, a correction must be applied to the measured value as shown by the equation

J J dn . dn d , n ^ air

dT ~- dT ^T—^ " " air

where the measured quantity is the first term on the right. Theoretical expressions for dn^^^/dT can

be obtained from the literature and these have been verified in the visible and the near infrared,

however, it is uncertain whether these expressions are correct further into the infrared. (3) The

technique depends on the subtraction of two numbers of comparable magnitude. If the accuracy of these

numbers for some reason is less than optimum, large errors in the values for dn/dT could ensue. For example, an error of 10"** in refractive index would lead to an error greater than lO'^K"'' in dn/dT.

The second method we use for measuring dn/dT overcomes many of the problems listed above. In this

method, we measure the shift with temperature of the Fizeau fringes generated by the reflections of

laser radiation from the surfaces of a flat plate of specimen material. This method has several advantages: (1) dn/dT data can be obtained over a wide range of temperature. We presently have two systems

that cover the temperature ranges -180 °C to 200 °C and 20 °C to 800 °C, respectively. (2) The value

obtained for dn/dT is for the material itself and is independent of and its temperature derivative.

The method has the following disadvantages: (1) Measurements can be made only at the discrete wavelengths of spectral lamps or lasers because coherent radiation must be used. In the infrared, we are presently limited to measurements at 1.15 um, 3.39 ym and 10.6 ym. (2) The method relies on accurate

values for the linear thermal expansion coefficient. Thermal expansion data are obtainable from the

literature or are measured by a Fizeau interferometer technique in this laboratory. The technique for

measuring the thermal expansion is similar to the technique for measuring dn/dT.

The results of our latest measurements of dn/dT by the interferometric method are given in

section 3.

2.3. Photoelastic constants

The photoelastic constants describe the effect of stress or strain on the refractive indices of a solid. In glasses and most cubic solids the coefficients qn and qi2 describe the absolute change of

refractive index with stress, whereas the coefficients qii-qi2 and ql^L^ (in glasses q^^l^ = qii-qi2) describe the stress-induced birefringence. We measure these coefficients by a variety of techniques

depending on the wavelength of measurement and the sensitivity of measurement required. These techniques, which have been described in the literature, are listed below.

Twyman-Green interferometer - we measure the shift of interference fringes for different polarizations of the radiation as a function of uniaxial ly applied stress.

Fizeau interferometer - we measure the shift of Fizeau fringes as a function of uniaxially applied

stress or as a function of hydrostatic pressure. These measurements together with the TwymanGreen interferometer measurements permit us to calculate the elastic constants as well as the

photoelastic constants.

Modified Twyman-Green interferometer - this interferometer permits us to measure fractional fringe

shifts in the infrared.

Modified Dyson interferometer - this interferometer permits us to measure fractional fringe shifts

in the visible.

Babinet-Soleil compensator or DeSenarraont cSompensator - these instruments are used for measuring

stress-induced birefringence in the visible and near infrared.

Stress-Birefringent compensator - this device is used for measuring stress-induced birefringence

in the infrared. In this technique a specimen subjected to a varying uniaxial load functions

in a manner similar to a Babinet-Soleil compensator.

The above techniques have been used to measure the photoelastic constants of the following materials in the infrared: AS2S3 glass [2], a chalcogenide glass (Ge 33%, As 12%, Se 55%) [2], fused

Si02 [8], Ge [9], KCl (RAP) [9,10], and ZnSe (CVD) [11]. Measurements are presently underway on CaF2,

BaF2, and SrF2.

-75-
3. Temperature Derivative of Refractive Index of Infrared Window Materials

We have obtained dn/dT of the following materials over the temperature range -180 °C to 200 °C: BaF2, CaF2, KBr (RAP), KCl (RAP), LiF, NaF, SrF2, ZnS (CVD) , and ZnSe (CVD) . Measurements were made

at the wavelengths 0.6328 ym, 1.15 pm, 3.39 pm, and 10.6 um with the exception that measurements on

ZnS were not made at 0.6328 vm and measurements on CaF2, LiF, and NaF were not made 10.6 um. The

results of these measurements are presented in figures 1-9.

We have also measured dn/dT of hot-forged CaF2 and KCl nominally doped with 1.5% KI , but these

measurements were indistinguishable from the measurements on the unmodified materials.

In table 1 we list values of dn/dT that we have obtained at 40 °C for comparison with earlier work. We compare our results only with the results obtained interf erometrically by other workers. We find

that our measurements on BaF2, CaF2, and SrF2 agree within experimental error with the values of

Lipson, Tsay, Bendow, and Ligor [12].

Material

Table 1. dn/dT (10 ''') of infrared transmitting materials at 40 °C

0.6528 pm 1.15 ym 3.39 ym 10.6 um Reference

BaFo -1.63±0.02

-1.67±0.04

-1.64±0.01

-1.66+0.03

-1.71±0.05

-1.68±0.01

-1.62±0.03

-1.68±0.04

-1.63+0.1

-1.48±0.03 Present work

[12]

[13]

CaF, -1.15±0.02

-1.18±0.07

-1.31±0.04

-1.16

-1.1810.02

-1.20+0.05

-1.34±0.04

-1.19

-1.14+0.03

-1.15+0.07

-1.28±0.04

-1.15

Present work

[12]

[13]

[13] corrected

KBr (RAP)

KCl (RAP)

-4.16±0.03

-3.65i0.02

-3.33±0.02

-3.64

-4.23±0.03

-3.68±0.02

-3.41±0.02

-3.72

-4.25+0.03

-3.69+0.02

-3.38±0.01

-3.69

-4.16±0.06

-3.54±0.04

-3.13±0.01

-3.43

Present work

Present work

[13]

[13] corrected

LiF 1.70±0.02 -1.73+0.04 -1.48±0.04 Present work

NaF

SrF^

ZnS (CVD)

-1.30+0.05

-1.25±0.02

-I.20i0.07

6.35±0.10

-I.34i0.06

-1.28+0.01

-1.27+0.05

4.6 iO.2

4.98i0.11

-1.27±0.05

-1.26i0.03

-l.30i0.0S

4.3 iO.2

4.59+0.10

-1.03+0.05

4.1 ±0.2

4.63i0. 12

Present work

Present work

[12]

Present work

[13]

ZnSe(CVD) 10.7 iO.l

g.lliO.ll

7.0 ±0.1

5.97i0. 12

6.2 ±0.1

5.34i0.11

6.1 ±0.1

5.20i0.12

Present work

[13]

Harris, Johnston, Kepple, Krok, and Mukai (HJKKM) [13] have recently reported values of dn/dT for

a series of materials including BaF2, CaF2, KCl, ZnS (CVD) and ZnSe (CVD). A comparison of our results

with theirs shows agreement within experimental error for BaF2 and reasonably good agreement for ZnS,

but larger deviations for the other materials. We will discuss the results for each of these materials

separately in an attempt to account for the discrepancies.

The values that HJKKM report for CaF2 appear to be larger than our values by about 12%. But, an examination of table II in their paper, which lists the parameters used in their calculations, shows

that the values of n for BaF2 and CaF2 are interchanged. If we recalculate their value of dn/dT by

assuming that the index for BaF2 had been mistakenly used in the original calculation, and furthermore,

if we change the value of a = 19. SxlO'^K"-' , the value they had used, to a = 19. 05xlO"^K"^ , the value we have used, the resulting value, as shown in table 1, is in excellent agreement with our value. A similar correction to their data on BaF2 makes a negligible change in the values.

-76-
The values that HJKKM report for KCl appear to be larger than our values by about 10%. This

discrepancy can be almost completely eliminated by changing the value of a = 35 . SxlO'^K"-' , the value

they had used, to a = 37 . 6xlO"^K" ^ , the value we have used. In table 1, a comparison of their corrected

values with ours shows deviations of no more than 3%. We believe our value of a is more nearly correct

because measurements of a made in our laboratory agree with values in the MP Handbook [14]

.

A comparison of HJKKM' s values for ZnSe with our values indicates a discrepancy of about 15%.

However, HJKKM have reported that they obtained different results on some materials when measuring

dn/dT in air and when measuring dn/dT in an immersion bath. We, therefore, decided to remeasure dn/dT

of ZnSe at 0.6328 ym using an immersion technique. We found no significant difference between the data

we had obtained previously and the data obtained by the immersion technique.

Tsay, Lipson, and Ligor [15] have recently reported measurements of dn/dT on Ca?2 as a function of

temperature. These data appear to be in good agreement with ours.

4 . Summary

We have enumerated the techniques we have used for measuring refractive index and the change of

refractive index with stress and temperature. We have listed the materials on which measurements have

been made. Data of dn/dT as a function of temperature have been presented for nine infrared transmitting materials. The results were compared with the results of other workers and found to be in general

agreement. We attempted to explain disagreements with other workers.

5. Acknowledgments

We thank the following for the specimen materials: E. Bernal G. of Honeywell, C. Pitha of the

Rome Air Development Center, P. Klein of the Naval Research Laboratory, J. Fenter and R. Harris of the

Air Force Materials Laboratory, P. Miles and B. DiBenedetto of Raytheon, and J. Fontanella of the U.S. Naval Academy. The support from the Defense Advanced Research Projects Agency is greatly appreciated.

6. References

[1] For a description of apparatus and the

experimental procedures see the proceedings

of these conferences starting in 1974. See

also, A. Feldman, Electro-Optical Systems

Design 8, 36 (1976) . In this reference the

caption in figure 7 is incorrect. The wavelength 10.6 vm should be changed to 0.6328

ym.

[2] A. Feldman, I. Malitson, D. Horowitz, R. M. Waxier, and M. J. Dodge, in Laser Induced

Damage in Optical Materials : 1974 , NBS

Special Publication 414, edited by A. J. Glass and A. H. Guenther (U.S. G.P.O., SD Catalogue No. C13. 10:414, 1974) pp. 141.

[3] M. J. Dodge, in Proceedings of the Fifth

Annual Conference on Infrared Laser Materials , Las Vegas, Nevada, December 1975

(1976) pp. 215.

[4] M. J. Dodge and I. H. Malitson, in Laser

Induced Damage in Optical Materials : 1975 , NBS Special Publication 435, edited by A. J. Glass and A. H. Guenther (U.S. G.P.O. SD

Catalogue No. C13. 10:435, 1975) pp. 170.

[5] M. J. Dodge, in Laser Induced Damage in

Optical Materials : 1976 , NBS Special Publication 462, edited by A. J. Glass and A. H. Guenther (U.S. G.P.O. , SD Catalogue No. C13. 10:462)

.

[6] A. Feldman, D. Horowitz, R. M. Waxier, M. J. Dodge, and W. K. Gladden, Optical Materials

Characterization , NBSIR 77-1030 (ARPA) (Aug.

1977)

.

[7] A. Feldman, D. Horowitz, R. M. Waxier, M. J. Dodge, and W. K. Gladden, Optical Materials

Characterization , NBSIR 77-1219 (ARPA) (March

1977)

.

[8] to be published.

[9] A. Feldman, D. Horowitz, and R. M. Waxier, in

Laser Induced Damage in Optical Materials :

.. 1975 , NBS Special Publication 435, edited by

A. J. Glass and A. H. Guenther (U.S. G.P.O.,

SD Catalogue No. C13. 10:435, 1975) pp. 164.

In table 1, the values for p^j^ and pi2 have

been interchanged.

[10] A. Feldman, D. Horowitz, and R. M. Waxier,

Appl . Optics, to be published.

[11] A. Feldman, R. M. Waxier, and D. Horowitz, in

Optical Properties of Highly Transparent

Solids , edited by S. S. Mitra and B. Bendow

(Plenum Publishing Corp., New York, 1975)

pp. 517. The following corrections should be

made: last line, page 519, change [111] to

[110]; page 520, eq. (3), change [110] to

[100].

[12] H. G. Lipson, Y. F. Tsay, B. Bendow, and

P. A. Ligor, Appl. Optics TS, 2352 (1976).

[13] R. J. Harris, G. T. Johnston, G. A. Kepple,

P. C. Krok, and H. Mukai, Appl. Optics 1_6, 436 (1977).

[14] American Institute of Physics Handbook , 3rd

edition (McGraw-Hill Book Company, 1972) p. 4-137.

[15] Y. F. Tsay, H. G. Lipson, and P. A. Ligor,

J. Appl. Phys. 48, 1953 (1977).

-77-
7. Figures

-2.0 I

I I I I I I I -200 -100 0 100 200

TEMPERATURE (°C)

Figure 1. dn/dT of BaF^ as a function of temperature.

-.4

-.8

-1.2

-1.6

-2.0

I'll '

I

' I CaF2 —

- \

• .6328 /i.m _

X 1.15 fjLm + 3.39^m ~

1,1,1 , 1 . 1 -200 -100 0 100

TEMPERATURE (°C)

200

Figure 2. dn/dT of CaF^ as a function of temperature.

-78-
0| \

^ \^

I

I

\

\ [

KBr ~ • .6328 /Lim

X 1.15 fj.m + 3.39 fj.m

^ -2 — A 10.6

in

-200 -100 0 100 200

TEMPERATURE {°C)

Figure 3. dn/dT of KBr (RAP) as a function of temperature.

-3

-4

-5

KCI • .6328 fjivn X 1.15 fj.m + 3.39;im

A 10.6 fj.m

J I L

-200 -100 0 100

TEMPERATURE ("0

200

Figure 4. dn/dT of KCI (RAP) as a function of temperature.

-79-
Figure S. dn/dT of LiF as a function of temperature.

Figure 6. dn/dT of NaF as a function of temperature.

-80-
Figure 7. dn/dT of SrF2 as a function of temperature.

8

- 4

ZnS

X 1.15 /i.m

+ 3.39 fjLm

A 10.6 fjLvn

X X,

-200 -100 0 100

TEMPERATURE (°C)

200

Figure 8. dn/dT of ZnS (CVD) as a function of temperature.

-81-
I_J ^ \ ^ \ ^ L -200 -100 0 100 20

TEMPERATURE (**C)

Figure 9. dn/dT of ZnSe (CVD) as a function of temperature.

COMMENTS ON PAPER BV FELPMAW, HOROWITZ, AWP WAXLER

Wo dLUctiiiion on thii papeA.

-82-
REFRACTIVE PROPERTIES OP CVD ZINC SULFIDE*

Marilyn J. Dodge

National Bureau of Standards

Washington, D. C. 20234

The refractive index of two samples of zinc sulfide made by

the technique of chemical vapor deposition (CVD) was determined

from 0.5'*6l to 11.475 pm. Measurements were made relative to air

by means of the minimum-deviation method on a precision spectrometer

at temperatures near 22°C and 34°C. Each set of experimental

data was fitted to a Sellmeier-type dispersion equation which

permits refractive index interpolation within several parts in

-S • 10 . The calculated data at the two temperatures was used to

determine the temperature dependence of the refractive index of

ZnS. The index and dn/dT of the ZnS will be compared with those

properties of CVD ZnSe.

Key words: Dispersion; refractive index; temperature coefficient

of refractive index; zinc sulfide.

1. Introduction

The performance of a high-power laser system is dependent on the optical and

mechanical stability of the optical components within the system. Optical distortion

of a component can occur as a result of absorptive heating, and if the distortion is severe enough the component can become unusable. New and better optical materials are being developed to help alleviate this problem.

Theoretical calculations of optical distortion in laser windows depends on the

knowledge of the absorption coefficient, refractive index (RI), change of index with

temperature Cdn/dT), thermal expansion coefficient, stress-optical constants, elastic

compliances, specific heat, thermal conductivity and density of the material under

consideration. An optical materials characterization program [1]''' is currently in

progress at NBS to determine their pertinent optical properties. The refractometry

laboratory is determining the RI and dn/dT of selected window materials over a limited

temperature range.

Chemical vapor deposited (CVD) ZnS is a possible candidate window material. A

comprehensive study of the refractive index of bulk ZnS is not readily available in

the open literature. This paper will present RI and dn/dT data on two samples of this

material. The prismatic samples, designated "A" and "B" were made available by B. A.

2 di Benedetto of Raytheon Company. Sample B appeared to be of much better optical

quality than sample A.

2. Transmission

This material has a practical transmission range from 0.6 to 12 ym, with the

exception of the 6 pm region where there is a strong absorption band. Figure 1 shows

a transmission curve of a sample of A-type material, 1.5 cm thick [2]. The 6 ym region

absorption band is considered to be due to a hydride [3], a common impurity in the CVD

ZnS currently available.

* This work was supported in part by the Defense Advanced Research Projects Agency.

1. The numbers in brackets represents references at the end of this paper.

2. The use of company and brand name in this paper are for identification purposes

only and in no case does it imply recommendation or endorsement by the National

Bureau of Standards and it does not imply that the materials used in this study

are necessarily the best available.

-83-
3. Experimental Technique

Each prismatic sample had a refracting angle near 32°, and two polished faces

approximately 2.5 cm square. The refractive indexes were determined by means of the

mininum-deviation method on a precision spectrometer shown schematically in fig. 2 [4].

In the visible and near infrared regions of the spectrum, the index was determined at

known emission wavelengths of mercury, cadmium and helium. Beyond 2 \im, a glo-bar was used for the radiant-energy source, and measurements were made at known absorption

bands of water, carbon dioxide, polystyrene, methocyclohexane , and 1, 2-4 trichlorobenzene. A series of narrow-band interference filters was also used between 3-5 and

10.6 ym. A thermocouple with a cesium iodide window was used for the detector. The

scale of this spectrometer can be read to 1 second of arc and the instrument is con- sidered to have an equivalent precision and negligible systematic errors. This will

permit refractive indexes accurate within a few parts in 10 over a wide wavelength

range, to be determined for good optical material. All measurements were made relative

to air under normal laboratory conditions.

4. Refractive Index

The RI of sample A was determined from 0.^h6l to 11.475 pm and from 0.5^61 to

11.862 vim for sample B. Measurements were initially made on each sample at a controlled-room temperature near 22°C and repeated near 34°C.

Each set of experimental data from 0.5^61 to 10.6 ym was fitted to a three-term

Sellmeler-type dispersion equation [5] of the form:

n^-l

3

I 3=1

A.X

J

2 2

The index of refraction is represented by n, X is the wavelength of interest in ym, the

X.'s are the calculated wavelengths of maximum absorption and the A.'s are the calJ J culated oscillator strengths corresponding to the absorption bands. The X.'s and A.'s

J J are not intended to have any physical significance. Primary emphasis is given to

procuring a mathematical fit of the measured data useful for interpolation.

The constants calculated for the dispersion equation for each specimen at two

temperatures, the number of wavelengths fitted and the average absolute residual (the

average difference between the experimental values and the calculated values) are given

in table 1.

Table 1. Dispersion equation constants for calculating refractive index

of CVD ZnS relative to air.

Sample

21. 6°C

A 33.6°C

Sampl

21.9°C

e E 33.4°C

^1 0 . 33904026 0 . 19924242 0 24199447 0. 79907896

^2 3 .7606868 3 .9029481 3 8575584 3. 3033398

^3 2 .7312353 2 .7620669 2 5433609 2. 8028661

^1 0 . 31423026 0 . 34058701 0 33005445 0. 28020665

0 . 17594174 0 .18057756 0 17899635 0. 16205174

33 . 886560 34 .059326 32 849275 34. 288073

No. of

wavelengths

fitted 25 25 30 27

Average

absolute

residual

X 10^ 5.5 5.0 4.6 4.2

-84-
The average absolute residual Is an Indication of the overall accuracy of the experimental data. The largest residuals were in t?ie region between 5.5 and 6.5 ym which

could be expected because of the strong absorption in this region. The Sellmeier

equation will fail when a fit is attempted too close to an absorption edge, but will

attempt to fit through a band when some data exists on either side of the band. It was possible to obtain experimental index data within the 6 ym region of absorption, but

the equation could not adequately fit the data in this region because of the anomalous |p index effect which occurs in regions of absorption. Experimental data were obtained for • both specimens beyond the 11 pm absorption band, but because of the natural cutoff of

the material not enough data could be obtained in this region to facilitate a fit

through the band. Instead, the anomalous index effect within this weak band put undo

influence on the IR parameters of the equation and an adequate fit was impossible at

all wavelengths. Therefore, a satisfactory fit could not be accomplished beyond

10.6 ym.

After a fit was obtained, the refractive index was calculated at regular wavelength intervals. Figure 3 shows the dispersion curve for specimen B. Figure 4 compares sample B with sample A which is represented by the zero line.

The index of specimen B is lower than that for sample A below 2 pm and higher at all

wavelengths beyond 2 ym. This behavior could be indicative of extended useful transmission, or less absorption, in sample B beyond the absorption edges of sample A.

5. Temperature Coefficient of Index

The calculated data for the two average temperatures at which each sample was measured were used to calculate dn/dT(°C)~"'' for each specimen. The resultant dn/dT

values are shown graphically in fig. 5. The data points in this figure represent the

dn/dT values which were calculated from the experimental data. With the exception of

between 6.5 and 10 ym the values for both samples agree within 5 x 10~^. This discrepancy is still well within the stated experimental errors for the index determinations from which the dn/dT was calculated. Corrections for the dn/dT of air have

not been applied in the determination of the dn/dT values for the ZnS samples in this

study. In table 2, the dn/dT of these two specimens are compared with those of

Feldman, et al. [6], and Harris, et al. [7] at specific laser wavelengths.

Table 2. Comparison of dn/dT x 10~^C°C)"''" for CVD ZnS

WAVELENGTH

(ym)

1.15

3.39

10.6

DODGE*

A B

(28°C)

5.0

4.6

4.7

5.2

4.8

5.0

FELDMAN

(30°C)

4.6

4.2

4.1

HARRIS

(45°C)

5.0

4.6

4.6

* These values have not been corrected for the dn/dT of air.

6. Comparison of CVD ZnS and CVD ZnSe

The refractive index and temperature coefficient of refractive index of CVD ZnSe

has been previously reported [8]. Because ZnSe and ZnS are similar materials, a com- parison of the RI and dn/dT of the two materials measured in this laboratory is made in

table 3.

Table 3- Comparison of the refractive properties (relative to air) of CVD ZnS

and CVD ZnSe.

WAVELENGTH ZnS ZnSe

(ym) REFRACTIVE INDEX

1.0 2.2923 2.4891

3.0 2.2572 2.4375

5.0 2.2462 2.4294

10.6 2.1921 2.4027

dn/dT X 10^ (°c)-i

1.0 5.2 8.9

3.0 4.9 7.2

5.0 4.3 7.3

10. 6 5.0 7.4

-85-
7. Conclusion

The refractive properties of CVD ZnS presented In this paper refer to the specific

samples in this study. Although these findings are probably representlve of the

material, caution should be used when applying any of these values to other samples of

ZnS. As growth techniques are improved, better material will result with possible

changes in the refractive properties, especially near the regions of absorption.

8. References

[1] Peldman, A., Malltson 1., Horowitz,

D., Waxier, R. M. and Dodge, M.,

Laser Induced Damage in Optical

Materials: 197^1, NBS Special Pub.

4l4, l4l, (197^^).

[2] Raytheon Company, In-Line

Transmission, Run ZS-157P,

PBN-7^-607; provided through

the courtesy of B. A. di Benedetto,

Advanced Materials Department.

[3] Personal communication with

D. A. di Benedetto,

Raytheon Company

.

[4] Stephens, Robert E.

,

Phyler, Earle K. , Rodney,

William S. , and Spindler,

Robert J., J.O.S.A. 43.,

110, (1953).

[5] Sutton, Loyd E. and

Stavroudls, Orestes N.

,

J. Opt. Soc. Am. 51,

901 (1961)

.

[6] Feldman, A., Horowitz, D.,

Waxier, R. M. , "Refractive

Properties of Infrared

Window Materials,"

Proceedings of the Ninth

Symposium on Materials for

High Power Lasers, 1977,

NBS Publication.

[7] Harris, R. J., Johnston,

G. T., Kepple, G. A.,

Krok, P. C, and Mukal, H. , Applied Optics 16, 436 (1977).

[8] Dodge, Marilyn J. and Malltson,

Irving H., Laser Induced Damage

in Optical Materials: 1975, NBS

Special Pub. 435, 170, (1976).

9 . Figures

100

w 80- o

0.3 0.5 1.0 3.0 5.0 10.0 30.0 WAVELENGTH (//m)

Fig. 1 - Transmission curve of CVD ZnS taken from "In Line Transmission Run" provided

through the courtesy of B. A. di Benedetto, Ratheon Company.

-86-
AMPLiFIER "ECOBOER

Fig. 2 - Schematic diagram of the modified Gaertner precision spectrometer showing

optical path. The prism is rotated at one-half the rotation rate of the

telescope assembly by gear system, thus maintaining the condition of

minimum deviation for any wavelength. The scanning device drives the

assembly which scans the spectrum to identify lines or bands and determine

their approximate scale positions.

Fig. 3 - Refractive index of CVD ZnS as a function of wavelength (logarithmic scale)

Data near 22°C were calculated from the dispersion equation.

-87-
Fig. 4 - Comparison of the refractive Index of sample B ZnS with sample A. The Index

of sample A Is represented by the zero-line.

10

8

-

« 6

—

4 -

2

-

0

-

0.3 0.5 _L

1.0 3.0 S.O 10.0 WAVELENGTH (fjm)

30.0

Fig. 5 - Temperature coefficient of refractive Index of CVD ZnS near 28°C. Sample A:

" dn/dT from Index data calculated by the dispersion equation;

# dn/dT from experimental Index data. Sample B: dn/dT from

calculated Index data; H dn/dT from the experimental data.

-88-
CRITICAL, ORIENTATIONS FOR ELIMINATING STRESS-INDUCED DEPOLARIZATION

IN CRYSTALLINE WINDOWS AND RODS*

R. E. Joiner, J. H. Marburger, and W. H. Steier

Departments of Electrical Engineering and Physics

University of Southern California

Los Angeles, California 90007

The amount of depolarization resulting from stress -induced birefringence in

crystalline materials can be extremely dependent upon orientation. In windows

and rods made of certain materials, critical orientations exist for which the depolarization effect is eliminated.

Key words: BaFg; CaF^; crystalline windows and rods; depolarization; optical

distortion; photo-elastic constants; stress-induced birefringence,

1. Introduction

Depolarization of beams transmitted through optical elements rendered birefringent by applied, induced, or frozen in stresses is a frequent nuisance in laser system design. In high power laser windows, this form of optical degradation effectively creates two distorted beams, one for each polarization. Each beam has a different focal point, and any attempt to focus one defocuses the other. In laser

rods, the effect appears as a loss in the cavity for polarized output. We have found that the depolarization effect can be eliminated in crystalline windows made of certain materials by properly orienting

the window. A similar critical orientation can also be found for a rod made of the same material,

where the strains are confined to the plane perpendicular to the axis of the rod.

2. Experiment

The experimental set-up shown in figure 1 was used to investigate the effect of crystal orientation

upon the depolarization resulting from thermally induced stresses in a window made of BaFg. A 25 W

Gaussian mode COg beam ( 10.6 |jm) was used to thermally induce the stresses, while a polarized and

collimated HeNe beam (632. 8 nm) with uniform intensity was used to probe the birefringence. Within

experimental limits, no intensity was observed in the orthogonal polarization of the HeNe beam when

the heating beam was blocked. With heating, a cloverleaf pattern in the orthogonal polarization was

observed as shown in figure 2. For normal incidence, the propagation ve_ctors for both beams were

along the (111) crystallographic direction, and the electric displacement D of the HeNe beam was

along some undetermined direction in the (111) plane. For this orientation, the resultant intensity

pattern is independent of crystallographic direction along D [l]^. The intensity pattern for the (111)

orientation is shown in figure 2(a). Other orientations were observed by rotating the window by the

angle cp about its normal ((111)) and by tilting the window about the horizontal axis of the mirror mount

holding the window, such that the propagation vector inside the window made an angle 9 with the window normal, as shown in figure 3. As the window was rotated by cp and Q, the intensity pattern in the

orthogonal polarization would rotate and change dramatically in intensity, as shown in figure 2(b), For

a particular orientation Oq, cpo), the pattern vanished, showing a null in the depolarization effect. This

null was also observed for (Qg, cp= cpo n(120°)) for n = 1, 2, consistent with the 3-fold symmetry at the

(111) direction,

3, Analysis (General Case)

For a given propagation_vector k in a naturally birefringent material, there are two directions for

the electric displacement D which result in a linearly polarized beam. In the index ellipsoid formalism [2], these directions correspond to the major and minor axes of an elliptical cross-section of the

index ellipsoid, perpendicular to k, as shown in figure 4j_ In phase plate terminology, these axes cor- respond to the fast and slow axes of the phase plate. If D is along one_of these axes, then the beam

remains linearly polarized as it propagates through the material. If D is along any other direction,

the beam becomes elliptically polarized.

If the birefringence is induced by stresses, the shape of the ellipsoid will depend upon crystal orientation and the amount of stress. For non-uniform stress, both the magnitude and direction of the

fast and slow axes will vary from point to point, and the window appears as a collection of differently

oriented phase plates, as shown in figure 5(a), In general, it is not possible for a beam to be polarized along a single direction over the entire cross-section of the beam. The resultant polarization

state is usually a complicated function of position, depending upon the distribution of stresses in the

window.

Work supported by DARPA under Contract No. F19628-77-G-0094.

1. Figures in brackets indicate the literature references at the end of this paper.

-89-
For the special case of a circularly symmetric heating beam, normally incident upon a (111) oriented window, the axes are along radial and azimuthal directions, as shown in figure 6, and the magnitudes are a function of the radial coordinate only. Hence, a cloverleaf intensity pattern in the orthogonal polarization is observed.

We have discovered that in certain materials, for certain critical orientations, the axes of the effective phase plates become uniform in direction, independently of stresses in the plane of the window,

as shown in figure 5(b). A beam passing through such a window will become distorted (because there are still index variations along the fast and slow axes) but not depolarized. To understand how such an

orientation could exist, we investigate the equation for the index ellipsoid.

In cartesian coordinates, the index ellipsoid for a cubic crystal is given by

(l/n^+ AB )xx + (l/n^+AB )yy + (l/n^+AB )zz + 2aB xy+2AB xz + 2aB yz XX yy xy ' " xz yz'

1,

where n is the index of refraction, and

AB. 2An. ./n'

(1)

(2)

is the stress-induced change in the ellipsoid. If the wave vector Ic is along the z direction, then the elliptical cross -section of the ellipsoid perpendicular to k is given by

(1/n^+AB )xx + (l/n^+AB )yy+2AB xy " XX " yy xy

1

.

(3)

If ABjy = 0, then equation (3) describes an ellipse with major and minor axes along x and y. For the existence of critical orientations as described, AB^^ must be zero, independent of stress.

The relationship between aB in the xy plane and stresses a in the xy plane is given by

AB'XX

AB'

yy

AB'

xy

"^11

^^21

^61

^12

*22

'16

^26

^'66

yy

xy

(4)

where the prime denotes a quantity which varies with crystal orientation. The q's are elements of the

stress-optic tensor [3]. The condition for ABjy = 0 independent of stress is

361 =^62=^66 (5)

Starting from some arbitrary orientation and rotating the coordinate system about the y axis by the

angle 9» one finds [4] that

2 2 2 2 q^ J = -sin9(q^^cos G + q^^sin g - q^gSin2e/2) + cos0(q^jCos g+q^^^^" Q - q^^^siaZQ/Z) (6)

' = -sine(q42) ^62 (7)

*566^'^44®^" ^'^%b^°^ e -((q4^+q^4)/2)sin29 , (8)

where the unprimed q's denote elements of the stress-optic tensor before the rotation. If the rotation

axis is a 2-fold axis or an axis perpendicular to a mirror plane, the unprimed q's in eqs.(6, 7) are

identically zero, and both q^j^ and qgj are zero independent of g. The angle 9 is determined by setting

qea= 0 in eq. (8).

4. 43m Cubic Symmetry Windows and Rods

4, 1 Windows

The window materials currently of interest have 43m cubic symmetry. The (100) and (110) directions satisfy the symmetry requirements in section 3. For z and y along (100) directions, q4a='3a4~

and qea=q44; hence from eq. (8), qe3=q44 independent of a rotation by 9 about the y axis, and no critical

orientation exists. For z along a (100) direction and y along a (110) direction, shown in figure 7, 184=146 = 0' 169=111-112. and q^ is given by

2 2 ^66^ ^^ir^l2'*^°® 9 + q44Sin 9- (9)

The value of qgg varies from (qn-q^a) when z' is in the (100) direction to q^ when z' is in the (110) direction. If (q^i-qia) ^'^'^ I44 are opposite in sign, then there exists a critical angle 9c fo^" which q^

vanishes, namely

tan^9,= (10)

where

-90-
(11)

is the stress-optic anisotropy ratio. Thus critical orientations exist in materials for which A is negative. For k along z' and D along either x' or y', no depolarization will occur, independent of stress

in the x'y' plane. To insure the plane stress approximation, the window should be thin, and the window

normal should be in the z' direction.

Stress-optic anisotropy ratios and critical angles were computed using experimental values for

stress-optic coefficients [5, 6, 7, 8] for several window materials at wavelengths where data was available. The results are shown in table 1. Experimental results in Ba-Fg [9], shown in figure 8, are in

good agreement with the theoretical estimate of Q^. The angle measured from the (111), g^, was 11°,

corresponding to a value of 65.7° for 9^ measured from the (100). Also note that the estimated value

of 9J for CaFg at 632. 8 nm is within 1 ^° of the (111) direction. Measurements by Bernal [lO] and

Detrio [ll] show negligible depolarization for (111) oriented CaFj windows, and significant depolarization for other orientations.

Table 1. Critical angles for stress -insensitive birefringence axes

NaCl KCl KCl CaFg BaF, SrF^ CdTe GaAs

X 633 nm 633 nm 10. 6 ^m 633 nm 633 nm 633 nm 10.6 ijm 10. 6 ijm

A +0. 68 -2. 16 -2. 12 -0.44 -0. 19 -0. 25 +0. 84 + 1. 10

Sc 34. 2° 34. 5° 56. 3° 66.4° 63. 4°

4. 2. Rods

Analogous to thin windows, critical orientations should also exist for beams propagating down the

axis of a long rod. For long rods, it is the strain rather than the stress that is confined to the plane

perpendicular to the rod axis. For this case

i^B'XX

^B'

yy

AB'

xy

P21

P61

P'l2

P22

P62

Pi6

P26

P66

yy

xy

(12)

where the Pu's are strain-optic coefficients, and the e's are strains. For z' in the (110) plane, pgj^ and pgg are zero independent of 9. and

^66-

1 2 2 ^(Pii'Pu''^"® 9 + P44sin 9 (13)

The factor of one half comes from the definition of p^= Pisig as opposed to q^- ^^laia' critical orientation 9^ is given by

tan^e = -1/A (14)

c p

where

is the strain-optic anisotropy ratio.

The p's and q's are related by

and the anisotropy ratios are related by

2P44/(Ph-P

^p= 2P44/(PirP 12'

12) = (q44/(qn-qi2))*(2c44/(c^j-c^2)'-

(15)

(16)

(17)

The c's are the elastic stiffness coefficients. Since the elastic anisotropy ratio is always positive, the

stress-optic and strain-optic anisotropy ratios have the same sign. So if a critical orientation exists

for a thin window made out of a certain material, a critical orientation also exists for a long rod made

of the same material,

Koechner, Rice, and DeShazer [12] first appreciated the orientation dependence of strain- induced

birefringence in YAG rods and investigated the depolarization of a HeNe beam (632. 8 nm) for rods oriented along symmetry axes. At this wavelength, A^ in YAG is positive, and therefore, only a minimum depolarization was observed.

-91-
5. Conclusions

The amount of beam depolarization resulting from stress-induced birefringence in thin windows and

long rods can vary significantly with crystallographic orientation. For certain critical orientations the

depolarization can be eliminated. In 43m cubic symmetry materials, the necessary condition is a negative photo-elastic anisotropy ratio. If such an orientation exists for a window made of a certain material, a critical orientation will exist for a rod made of the same material.

Apart from its application in the design of optical elements free from stress-induced depolarization,

the effect should also prove useful in connection with the measurement of photo-elastic constants. These

constants are relatively difficult to measure. A simple measurement of 9^ yields a ratio of constants,

and if one constant is known, the other is easily and accurately determined. A measurement of 9c while varying the wavelength would also give the dispersion in the stress-optic anisotropy ratio.

6. Acknowledgement

The authors are grateful to Professor Jan Smit for stimulating discussions of this effect.

7. References

[1] Bendow, B. , Gianino, P. D, , Flannery, M. , and Marburger, J. , Proceedings of the 4th

Annual Conference on Infrared Window Materials, compiled by C. R. Andrews and C. L,

Strecker (Air Force Materials Lab. , WrightPatterson Air Force Base, Ohio 1975), p. 299.

[2] Yariv, A. , Quantum Electronics , 2nd ed.

(Wiley, New York, 1975), pp. 354-355.

[3] Nye, J. F. , Physical Properties of Crystals , (Oxford University Press, Ely House, London W. 1, 1969), pp. 235-254.

[4] Joiner, R. E. , "The Elimination of Stress

Induced' Birefringence Effects in High-Power

Laser Windows," dissertation. University of

Southern California, Los Angeles, 1977.

[5] Pitha, C. , Friedman, J., Szcsensniak, J.,

Cutteback, D. , and Corielli, J. , Proceedings of the 5th Annual Conference on Infrared

Window Materials, compiled by C.R. Andrews

and C. L. Strecker (Air Force Materials Lab,

Wright-Patterson Air Force Base, Ohio,

1976), p. 927.

[6] Joiner, R. E. , Steier, W. H. , and Christensen, C.P. , in ref. [S], p. 937.

[7] Feldman, A. , Horowitz, D. , and Waxier,

R. in ref. [5], p. 943.

[8] Dickinson, S. K. , Infrared Laser Window

Materials Property Data for ZnSe, KCl, NaCl, CaF^ , BaF, , SrF^, (Air Force

Cambridge Research Labs, Bedford, Mass,

,

1975).

[9] Joiner, R. E. , Marburger, J., Steier, W.H,,

Appl. Phys. Lett. 30, 485 (1977).

[10] Bernal G.,E, and Leung, K. , private communication, Honeywell Corporate Research Center, Bloomington, Minnesota,

August 1977.

[11] Dietrio, J. A, private communication. University of Dayton, Research Institute,

Dayton, Ohio, October 1977.

[12] Koechner, W. , Rice, D. K. , and DeShazer,

L. G. , U.S. patent.

8. Figures

ANALYZER

1, Experimental set-up used to investigate

orientation effects upon depolarization

in BaF^.

-92-
3. Rotations used to investigate other

orientations.

2. Intensity pattern in the orthogonal polarization,

(a) k along <111>

(b) critical orientation

4. Major and minor axis of elliptical crosssection in index ellipsoid formalism.

-93-
<I00>

6. "Fast" and "slow" axes for a circularlysymmetric heating beam propagating in the (111) direction.

7. Critical orientations in windows made

material with 43m symmetry.

<I00> 9^ = 65.7° /

/<lll>

^ NULL

<IIO>

CRITICAL ANGLE IN BaFg AT 632.8 nm

THEORETICAL 66. 4°

EXPERIMENTAL 65.7*

8. Experimental value of 9^ in BaF

-9A-
COMMENTS OM PAPER BV JOINER, MARBURGER, AMD STEIER

Thz ipnakex wcw cu>kzd ^{j a cAaJxcoI ayxqlt deXMJnincution had been coAXltd oat ^OA VAG. He KUpondud

that the.y had looked don. a ctiticaZ anglz eiJiJeCxt in VAG at 6328 X, bat nom had been obieAued.

John diJyvio oi the, (Inivexditij od Vayton ReAza/ich Zmtttut^ pointzd oat that in zxpz/ujmnti caMJ.e.d oat

at UVRl, Kuulti iimiZoA. to thoiz pn.<u>zntzd in tkii papzA had bzzn obtaimd.

-95-
LATTICE DEFECT EQUILIBRIUM IN KCl:Eu*

J. B. Wolfenstine and T. G. Stoebe

Department of Mining, Metallurgical, and Ceramic Engineering

University of Washington

Seattle, WA 98195

A study of lattice equilibria in Harshaw-grown KCl:Eu laser window material has

been undertaken using optical absorption and ionic conductivity. Ionic conductivity

was determined using an A.C. method with a Wayne-Kerr bridge on 5 samples each taken

from four different regions containing Eu impurity contents between 30 and 60 ppm. Eu'^-ion concentrations were determined from the position of the "knee" in the con- ductivity plots. Optical absorption measurements were undertaken using a Cary 14 spectrometer at room temperature using samples from the same four regions of the

original crystal. The optical absorption coefficients, a, were determined at 343 and

243 nm and compared with the Eu"*"*" contents determined using ionic conductivity. The

results for the 243 nm band indicate a linear relationship between these two measurements such that

Mole % Eu"*^ = 4.3 X

10""* a.

For the 343 nm band, the proportionality constant is 4.5 x 10

This result disagrees with other work using different Eu-ion analysis techniques.

This may indicate the presence of additional Eu in some of the crystals, present in a state of agglomeration, which would not be measured using ionic conduction.

Key words: Eu impurity, ionic conductivity, KCl, laser window materials, optical

absorption.

1. INTRODUCTION

Recent advances in infrared laser technology have demanded the development of a suitable window

material for the high powered CO2 gas laser system. The window materials under consideration are primarily alkali halides, with the most promising of these being NaCl, KBr, NaBr and KCl [1].^ These

alkali halides are preferred in that they have very low optical absorption coefficients over a wide

portion of the infrared spectrum. However they do have the drawback that in their single crystal state

they are quite weak. Therefore in order to use these alkali halides for window materials it is important to increase their strength without decreasing their transparency. Some of the strengthening

methods being investigated are recrystallization, alloying and radiation hardening [1]. In KCl, one of

the main strengthening methods used involves alloying with europium (Eu)

.

In order for the europium-doped KCl (KCl:Eu) material to be used in a laser window system, either

in single crystalline or polycrystalline form, it is important to characterize its lattice defect state

and to relate the lattice defect structure to the optical properties of the material. This characterization should include the determination of the Eu impurity distribution throughout the crystal, the

nature of the charge and state of the Eu impurity content. These crystal characterization goals are

the basis of this present work in KCl:Eu single crystals.

2 . BACKGROUND + I [ +q Various studies in KClrEu have indicated the presence of Eu , Eu , and Eu ions in this material.

The predominant ion present at room temperature appears to be Eu"*^ present in the form of Eu'^-vacancy

dipoles [2-5].

I [ The Eu ion has two characteristic ultraviolet absorption bands that are due to electronic

transitions from the 4f^ ground state to the e and X.i components of the 4f^ 5d^ configuration, with

S § the former being at higher energy [6] . These bands occur at 243 nm and at 330 nm. A typical room temperature absorption spectrum of the KCliEu"*^ is shown in Fig. 1, where the more intense curve (H) is

from the heel section and the lower curve (C) is from the cone section Harshaw KCl:Eu crystal

KC.01ECH97. The 3300 X band consists of a staircase structure that can be partially resolved at room

temperature; at low temperatures it is composed of peaks at 3290, 3430 and 3640 A [7]. The 2400 A band

*Work sponsored in part by Air Force Office of Scientific Research under grant AFOSR-76-2977.

1. Figures in brackets indicate literature references at the end of this paper.
can only be resolved at low temperatures; at 77°K it consists of five individual peaks at 2340, 2400,

2440, 2510 and 2580 1 [7]. All of the samples from our KCl:Eu crystal showed the blue luminescent color

characteristic of Eu"^ [6].

The absorption spectrum of trivalent Eu ions has been observed only in europium salts [8] and in ^ LaCl3:Eu+^ [9]. The principal absorption lines that are of interest are at 5790, 5260, 4650 and 3960 A. Of these, the absorption at 4650 K has been reported as being about twice as intense as the others,

while the presence of the absorption of 5790 A seems questionable. Stoebe and Spry [10] made attempts

to observe the Eu"^^ absorption bands 5790, 5260, 4650 and 3960 A in as-received KClrEu crystals by

annealing the crystals at 650''C for 4 hours and then rapidly cooling them to room temperature (cooling

accomplished in about 30 sec.)- Honejrwell Corporation development work [1] had indicated that KCl:Eu

becomes brittle after annealing at 650°C in air; Honeywell speculated that this was caused by the oxidation of Eu"*"*" to Eu '

' ' . However, no Eu"*"^ absorption bands were observed by Stoebe and Spry [10] and any

change in the Eu"*"*" content (243 nm peak) was small and virtually undetectable. These observations

suggest that the concentration of Eu' '' as compared to the total Eu concentration is negligible.

The optical absorption bands of the Eu"*"*" (243 nm and 330 nm) , if calibrated, can allow for a non- destructive determination of the Eu"*"*" content. Stoebe and Spry [10] calibrated the peak absorption

coefficient of the 243 nm band against the Eu concentration as determined by the Honeywell polarographic

analysis [11]. The 243 nm band was chosen rather than the 330 nm band, since from fig. 1, the 243 nm

band has less structure at room temperature. A linear relationship was obtained as follows:

mole % Eu = 4.2 X

10"** a (1)

where a is the peak absorption coefficient of the 243 nm band in cm ^ at room temperature. The relative

comparison of optical absorption results and polarographic analysis using eq. (1) is shown in fig. 2 [10].

Using peak absorption coefficients and polarographic data, Stoebe and Spry [10] were able to determine the Eu impurity distribution in the edge, heel and cone regions of the KCl:Eu crystal

KC.01ECH97. They found that the Eu"*^ content through the crystal generally followed the expected

variations for an impurity with a distribution coefficient of less than one, except for the rise in con- centration near the surface. The initial Eu content in the melt was 100 mole ppm; this decreased to indicated concentrations in the crystal on the order of 30 mole ppm near the cone and 60 mole ppm near

the heel, as shown in fig. 2. The relative intensities of the absorptions in fig. 1 also indicate the

variation of Eu"'"'" content from heel (H) to cone (C) .

Ionic conductivity is another technique that may be used to investigate lattice defect configurations and divalent (or trivalent) ion concentrations. Ionic conductivity data can thus provide another

calibration check on the Eu"*^" content.

A simple theory of ionic conductivity, which is adequate for most of our work, has been well sum- marized by Lidiard [12] and Siiptitz and Tetlow [13] . The main point is that the variation of the con- centration of free cation vacancies as a function of temperature for doped crystals enables the

calculation of the impurity content. Of the three possible species of the Eu-ion present in KCl:Eu,

only the Eu"*""*" and the Eu"*^ can be detected by ionic conductivity, since only these two ion valences

increase the concentration of free cation vacancies. If Eu"*" is introduced into the KCl lattice to replace K"*", no extra cation vacancy is needed to maintain charge neutrality and hence Eu"*" cannot be

detected by ionic conductivity. However, if Eu"*""*" is introduced into the KCl lattice to replace K"*" an extra cation vacancy is also created to maintain charge neutrality, according to the charge balance

equation,

2k'^ J Eu"*^ + 2V^ (2)

where V indicates a cation vacancy. This extra cation vacancy can contribute to ionic conductivity ++ +3 +

and allows detection of Eu ions. If Eu is introduced into the KCl to replace K , 2 extra cation

vacancies are also created to maintain charge neutrality, according to the charge balance equation,

Z Eu"^ + 2V^ (3)

Hence these extra cation vacancies can also contribute to ionic conductivity and allow detection of the

Eu"^^ ion.

Ionic conductivity is usually plotted as Ln aT (ohm~-^cm ^K) versus 1/T(K ^). For our purpose,

only regions I and II of the conductivity plot need be considered. In region I the concentration of

cation vacancies is governed only by the thermal statistics of the lattice and is independent of the

impurity doping level. The slope of the Ln aT vs. 1/T plot in region I is (Hg/2 + Hj)) , where Hg is the

Schottky energy of formation, and H^j is the energy of motion of the cation vacancy. Region II is known

as the extrinsic region and is where the concentration of cation vacancies is equal to the concentration of divalent impurities; the slope in this region is H^^. The temperature at which regions I and II

-97-
intersect is known as the knee temperature and at this point the concentration of intrinsic cation

vacancies equals the concentration of extrinsic vacancies (concentration of divalent impurities) . Thus

the determination of the knee temperature and the slopes of regions I and II allows the calculation of

the concentration of divalent impurities present.

3. EXPERIMENTAL

The KCl:Eu crystals used in this study were from KCl:Eu ingot KC.01ECH97 [10], a 17-inch diameter

by 7-inch high single crystal grown by the Harshaw Chemical Company for the AFML-monitored laser window

development program. The sectioning plan for this ingot is shown in fig. 3. The KCl:Eu samples were

taken from locations shown in more detail in fig. 2; samples from the heel (last part to solidfy) are shown in fig. 2 [10], as H2 and H4, while samples from the cone section (first part to solidfy) are shown in fig. 2 as C2 and C4. Also available were several Harshaw-grown undoped KCl single crystals.

The techniques used to investigate the crystals consisted of optical absorption, electron paramagnetic resonance and ionic conductivity. Polarographic analysis of the Eu-ion content was obtained by

Honeywell, Inc. [11] on samples immediately adjacent to those used for the current work. The polarographic analysis results for heel and cone scans along with the earlier optical abosrption determinations of Eu-ion content have been shown in fig. 2.

The samples used for optical absorption measurements were cleaved to thicknesses 5 mm and 1 mm. Samples were used in the unpolished condition; however, care was taken so that the part of the crystal

in the beam contained no cleavage steps. Some of the crystals that were used in the unpolished condition were also mechanically polished to an optical finish using 0.6 ym alumina polishing powder and

compared to measurements made in the unpolished condition; no differences were observed. All optical

absorption measurements were made on a Cary-14 spectrophotometer at room temperature and the optical

absorption coefficient was calculated from the optical density and the thickness of the crystal. The

effect of reflectance in KCl is approximately 3.5% and therefore can be neglected; thus, a can be calculated from the peak absorption coefficient [14] using the relation:

^^2^(0.D.)

Here t is the sample thickness and CD. represents the optical density of the sample. Optical absorption measurements were also made on a few of the Harshaw-grown undoped KCl single crystals for com- parative purposes.

Ionic conductivity measurements were made using an A.C. method at 1592 Hz using a Wayne-Kerr

Universal Bridge which measured both conductance and capacitance of the sample. Conductivity values were determined by multiplying the measured conductance by L/A, where L is the length of the sample and

A is the cross sectional area of the sample. The crystal was coated with a colloidal graphite and then

mounted between two nickel electrodes in the conductivity jig [15]. The conductivity jig was placed

inside a horizontal Vycor tube which was heated by a Marshall furnace. The furnace temperature of the

crystal was measured by means of a chrome 1-alumel thermocouple placed next to the crystal and connected

to a digital volt meter. Before making the conductivity measurements, the crystals were annealed for

1 hr. at 420°C to allow for sample equilibrium. During the annealing treatment and the conductivity

measurements the furnace was continually flushed with dry helium gas. The conductivity measurements were made in equilibrium on heating from 420°C to 750°C at 25°C intervals.

4. RESULTS AND DISCUSSION

Ionic conductivity plots of the KCl:Eu crystals from samples H2, H4, C2 and C4 are shown in fig.

4 where the intrinsic slope is determined from that of the Harshaw crystals. The slope in region II,

Hj,, is O.SleV from crystal C2 [16]. This value seems high compared to some recent work [17-19] but

agrees fairly well with earlier studies [20-22], From the slope of region I and this value of H^,, Hg was determined as 2.56 eV. This agrees well with recent work. Using this value and the knee temperatures, the Eu"*^ content of these KCl:Eu crystals was then determined.

Assignment of a Eu"*"*" content from the above is valid only if no Eu^^ contributes to the ionic

conductivity data. Again the optical absorption bands at 5790, 5260, 4650 and 5960 k were checked for

Eu"*"^ absorptions. As with the work of Stoebe and Spry [10], none were observed.

The optical absorption coefficients at room temperature of both the 243 and 330 nm band were calculated for the KCl:Eu crystals H2, H4, C2 and C4. The optical absorption coefficient of the 243 nm

band is calibrated against the Eu++ content determined by ionic conductivity using a least squares

analysis, as shown in fig. 5. The result indicates a linear relationship between these two measurements such that:

mole % Eu"*^ = 4.3 x lO""* a (5)

-98-
This result agrees well with the work of Stoebe and Spry [10] and is probably more accurate than their

value. This result may also be compared to the polarographic analysis of the adjacent samples as in

fig. 6. The comparison agrees to within 10% in three of the locations but varies 25% at the fourth

(Sample HA). The reasons for this discrepancy are not clear.

I I When the optical absorption coefficient of the 330 nm band is calibrated against Eu content

determined by ionic conductivity, a linear relationship exists as follows:

mole % Eu"*""" = A.5 x lO"*^ a (6)

This compares well with the calibration of the 243 nm band given in eq . (5).

The result in eq. (5) may also compare to the results of Sill et al. [23] , who used atomic absorption spectroscopy to measure the Eu++ impurity content. Compared to the optical absorption coefficient

of the 243 nm band. Sill et al. obtained a proportionality factor that was 8 times higher than that in

eq. (5). This may indicate the presence of additional Eu in their crystals, present either as Eu"*" or in a state of agglomeration, neither of which would be measured using ionic conductivity. However,

this discrepancy is not seen in the polarographic analysis of our samples, noted above.

5. CONCLUSIONS

The 243 and 330 nm absorption bands in KCl:Eu, when calibrated by polarographic analysis and

ionic conductivity data, can yield a non-destructive determination of Eu"*"*" content in KCl:Eu laser

window materials. The concentration of Eu"*"^ is negligible when compared to the total Eu concentration

in the KCl:Eu samples studied. The results also indicate that at room temperature the Eu"*^ is in the

form of Eu"'^-vacancy dipoles.

The discrepancy between the Eu content determined by polarographic analysis, by atomic absorption,

and by ionic conductivity may be due to cation vacancies that are trapped by some type of complex that

would not allow detection by ionic conductivity, or by the presence of some of the Eu in the form of

Eu"*" ions. These possibilities are being investigated further.

6 . ACKNOWLEDGEMENTS

The authors wish to thank Dr. Robert Spry of the Air Force Materials Laboratory for his input in

formulating this program.

7 . REFERENCES

[1] Harrison, W. B. Hallde Material Processing

for High Power Infrared Laser Windows, Final

Technical Report AFML-TR-75-104, Honeywell,

Inc., July 1975.

[2] Rohrig, R. , Phys. Lett. 16, 20 (1965).

[3] Nalr, P. G., Llngam, K. V., and Venkataraman,

R., J. Phys. Chem. Solids 29, 2183 (1968).

[4] Aguliar, G. , Munoz, E., Murrieta, H. , Boatner,

L. A., and Reynolds, R. W. , J. Chem. Phys. 60,

4665 (1975).

[5] Maevskll, V. M. , Kalabokhova, E. N., Zhukov,

A. N., and Krulilovskii, B. N., Sov. Phys.-

Solid State 17, 150 (1975).

[6] Reisfield, R. , and Glasner, A., J. Opt. Soc. Amer. 54, 331 (1964).

[7] Chowdral, B. V. R. , and Itoh, N. , Phys. Stat.

Sol. (b) 46, 549 (1971).

[8] Kim, B. F., and Moos, H. W. , Phys. Rev. 161 , 869 (1967).

[9] DeShazer, L. G., and Dieke, G. H., J. Chem.

Phys. 38, 2190 (1963).

-99-

[10] Stoebe, T. G. , Spry, R. J., and Lewis,

J. F., Optical Properties of Europium-doped

Potassium Chloride Laser Window Materials,

Technical Report AFML-TR-76-20, April 1976.

[11] Harrison, W. B., Alkali Hallde Materials

for High Power Infrared Laser Windows,

Final Technical Report AFML-TR-76-106,

Honejrwell Inc., June 1976.

[12] Lidiard, A. B. , Handbuch der Physik,

edited by S. Flosse (Sprlnger-Verlag,

Berlin 1957) Vol. 20, p. 246.

[13] Suptitz, P., and Tetlow, S., Phys. Stat. Sol., 23, 9 (1967).

[14] Townsend, P. D. , and Kelly, J. C, Colour

Centers and Imperfections in Insulators

and Semiconductors , Sussex University Press,

London (1973)

.

[15] Vora, H., Ph.D. Thesis, University of

Washington (1974)

.

[16] Stoebe, T. G. , and Pratt, P. L., Proc. British Ceramic Soc. 9^, 181 (1967).
[17] Beaumont, J., and Jacobs, P., J. Chem. Phys.

45, 149 (1966).

[18] Chandra, S., and Rolfe, J., Can. J. Phys. 48,

412 (1970).

[19] Fuller, R. , and Marquardt, C. , Phys. Rev.

176, 1036 (1968).

[20] Beniere, M. , Beniere, F., and Chemla, M. , J. Chem. Phys. 67, 1312 (1970).

[21] Dreyfus, R. , and Nowick, A., J. Appl. Phys.

33, 473 (1962).

[22] Allnatt, A., and Jacobs, P., Trans. Faraday

Soc. 58, 116 (1962).

[23] Sill, E. L., Martin, J. J., and Al. Shaieb,

Z., Phys. Stat. Sol. (A) 39, K 131 (1977).

[24] Pawdey, S. D. , J. Chem. Phys. 47, 3094

(1967).

8. FIGURES

WAVELENGTH (MM)

Figure 1. Optical absorption spectrum of KCl:Eu

at room temperature. Spectrum H is

from the heel section and spectrum C is from the cone section of sample

KC.01ECH97.

-100-
Figure 2. Concentration vs. crystal location

determined using absorption and

polarographic results, from ref. 10. Schematic sections at heel and cone show sample locations and grain

boundaries (see also fig. 3).

-101-
•FMl S4MPLES

MECHOilCtl. SPECIUEXS

(2 ceouPSi

CMtHICAI. tAHPLC Figure 3. Sectioning plan for Harshaw-grown ingot

KC.01ECH97. See fig. 2 for specific sam- ple locations in cone and heel sections.

800 700 600 500 400 "C

rn 1 1 1 1—

I

C2 • C4

O H2

H4

IOVT (°K)-'

Figure 4. Ionic conductivity plots of pure KCl

and KCl:Eu crystals from the heel and cone sections of Harshaw-grown ingot

KC.01ECH97. Sample numbers from fig. 2. -102-
T

EUROPIUM CONCENTRATION (PPMM)

Figure 5. Absorption coefficient of the 243 nm

band versus Eu"*"*" concentration, as determined by ionic conductivity. The

slope, determined by least squares curve

fitting, is 0.234; the slope inverse

gives proportionality constant in eq.

(5).

-103-
Figure 6. Same as fig. 1 with addition of points

• , which represent Eu"*^ concentration

determined by ionic conductivity (fig.

5) compared to polarographic Eu-ion

content

.

COMMENTS ON PAPER BV WOLFENSTIWE AMD STOEBE

Jkz pfU.ncA.pcil point bfiought out -in thz dLiictUiton Men that thz dttMmtncution o{i thz choAgz itate. OjJ tht e.uA.op-ium -ion by abioKption mzcu>uA&ne,nt!> -6s vM.y dif^filciilt. The. oictUaton. itAzngth o(j thz doubly

Lonizzd zuAop-Lum Lis much gizatzA than that {,ofL thz tfUvatznt ton. A much mon.z izn^iAJU-vz mzthod o{j

dztzcttng Eu^^ muZd bz to look (,01 {^luofiziczncz o{i thz ion, bzcawiz thz iluofiziczncz ipzctna. o{, thz

divatznt and txivalznt itatzi o^ thz ion axz Mztl 6zpa/uitzd. Thz ipzakzA iz&pondzd tkoit thOi zxpz/u.-

mzYit had not bzzn canxizd out.

-104-
DETECTION OF OXYGEN IN CALCIUM FLUORIDE LASER WINDOW MATERIALS BY XPS

T.N. Wittberg, J.R. Hoenigman, and W.E. Moddeman

University of Dayton Research Institute, Dayton, Ohio 45469

and

C.L. Strecker

Air Force Materials Laboratory, Wright-Patterson AFB, Ohio 45433

The three most important criteria used in evaluating materials for laser

window applications are high mechanical strength, high optical transmission,

and the existance of applicable antireflective coating systems. In the case of calcium and strontium fluoride, oxygen impurities can play a strong role

in determining these characteristics.

XPS data are presented on CaFj substrates, ZnSe coatings, and ZnSe/

CaF2 interfaces. XPS data on CaFj show the presence of oxygen on surfaces

that have been argon ion sputtered and on surfaces that have been cleaved

in an inert atmosphere. Oxygen has also been noted at the ZnSe/CaF2 interface. These data can be related to previous results on the optical transmission of the coated laser windows.

Key words: Adsorption, atmospheric contaminants, calcium fluoride

coatings, laser window, oxygen contaminants, x-ray photoelectron spectroscopy (XPS), zinc selenide.

1. Introduction

A typical laser window consists of a substrate, such as CaF^, which is given an antireflective coating. The boundary region between the coating and the substrate is referred to as the coating/substrate

interface. Theoretical investigations have shows that two molecular layers of adsorbed water at the

coating/substrate interface would give rise to an optical adsorptance of lO"** which is unacceptable for

transmission applications at 10.6 pm [1]. Also, the chemical composition of the substrate's surface

influences the adhesion of laser window coatings. To date, little experimental information is available

as to, the role that surface-adsorbed impurities at the interface play in adhesion. In this paper, data

on changes in elemental surface composition are presented on CaF^ samples with different surface preparations. The preparations include: (1) polishing, (2) exposing to high humidity, (3) sputtering to a depth of approximately 30 nanometers, and (4) cleaving in different environments.

In addition, two ZnSe coated CaF^ specimens were analyzed for elemental variations in the coating

and at the interface. The technique used to analyze these materials was X-ray Photoelectron Spectroscopy

(XPS)*. The technique is well documented for its analytical capabilities [2,3].

2. Experimental Procedures

High purity polycrystalline CaFz materials were obtained from the Harshaw Chemical Corporation.

These samples were used in the analysis unless otherwise indicated. Emission spectroscopy were used to analyze for bulk impurities. Samples for XPS analysis were cut with a diamond bladed saw without the use of a liibricant. The samples were cut into rectangular specimens, approximately 10 x 5 x 1 mm. The surface of each sample was sequentially polished with 20, 9, and 1 pm alumina polishing compounds. Each

polished specimen was then washed with detergent (Alconox) , which was followed by three distilled water

rinses and three isoproyl alcohol rinses. Following the washing and rinsings, the samples were cleaned i with vapors. Each sample was then analyzed with XPS. J

Two of the polished specimens were removed from the XPS analyzer and exposed to a high humidity "

environment. This was accomplished by holding each specimen above boiling water for approximately one minute. These samples were then re-analyzed with XPS. Approximately thirty nanometers of the surface

of one sample were removed by argon ion sputtering and the saitple was re-analyzed by XPS. The ion sputtering and depth profiling were accomplished with a Varian 3 keV variable potential ion gun.

The two ZnSe coated CaF2 specimens were prepared by electron beam deposition [4] . The thickness of

each film was determined by a Sloan automatic deposition controller, model OMNI, which uses a quartz

crystal frequency oscillator for thickness monitoring. The thickness of each film was 50 nanometers

[4]. One coating was deposited at 0.1 nanometers/second and the other at 2 nanometers/second. Both of

these coatings survived a water immersion test [5]

.

* This acronym is often used interchangeably with Electron Spectroscopy for Chemical Analysis (ESCA)

.

-105-

i
The XPS spectrometer is a modified AEI ES-lOO instrument. This spectrometer with the accompanying

argon ion sputtering capability has been discussed in more detail elsewhere [6] . The system is pumped

by two 250J!./sec diffusion pumps and a 260J!./sec turbomolecular pump. Vacuum achievable in the sample

chamber is 10 torr. The chamber is bakeable to >150°C. Reactive vapors can be minimized in the system by baking, by a Ti sublimation pump, and by two liquid N2 cryostations. The anode used for all XPS

measurements was magnesium.

3. Results and Discussion

3.1. Calcium Fluoride Substrate

The first part of this study involved the examination of the calcium fluoride substrate itself.

The second part of the study concerned the zinc selenide coated calcium fluoride materials. Figure 1 illustrates the overall XPS spectra of a polycrystalline CaF^ sample polished and cleaned (la) , following exposure to high humidity (lb) , and following argon ion sputtering, (Ic) . These data are summarized

in Table I. The data represented in Table I are given in atomic percent [7]. The scan in Figure la

shows that the surface of the CaF^ is contaminated mainly with carbon and oxygen, and with smaller

amounts of other elements (Na, CI, Si, S, and P) . Upon exposure to high humidity, (Figure lb), the car- bon and oxygen signals were found to increase by approximately a factor of two, relative to the calcium

or fluorine. These data, for both the polished and exposed samples, suggest that CaFj has a high affinity for components of the atmosphere. Possible atmospheric sources for oxygen contamination are 0^ and

water. For carbon, possible sources include atmospheric dust and CO^ . The origin of the adsorbate

species has not been identified. From the results of Edelhock and Taylor [8] the increased oxygen con- tamination is most probably chemisorbed water and not atmospheric molecular oxygen.

The data following sputtering to a depth of <30 nm are also included in Table I. The decrease in

the carbon and oxygen signals relative to the calcium and fluorine signals indicate oxygen and carbon

are surface contaminants adsorbed from the atmosphere.

In order to examine these surface contaminants more closely, two experimental approaches were

taken. One involved a detailed, in-depth profile analysis of CaF^, and the second involved the comparison of cleaved surfaces*. Some of the samples used in these analyses had been annealed previously to approximately 1000°C.

The XPS data from the in-depth profiling for annealed and non-annealed samples are given in Table

II. Only four elements were monitored during sputtering. These in-depth profiles illustrate that oxygen is not only present on the surface of CaF^, but is a contaminant below the surface. Carbon is obviously a surface contaminant that is removed to less than one atomic percent, by sputtering.

A similar conclusion concerning the presence of oxygen in CaF^ can be derived from the comparison

of XPS data taken on cleaved surfaces with those of outside surfaces. These data are given in Table

III. The cleaving was performed in air and in dry nitrogen. All cleaved surfaces of CaF^ were found to contain approximately six atomic percent oxygen. The level of oxygen was found to be approximately a factor of two higher on the outside surface.

J. Estel et al [9] examined water adsorption on cleaved samples of alkali halides with Secondary

Ion Mass Spectroscopy (SIMS). He examined the (100) planes of LiF, NaF, and NaCl under a variety of

conditions: air and vacuum cleaved; with and without annealing; and with and without exposure to water

at various crystal temperatures. The main results were that cleaved crystal surfaces were found to be

free of water and hydroxide layers even in the submonolayer region under ultra-high vacuum conditions.

They found that water vapor would only adsorb on cooled cleaved samples, thus concluding that water

physically adsorbs to LiF, NaF and NaCl and the water is easily removed by evacuation.

Upon examining Estel' s spectra more closely, _one notes the presence of 0 and OH in the negative

SIMS spectra of a cleaved surface of LiF. These O and OH signal intensities were found to be '^'1/100 of the F intensity and the signals roughly decreased by only a factor of two upon in-depth profiling.

Estel rationalized from the low oxygen signals and the profiling information that the surface of LiF did

not contain large concentrations of O and OH . He attributed these 0 and OH signals as resulting

partly from oxygen bulk impurities.

Estel' s SIMS results, showing the presence of oxygen in alkali halide materials, agree qualitatively with the XPS results on CaFj. Since the quantitative capabilities of SIMS are questionable [10],

signal intensity comparisons made by Estel may have little correlation to actual sample composition.

* Preferential cleaving in CaF occurs along the (III) face.

2

-106-
Table 1 Qualitative and Seitii-Quantitatxve ESCA Results On

Nonexposed, Exposed and Ion Sputtered Nonallealed Calcium Fluoride*

Atom Percent

Surface After

Binding Scan Exposure

Energy Before Element to High Element After

Element Level (eV) Exposure C^ Humidity cl Sputtering Element

Fluorine Is 688 4.7 0.7 1.8 0.6 25.3 0.8

Oxygen Is 533 7.1 1.1 7.1 2.4 12.7 0.4

Calcium 2p 348 6.4 1.0 3.0 1.0 32.9 1.0

Carbon Is 285 75.6 11.8 85.5 28.5 2.5 <0.

1

chlorine, sulfur , phosphorus

,

sodium. and silicon were found present in

amounts on the Surface of these materials.

Table 2

In-Depth Profile Analysis of 0, C, F, and Ca In Annealed And

Non-Annealed CaF^ Polycrystalline Laser Window Substrate Materials

(Data Given in Atomic %)*

Sputtering

Time

(Min) **

Annealed Non -Annealed

F 0 Ca C F/Ca 0/Ca C/Ca F O Ca C F/Ca 0/Ca C/Ca

Surface 15 15 10 60 1.5 1.5 6.0 3 30 2 65 1.2 1.2 32. 0.5 56 9 23 11 2.4 0.40 0. 50 48 19 18 15 2.7 1.0 0.8

1 57 10 26 7 2.2 0. 38 0.27 54 15 22 7 2.4 1.70 0.32

2 59 12 24 5 2.5 0.50 0.21 57 15 23 3 2.5 0.65 0.13

4 56 13 26 4 2.2 0.50 0.15 58 12 27 3 2.1 0.44 0.11

8 56 12 28 4 2.0 0.43 0.14 57 13 29 1 2.0 0.45 0.03

16 58 10 30 2 1.9 0. 33 0.07 58 12 30 <1 1.9 0.40 0.03

32 58 11 29 1 2.0 0. 38 0.03 59 12 28 <1 2.1 0.43 0.04

Table 3 Qualitative and Semiquantitative ESCA Results on Cleaved Specimens Of

Single Crystal**And Polycrystalline Calcium Fluoride***

Single Crystal Polycrystalline

Cleaved in

Air Plus Cleaved in

Cleaved Baking at Nitrogen 'Outside " Cleaved

Element Level in Air =100°C t Gas t Surface t in Air t Fluorine Is 49.4 2 5 39.2 2.3 57.4 2.6 14.4 1.7 44.2 2.2

Oxygen Is 7.5 0 39 6.0 0. 34 6.2 0.28 13.7 1.7 7.9 0.40

Calcium 2p 19.5 1 0 17. 3 1.0 22.2 1.0 8.2 1.0 20.0 1.0

Carbon Is 23.6 1 2 37. 5 2.2 14. 3 0.64 63.6 7.7 27.9 1.4

** Single crystal from Harshaw Chemical Company

*** Fusion cast material from Raytheon Company, See, C. L. Strecker, "Analysis for Oxygen

in Calcium and Strontium Difluoride Utilizing X-ray Photoelectron Spectroscopy," Air

Force Materials Laboratory Report, AFML-TM-LP-77-1, January 1977, Wright-Patterson Air

Force Base, Ohio.

+ Ratio of elements relative to Ca.

3.2 Coatings

The data given in Table IV are the results of in-depth profiling two ZnSe/CaF^ systems. The data

are also plotted in Figure 2. The results show a zinc enrichment at the interface as well as a significant amount of oxygen (on the order of 12 atomic percent) and carbon (on the order of 8 atomic percent)

in the first few hundred angstroms of the CaFj substrate. The oxygen and carbon at the interface are probably due to atmospheric adsorption by CaFj substrate prior to ZnSe PVD deposition.

-107-
Johnston, et al [11] , measured the absorption of sputtered films of ZnSe on KCl. They found little

increase in film absorptance as the thickness of the film was varied from 0.4 to 2)Jm. They concluded

that the origin of the majority of the measured absorption was the interface region. These absorption

measurements are consistent with the XPS results on the ZnSe/CaF^ system which showed significant contamination and apparent stoichiometric deviations at the coating/substrate interface. Ritter [1] has

shown that preparation of substrate surfaces can have a marked effect on coating adhesion.

Table 4 ESCA Intensity Results of In-Depth Elemental Profile

Analysis On ZnSe Coatings Evaporated On CaF^

(Intensity Given in Atomic Percent)

Sputtering Element

Time Oxygen Carbon Zinc Selenium Calcium Fluorine

L/a\^a J- (J X 0.1 nm/sec.

Surface 17.2 42 8 19 4 20 6 30 sec 2.7 19 2 39 9 38 2 2 min 2.2 10 9 38 6 48 3 10 min 4.5 4 9 26 7 17 2 12.5 34.2

11 min 13.2 7 1 12 5 7. 9 17.6 41.8

12 min 8.2 7 6 9 8 5. 0 21.7 47.5

14 min 8.9 7 6 4 4 4 6 23.2 51. 3 20 min 13.3 4 5 0 5 <0 4 27.1 54.2

Data for 2 nm/sec

Surface 7.0 76 0 7 2 9 8 30 sec 1.6 34 6 45 7 17. 9

1 min 1.2 18. 8 50 0 30 0

2 min 0.9 8 7 58 9 31. 5 4 min 0.6 15. 6 53 9 29. 9

6 min 1.8 10 7 46 7 40. 8

4. Conclusions

The XPS data presented here show that there is a significant amount of oxygen present on fresh surfaces which have been formed either by argon ion sputtering or by cleaving in an inert atmosphere. For

a sample that has been sputtered to a depth of >6 nanometers, oxygen has been found present at a concen- tration of '^12 atomic percent. For cleaved polycrystalline CaFj specimens the level of oxygen was measured to be '^5 atomic percent. On coated samples in which the coating was removed by sputtering the

level of oxygen in the CaF^ was measured to be >10 atomic percent. In the coating itself the oxygen was measured to be 2 + 1 atomic percent.

The conclusion that can be drawn from the XPS data on the profiled and cleaved surfaces is that

either (1) a fresh surface of CaF^, created either by sputtering in a very high vacuum or by cleaving in

dry nitrogen, is extremely reactive to very low concentrations of environmental components (such as water), or (2) oxygen is present in the bulk of CaF2 as an impurity.

5. References

[1] E. Hitter, Physics of Thin Films, 8, 1 (1975).

[2] K. Siegbahn, C. Nordling, A. Fahlman, R. Nordberg, K. Hamrin, J. Hedman, G. Johansson,

T. Bergmark, S. Karlson, I. Lindgren, and

B. Lindberg, "ESCA: Atomic, Molecular, and

Solid State Structure Studied by Means of

Electron Spectroscopy," Almquist and Wiksells

Boktrycher: A.B. Uppsala, Sweden, 1967.

[3] C. D. Wagner, Anal. Chem . , 49, 1282, (1977).

[4] Samples supplied by Major David F. O'Brien,

Air Force Materials Laboratory, Wright-Patterson Air Force Base, Ohio to be published

as an Air Force Materials Laboratory Technical Report (AFML-TM- )

.

[5] D. A. Walsh, J. A. Detrio, P. J. Larger, and

C. E. Hill, "Water Immersion Adhesion Evaluation of Sputter-Deposited ZnSe Films," Monthly Research and Development Status Report No.

9, AFKL, Contract No. F33615-76-C- 5092

,

September 1976, Wright-Patterson Air Force

[6] T. N. Wittberg, J. R. Hoenigman, W. E. Moddeman, C. R. Cothern and M. R. Gulett,

'AES and XPS of Silicon Nitride Films of

Varying Refractive Indices," accepted in

J. Vac . Sci . and Tech.

[7] H. Berthou and C. K. Jorgensen, Anal. Chem. , 47 (3), 482 (1975).

[8] H. Edelhock and H. S. Taylor, J. Phys. Chem. , 58, 344 (1954)

.

[9] J. Estel, N. Hoinkes, H. Kaarmann, H. Nahr,

and W. Wilsch, Surf. Sci., 54, 393 (1976).
10] C. A. Anderson, Int. J. Mass Spect. and

Ion Physics, 3_, 413 (1970).

11] G. T. Johnston, D. A. Walsh, R. J. Harris,

and J. A. Detrio, University of Dayton

Research Institute Semi-annual Progress

Report No. 4, Contract No. F33615-74-C-5001,

June 1975, Dayton, Ohio.

6. Figures

0. CoSURFACE SCAN

CALCIUM

BINDING ENERGY (eV )

b. CoF; EXPOSED TO HIGH HUMIDITY

OXY ''^'^ CALCIUM

FLUORINE

CARBON

1

5x10" n < TT-rri-rrrrr; , , , 1 , , , , 1 , , , , 1 , , , , 1 , , , , I ^, , 1 1 1 , 1 1 iVv4.

900 800 700 600 500 400

BINDING ENERGY (eVl

300 200 100 0

Figure 1. XPS Scans on (a) freshly polished CaF^, (b) exposed to high

humidity, and (c) argon ion-sputtered to a depth >6 nanometers.

The two observed fluoride peaks can be associated with the

F Is photopeak at ~590 eV and with the F Auger peak at ~600 eV. The three calcium peaks are due to Ca 2p doublet at ~345 eV and

Ca 2s at ~440 eV. The oxygen and carbon are Is photoelectrons

.

The anode used was magnesium.

-109-
c. Co Fa AFTER SPUTTERING~ 6nm

• BINDING ENERGY (eV)

Figure 2. In-Depth Profile Analysis of Electron Beam Deposited ZnSe

Films on CaF^. (2a) Film Deposited at 0.1 nm/sec. {2b) Film

Deposited at 2.0 nm/sec.

-110-
COMMENTS ON PAPER BV WITTBERG, HOENIGMAN, MOVVEMAN, ANV STRECKER

Thz diicaii^on tka> papeA cdnZiAdd oAound tht fielaJxve. rrnAit^ AixgeA iptctKoicopy voJiMii, ESCA.

The. ipzakoA pointed out that QKeout coAe hoi, to be exeAct^ed In uAlng AageA ipectAoicopy on dtixontdz

mateAtoLi became o{j the poaibiZitij ojj de^onbinq ^iuoAx^dei duAing the AageA meai>vAeynent. Thii dt^icutty -Ui not encounteAed w-ith ESCA. TuAtheAmone, the chemlcaZ i>hA.(^ti, can be n.ei,olved^ESCA pn.ovldeA

moKe Indonmation on the chemlcat binding o^ the Individual. eZementi than Ia avaJJiabte uilng AugeA

ipectAoicopy, which ovity Identi^leA a ipecle^.

-Ill-
INTEENAL-REFLECTION-SPECTROSCOPY STUDY OF WATER ADSORBED ON CaF2

J. W. Gibson, R. T. Holm and E, D. Palik

Naval Research Laboratory

Washington, D.C. 20375

The infrared internal-reflection spectrum of CaF2 trapezoids in air

indicate the presence of an adsorbed water film. The absorption near 3 Vm has been studied in a vacuum system in which the trapezoids were cleaned

by baking and then water vapor was allowed to the surface. Measurements of

the unpolarized absorptance and the s/p polarization ratio of the absorptance

indicate that the index of refraction of the film and the product of the

extinction coefficient and film thickness can be obtained. Analysis implies

that the optical constants are significantly less than those of bulk water.

The simplest conclusion is that the film is porous.

Key words: adsorbed water; CaF surfaces; infrared spectrum; internal reflection;

optical constants; water absorption.

1. Introduction

The infrared (IR) absorption due to adsorbed water on surfaces of various materials has been

extensively studied [1,2,3]-'-. The sample was typically a polycrystalline material such as CaF2

precipitated from an aqueous solution [4] , a porous film of NaCl formed by deposition on a cold

substrate [5], or a porous glass with large surface area [1,2], One-angle ellipsometry has been used

to determine the monolayer thickness of an adsorbed water film on alkali-halide surfaces under the

assumption that the index of refraction of the film is the same as for bulk water [6]. We have used

the phenomenon of attenuated total reflection (ATR) to determine the optical constants of the adsorbed

water film on CaF2. They are found to be significantly less than those of bulk water. Inferences are drawn as to the structure of the adsorbed film.

2. Experimental Techniques

Trapezoids illustrated in the inset of figure 1 were used in a double-pass mode in a double-beam

spectrometer for experiments in air. The base angles were = 48°, 62 = 56.5°, the mean length 2. was

5.0 cm, the width w was 0.18 cm, the index of refraction was 1.42 and the number of reflections m was

25 for one pass and 38 for double pass. Trapezoids in air invariably have water and hydrocarbons on the

surfaces which give rise to characteristic absorption lines as illustrated in figure 1. Prominent OH,

H2O and CH absorption frequencies are indicated by the arrows even though this particular trapezoid may

not clearly display all the bands. Such trapezoids were also used single-passed in an ultra-high

vacuum (UHV) system and cleaned by baking to remove water. Then pure water vapor was admitted into the

chamber to saturated vapor pressure and unpolarized and polarized absorption measurements were made.

The trapezoids were [111] oriented on the large surface but mechanically polished, so the damaged

surface probably consisted of unoriented crystallites.

The spectra were analysed with a multilayer model [7], for which we assumed a layer of liquid

water on the surface of CaF2 and bulk optical constants.

3. Results

3.1. Measurements in air

We measured a number of double-pass trapezoids in air 18] and always found the peak unpolarized

absorptance aear 3400 cm"-*- to be (6.5 + 0.4)%. This is measured from the dotted background level

shown in figure 1. The laboratory temperature was 21°C and the relative humidity was about 50%.

Assuming the optical constants of bulk water [9], we computed the equivalent water film thickness to be

4.0 S. In the analysis of HF-laser calorimetry experiments, Horrigan and Deutsch JlOJ estimated from

an absorptance per surface of 0.029% for CaF2 a film thickness of about 6 X.

We placed a trapezoid into the UHV chamber at '^-5 x

10''"'^ Torr initial pressure and measured singlepass spectra after baking at 200°C for 24 hr. The absorption due to water was removed to the degree

1. Figures in brackets indicate the literature references at the end of tMs paper.

-112-
of experimental sensitivity. When the surfaces were exposed to laboratory air, an s absorptance

Ag + ag = 5.6% was obtained which upon evacuation decreased to ag = 2.3%. This residual absorptance

could not be decreased further by pumping but was removable by baking. This indicates {A] a thinner

chemisorbed layer about 1.8 A thick with absorptance ag and a thicker physisorbed plus chemisorbed

layer about A. 6 8 thick with absorptance ag + Ag.

As it was time consuming to bake the sample to clean the surface, we chose to measure the

polarized absorptance of the physisorbed film, since water could be removed from and returned to the

surface easily. This was done by s- or p-polarizing the radiation with the chemisorbed layer present

and then allowing water vapor into the chamber to saturation. Since we measured absorptance from the

level of the chemisorbed layer, we were actually measuring A^,pi»Ag^p/(l - ag p) . If the chemisorbed

absorptance is small, then Ag piyAg p. We can make a better assumption that ag = ap, which we did not actually determine for this type of film. Lastly, we could assume ag/ap = Ag/A^. An analysis

indicates that the A^/A^ ratio as well as the values of Ag p are not sensitive functions of the

various above approximations. We measured Ag/A^ = 0.75. The calculated value of this ratio is

Ag/Ap = 1.6. This discrepancy is well beyond the experimental error and implies that the film is not

bulk water. It also throws suspicion on the film thickness calculated from the observed unpolarized

absorptance.

3.2. Measurements in pure water vapor

The bulk of our experiments were done by exposing a clean surface to pure water vapor. The

unpolarized absorptance at 3335 cm"-'- is shown in figure 2 as a function of vapor pressure. Near

saturation the absorptance also saturates at A + a = 16.6%. Upon pumping, the absorption reduces

to a = A. 3%, but cannot be reduced by further pumping at room temperature. Baking does remove the

absorption. The calculated chemisorbed film thickness is 3.4 8 and the total film thickness

o I, (chemisorbed plus physisorbed) is 14.7 A. The Ag/A^ polarization ratio for the physisorbed film was measured to be 1.37, which corrects to 1.35 if we assume ag/ap = Ag/Ap. We have ignored this

correction in view of the overall experimental uncertainties in the measurements of Ag/A^. The values

of unpolarized absorptance a and A are determined by measuring from the clean-surface level first as shown in figure 3. Then, values of Ag and A^ are measured from the absorption level of the chemisorbed

film with the radiation s polarized and allowing saturated water vapor into the chamber. Then the

vapor is pumped out so that the absorptance returns to its previous (chemisorbed) level; the. radiation

is p polarized and the water vapor is re-admitted. The calculated Ag/Ap = 1.6 is in much better

agreement with A^/Ap for the water film formed from pure water vapor than for the water film formed

from air.

We measured the unpolarized and p-polarlzed absorption of the chemisorbed film at 3335 cm""*- and

found ag/ap = 1.1 + 0.1, However, there is a large uncertainty due to poor signal-to-noise ratio

and the small magnitude of the absorptance.

We next measured the wavelength dependence of the Ag/A^ ratio with the results given in figure Ab

by the experimental data points. The solid curve is calculated assuming the bulk optical constants

shown in figure Aa. Note that the calculation is for a water film on a CaF2 surface while the

experiment is for a thick physisorbed water film on a thin chemisorbed water film on a CaF2 surface.

If the chemisorbed film has the same polarization properties as the physisorbed film, the presence of

the chemisorbed film does not affect the calculated results for Ag/Ap significantly.

The data of figure Ab are in qualitative agreement with the calculation with the s/p ratio being

both larger and smaller than unity over the region of anomalous dispersion. This gives a clue as to why the water film formed from air gave A^/A^ = 0.75 at 3335 cm"-'- and the water film formed from pure

water gave a|/A^ = 1,37. If the air film were "dirty" with dissolved hydrocarbons, oxygen, nitrogen,

fete, it is possible that this solute effect II] could shift the band enough to lower frequency, so that at 3335 cm"-*- the Ag/Ap ratio is now less than unity. In effect we slide along the frequency

scale of figure Ab.

In the thin-film approximation it is easy to see that the Ag/Ap ratio is a direct measure of the . index of refraction q^.of the film and is independent of the extinction coefficient kj and film

thickness d. It depends only on the indices of refraction of the film and substrate and the internal

angle of incidence 13]. The calculated Ag/Ap ratio approaches 1.35 far away from the absorption band

where absorption is small. We measured this ratio only in the spectral range 3000-3800 cm~l because

absorption was too small elsewhere or absorption due to water vapor interfered. We feel that the

-113-
discrepancy in figure 4b is well beyond experimental or analytical error.

We analyzed the data of figure 4b to obtain the index of refraction of the film as shown in

figure 4d. Comparison with figure 4a shows that the index of refraction is generally smaller for the

film than for bulk water. In figure 4c is shown the unpolarized absorptance. By smoothing the

data of figure 4d, we could use the values of n^ to determine the product k^d from, figure 4c which is

shown in figure 4e. Thus, the analysis yields rif and k^d.

We now applied a Lorentz-oscillator model to analyze nj and k£d. This model is not strictly

appropriate for water, since at least two vibration modes contribute to the absorption, and when these

bands are separated in liquid HDO, a Gaussian shape fits better [12]. The form of the dielectric

function is

e = (n-ik)2 1 +

n2

2 2 - to - lyo)

(1)

2 where Eoo = nco is the high frequency dielectric constant in the visible, oJq is the resonant frequency,

Y is the damping constant (the full width at half maximum of nk is 2y) , and fi^ an oscillatorstrength term. Analysis of the data of figure 4a yields = 1.32, to^^ - 3390 cm y = 200 cm"-'-, and

u'^ = 1.56 x 10~3 cm~2. For the adsorbed film, the analysis of the data of figure 3b-3e gives n„ = 1.21, = 3405 cm~l, y = 185 cm~l and fi^ = 7^57 ^ 10"^ cm"^. These analyses suggest that there is a significant decrease in background index of refraction, a slight increase in resonant frequency, a slight narrowing of the line width, and a substantial decrease in the oscillator strength. With these

optical constants the total water-film thickness nearly doubles compared to the value obtained

assuming bulk optical constants.

4. Discussion

There are numerous references in the literature [13] indicating that evaporated solid films can have anomalous optical constants compared to the starting bulk material. This can be porosity, size

effect, anisotropy, surface roughness, for example. However, it is not obvious that an adsorbed

water film is analogous.

If the film is porous, we can determine from the Clausius-Mossotti equation

nl - 1 47T — ? N. a, (2)

n2 + 2 3 i i i

that the number N of electronic oscillators/cm^ must be reduced to about 0.7 of the niamber in bulk

water to allow the background index of refraction to decrease from 1.32 to 1.21. Here, we assume that

the atomic polarizability remains the same for both kinds of material. This is not at all obvious

for a film of water only a few monolayers thick because of the changes in bonding at the nearby

interfaces. In fact, the concept of bulk dielectric constant probably is suspect. This magnitude of

packing density is not unusual for solid evaporated films deposited on heated substrates 114], but

again, we do not know if an adsorbed water film behaves in a similar manner. In a porous water film

the hydrogen bonding is reduced, thus allowing the resonant frequency to increase and the line width

to narrow. Also, the oscillator streng,th term fl^ varies as N, where N is the number of vibrational

oscillators [15], so a porous film would lead to a weaker vibration band. To reduce fl^ by a factor of two in equation 1 requires a reduction of N by a factor of two, which is comparable to, but smaller

than the value of N suggested by the decrease in n„.

From the large body of work on the IR properties of bulk water [11] we can make some comparisons

with the adsorbed water film. In general, a solute perturbs the water band at 3390 cm"-'-. Various

molar solutions of alkali halides and other materials [11] cause the band to shift to higher frequency

and narrow or to shift to loser frequency and widen. However, the band always weakens in int^ensity,

and the background index of refraction always increases slightly. Solute effects could then be

invoked to explain our results except for the observed decrease in background index of refraction.

Solutes are a possibility since the film is no doubt a complicated mixture of Ca and F ions,

hydrocarbons, and perhaps chemical compounds such as Ca(0H)2.

There is a one-angle ellipsometry measurement of water adsorbed to alkali-halides which yields the

o film thickness assuming bulk optical constants for water [6]. A monolayer thickness is 3.7 A. There

are non-optical studies (SIMS) of water on low-temperature alkali halides which suggest irregular

films with clusters of water on the surface [16], But it is not obvious if this type of structure

persists at room temperature at high vapor pressures.

-114-
5. Conclusions

Analysis of ATR data indicate that for an adsorbed film, the absorption band of water shifts to slightly higher frequency and narrows a little. The peak extinction coefficient is reduced about a factor of two and the background index of refraction decreases significantly to 1.21. While the film

is probably a complicated mixture of ions, impurities and chemical compounds, the simplest way to explain these features is to assume a porous film with about 0.7 the water molecules of bulk water.

6. Acknowledgment

We thank P. H. Klein for useful discussions regarding adsorbed water on CaF2. J. N. Harrick

provided information aobut internal-reflection-spectroscopy techniques.

7. References

[J] Little, L. H. , Infrared Spectra of Adsorbed

Species , (Academic Press, New York, 1966).

[2] Hair, M. L. , Infrared Spectroscopy in

Surface Chemistry , (Marcel Dekker, New York,

1967).

[3] Harrick, N. J., Internal Reflection

Spectroscopy , (Wiley, New York, 1967).

[A] Barracough, P. B. and Hall, P. C, J. Chem.

Soc. Faraday Trans. I 71,2266(1975)

.

[5] Smart, R. St. C. and Sheppard, N. , J. Chem.

Soc. Faraday Trans. II 72., 707 (1976) .

[6] Bayh, W. and Pflug, H. , Z. Angew. Physik

_25, 358(1968).

[7] Holm, R. T. and Palik, E. D. , Appl. Opt.

(in press)

.

[8] Palik, E. D. , Gibson, J. W. , R. T., Hass,

M. , Braunstein, M. and Garica, B., Appl.

Opt. (in press)

.

[9] Downing, H. D. and Williams, D. , J. Geophys.

Res. 80,1656(1975).

[10] Horrigan, F. A. and Deutsch, T. F.

,

"Research in Optical Materials and

Structures for High-Power Lasers", Final

Technical Report. Contract No. DAAH01-70-C1251 AEPA, Sept. 1971, p. 82.

[11] Rhine, P., Williams, D. , Hale, G. M, and

Querry, M. R, , J. Phys. Chem. 28,238,1405

(1974), and references therein.

[12] Eisenberg, D, and Kauzmann, W. , The

Structure and Properties of Water , (Oxford,

New York, 1969) , Chap. 5.

113] Roberts, E. F. I. and Ross, D. , Surf. Sci.

_56,425(1976)

.

[14] Ritter, E., "Dielectric Film Materials for

Optical Applications" in Physics of Thin

Films . Edited by G. Haas, M. H. Francombe

and K. W. Hoffman (Academic Press, New York,

1975) p.l.

[15] Burstein, E. , "Optical Properties of a Dielectric Medium", in Dynamical Processes

in Solid State Optics , edited by R. Kubo

and H. Kamimura (W. A. Benjamin, New York,

1967) p.l.

[16] Estel, J., Hoinkes, H. , Kaarmann, H. , Nahr,

H. and Wilsch, H., Surf. Sci 54 , 393 (1976)

.

-115-
8. Figures

WAVELENGTH (^m)

4 5

2000 WAVENUMBER (cm-l)

o o a. u) z 5

Figure 3. The sequence used to measure the

absorptance at 3335 cm"''" of a water

film formed on CaF2 from saturated

water vapor.

Internal-reflection spectrum of a double-passed CaFp trapezoid in air.

The inset defines the trapezoid

parameters.

TOTAL ABSORPTANCE (PERCENT)

0 4 8 12 16

1—I—1—I—I—I—I—I—I10'

103 £

a.

(0

10'

10°

cr.

^10°

oNX

10-2

-10-3

0 02 0.4 0.6 ABSORPTANCE /SURFACE (PERCENT)

Figure 2. The unpolarized total absorptance of

a clean surface of CaF2 exposed to water vapor. When the water vapor is

pumped out, the absorptance decreases

to A. 3% indicated by the arrow, rather than 0% implying a chemisorbed

layer tightly bound to the surface.

WAVEUNOTM ifim 25 30 33 4.0

4000 3000

WAVENUMBER {cm"')

Figure 4. a) The index of refraction n and

extinction coefficient k of pure

water and the index of refraction of

CaF2.

b) The calculated and observed

absorptance polarization ratio for a water film on CaF2 surrounded by

saturated water vapor.

c) The experimental unpolarized

absorptance obtained in two separate

runs.

d) The index of refraction extracted

from the A^/A^ ratio given in Figure Ab

e) The kfd product determined from the

unpolarized absorptance in Figure Ac.

-116-
COMMEMTS ON PAPER BV GIBSON, HOLM, ANV PALIK

It wa4 polntud oat that the. ^uuZti pnu^nttd tn thi6 papeA on adioibtd MciXeA layeM have. bee.n con-

{jlnmzd on otheA 6ub6tAatei by eJULLpiomeXny . The. ipzake/t pointed out that the. izeming inva/U.ancz od

the^z ne^uZt^ on dx^de/ient iubit/uvte mateAtaZi wai due to the. (^acX thaX the p^vindLpai abioiption

deXected wcw atXJUbuted to a hyd/iogen vibfiatlon, io that the. bonding of, the oxygen end 0(5 the MoteA

molecuZe to the iu/idace, which Mould be e.xpe.cite.d to be. dA^{^{^eAent {^on. dA.{^{^eAenX 6ub.6tAateA, could not

be deXected by tki{> technique.

-117-
PULSED HF/DF LASER DAMAGE IN WINDOW MATERIALS*

E.W. Van Stryland, M. Bass, M.J. Soileau and C.C. Tang

Center for Laser Studies, University of Southern California

University Park, Los Angeles, California 90007

Laser damage thresholds are reported for several alkali-halides , alkaline

earth fluorides, ZnSe, As^S^, sapphire, spinel and quartz at HE (2.7 ym) and

DF (3.8 vim) wavelengths. A low pressure, transversly excited, double discharge

laser and two different focal length lenses were used. A Gaussian spatial beam

distribution was obtained by spatially filtering out high order modes in the

far field of an unstable resonator cavity. Sapphire was found to have the

2 highest bulk damage threshold of the materials tested, 100 GW/cm at 2.7 pm

(peak intensity on axis) . The damage threshold was found to vary as the inverse

of the spot diameter which supports the model of Bettis, et. al. (NBS Spec. Publ.

452) The measurements also show that all of the materials evaluated in this

effort are surface damage limited when exposed to pulsed HF or DF laser radiation.

Evidence concerning the roles of material manufacture, surface finishing and laser

irradiation conditioning in the damage process are presented.

Key words: Alkali-halides, alkaline earth fluorides, As^S^, DF , electric breakdown, HF, laser damage, quartz, sapphire, spinel, ZnSe.

1. Introduction

Measurements were made of the pulsed laser damage resistance of several candidate window materials

at HF and DF laser wavelengths. Of the materials tested sapphire and the alkaline earth fluorides have

the highest resistance to laser damage. A major part of the experimentation was devoted to modification and characterization of the HF/DF chemical laser. The damage irradiations were performed with to- tal powers less than 1/10 of the critical powers for self-focusing by tightly focusing the Gaussian la- ser beam. At 2.7 pm, the HF laser wavelength, a single triangular pulse waveform was used, however, at

3.8 ym, the DF wavelength, a double pulse was obtained because the laser oscillated on several lines

simultaneously. The occurance of surface damage set the practical use limit for the materials that

were studied and in general there was a considerable effect of laser preconditioning (N on 1 effect)

for the surface damage threshold. (1)

2. Experiment

In the damage threshold measurements a transversly excited HF (or DF) pulsed chemical laser was used as the irradiation source. The output wavelength of the HF laser was multiline and centered about

2.7 ym; the output is considerably more dispersed for DF and is centered about 3.8 ym. The only change

made in the system to go from HF to DF lasing was to switch from H^ to D^ gas. The power supply, laser

cavity and all other optics remained the same. The relevant laser parameters and cavity design employed are shown in figure 1

.

The laser was manufactured by Liamonics (2) and has been described previously, however, the cavity

has been modified as shown in figure 1. The unstable resonator employed a 100% reflector with focal

radius of 14 m and an NaCl output coupler of focal radius .8 m, The NaCl lens was uncoated and the 4%

reflection from the surface was the only feedback into the cavity. A series of Fresnel rings were observed in the near field of the laser output. These were sensitive to adjustment of either reflector

as well as to the size and position of the intracavity aperture. The output traversed the path shown

in figure 2. It propagated to a focus 5 . 76 m "downstream" where a 1 . 7 mm diameter spatial filter was positioned to block any off axis output modes. This spatially filtered output was then attenuated by

two pair of Brewster angle ZnSe slabs; the first pair of which was rotatable to vary the attenuation,

and the second was used to assure that the polarization in the beam reaching the target remained constant. After these, bulk attenuators such as quartz, Ge or As^Se^ were inserted as necessary. The energy in each pulse was monitored after the attenuators by a calibrated pyroelectric energy meter. At a total distance of 9.0 m from the laser output coupler lens the far field spatial distribution was the

Gaussian desired for meaningful damage experiments. At this point the output was focused by a ZnSe meniscus lens (2 different focal length lenses were used) on or inside the samples. A beam scan of the output of the HF laser at the position of the damage lens with a 0 . 4 mm aperture

gave the data points in figure 3. Each point represents the average of 10 laser shots. The drawn in

2 curve shows a fit of the data to a Gaussian. The 1/e full width of the Gaussian fit curve is 22.0mm.

*This work was supported by D-ARPA under contract No. F 19628-77-C-0094

.

^Current address: Aerospace Corporation, El Segundo, California.

''"Figures in brackets indicate literature references at the end of this paper.
A pinhole scan of the laser output was also performed in the focal planes of ZnSe lenses of focal

lengths 38.0 and 127 mm. A scan of the focal plane of the 38.0 mm focal length ZnSe lens is shown in

figure 4 using a 9. 1 um diameter pinhole. This pinhole was made in thin Al foil by the focused laser

output, and its diameter measured in an optical microscope. Each data point represents a single laser

shot measured with the pyroelectric energy monitor. The pinhole was moved by a differential micrometer

in 12.7 ym intervals. The data has been folded with respect to the maximum. Since the pinhole size was not small compared to the beam size, corrections were made to account for the finite size aperture.

(3) It was found by numerical integration (of a circular aperture with a Gaussian weighting function)

that the beam width appeared 5% larger using a 9.1 um diameter pinhole than if the scan had been performed with a much smaller aperture (e.g.: 1 lan) . The Gaussian, uncorrected for a finite size aperture is

2 shown as points in figure 4. Even using an aperture as large as one third the 1/e width causes very

little distortion of the observed beam profile.

The temporal pulse waveform was monitored at two points, one before the spatial filter and one after

transmission through the sample as shown in figure 2. The HF laser produced a triangular pulse with

some irregular spiking which has a full width at half maximum (FWHM) of 175 nsec as shown in figure 5. The HF waveforms were monitored using fast pyroelectric detectors with risetimes of ~1 nsec (Ge photon

drag detectors cannot be used at 2.7 ym) . The second pulse in the HF waveform is due to ringing in the

pyroelectric detector. The DF waveforms were monitored using Ge photon drag detectors and consisted of

the double pulse also shown in figure 5. Damage occured on the second part of the pulse as observed by

monitoring the waveform distortion upon transmission through the sample. An equivalent pulse width for

the DF pulses of 176 nsec was obtained by finding the total normalized pulse area (nsec) and multiplying

by the percentage of the area under the second part of the pulse. This is mathematically equivalent to the FWHM obtained for a triangular pulse of the same area. An example of the transmitted pulse waveforms when damage occurred is shown in figure 5 for both HF and DF pulses.

The pulse waveforms for both HF and DF operation were critically dependent on gas mix, pressure,

discharge voltage and laser repetition rate. The operating parameters listed in figure 1 were chosen

because they resulted in the best HF waveform and this could be reproducibly obtained from day to day. A single DF pulse could not be obtained by simply varying these parameters and, as demonstrated previously, the double pulse DF output is attributed to the multiline nature of the laser oscillation.

3 . Calculation

The definition of a damage threshold level of irradiation used in this paper is that flux which

produces damage at 50% of the irradiated sites. An example of data is shown in figure 6. The plus or minus values are a measure of the overlap that occurred in all of the samples tested. The 1 on 1 threshold corresponds to one irradiation per site. The n on 1 threshold is where a given site is irradiated

by a pulse or pulses with insufficient energy (or intensity) to damage prior to the pulse that produces

damage. From the measured energy which caused damage the intensity thresholds were determined in the

following way. The energy E in terms of the energy density e{r) at the focal plane is given by

=

j;

For the Gaussian beam used in these experiments

E = \ e(r)2iTrdr. (1)

0

, ^ -2(r/a) )^

e(r) = e e 0 (2)

0 2 where e is the peak on axis energy density and w is the 1/e half v;idth point in intensity. Thus,

0 "

, E = a)/eoTT/2 (3) giving the peak on axis energy density e as

0 2 e = 2E/TM (4)

0 2

0 This equation gives the damage threshold in J/cm . To obtain the intensity damage threshold I in watts/

2 cm , e is simply divided by the normalized area, T, of the temporal waveforms; 175 nsec for the HF

0 pulse, and 175 nsec for the DF as discussed previously. Thus

2

I = 2E/(ttio T) (5)

0 (Again, this is equivalent to performing the temporal integral for a triangular pulse waveform). The

transmission of the ZnSe lenses at both 2.7 and 3.8 um was measured to be 0.80. The energy E in the

previous equation was corrected for this lens transmission in the calculation. In addition for bulk or exit surface damage thresholds the energy was corrected for the front surface reflection, (i.e.:

2 4n/(l + n) of the incident energy is transmitted through an interface where n is the index of refraction of the sample at the appropriate wavelength) . In all the samples tested, with the possible excepttion of quartz at 2.7 um, absorption v;as entirely negligible.

4 . Data

Table 1 gives the damage thresholds at 2.7 \im (HF) as measured with a 38 mm focal length ZnSe lens

2 (measured 1/e full width spot size of 27 \im) . Also presented is the increase in threshold when a single site was preconditioned by pulses of insufficient energy (energy or intensity) to produce damage

2 (labeled N on 1 increase) . The number quoted multiplied by the 1 on 1 damage threshold in J/cm or

-119-
GW/cm gives the n on 1 damage threshold. The final two columns give our assessment of whether the bulk

damage was due to macroscopic inclusions or was intrinsic. This information was deduced from microscope

examination of the damage morphology. For example; if damage occurred at different positions along the

focal beam path the damage was deemed inclusion induced. If on the other hand as the sample was traversed perpendicular to the beam all the damage sites appeared similar and lay in a single plane, we deemed the damage to be intrinsic.

Table 1. Bulk thresholds with 27 Pm spot size (A = 2.7 m)

Specimen , 2 J/cm 2 GW/cm N on 1 Increase Inclusions Intrinsic

NaCl (Harshaw) 3.6 X

3 10 21 no X KCl (Harshaw) 2.9 X

3 10 16 no X KBr (Naval Research Lab) 1.2 X

3 10 6.6 X 1. 12 X HgF^ (Optovac) 13 X 10^ 75 no X BaF^ (Optovac) 13 X

3 10 78 no X SrF^ (Optovac) i J X xu 76 no X CaF^ (i Raytheon) L1 ^-> X 10^ 88 no X

car 2 riarsnawj 14 X 10^ PIo DR no YA

CaF^ (3 Harshaw) 0.47 X 10^ 2.8 X 1.7 X

Sapphire (Optovac) 17 X 10^ 103 no X

Spinel (Union Carbide) 12 X 10^ 68 no X

Quartz (1 NWC) 8.7 X 10^ 52 no X

Quartz (2 General Electric) 1.4 X 10^ 8.5 X 3.0 X

ZnSe (Raytheon) 0.46 X 10^ 2.7 X 1.3 X As^S^ (Servo) 0.29 X 10^ 1.7 no X

MgF^ (pressed, NWC) 0.05 X 10^ 0.29 X 1.7 X

*l/e full width intensity

2 Table 2 gives bulk damage thresholds in the same format at 2.7 um for a 1/e full width spot size

of 59 ym (the focal spot size of a 127 mm lens) . The greater than signs ( > ) in front of some of the

damage thresholds indicates that there was insufficient laser energy (or intensity) to damage the sample

and thus the numbers represent lower limits. No N on 1 data could be obtained for these samples.

Table 2. Bulk thresholds with 59 ym spot size* (X = 2.7 um)

2 2 Specimen J/cm GW/cm N on 1 Increase Inclusions Intrinsic

NaCl 1.7 X 10 10 no X KCl 0.84 X lO'^ 5.0 X 1.1 X

KBr 0.21 X 10 1.2 x 2.9 X

MgF^ >5.8 X 10'^ >34 X

BaF 6.1 X 10^ 36 no X

^

3 SrF 6.4 X 10 38 no X

^

3 CaF (1) >5.8 X 10 >34 X

^

3 CaP^ (2) 2.9 X 10 17 no X

Sapphire >6.0 x 10^ >35 X Quartz (1) 3.3 x 10^ 19 x 1.1 X

*l/e full width intensity

2 Table 3 shows surface damage thresholds again using a 59 ym 1/e full width focal spot size at the

2.7 ym wavelength. Surface damage was labeled inclusion induced since near threshold more than one damage site was observed within the focal area. The anomalously high threshold quoted for KBr (as com- pared to the bulk) may have been due to improper focusing.

-120-
Table 3. Surface threshold 59 pm spot size* (X =2.7 ym)

Specimen J/cm GW/cm N on 1 Increase Inclusions Intrinsic

NaCl 1 3 X 10^ 7 4 X 1 6 X KCl 0 27 X lO'^ 1 6 X 2 7 X KBr 0 44 X 10^ 2 6 X 1 5 X

MgF^

BaF,

SrF,

CaF^ (1) CaF^ (2)

1.3 X 10

0.36 X 10"

0.43 X 10"

2.6 X 10^

1.2 X 10"

7.4

2.1

2.5

15.2

7.0

X 1.6

X 1.0

X 1.2

X 1.2

X 1.3

X

X

X

X

X

Sapphire

Quartz (1)

0.65 X 10

'''l.O X 10^

3.8

^6.1

X 1.2 X

X

*l/e^ full width

^exit surface data

Table 4 shows damage thresholds obtained at 3.8 um (the DF laser wavelength) at the focus of the

2 38 mm focal length ZnSe meniscus lens. The 1/e full width focal spot size of 38 ym was calculated

from the measured spot size at the 2.7 ym wavelength and scaled according to wavelength. At the right

2 of the chart the damage threshold in GW/cm at the HF laser wavelength is compared to the damage

threshold at the DF wavelength after multiplying by the ratio of the spot sizes, 38 ym/27 ym. The

theory of Bettis, et. al. (4) predicts that the damage thresholds should be inversely proportional to the spot diameter. The HF and DF damage thresholds are nearly the same when scaled according to this

rule

.

Table 4. Bulk thresholds at 3.8 ym

38 |im Spot*

Specimen J/cm GW/cm

N on 1 Increase

Intensity

2 (GW/cm ) Scaled to

a 27 ym

Spot Size

HF Damage

Threshold

(GW/cm ) For a 27 ym

Spot Size

MgF^

(single crystal)

MgF^

(pressed)

Sapphire

Spinel

7.4 X 10"

0.12 X 10

13 X 10

6.9 X 10"

43

0.7

72

39

X 1.7

no

no

60

1.0

102

55

75

0.3

103

68

*l/e full width intensity

In table 5 we present a bar diagram showing the electric field at which breakdown occurred in the

bulk material at various wavelengths including DC. This RMS field was calculated from the intensity

damage thresholds, I , and the indices of refraction n, substituted in the equation

Field,^ ,^ , (Volt/cm) = 19.41 /l (Watts/cm^) /n (6) (breakdown)

The 2.7 ym data is from this work. Data at other wavelengths is from the referenced literature. No

attempt has been made to reduce the data to account for spot size or pulse width.

-121-
Table 5. Breakdown fields vs wavelengths

NaCl

KCl

KBr

2,7 fn

,69 /"M

Jl£_

2.7

69 /"M

DC 1 ) 1—

.6 /"M

06 /-M .m^H 1 i 1

The dotted lines indicate the experimentally determined breakdown field

while solid lines indicate estimated

errors (except for DC fields)

.

References 5,5,7,8 and 9.

Sapphire

Ouartz

2,7 ftn 1.06

2.7mh

1. 06 /.M

Field* (MV/cm ) *Peak on axis RMS field

2 The .69 ym Ruby laser data (7,9) used a focal spot diameter d/e" full width in intensity) of approximately 12 pm. The pulse durations were 14 nsec FWHM. At 1.06 \im, the damage threshold appeared

spot diameter independent using a spot diameter of from 16 pm to approximately 50 ym. The pulses were

about 10 nsec in duration (FWHM) . The 2.7 ym and 3.8 ym laser parameters are listed in this paper. The

10.5 ym damage data was taken using a CO^ laser focal spot diameter of ~100 ym and a pulse length of

^100 nsec. (7,8)

The estimated errors on damage threshold intensities quoted in this paper are~30%, which implies

an uncertainty of ~14% in the fields.

5. Conclusions

The damage data survey presented here shows that sapphire and the alkaline earth fluorides have

the highest damage thresholds at both HF and DF laser wavelengths. This is a fortunate result since

both are rugged materials. In particular sapphire is extremely strong and hard.

Bettis, et. al. (4) determined that the damage intensity threshold is proportional to the inverse

of the spot diameter. Although their model was developed for surface damage, they attempted to show

that their model also applied to bulk damage. From this we expect that the ratio of the damage intensities for the 27 ym and 59 ym spot diameters should be 2.2. The average value of this ratio determined

by this experiment is 2.5 which is in good agreement.

Using this spot diameter scaling and extending it to the 3.8 ym wavelength our DF damage thre-^ sholds are nearly the same as the HF thresholds (see table 4) . Damage thresholds for some of the mater2 als tested were obtained previously (2) at 3.8 ym using a 130 ym spot diameter (1/e full width). Scaling these values with spot diameter according to reference 4 also gives excellent agreement with the

2.8 xm data presented here. Thus it appears that the damage thresholds are insensitive to wavelength

between 2.7 >«m and 3.8 ^m. We should note, however, that due to the time structure of the DF laser

pulse (see figure 5) this data is less reliable.

As has been noted at other wavelengths optical materials are limited by surface damaae thresholds.

Our aata of taole 3 confirms this at 2.7,«m. The large laser conditioning effect (n on 1), the low

thresholds relative to the bulk values, and microscopic examination show that surface damage is defect

limited. Here again the fact that sapphire and the alkaline earth fluorides have high thresholds is

helpful since we expect that with careful surface preparation the damage thresholds of these hard materials can be substantially increased. (10) It is interesting to note that to within experimental error the raw data of table 5 shows a damage

field independent of wavelength.

-122-
6. References

(1) M. Bass and K.M. Leung, J. Quantum Electr.

QE-12 , 82 (1976).

(2) M. Bass, K.M. Leung and C. Tang, "Pulsed La- ser Induced Damage at the DF Laser Wavelength", NBS Special Publication 462 (1976).

(3) E.W. Van Stryland, J. A. Gunderson, G. Benedict, to be published.

(4) J.R. Bettis, R.A. House II, A.H. Guenther,

"Spot Size and Pulse Duration Dependence of

Laser Induced Damage", NBS Special Publication 462 (1976)

.

(5) A. Von Hippel, J. Appl. Phys . £,815 (1973).

(6) D.W. Fradin, E. Yablonobitch and M. Bass,

Appl. Optics, 12^, 700, April, 197 3.

(7) D.W. Fradin, Laser Focus, February, 1974.

(8) S.D. Allen, M. Braunstein, C. Guiliano and V. Wang, p. 65, NBS Special Publication 414.

(9) D.W. Fradin and M. Bass, p. 225, NBS Special

Publication 387.

(10) M.J. Soileau, A. Bennett, J. Porteus, P. Temple, "Laser Damage to HEL Window Materials as Related to Surface Damage" , Fifth Annual Conference on Infrared Laser Window Materials,

Las Vegas, 1975, page 351.

7. Figures

IIF/DF LASER CAVITY

Output Coupler (R » 0.8 m)

Cavity Aperture (27 mm)

H- .53 n

User Parameters:

2.8m

Pressure Discmarge Voltage

Gas nix: Hi/Dt

He Wavelength: HF

DF

0

Rear Reflector (R = M m)

60 torr

72 KV

13 liters/min*

2.2 liters/min*

2.6 liters/min* 2.7^M

3.8^M

•(STP)

Figure 1. HF/DF Laser Cavity.

EXPERIMENTAL ARRAHGETIENT

Fast Pyroelectric Detector

Spatial Filter

.7 m oiAM.)

HF/DF Laser

ZbSe Attenuators

<^ \ Enercy \ \Monitor

-tZhSe, flEHISCUS Lers 0"-x \^ Fast Pyroelectric Detector

nin K ATTCNiiATnRc Sample^ Recoixihatihg Lens H— 3.96 n

t |<- 5.76 M T-

-i.i9 M \r Bulk Attenuators

3.21 B

IIeKe Laser

Figure 2 . Experimental Arrangement

.

-123-
HF LASER SPATIAL PROFILE

(Prior to ZnSe Lens)

5

-12 -10 -8-6-4-2 0 2 1 6 8 10 12

R(mm)

Figure 3. HF Laser Spatial Profile (Prior to ZnSe Lens).

PINHOLE SCAN OF FOCAL PLANE

Figure 4. Pinhole Scan of Focal Plane.

-124-
PULSE WAVEFORMS

Figure 5. Pulse Waveforms.

THRESHOLD DEFINITION

Damaged

-) 4ttt -I- g o 00 1

Did Not Damage

, I -^t^ Q oc9j) , 1

(Relative Units)

\- 1-ON-l Threshold 2.0 + 0.2

0 N-ON-1 Threshold 3.0 ± 0.1

Figure 6. Threshold Definition.

-125-
COMMENTS ON PAPER BV fAN STRVLANV, BASS, SOILEAU, AW TANG

It Mcu. po-inti.d oat that the. ^-^eqaenci/ dtpzndnncz thz damage thAuhold, zvzn whm aUL otheA dactou

oAe heZd constant, aJi not expected to obe.y any iimpte. icating h.eXati.on. The. >iol<i Ojj abi>on.hi.ng -mpuJiitlti> and competUZive. pKocuiu at dLiUzAZYVt MaveZmgih!, tzndA to obicuAe. the. iimpte. iicaLing fieljition

pfLzdidtad by tht veAy nalvz the,o^y avalanche. lonA.zatA.on. The ipzaket indicated that Mhm he menZtoned

puAe matexiaJi he meant the beit RAP-gKom mateniaJL available, Mkich woi a6 d^ee. ^-^om vLiible icatteAing

<xi tt MOi poMihte to obtain.

-126-
OPTICAL ABSORPTION IN UV LASER WINDOW MATERIALS

James A. Harrington, Bradley L. Bobbs, and Morris Braunstein*

Hughes Research Laboratories

Malibu, California 90265

and

Rubin Braunstein, R.Y. Kim, and R. Stearns

Department of Physics

University of California

Los Angeles, California 90024

The requirements for low-loss optical components for high-power excimer lasers has

stimulated the investigation of optical absorption in a variety of highly transparent

materials at visible and uv wavelengths. The absorption coefficient S has been

measured, using laser calorimetric techniques, for CaF2, SrF2, BaF2, LaF3, CeFi, MgF2,

Si02, MgO, AI2O3, NaF, LiF, NaCl, and KCl at 3511, 3638, 4579, 4880, and 5145 A. The

absorption was found to decrease with increasing wavelength to a low value of

4 X 10~5 cm~l for SrF2 at 5145 A. In addition, wavelength modulation spectroscopy was used to obtain absorption coefficients for some of the samples. In this technique, the

energy derivatives of 6 are integrated, using the calorimetrically determined values of

6 as constants of integration to fix 6 at the laser wavelengths, to obtain absorption

coefficients continuously from 2500 to 5000 A. Key words: Extrinsic uv absorption; laser calorimetry; laser windows; Urbach tail; uv laser components; wavelength modulation spectroscopy.

1. Introduction

There has recently been considerable interest in low-loss optical components for uv excimer lasers.

To provide reliable coating and window materials that have high damage thresholds at uv laser wavelengths

requires making a systematic study of the optical absorption in a wide variety of transparent materials.

The materials with large band gaps that have been investigated include alkali halides, alkaline earth

fluorides, oxides, and some rare earth fluorides. The small, residual absorption coefficients (6's) were measured at five visible and uv wavelengths using Ar ion laser calorimetry; they were also measured con- tinuously from approximately 400 to 250 nm using wavelength modulation spectroscopy. A comparison of our data with published data (Urbach tail region) indicates that, although the spectral regions of the Urbach

tail data and our data do not overlap, our absorption data is significantly lower than a simple extrapolation of the "extrinsic" Urbach tail. These results suggest that the materials studied are considerably

more transparent around 350 nm than originally expected from published data.

2. Experimental Procedure and Techniques

The small optical absorption coefficients were measured in a variety of single- and polycrystalline

samples using laser calorimetric techniques [1]. A Coherent Radiation Laboratories model CR-12 Ar ion

laser equipped with visible or uv optics was used in conjunction with a simple air calorimeter to obtain

absorption results at 514.5, 488.0, 457.9, 363.8, and 351.1 nm. The schematic of the uv calorimeter

shown in figure 1 also contains the quartz half-wave plate utilized to maximize the power in each uv

line. The procedure yielded about 1 W at each uv wavelength and between 5 and 6 W for the strong bluegreen lines. Wavelength modulation spectroscopy was performed on some of the samples that had been

measured calorimetrically. The basic principles of this technique are discussed elsewhere [2^. Certain

refinements in the original apparatus have been made to provide the greater sensitivity required to measure these extremely small absorption coefficients [3].

3. Experimental Results

3.1 Calorimetric Measurements

Eleven different materials were measured at visible and uv wavelengths. In some cases, more than

one source or different forms of the same material were studied. Figures 2 and 3 display all the

calorimetric data for the various samples studied. In each case, absorption Increases with increasing

energy. The lowest absorbing crystals are the alkaline earth fluorides, KCl, NaCl, and Si02 (one

sample), where 3's in the low 10"'^ cm~l region were measured at visible wavelengths. The single crystal

Research supported in part by Hughes Independent Research and Development Projects.

1. Figures in brackets indicate the literature references at the end of this paper.

-127-
I KCl grown in the Hughes Laboratories, for example, had absorption coefficients that were unmeasurable in

the blue-green region (cf . fig. 3) but were estimated, based on the sensitivity of the calorimeter, to be no greater than 2 x 10~5 cm"-'-. The highest absorbing materials tended to be certain oxides, NaF,

and LIF, The NaF and LiF samples (obtained from Optovac, Inc., and Adolf Meller Co.) had surprisingly

high absorptions, especially considering the large band gap in these materials. The most probable

explanation is that these materials are not as pure as the alkaline earth fluorides and other alkali

halides, which have been more highly purified because of the emphasis on these hosts for applications as infrared laser windows. In no case, however, was it found that the absorption increased with increasing

exposure to the uv laser radiation, as might occur if color centers were being continuously produced.

3.2 Wavelength Modulation Spectroscopy

Absorption coefficients for some of the sample samples measured calorimetrically were obtained by

wavelength modulation spectroscopy from approximately 450 to 280 nm. The results of this work for

several materials are shown in figures 4 and 5. The raw data is returned as a derivative of the absorptance with respect to wavelength. These spectra are then integrated to yield the relative absorption

coefficients, shown as the upper curves in figures 4 and 5. An absolute calibration of these curves is

then obtained by comparing these to the calorimetric data of figures 2 and 3 (reproduced as the dashedbottom curves in figs. 4 and 5). In general, there is excellent agreement between the calorimetric and

X-modulation data for each host material studied.

The increasing absorption at uv wavelengths is clearly evident in figure 4 for LaF3, AI2O3, and Si02.

The high sensitivity of A-modulation methods in comparison to other forms of spectroscopy may also be

seen by noting the spectra of some older LaFj material measured on a conventional (Gary 14) spectrometer. Contrasting the conventional spectroscopic data and the calorimetric/X-modulation data, one can

see that some structure is visible in the integrated A-modulation spectrum that is impossible to observe

in the Cary 14 data because the Cary 14 is less sensitive than derivative spectroscopy. For AI2O3, one

observes another complementary feature of X-modulation spectroscopy. The peak in the absorption near

380 nm is missed by calorimetry because of the lack of laser lines at this energy. By comparison, Si02

data in figure 4 merely indicates a monotomic increase in absorption with increasing energy. This is

in agreement with the calorimetric results (i.e., no sharp absorption bands appear in the spectra).

CaF2 data (cf. fig. 5) exhibits a peak in the absorption near 370 nm. This is not entirely evident

from calorimetry alone, but derivative spectroscopy clearly delineates this band. A likely explanation

for this absorption is that it is due to F-centers. The absorption in SrF2, even though it is at an extremely low level, indicates some structure on an otherwise monotomically increasing absorption with

increasing energy.

3.3 Comparison with Band Edge Data

The alkaline earth fluorides were singled out for comparison with data in the Urbach tail region

taken from existing literature. Figure 6 includes the band edge data of Tomiki and Miyata [4] and our X-modulation/calorimetric data. Unfortunately, the two spectral regions do not overlap; however, the

data from this study is significantly lower than any extrapolation of the "extrinsic" Urbach tail data

would imply. Presumably, the purity of the samples has increased and, with more sensitive techniques

being used, this has led to the "extrinsic" absorption observed by us being less than that previously

observed. Experiments are underway to extend our measurements to 11 eV so that an accurate comparison

can be made. The attenuation due to scattering, calculated theoretically [s], is negligible when compared to the other absorption losses.

4. Conclusions

Our absorption studies of highly transparent solids for use as low-loss components on uv excimer

lasers has led us to conclude that the lowest absorbing materials near 350 nm are the alkaline earth

fluorides, NaCl, KCl, and Si02. More highly absorbing materials are AI2O3, MgO, alkali fluorides, and

rare earth fluorides. One reason for this is that the low absorbing uv materials are also those that

have been extensively studied and purified for low-loss at infrared laser wavelengths. A comparison

of our experimental data with "extrinsic" Urbach tail region data indicates less extrinsic absorption

than previously expected.

References

ri] Hass, M., Davisson, J.W., Klein, P.H., and [4] Tomiki, T. and Miyata, T. , J. Phys. Soc. Japan

Boyer, L.L. , J. Appl. Phys. 4^, 3959 (1974). 27, 658 (1969).

[2'| Welkowsky, M. and Braunstein, R. , Rev. Sci. pcn tu 1. r. j n • a a i ni. t Tr,^t- /x \qq nQ79^ L^J ^^^^» Pinnow, D.A., Appl. Phys. Lett.

Inst. 4J, J99 (19/2).

20, 264 (1972).

[3] To be published.

Figures

-128-
VERTICALLY

POLARIZED

HORIZONTALLY

POLARIZED

CRL Ar ION LASER

WITH UV OPTICS

QUARTZ HALF-WAVE PLATE

FOR 351.1 nm OR 363.8 nm

WITH OPTIC AXIS AT 45°

BARRIER TO BLOCK

ONE OF TWO BEAMS

THERMOCOUPLE LEADS

KEITHLEY

MICROVOLTMETER

HP

CHART

RECORDER

FUSED

SILICA

PRISM

MASSIVE Ai

BLOCK FOR

TEMPERATURE

REFERENCE -==

AIR

CALORIMETER

HEAD

SAMPLE

POWER

METER

HEAD

Figure 1. UV Calorimeter.

WAVELENGTH, /xm

500 450 400 375 350

10' — • FORGED. Rb ~ DOPED~

10 4 _

A - LESS THAN]

10-" IN VIS J

LSIMGLE CRYSTAL

E

uZ

UJO 10'

uiO

u

I

10-5^

o.oCO

00<

= NaCi

10-6 t ^

• SINGLE CRYSTAL B 182 ASINGLE CRYSTAL B-175

J L J L

2.4 2.6 2.8 3.0 3.2 3.4 3.6

ENERGY, eV

WAVELENGTH,

500 450 400 375 350

10' P r

10'3 ^

u 10'

o

o

9. 10 3 Jt Io.

DCOCO<

10'3 ^

10'

1 r

NaF

LiF

/ _

LaF,

> J

J I I \ L

2.4 2.6 2.8 3.0 3.2 3.4 3.6

ENERGY, eV

Figure 2. Calorlmetric measurements of uv-visible absorption in alkali halides and iLaF3.

-129-
u

O

o

z

o

a.ocOM

OQ<

101-3

WAVELENGTH, nm

500 450 400 375 350

=i^_,i I— "r

- CaF,

10'4 fenf10'^ = ^''^2

10'

10-5

10'

10'

^i/**^'" A FORGED

'y • SINGLE CR

-I- SINGLE CRCRYSTAL -=

CRYSTAL BAR =

SINGLE CRYSTAL FORGED

-4 -^ /

t

J \ \ \ \ L

2.4 2.6 2.8 3.0 3.2 3.4 3.6 3.8

ENERGY, eV

E

u

UJO

CO

CO<

WAVELENGTH, nm

500 450 400 375 350

10-

10'

10

— SiO,

u. 1 LUO

u

^ 10"3 l^* " o

lo-i =

10-2

• X CUT A MELLER UV GRADE

FUSED

0-4 tt

- Aj^oO 2^3

SINGLE CRYSTAL • MELLER UV GRADE "=

. (—

MgO / ~

±

2.4 2.6 2.8 3.0 3.2 3.4 3.6 3.8

ENERGY, eV

Figure 3. Calorimetric measurements of uv-visible absorption in alkaline earth fluorides and

various oxides.

INTEGRAL OF i -MODULATION

t -MODULATION

CALORIMETRV

<0='. CnvSTAL GROWTH FROM CRVSTAi CROMN

99 »7X PURE STARTING / FROM MATERIAL

MATERIAL / FURTHER PURIF lED BV ION EKCHANCE

0* SE

Z O

0«

A£03

INTEGRAL OF t - MODULATION

- MODULATION

CALORIMETRV

'integral of I' -MODULATION

-MODULATION

UV-auANTZ-SOj~MELL£R NO 1 f

i lOOtcm / // / / /

^ "^CALORIMETRV

6000 *M» MOO

WAVELENGTH.^

000 JGOO 3000

WAVELENGTH.

&

Figure k. Absorption in LaF^, AI2O2, and SiOj.

-130-
5000 4500 4000 3500 3000 2500 5000 4500 4000 3500 3000 2500

WAVELENGTH, A WAVELENGTH, A

Figure 5. Absorption in CaF„ and SrF_.

X.nm

100 110 120 130 140 150 160 180 200 220 250 300 350 450 550

12 11 10 9 8 7 6 5 4 3 2

PHOTON ENERGY. oV

Figure 6. UV and visible absorption in SrF2 and CaF

-131-
UV REFLECTANCE, TRANSMISSION, AND PHOTOLUMINESCENCE OF LIYF4

,

AND THE BULK LOSS COEFFICIENT IN CaF,*

Victor Rehn, David L. Burdick, and Vernon 0. Jones

Physics Division, Michelson Laboratories

Naval Weapons Center, China Lake, California 93555

Measurements have been made of the bulk loss coefficient in two CaF2 samples.

The measurements were made between 200 and 400 nm using the prism technique. The

results show a strong peak at 305 nm against a background loss of 10"^ to lO'^cm"-'-. The peak is tentatively assigned to Ce^"*". In addition, VUV measurements of the

transmission, reflectance, and photoluminescence of LlYFi, have been performed for

the first time. The fundamental absorption edge is at 10.42 eV. The transmission

and photoluminescence spectra show structure at 7.1, 8.0 and 8.6 eV.

Key words: Bulk loss coefficient; CaF2 ; Ce^"*"; LiYF^; photoluminescence; prism

technique; reflectance; transmission.

Materials which transmit vacuum ultraviolet (VUV) radiation without coloration or degradation are

in short supply. This is especially true for intense beams such as laser beams or beams of synchrotron

radiation from large electron storage rings. Hence, it was a pleasant surprise to find that a new

material, developed by Sanders Associates as a laser host, also has the third highest fundamental optical absorption edge energy among available optical materials. Here we report the first studies of the

VUV transmission, reflectance and photoluminescence of LiYFi,. The transmission measurements show that

the fundamental absorption edge is very sharp at about 10.42 eV (1190 A). Both transmission and photoluminescence measurements show that the particular sample we have contains an appreciable impurity

concentration. These impurities contribute to the absorption coefficient below the absorption edge,

which diminishes from 3 cm~^ at 10 eV to 0.1 cm~^ at 5 eV. The reflectivity and transmittivity in the

range 4 to 10.4 eV are used to deduce the optical constants for the polarization direction el c. The

reflectance spectrum is shown for the range 4 to 25 eV.

LiYFij forms in a tetragonal (scheelite or a-13) crystal structure with Li"*" ions arranged among YFi,

tetrahedra [1]-'-. Known physical properties include the density (3.99 g/cm^) , hardness (4 to 5 on Moh's

scale), strength (3.3 x 10^ N/m^) and Poisson's ratio (0.33) [2]. Optically, the crystal is clear and

transparent from 0.21 to 10.4 eV (0.119 to 5.88 um) . It is uniaxial and biref ringent , with (ng-nQ)/nQ = 0.0153 at 2 um, 0.0157 at 0.5 ym, 0.0167 at 0.3 um and 0.0175 at 0.25 um [3]. The refractive index for

the ordinary ray rises from 1.4424 at 2 um to 1.4567 at 0.5 um and 1.4754 at 0.3 um, slightly higher

than the refractive index of CaF2

.

The sample [4] was cut from an undoped a-axis rod about 2-cm diameter by 15-cm long. The c direction was marked by an unusual ridge along the side of the boule. The sample was oriented via x-ray

Laue photographs with the a and c directions in the plane of the coin-shaped disc 1.56-mm thick x 2-cm

diameter. It was mechanically polished on both sides using Syton [3] on a short-nap cloth as the final

polish. The surface appeared smooth and shiny, but slightly rounded. Before mounting in the UHV reflec

tometer [5] the sample was rinsed in electronic grade acetone and electronic grade methanol, and blown

dry with dry N2. The sample was mounted with c vertical. The measurements were carried out at the

Stanford Synchrotron Radiation Project [6] using the 8° VUV beam line. The synchrotron radiation (SR)

is 96 to 97% polarized in the horizontal direction as it emerges from the monochromator , so that the

optical properties measured are characteristic of the "ordinary ray," for which el c.

In figure 1 we compare the transmittivity of the sample with the photoluminescence excitation

spectrum. The transmittivity remains high to 7 eV, above which there is a gradual reduction to about

50% at 10 eV, and then a sharp decrease to 10% at 10.42 eV. The gradual decline is marked by dips,

most notably at 7.1, 8.0 and 8.6 eV. The photoluminescence excitation shows peaks at these same photon

energies

.

The photoluminescence emission was collected from 1.8tt sr in the backward direction and focussed on

a glass-fiber light pipe with a coating of sodium salicylate on the input end. The detector on the output end was an EMI 9514S photomultiplier . Hence, the spectral sensitivity for emitted photoluminescence

extended from less than 2 eV to the soft x-ray region. No effort was made to narrow this spectral range

Photons scattered by the sample were eliminated from the photoluminescence response by electronically

gating the photon counting system to accept photons only in the time period from 30 to 180 nsec after

* Work supported by NWC Independent Research Funds, the Office of Naval Research, and Defense Advanced

Research Projects Agency.

1. Figures in brackets indicate the literature references at the end of this paper.

-132-
the exciting light pulse. (The exciting light pulse is typically 0.3 nsec in duration.) In this way,

the total effect of stray light and dark count contributed only 50 to 60 Hz (1 to 10%) of the counting

rate in the photoluminescence spectrum. The spectrum shown in figure 1 has been approximately normalized to incident intensity, but because of uncertainties in phosphor efficiency, reflectance and

scattering losses, pulse pile-up and the relatively slow response of the photomultiplier , the absolute

luminescence yield shown is only an order-of -magnitude estimate. The relative yield across the spectrum

is probably accurate to + 10%.

Figure 2 shows the sample reflectivity in the 4 to 11 eV range, as well as the reflectance deduced

from the transmittivity and reflectivity data. Figure 3 shows the absorption coefficient in this range

as also deduced from the data of figures 1 and 2. In order to obtain the reflectance R and absorption

coefficient B, the equations for transmittivity T and reflectivity R' of a plane-parallel plate of

thickness d is incoherent radiation,

^ (1 - R)2exp(-6d)

1 - R2exp(-2Bd)

, 1 + (1 - 2R)exp(-2ed)

1 - R2exp(-26d) (2)

were combined into a quartic equation:

0 = ai^y"* + aay^ + a2y^ + ajy + , (3)

where y = exp(-ed) , ao = ai^ = - 1/2 a^ = 1/T, ai = -33 = 1 - (l-R'/T)^. This equation was solved by

successive .4 approximations in which the y and y^ terms were ignored first, then evaluated with the first

y value obtained from solution of the residual quadratic equation and added to ag before solving the

residual quadratic again. This procedure was very fast. With a programmable calculator about 10 suc- cessive approximations were made per second, and even with 6d as small as .02, convergence to six

significant figures was obtained within 200 successive approximations. As 6d becomes larger, less and

less approximations are necessary.

The reflectance, R, was obtained subsequently using the equation

p = Ay - 1 Ay - v2 (4)

where A = (1 - R')/T, obtained from eqs. (1) and (2) in a straightforward manner. Table 1 summarizes

the optical constants implied by the R and 6 values of figures 2 and 3, compared with the previous

results of ref. 3. Note that the kg and £2 values are characteristic of this sample and probably not

characteristic of pure LiYFi^..

The reflectance from 10 to 25 eV is shown in figure 4. Beyond 10.5 eV the sample is so opaque that

no back-surface correction is necessary, and R = R' . The dashed portions of figure 4 near 12 eV and

above 24 eV are regions where stray light or second-order problems reduce the accuracy of the data. In

the 12 eV region the problem is mainly second-order, which could not be filtered out by any available

filter. Above 24 eV the component of stray long-wavelength radiation becomes a significant fraction of

the monochromatic radiation.

Bulk Loss Coefficient Measurements in CaF2

We have made preliminary measurements of the total bulk loss coefficient of two CaF2 samples over

the spectral range from 400 to 210 nm. The measurements were performed at the Stanford Synchrotron

Radiation Laboratory in order to utilize the highly collimated monochromatic radiation available there

on the 8° beam line.

The measurement technique is based upon the use of prism-shaped samples. This technique has been

developed with both a double-beam configuration [7] (for differential measurements) and a single-beam

version. A schematic diagram of the single-beam system is shown in figure 5; this is the method used

at Stanford Synchrotron Radiation Laboratory for the preliminary CaF2 measurements.

Referring to figure 5, the beam passes through a BaF2 splitter, a tuning fork chopper, and the

sample. After reflecting from the mirror it retraces its path and proceeds to the photomultiplier tube.

The prism-shaped sample is placed on a translation stage such that it can be translated parallel to the

bisector of the prism angle, and so that the radiation is incident at the angle of minimum deviation.

This latter requirement prevents the beam from moving about on the mirror as the sample is translated.

The data analysis for the single-beam experiment is relatively simple. Let I(t) be the intensity

at the detector as a function of the beam path length in the sample. Then

-133-
I(t) = IoG(l - Ri - Ai - Si)2(l - R2 - A2 - S2)2e (5)

where R, A, and S are the surface reflectivity, absorption, and scatter coefficients. We assume that

the two polished surfaces are uniform but not necessarily identical; hence, the subscripts "1" and "2".

G is a factor which accounts for all the properties of the other optical elements, such as back-mirror

reflectivity, beam splitter reflectivity, and so on. B is the bulk loss coefficient, composed of

absorption and scatter. In highly transparent materials 6t << 1; therefore, to first order we find that

„ _ 1 1 dl

2 IoG(l-Ri-Ai-Si)^(l-R2-A2-S2)^ dt

'

Both dl/dt and the denominator in eq. (2) are directly measurable. Note that the surface parameters

need not be measured as they are eliminated by ratioing. Variations in Iq are eliminated by real-time

ratioing of the photomultiplier output to the storage ring current, a quantity directly porportional to

The bulk loss coefficients calculated from eq. (1) are shown in figures 6 and 7. The curves drawn

through the experimental points are least-squares fits to all but the points on the large peak at 305 nm. These curves have an approximate wavelength dependence of X~^-^.

In spite of the diverse origins of the two samples, the similarity of the spectra is striking. The

Raytheon material shows slightly lower losses overall. Based on the 305-nm peak, the tentative identification of the impurity responsible is Ce^''". The spectra of rare-earth ions in CaF2 have been extensively studied by Loh [8,9]. Loh has observed the lowest 4f -> 5d absorption band of Ce^"^ in CaF2 to be

located at 307 nm; the analogous bands in the remaining rare-earths are at shorter wavelengths (the next

closest is Pr^"^ at 219 nm) . The increase in absorption observed in figures 6 and 7 as one goes to wavelengths shorter than 305 nm may be due to the 4f - 5d transition associated with clusters of Ce^"*" ions.

In an interesting experiment on the concentration dependence of 4f - 5d absorption due to Ce^"^ in CaF2,

Loh finds peaks between 250 to 210 nm which he assigns to the Af - 5d transition of clustered Ce^"*" ions. He found that as the concentration of Ce^"*" increased, the 307-nm band decreased in intensity while the

higher energy clustered ion band increased. This suggests that Ce^"^ prefers to cluster rather than to exist as isolated ions.

References

[1] Shand, W. A., J. Crystal Growth (Netherlands)

_5, 143 (1969).

[2] Folweiler, R. C. et al. , Report //1-1413-76-L1025A, Sanders Associates, Inc., July 1976.

[3] Castleberry, D. E. and Linz, A., Appl. Opt.

14, 2056 (1975).

[4] We are grateful to R. C. Folweiler of Sanders

Associates for supplying us with samples. Mr. Folweiler has informed us that no special

precautions were taken to make an especially

pure crystal.

[5] The UHV absorption/reflectance/scattering

ref lectometer was designed and built by J. L. Stanford, Naval Weapons Center.

[6] Supported by NSF Grant No. DMR73-07692 in

cooperation with SLAC and ERDA.

[7] Burdick, D. L. , Rehn, V., Kyser, D. S. and

Jones, V. 0., in High Energy Laser Mirrors and

Windows, Semi-Annual Report No. 6, Sept. 1974-

March 1975, DARPA, Order #2175, pp. 32-101.

[8] Loh, Eugene, Phys. Rev. 147, 332 (1966).

[9] Loh, Eugene, Phys. Rev. 154, 270 (1967).

-134-
TABLE 1. Optical Constants for the Ordinary Ray

(e 1 c) in the Range 4 - 10.4 eV.

hv X

o koxlO^

±1.0

^1 ±.009

£2X10'

±.004'^ ±.0002^ ±2.1

A. 00 3100 1.472 2.6 2.165 7.7

A. 133 3000 1.473 1.4754 2.170

A. 50 2755 1.479 2.3 2.187 6.8

4.959 2500 1.487 1.4895 2.210

5.00 2480 1.487 2.1 2.212 6.3

5.50 2254 1.497 2.8 2.241 8.4

5.510 2250 1.498 1.5014 2.242

6.00 2066 1.506 3.4 2.269 10.2

6.50 1907 1.521 4.4 2.316 13.4

7.00 1771 1.540 6.9 2.372 21.3

7.50 1653 1.565 8.5 2.449 26.6

8.00 1550 1.604 13.7 2.573 44.0

8.50 1459 1.660 18.0 2.756 60.0

9.00 1378 1.724 22.4 2.972 77.0

9.50 1305 1.796 25.4 3.226 91.0

10.00 1240 1.883 27.7 3.546 104.0

10.20 1216 1.937 33.2 3.752 128.0

10.40 1192 2.080 124.0 4.326 516.0

This work. * Ref. 3.

Figures

Figure 1. Transmission and photoluminescence Figure 2. Reflectivity and reflectance

spectra of LiYF^ showing correspondence °^ ^'^^4"

between the structure in each spectra.

-135-
Figure 3. Absorption coefficient of LiYF, Figure 4. Reflectance of LiYF^ from 10 to as deduced from the data in 25 eV. The dashed portions are figures 1 and 2. regions where stray light or second-order effects reduce the

accuracy of the data.

STORAGE RING BEAM CURRENT

LOCK -IN VqutRATIO DVM I

monochromator

SPLITTER

Figure 5. Single-beam version of

the prism technique.

Highly collimated light

passes through the prismshaped sample at the

angle of minimum deviation.

Figure 7. Absorption coefficient of Raytheon

CaF2 (circles) with that of Harshaw

Figure 6. Absorption coefficient of Harshaw CaF^. CaF2 superimposed as a dashed line.

The large peak at 305 nm may be due

-136- to Ce"'^ ions.
FLUENCE DEPENDENCE OF THE ABSORPTANCE OF SOME ALKALINE EARTH FLUORIDES AT 0.36 iim*

P. A. Temple and D. L. Decker

Physics Division, Michelson Laboratories

Naval Weapons Center, China Lake, California 93555

The absorptances of MgF2, SrF2 and CaF2 have been measured at 0.351 to 0.364 urn. The absorptance is found to increase with each successive absorptance

measurement due to sample coloring by the 351 to 364 nm beam. This is found to be linear with /Pdt for a given beam diameter over the power range tested.

Increases in absorption of nearly two orders of magnitude to 1 x 10"^ were seen after a total central beam fluence of 3.5 x 10^ J/cm^. Under the beam and

sample configuration used all three materials were found to have an absorptance

of approximately 1 x 10"^ after approximately 3.5 x 10^ J/cm^ had been incident

upon the sample. Possible absorption mechanisms will be discussed, including

photochemically generated polymer films, impurities, and color centers.

Key words: Alkaline earth fluorides; color centers; laser calorimetry;

photochromic ; polymer films.

1. Introduction

This work was done on the Naval Weapons Center high sensitivity adiabatic calorimeter to be

described in another paper [l]-*-. In this instrument the sample is in thermal equilibrium with its

surroundings (a temperature-controlled can) and calibration is accomplished by observing the temperature

rise of the sample after laser irradiation and comparing it to the temperature rise caused by the

deposition of a precise amount of electrical energy. By using this technique it is not necessary to know the mass or specific heat of the sample [1] . The light source was the undispersed UV output of a Spectra Physics 171-19 argon ion laser. The two predominant lines are at 351 and 364 nm. For all

these measurements the power was limited to 50 mW and all exposures were for a period of 100 seconds.

This gave a total incident energy per exposure of 5 J. The Gaussian beam was condensed by two long

focal length lenses to a 1/e^ full width of 0.190 mm at the sample position [2]. The power density in

the center of this beam was then 350 W/cm^ and the energy fluence per exposure was 3.5 x 10** J/cm^.

All samples were 1-cm thick.

2. Experimental

The materials investigated were CaF2, MgFj and SrF2 • The CaF2 and SrFg were prepared by Raytheon

using a reactive atmosphere process and had very low absorptance at 3.8 um. The MgF2 was purchased

from Optovac. The raw data for SrF2 are shown in figure 1. Each data point represents a 5-J exposure.

The figure shows the temperature rises for 13 successive exposures at a fixed point on the sample. The

step changes in temperature are the result of individual 100-sec laser exposures. The horizontal

portions are equilibration times between laser exposures during which the sample came into equilibrium

with the surroundings. This figure shows the successively larger temperature rises for equal laser

exposure, indicating that the absorptance is increasing with each measurement.

As seen in the lower left of figure 1, the uncolored absorptance of the sample is small, and is

bracketed between zero and 1 x 10~^. This uncolored absorptance is consistent with the data reported

by Rehn et al . at this conference [3].

Figure 2 shows the data in figure 1 on a log-log plot. The solid line has a slope of 1. The data

follow this line closely, indicating that the absorptance is directly proportional to the fluence in

the range investigated.

Figure 3 shows the absorptance of CaFg, and figure 4 shows the absorptance of MgF2 . The higher

dose points in figures 2 and 3 were obtained by a factor of 10 higher power exposure (1/2 W) for the

appropriate period of time to obtain the indicated dose. The absorptance was then measured in the

usual way with 0.050 W power for 100 sec. The linear increase in absorptance, within experimental

accuracy, indicates that the sample darkening is proportional to fluence or total incident energy,

that is /Pdt, rather than power. The coloring rate is assumed to be proportional to the local intensity

I. If this is true, it can be shown rigorously that the measured absorptance is inversely proportional

to the beam area when the beam is Gaussian. Therefore, these data were all measured with the same beam

diameter in order to retain the same relationship between total power and central beam intensity.

* Work supported by Defense Advanced Research Projects Agency.

1. Figures in brackets indicate the literature references at the end of this paper.

-137-
Figure 3 contains points gathered the next day. These data indicate there had been no loss in

absorptance of the sample over that time period.

For the three materials shown in figures 2, 3 and 4, the absorptance after 20 exposures, or 7 x

10^ J/ cm fluence, are shown in table 1.

Table 1. Window absorptances after 20 exposures.

CaF2 2.7 X 10-2

MgF2 2.2 X 10-2

SrFj 2.0 X 10-2

3. Discussion

Two possible f luence-dependent absorption mechanisms will be discussed. They are photochemical

production of an absorbing polymer film and the generation of bulk absorbing sites due to impurity

atoms or vacancy-related sites (color centers)

.

3.1 Polymer Films

The growth of polymer films on plasma tube windows by UV-activated crosslinking of monomers is

well known in the laser industry [4]. These films are the result of an in situ photochemical reaction

on the window surface and are not directly related to prior cleaning procedure. If organic impurities

are present in the plasma tube the action of ion and UV bombardment results in a slow degradation of

the output due to this film production [4].

The pressure in the calorimeter during these measurements was 4 x 10-^ torr. The system is ion

pumped. There is a limited amount of organic material present in the system, including methane from

the ion pump. While data do not exist on the growth rate of such films, it is possible that there is

sufficient organic material present to cause the generation of absorbing polymer films. The fluence

dependence of the absorptance is consistent with film growth, if one assumes a linear growth rate with

incident energy.

3.2 Impurity and Color Center Absorption

As noted in a recent compilation by Sparks [5], there are a large number of atomic impurity species

which give rise to absorption in the UV. In addition, various color center bands exist in the UV. Color centers are generally associated with charge trapped at lattice vacancy sites.

Concerning the growth of absorption with dose, while lattice displacements would probably result

in the generation of color center sites, it is unlikely that atomic displacements can be caused by the ~ 3.5 eV photon beam. However, color centers may also be "activated" by the transfer of charge from

impurity atoms to already existing lattice vacancies. Photons of 3.5 eV are quite capable of such

charge transfer as color center bleaching experiments have shown [6]. In addition, charge transfer can

take place between various impurity species, resulting in the growth of absorption bands with dose. As notes by Sparks, if the oscillator strength is high, even ppb impurity concentrations can result in

absorption coefficients as high as 10-^.

3.3 Implications

The data shown above have serious implications to large laser behavior. For example, a laser of

10 MW/cm^ power density, 1 psec pulse length, and 1 kHz repetition rate has a fluence of 10 J/cm^ per

pulse, or a fluence of 10** J/cm^ per minute. With windows of CaF2 behaving as shown in figure 4, the

window absorptance will have risen to 2 x IQ-^ in 70 sec of operation.

Similar results have been observed empirically by Tachisto, Inc., who market a commercial UV laser

with an output power density of approximately 5 MW/cm^ with a total flux of 1000 J/hr [7]. CaF2 has

been used as a Brewster window material. The beam dimension is 0.4 cm x 1.5 cm which results in a Brewster window dose per unit area of approximately 8 x 10^ J/cm^ in 100 hr of operation (assuming the

output coupler has R = 0.7). Typically, 100 hr of use results in a drop in output of 20%. This is

equal to a Brewster window absorptance of 7 x 10"^. This is quite similar to the darkening with fluence

observed in the work reported here. It must be kept in mind, however, that there are tremendous power

density differences between the Tachisto device and that repeated here, and that the lack of dependence

on power which we report here was shown to be true over a limited power range. In addition, other

coloring mechanisms may be acting in the Tachisto laser where the window is exposed to hard UV,

electrons, and x-rays.

4. Conclusion

By proper calorimeter beam conditioning we have simulated the photon fluence anticipated in larger

laser systems. We report here the observation of a f luence-dependent absorption coefficient in CaF2,

-138-
SrF2 and MgF2 at 351 to 364 nm. Material exhibiting the f luence-dependent behavior discussed above

would be unsatisfactory for laser window material. We have discussed two possible darkening mechanisms,

polymer films and impurity or color centers. If the absorption is due to polymer film production it

should be possible to reduce the effect by appropriate care. If the absorption is due to impurity or color center sites, on the ohter hand, it will be necessary to determine the precise nature of the

absorption and then to reduce the concentration of the offending species.

5. References

[1] Decker, D. L. and Temple, P. A., "The Design

and Operation of a Precise, High Sensitivity

Adiabatic Laser Calorimeter for Window and

Mirror Material Evaluation," this conference.

[2] Yashida, A. and Asakura, T. , in 'Optics and

Laser Technology (December 1976), p. 273.

[3] Rehn, V., Burdick, D.. L. and Jones, V. 0.,

"UV Reflectance, Transmission and Photolumi- nescence of LiYFi, and Bulk Loss Coefficient

in CaF2," this conference.

[5] Sparks, M. , Vora, H. and Flannery, M. , Tabulation of Impurity Absorption Spectra—

Ultraviolet and Visible, Vol. 1, 9th Technical

Report, June 1977, Defense Supply Service,

Washington, DC.

[6] Schulman, J. H. and Compton, W. D. , Color

Centers in Solids (Pergamon, New York, 1963).

[7] Chase, Charles, Techisto, Inc., Needham

Heights, Massachusetts. Private communication.

[4] Wright, Dave, Spectra Physics, Inc., Mountain

View, California. Private communication.

6. Figures

Figure 1. Calorimetric data for SrF^

No. 101 at 351 to 364 nm

showing the increase in

absorptance with each

successive exposure.

Thirteen individual

absorptance measurements

are shown. The vertical

scale is an arbitrary

temperature scale.

Figure 2. The 351 to 364 nm absorptance

of SrF2 No. 101 as a function

of total Incident energy.

-139-
CaF2 NO. 24 (RAYTHEON)

351-364 nm CALORIMETRIC MEASUREMENTS

50 mW FOR 100 SECONDS PER MEASUREMENT

S

2

1 1

11

1 )

"^NEXT

/f DAY

0

0

1

0 0

1

1 1 12 1 1 5 10 20 50 100 200 BOO ACCUMULATED DOSE. X)ULES

Figure 3. The 351 to 364 nm absorptance of CaF2 No. 24 (Raytheon) as a

function of total incident energy.

MjFj OPTOVAC

351-364 nm CALORIMETRIC MEASUREMENTS

50 mW FOR 100 SECONDS PER MEASUREMENT

ACCUMULATED DOSE. JOULES

Figure 4. The 351 to 364 nm absorptance of MgFa (Optovac) as a function

of total incident energy.

-140-
COmEMTS OM PAPER BV TEMPLE AMV PECKER

T^e dlicmiion In tkii papM. cettte/ted oAound tha qutition Oj$ MheXheA a potymeA d^iZm on tko. iuA^acz

thz iampl& on. colon c^ntMA In thz baZk o{, thz iomplt mcus nuponiiblz don. th& obiznvzd ab&onption. It

wcu, note.d that potymeA d^Jjini Mkich couZd Itad to iuch an abionption o/lz mhJUL knoMn in iynchAotnon

nadlation itadiu and oAe due to the. polymeA-izajtion o^ hydn.ocan.bon compounds into h&avy hydnocaxboni,

.

A iijnitaA donmaZion o{j a ($-L5n -ti ieen in Imca tabu wham a loAgz amount od uuLtAavioZzt KadLation

dnom the. laicA dA^change. i6 pne^ent. BuZk cotonA.nQ iA ieen in the BnewiteA angle windom med in many

ga& ladcnji, e^pecA.aLty in quantz windom. In the expe/Umenti de^cAibed in tki!> papen the ipot iize

was veny maJUL, appnoxAjmateZy a tenth od a mUlLimeXeA in diameteA. It woi veAy diddicult theAedone to take the sample oat od the appaAata6 , make a poAaJLlet inveJitA,gati.on, and then neplace it in exacXZy

the (same locatxon. Tkii pnevented any investigation od the e^j^eai od anneoLing, which woutd indicate

that the discolonation wa& due to colon centeA donmation. The ipeakeA de^cnlbed the edd^(^ <^ peJmanent,

that, is, that no neduction in the attenuation was seen oveA a pe/iiod od ieveAoZ days. He indicated

that expeniment!) to deteAmine whethen on not the attenuation was neduced by anneating wauZd be cannled

out in the neon. dutuAe.

-141-
SURFACE STATISTICS OF SELECTED OPTICAL MATERIALS*

Jean M. Bennett and J. Merle Elson

Physics Division, Michelson Laboratories

Naval Weapons Center, China Lake, California 93555

In order to understand and predict the scattering properties of laser

mirrors and windows, it is necessary to know the statistical properties of the

surface microroughness . The rtns roughness of the surface determines the total

integrated scattering, while the slopes of the surface irregularities determine

the autocovariance function and hence the angular dependence of the scattered

light. The rms roughness, rms slope, height and slope distribution functions

and autocovariance function have been measured for materials of interest in

infrared, visible, and ultraviolet laser applications. These include fused

quartz, molybdenum, potassium chloride, copper, and silicon carbide. Variations between the statistics of different samples of the same material as well

as between samples of different materials have been found. The calculated

scattering to be expected from different samples will be discussed and related

to that measured for some samples

.

Key words: Mirrors; scattering; surface roughness; surface statistics.

1. Introduction

Scattered light from the surfaces of laser optical components can arise from various sources: (1)

Defects whose dimensions are large compared to the wavelength of light give rise to so-called macroseattering, which is governed by the laws of geometrical optics. (2) Isolated defects whose dimensions

are comparable to the wavelength of light can be considered as dipole scatterers whose scattering

characteristics are predicted by Mie (dipole) scattering theory. (3) Microlrregularities whose heights

are small compared to the wavelength of light produce scattering which is predicted by diffraction

theory. (4) From certain types of rough metal surfaces, for example, silver and aluminum, there can be

radiative excitation and subsequent decay of surface plasmons yielding second-order light scattering.

Although all of these sources of scattered light are important, the most prevalent source is (3) because

all surfaces have some sort of microlrregularities remaining from the finishing process, whether it be

mechanical polishing, diamond turning, electropolishing, chemical polishing, ion polishing, or some

other process. These microlrregularities are distributed over the entire surface and hence contribute

a larger fraction of the scattering, particularly in the visible and ultraviolet, than do isolated

surface defects such as scratches, digs, dust particles, etc. We are attempting to find ways to predict

the scattering from an optical component based on knowledge of its surface statistics. The theory

relating total integrated scattering (TIS) to surface statistics has proven to be valid in many cases,

and the advantages and limitations of this theory will be discussed. In order to predict the angular

distribution of scattered light, more information about the surface statistics is necessary, and the

theory is much more difficult. Progress is being made in this area, some of which will be discussed in

this paper. Finally, examples will be given of some of the wide variety of surface statistics which

can occur on optical surfaces.

2. Total Integrated Scattering

Figure 1 gives a schematic representation of scattering from a rough surface. The incident beam

is assumed to be normal (or nearly normal) to the surface and the specular beam is in the appropriate

direction so that the angle of reflection equals the angle of incidence. The rest of the light is scattered in all directions by the surface. For smooth but wavy surfaces the scattered light peaks about

the specular direction; rougher or matte surfaces scatter the light more equally in all directions.

For surfaces whose roughness is small compared to the wavelength and whose height distribution is

Gaussian, the TIS into a hemisphere is related to the rms roughness of the surface by the simple relation [l]! shown in figure 1. Most polished surfaces satisfy this relation and have height distribution

functions similar to that shown in the upper left of figure 2. (In this type of presentation the histogram is obtained from the measured height data. The length of each bar represents the fraction of the

total number of surface features which have heights equal to the value given on the abscissa, which is

measured relative to the mean surface level (dashed vertical line) . The smooth Gaussian curve encloses

the same total area and has a half width derived from the measured rms roughness of the surface.) Some

very soft materials such as polished KCl have proportionately too many large bumps or deep scratches.

This distorts the measured histogram (upper right of fig. 2), putting too much contribution in the

tails. The influence of the extrema on the surface makes the rms roughness proportionately too large.

* Work supported by NWC Independent Research Funds and Defense Advanced Research Projects Agency.

1. Figures in brackets indicate the literature references at the end of this paper.

-142-
so the half width of the Gaussian is also too large and the Gaussian does not fit the measured histogram

For surfaces of this sort, the agreement between measured values of the rms roughness and those calculated from TIS measurements is not very good. TXS-derived values tend to be higher. Relatively few

surfaces have the problems illustrated by the histograms shown in the lower part of figure 2. If a surface is extremely smooth but has rather large dust particles on it, one can obtain a distorted

height distribution histogram with the maximum shifted to negative values (because the particles have

influenced the placement of the mean surface level). Similarly, an etched or pitted surface which

is nominally smooth but has deep holes in it can have a distorted histogram with the peak shifted in

the other direction. Neither of these two types of surfaces would satisfy the assumptions in figure 1 and hence the simple relation between TIS and rms roughness would not be expected to hold.

Fortunately, most surfaces have gaussian height distribution functions and in this case it makes

sense to talk about an rms roughness for the surface. In figure 3 are shown the range of rms roughnesses, mostly derived from TIS measurements, which have been measured for polished optical surfaces.

In all cases it is easy to obtain larger roughnesses, but reducing the lower limit is much more difficult. (In the case of KCl, there is such a wide range of roughnesses that the difference between the

value measured directly and that derived from TIS measurements is relatively insignificant.)

Two unusual height distribution functions have been obtained from polished molybdenum and TZM

samples [2], and are shown in figure 4. The molybdenum sample on the left had a smooth but wavy surface

with a predominance of small bumps and absence of small holes. The TZM sample on the right had the

only bimodal height distribution we have seen with a relative absence of surface features whose heights were very close to the mean surface level.

3. Angular Scattering

Figure 5 shows the relation [3] between the scattering per unit solid angle dP/dfi as a function of

wavelength A, the optical constants of the mirror surface (given in terms of the dielectric constants

and £2), the scattering angle 9, the rms roughness 6, and the spectral density function g(k) , which is

the Fourier transform of the autocovar iance function measured for the surface. The autocovariance or autocorrelation function of the surface roughness is a measure of the average relationship between two values of surface height separated by a finite distance. If the two surface height values are sufficiently separated so as to be completely independent, then the autocovariance function is zero for that

separation distance. As the separation distance decreases, eventually one value of the surface height

will be influenced by the value of the other; then they are correlated. The autocovariance function has

argument p, the separation distance, and as p approaches 0 the surface heights become completely correlated. The autocovariance function is defined in a mathematical manner elsewhere [3]. In the relation

for dP/dfi given in figure 5, we are assuming that the light is normally incident and polarized, and

that the scattered light is measured in the plane perpendicular to the direction of polarization of the

incident beam. Note that the expression for dP/dQ is composed of two parts: an optical factor which

is a function only of wavelength, optical constants of the material and scattering angle, and a surface

factor which contains the statistical properties of the surface. In figure 5 the optical factor is

plotted for a silver-coated mirror for three selected laser wavelengths. The separation between the curves is caused by the wavelength dependence, while the curling over of each curve at large scatter

ing angles is caused by the cos^e term in the numerator. The entire shape of each optical factor curve

is primarily determined by the above-mentioned cos^G term. Since the contribution from the optical

factor is nearly constant for scattering angles from 0° to about 60°, all the variation in the angular

scattering is caused by the contribution from the surface structure by way of the spectral density

function, S^gCk) . A typical curve of a spectral density function for polished fused quartz is shown in

the lower right of figure 5. Here 6^g(k) is plotted versus k, the net change in the wave vector component along the surface between the incident and scattered photon. For normal incidence an incoming

photon has no wave vector component parallel to the surface and therefore k in this case represents the wave vector component, parallel to the surface, of the scattered light. Since k = (2tt/X) sin8 , the

value of 62g(k) can be obtained for any wavelength and scattering angle from the curve. The dashed

line marked FECO limit is the upper limit of k values for which 52g(k) can be obtained from interferometric measurements.

The calculated values of angular scattering are shown for three laser wavelengths in figure 6. These curves are products of the curves shown in figure 5. The vertical separation between the curves comes from the X~'* term, while the variation in the shapes of the curves for angles from 0° to about

60° is caused by the spectral density function. For larger scattering angles the downward curl is

caused by the downward curl in the optical factor curves.

From the curves in figures 5 and 6, it is clear that the measured surface statistics form the most

important part of the calculations of the angular scattering. Hence it is appropriate to look more

closely at the different types of spectral density functions produced by surfaces having different

types of statistics. In figure 7 are shown the three basic constituents of surface structure: short

range random roughness, long term waviness, and periodicity. The short range random roughness is the

predominant type on most polished surfaces and produces a large initial spike on the autocovariance

function. This is because the features on the surface are uncorrelated and the only correlation is

between points on a given feature (bump or hole) . The Fourier transform of this type of autocovariance

-143-
function is slowly varying with no initial spike, and produces the type of scattering curve shown in

the lower center of figure 7. Waviness on a surface produces an autocovariance function with a much

larger correlation distance (zero intercept) . The spectral density function and angular scattering

curves both show a very large spike for small k values and scattering angles. Periodicity in a surface

produces an oscillatory autocovariance function and a spike in the angular scattering curve at the

scattering angle corresponding to the relation sin9 = X/t, where t is the spacing between features on

the surface. This relation is a special case of the grating equation for normally incident light and

first-order diffracted light. For a periodic surface whose contour is close to a sine wave and whose

amplitude is small compared to X, such as the one shown, the second- and third-diffracted orders are essentially completely suppressed, even though the first-order is very strong [A].

If a surface having short range random roughness is considered to be composed of many periodic

components of different amplitude, spacing, and phase [4], one can calculate the separation of surface

features that will produce scattering at a particular angle and wavelength. This information, obtained

from the grating equation for normal incidence illumination, is contained in the nomogram in figure 8. The dashed lines show an example of how to use this nomogram: features which produce scattering at 5° from the specular direction at a wavelength of 0.5 um have separations of about 5.7 \m. This nomogram

emphasizes that for a particular wavelength, scattering between 1° and 90° can only be produced by

features on the surface having a finite range of separations whose ratio is less than 60. For example,

scattering between the angles of 1° and 90° at a wavelength of 0.5 um is produced by surface features

having separations ranging from 0.5 um (d = X) to 29 vm. Thus, evaporated films which consist of

particles having diameters in the 400 to 1000 A range should not affect the angular scattering in the

visible. Generally, the evaporated film contours the underlying substrate, so that the scattering is

produced by features on the substrate having larger separations.

4. Measured Surface Statistics

Based on the results of the preceding section, surface statistics should be measured with an instrument having a lateral resolution adequate to detect surface features which produce scattering at the desired wavelength and scattering angle. At Michelson Laboratory the instrumentation available for

the measurement of surface statistics includes a scanning interferometer employing multiple beam fringes

of equal chromatic order (FECO) [5], a profilometer with a diamond stylus [6], and stereo electron

microscopy [7]. The height sensitivity and lateral resolution of these various instruments is given in

figure 9 along with the maximum length on the surface for which statistical information can be obtained

at one time. Stereo electron microscopy is useful when one is considering angular scattering in the vacuum ultraviolet and x-ray regions. Conversely, FECO interf erometry is useful chiefly for predicting

microirregularity scattering in the infrared and near-angle scattering in the visible. In the infrared,

microirregularity scattering from many surfaces is so small that other scattering mechanisms, for

example, dipole scattering from isolated particulates, dominate. Thus, the Talystep profilometer

currently seems to be the best tool for measuring statistics of surfaces whose irregularities produce

scattering in the visible and ultraviolet. The remainder of this paper will be devoted to a discussion

of statistics for various types of surfaces.

4.1 Fused Quartz

One might think that the surface statistics for a given material might be quite similar for different types of surface preparation. That this is not so is well illustrated by polished fused quartz.

The surface profiles range from the smooth but wavy type for a bowl feed polished [8] surface to the

type with short range random roughness for a normal fresh feed polished surface. The former type of

surface profile produces an autocovariance function such as that shown in figure 10 (also, fig. 7,

left-hand column), while the latter type has an autocovariance function such as that shown in figure

11 (and fig. 7, center column). All variations in between are possible and have been observed, including an extremely smooth fresh feed polished surface that was wavy and had very fine polishing scratches.

All of these polished fused quartz surfaces had normal Gaussian height (and slope) distribution functions, such as that shown in figure 2, upper left.

The measured and calculated angular scattering curves for the two silvered polished fused quartz

surfaces whose autocovariance functions were shown in figures 10 and 11 are shown in figure 12. The

measured and calculated curves for the rougher, fresh feed polished surface were matched in the 20 to

40° range of scattering angles, but no other adjustments were made. The agreement between theory and

experiment is encouraging, but not yet quantitative. In particular, it appears that there may be

another unaccounted for scattering mechanism operating in the case of the smoother bowl feed polished

sample. One possibility is dipole scattering from isolated particulates such as dust on the surface

which may increase the entire scattering level without drastically affecting the shape of the scattering curve.

4.2 Molybdentim and TZM

Molybdenum [2] and one of its alloys, TZM (containing titanium and zirconium) [2], have quite

interesting surface statistics and predicted angular scattering properties. Two unusual height distribution functions were shown in figure 4. A third sample of polished molybdenum obtained from stock

-144-
made by a different process also had a skewed height distribution function similar to that of the

molybdenum sample shown in figure 4. The initial portions of the autocovariance functions for these

three polished molybdenum samples are shown in figure 13. With the exception of the very near angle

scattering (less than 1° from the specular direction) , the entire angular scattering curve in the

visible seems to be determined by the initial positive portion of the autocovariance function. Thus,

the predicted angular scattering from these three polished molybdenum samples should be quite different,

as shown by the three calculated curves in figure 14. However, the measured angular scattering for

these three samples is remarkably similar, as shown by the three additional curves in figure 15. The reason for the discrepancy between theory and experiment is not clear and is currently being studied.

4.3 Potassium Chloride

Potassium chloride is a very soft material, and polishing it has been likened to polishing peanut

butter. However, using a polishing process developed at Michelson Laboratory [9], it has been possible

to obtain surprisingly smooth surfaces. KCl is one of the few polished materials that does not have

Gaussian height and slope distribution functions (see fig. 2, upper right). A more unusual type of KCl

surface has been produced by Honeywell [10]. The sample has been made by pressing between optical

flats in a two-step process with no additional polishing or surface treatment following the second

pressing operation. The height and slope distribution functions for the piano surface of a planoconcave lens produced by this process [11] are shown in figure 16. These have a non-Gaussian character

similar to that of polished KCl. The corresponding autocovariance function for this material, figure

17, looks similar to that of a fresh feed polished fused quartz sample (fig. 11), although the surface

seems to be more wavy.

Since KCl is so soft, it is important that the diamond stylus used to obtain the surface statistics

does not gouge the surface. Extensive experiments have been performed to determine the magnitude of the

stylus loading which will not damage KCl surfaces when styluses having various radii are used [12]. All

the measurements reported here were made using loadings that did not permanently mark the KCl surface.

Surface scans on a diamond turned KCl sample produced the autocovariance functions shown in figure

18. For this sample tool chatter apparently was the dominant feature since the scan made parallel to the grooves did not look appreciably different from the scan made perpendicular to the grooves. In both autocovariance functions the dominant feature is the very short range correlation, ~ 2 ym, showing

that the machining grooves are much less important than are uncorrelated surface features.

4 . 4 Diamond Turned Copper

Single point diamond machining is much easier on soft metals such as gold, silver, and copper, and

the surface statistics of several diamond turned copper samples have been measured at Michelson Laboratory. Two examples will be given here. In figure 19 is shown the initial portion of the autocovariance

function for a diamond turned copper sample made at the Lawrence Livermore Laboratory. A periodicity

of about 14 ym is evident in parts of the surface scan and in the oscillations of the autocovariance

function. This periodicity would produce a spike in the angular scattering curve at about 2.7° for a wavelength of 0.6471 pm. In figure 20 a longer portion of an autocovariance function is shown for a diamond turned copper sample produced by the Moore Tool Company. As in figure 19, the scan was made

perpendicular to the grooves. Two other periodicities are present in addition to the ~ 14-ym one. The

submultiple, 6.9 pm, is not as distinct as the 14-vim periodicity. The very long periodicity, 290 ym, manifests itself as a beat frequency. As shown in the table in figure 20, this long periodicity would

only be important if the mirror were being used for very near angle scattering. We have not yet

measured angular scattering curves on diamond turned samples, but measurements of this type have been

reported elsewhere [4] . Most of the diamond turned copper samples whose statistics we have measured

have nearly Gaussian height and slope distribution functions [5].

4.5 Silicon Carbide

Silicon carbide is a promising low scatter material, particularly in the ultraviolet and at shorter

wavelengths where its reflectance is higher than that of platinum [13] and its scattering much lower

than that of molybdenum [13,14]. The surface profiles on the smoothest polished silicon carbide samples

are similar to those for bowl feed polished fused quartz, very smooth but wavy. Thus, the autocovariance function is also similar, as shown in figure 21. Angular scattering measurements have not yet

been made on this material, but they would be expected to have a shape similar to that shown in figure

12. Work in this area is continuing.

5 . Summary

In summary, the relation between TIB and rms roughness has been given, and the roughnesses of a variety of optical materials have been presented. We have shown that it is possible to predict the

shape of angular scattering curves for several optical materials using measured surface statistics in

the calculations. The most important surface statistic is the autocovariance function, and these have

been presented for several optical materials.

-145-
6. References

[1] Bennett, H. E. and Porteus, J. 0., J. Opt.

Soc. Am. 51, 123 (1961).

[2] Klugman, A., "Surface Finishing of Molybdenum

Substrates," presented at High Power Laser

Optical Components and Component Materials

Meeting, 3-4 October 1977, National Bureau of

Standards, Boulder, Colorado.

[3] Elson, J. M. and Bennett, J. M. , Appl. Opt.,

to be published.

[4] Church, E. L. and Zavada, J. M. , Appl. Opt.

14, 1788 (1975); Stover, J. C, Appl. Opt.

14, 1796 (1975).

[5] Bennett, J. M. , Appl. Opt. 15, 2705 (1976).

[6] Talystep Step Height Measuring Instrument

(Metric), Model No. 75-1001, obtained from

Rank Precision Industries, Inc., 411 East Jarvis Avenue, Des Plaines, Illinois 60018.

[7] Dancy, J. H. and Bennett, J. M. , "Investigation of Electron Stereomicroscopy as a Method

for Determining Heights of Features on Evaporated and Sputtered Films and on Polished

Surfaces," in High Energy Laser Mirrors and

Windows (Semi-Annual Report No. 7-8, Defense

Advanced Research Projects Agency Order 2175,

Naval Weapons Center, 1976), p. 166.

[8] Dietz, R. W. and Bennett, J. M. , Appl. Opt.

_5, 881 (1966).

[9] Soileau, M. J., Bethke, J. W. , Shaffer, J. and Bennett, H. E., "Polishing of AlkaliHalide Windows," in High Energy Laser Mirrors

and Windows (Semi-Annual Report No. 6,

Defense Advanced Research Projects Agency

Order 2175, Naval Weapons Center, 1975), p. 75.

[10] Anderson, R. and Bennett, J. M. , "Properties

of KCl Optical Elements Forged Between

Optically Polished Dies," presented at High

Power Laser Optical Components and Component

Materials Meeting, 3-4 October 1977, National

Bureau of Standards, Boulder, Colorado.

[11] Honeywell isostatically forged, flat-pressed

samples provided by Enrique Bernal G. and

Rogers Anderson, Honeywell Corporate Research

Center, Bloomington, Minnesota.

[12] Dancy, J. H. and Bennett, J. M. , "Study of

Diamond Styluses for a Talystep Prof ilometer ,

in High Energy Laser Mirrors and Windows,

Annual Report No. 9, April 1976-May 1977, NWC

TP 5988, October 1977, In press.

[13] Choyke, W. J., Partlow, W. D. , Supertzi, E. P., Venskytis, F. J. and Brandt, G. B., Appl.

Opt. 16, 2013 (1977).

[14] Rehn, V., Stanford, J. L. , Baer, A. D.,

Jones, V. 0. and Choyke, W. J., Appl. Opt.

16., 1111 (1977).

7 . Figures

Figure 1. Schematic representation of scattering

from a rough surface, and the relation

between total integrated scattering and

rms roughness. The assumptions implicit

in the expression for TIS are given.

I -146-

2. Effect of the surface profile on the

height distribution function for four

types of surfaces. The surface profiles

are shown above the corresponding

histograms.
HOUCHWESS65 OF POLISHED OPTICAL SURFACES

FUSED quart;

SILICON CARBIDE*

CALCIUM FLUORIDE

'1 1»»

,| |„ *LIMITED NUMBER OF SAMPLES

10, ,M

ELECTROLESS NICKEL "I

TITANIUM

COPPER TZM _

MOLYBDENUM

ALUMINUM

L

1 " »

- l-^ 1

141

11 1 |I1»

COPPER (DIAMOND TURNED)

BERYLLIUM COPPER "f »• " 1 : 1 " •

INVAR a, ,1.,

unNFI* If,—. ' . ^ n ^

POTASSIUM CHLORIDE "l 'Hi

STAINLESS STEEL*

0 20 40 BO 10 too RMS ROUGHNESS (jil

Figure 3. Range of rms roughnesses measured for polished optical surfaces.

HEIGHT DISTRIBUTION FUNCTIONS FOR POLISHED MOLY AND TZM

-100 -50 0 50 100 -200 -100 0 100 200 DISTANCE FROM MEAN DISTANCE FROM MEAN

SURFACE LEVEL (X) SURFACE LEVEL |X|

Figure 4. Surface profiles and height distribution functions for two

unusual molybdenum and TZM surfaces (see reference 2)

.

-147-
RELATE SURFACE STATISTICS TO OPTICAL SCATTERING

OPTICAL FACTOR SPECTRAL

DENSITY FUNCTION

(SURFACE FACTOR)

Figure 5. Expression for the angular scattering per unit solid angle for

normally incident polarized light. The symbols are explained in

the text. Typical graphs for the optical factor and surface

factor are shown below.

CALCULATED SCATTERING FROM POLISHED FUSED QUARTZ

T I I I I I I T

SCATTERING ANGLE. 0

Figure 6. Angular scattering curves for polished fused quartz

I calculated from the optical and surface factors in

figure 5.

-143-
EFFECT OF SURFACE PROFILE ON AUTOCOVARIANCE FUNCTION AND ANGULAR SCATTERING

PERIODIC SURFACE WAVY SURFACE NORMAL SURFACE

ISHORT RANGE RANDOM ROUGHNESS)

0° 30° 60° 90° 0° 30° 60° 90° 0° SCATTERING ANGLE

30° 60° 90°

Figure 7. Three basic types of surface profiles and their corresponding

autocovariance functions and angular scattering curves.

SEPARATIONS OF SURFACE FEATURES WHICH PRODUCE SCATTERING

NORMAL INCIDENCE LLUMINATION

d = X/sinS

1.0 10 SEPARATION, d l/im)

Figure 8. Nomogram giving separation of surface features that produce

scattering at a particular angle and wavelength. The dashed

lines are an illustrative example.

COMPARISON OF STATISTICAL METHODS

HEIGHT LATERAL MAXIMUM

SENSITIVITY RESOLUTION LENGTH

FECO INTERFEROMETER -3%. RMS ~2 (im 1 mm

TALYSTEP PROFILOMETER -lA RMS -0.1 iim 2 mm

STEREO MICROSCOPY ±20%. -I (im

Figure 9. Comparison of three techniques used to obtain surface statistics.

-149-
FUSED QUARTZ (TALYSTEP)

15p 15r

wavy bowl feed polished fused quartz surface.

FUSED QUARTZ (TALYSTEP)

500

500

400

475

300

0.05 mm

50 •L 200

AUTOCOVARIANCE FUNCTION

25

0

1 2 p(pm)

-25il 1 ^ 1 1—!—1 1 1 1 ^ 1 —' 1 I I 0 0.2 0.4 0.6 0.8 1.0 LENGTH p(mm)

Figure 11. Surface profile and autocovariance function for a fresh feed

polished fused quartz surface having short range random roughness.

-150-
SCATTERING FROM POLISHED FUSED QUARTZ, X=647lX

10-"

SCATTERING ANGLE. 0

Figure 12. Calculated (solid) and measured (dashed) curves showing angular

scattering for the polished fused quartz surfaces whose autocovariance functions were shown in figures 10 and 11.

20 30 LENGTH p(pml

Figure 13. Initial portions of the autocovariance functions for two

molybdenum samples and a TZM sample.

-151-
SCATTERING FROM MOLYBDENUM AND TZM

Figure 14. Calculated angular scattering curves for the molybdenum and

TZM samples whose autocovariance functions were shown in figure 13.

SCATTERING FROM MOLYBDENUM AND TZM

T

1 1 1 1 1 r

SCATTERING ANGLE, «

Figure 15. Same as figure 14 except that three measured scattering curves

have been added.

-152-
HONEYWELL PRESSED KCI LENS (TALYSTEP)

Figure 16. Height and slope distribution functions for the piano surface

of a pressed KCI lens (see reference 10)

.

HONEYWELL PRESSED KCI LENS (TALYSTEP)

LENGTH plmm)

Figure 17. Surface profile and autocovariance function for the same KCI

lens as in figure 16.-153-
Y-12 MACHINED POTASSIUM CHLORIDE (TALYSTEP) EUROPIUM DOPED

SURFACE SCANS

500 i. SURFACE SCANS SCAN PARALLEL

TO GROOVES

SCAN PERPENDICULAR TO GROOVES

20 40 LENGTH pdnlcronil

Figure 18. Surface profiles and autocovariance functions for a diamond

turned, europium doped KCl sample.

6aoA LLL MACHINED COPPER (TALYSTEP) SURFACE SCAN

SLOPE OOA/micron

300A

6.000

SO 100 LENGTH p (micronil

Figure 19. Surface profile and initial portion of the autocovariance

function for a diamond turned copper sample made at the

Lawrence Livermore Laboratory.

-154-
COMMERCIAL DIAMOND TURNED COPPER ITALYSTEPI AUTOCOVARIANCE FUNCTION

-J ^ 1 I I I 0.2 0.4 0.6 0.8 1.0 LENGTH p(mm)

Figure 20. Autocovariance function for a diamond turned copper sample

made by the Moore Tool Company.

SILICON CARBIDE ITALYSTEPI

40

20

GIokX^)

0

-10

0 0.2 0.4 0.6 0.8 1.0 LENGTH pimml

Figure 21. Surface profile and autocovariance function for polished

silicon carbide.

-155-
COMMENTS ON PAPER BV BENNETT ANV ELSON

Tht compoAcutivz v-Oitau FECO, taZyitzp pio {^ZtomiJyiij , and icaXteAlng mejziuAmznti weAe diicuiitd.

Both EECO and pno (^iZomeXfiy m&oAuAmzn^ gav& good aQh.tme.nt w-Lth ejxch othzA and yieZdzd thz iomz

>iuuJbU> {jOh. thz auutoc.onAzlatl.on function. In h.z6poMz to a qautlon 04 to wkich tzchniqaz muZd be

thz mzoMLfimznt of, cholcz, thz ipzakzn nzApondzd that, tn a nexUL iyitzm, onz u> KzaJUiy tntzAutzd In

iaattexing, thznzionz total. -intzQiatzd hcdttzftinq on. thz anguZaA icattznlng ipzctnum wouZd bz thz

mzoMinzmznti 0^ tntzAzit. HoMzvzn, {^on. thz dztznmlnation oi iuA^acz choAactzn and iuA{,acz itatJjitLcA

,

pn.o{^-UiomztA.ij aj> by ^oA thz but mzthod to uAZ. Thz qiiz^tion woa aAkzd -tj$ thz aatozonAzLoution {^unztion

woi uniqueJLy n.zJUitzd to thz iimdacz pn.o{j-ilz. Thz ipzakzA. n.zptizd that o^ coumaz thz izattzAying dpzctAum

and autozonxzlatlon {jU.nztA.on onz unlquzty n.zZatzd, but that dtHzAznt iuA^azz pno^U-U can gtvz vzAy

6.ijruJjxA., although poiitbly not IdzntxcaZ, autozonAzLatton {fUnztxond , on. zquivaJLzntZy vzAy itntAloA

&zattzAing ipzctAa. A gtvzn pKod-ilz 0^ zouA6z, mwit yizld a uniquz izattzntng ipzztAum, but thz

Azcomtnuction o£ thz 4UA^ace pAo{,'Ltz ^Aom thz izaXtzAtng ipzztnum on aatozonAzJLaution {jUnctyion may not

bz uniquzty dz^tnzd.

I

-156-
SURFACE ROUGHNESS STATISTICS OF FUSED SILICA AS

A FUNCTION OF SURFACE PREPARATION AND TREATMENT

R. A. House, II, and A. H. Guenther

Air Force Weapons Laboratory

Kirtland AFB, NM 87117

and

J. M. Bennett

Naval Weapons Center

China Lake, CA 93555

Samples of fused silica which had previously been laser damage tested on portions of

their surfaces were subjected to a detailed statistical topographic analysis. These samples

had been fabricated using a variety of surface preparation techniques, and the primary final

characteristic of interest was the rms surface roughness of each sample. The present study

focused on the height and slope distribution functions and an autocovariance length evaluation resulting from the differing surface treatments. The basic conclusion of the study is

that the type of surface finishing used strongly influences the surface structure, causing

a wide variation in light scattering properties and topography.

Key words: Correlation length; fused silica; polishing; roughness; slope; surfaces.

Introduction

Previously reported work [1] indicated a strong correlation between laser damage threshold and rms surface roughness. The experiments were performed on fused silica using a wavelength of 1.06 ym and a pulse length of 40.5 ns. A graphical summary of the experimental data is presented in Figure 1, where

the functional dependence is EaTM = constant. E is the threshold (in MV/cm) and a is the rms roughness

o (in A). For most of the surface treatments used on the substrates, the exponent m had a value of about

0.5, while the value of the constant varied considerably with surface finishing procedure. In the cases

of ion polishing and bowl feed polishing, the samples were polished as smooth as possible. Therefore,

these samples all exhibited low rms roughness values having insufficient spread to clearly delineate

a threshold-roughness relationship. The specific samples examined in detail during the present effort

are indicated in Figure 1.

Four significant observations from Figure 1 indicated that further surface characterization of the

samples would be desirable: 1) The value of m for the flame polished samples was considerably different

than 0.5; 2) The behavior of the etched samples was erratic; 3) The constants varied considerably with

surface preparation; 4) There was undoubtedly a wide range in the surface contamination levels introduced

during the final surface finishing. It was hoped that, as a result of detailed topographic evaluation,

the functional dependence of E on a would be clarified. It was also hoped that variations in the

structures of surfaces prepared in different manners could be assessed and correlated with previously

reported laser-induced damage results.

Procedures and Data

Selected samples that had previously been laser damage tested were subjected to detailed topography

measurements at the Michelson Laboratory, Naval Weapons Center. Measurements were made using Total

Integrated Scattering (TIS) and Talystep prof ilometry . The samples were also examined visually using

Nomarski microscopy [2].

Data resulting from the investigation of the six samples indicated in Figure 1 are presented in

Table 1 and in Figures 2-7. The symbols used in Table 1 are as follows: is the background roughness (i.e., the roughness excluding particles); a is the overall roughness including the particles;

t o a is the TIS-determined roughness (at the wavelength 5682 A) ; s is the rms slope of the surface

t o topography, in A/ym, with the equivalent angle given in parentheses; £ is the correlation length in

ym; h^ is the rms height of the gross surface features (i.e., particles or holes); f is the fraction

of the surface area covered by these gross features; E is the measured damage threshold of the surface

(previously determined); and a. is the initial roughness value for the surface (also previously

determined). Roughness values and a were determined from analysis of the Talystep data obtained

in the present study.

-157-
Table 1. Summary of surface statistics.

SAMPLE

TREATMENT

%O (A)''t

O (A)

^t

O (A)

s

o (A/um)

I

( pm)

h

P

O (A)

f E

(MV/cm)

a

.

1

o (A)

Flame 5.62 75.4 20.9 208.4

(1.19°)

231.8 0.0718 2.24 13.32

Conventional 2.27 16.27 12.4 63.93

(0.37°)

0.90 75.6 0.0422 1.60 13. 75

Bowl Feed 30.2 30.2 24.8 118

(0.68°)

0.89 0.86 13

Overcoated 2.68 2.68 9.3 28.85

(0.17°)

0.82 0.89 13. 75

Ion-Etched 60.8 60.8 16.5 260

(1.49°)

0.48 1.34 36

Acid-Etched 15 111 216 1870 1.5 0.41 243

CIO. 59°)

Figures 2-7 show typical raw and composite data for the samples. Part "a" of each figure presents

the surface height and slope distributions measured for the sample. Also shown are the fitted Gaussian

curves, the fit being made by requiring each Gaussian to have the same area and rms value as the

corresponding histogram. Note that the histograms represent the as-measured surface (i.e., they

include the measured particles or holes). Parts "b" for Figures 3-7 are the autocovariance functions

for the samples. The autocovariance function for the flame polished sample was not actually measured;

however, this sample's visual and surface scan characteristics were essentially the same as those of

the conventionally polished sample (Fig. 3), so that its autocovariance function is estimated to be the same as Figure 3b.

A feature typical of the samples investigated was the presence of large amounts of very small

o particulates (of the order of 1000 A or less in size). Some of these particles were very tenaciously

bound to the surfaces and could not be removed using dry nitrogen and a low-energy electron gun. This

cleaning process was the only one used on the conventionally polished and flame polished samples. Since

the particles were not removed by this treatment their statistics were examined. Figures 2b and 3c

show the measured (histogram) and fitted (Gaussian) distributions of the particles on these samples.

The other four samples were cleaned in soap and water, and most of the particles were removed. Parts

"c" of Figures 4-7 for these samples show typical Talystep surface scans. Note that several such scans were made on each sample and that the data were averaged to obtain the height, slope, and autocovariance

functions. Note also that the scan for the etched sample shows not only the background surface but also

one of the deep holes present in the surface.

Surface scan raw data consisted of 17,000 Talystep data points per scan length of 1.037 mm. The

resulting data-point spacing of 0.061 pm is much smaller than the typical spacing for FECO (2 pm) and

TIS (1 mm) measurements. That is, the Talystep stylus probes the surfaces in much finer detail than

do the other standard measurements. Each surface was scanned several times at various surface locations, and stylus pressures were kept light enough to avoid leaving tracks. The data were analyzed

by computer program and then plotted.

Conclusions

For the flame polished and conventionally polished samples, the height and slope distribution

functions were not Gaussian. Surface statistics were dominated by many strongly-bound particles,

although the background surfaces were extremely smooth. The overall distribution functions were considerably skewed by the particles, which themselves exhibited non-Gaussian height distributions.

o The majority of particles on the conventional sample were 20-30 A high, with the largest measured

o o o particle being 205 A. For the flame polished sample the corresponding numbers were 30-55 A and 906 A.

Although the particles could not be blown off, they apparently were easily removable using soap

and water. This technique was successfully applied to the other four samples.

The bowl feed polished sample was apparently free of particles, scratches, and surface blemishes

under Nomarski; but it exhibited surface scan characteristics of fresh feed surfaces (i.e., it cono tained considerable structure) . The height distribution was skewed because of small (100-500 A)

particles, although the slope distribution was Gaussian. The Talystep and TIS roughness values were

comparable (probably because of the particles), and there was little long-term waviness in the surface.

-158-
The Si02 overcoated sample was extremely smooth and Gaussian in its statistics. The surface

appeared to have no defects under Nomarski, and it exhibited the same character as the best bowl feed

polished samples. Although the surface was very smooth and showed very little long-term waviness, the

fact that its damage threshold was about half that of the conventional sample or the flame polished

sample clearly indicates that thin films introduce significant laser damage susceptibility at an optical

surface.

The Xenon ion polished sample had nearly Gaussian statistics and was flat but very rough. Talystep

roughness was significantly greater than TIS roughness and is probably explainable by the height variao tions being too closely spaced to be resolved by the wavelength used (5682 A) . Holes and bumps were of

o the order of 200-400 A in height or depth. There was also little long-term waviness.

The etched sample was very rough, primarily due to large numbers of holes in the surface. These

holes were as much as 1.5 ym in diameter and as much as 1-2 pm deep. However, the background surface

was fairly smooth. The holes clearly skewed the height and slope histograms.

Except for the overcoated and ion polished samples, the statistical distributions were definitely

not Gaussian. It is clear that the type of surface finishing technique used strongly influences the

structure and light scattering properties of the surface of a given type of optical glass. No definite

correlations have yet been found between laser damage threshold and rms slope or correlation length;

however, the sample space so far examined is quite small. Future measurements presently underway on other samples of a given type of surface treatment may yield such correlations.

One additional observation is warranted. If one normalizes damage thresholds to those characteristic of conventionally polished surfaces, then later treatments such as flame-polishing and acid or

ion etching afford more damage resistant surfaces while those other techniques, e.g., bowl-feed and

overcoating generally lead to increased damage sensitivity. This certainly appears to be the rule

of thumb for coatings.

In any event, considerable additional work is needed to fully describe this most complex situation

before we can predict and quantify the appropriate surface character /damage threshold relationship.

References

[1] House, R. A., II, Bettis, J. R. and [2] Bennett, J. R. , and Elson, J. M. , this

Guenther, A. H. , IEEE J. Quant. Elec. proceedings.

qE-13 , #5, 361-363 (May 1977).

Figures

.5 -

1.0 -

o.s -

^1 •

1 "—I ® FMDIN (lUUI) O CONVENTIONAL • FUm POLISH

ETCH

+ SIO2 COAT X HgFj CO«T

^> loH Polished

-0.5 -

0 -

2 3 4 m SURFACE muSHNESS

Fig. 1. Laser damage threshold vs. rms roughness

for 40.5 ns, 1.06 ym irradiations on fused silica.

Details of the sample preparation technique is

discussed in reference 1. Briefly, etch refers

to a dilute HNO2 and etch and Si02 and MgF2 coat refer to X/2 at 1.06 single film layers on substrates whose roughness is indicated on the

abscissa. The points indicated in the endorsed

diamond correspond to the samples examined in

this study.

-159-
SLOPE

DlSTRIttUTIOW FU.«TION

RMS Slope - :08.30 X/.1 n

To To SLOPE (X/O.Obl un)

HEIGHT

DISTRIBUTION

FUNCTION

RMS Roughneis TS.3S I

0-045 a;

DISTA,^CE FROM MtA.. SURFACE LtVEL (A)

Fig. 2a. Height and slope distribution: flame

polished sample

.

SLOPE

DISTRIBUTIO..

FU.^Cl 10,^

RMS Slope 05.1'j4 V .i

HEIGHT

DlSTRIBUTIO:>i

RMS Roughness - 10.27 S

0 10 20 -2S 0 2S SLOPE (X/0.061uBi) DIST. FROM MEAN SURF. LEVEL (X)

Fig. 3a. Height and slope distributions: conven- tional sample.

Fig. 3b. Autocovariance function: conventional

sample

.

-160-
Fig. 3c. Particle height distribution:

conventional sample.

Fig. 4a. Height and slope distributions: bowl

feed polished sample.

-161-
Fig. 4b. Autocovariance function: bowl feed

polished sample.

Fig. 4c. Typical surface scan: bowl feed

polished sample.

Fig. 5a. Height and slope distributions: SiO^-

overcoated sample.

-162-
: 4JTQC0VM>AHCf IWICImt 1

...

1

1 1 1 1

1

Fig. 5b. Autocovariance function: Si02-

overcoated sample.

1 —i

- !

lay

i

i

1 1 i 1

Fig. 5c. Typical surface scan: Si02-overcoated

sample.

1

15" *vum

i11

i N

—i- M-l

t f

1 —+— -i

1 1

! —8 —

h

1

i

1

1

.—

m

1 —1

i N

i

- =

| ij 1

'

1

1 1 ._ ML lb1 1 i +i -

- 1—

_

Fig. 6a. Height and slope distributions: Xenon

polished sample.

-163-
Fig. 6b. Autocovariance function: Xenon polished

sample.

>ll..lCi

! 1 1

Ml

-.1 ,. i

> - ..il- 1

i

'

Fig. 6c. Typical surface scan: Xenon polished

polished sample.

Fig. 7a. Height and slope distributions;

etched sample.

-164-
Fig . 7b. Autocovariance function: etched

sample

.

Fig. 7c. Typical surface scan: etched sample.

-165-
THE EFFECTS OF SURFACE ROUGHNESS ON 1064-nni, 150-ps LASER DAMAGE*

D. Milam, W. L. Smith, and M. J. Weber

Lawrence Livermore Laboratory

University of California

Livermore, California 94550

and

A. H. Guenther, R. A. House, and J. R. Bettis

Air Force Weapons Laboratory

Kirtland Air Force Base

New Mexico 87115

We report the first measurements of the influence of surface roughness on surface damage

thresholds for damage induced by subnanosecond 1064-nm laser pulses. Samples included carefully

characterized bare polished silica surfaces and glass surfaces which were used as substrates

for antiref lection coatings. Where possible, data is correlated with earlier work performed

with 40-ns, 1064-nm laser pulses.

Key words: Breakdown field; coating substrate; damage morphology; fused silica; Normarski

micrographs; pulse duration; surface damage threshold; surface roughness.

Introduction

Polished bare surfaces on optical components damage at flux levels below that which causes bulk

damage. Furthermore, exit surfaces usually damage more easily than entrance surfaces, provided the

damaging beam is collimated and normally incident on the sample. Serious efforts to understand the

vulnerability of polished surfaces began after Crisp, Boling, and DubS successfully explained the lower

threshold for exit surfaces. They showed [1] that the optical electric field strengths at the exit

surface were greater than those at the entrance surface because the wave reflected from the exit surface

is in phase with the incident wave as opposed to the entrance surface where the reflected wave is out

of phase with the incident wave.

Bloembergen [2] extended the notion of enhanced electric fields by calculating the local field

strengths near dielectric discontinuities whose shapes were similar to the surface disruptions which

result from polishing. The electric fields near grooves in a material with index of refraction n can

2 be n times greater than the field in the undisturbed material itself. Local intensities, which are

4 proportional to the square of the electric fields, could be enhanced by n . Accordingly, surface damage

thresholds, irrespective of the mechanism, should be reduced by surface irregularities. The exact

magnitude of threshold reduction would depend on whether the mechanism was linear or nonlinear, and on whether heat or electrons could diffuse away from the high-field regions during the pulse interval.

o Bloembergen estimated that defects less than 100 A in size would be unimportant because of electron

diffusion.

Two subsequent experimental studies produced disagreement as to the universality of Bloembergen'

s

model as an explanation of laser damage. First, Fradin and Bass [3] reported that the damage thresholds

of very smooth surfaces on fused silica and BSC-2 glass were equal to that of the bulk material. Second,

Boling, Ringlien, and DubS [4] demonstrated that chemical contamination could reduce the threshold even

on very smooth surfaces, and that leaching processes could yield rough surfaces with good damage

resistance.

The most comprehensive study to date on the effect of surface finish on surface damage threshold

was that conducted at the Air Force Weapons Laboratory (AFWL) by House, Guenther, and Bettis [5]. This

work included surfaces prepared by conventional polishing, bowl-feed polishing, ion polishing, flame

polishing, and acid etching. Damage tests were made on bare and coated surfaces using 40-ns, 1064-nm

-2 pulses focused to a spot size of = 150 \im at the e intensity level. It was invariably found that the

damage threshold increased as the quality of the finish improved. One obvious result was that cleaning

of the surfaces by flame, polishing, and ion or acid etching markedly improved the damage threshold at

a given surface roughness. This was probably due to the removal of easily ionized surface impurities.

The excursions of damage thresholds observed in that study are the largest ever reported in a single

o material. For example, reducing the roughness on conventionally polished fused silica from 335 A rms

° 2 to 15 A rms increased the damage threshold by a factor of 6.3 in breakdown field strength, or (6.3) =

40 in intensity.

*The work at the Lawrence Livermore Laboratory was performed under the auspices of the Materials Sciences

Programs of the U.S. Energy Research and Development Administration and Contract W-7405-eng-48.

-166-
A recent study [6] of surface damage induced by 150-ps, 1064-nin pulses suggested that surface

roughness might be less important in determining short-pulse thresholds. Near-threshold damage with

1064-nm, 125-ps pulses created a randomly distributed array of micropits. The initial surface

morphology was preserved intact, and the damage pits were only weakly associated with residual

polishing artifacts such as sleeks. Even when fresh scatches having widths of approximately 1-10 ym were scribed on polished surfaces, damage thresholds in the immediate vicinity of the induced scratches

were at least one-half as large as the threshold for undisturbed areas on the surface.

To quantify this apparent difference between the effect of surface roughness on damage induced by

40-ns and 150-ps pulses, replica samples from the AFWL AO-ns study were tested at LLL using 150-ps

pulses. Results of this comparison are reported below. In addition, measurements were made to determine whether substrate roughness was a dominant effect in damage to antiref lection films.

Samples

Four bare fused silica surfaces which had been prepared using a controlled grinding process to insure that each sample had approximately the same subsurface structure were tested. Different

roughnesses were obtained by removing the samples at various times during the final lapping. Barnsite

abrasives were used. Surface roughnesses reported here were determined by the FECO technique (Fringes

of Equal Chromatic Order) at the Naval Weapons Center.

Six 1102/5102 antiref lective (AR) films were also tested. Films of two designs were simultaneously

deposited on three substrates of varied roughness. Film deposition and measurement of substrate roughness were done by the Research Division of Optical Coating Laboratory, Inc. (OCLI) . Substrate roughness

was determined at OCLI by relative scatter measurements. Calibration of the scattering apparatus was

o done by testing a 10-A surface supplied by Dr. Jean Bennett of the Naval Weapons Center. The smoothest

o sample in the coating deposition test compared favorably with the 10-A rms sample. The roughest

o substrates had an estimated roughness of 100 A. These samples were part of a joint LLL/OCLI test of

the use of barrier layers with AR films. A description of this study by Apfel et al. is reported

elsewhere in these Proceedings.

The silica samples were dusted with a freon spray prior to testing. The smoothest silica sample

was washed with high-purity ethanol since it had been previously tested at 40 ns. Previously damaged

sites were carefully avoided during this experiment. The thin films were all cleaned using ethanolwetted strips of tissue.

Damage Experiments

Damage experiments at 150 ps were performed using an apparatus and techniques described in detail

elsewhere [6]. The laser beam was gently focused to spot sizes of 2-3 mm diameter at the e

"'" intensity

level. The beam was incident on the surface with measured roughness at 5° from the normal. Complete

spatial and temporal beam profiles were recorded on photographic film for each firing. The energy

density on axis was determined absolutely to within 7%. From the streak camera image, the pulse shape

and peak intensity was determined. The total absolute uncertainty in the intensity measurement is

± 15%.

In the 150-ps tests, three methods to detect damage were employed. (1) The sample was observed

during each irradiation. Near-threshold irradiation produced a faint, but visible, glow. (2)

Immediately after each firing, the site was inspected visually using a focused white light source. Since large areas were irradiated, the scattering from the few pits created by the laser was easily

visible, except for sites on the roughest sample in the set. (3) The final determination of damage

was made by Nomarski microscopy. The three tests agreed for all but the roughest sample. The surface

of that sample was so rough that scatter and microscopic examinations were of little value. The

40-ns experiments, performed previously at AFWL, are described in detail in Ref. 5.

Results

o Nomarski micrographs of damage sites on each of the silica samples are shown in figure 1. The 15-A

surface was free of the usual polishing sleeks when viewed at a magnification of 360 X. Sleeks could

o be seen on the 40-A surface. Damage on both of these surfaces consisted on randomly distributed

o o micropits. There was a weak correlation between sleeks and damage on the 40-A surface. The 140-A

sample was covered with raised polishing structure; damage was initiated at these structural features.

o We could not determine what specific features on the 350-A surface were related to the damage at threshold. There was no obvious difference in the microscopic appearance of the immediate surroundings

of damaged and undamaged regions of the sample.

-167-
Damage thresholds for the silica samples are reported in Table 1 in terms of energy flux, intensity,

and rms breakdown electric field strength. All beam parameters are those computed in air at the sample

surface; that is, the values have not been adjusted to account for the refractive index of the silica.

Damage thresholds of the four fused silica samples at both 150 ps and 40 ns are shown in figure 2. The primary feature of the earlier 40-ns data is the close agreement between the data and the functional

form

E , a = constant,

where a is the rms roughness and E^^^ is the rms breakdown electric field strength. Based on the limited

data available, it is not clear that the 150-ps data obeys a simple power law dependence on surface

o roughness since the 150-ps threshold of the smoothest surface (a = 15 A) is less than that of the sample

o o with 40-A roughness. It is obvious that a roughness > 100 A rms causes a reduction in damage threshold

at either pulse duration. But the total excursion in damage field strengths is much less for the 150-ps

data than for the 40-ns data. As regards Bloembergen' s hypothesis, it should be noted that surfaces with

o o an rms roughness of < 100 A can have topographical features >> 100 A.

Damage thresholds for the six antireflection films are shown in figure 3 as a function of relative

scattering intensity. In this case, there is no correlation between the roughness of the substrate and

the damage threshold of the film.

Table 1. 150-ps, 1064-nm damage threshold of fused silica as a function

of surface roughness.

Sample Roughness,

rms, A J/cm

Damage Threshold

2 GW/cm mV/cm

016 15 8.5 + 0 5 56 + 4 4.6 0.2

078 40 11 + 1 0 73 + 7 5.2 + 0.3

090 140 3.5 + 0 5 23 + 3 3.0 + 0,2

098 350 2.5 + 0 5 17 + 3 2.5 ± 0.2

Conclusions

o 1. Surface roughness greater than 100 A rms reduces the 1064-nm, 150-ps surface damage threshold of

conventionally polished fused silica.

2. Additional 150-ps tests will be required to determine whether a strong and convincing threshold

o vs. roughness correlation exists for the "smooth" surfaces (rms roughness < 50 A) on normally

available optics at 1.064 ym.

3. The 150-ps damage thresholds of Ti02/Si02 antiref lection films are not correlated with roughness

of the BK-7 glass substrates on which they were deposited.

Damage thresholds (MV/cm) at 150 ps are related by the fourth root of the pulse duration to the

thresholds found at 40 ns. Extrapolated breakdown fields at 40 ns agree with measured fields to within 30% for the smoother surfaces.

5. The extrapolation of Smith's [7] threshold for bulk breakdown at 30 ps in fused silica yields a predicted 150-ps breakdown threshold in air at the silica surface of 9.4 MV/cm. This, when compared

with the largest value observed here, 5.2 MV/cm, suggests that breakdown fields for surfaces are about half the bulk values. The average interatomic spacing (smoothest possible surface) for fused

silica is 3.57 A.

Acknowledgment s

We thank G. E. Murphy for his excellent technical assistance with the laser-damage measurements

and J. H. Apfel and E. A. Enemark of Optical Coating Laboratory, Inc., for their collaboration on the

substrate surface roughness—thin-film damage studies.

-168-
References

[1] Crisp, M. D., Boling, N. L. , and Dub6, G.,

Appl. Phys. Lett. 21, 364 (1972).

[2] Bloembergen, N. , Appl. Opt. 12, 661 (1973).

[3] Fradin, D. W. , and Bass, M. , Appl. Phys. Lett.

22, 159 (1973).

[4] Boling, N. L. , Ringlien, J. A., and Dubg, G.,

"Laser-Induced Damage in Optical Materials,"

in LcaeA-lnducdd Vamagi -in OpticaZ Uate/Uati

7974, A. J. Glass and A. H. Guenther, eds.,

Nat. Bur. Stand. (U.S.) Spec. Publ. 414,

p. 119 (1974).

[5] House, R. A., Bettis, J. R. , Guenther, A. H.

,

and Austin, R. , "Correlation of laser-induced

damage with surface structure and preparation

techniques of several optical glasses at 1.06 \im," in Loie/i- Induced damage, in Optical

MaJ:zniah>: ! 975, A. J. Glass and A. H. Guenthei

eds., Nat. Bur. Stand. (U.S.) Spec. Publ. 435,

pp. 305-320 (1975); also R. A. House, "The

effect of surface structural properties on laser-induced damage at 1.06 pm, " Ph.D.

dissertation. Air Force Institute of Technology, Wright-Patterson AFB, Ohio, 1975

(now available as report AFWL-TR-76-62

,

Kirtland AFB, NM 87117); also R. A. House,

J. R. Bettis and A. H. Guenther, "Surface

roughness and laser damage threshold," IEEE

J. Qu. Elect. QE-5, pp. 361-363 (1977).

[6] Milam, D., Appl. Opt. 16, 1204 (1977). I

[7] Smith, W. L. , Bechtel, J. H. , and Bloembergen,^

N., Phys. Rev. B12, 706 (1975).

Figures

Fig. 1. Nomarski photographs of damage induced on polished fused silica surfaces by 1064 nm, 150 ps

o laser pulses. a) Damage on a surface with roughness of 15 A consisted fo randomly distrio buted micropits. b) Damage on a 40 A surface. Finishing sleeks are visible, but there

is no strong obvious correlation between the sleeks and the damage induced pits. Damage

o on the 140 A surface occurred at rough features visible on sites which were not irradiated.

o Frames c) and d) show respectively damaged and undamaged sites on the 140 A sample.

-169-
Kb)
1(d)

-171-
Fig. 2. Surface damage thresholds of four fused silica surfaces as a function

of surface roughness for two different pulse durations.

-172-
8

CME

o

2

o

JZ

(/> 03

E 3

CD Q

2

I

t

±

All films simultaneously deposited

% Film design A

O Film design B

4 5

£n relative scatter

T

I

t

Fig. 3. 150-ps, 1064-nm damage thresholds of Ti02/Si02 anti-reflection coatings of

two designs as a function substrate roughness. Relative values of roughness were determined by relative scatter measurements. By comparison tests using

a sample of known roughness, the roughness of the smoothest substrate was

o inferred to be - 10 A rms.

-173-
LOW SCATTER FINISHING OF ASPHERIC OPTICS

W. P. Barnes, Jr., and R. R. McDonough

Itek Corporation

Lexington, MA 02173

Bowl-feed or settling slurry polishing operations are commonly used for low scatter

finishing of flat and shallow spherical optical surfaces. Aspheric surfaces are a powerfi

optical design option, but they do not permit the development of the closely fitting lap

and work surfaces required for effective bowl-feed polishing. Previous IR&D work at Itek

had indicated that short term polishing with colloidal silica and a non-woven poromeric 1; produced surfaces comparable to bowl-feed surfaces, as determined by scattered light and

1.06 \im damage threshold measurements.

To test the applicability of this approach for low scatter finishing of aspherics,

three f/2 paraboloids were given a short final polishing using colloidal silica and full

sized flexible laps. No degradation of surface figure was detected, and surface scatter

was reduced to values nearly equal to best practice in bowl-feed finishing.

Key words: Aspheric optics; laser damage; low scatter; optical fabrication.

(Manuscript not available at time of publication.)

-174-
LOW SCATTER

ASPHERIC FINISHING

• 12-INCH-DIAMETER, f/2 PARABOLOIDS

• FUSED SILICA, ULE, CER-VIT

• CONVENTIONAL HAND POLISH TO 1/2 TO 1

WAVE FIGURE

• ASPHERICITY IS 14.6 WAVES

• COLLOIDAL SILICA (SYTON, LUDOX) POLISH

• TWQ. 15-MINUTE RUNS

• ALUMINUM, 1/4-INCH CLOSED CELL

FOAM RUBBER, POLITEX LAP

Optical

Systems

-175-
CER-VIT

Interferogram Before Syton Polishing

Surface Deviations From True Paraboloid

(wavelengths at 633 nm) •

•

Before Aflcr

r^ component 0.45 0.38

Cylinder 0.14 0.03

Other 0.05 0.05

-176-
CER-VIT

o

Indicated Surface Roughness (A)

before/ Afkr Syton Polishing

Before .l/ur

Average 21.7 8.9

Standard Deviation 7.7 y.9

-177-
FUSED SILICA

Interferogram Before Syton Polishing

Surface Deviations From True Paraboloid

(wavelengths at 633 nm):

r^ component

Cylinder

Other

Before

0.83

0.29

0.07

Afki078

0.(6

oxn

-178-
FUSED SILICA

O

Indicated Surface Roughness (A)

Remove/After Syton Polishing

Before _A/h_

Average 26.9 //

5

Standard Deviation 4.2 4.3

-179-
ULE

Interferogram Before Syton Polishing

Surface Deviations From True Paraboloid

(wavelengths at 633 nm):

Before Afkr

r^ component 0.15 0.19

Cylinder 0.17 o.n

Other 0.03 003

-180-
ULE

o

Indicated Surface Roughness (A)

Before/ After Syton Polishing

Before Afttv

Average 38.9 iH.o

Standard Deviation 8.9 // /

-181-
NWC RESULTS

FOR CER-VIT

o

Indicated Average Surface Roughness = 14.8 A rms

(469 Points Measured)

-182-
MEASUREMENT OF MIRROR AND WINDOW CHARACTERISTICS FOR USE WITH 10.6 urn LASERS

S. Sharma, R.M. Wood and R.C.C. Ward

The General Electric Co. Ltd.,

Hirst Research Centre,

Wembley, England.

The absorption, reflectance and transmission characteristics of a range of

infra-red mirrors and windows have been measured accurately using apparatus

developed at the Hirst Research Centre based on a lOW CO2 c.w. laser. The above

characteristics have been related to the methods of fabrication (diamond turning,

polishing, etching, dielectric coatings) and to pulsed laser-induced damage

thresholds measured using a CO2 TEA laser, (60 ns FWHH)

.

The ref lectance/transmittance measurement technique will be discussed and the

results, together with the absorption data, correlated with the damage threshold

measurements. These latter measurements will in turn be shown to correlate with

surface quality (measurements made using optical and electron beam microscopy,

visible scatter and X-ray topographs) and with surface films or dielectric

coatings

.

Key words: absorptance; dielectric coated mirror; infrared laser windows;

laser mirrors; pulsed CO2 laser damage; reflectance; 10.6 um optical

conqjonents

.

1. Introduction

This paper is a continuation of the work presented last year [ 1 ]' when the programme of research on

10.6 ym laser induced damage thresholds at the Hirst Research Centre of The General Electric Company

Ltd. under DCVD sponsorship was outlined.

This paper reports work on the development of an accurate ref lectance/ transmittance measurement

apparatus and the correlation of these measurements with absorption, surface finish and laser induced

damage thresholds.

2. Ref lectance/Transmittance Measurements

A laser window or mirror can only be characterised fully when all the elements of the equation R+T+A+S= 1 have been measured accurately at the wavelengths of interest. As a logical sequence

of the development, use and experience gained with apparatus set up at HRC to measurance reflectance

and transmittance accurately at 1 pm it was decided to extend this facility into the 10 \im region in

order to back up the absorption measurements reported previously [l].

An apparatus has been developed capable of directly measuring both transmittance and reflectance to

a high degree of accuracy (i.e. ± 0.02%). This apparatus is shown schematically in figure 1 and

basically consists of a lOW CO2 c.w. laser, a chopper, beam splitter, two pyroelectric measurement heads

and associated electronics and ratiometric logic and display.

The c.w. laser, chopper and pyroelectric detector heads in conjunction with simultaneous digital

logic are used in order to facilitate the making of accurate measurements without the need for a highly

stable laser, optical bench etc.

The pyroelectric measurement heads are shown in figure 2. They consist of shot-blasted, gold

plated integrating spheres with pyroelectric detectors. This arrangement gives a spatial and angle

insensitive measurement. The outputs of these have been shown to be linear with respect to input

power.

The processing electronics are shown in figure 3. The beams are either directly compared, D/R, or subtracted (R-D)R, using a high quality analogue divider, depending on the relative values and

accuracy required.

The apparatus is first calibrated with the detector heads in positions R and D^ (see fig. I) with

no sample. The variable gain on the reference beam is changed until a signal ratio of 1.0000 has been

1. Figures in brackets indicate the literature references at the end of this paper.

-183-
sec. The sample is now put in the beam and the new signal (transmi ttance) read off (in the case of a window). The detector head D is now moved to Dj and the reflectance read off. Finally the same is

removed back to for a zero check. In the case of high reflectance mirrors the ratio (R-D)/R gives

even greater accuracy.

3. Absorption Measurement

The absorption apparatus previously described [l] has been further refined in order to give higher

accuracy and repeatability [2]. A schematic of the apparatus is shown in figure 4.

4. Damage Threshold Measurement

A schematic of the laser induced damage threshold apparatus is shown in figure 5. The laser is

sealed off CO2 TEA laser [3] with a 60 ns FWHH main pulse and is focussed to 100 ym 1 /e intensity

diameter using a 50 mm focal length germanium meniscus lens. The beam power may be varied either con- tinuously using a gas cell (propylene) or by means of thin polythene attenuators.

5. Surface Examination

Various techniques have been employed to characterise the substrate surfaces viz:

Optical microscopy - (up to 1000) using a Reichert MEF microscope with dark field illumination - a standard inspection technique on all samples.

Scanning electron microscopy - (up to x 60,000) - this technique has mainly been used on metal

mirror surfaces. The microscope has an energy dispersive attachment for detection of elements present

on the surface.

Visible scatter measurement - using a He/Ne gas laser probe and integrating sphere/photomultiplier

,

capable of making scatter measurements down to 0.005%. This has mainly been used to characterise and

evaluate dielectric coatings but has also been used with metal mirrors.

X-ray topography - used for evaluating the crystallographic perfection of the surface (controlled

penetration down to 20 ym for germanium) of single crystals.

Infra-red spectrophotometer measurements - (Perkin-Elmer Type 325 grating I-R spectrophotometer) on windows and dielectric coatings.

X-ray fluorescence measurements - analysing thin films and deposits on mirrors.

6. Metal Mirrors

The characteristics of a series of high quality metal mirrors have been measured. In order to eliminate variations in substrate quality etc. a number of mirrors were produced by single-point diamond turning OFHC copper blanks by Heliotrope Ltd. Some of these were tested bare, some were goldplated, by Mirror Techniques Ltd. and others were overcoated with ThFi^

The characteristics have been tabulated, see table 1, and correlation has been found between the

various surface measurement techniques and the laser damage thresholds. Table 1 shows the summary of

results made on two mirrors of each type, one as near perfect as has been obtained and one of slightly worse quality. These results illustrate the ways in which the surface quality affects the reflectance

and the damage thresholds of good quality mirrors.

The optical and scanning electron microscopy examination showed that the uncoated diamond turned

OFHC copper mirrors were of very good quality (no scratches visible by either technique). The SEM

micrographs (figure 6a and b) showed that the surfaces contain numerous very small shallow spherical

holes (approximately 500 A dia. , 250 A deep). These holes are typical of the diamond turning procedure. Under optical examination mirror number 2 exhibited a slightly patchy surface, attributed to a thin oxide layer. This slight difference between the samples is repeated in absorptivity, reflectance,

scatter and damage thresholds.

The gold-plated mirrors exhibited visible turning marks (not visible on unplated surfaces),

although the SEM micrographs (figures 6c and d) show that the plating procedure fills in the turning

'holes' and replaces them with a gently undulating surface. The height of these undulations is

estimated to be about 100 A. The visible scatter measurements were higher for these surfaces than for

the uncoated surfaces. Both gold-plated mirrors have higher absorptivity than the uncoated copper

mirrors in line with previous results [l]. Mirror 4 had a 'milky' looking surface and a higher absorptivity than mirror 3. The laser induced damage thresholds were shown to be uniform (~8 MW mm"^). The

lower damage levels for these mirrors (relative to the uncoated copper values of "lO MW mm~2) are due

to their greater absorptivities and are in agreement with previously measured results[l].

A ThFit dielectric coating on a diamond turned surface should, apart from microscopic coating

-184-
defects, allow the high reflectance and damage thresholds of uncoated copper to be realized whilst

giving protection from oxidation. In practice care must be taken both to coat the copper surface, as soon as possible after machining to stop a thin film of oxide forming, and also to coat with a thick,

enough layer to stop 'pinholes' penetrating through the protective layer. Table I includes results on mirrors 5 and 6, one of which developed a 'bloom' on the surface after exposure to air. The absorption,

reflectance and damage threshold values reflect the differences between the surfaces.

The composition of the surface film on degraded mirrors has been investigated and film thicknesses

of the order of 0.5 um have been examined using a scanning electron microscope fitted with an energy

dispersive system for the detection of elements present.

Oxide, nitrate and chloride layers have been identified. Figure 7 shows a SEM micrograph of a badly tarnished surface of a Be/Cu mirror. This layer was analysed using the energy dispersion

elemental analyser to be chloride and copper nitrate.

7. Window Materials

During the last year our attention has been mainly concentrated on measurements on CVD grown zinc

selenide and on surface improvement of germanium.

CVD grown zinc selenide from two sources (Raytheon and AWRE) has been tested. Table 2 illustrates

that both sources have reduced the absorption.

One of the principle areas of concern is the relative magnitudes of the possible surface layer

absorption and the bulk absorption.

Most of the surface absorption (of the order of 4 x 10"'* per surface for a diamond polished sample

[2!) may be attributed to a thin broken-up layer. This layer can be improved by polishing. Inspection

of figure 8 shows, for example, that measurements made on 1974 ex Raytheon ZnSe, after etch polishing

to a high standard indicated that the maximum surface absorption was less than 1.8 x lO""* per surface.

Other measurements [4] however, have indicated that it is possible to detect the effect of a water

layer on the surface and it has been suggested that one cause of discrepancy between measurements made

at different laboratories may lie in differences in relative humidity.

Inspection of the absorption and damage thresholds, table 2, indicates that the absorption is not

the dominating damage mechanism, the threshold being peak power density rather than energy density

dependent. The disappointing factor in this improvement is that the damage threshold has not improved

markedly over the years. This is presumed to be because of the microscopic structure of the ZnSe

(X-ray measurements show that the crystallite sizes are typically 20 um (max. 50 um) on samples from

both sources)

.

The relationship between surface finish and laser damage threshold has also been sought for

germanium. Previous results [l] indicated that the laser-induced surface damage threshold for 'Syton'

polished germanium was low and variable (0.7 to 4 MW mm~2). When ion-beam etched the damage threshold was raised to between 3 and 5 MW mm~2. SEM micrographs indicated that the etched surface looked

marginally better. X-ray topographs have been taken of the etched and unetched surface layers. Several

reflection topographs were obtained, using different Bragg reflections to vary the angle of incidence of

the X-ray beam with the crystal surface. These give differing pictures depending on the penetration

depths of the X-rays. The X-ray topographs reproduced in figure 9 show the difference between etched

and unetched crystal using two different Bragg reflections. Figure 9a is a 311 topograph (i.e. Bragg

reflection from {311} planes, penetration depth ~0.5 um) and figure 9b is a 511 topograph (Bragg

reflection from {511} planes, penetration depth ~10 um) . The surface etching was of the order of 1 to

2 um.

The unetched area (9b) shows a large number of polishing scratch images (not visible using an optical microscope) superimposed on images of grown-in dislocations. The etched area shows the con- tinuation of the strain associated with the most severe scratches superimposed on the dislocation

pattern.

In contrast to this the unetched area in Figure 9a, looking at the top 1 um on the surface of the

crystal, shows a greater scattering of the X-ray intensity than the etched area. The mottled appearance

of the etched area indicates that the surface is now no longer flat. The dislocation images are still

visible in this area but have a much reduced contrast compared with the 511 topograph (9b). The dislocation images are not visible in the unetched layer. Careful interpretation of the X-ray topographs

indicates that the unetched, Syton polished, surface is disturbed and a broken-up, physically deformed

layer (rather than amorphous) of about 0.5 um depth exists. Further investigation is being made into

the optimum technique of polishing/etching in order to arrive at a single crystal, flat surface.

8. Conclusions

-185-
The absorption, reflectance ana cransm ssion characteristics of a range of infra-red mirrors and

window materials have been measured and related to the methods of fabrication and to measured pulsed

laser-reduced damage thresholds. The mirror characteristics have been shown to correlate with surface

quality and surface films.

9. Acknowledgement

This work has been carried out with the support of Procurement Executive, Ministry of Defence,

sponsored by DCVD.

10. References

[l] Gibbs, R and Wood, R.M. , Laser-induced damage

of mirror and window materials at 10.6 ym. Proc. 8th Symp. on Laser Induced Damage in

Optical Materials. NBS Special Pub. 462, 181,

(1976).

[2] Gibbs, R and Lewis, K.L., Laser calorimeter

for the determination of the absorption coefficients of thin 10.6 ym windows (to be

published J. Phys. E.).

[3] Stark, D.S., Cross, P.H. and Foster, H. , IEEE

J.Q. Elec, QE.11(9), 774, (1975).

[4] Lewis, K.L. , (private communication).

Table 1. Characteristics of metal mirrors

Mirror

Percentage

Absorption

% A (3 10.6ym

J

Percentage

Reflectivity

% R @ 10.6ym

Percentage

Absorp tion

% A (a 0.63ym

Damage Threshold

MW mm-2

!

i

i Comment s } Defect 1 Site

Clear

Area

Di amond- turned

OFHC copper (1)

0. 7

i 99.2 ^ 0.27^.47

1 _ 1 1 Very good visual quality.

500 fi dia. , 250 g deep

' turning holes '

.

Diamond-turned

OFHC copper (2)

0.9 99.0 0.50*0.69 7.6 8.7^9.8 Patchy surface.

500 a dia. , 250 8 deep

' turning holes '

.

Diamond- turned

gold-plated (3)

OFHC copper

1.0 98.9 0.75^.81 7.8 Turning marks. Gently

undulating surface,

100 ft height.

Diamond- turned

gold-plated (4)

OFHC copper

1.2 98.7 0,59^.82 4.2 8. 1 'Milky' looking surface,

turning marks, gently

undulating surface,

100 S height.

ThFii overcoat

diamond turned (5)

OFHC copper

1. 37 98.5 1.0-* 1.6 2.0*3.4 3.5->-5.0 20 ym diameter coating

defects, 'bloom' on

surface.

ThFi^ overcoat

diamond turned (6)

, OFHC copper

1.241

i

i

98.8

1

' 0. !-> 0.5 3 >1 1 Very good mirror with

no visual defects.

Table 2. Absorption and damage thresholds for ZnSe

Source

r Date

i

Absorption Coefficient

cm" 2 Damage Threshold

MW mm-2

Raytheon 1974 4.9 X 10-3 3.2-^7.5

19 74 4.9 X 10-3 1 .5^3.8

1974

1 4.9 X 10-3 3.2^5.5

! 1976 2.1 X 10-3 3.5^7.0

AWRE 1 1975 20 X 10-3 0.8-^4.5

! 1976 5.0 X 10-3 2 . 2->-4 . 7

i 1977

I 1.6 X 10-3 3. 6^1 \. 2

-186-
-107-

PYROELECTRIC

DETECTOR

HEADS

AMP

T

6AIN

BAND

PASS

FILTER

RECTIflERl

D'

(pT-EOxG

AMP

BAND

PASS

FILTER

RCTIFIER

R'

D7 R'

(r^-dQ xg

R'

DIGITAL PANEL

METER

Figure 3. Reflectance measurement processing electrons.

He-Nc LASER

PERSPEX

BEAM DUMP

THERMAL

INSULATION DISC

CALORIMETER

ABSORPTION

CALORIMETER

SAMPLE

CO2 LASER

Ge.PHOTOOIODE

(o/p TO C.RjO.)

Figure 4. Absorption measurement apparatus.

-189-
V

CO

O

OO

to — O

£ o

> 2:

CO

5-1 nJQ-Cu

CO

+J

0)E

OJ

CO

0)

cubO

CO6

CO t3

CUCO

p.

O

o

•H

•P

CO6

0)o

<u3

•H

-190-
(c) & (d) Gold-coated, diamond-turned OFHC copper mirrors.

Figure 6. Scanning electron micrographs.

-191-
-192-
-193-
(a) Surface topograph [0.5 ym penetration]

unetched and etched (X30)

.

(b) Topograph [10 ym penetration] unetched

and etched (X30)

.

Figure 9. X-ray topographs of germanium mirror.

-194-
COmEHTS ON PAPER BY 5WARMA, WOOV, AND WARP

Thz ipejxksA Indicate-d that thz dtpth of, the. piti ieen on the. icdnninQ dLectAon mtcAoicopz p-tcXuAeA wcu, e^tunated by mmMiAlng the. angle Ojj the ihadow cMt into the hole, and that thil, meaiuAement technique

Moi good to about 10%. He wa6 oMked about the obie/ived damage leveZ od the gejmanium 4uA|$ace. He Indicated that fiecetved, the ge/mantum iiuA^aceM damaged at about 4 jOuleM pen. iquoAe centMneteA,

white a^teA etching, the damage leveZ Moi Kotied to about 25 jouJiei pen. iquoAe centimete/i.

-195-
POLISHING SINGLE POINT DIAMOND TURNED MIRRORS

R. E. Parks, R. E. Sumner and R. E. Strittmatter

Optical Sciences Center

University of Arizona

Tucson, Arizona 85721

A method of removing the high frequency microripple from single point diamond machined mirrors

has been developed. The process uses conventional

hand polishing techniques and laps of sufficient

rigidity to bridge the microripple yet soft enough

not to scratch the metal mirror surface. Substantial

material must be removed from the mirror surfaces in some cases and the surface figure may be degraded

unless careful testing is done during the process.

We present photographs showing the reduction in

scattering and improvement in mirror surface rough- ness as the result of the polishing. Phase-contrast

microphotographs also show that the polished surface

is almost as free of mechanical defects (scratches

and pits) as the original virgin turned metal surface.

Key words: Metal mirrors; microripple; polishing;

scattering; single point diamond turned mirrors.

1. Introduction

We present a series of pictures of 400mm diameter single point diamond turned copper off axis

parabolas before, during and after polishing to remove the microripple. Before polishing, the

scattered light in the visible is sufficiently intense to make proper alignment difficult to impossible. Double pass interferograms contain little useful information.

After polishing, the image forms a tight core, a double pass interferogram is easy tc interpret

and a microphotograph of the surface shows the absence of turning marks.

2. Test Methods

In order to assess the results of the polishing procedures, several methods of testing the off

axis parabolas were used. A point source microscope was used to examine the image formed by the

mirrors while a modified Fizeau type interferometer was used to measure the surface contour. In order to study the surface on a smaller scale, a Leitz metal lographic microscope with a lOX differential interference contrast objective was used.

Figure 1 shows the parameters of the mirror and its optical test. The focus of the test instrument is placed in coincidence with the focus of the parabola. The light, after being collimated by

the parabola, is autoreflected by an optical flat placed perpendicular to the optical axis of the

parabola. After a second reflection in the parabola the light returns to the original focus after

suffering twice from the defects in the surface of the parabola.

In order to examine the return image from the parabola, a point source microscope was placed

with its objective focus at the focus of the parabola. Figure 2 shows the design of the microscope

and the position of the camera used to photograph the image. A 20X objective was used to insure

filling the f/1.9 cone angle of the mirror. Illumination was provided by an optical fiber drawn

down to a diameter of 0.3mm and bent parallel to the axis of the microscope. The image size projected by the 20X objective was ISym.

In figures 4(a,b,c) the upper spot of light is light back scattered by the fiber and is not representative of the source size itself. The lower spot of light (elongated horizontally in figures

a and b) is the return image from the mirror at several stages of polish.

In order to assess the surface figure of the mirror, a modified Fizeau type interferometer

was used as illustrated schematically in figure 3. In this application the advantage of this interferometer is that the f/1.9 acceptance cone of the parabola can easily be filled without resorting

to a rather expensive fast diverging lens required by some interferometers. The pinhole spatial

filter in the interferometer as reflected in the beamsplitter cube is placed at the focus of the

parabola. Interference is then obtained between the return wavefront from the parabola and the

reflection off of the spherical reference surface on the beamsplitter cube. The fringes may then

be photographed directly using a 3Smm camera equipped with an f/1.5 or faster lens or by projection

through a large Erfle eyepiece onto Polaroid film. The results of this test procedure with the

-196-
interferometer are shown in figures 5(a,b,c). The improvement in fringe representation is a result

of polishing the mirror.

In order to examine the surface of the mirror itself, a Leitz metallographic microscope was

used. The microscope was equipped with a lOX differential interference contrast (Nomarski type)

objective. Results of this examination are shown in figures 6(a,b,c) all to the same scale.

3. Polishing Procedure

Before discussing the process worked out for polishing the microripple from these mirrors a possible misunderstanding should be clarified. The impression has perhaps been given that these

mirrors do not represent the state-of-the-art or are in some way much worse in terms of microripple

than can be generally expected of diamond turning. In fact these mirrors do represent something

very close to the state-of-the-art for what these mirrors are; namely they represent a parent

parabola of f/0.37 some 2 meters in diameter with a mass in the neighborhood of 300kg. This piece

was turned on a modified Excello N/C lathe accurate to 1 0.4ijm of the desired average contour over

an annular band 40cm wide. Components smaller in si-ze and whose contours are not aspheric have

vastly superior surface finishes. Small spheres and flats generally look as good and often much

better than can be obtained using conventional polishing techniques.

These remarks regarding the polishing of these mirrors are intended to illustrate that methods

exist to improve the quality of large aspheric mirrors which suffer from scattering in the visible

rather than degrading in any way the extremely powerful technique that diamond turning has shown

itself to be. In fact we consider the turning and polishing processes to be complimentary techniques

rather than competing ones. The polishing process we have developed is a two step procedure; the first removes the microripple with a firm or stiff lap but somewhat degrades the cosmetic appearance of the surface, the

second step restores the cosmetic appearance to that obtained from the turning lathe but less the

microripple.

The stiff lap was made by melting a quantity of beeswax and straining the same through cheesecloth. To the molten wax, an equal volume of 0.3pm alumina (Linde A) was added and thoroughly

mixed. If a lesser quantity of alumina is used, the mixture may be too thin to stay mixed and the

compound will settle out. The mixture is then poured onto a lap base; we used a 10cm diameter base.

After the mixture has cooled, the lap is beveled and channeled to give 2.3cm facets. The lap

is then softened by moistening with kerosene. After 5 minutes or so the lap is ready for use. We found that if the wax had been softened with a small quantity of turpentine when it was molten, the

lap would be almost soft enough that it could be used on copper with soapy water as a lubricant

without scratching. Extreme care was needed with this lap to avoid damage. This is why we simply

turned to kerosene as a lubricant. The lap slowly dissolves in use but it takes real effort to make the lap scratch copper deeply.

Before applying the lap to the work, the mirror should be carefully beveled with a clean, fine

file and the bevel polished out using Pellon or felt and Linde C. The risk of picking up a burr

from the edge is too great to forget this step. After beveling, the filled wax lap is rubbed perpendicularly to the turning marks using a generous amount of kerosene for lubrication. The slurry

should turn black immediately indicating a good cutting action. To judge progress, simply flush the

surface with clean kerosene. The surface will appear scuffed or lightly brushed and the evenness of

polishing is indicated by the uniformity of the brushed appearance. This surface degradation has no particular depth and will polish out quickly during the second step of the process.

The idea of a lap that slowly dissolves may not sound like a good idea at first but it has

several definite advantages. For one, it is very soft, something necessary for a metal as soft as copper. In addition, being an oil, the kerosene keeps the metal from oxidizing allowing one to leave

his work for a time without the worry of corrosion or etching. Further, one of the easiest ways

of picking up a scratch is for some of the metal that has been polished off the mirror to cake up

on the lap. This is avoided because there are no hard surfaces on the lap for the metal to affix

to. Finally, the lap continues to supply fresh compound at a uniform rate as the lap is used. We have found the alumina filled wax lap and kerosene to be a very successful method for removing

the microripple in diamond turned mirrors.

Once the ripple is reduced to the desired level and the figure is satisfactory, we proceed to

the second step of the process to restore the original brightness to the copper surface. First clean

up the surface by a thorough flushing with kerosene followed by acetone. It is difficult to completely

remove the wax film. Don't worry and don't be tempted to wipe it off with any sort of physical means. This usually spells disaster. Any remaining wax film will come off during the second step. For a lap we use 2 to 4mm thick open cell polyurethane sheeting which we obtain from upholstery

or fabric shops for something like $2 a square meter. Cut the sheeting into a piece 30cm square and

wash thoroughly with filtered water. The least contaminant at this point can ruin everything. Make

a paste of Linde A, dishwashing detergent (Liquid Ivory, etc.) and water. Make a ball of the foam

sheeting, dip into the paste and uniformly scrub the mirror surface. Use mild to moderate pressure

and a buffing motion. Too great a pressure will eventually scratch, too light a pressure will take

too long.

Very quickly the slurry will turn black indicating material removal. Continue a uniform polishing action while keeping the surface wet with additional filtered water. To inspect progress, flush

-197-
the surface with well filtered water. A clean piece of the polyurethane sheeting thoroughly wet may be used to help with the clean-up. The remarkable thing about the polyurethane sheeting is its

ability to be used as a cleaning cloth on soft copper without scratching.

Discretion must be used when polishing with the urethane sheeting. Ideally, polishing should

be continued until the brushed appearance from step one is gone but no farther. The second step

should not be used for removing heavier scratches but rather the first step carried on longer until

a uniform brushed surface is obtained with no heavy scratches. The problem with using the foam

sheeting too long is the same problem that arises whenever a "soft" lap is used, the softer areas

of the surface polish faster than the harder areas. This leads eventually to an orange-peel surface.

Before reaching this stage the copper begins to show evidence of low level wide angle scatter. A high intensity light focused on the surface will show a haze developing on the surface if the foam

is used too long. The only solution then is to go back to the soft filled wax lap to get a smooth

but brushed surface and then repeat the foam lap treatment for a shorter time.

If the wax lap finish on the surface is well done to start with, the finish obtained with the

foam lap can be spotless, not a sign of scratches or pits. The finish or specularity is at least

as good as the virgin turned surface except now of course all the ripple and consequent scattering

are removed.

After finishing with the foam lap, wash the mirror thoroughly (including all the edges) with

filtered water followed by distilled water. The water should bead off the clean surface. Then

flush with CP acetone or CP methanol and blow dry in a sheet with dry nitrogen. This final cleaning

step is important so that no water or other residue is left on the copper to potentially stain or etch the surface.

4. Results and Conclusions

Now that the polishing process has been explained we would like to return to the photographs

obtained during the testing of various stages in polishing. The photographs in Fig. 4 show the

reduction in scattering as the microripple is removed. Fig. 4a shows a mirror just as it came

from the SPDT lathe. The light, in fact, scattered far off the illustration, this being the central

third. The herringbone pattern is caused by the deeper of the turning marks. Because these mirrors

are off-axis sections, the turning marks are roughly unidirectional and thus the scattering is principally perpendicular to the grooves as in the case of a diffraction grating. This preferential

scattering is particularly bothersome in the visible because astigmatism is the principle aberration

caused by misalignment of an off-axis parabola. The astigmatism also causes a lengthened image and

it is very difficult to separate the effects of the scattering from the astigmatism due to misalignment. This makes it all but impossible to line up a SPDT off-axis section if the microripple

is too severe. Fortunately, the polishing procedure very quickly corrects this problem, the finest microripple

structure being the first to polish out. Fig. 4b shows the return image after polishing one of the

400mm copper mirrors for about an hour with the filled wax lap. Although the image is still con- siderably longer than it is high, the herringbone pattern is gone and there is a definite although

elongated core. After another 3 hours polishing with the wax lap and then cleaning up the surface

with polyurethane foam sheet, we get the image shown in Fig. 4c. Here the image is 3-4 times the source size of ISym and is in fact geometrically small enough that this mirror will give diffraction

limited performance at 10.6um.

The interferograms in Fig. 5a through c show the same mirror at the same stages described

above. Again in Fig. 5a notice the difficulty in trying to sort out astigmatism from other effects.

Even the interferometer is not useful for alignment purposes when used double pass off of the parabola. After the high frequency ripple has been polished off, the fringes become readable with some difficulty in Fig. 5b. Residual turning marks are still easily apparent in the surface of the

mirror as seen in the fringes. At the completion of the polishing in Fig. 5c the surface appears

smooth with only the slightest indication of residual ripple at the edges of the mirror.

The microphotographs in Fig. 6a through c show equally dramatic changes in the surface of SPDT

mirrors. Eig. 6a shows the untouched mirror while Fig. 6b is after 1 hour of polishing. On a small scale all the ripple is gone, there being just a hint of occasional residual ripple. Following further polishing including work with the foam lap, the surface looks almost as homogeneous as the turned except that the ripple is gone.

In conclusion, we have developed a method of removing the microripple from single point

diamond turned surfaces in an easily performed, conventional manner. The method leaves the surfaces

cosmetically as clean and bright as the originally turned surfaces yet free from the scattering

caused' by the microripple. Although the figure of the surfaces can be changed by the polishing,

carefr.l monitoring and correction can be used to avoid figure degradation and in fact if need be the

figr.re can be improved.

Acknowledgments

We would like to acknowledge the help of useful discussions with Drs. J. C. Wyant and Roland'

Shack and with R. C. Shagam.

This work was partially funded by LASL Contract #NP6-46285- 1

.

-198-
Reference

[1] Shack, R. V. and G. W. Hopkins, "The Shack Interferometer", SPIE Proc , Vol. 126, "Clever

Optics", 1977.

Figures

Figure 1. Parameters of the double pass autocollimation

test of a 400rmn diameter parabolic segment.

Figure 2. Schematic diagram of the point source microscope used to examine the return

image from the off-axis parabolic

segment. The source is a fiber optic

cane drawn down to a 0.3 mm point.

-199-
Camera 7

Beamsplitlei ^

cube

Microscope

Pinhole

Splierlcal reference surface

Figure 3. Schematic diagram of the Shack interferometer used to measure the wavefront errors produced by the off-axis

parabolic segments.

-200-
I

Figure 4. Return images off of parabolic

segments as photographed using

the point source microscope.

The upper light spot is scattered

light from the source itself,

a) Very elongated return image

from an unpolished SPOT off-axis : parabola, b) Return image from

the same mirror after removing

the majority of the microripple

using a Linde A filled wax lap.

The polishing time was about one

hour.

A(a)

4(b)

4(c)

-201-
Figure 5. Double pass interf erograms obtained

with the Shack, xnterterometer showing

the mirror surface ripple as a function

of polishing time, a) Typical surface

irregularity of an unpolished SPDT

off-axis parabola, b) Same mirror

surface after one hour of polishing,

c) Same mirror after the completion

of the second step of polishing.

5(a)

-202-
Figure 6. Microphotographs of the

surface of a SPOT copper

off-axis parabola at different polishing times. A lOX differential interference contrast objective

was used, a) A virgin

SPOT mirror. b) Surface

after polishing one hour

with filled wax lap.

c) Surface at the completion of polishing.

6(a)

6(c)

-203-
PULSED-LASER STRESS PHENOMENA ON HIGHLY REFLECTING METAL AND ALLOY SURFACES*

J. 0. Porteus, C. W. Fountain, J. L. Jernigan, W. N. Faith, and H. E. Bennett

Physics Division, Michelson Laboratories

Naval Weapons Center, China Lake, California 93555

Modification of bare metal surfaces by laser-induced stress has been observed

well below the threshold for melting with 100 nsec, 10.6 ym laser pulses. Resulting optical and mechanical properties may be very significant for the performance

of laser-optical systems and for structural components under stress. A multithreshold approach to damage testing is used to interrelate phenomena associated

with different damage mechanisms, and to help establish a relationship to material

characteristics. In general, the threshold for plastic deformation as indicated

by intragranular (banded) slip was found to increase with crystalline disorder,

while thresholds for melting and other types of catastrophic damage decrease. The

melt threshold calculated from heat-flow considerations is in reasonably good

agreement with experiment on a clean, annealed, single-crystal Cu target. Possible effects of elastic and inelastic strain on the melt threshold are considered.

Under certain conditions the target plasma may influence the observed slip via

shielding, "scrubbing" or compression. Transient reflectance data, singlecrystal slip-band patterns and air breakdown thresholds support a discussion of

these effects. Finally, multithreshold results are presented for a variety of

metal and alloy surfaces prepared by different methods. Target preparation techniques are evaluated in terms of their apparent effectiveness in providing resistance to slip, melting and other forms of damage.

Key words: Al 7050; crystalline disorder; Cu mirrors; dislocations; laser-induced

stress; melt threshold; Mo mirrors; plasma scrubbing; plasma shielding; slip;

surface preparation; Ti-6A-4V.

Introduction

When a metal target is exposed to a pulsed laser a variety of damage-related effects begin to appear as the fluence is gradually increased. In general, the thresholds for the various effects can be

quite different, and depend on the material, the condition of the surface and the conditions of irradiation. Much basic information concerning damage mechanisms can be obtained from the relative and absolute magnitudes of these thresholds as conditions are varied. The means of obtaining this multithreshold information are discussed in another paper at this conference [I]-'-. The present paper deals mainly

with results we have obtained by applying the multithreshold approach to the thermal stress problem.

The first type of damage usually to appear when a high quality bare Cu mirror is exposed to 100 nsec, 10.6 \nn laser pulses is plastic deformation resulting from thermal stress. Normally this appears

as one of the two types of slip shown in figure 1: (a) slip bands which occur as a result of displacement along crystallographic planes and (b) intergranular slip resulting from displacement along grain

boundaries. Such effects often appear well below the thresholds for melting and other forms of catastrophic damage. The Importance of slip for mirror technology is that it roughens the surface, which

leads to increased scattering and increased absorption via the anomalous skin effect [2] . Laserinduced slip is also found in structural alloys where it may influence the failure of thin metal components subject to mechanical stress.

Slip Versus Melting

Figure 2(a) shows the damage profile [1] of a high quality Cu mirror. Shaded bars represent damage

thresholds for the various observable damage-related effects, while the diagonally hatched boxes indicate standard deviations. Generally, the major contribution to the standard deviation is a threshold

variation over the sample surface rather than experimental uncertainty. Notice that the thresholds fall

into two groups, one consisting of the two types of slip, and another which includes melting and other

forms of catastrophic damage.

Figure 2(b) shows similar results from a clean single-crystal Cu target, sputter-cleaned and

annealed in ultrahigh vacuum. The disparity between the thresholds for slip and melting is even more

* Work supported by the Army Missile Command, the Naval Air Systems Command and NWC Independent

Research Funding.

1. Figures in brackets indicate the literature references at the end of this paper.

-204-
dramatic in the single crystal, amounting to almost a factor of 5 in fluence. This makes it quite clear

that the slip observed near threshold is not related to melting or other catastrophic damage, and must

be attributed to thermal stress. Furthermore, it is quite evident that slip and melt thresholds are related to the surface condition in different ways. The higher thresholds for catastrophic damage,

including melting, on the single crystal are evidently the result of lower absorption associated with

the higher degree of crystalline order and cleanliness. It is well known that a low yield stress is

also associated with order. This evidently offsets the effect of low absorption in producing a lowered

threshold for slip bands. There is, of course, no intergranular slip in a single crystal.

Intrinsic Melt Threshold

An intriguing question is whether or not the melt threshold indicated in figure 2(b) is representative of intrinsic single-crystal Cu. Using conventional heat flow theory, and temperature-dependent

thermal and optical parameters, we calculate a melt threshold of 74 J/cm^ for pure Cu. Although this is

considerably higher than the measured 45.3 J/cm^, the agreement must be considered reasonably good in

view of the uncertainty in the material parameters.

One possibly important effect which was neglected in the calculation is the influence of strain on the absorption coefficient. In the case of elastic strain the effect can be easily estimated, and turns

out to be quite small. Using the Hagen-Rubens relationship [3] between absorptivity a and electrical

resistivity p, one finds the relationship to the strain e is given by —Aa _ 1 Ap _ 1 dp , ... - V — — T X '^e , (1) a 2 p 2 pdE

where dp/pdc is the strain coefficient of resistivity (~ 1 for Cu) . With a fixed constraint the strain

is related to the temperature T through the linear absorption coefficient 6(~ 10~^ for Cu)

.

Ae ~ SAT . (2)

Assuming a temperature change AT ~ 10^ required to produce melting, we find Aa/a ~ 0.5%.

The effect of inelastic strain is much harder to evaluate since the dynamics of the problem are important and difficult to determine. We have attempted transient reflectance measurements on polished

Cu samples, but have not been able to detect a reduction in reflectance attributable to slip. However,

the sensitivity of our photographic data recording technique is limited to effects ~ 10% or larger. A

transient analyzer, which we will acquire soon, will provide much improved sensitivity to time-dependent

absorptance effects. It should be emphasized that the failure to observe a transient absorptance does

not necessarily preclude a residual change in absorptance resulting from slip damage. Unfortunately,

the latter is difficult to measure accurately because of the small damaged area (diameter = 629 pm)

.

Plasma Effects

One transient effect which can be easily observed is the reduction in specular reflectance produced

by the target plasma. Figure 3 compares incident and reflected laser pulses from a polished Cu target

where the fluence exceeds the thresholds for ion and light emission. The dropout in the upper trace

results from plasma interference. This part of the pulse, which is available for generating thermal

stress at lower fluences, is blocked by the plasma.

The above interpretation is supported by the single-crystal slip-band pattern, which undergoes a drastic change when the plasma forms. Figure 4 compares dark-field micrographs of three different sites

on a Cu (111) surface, each shot once at a fluence which increases proceeding from left to right. On

the left, only slip has occurred. In the middle, slight melting has also occurred. On the right, a crater has formed and emission was observed, indicating plasma formation. Notice that the six-foldsymmetric slip pattern, which appears in the first two micrographs, is replaced by a three-foldsymmetric pattern on the right. The six-fold pattern is produced by compressive thermal stress acting

along the target surface [4]. The three-fold pattern, however, is inconsistent with ordinary thermal

stress and requires a different mechanism [5,6]. Possible mechanisms for the three-fold pattern are pressure from the plasma or, as was recently suggested to us, by a surface tensile stress produced by

resolidif ication of the melt pool [7].

The most curious aspect of figure 4, which has also been observed with Nomarski [5,6], is the

absence of six-fold-symmetric slip when the plasma forms. Presumably, thermal stress should produce

this characteristic pattern early in the pulse, regardless of whether or not the plasma threshold is

reached at the higher temperatures which occur later on. The explanation apparently requires a non- equilibrium or dynamical mechanism. The one we propose is illustrated schematically in figure 5. Here

curves 1, 2 and 3 represent the magnitudes of the thermal stress versus time for the three fluences

represented in figure 4. Although these plots are qualitative, the essential features were verified by

calculation. The dashed curve represents the possible time dependence of the critical shear stress,

which is believed to be a decreasing function of time since dislocations have a nonzero response time [8].

-205-
Six-fold slip occurs at points labeled S, where the rising stress curves intersect a^. Points labeled

M and P indicate melting and plasma formation, respectively. We hypothesize that in the case of curve

3 vaporization occurs before slip. Plasma shielding then causes the stress to drop rapidly, as indicated, so that no intersection with is possible in this case. Vaporization is promoted by higher

strain rates, so that the plasma may actually precede melting. This has been observed with the target

at atmospheric pressure, as discussed below. If the vaporization is sufficiently prompt the assumption

of a Oc which decreases with time is, of course, not required.

An alternative explanation of the change in slip patterns is that slip due to thermal stress is

obscured by "scrubbing," a term used to describe laser-induced reduction of background scatter. An

example of this effect can be seen in the dark-field micrograph of figure 6, where an area of reduced

scatter appears outside the clearly defined zone of melting. Here the plasma has caused removal or redistribution of material, resulting in elimination of some of the scattering centers originally

present. However, the Nomarski micrograph shows no evidence of extensive surface smoothing in the

scrubbed area. In fact, careful examination reveals a subtle increase in texture due to slip outside

the melt zone on this fine-grained surface. This evidence suggests that obscuring of slip due to plasma scrubbing is minimal, especially when viewed with Nomarski.

An important result for practical applications is that the slip threshold is not appreciably

influenced by the presence of atmosphere. This was demonstrated on a mechanically polished Cu sample,

which was tested first in vacuum, and subsequently in one atmosphere of The results are shown in

figures 7(a) and (b) , respectively. Comparison of these figures shows a substantial increase in the

thresholds for melting and other catastrophic effects due to the additional shielding provided by the

N2 plasma. However, at the lower energy density where slip first occurs, no plasma could be detected

in either case. Judging from the near equality of the light-emission threshold in N2 and in vacuum,

the N2 plasma is triggered by the target plasma. Since the target-plasma threshold lies above the slip

threshold associated with thermal stress, the latter is not influenced by shielding in either case.

Specially Prepared Mirrors

In an effort to identify surface treatments which provide resistance to plastic deformation, as well as other forms of laser damage, we have tested mirrors prepared by less conventional methods. In figure 8 we show the damage profile of a diamond-turned (D-T) Cu mirror. Comparison with figure 2 shows that D-T Cu is qualitatively similar to single-crystal Cu in that thresholds for melting and

other catastrophic effects are high relative to that for slip bands when compared to the polished

sample. This strongly suggests that D-T Cu also has a high degree of surface order leading to a lower

inherent absorption, and also to a lower yield strength. The net effect is a threshold for slip bands

which is virtually identical to that of polished Cu. These arguments do not necessarily apply to the

thresholds for pits and intergranular slip, which may be influenced by random inclusions or segregated

impurities [1]

.

A more extensive multithreshold study of D-T metal mirrors has already been reported [9], including

samples where the metal to be turned was electrolytically deposited on bulk Cu substrates. Au and Ag

mirrors prepared in this way show very uniform damage thresholds (small standard deviations) , but are

not without problems. The chief difficulty with the D-T Au mirror, for example, is a high background

scatter, which can be seen in figure 6. The Ag mirror produced a slip threshold of only 12 J/cm^,

although the catastrophic thresholds compare favorably with D-T bulk Cu.

Unfortunately, we do not have multithreshold results on sputter-deposited Cu surfaces, and must

therefore base our discussion of this preparation method on Mo, which generally exhibits much lower

thresholds than Cu. Figures 9(a) and (b) show damage profiles of bulk and sputter-deposited Mo mirrors,

respectively, where the same polishing method was used in both cases. On the sputtered sample we find

no visible evidence of slip, but the melt threshold is much lower than on the bulk sample. Light

emission and, to some extent, ion emission are associated with pitting on the bulk sample and are not

representative of the surface in general. In view of our previous results the implications are that

the sputtered sample is highly disordered, resulting in a slip threshold which is indistinguishable

from the melt threshold. However, It is quite possible that slip simply cannot be observed on the

sputtered sample because of the very small grain size.

We have also tested Cu mirrors that have been ion-implanted and one that was ion-milled. No significant improvement in slip threshold was ever achieved by ion-implantation with any of various combinations of energy, dosage and ion type. Ion-milling, on the other hand, did produce a marked increase in

the slip threshold, but melting and other thresholds were low. However, since the purity of the Cu was

suspect on this sample, the possible benefits of ion-milling cannot be disregarded.

Structural Alloys

In order to evaluate laser-induced stress damage in structural alloys we polished and tested

samples of two materials. The damage profile of Ti-6A1-4V, shown in figure 10, indicates a slip threshold of only 0.6 J/cm^. This is the lowest we have seen on any well polished metallic sample. The

threshold for work function change, which is also very low, is attributed to surface oxide removal.

-206-
based on Auger results. The intergranular slip in this alloy can be seen quite clearly in Nomarski

(fig. 11) as an enhancement of the grain boundaries near the center of the field. Slip boards are difficult to observe on this sample because of the small grain size, but are faintly visible in the darkfield micrograph.

Figure 12 gives the damage profile of hand-forged Al 7050-T73652 from Alcoa. The lowest threshold

which could be observed was for work function change associated with a change in the state of surface

oxidation. No slip was visible, evidently as a result of the high dislocation density. Although hardening has minimized slip, this has been at the expense of the thresholds for melting and pit formation.

Pure, clean Al , for example, has a melt threshold of 14 J/cm^ [5,6]. The lack of homogeneity and resulting pits are apparent in figure 13. Auger analysis shows isolated Si, possibly from SiC which collected

in voids during polishing.

In summary, it has been demonstrated that thermal stress can damage mirror surfaces in the absence

of melting. Stress damage may be indicated by slip bands, roughening or displacement along grain boundaries, as seen by Nomarski or other types of microscopy. Surface condition, as influenced by the surface

preparation method used, can affect the thresholds for slip and for melting differently. Typically, slip

thresholds are low and melt thresholds high on clean, well ordered surfaces. The target plasma provides

shielding and scrubbing, but only above the threshold for slip due to thermal stress. Consequently, slip

thresholds are about equal in atmosphere and in vacuum. The plasma introduces another mechanism for slip,

which produces a different slip band pattern on a (111) single-crystal Cu surface. Results of testing

on specially prepared surfaces indicate that diamond-turning and ion-implantation are generally ineffective against slip. Sputtering, ion-milling and forging may be effective, but may also reduce thresholds

for other types of damage. The structural alloys tested have low damage thresholds. Ti-6A-4V is

extremely susceptible to slip damage.

Summary

Acknowledgments

The authors are indebted to W. R. Compton and J. B. Patton for heat-flow and stress calculations.

References

[1] Porteus, J. 0., Jernigan, J. L. and Faith, W.

N. , this conference.

[2] Bennett, H. E., Bennett, J. M. , Ashley, E. J.

and. Motyka, R. J., Phys. Rev. 165^, 755 (1968).

[6] Porteus, J. 0., Soileau, M. J. and Fountain,

C. W. , in Laser Induced Damage in Optical

Materials: 1976, A. J. Glass and A. H. Guenther, eds. (NBS Spec. Publ. 462), p. 165.

[3] Bennett, H. E. and Bennett, J. M. , in Optical

Properties and Electronic Structure of Metals

and Alloys (North-Holland Publishing Co.,

Amsterdam, 1966), p. 179.

[7] Glicksman, M. E. and Breinan, E. M. Private

communication.

[8] Steverding, B. and Dudel, H. P., J. Appl. Phys.

47, 1940 (1976).

[5] Porteus, J. 0., Soileau, M. J. and Fountain,

C. W., Appl. Phys. Lett. 29^, 156 (1976).

[4] Haessner, F. and Seitz, W. , J. Mater. Sci. 6^, 16 (1971).

[9] Porteus, J. 0., Decker, D. L. , Jernigan, J. L.

,

Faith, W. N. and Bass, M. , lEEE/OSA Conference

on Laser Engineering and Applications, held in

Washington, D.C., 1-3 June 1977.

-207-
Figures

SLIP BANDS AND INTERGRANULAR SLIP

BULK Cu. DIAMOND-TURNED

Figure 1. Nomarski and dark-field micrographs of a damage site on diamondturned bulk Cu below the threshold for melting. Crack-like

features are grain boundaries which have undergone intergranular

slip. The roughened appearance just left of center is due to slip bands.

BULK Cu, OPTICAL POLISH

(111) SINGLE-CRYSTAL Cu, UHV-PREPARED

SLIP BANDS

INTERGRAN. SLIP

MELTING

PITS

CRATERS

ION EMISS.

LIGHT EMISS.

WRK. FN. CHG.

(a)

20 40 PEAK THRESHOLD (J/CM^)

60

SLIP BANDS

MELTING

PITS

CRATERS

ION EMISS.

LIGHT EMISS.

WRK. FN. CHG.

(b)

20 40 PEAK THRESHOLD (J/CM^)

Figure 2(a). Damage profile of polished Cu, showing damage thresholds

and their associated standard deviations for eight damage-related

effects. All results in this paper were obtained with the target

in vacuum, unless otherwise indicated.

Figure 2(b). Damage profile of a (111) single-crystal target prepared and

tested in ultrahigh vacuum.

-208-
TRANSIENT REFLECTANCE POLYCRYST Cu - OPTICAL POLISH

REFLECTED

INTENSITY

INCIDENT

INTENSITY

120 180 TIME (nSEC)

Figure 3. Comparison of the laser pulse at normal incidence (bottom) with

the back-reflected pulse (top) from a polished bulk Cu target

above the threshold for plasma formation.

SLIP

SLIP BANDS

(111) Cu - UHV PREP.

MELT CRATER

200 iim 101 II

Figure 4. Dark-field micrographs of three sites on (111) Cu damaged at 47, 50 and 56 J/cm^, respectively, proceeding from left to right.

Bright lines are caused by scattering from slip bands.

-209-
PLASMA SHIELDING

Figure 5. Time dependences of thermal stress, illustrating the plasma

shielding effect. The dashed curve a^, represents the limiting

stress for elastic deformation.

MELTING, SLIP, SCRUBBING

ELECTRODEP. Au ON BULK Cu, DIAMOND-TURNED

NOMARSKI DARK FIELD

200

Figure 6. Nomarski and dark-field micrographs of a heavily damaged site

on diamond-turned Au, electrolytically deposited on bulk Cu. Slip, which is obscured by the small grain size and inherent

surface texture is indicated by a subtle increase in texture

in Nomarski. The term "scrubbing" refers to the area of lowscatter in the dark-field micrograph as explained in the text

-210-
POLISHED Cu - DAMAGE IN VACUUM

INTERGRAN. SLIP

MELTING

PITS

CRATERS

ION EMISS.

LIGHT EMISS.

100 200 PEAK THRESHOLD (J/CM^I

300

Figure 7(a). Damage profile of mechanically polished Cu damaged in vacuum. Note that the scale differs from previous figures.

POLISHED Cu - DAMAGE IN 1 ATM N,

INTERGRAN. SLIP

MELTING

PITS

CRATERS

LIGHT EMISS.

100 200 PEAK THRESHOLD (J/CM^)

300

Figure 7(b). Damage profile of the target of 7(a) damaged in one atmosphere

of N2 for comparison.

-211-
BULK Cu. DIAMOND-TURNED

SLIP BANDS

INTER6RAN. SLIP

MELTINQ

PITS

CRATERS

ION EMtSS.

LIGHT EMISS.

WRK. FN. CHG.

20 40 PEAK THRESHOLD (J/CM^)60

Figure 8. Damage profile of diamond- turned bulk Cu, for comparison with

figure 2.

BULK Mo, OPTICAL POLISH SPUTTERED Mo ON BULK Mo, OPTICAL POLISH

SLIP BANDS

INTERGRAN. SLIP

MELTING

PITS

CRATERS

ION EMISS.

LIGHT EMISS.

WRK. FN. CHG.

MELTING

PITS

CRATERS

ION EMISS.

LIGHT EMISS.

WRK. FN. CHG.

20 40 PEAK THRESHOLD (J/CM^)

—I

60 20 40 PEAK THRESHOLD (J/CM^)

(a) (b)

Figure 9(a). Damage profile of polished bulk Mo.

Figure 9(b). Damage profile of polished Mo which has been sputterdeposited on bulk Mo.

-212-
Ti-6AI-4V - MECHANICAL POLISH

INTERGRAN. SLIP

MELTING

PITS

CRATERS

ION EMISS.

LIGHT EMISS.

WRK. FN. CHG.

0 5 10 15 PEAK THRESHOLD (J/CM^)

Figure 10. Damage profile for mechanically polished

Ti-6A1-4V. Note the small scale compared

to previous figures.

SLIP TI-6AI-4V MECHANICAL POLISH (NWC)

NOMARSKI DARK FIELD

200 iim

Figure 11. Nomarski and dark-field micrographs of a damage site on Ti-6A1-4V at 1.6 J/cm^

peak fluence, showing intergranular slip

and faint slip banding.

-213-
Al 7050 - MECHANICAL POLISH

MELTING

PITS

CRATERS

ION EMISS.

LIGHT EMISS.

WRK. FN. CHG.

0 5 10 15 PEAK THRESHOLD (J/CM^)

Figure 12. Damage profile if mechanically polished Al 7050.

MELTING

POLVCRYSTALLINE Al 7050 - MECHANICALLY POLISHED (NWC)

200 mn

Figure 13. Bright-field, dark-field, and Nomarski

micrographs of a melted site on Al 7050

at a peak fluence of 10.9 J/cm^.

COMMENTS ON PAPER BV PORTEUS, FOUNTAIN, JERNIGEN, FAITH, ANP BENNETT

No cLUcuii4,011 on thJjs papeM.

-214-
LOW-EXPANSION LASER MIRRORS*

P. Pirooz, G. Dub^ and N. L. Boling

Owens-Illinois, Inc. Toledo, Ohio 45666

A unique concept for producing low-expansion IR laser mirrors was evaluated. This

concept involved the self-generation of thin copper films on very low-expansion

glass -ceramic substrates previously doped with copper. Exploratory studies of two

major glass -ceramic systems: LiaO-AlaOa-SiOs and ZnO-AlaOs-SiOa, doped with copper

have resulted in the development of mirrors with good film-to-substrate adhesion

and high specular reflectance at 10.6 ion. The effects of surface finish and process parameters on the film properties were also investigated.

Key words: Copper filmj film adhesion; glass -ceramics; IR reflectance; laser

mirrors; low-expansion materials.

1. Introduction

The recent increase in demand for high-power IR lasers has prompted a number of studies directed

towards the advancement of the state of the art of polished metal mirrors. Yet other workers have attempted to replace the metal mirrors with alternate materials in an effort to minimize the serious

thermal distortion problem encountered by the metal mirrors subjected to high-power laser irradiation.

H. E. Bennett [l]''" has concluded that, for a mirror, the thermaJ. distortion is given by the ex- pression,

where I is the irradiance, t the pulse length, R the reflectance, b the mirror radius, I the mirror

thickness, P the material density, C the heat capacity, and O- the thermal expansion.

If we assxme that all mirror materials can achieve the same reflectance, a figure of merit for

minimizing thermal distortion becomes the following expression

F.M. = ^

(2) a

Table 1 lists these parameters and the figure of merit for several couanon laser miiTor materials

which is a low-expansion glass-ceramic.

Table 1. Properties of laser mirror materials

Cu SIC Mo C-V

P 8.9 3.2 9.01 2.5

C 0.385 0.75 0.2i*.8 0.21

a(x 10-'^) 170 i^3 50 < k fC/a 1 2.5 2.2 > 6.5

The significantly higher F.M. for the CER-VIT material explains the interest in a mirror system

which consists of a reflecting film such as copper supported by a very low-expansion substrate such as CER-VIT material. Such a system oifers the potentied of a significant reduction in the thermal distortion.

This study was undertaken specifically to determine feasibility of producing low-expansion glassceramic mirrors in which the reflective copper film is self-generated on the s\irface by a unique diffusion controlled process. It was anticipated that the copper film generated by the subject process would

exhibit superior film-to-substrate adhesion and laser damage threshold. This was borne out by the results obtained in a limited study which is the subject of this report.

* This study was funded by the Defense Advanced Research Projects Agency, Department of Defense,

under the contract No. F33615-76-C-5343.

1. Figures in brackets indicate the literature references at the end of this paper.

® A registered trademark of Owens-Illinois, Inc.

-215-
2 . Background

The process of copper-film generation presented here is based on a phenomenon which results in the

selective migration of copper ions to the glass surface by diffusion.

In a previous study [2], attempts were made to delineate the basic mechanism for this migration.

The following discussion is based on the findings obtained from light microscopy and electron microprobe analysis of samples fired in oxidizing, reducing and neutral atmospheres.

Basically, these results have led to the conclusion that said migration is a diffusion-controlled

process as Indicated by the linear dependence of the depth of copper depleted layer on the square root

of the heat -treatment time.

The colors produced as the result of this migration support the opinion that the migrating species

are only the Cu"*" ions. A series of additional heat treatments have also strongly sviggested that the

driving force for the migration is the atmosphere in contact with the glass svirface which can readily

change the oxidation state of the Cu"*" ions. As depicted in figure 1, if the surface is exposed to an oxidizing atmosphere, the Cu+ ions are converted to CuO which forms a continuous black layer on the surface. If the immediate atmosphere is reducing, such as hydrogen, the Cu* ions will then convert to me- tallic copper which will form a continuous copper film on the glass surface. In either case, the reac- tion depletes the surface glass of Cu''' ions which are in turn replenished from the adjacent layers

driven by the resultant concentration gradient. This hypothesis was verified by the heat treatment of

samples in nitrogen and argon atmospheres which produced no detectable migration of the copper ions.

What is not readily explained by this hypothesis is the profile of the depleted layer as shown in

figure 2. This layer appears to increase in depth as a ^^anctlon of the heat -treatment time and/or temperature, but, as indicated by the color change and the electron microprobe data, it does not cause a progressive change in the copper ion concentration. This siiggests that each ion wovild be required to

travel the entire depth of the depleted region after it has come under the influence of the driving

force which is located at the surface. This type of mechanism is further supported by the diminishing

rate of the growth of the surface layer as a function of the heat -treatment time, or the depth of the

depleted layer.

It should be noted that the actual film generation behavior will eilso strongly depend on the host

material and process parameters.

3. Experimental

5.1. Preparation of the original glasses

The compositions of the host glasses were selected on the basis of their ability to crystallize

into a glass -cerajnic body possessing a very low coefficient of thermal expansion, near zero if possible.

For this purpose, two compositions were selected iVom totally different families of glass -ceramics; one iVom the Ll-Al-Si system and the other from the Zn-Al-Si system. Both of these were expected to produce

a high-quartz solid-solution crystalline phase resulting in very low thermal expansions. To these com- positions were added CuD at 2 and 44 levels (by weight). The experimental glasses were prepared from

these compositions by melting a 5000 g batch in a platinum crucible heated in an electric furnace for

approximately 25 ho\irs. At the completion of the melting process, each glass was cast in a steel mold

to obtain a relatively large billet which was then annealed.

3.2. Crystallization studies

In crystallizing the LiaO-AlsOs-SiOs glass, best results for the billet samples were obtained by

either an isothermal heat treatment at 625°C - I6 hours, or a two-stage heat treatment at 580°C - I6

hours plus 650°C - k hours, resulting in thermal expansion coefficients ranging from -1 to -3 x

10"''^.

3. 3- Film generation studies

The task of generating a copper filjB on the surface of these materials Involves two major processes: 1) migration of the copper ions to the glass siorface and their accumulation thereon, and 2) conversion of the copper oxide layers to the metallic copper.

To study these phenomena and the related process variables, flat samples approximately 4 x 4 x 0.

5

cm were sawed from the glass billet. The initial studies were made with the saw-cut surface, after

which surface ground and polished samples were used for the detailed property studies.

3.3'1' Cupric oxide generation

There are two basic process modes for generating the CuO layer: 1) the glass is subjected to the

necessary crystallization heat treatments in the form of raw stock, after which the crystallized bulk

material is shaped into the desired figure and subjected to a second heat treatment to generate the CuO

layer, and 2) the final figure is produced from the raw glass (noncrystalline) which will then see a combined heat treatment for bvlk crystallization as well as the CuO generation.

Since the majority of glasses has a tendency to undergo a significant volume change (l-5fo) due to

crystallization which can caiise disfiguring and warpage of plate samples, the first alternative discussed above is preferred and thus was used for most of the present studies.

-216-
All CuD generation heat treatments were conducted in air atmosphere. Samples were placed on a flat

ceramic plate inside an electrically heated muffle furnace. The top surface of the sample was labeled

so that it only would be utilized for all the subsequent tests and evaluations. This procedure was used

to eliminate the effect of the setter material on the metal film as an additional parameter.

The CuO film generated on these materials was extremely hard and durable which, combined with its

very strong bond to the substrate, made it diffictilt to remove. Hydrofluoric acid solution was found to

be the only medium capable of attacking this CuO layer. However, since this acid also readily attacks

the substrate material, it was not practical to make a determination of the thickness of this layer.

3.5.2. Copper metal film generation

The generation of the metal lie copper v/as done by the direct reduction of the CuO layer in a hydrogen bearing atmosphere at elevated temperatures. Once the critical parameters for the CuO generation

were determined and optimized, the metallizing process was controlled essentially by the following parameters: 1) firing temperat\u:e schedule, 2) firing time schedule, and 3) ambient atmosphere.

In this study the optimum conditions were determined by the study of a matrix which included a total of 21 samples. These samples, 12 mm wide, 2 mm thick and varying in length, were all cut from a block of LiaO-AlgOs-SiOs glass previously heat treated at 600°C (4 hrs ) + 650°C (32 hrs) + 700°C (32

hrs). At this point, all samples bore a surface layer of CuO. The metal generation heat treatments v/ere all conducted in a Lindberg muffle furnace equipped with

a conveyor belt and atmosphere control provisions. The temperature profile observed by the ssimple as it

traveled through the i\irnace weis programmed by tuning the three independent heating zones. Figure 3 shows a typical temperature profile that was used for the majority of the samples. The residence time

at the maximum temperature was determined by the speed of the conveyor belt.

The overall ranges of conditions used for said matrix studies consisted of: a) temperature: 275-

^kO°C, b) time; 5-30 minutes, and c) atmosphere: 8-25-"'- hydrogen mixed with nitrogen.

The primary criterion for the evaluation of these samples was film adhesion since no material was- thought to be worthy of consideration for the subject application without good film-to-substrate adhesion. The detail procedure for the adhesion test is described in a separate section.

Based on the film adhesion tests, the most favorable conditions for the metal copper generation on

this type of materisLl are: a) temperature: 275-300°C, b) time: 5-10 minutes, and c) atmosphere: B-ICP'- H2/N2. This information was later utilized to prepare the finsil samples for the film property evaluations.

3.'+. Film property evaluations

A relatively large number of samples were prepared and studied with respect to the manufacturing

process variables and physical properties such as: film adhesion, thickness, roughness and light reflectance.

^.k.l. Sample preparation

The final samples were plates 35 x 35 x 3 mm sawed from the precrystallized billet. Once the precrystallized plates were either ground or polished, the CxjD generation and the final conversion to Cu

was carried out by the procedures previously described.

The various surface finishes examined here are: as saw-cut, ground with No. 6OO grit silicon car- bide, grovmd with No. 1000 grit silicon carbide, polished with cerium oxide, and polished with diamond

paste.

3.4.2. Film adhesion test

The relative film-to-substrate adhesion was checked using the unsophisticated, but common, "Scotch

Tape Test." This test consisted of pressing a section of the tape (Scotch Magic Transparent Tape No. 810) on the copper film, making sure that no air remained trapped at the interface. The tape was then

peeled from the sample by pulling up slowly with a force normal to the film surface. The results were subjectively categorized as follows: excellent (no removal of the film), good (minor peeling), moderate

(partial peeling with moderate force), and poor (major peeling requiring no significant force).

3.4.3. Film thickness and surface roughness

A Clevite Brush Surf-Analyzer I50 was used to measure the roughness of the sample surfaces and the

thickness of the copper coating. This instrument drives a stylus across a horizontal surface and records the vertical motion of the stylus in microinches. It can record both profile and roughness.

Roughness refers to the short-range profile, but does not record longer-range slants or undulations of

the surface.

To measure the thickness of the copper coating, a small section of the copper coating was chemically removed by dipping in a solution of FeCla for 1 minute. The Surf-Analyzer was then driven across

this step to measure the thickness of the coating. Because the coating was fairly thin, only polished

samples could be used for the thickness meastirement . Three measurements were made on each sample.

-217-
3.h.k. Reflectance at 10.6 /im A Perkin-Elmer Model 621 Grating Infrared Spectrophotometer was used to measure the reflectance of

the samples using light with wavelengths from 2. 5 to 20 i/m. The reflectance reported is relative to

a silver-coated reference mirror. The absolute reflectance of this miiTor is not known, but is believed

to be 9^ ± 0.5^3. Because this instrument measures specular reflectance, only the polished samples gave

meaningful results. The relative reflectances at 10.6 ijfn of the polished samples ranged from 101 to 90'f:. Most samples were measured approximately three days after their manufacttire and a few days before

the laser damage tests. Many samples had their reflectivity measiired again about three weeks later to see if the oxidation, which was quite apparent on some samples, had lowered the reflectivity at 10.6 (fln.

k. Discussion of results

Table 2 summarizes the preparation conditions and the results of tests and measurements for some of

the flat plate samples made in this investigation. The three significant heat -treatment parameters tabi. ulated are: the initial crystallization heat treatment, the CuO generation heat treatment conducted in

air, and the Cu generation heat treatment performed in H^/Ns atmosphere. The surface finish shown is

that of the crystallized sample prior to the CuO and Cu generation processes.

Table 2. Process and physical properties data for

copper-coated glass-ceramic samples

^ Average

CuO Cu Electrical Reflectance Thickness

Sample Surface Generation Generation Resistivity at 10.6 Roughness (A) of Copper

No, t Finish °C (hrs) °C ( min) Adhesion Fresh Aged Substrate Copper Film ((bd)

1 G (6oo) 625 (16) 275 (5) 0.2 Exc. 65 6k ^ 15000 same ND

2

1 625 (32) 275 (5) 0.2 Exc. 63 15000 same ND

3 P [Ce) 625 (16) 275 (5) 0.2 Pr. 100 102 < 200 same 0.48

k

1 625 (32) 275 (5) 0.2 Pr. 101 90 » 500 same 0.36

5 G ;6oo) 625 (16) 300 (10) 0.2 Exc. 63 62 « 15000 same ND

6

t 625 (32) 300 (10) 0.2 Exc. 60 58 17000 same ND

7 P iCe) 625 (16) 300 (10) 0.2 Pr. 98 96 < 200 same 0.31

8 625 (32) 300 (10) 0.2 Exc. 100 100 < 200 same 0.46

9 G '600) 625 (16) 275 (6) 0.2 Exc. 67 72 a 15000 same ND 10 t 625 (32) 275 (6) 0.3 Exc. 54 6k « 15000 same ND

11 1 625 (16) 275 (7) ND Exc. ND ND ND ND ND

12 1 625 (32) 275 (7) 0.2 Exc. ND ND ND ND ND

13 P Ce) 625 (16) 275 (6) 0.3 Exc. 90 86 ^ 2500 same ND 14 1 625 (16) 275 (7) OA Exc. ND ND ND ND ND

15 G 600) 625 (16) 500 (10) 0.1 Exc. 76 76 ^ 15000 same ND

16 1 625 (32) 300 (10) ND Exc. 72 62 « 15000 same ND

17 P Ce) 625 (16) 300 (10) 0.2 Exc. 95 9^^ ^ 1200 < 200 0.24

l8t saw-cut 650 (32) 300 (16) 0.05 Md. ND « 3000 » 1000 17.1

Legends: G (600) = grovind with No. 6OO silicon carbide grinding powder

P (Ce) = polished with cerim oxide powder

Pr. = poor

Md. = moderate

Exc. = excellent

ND = not determined

t Crystallization heat treatment: 60°c/hr - 580°C(l6) + 30°c/hr -> 650°C(4) + 30°c/hr - 500°C(0)

+ furnace rate — room temperature.

=t New copper was electrolitical 1 y deposited on the original fi3jn.

4.1. Film adhesion

In general, film adhesion was exce3J.ent for the unpolished (ground) samples and poor for the polished samples with the excepition of samples 8, I3 and 17 which exhibited excellent adhesion. It is

noteworthy that all samples prepared from the ZnO-AlsOa-SlOa glass, whether ground or polished, showed

very poor film adhesion.

Preliminary studies conducted on a matrix of samples showed that the time, temperature eind atmosphere parameters of the Cu generation heat treatments are critical factors affecting the film adhesion.

-218-
In general, excessive time, temperature or Hs content of the atmosphere contributed to poor adhesion.

The optimian conditions were found to be: 8-10 minutes, 275-300°C and 6-10^ Hs. 4.2. Electrlceil sheet resistivity

Electrical sheet resistivity which Is a measure of copper film thickness and surface purity was measured on the majority of the samples. The resistivity values ranged from 0.05 to O.36 ii/G.

k. 5. Surface roughness

Surface roughness of the pwalished glass-ceramic substrate ranged from < 200 A to 500 A. Samples 13

and 17 were exceptions with 25OO and 1200 A roughness. The ground samples showed a range of I5OO-I70O

A. With the exception of sample 17, the surface roughness did not change significantly with the generation of the copper film. In sample 17, the avereige roughness was < 200 A with the copper film, as com- pared with 1200 A for the bare substrate. Sample I8 is a special case, in which a relatively large

amo\int of copper was electrolltlcally deposited on the original copper film. The substrate surface finish was obtained from a saw cut. The plated surface was lightly polished with alumina powder and diamond paste.

h.k. Film thickness

The average film thickness was determined only for the polished samples. These ranged from 0.2k to

0.51 (jm. That of the copper -plated sample was 17.1 iM, most of which was due to the electroplating.

Although the actual film thickness was not determined for the unpolished samples. It is also expected

to be < 1 /jm.

4.5. Reflectance at 10.6 ffa Light reflectance was measvired on ground as well as polished samples shortly after their preparation and again after about three weeks of aging. The first measurements on the polished samples re- sulted in 95-101'^ reflectance versus a silver-coated reference. That of the ground samples ranged from

54-76^: apparently due to considerable scattering. It is interesting to note that the values obtained

from two samples (not shown in the table) were extremely low, despite their bright appeeirsince. This is

believed to be due to a thin dielectric coating present on the copper filjn which is absorbing the 10.6 ^ radiation.

Although the visual appesirance of the samples indicated a major discoloration due to aging, the re- flectance data shown in the table do not present a clear direction. A mixed effect is shown by the data

including a significant drop in three samples, a significant rise in two samples and no significant

change in the remaining eight samples.

5. Conclusions

1. In a brief study, compositions from LiaO-AlaOa-SiOs and ZnO-AlaOa-SlO^ systems doped with copper at 2 to kfo levels were investigated.

2. Copper metal films were successfully generated on glass-ceramic plates exhibiting characteristics highly favorable to the laser mirror application. These included good film adhesion and high re- flectance at 10.6 ija.

3. It was demonstrated that, if desired, the copper film may be built up by electroplating of additional metal to several micrometers without deterioration of the critical properties.

4. It is conceivable that such mirrors can be repaired by refinishing and subsequent regeneration

of the copper film.

5. Extremely small thermal expansion/contraction coefficient of the substrate material will allow

the utilization of very steep temperature gradients across the wall for high heat transfer rates.

6. References

[1] Bennett, H. E. , NBS Special Publication No. [2] Pirooz, P. P., Owens-Illinois, Inc., Internal

462 {1976). Report (1973).

7. Figures

-219-
Glass CuO Cu°

Surface

FigTxre 1. Schematic presentation of copper ion migration in glass.

Cu

Cu , Cu

(RED)

\

'

++

Cu

(BLUE)

GLASS

SURFACE

DISTANCE FROM SURFACE

Figure 2. Copper ion concentration profile in a heat-treated

glass-ceramic.

-220-
300

200

LU

100

/

J L

10 15

J I I I I L_

20 25 30 35 40 45 50

TIME - MINUTES

Figure 5. Heat -treatment schedule for copper film generation.

COMMENTS OM PAPER 8^ VIV.0Q1, PUBE, KHV BOLZNG

Thz ipzaksA indiccuCe,d that although thz iuAl^acu the. iomplu weAe obizAved to toAn^k vLslbZy ovtn

a pzAlod o(i daifi a{^tvi thz pn.zpaAcutLon, no ddiztz/u.ouu> zd^jZct on thz nz{,Zz(itA.vitij at 10.6 niicAoni woi

obiZAvzd. Hz MOA oLio a^kzd id thz conducttvity o{i thz czAviX matAtx Moi, af^^zctzd by thz tncZivilon 0|$

coppzA torn. Hz tndlzatzd that tanz had not pzAmtttzd thz invz^fitlQaXlon o(^ thi6 point aitkough -it. wouZd zzAtoA^nLy bz an tntzAz^ttng a^d^zct to zxamlnz.

-221-
A NEW CHALCOGENIDE-GLASS ANTIREFLECTION COATING FOR KCl

A.D. McLachlan and W.E.K. Gibbs

Materials Research Laboratories

Defence Science and Technology Organisation

Jfelboume 3032, Australia

The design, preparation and performance of new low-absorption antireflection

(AR) coatings for KCl at 10.6-iiin wavelength are described. These coatings are of

the two- layer type and utilize the ehalcogenide glasses ^s^S^ and a newly-developed

thin-film material Ge^QAs-j^^Te^^Se^j (GATS) for the low-index and high-index layers

respectively. The compositional homogeneity, structure, refractive index and

optical dispersion of thin-films of the GATS material are reported.

The absorption of the AR coatings at 10.6-viro wavelength was determined by

laser calorimetry and found to be in the range 0.02% to 0.07^. Fine tuning of

the deposition procedures enabled coatings with reflectances of 0.025^ to be

produced. The damage thresholds for pulsed and cw COg-laser radiation were also

determined.

Key words: Absorptance; antireflection coatings; ehalcogenide glasses; laser

damage; potassium chloride; 10.6-ijm lasers.

1. Introduction

Potassium chloride is widely used as a window material in C02-laser systems due to its low optical

absorption at the operating wavelength of 10.6 pm. However, in many applications antireflection (AR)

coatings are required, and it is desirable that the absorption in such coatings be kept to a minimum.

Antireflection coatings of the two- layer type have a reasonable design flexibility that is illustrated

in the Schuster diagram [I]''' shown in figure 1. The shaded areas represent the range of refractive

indices of the layer materials for which zero reflectance may be obtained. Most coating designs

utilize a material (e.g. ZnSe, As^S^) for the inner layer with a refractive index, n^, of about 2.5 and

this is coupled with a material (e.g. ThF^, BaF2 ) with a low refractive index (n.^) to produce the

desired AR coating. However, there are difficulties in obtaining low refractive-index materials with

absorptions as low as the other materials used. Materials such as ThF . , Til and NaF have been used

[2,3,4-] with varying degrees of success.

The approach followed in this paper has been to develop a thin-film material for the inner layer

with a refractive index, n^, greater than 2.9 which can then be used in conjunction with an outer layer

of As2S^, a material that has been studied extensively in these laboratories [5]. An earlier example

[6] of this approach by the present authors was to use quarter-wave thicknesses of the ehalcogenide

glass Ge^^Se^j and As^S^ for the inner and outer layers respectively. The composition of the Ge-Se

glass was chosen so that its refractive index, n2, satisfied the zero-reflectance condition for quarterwave layers. Some test results on this AR coating were presented [7] at last year's Symposium by J.F. Lewis and M.C. Ohmer of the Air Force Materials Laboratory. In the present paper, the development is

reported of another ehalcogenide glass with a suitable refractive index, n2, greater than 2.9, that

results in an AR coating with an absorptance significantly lower than that reported previously [6,7].

Earlier measurements [8], which are summarized in table 1, at these laboratories have shown differences in the optical absorption between thin films and corresponding materials. In general, the reason

for the higher thin-film absorption is not known, although it has been suggested [9] that the presence

of water vapour in the pores of polycrystalline films may be a significant factor. Some support for this

explanation may be obtained by noting that the difference between thin-film and bulk absorption is least

for glassy materials, e.g. As2S^. It is well known [5] that films of this material are impervious to water vapour and thus protect KCl surfaces from attack by atmospheric water vapour. For these reasons, effort has been concentrated on low-absorption ehalcogenide glasses as candidate thin-film materials.

1. Figures in brackets indicate the literature references at the end of this paper.

-222-
Table 1. Thin-film absorption coefficient at 10.6 ym and corresponding bulk values.

MAiKHlAL

ABSORPTION COEFFICIENT ( cm )

THIN FIIM BULK

3.6 1.1

GeSe 11.9

BaF2 33 o.u

ZnSe 33 0.13

CdTe 59 0.08

The particular material investigated here is a ^--component Ge-As-Te-Se glass of composition

Ge^QA.s^,^Te^QSe2^ designated as GATS. This material has a refractive index - 3.1 (thus satisfying the

requirements for zero reflectance), and a low bulk absorption coefficient [10] - 0.0075 cm""''. It is

somewhat similar to the Texas Instruments ' glass "TI-20" in which some of the Se component is replaced

by Te. The addition of Te lowers the absorption in the wavelength region of 11.5 and 13.0 \m and in- creases both the refractive index and the reciprocal dispersive power [10]

.

The KCl substrates used in this work were prepared from forged polycrystalline blanks supplied by

the U.S. Air Force Materials Laboratory. The surfaces were mechanically polished on soft pitch laps

with sub-micron alumina abrasive in a glycerol medium.

2. Coating Preparation

2.1 GATS Thin Films

Samples of the GATS material were prepared by melting the four components in an evacuated, sealed,

silica ampoule that was rocked in a furnace at a temperature of 650° for 6 hours. Procedures were

followed that minimized oxygen contamination of the components during preparation [11]. The composition

of the resulting glassy ingot was foimd from electron microprobe measurements to be reasonably uniform.

Evaporated thin films of the GATS material adhered to the KCl substrates and appeared to be of good optical

quality. No evidence of crystalline structure was revealed by X-ray diffraction, and SEM micrographs of

transverse sections of fractured films showed conchoidal fractures characteristic of glassy materials.

A representative SEM micrograph is shown in figure 2. The composition of the thin films was found by

electron microprobe measurements to be Ge2gAs2-j^Te2gSe22 and this composition could easily be reproduced.

The slight difference in composition between the thin film and the bulk material could be due to the high

volatility of the As component.

The optical dispersion of the GATS thin-films was determined from measiirements of special transmittance and film thickness. The results are shown in figure 3 where it can be seen that the refractive

index at 10.6 um is 3.11, which is very close to that of the bulk material. The absorption of a number

of GATS thin films was determined by calorimetry at 10.6-ijm wavelength. The average value for the

absorption coefficient was found to be 2.5 cm""'" which, although considerably higher than the bulk value

of 0.008 cm"''", is suitable for low-absorption coatings. It may be noted that the absorption coefficient

of As2S^ thin films prepared under similar conditions was found to be 2.5 cm . This may suggest that

a limiting value has been obtained which is determined by contamination from the vacuum system rather

than by the intrinsic absorption of the bulk material.

2.2 Coating Design and Deposition

The thickness required for zero reflectance for a two-layer coating on KCl are obtained by solving

the relevant equations [12]. In general, there will be two solutions which result in the thicknesses

shown in table 2 and designated Designs I and II respectively. It was decided to investigate AR coatings specified by Design I since this design is less susceptible to errors in the thickness of the GATS

layer. With values of the thin-film absorption coefficients obtained above, the absorptance of the AR

coating is calculated to be 6 x lO"'* i.e. 0.06^.

-223-
Table 2. Design alternatives for GATS/As_S_ Antireflection Coatings on KCl

MATERIAL REFRACTIVE

INDEX

THICKNESS

DESIGN I DESIGN II

AS2S3 2.374 1.468 0.764

GATS 3.11 0.483 1.222

KCl 1.454

Deposition of the coating was performed in a turbo-pumped, liquid Nj-trapped vacuum system by an

electron-beam gun source under conditions specified in table 3.

Table 3. Coating deposition parameters

Vacuum system Turbo-pumped

Liq. N, trapped

0.1 mPa (1 X 10 Torr) base pressure

Evaporation source Electron Beam G\jn

Evaporation rate GATS 1 nm/s

As^Sj 2 nm/s

Knowledge of the shape of the dispersion oiorves of the two materials enabled the required layer

thicknesses to be deposited by monitoring the optical thickness at a wavelength of 1.62 um. The monitoring wavelength was chosen to avoid termination of the deposition on a transmission maximum or minimum.

Greater precision can be achieved in this way, since sensitivity to thickness change is greatest at thicknesses midway between transmission maxima and minima.

The spectral transmittance of the completed coating is shown in figure 4, together with values

computed from the measured optical constants and design thicknesses of the coating materials. The close

agreement indicates that the required layer thicknesses have been correctly deposited.

3. Coating performance

3.1. Absorptance

The coating absorptance was determined by calorimetry at a wavelength of 10.6 pm by means of a conventional experimental arrangement which has been reported previously [8]. The results obtained for

four AR coatings are shown in table 4. The values obtained are among the lowest ever reported for AR

coatings on KCl [4,13,4], and are lower than the expected value of 0.06%. The reason for the discrepancy between the absorptance of the AR coating and the measured absorptances of the coating

components is currently under investigation. Not all of the GATS/As^S^ AR coatings that we have produced have shown the low absorptance of the samples tabulated. However, for seven AR coatings deposited

at different times and on different KCl substrates, the highest absorptance measured was 0.07% and the mean was 0.035%.

Table 4. GATS/ASpS, AR Coating Absorption at 10.6 vm

COATING NO. SUBSTRATE ABSORPTION {%)

MRL 77-8 KCl 0.02

MRL 77 KCl 0.02

MRL 77-10 KCl 0.03

MRL 77-11 KCl 0.02

-224-
3.2. Reflectance

The reflectance of the AR coatings can be estimated from spectrophotometer transmittance charts;

however, for the commercial instruments available, this method is only accurate to about 1%. For more accurate measurements of the AR coatings, an experimental arrangement as shown in figure 5 was utilized.

A wedged (1 ) KCl substrate was used to eliminate interference in the reflected rays between the front

and rear surfaces, and was AR coated on one-half of the front surface as shown. This arrangement

spatially resolved the front and rear surface reflections so that a power meter could be positioned to record the front surface reflection. A sensitive measurement of the reflectance of the AR coating could

be obtained from the ratio of the power levels when the coated and uncoated portions of the samples are present in the beam.

The reflectances of a number of GATS/As2S^ coatings were measured and found to be less than 0.1%;

the lowest measured reflectance was 0.025%. Further improvement should be possible with fine tuning of

the coating-deposition procedures.

3 . 3 Damage threshold

The laser-induced damage threshold was determined for both pulsed and continuous-wave laser

radiation. The experimental arrangement for the measurement of damage threshold for pulsed radiation

is illustrated in figure 6. A TEA CO2 laser was operated in the TEM^^ mode and produced a typical

pulse, i.e. a sharp gain-switched spike of width 0.2 us followed by a decaying nitrogen-pumped tail

(= 3iJs). The energy in this pulse can be approximated by a square pulse of width 0.6 ys and height

equal to the height of the gain-switched spike. The intensity distribution at the sample plane was Gaussian with a(l/e ) diameter of 280 ijm. The damage threshold was defined as the peak-on-axis energy

density that just failed to damage the coating as revealed by subsequent Nomarski microscopy. The

2 damage threshold thus determined was found to be = 7 J/cm . For energy densities near the damage

threshold, the coating was damaged only at a few isolated points within the beam area, as shown in

figure 7. This behaviour suggests that the presence of inclusions may be the determining factor in this

assessment of damage. For conditions well above threshold the mode of failure was the complete removal

of the coating from the substrate.

Values of damage threshold for continuous-wave irradiation must be carefully interpreted since spot

size, time of irradiation, cooling, etc. all influence the value obtained. The damage threshold

determined in any particular experiment has the merit of ranking the performance of various coatings,

but cannot necessarily be taken as a firm design parameter. In our measurements, an area on the coating, 3' mm square, was irradiated uniformly for a fixed time of 3 seconds. This configuration is more representative of many practical situations than is the case with a smaller irradiated area. The GATS/

2 As2S^ coatings were observed to fail at power densities - 7 kW/cm . The mode of failure was the removal

of portions of the coating within the irradiated area and catastrophic cleavage of the KCl substrate did

not occur, although some melting of the film was evident. This behaviour suggests that induced stresses

2 in the films may be responsible for their removal. The figure of 7 kW/cm is not particularly high. However, it must be borne in mind that our previously reported GeSe/ASpS„ AR films exhibited [9] a damage

2 threshold - 60 kW/cm when irradiated with a spot size of 0.8 mm, yet their threshold in the large beam2 size experiment described above was somewhat less than 7 kW/cm .

4-. Conclusion

Two-layer AR coatings on KCl for 10.6-)Jm wavelength have been produced that utilize the chalcogenide glasses Ge^QAs-|^^Te^QSe22 and As2S^ as coating materials. The absorptance and reflectance of these

coatings have been measured to be as low as 0.02% and 0.025% respectively. The laser-induced damage

2 2 2 threshold has been measured to be = 7 J/cm for pulsed and = 7 kW/cm for cw irradiation (9 mm spot).

Whilst these damage thresholds are not particularly high, it should be noted that, in applications

where thermally-induced optical distortion rather than coating damage is the main limitation, the

extremely low absorptance exhibited by the coatings could be of advantage

.

5 . Ac knowle dgements

The authors would like to thank Mr. W.C. Schoonover of Air Force Avionics Laboratory and

Dr. M.C. Ohmer of Air Force Materials Laboratory for arranging for the supply of the substrate

materials used in this work.

-225-
6. References

Schuster, K., Ann. Phys. 6th series,

A, 352 (1949).

Braunstein, M., Allen, S.D., Braunstein, A. I., Giuliano, C.R., Turk, R.R., Wang, V., and

Zaccaro, D., Air Force Cambridge Research Laboratories Report No. AFCRL-TR-75-0429 (1975).

Baer, A.D., Donovan, T.M., Green, A.K. and

Turner, G. in Laser Induced Damage in Optical

Materials: 1976, NBS Special Publication 462,

p. 214.

Detrio, J. A., Petty, R.D., Ohmer, M.C. and

Swensen, O.F. in Laser Induced Damage in

Optical Materials: 1976, NBS Special Publication 462, p. 283.

Young, P. A., Thin Solid Films, 6., 423

(1970).

McLachlan, A.D., Appl. Opt. to be published.

Lewis, J.F. and Ohmer, M.C. in Laser Induced

Damage in Optical Materials: 1976, NBS

Special Publication 462, p. 279.

[8] Gibbs, W.E.K. and Butterfield, A.W.,

Appl. Opt., 14., 3043 (1975).

[9] Sparks, M. in Laser Induced Damage in

Optical Materials: 1976, NBS Special

Publication 462, p. 203.

[10] Savage, J., Royal Signals and Radar

Establishment (Malvern), United Kingdom.

Private communication.

[11] Webber, P.J. and Savage, J.A., J. of NonCiystalline Solids, 20, 271 (1976).

[12] Cox, J.T. and Hass, G., in Physics of Thin

Films, 2 (Academic Press, N.Y., 1964),

p. 257."

[13] Loomis, J.S. and Huguley, C.A., In Laser

Induced Damage in Optical Materials: 1974

NBS Special Publication 414, p. 94.

[14] Wang, v., Rudisill, J.E., Giuliano, C.R.,

Braunstein, M. and Braunstein, A. in

Laser Induced Damage in Optical Materials

:

1974, NBS Special Publication 414, p. 59.

7. Figures

AIR

Figure 1. Schuster Diagram for 2-layer AR coatings on

KCl. The shaded areas indicate areas for

which zero reflectance may be obtained. The

refractive indices of several candidate

coating materials are shown.

-226-
GATS

Film

Glass

Substrate

Figure 2. Scanning Electron Micrograph of

the transverse section of a fractured GATS film on a glass

substrate. The fracture surface

of the film is characteristic of

glassy materials.

3-4

Refractive

Index

Figure 3. The optical dispersion relationship for the chalcogenide

glass film Ge2oAs2., Te2gSe22

(GATS)

.

2 5

Wavelength, )um

10-6jum

SPECTROPHOTOMETER

COMPUTED

AS2S3 1 47)um

GATS 0-48 jum KCI

2-5 7-5 Wavelength, /jm

Figure 4. The spectral transmittance of

a GATS/AS2S2 AR coating on KCI

as measured by a spectrophotometer ( ) compared with values

(...) computed from the measured

optical constants and design

thicknesses

.

12-5

-227-
POWER

100 W

CO2

LASER

Figure 5 . Experimental arrangement for

the measurement of AR coating

reflectance.

Attenuators

Ge Lens Sample on

x-y stage

Figure 6. Schematic diagram of damagethreshold measurements.

Calorimeter

Single-mode TEA Laser

Photon -drag Detector

ABOVE THRESHOLD NEAR THRESHOLD

280 jum 280 jum

(1/e^) diameter d/e^) diameter

Figure 7. Nomarski micrograph of damage

sites within the irradiated

beam area. The site on the

right shows the damage near, but just above, the threshold

level. The site on the left

shows damage for power levels

somewhat higher than threshold.

-228-
COMMENTS ON PAPER BY McLkCHLAN ANV GIBBS

No cLiicuMion on tkU, papeA.

-229-
INTRINSIC AND THERMAL STRESS MODELING FOR THIN-FILM MULTILAYERS

Anthony M. Ledger and Roy C. Bastien

The Perkin-Elmer Corporation

Norwalk. Connecticut 06856

Abstract

Theoretical models of stress effects in thin films have been developed that can be used as a framework of a computer model for stress addition in multilayer films. A system of stress interferometers has been developed which enables intrinsic stress, thermal expansion coefficient, and Young's Modulus to be obtained for thin films. Data on these mechanical properties have been obtained for films of Thorium Tetrafluoride, Zinc Selenide, and Thallium Iodide

materials.

Key words: Film stress; infrared interferometer; stress interferometer; thin

film thermal expansion coefficient; Young's modulus; zinc selenide.

1 . Introduction

Multilayer dielectric coatings fabricated by thermal evaporation or sputtering exist in a stressed

condition. The stress levels often exceed the breaking strength of the bulk form of the material. The

total stress in such a deposited film is the sum of two basic contributions: an intrinsic stress characteristic of the thin film material; together with a thermal stress contribution caused by a difference

in thermal expansion coefficient between the thin film and its substrate. These stress levels can

greatly influence the durability of thin film structure, although a complete model of film durability

must include a description of the adhesive forces at each film interface and in particular at the boundary between the film stack and its substrate.

The initial impetus for the development of durability models for thin film structure was provided

by the difficulty in producing antireflection coatings for high expansion window material (KCjl, CaF2,

etc.) using traditional infrared coating materials. Although this particular difficulty has since been

overcome by the use of high expansion films of Thallium Iodide and Potasium Chloride-'-, the fundamental problem remains of predicting the durability of a thin film structure when exposed to intense

laser beams.

During the past year, a program was initiated to investigate durability models for thin films and

primary emphasis was directed towards the following areas.

• Stress addition models for multilayer coatings

• Development of a stress measuring interferomter system

• Mechanical property measurements of ThF4, ZnSe, T;^I infrared coating materials

A system of stable stress interferometers suitable for use at high temperatures was developed during

the program and utilized to provide stress measurements during film deposition, together with measurements of the thin film thermal expansion coefficient of several infrared coating materials. Before

discussing these specific measurement techniques, a brief review is presented in the following text

of the theoretical framework pertaining to stress effects in thin films.

2. Stress Measurements and Stress Effects in Thin Film Multilayers

Many different measurement techniques have been used for the measurement of intrinsic stress,

most of which depend upon detecting the minute deflections of a thin glass beam or disc upon which

the film is being deposited. Optical interference methods pro-vide a very sensitive method of measuring such small deflections although capacitative and electro -mechanical methods can also provide

comparable sensitivities. Comprehensive review of various stress measurement techniques have

been given by Hoffman^ and more recently by Kinosita^ and Hoffman'^.

1. Figures in brackets indicate the literature references at the end of this paper.

-230-
A stressed film depositing onto a flexible disc can be described in terms of a unit stress a(t) at some film thickness t as shown in figure 1. J£ the growing film produces a deflection a:(t) of the flexible disc due to the integrated effect of the unit stress function ait), up to a thickness t, the average

stress ^S(t)^ is given by the following expression.

s(t)

^h^2 ^^(t)

3(1-1.^) \Dj \ t s

(1)

The deflection of the disc is governed principally by the aspect ratio (h/D) and to a lesser extent by

the values of Poissons ratio j/g and Young's modulus Eg of the substrate material. A second order

term, 3 Ef (l-i/)t/hEg is included in the expression for the effective film stress and arises from

stress relief effects. When a stressed film is deposited onto an uncoated disc, the deflection of the

disc during the measurement produces a small amount of stress relief in the thin film material. Alternatively, if an external force is applied to the center of the disc so that the deflection produced by

a tensile film is removed, the film will elongate. In this case, the true value of film stress is larger

than if the plate were allowed to bend freely. The small magnitude of such stress relief effects in

practice (t/h «1) implies that the addition of a film to an existing multilayer stack will have little influence upon the stress levels of previously deposited films. In a similar manner, the deflection of a deformable mirror substrate will not produce measurable stress change in a mirror coating.

Although stress determinations using flexible substrates provide a measure of the total effective

film stress S(t) , nonuniformities in intrinsic stress as a function of film thickness are more

conveniently described in terms of the unit stress function a(t). This function cannot be measured

directly for a film material but can be computed from the measured data as follows:

(t) = ( S (t)> + t (S(t)) . (2)

The total film stress in a deposited film contains an additional contribution arising from thermal

stress caused by a difference in expansion coefficient between the thin film and substrate material.

If the film /substrate combination undergoes a uniform temperature change AT, the difference in

unit elongation of film and substrate is given by (OL^-a^) which leads to the existence of a thermally

induced stress a-pH gi'ven by

'^TH=^f^-V^^

where otf and are the expansion coefficients of the film and substrate material and E^ is the value of

Young's modulus for the film material.

The total stress in a thin film material which is deposited at a temperature T and subsequently

reduced to an ambient temperature Tq can be expressed as

oft) = (S(t)) +t^ (S(t) ) + Ej (a^-a^) (T-T^) (4)

and this expression forms the basis for the addition of stress effects in multilayer coatings.

When film materials are combined to form optical multilayer stacks, the stress levels in each

film combine to produce a total bending moment at the film /substrate boundary which tends to tear

the film from the substrate. This moment can be expressed in terms of the individual stress levels

in each film for a stack of m films as

t. m 1 ^STACK= I ^i^^)'^^

i=l Vl

where, the stresses a^iz) are defined according to eq. (4). The effect of this total moment on a flexible disc is such as to produce a deflection given by

3 ['-^Ad'

'^TOTAL 2 V E y , 3 ^STACK ^ s ^ h

In some cases this deflection can be minimized or reduced to zero resulting in so-called zero stress

multilayer designs but more properly denoted as zero moment multilayer structures.

-231-
From the preceeding discussion it is evident that a durability model for thin film multilayers requires, at the very least, experimentally measured data for intrinsic stress levels, the thin film ex- pansion coefficients, and the Young's modulus of each layer, together with adhesion and film breaking

stress levels. The present program addressed the measurement of the first three quantities using

a new stress interferometer system described in the following sections.

3. Interferometric Stress Measurements

The intrinsic stresses developed in metallic films have been measured by many authors to aid in

developing theories of film nucleation and growth. Stresses developed in dielectric films, however,

have not been investigated as thoroughly, particularly for thick films used in infrared coating designs.

The two most comprehensive investigations of stress effects in dielectric films were carried out by

Campbell^ and Ennos^. The stress behavior of various fluorides, bromides, and iodiodes, have been

measured by Campbell using a capacitance bridge whereas Ennos" utilized a laser interferometer to measure intrinsic stress in a large variety of coating materials as a function of the deposition conditions.

This type of laser interferometer configuration was found to be sensitive to vibration. Distortion

of the interferometer occurred at high temperatures. Recently, a high temperature Michelson interferometer designed for UHV applications has been described by Roll and Hoffman^. Thermal distortions are minimized in this design by obtaining an interferogram of the full aperture of the bending

disk and by deducing the deflection from successive photographs of the finge pattern. Tilts and axial

displacements of the substrate can be removed by computation, but the method requires substantial

data reduction time for each successive fringe pattern taken during deposition.

The problems of vibration and temperature disturbances have been minimized by designing a new

interferometer capable of continuous stress measurements during deposition at temperatures up to 250°C.

3. 1 Cats-Eye Interferometer

The cats-eye interferometer consists simply of a glass element whose front surface is polished to focus an incident laser beam onto two reflecting surfaces formed by the base of the cat's eye lens and

the substrate upon which the film is being deposited (figure 2). The base of the lens is coated with a hard dielectric reflector to reflect ~ 50% of the incident beam at 6328A and the deformable substrate

is coated on its top surface with a partially reflecting metal coating. In most experiments a gold

coating was used for this purpose since, when cold-deposited, gold has a low stress and has such

poor adhesion to the substrate that it can be easily wiped from the surface. In addition, gold films can withstand high temperature baking without the occurrence of excessive recrystallization.

This interferometer configuration acts in a manner similar to a retroreflecting mirror, and

alignment is a simple task. The reflected light consists of a set of circular interference fringes that

either expand or contract from the center as the substrate is deformed, depending upon the tensile or compressive nature of the film stress. A single silicon detector mounted in the center of the return

fringe pattern enables not only the amount of deflection to be measured but also allows the direction

of motion to be determined automatically from the fringe change recording.

O The interferometer is illuminated with a 4-mm diameter collimated laser beam (6328A) produced

by the interferometer laser source illustrated in figure 3. The polarized beam from a 2-mW He-Ne

laser is chopped by a synchronous chopper wheel and expanded to 4-mm diameter by a beam expander

and spatial filter. A polarizing beamsplitter reflects approximately 1 percent of the "p" polarized

laser beam to a silicon synchronous reference detector and transmits the remaining energy to the

cat's -eye interferometer. A quarter-wave plate located between the polarizing beamsplitter and interferometer rotates the polarization from "p" to "s" after two passes, and the return fringe pattern

is reflected by the polarizing beamsplitter to a fringe-cotint detector and display screen. The polarizing beamsplitter /quarter-wave plate system is used only to maximize the amount of light on the

viewing screen and is not fundamental to the operation of the interferometer system.

A system of four cat's-eye interferometers has been constructed to monitor film stress at four

radial positions in a 36" vacuum box coating system. An optical-thickness monitor is located on the

-232-
centerline of the chamber, and optical film thicknesses are monitored at any desired wavelength up to

1. OfJim by suitable choice of line filters. Optical thicknesses of the composite films of multilayers are individually monitored using fresh glass microscope slides for component films during an evaporation cycle. The entire assembly of interferometers, rigid substrate holders, and optical monitor

slides are enclosed in stainless -steel shields and can be treated to 250°C by a Calrod heater located

at the top of the vacuum chamber. Temperature control is obtained using an SCR proportional controller and a thermistor mounted close to the interferometer housings.

Stress changes in the films during deposition cause the bull's-eye fringe patterns at the fringecount detector to expand or contract depending upon the nature of the stress (tensile or compressive).

Changes in intensity of the central fringe are detected by PIN diodes and are fed to lock-in amplifiers

that synchronously demodulate the chopped signals using a reference signal from a second silicon

diode in the laser source housing. The outputs of the optical monitor and stress interferometers are recorded on a six-channel chart recorder and are also applied to the input of a data-logger /punched

tape recorder system. This stress interferometer system provides stable bull's-eye fringes at the

fringe count detector with all vibration effects eliminated. Stress measurements at equilibrium temperatures up to 250°C are reproducible although changes in temperature can cause erroneous fringe

counts during heat up or cooling cycles.

A unique feature of this type of interferometer is that of the magnitude and sign of the fringe

changes that can be detected using a single detector. The wavefront generated by the detector as the

fringes either contract or expand possesses waveform assymetry which occurs each cycle. The sign

of the assymetry depends upon the direction of motion of the flexible disc. Figure 4 shows this effect for a Magnesium Fluoride film deposited onto a Cervit disc at ambient temperature and illustrates the fringe covint waveform at the time the film undergoes cracking. Before the film deposition

point, the stress is tensile, and the positive going slope is greater than the negative going slope.

After the film cracks, the disc deflects in the reverse direction and the waveform assymetry changes

sign. This effect can be explained theoretically by the radial shearing which occurs in the focussed

beam at the two interferometer reflecting surfaces. ^

3.2 Stress Measurements

Stress measurements have been made for films of Thorium Tetrafluoride (ThF4), Zinc Selenide

(ZnSe)' and Thallium Iodide (Tjel) under a wide range of deposition conditions. The influence of vapor

incidence angle, evaporation rate, and substrate temperature have been investigated for ThF4. A

number of stress measurements have also been made for Thallium Iodide material evaporated from

an electron beam gun.

Data from the stress interferometer system is reduced using an IBM 70 and plots are obtained of

the sampled data, the average stress (S(t)) and the unit stress curve a(t) by best fit procedures.

The unit stress data can be well described in terms of three film stress coefficients A, B, and C ac- cording to

ait) = Ae'^* d-e'*^*) (7) Examples of the stress data obtained are shown in figures 5 and 6 for ThF4 and ZnSe materials as a function of the film mechanical thicknesses (microns). The experimental data for these materials

show a nonuniformity of stress with thickness for small film thicknesses. Examination of cracked

films of ThF4 and ZnSe deposited onto KCi substrates by microinterferometry show that flakes of

ThF4 material are usually highly curved whereas ZnSe film flakes are predominantly flat. This behavior conforms to the experimental stress curves since the nonuniform stress region is much wider

in ThF4 than ZnSe. Such stress nonuniformities may be caused by the flexible substrate being heated

by the source as the film is being deposited onto the surface and theoretical investigation of this effect are continuing.

Stress data obtained for ThF4 shows little dependence upon deposition rate, angle of incidence, or

deposition temperature, whereas the results obtained for Zinc Selenide films show a marked dependence upon evaporation rate. The possibility that this rate dependence is infulenced by source heating cannot be ignored at the present time. Thallium Iodide material exhibits extremely low stress

levels when deposited at 100°C from an electron beam gun source. Table I summarizes the stress

data obtained for these three materials in terms of the film stress coefficients A, B, and C defined

in eq. (7).

-233-
4. Experimental Measurement of The Thin Film Expansion Coefficients and Young's Modulus

Experimental values of the thermal expansion coefficient (cxj) and Young's modulus (Ej) for a thin

film can be obtained using the four-channel interferometer system by observing the total deflection

caused by thermally induced stress. Figure 7 shows the output from two interferometer channels during deposition of a film of ThF^ (X /4 at 10.6 \im) and during subsequent cooldown. This film is deposited onto Cervit and KCi discs and the thermally induced stress contribution is sufficient to produce a large tensile stress (+3400 kg/cm^) for the ThF4 on Cervit and a large compressive stress

(-1400 kg/cm'^) for ThF4 deposited onto KC£. When zinc selenide films are deposited onto KC;ein this

manner the large initial intrinsic stress - (1500 - 3000) kg/cm^ together with a thermally induced

stress of ~ 2400 kg/cm^ is usually sufficient to crack the films by compressive film failure at the

-4000 to -5000 kg/cm^ stress level.

Initial experiments using uncoated discs of various materials showed that substantial fringe

changes occurred upon heating and cooling presumable due to temperature gradients (radial and front

to back) existing in the flexible discs. Experimental measurements were therefore made using all four interferometers and coated and uncoated discs of each substrate material. Fringe changes were

utilized on cooldown since this results in a much lower cooling rate than diiring the heatup cycle.

The values of expansion coefficient and Young's modulus can be obtained from the expressions for

the thermally induced deflection of a thin disc, i. e.

,

3(1 -.^)t /2

CO 4E V h y ^f(«f-V('^i-V

s where (T^^ -T^ ) is the total temperature change, a^, j^^, and Eg being the expansion coefficient,

Poisson's ratio, and Young's modulus, respectively, of the substrate.

For two different substrate materials each coated with a film of thickness t, the thermally induced

deflections are

3(1 -V )t

s 4E^ \— J ^f^ f- 1

3(1 -u^)t

E, (a^-aj (T - T ) (9) o

^ 4E^ ^"h^; ^f^"f""2

Consequently, the values of the film constants and Ef are given by

(a. -Co) (T - T ) (10) o

"2^1^2 -^^2^1

= 7T~7, 1 (11)

W2

= . ,^ „ w ^ ) (12) o

Ag ("£-"2^* (T -T_)

3(1-.^) D\ 3(1-.^)

where

Ai = -IeT" ^2 = -417- ^ ^ ^

characterize the substrate properties.

In all experimental determinations the deflections and coo correspond to the differences in deflection between the coated and uncoated substrates, i. e.

,

u>l = (^i) - (^^) (14) c uc

c uc

The typical changes that occur during heating, cooling, and reheating of coated and uncoated discs

of different materials are shown in figure 8. Here, discs of Cervit and KCi are coated with ThF^

material, and the fringe changes as a function of temperature are used to compute the expansion coefficient and Young's modulus of the thin -film material as shown above.

-234-
Measurements were made of thin films of ThF4, ZnSe, T-^'I in this manner and the results are summarized in Table II.

Emphasis was placed on obtaining values of oc^ for thorium tetrafluoride since measurements of

this material in single crystal form by Van Uitert 9 show that the expansion coefficient is small and

negative in the region from 0 to 200°C. The experimental values for thin films consistently showed

a large positive value both for measurements made during cooldown and immediately after the film

was deposited and during subsequent reheating cycles. Different pairs of substrates (KCA+ Cervit

and KC£ + ZnSe) were also used for these experiments and gave similar results.

The expansion coefficient of thallium iodide material was so closely matched to that of the KCA

substrate that no discernible difference in the fringe changes could be seen for the coated and uncoated substrates as the temperature was varied.

5. Summary and Conclusions

The framework of a computer model for stress addition in thin film multilayers has been completed which uses measured data on film stress, thin film thermal expansion coefficients and Young's

modulus for each coating material.

A new cat's-eye stress interferometer has been developed which enables measurements to be

made at high temperatures (250°C) during film deposition. This interferometer offers simple alignment, immunity from vibrations, and fringe coimting sensitivity.

A system of four interferometers has been used to measure the stress levels in the ThF4, ZnSe,

and Tjll materials as a function of film deposition conditions (rate, temperature). Data has been presented for these materials in terms of three stress coefficients.

The interferometric system has also been used to measure the thermal expansion and Young's

modulus of thin films of the above materials. Thermal expansion values for ThF4 films in particular

have been shown to be much larger than the bulk single crystal values previously measured.

6 . Acknowle dgements

The authors are grateful for the dedicated assistance of M. R. Porier who conducted the experimental measurements and reduced copious amounts of data to significant essentials. This work was

sponsored by Captain Henry Winsor, under ARPA Contract DAA25-76-C-0410.

7. References

1. M. C. Ohmer, Laser Induced Damage in

Optical Materials Symposium, 1977, Bounder,

Colorado, ibid.

2. R.W. Hoffman, Physics of Thin Films, 3

211 (1966).

3. K. Kinosita, Thin SoUd Films, 12, 17 (1972)

4. R.W. Hoffman, Thin Solid Films, 34, 185

(1976)

5. R. S. Carpenter and D. S. Campbell, J. Mat.

Sci. , Vol. 2, p. 173, (1967)

6. A. E. Ennos, Applied Optics, Vol. 5, No, 1, p. 51, (1966).

7. K. Roll and H. Hoffman, Rev. Sci. Inst .

,

Vol. 47, No. 9, (1976).

8. A. M. Ledger, Applied Optics, to be

published

9. L. Van Uitert, et al. , Mat. Res. Bull. 11_, 699 (1976)

8. Figures

-235-
D

Film Thickness and

Unit Stress O (t)

Figure 1. Deflection of a thin flexible disc by a growing film of material

deposited in a stressed condition. The film stress is described

in terms of a unit stress a(t) acting in an element of thickness dt

at any thickness t.

Figure 2. Cat's-Eye stress interferometer configuration utilizing interference

effects in a focussed beam to generate bull's-eye fringes.

-236-
Spatial

Filter

Synchronous

Chopper

X

Laser

(2 mW He-Ne

Polarized)

/-Viewing Screen

-Fringe Detector

1[6

Beam

Expander

(4X)

Polarizing

Beamsplitter

Synchronous

Reference Detector

Figure 3. Cat's-Eye stress interferometer system. A chopped circularly

polarized laser beam is used to illuminate the interferometer

element which produces a set of circular fringes in reflection.

Fringe changes produced by deflections of the disc during coating

deposition are sensed by a synchronous detection system to provide

coating stress information.

Film Disruption Point

+ ve going slope > -ve going slope

Fringes expanding

out from center

+ ve going slope < -ve going slope

Fringe contracting into center

after film rupture

Figure 4. Fringe count for a 0. 5 -mm thick Cervit disc during deposition of

MgF2 (Note waveform asynnmetry indicating direction of fringe

motion before and after film cracking).

-237-
EXPONENTini MODEL:

R= 1057. 10

B= 0.000000050

C= 12.49699

STD. DEV.= 44.202

Average Stress (Best Fit)

Sampled Data

Unit Stress (j(t) =A exp (-Bt)[ 1 - exp(-Ct)]

t in fJ-m „ A = 1057. 1 (kg/cm ) B = 5x10-8

C = 12.

5

0.60 0.90 1.20 l.SO THICKNESS (MICRONS)I. BO 2. 10 2.110

Figure 5, Tensile stress produced in Thorium Fluoride film material deposited

at 37 A/s at 200°C. Stress measured from the deflection of 0. 5 -mm

thick Cervit substrate.

-238-
EXPnNENTini. MODEL:

n= -3300.00

B= 0. 199999600

C= 80.0000U

STD. DEV.- 83.205

0.08 0. 16 0.2M 0.32 0.40 THICKNESS (MICRONS)

0. so

-400 -

-800 -

S -1200

"bio

cn -1600

w

tn^ -2000

ui

Ul

<D

£-1 ^

-2400

oU

-2800

-3200

Average Stress (Best Fit)

Sampled Data

Unit Stress CT(t) = A exp (-Bt) [1 - exp(-Ct)]

-3600 .

-4000 _dt in M-m 2 A = -3300.0 (kg/cm ) B = 0. 199

C = 80. 0

Figure 6. Compressive stress produced in Zinc Selenide thin film material

deposited at 4. 4 A/s at 150°C. Stress measured from deflection

of 0. 25-mm -thick Cervit substrate.

-239-
M 3200

-2 3000 -

w 2800

t; 2600

COS 2400

w - 800

5 ^ 1000

a,a 1200

oP 14 00

stress I Buildup During

Coating

Deposition

ThF4

Deposited Onto

Cer-Vit (0.5 mm

at 2 00°C

ThF Deposite

Onto T<: CI

(1.0 mm) at

2 0,0°C .

1

Stress Changes Due to Differential Contraction

Between Film and

Substrate

Cer-Vit

ThF.

Increase in Tensile Stress

of IhF^ During Cooling

Temp. CO

00 \100 0

0.5 1.0 1.5 2.0

~ Mech. Film | \ KCl

\ ^^IlThF^

_ Thickness (m- ) 1

\ Compressive

\ Stress

\ Developed

\ in ThF4

VFilm During

\ Cooling

Figure 7. Intrinsic and Thermally Induced Stresses in ThF4 films deposited

onto Cervit and KCjl substrates at 200°C followed by cooling to ambient temperature.

-240-
Figure 8. Fringe changes for X /4 film of ThF4 deposited onto KC£and Cervit

during heating, deposition, and heat cycling. Full curves indicate

the fringe changes for cocked discs of KCi and Cervit

-241-
Table I. Film Stress Data for ThF ZnSe and T^^I Thin Films

Material

Rate

A /Sec

Temp

°C

Stress Coefficients

A

2 (kg/cm )

B

-1 (microns)

C

-1 (microns)

ZnSe 2. 6 100 -2 900 2. 0 25. 0

ZnSe 18. 0 100 -2100 0. 95 56. 0

ZnSe 4. 4 150 -3300 0.2 80. 0

ZnSe 9. 0 150 -2610 0. 4 75. 0

ZnSe 20. 0 150 o o n r\ -ZZUU 0. 9 DO. 2

ZnSe 24. 4 150 -2384 0. 9 74. 3

ZnSe 3. 0 200 -4640 ~0. 0 9. 0

ZnSe 19. 0 200 -3800 ~0. 0 21. 5

ZnSe 24. 0 200 -3020 0. 4 82. 5

ThF,

4

30A/Sec 150°C +1057 ~0. 0 12. 5

Til 100°C ~150

Table II. Expansion Coefficient and Young's Modulus Values for

ThF ZnSe, and Thallium Iodide Materials

Substrates

Utilized

Film

Material

Film Expansion

Coefficient (a^) Young's

Modulus (Ej)

KCji, Cervit ThF,

4

13. 3 X

10~^ /"C

5 , 2

4. 3 X 10 kg/cm

KCl, Cervit ThF,

4

11. 1 X 10"^/°C 3, 9 X 10^ kg/cm^

KCl, ZnSe ThF,

4

18. 1 X 10"^/°C

5 , 2 6.8x10 kg/cm

KCZ, ZnSe ThF^ 15. 1 X 10'^/°C

5 , 2

4. 3 X 10 kg/cm

KCJl, ZnSe Til 36. 0 X 10'^/°C 2. 3 X 10^ kg/cm^

KC^, Cervit ZnSe 8. 1 X 10"^/°C 4.2 X 10^ kg/cm^

-242-
COMMEWTS ON PAPER BV LEVGER ANV BASTIEW

Th^ quution wca icuAzd concM-rUng the. {^acJ: thaut thofvLum (^luo^dz ihowi a negative zxpan^lon cozif^lciie.YVt

yin the, bulk, wkiZ^ tht meM>uAe.d uoZue -cn the. tkln {^JJjn wcu, poi>ltLvz. The ipmkeA Indicated {^iut,

that the accuAacy o{j the eKpaiu-ion coeaiciewt meMu/ieimnt miu utimated to be app^oXAjnately 30%, and

that the dif^deAence in iign between the expani-Lon coe{t{jicyient in the thin {^iJbn {^onm and the bulk donm

Mai attributed to the {act that the iibn moa not pu^eZy ThV^. He indicojted. that it would be veny

inteAe.iting to peA^owt the expeAAjment u&ing a (,iMn o^ tkofu.m ^luoiidz d&poiited on a iubitAate

Thl=,.

-243-
HYDROXYL INFLUENCE AND REFRACTIVE INDEX DEPENDENCE

IN PICOSECOND THIN-FILM DAMAGE*

W. Lee Smith, D. Milam, and M. J. Weber

Lawrence Livermore Laboratory

University of California

Livermore, California 94550

and

A. H. Guenther, J. R. Bettis, and R. A. House

Air Force Weapons Laboratory

Kirtland Air Force Base

New Mexico 87115

Results of a study of the influence of OH content of silica substrates on the damage

resistance of thin films under 1.06-vim, 150-ps pulse illumination are presented. The OH

content variation was provided by the use of four types of commercial silica (Suprasil-Wl,

Optosil-2, Homosil, and Suprasil-1) as substrates. The OH concentrations, as indicated

by IR absorption spectroscopy, are approximately 5, 130, 130, 1200 ppm, respectively. For

each of the substrate materials, a set of four samples was made by depositing X/2 (at

1.06 um) films of ZrO^, Si02, ThF^, and MgF^ . Furthermore, an uncoated but otherwise

identical sample of four uncoated silica materials was tested to provide baseline surface

damage data. The results of this study are discussed in regard to earlier work of House

et al. concerning_hydroxyl chemistry at film-substrate interfaces. A new interpretation

of the role of OH in the breakdown process for nanosecond and picosecond pulses is

introduced

.

A second important aspect of this study is the correlation of thin-film damage threshold

with film material refractive index. On each substrate type, films covered the index range

of 1.37 to 2.0. The threshold range observed in this picosecond study is discussed with

regard to possible improvements in coating damage thresholds by use of low- index materials.

Key words: Thin-films; damage thresholds; hydroxyl content; half-wave films; SiO^; ZrO^,

MgF,; ThF,; refractive index.

Introduction

In this study two basic questions were addressed:

I. To what extent does hydroxyl chemistry at a film-substrate interface affect damage resistance?

By using a commercially available family of silica materials as substrates for the coated test samples, one has the ability to alter the hydroxyl (OH ) concentration of the substrate by a factor 250 and,

hence, to possibly alter the oxidation state of deposited thin-film materials at the interface. In a previous investigation, the possibility was entertained that the production of a different refractiveindex component at the thin-f ilm/substrate interface could significantly influence the damage threshold.

For example, when Si02 is deposited onto a substrate, SiO is formed to a small extent [1]. Because

SiO has a higher refractive index than Si02, its damage threshold should be lower than Si02 [2].

Increasing the oxygen availability (in the form of OH ) therefore allows the possibility of increasing

the damage resistance of the thin-film system by more complete oxidation of the silicon. Another

oxide depositant, Zr02, would be expected to behave in the same fashion. Conversely, MgF2 and perhaps

ThF^ thin-film damage resistance could be expected to fall with increasing oxygen availability, due to production of MgO or ThOF2 due to the production of higher index, less damage resistant coating.

Later in this report data gained from a 106A-nm, 150-ps pulse length damage study will be employed

to advance a new interpretation of the role of substrate OH content in the damage process.

II. To was extent can the high damage thresholds of low- index materials, e.g., fluorides, in

the bulk form be retained in thin-film systems? For several years data have been published which show

that materials with low refractive index offer generally high damage resistance. The bulk damage

thresholds of fluorides have been measured using both picosecond and nanosecond pulses, and fluoride

thin film thresholds have been measured using nanosecond pulses. Much of this data is summarized in recent publications which document the dependence of damage threshold on refractive index [2].

The work at the Lawrence Livermore Laboratory was performed under the auspices of the Materials Sciences

Programs of the U.S. Energy Research and Development Administration and Contract W-7405-eng-48

.

-244-
Yet to what degree the relative strength of the fluoride materials would be retained, or diminished

in thin-film form under picosecond pulse illumination was not yet investigated. Thus this picosecond

investigation of OH influence is an extension of work with 40-ns, 1.06-pm pulses by Bettis, House,

and Guenther [A], and the results of the two studies will be compared later in this report. As recent

progress in thin-film antiref lection (AR) coating design [3] has provided new possibilities for use of

lower index materials in practical AR coatings, the importance of the fluorides as coating constituents

can be expected to increase. One area of obvious concern is the production of homogeneous fluoride

coatings. In this sense homogeneous means uniform in refractive index throughout the film. The bigger

question is the development of understanding and preparation techniques that will allow materials in

thin film form to exhibit bulk-like behavior and characteristics.

Sample Characteristics

A. Substrate Materials

Five samples (3.8 cm diameter, 9 mm thick) of each of four types of fused silica (Suprasil-Wl,

Optosil-2, Homosil, Suprasil-1) were obtained from a commercial supplier [5]. The vendor's literature

lists the OH content of' these materials to be approj^imately 5, 130, 130, and 1200 ppm, respectively.

Spectrophotometer investigation between 1 and 3 ym verified that the relative concentrations of OH in our samples agreed approximately with the vendor's literature. Table 1 summarizes that data. The

absorption coefficients listed pertain only to OH absorption [6] and are therefore proportional to the

OH concentration in each sample. The ratios of coefficients of the various types agree with the

relative concentrations indicated by the vendor.

Table 1. Hydroxyl absorption coefficient (cm ) of silica substrates.

Silica Type

1.38 ym

Absorption Peak Wavelength

2.22 ym 2.72 ym

Suprasil-Wl 0.11

Optosil-2 0.02 0.08 4.18

Homosil 0.02 0.08 3.94

Suprasil-1 0.16 0.58

The twenty substrates were polished by identical bowl-feed procedures with Zr02 abrasives to specifications of X/20 smoothness in the visible and 0/0 scratch/dig. The possible implications of using

Zr02 abrasive in a test involving Zr02 as one of four coating materials is discussed in a later section.

Prior to coating, the substrates were rinsed with alcohol, washed in nitric acid and/or acetone.

Immediately before insertion into the coating chamber, an alcohol-wetted tissue was pulled across

each surface.

B. Coating Procedure

The coatings prepared by CVI, Inc., Albuquerque, New Mexico, employed in this study were single

layer films X/2 in optical thickness at 1.06 ym of Zr02, Si02, ThF^, and MgF2. In all cases, highpurity starting materials were employed. Prior to coating, the chamber was pumped down to 10 ^ torr. During coating, a pressure of 5 x 10 torr of oxygen was maintained as is standard practice. The

coating was electron-beam deposited at a rate of 16-20 minutes/half-wave. Substrate temperatures were

400°C; cooling to room temperature was slow (overnight). Just prior to damage testing, the samples were air-Jet dusted and dragged with an alcohol laden tissue.

C. Optical Characteristics of Coatings

Figure 1 exhibits the calculated optical standing electric fields for these A/2 coatings [7].

Along each vertical axis is plotted the time-averaged square of the electric field, E^, of a wave

(incident from the air) as a function of distance into the film-substrate composite. is normalized

to unity at the air-coating interface. The arrows point to the internal positions where various

extrema of are reached. Note that W = 0.666 occurs at each air/coating and coating/substrate

interface. Thus, all four of the coatings experience identical ¥^ values at the coating/substrate

interface—the location of primary interest in this experiment. Furthermore, to within 11%, the

maximum values of E are the same in all the specimens. The

^

values in the middle of the Zr02 and

ThF^ layers are reduced because their refractive index exceeds that of the Si02 substrate. E^ in

MgF2 is conversely enhanced by 11% over its interface value. This series of A/2 coatings is ideal

for providing uniform irradiation of the different samples at the interface of interest and minimizing

variations in the maximum of E^ reached throughout the thin films.

-245-
Experimental Apparatus and Procedures

Linearly polarized, 1.06-ym, 150-ps damage tests were performed with a laser system and techniques

described in detail elsewhere [8]. The beam was gently focused to a spot size of 2-3 mm diameter at the

e intensity level. A photographic recording of the sample-plane flux distribution is made for each

shot. Microdensitometric and computer analyses then yield the flux value at the spatial maximum of each

pulse. The single-shot flux uncertainty is ± 7%. Streak camera data of the temporal profile of each

shot were not available for this experiment. An average pulse duration (intensity FWHM) of 150 ps

should be used to convert flux to intensity [9]. However, for the laser used in the present experiment,

as is true of generally all lasers that are mode-locked by a saturable absorber, a distribution of

durations (with width ± 30% of the average) is actually produced [10].

Typically 30 sites were irradiated per sample. Identification of damage was made by breath fogging

the surface while illuminating it with a high-intensity microscope light. This method was more reliable

than observing a breakdown plasma, as some samples never yielded visible sparks unless irradiated

appreciably above the threshold indicated by fogging light-scattering. Nomarski microscopy was employed

occasionally on borderline sites, but consistent indications were always obtained with the fogging/light

scattering method.

The damage threshold for each sample was defined to be the flux value midway between the highest

flux which did not damage the sample and the lowest flux which damaged the sample. The uncertainty in

the threshold position (due to insufficient resolution from a limited number of shots) was summed with

the uncertainty for the individual shot flux to yield the final threshold uncertainty. For the

occasional situation in which overlap occurred between the damaging and non-damaging ranges of flux,

the midpoint of the overlap region was defined as the threshold position. The largest overlap width

encountered was ± 5% of the assigned threshold position, an amount less than our single-shot flux

uncertainty.

The complete experiment was performed twice in two consecutive weeks. Thresholds agreed for the

two separate runs for every material, thus indicating that potential changes in damage threshold due

to OH content alteration by exposure to air or breath-fogging were negligible.

Damage Results

Results for the 150-ps, 1064-nm damage thresholds are tabulated in Table 2 and displayed in

Figure 2. All flux values discussed herein are values achieved in air at the sample surface; no refractive index corrections have been applied. The notation for the coatings and substrate materials

is as follows: Z, S, T, M, SW, 0, H, and Sldenote ZrO^, SiO^, ThF^, MgF2, Suprasil-Wl, Optosil-2,

Homosil, and Suprasil-1, respectively. Note that the common logarithm of the substrate OH concentration

is plotted along the segmented horizontal axis in Fig. 2. The Optosil-2 and Homosil data are displaced

from each other for the sake of presentation only, as they have nominally the same OH concentration.

The large uncertainty bars on the SO and SSI data in Fig. 2 are due to unfortunate coarse bracketing of

the threshold resulting from less than optimum beam splitting ratios.

Table 2. Damage threshold flux data.

Coating-Substrate Flux (J/cm^) Coating-Substrate Flux (J/cm^)

ZSW 2 9 + 0.3 TSW 6 4 + 0.7

zo 3 6 + 0.3 TO 5 6 + 0.4

ZH 2 8

+ 0.4 TH 6 1 + 0.7

ZSl 2 9

+ 0.3 TSl 5 9+ 0.5

ssw 4 6 ± 0.5 MSW 6 0 + 0.6

so 5 6 + 1.3 MO 5 3

+ 0.5

SH 5 4

+ 0.5 MH 5 8 + 0.7

SSI 3 8

+ 0.9 MSI 6. 0 + 0.7

Bare Substrate Flux (J/cm^)

SW 12.7 + 2 7

0 8.7 + 0 6 H 8.2 + 1 5

SI 9.8 + 1 4

-246-
It was discovered after the completion of the experiment that the coating vendor used ZrO^ as the

polishing compound on all the substrates. Concern was expressed initially by us that the ZrO^ usage

might conceivably have affected the damage behavior of the samples In a significant way. However,

the different patterns of the data shown in Fig. 2 for the two oxide films (including ZvO^), the two fluoride films, and the bare substrates, probably rule out any major compromise in the behavior of

the various samples resulting from the use of Zr02 polishing abrasive.

In answer to our initial questions, specifically the first query, the data in Fig. 2 illustrates

that no obvious correlation is evident between the thin-film damage thresholds and substrate OH

concentration for 150-ps, 1064-nm pulses. This result is to be contrasted with the 40-ns, 1064-nm

work of Bettis, House, and Guenther [4]. In that earlier work the breakdown flux changed monotonically

in the Si02-coated samples by a factor of 4.8 as the substrate OH concentration was varied through

approximately the same range as in this study. In their MgF2-coated samples, a flux change of 40%

was observed. The apparent independence of the breakdown threshold on OH concentration in this

subnanosecond experiment is consistent with results in a different 150-ps experiment reported in these

proceedings [11]. There, a diminuition in the dependence of the damage threshold on surface roughness

was found for 150-ps pulses vis-a-vis that found for 40-ns pulses.

The results from these two 150-ps studies lead us to a new interpretation of the probable role

of the OH species at the coating-substrate interface. It is no longer clear that the manner in

which the OH affects damage resistance is via chemical production of a species with a different

refractive index (e.g., SiO in an Si02 film). In fact, from avalanche ionization considerations, it

is difficult to argue that a part per million SiO in a SiO„ medium would affect the overall lattice

impact ionization development. Only once in every 10 collisions would an SiO molecule be_involved in

the breakdown process. That is true, unless the breakdown initiation is facilitated by OH presence

as could be affected through a modification of the Si02 structure, i.e., a weakening. Consistent

interpretation of the damage dependence on OH concentration in the 40-ns and 150-ps experiments is

obtained only if one assigns the OH role to be assistance in the liberation of charge carriers in the

earliest stage of the breakdown process. The results of the 40-ns and 150-ps experiments indicate

strongly that at 40 ns the bottleneck to the breakdown process is its initial phase—the plasma

inception which depends on the promptness and difficulty with which initiating electrons are liberated

[12]. Hence, phenomena such as (1) OH concentration—which may function primarily in few ppm con- centration to influence the number of unsaturated bonds (states within the band "gap") —or (2) surface

roughness—which concentrates field distributions at all times during the pulse and thus enhances

initial carrier production—are probably of greater importance for nanosecond pulse durations than for

picosecond durations, if the proposed hypothesis is correct.

For the picosecond regime, the higher field strengths required to provide fast ionization and

sufficient incident flux for the heating stage (melting the lattice via the plasma) more effectively

drive initial nonlinearities [12] to provide numerous initiating carriers early in the pulse [13].

Hence, the importance of OH presence or surface roughness would be expected to decrease for subnanosecond pulses.

As regards our second question, the data displayed in Fig. 2 show a general correlation with Jjk refractive index. One should remember the graphs of Fig. 1 when comparing the MgF2 thresholds with ^

those of the other materials. According to the standing-wave analysis, the MgF2 films experienced 11%

greater electric fields than the other materials, per unit flux. Thus an correction would raise

the MgF2 thresholds by 11% to positions exceeding the ThF^ values, as one would expect from previously

reported refractive-index dependences of the damage threshold [2]. However, the thresholds of the

Si02 and ThF^ groups do not lie in the relative positions predicted by the refractive- index dependence.

The Si02 and ThF^ indices differ only by 0.04, however, apparently allowing other effects (e.g., porosity,

surface chemistry, inhomogeneity) to assume comparable importance.

The data displayed in Fig. 2 are encouraging for operational prospects of improved thin-film damage

resistance. In many current high-power laser systems, Ti02/Si02 are the coating materials used for thinfilm applications. It is generally observed that Ti02 and Zr02 have roughly the same damage resistance,

so we may view ZrO. and SiO, in Fig. 2 as representing the present wide-usage high- index and low- index

2 coating materials. The ZrO„ and SiO„ films damaged at average levels of about 3.0 and 5.0 J/cm , 2 respectively. As damage often occurs at the threshold of the high-index material, roughly 3.0 J/cm

characterizes the damage resistance of such films. Hence AR film designs which could employ exclusively

the low-index (n < 1.5) materials such as the fluorides present a distinct opportunity for improved

1.06-um damage resistance. For example, a three-layer [3] or alternate AR design employing ThF^ and MgF2

-247-
layers on a silica substrate may raise the 1.06-ym, AR damage resistance to the level of about 6 J/cm . In a field where even 30% improvements come only with struggle, such a 100% advancement would be

significant for many applications. Hence, the data shown in Fig. 2 are valuable in pointing out potential routes for improvement.

On the other hand, a look at the absolute damage thresholds in Fig. 2 relative to perhaps the

ultimate achievable thresholds—those of the bulk materials—reminds us of our primitive state of

understanding of (particularly) the thin-film damage mechanism details. Consider the SiO„-on-silica

2 series. The 30-ps bulk damage threshold inside silica (Suprasil-1) [14] is 13 J/cm which converts

2 to 35 J/cm in air for 150 ps by the square-root of the pulse duration [15]. Therefore, the bare

surface threshold average of our silica series from Table 2 is a factor of 3 to 4 lower than bulk,

2 but the thin-film average of about 5 J/cm is a factor seven lower than bulk. Much work remains to delineate convincingly the origins of these large differences between bulk and film behavior.

Conclusions

1. Within experimental uncertainty, no evident correlation of damage threshold with OH concentration

in silica substrates was observed for ZrO^, Si02, MgF2 and ThF^ films in 150-ps, 1064-nm experiments.

2. Two fluoride materials, ThF^ and MgF2, in thin-film form were found to be roughly twice as damageresistant as Zr02. Si02 was found to be 70% more damage-resistant than Zr02, in general agreement with an earlier oxide-film, picosecond study [16].

3. The absence of a strong correlation of damage thresholds with OH concentration at 150 ps and the

presence of correlation at 40 ns is a further indication that the initial stage of charge-carrier

buildup is the breakdown process bottleneck at 40 ns but not at 150 ps. This observation implies

that efforts to increase damage resistance by eliminating sources of initial electrons (by

annealing to reduce color center populations, ultrapurif ication, etc.) would be more effective

for supernanosecond pulses than for subnanosecond pulses.

Acknowledgments

We thank G. E. Murphy of LLL for his able technical assistance during the several hundred laser

firings for this experiment. Dr. J. H. Apfel of OCLI is acknowledged appreciatively for providing our

Fig. 1. For the manuscript preparation, we thank Ms. K. Wenzinger.

References

[1] Ritter, E. , in Physics of Thin Films, (Hass,

Froncombe, and Hoffman, Eds., Academic Press,

New York, 8^) p. 6 (1975)

.

[2] Bettis, J. R. , Guenther, A. H. , and Glass,

A. J., Nat. Bur. Stand. Spec. Publ. (U.S.)

414, p. 214; also Bettis, J. R. , House, R. A.,

Guenther, A. H. , and Austin, R. , NBS Spec.

Publ. 435, p. 289; also Bettis, J. R. , Laser

Induced Damage as a Function of Dielectric

Properties at 1.06 pm, Ph.D. dissertation

Air Force Institute of Technology, Wright

Patterson AFB, OH (1975).

[3] Baer, A. D. , NBS Spec. Publ. 462 (1976) p. 221.

[4] House, R. A., Bettis, J. R. , and Guenther,

A. H., NBS Spec. Publ. 462 (1976) p. 310;

Bettis, J. R. , Final Report AFWL-TR-76-61,

NTIS, Washington, D.C. (1976).

[5] Amersil, Inc., 685 Ramsey Ave., Hillside,

N.J. 07205, product literature.

[6] The absorption coefficients (a) listed in

Table 1 were obtained by inverting a Beer's

law relation, T^/T2 = e where T^^ is the

transmission (%) at the wavelength of a given absorption peak. T2 is a transmission

(%) obtained by constructing on the transmission graph a straight line tangent to the

flat base at the sides of the absorption

peak. The transmission denoted by the

straight line, at the wavelength of the

absorption peak, is T2. T^/T2 is independent

of Fresnel losses or intrinsic (non-OH ) silica absorption.

[7] Apfel, J., OCLI, Inc., Santa Rosa, CA,

private communication.

[8] Milam, D., Appl. Opt. 16, 1204 (1977).

[9] Apfel, J. H., Enemark, E. A., and Milam, d. , Smith, w. Lee, and Weber, M. J., this

proceedings.

-248-
[10] Smith, W. Lee, and Bechtel, J. H. , Phys.

Lett. 55A, 203 (1975).

[11] Milam, D. , Smith, W. Lee, Weber, M. J.,

Guenther, A. H., Bettis, J. R. , and House,

R. A.

[12] Milam, D. , NBS Spec. Publ. 462, p. 350 (1976).

[13] Smith, W. Lee, Bechtel, J. H. , and Bloembergen,

N., Opt. Comm. 18, 592 (1976).

[14] Smith, W. Lee, Bechtel, J. H. , and Bloember

N., Phys. Rev. B15, 706 (1975).

[15] Expressions of /x dependence of breakdown

threshold are found in a) Bettis, J. R.

,

House, R. A., and Guenther, A. H. , NBS

Spec. Publ. 462, p. 338 (1976), b) Laser

Program Annual Report, Lawrence Livermore

Laboratory Report UCRL-50021-76 (1977),

c) reference 13 above.

[16] Newnam, B. E. , and Gill, D. H. , NBS Spec.

Publ. 462, p. 292 (1976).

Figures

E2 0.350

1 M 0.666 on all

0.666 on all—

n: 1.0 2.0 1.45

1 -

Air Zr02 SIO2 substrate

0.631

1

1.0 1.49 1.45

Air ThF4

0.666

1

Si02

1.0 1.449 1.45

Air SIO2 SiOz

0.739

1.0 1.377 1.45

Air MgF2 SiOz

Fig. 1. Illustrations of the time-averaged electric field squared as

a function of distance into X/2 coated samples [7]. E^ is

normalized to unity at the air-coating interface. The arrows indicate positions where various values of are obtained.

The refractive indices of each material are included in each

illustration.

-249-
0123 40 123 40 123 40 1234

LoQio ^OH' in ppm)

Fig. 2. Damage threshold flux results for the A/2-coated samples. Note

that the common logarithm of OH concentration is plotted along

the segmented horizontal axis. The Optosil and Homosil samples

are displaced horizontally from each other only for clarity.

Their OH contents are essentially equal Z = ZrO„ ; S = SiO„

;

T = ThF^, and M = MgF^ for the films. SW = Suprasil-Wl;

0 = Optosil-2; H = Homosil, and SI = Suprasil-1 for the fused

silica substrates.

COMMEWTS ON PAPER BV SMITH, MUAW, WEBER, 6UEWTHER. BETTIS, ANV HOUSE

TheAz wcu> zxte-nilve. cU^cMi^on concM.ning the. quution on, to whzthzn. the. damage th/iuhold Man, cJLejxnJLy

ddf^lndd In the. ihonX puZiz mejuuxmewU , on MhhthoA the. damage me.(jU,u/Le/nent^ e.xhtbtted c/ioii-oveA.

Cfioii-oveA. -66 the tenm de^c/Llbtng the ^AJuiation tn which iome paZieA at a kigheA eneAgy do not pKoduce

damage, wkiZe otheM at a ZoweA eneAgy do. It woi Atpoitzd that at 40 nanoiecondi the cA06i-oveA woi

aiwayi ob.ieAve.d In damage testing. The ipexikeA Indicated that in te^ti at 30 and 150 picoie.cond6,

cAoi,6-oveA woi neveA obieA^icd. BfUan Wewnom, ^Aom LASL, indicated that in kti mza^uAementi aanAizd

oat in the. picosecond Kegime, cAoa-oveA woi aiwayi obieAved. TheAzf^onz the point lemalm unAeioZved.

The question o{i ihoAt puZ&e damage ve/u>ui long puZie damage wai Aeviewed, and the ipeakeA once again

indicated that 6ab-nanoiecond damage meaiuAementi seemed ^aA lei>i, semiJxve to extAaneouA iouAceA o^

eZecXAom than long puLi,e damage mexUiUAementM , and aLso that at ihont puLbed, damage thAeiholdi deemed

to be totaZJLy indendijtive to the tineoA abioiption o{j the medium. Howeven., he did acknowledge that iX

ti veAy dLi{i{iicaJLt to meoiuAe the lineoA abioAptxon o^ mateAiaJU, in thin {^ilm {^onm.

-250-
FURTHER STUDIES OF THE ROLE OF

ELECTRIC FIELD STRENGTH IN LASER DAMAGE

OF DIELECTRIC LAYERS

JOSEPH H. APFEL

Optical Coating Laboratory, Inc.

Santa Rosa, California 95402

In this study a silica/titania quarterwave stack

reflector, a four-layer antiref lection coating, and a combination of the two were prepared in a single evaporation coating run and damaged with 180-ps, 1.064um

laser pulses. The peak pulse intensity at the threshold

of damage was highest for the reflector stack and lowest

for the AR coating. Although relative thresholds of the

reflector stack and the combination are in the order predicted by the model of damage occurring at the location

of peak field intensity in the titania layers, the AR

coating damaged at an anomalously low value.

The results indicate that the substrate/coating

interface is vulnerable when illuminated by the laser

pulse as in the case of the AR coating.

Key words: Antiref lection coatings; dielectric films;

electric fields; laser damage; nonlinear absorption;

optical coatings.

1. Introduction

In studies of damage to optical coatings by 125-ps 1.064ym laser pulses, Milam

[1} , found that antiref lection (AR) coatings generally fail at lower pulse intensities

than dielectric reflectors. This is contrary to expectations based upon an analysis of

the internal electric fields [2] . In this study we evaluated the damage threshold of these

two types of coatings and a combination of them which were composed of silica and titania films prepared under controlled circumstances. These results provide for a quantitative comparison of the threshold versus the computed electric field intensity

in each of the coatings.

To explain the discrepancy between these results and the previously reported cor- relation between peak field intensity and damage £2] , we consider two models; nonlinear

absorption in the high index titania layers and substrate/coating interface vulnerability. The nonlinear absorption model is shown to be incompatible with experimental re- sults.

Another study reported at this conference [3] , which describes the effect of barrier layers at the substrate/coating interface, supports our conslusion that the interface is especially vulnerable to damage.

2. Experimental

Using equipment and procedures described previously £2] , we prepared three different multilayer coatings in a single evaporation run on super polished substrates of

BK-7 glass.

The three coatings were an eleven layer quarterwave stack high reflector (AR)

,

and a combination of the two in which the AR coating was deposited on top of the HR

coatings (see figure 1)

.

By means of externally actuated masks, the vapor from the electron beam heated

silica and titania sources was allowed to deposit onto substrates A and B for the eleven layer HR coating and then onto A and C for the AR coating. Although, the combination AR+HR coating has no practical application it was included to demonstrate that the

low threshold of the AR is not inherent in its design.

After coating, the three samples were delivered to the Lawrence Livermore Laboratory where they were tested for damage threshold by the techniques described pre-

-251-
viously £1,3]. For these samples only one laser beam was incident during each shot and

every shot was subjected to streak camera recording.

3. Experimental Results

Table 1 contains the results of damage testing. Thresholds are quoted as laser

pulse energy density (Joules/cm^) and laser pulse peak power density (GW/cm^ ) . From

the streak camera recordings which were used to convert the energy density data to

power density data, the average pulse width (FWHM) was found to be 180 picoseconds.

Table 1. Measured Damage Threshold

for 180-ps, l,064ym

Energy Density Peak Intensity

Sample Description ( J/cm ) (GW/cm^

)

11 Layer 7.0+0.5 45±5

B Quarterwave Stack

Reflector (HR)

4 Layer 3.2±0.3 17±3

Ant ire flection

Coating (AR)

Combination 5.1±0.5 2 8+4

A AR+HR

4. Discussion

Figures 2,3, and 4 are the electric field profiles of the three coatings at threshold. The incident intensities for these three profiles are proportional to the pulse

energy densities at threshold. Due to variations in the pulse length and shape, the

peak intensities are not exactly proportional to the pulse energies; if the profiles

were plotted using peak intensities the combination coating (Sample A) would have

slightly greater amplitude than shown.

Clearly, the three samples do not have the same peak electric field intensity in

the high index titania layers as observed by Apfel, et. al. [2]. In that study, four

assemblies of quarter and half-wavelength thick layers were prepared simultaneously and

found to have approximately equal peak electric field intensities at threshold.

Since the AR design contains non-quarterwave layers we sought an interference com- patible mechanism that would somehow make the three coatings similar under intense ra- diation. It was hypothesized that nonlinear absorption in the high index titania layers might cause sufficient absorption to modify the electric field profiles, and thus

the absorption, to a greater extent in the reflector (B) and the combination coating

(A) than in the AR coating (c)

.

Figures 5 and 6 show the results of the analysis. The high index layers of all

designs were subdivided and given absorption values proportional to the local field

intensity and the internal field was recomputed and then the process was repeated until no significant change in field occurred. Figures 5 and 6 resulted after five cy-^

cles. The proportionality constant was chosen to just cause melting of titania(1830 C)

in the sample with the highest peak field. The computed peak temperatures are shown in

the figures. Even with these rather high absorption coefficients the interference generated electric field profiles for the three coatings are not significantly changed

from the original curves which assume essentially zero absorption.

The AR coating, which failed at the lowest pulse energy density, is different from

the other two coatings in that there is intense radiation at the substrate surface. We

postulate that the substrate/coating interface is vulnerable to damage because of con- tamination from the substrate polishing process or from an undefined chemical reaction

between the substrate and the adjacent titania layer. Our experiments with barrier

layers of silica and alumina are reported at this symposium £3].

-252-
5. Conclusions

An antiref lection coating for 1.064ijm composed of evaporated silica and titania

has anomously low damage threshold relative to a guarterwave stack reflector and a com- bination coating consisting of the AR on top of the HR coating. It is proposed that

the low threshold is due to an unexplained vulnerability of the substrate/coating

interface

.

6. Acknowledgments

The damage threshold measurements were made by D. Milam and W.L. Smith of the

Lawrence Livermore Laboratories.

7. References

[l] Milam, D. (1977)

Appl. Opt. 16, 1204

[2} Apfel, J.H., Matteucci, J.S.,

Newnam, B. , and Gill, D. , NBS

Special Pub. No. 462, 301 (1976)

£3] Apfel, J.H., Enemark, E.A.,

Milam, D. , Smith, W.L., and

Weber, M.J., "The Effects of

Barrier Layers and Surface

Smoothness on 150-ps, 1.064ijm

Laser Damage of AR Coatings

on Glass." Presented to 1977

Boulder Damage Symposium.

Figures

1 .064//!

1.064,

II LAYER

aUARTER WAVE STACK

REFLECTOR (HR)

4 LAYER

ANTIREFLECTION

COATING (AR)

I.O6V1

SUBSTRATE

COMBINATION

AR + HR

Fig. 1. Schematic representation of the

three coatings.

Fig. 2. The time averaged square of the electric

field in an eleven layer quarterwave

stack high reflector (HR) coating at the

threshold of damage as a function of

distance measured normal to the surface

of the layers for radiation incident

from the left. The high index titania

layers are indicated by shading and the

incident laser beam intensity is indicated by the arrow.

-253-
--

AR COATIN6 (C)

e2

SUBSTRATE

AR + HIGH REFLECTOR (A)

SUBSTRATE

Fig. 3. The time averaged square of the electric

field in a four layer antiref lection (AR)

coating at the threshold of damage as a function of distance measured normal to the surface of the layers for radiation

incident from the left. The high index

titania layers are indicated by shading

and the incident laser beam intensity

is indicated by the arrow.

Fig. 4. The time averaged square of the electric

field in a combination (AR+HR) coating

as a function of distance measured

normal to the surface layers for radiation incident from the left. The high

index titania layers are indicated by

shading and the incident laser beam

intensity is indicated by the arrow.

4

--•400 5j

-30O

- - 200 j:^ I -too ^

200

lOO

r750"C

E'

Fig. 5. The electric field intensity profile pig.

for radiation incident on a high

reflector (HR) and an antiref lection

(AR) coating with nonlinear absorption in the high index layers as a bar graph overlay. The incident

laser beam intensity is indicated by

the arrow at the left and the peak

temperature is shown for a nonlinear

absorption coefficient adjusted to cause melting of Ti02 on the combination (AR+HR) coating. _7SA6. The electric field intensity profile for

radiation incident on a combination (AR+HR)

coating with nonlinear absorption in the

high index layers as a bar graph overlay.

The incident laser beam intensity is indicated by the arrow at the left and the

peak temperature is shown for a nonlinear

absorption coefficient adjusted to slightly

exceed the melting point of TiO..
THE EFFECTS OF BARRIER LAYERS AND SURFACE SMOOTHNESS

ON 150-ps, 1.064-ym LASER DAMAGE OF AR COATINGS ON GLASS*

J. H. Apfel and E. A. Enemark

Optical Coating Laboratory, Inc. Santa Rosa, California 95402

and

D. Milam, W. L. Smith, and M. J. Weber

Lawrence Livermore Laboratory

University of California

Livermore, California 94550

We observed that the maximum internal electric field at the threshold of

damage in antireflection (AR) coatings, as calculated from our measured threshold

flux, is lower than for high reflector (HR) coatings. The difference in damage

vulnerability is attributed to the coating-substrate interface which is illuminated

when AR coated but is protected by interference reflection when HR coated. To test

this postulate, experiments were designed using evaporated interference coatings

of silicon oxide (silica) and titanium oxide (titania) of different compositions

on glass substrates of different surface smoothnesses. These were damage-tested

with 150-ps, 1.064-pm laser pulses. We find that for sil ica/titania AR coatings

on BK-7 glass, a barrier or protecting layer of silica adjacent to the substrate

improves the damage threshold whereas the substrate surface smoothness is not

critical for short pulses. In one experiment, aluminum oxide (alumina) was used

for the barrier layer between the AR coating and the substrate.

Key words: Antireflection coatings; barrier layer; damage thresholds; glass

substrate; optical coating design; optical coating materials; substrate smoothness.

INTRODUCTION

In studies of damage to optical coatings by 125-ps, 1.064-ym laser pulses, Milam [1] found that

antireflection (AR) coatings generally fail at lower pulse intensities than dielectric reflectors.

This is contrary to expectations based upon an analysis of the internal electric fields. Previous

work [2] with several silica/titania multilayers demonstrated a correlation between 1 .064-ym pulsed

laser damage threshold and the maximum electric field strength produced in the titania layers.

Because the maximum internal electric field for AR coatings is a lower multiple of the incident

electric field than for quarter-wave-stack reflectors (HR), the damage threshold should be higher.

We suspected that the above differences are due to the damage vulnerability at the interface

between the substrate and the coating. This interface is subject to full pulse intensity for an AR coating but is protected by interference for a dielectric reflector. Coleman [3] found that the

slight optical absorption of sputtered titania films on soft glass microscope slides was reduced

when a silica layer was deposited prior to the titania. Although glass substrates, such as BK-7,

are compositionally different from the soft al kal i -bearing glass used by Coleman, the presence of

a titania first layer prompted our investigation of the effect of introducing silica and alumina

barrier layers on the substrate prior to depositing AR coatings. A series of coatings was designed

in which the barrier layer thickness, barrier material, and deposition process were varied. Damage

thresholds were measured using 150-ps, 1.064-Mm laser pulses.

For the initial experiments on barrier layers, coatings were placed on conventional polished

surfaces. However, we observed variations in thresholds for different experiments involving supposedly

similar coating. House et al. [4] have reported variations in damage thresholds with surface smoothness.

To investigate this, a second series of experiments were performed in which controlled, and widely

different, substrate' surface smoothnesses were used.

EXPERIMENTAL

Coati ngs

For glass, the simplest monochromatic AR coating which is composed of silica and titania films

is a two-layer V-coat consisting of a first-deposited thin titania layer and a thicker silica second

layer. This design is capable of zero reflectance at one wavelength for the design incidence angle.

The reflectance of a V-coat increases rapidly for wavelengths and incidence angles departing from the

design values. Accordingly, imprecise layer thicknesses or variations in layer thickness over a single

surface can result in unacceptably high reflectance values. A four-layer coating, similar to that

described by Rock [5], is more practical for large laser components because it maintains uniformly low

Work at the Lawrence Livermore Laboratory was performed under the auspices of the Materials Sciences

Programs of the U. S. Energy Research and Development Administration and Contract W-7405-eng-48.

-255-
reflectance for angle and thickness variations encountered in normal production of flat and gently

curved lens surfaces. The designs used in this study were composed of (1) four layers beginning with

high-index titania on the glass substrate or (2) five layers when a low-index silica or alumina barrier

layer was interposed between the glass and the first high-index layer.

Most of the AR coatings were prepared in a special vacuum facility equipped with externally actuated

masks which can be interposed in the coating vapor stream. In this way, both a control coating and a barrier layer coating were prepared in the same run with the four-layer AR coating simultaneously deposited

on both substrates. In the coating run made to compare the effects of silica and alumina barrier layers,

the vacuum was broken when the source material was changed.

To demonstrate the applicability of this work, three "production" runs were also made with regular

coating equipment and sample spatial filter lenses similar to those required for the LLL Shiva laser.

In one run the barrier layer plus a regular four-layer AR was coated on one surface of the lens and,

in another run, the opposite surface was coated with only the four-layer AR.

Substrates

The substrates for the research coatings were BK-7 glass disks of 38 mm diameter and 7 mm thick.

The disk faces were ground and polished by standard optical shop procedures to yield three degrees

of surface smoothness which will be referred to as commercial, conventional, and super-polish. All disks had one surface prepared with a commercial polish and the other polished to a higher degree.

The smoothness was characterized by applying a silvered surface and measuring the diffusely scattered

intensity of a He-Ne laser beam. With the polarized laser beam incident at 30°, the collimated detector

accepted a narrow cone of diffusely scattered light in the plane of incidence at an angle near the

specularly reflected beam. Relative roughness values are quoted in parts-per-mi 1 1 ion intensity

compared to a ground surface of boron nitride.

The silver coating was dissolved with hot nitric acid prior to applying the experimental coating.

Extended hot nitric acid immersion was previously shown to be beneficial to the damage threshold for

40-ns pulses [6] and is effective in removing all optical traces of the silver layer.

Damage Threshold Determination

The damage tests at 1.064 ym were carried out with the laser facility previously described [1].

Streak camera recordings of 37 laser shots made during one of the damage experiments show that the

laser pulse length (FWHM) varied from 110-210 ps with an average value of 149 ps. Experimental

results are quoted as maximum energy flux (J/cm^) measured at the surface in a plane perpendicular

to the beam. When streak camera pulse-length data were available, intensities (GW/cm^) are also

given.

An experimental configuration was used in which three beams having relative intensities 1:2:3 were incident on the sample for each firing of the laser. Several techniques were employed to detect

damage. The sample was visually observed during each irradiation to detect laser-induced light or

sparks. Immediately after each firing the focal sites were examined using a focused white light source. The damage experiment was continued until damage was detected by one of these two tests. Typically,

this procedure required four to seven laser firings and the irradiation of twelve to twenty-one sites

per sample.

Final determination of damage was made by using Nomarski microscopy to examine each focal site.

Since a fixed array of sites was used during sample irradiation, the spatial coordinates of each site were known. Damage was assessed by noting systematic differences in the morphology of irradiated

sites and non-irradiated portions of the film. Threshold damage consisted of a few micropits * 1 ym

in diameter or, in some cases, of a few areas 5-10 pm in diameter which appeared as raised "mounds."

The tops of these mounds were ruptured in some instances. There is an uncertainty in the microscopic

assessment because all dielectric films examined to date have a few small pinholes which resemble

laser damage. Mounds were never found except on irradiated sites. Depending on sample quality,

obvious damage was observed for sites irradiated at flux levels 10-20% above the assigned threshold.

The usual precision of threshold values was 10-12%.

Thresholds were defined as the midpoint between highest flux not producing damage and the lowest

flux causing damage. The assigned error accounts for uncertainties in the relative flux levels (~ 5%)

and the microscopic examination. The boundary between flux levels which cause damage and those which

cause no damage was always sharp to within experimental uncertainty. In no case did we observe large

scale variations of threshold over the sample surface.

RESULTS

Table 1 summarizes the results of three coating runs in which different thicknesses and materials

were used for the barrier layers. In every case the threshold was either unchanged or improved by the

addition of the barrier layer. Comparable benefit is seen for alumina and silica barrier layers.

-256-
Table 2 shows the results for three samples coated on both surfaces in production vacuum equipment.

Again, the coatings with a barrier layer demonstrated higher thresholds. Note that similar coatings

did not yield identical thresholds.

The control samples in Table 1 (566A, 565A, and 568A) were identical coatings made in separate

runs on conventionally polished substrates. Similarly, samples 566D and 568B had identical half-wave

optically thick silica barrier layers. Differences in the measured damage thresholds of these similar

coatings suggested that substrate surface roughness might be important.

Table 3 shows the effect of surface smoothness on the threshold for AR coatings with and without

barrier layers. In these experiments the laser pulse length was determined from streak camera data;

thus, it was possible to calculate pulse intensities (GW/cm^) at the threshold of damage. The data

in Table 3 are arranged into groups with similar coatings and in order of increasing roughness within

each group. Although the thresholds vary by a factor of two, there is no consistent correlation with

surface smoothness.

DISCUSSION

The present results in conjunction with previous results [1] show that for silica/titania films

the damage thresholds for 150-ps, 1.064-um laser pulses are lower for AR coating than for silica/titania;

multilayer reflectors. This is opposite to predictions based upon internal electric field strengths

in the film. As mentioned earlier, one obvious difference between the two types of coatings is the

intensity at the coating/substrate interface which is high for AR coatings and zero for HR coatings.

Since uncoated BK-7 disks damage at ^ 10 J/cm^, any interface vulnerability must be due to the presence

of the coating.

Table 1. Experimental damage thresholds (J/cm^) for coatings from research equipment.

Sample Barrier* Threshold Improvement

566A none 4 2

+ 0 3

566B X/50 Si02 4 0

+ 0 3 none

566C A/4 Si 02 4 0

+ 0 3 none

566D X/2 Si02 4 8

+ 0 5 15%

56 5A none 3 2

+ 0 4

5658 2 X x/4 Si02 6 3

+ 0 7 90%

565C 4 X x/4 Si02 6 3

+ 0 7 90%

568A none 3 2

+ 0 3 568B X/2 Si02 5 6

+ 0 6 75%

568C X/4 AI2O3 4 0

+ 0 4 25%

5680 A/2 AI2O3 5 1 + 0 4 60%

* Barrier layers stated in optical thicknesses. Barrier layers in 565B and 565C were deposited as an accumulation of several discrete thin films.

-257-
Table 2. Experimental damage thresholds (J/cm^ ) for coatings from production equipment

Sample Barrier Threshol d Improvement

Asphere A none 2.7 ± 0.3

Asphere B A/4 Si02 4.7 ± 0.8 70%

6974A none 3.5 ± 0.5

6974B A/4 Si02 4.3 ± 0.5 20X

6977A none 3.3 ± 0.4

6977B A/4 Si02 4.0 ± 0.4 20%

Table 3. Dependence of damage threshold on surface roughness of substrate.

Sampl e Surface

Preparation Barrier Layer Diffuse

Intensity (ppm)

Threshol

d

GW/cm^ J/cm^

600A

599A

Superpolish

Commercial

none

none

160

> 1000

27

37

+

+

7

9

4.0 ± 0.6

5.5 ± 0.7

599 B14

600 AAl

599 A16

599 AAl

Superpol i sh

Superpol ish

Superpol ish

Commerci al

2 (A/4) Si02

2 (A/4) Si02

2 (A/4) Si02

2 (A/4) Si02

20

40

70

> 1000

28

36

23

29

+1

+1

+1

+1

7

9

6

7

4.1 ± 0.6

5.4 ± 0.7

3.4 ± 0.5

4.4 ± 0.6

599 B8

599 A13

600 B6

599 B6

Superpol ish

Superpol i sh

Superpol ish

Commercial

A/2 Si 02 A/2 Si 02 A/2 Si02

A/2 Si02

20

70

230

> 1000

47

45

26

42

+1

+1

+1

+1

18

9

8

10

7.0 ± 2.0

6.7 ± 1.3

3.9 ± 0.8

6.3 ± 0.8

In the above experiments, several different barrier layer designs were tried. These layers did

not significantly affect the reflectivity of the AR coating or the electric field intensity at the

boundary. They do, however, provide physical and chemical isolation from the substrate and a different

surface for the deposition of the first titania layer. In Tables 1 and 2, the addition of the barrier

layers always yielded positive results in terms of increased damage thresholds.

From the limited data available, the thickness of the barrier layers seems to be important.

That is, up to the A/2 thicknesses investigated, the thicker layers had the higher damage thresholds.

We have chosen the A/2 barrier layer thickness for subsequent production applications.

The effects of depositing the barrier as (1) one continuous layer and (2) a series of discrete

A/4 layers separated by a time interval of several minutes are included in Table 1. The small

improvement by the latter technique evident in Table 1 was not repeated in Table 3. At present, the

interruption of layers appears to offer no benefit.

-258-
To investigate the effects of substrate surface irregularities, AR coatings were prepared with

and without X/2 barrier layers on substrates with various degrees of roughness. The roughness was determined by light scattering. The quoted scatter intensity of each disk is the average of five

measurements made at different locations within a 2. 5-mm-diameter central region. Two disks were evaluated at several locations within the central one-inch-diameter region to verify that the surface

scatter intensity was uniform. To correlate the scatter intensity measurements with surface roughness,

we examined three samples (generously provided by J. Bennett, Michel son Laboratory, Naval Weapons

Center) whose RMS roughnesses were all about loA as determined by TIS and FECO techniques. Our best

superpolish surfaces scattered slightly less than the Bennett samples and were, therefore, assumed

to be at least as smooth.

From the results shown in Table 3, the quality of substrate surface polish did not affect the

damage threshold for 150-ps pulses. House et al. [4], on the other hand, found a correlation between

damage threshold and surface smoothness for 40-ns pulses.

CONCLUSIONS

The addition of a silica barrier layer between a BK-7 glass substrate and the first titania layer

of a four-layer AR coating improved the average threshold for 150-ps, 1.064-ym laser damage. The

improvement varies from insignificant to nearly double when simultaneously coated samples were compared

There is a trend toward higher threshold with increased silica barrier layer thickness. On the basis

of results for the two samples, alumina barrier layers are as effective as silica in raising the damage

threshold. Contrary to the results of House et al. [4] for damage by 40-ns pulses, for 150-ps pulses

we found no correlation between damage thresholds and surface smoothness either with or without a barrier layer.

REFERENCES

[1] D. Milam, Appl. Opt. 16^, 1204 (1977).

[2] J. H. Apfel , J. S. Matteucci , B. Newnam, and

D. Gill, "The Role of Electric Field Strength

in Laser Damage of Dielectric Multilayers,"

NBS Spec. Pub. #462, 301 (1976).

[3] W. J. Coleman, Appl. Opt. U, 946 (1974).

[4] R. A. House, J. R. Bettis, and A. H. Guenther,

IEEE J. Quant. Electron. QE-13 , 361 (1977).

[5] U. S. Patent 3,432,225.

[6] R. A. House, "The Effects of Surface Structura

Properties on Laser-Induced Damage at 1.06 pm,

AFWL-TR-76-62 , Air Force Weapons Laboratory

final report, Kirtland AFB, NM (October 1976).

COMEUTS OU PAPER BV kPTEL, ENEMARK, MILAM, SMITH, ANV WEBER

The. ipmkeA indlcaXnd that the. damage tkxeAhold oi the bavUeA layeA. alone had not been Investigated.

He {^elt that the ei^^ecX which had been obieAved might be dae to the InteApoiltlon the baAntex layeJi

between the titanium dioxide ilbn and the ground and pollihed Bfe-7 iuA^ace, and Imptied that In the

absence oi the bannlen. layen. 4ome chemical InteAactlon might be taking place betuieen theAe tuo con- itituenti. Thl6 would item to be iupponted by flight Impfiovementi ieen when the <5uA^ace wai, clexined

by iputteA etching, pntoK to the depoiltlo

dxpoAimzviU hn had coMlzd oat in u)kicLk a boAAl^ layeA oi SIO^ uo(Li> applizd pnA^oK to putting dom a ApotteAed coating TiO^, no imp/iovmcnt duo, to tke, boAAioA layoA t^a6 -6een. It may be that the

6putteAtng pKoc^6 in iX^zl^ clmn6 the ^uAiace, and 4 0 the baAAi^A layeA becomes redundant.

NOTICE

"This report was prepared as an account of work

sponsored by the United States Government. Neither

the United States nor the United States Energy

Research & Development Administration, nor any

of their employees, nor any of their contractors, subcontractors, or their employees, makes any

warranty, express or implied, or assumes any legal

liability or responsibility for the accuracy, completeness or usefulness of any information,

apparatus, product or process disclosed, or

represents that its use would not infringe privately-owned rights."

-259-
USE OF NON- QUARTER -WAVE DESIGNS TO INCREASE THE DAMAGE RESISTANCE OF REFLECTORS

AT 532 AND 1064 NANOMETERS*

Dennis H. Gill, Brian E. Newnam and John McLeod

Los Alamos Scientific Laboratory

Los Alamos, New Mexico 87545

The damage resistance of multilayer dielectric laser reflectors has been increased

by using non-quarter-wave thicknesses for the top few layers. These designs minimize the

standing-wave electric field in the high-index layers, which are generally the weaker

layers. Algebraic equations have been derived for optimum film thicknesses and for the

resulting peak electric fields.

Five sets of reflectors for 532 and 1064 nm were fabricated according to these designs

by two vendors using two different material combinations. Each set contained one reflector

of standard all -quarter-wave design and three reflectors each with a different number of

modified layers. The damage thresholds of the modified designs were found to be higher

than the all -quarter-wave designs, in some cases by a factor greater than 2. The damage

thresholds have been analyzed and explained in terms of standing-wave electric fi.eld patterns.

Key words: Damage thresholds; electric fields; laser damage; non-quarter-wave designs;

picosecond pulses; standing waves; thin films.

1 . Introduction

A substantial increase in the damage resistance of multilayer dielectric laser reflectors has been

attained by using non-quarter-wave thickness designs for the top few layers. The principle of these

designs is to minimize the standing-wave (SW) electric field in the high-index layers, which are generally less damage resistant than low-index layers. Algebraic equations were derived for the optimum

film thicknesses and for the resulting peak electric fields. Reflectors for 532 nm and 1064 nm were fabricated according to these designs, and their measured damage thresholds were compared with those

of similar reflectors with all quarter-wave (QW) thicknesses.

To the knowledge of the authors, the first suggestion for using non-QW top layers was mad| by

Gary DeBell for the purpose of minimizing the total linear absorption in reflector stacks [1]. Newnam

suggested non-QW thicknesses to obtain minimum peak fields in high index layers in order to raise the

damage threshold [2]. Costich and Bloom designed and supplied test samples of TiO^/SiO^ reflectors to

LASL with one pair of non-QW layers, but these were not the optimum design [3]. Newnam and Gill presented the threshold data for these samples along with suggestions for improvement at Asilomar [4]

and reported on the first set of reflectors with optimum designs at CLEA [5] . Apfel did a very elegant'

job of mathematically deriving the optimum designs [6].

2. Theory

The standard dielectric reflector is composed of alternate layers of relatively high- and lowrefractive indices which have QW optical thicknesses. In this design, the SW-electric field maxima

occur at the layer interfaces and are largest in the top layers nearest the incident radiation (see

fig. la). Previous experiments [2,7,8] have shown that the damage threshold of QW stacks of Ti02 and

SiO^ is determined by the peak internal SW electric field in the high-index material (TiO^) . In the

visible and near infrared the high-index material is usually weaker than the low-index material, thus,

it is highly desirable to shift the position of the SW field maxima away from the film interfaces of

the top layers of a reflector and into the low-index (stronger) layers. This is accomplished by the

use of non-QW thicknesses for the top layers.

If the thicknesses of the top pair of layers are modified so that the high-index layer is thinner

and the low-index layer is thicker than a QW, the SW field maximum is shifted into the low-index

material, as shown in figure lb. Note that the amplitude of the peak has increased, so that the lowindex material is being stressed harder than for the all-QW case. The maximum value of E occurring in

high-index material has been reduced to 0.652/0.833 = 0.78 of its previous value. (These numbers

depend on the indices of refraction of the materials. This example uses 2.40 and 1.46.) The peak E occurring in low-index material has increased to 1.073/0.833 = 1.29 times its previous value. Further

shifting can be accomplished by modifying the thicknesses of the top two pairs of layers, as shown in

figure Ic. Here the peak E value in high-index material has been further reduced and the peak E in lowindex material has increased again. This procedure can be extended to the modification of as many

*Work supported by the U. S. Department of Energy.

1. Figures in brackets indicate the literature references at the end of this paper.

-260-
layers as desired. However, at some point the stronger material will break and no further improvement

can be obtained. This will be discussed in more detail below.

One can analyze the SW electric-field pattern in a reflector and derive equations for the peak

fields and the reflectance. To minimize the peak field in the high-index layers while maintaining

maximum reflectance, the thicknesses of the top two layers are given by

SIN e. (1)

and

where

TAN e H

(2)

(3)

and

2m . d

.

1 1

A (4)

Here, the subscripts L, H refer to the low- and high-index layers and d is the film thickness. Similar

expressions have also been derived by Apfel [6]. For the thicknesses specified by eqs. (1,2), the

peak E-field in the first high-index layer is reduced by a factor of

(S)

V2N^1

compared to the standard all-QW case. At the same time the peak E-field in the first low-index layer

is increased by a factor of

N

\/ 2N^-1

(6)

The equations above apply to the modification of the top pair of layers. For further improvement

one can modify additional pairs of layers. The thicknesses, however, are different for each layer. In order to generalize the equations, a convention is chosen for numbering the layers. Assume that some number of QW layers are deposited on the substrate (i.e., a standard reflector), then m pairs of modified layers are deposited on top of the QW stack. Number the modified layers outward, starting with

layer .1 adjacent to the QW stack and ending with layer 2m next to the air, as shown below.

AIR 2m 2m1

4 3 2 1 X X X A

4 4 4 4

H L H L H L H L H L SUBSTRATE

For this general case, the optimal thicknesses for the modified layers are given by

0 < i < m

Low Index Layers: SIN 0

1 2i-l

High Index Layers: TAN 9.

iN -(i-1)

N 2i

(N -1)

0 < i < m

< TT/2

(7)

(8)

where eqs. (3,4) still hold. The maximum E-field occurring in any high-index layer, relative to the

incident E-field in air, is

n. \/(m+l)P

Similarly, for a low-index layer.

mN -(m-1)

2 (m+l)N -m

(9)

(10)

-261-
Equations (9) and (10) can be used to determine the decrease or increase in the peak E-field

occurring in either material, relative to the field in a standard all-QW (m = 0) reflector. These

ratios are plotted in figure 2 as a function of N. Because of the relationship between damage threshold

and peak E-field, we define the reciprocal of the reduction in the peak E-field in the high-index

material as the improvement, I. This is the expected increase in the damage threshold, as compared to the all QW case, for a reflector with an m-pair modified design. This expected increase is given by

^ ^ ^ VCmn-DN^-m ,,,,

E N

H

^

This assumes that damage occurs due to failure of the high-index material. Figure 3 shows the improvement 1 as a function of N, as well as the ratio of the peak E-field in the low-index material to that

of the high-index material. Note that I relates to the E-field damage threshold. If one thinks of

2 damage threshold in units of joules per square centimeter, the improvement is I .

Whether or not these modified designs will yield higher damage thresholds than the all-QW design

depends on the ratio of the damage thresholds of the two materials. As has been shown, increasing the

value of m increases the E-field in the low-index material. As m is increased, at some point the

(stronger) low- index material will break. Define the parameter p to be the ratio of the damage

threshold (expressed in MV/cm) of the low-index film to the damage threshold of the high-index film.

Using eqs. (9,10) we can show that if

_ mN -(m-1) , (12)

then full advantage can be taken of an m-pair modification. For most practical cases in the visible

and near ir, the value of p limits the useful designs to m = 1, 2 or 3. If a value of m is used such

that eq. (12) does not hold, then the damage threshold will decrease from its optimum value. If m is

high enough, the damage threshold can actually be lower than for the all QW case.

3. Experiment

3.1. Test Procedure

In order to test the theory, five sets of reflectors were obtained from two vendors. Each set (except one) contained four reflectors, one each of m = 0 (all QW) , m = 1, m = 2 and m = 3. All four

reflectors were coated in the same coating run by using shutters, thus, all four reflectors have the

same stack of quarter wave layers. Then the modified layers are added, and as each reflector is com- pleted, it is shuttered off to protect it from the remaining modified layers. This rather complicated

procedure was used to assure that each reflector was as nearly identical to the others in its set as possible. In this way, differences in damage threshold should be due to design differences only,

assuming that the material in the outer layers is identical to that in the QW layers. The reflectors

obtained were TiO^/SiO^ and Zr02/Si02 at 1064 nm and 532 nm.

The reflectors were tested using a laser damage test facility that has been described elsewhere

[8,9]. The laser was a mode-locked Nd:YAG system. The fundamental or second harmonic was used as appropriate. Pulse lengths were 30 ps for 1064 nm and 21 ps for 532 nm. Considerable effort was taken

to assure a good gaussian beam profile, and the profile of the focal spot was measured on every shot

using a linear array.

3.2. Data at 1064 nm

Table 1 lists the data obtained on one set of Ti02/Si02 reflectors at 1064 nm. One can see that

the m=l design doubled the damage threshold compared to the m = 0 case. The m = 2 case was not quite

as good as m = 1, but still better than m = 0. The m = 3 case provided a damage threshold 2.6 times

as large as the QW reflector. The lower threshold for m = 2 followed by an increased threshold for

m = 3 does not follow the pattern predicted by the last column, using the exact design thicknesses.

At least part of the explanation lies in the lack of ability of the vendor to maintain accurate film

thicknesses on non-quarter-wave coatings. These modified-layer designs are much more sensitive to errors in coating thicknesses than are standard QW designs. By consulting with the vendor and carefully reading his coating run sheets, we were able to estimate the magnitude and location of several

thickness errors. Also, some of these could be verified by comparing spectral transmission curves

with theoretically predicted curves. The modified-layer designs have very distinct spectral signatures. Both of these methods can be used to estimate thickness errors. It was determined that there

were thickness errors in both the m = 2 and the m = 3 reflectors. Without ideal thicknesses, eqs.

(9-11) no longer apply. The actual (as best we could determine) thicknesses were used in a LASL

computer code that calculates the complete E-field distribution in an arbitrary stack of dielectric

films. The results were used to predict the improvement shown in the "Theoretical as Coated" column.

It should be noted that the theoretical improvements in all of the tables assume high-index layer

-262-
failure, unless otherwise noted. At higher m-values it is expected that the low-index layer will fail,

and these predicted improvements will be larger than those actually obtained. In table 1, the measured

improvement does not agree exactly with the "Theoretical as Coated" improvement, but the trends agree

well. Specifically, the lower improvement for m = 2 is apparently explained by the thickness errors. While these errors cause trouble in exact agreement between experiment and theory, all of the modifieddesign reflectors had substantially higher damage thresholds than the standard QW reflector.

Table 1. Ti02/Si02 reflectors at 1064 nm.

Measured

Damage E-field at threshold Improvement Relative to all QW

Threshold H- Layer L- Layer Measured Theoretical Theoretical

Layers Design

(J/cm^)

(MV/cm) (MV/cm) as coated ideal thick.

11 All QW 1.0 + .1 3.0 3.0

13 m=l 2.0 ± .2 3.4 5.4 2.0 1.59 1.59

15 m=2 1.7 ± .2 3.5 6.2 1.7 1.31 2.18

17 m=3 2.6 ± .5 3.1 6.7 2.6 2.47 2.77

Table 2 lists the data obtained on a set of Zr02/Si02 reflectors at 1064 nm. Here, again, coating

thickness errors play a major role. Note the rather dramatic drop in the threshold for the m = 2 case, which is explained by a thickness error. The m = 3 design provided a 74% increase in the damage

threshold. It is likely that the m = 3 design failed in the low- index layer.

Table 2 . Zr02/Si02 reflectors at 1064 nm.

Measured

Damage E-field at threshold Improvement Relative to all QW

Layers Design

Threshold

(J/cm")

H- Layer

(MV/cm)

L- Layer

(MV/cm)

Measured Theoretical

as coated

Theoretical

ideal thick.

17 All QW 1.33 ± .3 4.2 4.2

19 m=l 1.42 " 3.8 5.0 1 .05 1.27 1.42

21 m=2 1.0 ± .1 3.8 5.3 .74 .88 1.84

23 m=3 2.33 ± .6 3.8 6.7 1.74 2.10 2.26

Table 3 lists the data for another set of Ti02/SiO^ reflectors at 1064 nm. This set was chronologically the first set obtained. The results were somewhat dubious because of the larger range of

the damage threshold. Note that the lower end was constant for the three samples. This was attributed to defects. The samples had a high scatter level, which is often due to the presence of large

numbers of defects. The upper end of the range did show an improvement for the modified samples.

The improvement obtained for m=l is consistent with that predicted for either L- or H-layer failure,

and the improvement obtained for m=2 is consistent with that predicted for low-index layer failure.

The column predicting improvement if the L-layer fails appears in table 3 because we have a measured

value of p for this vendor and these materials. Recall that p is the ratio of damage thresholds for

the two materials. Previous to this experiment, single QW layers of Ti02 and Si02 had been obtained

from this vendor and damage tested. The value of p was found to be 1.5 - 1.6. The next-to-last

column in table 3 was calculated using 1.58 and seems to be consistent with the data. For other

vendors the value of p for Ti02/Si02 has been measured to be in the range 1.5 - 1.6, also. Because

of the constant threshold on the low end, it was decided to obtain samples from another vendor. All

of the other samples reported in this paper were from the second vendor.

Measured

Damage

Threshold

2 Layers Design (J/cm )

15

15

15

All QW 4.5-5.4

m=l 4.5-8.0

m=2 4.5-6.4

Table 3. E-field at

' Threshold

H- Layer L- Layer

(MV/cm) (MV/cm)

Ti02/Si02 reflectors at 1064 nm. Improvement Relative to all

7.0

7.1

5.6

7.0

11.1

10.7

Measured TheoreticaT Theoretical^

(Top of Range) H-Layer Fails L-Layer Fails

As Coated p = 1.58

QW

1.5

1.2

1.44

1.82

1.4

1.2

Theoretical

H-Layer Fails

Ideal Thick.

1.59

2.19

-263-
3.3. Data at 532 nm

Table 4 lists the data obtained from a set of TiO^/SiO^ reflectors at 532 nm. Once again coating

thickness errors reduced the expected improvement. As designed, the set would have shown a range of

improvements from 1.63 to 2.88, although the L- layer should have broken for m = 3. However, as coated,

the expected improvements were all roughly the same, and indeed this was observed experimentally. Even

so, a factor of 1.6 increase in damage threshold is well worthwhile.

Table 5 lists the data for a set of Zr02/Si02 reflectors at 532 nm. The maximum improvement of

30% was obtained with the m = 1 design followed by a slow decrease in the threshold for m = 2 and 3.

Table 4. TiO^/SiO^ reflectors at 532 nm. Measured

Damage E-Field at Threshold Improvement relative to all QW

Layers Design

Threshold

(J/cm^)

H- Layer

(MV/cm)

L- Layer

(MV/cm)

Measured Theoretical

as coated

Theoretical

Ideal Thick.

11 All QW 1.3 + .1 3.9 3.9 ---

13 m=l 1.8 + .1 4.0 5.6 1.4 1.31 1.63

15 m=2 2.2 ± .2 5.1 7.0 1.6 1 . 18 2.26

17 m=3 2.1 ± .2 4.7 7.2 1.6 1.38 2.88

Table 5. ZrO^/SiO 2 reflectors at 532 nm. Measured

Damage E-Field at Threshold Improvement relative to all QW

Layers Design

Threshold

(J/cm^)

H- Layer

(MV/cm)

L- Layer

(MV/cm)

Measured Theoretical

as coated

Theoretical

Ideal Thick.

17 All QW 1.9 ± .1 5.7 5.6

19 m=l 2.5 + .5 6.2 7.1 1.3 1.13 1.46

21 m=2 2.3 ± .4 5.5 6.7 1.2 1.21 1.91

23 m=3 2.1 ± .3 5.5 6.7 1.1 1.12 2.37

Sine

in these

TiO^/SiO^

reflector

errors, w of 2% to obviously

thickness

E-fields

e most of the samples

designs was studied,

with indices of 2.26

and for a 15-layer,

ith negligible field

% for the various 1 not insignificant,

errors always produc

in the L-layer.

3.4. Discussion of results

showed the effects of coating thickness errors, the tolerance for error

Table 6 presents the results of an error analysis for one specific case, and 1.44, respectively. The analysis is done for an 11-layer, all-QW

m=2 design. The all QW reflector is quite insensitive to thickness

changes for 3% errors. The m = 2 design has field changes of the order errors and field changes of 5% to 23% for various 3% errors. These are Since the designs are optimized with regard to the high-index layers,

e higher E-fields in the H-layer, but may produce either higher or lower

The effect of errors in index of refraction was also studied. The modified designs are not at all sensitive to an error in n if nd is correct, as might be expected. Thus, an accurate measurement

of the optical thickness nd is important, but an accurate knowledge of n is not so critical.

Tolerances of 1% in coating thickness are not unreasonable. Most coaters are capable of maintaining this tolerance. In some previous experiments using non-QW layers we have obtained samples from

vendors with thicknesses accurate to better than 1%. The primary difficulty is in the monitoring

techniques, which may require some changes. These changes should be worthwhile, however, considering

the possible gains (factors of 2 and more increase in damage threshold).

The amount of improvement that one can expect depends upon the relative damage thresholds of the

two materials, as has been shown earlier. It is expected that greater improvements can be obtained

in the uv and ir where one material tends to be much weaker than the other. Throughout this paper

it has been assumed that the high-index material was weaker. The theory presented here works equally

well for combinations where the low-index material is weaker. One simply modifies the thicknesses so

as to push the peak of the E-field distribution into the high-index layer. Equations for the optimal

thicknesses can be derived in a similar manner.

The reflectances for these modified designs will always be greater than the reflectance of the

corresponding all QW reflector, although not by very m.uch. This is true because one is adding the

modified layers to the QW stack.

-264-
Table 6. Coating thickness error analysis. Theoretical errors in E-field

due to thickness errors in coating, for Ti02/Si02, n^ = 2.26, n^ = 1.44,

N = 1.S7. Primes refer to fields in coating with errors.

.* .*

Design Thickness ^ ^

m=0

(All QW) All layers 3% thick 1,,003 1,,011

All layers 3% thin 1..006 1.,003

m=2 Mod., layers 1% thick 1.,036 1.,023

All layers 1% thick 1.,060 1.,033

Mod,, layers 1% thin 1.,058 0,,979

All layers 1% thin 1.,078 0,,973

m=2 Mod,, layers 3% thick 1,,124 1,,083

All layers 3% thick 1..225 1,,125

Mod,. layers 3% thin 1,.167 0,,948

All layers 3% thin 1,,218 0,.935

4. Conclusions

It has been shown both experimentally and theoretically that a substantial increase in the damage

threshold of a reflector can be obtained by using non-QW layers in the design. Factors of improvement

as large as 2.6 (in the energy density threshold) were obtained. The modified designs are sensitive to errors in coating thickness, and many of the samples showed less improvement than they might have due

to this. Even so, out of 5 sets of reflectors from 2 vendors, made of 2 different material combinations

for 2 different wavelengths, only one reflector had a lower threshold than its all-QW counterpart.

Non-QW thickness samples have been obtained with thickness errors less than 1%.

These experiments were done in the visible and near ir. It is expected that even greater improvements could be seen in the uv and ir where the weak layers are relatively weaker.

5. References

[1] DeBell, G., Ph.D. Dissertation, Univ. of [6] Apfel, J. H. , Appl. Opt. 1_6, 1880 (1977).

Rochester, University Microfilms, Ann Arbor,

Mich. (1971). [7] Apfel, J. H. , Matteucci, J. S., Newnam, B.

E. and Gill, D. H. , NHS Special Publication

[2] Newnam, B. E., Ph.D. Dissertation, Univ. Micro- 462 , 301 (1976).

films, Ann Arbor, Mich. (1973).

[8] Newnam, B. E., Gill, D. H. and Faulkner,

[3] Costich, V. R. and Bloom, A., personal communica- G., NBS Special Publication 435 , 254 (1975).

tion (1974).

[9] Newnam, B. E. and Gill, D. H., NBS Special

[4] Newnam, B. E. and Gill, D. H. , J. Opt. Soc. Am. Publication 462, 292 (1976).

66, 166 (1976).

[5] Gill, D. H. , Newnam, B. E. , and McLeod, J., 1977

lEEE/OSA Conf. on Laser Eng. and Applications

Digest of Tech. Papers, 54 (1977).

-265-
Fig. 1. Standing-wave electric field distribution in a multi-layer dielectric reflector. is the

incident electric field in air. Indices are 2.40 and 1.46. (a) Standard all -quarter-wave

reflector, m=0. (b) One pair of layers modified in thickness, m=l. (c) Two pairs of layers

modified in thickness, m=2.

-266-
-267-
-268-
RATIO OF INDICES N = Ru/n

Fig. 2. Peak E-fields occurring in high- and low-index materials, relative to an all-QW reflector, as a function of the index ratio N and the degree of modification m.

-269-
RATIO OF INDICES N= n /n,

n L

Fig. 3. Ratio of peak E-fields occurring in high- and low-index materials and the improvement

(E-field) , in the damage threshold, as a function of index ratio N and degree of

modification m.

-270-
CHARACTERISTICS OF R. F. SPUTTER-DEPOSITED

ZINC SELENIDE FILMS ON CALCIUM FLUORIDE*

D. A. Walsh and R. V. Bertke

University of Dayton Research Institute

Dayton, Ohio 45469

A study has been made of the optical, physical and chemical

properties of R. F. bias-sputtered CVD Zinc Selenide thin films

on polycrystalline CaF2 laser window material as a function of

sample preparation and deposition parameters. Optical performance of the coatings has been evaluated by measurement of coating absorption, physical integrity by topple- test and Scotch

Tape adhesion, and chemical properties by Rutherford Backscattering and Auger Spectroscopy.

A comparison is made between bias-sputtered and non biassputtered coatings. It is shown that a lower coating absorption is produced when the sample surface is sputter-etched insitu immediately prior to coating. A zinc to selenium ratio of

1.13 to 1 has been found for the initially deposited coating

material and the presence of oxygen at the coating/substrate

interface has been detected for bias-sputtered films.

Key words: Auger analysis; bias-sputtering; calcium fluoride;

Rutherford backscattering ; sputter-etching; zinc selenide.

1. Introduction

The deposition of coatings by the sputtering technique offers a number of potential advantages over resistive or electron beam methods. Inherent in sputtering is the

higher kinetic energy of the coating material as it arrives at the substrate. This

promotes tighter bonding to the substrate surface and should result in a coating of

higher density, improved adhesion and better durability. Along with these benefits

should also follow better homogeneity and lower absorption in the deposited film.

Lower deposition temperatures could also be possible and by utilizing production- type

equipment with vacuum interlocks, a decrease in coating cycle time might also be

realized. Of particular value is the ability to subject the substrates to a final

cleaning by means of in-situ sputter-etching.

2. Experimental

In this investigation three techniques were studied in a modified sputtering

apparatus equipped with an optical monitor for film thickness control. [1] Representative coatings of zinc selenide were fabricated on standard samples of CaF2 window material by straight RF sputtering and RF-Bias sputtering both with and without

sputter-etching of the substrate surface. The absorption of each coating was measured

at 5 . 2\i and representative films were evaluated by Auger Analysis to determine coating

composition and by Rutherford Backscattering to determine the stoichiometry of the

initially deposited ZnSe. All of the samples were subjected to the Scotch Tape Test

but in most cases were not given further adhesion tests to allow additional optical

measurements to be performed.

2.1 Sample Preparation

Substrates were 38 mm O.D. by 6 mm thick Harshaw Polytran CaF2 with a 3-5 fringe

flat standard IR finish. Prior to coating each sample was washed in a Liquinox detergent solution using a lens paper swab and given three rinses in both distilled water

and isopropyl alcohol. Samples were then stored in a Freon degreaser until ready to

coat. One set of samples was further cleaned by in-situ sputter-etching immediately

before coating. Coating conditions used in sample fabrication are shown in table 1.

The apparatus used in this work is equipped with a vacuum interlock which allows the

target to remain under vacuum at all times following its initial sputter-cleaning.

After the pre-heating of the specimen in the vacuum interlock, it is automatically

loaded into the sputtering chamber where it is outgassed immediately prior to coating

deposition or sputter-etching if this step is included.

* Work supported by the Air Force Materials Laboratory.

1. Figures in brackets indicate the literature references at the end of this paper.

-271-
2.2 Bias-sputtered Coating Absorption

The absorption values measured for three sets of ZnSe coatings as a function of

the optical thickness at 5.3vi are shown in figure 1. These were determined by subtracting the uncoated substrate absorption, A^, from the coated sample absorption, A^,

These observations were not compensated for coherence effects as only comparative

values were being sought in these studies.

Absorption in the non- bias sputtered coatings increases rapidly as thickness

becomes greater. The trend in the bias-sputtered coatings is also an increase in

absorption but at an overall lower rate. This could be attributed to the inherent

cleaning action of the bias-sputtering method.

VJhile a good bit of scatter is evident in the readings, the bias-sputtered films

clearly display lower values. It may also be seen that neither of the absorption

curves plotted for the two modes will extrapolate to zero. This could imply a higher

absorption in the initially deposited material or an appreciable contribution due to

interfacial effects between the coating and the substrate - or possibly a combination

of both.

2.3 RBS and Auger Analyses

Two experiments were undertaken to allow a closer look at the film/substrate

interface and the initially deposited film material.

A series of thin ZnSe coatings was deposited on CaF2 in the nominal range of

o o 25 - 175 A in 25 A steps. These were evaluated by the Rutherford Backscattering technique in an attempt to determine the stoichiometry of initially deposited ZnSe

material

.

The results of this evaluation are shown in figure 2, which is a plot of the

measured ratio of Zn to Se for each coating thickness. This ratio was found to be in

the range 1.05 to 1.18 with an average of 1.13.

This ratio may improve as the film becomes thicker; but since the RBS technique

can only be used for very thin films, an alternate method of measurement will have to

be found.

Another method of determining the composition of the interface is shown in figure

3. This is a plot of a series of Auger scans through a 4 300 A ZnSe film on CaF2. As

the interface is approached and penetrated, the presence of oxygen is clearly evident.

The Auger method can thus be used to detect the presence of this element but cannot

differentiate between an oxide, a hydroxide, (such as water), or some other oxygen

compound.

Regardless, the presence of any contaminant is undesirable and it must be removed

from the substrate to afford the coating material the cleanest possible surface on which to deposit.

2.4 Sputter-etching Results

One method of cleaning the surface which appears to produce a lower ZnSe film

absorption is in-situ sputter-etching. This portion of the study employed exactly the same coating and measurement procedures as used for the bias-sputtered coatings but

with a sputter-etching step added. Shown in figure 4 is a semi-log plot of the absorption coefficients of films deposited after the substrates had been sputter-etched

for the times shown. All of the coating thicknesses are the same and are approximately

half-wave at 5.3p. The initial data point represents a non-etched sample used as a control. _-|^ Following a 15 second sputter-etch the decreased rather sharply to 1.1 cm There was an increase with further etching up to about 2 minutes. A second decrease

was found at 4 minutes followed by a rapid rise. Visual inspection of similar CaF2

samples etched for the same times and aluminized for Total Integrated Scattering

measurements showed that after 16 minutes the surface became roughened and this would

contribute to the higher for the last sample.

All of the coatings produced in this study pass the Scotch Tape Test. Two

special test samples were fabricated using sputter etch times of 30 seconds and one minute. These were topple tested [2] but no qualitative results were forthcoming

since either the adhesive bonding material separated without removing the film from

the substrate or, as happened on two trials, a large chunk of the substrate material

was torn from the specimen. In no case was the film pulled away from the substrate.

-272-
3. Conclusions

The absorption of bias-sputtered films has been found to be consistently lower

than those deposited by non- bias techniques. RBS evaluation of the initially deposited zinc selenide material has shown that it is rich in zinc by approximately 11%.

Oxygen was found at the film/substrate interface of bias-sputtered specimens but its

form has not been determined.

In-situ sputter-etching of the specimen surface immediately before coating produced optimum film adhesion and the lowest absorption values measured to date.

4. Acknowledgements

The authors wish to gratefully acknowledge the assistance of Dr. Patrick Hemenger

of the Air Force Materials Laboratory and Dr. C. E. Christodoulides of the University

of Salford, England, for the Rutherford Backscattering Analyses and Dr. W. E. Moddeman

of the University of Dayton for the Auger Electron Spectroscopy evaluations.

5. References

[1] Walsh, D.A., Pukite, E.U. and Fenter, [2] Butler, D.W. , J. Phys. E. , 3, 979

J.R., Rev. of Sci. Instru. , £7 , 8, (1970).

932 (1976).

6. Figures

10-^ Zn Se ON CaF^ ® 5 3/i

30

Af vs X

20

10

NON -BIAS

BIAS^ /

/

A.\\

-I 1 1 1 I L. 74 72 3X'/4

OPTICAL THICKNESS

Figure 1. Absorption of sputter-deposited zinc

selenide films on non-etched substrate

surfaces.

-273-
BIAS SPUTTERED ZnSe ON CaF,

Nzn RBS N RATIO vs. t

{ } I { ^ { i _J I I I I I L_

25 50 75 100 125 150 175

FILM THICKNESS -A (NOMINAL)

ure 2. Ratio of zinc to selenium for thin layers

of zinc selenide on calcium fluoride by

Rutherford Backscattering analysis.

BIAS- SPUTTERED CVD ZnSe ON CaF,

AUGER PROFILE

3000 4000 5000

APPROXIMATE DEPTH A

Figure 3. Auger profile of zinc selenide film

on calcium fluoride showing presence

of oxygen at interface.

-274-
ZnSe ON SPUTTER -ETCHED CoFj, Q 5.3n

E 15

,25 ,5 I 2 4 8 16

SPUTTER-ETCHING TIME-min,

100

Figure 4. Comparative absorption coefficients for

equivalent coated samples based on sputteretching times.

Table 1. Processing conditions used in the

fabrication of coating test specimens

Precoating:

Preheat to 150°C, 5 min. @ lOy

_ g Outgas to 1 X 10 Torr

Coating

:

Target Voltage = lOOOV

Bias Voltage = 50V ^ R.F. Power = 520W (1.6 W/cm ) Pressure = lOp Argon

Spacing = 7 cm Etching

:

Platform Voltage = 350V ^ RF Power = 300W (1.0 W/cm ) Pressure/Spacing = as above

COMMEWTS OM PAPER BV WAL5H AWf BERTKE

IkzKi 0)04 dOMidzAjohtii du>cii&i^on on tkz condtm-ion that an 11% zxccM od zinc zxjj>te.d In the. ztnc

ieI.e.nA.dt {,ilm oi doXejmlmd {^fiom RuXheA{,on.d baak-icatteAing. In aniWM. to a qaution iiom the {jloon.,

tkz ipeafeeA indlcRttd that tht RutheA{,ofid back-icatteAing me.aiuAme.nti vioAZ caAAizd out with 4 Mel/ kutium lorn, and thaX the accuAacy 0(5 tkii technique wa6 eAtimated to be a j$ew peAcent. The zinc

exceii wai alio noted on the AugeA ipectAomttAy (,on thii mateAlat, the kugefi method howeveA li not

quantitative.

-275-
A STUDY OF INFRARED ABSORPTION IN ZINC SELENIDE THIN FILMS*

David F. O'Brien

Air Force Materials Laboratory

Wright-Patterson AFB, Ohio 45433

Zinc Selenide coatings for infrared laser windows have effective absorption

coefficients which are three to four ourders of magnitude higher than was predicted from bulk material optical constants. Prior growth studies and theoretical descriptions of binary compound deposition indicated that variations of the

stoichiometry of the zinc selenide films were a possible cause for the high absorption measured in these coatings. An experimental program was conducted in which the absorption in zinc selenide deposited onto calcium fluoride under a va- riety of deposition conditions was measured. It was shown that the infrared absorption decreased as the deposition rate was lowered. This directly correlated

with theoretical preductions based upon changes in stoichiometry caused by different film deposition conditions. Surface and chemical analyses using state-of-theart techniques indicated that the most probable cause for the anomalous absorption

in the films was an increase in the zinc to selenium ratio which could be controlled

to some degree by the proper selection of the vacuum deposition conditions.

Key words: Absorption; dielectric coatings; thin films; zinc selenide.

Introduction

It has become acknowledged that most commonly used infrared, low absorption coating materials display orders of magnitude differences between the effective absorption coefficients measured in the bulk

material and those measured in the thin film form of this same material. Several explanations for the

higher absorption in the thin films have been proposed [1]\ and all of these appear to have a certain

degree of validity. The greatest emphasis seems to have been placed on thin film contamination as being

a primary source of the increased absorption. It is somewhat surprising that although a large number of

these coating materials are binary compounds eg. AS2S2, l^s^Se^, KCl , ThF^, Tll, ZnSe, very little emphasis has been placed on investigating whether or not stoichiometric deviations of these materials could

be affecting the thin film absorption. This work was undertaken to determine if stoichiometric variations possibly occurring during the thin film deposition of ZnSe could be identified and related to increased infrared absorption in these coatings.

Karl-Georg Gunther has reported that for depositions involving evaporation of binary compounds

where the individual constituents have different vapor pressure, it can be expected that the deposited

coating will have a tendency to exhibit an excess of the less volatile material[2]. Zinc is known to have a substantially lower vapor pressure than selenium; therefore, for the particular case of depositions involving ZnSe, Zn rich coatings might be expected. In describing the deposiiton process for binary compounds, Gunther indicates that the degree of nonstoichiometry can be affected by three deposition parameters, substrate temperature, deposition rate, and component flux ratio. Specifically, the

following behavior can be inferred from his description:

High substrate temperatures tend to produce a more nearly stoichiometric film.

Low deposition rates tend to produce a more nearly stoichiometric film.

An excess of the more volatile component (Se) introduced into the vapor

stream increases the probability of producing a stoichiometric film.

Deposition Studies

The experimental program was designed to investigate if the calorimetrical ly measured infrared absorption in ZnSe thin film coatings could be varied by changing the above mentioned deposition parameters, and if so could these variations be correlated to the predicted stoichiometric variations.

* The material reported herein is based on portions of the author's dissertation submitted in partial

fulfillment of the requirements for the Doctor of Philosophy degree at the Air Force Institute of

Technology, Wright-Patterson Air Force Base, Ohio

1. Figures in brackets indicate the literature references at the end of this paper.

-276-
The substrates used in this investigation were 5cm diameter by 1cm thick polycrystall ine CaF^ man- ufactured by Harshaw. CaF^ was selected for its relative stability (as compared with alkali halides)

and for its low absorption coefficient. Each substrate was polished using successively a 1.0, 0.3, and

0.05um Al^ O3/H2O slurry and subsequently cleaned using a standard procedure described el sewhere[3]

.

Depositions were carried out in a diffusion pumped vacuum system which with liquid nitrogen trapping

maintained a vacuum of 1.33x10 ^Pa (IxlO'^Torr) . Electron beam evaporation was used since it provided

the easiest means for controlling the evaporation rate of the Raytheon CVD ZnSe source material. ZnSe was deposited onto substrates heated to a predetermined temperature between 20°C and 255°C and at rates

o which were varied from 55 to 2000 A/min. Additionally, some depositions were carried out with excess

Se in the vapor stream, accomplished by performing a separate thermal evaporation of high purity elemental selenium simultaneously with the electron beam evaporation of the ZnSe. Coating thicknesses of

nominally 0.15, 0.30, 0.60, and l.Oym were deposited for each of the separate sets of deposition parameters. Deposition rate was monitored by means of a quartz crystal rate monitor, and final film thickness was determined using a Sloan Dektak Surface Profile Measuring Systems.

The use of CaF^ as a substrate precluded calorimetric measurements at a wavelenath of 10.6um and

alternatives available were limited to a 5.3ym CO svstem or 3 l.Ofiym Nd:YAG system. The latter was se- lected when it was found that the CaF2 displayed a significantly lower absorption at 1.06ym thus offerInq a potential increase 1n the sensitivity of the thin film absorption measurement. The effective absorption coefficient of the ZnSe coating e^(cm ^) was determined by measuring the absorption A of the

uncoated and the coated substrates using standard calorimetry techniques [4] and dividing the difference

of these two values by the film thickness t. This is expressed as

A. A , , A

„ - _c _ coated - uncoated ^ "

t t

All calorimetry measurements on the coated substrates were made with the coated surface facing the incident laser beam, and no corrections for coherence effects were used since only comparisons of data from

measurements on coatings of similar thickness were made.

The first phase of experimentation was to determine if coating absorption would decrease when ZnSe

depositions were made onto heated substrates as would be expected from stoichiometric considerations.

Table 1 shows the results of this portion of the study. Comparisons of coating absorption for depositions performed at two different temperatures and at low, intermediate, and high rates were made.

Table 1. Effect of substrate temperature on coating absorption

Rate Substrate A^/t

(cm-1)

Coating

(A/sec) Temp(°C) Condition

0.9

0.9

76

181

5.5

6.3

Intact

Crazed

3.6

2.1

67

255

9.3

No data

Intact

Crazed

17.0

14.0

22

75

29.4

30.7

Intact

Intact

The data in table 1 show that the results of this phase were inconclusive. This was due to two factors. In the two cases where high substrate temperatures were used, 181°C and 255°C, thermal ex- pansion coefficient mismatch between the ZnSe coating and the CaF^ substrate caused crazing to occur

which either completely prevented a calorimetry measurement or rendered the data invalid. The third

entry in table 1 shows the results of an attempt to make a comparison between a room temperature deposition and a deposition onto a slightly heated substrate which presumably would have reduced the problems

caused by the thermal mismatch. The absorption data from these depositions were essentially identical.

It is possible that this negative result was caused by the two substrate temperatures being too close

to reveal the sought after trend.

The second phase of the experiment was designed to determine if a correlation existed between the

measured coating absorption and the predicted variations of film stoichiometry based upon differences

in deposition rate. A positive correlation would be indicated by a tendency for low deposition rates

to produce lower absorbing films. In this case, the CaF„ substrates were all heated to approximately

-277-
75°C, a value selected as being the highest temperature at which integrity of the film could be assured

and which provided a heated surface which would decrease the probability of having condensable impurio ties deposited in the films. Deposition rates between 55 and 2000 A/min were used and the measured

1.06um absorption coefficient for each coating was determined and plotted against the deposition rate. Figure 1 displays the results of this comparison and includes a third degree polynomial fit to the data.

Clearly evident from the figure is the definite decrease in absorption at the lower deposition rates. This agrees directly with predictions which attribute changes in stoichiometry to changes in deposition

rate.

As a final attempt at establishing the correlation between coating absorption and stoichiometry,

evaporations were made using excess Se in the vapor stream. Since predictions based upon the Gunther

model indicated that an increase in the Se/Zn flux ratio would aid in producing a more nearly stoichiometric film, it was felt that the addition of excess Se would result in a relative decrease in coating

absorption. Thermal evaporation of high purity Se from a tantalum boat was used to provide the excess

Se in the evaporant stream. Electron beam evaporation of the ZnSe was conducted simultaneously with

the Se evaporation, but because of geometry and control factors only a qualitative estimate of the

amount of excess Se could be made. It was estimated that from 10 to 30 percent excess Se was being introduced. Figure 2 shows the results of absorption measurements made on the coatings where, as in the

previous study, depositions were made at several different rates. Included in the figure is the curve previously shown in figure 1 for the case of no excess Se. The results show that except for the lowest

deposition rates an improvement in coating absorption was achieved by providing the excess Se. Again

this agrees with predictions based upon stoichiometry behavior.

Surface Chemical Analyses

The results of this study produced evidence supporting predictions that related theoretical stoichi

ometric deviations to absorption within the coatings. It was still desirable, however, to directly con- firm that these stoichiometric deviations were actually accurring within the films. Several surface

chemical analysis techniques were employed in an attempt to accomplish this. Prior to making these meas urements, estimates were made to determine, on an order of magnitude scale, what amount of stoichiometric deviation would be required to increase the ZnSe absorption to the levels observed in the thin film

coatings. The estimates were made using an approach described by Sparks [5] where, in this case, the

absorption was attributed to microscopic Zn inclusions existing in a stoichiometric ZnSe medium. Based

on this madel , it was estimated that from 0.01 to 0.1 percent excess Zn would be sufficient to increase

the ZnSe absorption coefficient from a value at 1.06vim of approximately 0.02 cm ^ to a value greater

than 1 cm'^ which was typical of the increase measured in the coatings.

Five different surface analysis techniques were used in an attempt to measure stoichiometric deviations in the films. These were x-ray fluorescence, electron microprobe, Rutherford backscattering(RBS)

,

electron spectroscopy for chemical analyses (ESCA), and Auger electron spectroscopy (AES); the latter

two methods included an ion sputtering attachment which provided a capability for performing profile

analyses of the films. One drawback existed, however, for all of the techniques. For each one the esti

meted lower limit of detection was at best 0.001 atomic fractions, and it could be presumed that it was

even higher than this for measurements of stoichiometric variations. Since this lower limit was the same order of magnitude as the highest estimated amount of excess Zn required to cause the observed level of ZnSe thin film absorption, measurement of the stoichiometric deviations were expected to be rather

difficult. Table 2, which lists the conclusions based on results of these measurements, bears this ex- pectation out.

Table 2. Summary of stoichiometry analyses

Dependence of Zn to Se Ratio on Technique Deposition Rate

X-Ray Fluorescence Possibly Higher for High Rate

Electron Microprobe Possibly Higher for High Rate

RBS No Dependence Detected

ESCA Higher for High Rates; Variation

Within a Single Film Noted

AES No Dependence Detected; Variation

Within a Single Film Noted

In each case measurements were made on two or more coatings deposited at different rates. Because

of ambiguities in the background corrections, interpretations of the x-ray fluorescence and the electron

microprobe results are very qualitative. Although still qualitative, the ESCA and AES measurements provided the most interesting results. Figure 3 gives an example of data from one AES measurement and re- veals perhaps one of the most significant findings resulting from the profile analyses of both the AES

and the ESCA measurements. The figure shows only the final portion of an AES analysis of a ZnSe coating

-278-
where the rapid decrease in the Zn and Se concentration is indicative of the fact that the coating/substrate interface had been penetrated by the ion beam. The fact that the overall Zn level in this figure

appears higher than that of the Se is only a result of the measurement technique and does not indicate

a difference in concentration. Evident in this data, however, is the indication of a significant point

to point variation in the Zn concentration which is far greater than any variation in the Se concentration. From these results it was felt that the nonstoichiometry of the ZnSe coatings existed not in a < uniform manner throughout the film but rather as local variations in the Zn concentration and that I changes in the deposition parameters affect the magnitude of these variations.

'

Conclusions

The observed behavior of ZnSe coating absorption at a wavelength of 1.06ym as a function of deposition parameters was found to be consistent with a model which explains the behavior on stoichiometric

variations. This conclusion is based on a comparison of changes observed in the measured thin film absorption with predicted stoichiometry deviations and on qualitative measurements of coating stoichiometry using several surface analysis techniques. The results of this study indicate that there are three controllable deposition parameters available which affect and, hence, can aid in minimizing the

absorption in ZnSe coatings. These parameters and their relationship to absorption are

Deposition Rate:

Se/Zn Flux Ratio:

Substrate Temperature:

Lowest rates produced

lowest absorbing films.

Inclusion of excess Se in vapor stream produced lower

absorbing films

High substrate temperatures

should aid in lowering absorption. Not confirmed

because of thermal expansion

problems

.

Finally, it should be noted that the AES and ESCA profile measurements of the Zn and Se concentrations within the coatings indicated that the stoichiometry variations were found to exist as point to point variations in the Se/Zn ratio and not as a uniform deviation throughout the film. This would

imply that the absorption of infrared radiation, which is likewise affected by the distribution of Zn and Se, occurs nonuniformly throughout the coating and is concentrated at localized high absorption

points.

References

[1] Sparks, M. , Theoretical Studies of High

Power Infrared Window Materials . Seventh

Technical Report 30 June 1976, DARPA Order

No. 1969 Amendment No. 7, Program Code

No. 6D10, (1976), pp 206-217.

[2] Gunther, Karl Georg, "Interfacial and Condensation Processes Occurring with Multi component Vapors," in The Use of Thin Films in Physical Investigations. J. C. Andrews ed..

Academic Press, New York (1966).

[3] O'Brien, David F., "Infrared Absorption in

Zinc Selenide Thin Films," to be published

as Air Force Materials Laboratory Technical

Report.

[4] See for example Kraatz, P. and Mendoza, P. J. "CO Laser Calorimetry for Surface and

Coating Evaluation," in Proc. Fourth Conf.

on Infrared Laser Window Materials , 1974,

C. R. Andrews and C. L. Strecker, eds.

Defense Advanced Research Projects Agency,

Arlington, Va., (1975), pp 214-230.

[5] Sparks, M. , Theoretical Studies of High

Power Infrared Window Materials. Final

lechnical Report December 1972, ARPA Order No. 1969-Program Code No. 2D10 (1972),

pp 90-110.

-279-
FIGURES

40 -

Figure 1. Coating absorption coefficient versus deposition rate of ZnSe.

5 10 15 20 25 30 35 40

DEPOSITION RATE (A/SEC)

Z2 -

>- - < -

Zn. .. -..

_

Se .............. ...

'•

T—i—I—mI I I—I I I—I r' i"i I Figure 3. Auger electron profile analysis of

TIME {iO Min/Div) ZnSe thin film showing Zn variations.

-280-
THE DESIGN AND OPERATION OF A PRECISE, HIGH SENSITIVITY ADIABATIC LASER

CALORIMETER FOR WINDOW AND MIRROR MATERIAL EVALUATION*

D. L. Decker and P. A. Temple

Physics Division, Michelson Laboratories

Naval Weapons Center, China Lake, California 93555

Important design details of a vacuum calorimeter intended for use with low

absorption materials and using only low power laser illumination are given.

Much attention has been given to the requirements for high sensitivity and to sources of systematic error including bulk and surface scatter, resulting in a number of improvements over existing instruments. Calorimetric measurements

are now possible in the visible and ultraviolet, and as a function of temperature. The calorimeter output is capable of absolute electrical calibration,

which is ordinarily an integral part of an absorption measurement sequence. The

system thermometry is sensitive to a change in temperature of 2 x 10~^°C. For a typical 1 . 5-inch-diameter sample, this corresponds to an absorptatice sensitivity

in the very low 10"^ range using one watt of laser power. The Assign of the

instrument permits the variation in absorptance across the sample to be determined. This paper presents examples of the operational capability of the instrument .

Key words: Absorption; calorimeter; laser; scattering.

This paper discusses the design philosophy, some construction details and actual performance of a very sensitive laser calorimeter. The goals in the design of this instrument were to provide accurate,

absolute absorptance data as a function of temperature in the 10~^ to 10~^ range for window or mirror

materials, using only low power (< 1 watt) laser sources over a wide range in wavelengths, and to provide

the capability to spatially scan the sample. Since the rate at which energy is dissipated in the sample

during laser irradiation can be as low as a few tens of microwatts, it is clear that very careful

attention to heat flow and to thermometry is mandatory. The design of necessity must approach an adiabatic ideal, requiring control over both radiation and conduction heat exchange from the sample to its

surroundings. Since the heat flows are so small, stray laser radiation reflected from calorimeter

windows, or scattered from the sample or baffles must be carefully accounted for and minimized to the

extent possible. In addition to minimizing systematic error sources, it was felt that some form of

absolute calibration was necessary to reach the accuracy levels required. This design approach thus

differs from the usual rate type calorimeter, and although much more difficult to construct, has capabilities and advantages which have not been heretofore available.

The design which has been implemented is based upon techniques similar to those employed in low

temperature specific heat calorimetry. The sample is in vacuum at a pressure of 10"^ Torr, and is

surrounded by a low emissivity isothermal enclosure which is temperature controlled to ±10~^°C. The

sample, which can be as large as 2 inches in diameter and 3/8- inch thick, is suspended in this can by a clamp ring supported by very fine steel wires. The sample temperature sensing thermistor and a calibration resistor are thermally connected to the clamp ring and hence the sample (see fig. 1). The thermal

link from the sample to the isothermal can has a conductance of < 10~^ watts/°C, with radiation being a factor of five or so more important than conduction for temperatures near room temperature. The input

window to the calorimeter is in fact a lens which focusses the laser beam onto the sample, the radiation

passing into and out of the isothermal can through small holes. The calorimeter output is taken as the

incremental change in sample temperature accompanying a laser irradiation of known beam power and

duration. This incremental response is heating rate independent to very high order. Absolute electrical

calibration is obtained by dissipating a known quantity of joule heat in the resistor attached to the

ring (see fig. 2). The transient response of the system to such a heat input is obviously different

than if the heat were deposited in the actual sample by laser irradiation. But since to high accuracy

both processes are adiabatic, the final equilibrium temperature increment is independent of the means,

rate or location of heat input. This is shown clearly in figure 3 where the same quantity of joule

heat is supplied to the calibration resistor on the clamp ring and to an identical resistor epoxied

into a hole in the center of a fused quartz disc. The fused quartz sample was chosen as a "worst case" since this material has a very small thermal conductivity. The incremental temperatures measured in

these two cases differ by only 2%, and in fact agree very well with a calculation of the theoretical

temperature increment using the calculated heat capacity of the sample and addenda and the sensitivity

of the thermistor (see Appendix) . Absolute electrical calibration of the calorimeter response is thus

possible, making it unnecessary to know the sample mass, specific heat or thermal diffusivity.

* Work supported by Defense Advanced Research Projects Agency and NWC Independent Research Funds.

-281-
Since the equilibrium temperature response is rate independent, it is unnecessary to irradiate the

sample only with a simple fixed geometry, and in fact the sample can be scanned point by point from one

side to the other by the beam, providing information on the spatial uniformity of the sample absorptance with a resolution limited only by the laser beam diameter.

Figure 4 shows the spatial response of a very uniform coated CaF2 window with an average absorptance of 4.8 x lO"'* and a maximum spatial variation of 0.6 x 10~'*/mm. Also shown on figure 4 is the

excellent repeatability of the measurement, the average absorptance difference between runs 1 and 2 being less than 1 x 10~^. The 3.8 ym laser power used in these measurements was less than 1 watt.

The spatial scan is actually accomplished by moving the isothermal can with respect to the beam.

This same motion permits the sample and can to be withdrawn completely from the beam, so that after

calibration of the losses in the windows, a direct measure of the laser beam power at the sample is

possible using a power meter at the output window of the instrument.

Figure 5 shows a typical run which includes a preliminary calibration. To compute the absorptance

one needs merely to ratio the incremental temperature response to laser irradiation ^Tia.ser ^° that of

calibration AT^^i.- One then computes the energy deposited during irradiation AQ^^ggj-

:

AT,laser

^^laser = -^f— ^^cal

cal

where AQj.^^ is just the calibration heat increment. If the laser power is Piaser> averaged over the

irradiation time ^tj^^ggj., the absorptance A is just:

AQ,laser

P, X At, laser laser

For the data in figure 5,

A = ill ,051 = 3 00 X

10-'* ^ 5.4 1.13 X 120

There are two sources of systematic error in this calorimeter design which are important: (1)

stray light which is absorbed on the clamp ring giving a false signal and (2) improper calibration of

the external power meter. The use of an electrically calibratable power meter such as the Scientech

351 is a step in the right direction, but the absorptance of the disc in this power meter is usually

taken more or less on faith as being 0.96. This assumption is not easily verified and constitutes

significant uncertainty in the present approach.

The stray light problem is present in calorimeters of all types, but especially in vacuum calorimeters because of the additional sources of scattered and reflected light coming from the entrance and

exit windows which are necessarily present. Carefully designed and aligned baffles can solve these

problems and the performance of the system can be directly verified by illuminating a sample which has

a hole through which the beam passes. Figure 6 is a run showing that under such conditions there is no detectable heating of the sample or ring. This test seems a trivial one, but is highly recommended as one of the important "proof tests" of any calorimeter.

The other major source of stray light is light scattered from the surface or bulk of the sample

itself. For many samples most of the scattered light comes from the surfaces, and it is possible to absorb much of this light directly in conical receivers attached to the isothermal can. However, bulk

scatter is potentially more of a problem, with light scattered possibly directly to the clamp ring.

Solutions to this problem include making the ring from material of lov/er absorption and limiting the area of the ring which looks into the sample. Sample scatter becomes much more serious as the wavelength is reduced, but as is demonstrated in a companion paper given at this conference by the same authors, the present instrument with no additional modification is capable of the same high performance

reported here at wavelengths into the near UV.

All measurements reported here were made at a temperature near 24.5°C. However, since the sample

"sees" primarily only its isothermal enclosure, similar measurements to those reported here can be made

at any temperature at which the enclosure temperature can be regulated. For the present apparatus this

temperature range extends from laboratory ambient (~ 24°C) to 100°C. This a capability not found in

the usual rate type calorimeter in which the sample is not thermally shielded. In such a device,

measurements only very near to the ambient temperature are possible.

The calorimeter described in this paper meets all original design goals, and has a sensitivity

using laser power levels of one watt of ~ 10"^ in absorptance. The calorimeter itself is capable of

absolute electrical calibration. The major sources of systematic error are associated with the calibration of the external laser power meter and uncontrolled sample scatter and limit the absolute

accuracy of the measurements to perhaps ±5Z for high quality, low scatter samples. Careful attention

-282-
to the stray light problem permits the use of this instrument into the near UV with the same accuracy

and sensitivity reported here for IR measurements.

Appendix

The following is a calculation of the theoretical equilibrium temperature response of the adiabatic

calorimeter with the quartz sample as reported in figure 3 of the text.

1) Calculation of the heat capacity of the sample and addenda

Addenda (mostly aluminum alloy) mass, 6.18 gm

Heat capacity of addenda, 6.18 gm x .90 J/gm°C = 5.56 J/°C

Sample mass, 23.52 gm

Heat capacity of sample, 23.52 gm x .787 J/gm°C = 18.50 J/°C

Total Heat Capacity, 24.1 J/°C

2) Calculation of thermistor bridge sensitivity

For an equal arm bridge with arm resistance R, the output voltage AV near balance is

1 AR

AV = — , where AR is the out-of-balance resistance of one arm, and is the

bridge supply voltage.

or

AV ^ 1 1 AR

AT 4 o R AT

for the particular thermistor used (at 35 °C)

III = 0.0400/°C

II

=

I- (1.35) (0.0400) = 0.0135 V/°C

3) Calculation of the system response

For the runs in figure 3 an average incremental voltage of 60 yV was seen

60 X 10-S X o~^35 = ^•'^'^ X 10"3°C

4.44 X 10"3 X 24.1 = 0.107 J

Computed Heat Input, 0.107 J

4) Calculation of the calibration heat input

5.856 V was applied across the calibration resistor (31.14 kfl) for 100 seconds.

The heat input was therefore

,p:f 3 X 102 = 0.110 J 31.14 X 10^

Calibration Heat Input, 0.110 J

5) Intercomparison of results

Computed heat input, 0.107 J Actual heat input, 0.110 J

Difference is then 0.003 J or 2.77%.

-283-
Figures

Figure 1. Two views of the adiabatic calorimeter with the vacuum container open.

The view on the right shows the isothermal can with the top in place,

showing the sliding top with beam exit aperture and the chain and

sprocket translation mechanism. The view on the left is with this cover removed, showing details of the interior including sample clamp

ring.

POWER METER

PRECISION RESISTOR

I LASER POWER

DETECTOR

.THERMISTOR

CALIBRATION CURRENT

SOURCE

AO CALIBRATION

BRIDGE AND

VOLTMETER

Figure 2. Operational schematic of the adiabatic

calorimeter. The calibration resistor

and temperature sensing thermistor are

in intimate thermal contact with the

sample rim. The system response to either a calibration pulse or to sample

absorption of laser energy is an incremental temperature change which

is as shown.

4.44 X 10-3 "C

8 16 24 32 TIME, MINUTES

Figure 3. This data indicates the effect of depositing joule heat at the center

of a fused quartz sample (a) or at the rim on the clamp ring (b) . The

equilibrium response differs by only 2% in these two cases, indicating

that the system response to high accuracy is Indeed adiabatic.

-284-
0.50 POSITION. IN.

Figure 4. Spatial variation of the absorptance of a Northrop-coated CaF2 window

sample 1-cm thick at 3.8 ym. The average absorptance is 4.8 x 10

and the maximum spatial variation is .6 x 10~'*/nim. Two different runs

are plotted which were made one day apart: © first pass; second pass.

Beam diameter (1/e^ points) was 1.35 mm.

A = 3.00 X 10'

TIME. MINUTES

Figure 5. Shown in this figure is a typical measurement run on a CaF2 sample at 3.8 urn which includes a preliminary calibration. The absorptance

computed directly from this data is 3.00 x 10"'* (see text).

TIME, MINUTES

Figure 6. No detectable heating of the sample or ring is evident in this data,

taken by allowing the beam to pass through a hole in the sample. This

is a very sensitive test for the presence of unwanted stray laser light

in the instrument originating at windows or baffles.

-285-
MODULATED LIGHT ELLIPSOMETER MEASUREMENTS OF STRAIN-INDUCED

ANISOTROPY IN THE REFRACTIVE INDEX OF As2Se3 AND AS2S3

FILMS ON KCl SUBSTRATES AT 10.6 ym*

M.E. Pedinoff and M. Braunsteln

Hughes Research Laboratories

Malibu, California 90265

and

O.M. Stafsudd

Engineering Department

University of California at Los Angeles

The measurement of strain-induced anisotropy in the refractive index of thin films by means of an Elasto-optic modulated light ellipsometer is discussed.

Ellipsometers of this type yield refractive index information indirectly in terms of

ratios of signals Vw/Vdc (R^) and V2a)/Vdc (R2) where u is the modulation frequency.

Computational routines have been developed that (differentially) relate changes in film

and substrate refractive index to changes in the signal ratios R^ and R2. These calculations

enable us to estimate the anisotropic change in the refractive index, An, of an optical film

due to a unidirectional stress applied to the substrate. Assumed anisotropics of 1%, give

signal ratio changes of 1% to 120%, depending on the experimental conditions. Preliminary

ellipsometric measurements show anisotropies of 0.6% generated in films of As„S, and As^Se,

on KCl by strains of 4 x 10-5 induced in the substrate.

An exact calculation of the change in signal ratios R^ and R2 was obtained using

D. Den Engleston's model of reflection from a uniaxial anisotropic film with the strain axis

in the film orthogonal to the plane of incidence.

The presence of large refractive index anisotropy cannot be discerned from measurements

made on a single film thickness sample by ellipsometric techniques. Experimental data is

presented for various thickness samples.

Key words: Anisotropy; ellipsometer; infrared; refractive index.

Introduction

The measurement of strain induced anisotropy in the refractive index of thin films by means of an Elasto-optlc modulated light ellipsometer will be discussed.

Ellipsometers of this type [l,2,3,4] yield refractive index information indirectly in terms of

ratios of signals Vw/Vdc (Ri) and V2a)/Vdc (R2) where o) is the modulation frequency. These ratios are used in a suitable set of equations [l,2] to calculate the equivalent sets of A and i|) which in turn are used in conventional computer programs [s] to calculate the refractive index of substrates and the

refractive index and thickness of films.

Computational routines [6] have been developed that (differentially) relate changes in film and

substrate refractive index to changes in the signal ratios Ri and R2. These calculations enable us to estimate the anisotropic change in the refractive index. An, of an optical film due to a unidirectional

stress applied to the substrate, from a measurement of the signal ratio changes AR^ and AR2.

An exact calculation of the change in signal ratios Ri and R2 has also been obtained using a variation of D. Den Engleston's [?] model of electromagnetic reflection from a uniaxial anisotropic film. In

this case, the optic axis lies in the plane of the film and is orthogonal to the plane of incidence.

Assumed anisotropies of 1%, give rise to signal ratio changes of 10% to 300%, depending on the film

thickness, film index, and the angle of incidence. Preliminary ellipsometric measurements on this effect

show that anisotropies of the order of 1/2% have been generated in films of AS2S3 on KCl by strains of

the order of 2.5 x 10-5 piezoelectrically induced in the substrate.

Sponsored by the Defense Advanced Research Projects Agency and administered by

RADC/ETSO, Hanscom Air Force Base, Massachusetts 07131.

1. Figures in brackets indicate the literature references at the end of this paper.

-286-
In those cases where the film is deposited on the substrate at high temperatures and then allowed to cool to room temperature, thermal expansion coefficient differences between the film and the substrate

can give rise to a uniaxial anisotropy in which the optic axis is orthogonal to the plane of the film.

In this "perpendicular" anisotropy, the signal ratios Rj^ and R2 will show a different dependence on film

thickness than the signal ratios of the isotropic case. We have found that the plots of versus R2 as

a function of angle of incidence as a parameter, were not useful in separating isotropic and anisotropic

effects. However, when the signal ratios Rj^ and R2 are plotted at the constant angle of incidence using

the film thickness as a parameter, the cases of an isotropic film and a film with 10% anisotropy can easily be resolved. This method of measurement of perpendicular optical anisotropy requires that the

measured signal ratio data be plotted on a theoretically calculated chart of the signal ratios for

several values of anisotropy and film thickness. The curve which best fits the data is then the best

estimate of the anisotropy. It is feasible but non-trivial to program the computer to find that set of

film thickness and refractive indices which best fit the data.

Analysis

Analysis of an anisotropic film has been approached by several workers 7,8 and in general is very

difficult due to the admixture of r and s polarized fields by the anisotropy of the film. However, a more tractible problem occurs if the film exhibits uniaxial properties and if the unique axis is either

perpendicular to the surface of the film (perpendicular case) or parallel to the plane of the film and

orthogonal to the plane of incidence (planar case). Using the coordinate system of figure 1 for these

two cases, the index tensor can be expressed as:

for the perpendicular case

for the planar case.

We will assume that the films and substrates are transparent in all cases.

Planar Anisotropy

Let us first examine the planar case (fig. 1). To calculate the reflection coefficients for the

r and s polarizations of the film substrate system requires calculating the fresnel coefficients of the

interfaces and the phase factor associated with traversal of the film for each polarization. At the

first interface (air - film), the reflection coefficients are of the form:

cos (()- - N cos d) ^(1) ^ ^1 ex ^ex

-L cos (t). + N cos <b 1 ex ex

cos t> - N cos <ti. or or 12 cos d) + N cos d>, or or ^1

r^^^ = or or ^1

at the film-substrate interface.

N cos ((i - N cos 4). (2) _ ex ex s ]_3^ -L N cos <t> + N cos ex ex s i

C2)

r,: N cos d)~ - N cos <t) or s or

N cos (!)_ + N cos cj) or ^3 s or

(3)

(4)

-287-
The phase factors associated with the two polarizations are

1/2

277 F

2lT

(n2^- sin\)

II X or ^or

The total reflection coefficients can then be calculated from

-2i3

and

(1) _^ (2) ^^^11

^11 + ^11 ^

1 + r(l) r(2) e-2i^„

(5)

^ N cos cj) . (6)

• + ^1 ^ (7)

1 ,^12 -2i6

1 + rj^ e

(8)

All angles other than 4>^ can be eliminated from Rj^ and R|| by using Snell's law:

sin ((). = N sin (b = N sin <t> = N sin <b^ . (9) 1 or ^or ex ^ex s 3 ^

'

The ratio of R

|| to Rj^ can be used to find the usual ellipsometric variables A and \l) from:

— = tan tl; e . (10)

The observable signal ratios R. , R„ can then be calculated directly from Rn and Ri or from A and i{) as defined above [l,2]. II -L

The results shown in figure 2 were obtained by calculating the R]^, R2 parameters of a typical isotropic film (such as ZnSe on KCl) as a function of thickness or angle of incidence and then comparing it

to a film of moderately strong anisotropy (e.g., Ng^ = 1.1 Ngr) • It is somewhat more enlightening, particularly for smaller anisotropies (e.g., 1%), to plot and R2 separately as functions of angle of

incidence, as shown in figures 3,4,5, and 6. The parameter most easily observed experimentally is the

change in R]^ or R2 (i.e., ARj^ or AR2) caused by stress-induced anisotropy. Therefore, we have also

plotted ARj^/Rj^ and AR2/R2 as functions of angle of incidence in figures 3,4,5, and 6.

If a stress orthogonal to the plane of incidence is applied to the substrate film combination,

anisotropy can be induced in the film index. The form of this anisotropy corresponds to our model of

the planar case.

In principle, an unstressed film could be measured, a clamp (or equivalent) used to stress the film

in a fixture, and then the film remeasured. The problem with this approach is that thermal effects in

the fixture and additional experimental uncertainties limit the accuracy of the results. However, if

the stress on the film is dynamically changed during the experiment, then the change in Ri and R2 can be

easily and unambiguously measured. The change in stress and associated strain can be produced by

piezoelectric transducers and measured by a strain gauge. Figures 3 and 4 represent a film of As2Se3

(X/4) thickness on KCl and figures 5 and 6 represent AS2S3 (A/4) thickness on KCl. Figures 3 and 5 and

figures 4 and 6 indicate the expected fractional change in Rj^ and R2, respectively, as functions of

angle of incidence. The assumed change in the index of refraction in each case is 1%. The curves show

that there exists an optimum range in the angle of incidence for the measurement of this effect.

The results of our first series of measurements on planar anisotropy generated by an externally

applied strain are shown in table 1. In this experiment, the strain was generated in the sample by a piezoelectric transducer and monitored by a piezoresistive strain gauge. One sample consisted of a film of AS2S3 on a KCl substrate. The mechanically induced strain in the substrate and the film was

2.45 X 10~5 and the attendent fractional change in refractive index was 0.006 ± 0.0003. Using these

values, the experimental strain optic constant [ll]defined as -2(AN/n3) (Ai/i) was found to be 89 ± 10. This thin-film strain optic value is more than two orders of magnitude larger than the bulk strain optic

values measured at 1.15 ym (0.31); similar results were obtained for the As2Se3 samples.

The typical maximum values for stress in optical films [9], observed by other methods, can be of

the order of 10^ dyne/cm2. By assuming a stress value of 107 dyne/cm2 and 1.6 x lO-'-^ d3me/cm2 for

-288-
Young's modulus in AS2S3 the strain in a typical film can be estimated to be as large as 6 x 10"^.

Using this value for the strain and the strain optic constant empirically determined above, the expected

change in refractive index AN/N due to the thermal expansion coefficient mismatch effect is equal to 0.14. This externally induced change in refractive index is much larger than the sample-to-sample variation observed in our conventional modulated light ellipsometer measurements of the refractive indices

of AS2S3 films. However, significant sample-to-sample variations were observed in the measured refractive index of As2Se3 films on KCl substrates. We have attributed these effects to strain- induced anisotropic, and we shall show that the existence of moderate to large perpendicular anisotropies are essentially undetectable elllpsometrically in a single-thickness film.

Perpendicular Anisotropy

A summary of the results of our conventional modulated light ellipsometer measurements* on AS2S3

and As2Se3 films is given in table 2. The variation of the measured refractive index from sample-tosample is less than 2% for the AS2S3 film and more than 14% for the As2Se3 film. The existence of

these variations in refractive index for different film samples from the same startup materials tends to indicate that the conditions prevailing during the evaporation play a significant role in the properties

of the film.

The fact that these films showed no significant variation in their refractive index when measured

at various angles has led to a theoretical investigation of the effects of perpendicular anisotropy

which arises when isotropic films deposited on isotropic substrates are internally strained. The ten- sion or compression forces that produce this strain are expected to be isotropic and planar. In this

case, the s-polarized wave always "sees" a constant index as a function of angle of incidence (mainly

Nqj.) . The polarization component, however, sees a varying index as a function of angle of incidence due

to the anisotropy of the film. Again, just as in the planar case, to calculate the reflection coefficient of the film-substrate system only requires calculating the fresnel coefficients at the interfaces

and the phase factors for traversal of the film.

The fresnel coefficients of the film air interface are

. cos <}), - N cos d) (1) _ ^1 or ^or

~| cos <t). N cos 4) 1 or or

r (11)

2 cos d), N N - (N - sin d),

)

j.(l) ^ ^1 or ex ex ^1

_

2 2 cos (b, N N + (N - sin i). ) ^1 or ex ex ^1

1/2

T72

The fresnel coefficients of the film substrate interface are

N cos (j) - N cos 6, ^(2)^ o ^o s ^3

I N cos (1) + N cos i)-, o OS ^3

r

2 2 (2) ^s <^ex

-

- ^or ^ex ^3

II - 2 2 N (N - sin (1), ) + N N cos (j). s ex 1 or ex 3

(14)

and the phase factors for traversal of the film are

2^ F 1/2

3j_ = (N^^ - sin^ (15)

2TT N , „ 1/2

" ""^ (N^ - sin-^ <t>,) . (16)

A N ^"ex '^V ex

Rj^ and Rn can be calculated as before:

R rf> + rf> e

i2e_L

(1) (2) "^i ^^'^

1 + r, r.| e i

These are not a result of squeezing the substrate.

-289-
R„ =

(1) ^ (2) "-^^ll

* +r G

"H II

T

, ,(1) ,(2) ^i26,,

1 + r r e II

r„ .... . (^8)

Inserting the expressions for the phase factors and the single surface fresnel coefficients into

eqs. (17,18) yields R and R as functions of Fj., Ng, Nj, (}), etc. As before, all angular variables

(4'or' '('ex' ^^'^ '^3^ '^^^ eliminated by using a modified Snell's law. Rj^ and R|| can then be used to calculate the ratios R^ and R2.

Figure 2 shows a plot of R-^ versus R2 at a fixed angle of incidence (70°). In this plot, the

parametric variable is the film thickness. The tick marks are spaced by film thickness increments of

\/20 (n2 - sin~2(f)-j^) 1/2. The solid curve indicates the increment of calculated values of R-^ and R2 for a film of ZnSe on KCl with no anisotropy. The A and 0 marks indicate the calculated values of Rj^ and R2

for anisotropic films of the same thickness on a KCl substrate. In the case of A, the film has planar

anisotropy with Nq^ = 2.45 and Ng^ = 1.1 N^^. For the 0 marks, the film is perpendicularly anisotropic

with Nor ~ 2.45 and Ng^ = 1.1 Nq^. The figure shows that even anisotropies of 10% give relatively small

shifts in the R-^ R2 plots. In fact, there are film thickness values for an isotropic film that give R-^ and R2 values identical to those of an anisotropic film of slightly different thickness values at a fixed

angle of incidence.

We have calculated the variation of R-^ and R2 with angle for a series of anisotropic films with

optical axes perpendicular to the surface of the film. This corresponds to the usual case of an optical

film deposited at high temperature on a substrate with thermal expansion properties than the film.

These calculations indicate generally that the presence of this type of anisotropy cannot be detected by

measuring a single film sample. For example, figures 7 and 8 show the theoretically expected values of

R]^ and R2 as functions of angle of incidence for isotropic films of index values 2.4 and 2.448. Also

plotted are the data points for an anisotropic film of Nqj- equal to 2.4 and Ngjj equal to 2.448. These

curves show that an anisotropic film exhibits R-j^ and R2 values as a function of the angle of incidence,

(J)]^, that are essentially identical to the behavior of an isotropic film of slightly different index

and/or thickness. This indicates that there will be very little variation in the experimentally determined refractive index as a function of the angle of incidence even though the film being measured is

highly anisotropic. This conclusion only applies to the perpendicular anisotropic case.

In principle, following the work of De Smet [s] and Den Engleston [7], the anisotropy can be evaluated using a series of optically identical films of vastly different thicknesses. With reference to figure 2 for the anisotropic case, the R^, R2 curves will spiral rather than form a closed figure. This

is analogous to the A, tjj plots for anisotropic film as shown by De Smet and Den Engleston. In practice,

however, this is not feasible with a 10.6 ym ellipsometer because the method requires films that range

In thickness over many wavelengths. This film thickness variation method for determining anisotropy is

viable for short wavelength ellipsometers provided the film is transparent at the measurement wavelength.

Conclusions

We have demonstrated the feasibility of measuring the anisotropy induced in a thin optical film by

piezoelectric or other external means. Measurements of these effects have been used to estimate the

strain optic constants of thin optical films at 10.6 ym. These constants, when used in conjunction with

typical strains associated with optical films, predict strong anisotropies of the perpendicular type

defined here. The existence of thermal expansion coefficient mismatch effects between the film and the

substrate can cause the films to have perpendicular anisotropy characteristics. These effects, combined

with the planar anisotropy effects that occur whenever an external stress is applied, can lead to biaxial optical anisotropy, which has not been analyzed here.

Theoretical calculations of the effects of perpendicular anisotropy on the measurement of the

refractive index of films show that these measurements cannot be used to determine whether or not the

film is anisotropic from a single sample. However, a series of measurements with films of various

thicknesses can be used to determine the perpendicular anisotropy, provided that the strain in the films

is independent of thickness. Experimental data taken on various films agrees with the theoretical

result that the perpendicular anisotropy alters the index but does not yield different index values for

different measurement angles.

-290-
Table of Symbols

Ft film thickness

X Wavelength In free air

\ index of substrate

N

or ordinary index of refraction of the film

N

ex extraordinary index of refraction of the film

angle of incidence

*or angle of refraction in the film of the ordinary polarization

*ex angle of refraction in the film of the extraordinary polarization

*3 angle of refraction in the substrate

h

the reflection coefficient of film substrate for s polarization

hi the reflection coefficient of the film substrate for p polarization

Kr phase factor for the ordinary wave

^ex phase factor for the extraordinary wave

References

[1] Pedinoff, M.E., Braunstein, M., and [6]

Stafsudd, O.M., "Modulated Light Ellipsometry at 10.6 pm," Proceedings of SPIE, San

Diego, CA, August 1977.

[2] Pedinoff, M.E., Braunstein, M. , and

Stafsudd, O.M., "Measurements of Refractive

Indices of IR Materials: 10.6 ym Ellipsometry," Appl. Optics, to be published

November 1977. [7]

[3] Allen, S.D., Braunstein, A.I., Braunstein, M,

Cheng, J.C., and Nefie, L.A. , "A 10.6 Micron

Modulated Light Ellipsometer , " Optical Prop- [8]

erties of Highly Transparent Solids , S.S. Mitra and B. Bendow (Plenum Publishing Co.,

New York), pp. 503-513, 1975.

L4J Jasperson, S.N., Burge, D.K. , and 0 Handley,

R.C., "A Modulated Ellipsometer for Studying

Thin-Film Optical Properties and Surface

Dynamics," 37, pp. 548, 1973.

[5] McCrackin, F.L., "A Fortran Program for

Analysis of Ellipsometer Measurements,"

Natl. Bur. Std. Technical Note No. 479 (1969),

(U.S. Govt. Printing Office, Washington,

D.C. 1969).

Pedinoff, M.E., Braunstein, M. , Stafsudd,

O.M. , "Modulated Light Ellipsometer Measurements of the Refractive Indices of ThF4

Single Crystals and As2Se3 and AS2S3 Films on

ICCl Substrates at 10.6 ym." Topical Conf. on High Power Laser Optical Components and

Component Materials, Boulder, Colorado,

October 3, 1977.

Den Engleston, D. , "Ellipsometry of Anisotropic Films," J. Opt. Soc. Am. 61, pp. 1460-

1466, 1971.

DeSmet, D.J., "Ellipsometry of Anisotropic

Thin Films," J. Opt. Soc. Am. 64, pp. 631-

638, 1974.

Ennos, A.E., "Stress Developed in Optical

Film Coatings," Appl. Opt. Vol. 5, pp. 51,

1966.

-291-
!

Table 1. Squeeze Experiment Results

Film Material AS2S3 As2Se3

Substrate KCl KCl

Nf (film index) 2.35 2.8

(substrate index) 1.455 1.455

Ft (film thickness) X 11610 9914

hill (strain) 2.45 X 10~^ ± 1.2 X 10"^ 4 X 10~^ ±2x6"^

AN/N (index change) 0.006 ± 0.0003 0.01 ± 0.0005

Strain optic coefficient

p - 2-^ / M

89 ± 40 64 ± 32

'^ll / ^

Table 2. Results of the Conventional Modulated Light Elllpsometer Measurements

MATERIAL

OPTICAL

THICKNESS

MEASURED

INDEX

MEASURED

THICKNESS

ACCEPTED

INDEX

AS2S3

KCi - X/8 2.34 ± 0.03 6826 ± 72 A° 2.38

AS2S3

KCi ~ X/4 2.35 ± 0.03 9916 ± 116 A°

AS2S3

K CJ2 ~ X 2.36 ±0.06 39210 ± 205 A°

AS2Se3

K Ci

- X/8 2.45 ± 0.05 6808 ± 120 A° 2.80

AS2Se3

KCi - X/4 2.66 ± 0.14 9683 ± 376 A°

AS2Se3

KCi - X/4 2.81 ±0.17 10170 ±570 A°

AS2Se3

KCi ~ X 2.85 ± 0.03 33700 ± 710A

-292-
Figures

INCIDENT 2 WAVE

\ ANGLE OF \ INCIDENCE

N = 1 \

; REFLECTED

i WAVE

N = Np

(FILM)

"or—

ANGLES OF

REFRACTION

N = Ns

(SUBSTRATE)

^3 ->IJ

Figure 1. Theoretical model of anisotropic film.

R2 Np = 2.45 <^ = 70°

NO ANISOTROPY

PERPENDICULAR

ANISOTROPY

PLANAR

ANISOTROPY

Figure 2. Plot of R]^ versus R2 for NF = 2.45, NS = 1.A55, x = 1.

-293-
Figure 4. R2 and AR2/R2 versus (j) with 1% planar anisotropy

-294-
Np =2.450 X/4

Ng = 1.455 x = 1

Figure 5. Ri and AR^ /R. versus (j)

with 1% planar anisotropy.

-295-
40 50 60 70 </) Figure 7. Perpendicular anisotropy R. versus <j).
COMMENTS ON PAPER 8/ PEPIMOFF, 8RAL/NSTEIN, ANV STAJ^SUW

The. ipmkzA. polntdd out that anti,otAoplu> up to J 51 kaue been mexUsUAzd by tkis ttchntquz.

-297-
!

DAMAGE RESISTANCE OF AR-COATED GERMANIUM

SURFACES FOR NANOSECOND CO2 LASER PULSES*

Brian E. Newnam and Dennis H. Gill

Los Alamos Scientific Laboratory

Los Alamos, New Mexico 87545

An evaluation of the state-of-the-art of AR coatings on gallium- doped germanium, used

as a saturable absorber at 10.6 ym, has been conducted. Both 1-on-l and N-on-1 laser damage

thresholds were measured with 1 . 2 ns pulses on bare and coated surfaces. Only front surface

damage was observed. With few exceptions, the thresholds for coated surfaces were centered

at 0.49 ± 0.3 J/cm^. Bare Ge had a threshold ranging from 0.65 to 0.70 J/cm^. No significant differences due to substrate polish, crystallinity or doping level were evident, and

multiple-shot conditioning resulted in the same threshold as for single shot tests. From

an analysis of standing-wave electric fields, damage of AR-coated Ge appeared to be limited

by the surface properties of Ge. Measurements at both 1.2 and 70 ns indicated that the

threshold (J/cm ) of both coated and uncoated Ge increases as the square root of the pulsewidth.

Key words: Antireflection coatings, germanium, laser damage, saturable absorber, standing- wave electric field.

1. Introduction

Gallium-doped germanium has been developed for use as a saturable absorber to prevent pre-pulse

gain depletion in the large CO2 amplifiers of LASL's eight -beam fusion laser [1,2]. For use at satu- rating intensities for pulses 1-nanosecond in duration, the damage resistance of the AR-coated surfaces

must be maximized. Accordingly, a careful evaluation of the state-of-the-art of AR coatings at 10.6 pm was performed.

Antireflection coatings comprising fourteen coating designs using eight film materials were obtained from nine coating manufacturers. Polycrystalline, p-doped Ge substrates polished by one vendor

were supplied to each. Substrates polished by a second vendor were also supplied for comparison.

Additionally, single-crystal Ge, p-doped and undoped, and undoped polycrystalline Ge were coated by one vendor to evaluate the effect of crystal structure and doping. The dimensions of the test substrates

were 25 mm in diameter and 6 mm thick. Coating depositions, however, were performed in chambers large

enough to eventually coat amplifier-size Ge discs (41-cm diameter and 4-cm thickness) with sufficient

uniformity to obtain a reflectance per surface of less than 1% at 10.6 ym and less than 3% from 9 to

11 ym.'

2. Experimental Procedure

Laser damage tests were conducted with 1.15 ± 0.05 ns pulses (FWHM) at the P(20) 10.6 ym wavelength. These short pulses were reliably carved out of a smoothed gain-switched pulse by use of a Pockel cell arrangement. The schematic of the laser diagnostics is shown in figure 1. Pulsewidth

measurements were made with a Molectron pyroelectric detector coupled to a 5-GHz bandwidth oscilloscope

of LASL design [3]. For supplementary tests with a 70-ns pulsewidth, a photon-drag detector was used.

Oscillograms of the temporal pulses are shown in figure 2.

The test samples were located prior to the focus of a 1 m F.L. ZnSe lens where the beam spot-size

radius was 1.1 mm. The peak value of the irradiance (J/cm^) at the sample plane was measured on each

shot by use of a 197-ym diameter pinhole (Optiraation, Inc.). The pinhole was located in a split-off

beam and placed at the same distance from an identical ZnSe lens as was the sample. The energy trans- mitted by the pinhole at the center of the reference laser beam was measured by a Laser Precision Energy

Meter (isolated from rf noise). Prior to each test series a calibration was performed with an identical

197-ym pinhole centered at the sample plane. During the damage tests the reproducibility of the spatial

profile was monitored by comparing the energy focussed through the pinhole reference with the total

energy measured by a Scientech calorimeter. The 197-ym diameter of the pinhole was chosen to minimize

the spatial averaging over the beam profile (at the pinhole perimeter the intensity of the Gaussian

profile dropped to 98% of the peak value) while transmitting an adequate amount of energy for easy

detection. Also, the ratio of the pinhole diameter to the wavelength was large enough to avoid significant diffraction effects. By use of the equation,

T = 1 . sin(2ka- ^) + . . . , (1)

VT(ka)5/2

^

*Work supported by the U. S. Department of Energy.

1 Figures in brackets indicate the literature references at the end of this paper.

-298-
where a = aperture radius [4], the transmittance of the 197 ym-diameter aperture was calculated to be

99.98%.

Damage was detected visually by the onset of increased surface scattering of a He-Ne laser beam

directed on the same site (back and front) as the pulsed laser and by examination under bright whitelight illumination using the He-Ne beam as the locator. After the tests, examination of irradiated

areas was also performed with a microscope (300X)

.

3. Damage Morphology

The characteristics of the laser-damaged sites viewed under 200 and 300 X magnification were very

interesting. Only front surface damage was observed in these tests. In figure 3, disruption of an AR-coated (Ge/PbFa/ZnSe/Air ) surface of p-doped, polycrystalline Ge, caused by a single 1-ns pulse

above threshold, is examined. The AR coating has been removed rather uniformly leaving a well-defined

perimeter. Linear interference ripples oriented normal to the laser polarization are grouped around

circular damage pits. Temple and Soileau have identified these ripples as perturbations in the

surface topography due to interference of the incident laser electric field with the time-varying

(laser frequency), induced surface charges on surface scratches, voids and inclusions [5]. The diameter

of the pits are mostly 8 to 12 \im and the ripple spacing is approximately 8.5 )jm which are close to the

laser wavelength. Damage sites in uncoated Ge (not shown) caused by 1-ns pulses did have faint ripples

with spacing exactly equal to the laser wavelength (± 0.2 nm)

.

The morphology of damage caused by 70-ns pulses was very different from the above as shown in

figure 4. On the coated surface, a random distribution of irregular sites was related to damage at defect sites, and extensive cracking of the AR coating is probably thermally-caused delamination. For bare Ge (fig. 5) the damage sites were all centered on circular pits accompanied by very tightly-spaced

(3.5 ym) interference fringes parallel to the laser polarization. The cause of these fringes has not been identified.

4. Results

The experimental results for coated and bare Ge substrates are presented in tables 1-3. These

thresholds are for pit formation or film disruption which occurred at much lower intensities than a breakdown plasma. Only the mean value of each threshold is listed for the coated surfaces since the

range, typically ± 0.02 or less, was unusually small. The absolute accuracy is considered to be ± 10%.

All thresholds listed are for front surface damage only as we were unable to damage any rear surface,

coated or uncoated. Further, we observed no difference between thresholds for 1-on-l and N-on-1 tests,

where N-1 shots (10 to 15) were fired below the single-shot threshold, before irradiating with a damaging intensity.

To compare the effects of two different conventional polishing methods, single-and polycrystalline

substrate material and Ga-doping level (undoped, R = 30 f2-cm; doped, 3 fi-cm), one coating vendor

deposited a two-layer ThFi,/ZnS Vee-coat on each different substrate during one run. As seen in table

1, no significant differences in thresholds caused by the two polishing methods were manifest. This

was surprising since Polish A qualified as better than "40-20" (scratch and dig code) and Polish B was slightly worse than "40-20". Likewise, no real differences were measured between coated single-crystal

and polycrystal Ge surfaces. Gallium-doping had no effect on coated single-crystal thresholds, and

only a minor 10-15% threshold reduction was measured for Ga-doped polycrystalline Ge.

Table 1. Damage threshold of AR-coated Germanium substrates

Coating

:

Germanium

Substrate

Single Crystal (undoped)

Single Crystal (p-doped)

Polycrystal (undoped)

Polycrystal (p-doped)

s/ThF^/ZnS/a

Polish A

0.47

0.47

0.48

0.41

Energy Density (J/cm^)

Polish B

0.46

0.46

0.51

0.46

The thresholds for fourteen coating designs on p-doped, polycrystalline Ge are listed in table 2. Multiple entries represent different samples of the same coating. The values ranged from 0.41 to 0.57

J/cm and the mean value was 0.49 ± 0.03 J/cm^. Even the two-layer coating of CaFa/ZnSe had the same threshold despite the fact that CaFz has a large absorption coefficient at 10.6 ym.

Due to the relative uniformity of thresholds for coated Ge surfaces, particular attention was paid

to the thresholds of uncoated Ge presented in table 3. The values for three different bare surfaces

were all greater than those of coated Ge by about 40%. In addition, it is noted that Ga-doping lowered

the threshold by - 10%.

-299-
Vendor

Table 2. Damage thresholds of AR-coated germanium

Single-layer coatings Ge/coating/Air

Design Energy Density

(J/cm^)

I ZnS 0.,51, 0,.46

E ZnS 0.,50, 0,.47

D Til 0.,46

Two-layer coatings

G CaF2/ZnSe 0,,50, 0,,48

H PbF2/ZnSe 0,,49

E ZnSe/ThF^ 0,.48, 0,,46

H ThP^/ZnSe 0.,44

A ThF^/ZnS 0.,46, 0,,41

Ge substrate. undoped, polycrystalline, poor polish

Three -layer coatings

I ZnS/Ge/ZnS 0.,53, 0,,50

D TII/KCI/TII 0,,50

F ThF^/ZnS/ThF^ 0.,51, 0.,48

F ThF^/ZnSe/ThF^ 0..50

ZnS/ThF^/ZnS 0.,48, 0,,47

Four- and Five-Layer Coatings

C ZnS/Ge/ZnS/ThF^ 0. 57, 0. 55^

B ThF^/ZnS/ThF^/ZnS/ThF^ 0..47, 0..45

Table 3. Damage thresholds of uncoated

polycrystalline germanium

(Front Surface Only)

Undoped

P-doped (Gallium)

P-doped -HNOj treated

Energy Density

(J/cm^)

0.70 ± 0.05

0.65 ± 0.05

0.72 + 0.05

5. Analysis of Electric Fields

The experimental results indicate that the damage threshold of AR-coated Ge surfaces is 1) independent of the design and materials of the AR coatings and is 2) lower than uncoated Ge. Furthermore,

damage occurred only at the front surface. These results may be explained by considering the electric

fields in the Ge, coated and uncoa|ed. Figure 6 represents the standing-wave electric fields,

normalized to the incident field E , in the vicinity of the front surface of an AR-coated and uncoated

Ge substrate. Although, the exact field distribution within the AR coating must be calculated for each

design, the gradual decrease of |e/E |^ from 1.0 at the air-film interface to 0.25 at the film-Ge

interface is the same for any design.

-300-
The normalized field in the AR-coated substrate is easily calculated using the relation

2

1n

(2)

where, for Ge, n = 4.0 at 10.6 pm. Likewise, the field-squared in the uncoated Ge substrate is 0.16

in the vicinity of the front surface as calculated from

(n^^l)

(3)

Since the power absorbed per cm^ is given by

= na|E/Ej|^ (4)

for linear absorption, damage by this mechanism (or any other involving the electric field) in Ge would

predictably occur at a lower incident laser intensity for an AR-coated surface. The ratio of the

fields-squared in uncoated-to-coated Ge is computed to be 0.64. The ratio of the measured damage

thresholds is 0.7 ± 0.1, which is consistent. These results indicate that the damage probably occurred

in the Ge at the Ge/film interface. Thus, for all but the lowest thresholds measured, it is apparent

that the coatings had at least as high a damage resistance as the substrate itself.

It is informative to calculate the peak electric field present in the various coating materials at the threshold of damage of the coated Ge surfaces. These fields, listed in table 4, represent a number

of samples and coating designs. In terms of electric field, it is seen that the same value, 0.2 MV/cm,

existed in the coated and uncoated Ge substrates at the threshold, again implicating the substrate as the limiting factor. We also note that this breakdown field is comparable to the dc value of ~ 0.08

MV/cm [6].

Table 4. Peak electric field at damage threshold

of coated Ge surfaces

10.6 ym, 1.15 ns pulse

Coating Material Number of E„,,„CMV/cml " _ , RMS

Samples

ZnS 17 0.33 to 0.40

ZnSe 7 0.33 to 0.40

Til 2 0.32 to 0.39

ThF^ 15 0.24 to 0.40

PbF2 1 0.33

CaF2 2 0.30

KCl 1 0.23

Ge 2 0.22 to 0.26

Ge substrate 26 0.18 to 0.21

coated

Ge substrate 7 0.18 to 0.19

uncoated

Interestingly, the Ge films withstood 20 to 25% more electric field than the substrate. In most

other cases, thin films are generally less damage-resistant than bulk material. Given a more damageresistant substrate than Ge, it is likely that the various coating components could survive even higher

electric fields than attained in these tests.

6. Pulsewidth Dependence

A second set of tests at a longer pulsewidth, ~ 70 ns, produced thresholds of 4.2 J/cm^ for

PbFz/ZnSe AR-coated Ge and 6.4 J/cxtr for uncoated Ge. The ratio of these thresholds, 0.66, is still in consonance with the SW electric-field explanation. Although only two points are used in figure 7 to draw a straight line, the threshold dependence of both coated and uncoated Ge goes nearly as the square

root of the Ge pulsewidth:

-301-
AR-coated Ge

£p = 0.455 T (5)

Uncoated Ge

= 0.67 T 0.53

(6)

This scaling relationship is the same as that for free electron absorption in metallic surfaces and

for electron plasma absorption initiated by avalanche breakdown in dielectrics [7]

.

7. Discussion

It is a general observation that the rear surface of a transparent dielectric is damaged prior to the front surface by a collimated laser beam, assuming the surfaces have identical surface properties.

For substrates like glass, this effect has been clearly demonstrated to be due to the greater standing-wave electric field at the rear surface [8]. Figure 8 illustrates the 180° phase reversal of the

reflected wave at the front surface and the in-phase reflected wave at the back surface. The resultant

ratio of the total electric fields at the two surfaces is

E (rear

)

_ 2n

E (front) "

'

a quantity greater than unity.

The magnitude of this field ratio for several dielectrics, transparent at 10.6 ym, is listed in

table 5. As reported in section 4, only front surface damage of Ge was observed for all surfaces,

AR-coated or uncoated, polycrystalline or single-crystal, doped or undoped. This was very surprising

considering the large ratio, 1.60, for the fields. Even for a thinner (3 mm) substrate, no rear damage

could be caused. To clarify this observation, three other substrates, NaCl, CdSe and CdTe, were

irradiated. The expected early rear-surface damage was observed for NaCl and CdSe, but like Ge, only

front surface damage could be produced on CdTe. This was especially intriguing since the band gap of

CdTe and CdSe are nearly the same, 1.50 and 1.74 eV, respectively, at 300°K [9].

Table 5. Rear versus front surface damage at 10.6 microns

Substrate

NaCl

CdSe

CdTe

Ge

Refractive

Index

1.49

2.43

2.69

4.0

Ratio of

Electric Fields

Rear to Front

1.20

1.42

1.46

1.60

Damage

First Observed

Rear

Rear

Front only

Front only

A first order examination of this phenomenon requires taking into account the gentle convergence

of the laser beam at the sample and the linear absorption of the Ge using the relation for an uncoated

substrate

E- - w, -ax/2

2 o 2n e , E^ ~ n+1 W2 (6)

where 1,2 refer to front and rear surfaces, w is the spot size radius, a is the absorption coefficient

and X is thickness. For our experiment wi/wa = 1.008 and sample thicknesses were 5.6 mm. For the

p-doped Ge, of resistivity 3 f2-cm, the weak signal absorption coefficient was approximately

0.6 cm ^ at 10.6 pm; for undoped Ge it was 0.005 cm

Substituting, we find that for p-doped Ge E2 was 1.15 times Ei for weak intensities and larger

for higher saturating intensities. For undoped, AR-coated Ge E2 was 1.61 times Ej. Only in the case

of AR-coated, p-doped Ge was E2 less (between 0.72 and 1.0) than the Ei . However, attempts to damage the rear surface of these samples by increasing the incident laser intensity were unsuccessful.

One promising explanation for the absence of rear surface damage was suggested by Phipps [10].

In a darkened room he observed a faint blue surface corona on the front surface of a Brewster Ge plate

well below the threshold for damage. This corona was distinctly different from a spark associated

with an electron avalanche, and no traces of damage were found. It is possible that this visible light

-302-
was emitted during carrier recombination. Further, absorption of this visible light by the Ge could

increase the free carrier density (for intrinsic Ge n. = 2.4 x lO'^/cm' at 300°K [6]) to obtain a sufficiently absorbing plasma for higher laser intensities to cause surface damage. Further experiments are obviously needed to examine this possibility.

8 . Summary

This study has produced a number of results which are best listed under three headings.

8.1 General

a. The damage threshold was limited by germanium surface properties.

b. Front surface damage, only, occurred.

c. A pulsewidth dependence (J/cm ) ~ x , was observed between 1 and 70 ns.

8.2 Bare Ge

a. The damage threshold at 1.2 ns ranged from 0.65 to 0.70 J/cm^.

b. An HNOa treatment raised the threshold ~ 10%.

8.3 AR-Coated Ge

a. Damage thresholds at 1.2 ns were 0.49 ± 0.03 J/cm^ with a total range from

0.41 to 0.57 J/cm^

b. Multiple-shot conditioning (N-on-1) produced the same threshold as singleshot tests (1-on-l).

c. No variation in threshold for 3 different conventionally polished substrates

was observed.

d. No large difference of the threshold between substrates with varied doping or crystal linity was observed.

9. Acknowledgments

The authors wish to thank the optics personnel of the optical companies listed in table 6 for

producing, polishing and AR-coating the Ge test substrates. Also due credit are Ed Foley and Andrew

Nowak for constructing and maintaining the CO2 laser used in these tests and George Faulkner for

technical assistance with the experiments. We add our appreciation to Claude Phipps for several help:

discussions

.

Table 6. Participating Optical Companies

Germanium Substrates :

Eagle-Picher

Coatings :

Coherent Radiation

Design Optics

Exotic Materials, Inc. Honeywell, Inc. Optical Coating Laboratory, Inc. Perkin-Elmer Corporation

Santa Barbara Research

II-VI, Inc. Valtec Corporation

Polishing :

Design Optics

Optical Systems 5 Technology, Inc.

-303-
10. References

[1] Phipps, C.R., Thomas, S.J. and Figuera, J.F.,

paper WA6, 1976 Conference on Laser and Electro-optical Systems, Digest of Technical

Papers, 30 (1976).

[2] Phipps, C.R., Thomas, S.J., Ladish, J.,

Cuzuchlewski , S.J., and Figuera, J.F., paper

8.8, 1977 Conference on Laser Engineering and

Applications, Digest of Technical Papers, 36

(1977)

.

[3] McLellan, E.J. and Lunsford, J.S., paper 8.10,

ibid, 38 (1977).

[4] Jackson, J.D., Classical Electrodynamics , (John

Wiley S Sons, New York, 1962), p. 295.

[5] Temple, P. A., and Soileau, M.J., NBS Spec. Pub.

462, 371 (1976).

[6] Grove, A.S., Physics and Technology of

Semiconductors , (John Wiley, New York,

1967), pp. 102-103.

[7] Bettis, J.R., House, R.A., and Guenther,

A.H., NBS Spec. Pub. 462, 338 (1976).

[8] Crisp, M.D., NBS Spec. Pub. 387, 80 (1973).

[9] Pankove, J. I., Optical Processes in Semiconductors , (Prentice-Hall, Englewood

Cliffs, NJ, 1971) pp. 412-413.

[10] Phipps, C.R., private communications, 1977.

11. Figures

LASER PRECISION METER

GET

LOST

BOX

SCIENTECH CALORIMETER

CO2 LASER DAMAGE TEST FACILITY

Figure 1. Schematic of the CO2 laser damage test facility.

-304-
0»S ns€c peir

division

ZO nstc fi€r

divisioi^

Figure 2. Oscillograms of CO2 laser pulses of duration (FWHM) (a) 1.2 ns measured with a pyroelectric

detector and (b) 60 ns measured with a photon-drag detector.

-305-
I o

fx "-Hni a Eo 0 q 1 T3 a, <u

4->

o (U •H

Jh

o

O

ni

4-1

E

U 3. 3

LO

00

u

•H I < cn

<U •H c bOC

•H

CM

O

ClJ &,

(n

<U t—t *—' &&,

T3 •H

0) ^1 J

CB T3 O c o CB 1 a: E < a. c r\) rtc O o 4->

<u 00

tn

X

a r-H O c1 E

1—(

0) • ^1

ni nJ 3O X to U u u

t-> ^

tu

UC

o

•H -H

O

cU

H -rt lU

ni 1—

t

am • o a

<u <u C in H 01 rH i-H I—(

<u n) lU u +-> J= 3

60

-H

O

PL,

-306-
•'J

A

-307-
o

O

in

CO

COCM

Oh©

•au

0)o

I <

M

0 o

I—

I

u cd ui

.-I <n 3 -H

m o

C -H

1 *-> o rt •H

CS h ^ o

p. a<u u

3

t3O -J

0) I.S

n)

• (-"

u o

3 X 00 i-H •H O

u. cx,
-309-
I

00 0oGo

<uuo

+Jc fm

•a

m0 d nj •M

tf)

o

X Ph

JiO !-i X D

I—

tw

O nj I—f - <u 13 X <a *-> P.o o3 - o

n) n! X3 ^C ^ 0 (U (—)

(1) t-H V) ca

P. P. W T3 c a> 1 4-> O C

•H

01 ^1o

i-i 3 3. O Ln u •

rt -H a ni Q P,o m

tn

(U <u 3 &,

B£ P,

-310-
I

o

-311-
STANDING -WAVE FIELDS

Figure 6. Standing-wave electric fields for AR-coated and uncoated Ge substrates.

-312-
-313-
AIR SUBSTRATE AIR

E (REAR) 2n

E (FRONT) n + 1

gure 8. Electric-field vectors of a linearly-polarized laser beam at front and rear surfaces ( transparent dielectric.

-314-
COMMENTS ON PAPER BY NEWWAM ANV GILL

V^o^jUiiOK Mamnkov pointtd oat that thi icxiting ojS thz damage thKUhotd M-ith thz iquoAt Hoot OjJ the. puLit duAxition Li not coYii.ifite.nt Mtth atl theonteA o^ avalanche Ionization, oi Ls bon.ne out by kid

iubicquent pn.uentation. Reiponding to qaeMtiom about the moiphology 0(5 the damage, the ipeakeA

indtcated that ^01 iihonJ: puLi>e.i, that ti Ojj the on.deA of, 1 nanoiecond, the damage at th/ieihotd took

the )$0Ajn o{j weZt localized mtcioicopic ptti,, but 04 the pulie duxation wa^ incAea^ed to 70 nanoiecond^

a moA.e dtithtbuted (,o>m o^ damage wai ieen oveA the ^uAjJace. Thii could be ottAtbuted to the ex.cava- tion od the iuA^^ace in the vicinity 0^ damage iltei by the longeA puLse. He indicated that at powe/i

teveJU, approximately 20% hlgheA than that at which the malt piti (^iut occuAAed, damage woi ieen to occuA oveti the total aAea o^ the illuminated Aeglon, Mhich might be ottAlbuted to bulk abioAption in

the mateAlal.

-315-
10.6 m LASER DAMAGE IN COATINGS CONTAINING As^S, AND As^Se

C.C. Tang*, M. Bass, M.J. Soileau and E.W. Van Stryland

Center for Laser Studies, University of Southern California

University Park, Los Angeles, California 90007

The laser damage properties of 10.6 ym coatings containing As^S^ and

As^Se^ are reported. A TEM^^^ mode CO^ TEA laser with an intracavity CW CO^

discharge section was the irradiation source in these experiments. This

enabled the study to include a test of the role of mode locked pulses on the

damage thresholds of the sample coatings.

Coatings containing As^S^ were damaged only after an incandescence or spark was observed during the irradiation. There was no difference in the

2 intensity threshold, ~300 MW/cm (peak intensity on axis) for damage in half

wave or full wave thick As^S^ coatings, with or without mode locked pulses in

the laser waveform. However, evidence for laser irradiation conditioning was

found in certain areas of the ^s^S^ coatings.

Two different thresholds were observed for coatings containing As^Se^; one where a spark was observed and a large damage site produced and a second,

at ~30% lower intensity which produced a very small damage site even though

2 no incandescence was detected. The latter occurred at~12 MW/cm in both the

full and half wave As^Se^ coatings. Otherwise, the qualitative behavior of

the threshold was similar to that of the As^S^ coatings.

The intensity of threshold for damage to As^S^/KCl/As^S^ anti-reflection

coatings on KCl substrates was as high as 430 MW/cm^ and did not depend on

the presence of mode locked pulses. Three layer AR coatings containing

2 As^Se^ damage at ~20 MW/cm . The relationships between the coating damage

thresholds and microstructure , design and measured absorption are discussed.

Key words: As^S^; As^Se^; coating damage; coating design; defects; laser

damage

.

1. Introduction

Thin film coatings are often the most easily damaged components of an infrared laser system. The

2 10.6 |j.m, pulsed damage thresholds of films containing As^S^ and As^Se^ are typically '^325 MW/cm and

'''15 MW/cm^ respectively. These thresholds were found not to depend significantly on the coating design,

the details of the laser pulse waveform or the sequence of the irradiation employed. Layered coating

designs used to reduce coating absorption by redistributing the fields in the layers (1)^ do not

significantly reduce the energy absorbed per unit mass of absorptive material. Thus the heating of the

coating on exposure to optical irradiation and any failure properties governed by uniform absorption

should be independent of the coating design. However, an examination of the morphology of the damage

sites reveals that microscopic defects or inclusions are responsible for the failure of the coatings

studied to withstand intense laser irradiation. Initial "survival curve" data is consistent with this

model since it rules out uniform linear absorption as the dominant pulse damage interaction.

2 . Experimental

The experiment is shown schematically in figure 1 and the properties of the laser system are listed in table I. The low pressure CW discharge in the TEA laser cavity was used to restrict oscillation

to a single longitudinal mode (2) and thus to supress mode locked pulses in the output. Figure 2 shows the two waveforms used in these experiments

.

Coatings were prepared at the Hughes Research Laboratory, (HRL) , in Malibu, California, on single

and poly-crystalline KCl substrates. The substrate surfaces were polished and etched. The three layer

coatings were designed as anti-reflection coatings in the manner discussed later in this paper. Half

^This work was supported by D-ARPA under contract No. F 19628-77-C-0094

.

*Current address: Aerospace Corporation, El Segundo, California

^Figures in brackets indicate the literature references at the end of this paper.

-316-
Table 2. Damage Thresholds.. . (continued)

Material

Absorption

% / Coating

Energy Density*

2 (J/cm )

Power Density*

2 (MW/cm ) As.S - X/2 0.067 60 311

3 on 1) (88) (462)

As S,i " ' 0.12 56 293

riq on 1

)

(56) (293)

''^As !3 /KCI/AS S 0.029 83 433

(!<l on 1) KlZU)

^As !B^/KCl/As^S^ 0.045 O . X AO

TlSI on 1) (8.1) (42)

*The thresholds quoted are the peak on axis values of the indicated quantity, and

2W = o

= 125 ym is the diameter at the

2 focus measured to 1/e of the on axis intensi

The pulse duration was 180 nsec.

The three layer coatings are anti-reflection coatings at 10.6 ym.

4. Coating Design

Figure 3 shows the field distributions in the half wave and the three layer anti-reflection coatings containing As^S^ which were studied. If the As^S^ layers are responsible for absorption in the

coating then the three layer design will have less absorption. This is so because there is less of the

absorptive material exposed to the light field in the three layer design. However, in the three layer

design less material is responsible for proportionally less absorption. Thus, the energy absorbed per

unit mass of coating material is unchanged and each coating will be heated to the same temperature by

the same light flux. By this argument it is possible to interpret the measured damage thresholds as being caused by uniform linear absorption. The presence of defects or inclusions in the coatings will

also give rise to a damage process which is independent of coating design and as shown in the next sec- tions is in fact the principal failure mechanism.

5. Morphology of Coating Damage and Coating Uniformity

In figures 4 through 9 several representative examples of coating damage morphology are shown.

Figures 4 through 9 show that in the irradiated area ('^-125 ym in dia.) there were one or more randomly located sites which were easily damaged. The crazing in figures 8 and 9 is due to the polycrystalline nature of the substrate on which the coating was placed. Figure 9 shows that when a spark was produced on an As^Se^ coatings, a significant portion of the coating was completely removed. In this

context it is important to note the value of using more than one type of microscope illumination to examine laser damage.

In figure S, a case of spark damage to As^Se^ coating, there are interference "ripples" with spacing X/n KCl similar to those described by Temple and Soileau. (4) These are due to light scattered from

a defect in interfering with the incoming laser beam. The coating to substrate interface is an "exit"

surface in the same sense as in reference 4 and the "ripples" are found, as expected, on the KCl substrate surface.

Figure 10 is a plot of the damage threshold as a function of position on an As^Se^ coating. This

data demonstrates the point-to-point non-uniformity of the coating and is additional evidence for a damage mechanism dominated by the presence of defects or inclusions.

5. Survival Curve Data

A survival curve is a statistical means to examine the mechanism causing laser damage. (5) A uniform intensity pulse is used to irradiate several sites on or in a sample and the statistics of the

period of time each site can survive this irradiation is used to determine the damaging interaction.

If the dominant interaction is uniform linear absorption, all sites will be identical and all will sur- vive exactly the same period of time. Figure 11 shows the distortion of a "flat- topped" laser pulse

which occurs when three different sites on a ZnSe containing coatings were damaged. Clearly the three

sites survive different times and so these data rule out a significant role for uniform linear absorption in the pulsed laser damage process for such a coating.

More extensive survival curve studies for bulk material and coatings are planned. The initial data

presented here shows the potential for this technique to sort out damage mechanisms.

7. Summary and Conclusions

The principal results of this work are summarized in table 3. Included is data from 1976 on ZnSe

containing coatings (6) which show that the best currently available coatings are those containing As^S^

or ZnSe. The damaging interaction in all the coatings tested is determined by the presence of microscopic defects or inclusions

-317-
and full wave layers were studied to test the role of coating design in determining the damage threshold.

The occurrence of damage was detected by an observer viewing the irradiated site through a long

working distance microscope. Both white light and HeNe laser light scattering were used with comparable

results. The coatings which contained As^Se^ often damaged with no visible sparks or incandescence at

f luxes 30% less than required to produce a spark, However, As^S^ coatings only damaged after the irradiation produced a spark.

Both 1 on 1 and N on 1 irradiation sequences (3) were used but only the uncoated substrate showed

a significant irradiation conditioning effect. The irradiated sites were inspected with visible microscopy to determine the damage site morphology. Normal incidence reflected, bright field transmitted,

and phase contrast transmitted illumination were used to maximize the effectiveness of the morphological

examinations

.

Table 1. CO^ laser parameters & performance data. TEA Section:

Discharge Length

(Double Rogowski Electrodes)

Energy Storage Capacitance

Flow Rate Ratio

(HeiN^iCO^)

Typical Input Energy into

Pulsed Section

CW Section:

Discharge Length

Gas Mixture

Gas Pressure

Flowing gas, water cooled

Mirrors

Brewster Windows

Intracavity Aperture Diameter

for TEMpQ Mode

Total Cavity Length

Maximum TEM^^^ Energy Available

at the Target

Typical Width of Pulse (FWHM)

41 cm.

0.08 yf

8.7:2.4:2.6 Vmin.

21 J

104 cm. 86%:7%:7%

3 Torr

100% R - flat/Si

80% R - 10 meter/Ge

KCl

8 mm.

307 cm.

30 mj

180 nsec

3. Measured Damage Thresholds

The damage thresholds obtained in these experiments are listed in table 2. The values for coating

absorption were measured by H.R.L. as part of the coating characterization. Clearly coatings containing As^S^ can withstand 20 times the flux that will damage one containing As^Se^. The data also demonstrate that the damage flux for each type of coating does not depend strongly on the coating design.

The last entry for an As^S^/KCl/As^S^ three layer coating represents data for a poor quality coating.

No significant laser conditioning (N on 1) effect was observed except for the uncoated KCl substrate

surface. In addition, the threshold for all the listed samples did not depend on which pulse waveform

was used (fig. 2)

.

Table 2 . Damage thresholds for As^S^ and AS2Se^ containing coatings.

Absorption Energy Density*

2 Power Density*2 Material % / Coating (J/cm ) (MVV/cm ) Surface of KCl

single crystal 171 871

(N on 1) (329) (1720)

As Se - X/2 0.02 2.40 12.7

(N on 1) (2.40) (12.7)

As Se - X 0.018 2.60 13.5

(N on 1) (3.03) (15.8)

As Se /KCl/As Se 0.03 3.85 20.1

(N on 1) (3.85) (20.1)

As Se /KCl/As Se 0.02 3.3 17.2

(N on 1) (3.3) (17.2)

-318-
Table 3. Summary of Coating Damage Thresholds

2 KCl Substrate Surface 1000 MW/cm

Coatings on KCl Coatings:

^ As Se 15 MW/cm

2 As S 325 MW/cm

2 ZnSe 450 MW/cm

Cause of Coating Failure:

Mechanical Defects and/or Inclusions

8. Acknowledgments

We wish to acknowledge the assistance of J. Menders, A. Stewart, K. Moravvej and M. Harvey in the

damage measurements.

(1)

(2)

(3)

M. Braunstein, et. al., "Laser Window Surface

Finishing and Coating Science", Semi-Annual

Technical Report 2, AFCRL - TR - 76 - 0048,

Hughes Research Laboratory (1976)

.

A. Gondhalekar, N.R. Heckenberg and E. Holzhauer, J. Quantum Electr. QE-11, 3 (1975).

References

(4)

M. Bass and K.M. QE-12 , 82 (1976)Leung, J. Quantum Electr.

P. A. Temple and M.j. Soileau, NBS Special Pu- blication 452, p. 371.

(5) D. Milam, R.A. Bradbury, R.H. Picard and M. Bass, NBS Special Publication 414, p. 169.

(5) D. Zuccaro, M. Braunstein, M. Bass, K. Leung

and C. Tang, "Status Report of 3 Layer 10.6

]im A-R Coatings on KCl", Proceedings of In- frared Laser Window Materials Meeting, July

12, 1976, Boulder, Colorado.

10. Figures

REAR

REFLECTOR

LOW PRESSURE COp

LASER SECTION

VACUUM

CHAMBER

PD,

APERTURE

COg TEA I LASER SECTION

^MICROSCOPE

BEAM

SPLITTER

APERTURE

HeNe

OUTPUT

MIRROR

ROTATABLE

ZnSe POLARIZER

FIXED ZnSe POLARIZER

TO SCOPE AND D/A SYSTEM

Figure 1. Schematic of the equipment.

MODE-LOCKED PULSED LASER OUTPUT

TEMPORAL PROFILE OF PULSED LASER OUTPUT

USING CW SECTION

Figure 2. Temporal waveforms used

in the damage measurements.

-319-
X/2 As S- Coating on KCl at 10.6 um

1

1 1

1 2 Distance in Coating { u m)

3 Layer 10.6^imA-R Coating on KCl

KCl KCl substrate

1- 11

1

Figure 3. Electric

coatings

field distributions in

containing As^S^. The

^ 2 electric field squared is normal

Distance in Coating (pm) ized to the electric field in

the substrate.

DAMAGE TO - THICKNESS AsoSr ON KCl SUBSTRATE

TRANSMITTED PHASE CONTRAST ILLUMINATION

Figure 4. Damage near threshold to a A/2 As^S

coating on KCl at 10.6 pm.

-320-
DAMAGE TO V2 THICKNESS AS2SE3 ON KCl SUBSTRATE

50>iM

I 1

50>(M

REFLECTED NORMAL INCIDENCE ILLUMINATION

Figure 5. Damage near threshold to a X/2 As^Se^

coating on KCl to 10.6 \im. (Upper

photo is for damage with no spark and

lower is a case where a spark was

observed.

DAMAGE TO AS2S3/KCL/AS2S3

ON KCl SUBSTRATE

DAMAGE TO A THICKNESS AS2SE3 ON KCl SUBSTRATE

I 1

50^M

I

1

REFLECTED NORHAL INCIDENCE ILLIWINATION

Figure 6. Damage near threshold to a X As^Se^

coating on KCl and 10.6 pm.

I 1

Figure 7. Damage near threshold to an As Se/KCl/As Se 10.6 ym

anti-reflection coating on

KCl.

TRANSMITTED PHASE CONTRAST ILLUMINATION -321-
DAMAGE TO AS2SE3/KCL/AS2SE3

ON POLYCRYSTALINE KCl SUBSTRATE

Figure 8. Damage near threshold to an As-Se./KCl/As.Se^ 10.6 ym

REFLECTED NORMAL INCIDENCE ILLUMINATION 2 3 2 3 anti-reflection coating on polycrystalline KCl when no spark was produced during

the irradiation.

DAMAGE TO AS2SE3/KCL/AS2SE3

ON POLYCRYSTALINE KCl SUBSTRATE

REFLECTED NORMAL INCIDENCE ILLUMINATION

Figure 9. Damage to an As2Se2/KCl/As2Se^ 10 ym

anti-reflection coating on polycrystalline KCl when a spark was produced during the irradiation. TRANSfllTTQ BRIGHT FIELD ILLU1INATI0N

SOjJM

-322-
EFFECTS OF COATINP, NON-UNIFORMITY ON HAMARE THRESHOLD

. In X Thickness AsoSet

r ^ ^ (Not Damaged)

30

25 J

20

15

10

-

-\ l<-0,635 MM

Q

I I I I I I I I L

POSITION OF IRRADIATED SITE

Figure 10. Laser damage threshold versus position on an ASjSe^ coating

This demonstrated the site-to

site non-uniformity of the

coating.

TRANSMITTE!) WflVEFORtl niSTORTinH DUE TO 10.6^m

LASER DAMAfiE ON A ZnSe/KCl/ZnSe THIN FILM COATING

A Flat Topped Pulse Was Used

DAMAGE

DAMAGE

Figure 11. Transmitted waveform distortion

due to 10.6 nm laser induced

damage in a Anse/KCl/AnSe antireflection coating on KCl. The

incident waveform was a flat

topped pulse 30 pnsec long.

This demonstrates the site-tosite non-uniformity of the

coating.

DAMAGE

SURVIVAL CURVE PROOF THAT DAMAGE MECHANISM IS

NOT UNIFORM ABSORPTION

-323-
COMMENTS ON PAPER BV TANG, BASS, SOILEAU, ANV VanSTRVLANV

No cLcicuM-ion on tkti pcipeA.

-324-
LARGE-SPOT DF LASER DAMAGE OF

DIELECTRIC-ENHANCED MIRRORS*

D. B. Nichols and R. B. Hall

Boeing Aerospace Company

Seattle, WA 9812A

and

R. A. House II Air Force Weapons Laboratory

Kirtland AFB, NM 87117

Large-spot DF laser damage thresholds have been measured for several

dielectric-coated mirrors. Five coating designs were tested, using both

molybdenum and Cer-Vit substrates. Chemical laser pulses with FWHM of 4 ysec

and energies up to 70 J were focused to a target-spot diameter of 1 cm. Each

test site was exposed to pulses of successively larger energies. Measured

thresholds in terms of pulse peak irradiance ranged up to 21 MW/cm^, which is

above the plasma formation threshold for bare aluminum. The mirrors exhibited

no surface plasma during small-scale damage, but a well-defined laser-supported absorption wave during catastrophic damage. No threshold differences due

to substrate material were apparent. Factor-of-ten spot-size tests were used

to investigate the range of validity of the spot-size scaling laws which

have successfully correlated damage data over smaller areas. Whole-target

and microscopic damage morphology are examined briefly.

Key words: damage thresholds; dielectric-enhanced mirrors; large-spot laser

damage; pulsed DF laser damage; substrate dependence; surface plasmas; spot

size dependence.

1. Introduction

The emergence of high power chemical lasers has led to an increasing concern with optical com- ponents for the 2 - 5 vim wavelength region. Experimental laser damage data, however, are still

heavily concentrated at other wavelengths. Damage studies with pulsed chemical laser radiation have

typically involved spots only 10 to 100 pm in diameter due to limited total energy per pulse [1]*.

We have measured damage thresholds for dielectric enhanced mirrors with a 1 cm diameter beam spot.

One objective was to investigate the large-spot limit for the size-scaling laws which have been very

useful in describing dielectric damage for small spots [2]. Five coating designs were compared, with

each coating on a metal and a glass substrate. Each of the ten mirrors was exposed to pulsed beams of two different diameters, giving a total of twenty damage sites. Each site received several exposures.

2. Laser Beam Characterisitics

The laser used in this work was the Boeing PHOCL-10 photoinitiated HF/DF device Esl. Figure la

shows the far-field burn pattern at the focus of a 200-cm mirror. The lighter curve is an analytic

approximation. The overall circular shape, with four indentations, is due to the geometry of the laser

itself. Figure lb shows the configuration of a detailed beam scan. The results of this scan are shown

in figure 2. The central part of the beam is quite uniform. Figures 1 and 2 are from reference [4],

which includes further discussion of the spatial profile.

Figure 3 is the DF time profile, which has a generally smooth shape, and has a FWHM of 4.0 ysec.

This profile, sampled from the entire 10 cm diameter near-field beam, was measured for each laser shot

and was essentially unchanged throughout these tests. Beam energy was varied by attenuation external

to the laser, so that the relative time profile was independent of beam energy.

3. Mirror Samples

The ten mirror samples are listed in table 1. Five coating designs (designated by numbers 2 to 6) were each deposited on both molybdenum and Cer-Vit substrates (denoted by "M" and "C"). The samples were fabricated in 1975 by Coherent Radiation (coatings No. 2-4) and Northrop (coatings No. 5 and 6).

* Work supported by the Air Force Weapons Laboratory under Contract F29650-77-90202

.

1. Figures in brackets indicate the literature references at the end of this paper.

-325-
Table 1. Coating designs of ten dielectricenhanced mirror samples. The tabulated reflectances are from absorption

calorimetry measurements. Each

coating design was deposited on both

a molybdenum (M) and a Cer-Vit (C)

substrate

.

SAMPLE

NUMBER REFLECTANCE COATING DESIGN

Cr + (Al^O^/Si)^

Or + Ag + (SiO/Si)^

2M 0.9905 ±0,,0007

2C 0.9792 ±0.,0007

3M 0.9890 ±0.,0007

3C 0.9880 ±0.,0007

4M 0.9916 ±0. 0007

4C 0.9918 ±0.,0007

5M 0.9961 ±0. 0003

5C 0.9909 ±0.,0003

6M 0.9978 ±0,,0003

6C 0.9937 ±0.,0003

Cr + Au + (SiO/Si)^

3 Ag + (ThF^/ZnS>

Ag + (ThF^/ZnSe)'^

The tabulated reflectances are from absorption calorimetry measurements made in early 1977 at the

University of Alabama in Huntsville. At least two separate measurements were made for each sample.

The two uncertainties indicated in table 1 reflect the maximum observed scatter between duplicate

measurements within the two groups.

4. Test Configuration

These tests were carried out in the focal plane of an f = 76 cm mirror. The laser pulse energy

and time profile were monitored by two sample beams from a CaF^ wedge. Return of laser radiation from

the target mirror to the laser and calorimeter was prevented by orienting the target mirror normal at a 12° angle from the incident beam axis; this angle is a function of the f number of the final focusing

mirror.

Each sample was mounted with a sealed partition along a diameter, providing two isolated test

areas. One side was used for 1 cm diameter beam-spot tests and the other side for 0.1 cm diameter

beam-spot tests. To obtain the 1 cm diameter beam, the 45 relative profile of figure 2 was truncated

at about the half-maximum point by introducing a 1-cm diameter aluminum aperture 0.1 cm in front of the test mirror. Eighty-three percent of the total energy passed through this hole. The 0.1 cm diameter

beam was formed by a 0.097 ± 0.002 cm aperture in a 0.025 cm-thick aluminum plate situated 0.05 cm in

front of the test mirror. This 0.1 cm hole was beveled to give an effective aperture thickness of

0.01 cm.

5. Procedure and Results

For each test site, exposures began below 4 J/cm^ , or 1 MW/cm^ peak intensity, and proceeded on

the same site with nominal intensity increases of 50 to 100%. At least ten minutes elapsed between

shots for the Cer-Vit samples. Small-scale and finally catastrophic damage was determined by

examination of the site between each shot with strong illumination. For this inspection, the 0.1 cm aperture was removed and repositioned by a translation stage.

The tests and results are summarized in figure 4. The five groups of data correspond to the five

coating designs. Molybdenum or Cer-Vit substrates are denoted by "M" or "C." A large or small circle

in figure 4 indicates a 1.0 or 0.1 cm beam spot; this distinction is made by the letter "1" or "s" in

site designations. Test sites are designated in the text by coating, substrate, and spot size; e.g. "site 6C1."

For each test site the successive laser shots are indicated in figure 4 by horizontal lines. The

percentage of area damaged after each shot is roughly indicated by the extent of cross hatching from

that exposure down toward the previous exposure. Shading which extends completely from one shot down

to the previous shot indicates that the entire central area of the mirror coating was destroyed.

The mirror surfaces were monitored for plasma formation during exposure by open-shutter photography. The large double XX in figure 4 indicates plasma formation with the nearby aperture in place,

leaving in doubt whether initiation occurred on mirror or aperture or both. In several cases, plasma

was anticipated due to high intensities, and the 1 cm aperture was removed before the final shot, to preclude plasma initiation on the aperture edges. In these cases, mirror surface plasma formation is

indicated by a single X. The only plasma observed with the 0.1 cm aperture was a very weak event

indicated by the small double xx. -326-
6. Damage Site Morphology

Coatings No. 5 and 6 exhibit the highest thresholds in figure A. Figure 5a and 5b show the sample

in situ during and after an exposure with beam-axis fluence of 54 J/cm^. Figure 5c and 5d are during

and after a 100 J/cm^ shot. Figure 5a and 5c are open-shutter with room lights; figure 5b and 5d have

additional illumination. The damaged area is shown in relief by oblique lighting in figure 5e. For the same substrate and spot size figure 6 shows site 6C1 after 50 J/cm^ (fig. 6a) and 63 J/cm^

(fig. 6b). Comparison with site 5C1 shows that the replacement of ZnS with ZnSe reduces the smallscale damage threshold from 50 J/cm^ to 30 J/cm^ , Figure 7 shows an area along the edge of the 1 cm diameter damage zone of site 5M1 after an exposure of 88 J/cm^ . At the bottom of the figure is an undamaged region. Next is a zone where the

coating is fractured but still in place. The relief apparent in the next region indicates major

disruption of the coating, and this zone has an overall shape which suggests blow-off effects radiating

from the spot center above. Finally, the inner region at the top has the coating entirely removed.

Site 5Ms sustained the highest intensity with the least damage. The Nomarski micrograph of

figure 8a shows the undamaged mirror roughness, as well as several major damage points. The peak

intensity here was 23 MW/cm^ . Darkfield (fig. 8b) shows additional small damage points.

Figure 9 is the small-spot damage of the mirror shown in figure 5. The two major damage areas of

figure 9a are connected by a region of fringes which is more apparent in darkfield or the Nomarski view

of figure 9b. We attribute these fringes to interference between coating layers which have been

delaminated by the damage event.

7. Conclusions

Figure 4 allows direct comparison of threshold data as a function of several variables.

7.1 Coating Materials

The highest damage thresholds were observed with coating designs No. 4, Ag + (ThF^/ZnS) , and

No. 5, Ag + (ThF /ZnSe) . These two coatings also exhibited low absorption in calorimetry measurements

(table 1).

7.2 Substrate Effect

Threshold results are similar for molybdenum and Cer-Vit substrates. The thermal conductivity of

the metal substrate gives no significant advantage for single pulses with FWHM of 4 ysec.

7.3 Spot Size Effect

Damage intensities q , , are similar for 1.0 cm and 0.1 cm beam spot diameters. Under the * ^thresh _^ present conditions, the dielectric-damage scaling law [2] , Ij-j^j-ggj^ ~ '^gpot

does not extend to spot

diameters larger than 0.1 cm.

7.4 Threshold Intensities

The absolute values of damage thresholds for the best mirror coatings, 12-22 MW/cm^, are high, and

comparable to the thresholds for formation of surface plasmas on bare metals; the threshold for

unpolished aluminum under these conditions is 15 MW/cm [4].

Significant small-scale damage often occurred without mirror-surface plasma; the occurrence of

plasma, on the other hand, was always associated with catastrophic surface damage.

8. References

[1]. Bass, M. , Soileau, M. J., Tang, C. C,

and Van Stryland, E. W. , "Pulsed HF/DF

Laser Damage in Window Materials," these

Proceedings.

[2]. Bettis, J. R. , House, R. A., and Guenther,

A. H., in Laser Induced Damage in Optical

Materials: 1975 , A. J. Glass and A. H. Guenther, eds. (NBS Spec. Publ. 462,

1976) p. 338.

[3]. Nichols, D. B. , Hall, R. B., and McClure,

J. D., J. Appl. Phys. 47^, 4026 (1976).

[4]. Hall, R. B., Maher, W. E. , Nelson, D. J.,

and Nichols, D. B., Report AFWLTR-77-34 (Air Force Weapons Laboratory,

Kirtland AFB, NM, 1977).

9. Figures

-327-
Figure 2. Results of far-field beam profile scans.

DF Laser Pulse

(70 J)

TIME (MICROSECONDS)

Figure 3. Time profile of the DF laser pulse.

-328-
DF Laser Damage: Dielectric-Coated Mirrors

100

80

60

FLUENCE

(J/cm2)

40

SPOT SIZE SUBSTRATE

COATING

VENDOR

NUMBER

I

o O

M

i

o O

Cr + A9 +

(Si 0/Si)1

CR

3

o O

M

oO

Cf + Au + (SiO/Si)1

CR

4

o O

M

o O

A9*

<Th F4/

ZnS)3

o O

M

o O

A9*

(Th F4/ Zn S«)3

- 25

- 20

- 16 PEAK

INTENSITY

(MW/cm2)

10

Figure 4. Summary of test sites, exposure histories, and damage results. Five

coatings (No. 2 to 6), each on two substrates (molybdenum "M"

or Cer-Vit "C") , were exposed to beam spots of 1 cm (large circle)

or 0.1 cm (small circle) diameter. Laser shots are indicated by

horizontal bars and damage is shown by shading (see text).

Occurrence of surface plasma is indicated by "X."

-329-
-330-

-332-
13

-333-
-334-
-335-
-336-
Figure 7. Edge region of 1 cm diameter damage at Site 5M1, after an 88 J/cm'

exposure. The width of this micrograph is 0.1 cm.

-337-
-338-
-339-
-340-
-341-
DEFECTS AND IMPURITIES IN AS2S3, As^Se^, AND NaF COATINGS*

T. M. Donovan, A. D. Baer, J. H. Dancy, and J. 0. Porteus

Physics Division, Michelson Laboratories

Naval Weapons Center, China Lake, California 93555

The damage thresholds of thin films of chalcogenide glasses are known to be lower than the thresholds of thin films of NaF. On the other hand, the

absorptance of NaF thin films is high. The mechanisms responsible for this

performance are discussed. It is suggested that the low damage threshold of

chalcogenide glasses is due to the presence of micron-sized crystallites.

These crystallites are not present on a fresh film, but grow on the films over

a period of days, and they are an arsenic-rich sulfide, and not an oxide as has

been reported. It is suggested that the high absorptance of the NaF thin films

is due to a thin layer of water adsorbed on the surface, and that this water

layer forms quickly when the coating is exposed to atmosphere.

Key words: Arsenic triselenide; arsenic trisulfide; Auger; damage; defects;

electron microscopy; ESCA; optical coatings; thin films.

1. Introduction

Deposition, absorption, and C02-laser damage threshold for AS2S3, As2Se3, and NaF coatings and the

use of these materials in three-layer antiref lection designs for KCl and NaCl have been reported [1,2,3,

4]-*-. Such coatings have been made with absorptions of 5 x 10""* coated surface. The chalcogenide

glasses have low absorption (6=1 cm~^ at 10.6 ym) and AS2S3 is the only material reported [2] to have

been deposited in thin film form with bulk-like absorption coefficients, ~ 1 cm"-^ at 10.6 ym, which

is also among the lowest absorption reported in coatings. Films of AS2S3 have survived unpulsed CO2-

laser radiation with power levels in excess of 100 kW/cm^ [5]. Being insoluble in water, they form

protective layers for hygroscopic KCl and NaCl [6]. A potentially serious problem with AS2S3 and

As2Se3 coatings, however, is their relatively low threshold for damage by pulsed C02-laser radiation

[A]. NaF films have absorption levels greater than bulk NaF as well as AS2S3 and As2Se3 coatings, and

this excess absorption limits the performance of the three-layer design.

Damage-threshold limitations of a film become apparent when one examines how the damage threshold

of a sample is degraded when it is coated with the film. Figures 1 and 2 show the damage thresholds of

uncoated NaCl and coated NaCl, respectively. The samples, 1.5-inch diameter, single crystal, were

obtained from Harshaw Chemical Company and prepared by etching for 3 min in HCl. The AS2S3 film was

deposited at 25°C in a trapped, diffusion-pumped system. The presence of a large number of defects on

the films (3 x 10'*/mm^) was verified before damage testing. The damage threshold was measured by

Porteus [7] using pulsed 10.6 ym radiation, 100 ns pulse length, and 620 ym spot diameter. Damage was identified according to a number of different criteria and plotted, as shown in figures 1 and 2. The

cross-hatched bar on these figures indicates the average energy for damage, while the overlapping

lighter bar shows the range of levels at which damage was observed. Pit formation was not observed on

the uncoated sample but was observed on the coated sample. As shown, light and ion emission from the

film occurred at one-third (or less) the energy density required for uncoated sample damage. The light

appears to be emitted from tiny points, which could be associated with the micron-sized defects. These

data demonstrate that the damage threshold of an As2S3-coated sample is much lower than that of uncoated

samples, and that the damage mechanism for coated samples is different from that for uncoated samples.

Defects in films of chalcogenide glasses could cause the damage thresholds of the films to be

reduced. Blisters and crystallites a few microns in diameter grow in fresh films of AS2S3 over a period of several days. The crystallites have diameters up to a few microns and rest on the surface of

the film, and could cause a reduction in damage threshold. A few microns is large enough compared to the light wavelength of interest that interference of the incident light with light reradiated from the

crystallites can increase the electric field in regions around the defect [8,9]. Consequently, dielectric breakdown can occur at lower intensities of incident radiation. We regard this model for laser

damage of chalcogenide films as highly plausible but not fully verified. If the model is correct, then

performance of chalcogenide films can be increased by eliminating the defects or greatly reducing their

size. Much of the following discussion is a report of studies of the defects and attempts to eliminate

them. The size and shape of the defects are discussed in section 2. Their chemical nature and the

chemical nature of the films are discussed in section 3, and the results of some attempts to eliminate

the defects are reported in section 4.

* Work supported by Defense Advanced Research Projects Agency and NWC Independent Research Funds.

1. Figures in brackets indicate the literature references at the end of this paper.

-342-
For coatings comprised of chalcogenide glass and NaF layers, the damage threshold is limited by

that of the glass but the absorptance is limited by that of the NaF [1,2,3]. The absorption coefficient

of thin films of NaF is about 5 cm~^, six times that of the chalcogenides and ten times that of bulk

NaF [10]. The absorptance of NaF is so high that the three-layer design with the lowest absorptance is

the one which minimizes the thickness of the NaF [1] . The extra absorptance could be related to the

structure or porosity of the films, or impurities in the NaF. NaF films are polycrystalline and

infrared-absorbing states could exist at interfaces between the NaF crystallites; however, NaF is

hygroscopic and Sparks [11] has estimated that two monolayers of water in or on a material will produce

an absorptance of 1 x 10""* [11]. Other impurities may produce similar increases in absorptance. In section 5, the observation of impurities in NaF, attempts to eliminate them and reduce absorption from

the NaF films are reported.

2. Crystallite Morphology and Formation

It has been known for some time that AS2S3 films deposited under standard vacuum conditions are unstable in the sense that small crystallites on the order of 1 ym in diameter grow on the surface.

The crystallites have been reported to be AS2O3 [12,13] or AssSi^ [14]. The decomposition of AS2S3 is

reported to be catalyzed by the presence of water vapor and enhanced by visible (band gap) light [12].

Figure 3 shows electron micrographs of the surface of an AS2S3 film before and after it was scrubbed

lightly with cotton soaked in benzene. The film was roughly 0.5-um thick and was deposited on a singlecrystal KCl sample at 75°C using a rate of about 1.3 nm/sec in a trapped, diffusion-pumped system. The

sample was etched for 3 min in HCl prior to deposition. Since the replication required for the micrograph is destructive, the sample was broken into several pieces. One piece was replicated 3 days after

deposition without washing, while a second piece was washed (lightly scrubbed with cotton soaked in

benzene) and replicated 7 days after the first samples was replicated (10 days after deposition). The

micrographs show two types of defects: crystallites and blisters. On the "before" micrograph, the

crystallites can be seen to agglomerate on the blisters. The "after" micrograph shows that more blisters appeared during the intervening 7 days. This illustrates how the defects grow over a period of

days. Notice that after the films are washed, no evidence of the crystallites remains. There are no jagged edges of the crystallites protruding from the surface. Reflection electron diffraction patterns

showed crystalline structure before, but not after, washing. Evidently the crystallites are resting on

an amorphous matrix, and can be washed off.

Figure A gives a closer view of the shapes of defects in the AS2S3 and As2Se3 films. The AS2S3

film shown here is the same as that shown in figure 3, but the film has been aged 86 days prior to replication. The As2Se3 film was 0.6-pm thick and was deposited on single-crystal, etched KCl at 75°C

in a trapped, diffusion-pumped system; the replica was made 7 days after deposition. Notice that

blisters are visible under the AS2S3 crystallites shown in both figures 3 and 4, but no blisters are visible under the As2Se3 crystallites. The AS2S3 crystals have a distinct, pyramidal habit while the

As2Se3 crystals have a more agglomerated, triangular prism habit. The crystallites are clearly different on the two types of film, but the implications of this difference have not yet been explored.

3. Analytical Results

There is disagreement as to the chemical form of the crystallites appearing on AS2S3 films; they

have been reported to be both AS2O3 and AsitS3 [12,13,14]. The chemical nature of the crystallites is

closely related to the cause of their formation, and hence to the means for preventing their formation.

If the crystallites are an oxide being formed by oxidation in contact with atmosphere, then an oxygenimpervious protective coating combined with water-free deposition should stop their formation. If the

crystallites are a sulfide or selenide which form because the amorphous matrix is unstable, then some means of stabilizing the matrix must be found. Stabilization could involve altering the stoichiometry

of the matrix or introducing a stabilizing impurity. Some of the passivation attempts reported in

section 4 were based on the assumption that the crystallites are formed by oxidation. This assumption

is not borne out by the more recent data reported in this section.

A general idea of the chemical form of the crystallites can be obtained by considering the composition and form of likely materials. Crystallites form on AS2S3 films whether the films are deposited on NaCl, KCl, quartz, Nesa glass, AI2O3 or Al [2,4,13,14]. Evidently, constituents of the crystallites

are not coming from the substrates, but must be coming from the atmosphere or the film. The list of

compounds likely to form on an AS2S3 film includes AS2O3 (cubic and/or monoclinic) , AS2S5, AS2S3

(monoclinic) , Ast^Ss (orthorhombic) and AS2S2 (monoclinic) [15,16]. The shape of the crystallites may

give some clue as to the material involved. The crystallites shown in figures 3 and 4 are shaped

roughly like octahedrons; but this by itself is not enough to discriminate between materials since the

habit of As[,S3 has been described as "dipyramidal , " while that of AS2O3 was described as "octahedral."

Either description could apply to the crystallites; one cannot discriminate between the oxide and the

sulfide with this information.

In order to identify the chemical nature of the crystallites, samples were examined at Physical

Electronics, Inc. (PHI) using the Model 590 high resolution scanning Auger spectrometer. This instrument has a lateral resolution of 0.25 um. AS2S3 films were deposited under standard deposition conditions (75°C) on single-crystal etched KCl and sent to PHI. The crystallites were present on the films

-343-
and were easily located and identified in the scanning electron microscopy mode of the Model 590.

Auger spectra of a large area in the sample were taken of the as-grown AS2S3 coating. Auger spectra

then were recorded with the electron beam focused on a crystallite and with the beam focused on a region free of crystallites (matrix) . The sample was then sputter-etched in purified argon to remove

about 10 nm of surface film and the measurements repeated. Figure 5 compares the Auger spectra measured

before and after 10 nm was removed by sputtering. The composition of the average area and the crystallite are compared in figure 6 and both show C and 0 contamination in the as-received conditions. After

10 nm of the crystallite surface was sputtered away, the C and 0 contamination is greatly reduced, and

Cl~ and K"*" appear. In contrast to this, in the crystallite-free region, shown in figure 5, sputteretching does not result in the appearance of Cl~ and K"*", although C and 0 contamination is removed.

The data indicate that these crystallites do not consist of AS2O3 as suggested in some studies. This

result is consistent with data reported by Hughes Research Laboratories on films deposited by vapor

transport, and with results from enhanced diffusion of the id" and Cl~ possibly results from enhanced

diffusion of the K"*" and Cl~ from the substrate due to local heating of the electron beam in the defect

or blistered regions. Again, this result is consistent with our electron microprobe results which also

indicated enhanced diffusion o f K+ and CI- in the crystallite region. It Is of interest to note that

x-ray photoemission spectroscopy (fig. 7), which does not introduce local heating or electron damage,

confirms the result that 0 and C are present as surface contaminants. Also, there is no indication of

K"*" or CI" in what amounts to a broad area analysis. Notice also that this sputter-etch, lighter than

that used for the Auger spectra, removed most of the C and 0. Evidently, the C and 0 are in a layer

roughly 2-nm thick on the surface. Previous workers also have found the 0 layer to be 2-nm thick [13]

.

Some information about the relative stoichiometry in the crystallite and crystallite-free regions

can be obtained from peak height measurements of both Auger and electron microprobe spectra. The

height of a particular peak is roughly proportional to the density of atoms of that material in the

sample volume. By taking the ratio of peak heights on a given curve, say the S peak to the As peak,

one can gain an idea of changes in the relative number of As and S atoms present. Table 1 compares

peak-height ratios for an Auger spectrum measured from a crystallite with the ratios for a spectrum

measured from the matrix. Ratios derived from both Auger and microprobe measurements are shown. For each type of measurement, the ratios are normalized so that the S/As ratio comes out 1.50 for the film

matrix. This was done so that changes in the S/As ratio could be more easily compared with that of

perfect AS2S3, 1.50. For both Auger and electron microprobe data, the ratios on the crystallites are

less than 1.50. Since the stoichiometry of the matrix is not definitely known at present, and the

extent to which the electron-beam sampling is limited to the crystallites is not known, one cannot

accurately estimate the stoichiometry of the crystallites. While the estimate is probably not numerically accurate, the data clearly show that the crystallites are more arsenic-rich than the matrix.

Table 1. Comparison of peak height ratios for structure associated

with the film with that for structure associated with the

crystallites. The sulfur 150 eV peak and the arsenic

1240 eV peak of the Auger curves were used.

S/As peak ratio^

Ratio Film Crystal

Auger (10 nm sputtered) 1.50 1.27

As-received Auger 1.50 1.20

Electron microprobe 1.50 1.36

Arbitrarily normalized to 1.50 on the film.

These analytical results presented above resolve the disagreement concerning the composition of

crystallites on AS2S3. The crystallites are a sulfide, not an oxide. Moreover, the crystallites are more arsenic-rich than the matrix. This is consistent with the suggestion by other workers that the

crystallites are Asi^S3. It now appears that crystallite formation is due to a basic instability of

AS2S3, although the details of formation are not clear at present.

4. Passivation Attempts

In attempting to passivate the As2Se3 and AS2S3 coatings against defect growth we have made the

assumption, based on the literature, that crystallite formation involved the presence of water vapor

and visible light; if oxygen or water could be excluded from the surface, crystallites would be prevented from forming. Two methods of excluding oxygen or water were tried. In the first, we obtained

water-free starting material from the Catholic University and deposited coatings in an oil-pumped

vacuum using this material. Figure 8 shows transmission traces of a commercial AS2S3 sample and a -344-
research grade sample fashioned from a boule of high-purity As2Se3 obtained from C. Moynihan of the

Catholic University. The commercial glass clearly shows the presence of adsorbed and chemically com- bined 0-H in the region of 2.8 \im. There is also a small absorption band indicating the presence of HS, which was probably introduced in the formation stage of the AS2S3. The research grade glass, on the

other hand, has an almost clean spectrum. There is a slight Indication of 0-H at 2.7 ym, and this may

be the surface contamination observed in the Auger /ESCA analysis. Figure 4 shows the surface of the

As2Se3 coating made using this material. Crystallites are present, but are smaller and not as well

formed as those of the AS2S3. Further efforts to reduce the presence of these defects will involve

deposition in ultraclean vacuum.

The second attempt at passivation was to overcoat the AS2S3 prior to air exposure with a thin

layer of either Ag or NaF in order to prevent oxygen or water vapor from contacting the chalcogenlde

surface. Each of these overlayers failed to prevent defect formation. A Z-ym-thick film of AS2S3

was deposited on etched, single-crystal NaCl at 75°C in a trapped diffusion-pump system. Half the

surface was subsequently coated with 100 nm of NaF at 140°C. As seen in figure 9, crystallites are growing on both halves of the surface. Thus the presence of NaF did not significantly retard growth of

the crystallites.

5. Impurities in NaF

In order to isolate the cause and reduce anomalously high absorption in NaF we have studied the

effect of vacuum on the chemical contamination and absorption. Films have been deposited in standard

oil-pumped vacuum, as well as baked in ultrahigh vacuum (UHV) . The chemical purity has been monitored

using Auger/ESCA and we have looked for the presence of surface or interface absorption using ellipsometry and total internal reflection.

We find that by using optical grade (no impurity analysis) NaF obtained from Harshaw Chemical Co.,

and evaporating as described previously [4], in either standard or UHV the resulting absorption was the

same, ~ 6 cm~^ at 10.6 pm. Measurements made at 3.8 pm yield a slightly lower absorption, 4 to 5 cm~^.

Auger and ESCA measurements have been made on these coatings in order to identify possible sources for

this residual absorption. We found that the electron beam used in the Auger measurement alters the

surface composition of the NaF, and therefore we limit our discussion to the ESCA results. Figures 10

and 11 show ESCA traces of a UHV-deposited film taken after exposure to air and the same film after

approximately 30 A of surface layer had been removed, respectively. It is clear from these figures

that oxygen and carbon are present as surface contamination layers only, and can be removed by a light

sputter-etch treatment. Since the calorimetry measurements were made on air-exposed samples, it is

possible that 0, present in the form of OH or CO2, may be contributing to the absorption at 10.6 and

3.8 pm. Even though transmission spectra do not reveal any OH or CO2 absorption bands at 10.6 and

3.8 ym, both Auger and ellipsometr ic measurements indicate surface or interface contamination. These

contaminants could be responsible for the residual absorption observed at 3.8 and 10.6 ym.

6. Summary and Conclusions

Crystallites which form in air on AS2S3 and As2Se3 coatings are not AS2O3, as previously suspected,

but some form of sulfide or selenide. Attempts to stabilize the coatings by depositing thin layers of

Ag or NaF in situ failed, as did attempts using water-free As2Se3 evaporant

.

NaF as-deposited in UHV using optical grade evaporant has an absorption ten times that of bulk NaF

at 10.6 ym. This absorption may result from oxygen impurities in the form of CO2 or OH which have been

found limited to the surface region. This suggests that NaF forms a dense, nonporous coating which

does not take up water vapor on exposure to air. Further, it suggests that by eliminating surface oxygen, film absorption approaching that of bulk NaF might be obtained.

7. References I

[1] Baer, A. D. , "Design of Three-Layer Antiref lection Coatings," in Laser Induced damage

in Optical Materials: 1976, A. J. Glass and

A. H. Guenther, eds. (NBS Spec. Publ. 462),

p. 221.

[3] Baer, A. D., Donovan, T. M. and Soileau, M. J.,

"AS2S3 Coatings on KCl," in Laser Induced

Damage in Optical Materials: 1975, A. J. Glass

and A. H. Guenther, eds. (NBS Spec. Publ.

435), p. 244.

[2] Donovan, T. M. and Baer, A. D., "AS2S3 and

ThFL, Coatings on KCl and NaCl Windows,"

Proceedings of the Fifth Annual Conference

on Infrared Laser Window Materials: 1975,

C. R. Andrews and C. L. Strecker, eds.

(Defense Advanced Research Projects Agency,

Arlington, VA, 1976), p. 241.

[4] Baer, A. D., Donovan, T. M. , Green, A. K. and

Turner, G. , "Alternate Materials for Infrared

Window Coatings," in Laser Induced Damage in

Optical Materials: 1976, A. J. Glass and

A. H. Guenther, eds. (NBS Spec. Publ. 462),

p. 214.

-345-
[5] Air Force Weapons Laboratory. Private

communication

.

[6] Young, P. A., Thin Solid Films 6^, A23 (1970).

[7] Porteus, J. 0., Michelson Laboratory. Private

communication.

[8] Temple, P. A. and Soileau, M. J., "Resonant

Defect Enhancement of the Laser Electric

Field," in Laser Induced Damage in Optiaal

Materials: 1976, A. J. Glass and A. H. Guenther, eds. (NBS Spec. Publ. 462), p. 371.

[9] Bloembergen, N. , Appl. Opt. 12^, 661 (1973).

[10] Klier, M. , Z. Physik 150, 49 (1958).

[11] Sparks, M. , "Materials for High-Power Window

and Mirror Coatings and Multilayer Dielectric

Coatings," in Laser Induced Damage in Optical

Materials: 1976, A. J. Glass and A. H. Guenther, eds. (NBS Spec. Publ. 462), p. 203.

[12] Berkes, J. S., Ing, S. W. , Jr. and Hillegas,

W. J., J. Appl. Phys. 42, 4908 (1977).

[13] Trubisky, M. P. and Nevihart, J. H. , Appl.

Opt. Suppl. 3, 59 (1959).

[14] Braunstein, M. , Laser Window Surface Finishing and Coating Technology (Semi-Annual

Technical Report, Hughes Research Laboratory

Malibu, CA, 1974), AFCRL-TR-74-0425

.

[15] Palache, C, Berman, H. and Frondel, C,

Dana's System of Mineralogy , 7th Edition

(John Wiley and Sons, New York, 1944).

[16] Handbook of Chemistry and Physios, 50th Edition, R. C. Weast, ed. (Chemical Rubber Co. Cleveland, OH, 1969).

8. Figures

SINGLE-CRYSTAL N«CI, HARSHAW. FRONT SURFACE

RIPPLES

CRAZING

MELTING

ION EMISS

LIGHT EMISS

SCRUBBING

mm

10 20 30 40 PEAK THRESHOLD, J/cm^

Figure 1. Laser damage at 10.6 ym of uncoated single-crystal NaCl.

-346-
ajjSj coating on n«ci

RIPPLES

PITS

DELAMINATION/

ION EMISS.

LIGHT EMISS.

SCRUBBING

Figure 2. Laser damage at 10.6 ym of single-crystal NaCl coated with AS2S3.

Figure 3. Defects on an A82S3 film before and after a light scrubbing with

benzene

.

J]

PEAK THRESHOLD IJ/CM^l

-347-
^

A«2«3 I

' *^ 1 ,M

Figure 4. Defects formed on AS2S3 and As2Se3 films.

MATRIX

AS RECEIVED

f

MATRIX

100 NM REMOVED

600 800 0 200 ELECTRON ENERGY, eV

Figure 5. Auger spectra measured on an AS2S3 film before and after 10 nm

of the surface was sputtered away.

-348-
AVERAGE AREA

rrr

CRYSTALLITE

AS RECEIVED n

-r

CRYSTALLITE

10 NM REMOVED

200 400 600 0 ELECTRON ENERGY, eV

Figure 6. Comparison of Auger spectra representative of the average area of an AS2S3 film, a crystallite, and a crystallite with 10 nm removed.

—I I I

1 1 1

X-RAY PHOTOELECTRON SPECTRA: A^Sj FILM

Figure 7. X-ray photoelectron spectra measured before and after 1.5 nm of

AS2S3 film was sputtered off.

-349-
WAVELENGTH (MICRONS)

2^5 3 4 5

_ 80

20

-

1 1

'

HF DF

ASjS, GLASS (COMMERCIAL) THICKNESS 1/8"

I, 0-H

-

/

-H H-S

0-H \

ASjSej GLASS CATHOLIC U. THICKNESS 1/8"

-

4000 3500 3000 2500 2000 WAVENUMBER (CM"^)

Figure 8. Transmission of conmercial AS2S3 and high purity As2Se3.

WITH NiF I I WITHOUT MiF

1 m

Figure 9. Micrographs of crystals on a 2-ym AS2S3 film deposited on a NaCl

window. Half of the window was overcoated with NaF, 100-nm thick.

-350-
T

1 1 1 1 1 1 1 r

NaF (AS RECEIVED)

BINDING ENERGY. eV

Figure 10. X-ray photoelectron spectrum recorded on a NaF film after exposure

to atmosphere.

1 1 1

f

1— 1 1 1 T 1

Na F (3 NM REMOVED)

Na

1 1 1

Ni

. .? III!C

1,000 500 BINDING ENERGY. aV

Figure 11. X-ray photoelectron spectrum recorded on a NaF film after 3 nm were

removed from the surface by sputtering.

-351-
OPTICAL CONSTANTS OF As^S^ BY A WEDGED-FILM TECHNIQUE*

David L. Burdick

Physics Division, Michelson Laboratories

Naval Weapons Center, China Lake, California 93555

A wedged-film technique has been developed to measure the optical constants

of thin films. The technique has been applied to an AS2S3 film. Measurements

in the visible spectrum have produced absorption coefficients ranging from 3 cm~^

to greater than 10** cm~^, and refractive indices from 2.47 to 2.72 with precisions ranging from 0.3 to 1%. Key words: Arsenic trisulfide; optical constants; visible spectrum; wedged film.

Introduction

Techniques for measuring the optical constants of thin films abound. This paper describes yet

another method which admits of certain advantages, and the method is applied to AS2S3 films.

The use of a wedged film on a transparent substrate is a direct descendant of the prism technique

for bulk materials and, as such, shares one of the main advantages of that technique. That is, by

properly ratioing the transmitted intensities one can, in principle, eliminate surface effects. It has

the further advantages that film thickness need not be measured independently and that the optical

constants can be measured over a wide spectral range on a single film.

Experimental Arrangement

The experimental apparatus is shown schematically in figure 1. The wedged film on a transparent

substrate is placed normal to a monochromatic beam which is focussed onto the film. The transmitted

light is monitored by a detector as the substrate is translated through the beam so that the wedged

film is scanned as a function of thickness. A position readout device (a linear potentiometer) measures

the film position. The detector output and the position signal are ultimately fed to the x-y recorder.

Theory of Operation

The transmission coefficient for light passing through a thin film on a transparent substrate is

16n (n2 + k2)T

s s

r2[(e'^ - p/r e"^)2 + 4 p/r sin2(2Tint/X + l/2(((> - e))]

T

= : § ^

, (1)

where

X = 2iTkt/X

p2 = [(1 - n)2 + k2][(n - n)2 + k2]

s r2 = [(1 + n)2 + k2][(ng + n) 2 + k2]

tan<\> = -k(2n + 1 + ng)/[(l + n) (n^ + n) - k2]

tane = -k(2n - 1 - ng)/[(l - n) (n^ - n) - k2].

The extinction coefficient is k and n is the refractive index of the film, n^ is the substrate index,

X the wavelength, and T^ a transmission factor which describes how the beam leaves the substrate.

The simplest case to treat is that encountered when kt << 1. Assuming that n > n^ we find in this

case that

= r2 . (2)

In this approximation both p and r are simple functions of n and eq. (2) can be solved for n to give

2n = (1 + n )R + /r2(1 + n )2 - 4n , (3)

s s s

* Work supported by Defense Advanced Research Projects Agency.

-352-
This result is important for two reasons. First, it allows us to calculate n from the directly acces- sible ratio R, and surface effects are eliminated in the process. Second, n can be obtained without

first measuring the film thickness.

Having determined n at a wavelength where the film is not absorbing, one can then calculate the

film thickness profile as a function of position by using the normal interference conditions at the

extrema. For instance, at the maxima.

MX

2n (4)

where M is the order of the fringe.

Once n and the thickness profile have been determined, the extinction coefficient (and absorption

coefficient) can be determined by using differences in T^^^^^ at different film thicknesses Thus, in the

lowest approximation, and using Tjj we have

Ti

Solving for the absorption coefficient a we have

4Trk

\

(

l + p/r \

At (5)

(6)

and this coefficient is determined entirely in terms of ratios, which again eliminates surface effects.

The analysis of the data takes on a much different character when kt is not small. In essence,

one must use the thickness profile obtained as above and perform a least-squares fit to extract both n and k.

Experimental Results

The wedged-film technique has been used to obtain the optical constants of an AS2S3 film on fused

quartz at six laser wavelengths. A photograph of the film is shown in figure 2 as obtained on a Zygo

interferometer. The film is illuminated by 632.8 nm light to reveal the interference fringes in the

film. The fringes are highly linear and uniform, a circumstance which is pleasing but not essential to the method. The film was fabricated by placing a shutter between the substrate and evaporant source. After a uniform deposition rate is established (with the shutter closed) the shutter is opened and

closed at a uniform rate until the desired film thickness has been obtained.

The thickness profile of the film is shown in figure 3. The thickness was calculated from interference conditions such as given in eq. (4) at two wavelengths; the result confirms the linearity of

the wedge. The greatest thickness of this film was about 1.5 ym.

Figure 4 shows the intensity variation observed in the transmitted light at 0.6471 and 0.5682 um

as the film is translated through the focussed beam. The film absorption at these two wavelengths is

very small as evinced by the fact that the extrema do not decrease in amplitude as the film becomes

thicker. The situation is markedly different, however, in figure 5. Here we show the transmission

patterns obtained at 0.5208, 0.488, 0.4825, and 0.4762 um. The evidence for increasing absorption with

increasing energy is clearly visible, ranging from a linear decrease in intensity of the extrema at 0.5208 ym to a strong exponential character in the decrease at 0.4762 ym.

The results obtained at 0.6471, 0.5682, and 0.5208 ym are easily treated by eqs. (3) and (6). At 0.488, 0.4825, and 0.4762 ym one must resort to curve-fitting. The results obtained for the optical

constants are shown in figures 6 and 7. Figure 6 shows the absorption coefficient as a function of

photon energy, together with the results obtained by Young [I]-'- for comparison. The agreement is moderately good. The refractive index is shown in figure 7, again with Young's results. Our numbers are about 3% lower. It is not uncommon to find such differences in the refractive indices of AS2S3 films

grown under different conditions. Our precision is estimated to vary from 0.3% at the longer wavelengths up to 1% at the shorter. Variations in the precision are due primarily to differences in how

the locations of extrema were measured.

Conclusions

The wedged-film technique appears to offer a number of advantages when compared with other methods

of measuring optical constants of thin films. First, no independent measurement of film thickness is

1. Figures in brackets indicate the literature references at the end of this paper.

-353-
required. Second, one need not worry about dispersion in the index over some spectral range; some methods require the assumption that the index is constant over an extended spectral range. Third,

measurements can be made over a large spectral range on a single film. Fourth, absorption coefficients

down to 1 to 10 cm"-^ are measurable and high precision index measurements are possible. Finally, the

method eliminates surface and interface effects.

References

[1] Young, P. A., J. Phys. C: Solid State Phys. 4_, 93 (1971).

Figures

detector

- •) 1 lock-in"1

POSITION READOUT

XY RECORDER

X

Figure 1. Schematic diagram of wedged-film apparatus. Interference fringes

are observed as a wedged film is translated through a focussed

monochromatic beam.

Aa,*! *KOGEO FILM

Figure 2. Photograph of AS2S3 film in Zygo interferometer showing fringes

at 632.8 nm.

-354-
Figure 3. Thickness profile of AS2S3 film as determined from measured indices

and the interference condition.

G6S2

FILM THICKNESS (pml

1.45

Figure 4. Intensity variations in transmission at 0.6471 and 0.5682 ym. No

absorption increase with thickness is observed at these wavelengths.

-355-
FILM THICKNESS (fiml 1.45

Figure 5. Intensity variations in transmission at 0.5208, 0.488, 0.4825 and

0.4762 ym. Increases in absorption with thickness are clearly

visible

.

10.000 :

1000 PRESENT WORK

2.0 2.2 2.4 PHOTON ENERGY (eV)

Figure 6. The absorption coefficient of AS2S3 by the wedged-film technique

(six points). Also shown are Young's results [1].

-356-
Figure 7. The refractive index of AS2S3 by the wedged-film technique, and

Young's results [1].

-357-
DIELECTRIC COATINGS FOR DEFORMABLE MIRRORS*

J. M. Rowe

Northrop Research and Technology Center

Hawthorne, California 90250

Results of an experimental program to determine the optical and physical

properties of high-reflectance dielectric coatings subjected to a large number

of deformation cycles are reported. The effects of deforming mirrors with

film stacks are difficult to predict since too many parameters are involved.

Therefore, an experimental evaluation of promising designs was undertaken.

Measurements included residual coating stress immediately after deposition,

spectral reflectance, absorptance at the design wavelength, and examination

by Nomarski microscopy. Mirror properties were evaluated at several points

in a schedule of controlled deformation cycling. Coating designs consisted of

dielectric enhanced silver at 3. 8 \im deposited on molybdenum substrates.

The data obtained provides a basis for choosing multilayer coating designs

and deposition methods that may be applied to the fabrication of high-reflectance, deformable mirrors for mid-IR lasers.

Key words: Coating stress; deformable mirrors; dielectric enhanced reflectors; laser mirror coatings; stress cycling of coatings; thin films.

1. Introduction

The performance of deformable mirrors used with high power IR lasers could be improved by

coating the mirrors with dielectric layers to increase reflectivity. The ability of these coatings to maintain optical and physical integrity when the mirror is deformed was investigated in a quantitative

manner and results obtained to date are reported. The coatings consist of silver with reflectivity at 3.8 |Jm enhanced by dielectric quarter-wave stacks. Four candidate designs were chosen for evalu2 2 3 3 ation: (SiO/Si) ; (Al^O^/Si) ; (ThF^ /ZnS) ; and (ThF^ /ZnSe) . Although the mirror s tested were

not part of an adaptive optical system, the results will find application to that field.

2. Experimental

In order to provide quantitative results the coated mirror substrate must undergo only controlled,

well characterized deformations. This was accomplished with the mirror and driver assembly shown

in figure 1. The mirror and driver housing are molybdenum. This material was chosen because of

its high elastic modulus and fatigue stress, low thermal coefficient of expansion, and because it is

capable of a high quality optical polish [l]^. The polished diameter of the mirror is 3. 81 cm and the

web thickness is 0.762 mm. The mirror may be deformed in two ways. A stack of piezoelectric

wafers is used to produce AC deflections electrically and a differential screw provides purely mechanical deflections of ~ ll|am per turn.

The differential screw is used to apply mechanical bias to the mirror during what has been called

"DC" cycling. By this is meant deflections of one sign only, e. g. flat -to -convex-to -flat equals one

"+" cycle. The concept is illustrated in figure 2. After application of mechanical bias, the differential screw lock nut is tightened and AC drive applied to the PZT stack with peak-to-peak deflections

of twice the bias level. Zero-to-peak "DC" displacements of 10. 9 |Jm at 10 Hz were used in testing

7 the mirrors. Under these conditions a peak stress of 9. 89 x 10 Pa occurs at the mirror surface in

the anular region directly above the edge of the central boss. All optical tests were made in this

region of maximum stress. Stress induced in the coating is proportional to the ratio of coating modulus to substrate modulus since the strains are equcd. For the coating materials used this ratio varies

from 0. 15 to 1.2. It is noted that the mechanically induced coating stresses were comparable to the

residual stresses commonly produced as a result of the deposition process itself.

*This work was supported by the Air Force Weapons Laboratory under Contract No. F29601-77-C

0015.

1. Figures in brackets indicate the literature references at the end of this paper.

-358-
A fringe counting interferometer (fig. 3) was used to measure mirror deflection. Interference

between a He/Ne laser beam incident on the center of the mirror under test and a reference beam

from a stationary mirror produced two maxima when the deformable mirror was deflected by one wavelength of the He/Ne light. A photodiode detected this signal which was then amplified and displayed on an oscilloscope. Drive voltage to the PZT stack was simultaneously displayed.

The following procedure was used in evaluating the mirror coatings. Coating and optical tests were done with the mirror detached from the driver. Before coating deposition, each mirror was

calibrated in the driver to determine the PZT drive voltage required to produce the desired 10.9 Mm

zero-to-peak displacement. The coatings were then deposited. During each coating run two stress

witness samples, thin disks of fused silica, were also coated and the residual coating stress determined by well known techniques [2]. A scan of mirror reflectance vs^ wavelength was then made.

Finally each mirror underwent a sequence of testing and controlled deformation cycling. Mirrors were examined by Nomarski microscope, double-bounce reflectometer , and DF laser calorimeter

N

before any cycling and upon each completion of 10 cycles, where N = 1, 2, 3, 4. When not undergoing controlled cycling the mirrors were kept flat to X/5 at 3.8 n m. In particular, the mirror flat- ness was monitored with an optical test flat during mounting and demounting to the driver.

3. Results

Two coating designs have completed the "DC" tests. For each design two mirrors were coated

simultaneously, one for + and the other for - "DC" testing. Changes in optical performance with

stress cycling were small and examination by Nomarski microscope at 167X revealed no physical

2 degradation such as cracking or debonding of the films. The results for the (SiO/Si) design are

7 shown in table 1. The residual tensile stress in this coating was 8. 65 x 10 Pa. The difference in

thermal expansion between the fused silica stress witness and molybdenum mirror substrate has been

taken into account and the number given is for coating stress on molybdenum. It is evident from the

table that the double-bounce reflectometer precision (±0. 1%) was not sufficient to measure the small

changes in reflectance due to deformation cycling. The DF laser calorimetry measurements made

after + "DC" cycling show a gradual increase in absorptance with number of cycles amounting after

4 -3

10 cycles to 28% of the original value of 2, 14 x 10 . Some of the added absorption may be due to handling of the sample although great care was taken to avoid damaging the mirror surface. It is

noted that + "DC" deformation increases the tensile stress already present in the coating while - "DC"

deformation reduces the coating stress. Absorptamce of the - "DC" sample shows no systematic variation with cycling and equals 2.42 x 10 ^ with a standard deviation of 3%.

2 Results for the (Al O /Si) coating are shown in table 2. The residual stress in this coating was

8 3 1.29 X 10 Pa. The measured absorptance of both the + and - cycled samples decreased through 10

cycles, an unexpected result. This apparent decrease may be within the range of experimental error,

however, since the standard deviation for all absorptance measurements on either one of these samples was only ~ 8 % . No physical mechanism is proposed that would lower absorptance by stress

cycling.

4. Work in progress

3 3 Two more coating designs, (ThF^/ZnS) and (ThF^/ZnSe) , are currently being evaluated. Once

"DC" testing of all four designs is complete, two of them will be selected for "AC" testing with ±5. 5 7 [Jm deflections at 1 kHz for up to 10 cycles. Adherence and humidity testing will also be done on

these two designs, and the results of all measurements will be compared with a calculation of coating

stresses.

5. Conclusion

The results obtained to date indicate that dielectric enhanced metal coatings suitable for use on

high reflectance IR laser mirrors are able to withstand a large number of substrate deformation

cycles without degradation. In particular, changes in absorptance were small and no physical disruption of the coating occurred. The effects of deformation cycling on the ability of the coatings to pass adhesion and humidity tests is still under investigation.

-359-
6. Acknowledgments

The mirror driver for this experiment was designed and fabricated by W. J. Schafer and Associates. Polishing and coating of the mirrors was done at Northrop by A. Klugman and S. Holmes. Able

technical assistance was provided by M. Evangelista and F. Towns.

[l] A. Klugman, "Surface Finishing

of Molybdenum Substrates", 1977

Proc. High Power Laser Optical

Components and Component Materials, to be published.

7. References

[2] K. Roll, J. Appl. Phys. 47 , 3224 (1976).

Tables

Table

9

1. Results for (SiO/Si)'^ coating design at 3. 8 /im.

+ "DC" TtST "DC" TEST

Reflectance Absorptance Reflectance Absorptance

Cycles (%) (10"^) Cycles (%) (10"^)

0 99.7 2. 14 0 99.7 2.43

10 99.8 2.24 10 99.8 2.46

99.8 2.58 10^ 99.8 2.52

lo' 99.8 2.72 99.7 2.33

10^ 99 J 2.75 99.7 2.35

Table 2. Results for (A ^2^3/51)^ coating design at 3. 8 /im.

1 "DC" Test "DC" Test

Reflectance Absorptance Reflectance Absorpta

Cycles (%) (10"^) Cycles (%) (10"^)

0 99.8 2.49 0 99.8 1.71

10 99.8 2.21 10 99.8 1.59

99.8 2.17 10^ 99.8 1.48

lo' 99.8 2.10 99.8 1.41

10^ 99.8 2.16 10" 99.8 1.68

-360-
9. Figures

MimtoK

Figure 1. Driver assembly.

Dellerlion

Mechanical Bias

-rf- or Time

Pliolo Diode

Mirror

Figure 2. "DC" cycling.

Driver

I'ower

Aiiiplilier

Function

Generator

Figure 3. Fringe counting interferometer.

-361-
COMMENTS ON PAPER 8/ ROWE

The. ipzakeA >imanke.d thout the.y had not meoAoted thz it/LZii> cAmtzd In thz cocuting dad to the. dz(^onmation o{j the nuAAo^, bmt {^eJLt that it. MoaJLd tui, ixndeA any ctAcmitanceA than the. it/teM, In the ^ace

plate. St/ieM in the (^ace plate Moi e^tijmated to be no g^eateA than 14, 000 pit.

-362-
QUANTITATIVE CHARACTERIZATION OF COATINGS:

Part I: Adhesion*

Stephen R. Scheele and James W. Bergstrom

Hughes Aircraft Company

Culver City, California 90230

Methods for quantifying the adhesion of optical coatings have been investigated with the goal of replacing the current pass-fail MIL-Spec cellophane tape

tests. Three mechanical pull-off tests were compared on a large number of glass

and infrared substrates with various reflection and antiref lection coatings. The

tests included: the direct pull-off test, in which a plug is bonded to the coating and then pulled perpendicular to the surface; the topple test, in which the

top of the bonded plug is pulled parallel to the surface; and the peel test, in

which an aluminum strip is bonded to, and then peeled from the surface.

Key words: Abrasion; adhesion; optical coating testing; peel test; plug-pull test; topple test.

1. Introduction

Efforts to predict the lifetime of optical coatings when exposed to various environments have historically been hampered by the absence of accepted quantitative testing procedures; procedures which

could be used in coating development and also in quality control of production coatings. Though there

have been many quantitative tests developed for nonoptical coatings such as paints and deposited

electrical leads for integrated circuits, the only durability tests currently accepted throughout the

optical coating industry are the qualitative MIL-Spec tests of MIL-C-675, MIL-C-48497 and MIL-M-13508.

The tests include the moderate abrasion or cheese cloth test , the severe abrasion or eraser test and

the adhesion or cellophane tape test. In addition to being qualitative (pass/fail) the MIL-Spec tests

are also subjective; i.e., the criterion for passing the abrasion and adhesion tests is based on visual

inspection for damage. Furthermore, the test procedures are not well controlled. For example, the

speed at which the tape is pulled is "specified" only as slow (MIL-M-13508) or quick (MIL-C-48497)

.

Since the pulling force is a function of the pulling speed (see sec. 5), variations in pulling speed can

lead to a wide variation in results. Another example is the eraser test which is deficient in that the

abrasiveness of the eraser is not well controlled and the speed and method of rubbing the surface is not

specified (see part II)

.

The objectives of this program are to develop quantitative methods to test the adhesion and abrasion

resistance of optical coatings, with particular emphasis on coatings for high powered laser windows.

The tests should be relatively inexpensive, that is, inexpensive with respect to the resources of a small coating company. This includes the cost of any equipment (purchased or fabricated), the cost to develop the testing procedure (operator training, etc.) and the cost per test to run. The tests should

also be accurate, i.e., repeatable (intra-laboratory) , and reproducible (inter-laboratory). The tests

should, if possible, simulate the action of environmental stress and the results should be relevant to optical systems.

2. Adhesion Testing

2 . 1 Background

To aid in the discussion of adhesion and adhesion testing, it is useful to define three types of

adhesion: basic adhesion, which is the summation of the intermolecular interactions; experimental or testing adhesion, which is measured in an adhesion test; and operational adhesion, which is a measure

of the resistance of the coating to deadhesion in its operating environment. The latter two types of

adhesion are similar to Mittal's [I]''' definition of practical adhesion or bond strength. Though it is

certainly difficult to disagree with Chapman's [2] contention that "we should try to devise tests of

durability in the service conditions," it turns out to be even more difficult to devise such tests. That statement was made in an article written in 1973 and no article written before or since was uncovered which references an adhesion test that simulates the operating environment of optical coatings.

If it were possible to calculate basic adhesion from measures of experimental adhesion, then perhaps

the environmental adhesion could be calculated from it. While Kendal [3-6] has made some progress in

relating experimental with basic adhesion by considering simple systems such as plastic in contact with

gelatin, the problem of measuring basic adhesion in complicated systems remains unsolved.

The relation of practical adhesion to basic adhesion is unclear, as is the relation between various measures of practical adhesion. There is considerable analytical and experimental evidence to indicate

* This work funded under contract to Air Force Materials Laboratory, Wright Patterson Air Force

Base, Ohio. Contract No. F33615-76-C-5285

.

1. Figures in brackets indicate the literature references at the end of this paper.

-363-
that all known adhesion tests are influenced by factors extraneous to basic adhesion such as coating

thickness, substrate elasticity, thickness of peeling strip etc. These extraneous factors cannot be

eliminated from the experimental apparatus and they cannot be analytically eliminated from the results.

The best that one can do is to control these parameters so as to make the results reproducible.

2.2 Review of Adhesion Tests

There are good reviews of the various adhesion test by Bullett [7], Campbell [8], Chapman [9],

Jacobson [10], Mittal [1,11] and Weaver [12]. The tests generally fall into two classes; mechanical and

non-mechanical (nucleation) . The latter tests are not applicable to this program. The mechanical tests,

which are all destructive, usually involve either bonding an external element to the coating such as a plug or strip and then pulling the coating off, or the acceleration of the coating so that the momentum

of the coating will tear it away. In the latter methods which include ultracentrifuge , ultrasonic, and

impact tests, the results depend directly upon the thickness and density of the coating making it

difficult to compare results from coatings of differing designs. There is a non-contact test called the

blister method in which a hole is drilled through the sample to the film and then air pumped through

until the coating fails. This technique is obviously not applicable to optical coatings. External

strips or plugs can be eliminated if the coating can be gripped by its edge and peeled away. This has

been done on some metal coatings [13] but cannot be done on all optical coatings and would require

special coating samples which would be more expensive than the standard samples now in use. The one test which falls into none of the above categories is the stylus test. Whether or not it is a true test of

adhesion has been the subject of much debate [14,15].

Thus, the available adhesion tests which meet the requirements of this program all involve bonding

a member to the coating and pulling the coating off. This can be done by peeling, toppling, twisting

or pulling as illustrated in figure 1. The tests that were implemented for this study were the plug

pull test, topple test, and the peel test. They all have various features and drawbacks as discussed

in the following paragraphs.

Plug Pull Test

The plug pull test (variously known as the direct pull-off and flat-wise tension test) is geometrically the simplest test and would thus appear to be the easiest test to relate to basic adhesion.

Detailed analysis, however, shows the test to be quite complicated. Kendall [3] studied the pull test

at equilibrium, i.e., under conditions where the pulling force is just sufficient is slowly propagate

cracks.* Using two simple systems, he derived two relationships between the pulling force P and the

surface free energy y. For the case of a rigid disk of radius a in contact with an infinite elastic

material of Young's modulus E and Poisson's ratio v, the relation is

„ _ 3/2 1/2

Alternately, for a rigid disk bonded to an infinite, rigid substrate by a thin elastic film of thickness

t and bulk modulus K, the relationship is

2Tr^K 2 1/2

-J- a Y

Kendall tested these equations using plastic disks (Perspex) and gelatin (for the adhesive) and found

them to be accurate. However, the systems tested in this study, consisting of multi-layered coatings

on flexible substrates, bonded to flexible disks, are much more complicated and no attempt was made to relate pulling force to adhesive energy.

As pointed out by Jacobssen and Kruse [16], misalignment of the plug and sample can cause additional

stress a„ at the bond edge due to bending moments. They give the relation as:

El

''e =-34M

where the moment M is

:

Fa

1 + FL^/3EJ

- 64

where E is Young's modulus, a the misalignment, L the plug length, F the pulling force, and R the plug

radius. Care was taken in implementing this test to assure good alignment.

* If the fracture proceeds at a slow enough rate, no energy will be dissipated in viscous processes.

-364-
Topple Test «

The requirements for precise alignment are relaxed in the topple test of Butler [17]. The other

advantage pointed out by Butler is that there is no net force normal to the substrate and therefore less

substrate distortion than in the plug pull test. There is one other possible advantage which lacks

analytical confirmation, but could be important. That is, the normal mode of failure of butt joints is

crack propagation. In the plug pull test the location, size and direction of propagation of the starting

crack is unpredictable, varying from sample to sample. This is undoubtedly partially responsible for

the observed spread in data. In the topple test the maximum tensile stress is at one edge of the sample,

greatly increasing the probability that the crack will form there and propagate in the direction of pull.

Peel Test

The peel test, developed initially by Jacquet [18] after whom the test is sometimes called, is

attractive if for no other reason than for the existing theoretical foundation. There are two quite

different models for this test. The first model developed by Spies [19] and Bikerman [20] and later

modified by Jouwersma [21] , is based upon small bending theory applied to an elastic beam on an elastic

(Winkler) foundation. The second model was derived from energy balance considerations by Kendall [3-5].

Apparent contradictions in the two models were resolved by Gent [22]

.

One possible soured of error is energy dissipated in bending the adhering strip. This can be mini- . mized by using thin strips or by using a roller to control the radius of bend as reported by Gent and

Hamed [23]. Another source of error is energy dissipated by viscous processes. This should be a monotonically increasing function of peel speed and thus should be minimum at low speeds. Kendall [24]

showed that peeling force of a rubber strip on a glass plate was independent of speed below 10 um/s.

This is a factor of four below the slowest speed available on the Chatillon tester used for the peel

tests, and therefore no measurements were made of peel force at this low speed. Also, it is impractical

to set up a QC peel test at this low speed because it would take forty minutes to test a one-inch

sample. This is just another example of the many extraneous factors in adhesion testing that are more

amenable to control than elimination.

The results of the peel tests are given in units of peel energy per unit area (or simply referred

to as peel energy) . The peel energy E is related to the peel force F and length of peel L by

L

E = J

Fdx

o

but the peel force, averaged over the length is

;=!/

o

thus dividing the average peel force by the peel width

1 = A

W LW

is equivalent to the peel energy per unit area.

2.3 Apparatus

The key device used in all of the adhesion tests is the universal tester, capable of measuring

either tensile or compressive forces. The tests were performed with three different universal testers,

all operating in the tensile mode. The Baldwin 68-TE-1300 universal tester was used only when crosshead speeds less than 0.25 cm/min or forces greater than 2000 Nt were required. Since the force

sensing device and the display are mechanical, this machine has an inherently slow response. Therefore, most of the adhesion tests were performed on a Chatilion UTSM-EL electronic universal tester. Its speed range was modified from the standard model to cover the lower range of 0.25 to 3.0 cm/min. With

the standard and additional load cells, several ranges were available with the following full scale

force values: 5, 50, 100 and 500 pounds. An Instron TTC-M1-M4 universal tester was used for one set

of tests because a crosshead speed of up to 50 cm/min was available.

With forces greater than 4000 Nt in some instances, the plug pull was the most severe test of the

fixturing and universal tester. The Baldwin tester was used in all of these tests because of the high

forces and slow speeds necessary.

In the plug pull and the other two adhesion tests, the bonding surfaces of the test fixtures were

made of aluminum since it is easy to machine, and especially because it is a preferred surface for

cleaning and bonding. The most important consideration in the design of the plug pull fixtures was maintaining perpendicular loading on the test surface. Three methods were used to help meet this

criterion

:

1. close tolerances in fixture fabrication

2. careful alignment during bonding

3. connections to the universal tester which only allow axial loads.

-365-
Figure 2 illustrates the design for the upper bonding plug which is bonded directly to the coating

under test. Note that the bonding surface perpendicularity and roughness are specified to obtain a surface free of large stress concentrations. The lower bonding plug, which is bonded to the opposite

side (bottom) of the substrate, is similar except that it has a constant 1-inch diameter over its entire

length.

Alignment during bonding was made possible by a simple piece of aluminum angle as pictured in figure 3. Tests were made on a number of bonded samples to determine the concentricity of the two plugs.

The maximum deviation from the axis of rotation was 0.03 cm, only slightly larger than the ellipticity

of the plugs (approximately 0.018 cm). The maximum possible edge stress was calculated to be 2.4 percent higher than the average stress. This is probably less significant than the stress concentrations

caused by other factors such as irregularities in the coating, and the use of glass beads in the

adhesive (sec. 4). In figure 4 the entire bonded assembly can be seen held in place by two universal

joints used for removal of all non-axial forces.

The advantage of the topple test is the lack of the close tolerances required in the plug pull test. The important criterion for a proper topple test is to apply only a moment at the coating surface.

The topple bar /sample holder is shown in figure 5 and the entire apparatus including the turnbuckle

(used to eliminate shear stress) is shown in figure 6. Figure 7 shows the adjustable sample holder for the peel test with a one-inch diameter sample in

place on a circular shim. The holder was designed to accommodate samples up to 0.5 inch thick and up

to two inches in diameter. The sample holder rests on a low friction slide table so that the peeling

area remains directly under the load cell and peel strip, thus maintaining the 90° angle. Figure 8 shows

the entire peel test apparatus, including the Chatillon Universal Tester, and the Hewlett-Packard X-Y Recorder used for generating the peel test plots.

3. Coatings*

Coatings on glass substrates were used in the initial test development phase. One hundred durable

and non-durable antiref lection coating samples (AR) and 100 durable and non-durable mirror coating

samples were obtained for a total of 400. The durable mirror coating was SiO -overcoated aluminum and

X o the non-durable mirror was gold, undercoated with lOOOA of chrome. The durable AR coating consisted of

a layer of SiO^ (QWOT** = 3 ym) followed by a layer of TiO^ (QWOT = 3 pm) . The non-durable AR coating

was a single layer of cryolite (QWOT = 3 ym)

.

Since these coatings on glass were to be used only for test development, there were no durability

specifications and the only performance specification was for the two-layer AR coating, which was to have maximum reflectance of four percent at 3.0 um ± 0.2 ym. AR coated infrared windows were used in the final stages of test procedure development. Fifty CaF^

windows and 44 KCS, windows (25.2-mm diameter by 5-mm thick) were obtained from the Harshaw Chemical

Company. These samples were packed in a desiccant and arrived at Hughes with no apparent moisture

damage. In addition, 16 ZnSe windows (25.4 mm diameter x 3.2 mm thick) were purchased from the II-VI Corporation. The material was Raytheon CVD-type ZnSe. Two different AR coating designs were chosen for each substrate type for a total of 6 different

coating/substrate combinations as shown in table 1. The coating materials were chosen on the basis of

the ease of coating and suitability for IR laser windows.

The KC5, sample coatings of PbF^ + ZnSe were received in a severely deteriorated condition due to humidity exposure during shipment. It was also noted that the ThF^ + ZnSe coatings on KCil were

extremely sensitive to humidity. If left exposed for several minutes to the ambient laboratory

humidity level of 55 to 65 percent at room temperature, blistering of the coating would occur. Because of these coating failures, several samples of each coating type were exposed for 24 hours to ambient humidity at room temperature, and other samples of each type to 100 percent humidity at approximately 50°C. Results from this test indicated that only three of the coatings procured would withstand 24-hour preconditioning at 100 percent humidity. These coatings were MgF^ + Ge and SiO + Ge on

CaF2 substrates, and ThF^ coatings on ZnSe substrates. The ZnS + Ge coatings on ZnSe failed the 100 percent

humidity test in only 4 hours. The SiO^ + Ge coated CaF2 samples are listed as marginally passing

the humidity test because three failed on one side only while one sample failed on both sides. Additional samples were supplied by AFML. They included ZnSe AR coatings on CaF2 and KCl (4 each) and

8 TJlI-KCi!.-TS.I AR coatings on KC5,.

* The coatings described in this section were used in both the adhesion and abrasion (following paper)

phases of this program.

** Quarter-wave optical thickness or the wavelength at which the phase shift upon the passage through

the layer is 90 degrees. This is the accepted measure of coating thickness.

-366-
Table 1. AR coated laser windows.

Substrate

Coating Design: Material

(index) QWOT, ym Transmittance,

Center

Wavelength, Humidity

l^UaU L X L y Outer Layer Inner Layer fa )jin iest Ke suits

19 SiO^ (1.75) Ge (4.0) 96 3.5 Marginally

(1.41) 5.63 0.45 Passed 100%

tiuniia icy

20 KCl PbF2 (1.72) ZnSe (2.4) 10.6* Failed

(1.46) 14.1 4.71 Shipping

Env ironinent

ThF^ (1.5) ZnSe (2.4) inn r aiiea

(1.46) 14.4 3.06 Ambient

Humidity

19 CaF^ rp (L 11 98 3.3 Passed 100%

(1.41) Humidity

7 ZnSe

(2.40)

ThF^ (1.5)

10.6

96 10.0 Passed 100%

Humidity

7 ZnSe ZnSe (2.15) Ge (3.8) 98 10.0 Failed 100%

(2.40) 12.0 4.77 Humidity

*Design goal, measurement not performed due to coating failure.

4. Adhesives

Several adhesives were used in the early phases of the adhesion tests in order to pick the best one. The adhesives (all epoxies) were chosen for strength, good adhesion to glass surfaces and low curing

stress (low shrinkage). Table 2 lists the adhesives used, the manufacturer, estimated yield strength,

relative elasticity, curing temperature, and relative curing stress.

Table 2. Adhesives

Designation Manufacturer

Estimated

Nominal

Yield

Strength,

Nt/cm^

Relative

Elasticity

Curing

Temperature

,

op Curing Stress

EPON 828* Shell 4800 very

stiff

70 Low

A1177-B B.F. Goodrich 5500 stiff 70 Low

C7/W Armstrong 5200 flexible 70 None

EC 2216 3M 3400 very

flexible

70 None

EC 2214 3M 10000 stiff 225 Mod-Low

*Catalyzed with TETA.

-367-
One of the mechanisms by which coatings fail in use is internal stress developed during coating.

Additional stress due to the adhesive cure cycle would add to the internal stress and affect the test results. For this reason most of the bonding was done with room temperature curing adhesives to reduce the thermally induced curing stress. For comparison one high temperature curing adhesive was

used: 3M type EC 2214 epoxy which cures between 100 and 120°C.

The aluminum plugs and strips were cleaned just prior to bonding in a hot (66°C) acid etch for

10 to 12 minutes. The acid was sulfuric with sodium dichromate added. Following the etch were two water rinses, the last in de-ionized water. The parts were then dryed by placing them in a dry 66°C

oven.Most of the samples were cleaned with flowing reagent grade acetone. An ultrasonic bath was

tried with no improvement in the results. In addition, a relatively mild alkaline bath was tried

(Oakite 164) but was found to attack the coatings and therefore its use was discontinued.

In many of the early tests the failure occurred at the sample-adhesive interface or the plugadhesive interface, evidence of an improper bond.* To improve the interface, the aluminum was primed

with a corrosion-resistant primer EC 3924 while the samples were primed with a silicone primer

EC 3901.

The bond thickness was controlled through the use of small (0.1 mm diameter) glass beads, 0.2%

by weight. Teflon shims were also tried but the results were poor. Also the shimming technique is

very difficult to apply to the peel strip bonding process.

5. Results

5.1 Plug Pull Test

The results of the plug pull tests are presented in table 3. Comparing the first two entries one

can see that the A1177B forms a good bond with glass. The results for the aluminized mirrors show

that a good bond was formed between the plug and sample but that the yield strength of the adhesive

was insufficient (for the most part) to overcome the adhesion of the coating to the glass.

Table 3. Plug pull tests*

Sample Quantity

Average

Yield

Force, Nt Standard

Deviation, %

Fraction

of Coating

Pulled, % Type of Failure

None** 5 5400 10 Cohesive

Glass only 5 5150 19 Cohesive

Al + SiO on Glass5 4840 7 5 Mainly cohesive

Cr + Au on Glass 5 2150 30 0 Mixed adhesive and cohesive

Cryolite on Glass 5 3785 15 0 Mixed adhesive and cohesive

* A1177-B adhesive: pulling speed = 0.025 cm/min.

** Direct plug to plug bond.

The tests on the gold- and cryolite-coated samples resulted in partial adhesive failure and thus low

yield force. There was a strong correlation between the fraction of cohesive failure and the yield

strength. This is illustrated in figure 9 which shows the results of testing two gold mirrors. The

dark areas (gold) indicate adhesive failure while the light areas (adhesive) indicate cohesive failure.

The measured yield force for the sample on the left was twice that of the sample on the right.

Due to the inability of the plug pull test to consistently pull coatings, the program emphasis

was shifted to the topple and peel tests.

5.2 Topple Test

The topple test was performed on the plain glass samples, coated glass samples, and AR-coated IR

substrates. Two deflection speeds (1.3 cm/min and 0.28 cm/min) were compared during the tests on plain glass. It was found that the 1.3 cm/min speed had better results. The average force was approximately ten percent higher than for the slower speed, and the standard deviation was six percent of the

average as opposed to 20 percent for the 0.28 cm/min speed. Thus the 1.3 cm/min speed was used on most of the gold-, aluminum-, and cryolite-coated glass samples. From the results in table 4, it

is seen that the test was successful in removing significant amounts of each coating type.

* According to Bikerman [25], failure in a properly formed bond is always within the adhesive (cohesive

failure) . A failure at the boundary (adhesive failure) is indicative of an improper bond which he

calls a weak boundary layer.

-368-
Table 4. Topple test results on coated glass

Quantity Coating

Average Force,

Nt. Standard Deviation,

% of Average

Average Percentage

Coating Pulled Type Failure

5 Al + SiO

X 273 25 30 Cohesive

5 Cr + Au 118 30 55 Adhesive

5 Cryolite 173 24 30 Adhesive

*A1177-B adhesive, deflection speed = 1.3 cm/mln for most samples.

The AR coated, laser substrates were all topple tested at the 1.3 cm/min speed. A summary of the test results is listed in table 5. The test was successful in removing either the coating or part of

the substrate itself for every sample tested. Figure 10 depicts the unexplained destruction of the KCl

substrates at very low topple forces. For comparison, the "slow" tape test (MIL-M-13508) was performed

on the remaining coating of all of the coated samples tested by the topple method. The only samples

to fail were three of the Si02 + Ge on CaF^ samples, four of the AFML samples, and all of the ThF^ +

ZnSe on KCl samples. It has not been explained why the ThF^ + ZnSe on KCl samples failed the cellophane tape test although the substrate failed in the topple test before the coating was removed.

Table 5. Topple test results on AR coated IR substrates*

Quantity Substrate Coating

Average Force,

Nt.

Standard Deviation,

% of Average Comments

10 CaF^ SiO^ + Ge 99.6 36 coatings entirely

removed, slight

substrate damage

6 CaF2 MgF^ + Ge 125.4 24 substrates destroyed

3 ZnSe ZnS + Ge 43.6 28 all coatings removed

A ZnSe ThF,

4 64.1 12 substantial coating

removed

9 KCl ThF, + ZnSe

4 32.9 27 substrates destroyed

•7 KCl T1I+ KCI-HTII 55.4 15 parts of coating

layers removed

1 CaF^ inSe 68.1 substrate and coating removed

1 KCl ZnSe 52.0 substrate and coating removed

1 CaF2 ZnSe 14.7 **coating completely

removed

1 KCl ZnSe 52.5 **coating completely

removed

* A1177-B adhesive, deflection speed = 1.3 cm/min.

** These coatings were deposited by sputtering. All the other coatings were thermally deposited.

-369-
5.3 Peel Test During the development of the peel test, peel strips of varying material, thickness and temper were tried. It was found that thin (0.15 mm) aluminum strips of low temper gave the highest peel

energies and were the most consistent at pulling coatings. It was also determined that a more

elastic adhesive (3M EC 2216) gave better results. A summary of these results is shown in table 6 for a range of pulling speeds from 0.3 to 3.0 cm/min. There was no direct correlation between speed

and the results observed, therefore a pulling speed of 1.3 cm/min was arbitrarily chosen for the

subsequent tests.

Table 6. Peel test on coated glass*

Quantity Coating

Average

Peel Energy,

Nt/cm

Standard Deviation,

% of Average Coating Pulled Comments

3 Cr + Au 4.8 11 all

3 Al + SiO

X 19.1 12 none Adhesive failure

3 Cryolite 10.3 10 none Adhesive failure

3 SiOj + Ti02 19.4 17 5% on 2 samples Adhesive failure

*EC 2216 adhesive, 0.15 mm thick aluminum strip, various speeds.

Table 7 lists the results for these tests. For comparison the "slow" tape test was also performed on the remaining coating on these samples. All of the samples passed with the exception of the

KCl windows. The peel test was therefore capable of removing coatings which had passed the tape test,

and with forces low enough to avoid substrate damage.

Table 7. Peel test results on coated IR window substrates

Substrate Coating

Average Peel Energy

Nt/cm Comments

CaF2 Si02 + Ge 0.76 all coatings removed completely

CaF^ MgF^ + Ge 13.5 nothing significant removed

ZnSe ThF,

4 0.70 two samples only, all coatings removed

ZnSe ZnS + Ge 2.5 one sample, 90% coating removed

KCl ThF, + ZnSe

4 0.61 two samples, >60% coating removed

5.4 Cellophane Tape Test

In order to compare the peel test to the established MIL-Spec cellophane tape test, measurements

were made of the peel energy of the cellophane tape (Type I, Class A; Federal Specification L-T-90D)

at various pulling speeds (fig. 11). Four people were asked to perform the slow pull tape test and

resulting range of pulling speeds are also shown in the figure. The "quick" or snap test of MILC-48497 was also performed. The corresponding peel energy could not be measured because of the slow

response of the tensile test machine so the value was estimated by linear estrapolation. It has been

shown by Kendal [5] that the peel energy should approach a constant at high peel rates and thus the

estimated value of 7.7 Nt/cm should be considered an upper limit.

Since all coating vendors do not use the approved cellophane tape, a comparison test was performed using 3M No. 6201 (Highland®) mending tape. The resulting curve in figure 11 is quite different from the cellophane curve. The tape exhibited a slip-stick peel pattern which is the reason

for the relative insensitivity of the peel energy to peel rate. All the infrared samples were tape tested and the results were compared to the results of the

topple test (fig. 12) and to the results of the peel test (fig. 13).

Despite the dispersion of results there appears to be good correlation between the pull tests and

the tape test. The Wilcoxon Two Sample Test [26] gives a confidence greater than 99.7% that the "passed"

group of samples is really different from the "failed" group. Thus the new tests and the older tape

test are testing related, if not the same, adhesion phenomena.

-370-
6 . Summary

The basic objective of this program was to develop quantitative adhesion tests that could be used

to replace the qualitative MIL-Spec tests. Though it proved difficult to remove coatings with good

adherence to glass substrates, it was possible to remove coatings from infrared substrates and therefore to quantify their adhesion.

The features of the various tests are summarized in table 8. The plug pull test is perhaps the

least desirable of the three because of relatively poor results and high forces involved. The topple

test gave better results and is inexpensive to implement but resulted in still unexplained substrate

damage. The peel test is expensive to implement but has the lowest force of the three tests; it has

the advantage that it gives localized adhesion values.

Table 8. Comparison of tests

Ability to Type Force, Nt. Pull Coatings Repeatability Cost to Implement Cost to Perform

Plug pull A, 400 Poor Fair High Low

Topple 264 Fair Good Low Low

Peel 22 Fair Good High Moderate

7. References

[1] Mittal, K.L., "A Critical Appraisal of the

Methods for Measuring Adhesion of Electrodeposited Coatings," in Properties of

Electro-deposits: Their Measurement and

Significance , Ed. R. Sard, H. Leidheiser,

Jr. and F. Ogburn, Chapt. 17, Electrochemical Society, Princeton, NJ, April

1975.

[2] Chapman, B.N. , "Thin Film Adhesion,"

J. Vac. Sci. Technol., 11, 106, 1974.

[3] Kendall, K. , "The Adhesion and Surface

Energy of Elastic Solids," J. Phys. D.

,

4, 1186, 1971.

[4] Kendall, K. , "Shrinkage and Peel Strength

of Adhesive Joints," J. Phys. D. , 6^, 1782,

•1973.

[5] Kendal, K. , "The Dynamics of Slow Peeling,"

Int. J. Fracture, 11, 3, 1975.

[6] Kendal, K. , "Thin-Film Peeling - The

Elastic Term," J. Phys. D. , 8, 1449, 1975.

[7] Bullett, T. R. , and Prosser, J.L., "The

Measurement of Adhesion," Progress in

Organic Coatings, 1, 45, 1972.

[8] Campbell, D.S., "Mechanical Properties of

Thin Films," Handbook of Thin Film Technology , Edited by L.I. Maissel and R. Glang,

Chapter 12, McGraw-Hill, New York, 1970.

[9] Chapman, B.N. , "Adhesion of Thin Metallize

Films," Aspects of Adhesion , Ed. D. J. Alner, 43, CRC Press, 1969.

[10] Jacobson, R. , "Measurement of the Adhesion

of Thin Films," Thin Solid Films, 34, 191,

1976.

[11] Mittal, K.L., "Adhesion Measurements

of Thin Films," Electrocomponent

Sci. Technol., 3, 21, 1976.

[12] Weaver, C. , "Adhesion of Thin Films," J. Vac.

Sci. Technol., 12, 18, 1975.

[13] Foley, N.M. and Whitaker, H.L., "Adhesion of

Chromium Films to Soda Lime Gloss," J. Vac.

Sci. Tech. , 11, 114, 1974.

[14] Butler, D.W., Stoddart, C.H. and Stuart, P.R.

,

"The Stylus or Scratch Method for Thin Film

Adhesion Measurement : Some Observations and

Comments," J. Phys. D. , 3^, 877, 1970.

[15] Kuwahara, K. , Nakahara, S. and Nakagawa, T.

,

"The Adhesion of Thin Evaporated Films," Int. Conf. on Strength of Metals and Alloys - Proc, 9^ Supplement, 1034, 1968. ^

[16] Jacobson, R. and Kruse, B. , "Measurement of

'

Adhesion of Thin Evaporated Films on Glass

Substrates by Means of the Direct Pull Method,"

Thin Solid Films, 15, 71, 1973.

[17] Butler, D.W. , "A Simple Film Adhesion Comparator," J. Phys. E, 2. 979, 1970.

[18] Jacquet, P.A. , "Adhesion of Electrolytic

Copper Deposits," J. Electrochemical Soc,

66, 393, 1934.

[19] Spies, G.J., "The Peeling Test on ReduxBonded Joints," Aircraft Eng., 25, 64, 1953.

[20] Bikerman, J.J., "Theory of Peeling through a Hookean Solid," J. A. P. 28, 1484, 1957.

[21] Jouwersma, C, "On the Theory of Peeling," J. Polymer Sci., 45, 253, 1960.

[22] Gent, A. N. and Hamed, G.R. , "Peel Mechanics,"

J. Adhesion, 1, 91, 1975.

[23] Gent, A.N. and Hamed, G.R. , "Peel Mechanics

for an Elastic-Plastic Adherend," J. Appl.

Polymer Sci.

[24] Kendal, K. , "Crack Propagation in Lup Shear

Joints," J. Phy. D. : Appl. Phy., 8, 12, 1975.

[25] Bikerman, J.J., "The Science of Adhesive Joints

2nd Ed., Academic Press, N.Y. , 1968.

[26] Noether, G.E. Introduction Statistics , Houghton Mifflin Co., Boston, 1976.

-371-
8. Figures

Figure 1. Various pull-off tests.

Figure 2. Upper bonding plug.

-372-
Figure 3. Bonding alignment fixture.

Figure 4. Bonded plug pull assembly in test fixture.

TOPPLE TEST SETUP

t

TO LOAD CELL

II II .5" X .5" BAR

. V DIAMETER ROD

///////////////////////////////////////////////^^^

Figure 5. Topple bar/sample holder.

-373-
Figure 6. Topple test arrangement.

Figure 7. Sliding sample holder.
Figure 10. KCl sample damaged during topple test.

O CELLOPHANE TAPE

7. 7 NT/CM^-|;;5>.|"^' 3M NO. 6201 MENDING TAPE APPROXIMATE RANGE OF

"SLOW" HAND PULL SPEEDS ^-^^"^ APPROXIMATE SNAP TEST

r

1 1 1

IS25

1

I I 10 100 1000 10,000

SPEED, CM/MIN.

Figure 11. Tape test results.

-375-
* u s 5 u

c g t

"5. 6 o O e o E5

^

•s AFM S

<

o

So

_ MEAN ° CD

= 83.2 n n O

STD OEV O

- 38 o 0 - ° e o CP O o 0 MEAN = 38.3

o

STD DEV = 1B.2

O

o

-

PASSED FAILED

TAPE TEST 8ESULTS

Figure 12. Comparison between topple and tape

test on AR coated IR windows.

CoF KCI

o o O o

N

0^ f

o

o&

o

MEAN = 6.65

STO DEV = 6.8

o

O

OO CO o

o

MEAN = 0.40

STD DEV = 0.26

CD

PASSED FAILED

TAPE TEST RESULTS

Figure 13. Comparison between peel and tape test on AR coated IR windows.

-376-
COMMENTS ON PAPER EV SCHEELE kW BERGSTROM

Stvzfial quutioya weAe nxuAe-d conzexyiLng thz natuJiz th(i adhulvu, aied -cn thuz teAti. Thz 6peM.keA

incUcatzd that thz. adhuivsA ixMid Mejie. noorn tmpeAxutuAn ieJXing tpoxlu, choim i>o cu> to avoid iwUioducA.nq thsAmaZ itAUi in tht bond. Some tutis weAe n.a.n ming high tmptnatuAii tpoxlu bmt no ilgni-

{iicantZy di^eA^nt nozuJUns weAe obtain&d. Tht chmical ej^^ecX o{^ thz adhuivz on the. coating iuA^ace

Moii not te^tzd, aZthoagh >iz{^(iAznc.u In the titoAatuAz Indicated no chemlcaZ InteJiactlon {^on. the

adheilve^ that we/te employed. The epoxxei Lued iuppoicdZy have veny low iheoAlng itAeii and 6o any

ihAlnkage o{j the epoxy ihoald not have Introduced i,lgnl{^lcant itAeAi In the coating, howeveA, no meoMViementii weAe made o{j thLi> e^^eci; dlnectZy. 0(5 the thn.ee teMti> examined, the ipeakeA Indicated

that they had a pKe^eAence {^on the topple te6t, despite the ^act that on iome occoilom, It lend to 6ubit/iaXe dalZuAe. Tkii {jOlZuAe woi delX to be due to bleeding o^ the adhesive thAough the coating,

bonding the topple plug to the 6ub6tAate. Due to thld and othen unceAtxu.ntJ.eji,, the ipeakeA ielX that

It Moi pfiematuAe to Indicate a de{^lnlte pAef^eAence oi one tut oven, anothen at this time.

-377-
QUANTITATIVE CHARACTERIZATION OF COATINGS:

Part II: Abrasion Resistance*

R. A. West and C. W. Nichols

Hughes Aircraft Company

Culver City, California 90230

A new method of quantification of abrasion resistance of antiref lection coatings has been developed. The Falling Sand Test of ASTM was modified to abrade

optical coatings on witness samples in a controlled manner. Quantification of

the abrasion damage was accomplished by measuring transmittance degradation in

the abraded area. Approximately 170 samples of antiref lection coatings on various substrates including glass, KCS,, ZnSe, and CaF2 , were tested using the new method. The existing standard military eraser and cheesecloth abrasion tests were also performed on many of the samples and the degree of correlation

between the existing tests and the new method was established.

Key words: Abrasion resistance; coating degradation; coatings; hardness; transmittance degradation.

Introduction

Whether intended for use in laboratory optical systems, military, or space applications, most coated

optical elements are required to exhibit some degree of durability to insure a reasonable useful lifetime for the element. One of the parameters commonly specified as a measure of this anticipated lifetime is abrasion resistance of the coated surface. The resistance of a coating to abrasion allows the

element to be cleaned as necessary using standard optical cleaning procedures and, in extreme cases,

to operate for a reasonable period in an environment where dirt and dust impinge and accumulate on

the surface.

The current tests of abrasion resistance in use throughout the optical industry include the 20-rub

eraser test of MIL-C-675 and MIL-C-A8A97 and the 50-rub cheesecloth test of MIL-M-13508 and MIL-C-48A97.

Both of these tests call for a specific number of "complete strokes"** ofthe eraser or cheesecloth pad

over the surface. The force on the surface specified for the eraser test is 2.0 to 2.5 lb while that

for the cheesecloth is 0.75 to 1.25 lb. There are several problems associated with these tests. They provide only a qualitative "passfail" measure of a coating's abrasion resistance and the criterion for judging whether a coating is

passing or failing is that "no visible deterioration" of the coating be apparent after the test. For

infrared optical elements, the visible degradation criterion can be excessive since surface deterioration at infrared wavelengths is not as critical to normal performance. Furthermore, since many infrared optical elements are visually opaque and have high refractive indices, the judgement of visible

degradation is highly subjective, being dependent on lighting conditions, and on the eyesight of the

inspector

.

Specific problems with the tests as they are currently structured include variability due to manufacturing variations in the "MIL-Spec" erasers, and lack of detail in specifying procedures to be used

in preparing the tester (especially the eraser surface), rubbing speed, and limitations on useful life

of erasers or cheesecloth.

Other Test Methods

A quantitative test of abrasion resistance of optical coatings requires two parts: first the actual

abrading of the sample and then the evaluation of that abrasion. In developing a new method to provide

a quantitative measure of abrasion resistance, emphasis was placed on repeatability, relevance to actual

operational parameters, and ease of implementation and performance. A survey of the literature

revealed many types of abrasion tests for paints, linoleum, refractory materials, etc., although few

could realistically be adapted to optical coatings. Many of the tests encountered required costly

equipment and/or large sample sizes, i.e., larger than 3.8 cm (1.5 in.) in diameter.

* This work funded under contract to Air Force Materials Laboratory, Wright Patterson Air Force

Base, Ohio. Contract No. F33615-76-C-5285

.

** The phrase "complete strokes" is emphasized here due to historical confusion over the definition of

a complete stroke as one pass over a point on the surface as opposed to a "complete cycle" or two passes over the point

.

-378-
Falling Sand Test

The approach selected as the most readily Implementable and easily variable test method to cover a wide range of coatings was the falling sand test modified from ASTM literature by Johnston and

Walsh [1,2]"'". Figure 1 shows the apparatus used to abrade the samples. A pyrex funnel emptied into a lOcc disposable adhesive syringe with a nominal 1.9 mm (0.075 in.) diameter opening cut in the tip.

(The syringe was incorporated early in the test development to control. the rate of sand entering the

fall tube independent of the amount of sand in the funnel.) The syringe was inserted into a vertical

61cm (24 in.) long, glass fall tube (8.0 mm I.D.) with the bottom end located 2.54cm (1.0 in.) above the

sample surface. | The sample was mounted on a rotating teflon block, belt driven by a small synchronous motor at approximately 60 rpm. (The rotation of the sample as the sand impinged on the surface was incorporated

to eliminate any effects due to anisotropy of the coating, substrate material, or of the measurement

technique.) The teflon block and sample were oriented at a 45° angle from the horizontal to enable each

sand particle to theoretically impact the surface only once. The center of rotation of the teflon block

was offset from the center of the sample by 6.4 mm (0.25 in.) to allow for abrading only one-half of

the sample. A mask was designed to cover the remaining half to retain an undamaged reference area to perform spectral comparison tests. The abrasive material selected for the abrasion process was silica sand from the Ottawa Silica

Company of Ottawa, Illinois. This material was selected because it was readily obtainable, inexpensive,

and was made up of relatively smooth, regular shaped particles. It is the same type of sand approved for

various ASTM tests, except the range of particle sizes was smaller than the standard ASTM grades. An extensive sieving process was used to generate a known particle size distribution and to verify

that this distribution remained unchanged throughout the tests. The distribution obtained from the

sieving process was as follows: 88.1 percent (by weight) of the particles were in the 180-212 um range

(#80 sieve), 2.8 percent in the 212-250 pm range (#70 sieve), and 9.1 percent in the 150-180 pm range

(#100 sieve).

The method used to quantify the amount of abrasion damage achieved was the measurement of transmittance degradation. Transmittance measurements were performed on a Perkin-Elmer Model 180 spectrophotometer at the wavelength of peak transmittance for each sample. Figure 2 shows the sample holding

fixture designed for mounting the samples. With this fixture, measurements could be performed on both

the untested reference area and the abraded area simply by rotating the holding fixture through 180°.

By using a lOx ordinate expansion and long integration times (8-16 seconds) , transmittance differences

as low as 0.2 percent could be accurately measured. The figure shows the abraded area of a typical

sample in position in front of a 3.8 mm (0.150 in.) diameter aperture. The mount utilized indexing

holes and scribe marks for proper alignment and repeatable locating of the sample for successive

measurements as damage was accumulated.

Test Results: Glass Samples

Early testing and equipment checkout was performed using glass Balzer witness plates with single

layer cryolite and two layer Si02/Ti02 antiref lection coatings.* The first tests utilized measurements

of specular reflectance increase instead of transmittance decrease as described above. However, erroneous results were obtained since the increase in diffuse reflectance caused a corresponding

decrease in specular reflectance, counteracting the anticipated increase with coating removal. By measur

ing transmittance loss instead, these anomalies were eliminated since increases in diffuse and/or

specular reflectance will both result in a transmittance loss.

The next variable to be quantified was the sand charge size to abrade the different coating types

and measure the transmittance degradation of each type. Figures 3 and 4 show the transmittance

degradation vs. sand charge for both antiref lection coatings in increments of 5 grams. The points on both of these curves where peak degradation occurs represent the maximum transmittance loss due to scattering in the abraded area and correspond to the points at which coating "break through" occurred

exposing the glass substrates. In order to select one fixed sand charge size to use for quantitatively

ranking several coating types with wide ranging abrasion resistance characteristics it would be necessary to remain on the linear portion of the degradation curve for each coating type. It was apparent

for the two coating types tested here that no one fixed charge would be appropriate for both types, since

too large a charge would remove the entire cryolite coating before producing significant results on the

SiO^/TiO^ coating.

Hence, the method finally adopted was to select, as the independent variable, a fixed transmittance

degradation level of 2 percent (considered a realistic deterioration factor from a system standpoint and

also large enough to insure relative accuracy in the measurement). The dependent variable was then the

amount of sand necessary to produce the 2 percent transmittance loss, which could vary widely among

coating types to provide a quantitative measure of abrasion resistance. This provided a standard procedure useful for all types of coatings, visible and infrared, "soft" and hard."

1. Figures in brackets indicate the literature references at the end of this paper.

* Details of coatings and their designs are included in Part I.

-379-
Using this method, abrasion tests were performed on a number of the glass samples with Si02/Ti02

and cryolite coatings. Figures 5 and 6 show histograms of the results obtained. In figure 5, the

sensitivity and repeatability of the falling sand test can be demonstrated. The Si02/Ti02 coatings were

deposited in a planetary coating chamber utilizing three separate plates on which samples were mounted.

The figure shows that, in general, samples on a given plate were grouped together in the histogram

indicating good repeatability in the test, although a wide spread between groups is apparent, indicating

high sensitivity. It should be noted that all these samples passed the standard eraser test yet the

difference in measured abrasion resistance from the "softest" to the "hardest" is more than a factor of

ten. The results for the cryolite coating in figure 6 were not as revealing since all samples were from

the same plate in the coating chamber. However, since the cryolite samples also passed the standard

eraser test, the ability of the falling sand test to provide useful quantitative data with which to rank

samples all of which pass the eraser test was further demonstrated.

Test Results: IR Samples

Abrasion resistance tests were performed on laser window samples of KCl, ZnSe, and CaF2, some purchased by Hughes from various vendors and others furnished by the Air Force Materials Lab. Table 1 summarizes the results of these tests. In order to determine if the environment in which a sample

was stored prior to abrasion testing would have any effect, several samples of each coating type were

stored in the ambient laboratory atmosphere (50-60%) and several in a 100% humidity environment for

24 hours prior to testing. The results show that there was indeed an effect on the test results for the

CaF2 substrates and the ThF^-coated ZnSe substrates. The other samples could not be preconditioned at

100% humidity since the coatings were not sufficiently durable to withstand that environment. Furthermore, the coatings on the KCl substrates were not able to withstand the normal lab humidity and were

therefore tested by enclosing the test apparatus in a plastic bag and purging it with dry nitrogen gas

(10-15% humidity). The AFML samples were evaluated in this same environment.

Many of the samples were also tested using the standard eraser or cheesecloth tests for reference.

Figure 7 summarizes the sand test results compared to the eraser test results. For purposes of this

comparison, samples which failed or were marginal in the cheesecloth test were included in the "failed eraser test" column.

The first thing that is apparent from the graph is the wide spread in data which results from the

variations in coating type and substrate. Due to the widely varying nature of the population from

which these statistics are drawn, it would require a much larger number of samples to reach any definite

conclusions. This can be stated more quantitatively by testing the hypothesis that the pass and fail

groups are the same. Using a test from non-parametric statistics, the Wilcoxon Two-Sample test the confidence level for rejecting this hypothesis was found to be 76 percent. It is quite apparent however that if a difference exists it is not large. There are two possible reasons for this low correlation.

One is the subjective criterion for passage of the MIL-Spec tests, i.e., "no visible degradation." There is

probably little correlation between this criterion and the falling sand criterion of 2 percent change in IR transmittance. The other is that the eraser or cheesecloth tests and the falling sand test might actually measure different aspects of a coating's hardness.

Table 1. Results of falling sand test

Peak

Mean Charge Size (grams)

Wavelength Ambient No. of 100% No. of

Substrate Coating (vim) Humidity Samples Humidity Samples

CaF2 MgF2+Ge 3.3 62.8 9 44.9 8

Si02+Ge 3.5 22.1 9 27.9 6

ZnSe ThF, 10.0 95.7 3 76.7 3

ZnS+Ge 10.0 11.4 4

KCl ZnSe+ThF,

4 10.0 15.7* 14

KCl TII+KCI+TII 10.6 2.43* 7

CaF2 ZnSe 4.8 1.2* 1

CaF2 ZnSe 3.7 0.7* 1

KCl ZnSe 5.0 0.3* 1

KCl ZnSe 6.7 2.8* 1

*Ambient Humidity 50-60%. Those marked with (*) 10-15%.

-380-
Summary

The falling sand test has proven to be an easily implemented, sensitive, repeatable, and quantitative

test of abrasion resistance. In many cases, the test results indicated a variation among several

samples within the same coating run, traceable to various locations or mounting plates within the coating

chamber. Samples near each other on a given plate tended to have results close to each other indicating

that test repeatability is quite good.

The lack of a definite correlation between the falling sand test and the existing tests is probably due

to the fact that the two types of tests measure different damage parameters and test different damage

mechanisms. The sand test is an indicator of a coating's ability to withstand a blowing grit or dust

environment while the existing tests indicate scratch resistance and cleanability

.

References

[1] ASTM D968-51, "Abrasion Resistance of Coatings [2] Johnston, G.T. and Walsh, D.A. , "Some Effects of

of Paint, Varnish, Lacquer, and Related Environmental Factors on the Performance of

Products by the Falling Sand Method." Coatings for High Power IR Lasers," Interim

Technical Report AFML-TR-75-15 , February 1975.

Figures

Figure 1. Falling sand apparatus. Figure 2. Sample holder for transmittance measurements.

-381-
SAND CHARGE GHAMS

Figure 3. Transmittance degradation curve for Si02/Ti02 coating on glass substrate.

I'M 1

'

'

iliinilll l< ^ .

. . . ,n.i.i nnnnn n

CHARGE SIZE. GRAMS

0 20 SAND CHARGE. GRAMS

g TEST SAMPLES FROM PIATE A Q TEST SAMPLES FROM PLATE B I

TEST SAMPLES FROM PLATE C

Figure 5. Histogram of 2 percent transmittance degradation

results for SiO^/TiO^ coating on glass substrates.

Figure 4. Transmittance degradation

curve for cryolite coating

on glass substrate.

-382-
48 5,4 CHARGE SIZE. GRAMS

Figure 6. Histogram of 2 percent transmittance degradation results

for cryolite coating on glass substrates.

MEAN • 31.

B

STD DEV 23.2

z z o < a g8 i

STD DEV • 40-4

MEAN - 13,8

ERASER TEST RESULTS

Figure 7. Comparison of sand test results to eraser

test results for IR samples.

-383-
I

I COMMEWTS ON PAPER BV WEST ANV NICHOLS

Thz iptakeA -incLiccute.d that tz&ti wz/ie. caMA.zd out taith Ottawa iand, which -ii an A5TM Abfuii-ion Standard.

Tht iand moa poM^d thAough a No. SO muh and d/ioppzd ^lom 24 inchu heJ-ght. Thz attmpt woi made to reduce thz amount of, anQuloAtty in thz ab>iaAlvz compound. It woa iuggzitzd ^Kom thz dlooi that it

might bz advi^ablz to fiy glaii bzadi oi an abiaiivz mzdium. Thz quzition thz z^zct OjJ thz paAtlclz

6izz on thz abnxJU>ion tzAt had not bzzn tookzd at in tkli won.k, but thz ipzaksA indicatzd an intzKzi>t

in invzitigaJxng that 'izZationihlp.

-384-
DIELECTRIC ENHANCED MIRRORS FOR THE 2-6 lam REGION*

S. J. Holmes

Northrop Research and Technology Center

3401 West Broadway

Hawthorne, California 90250

This paper reports on work done to define the performance characteristics

of a large variety of multilayer dielectric enhanced mirror designs. In particular, dielectric enhanced metallic mirrors have been designed and fabricated

which demonstrate reflectivities of 99. 9 % or greater for the 2. 8 Mm, 3. 8 |am,

and 5. 3 \Jm wavelengths. Peak reflectance and description measurements are presented along with environmental testing to demonstrate the performance of

these mirror coatings.

Key words: Absorptance; Cleaning procedures; Coating

materials; Dielectric enhanced mirrors; Deposition

parameters; Reflectance.

1. Introduction

Until recently the high power mirrors used on laboratory lasers have been bare metal, copper

or molybdenum mirrors, which are either heat sunk or water cooled. This is adequate for short duration laboratory tests, when only a few optical elements are needed.

The difference in reflectance for uncoated versus coated mirrors represents a substantial fraction of the total laser beam output from an optical system where several mirrors are required. The

increased reflectance of a coated mirror also has important implications for beam quality and component cooling. A water-cooled mirror with high reflectance will require less cooling for a given

temperature induced distortion or alternatively will have less distortion for an equivalent water cooling flow. This can be important in simplifying cooling hardware or for improving the laser beam

quality as well as increasing power output. The development of dielectric enhanced mirror coatings

for thermally conducting substrates will provide the basis for water-cooled laser resonator mirrors

required to keep pace with the high power laser technology. Similar coatings on thermally insulating

substrates are needed for use as uncooled mirrors external to the laser.

Reflectivities between 98% and 99% for the 2-6 Mm region can be realized with metallic mirrors

of copper, gold, or silver. When reflectivities greater than 99% are required, multilayer interfer- ence coatings are required. The most practical design for the infrared region is the enhanced metallic mirror surface coated with pairs of alternating quarter wavelengths of dielectric layers of low

and high refractive indices. In principle this type of mirror can achieve reflectivity of greater than

99. 9%.

2. Theoretical mirror designs

There are three basic multilayer dielectric enhanced mirror designs: (1) all dielectric multilayer mirrors, (2) multilayer dielectric enhancement of a highly reflecting metallic thin film deposited on a suitable substrate of metal. It is the latter design that was chosen for this work.

For the initial design of a multilayer dielectric enhanced mirror, several simplifying assumptions were made. It was assumed that all of the dielectric layers were nonabsorbing, homogeneous,

plane-parallel and were illuminated by a parallel beam of linearly polarized light having an electric

field strength of unity. Because of their simplicity, the equations derived by Young [l] are used

here. The reflectance R of a metallic film is related to its complex reflective index (n - ik) by m

R =1- f^-j- (1)

(l+n)2+k^

* This work was supported by AFWL, Contract No. F2 960 1 -76 -C -000 3.

1. Figures in brackets indicate the literature references at the end of this paper.

-385-
When a multilayer coating of 2N nonabsorbing quarterwave layers having refractive index n

ri (high) and n^^ (low) is deposited on the thin nnetallic film, it can be shown that the reflectance is increased to R=i - (2) (x + n) + k

where

To obtain the maximum reflectance from a multilayer dielectric enhanced mirror, the metallic

film is first coated with the low refractive index material until its reflectance decreases to a minimum at the wavelength for which the highest reflectance is desired. The optical thickness of the

layer should be [2]

+ 1) -X

2 4n arc tan

2n^ k

n - k

(4)

where j is an integer or zero.

Under this condition, the low index film adjacent to the metal is effectively one quarter-wavelength. In most cases, (i. e. , the infrared region), the simple quarter-wavelength coating gives adequate results. The high refractive index material is added until the reflectance reaches a maximum,

in other words, until a quarter -wavelength thickness is added. For further increase of reflectance,

additional quarter-wavelength film pairs must be added in the same sequence.

The reflectance of such a mirror depends on the refractive index ratio of the dielectric materials,

the numbers of layers, the reflectance of the metallic thin film, and the absorption in the dielectric

layers. For optimum performance, one desires coating materials with the lowest possible absorption and the highest possible refractive index ratio. The refractive index ratio which one can achieve

depends on the availability of suitable coating materials. To keep the number of dielectric layers to a minimum and hence minimize the effect of absorption in the layers, the reflectance of the metallic

thin film should be as high as possible.

* Table 1 lists the initial designs selected for the experimental coatings. Ideally one coating design should be usable for all three wavelengths with the appropriate adjustments of optical thick- nesses. The effect of absorption in the dielectric layers has not been taken into account.

3. Experimental

The experimental investigations required the development of optimum cleaning and deposition

techniques. Several experiments were conducted to develop the optimum substrate surfaces for the

mirror coatings. The candidate coating designs were deposited on small scale (1.52 inch diameter)

molybdenum substrates. The substrate surfaces were characterized for surface figure and finish

before coating deposition. Established deposition techniques and parameters were utilized for all coating designs whenever availailable.

Molybdenum was chosen as a consequence of its thermal and mechanical properties. These

properties are summarized in Table 2. The molybdenum used for the mirror substrates on this program was ASME 7805. This is a vacuum arc cast, low carbon content molybdenum readily available

in bar stock from American Metal Climax Corporation.

* Tables 1, 3, 4, 5, 6, 7, 8, 9, and 10 are shown following the end of the text.

-386-
Table 2. Physical properties of Molybdenum

3 Density 10. 2 gm/cm

Melting point 2895°C

Specific heat 0.27J/gm°C

Thermal conductivity 1.3 J/sec cm°C

2 Thermal diffusivity 0.52 cm /sec

Thermal expansion 4. 9 x 10 ^/°C

The quality of the surface is the most important property of the substrate since it is here that

the coating-substrate interaction occurs. It is desirable to have surfaces as smooth as possible (i. e,

, free of scratches and digs and with a low rms roughness). These defects are replicated by the

films or propagated into the coatings. That is, each imperfection in the substrate is reproduced in

each layer that is deposited on top of it which can affect both the optical and mechanical properties

of the films. Surface smoothness is also important from the standpoint that it improves film adherence, decreases absorption, and minimizes light scatter.

Two polishing techniques have been developed in our optical facility which yield well figured optical surface on molybdenum mirrors. The first, based on conventional optical polishing methods

(fresh feed polishing), employs a pitch lap, Al^O^ abrasive and water as a polishing vehicle. We

have achieved excellent results employing this technique. The second technique is continuous feed

and is intermediate between the normal method of applying abrasive during the polishing process and

the bowl feed technique.

I

i

The procedure that gave the best combination of smoothness and surface figure was the fresh

feed technique using 0. 3 pm alumina and water as the polishing vehicle on a pitch lap. The basic polishing procedure used to polish the molybdenvim substrates is summarized as follows. In the grinding operation the substrates were ground flat on a cast iron tool with a slurry of #320 silicon carbide

and water followed with an intermediate grind with #225 aluminum oxide and water and a final grind

with #95 aluminum oxide and water. Typical grinding time required approximately 10 hours with a

10 pound overarm weight. The intermediate and final grind, typically, took one hour each using a 10

pound overarm weight. In all cases the polishing was achieved using the fresh feed technique to apply

the polishing slurry to the lap. The polishing lap was of medium-hard pitch and the polishing slurry was Linde A (0. 3 |-im •'^2*^3^ ^^'^ water. Typical polishing time required approximately 40 hours with an overarm weight of 5 pounds with an upper spindle speed of 40 rpm and a lower spindle speed of 30

rpm. The substrates were polished until optimum surface finish was obtained as judged by Nomarski

examination.

Clean substrate surfaces are a prerequisite for successful coatings. The slightest amount of

contamination can cause an immense amount of harm in the coating deposition process as well as an increase in absorption and a decrease in surface adhesion. Traditionally, cleaning has been an art

and there is a great diversity of opinion on what constitutes a "good" procedure for cleaning the substrates prior to coating them. A quantitative evaluation of the various cleaning methods is difficult

because slight differences in procedures of apparatus can influence the results.

There are innumerable cleaning procedures which vary from one laboratory to another. Since

cleaning procedures and philosophies vary widely, it is not expected that the views expressed herein

will be met with universal agreement. However, it is safe to say in general, that cleaning procedures should be kept as simple as possible. With this in mind, the following procedure has been developed for cleaning metal mirror substrates.

1. Remove gross contajninants from the as-received substrates by soaking in acetone or xylene.

Five to ten minutes of ultrasonic agitation will increase the rate of contaminant removal and in- sure a more thorough cleaning.

-387-
2. Wash with warm tap water and Liqui-nox detergent. Scrubbing the polished surface must be

avoided, however, the cleaning can be assisted by ultrasonic agitation.

3. The substrate should be thoroughly rinsed in distilled water after detergent cleaning.

4. Final rinse with a reagent grade alcohol.

5. Final drying of the substrate is accomplished by inserting it in a vapor degreaser working with

isopropyl alcohol. The substrate should remain in the vapor degreaser until it has reached the

temperature of the vapor. The residual alcohol on the surface is immediately evaporated when

the substrate is withdrawn and it is ready for loading into the coating chamber. It is a good

practice to load the substrate into the chamber immediately after the cleaning process. If storage cannot be avoided, dust-free container with a lid or a desicator should be utilized.

Clean lint-free gloves should be worn during the placement of the substrate into the coating

dhamber.

6. Before closing the chamber, the surfaces must be checked for dust particles. If dust is evident,

a small air syringe to blow off the particles can be used.

All coating depositions required for this program were carried out in a commercial vacuum system (Balzers - 710) which is equipped with an oil diffusion pump and a liquid nitrogen trap. This system is capable of routinely maintaining pressures of the order of 10 ^ Torr and is equipped with a substrate heater and a thin film monitor to control deposition rate and film thickness. Proper control of the thickness of each layer is afforded by observing the reflectance at the control wavelength

of a suitably positioned monitor plate in the coating chamber and stopping deposition when the reflectance reaches a predetermined level. In all cases, care is taken to ensure the uniformity of the

thickness of the layers. In order to obtain uniform layers, the window substrates are rotated above

the evaporation source during the deposition process.

Deposition parameters utilized in this work are summarized for each material in Table 3. Coating materials employed in this work are listed and identified as to source and nominal purity in Table

4. No attempt was made to further purify any of these materials.

4, Experimental results

Ten different dielectric enhanced mirror designs were evaluated for each of the three laser

wavelengths. All of the mirror coatings were fabricated on 1. 52 inch diameter molybdenum substrates. Each mirror substrate was polished as outlined in Section 3, and all of the coatings were

deposited using the paraments listed in Table 3 of Section 3. Each mirror coating was evaluated for

peak reflectance and absorption at the design wavelength. Environmental evaluation included tests according to Military Specifications MILi-M-13508C for durability, adhesion, and humidity resistance.

The experimental results for the mirror coatings are presented in Tables 5-10.

-388-
5. Acknowledgments

The author acknowledtes the assistance of M. Evangelista for the deposition of the mirror coatings cind A. Klugman for the polishing of the mirror substrates. The absorption measurements were

taken by F. Towns with the help of P. Kraatz and J. Rowe.

6. References

[l] Young, L. , "Multilayer Reflection Coatings on a Metal Mirror", Appl. Opt., Vol. 2, p. 445

(1964).

[2] Parks, K. C, "The Extreme Values of Reflectivity and the Conditions for Zero Reflection from

Thin Dielectric Films on Metals", Appl. Opt., Vol, 3, p. 877 (1964).

-389-
Table 1. Theoretical coating designs

Theoretical reflectance

2. 8 i-im 3. 8 Mm 6. 3 Mm

1. Ag +

3 (ThF /ZnS)

4

0. 9995 0. 9995 0. 9995

2. Ag +

3 (ThF /ZnSe)

4

0. 9997 0. 9997 0. 9997

3. Ag +

,2 (SrF^/ZnS) 0. 9993 0. 9993 0. 9993

4. Ag +

2 (SrF^/ZnSe) 0. 9995 0. 9995 0. 9995

5. Ag + (PbF^/ZnS)^ 0. 9988 0. 9989 0. 9989

6. Ag + (Al^O^/Si)^ 0. 9996 0. 9997 0. 9997

7. Ag + (SiO/Si)^ 0. 9996 0. 9996 0. 9997

8. Ag + (ThF /Si)^

4

0. 9998 0. 9998 0. 9998

9 Ag + (MgO/Si)^ 0. 9997 0. 9997 0. 9997

10. Ag + (PbF^/ZnSe)^ 0. 9993 0. 9993 0. 9993

* Dielectric layers are quarterwave optical thickness at the design wavelength.

* Ag layer is approximately loooA thickness.

-390-
Table 3. Deposition parameters

Method of

Evaporation

E-Beam

E-Beam

E-Beam

E-Beam

Ta Boat

Mo Boat

E-Beam

Ta Boat

Ta Boat

Ta Boat

Material

A1^03

MgO

PbF^

Si

SiO

SrF^

ThF,

4

ZnS

ZnSe

Ag

Substrate

Temperature

ZOO°C

200°C

200° C

200°C

200° C

200° C

200°C

150°C

150°C

25°C

Deposition

Pressure (Torr)

5 X 10

-5

6 X 10

7 X 10

-6

2 X 10

1 X 10

8x10

4x10

-6

-6

8x10

4 X 10

-6

5 X 10

Deposition

Rate

~600A/Min

~ 1200A/Min

~ 1800A/Min

~3000A/Min

~3000A/Min

~ 1800A/Min

~ IBOOA/Min

~ 1800A/Min

~ 1800A/Min

~ lOOoA/Min

-391-
Table 4. Coating materials

Ag Shot, 99. 9999% Purity, American Drug and Chemical Company, Los Angeles, CA

Al^O^ Random Chunks of UV-Grade Sapphire, Union Carbide Corporation, San Diego, CA

MgO Hot -Pressed Tablets, 99. 95% Purity, Balzers High Vacuum Corp. , Santa Ana, CA

PbF^ Fused Granules, 99. 99% Purity, Balzers High Vacuum Corp. , Santa Ana, CA

Si Random Chunks, 99.999% Purity, Balzers High Vacuum Corp. , Santa Ana, CA

SiO Linde Select Grade, R. D. Mathis Company, Liong Beach, CA

SrF^ Random Chunks, Optical Grade, EMCO Sales, Anaheim, CA

ThF^ Fused Granules, 99.9% Purity, Balzers, High Vacuum Corp., Santa Ana, CA

ZnS Hot-Pressed Tablets, 99% Purity, Balzers High Vacuum Corp., Santa Ana, CA

ZnSe Granules, 99. 99% Purity, Balzers High Vacuum Corp. , Santa Ana, CA

-392-
Table 5. Experimental results for 2.8 |Jm mirrors

Absorption

Sample no. Coating design Peak reflectance * A Std. dev

170 Mo T Ag + (ThF /ZnS)^

4

A O O C U. Vo5 11 . 5 0 X 1 u Z . 7 1 7o

lie M.O Ag + (ThF /ZnSe)^

4

n n o Q T

C. . X 1 u

7 A C <77 3. 95 70

i 0 J JVIO J.T Ag + ( AX O /Si)^

c i

A A QQ 11 . r> c. V 3 X

10-^ 7 A 1 07 J . U 1 /o

100 IVIO J.T Ag + (SiO/Si)^ nU. Q oo 777 1i . OA X

10-2 ^0.114 1 0770

loo IvlO T Ag +

2 A n O A U . 7oU 6 . li 3 X

10-2

i u 0 . 0 U 70

173 Mo + Ag + (PbF^/ZnSe)^ 0. 996 4. 42 X 10 11. 15 %

179 Mo + Ag + (BaF^/ZnS)^ 0. 950 5. 15 X 10-2

6. 65 %

181 Mo + Ag + (ZnS/Si)^ 0. 973 Z. 87 X

10-2 7.97 %

183 Mo + Ag + (ZnSe/Si)^ 0. 997 2. 76 X

10-2 2. 84 %

176 Mo + Ag + (BaF^/ZnSe)^ 0. 986 1. 42 X

10-2 2. 43 %

* Double-Reflection Absolute Reflectometer (± 0.001)

** HF Laser Calorimeter

-393-
Table 6. Experimental results for 3. 8 um mirrors

Absorption

Sample no. Coating design Peak reflectance * A Std. dev.

107 Mo + Ag + (ThF /ZnS)^

4

0. 998 1. 68 X 10-2 3. 93 %

100 Mo + Au (ThF /ZnS)^

4

0. 996 3. 18 X

10-2 3. 67 %

105 Mo + Ag + (ThF^/ZnSe)^ 0. 999 6. 31 X

10-^ 1. 45 %

018 Mo + Au + (ThF /ZnSe)^

4

0. 999 6. 47 X

10-^ 4. 06 %

103 Mo + Ag + (AW^/Si)^ 0. 999 9. 99 X

10-^ 1. 79 %

113 Mo + Ag + (SiO/Si)^ 0. 998 9. 64 10-^ 1. 37 %

125 Mo +

2 Ag + (ThF /Si)

4

0. 998 1. 34 X 10

^ 2. 60 %

132 Mo + Ag + (SiO/ZnSe)^ 0. 993 6 15 X

10-2 6. 14 7o

133 Mo + Ag + (PbF^/ZnS)^ 0. 998 1 77 X

10-2 7. 04 %

111 Mo + Ag + (P^F /ZnSe)^

b

0. 998 1 02 X

10-2 1. 51 %

127 Mo + Ag + (BaF^/ZnS)^ 0. 997 2 22 X 10-2 1. 45 %

1.29 Mo + Ag + (BaF^/ZnSe)^ Failed

before test

* Double -Reflection absolute reflectometer (± 0.001)

** DF laser calorimeter

-39A-
Table 7. Experimental results for 5. 3 nm mirrors

Absorption

Sample no. Coating design Peak reflectance ^ A Std. dev.

001 Mo + Ag + (ThF /ZnS)^

4

0. 999 9. 11 X lO''^ 3. 98 %

003 Mo + Ag + (ThF /ZnSe)^

4

0. 999 6. 40 X lO"^ 6. 35 %

005 Mo + Ag + (SrF /ZnS)^

c

0. 995 1.43 X lO"'' 3. 04 %

007 Mo + Ag + (SrF /ZnSe)^

2

0. 992 3. 13 X 10'^ 3.29 %

010 Mo + Ag + (PbF^/ZnS)^ 0. 987 4. 11 X lO"'^ 9. 02 %

on Mo + Ag + (Ai^Oj/Si)^ 0. 996 1.72 X lO""' 3.42 %

013 Mo + Ag + (SiO/Si)^ 0. 999 1.81 X lO'^ 5. 38 %

136 Mo + Ag + (ThF /Si)^

4

0. 997 1. 96 X lO"'^ 5. 15 %

019 Mo + Ag + (MgO/Si)^ Coating failed during deposition

021 Mo + Ag + (PbF^/ZnSe)^ 0. 990 1. 14 X

10'^ 2. 86 %

* Double -Reflection absolute reflectometer (± 0.001)

** CO laser calorimeter

-395-
Table 8. Environmental tests -2.8 Mm mirror coating

Coating design Durability Adherence

Mo + + (ThF /ZnS)^

4 P p

Mo + Ag + (ThF /ZnSe)^

4 P X Mo + Ag + (Ai O /Si)^

2 3 P P

Mo + Ag + (SiO/Si)^ P P

Mo + Ae + (PbF^/ZnS)^ p P

Mo + Ag + (PbF^/ZnSe)^ P P

Mo + Ag + (BaF^/ZnS)^ P F

Mo + Ag + (ZnS/Si)^ P- P

Mo + Ag + (ZnSe/Si)^ P P

Mo + Ag + (BaF^/ZnSe)^ P P

Durability - 50 Rubs with cheesecloth 1 pound force.

Adherence - Scotch tape MIL - 13508 B.

Humidity - 95% at 120° F for 24 hours.

P - Passed

F - Failed

-396-
Table 9. En vir onxnentcLl tests - 3,8 [itn mirror coatings

Coating design Adher enc e Durability Humidity

Mo + Ag + (ThF /ZnS)^

4 P i-" F

Mo + Ag + (ThF /ZnSe)^

4 P D P

Mo + Ag + (Ai O /Si)^

2 3 P f F

Mo + Ag + (SiO/Si)^ P F

Mo + 6 + (ThF /Si)^

4

r P P

Mo + Ag + (SiO/ZnSe)^ F F F

Mo + Ag + (PbF^/ZnS)^ P P P

Mo + Ag + (PbF^/ZnSe)^ P P P

Mo + Ag + (BaF^/ZnS)^ P P F

Mo + Ag + (BaF^/ZnSe)^ * *

Durability - 50 Rubs with cheesecloth 1 pound force.

Adherence - Scotch tape MLL - 13508 B.

. H\amidity - 95% at 120° F for 24 hours.

P - Passed

F - Failed

* - Failed before test

-397-
Table 10. Environmental tests -5.3 urn mirror coatings

Coating design Adherence Durability Humidity

Mo + Ag + (ThF /ZnS)^

4 P P P

Mo + Ag + (ThF^/ZnSe)"' P P F

Mo + Ag + (SrF^/ZnS)^ F F F

Mo + Ag + (SrF^/ZnSe)^ F F F

Mo + Ag + (PbF^/ZnS)^ P P P

Mo + Ag + (AX^Oj/Si)^ P P P

Mo + Ag + (SiO/Si)^ P P P

Mo + Ag + (ThF /Si)^

4 F P F

Mo + Ag + (MgO/Si)^ * * *

Mo + Ag + (PbF^/ZnSe)^ P P P

Durability - 50 Rubs with cheesecloth 1 pound force.

Adherence - Scotch tap« MIL - 13508 B.

Humidity - 95% at 120° for 24 hours.

P - Passed

F - Failed

* - Failed during coating deposition

-398-
Temperature and Concentration Dependence of Laser Bulk Damage

to Neodymium Glass*

J. G. Sliney Jr. and L. G. DeShazer

Department of Electrical Engineering, University of Southern California

Los Angeles, CA 90007

Bulk damage thresholds of Nd doped materials were measured

using a Q-switched Nd:YAG laser with a single transverse and

longitudinal mode having 7-ns pulsewidth. Damage was studied in

a series of glasses of differing Nd concentrations: 0% Nd (ED4),

1% Nd (ED2.1), 2% Nd (ED2.2) and 3% Nd (ED2.3). At room temperature, the threshold decreased linearly by eleven percent as the

Nd concentration increased from 0 to 3%. This variation could be

explained by any one of three processes- single-photon absorption

of Nd from the excited state I n / 2 ^3/2' two-photon absorption

of Nd from the ground state ^19/2 ^'^7/2 ^"^^ absorption by impurities such as Sm'^"'' which typically accompany Nd doping. However, we believe the single-photon absorption to be applicable because when

the glass temperature is lowered to 110°K, the damage thresholds for

all concentrations are nearly equal. Bulk damage thresholds wera

also measured for a variety of Nd doped crystals. Nd:LiYF4 had the

highest damage threshold with Nd:La2Be205 (BeL) at 55% of LiYF4's

threshold, TAG at 53% and GGG at 30%.

Key words: Damage temperature dependence; damage thresholds; GGG;

La2Be205; LiYF4; Nd glass; YAG

.

1 . Introduction

The laser damage resistance was measured for seven Nd laser materials relevant to

operation in a 10-ns Q-switch laser system. We used a Nd:YAG laser amplified by Nd:ED2

glass to damage crystal and glass cubes prepared from state-of-the-art material of Nd

doped YAG, BeL, YVO4, GGG, YLF and the ED2 series of glasses.

Prior to this investigation, published reports on 1 as er - indue ed bulk damage thresholds of Nd laser crystals have been surprisingly meager. The pertinent values obtained from the scientific literature are listed in Table 1. Since laser-induced damage

thresholds depend on the laser pulse duration for the usual damage mechanisms, damage

threshold values for different materials can be compared only at nearly equal pulsewidths of the damaging laser. The pulsewidth chosen for Table 1 is 10-ns which pertains

to the usual Q-switched Nd laser.

Table 1. Comparison of Previously-published Laser Damage

Thresholds under Q-switched 1.06 ym Irradiation

Mat erial Damage Threshold Damage Threshold Ref

.

Front Surface Bulk (GW/cm^)

(GW/cm^)

Nd : YAG 0.8 - 5.3 3.4 - 8.2 Cl]l

0. 9 [2]

Nd:YA103 1.1 - 2.6 [1]

Nd : CaW04 1.0 - 3.0 1.0 - 5.0 [1]

Nd : BeL 1 . 3 [2]

A majority of the reported values of damage threshold were determined in an investigation on 1.06 ym laser damage to optical materials at the Hirst Research Centre,

General Electric Co. Ltd. [l]. The GEC group used an el ec tro - op t i cal ly Q-switched Nd

:

YAG laser operating in the TEMqq mode up to 10 mj in 10 ns. The range of threshold

values reported in reference [IJfor the surface damage resulted from the surface damage

thresholds being markedly affected by surface treatment and polish. The laser used in

reference [2] was also a single-mode Nd : YAG laser, e 1 ec tro - op t i ca 1 ly Q-switched with a 10-ns pulse. Only damage thresholds for the surface were measured by R. Byer and R. Fleming [2].

* Research supported by NASA contract number NAS 5-23698

1. Figures in brackets indicate the literature references at the end of the paper.

-399-
2 . Experiment

2.1 Laser System Performance

A well-defined pulsed Nd:YAG oscillator was the source of the laser pulse which was amplified by a Nd:glass amplifier as shown in figure 1. The Nd:YAG oscillator was Q-switched passively by the Eastman

Kodak dye 14015 (bis 4-dimethylaminodithiobenzil nickel). The use of a Mendenhall aperture of radius

0.5 mm and 27 cm cavity length gave a Fresnel number of 0.86 and a single transverse TEM mode. The

amplifier used a Brewster-faced Nd:glass rod (1.11 x 15.24 cm) pumped by four flashlamps m a quadruply

elliptical gold-coated cavity. Two Glan-Kappa calcite polarizers were used as a variable attenuator.

The pertinent operating parameters for the laser system are listed in Table 2. When the amplifier was double passed, the energy output was 6 mj.

Table 2. Operational Characteristics of Oscillator and Single-passed Amplifier

Parameter Oscillator

(Nd:YAG)

Amplifier

(Nd:glass)

Energy

Pulsewidth

Peak power (unfocussed)

Beam spot size

Peak energy density (unfocussed)

Peak power density (unfocussed)

Peak power density (focussed)

2 mJ (±5%)

4.6-7 ns 0.44 MW

0. 5 mm

0.26 J/cm^

106 MlV/cm^

44 GW/cm^

4 mJ 5%)

4.6-7 ns 0.87 MW

1 mm

0.127 J/cm2

51.9 MW/cm^

347 GW/cm^

In order to measure accurately the damage thresholds, three beam quantities are needed: the total

energy E, the temporal pulsewidth At and the focussed spot size w. When the laser pulse is well approximated by a Gaussian intensity distribution, both in space and time, as is our case, the peak power density Pq is

Pq = 0.598 E/(w^At)

The total energy E was measured using a ballistic thermopile (Hadron model 9) positioned after the beamsplitter and before the lens as shown by the dotted lines in figure 1. The pulsewidth At is defined as the full width a half maximum (FWHM) and was measured using fast photodiode with Tektronic 519 oscilloscope. Oscillograms of laser pulses from the oscillator and amplifier are given in figure 2 showing

their smooth temporal profile and near Gaussian shape (within 15%).

The spatial intensity profile of the oscillator and the oscillator-amplifier system was measured

using a television monitor system with a vidicon camera (Javelin NT-7310), a TV receiver (Panasonic

TR910MN) and a dual-beam oscilloscope (Tektronic 555). The oscilloscope traces in figure 3 are for a single laser pulse monitored by the TV system at a distance of 135 cm from the oscillator (2.9 Rayleigh

distances). As determined by the TV monitor, the spatial intensity profile was very close to the Gaussian distribution lQexp(-2r^/w^) . The system was calibrated by a slit mask where the flat top is 1.1 mm; the slanted portion of the calibration trace is the penumbra of the shadow due to the extended size of

the light source illuminating the slit.

A plano-convex lens (25.4 mm focal length) was used to concentrate the power sufficient to cause

bulk damage without causing surface damage at the entrance surface. The determination of the focussed

spot size is subject to the most inaccuracy arising from lens aberrations. For the diffraction limited

where f is the focal length of the lens and Wq is the spot size incident to the lens. For Wq = 1 mm, w = 8.6 ym. However, when NaCl was damaged at 1.06 ym using our system, the value of 40 pm was found

to be the approximate value for the spot size by assuming the literature value of the damage thresholdtS]

of NaCl. The major cause of the difference between the diffraction-limited calculation (8.6 ym) and

the spot size (40 ym) estimated by the NaCl damage is spherical aberration. Since our lens had a shape

factor of +1.0, the lateral spherical aberration was calculated to increase the spot size from 8.6 ym to

9.3 ym, but the longitudinal spherical aberration was calculated to be 32 ym. The effect of the longitudinal aberration is to spread out the energy of the laser pulse along the optical path of the focussing

lens, so that the peak intensity of the pulse is reduced to the intensity level one would measure if the

lateral spot size had been 40 ym.

2 . 2 Low Temperature Damage Setup

For the damage measurements made at 110°K, the test samples were mounted on a copper cold finger

and affixed with a highly conductive silicone heat sink compound (Dow Corning 340) as shown in figure 4. The other end of the cold finger was immersed in a bath of liquid nitrogen. The 19x19x24 cm chamber

enclosing the test samples was flushed with dry nitrogen gas. The temperature was measured by a thermocouple (Omega MCJ-T copper-constantan) which was attached to the cold finger next to the sample. After

the cooling down period, the samples remained at 110 ± 5°K for a period of one hour or more.

-400-
2.3 Damage Detection Techniques

In every case of bulk damage, the laser-induced scatter method and laser-induced change in transmittance was used to detect damage. For the case of induced scatter, evidence of damage was manifested

by scattering of probe He-Ne laser light. The beam from the He-Ne laser was simultaneously used as an alignment laser for the entire laser system so it was collinear with the Nd laser pulse. The transmittance method sampled the pulse before and after the test sample so that comparison of these incident and

transmitted pulses would show the occurrance of laser damage by a sharp reduction in power at the pulse

peak for the transmitted pulse. Examples of the three pertinent cases above, below and at threshold for

Nd:YAG are shown in figure 5. Notice that the transmittance method does not clearly mark the damage

threshold since the transmitted pulse is imperceptibly distorted at threshold. Only a small portion of

the transmitted beam is blocked by a small damage site near threshold. However the scattering method

clearly delineated this threshold by the scatter from the damage site.

3. Neodymium Glass Damage

3.1 Concentration and Temperature Variations

Measurement of laser- induced bulk damage for ED2 laser glass showed that they were slightly concen- tration dependent. The samples were obtained from Lawrence Livermore Laboratories, and were of square

cross section 1 cm on a side and of several different thicknesses ranging from 5 mm to 10 mm. The con- centrations of Nd in the ED2 glass host include 1, 2 and 3% Nd which are designated ED2.1, ED2.2 and

ED2.3, respectively.. In addition, the threshold of undoped ED4 glass was measured as the 0% concentration limit. At room temperature the threshold decreased linearly by eleven percent as the neodymium

concentration increased from 0 to 3%. This decrease is illustrated graphically in figure 6 in the lower

set of data points which are normalized to the ED4 bulk damage threshold at room temperature. Also, the

laser induced bulk damage thresholds of ED2 laser glass were measured for the several concentrations at

110°K. At this low temperature, the damage thresholds of ED2.1, ED2.2 and ED2.3 became equal at a value

20% above the room temperature threshold of ED4 glass as shown in figure 6 in the upper set of data

points. The observed temperature variation of the thresholds as well as the variation of threshold with

concentration could be explained by three processes: single-photon absorption by Nd from the excited

state '*I|i^2 '^^3/2' two-photon absorption by Nd from the ground state ^^9/2 ^^1/2 absorption

by impurities such as Sm^"*" which typically accompany Nd-^'*' doping.

3.2 Two Photon Process

In order for absorption to take place by a two-photon process, the Nd^""" would have to absorb two

1 . 06-pm photons starting from the '^19/2 ground state and terminate at the excited level of Nd-'"''. The previous work of Penzkofer and Kaiser [4] and Milam [5] does not rule out the possibility of the two photon process being operative. However, the temperature effect leads to the conclusion that the two photon process is not responsible, because if the two photon process were, then lowering the temperature

from 300 to 110°K would not hinder two-photon absorption because it starts at the ground level. It should be pointed out that Penzkofer and Kaiser did light transmission experiments at 3000K and at 80°K

and did not observe any change in the transmission of light at 1.06 ym. Assuming a Boltzmann distribution, at 300°K nearly 85% of the Nd ions are in the three lowest levels of the '^19/2 ground state, but

at SO^K the percentage has increased to 99%. It would seem that a two-photon process in Nd glasses

should proceed even better at lower temperatures, because of the greater abundance of Nd ions in the

ground state compared with the number at room temperature.

3.3 Single Photon Process

In order for absorption to take place by a single photon process, the Nd-^"^ would have to absorb one 1.06 ym photon starting at the '^Iii/2 excited level and terminating on the ^'^1/2 excited level. The

population of the ^lii/2 level of Nd in ED2.3 laser glass was calculated at 300 and 110°K; the other

spectroscopic parameters were taken from the work on Nd doped ED2 laser glass by DeShazer et^ al_. [6].

For the low temperature it was assumed that in the case of Nd doped ED2 glass that the line centers and

linewidths do not change with temperature, which is supported by the work of Mann and DeShazer [7]. The

absorption lines for the ^^n/j " ^^^3/2 transitions of Nd in ED2.3 laser glass have been calculated

theoretically and plotted for 300°K and is shown in figure 7. The smooth curve above the individual

absoprtion lines in this figure represents the envelope of all the lines. For the Nd:YAG laser line

(1.0642 vim] at 9397 cm"^, the absorption coefficient at 300°K is 3.42 x 10"^ cm"^ but at 110°K it is

4.78 X 10" cm"-'^ which is a reduction of seven orders of magnitude. This supports the belief that the

single photon absorption process is responsible for initiating a portion of the laser-induced bulk damage in Nd doped ED2 laser glasses.

4. Bulk Damage of Neodymium Crystal Hosts

The laser- induced bulk damage thresholds of five Nd crystal hosts are listed in Table 3 along with

Nd:ED2.3 glass and undoped NaCl. The 1-cm cube sample of the 1.2% Nd^"^ in lanthanium beryllate,

La2Be205 (symbolized BeL) , was obtained from Allied Chemical Co. Since the polarization of laser emis-

-401-
sion from Nci:BeL is parallel to the Y-axis, the laser damage measurements were made with the incident

laser pulse polarized parallel to the Y-axis. The bulk damage threshold for the Y-polarization was measured to be 4% higher than the threshold measured for Nd:YAG as shown in Table 3 . Morphology of

bulk damage looking down the path of the laser is shown in figure 8. The damage pattern is a six-pointed star in contrast to the damage pattern of Nd:YAG which is a four-pointed star as shown in figure 9. One interpretation of the pattern is that the fracture planes are due to the outward propagation of

phonons as result of the damage process [8]. For the case of Nd:BeL with propagation in the X-direction

the angles between the fracture planes measured 54°, 44° and 82°. These angles are undoubtedly a con- sequence of the monoclinic crystal structure of BeL (space group C2/c) but as yet we have not made a crystallographic directional analysis of these fractures.

Tables. Bulk Laser Damage Thresholds under Q-switched 1.06 pm Irradiation

(4.65 ns pulsewidth FWHM]

Material Power Density Electric Field Power Density

(GW/cm2) (MV/cm) Relative to Nd:YAG

Nd:YLF 18.9 2.21 1.87

Nd:ED2.3 glass 16.9 2.2 1.67

Nd:BeL (E//y) 10.5 1.4 1.04

Nd:YAG 10.1 1.45 1.00

Nd:BeL (E//z) 9.33 1.33 0.92

NaCl (undoped] 7.64 1.39 0.76

Nd : YVO4 <6.43 <1.14 <0.64

Nd : GGG 5.56 1 .08 0.55

The laser induced bulk damage threshold of Nd:YAG was measured to be third in rank after Nd:YLF and

Nd:BeL as shown in Table 3. However, Nd:YAG was less than 5% lower than Nd:BeL so it is still very

competitive. Examples of laser-induced bulk damage are shown in figure 9. The upper picture looks down

the path of the laser and the lower figure is looking from the side. The sample of Nd:YAG was obtained

from Crystal Products Department, Union Carbide Corporation.

The test sample of NdrYLF was donated by Hughes Research Laboratories, Malibu, CA. This Nd doped

host had the highest threshold, 18.9 GW/cm^, of any material tested including the Nd glasses. The Nd: YLF sample exhibited a large number of defects, mostly voids, which seemingly did not inhibit its damage

resistance. In contrast to Nd:YLF, which manifested the highest bulk damage thresholds despite the

obvious presence of scattering centers, Nd:YV04 placed a poor fourth among the Nd laser hosts. Apparently the iridium inclusions of the YVO4 host were more favorable to the initiation of bulk damage than were

the voids of the former. Allowing for the poor quality of the Nd:YV04 crystals available for these damage experiments, it is reasonable to expect that Nd:YV04 crystals of the same good optical quality as the

YAG and BeL used in this research will show a higher bulk damage threshold than that which is presently

being reported. Nd:GGG exhibited a low threshold, which came as a surprise because the crystal was of

the highest optical quality. It showed no scattering centers before damage, when it was probe with a HeNe laser. Neither did it show any strain patterns when viewed between crossed polarizers.

This research included laser-induced surface damage experiments on Nd doped crystals of YAG and BeL. The surface damage to Nd:BeL was conducted with the E-field of the laser parallel to the Y-axis of the

crystal which is the same polarization used in the Nd:BeL laser. Both laser-quality surfaces of the BeL cube were damaged; one surface had been AR coated and the other had not been AR coated. The techniques

previously used in reducing the bulk damage are not entirely applicable in the case of surface damage.

The He-Ne laser scattering that was readily applicable in detecting bulk damage sites was found to be

unreliable in the case of surface damage. Consequently the catastrophic break at the peak of the transmitted damage pulse has been adopted as the criterion for surface damage threshold. The values for the

surface damage thresholds of the two Nd crystals were both nearly 90% of their respective bulk damage

thresholds.

5 . Acknowl edgement s

One of the authors (LGD) acknowledges conversations with Roger Wood, GEC, and Professor Robert Byer,

Stanford, and both of the authors acknowledge the interest and support of Milton Birnbaum, Aerospace Corp.

Experimental assistance on the low temperature measurements by J. A. Wunderlich was appreciated.

6. References

[1] Wood, R.M., Taylor, R.T., and Rouse, R.L.,

Opt. and Laser Tech. 1_, 105 (1975). This

paper is summary of annual reports on "Laser

damage investigation" by Gen. Electric Co. Ltd. to UK Ministry of Defence, CVD Research

Project RP4-67 (1970-73).

[2] Byer, R., and Fleming, R., Stanford Univ. as reported in Jenssen, H.P., et al., J. Appl.

Phys. 47^, 1496 (1976).

Byer, R. , private communication.

-402-
[3] Wang, v., et^ al^. , Laser Induced Damage in

Optical Materials: 1975 , NBS Spec. Pub. No

435, 118 (1975).

Leung, K.M., et_ a/\_. , Laser Induced Damage

in Optical Materials: 1975 , NBS Spec. Pub.

No. 435, 107 (1975).

[4] Penzkofer, A., and Kaiser, W., Appl. Phys.

Lett. 21, 427 (1972).

[6] DeShazer, L.G., et_ al_. , "Spectroscopy of

Neodymium in Laser Glasses," USCEE Report 479,

Univ. Southern Calif., Los Angeles, CA (1974).

[7] Mann, M.M., and DeShazer, L.G., J. Appl. Phys.

41_, 2951 (1970).

[8] Draggoo, V.G., et^ a]_. , IEEE J. Quan. Elect.

QE-8 , 54 (1972).

[5] Milam, D., in Laser Program Annual Report , Lawrence Livermore Laboratory, Livermore,

CA, pp. 22-28 (1976).

7. Figures

Q-SW ITCHED ATTENUATOR

NdrYAG LASER (Calcite polarizers)

DETECTOR

TEST

SAMPlf

LENS (25 mm fl)

3 BEAM SPLITTER

\ Nd GLASS AMPLIFIER MIRROR

^^^^

PHOTO DIODE

OSCILLOSCOPE

Figure 1. Schematic of Nd:YAG Oscillator, Nd:glass Amplifier, Detectors and Optics

for Laser Damage Experiments

-403-
10 ns

Figure 2. Temporal Waveforms of the Output from a) Nd:YAG Oscillator and b) Nd:glass

Amplifier.

-404-
Figure 3. Transverse Intensity Profile of the Beam as Determined

by TV Monitor System. a) Vertical spatial profile,

b) Horizontal spatial profile at y = 0, and c) Radial

distance calibration by slit.

-405-
1.064/im

LASER

Figure 4. Schematic o£ Apparatus for Low Temperature Laser Damage Experiment.

-406-
10 ns

BELOW

THRESHOLD

ABOVE

-| \ r

/V A

-1 r

t I

'in 'trans

Figure 5. Distortion of Transmitted Laser Pulse by Bulk Laser Damage in Nd:YAG for

the Cases Above, At and Below Damage Threshold.

1.2

"5 1,1

roE

i_

o

1

- i.oO0.1

CO

1 2 3

PERCENT Nd CONCENTRATION

Figure 6. Laser Bulk Damage Threshold of ED2 Laser Glass as a Function of Concentration

and Temperature, a) Upper data points at 110°K, and b) Lower data points at 300OK.

-407-
-40G-
A

|-545

Figure 8. Nd:BeL Bulk Damage Looking Along the Optical Path of Laser Beam.

Figure 9. Nd:YAG Bulk Damage Morphology Looking a) Down the optical path and b) From

the side.

-409-
Direct Measurement of Inversion Density in Silicate and Phosphate Laser Glass

Joseph A. Abate, David C. Brown, Christopher Cromer, Stephen D. Jacobs, John Kelly, James Rinefierd

Laboratory for Laser Energetics

College of Engineering and Applied Science

University of Rochester

Rochester, New York 14623

The first direct measurements of the inversion density in ED-2, EV-2, LHG-7,

and Q-88 laser glasses are reported. The data presented qualitatively confirm

the validity of the Trenholme-Emmett Xe flashlamp Nd:glass pumping model

and its extension to phosphate compositions. Comparison of the measured results

with previously obtained analytical and computer code formulations of the

variation of inversion density with optical thickness are discussed.

Key words: Inversion density; Parasitic oscillation; Phosphate laser glass;

Silicate laser glass; Stored energy profiles.

Introduction

The calculations presently used to predict the onset of parasitic oscillations in laser amplifiers depend both on the energy storage profile of the active medium as well as the geometry of the

amplifier. An energy storage profile for silicate Nd laser glass has been reported by J. F. Holzrichter and T. R. Donich [1]. Their method consisted of measuring the total flashlamp light

absorption, i.e., stored energy plus heat, by silicate laser glass samples of varying optical

thickness. Corrections for fresnel reflections and bulk glass absorption were made to the total

absorption measurements, however they could not directly measure or correct for the heat due to the quantum defect. Instead they relied on computer code corrections generated by GENEFF [2], a J. B. Trenholme and J. L. Emmett code. The GENEFF corrections were used to scale the absorption

versus optical thickness data. The energy storage profile was obtained by taking the derivative

of the corrected absorption versus optical thickness curve. Certain questions arise because of this correction procedure. Some of the assumptions used in GENEFF have either not been explicitly tested or only tested in silicate laser glass. These include

the assumption that the quantum efficiency is independent of the wavelength of the absorbed light.

Also the model used in GENEFF did not take into account the time evolution of the flashlamp-glass

system, but rather used instantaneous or averaged values of the key parameters to make its

predictions. Finally the absorbtivity per ion has been determined to be a constant in silicate

laser glass independent of the wt-% doping up to 3% by W. Krupke [2] and no extension to other glass

compositions have been made before now. It was our hope that by directly measuring the stored

energy profiles, the measurement presented here will provide evidence to the validity of the GENEFF

assumptions

.

Experiment

A block diagram of the apparatus used is shown in figure 1. The light from a 19mm bore water

cooled Xe flashlamp was collimated by set of three pinholes and a fused quartz lens. The measured

divergence of the resulting beam is smaller than .5 milliradians. The flashlamps were driven at 7Kv by a single mesh pulse forming network (PEN), and typically fired every 20 seconds. A beam

splitter sent a portion of the collimated flashlamp light to a photo diode which was used by the

boxcar integrator to monitor the shot to shot variation in the flashlamp output. The collimated

flashlamp light struck a glass sample with an energy density typically of 40 mJ/cm^.

The glass samples used measured 1 x 1 x 16cm and were polished on all six sides. Three of the

large faces were painted with black paint to minimize the effect of reflections inside the sample.

A Wood's horn, filled with index matching fluid, was mounted as shown in figure 1, to minimize the back

reflection of the flashlamp light. Four glass compositions were investigated; one silicate, ED-2

produced by Owens-Illinois and three phosphate EV-2, Q-88 and LHG-7 produced by Owens-Illinois,

Kigre Inc. and Hoya Glass respectively. Two fiducial marks were placed on each sample to define the

distance from the front edge of the sample.

A precise determination of the neodymium doping level was required in order for it to be possible

to compare the stored energy profiles of the various glasses in terms of the optical thickness, i.e.

weight percent doping x distance into the sample. Wet chemical and x-ray fluorescence analyses,

performed by glass vendors and independent testing laboratories have been obtained to establish the

doping in weight percent of Nd203 for selected samples. Optical density measurements obtained with

a Cary 14 spectrometer, permitted the calibration of additional samples from these primary standards

(see figure 2). Glass density measurements (ASTM C69374) were subsequently used to generate plots

of glass density versus Nd203 weight percent doping, as shown in figure 3, so that Nd ^ ion concentration may be calculated from the following:

-410-
confirms the independence of Nd ^ ion absorption versus doping level for ED-2, shown previously by

W. Krupke [2]. Figure 3 also extends this result to the phosphates out to 4.8 wt%. This graphical

data was used to determine the Nd203 doping levels via optical density or physical density measurements in the four glass samples used in this experiment.

The 1.06m fluorescence detector consisted of a microscope objective which imaged a 2mm aperture

into the glass sample and as a result collected the fluorescence over an area .6mm in diameter. The

fluorescence passed through a 1.06y filter and then fell upon an InGaAsP photocathode photomultiplier

tube (PMT). The InGaAsP photocathode was chosen for its high quantum efficiency, typically four

to five percent at 1.06y. The entire detector assembly was capable of being moved under stepping

motor control over the entire length of the glass sample. The movement of the detector by the

stepping motor was calibrated and always checked with the known position of the two fiducial marks

on the glass sample.

The signal from the PMT was amplified and fed to the boxcar integrator which was gated on 300u

seconds after the flashlamp was triggered and remained on for 500p seconds. The 1.06y fluorescence

from 10 to 20 flashlamp shots were averaged for a given position on the glass. A typical experimental

run began with the 1.06y detector positioned just in front of the edge of the glass sample, i.e.,

closest to the flashlamp. The detector was then moved along the glass sample measuring the fluorescence

at greater and greater optical thickness. For a consistency check at the conclusion of a scan, the

detector was moved back to one of its earlier positions and the fluorescence intensity was remeasured.

In all cases the two measurements agreed well within the stated error.

In this experiment we have assumed that the stored energy density is strictly proportional to the 1.06u fluorescence intensity as measured by the photomultiplier. This assumption is valid because

the l.OGy fluorescence is the spontaneous emission corresponding to the ^f^^ii-''*^-] yi manifold transitions, and for all practical purposes, stimulated emission is absent in this experiment. This is expected to be the case due to the low energy flux incident on the samples, typically 40 mJ/cm^. As an experimental check on this assumption the observed fluorescence decay curves were checked and did

not show any shortening or distortion as a function of the voltage applied to the PFN or of position

in the glass sample. These effects would be expected if significant stimualted emission were

present [3].

The measure stored energy density as a function of optical thickness is shown in figure 4 for

all glasses measured. Each curve has been normalized to the fluorescence at the front edge of the

glass sample. To obtain this normalization, it was necessary to extrapolate the fluorescence to zero optical thickness. We assumed an exponential fall off of the fluorescence to be a good approximation over the first few millimeters of the sample. This can be seen in figure 5 in the top portions

of all curves. A least squares fit to an exponential was used to determine the value at zero optical

thickness. The representative error bars shown in figures 4 and 5 are one standard deviation.

From figure 4 it can be seen that in general the phosphates (EV-2, Q-88, LHG-7) have a steeper

fall off in their stored energy profiles than does the silicate ED-2, a result which has consequences

for parasitic oscillations. The non-exponential character of the energy storage profiles of all four

glasses is apparent from figure 5. The normalized shapes of the profiles of Q-88 and LHG-7 can be seen to be quite similar and differ most from that of ED-2.

This experiment also allowed. the measurement of the relative pumping efficiency of the mentioned

glasses as a function of optical thickness. Following the detailed scans of all the glasses, a measurement of the fluorescence intensity was made at a known optical thickness for each glass and

used to scale the data shown in figure 4. In figure 6 the relative stored energy density profiles

for the glasses measured are shown relative to that of ED-2. Q-88 was found to have a stored energy

density as high as 1.45 times that of ED-2 in the surface region. Both EV-2 and LHG-7 were found to have a stored energy density 1.2 times that of ED-2 near the surface. In general these differences

became less at optical thickness larger than 3 cm-wt%.

We will consider now previous predictions for the energy storage profile for ED-2. There have

been several analytic expressions proposed to represent this profile. The purpose of these expressions

is to aid the laser designer in the calculation of bulk and surface parasitic thresholds. In figure

7 we show our measured energy storage profile for ED-2 and the analytic expression used by Glaze et al.

Results

Comparison to Theory

[4],

1 (2)

(1 + .7708y)

1.25

-411-
Here is the normalized stored energy and y is the optical thickness (cm-wt%). There is substantial

disagreement in that equation (2) predicts a less steep profile than observed.

Recently Brown et al. [5] have suggested an expression

^ (1 + .7708y)^-^^^ '

for ED-2 in their study of parasitic oscillations in active mirror and disc amplifiers. Equation (2)

is compared to the measured profile in figure 8. It can be seen that equation (3) is the better

approximation of the energy storage profile for ED-2. In this same paper Brown et al. [5] discussed

a computer code (FLASH-STORE) which models the instantaneous intensity output of a Xe flashlamp, from

known operating conditions (FLASH) and then calculates the stored energy profile using measured

absorption data (STORE). The FLASH-STORE code is equivalent to the Lawrnece Livermore code

GENEFF. In figure 9 the measured values for the stored energy are shown along with the FLASH-STORE

predictions. The agreement is seen to be quite good.

Conclusion

The data presented here confirms the predictions of Brown et al . [5] that the phosphate laser

glasses should display a more rapid fall off in stored energy density than in silicate laser glass.

The higher stored energy density at the glass edge shown in figure 6 leads one to conclude that it

will be important to take individual glass compositions into account in future studies of laser

amplifier parasitics. The expression in equation 3 has been shown to give a very good approximation

for the energy storage profile ED-2.

Acknowledgment

The authors would like to thank Owens-Illinois, Kigre Inc., and Hoya Glass for providing us with

the glass samples used. We also would like to thank Owen Lewis for his helpful discussions during the course of this work. The support of the Laboratory for Laser Energetics at the University of

Rochester is gratefully acknowledged.

References

[1] J. F. Holzrichter and T. R. Donich, [3]

UID-16860, November (1973).

[2] W. F. Krupke, LLL Report UCLR-714, [4]

December (1973).

[5]

J. B. Trenholme, NRL Memorandum Report, 2480,

July (1974).

J. A. Glaze et al . , Appl. Optics 13, 2808 (1974).

D. C. Brown et al . , Appl. Optics, January (1978).

Figures

19nim ^FLASHLAMP

COLLIMATI^ PINHOLES "-^l*^ .-J---! -Jj-'v

PMT

HOUSING

I— —"—

PREAMP —

ICROSCOPE

OBJECTIVE =^

PHOTOOIODE

PFN

i^acn WOODS

^6cm ^1 HORN- 1

STRIPCHAR

DIGITAL VOLTMETE

TRIG

BOXCAR

4TEGRATER

T

It

3ER CONTROLLER

Figure 1 Block diagram of experimental apparatus.

-412-
Fig. 2. Glass density for various Nd203 doping Fig. 3. Optical absorption per ion for

levels. corrected dopings.

OPTICAL THICKNESS (cm-wt»/.) OPTICAL THICKNESS (cm - wW.)

Fig. 4. Inversion profiles for silicate and p^g_ Non-exponential nature of stored

phosphate laser glasses. ' energy deposition.

-413-
OH 1 1 1 , r

0 12 3 4 5 OPTICAL THICKNESS (cm - wfh)

Fig. 6. Relative stored energy density profiles

for silicate and phosphate laser glasses.

0 T 1 1 1 1 1 1 1

r

1 1 1

1 0 ID 2.0 aO 40 so 6.0 T-OPTICAL THICKNESS (cm - wt°/>)

Fig. 8. Inversion data for ED-2 and the prediction of Brown et al. [4].

0-1 1—1

,—,,—,—I—,—,—,—,—

,

0 10 20 aO 40 50 6.0 7-OPTICAL THICKNESS (cm - wl"/o)

Fig. 7. Inversion data for ED-2 and the prediction of Glaze et al . [3]

.

0-^ 1 1 T

1 1 1 ' ' 1 1 1 ' 0 10 20 aO 4.0 5.0 60 7-OPTICAL THICKNESS (cm - wf/o)

Fig. 9. Inversion data for ED-2 and the prediction of FLASH-STORE [4].

-414-
COMMENTS ON PAPER BV ABATE, BROWN, CROMER, JACOBS. KELLY, LEWIS. ANV RINEFIERP

Ailde. {jiom polwti, od claAA.{,iccvtLon, thzn.z woi no diicuM-ion o{j this papzu.

-415-
FIGURES OF MERIT AND CORRELATIONS OF PHYSICAL AND OPTICAL PROPERTIES IN LASER GLASSES

David C. Brown, Stephen D. Jacobs, Joseph A. Abate, Owen Lewis, James Rinefierd

Laboratory for Laser Energetics

University of Rochester

Rochester, New York 14623

We propose a figure of merit which suggests that phosphate laser glasses

have advantages over silicate compositions in that they exhibit a lower

nonlinear refractive index, higher specific gain, lower thermo-optic distortion

and less sensitivity to stress induced birefringence. Disadvantages of the

phosphates are their tendency to heat to a greater extent under pumping, a reduced capacity to dissipate this heat, softness, high thermal expansion,

low mechanical strength and reduced thermal shock resistance. From physicaloptical properties correlations for a broader sampling of 120 commercially

available optical glasses, we infer that one might expect lower index glasses

to have poorer physical properties than higher index ones, based in part on the following:

1. there is a strong linear decrease in glass refractive index with

glass density.

2. appropriate functional dependences exist which relate a low glass

density to a small value of Young's modulus, and a small value of

Young's modulus to a reduced hardness and rupture strength.

Preliminary data for the fluorophosphates and their high densities suggest

that glasses from this composition group will be more durable than the

phosphates. This trend is not well understood in light of the low refractive

indices of these glasses.

Key words: Birefringence; f luorophosphate laser glass; glass density; nonlinear refractive index;

phosphate laser glass; physical-optical properties; rupture strength; silicate laser glass; thermooptic distortion; Young's modulus.

1. Introduction

Experiments in laser induced thermonuclear fusion are in progress at government, university, and

private research facilities. Two of the largest glass laser systems in the United States are Shiva, a twenty beam silicate glass system in operation at Lawrence Livermore Laboratories, and OMEGA, a phosphate glass system under construction at the Laboratory for Laser Energetics of the University of

Rochester. Fundamental to the design and performance of these and any future large, high peak power

glass systems is the availability of glass compositions whose properties have been optimized for specific

amplifier and staging designs.

The refractive index n^ and the nonlinear refractive index n2 of a glass set a limit on the peak

power that can be propagated within the material without causing a permanent degradation in glass

quality. This glass damage, caused by self-focussing of laser pulses, can be minimized for a given

power level by maximizing the ratio nj/n2 [1]^. A relationship among n^, the Abbe value v^, and n2

proposed by N. Boling, A. Glass, and A. Owyoung [2] has provided the glass industry with a guideline

for the development of compositions more suitably tailored for high power glass systems. The phosphates

commercially available today are the result of an optimization program which succeeded in raising the

value of n^/n2 by roughly 50 percent when compared to established silicate compositions.

Glass development programs have suffered from a lack of attention to changes in other physical

properties as compositions are optimized for low n2. Of necessity, thermal expansion and chemical

durability are adjusted so that promising compositions can be fabricated in useful sizes; but numerous

other glass properties go unmonitored due to a lack of guidance as to what is important for high power

glass systems. The purpose of this paper is to suggest a figure of merit for laser glasses which is sufficiently broad in scope to define desirable glass attributes for each kind of amplifier configuration required by these systems.

In what follows we present our figure of merit (FOM) formalism, justifying the importance of each

component. We then turn to a comparison of silicate and phosphate laser glass compositions, using

physical properties data generated at the Laboratory for Laser Energetics (LLE) and elsewhere for our

FOM calculations. Advantages offered by the phosphates for medium repetition rate (2 shots/hr.

)

systems like OMEGA are identified, as are the potential drawbacks.

We try to understand the observed trends in the silicate to phosphate transition by plotting

physical-optical properties correlations for a broad cross-section of commercially available optical

glasses. We conclude with some predictions for the newly developed fluorphosphate laser glasses, based

in part on the correlations presented in this paper.

1. Figures in brackets indicate the literature reference at the end of this paper.

-416-
2. Figure of Merit Formalism

2.1 Definition of Terms

The figure of merit we wish to consider may be expressed in its most general form as

n = II (J..)*^^ (1)

1=1 ^

where the are product factors and is a weighting factor (0<W^£l). The individual J factors that we identify as being important are:

J factor Origin

= n/n2 (minimize B integral)

= ct^ (maximize specific gain)

= (E/H)jj (maximize inversion/heat)

= (maximize quantum efficiency)

= k/pc (minimize thermal decay time constant)

Jg = pc (minimize induced temperature change)

^7 ^

I ^P^^^Pg T''^ (minimize average change in optical pathlength)

Jg = |AP^-APg|"'^ (minimize stress-induced birefringence)

The derivations of these J factors, discussed elsewhere [3], are based on laser rod geometries in the

short pulse regime. An abbreviated description of the significance of each factor is given below:

{n/n2)Jy The ratio of glass refractive index, n, to the nonlinear refractive index, n2, should

be maximized to minimize the accumulation of B integral in a glass laser system.

(a )J„: It is of interest to maximize the gain per unit length in a laser glass. Small

signal and saturated gain considerations lead to a desire to maximize the specific

gain coefficient, a^.

{E/H) Jy This is the ratio of flashlamp pump energy converted to useful inversion, E, over

that converted into unwanted heating of the laser glass, H, evaluated for zero

optical thickness.

(riQ)J4: To maximize the obtained stored energy density, E, one would like to maximize the

quantum efficiency, (assumed wavelength independent).

(k/pc)Jg: For a rod geometry and an assumed parabolic temperature profile (center to edge) a characteristic thermal decay time constant can be derived which gives a measure of

the time required for a glass to dissipate absorbed heat. To minimize this thermal

decay time constant we want to maximize the glass thermal diffusivity, defined as thermal conductivity, k, divided by the product of glass density, p, and specific

heat, c.

(pc)Jg: One wants to minimize the induced temperature change in a volume of glass due to heat input, H, by maximizing the product of glass density and specific heat.

|aP +aP I'^^J^: In the absence of stress, the change in optical pathlength in a rod of length L for

a given uniform temperature increase AT of the glass is given as

AP' = L [6 + a(n-l)] AT (2)

where B=dn/dT, or the temperature coefficient of refractive index; and a=linear

coefficient of thermal expansion. The quantity in brackets is referred to as the

thermo-optic coefficient. In the presence of stress caused by an assumed radial

temperature gradient, one can derive a comparable expression in terms of the change

in optical pathlength per unit length per unit temperature for light polarized

radially (aP^) or tangentical ly (aPq) within a glass rod. The average of the

optical pathlength change for r and e polarizations with thermally induced strain

present is then

aP^+aPq = 26+ (3Bj_+B|

I

) (3)

where Y is Young's modulus, P is Poisson's ratio, and B|

| are photoelastic con- stants for light polarized perpendicular or parallel to the direction of internal

strain. A factor of 2 has been neglected in obtaining this equation. One wants

to minimize the amount of thermo-optic distortion, and thus J, is expressed as the

inverse of equation 3. -417-
|aP -APgl'^^Jg: The difference between aP^ and AP^ can be shown to give

where B is the stress-optic coefficient. Thus Jg is related to the susceptibility

of the glass to stress induced birefringence, being large for glasses possessing a small stress-optic coefficient.

2.2 Special Cases

In most of what follows, no attempt is made to specify the weighting factors, W^. . There are, however, a number of limiting cases where certain W^. may be specified. If only optical effects are con- sidered to be important, then by setting W2=W^=Wg=Wg=W-,=Wg=0 and W^=W2=1, the FOM reduces to na^/n2,

the familiar pump limited case. Likewise, one can obtain the parasitic limited FOM, n=n/np, by

setting W2=0.

2.3 Physical Properties Data The calculations presented in the following sections depend upon numerous measurements of basic

physical properties. This data is readily available from vendors for their respective commercial

products. It is, however, preferable to measure as many properties as possible at one facility under a given set of operating conditions, thereby improving the consistency of accumulated data and the relative

comparisons which can be made. In house measurement techniques used to obtain physical properties of

interest, as well as data supplied from other sources are identified as follows:

n: Refractive index measurements in the visible (dye laser), at .6328ym (HeNe), and at 1.06ym

(Yag) are obtained with a Hi Iger-Chance ref ractometer [4] constructed from BK-7 glass. The

range of values measurable with this device is 1.3 to 1.8 ± .0005, as presently

configured in our laboratory.

The nonlinear index of refraction is calculated from vendor supplied data for n^C. 588pm) or np{.589um) and Abbe value v^ or Vq, using an empirical relationship [2].

a : Specific gain is derived from stimulated emission cross section calculations and peak

° emission wavelength measurement data found in vendor product literature.

(E/H) : Calculations [5] from glass absorption band profiles predict that the ratio of stored

energy to heating will be lower for phosphates versus silicates. Actual measurements

of the stored energy profiles in several commercial glasses are presented elsewhere

in these proceedings [6]. Until a technique for measuring heat density is developed

as an alternative to calculations which convolve xenon flashlamp spectra with glass

absorption spectra, this quantity will be set equal to 1 for all glasses.

n : No absolute or relative measurements of quantum efficiency have been performed for the commer-

° cial laser glasses of interest here (except ED-2[7]). Thus is set equal to one in the

following calculations.

k: Thermal conductivity at room temperature is measured with a commercial thermal comparator [8].

This device records the rate of cooling at the tip of a probe heated to 20°C above ambient,

upon contact with the polished surface of interest. A thermocouple pair responds to the temperature change by generating a millivolt signal which can be converted to thermal conductivity

by calibration with known reference standards. Precision achieved is ±2 percent.

p: Glass density is measured according to ASTM (693 74)-Density of Glass by Buoyancy.

c: Glass specific heat near room temperature is taken from the product literature, except for

EV-2 (measured in house) and Q88 (calculated from compositional and additive coefficient data

[9]).

a: The linear coefficient of thermal expansion is measured at the room temperature point on the

expansion versus temperature curve obtained with a commercial quartz dilatometer [10].

The measurement sensitivity is ±5 x 10'^/°C.

6: The change in refractive index with temperature is measured on 60° equilateral prisms of glass

between 10°C and 40°C at x=1.064ym. 2 cm high by 1.2 cm wide prismatic samples are oriented

for minimum angle of deviation; upon heating the deflection of the laser spot is measured over

a folded pathlength equivalent to 30 meters. The precision of this technique is ±6 x 10"^/°C.

Y: Young's modulus is measured with an ultrasonic pulse technique [11]. 20mm by 4mm diameter

cylindrical rods are cemented to the end of a magnetostrictive nickel alloy rod. A magnetostrictive transducer creates short duration extensional wave pulses within the rod. The velocities of these pulses within and along the rod axis together with glass density permit one to

2 calculate Y. Measurement precision is ±130kg/mm for the present setup.

-418-
p: The technique described above is also used to measure Shear modulus. The same precision limit

Y applies, however, leading to a large experimental error in Poisson's ratio (p=25- -1) when

calculated from these measured quantities. An optical technique [12] capable of greater

precision is being investigated, but for the present, vendor data for Poisson's ratio is used.

B: Stress-Optic coefficient measurements are performed in a compression tester [13] at low values

2 of applied stress (9 Kg/cm ), using a Soleil compensation technique [3] at a wavelength of

2 1.152ym. Data is good to ±0.1 nm/cm/kg/cm .

B| B, , : Fizeau interferometry is required to distinguish between the change in glass refractive index

due to stress applied parallel versus perpendicular to the polarization direction of light

propagating through a test sample. Experimental data taken at a wavelength of .6328um is good

to no better than 20 percent, due partly to a correction factor containing Young's modulus and

Poisson's ratio, which is required to reduce the interferometric results.

Measured values for the above physical properties are given in table 1. The number of glasses con- sidered is small, only because of the unavailability of photoelastic constant data at the present time

for other commercial compositions (e.g. LG-700/710[17])

.

Table 1. Glass physical properties data

Data Property

Glass Source

refractive index, n l.Oeym

- 13 nonlinear index, n„ (10 esu)

2 specific gain, a cm /J

th. conductivity, k density, p gm/cc

specific heat, c cal/gm

cal/cm-sec-°C(10

C

-3x

th expansion, a 10'''/°C / v _-. temp, coeff. refractive index, 10" /°C

7, thermo-opt. coeff.

Young's modulus, Y Poisson's ratio, p stress-optic coeff.

photoelastic const. photoelastic const.

6+a(n-l), 10"7°C

2 kg/mm

B, nm/cm/kg/cm

^ B|, nm/cm/kg/cm

_L 2

B , nm/cm/kg/cm

ED-2 EV-2 LHG-5 LHG-7 Q88 Source

[14] [14] [15] [15] [16]

1.5554 1.5037 1.5291 1.5050 1.5310 LLE

1.4 1.0 1.2 1.0 1.2 vendor

.16 .25 .21 .20 .23 vendor

3.00 1.08 1.75 1.46 1.76 LLE 2.51 2.72 2.64 2.60 2.67 LLE

.22 .15 .17 .17 .21 vendor

11 141 77 91 90 LLE

31 -100 -2 -26 -16 LLE

74 -28 39 20 32 LLE 9620 4258 7258 6049 7123 LLE

.24 .24 .19 .24 .24 vendor

1.92 1.45 2.17 2.14 2.07 LLE 2.6 4.9 3.5 3.5 2.9 LLE -0.2 3.1 2.2 2.0 0.0 LLE

2.4 Figure of Merit Calculations

The individual J factor components for our FOM, calculated from the data in section 2.3, are given

in table 2. Results are normalized relative to the silicate laser glass ED-2. The last column gives

an average component FOM for the four phosphates evaluated. Based on a comparison of these averages

with ED-2 we conclude the following:

Jp J2:

^8^

J-,:

>^5' ^6 =

Clearly the phosphates offer higher gain/lower performance advantages over the silicates.

Thermally induced birefringence is less severe for phosphates versus silicates.

Thermally induced wavefront distortion is much smaller for phosphates than silicates. This

may be attributed to the negative value of temperature coefficient of refractive index for

these glasses. In fact, dn/dT is so negative for EV-2 that one can predict convergence for a beam propagating through an amplifier rod of this composition[18] versus predicted divergence

for all other glasses in table 2. The one disadvantage for phosphates is their tendency to heat to a higher level under flashlamp pumping, and their diminished capacity to rapidly dissipate heat deposited in them. In practice, however, these somewhat unfavorable thermal properties do not prevent

90mm diameter by 370mm long laser rods from being fired in amplifier heads at a repetition

rate of twice per hour in the Glass Development Laser system at LLE.

-419-
Table 2. Component figures of merit

J Factor

n/n2, J-^

^0' "2

(E/H

J

0' ^3

0' ^4

k/pc, Jg

PC, Jg

(AP^+APg)

lAP.-APgl

Glass:

Silicate

D-2

Phosphateb Phosphate

EV-2 LHG-5 LHG-7 Q88 Average

1.4 1.2 1.4 1.2 1.3

1 K 1 ^ 1 A

11111X1111

1 1 1 1 1 0.5 0.7 0.6 0.6 0.6

0.7 0.8 0.8 1.0 0.8

-2.3 1.6 3.5 3.3 2.7

1.6 1.3 1.2 1.1 1.3

Data normalized to ED-2.

'^Because of experimental uncertainties, this data is not to be construed as favoring

one phosphate glass composition over another.

""Absolute value deleted to reveal special phosphate effects; see text.

3. Optical-Physical Properties Correlations

3.1 Properties Neglected by the FOM Formalism

When phosphates were first introduced as commercial laser glass products, their biggest drawback

was thought to be poor durability, characterized by softness and inadequate chemical resistance to water.

In using large phosphate rods we have discovered that these are important concerns for some compositions,

as are thermal shock resistance and the presence of certain types of impurities. The above are not included in the FOM formalism, but are discussed here.

Evidence of poor chemical resistance has been seen in phosphate glasses at LLE. Permanent grooves

have upon occasion been etched into rod barrels by the interaction of cooling water, flashlamp radiation,

and 0-ring sealing materials. AR coated oscillator rod surfaces have shown evidence of moisture induced

glass substrate erosion, resulting in wrinkling and cracking of the surface thin films. From chemical

durability tests we have found that the chemical resistance of phosphates can be increased to a level

comparable with that found in silicates by the addition of at least 50 percent by volume of ethylene

glycol to any recirculating coolant cycle. The dramatic decrease in weight loss for selected phosphate

glasses is shown in figure 1. Each test plotted therein was conducted on immersed glass blocks, agitated

by magnetic stir bar for 18 hour time periods at 50°C.

One can calculate the thermal shock resistance of glass from its rupture strength, R, thermal diffusivity, Young's modulus, and thermal expansion [9],

X _ R (k/pc)^=

'SR Ya

(5)

A relative measure of rupture strength has been obtained at LLE using Shand's method [19], wherein flat

circular specimens (1.2 mm thick by 25mm in diameter) are placed on a 19mm diameter steel ring. A load

is applied from above with a concentric ring 6mm in diameter until fracture occurs. All samples in

this test were fine annealled specimens whose surfaces had been uniformly damaged by application of a fine grind. The resulting rupture strength data and subsequent calculations from equation (5) show

that increased care is a wise practice when subjecting phosphates to sudden temperature drops that might

not harm silicate compositions (see table 3).

Table 3. Thermal shock resistance

# samples tested „ R, rupture strength, kg/mm

Tgj^, thermal shock resistance

from equation (5) (relative)

Glass

Silicate Phosphate

ED-2 EV-2 LHG-5 LHG-7 Q88

20 20 26 33 24 29 14 20 17 14

1.0 0.4 0.8 0.6 0.4

Two particularly troublesome impurities in phosphate glasses are platinum and water. There is

a correlation (fig. 2) between increasing platinum level [20] and increasing glass optical absorption

in the UV. The high solubility limit for platinum in phosphate glasses imparts to them an orange

coloration (seen easily at low doping levels), a direct cgnsequence of this shift of the glass UV

edge toward the visible (fig. 3). A masking of the 3550 A Nd pump band can occur, causing a reduction

in energy storage and rod amplifier performance (fig. 4).

-420-
of the neodymium fluorescence which

; just as severe as concentration quenching. This effect is illus

of „" "

rpti

less than about 15%.

3.2 Laser Glass Properties Correlations

One can ask whether there is any fundamental reason why glasses optimized for high gain and low

must therefore suffer from a degradation of other important physical properties. In an attempt to answer

this question we look first at correlations among selected physical and optical properties for commercial

laser glasses.

Plots of four physical properties versus calculated nonlinear refractive index are presented in figure 6. Two commercial silicate, six commercial phosphate, and two commercial f luorophosphate laser

glass compositions are considered, as well as an experimental Nd doped beryllium fluoride glass [21].

Observed behavior can be summarized as follows:

1. There is a trend toward higher thermal expansion as nonlinear index decreases. All composition

groups fit this pattern well.

2. There is a reduction in glass hardness with decreasing nonlinear refractive index. This is also

seen for all glass groups.

3. For the silicates and phosphates, there is a correlation between reduced Young's modulus, thermal

conductivity and decreasing n^- These trends are reversed for the fluorophosphates.

The above observations clearly confirm the degradation in physical properties of phosphates versus silicates, as alluded to in the FOM discussions of section 2. Because nonlinear index depends upon

linear index and glass dispersion in a complicated and somewhat empirical way and because of the exceptional behavior of the fluorophosphates, it would be useful to consider correlations of physical

properties with more basic optical ones.

3.3 Optical Glass Properties Correlations

At the ERDA review of the LLE Beta Laser System held at the University of Rochester in January,

1977, Alexander Glass of LLL suggested that we evaluate the physical properties trends we observe for

silicate and phosphate glasses by plotting them on a standard glass diagram. In this section we discuss

the results of his suggestion.

Using the Schott Optical Glass Catalog we have selected approximately 120 optical glasses from the

following composition groups: FK, PK, PSK, BK, K, ZK, BaK, BaLF, SK, KF, LLF, F, TiF, KzF, KzFS, LaSF.

In addition we have included data from the experimental series E of fluorophosphates investigated at Owens Illinois [22]. The refractive index versus Abbe value for each glass has been plotted on a glass

diagram. A given physical property may be examined by circumscribing the majority of glass data points

possessing a common value of the property of interest. Deletion of the data points gives the type of

plot shown in figures 7a, 7b, and 7c which can be analyzed as follows:

7a: The range of glasses examined is broad enough to illustrate the well known decrease in non- linear index, with decreasing refractive index n^ and increasing Abbe value v^.

7b: There is a strong correlation between Young's modulus and refractive index for Abbe values between 50 and 70, in agreement with the trend seen in figure 6. The lack of correlation for

high index, large dispersion glasses (the region for Y=6000 kg/mm ) can be linked via the high

density of these glasses with the anomalous behavior of the fluorophosphates. This topic

will be addressed in the next sections.

7c: Glass density appears to strongly follow refractive index, exhibiting little or no relationship to Abbe value.

Because of 1) its ease of measurement, 2) its fundamental nature, and 3) the strong correlation with

the optical refractive index, glass density would seem to be one key to understanding the durability

versus problem.

3.4 The Importance of Glass Density

The strong dependence of glass refractive index on glass density is obvious from figure 8, wherein

a least squares fit to data plotted from the glass groups used in section 3.3 gives

n^ = 0.1176 p + 1.206 (6)

This equation is reminiscent of the Lorentz-Lorentz law under the approximation of small refractive

index [23],

2

n 1 n 1 = constant iTZI^^~Z\ —— ' constant (7)

.2 „x (small n) p (n"+2: p

A firm relationship between n and p enables us to qualitatively understand other trends in measured

attributes.

Young's modulus is derived from a measurement of ultrasonic extensional wave velocity and glass

density [11],

-421-
(8)

Thus as density drops with decreasing refractive index, Y should similarly show a decline. The correlations presented in previous sections support this relationship for glasses whose densities are less

than about 3 gm/cc.

The link among refractive index (glass density), hardness, and rupture strength is accomplished by

referring to the following relationship [24]

(Hardness)H = (2/3)R [1 + In (9)

Glass hardness depends explicitly on rupture strength, and to a lesser degree on Young's modulus. But,

because data presented here (tables 2 and 3) and elsewhere suggest that Y and R are directly proportional to each other (and frequently equated in the theoretical limit [25, 26]), we use equation (9) to support the trends observed in figure 6.

3.5 Exceptional Behavior of the Fluorophosphates

Preliminary results show the fluorophosphates to be more chemically durable than the phosphates

[27, 28, 29]. The finding J_s consistent with the high density-good durability (hardness, stiffness,

rupture strength) correlations presented in this paper, for the fluorophosphates have densities of

around 3.6 gm/cc. But the further expectation that high density implies high index is invalid.

Refractive indices for these glasses are around nj=1.45. One is led back to a consideration of the

Lorentz-Lorentz law.

The quantity specified by equation (7) is constant only under special circumstances. If multiplied

by the molecular weight (M) of a substance, equation (7) becomes the molar refractivity [n] or [23]

[n] = (10)

(n^2)p

In a glass [n] depends on the molecular ref racti vi ties of many constituents. This dependence can be

simply additive for constituents whose electrons do not strongly interact; or it is quite possible that

in for example dense fluorophosphates, electron-electron interactions are complicated by different

chemical bonding, invalidating simple additivity laws and thereby breaking the n-p correlation observed

for other optical glasses.

4. Conclusions

We have presented the components to a figure of merit for evaluating neodymium doped glass compositions intended for use in large laser systems. Measurements of physical and optical properties and

subsequent calculations point out the general attributes of phosphate versus silicate compositions.

The former can offer higher gain, lower nonlinear refractive index, less sensitivity to stress induced

birefringence, and lower thermo-optic distortion, while sacrificing thermal properties and durability.

Correlations among physical and optical properties of other optical glasses show that some tradeoffs are

a necessary consequence of low glass density. The high density fluorophosphates are an important ex- ception to predicted trends for low ng glasses, and illustrate the need for including a consideration

of molecular polarizability in future studies.

5. Acknowledgements

The authors gratefully acknowledge Dr. Tetsuro Izumitani for technical discussions on this subject,

and for his suggestion that we consider the link among refractive index, molar refractivity, and glass

density.

-422-
L Annual Report (1974).

N. L. Boling, A. J. Glass, and A.

Owyoung, report UCRL 75628 (1974).

LLE Annual Report (1977).

J. V. Hughes, J. Sci. Inst. 18, 234

(1941).

D. C. Brown, S. D. Jacobs, N. Nee, to be published in Applied Optics , Jan. (1978).

J. A. Abate, D. C. Brown, C. Cromer, S. Jacobs, J. Kelly, J. Rinefierd, to be

published in NBS Damage Symposium (1978).

L. G. DeShazer and L. G. Komai, J.O.S.A.

55, 940 (1965).

Lafayette Instrument Co., Lafayette,

Indiana.

M. A. Matveev, G. M. Matveev, B. N. Frenkel , Calculation and Control of

Electrical, Optical, and Thermal

Properties of Glass , Ordentlich Press,

Israel (1975).

Perkin Elmer Corporation, Norwalk, Conn.

L. C. Lynnworth, J. of Testing and

Evaluation 1, 119 (1973).

W. T. Szymanowski, J. Appl . Phys. 15^, 627 (1944).

Comten Industries, St. Petersburg, Fla.

Owens Illinois, Inc., Toledo, Oh.

Hoya Optics, USA Inc., Menlo Park, Ca.

[16] Kigre Inc. , Toledo, Oh.

[17] Schott Optical Glass, Inc., Duryea, Pa.

[18] text prepared for ERDA Review, Jan. 6-7

(1977).

[19] E. B. Shand, Glass Engineering Handbook , ch. 6, 132 (1958T:

[20] platinum levels provided by P. Vergano, Owens-Illinois, and T. Izumitami, Hoya Optics.

[21] type 816KY, provided by W. Dumbaugh

and D. Morgan, Corning Glass Works,

Corning, N. Y.

[22] P. J. Vergano and C. F. Rapp,

quarterly progress report to ERDA,

EY-76-C-02-4080, Jan. (1977).

[23] R. S. Longhurst, Geometrical and

Physical Optics , 459 (1967).

[24] T. Izumitani and I. Suzuki, Glass

Technology 14, 35 (1973).

[25] W. C. LaCourse, Introduction to Glass Science , 454 (1972).

[26] R. J. Charles, Progress in Ceramic

Science, U 4 (1961).

[27] Owens-Illinois EVF-1, product literature June (1977).

[28] Hoya LHG-10/11, product literature

June (1977).

[29] Schott LG-800, product literature

Sept. (1977).

-423-
7. Figures

1007o 50/50 100%

E.G. H2O

SOLUTION COMPOSITION

Figure 1 Laser glass durability in ethylene glycol -water solutions.

-424-
/

LHG-5

1—I

1 1—I1—I—I—T"

0 40 80 120 160

PLATINUM CONCENTRATION (ppm)

Figure 2 Platinum concentration versus absorption for EV-2 and other glasses.

-425-
Figure 3 UV-VIS absorption in laser glasses from dissolved platinum.

-426-
.6

Figure 4 Gain profiles for high platinum and low platinum laser rods.

-427-
OD/cm @ 2.2 nm

CO

400-1

350-

300-

It: 250-

.05

_1_

.1 .15 .2 L

\

O CONCENTRATION

QUENCHING ^

O HiH20

\

\H

\\

\\\

\

HI DOPING

WATER

QUENCHING

\

\

\\

-\

400

-350

-300

-250

200- I2

"-200

Figure 5 Fluorescent lifetime effects in LHG-5/7 glasses.

-428-
u

o

a>wIEu

a

u

I

I

4-

2-

0-

180-

140-

100-

60-

8000-

4000-

0-

700-

500

300

100

THERMAL CONDUCTIVITY

A - SILICATE O -PHOSPHATE

- FLUOROPHOSPHATE

• - BERYLLIUM FLUORIDE

LINEAR COEFFICIENT©

THERMAL EXPANSION O M*

YOUNG'S MODULUS

KNOOP

HARDNESS

o o

~i—I—I—I—I—I—I—I—I—I—

r

.4 .6 .8 1.0 1.2 1.4 CALCULATED Oj, 10 " ESU

Figure 6 Laser glass physical properties correlated with r\2-

-429-
1.65

1.60

^ 1.55

ISS

^ 1.50

p

i

1.45

1.40-

J 1

CALCULATED NONLINEAR

REFRACTIVE INDEX, n^(W^^ESU)

1.35-1 ^ y ^ y r 100 90 80 70 60 5 0 4 0 30

ABBE VALUE Z/fj

Figure 7a Regions with common n- on the optical glass diagram.

-430-
100 90 80 70 60 50 40 30

ABBE VALUE Z/^

Figure 7b Regions with common Young's modulus on the optical glass diagram.

-431-
1.35-1 ^ ^ r 100 90 80 70 60 50 40 30

ABBE VALUE 1/^

Figure 7c Regions with common density on the optical glass diagram.

-432-
2.0 n

LEAST SQUARES FIT: 152 PTS.

1.8

2

5 1.6 H

o 1.4

E

la 1.2

1.0

= .1176p + 1.206

—

I

1 1

3.0 4.0

p, DENSITY (gm/cc)

2.0 5.0

Figure 8 Refractive index versus glass density for optical glasses.

-433-
PROGRESS IN CLADDINGS FOR LASER GLASSES*

R. B. Bennett, K. R. Shillito

Battelle, Columbus Laboratories

Columbus, Ohio 43201

and

G. J. Linford Lawrence Livermore Laboratory

University of California

Livermore, California 94550

Cladding glass compositions matched in refractive index have been developed

for two commercial neodymium-doped silicate laser-glass-disk amplifiers for the

suppression of parasitic oscillations. Coatings based on a copper oxide-doped

borate glass system satisfied specifications for single-pass optical transmission,

xenon flashlamp damage threshold level, scatter, and thermal -expansion match.

Commercial application for Owens-Illinois' ED-2 laser glass and Hoya Optics' LSG-91H

laser glass achieved gain levels surpassing those of previously tested commercial

glass claddings and equivalent to that of liquid cladding systems. Compositions

and cladding design criteria are presented.

The feasibility of glass claddings for neodymium-doped fl uorophosphate laser

glass is also reported. Claddings based on copper oxide-doped alkali phosphate

glass systems having softening temperatures below 400 C, linear thermal -expansion

coefficients between 14 and 18 x 10"6/c, refractive indices of 1.48-1.50, and good

chemical durability have been identified. Technology is being developed to meet

specific design and performance goals and to reveal the nature and extent of

potential problems associated with claddings for fl uorophosphate laser glass for

advanced laser systems. Compositional ranges and pertinent data along with design

considerations are presented.

Key words: Fl uorophosphate laser glass; glass cladding; laser fusion; microstructure;

parasitic oscillation; refractive index; residual stress.

1. Introduction

As is well known, the Lawrence Livermore Laboratory has been constructing a large neodymium glass

laser system as a part of their laser fusion program. Some of the principal components of this laser

glass system are disk amplifiers. The Shiva system, currently nearing completion, includes 20 laser

chains of progressively larger disk amplifiers. Most of our own particular development has been

targeted toward the C-size amplifier which has major and minor axes of 40 and 20 cm, respectively.

The main reason for applying the claddings on these disk amplifiers is the suppression of parasitic

oscillations which arise within the volume of the laser disks.

2. Discussion

Figure 1 illustrates the parasitic ray geometry of a laser disk with a partially transmitting

cladding, illustrates several important considerations in cladding technology. In the schematic, a light ray in the laser glass disk meets the interface of the cladding glass and laser glass and is

" refracted by an amount varying with the refractive index mismatch. During that refraction, a portion

of the light is reflected back into the laser glass, and the amount of the reflected light would be lessened, or perhaps eliminated entirely, if the refractive index match between the cladding glass

and the laser glass is perfect. The cladding glass is doped with absorptive ions, usually by adding

cupric oxide, which absorb the parasitic rays and dissipate the energy thermally. Any unabsorbed

light will be reflected at the cladding-glass/air interface, and may reenter the laser glass after

traversing the edge coating. Thus, the thickness of the cladding glass and the doping level must be coordinated so as to achieve the desired sufficient amount of light absorption, yet prevent excessive

heating of the cladding glass. The cladding glass microstructure is also important. The presence of

bubbles or second solid phases must be minimized, since either type of microstructural feature would

be expected to reflect parasitic light back into the laser glass.

Figure 2 is another schematic which shows the alternate approach to edge cladding. This method

was developed earlier because it was uncertain whether satisfactory solid claddings could be developed.

Basically, this liquid cladding system involved a sealed containment ring through which a chemical

solution designed to have a refractive index matching that of the laser glass was pumped. The

additional complications of this approach make the liquid systems less desirable than a satisfactorily

performing solid edge cladding.

*Work at the Lawrence Livermore Laboratory performed under the auspices of the U.S. Department of

Energy under Contract W-7405-eng-48.

-434-
The principal objective of the cladding compositional development has been to match the refractive

index of the laser glass, while maintaining compatibility with commercial methods for cladding preparation and processing. The glass cladding technique is basically a glass enamel process. Preparation of

the cladding glass includes several steps. The glass batch is weighed, melted, poured and cast, and an- nealed. After samples for optical and thermal property measurements are cut from the annealed glass,

the remainder is pulverized to powder (usually -325 mesh) in preparation for its use in an alcohol-based

slurry suitable for either spraying or flow coating onto the laser glass samples. Of course, the laser

glass substrates require some preparation. Including cutting samples of satisfactory size, polishing

the surfaces to be coated, and cleaning.

Several compositional design criteria must be adhered to in developing the cladding glass. The absorption coefficient must be maximized in order to absorb parasitic light, because the commercial application process limits thickness to about 0.5 mm. The refractive index must be matched to that of the

laser glass to minimize the reflection at the laser glass interface. The softening point must be minimized so that the cladding can be fused onto the laser glass at a temperature at which the laser glass

itself is not distorted. Thermal contraction must be matched to that of the laser glass to prevent the

formation of excessive residual stresses which would both harm lasing characteristics and. also result in

mechanical damage to the laser glass or the cladding, Mlcrostructural inhomogeneities must be minimized to prevent light scatter and back reflection. Stability with respect to devitrification phase

separation or possible hydrolysis by atmospheric or processing moisture must be maximized.

Table 1 shows the composition and property data for an optimized cladding applied to Hoya Optics'

LSG-91H silicate laser glass. One of the significant aspects is the BaO:ZnO ratio which was balanced

in order to minimize residual stress. The copper oxide was included for its absorption function.

Fluorine is present to minimize the softening point of the cladding glass within an acceptable range.

AI2O3 was added for stability. Regarding the properties listed, the optical transmission is within the

LLL specification, devitrification was avoided thus minimizing light scattering, and refractive index

was within the specification of 1.56-1.60 (the refractive index of the silicate laser glasses was 1,555). The other listed properties are pertinent to the ability to apply the cladding glass by com- mercial practice.

Table 1. Composition and property data for cladding glass LBH-30.

Batch Composition: 5.80 NaF

2,05 KF

3.83 K2CO3

1.01 A1(0H)3

58.97 H3BO3

12.82 PbO

3.52 BaC03

12.01 ZnC03

4.93 CuO

Oxide Composition: 6.5 Na20

6.5 K2O

1.0 AI2O3

50.5 B2O3

19.5 PbO

4.15 BaO

11.85 ZnO

7.5 CuO

5.0 F

Smelting Temperature - 815 C Softening Point - 499 C Thermal Exp Coeff - 10.9 x 10"^ cm/cm/C

Refractory Index - 1.5781 at 1.06 ij,m Residual Stress - 3 nm/cm

Devitrification - None

Optical Transmission - 1.1% at 1.06 [j,in 5.5% at 1.35 ij,m

Compositional dependency of the residual stress thermally induced in the laser glass by application

of the cladding glass is shown in figure 3 for both 6 percent and 8 percent copper oxide doping. Adjustment of the relative amounts of BaO and ZnO, while maintaining their total at 16 weight percent,

permitted residual stress minimization. When CuO level was adjusted, a corresponding modification in

the BaO/ZnO ratio maintained the low residual stress level.

The typical microstructures of the two previously available commercial coatings for silicate laser

glasses are shown in figures 4 and 5, The BSDL-6 cladding section shown in figure 4 is about 0.15 mm

-435-
thick. It has a solid second phase which results in surface roughness, which is undesirable from the

standpoint of mounting the claddings in the retaining rings by LLL, and also would be expected to cause

some back-reflection. In addition, it contains porosity which is preferentially oriented at the interface and, therefore, would tend to maximize the amount of back-reflection off the porosity.

The competitive commercial edge cladding, as applied to ED-2 laser glass, is shown in figure 5. Typically, EI-4 is about 0.25 mm in thickness and has a significant amount of porosity, which is homogeneously dispersed in the cladding glass. A typical mlcrostructure of LBH-30 cladding is shown in

figure 6 for comparison. The cladding section shown is about 0.42 mm thick, has no second solid phase,

and has less porosity than the commercial claddings. The pores in the LBH-30 cladding are homogeneously distributed through the coating. This is typical of the optimum coatings developed for both LSG91H and ED-2.

A plot of the gain data obtained by LLL for the LB-615I cladding, which was developed for ED-2

laser glass, is given in figure 7. This is some of the early data reported by LLL and was important because it showed that when the desired properties and specifications for the cladding glass were achieved,

desirable gain characteristics resulted. The data for a previously available commercial solid edge

cladding, that for liquid edge cladding systems, and the data for the LB-615I which was developed and

applied to ED-2 laser glass are plotted. There is a slight superiority over the liquid cladding system

which, of course, allows the elimination of considerable hardware and maintenance which were needed for

liquid claddings. However, there is also at least a 20 percent increase in gain over that measured for

previously available solid edge claddings, resulting in quite a substantial increase in the achievable

power in the laser system. As a matter of fact, using this type of solid edge cladding in the system,

LLL recently achieved their first major program milestone of 500 Joules output for a single laser

chain. Thus, the development of claddings for silicate laser glasses was concluded in a satisfactory

fashion and both of the commercial suppliers have adapted such compositions for use on their silicate

laser glasses. More recently, work has been directed toward the advanced laser glass systems being developed under the direction of ERDA, Specifically, claddings have been sought for the f luorophosphate

glass compositions now being developed by at least four of the glass vendors.

Table 2 lists the cladding glass properties which would be needed for the f luorophosphate laser

glasses. The refractive indices need to be lower than for silicate laser glasses, probably below 1.48.

The thermal expansion coefficient will likely be in the range of 15 to 18 x 10"^ cm/cm/C. Residual

stress again will need to be quite low, <5 nm/cm. The softening point will need to be less than 425 C in order to apply the claddings at a satisfactory low temperature. Absorption coefficients again will

need to be maximized and scattering minimized for efficient operation of the claddings.

Table 2, Cladding glass design criteria for advanced laser glass.

Refractive Index - <1.48 at 1.06 p-m Thermal Expansion Coefficient - 15.0 - 18.0 x 10"^ cm/cm/C

Residual Stress - <5 nm/cm

Softening Point - <425 C Absorption Coefficient - Maximize

Scattering - Minimize

The compositional range for the f luorophosphate glass claddings developed to date is given in

table 3. These are phosphate glasses, essentially, and they are doped with 4 to 8 percent copper oxide.

Table 3. Cladding glasses for f luorophosphate laser glass

Oxide compositions (w/o)

1.0 - 3.0 AI2O3

3.0 - 8.0 B2O3

55.0 - 65.0 P2O5

5.0 - 15.0 Na20

5.0 - 12.0 K2O

0

- 7.0 ZnO

0

- 5.0 BaO

0

- 7.0 MgO

5.0 - 9.0 F 4.0 - 8.0 CuO

Data which have been obtained thus far for claddings developed for fluorophosphate laser glass are given in table 4. The thermal expansion coefficients are in the desired range, since the more promising laser glasses obtained from different suppliers have ranged in thermal expansion from 14 to 18 x

10"^ cm/cm/C. Residual stress of 1.9 MN/ri^ (280 psi) is excessive, but the final adjustment will be

made only when the desired laser glass composition (s) have been selected. Index of refraction is 1.515;

-436-
Table 4. Typical cladding data for advanced laser glass.

Thermal Expansion Coefficient

Residual Stress

Index of Refraction

Transmission

Chemical Durability

(H2O, 100 C, 1 hr)

15.5-17.5 X 10-6 cm/cm/C

1.9 MN/m2 (280 psi)

1.515 at 1.06 M,m 0.107. at 1.06 (j,m 1.47« at 1.35 ij,m 0.0177o min-1

and that is acceptably close to the target. Single-pass optical transmission is well within specifications, having 0,1 percent at 1,05 microns and 1,4 percent at 1,34 microns wavelength.

In conclusion, a solution was obtained for the problem of index matching a satisfactory glass solid

edge cladding for the silicate glass laser systems, thus helping LLL to achieve a major laser program

milestone. The more recent work on the f luorophosphate glass compositions has progressed sufficiently

that the feasibility of obtaining a fully satisfactory cladding for the f luorophosphate glass systems

has been demonstrated. Although there is work proceeding on beryllium fluoride based laser glasses, at the present time no development effort has started on claddings for these systems,

3. Figures

Gtau tufaitrat* Glau cladding

-

Figure 1, Schematic representation of the cladding-glass function

Extrinsic (Chrome Black) Absorber

"0"-Ring Seal

Pump Light Disk' face

\ Laser Disk

Imnersion Fluid

Liquid-Clad Disk Holdtr

Totally-Reflected Ray

Figure 2. Cross sectional schematic of

liquid -clad laser disc fixture

-437-
Figure 3. Effect of compositional variation on stress (annealed)

• • •

El-4

cladding

ED -2 laser

glass

Figure 4. Typical cross-section of Owens-Illinois' EI-4

cladding, approximate 0.010-in. thickness

^. 'r-fL. . ^T.>• -iSi " i- cladding

LSG-91H

laser

glass

Figure Typical cross section of Hoya BSDL-5 cladding, approximately 0.006-in. thickness

-438-
LBH-30

cladding

LSG-91H

laser

glass

Figure 6. Typical microstructure of LBH-30 cladding

' I

I 1 1 1 I U

0 so 100 130 200 230 300 330

InpuC Energy, kj

Figure 7. Comparison of gain coefficients for LB -6151

experimental cladding with those for liquid

cladding and commercial solid-edge claddings

COMMEMTS ON PAPER BV BENWETT, SHILLZTO. AMP LWTORV

The. ipeafee/L mcus cak&d why paAoi-LUc ivcppttuislon iMU ipzcA-ilzd cut 1.06 mlcAomztiVU, and 1.35 mlcAormJ:zu

bat not at O.SS micAomeXzu, which u, a tKOMiXJion inom th^ mztoitablz itate. to thz ground itate.. It um pointzd out by MaAvln We.beA i$/Lom LLL, that mliilon at O.&i mlcAomzXeAi, lt> neabionhzd by ground

itatt ndodymuxm torn, itpldnlihlng the. ln\jeAf>-ion and theJizione. dou not fLe.pn.eJiZnt a net loi>i>. Conie.quzntly tkexz -u no nze.d to mppne^i nadAMlon o^ ilaofieAcznce at tJnat waveZdngtk.

-439-
DAMAGE HISTORY OF ARGUS, A 4TW Nd Laser System

Irving F. Stowers and Howard G. Patton

Lawrence Livermore Laboratory

Livermore, California 94550

Argus is a two arm fJd:glass laser built for laser fusion experiments.

It operates at a power level of 4 terawatts with 100 ps pulses and has

placed 2KJ on a fusion target with 1 ns pulses.

To minimize spatial modulations on the beam and damage to coated and

uncoated optics, the system uses five spatial filters on each arm and the

beam fluence is maintained below 2 j/cm . Despite these precautions,

coated optics and uncoated disks occasionally still damage. Damage limits

the useful life of the system and determines the frequency of maintenance.

This paper discusses the several forms of damage that have been

observed, their morphology, severity and frequency of occurrence. Three

distinctly different forms of damage have been observed in the system.

They are: A) contamination induced damage to Nd:glass disks caused by

broad-band xenon flashlamps, B) optically obscuring films on disks and

C) damage to coated optics by the laser beam.

Argus History

Each Argus arm is composed of a 4-cm rod amplifier, three 8.5-cm aperture amplifiers, (shown in figure 1) and four 20-cm aperture disk amplifiers. The disk amplifiers have 3%-Nd-doped silicateglass disks set at Brewsters angle to minimize reflection and are enclosed in a quartz shield tube to create a clean beam cavity. The beam cavity and the flashlamp covers are flushed with dry nitrogen to exclude particulates and to cool the disks.

The first arm was placed in operation in December, 1975 and since then the system has been fired

approximately 600 times. In August, 1976, after 150 shots, the disk amplifiers were disassembled,

examined, and found moderately damaged and obscured by a light scattering film. The entire system was refitted in three weeks, replacing badly damaged disks with new and recleaning the remainder. In

October, 1976, a planned periodic maintenance schedule was initiated ^ with one amplifier being rebuilt

every two weeks. During this periodic maintenance, the following information was gathered on severity

and frecuency of damage in order to determine system reliability and to predict optimum maintenance

intervals.

A Disk Amplifier Damage Morphology

Damage to uncoated disks occurs in a size range from 1 pm to 1 mm and the morphology is con- sistent and repeatable enough to allow certain patterns to be identified.

The single most outstanding pattern is that damage sites are round, or at least circularly

symmetric. The sites are always located on the surface of the glass and damage to the bulk of the

glass has not been observed. Most large damage sites, 100 ym to several millimeters in diameter, are accompanied by extensive fracturing of the glass surface. Figure 2 shows a typical large damage site

which is characteristically round and fractured at the edges. In almost all cases, glass fragments

from the center of the site are ejected or melted and only those attached to the edge of site remain

intact.

The fractured glass produces loose chips which when ejected from the initial location fall

back onto the disk and cause subsequent damage. Figure 3 show the area immediately surrounding a large

damage site, and it appears that the smaller damage sites were started by debris thrown from the

initial site.

Many damage sites have melted fragments of glass attached or partially covering the damage

site. These most likely represent fragments of glass from the disk itself, which have melted and

resolidified, but they may also be glass fragments from the encircling quartz shield tube shown in

figure 1, which have fallen onto the disk and initiated the damage. Figures 2 and 4 show typical large

damage sites with melted fragments of glass of unknown origin attached to the surface.

Small damage sites, 5-50 ym in diameter, are also very circular as shown in figure 5, and

generally appear to have contaminants or foreign particles fused into their surface. Fracturing of

glass is generally found to be correlated with disk position within the amplifier. That is, disk

-440-
surfaces facing upward generally sustain more damage than downward facing surfaces and several amplifiers have been found to have from 2-5 times more damage on their upward facing surfaces. This effect

is attributed to gravitational settling of particles either airborne in the nitrogen or ejected from

damage sites on the disks or on the quartz shield tube. It has also been noted that the first disk

surface, in the direction of gas flow, generally has more damage, by as much as a factor of five, than

any other disk surface in the same amplifier. This is attributed to its being the first surface upon

which nitrogen carried particles would come to rest.

Although foreign contaminant particles are rarely found attached to large damage sites, they

are the cause for all of the forms of damage mentioned above. Any dielectric or metallic particle

remaining on disk surfaces is rapidly heated by the flashlamp radiation and the laser pulse. The

2 cerium-doped-quartz flashlamps, pumped with 9KJ in 600 ps, supply in excess of 20 J/cm at the disk

surface. This is sufficient energy to heat and evaporate most particles smaller than 10 ym. The

exact mechanism by which the heated particles damage the disk is not fully understood but it is proposed that thermal diffusion of the contaminant may cause devitrification of the glass. Alternatively, a thermally induced stress caused by rapid heating and cooling of the contaminant particle in con- tact with the disk may be causing the cracking.

Optical Severity of Damage

A quantitative measure of the optical severity of damage is needed to ascertain when a disk

has sustained sufficient damage to warrant regrinding and repolishing, A quantitative measure of damage

is also useful for correlation with proposed causes for the damage such as number of amplifier firings

or incident energy.

The fraction a of the optical surface obscured by damage sites is one measure of the amount

of damage to the disk. It may be obtained by integrating the product of the area obscured by a damage

site and the size distribution of the damage sites. The cumulative size distribution of damage sites

found on both lightly and heavily damaged Ndrglass disks, shown in figure 6, follows a power function

of the form,

C = a d".

The count density C may be expressed as the number of damage sites per unit area greater than diameter

d; the parameter a determines the concentration and the constant n has been experimentally found to be

-1.64. The fraction of the optical surface obscured by damage is thus given by

-aniT r.n + 9 ^ n + 2 1 ° = 4(n + 2) [\ - '^s J

where d^^ and d^ are the largest and smallest damage sites on the surface, respectively. We have found

that with Nd:glass, d may generally be taken as 1 pm. To evaluate the function, the value of d. is

S determined by finding and measuring the largest damage site on a given disk surface. The parameter

a is obtained by microscopically observing several representative areas and counting the number of

damage sites greater than diameter d, called C^, and calculating

a = d-"

Since January 1977, all amplifiers overhauled have been examined to determine the extent of

damage. Figure 7 shows the unaveraged data pertaining to the quantity of damage sites per disk surface

as a function of the number of anplifier firings. There aopears to be an upper bound of 1 damage site/ cm > 100 ym corresponds to 0,1% obscuration per surface. The average level of damage is ,20 damage

sites/cm > 100 ym corresponding to .013% obscuration per surface or .92% per arm for the Argus system;

a very acceptable level.

The low correlation of damage with number of firings, shown in figure 7, implies that only

initially present contaminants or defects in the surface initiate the damage. The data also implies

that there is no long term surface fatigue effect and that disks examined after the first 10-20 firings

will have as much damage as after several thousand firings. This, of course, excludes the possibility

of inadvertent recontamination of the optics. It also implies that, at low repetition rates, very long

lifetimes can be expected if all surfaces are initially clean and remain clean.

B. Films on Disks

In addition to damage sites found during rebuilding, many disks have been found to have a light

scattering film covering their surface. Figure 8 shows this film on a 10 cm aperture disk. To

-441-
determine its composition, samples of the contaminant were collected using methyl chloroform and

ethanol rinses and analyzed in a gas chromatograph mass spectrometer (GCMS). The analysis found

dioctylphthalate (OOP), an unidentified terpene and a branched hydrocarbon. The concentration was

2 quite low and corresponded to 14 ng/cm of area rinsed. The cooling gas, obtained from liquid nitrogen

boil-off, was also sampled to determine if it was the source for any of the contaminants. Resin-filled

columns were used to sample nitrogen from the operating system and any absorbed organics were eluted

with ethyl ether and analyzed with the same GCMS. Very low concentrations of DOP were detected along

with the same terpene detected on the disks. A homologous series of hydrocarbons was also detected

which was similar to that found on the disks. The total concentration of organics measured was very

low, and corresponded to only 60 ng/J,. While it was at first thought that the film was a condensed

gaseous contaminant brought into the amplifier with the nitrogen cooling gas, it is unlikely that these

organic compounds are responsible for the film observed on the disks.

Subsequent microscopic examination of the film showed it to be composed of small spots and

crystalline fibers. The largest fibers are typically 2-5 ym in diameter and up to 500 ym long. Figure

9, a photomicrograph of the crystalline film, shows the large fibers and smaller spots characteristic

of the film. It is of interest that the fibrous film has been found on unfired as well as fired amplifiers and that the radiant energy from the flashlamps does not appear to diminish growth nor does the

film appear to aggravate damage.

The film is readily removed using ethanol and does not appear to be chemically attached to the

glass surface. Films similar to those found on disks removed from amplifiers have been artificially

grown by simply leaving a Nd:glass disk in an air-conditioned clean room at 50% relative humidity for

30 days. First evidence of a film becomes visible in as short a time as 2-3 days. All growth of the

film can be prevented in a vacuum dessicator. This evidence leads to the conclusion that the film is

the result of an atomospheric attack, possibly water vapor, of the glass surface.

The first evidence of the compositon of the fibers was obtained from an index of refraction

measurement. The fibers were found to be anisotropic and one of the indexes was determined to be 1.454,

using Cargille fluids. Anhydrous lithium hydroxide LiOH is a birefringent tetragonal crystal with

indexes of 1.464 and 1.452. The source of the lithium is the Nd:glass which contains almost 30 mol%

Li20.

Direct examination of fibers using an ion probe (IMMA) verified the existence of lithium

and nitrogen in high concentrations in the fibers. This implies lithium nitride Li^N, the nitrate

LiNO^, the trihydrate form LiN02*3H20, or the nitrite LiN02'2H20. Since lithium nitride decomposes

to LiOH in the presence of water vapor, there appears to be more than sufficient evidence to support

the theory of atmospheric attack of the Nd: glass. Further tests are planned to verify the composition

of the film using scanning Raman spectroscopy.

To determine the optical severity of the film problem, a single small sample of Nd:glass

wrapped in a lens tissue for over one year was examined with a Gary 17 Spectrophotometer. The film

which had grown on the surface reduced the transmission by over 0.3% per surface as compared to an average damaged disk of 0.013%. A heavily filmed disk can, therefore, reduce transmission much more

than a damaged disk. Removal of the film requires that the amplifier be removed from service, com- pletely disassembled for cleaning, and then reinstalled and realigned. Approximately forty man-hours

are required for this maintenance and this is significant for a large laser system such as the 20 arm/

100 amplifier SHIVA system.

C Damage to Coated Optics

Damage to coated optics on Argus has been minimal. On two occasions, coated spatial filter

lens have been found damaged. In both cases the damage occurred on the vacuum side of the lens,

suggesting damage caused by contamination due to vaporization of material around the pin hole or con- tamination from the ion pumps. The pattern of damage shown in Figure 10, appears as discolored areas forming several rings similar to large scale beam spatial modulations. The morphology of the damage

was examined in a SEM and photomicrographs are shown in figure 11. The spatial filter coating is com- posed of alternating layers of Si02 and Ti02, and photomicrographs clearly show that the discolored

areas are not surface deposits but regions in which the coating has delaminated. The delamination caused

the top three layers to be removed exposing the fourth Ti02 layer. The composition of the individual

layers was verified by the use of ESCA. Contaminants were not found, though, from either materials

around the pin hole or materials from the ion pumps. Currently damage testing is being conducted on coated optics under vacuum conditions to determine if the coating is drying out on the vacuum side.

It has been theorized that an inadequate bond, perhaps due to contamination during coating, resulted

in the delamination at this particular interface. Contaminants were not found on the coated surface,

and the reason for the damage occurring on the vacuum side of each lens has not yet been determined.

-442-
The circular pattern is due, apparently, to spatial modulations (noise) on the beam.

Concl usion

It appears that the most severe (i.e. expensive) and continuing damage problem on Argus is to the surfaces of laser disks and that this damage is due to flashlamp heating of contaminants. Damage

appears to create loose glass particles which can catalyze additional damage. Current cleaning and

surface preparation techniques have been able to produce amplifiers with sufficiently low damage

quantities that an entire laser arm will sustain less the 1% beam obscuration after 500 shots. The film

crystallizing out onto the disk surfaces is currently the most significant problem from the beam

obscuration point of view and an immediate solution is not available. It may be possible, though, to deplete the lithium from the glass surface and thus substantially extend the time between cleaning

periods and thus increase system reliability. Damage to coated optics is rare, given that beam fluence

is maintained below damage thresholds.

REFERENCES

1. Stowers, I. F.; Patton, H. G. ; Jones, W. A.; Wentworth, D,E,

Proceedings of 9th Annual Electro-Optics/Laser 77 Conference

Anaheim, California - October 25 - 27, 1977

-443-
FIGURES

TZl +J O CL 3 i- E O s- ro

t/i ^ o ro QJ

+-> D>

>+-

O

o S_ 0) 3 +-> ^

<D (/I !-> Q- Ol O JD C >>

Z3

o s1- -!-> 0) 4-> o ro

s- !-> OJ 0)O- 3

a. (/I

o

E -D

to NJ Ol

+->S~

ro s- ij^

o

(D +-> •(

—'

S_rs >.

4-> !-> S- (/I 0) >

O- 0)o ro

ro

o

l/l

E to

E

o ID ro ro C OJ LO ra Ol J3

CO f/1a -o

o.

c

t/1

E ro

fO

X

cn.— t/i s_ ^ (/I s- <:

ro > C ou

s-

-AA4-
s_

OJ

c -u

la OJ u E

ro U1 -r- +J XJ c

l/> QJ

E

Ol

a; ro I

S-

—

4-

Ol to D C

0) OJ

o ^ +-> •.- 1— <J -t-J OJ ro •-> U • Ol O ^ 1—

t/l (1) •r- ^ S_ o +-> cu

1/) "O +-> 01 c O

ro ro 1—D1T3 ro • •> (U O S- QJ

»-> ro

C

O

E

O ro ro

s- -o

QJ 4- M r— r- >^ ro

(/I rC

•—

o

<D ro

CT)

o 4-> ro •!- >-

ro CO "O O •!- ro

Si- Oi 0)

<a <-> -tJ 1— U ro 3 ro •!— O t. 4-> i- ro -r-

•r- ^ C o o •<-

0)

-445-
<u o• <u

+-> S- 0 O)

S- CD

1- fO 3 E

1/1 (D ' x;

01 ^ cu

!- oi

u I— (/I CD (O -c E s-

-t-j Lo aj a r— Sc: (o fo

rtJC 1

o

tu •- OJ

4-> +-' JC

•r- +J

•o

E

Qj rci

o

a> s- E o

ra XJ a s- (u

O) +-> '-^-Q (J E E OJ E = •-J C OJ

<D i/>

Ol &- &.

S. ,- <U

(0 TD

I— OJ ^ o < h- 4->

01

s- 0 4

-446-
OJE

fo ss- tt) ' 01 o E

Ol ro +J •!- .— -a

Ol

(/> •I- <u T3 U

(/I 4-

i/l S- nj u I— to CD

.. 01 o ^ Z •!-> c o O

0) TJ

•I- ^V) O

Ol +J

a)+->

ro ID E

<T3 to "O 10

0) 1—

cn cn

s. —I

O

-447-
(V 4- •.- O .— +-> 1— S- E fT5 fT3 (/) S- E a. on O LA rt3 E I O

O) OJ > u

x: 4-

s- . O 13

O cn

+-> <u I— ^3 +->

OJ "O

sc

S- <D

Q.QO

n3 +->c

I— -o

CD -rO) Clio

OJc

cn-.-

ro

E

O) 3 ro C 4- "D

C

cn OJ

I/)u

OJ -ro

Ol Qt/1

•r- SC S1/1 fO 3 ^ O) o- o o +J

OJ S- OJ crn-> n3 c

roc

E

ro

C

TZI .-E

.— fO .— +-> 4-* O-O ra

t/l -r— -r- O t- E

ro

EO

oo

u o. Ol

—

-448-
1000 E—I I I 1

1

E

CO Q 100

A

CMI

a

CO

1.0 -

3

N

0)

>

0.1 ECO I

0.01

o

0.001

1

I—I I I 1 1 lll|—I I I mil:

Typical size distribution

of damage sites

- 1.64 Slope

Typical \

contaminant o ^ Standard

particles flushed Arizona

from amplifiers road dust

Damage

Contaminants

I I I 1 1 ml I I I I mil I I I I I III! ' I'll' 10 100 1000

1 mm

10,000

1 cm

Diameter nm

Figure 6. Cumulative size distribution of damage sites measured on several Nd:glass disks. The average slope of the curves

is -1.64. The size distribution of typical contaminant

particles follows the same general form as the size dist'-ibution of the damage.

-449-
lO"" p 1—I—I—I

I I I

1

1

08.5 cm aperture disks)

^« } Argus

. a20 cm aperture disks ; ^ •10 cm aperture disks — Shi^a

1—I—II I I U

E

3. o

o

f—

A

CME

u

(/> 0)

10°

0)

CQE

Q

10 1 -

-•

10-

10

o

o

o

o

o

o

o

S3

0 _

8

0

a 0

0

53

0

000

000

000

0 0

i 0 00

> 0000

O sp

o

o

o o

JO.

o

JDL QIQ I I I I I 100

Amplifier firings

1000

Figure 7. Damage site density on Argus and Shiva Nd:glass disks as a function of the number of amplifier firings.

-450-
1
>i

+-*

•(

•a X)

S- I— o+J

SL| CTH-J >

c •1

o -t-J O) E n3 c d)

S- S- (U >

-l-> H- o ra

S- 0) o +-> u Hra 4- s- s_ s_ <u fO Hon

1/1 fO CL 3

lO E 00

S- !-

C7> O) Ol T3 4- -o

4- c O- E o -o E 3.

Ol rt3

t/) s_ "3- S- I <D ll- c +-> 1-

T3

OJ C o

c to OJ cu

s- o <u n3

0) _c 01

<a on Q. 00 O ono c

£^ E O)

4-> o la to S

i.

C31
0)

s- a> ra -Q

S- !-> o co

<u s: (/) t— co

•r— • +J

(/) <a c r— I— -oo

!-

ec

t— O)

ro

C

n3 0)

CO 4->

I/I <U 2o

<D I— O I—

<J

O

c0 01

•I- D>

+J fO U E

QJ 03 t— -o

4-

01 CU C 4-

-453-
Figure 11. Scanning electron micrograph of damage on AR coated spatial

filter lens. The individual layers of the coating are visible

at the higher magnifications.

C0MMEMT5 ON PAPER EV STOUERS ANV PATTON

The. 4peafeeA wcu, cu>kzd -c($ oveAcoaZinQ tkz iuA^ace the. EV-2 gtca-i wouZd iappn.e^i the gioMth Oj$ tht

tiikium YiUMitQ. {lilm. He nupondtd that thzy had tnA.e.d tivu, zxpzAAjmnt bij placing Aomptu o{j EV-2

glcuii OK)tfLC.oate.d w-Lth a. pn.otzcJU.vt ^-ilm, in a. highly humid atmoipheAz, thz LiN izmzd to {jOm eJjthnA

undeA on on top Oj^ the. pKottctive. coating, bat the. imaJii {^ibeM o{, liH did not ieem to appea/i. The

ipeakeA. commented {jtwtheA. that a conclusion one dfuxwi {^n.om thl& wonk ii that given a choice, {^on. a

cLiic omptL^ieA, one woutd pn.e{^eA to iet it on edge, io that the/ie we/ie no upwaAd {^acing i,Lin.{,acu.

-454-
NEW RESULTS ON AVALANCHE IONIZATION AS A LASER DAMAGE

MECHANISM IN TRANSPARENT SOLIDS

A. A. Manenkov

P. N. Lebedev Physical Institute

of the Academy of Sciences of the USSR

Recent theoretical and experimental investigations carried out in the P. N. Lebedev

Physical Institute of the Academy of Sciences of the USSR on electron avalanche ionization

as a laser damage mechanism of solid transparent dielectrics are discussed. Formulas

obtained on the basis of solution of the kinetic equation for electrons in the conduction

band are given for the threshold (critical) fields E^,. The fundamental characteristics

of the avalanche mechanism, namely, dependence of E^ on the sample temperature, radiation

frequency and pulse duration are noted. Experimental results on the temperature dependence

of damage thresholds for a number of alkali-halide crystals and other crystals used in

laser optics are analyzed on the basis of avalanche ionization theory. These include NaCl,

KCl, KBr, Csl, KI, RbCl, CsPr, AI2O2, YAG, Si02, CaF2, LilO^, BaF2, NaF and LiF. Damage

results were obtained with CO2 (10.6 ym) , NdrYAG (1.06 pm, 0.53 pm) and ruby (0.69 m) laser

radiation, in pulses with duration of a few ns.

This analysis shows that for those crystals of NaCl, KCl and KBr having the highest

damage thresholds, the avalanche ionization can be responsible for laser damage in the

range of A = 0.69 - 10.5 pm. This conclusion is supported by a rather good agreement

between the theory and the experiment for the frequency dependence of the damage threshold

over that frequency range at room temperatures and for the temperature dependence in the

100°K to 800°K range at high frequencies (\ = 1.06 pm and 0.69 pm) . However, the theory

does not explain the temperature dependence of the damage thresholds observed above 300°K

at low frequency (A = 10.6 pm)

.

It is shown that at A = 0.53 pm, the laser damage of alkali halide crystals is not

explained by avalanche ionization, and it is probably attributed to multiphoton ionization.

For the other crystals investigated (in particular, for AI2O2, NaF, and LiF with

high damage thresholds) the observed frequency and temoerature dependence of the damage

threshold at A = 0.69 pm and A = 1.06 pm do not correspond to values predicted by

avalanche ionization theory. The question of the nature of the damage mechanism of

these crystals is still open.

Finally, we shall discuss the previously published results on the dominant role of

avalanche ionization in laser damage of the alkali halides and some other crystals. The

qualitative arguments on which this conclusion was based, are shown not to be confirmed

by our data, which was obtained in more detailed experiments, and analyzed using a con- sistent theory of avalanche ionization.

Key words: Laser damage; avalanche ionization; optical materials; damage thresholds;

temperature dependence of damage thresholds.

1. Introduction

Electron avalanche as a laser damage mechanism of transparent solids has been discussed for over

10 years. It has been assumed that the process of avalanche ionization should be basic, and that it

defines the limiting resistance of pure transparent materials to laser radiation in the nanosecond range

of pulse durations. However, in spite of the rather large number of theoretical and experimental

efforts which were devoted to this process, the question of whether electron avalanche is the effective

damage mechanism of real optical materials was open, up to now. The reason was, that an adequate

experimental study supported by a consistent theory of electron avalanche in solids was not undertaken.

Early experimental studies were restricted to measurement of the damage threshold in somewhat

accidental experimental conditions (often using multimode lasers), and by quantitative analysis based

on insufficiently adequate theoretical models.

Later on, more careful and informative measurements have been carried out in appropriate experimental

conditions which resulted in more reliable and reproducible data (using single-mode lasers, and taking

-455-
into consideration the effects of self -focusing and absorbing inclusions). However, lack of sufficiently

full and appropriate experimental data, as well as a consistent theory of the avalanche ionization process

in solids did not, until recently, allow one to make well-founded conclusions on the role of this mechanism in laser damage. This, we believe, refers also to some recently published papers of American,

Japanese and Soviet authors [1-5] in which it was claimed that in the alkali halide crystals investigated,

intrinsic damage caused by electron avalanche ionization has been observed. Research carried out at the

Lebedev Institute has shown that the qualitative arguments on which the quoted authors based their con- clusions are not confirmed by more complete experimental data and an adequate theoretical analysis.

In our experiments, we proceeded from the basic assumption that for a well-founded solution of the

avalanche mechanism problem in laser damage, it is necessary to develop a consistent theory, which could

give not only the numerical estimate of the damage thresholds, but what is more important, makes it

possible to elucidate which characteristic features, specific to the mechanism, should be investigated

experimentally. The developed theory [8-10] shows that the dependence of the damage threshold on frequency, pulse duration and initial temperature of the sample turned out to be characteristic for the

avalanche mechanism. We studied these dependences for a great number of alkali halides, and other

3+ crystals used in laser optics (such as sapphire, YAG.Nd , LilO^, SiO^) at the CO^, Nd:YAG and ruby

fundamental laser frequencies, and at the second harmonic of the Nd:YAG laser in a wide temperature

range (100 to 1000°K) . The analysis of the experimental data on the basis of our developed theory has

allowed us to make, for the first time, well-founded conclusions regarding the role of electron avalanche

in laser damage.

The main results of our theoretical and experimental investigation as well as a comparison with

data from the other work mentioned above are given below.

2 . Theory

The problem of determining a critical field (or threshold intensity) of laser radiation producing a breakdown in a transparent dielectric, caused by a process of impact ionization, reduces to that of

finding the electron avalanche rate y (E) as a function of the electric field E, and of defining a physically adequate damage criterion. An effective procedure for finding y (E) has been given by

Epifanov [8]. It is based on the solution of quantum kinetic equation for the electron energy

distribution function f(e, t), which has the following form:

^llS^.iL ^B2(q) I J

2

3t h q

eEqT(p) {[f(p+q)(N^+l)

-f (p)N^]6(£(p+q)-e(p)-tia;^-nhn) (1)

-+[f (p+q)N^-f (p) (N^+l)]6(e(p+q)-e(p)+fiu^-nhfi)}

The damage criterion has been determined by consideration of the kinetics of lattice heating due to

absorption of the electromagnetic field energy by free electrons, which is described as follows:

The following designations were used in (1) and (2): e and m are the electron charge and mass, respectively. E and Q are the strength and frequency of electromagnetic field. P(e) is the momentum

of an electron with energy e. B(q) is the matrix element of the electron-phonon interaction. is

the number of phonons with wave vector q, -Ku is the phonon energy, T(e)is the relaxation time for the

longitudinal component of electron momentum 3n zero field, and T is the lattice temperature.

Following this research we have considered [9, 10, 7] the electron avalanche process in detail,

taking into account in electron-phonon scattering, both acoustic and optical phonons. As a result,

rather simple formulas have been obtained for the critical field. They describe the characteristic

properties of electric breakdown in solids in a wide range of frequency from zero (the dc field) up

to the frequency of visible light. Formulas in the case of electron scattering by acoustic phonons*

for various ranges of radiation frequencies and lattice temperatures are written as follows:

*Allowance for scattering by optical phonons for typical crystal materials only slightly changes the

E value.

c -456-
6elm2v 2 s

kTe2

mi ^

ac

'Rn<£ , T>T (3)

E

2 c

3(hf2)2in2v^2

kTe^el

[ch

fin

el -1], nT>>l, T>Ti n

(4)

15mVg(2inI)

2e2

21

5ml 2 ac

-fin<e, T<T (5)

E

2 c

15mVg/2mI(hn)2fi2

YE I2e2

[ch £l

-1] T<T (6)

Here e is the parameter related to the damage criterion yt L by

= In L6

where t is the radiation pulse duration, and L and 6 are parameters whose numerical values typically

-15

are L = 15, and 8 =4.10 sec [10]. A transient temperature separating the high and low temperatures

regions is determined by the relation:

T = h V /2^ n s

where is the sound velocity in the crystal, and I is the ionization energy. Other values in (3) -

(6) have the following meanings: e = el is the average electron energy, 1 - is the free path of an

ac electron with the energy of e = I. Formulas (3) - (6) are justified in the frequency range of

JtSI/4fi. For higher frequencies one must use a numerical solution of the kinetic equation.

From the given formulas it is evident, that the fundamental laws of the avalanche ionization process

which can be used for experimental confirmation of its role in laser damage give rise to specific dependences of the critical field on frequency and pulse duration and on the temperature of the sample.

The frequency dependence of the critical field has a rather complex, non-monotonic form. It remains practically constant with field frequency, up to = V^^^, it grows, at first rapidly, then more slowly. (see fig. 1).

Our theory predicts very specific features for the temperature dependence of the critical fields.

It is quite different in the two cases when the field frequency is higher or lower than the electronphonon collision frequency, as it is shown in fig. 2.

The dependence of E on the pulse duration in the whole region where the developed theory is

-7 -11 applicable (t^ = 10 -10 sec) turns out to be very weak.

The indicated characteristics, which are predicted by avalanche ionization theory, have been used

for experimental elucidation of the question of whether laser damage in optical materials is associated

with electron avalanche for nanosecond light pulse duration. The frequency and temperature dependence

of the damage thresholds were studied most carefully.

3. Experimental Results aad discussion

3+

In the experiments we have used the TEMoo single-mode COj . Nd:YAG and ruby lasers as well as the

Nd:YAG laser with frequency doubling in a LllO^ crystal. The output characteristics of the lasers are represented in Table 1.

-457-
To produce bulk damage, the laser beam was focused into the sample by short focal length lenses.

The spatial intensity distribution in the local plane of the lens was carefully measured. [7] It was

close to gaussian at all laser frequencies used. Table 2 shows measured focal spot diameters (at the

level of 1/e of maximum intensity).

In the experiments, the effects of lens and sample aberration, optical inhomogeneity of the

crystal and self-focusing were controlled. These effects proved to be negligible under our experimental

conditions, which gives us assurance that the real damage threshold intensity is being measured. Such assurance is important in reaching reliable conclusions on damage mechanisms, because it is based on

a quantitative comparison of the thresholds measured in experiments with widely varied parameters, such

as radiation frequency and sample temperature. Measurement errors in our experiments were about 40%

for the threshold frequency dependences, and 10% for the temperature dependence and relative threshold

values of different samples at fixed laser parameters.

Similarly, to obtain reliable data, the experiments were conducted with a great number of crystal

samples. Thus, for NaCl, we have investigated more than 100 samples grown by different techniques.

These experiments showed that from sample to sample there are considerable variations of the damage

threshold, indicating impurity effects.

As an example fig. 3 represents a distribution function of the damage thresholds of crystal samples

of NaCl measured at the Nd:YAG laser frequency, at room temperature. The majority of samples are seen

to fall into the range corresponding to a rather low optical resistance. There are some samples having

a much higher threshold, which appears to be a limiting value. Note that in previously published work

such high damage thresholds of NaCl were not reported, and the values represented by our first group of

samples were typical. [1-6]

Investigation of the damage mechanism in low-threshold and high-threshold samples has shown that

for the former, the temperature dependence of the threshold behaves typically in a non-reproducible

fashion, which, in general, does not correspond to that predicted by electron avalanche theory.

Figure 4 shows some typical examples of the temperature dependence, at the ruby laser frequency, for

NaCl crystals. While investigating the reason for this temperature dependence non-reproducibility the

effect of thermal strengthening was observed. At all frequencies under investigation except the

C02~laser, for many samples of NaCl, KCl, and KBr, the damage thresholds increased considerably with

appropriate thermal treatment, in which the sample was held for a long time at high temperature close

to the melting point, followed by rapid cooling. This effect is more pronounced for the samples with

low initial thresholds. The samples with maximum damage resistance are not affected by the thermal

treatment. This effect of thermal strengthening is reversible. Thus, prestrengthened crystals after

long annealing return to their initial threshold values.

The analysis of the described thermal effects, in the case of laser damage of low-threshold samples,

showed that the latter involved the influence of impurity atoms in the crystals. Impurity clusters in a crystal can be either formed or dispersed, depending on the thermal treatment i.e., heating duration

and cooling rate, and this can substantially affect the damage threshold.

The study of the frequency dependence of the damage threshold has shown that it has a considerably

different character for various samples. One observes either an increase, constancy, or a decrease of

the threshold with increasing frequency. Constancy or a decreasing threshold are observed, as a rule,

for low-threshold samples. Figure 5 illustrates the frequency dependence observed for several NaCl

samples. Obviously, the observed variation of the frequency dependence of the damage threshold, like the

anomalies in temperature dependence, arises from the influence of impurities and absorbing defects.

The temperature and frequency dependence typical of intrinsic damage can be expected in samples

with the highest damage thresholds at all frequencies, without thermal treatment. Accordingly, we have

especially selected samples of NaCl, KCl, KBr, Csl, NaF, LIF and AI2O2 with the desired properties, and

carefully studied the temperature and frequency dependence of their damage thresholds. The damage

thresholds observed for these samples at T - 300°K are shown in table 3. For NaCl, KCl, KBr, and Cal,

the observed dependence on frequency and temperature proved to be similar, and to agree qualitatively

with those predicted by avalanche ionization theory, as it is seen from the data for NaCl shown in

figs. 6 and 7. In fact, in the wavelength range of 10.6 to 0.69 pm, one observes an increase of the

damage threshold as well as a change of the temperature dependence character, with an increase in laser"

frequency. Temperature dependence is practically absent at A = 10.6 ym, while at X = 1.06 ym and

0.69 ym the damage thresholds drop with increasing temperature. This allows us to assume that in the

selected, high- threshold samples of alkali-halide crystals, laser breakdown is due to electron avalanche.

To support this assumption, we have analyzed the experimental data on the frequency and temperature

dependence by more careful and quantitative comparison with theory. The comparison has shown that there

is rather good agreement between the theory and experiment for the frequency dependence of damage

thresholds from 10.6 ym to 0.69 ym, at room temperature, if for the electron-phonon collision frequency,

one assumes certain values, which seem to be rather reasj^able^^ For NaCl crystals, as an example, the

collision frequency has been found to be equal to 6 x 10 sec . This number fits the theory and

experiment rather well for both the frequency dependence of the damage threshold in the range from

-458-
10.6 ym and 0.69 pm at room temperature, and the temperature dependence of the thresholds observed at the wavelengths 1.06 pm and 0.69 pm. It gives also absolute values of the critical field, estimated

from formulas (3) - (6), which are very close to the thresholds measured at 1.06 \im. However, the

14 -1 collision frequency magnitude of 6.10 sec does not correspond to the temperature independence of the

damage threshold observed at 10.6 ym (see fig. 7). In this case the condition n<v is satisfied and ef f hence the damage threshold should rise with increasing temperature above 200°K. We have not yet a good

explanation, supported by consistent theoretical considerations, or confirmed by appropriate experiment,

for this disagreement between theory and experiment. It is reasonable to note here that the temperature

independence of the critical field for dc breakdown previously observed [12], is also not readily

explained. One may suppose that the dc case and low optical frequency case arise from similar causes.

It is obvious that further theoretical and experimental studies have to be carried out to clarify the

peculiarity of low frequency breakdown.

Another result which also cannot be explained in terms of avalanche theory is the temperature

independence of the threshold observed for alkali halide crystals at 0.53 ym. (fig. 7). For that

wavelength, the condition n>v^^^ is definitely fulfilled, and according to the avalanche theory, the

damage threshold has to be a decreasing function of temperature above 200-300°K. One can reasonably

suppose that at this high laser frequency, damage is associated with the multiphoton ionization

mechanism, rather then with the electron avalanche process.

We have also found that experimental data obtained on the temperature dependence of damage

thresholds for NaF, LiF and ^120^ crystals, over the range of wavelengths investigated (1.06 ym, 0.69 ym and 0.53 ym), are not fully explained by avalanche ionization theory. Thus, damage thresholds

of NaF and LiF at 1.06 ym and 0.69 ym are practically independent of temperature, but for the avalanche

mechanism such a situation can take place only in a very narrow frequency region, when n = ^eff"

sapphire, one observes frequency independence of the damage threshold in the whole range investigated

at T = 300°K (see table 3), as well as a decrease in threshold with temperature at T = 300°K, at all

frequencies. Such dependences do not agree with the prediction of avalanche theory. Thus, the nature

of the laser damage mechanism in these crystals is still an open question.

We have also analyzed data obtained previously [1-6] on laser damage of alkali halide crystals,

and compare them with our results. Such an analysis has led us to a conclusion that the simple arguments on which the authors of these cited papers based their conclusions about the avalanche damage

mechanism of the crystals investigated, are not confirmed by our data. For example, the lack of dependence of the threshold on frequency up to = 2.7 . 10"'"^sec ^ (X = 0.69 ym) '^'^'^ or even up to

= 5.4 X 10'''^sec ^ (\ = 0.35 ym)'-^^ was taken as one of the decisive arguments in favor of the avalanche

mechanism. But as we have shown, such independence indicates that the avalanche mechanism has not been

realized, since it is typical of samples with relatively low optical resistance. In high threshold

samples, as has been discussed above, the frequency dependence (increase) of the damage threshold is

already observed on going from \ = 10.6 ym to X = 1.06 ym, corresponding to the condition ^^^^^^ for

frequencies higher than that of the Nd laser. This condition is additionally confirmed by the character

of the temperature dependence, as discussed above. Figure 8 illustrates the divergence of data on the

frequency dependence of damage thresholds in NaCl, obtained by three groups: Lebedev Physical Institute,

Moscow State University, and Harvard University. It seems likely that the lower thresholds measured

at Moscow State University and Harvard University are associated with the effects of impurities and

absorbing inclusions in their samples.

Some authors have claimed [4, 6] that they succeeded rather confidently in distinguishing between

inclusion and intrinsic damage by morphological studies. Detailed investigation carried out in our laboratory by Gorshkov [11] showed, however, that a damage morphology observation could not be used as test of intrinsic damage. This is illustrated by the following experimental fact. In the samples with

rather high optical resistance, the damage character is found not to change, even with considerable

variation of the damage threshold. (For example, for LiF at 1.06 ym, from 1 x lO"'"''' W/cm^). The

damage observed in these cases was similar to that referred to previously as intrinisic.

4. Conclusion

The results discussed in this paper show that the problem of avalanche mechanism in laser damage

is very complex, and a well-founded conclusion on its role in real crystals can only be made based on detailed investigation of the fundamental damage characteristics, and by comparison with an adequate

theory. The experimental data, analyzed on the basis of a consistent theory of avalanche ionization in

solids, obtained by solution of the quantum kinetic equation, shows that in those alkali halide crystals

with high damage thresholds, electron avalanche is the probable mechanism of laser damage for nanosecond light pulses in the wavelength range of 10.6 ym to 0.69 ym. The theory satisfactorily explains

most of the observed characteristics concerning the frequency and temperature dependence of the damage

threshold, and yield reasonable magnitudes of the principal parameter, the electron-phonon collision

frequency. However, the theory does not explain all the experimental data concerning the frequency

-459-
TABLE 1.

CHARACTERISTICS OF LASERS USED IN DAMAGE EXPERIMENTS

Laser

CO2

Nd:YAG

Ruby

Nd:YAG with

LilO^ frequency

doubler

Wave length

(um)

10.6

1.06

0.69

0.53

Output Energy

(mj)

60

15

25

2.5

Pulse duration

(nsec)

60

1.5-15

10

TABLE 2.

FOCAL SPOT DIAMETER (ym) AT 1/e IN INTENSITY

Wavelength (ym) 10.6 1.06 0.69 0.53

Focal length (mm)

15 - 4.6 3.6 4.0

38 - 12 10 11

100 51 -

TABLE 3.

ROOM TEMPERATURE DAMAGE THRESHOLDS (xl0''"°w/cm2)

Javelength

(ym)

Pulse Width

(nsec)

NaCl KCl KBr Csl NaF LiF AI2'

0.53 8 13 2.5* 6 0.9 9 24 40

0.69 10 15 8 5,8 1.1 14 36 40

1.06 10 12 7 5 1.2 14 36 40

10.6 60 2.5 0.1 1.2 0.05*

* There is some indication that the thresholds marked are not the highest at particular

frequencies.

-460-
and temperature dependence over the whole range studied. A further development of the theory, which

will consider, in particular, the real structure of the conduction band of dielectrics is required to explain the remaining disagreement.

Thus, the results discussed in this paper show that the problem of avalanche ionization as a laser

damage mechanism proves to be more complex than previously assumed. It becomes clearer, however, since more detailed and informative experimental data and sufficiently developed theory are available, that

further investigation is necessary to solve this problem, which is important both for physics, and for

high power laser engineering.

5. References

[1] Yablonovitch, E. , App. Phys. Lett., 19, 495 [7]

(1971).

[2] Fradin, D. W. , Yablonovich, E. and Bass, M.

,

Appl. Opt., 12, 700 (1973].

[3] Fradin, D. W. and Bass, M. , NBS Spec. Pub. [8]

387 , 225, (1973).

[4] Bloembergen, N. , IEEE J. Quant. Electr., [9]

QE-10 , 375 (1974).

[5] Yosojima, Y. , Ohmari, Y. , Okumura, N.

,

and Inuishi, Y. , Jap. J. Appl. Phys., 1^, [10]

815 (1975).

[6] Aleshkevich, V. A., Akhmanov, S. A., Zhdanov,

B. v., Kovrigin, A. I., Kuznetsov, V. I., [11]

and Sukhorukov, A. P. , Zhurnal tekhnicheskoi

fiziki, 46, 1693 (1976). [Soviet Physics,

Technical Physics 21, 975 (1976)]. [12]

Gorshkov, B. A., Danileiko, Yu. K.

,

Epifanov, A. D. , Lobachev, V. A.,

Manenkov, A. A., and Sidorin, A. V., JETP,

72, 1171 (1977). [Translation to be

published ]

.

Epifanov, A. S., JETP, 67, 1805, (1974).

[Soviet Physics JETP 40, 897 (1975)].

Epifanov, A. S., Manenkov, A. A., Prokhorov,

Prokhorov, A. M. , JETP Pis'ma, 21, 483

(1975). [JETP Letters, 21, 223 (1975)].

Epifanov, A. S., Manenkov, A. A., Prokhorov,

A. M., JETP, 70, 728 (1976). [Soviet Physics

JETP, 43, 377 (1976)].

Gorshkov, B. G. , Thesis, P. N. Lebedev Physical Institute (1977).

Kuchin, V. D. , FTT (Fizika tverdovo tela),

1, 450 (1959). [Soviet Physics, Solid

State 1, 406 (1959)].

Figures

o

15

10 -

0.05 OA O.IS 0.2 0,25

Re/otive Photon Eiwrgy (fiQ/l)

1

0.5 I 1.5 Z

Rehtne Temperolure (t/T„)

Figure 1. Dependence of critical field upon

radiation frequency as predicted

by avalanche ionization theory. A

collision frequency of 6xl0'''^sec has been taken in the calculations.

Figure 2. Dependence of critical field on lattice temperature in two limits

where the radiation frequency, fi, is much greater or much less than

the electron-phonon collision

frequency, '^g££'

-461-
n

I. . I W 20 50

Threshold. QW/cm^

Figure 3. Distribution of laser damage

thresholds at 1.06 ym for NaCl

samples.

-462-
— - Theory

' ' I 2 5 4f Frequency, " fQ radn s

1.06 0.69 0,53 • Wavelength, um

Figure 6. Frequency dependence of damage threshold

for selected high optical resistance

samples of NaCl at T = SOO^K (experiment

and theory)

.

3 A - m^m

iOO T K

Figure 7. Temperature dependences of damage

threshold for selected high optical

resistance samples of NaCl at various

frequencies

.

• - Le6edev Inst,

Moscow tin'-

-Harvard Univ.

Figure

2 3

Frequency, '10 radn/s.

8. Dependence of damage threshold upon

frequency for NaCl after measurements

by three research groups: Lebedev

Institute [?], Moscow University [6],

and Harvard University f3,4].

-463-
COMMENTS OW PAPER 8/ UANEMKOV

Thz ApeafeeA jLHitA^^^-izd the, n2.gZe.ct heat tn.anipoHt In the theofLeticaZ model, by cuigatng that {^on. nanoiecond puZiej> the dt&tance tn which heat couZd be expected to diHaie, uAtng fioom tempexatuxe

dl{j(iiU,ton CO eiilcienti, , Ma^ on the oxdeA JO centAjmete/u, , much maJUieA than the docal volume.

Lee Smith o{j LLL pointed out that expefiimentii done with picosecond pulses ihow that the Incipient

pla&ma that onlglnaZty {,onmi In the breakdown pxoceM is onZy a ^ew urn In dimension, and then.e{^one

that heat tfiampoxt oveA dlitanceis of, the onden. o{, a ym couZd be ilgnl{,lcant In the eaxZy itageJ, 0(5 the (jOHmatlon o£ the ptaima. In addition it ihoutd be noted that the pieAence 0(5 conduction etectAoni

In loAge numbeAA In the vicinity o{j the bA.enkdown may aZso Increase the thenmal dif^duAlon coeHletent.

In conttnulng dticuiilon 0(J the puLie length dependence quu-tion, the ipeakeA Indicated that In expentmenti he had coMted out uAlng VAG iMeA iMadtation, only a veAy weak puLi>e duAotion dependence wa4

ieen ^on the bKeakdom thfie^hold In vefiy puAe matentaLi,

.

-464-
tJULTIPHOTON THEORY OF OPTICAL BRE/JCDOWN IV. ALKALI HALIDES

A. Schmld and P. Kelly

Division of Physics

National Research Council of Canada

Ottawa, Ontario KIA ORl

and

P. Braunlich

Department of Physics

Washington State University

Pullman, Washington, 99163

A theory of optical breakdown in dielectrics is presented; based on multiphoton carrier generation and energy transfer from the radiation field to the

lattice via polaron-photon absorption. This mechanism represents a clear

alternative to avalanche breakdown models as no lattice impact ionization is

involved. Breakdown field strengths calculated for various laser pulse lengths

and frequencies, are fully corrected for the nonlinear refractive index nj as well as for contributions to the refractive index resulting from a sharp increase

in polaron density during a damaging pulse.

Key words: Breakdown flux and field strength; frequency dependence; intrinsic

laser damage; linear and non-linear refractive indices; multiphoton

absorption; polaron; pulse length dependence.

On behalf of my colleagues Drs. Peter Braunlich and Ansgar Schmid of Washington State University,

I will present the main points of our new theory of optical breakdown in the Alkali Halides. A further description will be published shortly^.

Basically we will show that intrinsic damage is caused by polaron joule heating. It is necessary,

then, to define criteria for intrinsic breakdown.

As in the past^ we find it useful to represent the temporal profile of a laser pulse by the

relation

2 F = A* sin (TTt/T ) P

with the pulse duration. We consider that breakdown occurs at the peak of the pulse i.e., when

t = T /2 such that F = A? when a small volume of material has reached its melting point T, P B M.

Band-to-band excitation of electrons can take place by single or multiphoton absorption. Consider

22 -3

an initially empty conduction band n (0) = 0 and a full valence band n (0) w 10 cm . If a,, is the

generalized absorption cross section

c then the conduction band concentrationon

is given by

n (t) = n (0)

c v

l-expC-Oj^ j

r^dt)

An electron generated in this manner carries with it a lattice polarization field. It is necessary

to consider in some detail the polaron, the composite particle of electron and phonon field, and its

interaction with photons and the lattice. Multiphoton absorption by polarons has received much

attention and most recently Pokatilov and Fomin^ have derived expressions of the total absorption

coefficient, K, for a variety of interaction mechanisms using the path integral method of Fejmman

et al^

In anticipation of our results let me point out that when one considers a photon-polaron-phonon

interaction whether it be with acoustical or optical modes the dominant term (j99%) in

N= 1

-465-
is that for single photon absorption so K~K^j and aside from material parameters

K=iK, (T, n , -fto), fioj , V, E^)

i c o

where "Ruj is the laser photon energy, -Rtu^ the respective phonon energy (dispersion is ignored), E the

RMS local field intensity and V depends on the optical coupling constant a or on the deformation

potential constant 3^ dependent on the mode involved (Note well, the presence of E^) . Generally,

the local refractive index n^, contains a non-linear contribution for high fields i.e.

2 n = n + n. E

r o 2 2 where n^ is the linear refractive index (Again note the presence of E )

.

In general the local field strength is unknown although it is related to the flux since, with c a constant, one has

E^ = cF/(Xnp

2

, preceding terms in E can be related to the Flux.

It remains then to ascertain the effect of a large polaron concentration on n^ and then to compute

the temperature rise of the sample produced by polaron joule heating since

dT/dt = F.K. iiuj/pc o with c^ the specific heat and p the mass density.

Quite generally, the permittivity of Alkali Halides consists of a polaron contribution and a lattice contribution i.e..

'P

The lattice contribution is given by

2 2

f

((jj - 0) - iyu)

2 2

(to^ - 0) - iyoj)

where y is the damping parameter for optical vibration (and is considered negligibly small).

In the case of optical phonons the phonon contribution has been analyzed by Gurevich et al^

in the limit of small coupling resulting in

e (oj) = e , + ie - = 4it i a(a)) /u

P pl p2

where the real part of the conductivity is given by^

2

R a(uj)

We have followed their procedure but we have replaced the matrix element for free-electron optical

phonon scattering by that for free-electron acoustical phonon scattering and obtained

2 2 3/ pU

where is the speed of sound. In either case.

oM = ^

0^)^ [1- (exp (*. /kT) . 1)

-A66-
Im a(a)) = n e /mo)

The linear refractive index is then given by

n + e 11

2

Resul ts

A comparison of optical and acoustical modes of scattering, with all the parameters the same except for the interaction term, V, shows that the dominant mechanism in NaCl is photon-polaron- acoustic phonon scattering.

Figure 1 shows the behaviour of the local refractive index as a function of time for a 30 psec

Ruby pulse. The insert illustrates the self-focussing effect, n2E^ and the self-defocusing effect due

to the rapid increase in polaron concentration, n . The corresponding temperature increase is also

included.

Since in general the value of the refractive index has not been measured in damage experiments

we show in figure 2 the breakdown Flux vs The slopes are here given by (M+1) /2 where M is the

order of the multiphoton generation term. Please note that we used the smallest values of available

in the literature.

Since we know the refractive index we can compute E and in figure 3 we show E vs t Now, at

D Bp

least in the nanosecond regime, the slopes are given by (M+1). The ND line shows a large deviation

from linearity.

Figure 4 illustrates the frequency dependence of the breakdown flux for different pulse lengths

and in figure 5 we show the nanosecond behaviour of E vs -fiui and compare it with existing experimental

data.

In conclusion, then, we have presented a theory which fits experiment and which is a clear

alternative to the avalanche mechanism.

References

(.1) Schmid, A., Kelly, P. and Braunlich, P. to be published (Phys. Rev. B Nov 15, 1977)(4) Feynman, R. Hellwarth, R. , Iddings, C. and

Platzman, P. Phys. Rev. 127, 1004 (1962)

(2) Braunlich, P. , Schmid, A. and Kelly, P. Appl. Phys. Lett.' 26, 150 (1975)

(5) Gurevich, V., Lang, I. and Firsov, Y. Sov. Phys. - Solid State 4^, 918 (1962)

(3) Pokatilov, E. and Fomin, V. Phys. Stat. Solidi, B _73, 553 (1976)

-467-
Figures

time ^\

Figure 1. Time evolution of the refractive index n, the density of polarons

n and the temperature AT = T - 300°K during a 30 psec ruby laser

pulse of the form F = 8.4 x 10 sin ( nt/tp) [photons cm sec J

for which breakdown occurs at t = = 15 psec. The insert

shows details of the initial increase of the refractive index due

to the non-linear refractive index n^.

-468-
Figure 2 , Breakdown photon flux Ag* in NaCl vs. inverse laser pulse length.

t calculated for different laser frequencies:

P Nd: neodymium laser frequency

2Nd: doubled frequency of neodymium laser

Rb: ruby laser frequency

2Rb: doubled frequency of ruby laser

Xen: xenon laser frequency

-469-
Figure 3. Breakdown field strengths Eg under the same conditions as in

figure 2.

-470-
Figure 4. Breakdown flux A^* in NaCl vs. the energy of the laser photons for

D different pulse length t

-471-
Figure 5. Breakdown field strength E„ in NaCl vs. photon energy for a

D

tp = 60 nsec pulse. Experimental data from D.W. Fradin Harvard

Univ. Tech. report No. 643, 1973 (ONR-372-001 2) unpublished.

COMMENTS ON PAPER BV SCmW, KELLV, kUD W.kUUllCH

It Mcu> gzneAciity agizzd that -tn on.deA to take. duIZ acc.ou.nt o{^ ipatiat e^^ecXi a much mote. iiopka>ticaZe.d

caZcaZation Mould be nexiuAjizd. The authofu -ind^ccuted that In thii calculation thzg we/ie onJLij intoAutzd

In the tmpofuxl dcvtlopmcnt o{^ the lontzatyion, and that they axe pfie^ently extending theJji calculation

to Include ipatlal ed^cct^, Including pn.opagatU.on thAough an Ionized region o^ {^ImXe e.xtent.

-472-
THE ROLE OF CARRIER DIFFUSION IN LASER DAMAGE OF SEMICONDUCTOR MATERIALS

M. Kruer, L. Esterowitz, F. Bartoli and R. Allen

Naval Research Laboratory

Washington, D. C. 20375

The role of carrier diffusion in laser damage of semiconductors is

investigated for various laser wavelengths. The results indicate that carrier

diffusion significantly affects the magnitude of the damage threshold when the

optical absorption coefficient is greater than 10^ cm ^,

Key words: Laser damage; semiconductor damage; carrier diffusion; time

dependence; wavelength dependence.

1. Introduction

The response of a semiconductor to laser pulses depends on the optical, thermal, and electronic

properties of the semiconductor as well as the wavelength, duration, intensity and spatial characteristics of the laser radiation. If the photon energy is larger than the bandgap , the irradiated

surface undergoes thermal damage when the laser flux is increased beyond some threshold value. In this paper such thresholds are analyzed to determine the role of carrier diffusion in laser damage of

semiconductors. Thresholds for surface melting are examined since the experimental criteria for

damage is well defined, the threshold temperature is accurately known and the data analysis is not

complicated by phase changes. A thermal analysis is performed to calculate melting thresholds as a function of irradiation time. To investigate the role of carrier diffusion in laser damage in semiconductors it is desireable to analyze threshold data for laser pulses sufficiently short that thermal

diffusion effects can be neglected. For such pulse durations, the material is heated to a depth 6 determined by the optical absorption depth and the carrier diffusion distance [1]'. The carrier

diffusion distance, however, is not well known under the conditions of laser damage since both the

diffusion coefficient and lifetime decrease during laser heating. Similarly the optical absorption

coefficient can change for photon energies equal to or less than the bandgap, because of band shifting

or free carrier effects. In this work & was treated as an adjustable parameter in the analysis and

obtained empirically from experimental thresholds.

2. Thermal Analysis

We consider a homogeneous semiconductor irradiated by a Gaussian laser beam of the form: P(r) =

2 2 exp(-2r /R ) . We assume that values of the material properties may be treated as constants. These

are to be considered "effective values" over the wide range of temperatures and carrier densities

encountered during laser heating. If heat does not diffuse to the edges of the material, the thresholds

can be approximated using a thermal model in which the sample is treated as a semi-infinite solid. We

have shown previously that the power density threshold as a function of irradiation time t can be

approximated by the expression [2]

:

^o = ^AT

1 + A2t:V^

R 6 tan (Ski/R^)

(1)

where k is the thermal diffusivity and 6 the depth of material heated in the absence of thermal

conduction. The T-independent value E^^ is simply the energy density required to raise the crystal

surface to the melting temperature assuming thermal conduction is negligible. Its value is given by

E^^ = AT pc6/(l-R') where p is the density, c the specific heat, R' the reflectance, and AT is the

difference between the melting temperature and ambient temperature . The energy density threshold is

obtained using the expression: E^ = P^t. The values of the parameters used to evaluate the model fo

all materials discussed in this paper are given in table 1.

1. Figures in brackets indicate the literature references at the end of this paper»

-473-
Table 1. Material properties of selected semiconductors.

Property Germanium S 1 1 1 nn InSb

Densxty, p(g/cm ) 5.33 [3] 2.33 [7] 5.78 [8] Specific heat, c (J/g — K) 0.4 [31 0 72 M\ 0 26 r«i Thermal Conductivity, K (W/cm-K) 0.2 [4,5] 0.33 [4,7] 0.08 [4,9]

Melting Temperature, T (OR) m

1210 [3] 1683 [7] 788 [8] neat or rusion, n ^,j/g^ 40/ [3J 1650 [7J Vaporization Temperature, T (°K) 3107 [3] 3060 [7] Heat of Vaporization, (J/g) 3921 [3] 10600 [7] Reflectivity, R' A = 0.69 um 0.43 [6] 0.3 [7] 0.46 [8]

\ = 1.06 pm 0.38 [6] 0.3 [7] 0.35 [8]

X = 10.6 pm 0.35 [8] Radius of laser beam, R (cm) 0.045 0.085 0.054

3. Presentation of Results

3.1 Germanium

Damage in germanium has been studied extensively with 0.69 pm and 1.06 ym lasers [10-14].

Thresholds for melting are plotted as a function of t in figure 1. Melting thresholds at short times were obtained by monitoring the sample reflectance which increases abruptly at the melting point [15]

.

The thresholds at 0.69 um are represented by circles in figure 1. The scatter in the data is not much

larger than the experimental uncertainty which is often as large as ± 30%. The scatter is attributed

to differences in samples and in criteria for threshold. Thresholds for 1.06 ym radiation are presented as triangles in figure 1 and are approximately equal to the 0.69 ym thresholds. The curve is

obtained by fitting eq. (1) to the data using 6 as an adjustable parameter. The model describes the

time dependence of the damage thresholds quite accurately. The value of 5 was found to be 0.5 ym for

both 0.69 ym and 1.06 ym.

The optical absorption depth (a "*") of germanium at room temperature and low optical flux levels

is presented in figure 2 as a function of wavelength. The values of 5 obtained from the damage

thresholds are plotted as filled circles. The value of 6 at 0.69 ym is about 5 times a while the

value of 6 at 1.06 ym is approximately equal to a The difference between 6 and a

''' at 0.69 ym is

attributed to carrier diffusion. This difference cannot be due to saturation of the optical absorption

because saturation should not occur at such low flux levels. Studies of the optical absorption at 1.06 ym using picosecond laser pulses show no deviations in linearity for flux densities below

8 2 9x10 W/cm [16]. The absorption at 0.69 ym is expected to saturate at even higher flux levels.

3.2 Indium Antimonide

Laser damage thresholds of ambient tempersture indium antimonide [17-20] are plotted as a function

of irradiation time in figure 3. The thresholds at 0,69 ym (circles), 5 ym Ctriangles) and 10.6 ym

(squares) correspond to surface melting [21] . The thresholds at short irradiation times were determined

from the abrupt increase in sample reflectance which occurs at melting. For long irradiation times,

the thresholds were obtained by microscopic examination of the damaged surface. The damage thresholds

for short irradiation times increase with laser wavelength as shown in figure 3. Equation (1) was fit

to the 5 ym thresholds with 6 used as an adjustable parameter. As shown in figure 3, the calculated

thresholds agree with the experimental data. Using the same values of the material parameters,

thresholds were calculated for 0.69 ym and 10.6 ym by varying 6 to obtain agreement with the data.

The values of 5 obtained in this way were 0.8, 30 and 50 ym for laser wavelengths of 0.69 ym, 5 ym

and 10.6 ym respectively.

The empirical values of 6 for InSb are compared to a as a function of wavelength in figure 4.

The value of 6 at 0.69 ym is almost an order of magnitude larger than a Since the absorption depth

at 0.69 ym is expected to remain constant during irradiation, this difference is attributed to carrier

diffusion. We conclude from the data that the diffusion distance is approximately 0,8 ym. The value

of 6 for 5 ym radiation (i.e. 30 ym) is much larger than a However this value is much greater than

the diffusion distance (0.8 ym) and hence cannot be related to carrier diffusion. The 30 ym value of

6 results from an increase in the absorption depth due to the Burstein effect. The Burstein effect

is observed at 5 ym in InSb at flux levels less than 10^ W/cm^ [22] . The value of 6 at 10.6 ym is much

less than a indicating that the optical absorptance increases greatly during laser heating. Increases

in the absorptance of InSb at 10.6 ym due to free carriers have been observed [23] previously using

lower flux levels for which sample heating was negligible. Here the optical absorptance at 10.6 ym

-474-
is further increased by shifting of the band-edge to longer wavelengths as the temperature increases.

Since the value of 6 at 10.6 urn is much larger than the diffusion distance obtained above, carrier

diffusion does not significantly affect the damage thresholds.

3.3 Silicon

The melting thresholds of silicon [24-31] are plotted as a function of irradiation time in figure 5 for 0.69 pm (circles) and 1.06 pm (triangles) radiation. The theoretical curves were obtained from

eq. (1) using the material properties in table 1 and varying 6 to fit the data at short irradiation

times. The values of 6 were found to be 3 pm and 200 pm for 0.69 pm and 1.06 pm radiation respectively.

These empirical values of 6 are compared to a

"'" as a function of wavelength in figure 6. The values

of 6 and a

^ are approximately equal at both 0.69 pm and 1.06 pm. These results imply that at these

wavelengths the absorptance of silicon does not change appreciably during laser irradiation and that

carrier diffusion does not significantly affect the damage thresholds. Grinberg, et al. have shown

previously by direct measurements that the absorptance of silicon at 1.06 pm remains essentially

constant as the incident laser flux is increased to the damage level [25]

.

3.4 Other Semiconductors

The value of 6 determined here for gallium arsenide irradiated by 0.69 pm radiation is 0.26 pm

which is about twice a Measurements of photoconductivity and absorption by nonequilibrium carriers

in GaAs excited by 0.69 pm radiation show that the thickness of the layer containing nonequilibrium

carriers is 0.3 pm [1]. These results indicate that carrier diffusion must be considered in calculating

damage thresholds of GaAs at 0.69 pm. The values of 6 that we determined from the thresholds of

-3 -1

HgCdTe and PbSnTe at 10.6 pm are both 10 cm. For these materials 6 = a at 10.6 pm and the damage

thresholds do not show effects of carrier diffusion.

4. Discussion

The comparisons of 6 and a for the semiconductors studied here show that 6 is equal to the

optical absorption depth when a is significantly less than 10^ cm When a 10^ cm 5 can be

greater than the absorption depth due to carrier diffusion. If 6 >> a then 6 is approximately the

carrier diffusion distance. Values of diffusion distances were found to be on the order of 0.5 pm. The carrier diffusion distance L^^ is approximately given by the expression L^^ = /Dt^ where D is the

ambipolar diffusion coefficient and t is the carrier lifetime. Previous studies have assumed that

c carrier diffusion distances of the order of 1 mm determine the thresholds for laser damage in semiconductors [32,33]. These large distances are obtained when the expression for L_ is evaluated using

2 -1 -3

initial values of the material parameters (e.g.. For Si: D = 16 cm s , t^ = 10 s) . However the

values of D and t^ in a semiconductor heated by laser radiation are expected to differ from those of

the unexposed material. The high carrier densities generated by the in-band laser radiation shorten

the carrier lifetime due to Auger recombination [34,35]. Recombination rates approaching 10"'"'^ s

''"

20 -3

have been measured when carrier densities of 10 cm are generated by short laser pulses which produce

little heating [16] . These high recombination rates imply small carrier diffusion distances. For laser

heating the diffusion distance is further reduced since both D and t decrease with increasing

temperature.

5. Conclusion

The threshold laser flux for thermal damage in opaque semiconductors agrees with the predictions

—8

of the thermal model discussed for pulse durations between 10 s and several seconds. The model

assumes that the material is heated to a depth 6 for pulse durations sufficiently short that thermal

conduction is insignificant. The magnitudes of the experimental damage thresholds indicate that 6 can

be greater than the optical absorption depth if a 10^ cm This is attributed to carrier diffusion.

When a is appreciably greater than 10^ cm 6 equals the carrier diffusion distance. For the materials

investigated diffusion distances were found to be on the order of 0.5 pm.

6. References

[1] L.M. Blinov, V.S. Vavilov and G.N. Galkin, [3] Handbook of Eleatronio Materials, 5_, M. Sov. Phys. - Semicond., H9) , 1124 (1967). Neuberger, (IFI/Plenum, Washington, 1971).

[2] F. Bartoli, L. Esterowitz, M. Kruer and R. Allen, J. Appl. Phys. 46(10), 4519 (1975).

-475-
[4] Determined empirically from the damage

thresholds of large samples at long

irradiation times. The procedure is

discussed in detail in ref. 23. Second

reference included in the brackets shows

the temperature dependence of K.

[5] Ameriaan Institute of Physios Handbook, Ed.

by D.E. Gray (McGraw-Hill, New York, 1972),

pp 4-154.

[6] Handbook of Electronia Materials, 1^, A.J. Moses (IFI/Plenum, Washington, 1971).

[7] Silicon, M. Neuberger and S.J. Welles

(National Technical Information Service,

Springfield, VA, 1969).

[8] Handbook of 'Electronia Materials, 2_, M. Neuberger (IFI/Plenum, New York, 1971), p 77.

[9] Thermophysiaal Properties of Matter, 10 , Y.S. Touloukian, R.W. Powell, C.Y. Ho and

Nicolaou (IFI/Plenum, New York, 1974) p 370.

[10] M. Birnbaum and T.L. Stocker, J. Appl. Phys.

39(13), 6032 (1968).

[11] L.M. Blinov, V.S. Vavilov, and G.N. Galkin,

Sov. Phys. Solid State, 9 0), 666 (1967).

[12] M. Bertolotti, L. Stagni, and G. Vitali, J. Appl. Phys. 42(13), 5893 (1971).

[13] L. Muzii, L. Stagni, and G. Vitali, Nuovo

Cimento, 14(2), 173 (1973).

[14] A.M. Bonch Bruevich, V.P. Kovalev, G.S. Romanov, Ya. A. Imas and M.N. Libenson,

Sov. Phys. Tech. Phys., 13(4), 507 (1968).

[15] J.N. Hodgson, Phil. Mag. 6, 509 (1961).

[16] D.H. Auston, C.V. Shank and P. LeFur, Phys.

Rev. Lett., 35(15), 1022 (1975).

[17] M. Birnbaum and T.L. Stocker, J. Appl. Phys.

39(13), 6032 (1968).

[18] M. Kruer, L. Esterowitz, R. Allen and F. Bartoli, Infr. Phys., 16, 375 (1976).

[19] R.E. Slusher, W. Giriot and S.R. J. Brueck,

Phys. Rev. 183(3), 758 (1969).

[20] M. Kruer, L. Esterowitz, R. Allen and F. Bartoli, unpublished results on InSb at 0.69 urn and 10.6 ym.

[21] We include a 10.6 ym threshold for sparking

in a vacuum. Since InSb decomposes in a vacuum only 20K above its melting temperature,

this threshold is approximately equal to the

threshold for melting.

[22] The Burstein Effect in InSb at 5 ym is

discussed by A.V. Nurmikko , Opt. Comm. 16 (3)

,

365 (1976). For laser heating the increase

in absorption depth is partially counteracted

by the temperature increase which shifts

the bandgap to longer wavelengths.

[23] H.J. Fossum, W.S. Chen, and B. AnckerJohnson, Phys. Rev. B, 8^(6), 2857 (1973).

[24] M. Kruer, R. Allen, L. Esterowitz, and F. Bartoli, Opt. and Quant. Elect., 8^, 453 (1976).

[25] A. A. Grinberg, R.F. Mekhtieo, S.M. Ryvkin,

V.M. Salmanov and I.D. Yaroshetskii , Sov. Phys. Sol. State 9^, 1085 (1967).

[26] Y. Matsuoka, J. Phys. D. 9, 215 (1976).

[27] S.M. Bedair and H.P. Smith, J. Appl. Phys. 40,

4776 (1969).

[28] Y. Matsuoka and A. Usami, J. Phys. D. ]_, 1259

(1974).

[29] M. Bertolatti, F. Depasquale, P. Marietti,

D. Setti and G. Vitali, J. Appl. Phys. 38,

4088 (1967).

[30] Y. Matsuoka and A. Usami, Appl. Phys. Lett. 25(10), 574 (1974).

[31] M. Birnbaum and T.L. Stocker, J. Appl. Phys.,

39(13), 6032 (1968).

[32] Y. Matsuoka, A. Usami and K. Shimazaki, J. Phys. D (Appl. Phys.) 8^, IL (1975).

[33] L. Stagni, Appl. Phys. 12, 31 (1977)

[34] L.M. Blinov, E.A. Bobrova, V.S. Vavilov and

G.N. Galkin, Sov. Phys. Sol. State £(11)

,

2537 (1968).

[35] N.G. Nilsson, Phys. Scripta 8^, 165 (1973).

8. Figures

-476-
Fig. 1. Damage thresholds of germanium as a function of Irradiation

time. Curve A Is threshold for melting and curve B Is threshold for vaporization.

GERMANIUM

10' 1 10 lO' lO^

1 Ijiml

Fig. 2. A comparison of a and 6 for germanium as a function of

wavelength.

-477-
10'

10' r

i

10^

0? 10=

10"

lO'^

1 1 Illllj 1 1 ;^li| 1 1 r iiilj 1 1 1 Mil

1

1 1 IIMIIj 1 1 Illllj 1 1 Illllj 1 1 Illllj 1 null

111

ml11

1

InSb

ID1

1

1 1

nil 1

1

\ O 0.69 [ivn _ V 5 ^jm

: 10.6

1 1 1 mill 1

1

1 "111 1 1 mill 1 il il 1 1 1 1 mill 1 10'^ 10-^ 10-^ 10-^ 10'^ 10'" 10-^ 10-2 10'^ 1

r (s)

Fig. 3. Damage thresholds of indium antimonide as a function of

irradiation time and laser wavelength.

I02

10*

'I ' ' "Irn 1—I I i| 1—I

' 'I r

10^ 10^

Fig. 4. A comparison of a" and 6 for indium antimonide as a function

of wavelength.

-478-

COMMEMTS OW PAPER BV KRUER, ESTEHOWITZ, 5ART0LI, AMP ALLEW

Mo (LUcuM'ion on thii, papzn..

-480-
MULTIPHOTON-INDUCED CONDUCTIVITY IN ULTRAVIOLETTRANSMITTING MATERIALS 1. FREE CARRIER LIFETIME*

R. T. Williams, P. H. Klein, and C. L. Marquardt

Naval Research Laboratory

Washington, D.C. 20375

Fast photoconductivity measurements are being used to characterize freecarrier recombination and trapping rates in insulators. Carriers are generated in

these experiments by multiphoton absorption of a 20-ps, 347-nm laser pulse. At low carrier densities, lifetimes governed by extrinsic traps are measured to be

typically 10-20 ns. Material preparation and subsequent treatment are shown to have measurable effects on carrier lifetime in the extrinsic limit. Published

optical data are cited as evidence that the free carrier lifetime in alkali halides

is strongly dependent on carrier concentration, changing from the extrinsic nanosecond range to less than 10 ps when the concentration is increased to about

5 X 10 '^ carriers per cm^ . Physical mechanisms giving concentration-dependent re- combination times are discussed. An investigation by photoconductivity of the

carrier concentration range intermediate between the two cases just cited has been

attempted within limitations imposed on the present technique by space charge buildup. Key words: Dielectrics; free carrier; laser damage; lifetime; multiphoton;

ultraviolet; window materials.

1. Introduction

Free carriers are central to many of the failure modes of high energy laser windows and components,

particularly those being considered for applications in the ultraviolet. As discussed by Sparks and

Duthler [1,2]^, wavefront distortion in components subject to high optical flux can result from freecarrier contributions to the refractive index and from expansion associated with heating of the electron-hole plasma. If the heating is severe, and particularly if there is avalanche multiplication of

free carriers, the material fails by fracture or melting. A key number entering calculations dealing

with these failure modes is the time-dependent density of free carriers during the laser pulse. This

is governed generally by a rate equation expressing a competition between source terms such as multiphoton absorption, impurity ionization, or avalanche ionization, and carrier loss terms associated with

recombination and deep trapping. In recent discussions or calculations of failure thresholds by

Bloembergen [3], Sparks and Duthler [i|3, and Braunlich, Schmid, and Kelly [5]» the conduction electron

lifetime due to recombination and trapping was assumed to be longer than most laser pulses and therefore

the carrier loss term was neglected. Sparks and Duthler included carrier loss terms in a more recent

calculation [2], but treated the conduction electron lifetime as a free parameter due to lack of experimental data for materials and temperatures of interest. Recombination times of 10"^ to 10"'* s in

germanium were cited as an available example [2].

Catalano, Cingolani, and Minafra reported a conduction electron lifetime of about 1 usee in alkali

halides at room temperature as measured in their experiments on multiphoton-induced photoconductivity [6].

This value was used as the constant carrier lifetime in their analysis of data on laser power dependence

of integrated photocurrent . On the other hand, observations by Bradford, Williams, and Faust using

picosecond optical spectroscopy of defect formation [7,8] and exciton formation and relaxation [8] in

alkali halides following two-photon generation of free carriers yielded strong evidence that electronhole recombination occurs in a few picoseconds or less when the initial carrier density is about

5 x 10^'' cm-^ . In comparison, laser damage typically involves carrier densities in the range lO^^-lO'^

cm~^ . Resolution of this apparent conflict in experimental conduction electron lifetimes and progress

toward an understanding of carrier annihilation under conditions of interest in laser damage were set

as two of the early goals in the present study. This ordering of emphasis is also appropriate to the

measurement of carrier generation cross sections by photoconductivity because the carrier lifetime must

be known accurately under the relevant range of conditions if generation cross sections are to be

deduced from the photoconductivity.

In addition to addressing fundamental questions of carrier lifetime and intrinsic multiphoton cross sections, photoconductivity may be developed as a diagnostic tool to be used in improving uv window

materials for high-energy applications. The potential strength of this approach is that it focuses on parameters (free carrier generation and annihilation) which are known to be central to material failure.

We are in the process of identifying extrinsic sources and traps with their specific effects on photoconductivity, where possible, and correlating these parameters with experimental failure thresholds to test predictive ability. The problem on which we have concentrated first is the demonstration of observable effects of material treatment and/or purity on carrier lifetime in wide-gap optical materials.

*Work supported in part by the Advanced Research Projects Agency of the Department of Defense.

1. Figures in brackets indicate the literature references at the end of this paper.

-481-
2. Experiment

Although photoconductivity as a measurement technique has been developed to the point of routine

technology in some classes of semiconductors, its application to extract the kind of information we

want about insulators is comparatively undeveloped. The reason lies possibly in a lack of prior device

motivation arid in the special difficulties presented by insulators at the relatively high carrier

densities encountered in failure of laser components. Problems include space charge effects, difficulty

of bulk carrier generation by ordinary excitation, and short lifetimes in most of the materials studied

at relevant carrier densities. Laser windows are not usually situated in a steady electric field, so ultimately we must be able to extract parameters of photo-carrier evolution in the limit of zero applied d.c. field but appropriate optical fields.

Several prior experiments on multiphoton conductivity of insulators, particularly alkali halides,

have been carried out [6,9,10]. These usually involved low densities of carriers excited by a Qswitched laser pulse and did not deal extensively with the carrier lifetime. In the present work we

used for excitation the second harmonic of a single 20-picosecond pulse from a mode- locked ruby laser,

whose wavelength (347 nm) is reasonably close to that of the XeF* laser (354) nm) . The very short

pulse permits both the resolution of shorter carrier lifetimes and the use of higher working intensities

below thresholds for catastrophic failure.

The apparatus used in the present experiments is diagrammed in figure 1. The oscillator com- prises a 3/8-inch Brewster-cut ruby rod in a 1.2-meter cavity with a 2.8-mm aperture selecting the

lowest-order transverse mode. A contact cell with a flowing dye solution of DDI in methanol provides

passive mode- locking. A single pulse is selected, amplified, and frequency-doubled (in KDP) . The

ultraviolet pulse then passes into the screen room where all electrical measurements are made. A pyroelectric energy meter samples the incoming uv beam and a photodiode registers the transmitted uv. Between shots, white light from a super-pressure mercury lamp is injected along the optical path, both

to facilitate inspection of a magnified image of the sample for damage and to bleach trapped charges.

The sample, typically 18 x 4 x 3 mm, is placed between electrodes in a shielded coaxial housing which

can be flushed continuously with dry nitrogen. Light travels down the 18-mm dimension and a potential

between 500 and 3000 v is applied across the shortest ( ~ 3 mm) dimension a few seconds before the laser

shot. As shown in the enlarged view in figure 1, the sample was typically several millimeters longer

and wider than the corresponding electrode dimensions so that the illuminated end surfaces and two side

surfaces were outside the high-field region.

From known approximate mobilities, applied fields, and measured lifetimes, we find average carrier

displacements of the order of 10"^ cm or less for these experiments. Since the sample is not illuminated within about 0.5 mm of any electrode, the free charge reaching the contact areas is negligible. We

are measuring charge induced on the electrodes by displacement current within the dielectric. Thus

blocking contacts are appropriate. Experiments to date have utilized polished flat copper or brass

electrodes pressed against the insulator sample surfaces. A thorough study of effects of specific

electrodes and fused or evaporated contacts on the measurement of photoconductivity is planned. This

must be investigated before we attempt to extract accurate multiphoton absorption coefficients because

the nature of the contact and associated space charge is critical for deducing the true internal

electric field, and because of the possibility of photo-injection from the electrodes. However, the

present results on carrier lifetimes in regions of the sample well removed from the contacts should not

depend significantly on the type of contact used.

We have made direct measurements of photocurrent using a Tektronix 7844 oscilloscope and 7A19 preamplifier for a rise time of about 0.8 ns. The data presented here are in the form of integrated photocurrent, measured by attaching a Hewlett-Packard 1120A active probe (500 MHz bandwidth) to the output

electrode. From the known resistance to ground (e.g. 510 kfi ) and the measured RC time constant for

decay of the induced charge, the output capacitance to ground (integrating capacitor) was determined

to be 3 pF. The ability to measure a photocharge rise time of 1 ns consistent with instrument bandwidth was verified experimentally.

3. Carrier Recombination

We have already cited the apparent discrepancy between microsecond lifetimes reported for free

carriers created in alkali halides at relatively low density [6] and for picosecond recombination times

for carriers at higher density £7,8] . The picosecond recombination times are indicated by short-pulse

laser studies undertaken principally to elucidate processes of exciton relaxation and photochemical

defect production in KCl, NaCl , and NaBr [7,8]- The efficiency for reaching a deep bound state of the

electron-hole pair was found to be of order unity, and there was no evidence of continued reaction or recombination of free carriers beyond about ten picoseconds at room temperature. It is concluded that

the observed recombination time is characteristic of essentially all free carriers at initial density

5 X 10^'' cm~^ in these halides [8]. Theoretical treatment of material failure requires an understanding of why the free-carrier lifetime decreases at high density, and in particular, the functional form

specifying carrier lifetime at intermediate densities. Two possible mechanisms for a density-dependent

free-carrier lifetime are outlined below.

3.1 Simple Kinetic Model

Consider an idealized semiconductor or insulator having a single type of recombination site and a single type of deep electron trap. Let n be the density of photo-generated free electrons and ne be

-482-
the equilibrium density of electrons in the dark. Assume that all holes, generated as counterparts to free electrons , are trapped at recombination sites, and assume further that there is no re-ionization

out of the recombination centers or deep electron traps. If n^ free electrons are initially created by an infinitely short optical pulse at time t = 0, then subsequent evolution of free electron concentration will be governed by a set of coupled differential equations. — /-I IN / ll\ = - van(n + n - n ) - va n(n - n ) at e o t t

dn —* = + von(n^ + n^ - n') (1)

n = n - n' - n"

o Here v is the average thermal velocity of conduction electrons, a and are the recombination and

trapping cross sections, respectively, n' and n" are the numbers of conduction electrons annihilated

by recombination and trapping, respectively, and n,. is the concentration of deep electron traps. The

effect of shallow electron traps is also included implicitly in equation (1), since shallow trapping

influences only the average velocity v.

There are several cases in which eq. ( 1) reduces to a very simple form. For example, in an intrinsic semiconductor nj.= 0, and at ordinary excitation levels >> n^ . Annihilation of the photoelectrons

occurs predominantly by recombination, and is governed approximately by first-order kinetics.

n = n^ exp(-t/T), x = (von^)"!

Therefore, it is possible to specify a free-carrier lifetime t which is characteristic of the material

and essentially independent of n . o First-order kinetics may also be obtained from eq. (1) if the density and cross section of deep

electron traps are sufficiently large, such that j n^. >> a (n + n ). In this case, eq. (1) yields

n = n^ exp(-t/T^), = (va^n^.)-!

(3)

This behavior can be observed in insulators as well as in semiconductors, but in both cases it indicates an extrinsically determined free-carrier lifetime not characteristic of a perfect material.

If, on the other hand, we consider an intrinsic insulator, deep electron traps can be neglected

(n^ = 0), and at photo-excitation levels currently of interest n^ >> ng. Annihilation of photoelectrons

occurs by recombination as in the intrinsic semiconductor, but since n^ >> ng, eq. (1) leads to secondorder kinetics.

n = n^ (1 + t/T)-l, T = (van^)-l

In this case it is only possible to specify a carrier lifetime which is a function of carrier density,

nQ, as well as being characteristic of the material itself. Note also that the definition of "lifetime"

in eq. (4) differs mathematically from that given in eqs. (2) and (3).

Experiments to determine free carrier lifetimes in available pure insulators would be expected, in

principle, to encompass two extremes. At sufficiently low carrier concentrations, trapping would be

the dominant effect, and a concentration-independent lifetime would be observed, in accordance with eq. (3). At sufficiently high concentrations, however, intrinsic recombination would become dominant, and

an inverse dependence of lifetime on concentration should be observed according to eq. (4). Detailed

behavior of lifetime as a function of concentration depends strongly on the chemical and physical perfection of the sample. Concentration and cross section of deep electron traps are the principal factors

influencing the lifetime in the trap-controlled region. They also determine the free-carrier concentration range in which the transition occurs from the trap-controlled region to the intrinsic recombination

region. Shallow traps influence the lifetimes in both regions by controlling the average thermal

velocity of the electrons.

3.2 Auger Recombination

Band-to-band Auger recombination is a well-documented channel for decay of high-density electronhole plasmas in semiconductors [11], although very little is known of the corresonding process in

insulators. Basically, an electron-electron collision in the presence of a hole (or vice-versa) results

in nonradiative electron-hole recombination with the third particle gaining kinetic energy equivalent

to the recombination energy. The kinetic equation for plasma recombination by this three-body interaction can be written

8n V 3 = - ^3 " (5)

where n is the concentration of electrons and holes. Hence the onset of Auger recombination is very

-483-
rapid with increasing carrier density, and it eventually dominates all other recombination channels.

Threshold densities for onset of significant Auger recombination in semiconductors are typically quite

high even in comparison to 5 x 10^'' cm-^. Auston, Shank, and LeFur found that in germanium the Auger

process is clearly observable above radiative recombination when the plasma density is 3.4 x 10^" cm~^

but not when it is 1.4 x lO^O cm"^ [12]. However a significantly lower threshold in HgCdTe has been

reported [13]

•

Predicting an Auger recombination rate for plasmas in alkali halides is complicated by self -trapping of the hole. However the immediate question of whether the Auger process accounts for the rapid

recombination rate in alkali halides as reported in refs. 7 and 8 can be addressed by more general

arguments. In the first place, a band-to-band Auger recombination would bypass the defect production

channel [1^1,15] with the energy going instead to the escaping electron. Thus even if the Auger process

occurs appreciably at 5 x 10^' carriers per cm^ in KCl, it cannot account for the observation [7,8] of

fast F-center production. An alternate mechanism of fast, density-dependent recombination with subsequent vibrational relaxation is still needed. In fact, the onset of Auger recombination is so sharp

that in the Auger-dominated regime all F center production or population of self-trapped exciton states

would be wiped out by a moderate increase in carrier density. This was specifically not observed [7,8],

so we conclude that Auger recombination is probably not important in alkali halides at 5 x 10^^

carriers per cm^ . The alternative fast recombination mechanism combines bimolecular kinetics as discussed in section 3.1 with very rapid vibrational relaxation of the s61f-trapped exciton to a deep

level after initial electron capture.

4. Photoconductivity Data

4.1 Extrinsic Trap-Limited Regime

Part of our expected use of photoconductivity for characterizing wide-gap materials presupposes

that some of the measured quantities will depend in a fairly sensitive way on extrinsic, hence controllable, parameters of the material. As one of the early stages of the present work, we have surveyed effects of a number of different material preparations and treatments on photoconductivity— again

looking principally at effects on carrier lifetime. Because of our shorter excitation pulse, we can

look for carrier lifetimes in the range 1-20 ns which has been inaccessible in past experiments on halides. In fact most of the extrinsic trapping of carriers we have observed occurs in this range.

In figure 2 we show the integrated photocurrent in NaF from two sources. (The free-carrier density

is roughly proportional to th^ measured photocharge . ) The order-of-magnltude difference in carrier lifetime in these two samples is tentatively attributed to a reduction in the concentration of electrontrapping impurities in the highly-purified material (NRL #107E) . The traps associated with these impurities are assumed to be shallow; decreasing the impurity concentration would then decrease

carrier lifetime by increasing the average drift velocity.

A second dramatic effect of chemical treatment on lifetime was observed in commercial-grade sapphire, as illustrated in figure 3. A mild anneal (20 min at 650°C) in a reducing atmosphere introduces

a long carrier lifetime (t^ ~ 100 ns) ; subsequent annealing in an oxidizing atmosphere removes this

long component. These observations suggest that chemical reduction of sapphire also introduces (presumably shallow) electron traps. Electron-spin-resonance experiments have demonstrated that this

effect is not related to changes in the valence state of iron, which is the principal impurity. It is more likely that oxygen vacancies produced in the reducing anneal could act as electron traps.

In order to study the transition from the low-density, extrinsically-controlled regime to the

higher-density regime of intrinsic recombination, we have used the unfocused uv pulae to generate carriers by two-photon excitation in high-purity KBr, varying the intensity of excitation at 347 nm from

0.05 to 50 MW/cm^. In figure 4 the maximum induced photocharge is plotted versus uv intensity, and

oscilloscope traces of the build-up of integrated photocurrent are shown for three points along the

curve. To stay within the dynamic range of the FET probe, the circular points in figure 2 were taken

with 500 V applied as opposed to 2000 V for the square points. Independent measurements showed that

the measured carrier lifetime did not depend significantly on applied voltage within this range. The

ordinate values of induced charge are scaled with respect to the 2000-V data.

Consider first the lower end of the curve, where calculations indicate negligible space charge

effects and where the approximate constancy of the 20-ns carrier lifetime with carrier density suggests

that this is the extrinsic trap-controlled limit. The induced charge of 10~^^C corresponds to an

initial carrier density of about lO^'' cm~^ under these conditions. The 20-ns carrier lifetime we find

in KBr is substantially shorter than the 1-ys lifetime found for some alkali halides by Catalano et al.

[6] in the same or higher range of carrier density. (In ref. [6], the 34 7-nm pulse intensity was in

the range 0.1 to 12 MW/cm^, but the duration was 20 ns, or 10^ times the duration of our pulse. One can deduce from the known two-photon absorption coefficient and indicated carrier lifetime that

photoinduced carrier densities were in the range 10^^-10^^ cm~^.) In view of the chemical effects

mentioned above, this result suggests that the longer lifetimes measured in previous work may be due to a high concentration of shallow traps.

4.2 Space-Charge/Bimolecular-Kinetics Regime

When a dielectric containing an electron-hole plasma is subjected to an electric field, the plasma

will polarize to produce internal cancellation of the applied field in a time t^ = e (n^ey)" ^ , some-

-484-
times called the dielectric relaxation time. Here is the carrier pair density, \i is the sum of

electron and hole mobilities, e is the electron charge, e the dielectric constant, and Eq is the permittivity of free space. A criterion for whether useful transient photoconductivity measurements can be made without serious distortion by space charge effects is that either the carrier lifetime or the

time for which the external field has been applied must be significantly less than t^.. The dielectric relaxation time for a plasma density of 10^" cm~^ in KBr is about 3 \is. Thus our measured carrier lifetime of 20 ns at this density should be independent of space charge effects. How- ever the upper half of figure A, corresponding to carrier densities above lO'^ cm~^, is getting well

into the space charge region. This introduces ambiguity into interpretation of the present kind of

experiment. Figure 4 indicates that the decay of the measured photocurrent becomes faster as the density of carriers increases. Extrapolation of the data in figure 4 according to eq. (4) (second-order

kinetics) predicts picosecond recombination times at lO'^^ carriers per cm^ , in reasonable agreement

with the optical experiments cited earlier. The induced photocharge in figure 4 falls below the

expected quadratic dependence on laser intensity (dashed line) in the high density range, as would be

expected for shorter carrier lifetimes. These trends can be fit rather well within the kinetic model

of section 3.1.

Similar behavior could also be attributed to space charge build-up. In that case, an additional

mechanism would be required to account for the short lifetimes inferred from the previous defect

generation experiments [TjS]. Although an Auger mechanism cannot be completely ruled out, it seems un- likely, as shown in section 3.2. In an attempt to overcome this ambiguity in interpretation of data

for carrier densities in the range 10^'* - 10^° cm~^, we are undertaking experiments in which the electron-hole plasma evolves in zero field and is then probed by a transient electric field much shorter in

duration than the dielectric relaxation time.

5 . Summary

We have made time-resolved photoconductivity measurements on several wide-gap materials. Three

major conclusions from this work are:

1. At low excitation densities, lifetimes on the order of 10 ns were found in all alkali

halide samples measured at room temperature. Although extrinsically controlled, these times provide an upper limit for free carrier lifetimes which may be expected in state-of-the-art alkali halide materials.

2. Intensity dependence measurements in high purity KBr have suggested that, in the range of

interest for laser damage studies, bimolecular recombination kinetics may give rise to an inverse

dependence of carrier lifetime on carrier density. Extrapolation to higher intensities on the basis of this model is in agreement with results of previous defect production studies [7,8], which indicated

that the free carrier lifetime in alkali halides is shorter than 10 picoseconds for carrier densities

well below typical damage thresholds. Possible space charge effects in the present experiments have

precluded a completely unambiguous interpretation of our initial photoconductivity data at intermediate carrier densities. More refined experiments are in progress to eliminate this ambiguity.

3. Two effects of material preparation techniques on carrier lifetimes in the extrinsic- trapcontrolled regime have been observed. Chemical purification of NaF was found to shorten the carrier

lifetime, presumably by lowering the concentration of shallow traps. In aluminum oxide (sapphire), the

lifetime was found to be highly sensitive to oxidation-reduction conditions. In combination with electron spin resonance experiments, these data suggest that lifetimes in oxides may depend critically on small deviations from stolchiometry . These observations support the view that photoconductivity can be developed as a useful diagnostic technique for evaluation of uv laser window materials.

6. Acknowledgements

We wish to thank D. G. Frazier, Sr., for preparing many of the samples used in these experiments.

Comments by A. Defonzo and M. N. Kabler have been very helpful.

7. References

[1] Sparks, M. and Duthler, C. J., Xonics

Seventh Technical Report, Contract

DAHC 15-73-C-0127, (June, 1976).

[2] Sparks, M. and Duthler, C. J., Xonics

Eighth Technical Report, Contract

DAHC 15-73-C-0127, (December 1976).

[3] Bloembergen, N. , IEEE J. Quantum Electron.

QE-10 , 375 (1974).

[4] Sparks, M. and Duthler, C. J., Xonics

Fifth Technical Report, Contract

DAHC 15-73-C-0127, (June, 1975).

[5] Braunlich, P., Schmid, A., and Kelly, P.,

Appl. Phys. Lett. 26, 150 (1975).

[6] Catalano, I. M., Cingolani, A., and

Minafra, A., Phys. Rev. B5^, 1629 (1972).

[7] Bradford, J. N. , Williams, R. T. , and

Faust, W. L. , Phys. Rev. Lett. 35, 300 (1975).

[8] Bradford, J. N. , Williams, R. T. , and

Faust, W. L., to be published.

[9] Dneprovski, V. S., Klyshko, D. N. , and

Penin, A. N. , JETP Lett. 2. 251 (1966).

[10] Bolte, Steven B. and Brown, Frederick C,

in Proceedings of the Third International

Conference on Photoconductivity , Pell, Erik

M., ed., (Pergamon, New York, 1970), p. 139.

-485-

I
[11] Blakemore, J. S., Semiconductor Statistics

(Pergamon, New York, 1962) pp. 214-248.

[12] Auston, D. H., Shank, C. V., and LeFur, P.,

Phys. Rev. Lett. 35, 1022 (1975).

[13] Bartoli, F., Allen, R. , Esterowitz, L. , and

Kruer, M. , J. Appl. Phys. 45, 2150 (1974).

[14] Williams, R. T. , Phys. Rev. Lett. 36,

529 (1976) .

[15] Kabler, M. N. and Williams, R. T., to be

published.

8. Figures

AMPLIFIER KDP 694 nm ; 20 psec

1 I

SCREEN ROOM

VIEWINGi SCREEN r

PD I

SCOPE —1

1 PD2

-4r

Figure 1. Apparatus for measuring photoconductivity.

NaF Commercial (~1970)

NaF NRL 07E - Pure

10 nsec/div

Figure 2. Photoconductivity data on samples of commercial and

ultrahigh purity NaF. The lower trace in each photo

shows integrated photocurrent vs. time after multiphoton excitation.

-486-
TXEWWL HISTORY; OXIDATION-REDUCTION

AL2O3 (Verneuil-Grohn, Commercial Grade)

kj B, D: 10 nsec/div

C! 20 NSEC/DlV ^Si£>

Figure 3. Integrated photocurrent vs. time after three-photon

excitation of a sapphire sample as received (A)

,

after annealing in hydrogen (B,C), and after

subsequent annealing in air (D)

.

Figure

.01 .10 1.0 10 LIGHT INTENSITY (MW/cm^)

Integrated photocurrent in ultrahigh purity KBr

as a function of peak intensity of the 347-nm

excitation pulse. The ordinate (induced charge on the electrodes) may be scaled to free-carrier

density within an order of magnitude by the multiplicative factor 10^^ carriers cm~^ (coulomb

induced)"-'. The inset photographs show the time

dependence of integrated photocurrent (20 ns/div)

for the indicated carrier densities.

-487-
COmENTS ON PAPER BV WILLIAMS, KLEIN, ANV MAROUARPT

In /teiponie to a qautlon conceAnlng the. fiole. ojS ihoLtow t/iapi in tibzAattng elzctAonA, the. ipeakeA

A-ndicated that in the. coie Ojj twophoton photocondacitivity, the twophoton p^LoaeAi dLeafitij dominatti the.

<Lx.CAJjcition 0(5 eX&cJyiom, liom tA.api,, and this can be ie.zn by the intemtty dependence the condactiviXy.

HoweveA -in htgheA ofideA pfioceMeA , MheAe the multlphoton pnocui Li weakeA, the two iouAoii e.lectA.on6 compete. He tndtcaXed that among the diAecXtom -in whtch thti won.k mutd be taken MoaZd be meaiuAe.-

ment^ Oj$ the tempeAoXuAe dependence o^ the photoconductivity, the exteAnal photo-emiMion o(, electAom

and photo-Zumine^cence.

-438-
ABSOLUTE TWO-PHOTON ABSORPTION COEFFICIENTS AT 355 AND 266 nm*

Liu, W. Lee Smith, H. Lotem, J. H. Bechtel and N. Bloembergen

Gordon McKay Laboratory

Harvard University

Cambridge, Massachusetts 02138

and

R. S. Adhav

Quantum Technology, Inc. Grand Island, New York 14702

The absolute two-photon absorption coefficients of ultraviolet-transmitting

materials have been measured using well -cal i brated single pulses at 355 nm and

266 nm from a mode-locked NdrYAG laser system. Two-photon absorption coefficients

of the order 10"^ cm/MW were found for alkali-halides, and of order lO""* cm/MW for

KDP and its isomorphs. Several other materials - A1203, CaC03, Si02, and CaF2 -

were also tested. In materials with band gap greater than 2'liw, no nonlinear

absorption was detected up to the threshold of surface breakdown. The effects of

multiphoton absorption on the breakdown of ultraviolet window materials is discussed.

Keywords: Alkali-halides, avalanche ionization, breakdown, calcite, KDP,

multiphoton absorption, sapphire, silica, transparency, two-photon

absorption, ultraviolet windows.

INTRODUCTION

The study of two-photon absorption (2PA) [1] will attract increasing scientific and technological

interest in coming years. Because the selection rules for 2PA transitions are, in general, different

from those familiar with linear spectroscopy, 2PA studies will provide much new information about the

band structure and energy levels in solids, liquids, and gases. Due to widespread and growing use of

high-power ultraviolet (UV) and vacuum ultraviolet (VUV) lasers, the impetus is present to understand

the ramifications of multiphoton ionization for breakdown processes in large band-gap optical materials.

The interplay between multiphoton absorption and avalanche breakdown mechanisms in solids has been

the subject of considerable theoretical discussion in the recent literature [2-5]. A recent experimental

study [6] addressed the frequency dependence of dielectric breakdown and discussed the implications of

multiphoton absorption for breakdown experiments.

This report relates a subset of aspects — dealing directly with breakdown processes — treated in

a major study [7] of 2PA at 355 nm and 266 nm to be presented elsewhere. In that study, attention is given in addition to the questions of spatial anisotropy of 2PA, the relation of the 2PA coefficient to other nonlinear optical parameters, techniques for accurate relative measurements of 2PA, and comparison

of the frequency dispersion of the 2PA coefficient, using Keldysh multiphoton absorption theory, with

experimental results.

THEORY

In materials with optical band gap greater than the energy of a single photon but smaller than

twice the photon energy of an interacting light wave, it is possible for transitions to occur by

simultaneous absorption of two (or more) photons from a single, sufficiently intense laser beam [1].

The expression for the 2PA transition rate may be written down straightforwardly from second-order

perturbation theory. However, one's ability to proceed to the point of making useful predictions

or precise calculations with that direct approach has been hampered in 2PA studies [1]. Linear

spectroscopy theory has been similarly impeded and for the same reasons — lack of sufficient knowledge

of energy band parameters, etc., of the materials.

*This work was supported by the Advanced Research Projects Agency (F-44620-75-C-0088) and by the

National Aeronautics and Space Administration (NGL-22-007-1 1 7)

.

"""Present address: Lawrence Livermore Laboratory, Livermore, California 94550.

''"Present address: General Motors Research Laboratory, Warren, Michigan 48090.

-489-
A different approach to the problem was developed by Keldysh [8], who abandoned the standard

method of calculating rates using unperturbed Bloch wave functions and directly modified the wave

functions to account for the strong electric field strengths required for the occurrence of 2PA.

Although imperfections in the multiphoton ionization transition rate formulae derived by Keldysh have

been pointed out [9] and the absolute accuracy of the theory is no doubt limited due to its simplified

band structure, the Keldysh formulae are appealing in their tractibility. In a solid with optical

band gap $g, the Keldysh expression for the multiphoton transition rate per unit volume is

where A = $^ + 9 " 4m*w'

I* and <x> is defined as the integer part of x. The function $ in Eq. 1 is the Dawson integral, and m' is the electron-hole effective reduced mass. In Eq. 1, E is the peak electric field rather than the

rms electric field tabulated in many dielectric breakdown papers.

The total attenuation of a light wave as a function of distance z into a medium with linear and

nonlinear absorption may be written

dl/dz =E I^ (2)

N

where a^*^^ is the N-photon absorption coefficient. The Keldysh two-photon transition rate is related

to a(2) with the relation

Consider a material which supports only one- and two-photon absorption. If a planar slab of such

a material with thickness £ is traversed by an intensity packet

Kr.o.t) = lo e-(^/P)^ e-(^/^)'

at the entrance plane, then we may write [10] for the transmitted intensity at the exit surface

I„(l-R)^e-(^/p)^ e-(t/^)^ e-'"^

'^''"^ "

1 +a(0 1,(1. R) e-(^/P)%-(t/T)^(i.e-«f'>^)[a^'M-^

"

In the above equation R is the intensity reflectivity of a single surface of the medium. The energy

transmission T is therefore

out 2tt / rdr / Io(r,5,,t) dt

J

^ = 0

|°°,|,,.(OMl-R)(l-e-- \.-y^^

At low intensity we obtain

(2)

lim d(T~') ^ g ^ ^ (6)

Iq-^O d lo 2V2 (1 - R)

-490-
as long as Iq (1 - R) (1 - e " ^) a^^ Vot is much less than unity and a^' ^

2. << 1 as well . Hence,

the slope of experimental data of T"' plotted vs. intensity Iq yields the 2PA coefficient a^^).

In actuality, we found the approximate value of a(')from Eq. 6 for each material, and then generated

d family of curves using Eq. 5 with values of a(^) close to the approximate value. The final 2PA

coefficient was then chosen according to optimum fit of the data.

EXPERIMENTAL EQUIPMENT AND PROCEDURE

The Nd:YAG picosecond laser oscillator - amplifier system is described in Refs. 6 and 11. Passive

mode locking is employed with a laser-triggered electro-optical shutter to produce single pulses with

energy up to 10 mJ. The pulses have a smooth, temporally Gaussian profile with an average full-widthat-hal f-intensi ty duration of 30 ps During the 2PA measurements, a fast vacuum photodiode/oscilloscope

was employed to eliminate data resulting from laser pulses that were not perfectly excised from the

pulse train. A CDA crystal, 90° phase-matched by temperature tuning, was used to convert the 1064-nm

light to 532 nm. An angle-tuned KDP crystal was used to mix the two waves to produce 355-nm pulses.

A second 90° phase-matched crystal (ADP) was used to convert 532-nm light to 266 nm. Unwanted wavelengths, were separated out by filter glass or by a dispersive prism set at the minimum deviation angle to avoid

spatial profile distortion.

A fused silica lens (24.7 cm focal length at 355 nm) was used to increase the intensity, per unit

energy, of our pulses in the sample and a second lens after the sample was used to recollimate the pulses

for final energy detection. The samples were always placed behind the focal point of the first lens by

at least 2 cm, i.e., many Rayleigh lengths. There are two advantages to this placement.

First, since the sample positions were well into the region describable by geometrical optics, the

calculation of the area of the pulses in the sample is less susceptible to error arising from lens

aberration. In these experiments the pulse area was measured at the focusing lens plane by a photodiode

array with 50-ym resolution. This measured area was used along with the carefully measured (+ 0.6%)

focal length of the focusing lens to calculate, using diffraction-limited theory, the pulse area in the

samples. The area at the sample plane could not be measured with the photodiode array due to insufficient

resolution and susceptibility to damage of the detector array.

Second, the placement of the samples far beyond the lens focal point has an advantage for the

reduction of harmful sel f-focusi ng effects in the experiment. If a high-power experiment is conducted

in a medium other than vacuum at (or within a Rayleigh length or so of) the focal point of a lens,

then strict attention must be given to whole-beam sel f-focusi ng [6]. The input power must be kept

well below the sel f-focusi ng critical power for each material in order to prevent nonlinear focal area shrinkage.

However, far away from a focal region and in thin samples such as were used in these experiments, pulses

with many times the critical power may be propagated through the sample without catastrophic sel f- focusing

In the unfocused, thin sample geometry, small-scale sel f-focusi ng [12] is the phenomenon to be avoided.

In a medium of length I with a nonlinear refractive index n2, input spatial profile modulation

(noise) is augmented in intensity during propagation at most by a factor e^. The so-called B-integral

[12] is

X 10^ TT^ r\2 (esu)

/

I(z)dz. (7)

Values of n2 are obtainable from the literature [6] for several of the materials used in this study, and

n2 of the other materials is estimable from the values in the literature. Calculations using Eq. 7 state that in these experiments it was possible, with only two exceptions, to obtain sufficient absorption and measure accurately the 2PA coefficients without incurring appreciable noise enhancement and concomitant error introduction via small-scale sel f- focusing (sssf). This conclusion was verified by measuring the

2PA coefficient in samples of the same material and orientation but different length I. The coefficients

showed no variation with I greater than the experimental uncertainty (from all sources other than sssf)

in a(^). A further check was made by observing, under a microscope, the stable color pattern formed in KCl doped with U-centers by irradiation with pulses at the upper limit of intensity used in these experiments.

In Si02 at 355 nm and in LiF at 266 nm, the 2PA coefficients were so small that considerably higher , intensity was passed through the samples in attempt to get measurable 2PA. In these two cases, noise

fluctuations of the worst-case spatial frequency may have undergone intensity growth by as much as a 1 factor e^ or about 3. Nevertheless, 2PA coefficients were not large enough to be measured in these two cases and upper bounds only were determined. The reader should note that the n2 values employed in the

above discussion were measured at 1.06 ym [11]. At the shorter wavelengths of 355 and 266 nm, the true -491-
r\2 values may be somewhat different due to positive or negative contributions due to the nearby resonant

two-photon energy bands. Resolution of that question must await n2 measurements in the UV.

In order to eliminate possible accumulative effects on the measurements, such as F-center generation,

each transmission data point was performed at a fresh site in the sample. This was accomplished by

translating the sample after each laser shot.

The uncertainty in the values of a(^) measured here is ±43%, comprised of ±7% uncertainty in

absolute energy calibration, ±8% in pulse area at the sample, and ±28% in the average value at low

intensity of the tA pulse-to-pulse fluctuation parameter [13]. Hence, the absolute uncertainty sums

to ± 43%.

RESULTS

At 355 nm, we measured the absolute 2PA coefficients in three al kal i-hal ides , and in KDP and nine

other harmonic generating crystals. These are all listed in Table 1. Upper limits for a{^) in the

commonly used UV window materials, sapphire and fused silica, with band gaps greater than 2fia) (7.02 eV)

are also tabulated. For the materials checked with band gaps greater than 2^1(1), the transmission was essentially constant to intensities up to the region of about 2 x lo'" W/cm^. Higher beam intensities

could not be used due to the onset of surface damage.

At 266 nm, we measured the 2PA in eight al kal i -hal ides with band gap less than 2ha3 (9.36 eV) and

in KDP, ADP, AI2O3, Si02, and calcite. The window materials LiF and CaF2 (in which no 2PA is expected)

were also measured. The measured values are listed in Table 2. For alkali-halides the a(^) values

measured are of the order of 10"^ cm/MW while for KDP, ADP, A1203, and calcite they are of the order

lO""* cm/MW. For the materials with band gaps greater than 2fiw, an upper limit for a(0 is given. It is interesting to note that fused silica and quartz have quite small 2PA at this wavelength, even though

2'fiw considerably exceeds ^g.

Fluorescence is observed in all the alkali halides along the path of the beam [14]. Coloring can

be seen with the bare eye after a few shots. In KCl the absorption of a monitoring beam at 532 nm by

the color track is observable and can be explained by the spectral position of the absorption band of

the F-center [15]. The physical process involves the generation of electron-hole pairs by two-photon

excitation; the pairs recombine to form excitons with the emission of fluorescent light. The exciton

energy is given up to form and F-centers [16]. We have found that the F-centers generated by several

hundred shots of the laser at the same site of the crystal did not influence the transmission of the

crystal at the laser frequency after they had relaxed for a few seconds.

We may consider the effects of multiphoton absorption on the breakdown process via the Keldysh

model. Such considerations will not be quantitative, although we may expect the Keldysh model to predict

trends in the behavior of the 2PA contribution to breakdown.

The major difficulty in accurately calculating breakdown features is the diversity of the processes

involved. Even if one neglects lossy facets of plasma recombination and electron and thermal diffusion,

he is still faced with the coupled avalanche — 2PA dynamical problem. At 1064 nm in typical wide band

gap insulators, picosecond breakdown has been interpreted in terms of an avalanche model [11]. Avalanche

ionization rate data at 1064 nm has even been extracted for several materials from such breakdown

experiments, yet to what extent the 1064-nm avalanche rate differs from 355-nm or 266-nm avalanche

rate data is unknown.

When an experimental breakdown threshold is measured, the result may be due to combined avalanche

multiplication and two (or higher-order) photon absorption. The plasma growth by the cooperative effect

of 2PA and avalanche multiplication of the 2PA-generated charge carriers and avalanche-generated carriers

cannot be dealt with by calculation unless the proper avalanche rate is known. Hence, even though the

2PA coefficient may be known, calculations on breakdown phenomena [3] are made difficult by our lack of

data concerning the avalanche process by itself in the ultraviolet.

-492-
One approach, with perhaps the single redeeming quality of tractabi 1 i ty , is simply to delete the avalanche considerations altogether and to model a plasma production by sole 2PA. We have used the Keldysh [8] model for such a first look at 2PA breakdown in CaF2. Figure 1 illustrates the results.

The three picosecond breakdown thresholds [6,11] for CaF2 are plotted versus photon energy. The

other bars mark the electric field necessary to generate 10'^ charge carriers per cm' by multiphoton

absorption in CaFg. For these calculations, Eq. 1 was used with t^Q = 10 eV and m* = 0.1 to 10 m. The range of m* produces the range of electric field for 3PA and 5PA in Fig. 1. (Note that rms electric

fields are plotted in Fig. 1, whereas peak fields actually must be used in the Keldysh expressions.)

Although the absolute positions of the multiphoton bars are not significant, the relative positions

are more worthy of note. In Ref. 6 it Was stated that the reduced 355-nm threshold compared to the 532-nm threshold in CaF2 was probably due to the assistance of three-photon absorption to the breakdown

avalanche process. Figure 1 illustrates that the Keldysh model predicts a decrease in the electric

field, necessary to make a plasma density of 10'^ cm\ by a factor of 2 to 3 depending on m*. This

result supports the interpretation of Ref. 6 that the three-photon absorption assistance to the avalanche process served to lower the 355-nm breakdown threshold.

One further point merits discussion here regarding 2PA and breakdown. In Ref. 6 it was surmised

that in KDP at 355 nm, 2PA had caused beam depletion in front of the focal point to such an extent that

an artificially high damage resistance was recorded. With the measured KDP a(^) of 6 x 10"^ cm/MW

from Table 1, we see now that, indeed, at the 2 x 10'' W/cm^ intensity level, an energy loss of as much as 12% per lOOym will occur. Therefore, the often-used bulk breakdown geometry of tight focusing

inside a sample surface will perhaps give way entirely to surface breakdown testing in the ultraviolet

and vacuum ultraviolet.

SUMMARY

Absolute coefficients for two-photon absorption at 355 nm and 266 nm were presented for a variety

of wide-band-gap optical materials. The influence of multiphoton absorption on the breakdown process

was discussed. It was shown that pulse depletion due to two-photon absorption is a serious obstacle

to the use in the UV and VUV of the classic bulk breakdown geometry of tight focusing inside a medium.

ACKNOWLEDGMENTS

We express our indebtedness to Mr. S. Maurici for his preparation of samples and to Dr. L. F. Mollenauer for a U-center doped sample. We also thank Dr. C. B. de Araujo for sharing equipment and

useful discussions, and Mr. Richard Yen for assistance with the measurements. Ms. K. Wenzinger of

LLL is much appreciated for preparation of this manuscript.

REFERENCES

[1] Recent reviews of 2PA are found in J. M. Worlock, Laser Handbook, eds. T. Arecchi and F. SchulzDubois (North Holland, Amsterdam, 1972); and in

H. Mahr, in Quantum Electronics, eds. H. Rabin

and C. L. Tang, vol. 1 (Academic, New York,

1975).

[2] N. Bloembergen, IEEE J. Quant. Electron. QE-10 , 375 (1974).

[3] P. Braiinlich, A. Schmid, and P. Kelly, Appl

.

Phys. Lett. 26, 150 (1975).

[4] P. Kelly, P. Braiinlich, and A. Schmid, Appl.

Phys. Lett. 26, 223 (1975).

[5] N. L. Boling, P. Braiinlich, A. Schmid, and

P. Kelly, Appl. Phys. Lett. 27, 191 (1975).

[6] W. Lee Smith, J. H. Bechtel , and N. Bloembergen,

Phys. Rev. Bi5, 4039 (1977).

[7] P. Liu, W. Lee Smith, H. Lotem, J. H. Bechtel,

N. Bloembergen and R. S. Adhav (to be published).

[8] L. V. Keldysh, Zh. Eksp. Teor. Fiz, 47, 1945

(1964).

[9] P. Kelly (to be published).

[10] J. H. Bechtel and W. Lee Smith, Phys. Rev. B13, 3515 (1976).

[11] W. Lee Smith, J. H. Bechtel, and N. Eloembergen,

Phys. Rev. 622^, 706 (1975).

[12] E. S. Bliss, J. T. Hunt, P. A. Renard, G. E. Sommargren, and H. J. Weaver, IEEE J. Quant.

Electron. QE-12 , 402 (1976).

[13] W. Lee Smith and J. H. Bechtel, J. Appl. Phys.

47, 1065 (1976).

[14] J. N. Bradford, R. T. Williams and W. L. Faust,

Phys. Rev. Lett. 35, 300 (1976).

[15] W. B. Fowler, Physios of Color Centers

(Academic, New York, 1968).

[16] Y. Toyozawa, in Proc. VI Int'l Conf. on VUV Radiation Physics, ed. E. E. Koch (Pergamon,

Vieweg, Hamburg, 1974).

-493-
Table 1. Experimental results for two-photon absorption coefficient a ' at 355 nm. The orientations of sample and electric field are tabulated, along with the sample thickness I.

Material

<^'g, eV (approx.

)

Surface

Normal

1- / 1 1- 1 E/|E| I, cm

f 9 1 /tit 1 a^"^', cm/MW

KI 5.0 a s [100] b E [001] 0 47 (7.3 ± 1.5) x 10''

RbBr 5.4 a b 1 0 (2.4 ± 0.5) X 10''

Rbl 5.0 a b 1 0 (5.1 ± 0.8) X 10"'

ADA c E 45° z-cut d E 1 z axis 1 6 (3.5 ± 1 .1 ) X iC

ADP 6.8 c d 2 0 (6.8 ± 2.4) X 10''

CDA c d 1 4 (8.0 ± 2.4) X 10"^

CD*A c d 1 15 (2.8 ± 1.0) X 10'^

KDA c d 1 0 (4.8 ± 1 .4) X 10"=

KD*A c d 1 0 (2.7 ± 0.9) X 10"=

KDP 6.9 c d 1 0 (5.9 ± 2.1) X 10"'

KD*P c d 1 0 (5.4 ± 1.9) X 10-'

RDA c d 1 25 (5.0 ± 1.5) X 10"=

RDP c d 1 0 < 5.9 X 10"'

AI2O3 1 . 3 c-axi s 0 5 < 1 .6 X 10

^

SiOg (fused) 7.8 0 64 < 1.3 X 10"^

Table 2. Experimental results for two-photon absorption coefficients a at 266 nm. The orientations of sample and electric field are tabulated, along with the sample thickness l'.

^^^-•^^ (t?;of) ^m^aT E/IEI .,cm a^^), cm/MW

NaCl 7-8 a E [100] b E [001] 0 74 (3.5 ± 0.9 X 10

NaBr 7.7 a b 0 74 (2.5 ± 0.4 X 10

KCl 6.5 a b 0 74 (1.7 ± 0.3 X 10

KBr 6.0 a b 0 74 (2.0 ± 0.6 X 10

KI 5.1 a b 0 47 (3.8 ±1.1 X 10

RbCl 7.3 a b 0 60 (1.0 ± 0.2 X 10

RbBr 5.4 a b 1 0 (2.2 ± 0.4 X 10

Rbl 5.0 a b 1 0 (2.5 + 0.8 X 10

CaC03 5.9 0 43 (2.4 ± 0.7 X 10

AI2O3 7.3 c-axis 0 50 (2.7 ± 0.8 X 10

Si02 (fused) 7.8 0 57 i 4.5 X 1 0" =

Si02 (crystal) 7.8 c-axi s 0 50 < 4.5 X 1 0-^

CaF2 10.0 a b 0 47 < 2.0 X 1 0" =

LiF 11.6 a b 0 47 < 2.0 X 1 0-=

KDP 6.9

1 . 06 um 0.53 ym cut

0 57 (2.7 ± 0.8 X 10

ADP 6.8 c-axis 0 50 (2.4 ± 0.7 X 10

-494-
100 -

E 80 -

o

« 60 -

40 -

20 - o

5-photon

C)

CaF2

m* = 0.1 -10.0 m

O Experimental

breakdown

thresholds

3-photon

fico

1.165 eV

(1.06 jum)

2.33 eV

(0.532 Mm)

3.495 eV

(0.355 Atm)

Fig. 1. Breakdown parameters versus photon energy for CaF2. The rms electric field breakdown

threshold is plotted along the vertical axis. The three round data points are experimental

breakdown thresholds from Refs. 6, 11. The other data bars represent the range of electric

field strength needed, according to the Keldysh model, to produce 10'' electrons/cm^ in

CaF2. The large uncertainty in the Keldysh bars reflects the range of m* (0.1 to 10.0 m)

considered.

COMMENTS ON PAVER BV LIU, LOTEU, BLOEMBERGEW, SMITH, 8ECHTEL, ANV AVHAV

ExtzMivz dLiicuM-ion Mcu, ca/ou-dd oat concnAninQ the. dtfilnitxon tka band gap appKopfUatz to miMxphoton dXcJXatloni . Ihz quution utaJLtij one. the dznA-iXy OjJ itate^ accui-Lble to muLtipkoton

t>uinitiXA.oni, . It woi poi.nte.d oat that -in atkaJU. koLideA, txanittton^ to the Zowut tying zxciton itatz

axe. alZoMzd {^on. odd numbeMA o^ photom, and {^on. even nambeA o(, photons the loMe^t lying accsAiibZe

itate matd be the ^iut odd-pcuiity itate above the zxciton peak. In tkii expenAjnent the band gap MOi

inleXAzd l>iom one-photon abioxption data. The ip&akeA indicated that theAz Mat, no Ke.aion to expzcX

any paL&e da/iati.on dzpe.ndence. (^on. tMo-photon abio^ption, bat that no expeAimtnti, have, been cajwied out

to i>ze ii thii woi, Keatiy Piue. The apptiaxbiLUty Oj$ the KeZdyih theon.y in deAc/Ubing maltiphoton

abioKption wa6 chaZZenged. The. ipeakeA pointzd oat that the KeZdyih the.on.y had be.en oied iucceM^alty

in thz poit to dzicAibz mattiphoton abion.ption and woi employed heAe 06 a heix/uj,tic toot.

-495-
STATISTICAL CONSIDERATIONS IN LASER DAMAGE MEASUREMENTS'

J, A. Detrio and A. P. Berens

University of Dayton Research Institute

Dayton, Ohio 45469

The measurement of the laser damage threshold for a given material or

optical component is usually accomplished by testing at predetermined power

levels and irradiation times and then observing whether or not damage occurs.

This experimental procedure is known as sensitivity testing which attempts to determine the percentage of specimens which are damaged at the various power

levels. It is not practical to determine the complete cumulative distribution curve for a given sample population because of the limited number of specimens

normally available. The variability in damage level for a given optical component will influence the number of specimens required to measure the damage

threshold with a given precision and with a known level of confidence or to achieve a desired degree of discrimination in comparing two different materials,

preparation methods or production runs. A statistical method applicable to sensitivity testing is analyzed to permit the experimentor to determine the number of specimens required in accordance with the desired level of precision and

confidence. A novel nonparametric method which is applicable to damage testing

and requires few samples is also analyzed.

Key words: Laser damage testing; statistics; sensitivity testing; up and down

method; probit analysis.

1. Introduction

Statistical considerations should be included in the planning of laser damage experiments when

the purpose of the test is to select one material, component, or fabrication process over another or if the damage threshold and its standard deviation must be determined with a given precision and level

of confidence. If the materials or components being tested always fail at a precise power the problem

is simplified and statistics are not required. In practice, the test specimens tend to fail at random

over a range of incident powers. This finite spread in the measured damage threshold makes it neces- sary to carefully consider the number of specimens selected. If the experimental results are used to test a hypothesis concerning the magnitude of the damage threshold for the entire population of materials from which the samples were selected, it is important to understand the consequences of an

error in judgment being made on the basis of the results of the experiment.

Several constraints on the conduct of CW damage tests have encouraged a search for test strategies that are economical in the use of samples and that are compatible with the method of operation

of high energy lasers (HEL). Some of the constraints are the limited numbers of samples available

for testing, the expense of HEL operation, and the increase in test time which is required when frequent power level changes are used. In addition to these constraints, there are factors which influence

the results and the test planning that are a consequence of the nature of the samples themselves.

Some materials are sensitive to environmental conditions, others are affected by the previous history

of laser irradiations and almost all samples show a spatial distribution of defects which make damage

results a fvinction of the irradiated area.

The objective of this work is to analyze a test method which is capable of yielding the required

parameters and which is compatible with some of the limitations of HEL testing. A nonparametric

technique for estimating the threshold with known confidence is presented. This latter method is compatible with HEL operation and is economical in the use of samples and test time.

The probabilistic nature of laser damage, which has been addressed by Bass and Fradin [l]\

is of considerable interest in the study of the fundamentals of damage processes but it is not within

the scope of this paper.

* This work supported by the Air Force Materials Laboratory, Wright- Patterson Air Force Base.

1. Figures in brackets indicate the literature references at the end of this paper.

-496-
2. Problem Definition

We will define the damage threshold in accordance with conventional practice in agricultural

research. It is common in the testing of pesticides to define the dosage at which 50 percent of the subjects expire as the LD50, (lethal dose 50 percent) level [2] which also corresponds to the center of the

distribution of fatalities (failures) versus dosage (power). The power which corresponds to a 50 percent probability of failure is the damage threshold.

In many laser damage experiments, it is not possible to determine the power level at which

damage occurs on a continuous scale of power. Rather, a power level is set, a specimen is tested at that level for a fixed time, and it is either damaged or not damaged according to the criteria selected

to define "damage". This experimental procedure is called sensitivity testing. Statistical methods

must be used to properly interpret such data if the required precision of the damage threshold is less

than the variations in observed damage threshold. Thus, even before applying formal statistical

methods to the data, the separation of the power levels at which the testing will be done must be

rationally established with respect to the required precision. For example, if one wishes to resolve

differences in damage threshold of ± 10 percent, then the test power levels should be spaced at no more than 10 percent increments. If the variability in the material' s response is considerably greater

than 10 percent, then statistical methods are necessary, and the power levels should be selected to span the region from below threshold to above the threshold.

If the required precision is stated as a percentage as in the example above, then the test levels

will be spaced in a logarithmic sequence. The methods developed below are applicable to log-normal

distributions and are insensitive to the assumption of a normal (Gaussian) distribution. Because the

accuracy and precision of power measurements is usually expressed in percentages as are safety

factors, the log sequence is in general a logical choice.

In damage experiments, the damage thresholds of the individual test specimens are the random

variables and the damage threshold of the total population of the material must be defined in terms of

the distribution of the random variable. The cumulative distribution displayed in figure 1 contains all the necessary information for describing the damage threshold for the particular population of speci- mens. Given a sufficiently large number of test samples from the population, the probability of damaging a specimen at any power level could be precisely determined. It will be assumed, that either

the mean or the median of the distribution is the desired damage threshold parameter which describes

a particular population of material.

The existing statistical techniques for sensitivity testing require the assumption of a normal

distribution of the random variable being characterized. When the random variable is not normally

distributed, transformation to an approximately normal distribution may be performed. The commonly used analytical methods require that the fixed levels of the random variable be set at equally

spaced increments. Therefore, in the damage tests for which irradiation levels are set at equally

spaced intervals on a logarithmic scale, it will be assumed that the power levels at which damage of

the specimens is initiated will have a log- normal distribution.

The use of the mean value of the distribution as the definition of the damage threshold softens

the requirement for a normal distribution and improves the efficiency of the experiment with respect

to the number of specimens required.

3. Experimental Design

A variety of considerations affect the planning of a damage experiment. In the statistical portion of the experimental design, certain questions should be addressed. For example, are the parameters of the distribution of damage threshold as a function of power to be estimated? What is the

required accuracy of the estimates? Is a hypothesis being tested? What confidence limits are

required? What is the consequence of a faulty conclusion being drawn from the data? Many of these

questions have a formal definition and statistical methods exist for a quantitative stating of these

questions and for obtaining the answers.

There are obvious advantages to the re-use of specimens that are not damaged. The difficulty

in the re-use of specimens lies in the mixing of the environmental conditions of the population being

characterized. These environmental factors can be significant and could influence the estimate of a

-497-
damage threshold or may lead to a wrong conclusion in comparing two materials. This problem is normally referred to as the one- on- one versus n- on- one threshold. The effect of previous irradiations below threshold is called conditioning.

If a given material has been demonstrated to have no conditioning effect due to previous irradiation exposure, then re-use of the specimens would be possible. The verification of the presence or absence of a conditioning effect for a given material would probably be conducted during a preliminary

characterization experiment to determine the appropriate damage threshold. It will be assumed the

specimens are virgin and that any retest of specimens is conducted to determine a conditioning effect.

The three approaches to sensitivity testing under the restriction of testing at fixed power levels are the probit method [3], the up-and-down method [4], and a nonparametric (multilevel 3 of 4) method. The probit method is mentioned primarily because of its prominence in sensitivity experiments. In this approach, specimens are tested at several power levels and the percent damaged at each level is obtained. Assuming the data are from a normal distribution, the mean and standard

deviation are derived from the best curve fit through the observed cumulative distribution. Although

this method has been extensively developed [3], a minimum of 50 specimens are recommended.

Since 50 specimens will probably not be available for a typical laser damage test, the probit method

will not be discussed further. The up-and-down method and the nonparametric method for conducting

the laser damage tests will be presented in the following.

4. The Up- And- Down Method of Sensitivity Testing

4. 1 General Approach

The up-and-down method of testing is a sequential procedure in which the power level of the

i+1 test depends on the result of the ith test. If damage does not occur, the next specimen is tested at a higher level. If damage does occur, the next specimen is tested at a lower level. The effect of this

sequential procedure is to provide more observations around the center of the population which allows

inferences concerning the mean to be made with fewer samples. This efficiency is achieved at the

cost of testing samples sequentially with a change in power between each pair of samples. A graphical display of the results of a series of up-and-down tests would appear as in figure 2.

The mean threshold is given by:

X = X(min) + AX (^+ j)

k

where A = 5^ in and X(min) is the lowest power level at which failures occur, n. is the number of ^ i

1 i=0 k

times failures occur at the i test level and N = • have assumed slightly fewer failures

i=0

than survivors. The precision of the estimates of the mean and standard deviation is based on the

minimum of the number of damaged or nondamaged specimens. Since the number of damaged specimens will be approximately equal to the number of nondamaged specimens, the effective sample size

N, will be approximately half the number of specimens tested.

The standard error of the mean is given by the formula:

X VN

where S is the standard deviation of the population, and G is a factor which depends on the ratio of

the increment of power (Ax) to S and on the difference between the true mean and the nearest Xj.

For values of Ax/S£ 2. 5, G is independent of the location of the true mean and is approximated by

the equation

G = 0. 85 + 0. 15 .

When Ax/S> 2. 5, the location of the true mean with respect to the test intervals becomes important

and the maximum and minimum G values are available as a function of Ax/S.

-498-
In order to determine the number of specimens required in the experiment, the objective must

be stated in terms specifically related to the sampling variation of the experiment. Assume first that

the objective is to estimate the median of the damage threshold distribution and that the one-sided

100 P percent confidence limit on the median will be less than 100 q percent of the estimate. Without

loss of generality, assume that the upper confidence limit is being considered. The upper confidence

limit on the mean of the X distribution is given by — *^r^^^ U. C. L. = X + "

th where t is the p percentile of the Student' s t distribution. (In the calculations which follow, t values

P were selected for a t- distribution with 10 d. f. and a log distribution of power levels is used.) The

width of the confidence interval in power units is given by X and the upper confidence limit and, thus,

the desired precision statement is

t GS

_E < log ( 1 + q) .

Therefore, if

the desired precision will be achieved.

N > [log (1 + q) J

'

The sample size formula requires an estimate of the standard deviation of the power thresholds.

Prior to testing and in the absence of experience for specific materials, this standard deviation is unknown. Three values which span a wide range of possibilities were assumed in the calculations.

They were arrived at by assuming the range of damaging power that will be realized in the experiment

span 1, 0. 5, and 0. 25 orders of magnitude. Assuming that the range is equal to 4 standard deviations

results in log- transformed values of S = 0. 25, 0. 125, and 0. 0625.

Figure 3 presents N (one- half required number of specimens) as a function of q to compare

sample size requirements for different levels of P at the fixed values of S. Similarly, figure 4 presents N versus q to compare values of standard deviations at a fixed level of P. Figure 3 shows that

large sample sizes will be required if the width of the confidence interval is to be 10 percent or less

than the estimate of the median. If the practical limit is 24 specimens and damage thresholds that

have a one- half order of magnitude range (S = 0. 125), then an 80 percent confidence limit would be 13 percent of the estimate of the median and a 95 percent confidence limit would be 22 percent of the

estimate of the median. The other two values of the standard deviation give less precision.

Figure 4 indicates that a smaller sample size is required for the same precision at the middle

of the three standard deviation values. This result occurs due to the G factor which is a function of

sample size and the spacing of the power level settings. The optimum is reached when S = a, but S is generally not known in advance so it is generally not possible to design the test to minimize sample

size requirements due to this factor.

For constant values of S, figure 5 presents P as a function of q for N = 3, 4, 6, and 12. This

figure indicates that for 80 percent confidence and S = 0. 125, precision for N = 6 is 12 percent of the

estimate while for N = 12 the precision is 8. 5 percent. Comparisons of this nature should be made

for any prospective sample sizes to insure that the gain in precision is worth the gain in added tests.

4. 2 Testing Hypothesis

If the objective of the experiment is to compare the damage thresholds of two materials,

sample size evaluations can be conveniently made in terms of the probability of finding a significant

difference as a function of the true difference between the population means. This probability is a function of the level of significance of the test, the standard deviation of the mean, and the sample

size. In particular, if X ^ and X^ are the means of the population to be compared then the hypothesis

to be tested is,

H^:^^ = vs Hj:,x^ > (.^ .

-499-
H is rejected if Z > Z , where

O Q

Z = and S— — = —

and Zf^ is the (l-a)th percentile from a standard normal distribution. It is assumed in the following

computations that the standard deviations of the two material populations are equal and that equal

sample size will be tested from both populations. The normal distribution is being used in place of

Student' s t to facilitate calculations, but this approximation will have little effect on the comparisons

to be made.

The probability of accepting H as a function of D = fj.^ - [jl^ is given by

P I Z < Z^

I D I = P

'^1 - ^2

Figure 6 displays the probability of accepting H as a function of D for a 90 percent level of confidence

o

test (O! = 0. 10), for the three assumed values of standard deviation and for N = 3, 4, 6, and 12,

Let P denote the probability of accepting when is true. When a and P are fixed, then

sample size requirements to achieve a specified degree of discrimination can be determined. In particular, if 2 2 2 2(Z - ZJ G S

N > —-

then the probability of accepting p.^ = y.^ is 1-a if [Jtj - |J-2 — ^' ^'^^ probability of rejecting

[J.J = [1^ is 1-P if [jLj - |J-2 ^ Figure 7 presents a plot of sample size versus D = fji^ - fo.^ for

o = P = 0. 1 and the three assumed values of S.

5. Nonparametric 3- of- 4 Approach

The problem of estimating the damage threshold of a material can also be formulated in a manner that does not require an assumption regarding the underlying distribution of damage levels of

the specimens. If the population damage threshold is defined as the average of the two test irradiation levels which fall on either side of the median, then the problem is to establish these two levels

with sufficient confidence. Specimens are tested at a power level and an inference would be made

about the probability of damage (p) in any single test specimen. When the analysis has indicated that

the median (p = 1/2) of the distribution has been bracketed by two of the power levels, the experiment

is stopped. This approach was stimulated by a comment by Loomis [5] that 3 of 4 failures indicated

a power level above threshold.

There are two methods of analyzing the results of the experiment; in the traditional approach

which is the probit method, the results of experiments are used to place a lower (or upper) confidence

limit on the probability of damage at that level. For example, if n specimens are tested at a level

and k of the specimens are damaged, then the lower confidence limit on the probability of any one specimen being damaged is the solution for p, of the equation

n\ y /I vfi-y -» "Ml - Pj) ' = « y=k

where a 100 (l-o) percent confidence level is sought. Values of p, are tabled in standard texts [2],

-500-
i

I

Assuming that, at most, 6 specimens could be tested at any one level, a lower 90 percent con- fidence limit is desired, a summary of inference for n <^ 6 is as follows:

a) For n <^ 3, regardless of result cannot be 90 percent confident that p > 1/2,

b) For n = 4, if all four are damaged, lower 90 percent confidence limit is p^ = 0. 56

and hence 90 percent confident that p > 1/2, If only 3 of the 4 are damaged, p^ = 0, 32.

c) For n = 5 and 6, all specimens must be damaged for 90 percent lower confidence limit

to be greater than 0, 5, In particular, if n = k = 5, p^ =0.63 and if n = k = 6, p =0. 68.

Again, if only one specimen is not damaged, there is not 90 percent confidence lhat p is greater than 1/2.

The above summary indicates that if this analysis method is adopted, then four specimens at each power level would be the most efficient design. Note, however, that if all specimens out of 5 or

6 are damaged, then there is 95 percent confidence (rather than 90) that p > 1/2. Even if this higher

confidence level is desired, 5 rather than 6 specimens would be tested at each level.

The second method of analyzing the data from this experimental approach is the Bayesian

approach. In this approach a distribution of possible values of the probability of damage at the particular power level is assumed and the results of the test are used to re- evaluate the distribution. In particular, the probability of obtaining the result is

1

f (k.n-k) = r p (l-p)*"' g (p) dp .

Now assume that g (p) has a beta distribution.

.. (a+b+1)! a-1 , .b-1

= (a-1): (b-1): P

This family of distributions covers a wide range of shapes and, in particular, contains the uniform

distribution (a = l, b = l) which will be assumed shortly.

Given f (k, n-k) and g (p), the distribution of p given the result k and n-k from the experimental

tests, the modified distribution of p is

, ,, ,, (n+a+b-l)I k+a-1,, ,n-k+b-l

S (pI'^' ""-^^ = (k+a-1) ! (n-k+b-D! P '^"P^

Now before tests are begun at a particular level, very little is known about the likelihood of the values

of p. Thus, it will be assumed that prior to the experiment, any value of p between 0 and 1 is equally

likely; i. e. , p has a uniform distribution. Under this assumption

/ ii i\ (n+1)! k n-k g(p|k, n-k) =

^, p (1-p)

Given the distribution of p as interpreted in the light of the experimental data, the question of the

possibility of p being greater than 0. 5 is easily answered.

1 P [p> l/Z] =J g(p|k, n-k)dp

1/2

This probability is the measure of assurance that the power level setting is greater than the median of

the threshold distribution.

Consider the following examples. Assume 4 specimens are tested at a level and three of them

are damaged; n = 4, k = 3. Then

-501-

I
g(p|3, 1) =3177 P^(l-P) = 20(p^-p^

and

P (p > 0. 5)

Thus, there is 81 percent confidence that p >^ 1/2.

If all four specimens are damaged, then

I 5; 4 o 4

g (P|4,0) = p (1-p) = 5 p^

and

P (p > 0. 5) =y 5

p"* dp = 0.96875,

0 Thus, if all four are damaged, the confidence that p _> 1/2 is definitely increased.

The results of the two methods of analysis are in close agreement. The Bayesian analysis provides a sharper insight. For example, in the classical analysis with lower 90 percent confidence

limits being calculated there was no advantage in testing 5 or 6 specimens rather than 4 if the objective was to determine if Pj > 1/2. In the Bayesian analysis, however, the confidence level is not preset and the probability of p being greater than 1/2 is calculated after the experiment is conducted.

Thus, the results are given different interpretations. To design the experiment for a Bayesian

analysis, note that only two samples must be tested at any level below threshold for which no failures occur. Additional samples must be tested only if needed to verify that 3 of 4 fail- -if 3 of 3 fail, the

test is over unless one seeks the higher confidence.

J

20 (p^ - p"*) dp = 0. 8125 .

0. 5

In comparing two populations of materials, there is no method of evaluating sample sizes with

respect to the ability to discriminate between median threshold values.

6. Summary

The up-and-down method of sensitivity testing can be used to obtain laser damage threshold

data when the required changes in power between tests presents no great difficulty. Information on

the confidence limits, sample size, test level spacing, and the ability to discriminate between dif- ferent damage thresholds is presented. The curves are useful in making the appropriate trade-offs

during the planning of a damage experiment. An analysis of the 3- of- 4 method showed the most

economical method for estimating the median of the damage threshold at an acceptable level of

confidence.

7. References

[1] Bass, M. and Fradin, D. W. , IEEE J. Quantum Electronics, QE9 (197 3) 890,

f2] Dixon, W.J. and Massey, F. J. , Introduction to Statistical Analysis, McGraw-Hill,

New York, 1957.

[3] Bliss, C. I., Ann. Appl. Bio. , _22, (1935)

1 34 and Finney, D. J. , Probit Analysis , Cambridge Univ. Press, Cambridge 1947.

[4] Dixon, W. J. and Mood, A. M. , J. Am.

Statistical Assn. , _43 (1948) 109.

[5] Huguley, C.A. and Loomis, J. S. , Laser

Induced Damage in Optical Materials: 1975,

NBS Special Pub. 435, pl89.

-5C2-
8. Figures

O

UJ<

<

o

o

tr.

Q.

1.0

0 1

POWER LEVEL

Figure 1. Example Result of Large Sample Sensitivity Experiment,

bJo

Q.

UP-DOWN

0 0 ® O (gi ®0(giOOO<S>0®

O O O O ®

O O

O

SAMPLE

Figure 2. Example Result of Up-and-Down Testing.

-503-
S = 0.25

P=0.95 10

P=0.9

P = 0.80

10 20 30 40

lOOq

S= 0. 125

10 20 30 40 50

lOOq

10 20 30 40 50

lOOq

Figure 3. Comparison of Sample Size for Three Confidence Levels.

P = 0.9

10 20 30 40 50

lOOq

Figure 4. Comparison of Sample Sizes

for Three Standard Deviations.

1 .0 r

0.9 -

N= 12 6 4 3

0.8 -

0.7 -

0.6 -

0.5

0 10 20 30 40 50

lOOq

Figure 5. Comparison of Confidence Levels

for Four Sample Sizes.

-504-
Figure 6. Probability of Accepting = as Function of fx^ = lo.^ for

for a = 0. 10 and N = 3, 4, 6, and 12, Also shown on the

abscissa scale is the ratio of damage thresholds that corres- ponds to the respective values of D. As an example, if it is assumed that S = 0. 125 (center graph) and 24 specimens are

tested (N 12), if X - X > 0. 14, then the probability of

rejecting X = X is greater than 0.90. That is, if the ratio

^ 0. 14

of the damage thresholds is greater than 1. 38 ( = 10 ), there

will be greater than a 90 percent chance of concluding that (x^

is greater than [a^- On the other hand, if 6 specimens are

tested (N 3), the ratio would have to be greater than 1.92

0. 282

( = 10 ) to be 90 percent sure of rejecting equality.

lOOr

0 01 0,2 0 3 0.0 0 6 0

10 126 191 200 2 51 3.16 RATIO OF OflMflGE THRESHOLD

Figure 7. Sample Size Required to Yield a and P as a Function of [i - p. X -505-
COMMENTS ON PAPER BV VETRW ANV 8EREW5

Thi. ipmkeA mphoiizzd that thLi mzthod -a, a itcutiiticcLl method and can be apptidd to any typz

damagz mejuuAmznt. That -06, -ut can be apptizd to itngtz ihot damagz on. to damage cxpzHAjmcnti uiheAe. poAticuLoA 6itc ti> lA/iaduiatcd many tmu. In thz ZattcA caiz, duz to cumuZaJxve. c^ccti, damage may

not occuA on thz {^iA^t ihot bat may occuA on iubizqucnt ihoti. The. method advanced tn thii papeA -Li meani wheAeby the thAe^hotd (JoA. damage [hoMeveA damage ii> defined] can be deteAmtned. He {^uAthiA

po-inted out that td a dtitAibixtion 0(5 damage vaZueJs oveA a laxge nmbeA 0^ iampte^, -U, obtatned, then

one can oMtgn a pKobabtLity o{, {jOUZuAe at any given poweA level 0^ incident nadtation. Tka> -c4 the

chaAactentzatton wkich -c4 needed, in oKdeA to emuAe !>a{^e openatlon oven a loAge numbeA oi ihoti>.

-506-
MULTITHRESHOLD MEASUREMENT AND ANALYSIS OF PULSED LASER DAMAGE ON OPTICAL SURFACES

J. 0. Porteus, J. L. Jernigan, and W. N. Faith

Physics Division, Michelson Laboratories

Naval Weapons Center, China Lake, California 93555

The methodology of multithreshold analysis, a new approach to laser-damage

research, is described. The comparison of thresholds for various damage-related

effects identifies dominant failure mechanisms and provides better guidance for

laser-materials technology. After a brief description of apparatus, the procedure for routinely measuring up to eight different thresholds per sample is

given. The maximum-likelihood principle is used to derive an algorithm for

computing thresholds and standard deviations. The use of a standard gold sample

to verify reproducibility and to maintain long-term calibration is discussed.

Examples of multithreshold results on uncoated and coated infrared optical

components are presented. The following are some of the effects for which

thresholds are compared: slip, roughening, cracking, pits, melting, craters,

delamination of coatings, ion and light emission, and work function change.

Key words: Damage thresholds; failure modes; laser-optical components; Mo mirrors; optical coatings; pitting; ripples; standard sample Au mirrors; surface characterization; surface uniformity; threshold estimation; windows.

-1. Introduction

The damage resistance of optics for lasers may be improved with an understanding of important

damage mechanisms and their relationship to material characteristics. The comparison of thresholds for

the various observable damage-related effects provides a good basis for this understanding. Some of

the effects typical of a bare metal mirror may be seen in figure 1. Slip bands and intergranular slip

indicate susceptibility to stress damage, which can be related to yield strength, as shown in a companion paper [l]-*-. Melting, which produces the dark area in the dark-field micrograph, is an indicator of

optical absorption. Laser-induced pitting, which occurs mainly on grain boundaries in figure 1, can be

caused by segregated impurities. Cratering, identified by a raised rim around the melt zone, is usually

caused by pressure associated with vaporization. Other effects measured by other techniques may also be

informative. A change in surface work function, for example, can signal changes in surface composition

or topography. Ion and light emission indicates plasma formation.

The present work presents the methodology of obtaining thresholds routinely and accurately for as many as eight different laser-induced surface effects. After a brief description of the apparatus,

details of which are given elsewhere [2] , the procedure for acquiring and reducing the data is discussed.

The derivation of an algorithm based on the maximum-likelihood principle, which is used to compute

thresholds and standard deviations, is also given. We then discuss the use of a standard gold sample to verify reproducibility and to maintain calibration. Finally, we present typical results and their

interpretation on various infrared optical components.

2. Apparatus

Measurements are made in a previously described laser damage facility [2], where comprehensive

target monitoring and careful beam characterization are key features. Our ultrahigh vacuum (UHV) test chamber was equipped with the following monitoring apparatus for the results presented here: (1) 20-

power microscope; (2) Faraday cup with grounded entrance grid (10-mA/sr detection sensitivity at 35° off

the laser beam axis) for sensing of target-emitted ions and neutrals having sufficient energy to produce

secondary electrons; (3) an Auger analyzer with electron imager [2]. The electron images are generated

by work function variations -5-0.1 eV over the target surface caused by variations in surface composition

and topography.

Figure 2 illustrates the Gaussian spatial intensity distribution of the 10.6 ym laser beam when

focussed on the target by a ZnSe lens having a focal length of 24 cm. Temporally, each pulse consists

of a train of mode-locked spikes which form an envelope of 100 nsec nominal duration. Thresholds given are peak thresholds, i.e., they refer to the time-integrated fluence of the damaging pulse at the point

of maximum spatial intensity.

* Work supported by the Naval Air Systems Command, the Office of Naval Research and NWC Independent

Research Funds.

1. Figures in brackets indicate the literature references at the end of this paper.

-507-
The data reduction equipment consists of a Hewlett-Packard Model 9821 calculator interfaced with a Tektronix Model 4662 interactive digital plotter. Data reduction is performed entirely in the calculator, while the plotter is used only for display of final results.

3. Experimental Procedure

The sample is exposed to one pulse at each of approximately 50 sites spaced at l-mm intervals

along its surface. Pulse energies, which are measured for each pulse [2], are uniformly distributed

over a range which includes the thresholds for each of the damage effects of interest. The occurrence

of light emission (flash) and pit formation are noted by an observer as they occur, while damage features which are more difficult to identify, e.g., slip, melting and cratering, are categorized with the

aid of more sophisticated optical microscopy after the sample has been removed from the test chamber

(fig. 1). The damage observed at each site is summarized by a set of code numbers, each number representing a specific effect. The data is reduced by entering the damage summary and corresponding peak

energy fluence for each site into the calculator and applying the algorithm derived below.

3.1 Threshold Algorithm

The routine determination of up to eight thresholds per sample tested requires an algorithm permitting rapid data reduction. On the other hand, effective use of available data is important, since the

fluence range of thresholds is often wide, and therefore incompatible with a high density of data points.

In this section we develop an algorithm which yields reproducible results with a minimum of computational

complexity.

Consider the probability of obtaining a certain type of damage X-j^ times out of Nj[^ shots on fresh

sites (1-on-l damage) [3,4], all at the same fluence 4 tion

This is represented by the binomial distribuP^(X^;N^;p.) (1 (1)

where

^i " Pi^^i'*'^) (2)

the probability of single-shot damage is a function of the damage threshold ((> and its standard deviation

a, in addition to the fluence (jij^. The problem is to obtain statistical estimators for <|) and a in terms

of the damage data X^, N-^ and iji^. Our solution is based on the method of maximum-likelihood [5]. Maximizing the logarithm of the likelihood with respect to the parameters e = (fi or o, leads to two expressions of the form

3P.

y — N,P,

P^d - P,),

(3)

Solving simultaneously for ifi and a yields the corresponding maximum likelihood estimators $ and a. However, the functional form of p^ is required. For computational simplicity we assume that is uniformly

distributed, i.e..

Pi =

where

"i + 2

"1-2

"i ^ 2

(4)

_ "^i "i = "

2/3

(5)

Substituting eq. (4) in eq. (3), one finds that only those terms j for which |u. | < 1/2 are nonzero. To further simplify the computation we neglect u^^^ terms in the denominators, which are of the

form 1/4 - ni^. As a result, our estimators differ from the true maximum-likelihood estimators in that

data points near u-^ = 1/2 are less heavily weighted relative to those near u^ = 0. This compensates to some degree for the fact that the distribution given by eq. (4) generally represents the sample distribution less accurately near u^ = 1/2. Introducing eq. (5) and solving for <() yields

-508-
Solving for a gives — ; I(X. -^.)^0 , (7a)

2/3 y(x. - ^.) J ^

r i but gives the indeterminate form 0/0 when /,(Xj - -^N.)

of (fi^ under the latter condition, we infer that

It follows from the definition of j that eqs. (6) and

2/3 a

= 0. Noting that eq. (6) defines the sample mean

; l(X. - = 0 . (7b)

(7) apply only to data within the interval

< 4 • (8)

While the estimators (f and a are expressed entirely in terms of the damage data, the data interval given

by eq. (8) involves the unknowns (t> and a, and must be determined by trial and error. A satisfactory

method for this has been developed, and is included in the routine data reduction procedure described

below.

3.2 Data Reduction

Equations (6) through (8) are applied to each damage effect in turn via the calculator as follows.

Fluences associated with the code number corresponding to a particular effect are sorted into a damaging, and the remaining fluences into a nondamaging category. Damaging fluences are then arranged and

listed in order of ascending value, while the nondamaging fluences are listed in descending order. Data near the head of each list is examined for consistency by the operator and reclassified or discarded as required. Equations (6) and (7) are then applied to the revised lists. The j summations are initially

taken over all data in the smallest interval which (a) includes the lowest damaging and highest nondamaging fluence, and (b) includes a damaging and nondamaging fluence as upper and lower limits, respectively.

The interval is then extended as required to provide the minimum value of a, determined by applying eq.

(7) to each new interval. This minimum a and the $ obtained by applying eq. (6) to the same

interval are taken as the final results for the effect in question. In practice, it is found that $ is

rather insensitive to the data interval. However, a depends more strongly on the number of data points

used and their separation. Given adequate data the major contribution to a usually comes from the variation of the damage threshold across the sample surface, with a relatively small contribution coming from

the experimental uncertainty.

Repeated application of the algorithm gives an average threshold, together with its standard deviation, for each effect. The resulting thresholds for the target are presented in the form of a bar graph

or damage profile, examples of which are presented below.

3.3 Reproducibility and Long-Term Calibration

The reproducibility of the threshold results can be demonstrated by repeated application to a sample with uniform damage characteristics over its surface, as indicated by relatively small standard

deviations of the damage thresholds. A mirror produced by electrodeposition of Au on diamond-turned

bulk Cu has this desirable property. The oxidation resistance is an added advantage, which makes this

sample especially attractive as a standard for calibration maintenance. The ability to directly

standardize damage thresholds is an invaluable asset in the long-term development of damage-resistant

optical components.

Figure 3 presents the results of two independent multithreshold measurements on the standard gold

sample. Figure 3(a) was obtained before, and figure 3(b) after replacing the output coupler on the

laser over one month later. After correction for a 10% change in focal spot diameter, the thresholds

for all effects listed except one agree within the combined standard deviations given. Statistically,

approximately half the thresholds should agree this well when the effects are independent. It is clear

that changing the output coupler had negligible effect on the calibration. Furthermore, we may conclude

that the threshold algorithm gives consistent and reproducible results, the accuracy of which is

conservatively represented by the standard deviations.

-509-
3 . 4 Examples

Multithreshold analysis has been applied to a variety of infrared mirror and window surfaces prepared by various techniques. Both coated and uncoated samples have been studied. The present section

gives a few representative examples of results obtained.

The application of multithreshold analysis to relate damage mechanisms to surface characteristics

of bare metal mirrors, particularly Cu, has been amply demonstrated [1]. Certain aspects of the Au mirror represented in figure 3 were also discussed. To complete this discussion it should be pointed

out that the relatively high slip threshold indicated in figure 3 may be influenced by the difficulty in

observing slip on this sample [1]. In fact, the lowest threshold is for work-function change, which is

attributed to evolution of water by the Au layer. This result is supported by a laser-induced diminution of oxygen as observed by Auger.

The value of multithreshold analysis in evaluating nominally equivalent mirror finishes is illustrated in figure 4, where we compare damage profiles of Mo mirrors polished by two different vendors.

While mirror A exhibits higher thresholds for most types of damage, including melting, it is far more

susceptible to pitting and associated plasma formation. The superior uniformity of mirror B is also

evident in the smaller standard deviations.

As an example of multithreshold results from a window material we compare profiles for damage

occurring on the entrance (front) and exit (back) surfaces of a polished NaCl blank (fig. 5). Only

those sites where no simultaneous damage was observed on the front surface are represented in figure 5(b).

All thresholds on the front are essentially the same, indicating breakdown as the only damage

mechanism. Thresholds at the back surface are generally lower because of the constructive phase relationship between incident and reflected fields. Effects peculiar to the back surface are ripples, pits

and fracture. Ripples result from the interference of incident and scattered radiation [6]. Present

evidence shows that ripples occur only in the presence of a plasma. The pits may represent scattering

sources for the ripples. The fact that pits occur only on the back surface suggest that they result

from bulk, rather than from surface inhomogeneities . Scrubbing associated with the plasma, which was observed in mirrors [1], has also been observed on this sample.

A damage profile from an enhanced-reflection coating on polished bulk Cu is shown in figure 6. The

thresholds are well below those for bare Cu. Pitting due to coating defects is the dominant failure mode

Almost identical damage profiles were obtained when the same coating design was applied to Mo and to ULE glass

.

Figure 7 compares damage thresholds from AS2S3 in bulk form, and as a coating [7] on the NaCl. In

the bulk material the coincidence of thresholds for surface pitting and bulk damage suggest that these

result from the same mechanism which, surprisingly, is active below the threshold for pitting in the

coating. Also, pitting in the coating tends to associate with a plasma, which is not true in the bulk

material

.

4 . Summary

An efficient and reliable method has been developed for simultaneously measuring the thresholds and standard deviations for up to eight laser-damage-related effects on a single sample. An algorithm based on the maximum-likelihood principle has been derived for computing results directly from

the experimental data. Reproducibility has been demonstrated by repeated measurements on a standard Au

sample. The use of this sample to maintain long-term calibration was described. Examples of multithreshold results on representative optical components of each type have been given and the principal

weaknesses of these components identified. The comparative analysis of such results from well characterized samples prepared from different materials and by different techniques yields the relationship

between damage thresholds and material characteristics. These comparisons can also provide a better

understanding of laser-damage mechanisms.

5. Acknowledgments

The authors are indebted to S. D. Sorenson for programming the plot routine for the damage profiles.

6. References

[1] Forteus, J. 0., Fountain, C. W. , Jernlgan,

J. L., Faith, W. N. and Bennett, H. E., this

conference

.

[2] Forteus, J. 0., Soileau, M. J. and Bennett,

H. E., in Laser Induced Damage in Optical

Materials: 1975, A. J. Glass and A. H. Guenther, eds. (NBS Spec. Publ. 435), p. 207.

[3] Bass, M. and Leung, K. M. , IEEE J. Quantum

Electron. QE-12 , 32 (1976).

[4] For n-on-1 damage the situation is obviously

equivalent if nondamaging pulses leave an equivalent damage probability at each site.

[5] Mood, A. M. , Introduction to the Theory of

Statistics (McGraw-Hill, New York, 1950), Ch. 8.

-510-
[6] Temple, P. A. and Soileau, M. J., in Laser

Induced Damage in Optical Materials: 1976,

A. J. Glass and A. H. Guenther, eds. (NBS

Spec. Publ. 462), p. 371.

[7] Donovan, T. M. Porteus, J. 0.

Baer, A. D., Dancy, J. H. this conference.

and

Figures

CRATER. SLIP. PITS

BULK Cu. OPTICAL POLISH

NOMARSKI DARK FIELD

200 iim

Figure 1. Micrographs of visible damage on polished bulk Cu at a peak

fluence of 68.3 J/cm^. In Nomarski slip bands appear outside

the crater at the 5 o'clock position, while intergranular slip

is responsible for prominent crack-like features.

T r

DISTANCE FROM BEAM CENTER

Figure 2. Gaussian spatial profile of focussed laser pulse, showing 1/e

width and peak energy density (fluence) at which thresholds

are determined.

-511-
GOLD STANDARD. CALIBRATION A

SLIP

MELTING

PITS

CRATERS

ION EMISS.

LIGHT EMISS.

WRK. FN. CHG.

10 20 30 40 PEAK THRESHOLD (J/CM^)

GO

Figure 3(a). Damage profile of standard gold sample, measured for calibration

with the original laser output coupler. Thresholds are indicated

by the shaded bars. Standard deviations (+ or -) are indicated

by the semishaded boxes at the ends of the bars.

GOLD STANDARD. CALIBRATION B

10 20 30 40 PEAK THRESHOLD (J/CM^)

Figure 3(b). Damage profile to be compared with figure 3(a), measured for

calibration with new laser output coupler.

-512-
BULK Mo, OPTICAL POLISH A

SLIP BANDS

INTERGRAN. SLIP

MELTING

PITS

CRATERS

ION EMISS.

LIGHT EMISS.

WRK. FN. CHG.

5 10 IB 20 PEAK THRESHOLD (J/CM^)

Figure 4(a). Damage profile of polished bulk Mo from vendor A.

BULK Mo. OPTICAL POLISH B

SLIP BANDS

INTERGRAN. SLIP

MELTING

PITS

CRATERS

ION EMISS.

LIGHT EMISS.

WRK. FN. CHO.

1

3

B 10 16 20 PEAK THRESHOLD M/CM*|

Figure 4(b). Damage profile of polished bulk Mo from vendor B.

-513-
SINGLE-CRYSTAL N«a, HARSHAW, FRONT SURFACE

CRAZING

MELTING

ION EMISSL

LIGHT EMISS.

SCRUBBING

10 20 30 40 PEAK THRESHOLD (J/CM^)

60

Figure 5(a). Front surface damage profile of a NaCl blank, 6.4-mm thick, polished

on both surfaces.

RIPPLES

CRAZING

MELTING

PITS

FRACTURE

LIGHT EMISS.

SINGLE-CRYSTAL NlO. HARSHAW. BACK SURFACE ONLY

10 20 30 PEAK THRESHOLD M/CM^)

GO

Figure 5(b). Profile of damage occurring only on

the back surface of the same blank.

The laser beam was focussed on the

front surface.

-514-
BULK Cu. CMTICAL POLISH. E-R COATED

Figure 6. Damage profile of a 10.6 \im enhanced reflection coating on bulk

Cu. Coating consists of alternating layers of Ge and ZnS on a thin Ag layer.

AijS, COATING ON NM, FRONT SURFACE BULK f^Sf, FRONT SURFACE

PEAK THRESHOLD (J/CM^)

Figure 7(a). Damage profile of an AS2S3 coating Figure 7(b). Damage profile of polished bulk AS2S3.

on the front surface of NaCl.

-515-
COmENTS ON PAPER BV PORTEUS, JERMIGAM, AMP FAITH

diicuM-ion on tkli papeA.

-516-
APPENDIX I

PARTICIPANTS

Joseph A. Abate

Laboratory For Laser Energetics

University of Rochester

River Station

Rochester, N. Y. 14627

George J. Balaschak

Pratt & Whitney Group

Bee Line Highway

West Palm Beach, PL 33410

Norman Barnes

Los Alamos Scientific Labs.

Thomas H. Allen M/S 564, P. 0. Box 1663

Optical Coating Labs. Los Alamos, NM 87545

P. 0. Box 1599

Santa Rosa, CA 95401

C. M. Allred

Division 276.00

National Bureau of Standards

Boulder, CO 80302

Robert J. Andrews

University of Dayton Res. Inst. AFWL/LRE (UDRI)

Kirtland AFB, NM 87117

Joseph H. Apfel

Optical Coating Laboratory, Inc.

P. 0. Box 1599

Santa Rosa, CA 95403

L. J. Aplet

Hughes Aircraft Co. Centinela & Teale Sts. Culver City, CA 90230

Adrian Donald Baer

Naval Weapons Center

Code 3818, Research Dept.

China Lake, CA 93555

D. N. T. Beauchamp

Optical Coating Laboratory Inc. 2789 Giffen Ave. Santa Rosa, CA 95401

Harold E. Bennett

Michelson Laboratory, Code 38101

Naval Weapons Center

China Lake, CA 93555

Jean M. Bennett

Michelson Laboratory, Code 3818

Naval Weapons Center

China Lake, CA 93555

Russell B. Bennett

Battelle-Columbus Labs.

Materials Technology Dept.

505 King Ave. Columbus, OH 43201

Enrique Bernal G. Honeywell Corporation

Material Sciences Center

10701 Lyndale Ave. So. Bloomington, MN 55420

John F. Blais Jr. Spectrum Systems Division of Valtec

211 Second Avenue

Waltham, MA 02154

-517-
W. T. Boord

Honeywell Systems & Research Center

MN 17-2340

2600 Ridgway Parkway

Minneapolis, MN 55413

David Burdick

Naval Weapons Center

Code 3813

China Lake, CA 93555

E. C. Borsare,

W. J. Schafer Assoc.

10 Lakeside Office Park

Wakefield, MA 02090

Charles A. Burke

Spectrum System Div.

211 Second Ave. Waltham, MA 02154

Valtec Corp.

Capt. Dan L. Boulet

Air Force Materials Lab.

USAF, AFML/LPO

Wright-Patterson AFB, OH 45433

Pat Burke

University of Dayton Res. Inst. 300 College Park

Dayton, OH 45469

Gordon Boultbee

OCLI

P. 0. Box 1599

Santa Rosa, CA 95401

Chalot Serge, S.C. Laboratories de Marcoussis

Route de Nozay

Marcoussis, FRANCE

C.R. C.G.E.

Morris Braunstein

Hughes Research Labs.

Dept 30-23

3011 S. Mallbu Canyon Road

Malibu, CA 90265

George E. Chamberlain

Division 276.04

National Bureau of Standards

Boulder, Colorado 80302

R. E. Brocklehurst

AF Materials Lab.

AFML/LP

Dayton, OH 45433

John W. Childress

Northrup/N.T.C.

MS 300-62

3401 W. Broadway

Hawthorne, CA 90250

David C. Brown

Laboratory for Laser Energetics

Hmiversity of Rochester

River Road

Rochester, NY 14627

Aaron Budgor

Lawrence Livermore Labs

.

P. 0. Box 808-L488

Livermore, CA 94550

Terence M. Christmas

OCLI Optical Coatings LTD. Hillend Fife Scotland

Dunfermline SCOTLAND

Patsy Collins

University of Dayton Res. Inst. 300 College Park

Dayton, OH 45469

Verne R. Costich

Design Optics

155 Moffett Park Drive

Sunnyvale, CA 94086

-518-
Tony Damonmiio

Physics Dept.

University of Dallas

Braniff Rm. A2, Northgate Blvd.

IRving, TX 75060

David Eimerl

Lawrence Livermore Lab.

P. 0. Box 808

Livermore, CA 94550

Donald Decker

Naval Weapons Center

Code 3818

China Lake, CA 93555

Edward A. Enemark

Optical Coating Laboratory, Inc. 2789 Giffen Ave. Santa Rosa, CA 95A01

Louise Decker

Lockheed Research Labs.

Dept. 52-54, Bldg. 202

3251 Hanover St. Palo Alto, CA 94304

John A. Detrio

University of Dayton Res. Inst. KL 541

300 College Park

Dayton, OH 45469

Marilyn J. Dodge

Division 232.03

A251, Physics Bldg.

National Bureau of Standards

Washington, D. C. 20234

Richard R. Esposito

International Laser Sys.

Optical Coating Lab. Inc. Box 9316

Albuquerque, NM 87119

Albert Feldman

A259, Materials Bldg.

National Bureau of Standards

Washington, D. C. 20234

Nics Ferneliua

University of Dayton Res. Inst. 300 College Park

Dayton, OH 45469

John Fielman

University of Dayton Res. 300 College Park

Dayton, OH 45469

Inst.

T . M . Donovan

Naval Weapons Center

Code 3818

China Lake, CA 93555 Dennis Fischer

Coherent, Inc., Optics Dept.

374 Fay

Mountainview, CA 94043

C. J. Duthler

Xonics, Inc. 6849 Hayrenhurst Ave. Van Nuys, CA 91406

David F. Edwards

Los Alamos Scientific Lab.

MS-564

Los Alamos, NM 87545

Douglas B. Flint

ITEK Corp. - Optical System Div.

Dept. 535

10 Maguire Rd.

Lexington, MA 02173

Alan M. Frank

University of California

Lawrence Livermore Labs

Optics Group

P.O. Box 808

Livermore, CA 94550

-519-
W. G. D. Frederick

AF Materials Laboratory

AFML/LPO

Wright-Patterson AFB, OH 45433

Charles Greninger

Rocketdyne

521/M S BA 30

6633 Canoga Ave. Canoga Park, CA 91301

W. J. Fredericks

Oregon State University

Chemistry Dept.

Corvallis, OR 97331

Arthur H. Guenther

Air Force Weapons Lab.

CA

Kirtland AFB, NM 87117

Francois Garaude

CEA - CEL _____

BP 2f 94190 Villeneuve

St georges, FRANCE

Center France

Harold S . Gurev

Opt. Sci. Ctr. University of Arizona

Tucson, AZ 85721

R. E. Gaylord

E-0 Program Office

U.S. Naval Material Command

MAT 08TE

Washington, DC 20028

Douglas Hamilton

University of So. California

Dept. of Physics

University Park

Los Angeles, CA 90007

Dennis H. Gill

Los Alamos Scientific Lab. Los Alamos, NM 87545 Norman F. Harmon

The Mitre Corporation

P. 0. Box 208

Bedford, MA 091740

Alexander J. Glass

Univ. of California

Lawrence Livermore Lab.

P. 0. Box 808

Livermore, CA 94905

Nora-Jean Harned

AVCO Everett Research Lab.

2385 Revere Beach Parkway

Everett, MA 02149

Ann Glassman

University of Dayton Res. Inst. Applied Physics Dept.

300 College Park

Dayton, OH 45468

Robert D. Harned

AVCO Everett Research Lab.

2385 Revere Beach Parkway

Everett, MA 02149

Helen Gourley

ILC Technology Corp.

164 Commercial St. Sunnyvale, CA 94070

Robert J. Harrach

Lawrence Livermore Lab.

T-Division

P. 0. Box 808, Mail Code L-71

Livermore, CA 94550

George A. Graves

University of Dayton

1930 Knoll Dr. Bellbook, OH 45305

James A. Harrington

Hughes Research Lab.

Malibu, CA 91361

-520-
James Harris

University of Dayton Res. Inst. 300 College Park

Dayton, OH 45469

Samuel J. Holmes

Northrop Corp.

3401 W. Broadway

Hawthorne, CA 90250

Marvin Hass

Naval Research Lab.

Code 6408

Washington, DC 20375

Capt. Richard A. House II AFWL/LRE

Kirtland AFB, NM 87117

Gerard Hasserjian

Northrop Corp.

3401 W. Broadway

Hawthorne, CA 90274

Richard S. Hughes

US. Naval Weapons Center

Code 3815

China Lake, CA 93555

Kenneth J. Hebert

Design Optics Inc. 155 Moffett Park Drive

Sunnyvale, CA 94086

Florence M. Indorf

Division 276.00

National Bureau of Standards

Boulder, Colorado 80302

Don Helman

University of So. California

Dept. of Physics

University Park

Los Angeles, CA 90007

Robert B. Hemphill

Vought Corp., Advanced Tech. Center

Box 6144

Dallas, TX 75222

Gilbert 0. Hendrickson

Honeywell Inc. Ceramic Center

1885 N. Douglas Drive

Golden Valley, MA 55422

Graeme L. Hennessey

Valtec Corp.

1244 Highland St. Holliston, MA 01746

Stephen D. Jacobs

Laboratory for Laser Energetics

University of Rochester

River Station

Rochester, NY 14627

Robert J. Jelley

E-A Industrial Corp.

4500 North Shallowford Rd., NE

Atlanta, GA 30341

Carl J. Johnson

II-VI Incorporated

207 Garden Lane

Glenshaw, PA 15116

George T. Johston

University of Dayton Res. Inst. AFWL/LRE

Kirtland AFB, NM 87117

R. E. Joiner

University of So. California

University Park, 620 PHE

Los Angeles, CA 90007

Richard A. Hoffman

Westlnghouse Electric Corp.

R&D Center, Beulah Road

Pittsburgh, PA 15235

-521-
Paul Kelly

National Research Center

Physics Division

310 Smyth Road

Ottawa, CANADA

Neil Larsen

Division 276. OA National Bureau of Standards

Boulder, Colorado 80302

John C. Kershenstein

Naval Research Lab.

Code A104

4555 Overlook Ave. SW

Washington, DC 20375

Claude A. Klein

Raytheon Research Division

28 Leym Street

Waltham, MA 02154

Hubert P. Lebodo

Laboratoire Nationiel ou Enoies

K015 Paris, France

Karl G. Leinberger

University of Dayton

KL 163C

300 College Park Ave. Dayton, OH 45469

Philipp H. Klein, Code 5222

Naval Research Lab

.

Washington, D. C. 20375

Kang M. Leung

Honeywell Inc. Corporate Material Sciences Center

10701 Lyndale Ave. S. Blooming ton, MN 55420

Abe Klugman

Northrop Research & Tech. Center

3401 W. Broadway

Hawthorne, CA 90250

Capt. Steven R. Lindle,

USAF/AFWL/LRE

9801 Pitt PL. NE Albuquerque, NM 87111

Lawrence Kopell

Wright-Patterson AFB AFMC/LTE

Dayton, OH 45433

Herbert G. Lipson

RADC(ESD)ESE

Hanscom AFB

Bedford, MA 01731

Paul Kraatz

Northrop Research & Tech. Center (375/61)

3401 W. Broadway

Hawthorne, CA 90250

Jim R. Logsdon

Electronics Metal Finishing Corp.

P. 0. Box 848

Oak Ridge, TN 37830

Melvin R. Kruer

Naval Research Lab.

Code 5554, Opt. Sci. Div.

Washington, DC 20022

Ron. W. MacPherson ~X

Defence Research Establishment Valcartier

Electro-Optics Division

P. 0. Box 880

Courcelette, Quebec, CANADA GOA IRO

G. Edward Kuhl

USAF, Air Force Materials Lab.

AFML/LPO

Wright-Patterson AFB, OH 45433

Robert J. McBroom

UDRI

AFWL/LRE (UDRI)

Kirtland AFB, NM 87117

-522-
Richard Macklin

EM Laboratories Inc. 500 Executive Blvd.

Elmsford, NY 10523 10523

Thomas J. Moravec

Honeywell Corporate Material

Sciences Center

10701 Lyndale Ave. South

Bloomington, MN 55420

William M. Madden

Pratt & Whitney Aircraft Group

Gov. Prod. Div.-Dept 713

Mail Stop E-25, P. 0. Box 2691

West Palm Beach, FL 33402

Prof. Manenkov A. A. Lebedev Physics Institute

Moscow, USSR

Dennis Morelli

Optical Coating Lab. Inc.

P. 0. 1599

Santa Rosa, CA 95403

Mouchart Jacques

Laboratoires de Marcoussis

Route de Nozay

Marcoussis, FRANCE

I

C.R. C.G.El

Sam D. Meyerson (21t)

USAF/AFWL/IRE

Kirtland, AFB

Albuquerque, MM 87117

Donald L. Mullen

University of Dayton Res. AFWL/LRE (UDRI)

Kirtland AFB, NM 87117

Inst,

David Milam

Lawrence Livermore Labs

y Div. , P. 0. Box 808

Livermore, CA 94550

C. T. Myers

Northrop Corp.

Electro Mechanical Div.

Dept. 6620

500 E. Orangethorpe Ave. Anaheim, CA 92801

Perry A. Miles

Raytheon Co. Opt. Systems

Hartwell Road

Bedford, MA 01730

Brian E. Newnam

Los Alamos Scientific Laboratory

MS 552, P. 0. Box 1663 Los Alamos, NM 87545

S. S. Mitra

University of Rhode Island

Dept. of Electrical Engineering

Kingston, RI 02881

Davis B. Nichols

Boeing Aerospace Co. MS 88-46, P. 0. Box 3999

Seattle, WA 98124

W. E. Moddeman

University of Dayton

300 College Park Drive

Dayton, OH 45469

Novaro narc n" CEA Limeil Center

BP 2f 94190 Villeneuve St georges

FRANCE 96790

Michael J. Monsler

Lawrence Livermore Lab

.

L-488, P. 0. Box 88

Livermore, CA 94550

Major David F. O'brien

USAF, Air Force Materials

Laboratory /LP

Wright-Patterson AFB, OH 45433

-523-
Melvin C. Ohmer Perry P. Pirooz

AFML/LPE Owens Illinois

Wright-Patterson AFB, OH 45433 1700 N. Westwood

Toledo, OH 43607

Jerry R. Oldenettel

Maxwell Labs. Inc. B. C. Piatt

9244 Balboa Ave. Optical Technology Section

San Diego, CA 92123 AFWL/LRE (UDRI)

Kirtland AFB, NM 87117

Russell M. Ono

TRW

One Space Park, 41A/CTS

Redondo Beach, CA 90278

James 0. Porteus

Naval Weapons Center

Code 3817

China Lake, CA 93555

Carlo G. Pantano

University of Dayton Res. Inst.

ICL542 300 College Park

Dayton, OH 45469

Lt. Col. Ronald F. Prater

AFWL/LRE

Kirtland AFB, NM 87117

Roger A. Paquin

The Perkin-Elmer Corp.

MS 420, Main Ave. Norwalk, CT 06856

Diamantis Preonas

University of Dayton Res. Inst AFWL/LRE (UDRI)

Kirtland AFB, NM 87117

Robert E. Parks

Optical Sciences Center

University of Arizona

Tucson, AZ 85721

Victor Rehn

Lawrence Berkeley Labs.

Bldg. 70A, Room 1115

Berkeley, CA 94720

Melvin E. Pedinoff

Hughes Research Labs.

30-23-00

3011 Malibu Canyon Rd.

Malibu, CA 90265

Harold C. Reynolds Jr. Pratt & Whitney Aircraft

P. 0. Box 2691

West Palm Beach, FL 33410

Roger D. Petty

University of Dayton Res. 300 College Park Road

Dayton, OH 45469

Inst.

Richard Pierce

Spawr Optical Research

1527 Pomona Road

Corona, CA 91720

Stephen G. Reynolds

University of Dayton Res. Inst AFWL/LRE, Bldg. 401

Kirtland AFB, NM 87111

Mort Robinson

Hughes Research Labs.

Chem. Phys. Dept.

Malibu, CA 90265

-524-
Roger E. Rondeau

WPAFB, AFML/LPJ

Wright Patterson AFB, OH 45433

Dave R. Segawa

Hoya Optics, USA, Inc. 2180 Sand Hill Road

Menlo Park, CA 94025

Terry Rosshart

TRW Space & Defense Systems

Hel. Optics Dept.

One Space Park 01/1271

Redondo Beach, CA 90278

Glenn H. Sherman

II-VI, Inc. 207 Garden Lane

Glenshaw, PA 15116

James M. Rowe

Northrop Research & Tech. Center

375-61

3401 W. Broadway

Hawthorne, CA 90250

Keith R. Shillito

Battelle-Columbus Labs.

Materials Application Section

505 King Ave. Columbus, OH 43201

J. Earl Rudisill

Hughes Research Labs.

3011 Malibu Canyon Road

Malibu, CA 92650

Frederick M. Shofner

Union Carbide/K-25

Box //P Oak Ridge, TN 37830

Aaron Sanders

Division 276.04

National Bureau of Standards

Boulder, Colorado 80302

Philip A. Simpson

Division 276.04

National Bureau of Standards

Boulder, Colorado 80302

Stephen R. Scheele

Hughes Aircraft

Culver City, CA 90230

James G. Sliney, Jr. University of So. California

c/o Physics Dept. SSC 302

University Park

Los Angeles, CA 90007

Ansgar Schmld

Washington State University

Dept. of Physics

Pullman, WA 99164

T. E. Schmid MS BOS

Pratt & Whitney Aircraft

Box 2691

West Palm Beach, FL 33402

R. I. Seddon

Advanced Products Dev. Optical Coating Lab. Inc. 2789 Giffen Ave. Santa Rosa, CA 95404

Durand M. Smith

General Electric, RESD

P. 0. Box 8555, 20A10

Philadelphia, PA 19460

W. Lee Smith

Lawrence Livermore Lab.

Mail Code L479

Box 808

Livermore, CA 94550

John B . Sonderman

Lawrence Livermore Lab.

P. 0. Box 808

Livermore, CA 94550

-525-
Marshall Sparks

Xonics, Inc. 1333 Ocean AVe. Santa Monica, CA 90401

P. A. Temple

Naval Weapons Center

Code 3818

China Lake, CA 93555

J. L. Stanford

Naval Weapons Center

Code 3818

China Lake, CA 93555

E. Theron

CSIR

Optical Sciences Div. NPRL

P. 0. Box 395

Pretoria, Republic of SOUTH AFRICA

Major James L. Stapp

Air Force Weapons Laboratory

AFWL/LRE

Albuquerque, NM 87117

Normal Thomas

Optical Coating Lab, Inc.

P. 0. Box 1599

Santa Rosa, CA 95403

Thomas G. Stoebe

University of Washington

326 Roberts, FB-10

Seattle, WA 98195

Irving F. S towers Lawrence Llvermore Lab

.

L-474, P. 0. Box 808

Livermore, CA 94550

Charles Strecker, AFML/LPO

Air Force Materials Laboratory

Wright-Patterson AFB, OH 45433

James H. Tillotson

Maxwell Labs

.

Research & Development

9244 Balboa AVe. San Diego, CA 92064

Michael J. Treadaway

IRT Corp.

P. 0. Box 80817

San Diego, CA 92138

H. L. Wain

Embassy of Australia

1601 Massachusetts Ave,

Washington, DC 20036

Eric W. Van Stryland

Center for Laser Studies

University of So. California

Los Angeles, CA 90007 Thomas W. Walker

USAF

Wright-Patterson AFB, OH 45433

Paul M. Sutton

Electro-Optics Dept., L270

Ford Aerospace & Communications Corp.

Ford Road, Newport Beach, CA 92660 David A. Walsh

University of Dayton Res. Inst. 300 College Park

Dayton, OH 45469

Wayne A. Taggart

Rocket Dyne

6633 Canoga Ave. Canoga Park, CA 91304 Marvin J. Weber

Lawrence Livermore Laboratory

L-479, P. 0. Box 808

Livermore, CA 94550

-526-
R. A. West

Hughes Aircraft

Centinela & Teale Sts. Culver City, CA 90230

R. M. Wolff

Western Sales

Valtec Corp.

Box 2311

Newport Beach, CA 92663

Richard T. Williams

Naval Research Lab.

Code 6440

Washington, DC 20375

C. B. Willingham

Raytheon Research Div.

28 Seyon Street

Waltham, MA 02154

James M. Wimmer

University of Dayton Res. Inst. 300 College Park

Dayton, OH 45469

Capt. Harry V. Winsor

DARPA/MSO

1400 Wilson Blvd.

Arlington, Washington 22209

G. Richard Wirtenson

University of California

Lawrence Livermore Labs.

Laser fusion?ME Dept. L-474

P. 0. Box 808

Livermore, CA 94550

Shew M. Wong

Rockwell International

Rocky Flats Div. B/779

P. 0. Box 464

Golden, Colorado 80401

Roger M. Wood

G.E.C. LTD., Hirst Research Centre

Eastland, Wembley

Midd , England

Robert W. Zimmerer

Scientech, Inc. 5649 Arapahoe Ave. Boulder, CO 80303

David Zuccaro

Hughes Research Labs.

3011 Malibu Canyon Road

Malibu, CA 90265

-527-

NBS-114A (REV. 7-73)

U.S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET

1. PUBLIC AI ION OR RHPORT NO.

NBS SP-509

4. Tni,t-: AND SUBTITLE

2. Gov't Accession

No.

Laser Induced Damage in Optical Materials: 1977

7. AUJ|10R^S) Edited by Alexander J. Glass and Arthur H. Guenther

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

3. Recipient's Accession No.

5. Publication Date December 1977

6. Performing Organization Code

8. Performing Organ. Report No.

10. Project/Task/Work Unit No. 2760581

11. Contract/Grant No.

12. Sponsoring Organization Name and Complete Address (Street, City, State, ZIP)

Electromagnetics Div. , National Bureau of Standards, Boulder, CO. American Society for Testing & Materials, Phila.,PA. 19103

Dept. of the Navy. Office of Naval Research, Arlington, VA 22217 Energy Research & Development Admin., Washington, DC 20234

Defense Advanced Research Project Agency, Washington, DC 20234

13. Type of Report & Period

Covered

FINAL

14. Sponsoring Agency Code

15. SUPPLEMENTARY NOTES

Library of Congress Card Catalog Number: 78-600021

16. ABSTRACT (A 200-word or less factual summary of most si^ificant information. If document includes a significant

bibliography or literature survey, mention it fiere.) The Ninth Annua1 S ympo s ium on Optical Materials for

High Power Lasers (Boulder Damage Symposium) was held at the National Bureau of Standarc

in Boulder, Colorado, from 4 to 6 October 1977. The Symposium was held under the

auspices of ASTM Committee F-1, Subcommittee on Laser Standards, with the joint sponsorship of NBS, the Defense Advanced Research Project Agency, the Department of Energy

(formerly ERDA) , and the Office of Naval Research. About 185 scientists attended the

Symposium, including representatives of the United Kingdom, France, Canada, Australia,

Union of South Africa, and the Soviet Union. The Sjrmposium was divided into sessions

concerning Laser Windows and Materials, Mirrors and Surfaces, Thin Films, Laser Glass

and Glass Lasers, and Fundamental Mechanisms. As in previous years, the emphasis of th^

papers presented at the Symposium was directed toward new frontiers and new development

Particular emphasis was given to materials for use from 10.6 ym to the UV region. Highlights included surface characterization, thin film-substrate boundaries, and advances

in fundamental laser-matter threshold interactions and mechanisms. The scaling of dama^

thresholds with pulse duration, focal area, and wavelength were also discussed. Alexanl

J. Glass of Lawrence Livermore Laboratory and Arthur H. Guenther of the Air Force Weapon

Laboratory were co-chairpersons of the Symposium. The Tenth Annual Symposium is

scheduled for 12-14 September 1978 at the National Bureau of Standards, Boulder,

Colorado.

7. KEY WORDS (six to twelve entries; alphabetical order; capitalize only the first letter of the first key word unless a proper name; separated by semicolons)

Laser damage; laser interaction; optical components; optical fabrication; optical

materials and properties; thin film coatings.

18. AVAILABILITY Unlimited

I ! For Official Distribution. Do Not Release to NTIS

Py] Order From Sup. of Doc, U.S. Government Printing. Office

Washington, D.C. 20402, SD Cat. No. CI 3 .10; 5U9

I ! Order From National Technical Information Service (NTIS)

Springfield, Virginia 22151

19. SECURITY CLASS

(THIS REPORT)

UNCL ASSIFIED

20. SECURITY CLASS

(THIS PAGE)

UNCLASSIFIED

21. NO. OF PAGES

561

22. Price $6.75

o U. S. GOVERNMENT PRINTING OFFICE : 1978 261-238/62 USCOMM-DC 29042-P7<
NBS TECHNICAL PUBLICATIONS

PERIODICALS

JOURNAL OF RESEARCH—The Journal of Research

of the National Bureau of Standards reports NBS research

and development in those disciplines of the physical and

engineering sciences in which the Bureau is active. These

include physics, chemistry, engineering, mathematics, and

computer sciences. Papers cover a broad range of subjects,

with major emphasis on measurement methodology, and

the basic technology underlying standardization. Also in- cluded from time to time are survey articles on topics closely related to the Bureau's technical and scientific programs. As

a special service to subscribers each issue contains complete

citations to all recent NBS publications in NBS and non- NBS media. Issued six times a year. Annual subscription:

domestic $17.00; foreign $21.25. Single copy, $3.00 domestic;

$3.75 foreign. Note: The Journal was formerly published in two sections: Section A "Physics and Chemistry" and Section B "Mathematical Sciences."

DIMENSIONS/NBS

This monthly magazine is published to inform scientists,

engineers, businessmen, industry, teachers, students, and consumers of the latest advances in science and technology,

with primary emphasis on the work at NBS. The magazine

highlights and reviews such issues as energy research, fire protection, building technology, metric conversion, pollution

abatement, health and safety, and consumer product per- formance. In addition, it reports the results of Bureau pro- grams in measurement standards and techniques, properties

of matter and materials, engineering standards and services,

instrumentation, and automatic data processing.

Annual subscription: Domestic, $12.50; Foreign $15.65.

NONPERIODICALS

Monographs—Major contributions to the technical liter- ature on various subjects related to the Bureau's scientific and technical activities. Handbooks—Recommended codes of engineering and industrial practice (including safety codes) developed in cooperation with interested industries, professional organizations,

and regulatory bodies.

Special Publications—Include proceedings of conferences

sponsored by NBS, NBS annual reports, and other special

publications appropriate to this grouping such as wall charts,

pocket cards, and bibliographies.

Applied Mathematics Series—Mathematical tables, manuals, and studies of special interest to physicists, engineers,

chemists, biologists, mathematicians, computer programmers,

and others engaged in scientific and technical work.

National Standard Reference Data Series—Provides quantitative data on the physical and chemical properties of

materials, compiled from the world's literature and critically

evaluated. Developed under a world-wide program co- ordinated by NBS. Program under authority of National

Standard Data Act (Public Law 90-396).

NOTE: At present the principal publication outlet for these

data is the Journal of Physical and Chemical Reference

Data (JPC'RD) published quarterly for NBS by the American Chemical Society (ACS) and the American Institute of

Physics (AIP). Subscriptions, reprints, and supplements

available from ACS, 1155 Sixteenth St. N.W., Wash., D.C.

20056.

Building Science Series—Disseminates technical information

developed at the Bureau on building materials, components,

systems, and whole structures. The series presents research

results, test methods, and performance criteria related to the

structural and environmental functions and the durability

and safety characteristics of building elements and systems.

Technical Notes—Studies or reports which are complete in themselves but restrictive in their treatment of a subject.

Analogous to monographs but not so comprehensive in scope or definitive in treatment of the subject area. Often

serve as a vehicle for final reports of work performed at NBS under the sponsorship of other government agencies.

Voluntary Product Standards—Developed under procedures

published by the Department of Commerce in Part 10, Title 15, of the Code of Federal Regulations. The purpose

of the standards is to establish nationally recognized requirements for products, and to provide all concerned interests with a basis for common understanding of the characteristics

of the products. NBS administers this program as a supplement to the activities of the private sector standardizing

organizations.

Consumer Information Series—Practical information, based on NBS research and experience, covering areas of interest

to the consumer. Easily understandable language and

illustrations provide useful background knowledge for shopping in today's technological marketplace.

Order above NBS publications from: Superintendent of

Documents, Government Printing Office, Washington, D.C.

20402.

Order following NBS publications—NBSlR's and FIPS from

the National Technical Information Services, Springfield, Va. 22161.

Federal Information Processing Standards Publications

(FIPS PUB)—Publications in this series collectively constitute the Federal Information Processing Standards Register.

Register serves as the official source of information in the

Federal Government regarding standards issued by NBS

pursuant to the Federal Property and Administrative Services Act of 1949 as amended. Public Law 89-306 (79 Stat. 1127), and as implemented by Executive Order 11717

(38 FR 12315, dated May 11, 1973) and Part 6 of Title 15

CFR (Code of Federal Regulations).

NBS Interagency Reports (NBSIR)—A special series of

interim or final reports on work performed by NBS for outside sponsors (both government and non-government).

In general, initial distribution is handled by the sponsor;

public distribution is by the National Technical Information

Services (Springfield, Va. 22161) in paper copy or microfiche

form.

BIBLIOGRAPHIC SUBSCRIPTION SERVICES

The following current-awareness and literature-survey bibli- ographies are issued periodically by the Bureau:

Cryogenic Data Center Current Awareness Service. A literaI tare survey issued biweekly. Annual subscription: Domesi' tic, $25.00; Foreign, $30.00.

I Liquified Natural Gas. A literature survey issued quarterly.

Annual subscription: $20.00.

I

Superconducting Devices and Materials. A literature survey

issued quarterly. Annual subscription: $30.00. Send subscription orders and remittances for the preceding bibliographic

services to National Bureau of Standards, Cryogenic Data

Center (275.02) Boulder, Colorado 80302.
U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

Washington. D.C. 20234

OFFICIAL BUSINESS

Penalty for Private Use. S300

POSTAGE AND FEES PAID

U.S. DEPARTMENT OF COMMERCE

COM-2 1 5 U.&MAII

SPECIAL FOURTH-CLASS RATE

BOOK

1298



I