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Symposluin on Optical/Laser Induced damag

QC100 .U57 N0.462, 1976 C.2 NBS-PUB-C 19

I/) J

NBS SPECIAL PUBLICATION 462

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

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OCT 1 1981

Laser Induced Damag

in Optical Materials:

1976

STP 622
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W 2 6 1977 Laser Induced Damage

In Optical Materials: 1976

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

July 13-15, 1976

NBS Boulder, Colorado 80302

Edited by

Alexander J. Glass

Lawrence Livermore Laboratory

I Livermore, California 94550

!!

I and

Arthur H. Guenther

Air Force Weapons Laboratory

Kirtland AFB, New Mexico 87117

U.S. DEPARTMENT OF COMMERCE, Elliot L. Richardson, Secretary

Edward 0. Vetter, Under Secretary

Dr. Betsy Ancker-Johnson, Assistant Secretary for Science and Technology ^ C NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Acting Director

Issued December 1976
Library of Congress Catalog Card Number: 76-600074

National Bureau of Standards Special Publication 462

Nat. Bur. Stand. (U.S.), Spec Publ.462,410 pages (Dec. 1976) CODEN: XNBSAV

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Foreword

The Proceedings contain the papers presented at the 8th Annual Symposium on Optical Materials for High Power Lasers held at the National Bureau of Standards in Boulder, Colorado, on July 13-15, 1976.

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 160 scientists from the United States, the United Kingdom, France, Canada, and Brazil. It was divided into

half-day sessions devoted to the following topics: bulk material properties and thermal behavior,

mirrors and surfaces, thin film properties, thin film damage, and scaling laws 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 Symposium 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 9th Annual Symposium on this topic will be held in Boulder, Colorado, from October 4 to 6, 1977.

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 in 1977 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.

The purpose of these Symposia 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.

A. H. Guenther

A. J. Glass

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 recommendation or endorsement by the National Bureau of Standards, nor does it imply that the material or equipment 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

Ill
I

I
CONTENTS

Section Page

Foreword

A. H. Guenther and A. J. Glass

Summary and Conclusions viii

A. J. Glass and A. H. Guenther

0.1 Introduction and Welcome 1 A. J. Glass

0.2 SI Conversion Units 3

0. 3 Advanced Optical Materials for Laser Fusion A J. W. Weiss

BULK PROPERTIES and THERMAL BEHAVIOR

1.1 Thermal Distortion Thresholds for Optical Trains Handling High Pulse Powers

H. E. Bennett 11

1.2 Electric Fields and Power Dissipation in Windows at Oblique Incidence 25

J. H. Apfel

1.3 Analysis of Laser-Induced Thermal Stresses in Laser Windows 29

R. E. Rondeau & D. Ford

1. A Interferometrlc Measurements of Laser Heated Windows 36

E. Bernal G. & J. S. Loomis

1.5 New Materials for Chemical Laser Windows 45

J. A. Harrington

1.6 Studies of Absorption in Mid IR Laser Window Materials 50

A. Hordvik, B. Bendow, H. G. Lipson, L. H. Skolnik, & R. N. Brown

1.7 Effect of Temperature and Stress on the Refractive Index of Window Materials 58

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

1.8 Refractive Index and Temperature Coefficient of Refractive Index of Hot-Forged Calcium

Fluoride 64

M. J. Dodge

1.9 Fabrication and Properties of Laser Window Materials 69

R. M. Cannon, H. K. Bowen, A. M. Glaeser, H. J. Mayson, F. A. McClintock, W. M. Sherry,

J. B. VanderSande, & M. F. Yan

1.10 Double-Doped Alkali Halide Laser Windows 82

H. Posen, N. Klausutis, J. Bruce, J. A. Adamski, J. R. Weiner, & S. A. Kulin

1.11 Intermediate Temperature (0.5 to 0.7 T^p °K) Forging of Calcium Fluoride Crystals 87

R. H. Anderson, B. G. Koepke, & E. Bernal G. 1.12 Forging Finished Halide Optical Components 95

R. H. Anderson, E. Bernal G. , & R, J. Stokes

1.13 Vapor Deposition and Mechanical Properties of CdTe 100

H. L. Tuller, K. Uematsu, J. B. VanderSande, E. L. Hall, & H. K. Bowen

lolA Absorbing Tellurium Inclusions in Polycrystalline Cadmium Telluride: Estimates for Extrinsic Damage Thresholds 119

H. S. Bennett & C. D. Cantrell

1.15 Hot-Pressed CdCr2Si, for Faraday Effect Isolation at 10.6 ym: Preliminary Damage Tests. . . 126

S. D. Jacobs & K. J. Teegarden

1.16 Pulsed Laser Induced Damage at the DF Laser Wavelength 136

M. Bass, K. M. Leung, C. Tang, & M. J. Soileau

V
MIRRORS and SURFACES

2.1 Naval Weapons Center Diamond Turned Optics Facility 1A5

D. L. Decker & R. E. Cram

2.2 10.6 ym Pulsed Laser Damage Studies of Diamond Turned KCl Window Surfaces . . . 149

M. J. Soileau, J. M. Bennett, J. 0. Porteus, W. Faith, J. Jernlgan, & T. T. Salto

2.3 Surface, Optical and Laser Damage Characteristics of Diamond-Turned Metal Mirrors .... 158

D. L. Decker, M. J. Soileau, J. 0. Porteus, & J. M. Bennett

2.4 Character of Pulsed Laser Damage to Al at 10.6 ym Inferred from Single-Crystal Targets

In Vacuum 165

J. 0. Porteus, M. J. Soileau, & C. W. Fountain

2.5 Measurements at 10.6 ym of Damage Threshold In Germanium, Copper, Sodium Chloride, and Other Optical Materials at Levels up to 10^° W/cm~

J. J. Hayden & I. Llberman

2.6 Laser-Induced Damage of Mirror and Window Materials at 10.6 ym 181

R. Glbbs & R. M. Wood

2.7 Absorption of Dielectric Enhanced Mirrors at 5.3 ym 189

P. Kraatz, J. R. Buckmelter, & S. J. Holmes

2.8 Optical Properties of Mirrors Prepared by Ultraclean dc Sputter Deposition 195

P. A. Temple, D. K. Burge, & J. M. Bennett

THIN FILM PROPERTIES

3.1 Materials for High-Power Window and Mirror Coatings and Multilayer-Dielectric Reflectors. 203

M. Sparks

3.2 Alternate Materials for Infrared Window Coatings 214

A. D. Baer, T. M. Donovan, A. K. Green, & G. Turner

3.3 Design of Three-Layer Antiref lectance Coatings 221

A. D. Baer

3.4 Characteristics of Plasma Polymerized Ethane for Laser Window Coating 230

T. A. Rels, H. Hlratsuka, A. T. Bell, & M. Shen , 3.5 Epitaxy and Optical Scattering in T£I Films 239

R. A. Skogman

3.6 Thin-Film Coating Evaluation by Attenuated Total Reflection 246

R. T. Holm & E. D. Pallk

THIN FILM DAMAGE

4.1 ZnSe Sputtered Coatings 253

D. A. Walsh & J. A. Detrlo

4.2 Laser Damage Studies of Low Index Coating Materials at 10.6 ym 264

M. J. Soileau, A. D. Baer, J. 0. Porteus, J. L. Jernlgan, W. N. Faith,

T. M. Donovan, & A. K. Green

4.3 The Use of Rare Earth Fluorides as 10.6 ym Antiref lectlve Coatings 271

A. Golubovlc, J. Fitzgerald, I. Barman, J. Bruce, R. Bradbury, J. J. Comer,

W. S. Ewlng, & H. G. Llpson

4.4 Performance of Geg, 45Seo, 55/AS2S3 Antiref lection Coatings at 3.8, 5.3, and 10.6 ym . . . 279

J. F. Lewis & M. C. Ohmer

4.5 AR-Coated KCl Damage at 10.6 ym 283

J. A. Detrlo, R. D. Petty, M. C. Ohmer, & 0. F. Swenson

4.6 Spectral Dependence of Damage Resistance of Refractory Oxide Optical Coatings . . o . . . 292

B. E. Newnam & D. H. Gill

4.7 The Role of Electric Field Strength in Laser Damage of Dielectric Multilayers 301

J. H. Apfel, J. S. Matteucci, B. E. Newnam, & D. H. Gill

4.8 Preparation Techniques and Hydroxyl Concentration vs Surface Damage Threshold 310

R. A. House, J. R. Bettls, 5, A. H. Guenther

4.9 Laser Induced Damage to Thin Films Immersed in Liquids 315

A. Balbln-Vlllaverde & L. G. DeShazer

4.10 Dielectric Coatings on Metal Substrated 331

S. S. Glaros, P. Baker & D. Milam

VI
SCALING LAWS and FUNDAMENTAL MECHANISMS

5.1 Spot Size and Pulse Duration Dependence of Laser-Induced Damage 338

J. R. Bettis, R. A. House II, S, A. H. Guenther

5.2 Re-Examination of Laser Induced Breakdown in the Alkali Halides at 10.6 ym 346

C. C. Tang, K. M. Leung, & M. Bass

5.3 Can a Model Which Describes Gas Breakdown also Describe Laser Damage to the Bulk and Surfaces of Solid Dielectrics 350

D. Milam

5.4 Laser Damage to Semiconductor Materials from 10.6 ym CW CO2 Laser Radiation 357

S. K. Gulati & W. W. Grannemann

5.5 Phototransport Damage of Optical Materials 365

S. M. Wong & D. L. Olson

5.6 Resonant Defect Enhancement of the Laser Electric Field 371

P. A. Temple & M. J. Soileau

APPENDIX I Participants 379

VII
Laser Induced Damage in Optical Materials

8th ASTM Symposium

July 13-15, 1976

The Eighth Annual Symposium on Optical Materials for High Power Lasers, the Boulder Damage Symposium, was held at the National Bureau of Standards in Boulder,

Colorado, from July 13th to 15th. 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 Energy Research and Development

Administration, and the Office of Naval Research.

About 160 scientists attended the Symposium, including representatives of the United Kingdom, France, Canada, and Brazil. The Symposium was divided into five

half-day sessions, concerning Bulk Material Properties and Thermal Behavior, Mirrors

and Surfaces, Thin Film Properties, Thin Film Damage, and Scaling Laws and Fundamental Mechanisms,

As in previous years, the emphasis of the papers presented at the Symposium was directed towards new frontiers and new developments. Particular emphasis was given

to new materials for use at 10.6 pm in mirror substrates, windows, and coatings. New techniques in film deposition, and advances in diamond turning of optics were

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

wavelength were 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 Ninth

Annual Symposium is scheduled for October 4-6, 1977, at the National Bureau of Standards, Boulder, Colorado.

Key words: Avalanche ionization; infrared windows; laser damage; metal mirrors;

optical fabrication; optical materials; thin films.

1. Principal Conclusions

The Eighth Annual Symposium on Optical Materials for High Power Lasers, the Boulder Damage

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

July 13 to noon on July 15, 1976. 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 for the first time an effort was made to coordinate the Damage Symposium with the DARPA Laser Window Conference. The Window Conference was held on Monday, July 12, 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, July 15 and 16. About 160 scientists attended the Symposium,

including representatives of the United Kingdom, France, Canada, and Brazil, The Symposium was divided

into five half-day sessions concerning bulk material properties and thermal behavior, mirrors and surfaces, thin-film properties, thin-film damage, and breakdown theory and experiments. For the first

time in the eight year history of the Symposium two poster sessions were held. The general consensus of both those presenting poster papers and those viewing them was highly favorable. In all, approximately

50 technical presentations were made. Alexander J. Glass of Lawrence Livermore Laboratory and Arthur H. Guenther of the Air Force Weapons Laboratory were co-chairDersons of the Symposium. The Ninth

Symposium is scheduled for October 4 through 6, 1977, at the National Bureau of Standards, Boulder.

As in previous years, one of the major concerns of the Symposium was recent advances in fabrication

and testing of optical elements. Concern was expressed over the capacity of the special optics industry

to supply the large volume of high quality optical elements that would be needed if influences like

various DoD laser programs or the laser fusion program continue to increase at its present rate. Several new fabrication techniques have been proposed in order to remedy this problem. One of these is diamond

turning or micromachining. Diamond turning of metal optics has been reported in previous years at the Damage Symposium and a number of high quality reflecting flats and ellipsoids of large aperture have

been made by this process. This year for the first time reports were presented on the diamond turning

of dielectrics. Of particular interest is the diamond turning of the following materials: NaCl; KCl; CaFz, SrF2, BaF2, ZnS, ZnSe, and MgF2. Among the metals which have been diamond turned are Pb, Ni, Al, Au, Cu, and various Cu alloys. To date, certain materials, most notably glasses and ferrous materials,

have not been successfully diamond turned. New facilities have been established, most notably at the Naval Weapons Center, for investigation of diamond turning technology and the extension of the process to new materials. Large scale, volume production facilities have been set up at the Y-12 plant in Oak Ridge

Tennessee. Another process that has been advanced for volume production of transparent dielectric optics

is hot forging. This technique has recently been applied to divalent fluorides for the fabrication of both lenses and windows.

VIII
There is increased emphasis on the importance of process control in the fabrication of both

windows and thin-film coatings. It is now relatively well known that it is essential to use ultrapure materials, such as those fabricated by chemical vapor deposition, and to insure reduced surface

absorption by paying particular attention to substrate cleaning. The chemistry or chemical activity of

the substrate surface is also of paramount importance in controling surface absorption. If substrates

are subjected to optical polishing, it is, of course, important to use an appropriate cleaning or even

etching technique in order to remove the residual polishing material. In the thin-film optics area the

use of ultra-high vacuum techniques in the film deposition is one way of achieving high purity in the film structure. Because of the frequently reported observations that sputtered films exhibit higher

damage thresholds than do vacuum deposited films, there is continuing interest in both rf and dc sputtering for metals, dielectric films, and even polymerized organics. However, great care must be

exercised in the sputtering process due to the fact that the surface can be contaminated by the sputtering gas. Another area of continuing advance is the introduction of new materials for a variety of purposes

in high power laser optics. The Proceedings of the Damage Symposium continue to record the expansion

of the data base regarding new materials. The development of new materials can have a significant

impact on system performance; thus in the high power glass program of the Energy Research and Development Administration the Introduction of materials of low index nonlinearity and high damage

resistance, both in active and passive elements, can lead to significant improvement in the output and

efficiency of large glass lasers. In considering the advantages resulting from the introduction of a new and improved material in a large laser system, it is essential that a total system evaluation be

carried out, rather than simply an evaluation of an individual element. The presence or absence of a correlation of figure errors and laser-induced distortion between system elements must be considered in

evaluating the benefit wnich accrues from the introduction of a new material.

With these caveats in mind we can review the new materials which are currently under investigation.

Silicon carbide was introduced at this year's symposium as a useful substrate material for 10.6 \im mirrors. SiC can be electroplated with a metal such as copper to yield a highly reflective surface

with a high damage threshold. Thresholds as high as 11 joules per square centimeter have been observed

in 1.4 nanosecond 10.6 ym pulses on copper-plated SiC mirrors. These mirrors exhibited surface rough- nesses as low as 5 A rms. In previous years speakers had expressed concern over the high absorption

values seen in ThFi, coatings used at 10.6 ym. This year several new coating materials were described

for possible use as low index materials at 10.6 ym. These include monovalent and divalent fluorides

which exhibit low refractive indices. NaF is especially attractive as an alternative to ThFi, since it exhibits low absorption, a high damage threshold, and can be used as a single layer anti-reflection

coating on NaCl. Novel designs for anti-reflection coatings, such as a three-layer coating consisting of only low index materials (BaF2, NaF) can lead to a greatly increased damage resistance due to the absence of normally encountered high index coating materials. Thus, not only new materials, but new

designs are leading to improved performance. Additionally, rare earth fluorides, including SmFs, EuFa, and NdF3, can be used in conjunction with higher index substrates in single layer anti-reflection

coatings and in two-layer designs along with a higher index material. The properties of these materials

are just beginning to be investigated. It has been observed that they are particularly sensitive to the presence of water, even at very low pressures in the deposition process. Considerable interest is evident in the use of Til as a high index coating material for use at 10.6 ym. This material exhibits

an index of 2.3 at 10.6 ym, is insoluble in water, and adheres well to KCl. A totally new class of materials was introduced this year for the first time as an optical coating

material. Plasma poljrmerized ethane (PPE) coating has been discussed as a possible protective coating

for alkali halide windows. A substantial amount of research is still required in order to reduce the residual absorption in this material. Another attractive option which has been presented is that of using

mixed films of germanium and selenium in order to create films of continuously variable refractive index.

Finally, the use of hot pressed chalcogenides as Faraday rotator materials at a variety of wavelengths

in the infrared has been suggested. The hot pressing process offers the possibility of developing

large aperture components.

Advances in the laser damage field have generally required corresponding advances in the area of diagnostic instrumentation technology. The use of Auger spectroscopy to probe the concentration of impurities near surfaces is an example of a sophisticated research tool being brought to bear on an optical materials problem with great effect. This year saw an increasing use of Auger techniques in

diagnosing surface contamination, thin film deposition phenomena and the role of impurities. Another

technique advanced for thin-film studies was attenuated total reflection (ATR) spectroscopy. By depositing thin film specimens on ATR witness plates it is possible to probe for the effects of surface

absorption with a very high sensitivity, and to distinguish near surface absorption in the bulk of the film or substrate. Another technique advanced for distinguishing between surface and bulk absorption

was photo-acoustic spectroscopy.

IX
In order to provide the systems designer with useful relationships concerning the utility of specific materials in a variety of laser systems, there has been a tendency to organize data in the form

of empirical scaling relations. This year has seen an advance in the sophistication and in the scope of these scaling laws. It has been pointed out that a significant amount of relevant work exists in the literature, particularly in the literature of the Soviet Union, on such scaling relations. This year at the Damage Symposium scaling laws for both the area dependence and time dependence of laser damage were

put forth. It has been observed that the power density at which damage occurs increases as the reciprocal of the diameter of the focal spot size and as the reciprocal of the square root of the pulse

duration. These scaling laws have been seen to apply over a wide range of pulse durations from 10"'°

to 10~^ seconds and over a range of spot sizes from 9 to 140 pm in the bulk and 180 to lo"* ym on surfaces.

The fundamental mechanisms underlying these simple scaling laws and the reasons for the extensive range

of applicability of these relationships are not immediately apparent. Nevertheless, within this range

of parameters the use of the scaling laws as an empirical guide seems fully justified. Another

parameter which can be varied in order to obtain scaling laws is the wavelength or frequency of the incident light. However, in this case great care must be exercised because of the existence of resonant

phenomena. Thus, it has been observed that in a number of oxides of refractory materials the damage

threshold increases as the operating wavelength goes from 1 pm to 0.5 ym, but that the damage threshold

is further reduced by moving the operating wavelength further into the ultraviolet. The reason for this

is undoubtedly the onset of direct absorption in the material, either by one photon process or possibly by a two-photon process. There are other cautions that must be observed regarding the application of scaling laws. One of these relates to the Achilles Heel nature of damage phenomena.

Damage at a surface or in a thin film structure, or even in bulk material, typically occurs at a weak

point in the material. Scaling laws, of necessity, typically relate to average properties of the material.

It is not always expected that failure at a weak point will follow a simple scaling relation, and therefore, it is only expected that empirical scaling laws will be obtained for relatively pure homogeneous

and well characterized materials. Additionally, other factors such as standing electric waves from

constructive and destructive interference must be taken into account. This is most evident, of course,

in multi-layer dielectric films, and experiments were described in this year's symposium which bear out both the Achilles Heel nature of laser damage and the importance of calculating the detailed local

electric field in more fully understanding damage phenomena.

Finally, advances continue to be made in the fundamental understanding of damage processes. The role

of impurities is continually being elucidated. An abundance of data has been generated regarding such

properties as bulk absorption in transparent dielectrics. Good care must be excerised in the intercomparison of these data due to the mediating effect of impurities in determining bulk absorption. The damage threshold of hard drawn copper mirrors, for example, was found to be much lower than the damage

threshold of OHFC copper mirrors. The cause of this discrepancy was identified as the presence of oxide

impurities at grain boundaries in the hard drawn material. Impurities often play an important role in improving the quality of laser materials. The introduction of divalent impurities in alkali halides,

for example, increases their fracture strength and also stabilizes micro-crystals preventing the migration of grain boundaries. So, for example, the use of strontium chloride, SrCl2, in stabilizing

KCl. The presence of impurities, either those introduced on purpose or those that come into the material

accidentally, greatly complicates the problem of materials characterization.

The avalanche ionization process continues to be investigated. It has been observed that ionization

damage in the subnanosecond regime appears to be deterministic while in the nanosecond time range the process appears to be statistical in nature. As in gas breakdown at high pressures, avalanche

ionization appears statistical when it occurs at a lower field strength. In short pulse experiments in solids or low pressure experiments in gases, breakdown occurs at high field values where the generation

of the initial electrons is highly deterministic. T-rtiereas, at low pressures in the gas or at long pulses

in solids where low field values are involved the statistical nature of the initial electron generation

is more in evidence. Other points of fundamental interest that come to the fore in this year's discussion

include the identification of an interference of the electric wave with near-field scattering from dielectric or metallic inclusions giving rise to a ripple-like damage pattern oriented along the electric vector of the incident light wave. This effect has been noticed by several observers in the past, but it was only this year that a coherent explanation of the observation has been provided. Finally, a note of caution has been introduced in regard to the role that defects play in determining the damage process.

No matter what the fundamental nature of the damage phenomenon is, whether it is avalanche ionization,

thermal, multi-photon absorption, or field intensification, all of these processes are enhanced in the vicinity of a defect or discontinuity in the material. Therefore, the fact that damage occurs in the vicinity of a discontinuity or defect in no way indicates that mechanism is responsible for the damage.

Uncovering of the intrinsic damage process must thus be accomplished in the absence of extrinsic

influences

.

X
2.0 Summary of Papers

The 1976 Symposium was divided into five sessions, each occupying roughly half a day. The formal

presentations were supplemented by poster sessions, at which several papers, either of special interest

or consisting primarily of data tabulation, were presented. The general topical areas were (1) Bulk

Material Properties and Thermal Behavior, (2) Mirrors and Surfaces, (3) Thin Film Properties, (4) Thin

Film Damage, and (5) Scaling Relations and Fundamental Damage Mechanisms. In the following summary a concise statement of the principal results of each paper is presented. Readers are thereby directed to the topics of their particular interest, and at the same time, presented with an overall view of the principal concensus of the Symposium.

2.1 Bulk Properties and Thermal Behavior

In high average power systems, thermal effects are the dominant damage considerations. At average

power densities of tens of kW/cm^, absorptance values of 10""* cm"' can easily lead to an unacceptable de- terioration in system performance. Accordingly, one of the topics of principal interest in this session

was weak absorption and its consequences on overall system performance. Thermal effects can also be

important for high peak power systems, leading to transient optical distortion or even thermal fracture.

In the first paper of the Symposium H. E. Bennett of the Michelson Laboratories at the Naval Weapons

Center presented a detailed multi-component system oriented discussion centered about thermal distortion

thresholds for optical trains handling high pulse powers. A diffraction approach based on the Marechal

criterion for the multi-element system was employed. This analysis included the role of figure errors, thermal distortion in windows and mirrors, energy transmission limits for optical trains and a comparison

of the influence of window and mirror material properties. In this presentation emphasis was placed not

on the catastrophic damage limit, but rather on the allowable degradation in beam focusability and the impact of this latter limit on optical tolerances based upon fabrication specification and material

properties. Detailed consideration of the utility of adaptive optics to correct or ease these constraints

was not discussed, but rather the development of frequency dependent figures of merits which would ease

the correction requirements, by allowing for optimum material selection and component specification.

He concluded that for most multi-component optical trains, thermal distortion, initial figure error

and optical homogeneity must be considered with initial figure errors frequently a fraction of a visible

wavelength even at infrared wavelengths. Thus for materials of low index (alkali halides and alkaline

earth fluorides) , tolerances can be set at much more relaxed levels than for high index materials such

as ZnSe, However, correlation of errors must be considered in assigning an error budget to each element

in a multi-element system. For example, initial figure errors and homogeneity error are usually un- correlated with each other or with thermal distortion, while figure errors resulting from thermal distortion are correlated. The sign of the variation must also be considered. From the materials selection standpoint, calculations suggest that selected window materials can handle much higher single pulse energy densities than mirrors, while for high average power cw or repetitively pulsed systems metal mirrors are easier to cool. For the materials considered, coated

molybdenum is preferred for mirrors used in microsecond or larger pulsed systems, while uncoated mirrors

are best for shorter pulse lengths where catastrophic damage evolved. Here copper is preferred over molybdenum or aluminum.

J. H. Apfel of the Optical Coating Laboratory presented formulas for calculating the polarization

dependent electric field strength and power dissipation due to weak absorption in transparent optical

elements at non-normal incidence. Although not of an exact description, power dissipation per unit

value can be assumed to vary as the cosine of the incident angle.

Of obvious interest to individuals concerned with the system design and performance of high power cw

gas laser systems is the induced thermal stress in laser window elements. This subject was the topic

of a paper by R. E, Rondeau of the Air Force Materials Laboratory and Dale Ford of the University of Dayton Research Institute. Their paper discussed in detail several coupled two-dimensional computer

programs which model in-depth optical absorption, resulting heat transfer, and induced stress in disc

geometries. The effect of window thickness, incident power level, absorptance and duration of exposure

of a ZnSe output coupler employed in a 15 kW CO2 laser was studied. Stress resulting from radial and

axial temperature distortions was calculated and compared to experimental results. Although not conclusive, this comparison indicated the possible degradation of ThFi, coatings by hydroscopic action which

led to increased surface absorption resulting in intolerable thermally induced tangential tensile

stresses occurlng near the edge of the beam. This possibly was suggested by the abnormally high value

measured for the coating absorption.

In their continuing exploration of improved techniques for interf erometric analysis of laser heated

windows E, Bernal of the Honeywell Corporate Research Center and J. S. Loomis of the University of Arizona reported on several new developments. In particular, they investigated the contributions of anisotropic aberrations, due to a polarization dependence, to resultant window response. Results indicated this dependence to be quite small for many normally employed materials. In order to make the re- quisite measurements with accuracy and facility, elegant techniques involving the generation of synthetic interf erograms and semi-automatic digital analysis were reported.

XI
Their results concluded that signified anisotropic effects are present in BaF2, SrF2, and CaF2

at 10.6 ym while alkali halides (of considerably less absorption at 10.6 m) exhibited essentially in- significant anistropic effects. In fact, these materials are 50 times less sensitive than the alkaline earth fluroides at 10.6 ym. As laser technology develops and new laser wavelengths become the center of interest, a search for transparent materials exhibiting desirable characteristics and dai?.age resistance evolves. Such was the subject of a report by J. A. Harrington of the University of Alabama in which he assessed the potential

of several alternate optical materials for use with HF (2.8 ym) and DF (3.8 ym) chemical lasers. The

main thrust of his investigation concentrated on an evaluation of the optical absorption at the chemical

laser wavelengths of LiYFi,, Yttralox [Y2O3 (90%) :Th02 (10%) ] , Zns, CdTe, KBr, SrF2, and KRS-5 window

materials. Unfortunately, the optical absorption of all the materials studied was in a limited range. However, Yttralox and LiFi, should be investigated for achieving lower absorption through improved

materials processing because of their otherwise appealing physical properties (strength and environmental resistance). As an ancillary to this study, water repellent pol5rmeric coatings were evaluated

as a protection for hydroscopic materials such as KCl. Initial tests relating to film absorption are sufficiently encouraging to warrant further investigation using higher quality KCl substrates.

The group at Hanscora Air Force Base has been a vital contributor to the development and assessment

of low loss infrared optical materials, particularly windows and coatings. In a continuation of their

work A. Hordvik, B. Bendow, H. G. Lipson, L. H. Skolnik, and R. N. Brown have recently completed a rather

thorough and comprehensive investigation of bulk and surface optical absorption in several mid IR laser window materials at CO and HF laser wavelengths. The materials investigated included ZnSe, AI2O3, MgO,

LiF, MgF2, CaF2, SrF2, and BaF2. Experimental techniques included emittance and Fourier transform

spectroscopy as well as laser calorimetry and a new photo-acoustic technique which allowed simultaneous

determinations of both bulk and surface absorption to be made. The latter technique, with an estimated

sensitivity of 10~^cm~^determines the resultant expansion of a material after short duration exposure to an appropriate wavelength optical pulse. Any material absorption leads to the generation of an acoustic

or elastic wave, which is measured and related to the absorption coefficient. Tabulations of both the bulk and surface values as a function of wavelength were presented. Interestingly at 2.7 ym surface

absorption may become dominant over bulk losses. Sapphire emerged as an excellent candidate at the HF laser wavelength providing its large surface absorption can be substantially reduced. Included in the presentation was an indication of success in calculating the intrinsic absorption level in ZnSe from

first principles.

In their continuing program on establishing quantitative values for optical and mechanical

properties of important optical materials, the group at the National Bureau of Standards consisting of

A. Feldman, D. Horowitz, and R. M. Waxier reported on the temperature and stress dependence of the re- fractive index of CaF2, BaF2, CVD ZnSe, and fused silica at 632.8 nm, 1.15 ym, 3.39 ym, and 10.6 ym, typically over the range -180° to +200°C. This information is coupled with thermal expansion data to allow a quantitative evaluation of the response of these materials in window form to high laser intensities. Thermal expansion is normally measured by the method of Fizeau interferometry while

piezo-optical constants are made using a modified Dyson polarizing interferometer, exhibiting a precision of X/500 at 632.8 nm. Expansion parameters from -100°C to +120°C are as follows: for

CVD ZnSe At/t = (7.29 ± 0.02) x 10-^ T + (5.26 ± 0.17) x 10-^ T^ - (1.45 ± 0.07) x 10-^ ^T^ for BaFz the coefficients are + (IS.l ± 0.1) x 10"^ T + (11.5 ± 0.3) x 10"^ T^ - (2.0 ± 0.3) x 10-^ ^T^ and for CaFz + (18.4 ± 0.1) X 10-^T + (16.4 ± 0.3) x 10"' T^ - (3.7 + 0.3) x 10"" T^ At 632.8 nm the dn/dl of CVD

ZnSe ranges from 9.3 x lO-^T at -100°C to 11.2 x lO-^T at 120''C. The piezo-optic coefficients are in essential agreement with earlier investigations.

In a closely allied paper M. J. Dodge also of the National Bureau of Standards investigated the re- fractive index and the temperature coefficient of refractive index of hot-forged CaF2. Using a minimum

deviation method and precision spectrometer, she reported the refractive index from 0.25 ym to 8.0 ym at 21°C and 34°C. Data was fitted to a three term Sellmeier-type dispersion formulation

- 1 =

I [AjXV(X^ - \'.)]

which permits interpolation within a few parts in 10~^. The index of refraction is n, X is the wavelength

of interest, -^j's are the calculated wavelengths of maximum absorption, and the A

j

' s are the calculated

oscillator strengths. At 20.8°C the constants are Aj = 0.98584551, A2 = 0.05290246, A3 = 4.2816899,

Xi = 0.07218116, X2 = 0.14114719, and X3 = 36.465937, while An/AT(°C)"^ in the stated temperature range varies between -2.2 x 10"^ and -4.5 x 10~^ over the wavelength range studies. These values for the hot

forged polycrystalline CaF2 are between one-third and equal to the corresponding values for single

crystal CaF2. New materials and fabrication processes are continually evolving as laser technology progresses. Recent interest in the hot forging process as a technique for fabricating laser windows with high

strength and low absorption was evidenced by four papers relating to this process.

XII
Some critical questions relating to the use of hot forged monovalent and divalent halides as laser window materials were addressed in a paper by R. M. Cannon, H. K. Bowen, A. M. Glaeser,

H. J. Mayson, F. A. McClintock, W. M. Sherry, J. B. VanderSande, and H. F. Yan, all of the Massachusetts

Institute of Technology. They concerned themselves with two aspects of hot forged crystalline materials,

micro-structure formation during the forging process, and microcrack coalescence, leading to mechanical

failure. They observed that dynamic recrystallization is significant in CaF2 at temperatures as low as half the melting temperature, while in monovalent halides, it generally is limited to temperatures

greater than 80% of the melting temperature. An analytical model has been developed for grain boundary

mobility in alkali halides, which relates the strengthening effect of divalent impurities to the in- creased electrostatic interaction between the impurity and the grain boundary. Techniques for microcrack detection using both acoustic emission and light scattering were discussed.

Due to' the untimely illness of Harold Posen of Hanscom Air Force Base, the paper by him, N. Klausutis,

J. Bruce, J. A. Adamski, and J. R. Weiner on double doped halide laser window materials was not formally

presented at the Symposium; however, it is included in the Proceedings proper. In this interesting

discourse, the grain boundary stability of alkali halide materials alloyed with 1.75% RbCl was investigated. RbCl is usually added to RAP (reactive atmosphere processing) grown KCl to produce a considerably

strengthened polycrystalline window material by hot forging with little degradation in other important

optical properties. However, instabilities in the micro-structure of these KCl-RbCl alloys is usually

observed, leading to secondary recrystallization and grain growth. Thus these materials tend to revert

to a single crystal state causing a considerable reduction in material strength. To prevent this un- desirable state of affairs, the addition of 10 ppm to 250 ppm SrCl2 as a stabilizing agent was studied.

Results of this investigation led to the following conclusions: polycrystalline KCl-RbCl alloys

stabilized with low concentrations of SrCl2, yield forged billets which (1) have small grain size

(< 10 ym) , (2) have high strength when complete recrystallization is achieved (5840 psi), (3) have low

10.6 ym optical absorption (2-6xl0~'*cm~^) ,and (4) show greater grain boundary stability with increasing

ppm SrCl2 .Billets containing 50 ppm SrCl2 or greater show no secondary recrystallization or grain growth

after a 350°C anneal.

In an additional contribution from the Honeywell group, R. H. Anderson, B. G. Koepke, and

E. Bernal G. discussed a related subject, namely, the Intermediate temperature (0.5 to 0.7 Tm)

Isostatic forging of calcium fluoride crystals. Their results conclude that isostatic forging is a viable process for producing crack-free fine grained (< 10 ym) material from CaF2 single crystals with

significantly improved fracture energies, providing forging is done at temperatures greater than 500°C. However, from birefringence measurements there resides a considerable internal stress. This problem will

require further study. If a positive result can be attained without increased grain size, an additional

Increase in the fracture energy is expected. Observations of the present developmental forgings indicate

a certain amount of "veiling" (internal cloudiness). Higher temperature forging should greatly reduce

this annoying feature.

In a poster paper R. H. Anderson, E. Bernal G. , and R. J. Stokes of the Honeywell Corporate

Research Center discussed an evaluation they performed on the forging of completely finished halide

optical components. This investigation was initiated to address the possibility of eliminating or reducing optical shaping and finishing operations. In this work employing KCl billets (3.81 cm in

diameter x 3.81 cm in height), forging is done isotatically using a closed X/10 quartz die technique to produce the desired surface shape and finish at 275°, Normally two separate forging operations are necessary, one to produce uniform strain distribution and then, after a small amount of preshaping by water polishing, the major shaping operation is accomplished by a second forging, A major conclusion

of this work was that the forged surfaces appeared superior to mechanically prepared surfaces as far as coatabillty is concerned, particularly for protective and thin films of thallium iodide. Work on this

Interestlr,^ approach to volume production of high quality optical elements is proceeding.

Two papers were presented concerning the properties of CdTe, a promising material for Infrared

windows. Other papers dealt with new materials in the Infrared for use as Isolators and windows.

H. L. Tuller, K. Uematsu, J. B. VanderSande, E. L. Hall, and H. K. Bowen of the Massachusetts

Institute of Technology presented results from a detailed materials study of the mechanical properties of vapor deposited CdTe. The Intent of this study was to evaluate the feasibility of growing large, high

quality plates of CdTe for laser window applications. Heretofore, previous attempts have results in material whose absorption coefficients were greatly in excess of theoretical expectations ('^ 10~'*cm~'). It was pointed out that this difficulty can arise because (CdTe) exists over a range of compositions

around the stoichiometric composition, and thus the concentration of defects will be very sensitive to the original growth conditions, e.g., substrate temperature and Cd/Te ratio, and to the cooling path

followed after growth. For this reason vapor deposition from elemental Cd and Te sources was chosen

as the best approach.

Several Important results were obtained. Mechanical tests indicate that CdTe undergos a considerable

amount of plastic strain at 300°C before failure, thus forging is Immediately suggested for preparing

large blanks. Additional effort on the development of large sized, high optical quality, strong CdTe

elements are contemplated.

XIII
From the previous paper it Is obvious that when substances are proposed as new candidate optical

j materials, growth and fabrication procedures together with resultant optical and mechanical properties I must be assessed and correlated to fully appreciate the details of failure modes. Frequently the specifics of the failure mechanism and an appropriate line of analysis have previously been detailed for another related case. Cubic CdTe, as studied by H. S. Bennett of the National Bureau of Standards and

C. D. Cantrell of the Los Alamos Scientific Laboratory, is a promising infrared material which frequently

fails extrinsically due to the deleterious heating of absorb in& tellurium-rich inclusions. Resulting SI thermal expansion leads to the production of unacceptable stresses within the host materials. This is ^l' not at all unlike the care of absorbing Pt or Sb inclusions in laser glass as studied by Hopper and i

j Uhlman, as well as Feldman and detailed in the Proceedings of the 1970 Symposium on Damage in Laser

Materials. In that case '^j 1 ym spheres were most susceptible to damage in 40 ns pulsed 1.06 ym

jj irradiations. Here for Te conclusions, computations suggest that heating of 0.1 ym to 10 ym Te ! aggregates when subjected to power densities of 100 MW/cm^ and pulse widths of 200 ns could produce j[ stresses comparable to the fracture strength of the CdTe host. ' Because of the great advantages of using a saturated ferromagnetic material in a Faraday rotator, ! the material CdCr2Si,, a ferromagnetic spinel, has attracted attention as an Isolator material at 10.6 ym. :

i

As reported by S. D. Jacobs and K. J. Teegarden of the University of Rochester, large aperture components ill' can be made by hot-pressing polycrystalllne CdCr2Si, material. Unfortunately, the observed absorption in ( the spinel is high, typically 1 cm"' in the most favorable spectral range. However, the authors feel i that this absorption is principally due to impurities, and that it can be reduced by purifying the starting material. Due to the high Verdet constant of the material, only thin samples need be used.

Thus, in a saturating field of 3.5 KOe, a thickness of 0.375 cm will yield 45° rotation, providing from i 20 to 30 dB isolation with a 30% insertion loss. Preliminary damage tests on the material indicate that ( under 50 ns, multimode irradiation at 10.6 ym, the damage threshold is about 6.5 J/cm^, about half that

of Germanium under similar test conditions.

There is a paucity of Information regarding laser damage in the midrange of the Infrared from 3 to

6 ym. M. Bass, K. Leung, and C. Tang of the University of Southern California, along with M. J. Soileau

of the Naval Weapons Center, have made a first attempt at remedying this deficiency. They investigated

laser damage in a variety of midrange window materials, namely, divalent and monovalent fluorides and sapphire, using a multiline, transverse-excited DF laser. The output pulse shape of the laser was not

simple, but an equivalent pulse width was about 75 ns. The samples irradiated were commercially prepared, and specified as having "high power laser" finishes

but showed surface damage thresholds typically a factor of ten below the corresponding bulk values.

Surprisingly, although AI2O3 exhibited a bulk absorption of 4 x 10"^ cm"', 100 times that of the fluorides, its bulk damage threshold was 1.6 times that of NaCl, and within 20% of the highest values

obtained for any material. The authors feel that the observed bulk damage was impurity-mediated. The

high value obtained for sapphire is undoubtedly correlated with its very high melting temperature, which

is 1.5 to 2.0 times that of the fluorides. This is especially valid if the bulk absorption In AI2O3 is actually of the order of 10""* cm"' rather than 10"^ cm"' as is reported elsewhere in these Proceedings

by investigators at Hanscom Field. The discrepancy in reported absorption values may be due to high 1 surface absorption for sapphire. j 2.2 Mirrors and Surfaces j As in last year's Symposium, there was considerable interest evidenced in diamond turning, both of

j metals and new dielectrics. An extensive catalogue of materials successfully finished by this process ! is presented below. The high optical quality and damage resistance shown by micro-machined mirrors has , generated strong interest in metal mirrors for laser use, as well as in diamond-turned dielectric

windows

.

D. L. Decker and R. E. Cram of the Michelson Laboratories reported on a highly precise diamond

| turning facility for the machining of optics now under development at the Michelson Laboratories. A brief,

review of the advantages of diamond turning for optics was given, with special emphasis on the require- ' ments for a compatibility between tool and work piece for successful machining. The current capabilities

of diamond turning were summarized as follows: mechanically polished metal surfaces are accurately fab- ricated to a random surface error of 25 to 40 A rms. Small diamond-turned pieces (< 15 cm diameter) are ' fabricated to a surface finish of < 40 A rms for flat pieces, 50 to 100 A rms for figured pieces. Large

pieces display surface finish of the order of 180 A rms, typically. Diamond turned optics shows lower

surface absorption, and higher laser damage values than conventionally polished optics. Among the materials successfully diamond-turned to date are metals; including Cu, Cu-alloys, brass, Au, Ag, Al,

Al-alloys, Pb and electrodless nickel; insulators, such as NaCl, KCl, CaF2, SrF2, BaF2, ZnS, ZnSe, MgFa,

and acrylic plastics.

The proposed machine at NWC incorporates an air bearing spindle, and air bearing slide ways in both

X and Y directions,, mounted on an air suspension Isolated, block granite layout plate. A slide position

interferometer is mounted at tool height, thereby reducing the offset errors. Work pieces up to 35 cm

in diameter and 100 pounds weight can be accommodated. The machine is viewed as a research facility to explore both the machine technology itself and the capabilities of diamond turned optics. The facility

will be operational by February 1977.

XIV
A detailed investigation of the surface properties of diamond-turned KCl windows was reported by

M. J. Soileau, J. M. Bennett, J. 0. Porteus, W. Faith, and J. Jernigan of the Michelson Laboratories,

and T. T. Saito of the Lawrence Livermore Laboratories. They examined specimens of polycrystalline KCl,

doped with EuCl and RbCl, hot-forged into laser window blanks. The windows were diamond turned at the Oak Ridge Y-12 facility, using an off-center fly cutter without lubricant, to generate a smooth surface

free of contamination. The off-center cutting produces grooves which are relatively straight and

parallel. The samples were then sent to the Naval Weapons Center, Michelson Laboratories for characterization and laser damage tests. Among the tests carried out were laser absorption calorimetry, total integrated scattering, surface

prof ilometry, using a Tal^step instrument and CO2 laser damage threshold measurement. Surface roughness

was measured to be 135 A rms, b^ Talystep profilometry , 180-200 A rms by use of a polarization interference microscope, and over 300 A as measured by total integrated scattering at 10.6 ym. Similar discrepancies are seen between TIS and Talystep measurements of mechanically polished surfaces, and may be

due to the contribution of bulk scattering to the TIS value.

Surface absorption was estimated to be about 3.7 x 10""* per surface, obtained by measuring total

absorption before and after etching. This measurement was made on the KCl: EuCl sample; no satisfactory

etching could be obtained with the KCl: RbCl sample. The high value of surface absorption may have been

due to atmospheric contamination or poor handling after machining, rather than to the machining process

itself. Laser damage thresholds were observed to be as high or higher than on conventionally polished

and etched surfaces. A further investigation into the properties of diamond-turned metal mirror surfaces was reported by

D. L. Decker, M. J. Soileau, J. 0. Porteus, and J. M. Bennett of the Michelson Laboratories. The samples

investigated included diamond turned copper surfaces from the Lawrence Livermore Laboratory, and from a commercial source (Heliotrope) as well as electroplated silver and gold mirrors from LLL. Surface

roughness, laser damage, and surface composition were studied. The surfaces were all very smooth, with

rms roughness is from 30 to AO A. Profilometry revealed a regular topographical structure corresponding

to the tool advance. Light scattering measurements did not correlate with the measured roughness, and

measured scattering in the visible was much lower than the measured surface roughness would imply. No explanation for this discrepancy was advanced. Laser damage tests implied that damage was due to absorbing centers congregated at grain boundaries, or in isolated particulates. Laser- induced delamination

was observed in electroplated samples.

Damage at optical surfaces remains a significant problem in high power laser systems. Due to the prevalence of defects and impurities of exposed surfaces, from both environmental contamination and the polishing process, surfaces fail more readily than does the bulk material. A detailed study of laser-induced thermal damage on single-crystal, Al targets, irradiated in vacuum

with 100 ns pulses of 10.6 ym radiation, was carried out by J. 0. Porteus, M. J. Soileau, and

C. W. Fountain of the Michelson Laboratories. The objective of their study was to differentiate between

stresses generated by thermal gradients in the material and compressions resulting from the vaporization

of target material. Both ion and neutral particle emission from the targets were monitored. The authors

concluded that evaporative compression is responsible for stress generation at illumination levels above

the threshold for local melting, while below that threshold, surface roughening can be generated by

thermally induced stresses. Thresholds for damage by melting were about 14 J/cm^ on these single-crystal

targets, which is almost twice the threshold observed on polished 2024 polycrystalline targets. This

apparent inconsistency is probably due to a variation (reduction) in surface absorption.

Damage at 10.6 ym resulting from plasma formation was studied by J. J. Hayden of the Los Alamos

Scientific Laboratory and I. Liberman of the Westinghouse Research Labs. Using a mode- locked TEA laser

emitting pulses of 1.4 ns FWHM, they measured the threshold for plasma formation on OFHC copper mirrors,

both micro-machined and conventionally polished, on NaCl windows, with and without single layer CaF2 AR

coatings, and on a variety of other window and mirror samples. Their principal results are summarized

as follows.

Micro-machined Cu mirrors showed slightly higher damage thresholds than conventionally polished

samples, when tested both in air and in vacuum. At both 90° and 45° angles of incidence, damage thresholds varied from 7 to 10 J/cm^ in the indicated pulse duration. The morphology of the damaged surface

was different for the two methods of preparation; conventionally polished surfaces showed damage in the

form of small pits, while micro-machined surfaces showed a more uniform surface melting.

AR-coated, NaCl windows showed damage thresholds almost twice as high as uncoated surfaces probably

due to the significantly lower index of refraction of the coating material. The threshold at the coated

surfaces varied from 7 to 10 J/cm^, typically. These were very good, single crystal specimens of NaCl.

Among the other materials tested, copper-electroplated SiC mirrors were of great interest. Repeated

exposures at 11.4 J/cm^ failed to produce damage on these surfaces.

XV
Another damage study at 10.6 vm was reported by R. Gibbs and R. M. Wood of the General Electric

Company, Ltd. Laboratory in Wembley, England. Operating at a pulse duration of 60 ns FWHM, they observed

thermal damage in metal mirrors, ir window materials, and dielectric coatings. They also observed com- parable thresholds in conventionally polished and diamond- turned Cu. They reported, however, that harddrawn Cu showed a much lower threshold than OFHC Cu due to oxide formation at the grain boundaries. In metal mirrors, damage thresholds were shown to correlate with surface absorption, and with surface

scratches from the polishing process. On coated elements, the presence of absorbing defects was shown

to reduce the damage threshold significantly. Plated gold surfaces showed significantly higher thresholds than evaporated gold surfaces, as well as higher reflectivity and greater durability. A method for the measurement of the absorptivity of dielectric-enhanced mirrors was presented by

P. Kraatz, J. R. Buckmelter, and S. J. Holmes of the Northrop Corporation. They investigated a number

of dielectric coatings on Mo, Cu, CerVit, and CaF2 substrates, using 5.3 ym radiation from a cw CO laser. Measurements were made calorimetrically from measurements of the energy reflected by and transmitted

through the sample, and a direct measurement of the sample's temperature rise. The calorimetrically

determined absorption was compared to ref lectometer values obtained at the Naval Weapons Center. Agreement in three samples was within 4 x 10""*, while in a fourth sample the discrepancy was 2 x 10~'. The reasons for this discrepancy were unclear. Absorptivities on the order of 1 to 5 x 10~' were

measured for ThFii/ZnS multilayer coatings on bare Mo, Ag-plated Mo, Cr-plated Mo, Ag-plated CerVit and

CaFa.Absorptivity values of 1 x 10"^ or lower were observed in multilayer ThFi,/ZnSe coatings on all of

the above substrates. Coatings of SiO/Si and SiO/Ge exhibited absorptivities of 1 - 9 x 10~^ on most

substrates. Al203/Si multilayers exhibited much higher absorptivities. The best overall combination

appeared to be ThF/ZnSe, with 3 or 4 pairs of layers. Several vendors provided reflectors with

absorptivities less than 1 x 10-^ and reflectivities of 0.999 on numerous substrate materials.

The effect of sputtering gas entrapment on the reflectance of metallic films was studied by

P. A. Temple, D. K. Burge, and J, M. Bennett of the Michelson Laboratories. Cu, Au, Ag, and Al films were deposited via dc sputtering, using Ar and Xe sputtering gases. Films were prepared using various

combinations of gas type, gas pressure, and sputtering rate. The best films prepared compared favorably

with those prepared by UHV evaporative deposition. In general, in accord with the model advanced by Lee

and Oblas, higher gas pressure led to lower gas entrapment, and consequently higher reflectance films.

However, effects due to the reactivity of the metal, as in the case of Al, and the presence of residual

impurities, especially oxygen, can greatly outweigh the direct deleterious effect of sputtering

gas entrapment.

2.3 Thin Film Properties

Thin film multilayers represent the weakest link in any high power laser systems. TJhatever weaknesses surfaces are heir to are manifested many times over in multilayer dielectric coatings. Such

coatings, however, are essential ingredients of the laser system. A continuing effort to find new

materials, characterization tools, methods, and designs for thin film optics is underway, especially in

the infrared, where thin-film optics is less advanced than in the visible.

Multilayer dielectric coatings for polarizers, reflectors, beam splitters, and AR coatings, remain

a critical area at 10.6 ym. Currently, there are significant problems encountered in obtaining satisfactory coatings for high-power, infrared laser use. These problems, their elucidation, and potential

resolution in the next few years, were the subject of a report by M. Sparks of Xonics, Inc. Sparks

identified a set of guidelines for the selection of materials for thin film use at 10.6 ym, including;

bulk absorption less than 0.5 cm-^, appropriate refractive indices, non-hygroscopic character, appropriate mechanical characteristics, and good properties in the thin film state, such as high packing

density, good adhesion, stable stoichiometry, and chemical inertness. Materials satisfying most, if not

all of these criteria, include ThF^ , NaF, NaCl, KCl, BaFa, SrFa, and KGaFi, for low-index layers, and AS2S3, AsaSea, ZnS, ZnSe, and Til for high index layers. Specifically excluded were materials containing elements lighter than flourine since they generally exhibit a fundamental resonance near 10.6 ym. This exludes oxides, carbon compounds, and some fluroides (LiF, CaFa, MgFa, AIF3, and cryolite) which

absorb at 10.6 ym. Polymer films, discussed elsewhere in this Symposium (see paper by Reis, Hlrtsuka,

Bell, and Shen) have a potential as protective coatings for hygroscopic materials, assuming the problems

of pinholes and H2O vapor transmission can be solved.

XVI
One of the major problems of thin films today is the presence of H2O in the film. Water is absorbed on the surface of the film, and also forms as liquid H2O in the pores of the film. The latter

has a doubly deleterious effect in high index materials, since the index of H2O is low, and the absorption coefficient for liquid H2O is uniformly high at all A > 4.5 ym. Film porosity is a problem

of major importance in improving thin film performance at all wavelengths, not only at 10.6 ym. Other

impurities can be equally deleterious. Sparks suggests a near-term process development program, using

only ultra-pure materials, UHV vapor deposition (although sputtering, for example, is still interesting

as an alternative process), careful characterization of the films, calorimetric measurements of film

absorption (spectroscopic measurements are not sufficiently sensitive) and standardized use of attenuated

total reflection spectroscopy (ATR) to probe the location of absorbing layers in multilayer films. Laser damage measurements could complete an appropriate characterization program.

Sparks report represents an effort to delineate a near-term development program, carefully bounded

for maximum effect, to achieve a particular goal, better coatings at 10.6 ym. As will be seen elsewhere

in this Symposium, the ideas he presents represent the consensus of informal opinions among researchers

in the field. Sparks has drawn these ideas together, along with their supporting arguments, in a concise

summary. Realizing the need for better anti-reflection coating materials in the infrared, A. D. Baer,

T. M. Donovan, A. K. Green, and G. Turner of the Michelson Laboratory studied several dielectric materials

for this application. Specifically addressed were films of NaF, SrF2, and As2Se3. Correlation of optical

and other physical properties, such as absorption at 10.6 ym, scattering, structure, index of refraction,

and laser induced damage thresholds, with processing variables are reported. General conclusions were that NaF is a very promising low index (n = 1.23) material since it has relatively low absorption and high

damage threshold which is quite suitable as a single layer anti-reflection on NaCl at 10.6 ym. Furthermore, its solubility is only 0.12 of NaCl, which , however, would still be a concern under high humidity.

SrF2 films exhibit higher absorptions, but compared to the previously proposed material BaF2, it is more

damage resistant. Finally As2Se3 appears to be worse than AS2S3 for IR coating applications. Compared

to the solubility of NaCl,SrF2 is only 3 x IQ-"*, while As2Se3 is insoluble. Measured absorotion coefficients of X/4 films of NaF, ThFi,, SrF2, and BaF2 on KCl were 6.0, 10-12, 25 and 15 cm"', respectively,

while corresponding films of AS2S3 on KCl and As2Se3 on NaCl were 1,2 and 1.1, respectively. Reported

damage thresholds for 100 ns pulses of a CO2 TEA laser indicated levels of 38 ± J/cm^ for the NaF films

and 194 and 76 J/cm^ for the SrF2, and BaF2 films, respectively, while for the above described films of AS2S3 and As2Se3 the values were 38 and 12 J/cm^, respectively.

The problem of designing 3-layer antiref lectance coatings was analyzed by Ao D. Baer of the Michelson Laboratories. He wrote down the conditions which must be satisfied by the thicknesses and indices of the materials used in the various layers of the coating in order for maximum transmission to occur. The effect of losses was also examined. Solutions were presented for both combinations of high

index- low index- high index materials, such as As2Se3/NaF/As2Se3 on CaF2, which is currently used at 10.6 ym, and for low index - medium index - low index combinations , such as NaF/ThFi, /NaF on KCl, which

represents a novel class of coatings. The latter type of coating has no two-layer solution, while two- layer versions of high-low-high coatings (where one layer goes to zero thickness) do exist. The low index coatings are attractive due to the possibility of higher damage thresholds.

In the search for high performance, high quality damage resistant thin films in the infrared, an intriguing approach is being pursued by T. A. Reis, H. Hirtsuka, A. T. Bell, and U. Shen of the Department of Chemical Engineering at the University of California at Berkeley. They reported on an investigation of plasma polymerized ethane (PPE) as a possible organic coating material. The films are prepared by passing ethane gas through a radio frequency glow discharge. Infrared absorption properties

havebeen correlated with deposition conditions, and as expected, 3.5 ym and 7.0 ym are regions of high

absorption due to C-H stretching and bending motions. Outside of these regions film absorption is en- couragingly low, exhibiting about 4-7 cm"'' at 10.6 ym when PPE is deposited on KCl substrates. PPE

has an index of refraction of 1.51 at the sodium D line. Films of ^ 1 ym thickness have not shown any marked deterioration when exposed to a relative humidity of 55% - 60% for 100 hours. Deterioration,

however, was noted over the same duration at 84% RH, when moisture attack was confined to regions of scratches or other imperfections on poor quality KCl substrates. Microscopic results indicate that PPE

can coat surfaces of high quality and of gentle variations while sharp edges and small holes cannot be

suitably coated. In all cases, high humidity deterioration was noted on elements containing these later

type of characteristics. Polymerized coating is a promising avenue of approach which demands additional

and increased attention.

XVII
I

I " I It has long been noted that a need exists for a durable high index coating material at 10.6 ym for use both in multilayer designs or as a protective coating for the alkali halides, such as KCl. I R. A, Skogman of the Honeywell Research Center addressed this quest reporting on a study of epitaxy and | scattering in the Til films. It had been reported that Tll has an absorption coefficient of less than

1 cm~^ at 10.6 ym, and in addition is insoluble, while exhibiting excellent adhesion to KCl surfaces. This tenacious adhesion is primarily due to its epitaxial growth on KCl. He reported that the Tll film structure was strongly dependent upon the specific KCl crystalline orientation. The correlation of film structure on KCl orientation has allowed an understanding of the optical scattering in Tll films.

It was reported that the alignment of the Tll axis parallel to the KCl <110> direction determined the ' epitaxial growth habit. As a consequence, on a <100> crystal surface, two <110> directions lie in the plane and thus Tll nucleates in two orientations 90° from one another. The resulting film structure

j manifests itself as increased light scattering from diffraction of light passing from one biaxially

birefringent orientation to another. Since this epitaxial occurrence is not amenable to solution for i coating polycrystalline materials an alternate solution (opposed to only using that crystal orientation

which did not produce birefringent behavior) had to be found. This was accomplished as follows: Tll

was condensed in its cubic (6) phase on KCl and allowed to transfer to its stable orthorhombic (a) phase

resulting in a clear polycrystalline, randomly oriented film independent of substrate orientation. This

is done by maintaining the substrate at 140°C or above during deposition. The phase change occurs [ during substrate cooling with the resulting a grain size dependent upon the condensation temperature and ; cooling rate. High evaporation rates are necessary ('\' 2000 A/min)

.

It is now abundantly clear that much of the near-term effort in the field of laser induced damage to optical materials will concentrate on the multi-faceted area of thin-film coatings. To aid in unraveling

the details of the response of thin films to high power laser irradiation, improved film characterization

is necessary. One technique which is receiving considerable attention as an evaluation aid is attenuated

total reflection (ATR) spectroscopy. This subject was discussed in detail by R. T, Holm and E. D, Palik

of the rfaval Research Laboratory. Normally, thin films are in part evaluated by either single or double

pass transmission, absorption or reflection spectroscopy. However, with the need for producing very low absorption coatings a much more sensitive technique was demanded. ATR is one such promising alternative,

which is both sensitive and frequency dependent as opposed to the normal non-spectroscopic calorimetric

or non-sensitive simple spectroscopic techniques. Consider the investigation of AR coatings. Typically

a trapezoid is made from the laser window material, then thin films of thickness d, are deposited on both sides. Light is then multiply-reflected down the trapezoid at an appropriate angle 6 which results

in total reflection. If there were no absorption in the film the reflection would be unity. If, however, there is some absorption the reflection is reduced, in other words attenuated. By changing the frequency of light, absorption bands in the film can be identified. This technique has the sensitivity

of determining thin film absorption coefficients of 1 cm-' or less by increasing the number of internal

reflections. Furthermore, by changing the angle of incidence, probing of a thin film in depth (cross

section) is possible, thus thin film homogeneity, etc., can be evaluated. This technique should become

an important adjunct to the science of the film characterization.

2 . 4 Thin Film Damage

As indicated above, the need for new materials for dielectric coatings is most strongly felt at 10.6 ym. A number of alternative materials, both for high and low index coatings, were proposed at this

year's S3rmposium, together with quite encouraging preliminary damage test results. In addition, the well- known dependency of damage threshold on the method and circumstances of deposition was explored in greater detail. Extensive use of Auger spectroscopy continues to be of great value in elucidating the role played by impurities.

D. A. Walsh and J, A. Detrio of the University of Dayton Research Institute (URDI) reported results

of a detailed study of rf-sputtered ZnSe films. Single-layer films of ZnSe were deposited on both ZnSe

and KCl substrates. Two target materials were used for the sputtering, being hot-pressed (HP) Kodak

Irtran IV ZnSe, and chemical vapor deposited (CVD) Raytheon ZnSe. Calorimetric measurements indicated

three to six times greater absorption in films deposited from HP material. Auger analysis revealed a measurable sulfur impurity in the HP ZnSe, which was absent in the CVD material. Another observation

was the presence of impurities in the sputtering gas, in this case, argon. Using as-received argon, deviations from stoichiometric composition were observed, while using gettered argon, stoichiometric

films resulted. The sputtered films showed excellent environmental stability, when deposited on CaFa. Using a water

immersion test, sputtered films were found to be more stable than e-gun deposited films. Films deposited

on a freshly cleaned surface showed the greatest durability.

XVIII
An exploration of the use of NaF, SrF2, and BaF2 monolayer films as opposed to the normally

employed ThFu, as low Index coatings on KCl and NaCl windows was presented by M. J. Soileau, A. 0. Baer,

J. 0. Porteus, J. L. Jernigan, W. N. Faith, T. M. Donovan, and A. K. Green of the Michelson Laboratories.

NaF is of particular interest, since at 10.6 pm, its refractive index is very nearly the square root of that of NaCl, allowing a simple single-layer AR coating to be made.

Films of the materials indicated were vacuum-deposited on NaCl and KCl substrates. Bulk absorption

of the coating materials was measured, as was the absorption of the coating substrate and the final

coated element. Film absorption values were typically 10 to 200 times bulk values. In the bulk, each

of these low index materials showed a higher damage threshold than the substrate materials, as expected.

The coated elements failed under intense CO2 laser illumination due to delamination of the coating in

the vicinity of surface defects at the coating-substrate interface. Ostensibly, these defects could be

prevented by suitable substrate preparation and handling.

As part of a continuing search for materials to use as alternatives to ThFi, in multilayer dielectric

coatings at 10.6 pm, A. Golubovic, J. Fitzgerald, I. Berman, J. Bruce, R. Bradbury, J. J. Comer,

W. S. Ewing, and H. G. Lipson of the Deputy for Electronic Technology of RADC reported on the properties

of several rare earth fluoride films. The films studied were deposited as single-layer coatings on

ZnSe, and in combination with ZnSe layers as two-layer films on KCl. Both vacuum evaporation and rf sputtering were used. Detailed parametric studies of the film deposition process were carried out. Starting materials were carefully analyzed, and resulting film morphology was correlated with deposition

parameters.

Sputtered ZnSe films were deposited in cubic form with a preferred orientation of the {111} planes

parallel to the substrate surface. Thermally deposited ZnSe showed the same preferred orientation. The

quality of the thermally deposited films was strongly dependent on the deposition rate. High deposition

rate films produced coloration, and crazed immediately when removed from the deposition chamber,

while the sputtered films showed good adhesion, and resisted crazing.

The fluoride films were deposited on ZnSe with cubic, orthorhombic, or hexagonal structure, depending

on the coating material. Hexagonal films showed the greatest OH absorption, as evidenced by absorption

bands at 3 and 6 pm. The effect of adsorbed OH was reduced under ultra-high-vacuum (UHV) deposition,

for LaFj, which exhibited pronounced OH absorption. SmF^ and CeT 3 films, which showed only weak OH bands,

did not improve perceptibly under UHV deposition.

These films were damage tested at 10.6 pm, using pulses of 100 ns FWHM from a TEA laser. Absorptances of 3 to 6 percent were observed. Observed damage thresholds were generally high. In combination with ZnSe, on KCl substrates, all the materials showed damage at the same level, about

6-7 J/cm^, probably corresponding to failure of the high index material. The fluoride single layers on KCl damaged at 10 to 15 J/cm^. Sputtered ZnSe on KCl damaged at 20 J/cm^, comparable to values obtained

from thermal deposition.

John Lewis and M. C. Ohmer of the Air Force Materials Laboratory discussed the use of variable index

films of the form GejjSe-]^_^ in combination with AgaSea for AR coatings at 3,6 pm, 5.3 pm, and 10.6 pm. A two-layer film, deposited on substrates of Eu-doped KCl (10.6 pm) , BaF2 (5.3 pm) , and CaF2

(3.8 pm) with an x = 0.55 composition. Absorption, transmission, and laser damage thresholds were

measured. No evidence of H2O or OH absorption bands was observed. The films were deposited by vacuum

evaporation and were observed to be physically robust. At 10.6 pm, the film showed the lowest

absorption the investigators had ever seen on coated KCl substrates, on the order of 0.1% per surface. A possible disadvantage of these films is their poor transmission in the visible. The 10.6 pm coating

had an opaque, metallic appearance and the thinner coatings were observed to be reddish in color. The

3.8 pm coating exhibited an optical density of 1.0 in the visible.

In the continuing search for materials to use as dielectric coatings at 10.6 pm, J. A. Detrio and

R. D. Petty of the University of Dayton Research Institute, and M. C. Ohmer and 0. V. Swenson of the Air

Force Materials Laboratory reported on a study of Til coatings. These coatings, either in monolayers or

in combination with ThFi,, were deposited on KCl windows by vacuum deposition. Samples were irradiated

at increasing power levels with cw CO2 laser radiation at 10.6 pm. The damage threshold, either

identified by disruption of the film or severe stress-induced birefringence, was defined as the lowest

power at which damage was visually observed. Single layer Tll films generally exhibited damage levels

in excess of 100 kW/cm^. Under similar test conditions ThFi^ films damaged at levels of 30 - 50 kW/cm . Auger scans of the ThF^ films showed significant oxygen impurities, whereas no impurities were detected

in the Til films.

XIX
The art of optical coating at near-visible wavelengths is further advanced than in the infrared, but

studies of the damage mechanisms at shorter wavelengths continue to be of great value. In fact, the greater reproducibility of coatings prepared with established methods and materials leads to more meaningful results in damage studies, as well as forming an important data base as laser activity moves to shorter wavelengths. This year studies were reported of the effects of wavelength, standing wave effects

substrate preparation, and liquid immersion on film damage tViresholds. Finally, the totality of design

considerations for a dielectrically enhanced, diamond turned, metal mirror for high power laser use was

presented.

Brian Newnam and Dennis Gill of the Los Alamos Scientific Laboratory presented a study of the spectral dependence of sub-nanosecond damage in refractory oxide optical coatings. Quarter-wave thick

films of TiOa, Zr02, Hf02, and SiOa were deposited on fused silica substrates using electron-gun

evaporation. The first three materials were chosen as representative of high-index materials used in laser coatings, while Si02 is frequently used as a low-index component. These films were tested at 1.064 ym, 0.532 \im, and 0.355 ym, using the fundamental, second and third harmonic of the output of a mode-locked, Nd-YAG laser. The pulse duration was 30 ps at 1.064 ym, 20 ps at 0.532 ym, and 17 ps at 0.355 ym. Forty exposures on a sample were used to determine damage thresholds. Damage was identified

by spark emission detection at the longer wavelengths and by laser-induced scattering. Since all the tested materials fluoresced under 0.355 ym irradiation, spark detection could not be used at this

wavelength. No significant spot-size dependence was observed for focal spots from 70 to 500 ym in radius

at these pulse lengths.

Damage thresholds were corrected for the standing wave pattern in the coated samples. All of the materials showed the same general behavior, an increase in threshold electric field from 1.064 ym to 0.532 ym, and an abrupt reduction of threshold at 0.355 ym. This reduction in threshold was especially

pronounced for Ti02. Absorption spectra were taken on thin film samples of the four oxides tested.

Ti02 showed a strong absorption edge at 0.350 ym, while Hf02 and Zr02 showed an edge between 0.220 and

0.230 ym. Good transmission was seen for Si02 to 0.200 ym. Thus, one-photon absorption is expected to dominate in Ti02 films at 0.355 ym, sharply reducing the damage threshold. Two-photon absorption is identified as leading to reduced damage thresholds in Hf02, Si02, and Zr02 at short wavelengths. The slight increase in threshold from 1.064 ym to 0.532 ym is consistent with the electron avalanche picture

of laser damage. An elegant experimental study on damage in dielectric multilayers was reported by J. H. Apfel and

J. S. Matteucci of OCLI, and B. E. Newnam and D. H. Gill of LASL. Nine-layer stacks of alternating

quarter-wave layers of Ti02 and Si02 were deposited on fused silica substrates, with the high index

(TiOa) layer always at the substrate-film interface. Similar stacks were prepared with the fourth, fifth

and sixth layers respectively eliminated, resulting in half-wave thicknesses of low or high index

material in the middle of the stack. The standing-wave pattern in each design was then analyzed, to relate the peak field inside the stack to the external field associated with the incident light wave. Damage studies were carried out using a 30 ps, 1.064 ym mode-locked Nd:YAG laser. The observed

damage thresholds provide strong confirmation for the hypothesis that multilayer coatings fail in the high index material, when the local field in that material exceeds its damage threshold. Failure in Ti02 occurred at about 8 >fV/cm local field strength. The SiOa coating withstood damage to field

strengths as high as 12 MV/cm, a value comparable to that previously reported for bulk fused silica.

This study strongly supports the guideline that multilayer coatings should be designed to keep the field

strength in the high index material as low as possible as a means of raising the damage threshold.

The effect of substrate contamination on coating performance is generally considered to be

deleterious. The fact that this is not always the case was demonstrated in a report given by

R. A. House, J. A. Bettis, and A. H. Guenther of the Air Force Weapons Laboratory. They studied the effect of the hydroxyl impurity content in fused silica on damage threshold of films of Si02 and MgF2,

deposited on these substrates. The principal effect of the OH lies in the chemistry of the film. For

StOz films, the oxidizing action of the OH greatly reduces the formation of SiO which damages more

easily than Si02. For MgF2, the presence of OH leads to the formation of some MgO, lowering the damage

resistance of the film. In short, the presence of OH is deleterious in these cases where the desired

film composition is in competition with the simple oxide.

In these experiments, OH concentration was monitored by absorption spectroscopy at 1.4 and 2.23 ym. As these absorption bands, associated with the OH radical, increased in strength, the damage threshold

of the Si02 films increased, while that of MgF2 films decreased. The effect of surface roughness on the damage threshold was taken into account by normalizing the damage threshold electric field to E, where a is the rms surface roughness. This allowed a detailed comparison of results from samples of different surface character to be made.

XX
A. Balbin-Villaverde of the Universidade Estadual de Campinas in Brazil and L. G. DeShazer of the University of Southern California reported on laser-induced damage in thin film immersed in liquids of varying dielectric and optical properties. Samples included a variety of quarter- and half-wave monolayers, multilayer reflectors, and AR "vee" coatings. Eight different liquids, with visible light

indices ranging from 1.32 (methanol) to 1.71 (di-iodomethane) were used. Damage testing was carried out with a pulsed ruby laser, operating in a single longitudinal and transverse mode with a pulse duration

of 12 ns FWHM.

In general, one might expect the damage threshold to be affected by the alteration of the internal

field distribution brought about by immersion in the liquid. The authors calculated the exact field

distribution throughout the liquid-film-substrate system, and searched for correlation between observed

thresholds and calculated field strengths. This correlation was not generally found. A correlation was

observed between laser damage threshold and low frequency dielectric constant. The authors propose that

this effect may be due to a change in the photoemission barrier (work function) at the immersed surface,

or due to a filling-in of the cracks, pores, and defects at the surface, reducing the field intensification effect. An extensive tabulation of data is given.

Large aperture, highly damage resistant reflective optics are required for the laser fusion program.

S. S, Glares, P. Baker, and D. Milam of the Lawrence Livermore Laboratory reported on the choice of material and processes for the fabrication of 50 cm aperture reflecting optics for use with the Argus

laser. Three design decisions were considered, the choice of the substrate material, the choice of the surface coating, and the choice of the dielectric overcoating of the optical surfaces. An analysis of the relative merits of BK-7, CerVit, and beryllium led to a choice of beryllium for the mirror substrates.

Beryllium is favored over the glassy materials due to its greater strength, stiffness, thermal

conductivity, and convenience of figuring. The substrates were chemically plated with copper and nickel

layers. The copper plated substrates were diamond turned to the desired shape, and the final nickel

plated surfaces were lapped to the requisite optical figure. Various dielectric multilayer coatings were tested for laser damage and maintenance of optical figure. A 12-layer stack of Ti02/Si02 was the final choice, exhibiting a damage threshold of 6.5 J/cm^ at 1.064 ym, in 125 ps pulses.

2.5 Scaling Laws and Fundamental Mechanisms

In recent years, a number of simplified empirical scaling relations have evolved for the evaluation

of materials for use in high power lasers. The expected dependence of nonlinear index and damage

threshold on the linear index, for example, represent useful rules-of-thumb of this type. Such scaling

laws are of great use to the laser designer in selecting materials for particular applications, and also

represent intriguing simplifications of an otherwise overwhelming abundance of experimental data,

challenging the theorist for a cogent explanation.

In a continuing effort to establish empirical scaling laws for laser damage, J. R. Bettis,

R. A, House, and A. H. Guenther of the Air Force Weapons Laboratory presented a model for the pulse

length dependence and spot size dependence of surface damage thresholds. The model was also extended to bulk damage. The starting point is the assumption that the surface damage threshold corresponds to the Investment of a critical amount of energy in a plasma layer at the surface of the material. Using the expression for the classical inverse bremsstrahlung absorption, they argue that the plasma absorption

increases linearly with increasing laser power density, so the heating rate goes like I^, where I is the incident power density. Thus they arrive at a scaling law (assuming the evolving plasma fills the entire

beam) that at threshold, tpAI^ is constant for a given material. Here tp is the pulse duration and A is the beam area. Considerable evidence is adduced from the literature showing that for a large class of materials, both bulk and surface damage thresholds vary according to the rule, tpl^ is constant. Evidence for the area scaling rule is somewhat less convincing.

As is pointed out elsewhere in these Proceedings (see, for example, the paper by Gibbs and Wood),

any diffusion model, like absorption heating, will yield a relation, t I^ is constant. This scaling

law appears to hold from 30 ps to 100 ns pulse durations, and is a useful rule-of-thumb, as long as new phenomena do not come into play. Thus, scaling from the realm of avalanche ionization to that of thermal

damage may be erroneous. Further explanation and justification of scaling relations remains to be done. It is important to establish the realm of validity of such relations, due to their potential use in scaling up large laser

systems or correlating damage testing results.

XXI
The question of spot size dependence was further investigated by C. C. Tang, K. M. Leung, and M. Bass

of the University of Southern California. They re-examined the bulk breakdown threshold of alkali

halides, both RAP- and conventionally grown, using a CO2 laser emitting 92 ns bursts of mode-locked

pulses of about 2.5 ns duration, spaced 8 ns apart. The pulses were focused to two spot diameters,

59 ym and 89 "Um. Damage thresholds were recorded both for single shots and for "N on 1" experiments.

In the latter, the pulse energy is incrementally raised from far below threshold until damage fully appears. The difference between single-shot and N on 1 thresholds is taken as evidence of a pre- conditioning of the material. It is not clear what the mechanism of pre-conditioning in bulk damage

entails, unless it is the dispersal of electrons from shallow traps. However, in the samples studies,

damage was highly impurity-mediated, as evidenced from the spot-size dependence. Thus, these samples

for which the N on 1 threshold was significantly greater than the single-shot threshold also showed

strong spot-size dependence, and exhibited defects upon subsequent mophological investigation.

An interesting side light of this investigation was the fact that RAP-grown materials used in this

study showed high concentrations of defects and inclusions. Apparently, the phrase "RAP-grown" does not automatically assure high quality in alkali-halide materials.

The fundamental mechanism for intrinsic failure of optical materials at high flux levels is avalanche ionization. This subject has been widely explored in recent years, but discrepancies remain

in the model generally proposed for this process.

The subject of avalanche ionization was further explored by D. Milam of the Lawrence Livermore

Laboratory who proposed to explain essentially all features of existing avalanche ionization experiments

on the basis of a three-part model. In his model, one distinguishes three phases of the avalanche process, the initiation, by photoionization of defects or impurities, or multiphoton ionization, the generation of a dense plasma, by the avalanche process itself, and the damage of the material, by the deposition of heat in the dense plasma. By analogy with gas breakdown at various pressures, Milam

argues that short pulse avalanche ionization (tp < ns) differs from long pulse ionization primarily,

because in order to have enough energy in a short pulse to heat the plasma, the electric field strength must be very high. At high field strength, the initiation process is highly probable, and behaves in a deterministic fashion. For longer pulses, however, where the requisite energy density can be achieved

with a low electric field value, the initiation process becomes statistical in nature, depending on the availability of "seed charge" in the focal volume. Evidence of this is the appearance of explosive

damage on carefully prepared surfaces, such as those prepared with ion-etching, chemical etching or flame-polishing. On such surfaces, the sites for the initiation of the avalanche at low field strength

are widely spaced. Thus, the pulse will rise well above the threshold for the avalanche buildup before

the plasma is initiated, and an explosive avalanche will ensue. Milam offers corresponding explanations

of many other observations relating to both bulk and surface damage. He addresses himself to the questions

of breakdown times (why are there no breakdowns on the tail of the pulse?), damage statistics, and micropit formation. The arguments he presents and the data he introduces are very convincing, although some contradictory evidence does exist. Milam's careful reasoning suggests that overly simplified models of the avalanche process, such as Bettis et al. , had earlier proposed, may only be approximate guidelines

and must be applied with considerable caution.

In an intriguing report which generated considerable discussion, S. K, Gulati and W. W, Grannemann of

the Department of Electrical Engineering and Computer Science at the University of New Mexico presented

results on 10.6 ym cw laser induced damage in Si, Ge, GaAs , GaAsP, and GaP semiconductors. Laser induced

lattice damage was ascertained from the alteration of low temperature carrier mobility measurements.

This modification was analyzed on the basis of a parametric instability in solid-state plasmas previously

formulated by DuBois and Goldman, after discounting possible competing processes such as two stream

instability, avalanche breakdown, and simple Drude absorption or joule heating. While their experimental

damage results are not in exact numerical agreement with analytical predictions based on the proposed

parametric instability, they do approximate the correct order of damage sensitivity. An intriguing hypothesis was put forward for the consideration of the Symposium by S. M. Wong and

D. L. Olson of Rockwell International. They described the phenomenon of electro-transport, well-known

in electrical conductors, in which the transfer of momentum from dc current carriers such as electrons,

to atoms in the conductor can lead to the migration of selected species of atoms or even of vacancies

towards one of the electrodes of the circuit. In situations in which the current density is nonuniform,

localized concentrations of vacancies, and even macroscopic voids can be formed. The authors propose a corresponding effect, photo-transport, where in a light-carrying medium the transfer of momentum from photons to the medium, either through absorption or dielectric polarization can lead to a migration of ions or defects along the direction of light flow. Although no detailed calculations were presented in support of this hypothesis, it is interesting as a possible source of progressive deterioration of an optical material under prolonged use. Further investigation of this effect is, of course, only warranted if calculations indicate that the magnitude of the effect is observable.

XXII
A number of investigators have reported that in the vicinity of small defects at optical surfaces,

the occurrence of laser damage is accompanied by the formation of a pettern of ripples extending away

from the damage site in the direction of the electric field vector of the incident light wave. In a paper by P. A. Temple and M. J. Soileau of the Michelson Laboratories, this ripple pattern is attributed

to the interference of the incident light wave with the electrostatic field induced in the vicinity of

the polarizable defect. This hypothesis is given support by the fact that the ripple spacing is observed to be X', the optical wave length in the medium rather than X'/2, which would be the spacing if the effect were purely due to the radiation field and oriented along the E-vector where no dipole

radiation can propagate. The authors propose that the effect should be a maximum for defects of an odd

number of half-wavelengths in size due to resonant enhancement of the field. In the discussion of the paper, it was suggested that tests with circularly polarized light would be of interest. It is expected that circular ripples would occur in this case.

Recommendations

Over the last ten years, the subject of laser damage has moved steadily from primarily a reporting

of empirical observations to one of genuine scientific study. The major phenomena of laser damage have

been catalogued, and are understood to varying degrees. Scaling laws of increasing sophistication

provide the system designer with convenient and generally reliable rules-of-thumb governing the selection and employment of materials for windows, coatings, and laser media as well as pointing the way

for material development. The study of laser materials closely parallels other aspects of materials

science, with the added complications imposed by the interaction of intense light with optical materials.

If there is any one word which describes the essence of studies of this kind, it is characterization . This encompasses the detailed description of the material, the conditions of the experiment, and a full diagnosis of the results. All of these are essential in any laser damage study carried out today. The

characterization of bulk material for optical use should be in terms of its residual absorption.

Materials of interest for laser components are generally used in the region of high transparency,

between the Intrinsic Urbach tall and the reststrahl edge, in a spectral region dominated by extrinsic

Impurity absorption. The absorption in this region, like the resistivity of semiconductors, is a direct measure of the impurity level in the material. In order for reported results to be compared, or applied

to device design, such a characterization is essential. A comparable characterization of the structural homogeneity of bulk material is provided by optical

scattering measurements. Scattering may be due to particulate inclusions and voids, or to the presence

of micro-crystallites or phase separation. A scattering measurement provides the same overall

description of the quality of a sample, from the standpoint of structural homogeneity, as is provided by

absorption data for compositional homogeneity. If it is necessary to characterize the actual source of the scattering or absorption, microprobe analysis of some variety is generally required.

The characterization of the optical surface is even more demanding than the bulk description.

Surface contamination, either from embedded polishing materials or environmental factors, greatly in- fluences the damage properties of the materials. Surface roughness is now well established as a factor

in damage studies. Consequently, where optical surfaces are involved, in addition to bulk properties,

surface absorption and scattering data are required to complete the characterization of the experiment.

Again, if it is necessary to detail the origin of the absorption or scattering, microprobe analysis.

Auger spectroscopy for example, and prof llometry, either mechanical or by optical means, must be carried

out. When one considers the characterization of thin film structures, to the degree indicated above, the mind boggles. The identification of bulk vs surface properties becomes difficult in layers of material

of thickness less than one ym. A full characterization of even a single, quarter-wave layer goes beyond a simple statement of absorption and scattering. The means and rate of depositing, substrate

temperature and background environment during deposition, substrate preparation, composition and

morphology of the film, porosity, complex index, residual stress, electric field distribution, film

and substrate roughness and aging or film stability have all been shown to Influence the damage

resistance of the film. If a multilayer structure is involved, the list of relevant parameters grows accordingly.

XXIII
Clearly, it is not possible to specify all of these parameters for every study of laser damage on thin films. Parametric studies must be carried out to isolate the effects of each of the relevant

variables, holding the other factors constant as far as possible. This requires carefully controlled

deposition conditions, specified as fully as possible, extreme cleanliness, care in preparation and handling, and relevant film characterization. Damage tests on production films, although of empirical

value in defining the current state of the industrial art, are of minimal value in isolating the factors

responsible for the failure of the film. The reluctance of manufacturers to characterize their products

even in terms of composition must be 'overcome, and a policy of full disclosure instituted, if not in production films, at least on samples supplied for damage testing. In this way, variations in the parameters of the film fabrication process can be related to variations in observable properties, such

as absorption, scattering, structure, and residual stress, and these, in turn, to the obseirved damage

phenomena. These remarks apply to both vacuum-evaporated films and sputtered films.

At present, given the intense, goal-oriented pressures in the large laser programs, no one has the time and money to pursue basic research on optical films for high power laser use, with the instrumentation and detail of investigation required for full fundamental understanding. However, if laser fusion,

for example, becomes a reality, and systems with hundreds of coated surfaces proliferate, the demands of

^ the market will be sufficient to warrant the kind of heavily instrumented, highly detailed research that | characterizes the study of metallic films in the electronics industry today. Basic research I investigations, and funding agencies, should be moving in that direction now, trying to develop a first i principles understanding of the relation between deposition parameters and the physical, chemical,

mechanical, structural, and optical properties of the finished dielectric film. As indicated above, characterization of samples is all important. No less important is the full

characterization of the laser output used in the experiment. The actual profile of the beam at the sample and its Z direction variation should be reported, wherever possible, in conjunction with damage

results. This goes beyond the description, "TEM^^", since aberrations due either to high power propagation effects in the laser, or figure errors in the lenses can change the focal distribution

significantly. With this emphasis on characterization, the subject of diagnostics must be addressed.

The relation of light scattering to the nature of the surface remains unsatisfactory. Discrepancies

among different methods of measuring surface profiles remain, and should be understood. Measurements

must be correlated with exact vector theory, and wherever possible, scalar approximations for light

scattering should be developed. There is an extensive literature of radar scattering which should be

valuable in guiding this work.

In the damage area, the reason for the correlation between surface roughness and damage threshold

remains open to speculation. It has been proposed that increased roughness yields a larger effective

surface area, or higher electric field values, or increased electron scattering below the surface. Experimental investigations of these hypotheses are needed. The use of ATR and ellipsometry can provide a sensitive tool for these investigations.

Improved calculations of thermal stresses in windows are also required, taking account of the antistropy of crystalline materials in an exact way. The analysis of thermally induced stress- birefringence may yield a sensitive measure of bulk absorption for transparent materials.

Much remains to be done in the development of scaling laws. The recently enunciated scaling with

area and pulse duration, apparently valid over many orders of magnitude,, must be understood on a more

fundamental basis. The sensitivity of these scaling relations to material quality (impurities,,

defects, etc.) is also in need of investigation. The scaling of avalanche ionization with frequency is still largely unresolved. Resonant processes, including multi-photon absorption, complicate the picture, where frequency scaling is involved, and must be taken into account, as reported at this year's

Symposium. The temperature dependence of avalanche ionization should be further investigated, to elucidate the role of phonon processes. Some investigations of the temperature dependence of avalanche ionization have been carried out by Manenkov and co-workers in the USSR. A new kind of scaling relation is needed. One needs to know, how long can an optical element be

expected to last if operated at a certain fraction of the normal damage level? This kind of information

is known for capacitors, flashlamps, and other high voltage equipment. High power laser components need

to be characterized in the same way. This area is closely related to aging or film stability under a variety of environmental conditions. It may be due to effects, such as H2O entering the porous regions

of the film, or structural effects, such as from growth, or some other cause.

XXIV
There are a few practical questions which need immediate attention. One is the problem of surface

contamination during substrate preparation for thin films. Diamond turned metal surfaces must be

entirely free of oil or other contaminants to ensure adequate adherence of dielectric over-coatings.

More investigations of novel coating designs, such as that presented by Baer this year are needed,

especially in the uv, where the variety of available coating materials is limited.

Finally, a need remains for the consolidation of data on window and coating materials, along the lines of that presented in the Handbook of Military Infrared Systems, but extended to high power laser

uses. Wide gap materials such as alkali halides are useful from the uv to the IR,, and new materials

are continually evolving. Some effort of this kind of compilation has been made by the DARPA laser

window program, but a broader compilation of data is needed, with a provision for continuing expansion,

a sort of laser materials documentation project.

4. Acknowledgment

The editors would like to acknowledge the invaluable assistance of Dr. Harold S. Boyne, Mr. Wilbur

Anson, and Ms. Florence M. 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. The continued

success of the Damage Symposia would not have been possible without the enthusiastic support of those

named above.

5. Bibli

[1] "Damage in Laser Glass," A. J. Glass and

A. H. Guenther, Editors, ASTM Special

Technical Publication 469, ASXM,

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).

[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).

igraphy

[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 to 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 Otpics 15, No. 5, pp. 1510-1529 (1976).

A. J. Glass

A. H. Guenther

XXV
/
0.1 INTRODUCTION AND WELCOME

AT exander J . Gl ass Lawrence Livermore Laboratory

P.O. Box 808

Livermore, CA 94550

On behalf of Dr. Guenther and myself I'd like to extend a personal welcome both to our old friends and new acquaintances here at the Symposium on Optical Materials for High Power Lasers.

The meeting is subtitled the Eighth Annual Boulder Damage Symposium, a name which evokes images

of people involved in crushing rocks. This image may not be entirely inappropriate to the task before us.

The previous speakers have alluded to the fact that we've made outstanding progress over the

last eight years in the field of laser damage and I heartily agree with that. I am, however,

somewhat unnerved to look at our agenda which breaks down into the following five topics: (1) Bulk

Absorption, (2) Surfaces and Mirrors, (3) Thin Film Properties, (4) Thin Film Damage, and (5) Avalanche

Ionization. This is hardly a radical departure from previous years. The only difference is that

we have, at least for the time being, set aside self-focusing and the nonlinear index. It behooves

us to ask ourselves why the same subjects year after year. In part it may be because those

who fail to study history are doomed to repeat it. Certain problems continually re-emerge in new guises, and for those who have been coming to Boulder every year for eight years there is a familiar

ring to some of the papers. From time to time we ask ourselves, didn't that come up in '69 or

'70 in a slightly different form? It's true, we do continually work away at the same old problems

in some ways, but that's because these problems are difficult and because they are important.

Our concern in this Symposium is the limitations imposed by optical materials, real optical materials,

on high energy laser systems. These limitations are what stand between the brilliant conception

of the device and its realization. Somehow we all come out of graduate school imbued with the concept of ideal materials, materials which are perfectly transmitting dielectrics and perfectly

reflecting metals. We need to be continually reminded of the limitations that real materials

impose on us. Nature takes care of that; the reminders are ever present. We continually report

progress at this Symposium even though the agenda doesn't change, and it doesn't always look like progress. It takes a long view to recognize progress in its various forms. '

The Talmudic scholars have a metaphor for the truth, which is that truth appears like an

onion. If you peel away one layer, there's another layer that looks just like it underneath and

if you peel away that layer, still another appears. Our onion, unfortunately, exists in Hilbert

Space. Problems which are simply stated at first, upon subsequent investigation turn out to be much more complex than anyone originally thought, and on further investigation seem to be totally

unclear. The best example of that which comes to mind in our present agenda is the question of avalanche ionization. At least two papers have appeared in the literature in the last five years

which purported to be the last word in avalanche ionization, summing up and stating the received

opinion on what was now a well understood and relatively simple phenomenon. Of course, subsequent

investigation showed that was indeed not the case and more work remained to be done. Thus, the old

subjects continue to be unraveled and sometimes look more complex for having been investigated.

As Harry Winsor just mentioned, we also keep moving into new operating ranges and new wavelength regions and introducing new materials in the systems. The advent of the carbon monoxide,

HF, or DF lasers for long wavelength applications, and the advent of shorter wavelength systems

both in the ARPA applications and in laser fusion and laser isotope separation keep confronting

us with new aspects of the old problem areas. These subject headings are so general that they

essentially comprise the total subject of laser materials for high energy use; bulk absorption,

surface effects, thin film properties, and of course, the ultimate limitation of dielectrics,

avalanche ionization. We have made continuing progress in this field and can look forward to further progress being made as long as someone is interested in what we have to say.

We must ask ourselves the burning question, "Who cares"? The answer is to take a look at

our ever expanding list of sponsors to get some idea of who cares. I feel that the work of this

Symposium continues to be relevant because it continues to be supported, and the day when it ceases to be supported is the day when the Symposium will be out of business. We're very happy

this year to acknowledge the expanded support that we have received and I want to acknowledge

the continuing support of the ASTM and thank Haynes Lee for his efforts on behalf of the ASTM

in helping us move this Symposium along. From NBS, whose hospitality we enjoy and from whom we receive tremendous administrative support, we have to thank Hal Boyne. The Office of Naval Research

has supported the Symposium for several years now, and we are indebted to Hershel Pilloff for his

help, cooperation, and support. We are very pleased this year to have a speaker on our agenda from

ERDA, Joel Weiss from the Washington office, who will review some aspects of the laser materials

program in ERDA. Let me again thank Harry Winsor for his support and cooperation in coordination

-1-
with the laser window program. I am particularly glad to see full participation by ERDA, because

I work for ERDA and I consider it a good sign for continuing employment to see a manifestation of interest from on high. I think everyone is aware of the fact that Martin Stickley, who has a long and abiding interest in optical materials, is now in ERDA leading the new laser division.

If you are a reader of portents, you might consider that a good sign for those of us who are interested in further investigation of optical materials.

We're trying a number of innovations this year. The first and most important one is that

we are trying to coordinate this meeting with the ARPA Window Conference. I want to give my particular thanks to Bob Andrews and Patsy Collins, who organized the agenda for the Window

Conference and with whom Art Guenther and Harry Winsor worked very closely in setting up dual agendas for these two meetings. Secondly, we are trying something new in the way of having poster

papers. From the response to the poster papers yesterday and the response I anticipate for the two poster sessions we have in this meeting, I think that will turn out to be a very nice vehicle for allowing us to present a large number of papers without having recourse either to ten minute

papers or multiple sessions. The poster papers, I want to make clear, are not intended to be a form of second class citizenship in this meeting. Some papers are more appropriately given as poster papers. By the way. Art Guenther and I would welcome comments and yes, even criticism (if

properly tendered) with regard to all aspects of the Symposium, but particularly with regard to the poster sessions. Any suggestions you might have of how these can be improved, expanded, or deleted will be received with interest.

Once again, we're enjoying the hospitality of NBS. I want to acknowledge the steadfast

support we have received from Florence Indorf, Wilbur Anson, and the staff of the Bureau in making

these superb facilities available to us, for helping us in the preparation of the manuscripts,

and in making the whole enterprise both enjoyable and productive.

-2-
0.2 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 conversion

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

1 microinch = 2.5400* x 10~" meter

Area

1 in^ = 6.4516* x lO"" 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, K

1 cal (thermochemical) cm.s. °C = 418.40* watt/meter kelvin

specific heat, C

1 cal (thermochemical) /gm. °C = 4184.00* j oule/kilogram- kelvin

*Exact Value

-3-
0.3 ADVANCED OPTICAL I4ATERIALS FOR LASER FUSION

Joel A. Weiss

Division of Laser Fusion

U.S. Energy Research and Development Administration

Washington, D.C, 20545

I appreciate the opportunity to make a few remarks this morning about the optical materials needs

of the laser fusion program. ERDA's Division of Laser Fusion has the distinction of being not only the newest of the sponsoring organizations for this symposiiim, but also the newest of the sponsoring

organizations in terms of length of time of existence. As Alex Glass mentioned in his remarks,

Dr. C. Martin Stickley has been appointed the Director of the newly created Division which became a full division in ERDA on March 22, 1976. The Division has the mission of demonstrating, developing,

and utilizing inertial confinement fusion, which includes laser, e-beam, and ion-beam fusion processes.

The laser isotope separation program, which had been coupled with the fusion program is now part of

the Division of Nuclear Research and Applications, under Dr. Richard Roberts the Assistant Administrator

for Nuclear Energy.

From talking with people over the past several days I have noticed that to many of you ERDA is a large amorphous organization. I would like to take a moment to point out some of the features of this new and evolving agency because there are divisions other than my own to which some of you may

wish to submit research proposals.

The basic ERDA structure is as follows: There are six scientific Assistant Administrators who

report to the Administrator, Dr. Seamans . Of these six there are three who would be of interest to this audience. Dr. Robert Hirsch is responsible for the Division of Physical Research, ERDA's basic

research unit as well as for solar, geothermal, and magnetic confinement fusion research. I have

already mentioned Dr. Roberts whose domain includes all nuclear energy research. Lastly, there is the Assistant Administrator for National Security, Alfred D. Starbird, whose responsibilities include

the Division of Military Application, the parent organization of the present Division of Laser Fusion,

which remains under his direction.

At present our division has an extremely small staff, something that will most probably change

over the next year. My area of responsibility is solid state laser development and related optical

materials research. This area covers the full range of materials and coatings for use in high power pulsed lasers for fusion.

Because the laser fusion program is a young and evolving program, in contrast to the mature program which Harry Winsor discussed yesterday, the projected needs and experimental objectives of the research have continued to change over the past few years. This behavior is probably typical of most state-of-the-art technology efforts in their early stages where the direction of the program is continually being driven by the results of ongoing experiments and calculations. However, our program

has now matured to the point where we feel we can map out a well-coordinated plan for development over

the next 25 years and can attempt to guide a research program to meet our program objectives.

Because the program has needs in both the near and long term time frames, the materials research

efforts sponsored by the Division have two relatively distinct facets. We are currently supporting

efforts to develop materials needed to bring on-line laser systems now in the planning stages and at the same time we are sponsoring research on more basic, long term efforts required to permit realization

and development of the next generation of laser fusion facilities, such as a materials test reactor

or an experimental power reactor. As an example of this two-fold approach, we will be supporting

research on dielectric coatings for use on glass lasers at Livermore while at the same time we are interested in supporting some efforts oriented towards developing a fuller understanding of coating

science

Yesterday, Walt Reichelt gave an overview of the materials program as it affects the Los Alamos

CO2 laser program. Today, I plan to limit my remarks to the materials effort at 1.06 ym and visible wavelengths. However, let me point out that the Division of Laser Fusion has prime responsibility for coordinating all aspects of inertial confinement fusion be they Nd: glass, CO2 , e-beam,

or ion beam. So my omission of CO2 from my discussion does not indicate a lack of interest in that subject.

In his talk Walt mentioned that at 10 pm laser design is limited by intrinsic materials

breakdown considerations. These limitations drive the laser designer to develop large aperture systems.

At 1 vim the laser designer is also materials limited, but not principally^ by intrinsic breakdown.

-4-
but rather by nonlinear propagation effects, that is, small scale self-focusing (the B-integral).

Considerable progress over the past several years in controlling small scale self-focusing through

spatial filtering has permitted development of lasers such as the ARGUS system at Livermore which has recently begun operation at better that 1 TW per beam line. This technology is basically the same

as will be used in the SHIVA laser now under construction at Livermore which will have 20 such beam

lines

.

However, despite the fact that spatial filtering can reduce small scale self-focusing and thereby reduce point-to-point variations in laser intensity across the aperture the overall limitations

imposed by the nonlinear index of refraction still force the laser designer to ever larger apertures.

An example of this trend can be seen on the first slide (fig. 1) .

This slide is a picture of a Livermore D-size disk amplifier. This device has a 30cm clear

j aperture and Is the largest disk amplifier ever built. Although not planned for use on the present

j ARGUS or SHIVA laser systems this amplifier is being considered for use on an upgraded SHIVA laser.

Obviously, when one gets to lasers of this size, it is clear that a better way must be found to

' increase power or else things will soon get completely unmanageable. One approach to achieving more

laser power would be simply to increase the number of beam lines without Increasing apertures , but

this is an extremely expensive way to go and leads to complexities in the area of pointing and

focusing. A much simpler and cheaper approach to higher power laser systems would be to increase the power that can be obtained from any one beam line. If this can be achieved in an existing system by

replacement of the optical glass , there can be substantial savings since the cost of glass for a "glass"

laser is usually less than 25% of the total outlay.

I This then represents the central theme of the present DLF materials program: To develop low

n2 optical materials and related technologies, such as coatings, to improve the performance of solid

! state laser systems.

One might ask at this point: Why spend all this money on glass lasers since they can never be

used as practical fusion lasers. One answer is that glass lasers are the most advanced high power

short wavelength (<< 10 ym) lasers now in existence, and therefore, represent the most direct, and

I probably least expensive approach to demonstrating the scientific feasibility of laser ignited

j thermonuclear fusion. These lasers are the only type of short wavelength laser capable of being

brought to the 100-200 TW level in the next five years.

Another answer to the question of why pursue glass development is that only about one-half of

the glass in a "glass laser" is actually laser glass. The remainder goes into lenses, polarizer

substrates, Faraday rotators, etc. Even when we discard glass lasers, we will not discard all of the glass

.

Furthermore, there is no question but that when we discard glass lasers we will still be every

bit as concerned with reducing the n2 of the glass that remains as we are now. In fact, if it proves

I to be true that n2 increases as we go toward shorter wavelengths, we will be even more interested in reducing it in the future.

For these reasons then, we are pursuing a program to develop low n2 optical materials.

The next slide (fig. 2) , which is courtesy of Alex Glass and Marv Weber at LLL, is a plot of refractive index versus Abbe number for a number of common optical materials. The dotted lines are

lines of constant n2. Notice that all presently available laser glasses fall in the upper right

region of the plot. A tremendous reduction in n2 can be achieved by developing glasses that lie

in regions toward the lower left. Notice also that some of the most attractive materials, from an

n2 standpoint, are the alkali halide and alkaline earth fluoride materials with which many of you

j here have been dealing as DoD window materials

.

Although the basic parameter of importance in our research program is the nonlinear index, n2,

the actual figure of merit for an optical material may, in addition, be independent on other parameters

and the functional dependence may vary depending on the role played by the material in the laser

system. The next slide (table 1) shows the functional form of the figure of merit for laser rods,

disks, lenses, polarizer substrates, and Faraday rotators. As you can see, the choice of a material

is highly dependent on where it is in the laser system and what purpose it will serve. For example,

laser disks operating in the parasitic-limited regime have a figure of merit which is equal to n2/n2, whereas, if parasitics are not a problem, the figure of merit includes gain and can be written

as n2a/n2. Notice also that it is difficult to improve the figure of merit for a lens because of

j the n(n-l) dependence since n decreases with decreasing n2. The data is valid for 1 ns pulses.

! -5-
On this table are also given representative figures of merit for silicate, phosphate, fluorophosphate and f luoroberyllate glasses, and for fluoride crystalline materials. It is apparent that the

use of f luoroberyllate glasses and crystalline materials is the most attractive course to follow even

if it is a somewhat more difficult path.

In our research program we are obviously not assuming that one class of materials will be

suitable for all purposes in a laser system. The next slide (table II) is an outline of the roles as we see them for the various optical materials on which we are supporting research. We are counting

on f luorophosphate materials to be available as laser hosts, polarizer substrates, and rotator hosts. Crystalline materials will be used principally for lenses, possibly for rotator hosts and almost

certainly as oscillator materials. (The Nd laser wavelength for fluorophosphate and beryllate glasses

lies at 1,047 ym, requiring the development of new oscillator materials.) Lastly, as a high

risk, high payoff option we are supporting research on f luoroberyllate glasses to be used in the same

roles as the f luorophosphates

As we support research designed to increase the power output per unit area for a laser beamline

we find ourselves suddenly pressing another area of technology to the limit —dielectric coatings.

The next slide (table III) shows typical coatings requirements for an upgraded SHIVA laser operating

at the 100-200 TW level. As you can see we are seriously deficient in the area of AR coatings while

the situation is not nearly bad for polarizers and dielectric reflectors. Notice that even with such

a laser we are still operating well below the intrinsic bulk damage level for the optical glasses

themselves. Although we can dispense with some coatings in these laser systems we can do so only

with a loss of output power. For this reason we are interested in supporting development of improved

coatings for high power applications.

We are attempting to have the materials which I have discussed today developed within 2-2% years

to the point where purchase orders can be placed for production quantities and sizes. This will

require that pre-production prototypes be produced by the end of this time period. This is a big job,

but we are getting responses of interest from many of the major glass manufacturers and from quite a few companies that have been involved in the ARPA window program. We feel that this is an important

job and one which can and will be accomplished.

I appreciate the opportunity to speak here this morning to present our program and our needs, and

I encourage you to contact me if you have any questions about our materials research effort.

-6-
Tables and Figures

0)M

•H

U4

<U I—

•O <T3 -.- +J

S_ lO 2 b

I *>> l/> S_ CO

0) 03

O C5

i-O O)

JC .

o

01 M c/1 -c: m

Ol 1T3

I/) r— O O

3 M- --

C71 O SE O •!- O _)

a:

CL

d; e

LU 3

CO Q.

•a: <:

o

CO +->

LU nj CO Q. a: oQ

Oh-

-7-
Table II. Roles for candidate optical materials.

FLUOROPHOSPHATE GLASSES

e LASER HOSTS

0 SUBSTRATES, FOCUSING LENSES, WINDOWS

0 FARADAY ROTATOR HOSTS

CRYSTALLINE MATERIALS

• FOCUSING LENSES (CAF2)

9 WINDOWS (CAF2)

» OSCILLATOR MATERIALS (Nd:YLF)

« FARADAY ROTATORS

FLUOROBERYLLATE GLASSES

• LASER HOSTS

• FOCUSING LENSES

e FARADAY ROTATORS

Table III. Coating requirements for an upgraded shiva laser.

TOO ps porforn'.ance Ant i reflect ion High reflection Polarizers Substrates

Past Experience 2 J/cni2 2 J/cffl2 3 J/cm2 15 J/ciii^

Best Toddy 4 to 6 J/cnv- 4 to 9 J/Gin2 4 to 7 J/cm2 30 J/cm2

Upgrade Require:. 10 J/cm2 8 J/ci:i2 8 J/cni2 15 J/ci:i2

-8-
-9-
Nonlinear index _ n^ dO^^esu)

Silicate and

phosphate

glasses

FR-5

'BeF

• NaF

Fluoroberyllates

--0.25

Fluorosilicates ^10

0.5

110 100 90 80 70 60 50

Reciprocal dispersion — Abbe number

40

Figure 2. Refractive Indices of laser system optical materials.

COMUEMTS ON PAPER BV MEISS

No diicuMion on tha> papzfi.

-10-
1.1 THERMAL DISTORTION THRESHOLDS FOR OPTICAL TRAINS HANDLING HIGH PULSE POWERS

H. E. Bennett Michelson Laboratories, Naval Weapons Center China Lake, California 93555

The maximum pulse power level which components in the optical train of a high

energy laser can pass is determined by the catastrophic damage threshold for the system. Before that level is reached, however, thermally induced optical distortion may make the beam unfocusable and hence for most practical purposes unusable.

The amount of thermally induced optical distortion which may be tolerated depends

strongly on the initial optical figures of the various components, and in the case of windows, on their optical homogeneity. Requirements for these parameters

in a multicomponent infrared optical train can be surprisingly severe, and are frequently comparable to those commonly used for high quality optics designed for the visible region of the spectrum. Figure requirements for mirrors are more

severe than for windows unless their index of refraction is above about 2. A. Adaptive optics may be used to remove some of the resultant wave front error, but

the lower the requirements made by the optical train the more easily the adaptive

optics system can correct for wave front distortions arising from the laser medium

and from the optical path. A detailed analysis is required to set optimum tolerances for the various components in the optical path of a particular system.

Simple relations can be developed, however, which can be used to predict the approximate pulse power handling capability of the optical train for short periods

of time in terms of the material parameters of the various components in the system,

their optical figures, and in the case of windows, their homogeneity. These relations can be used as figures of merit to select between materials and preliminary

calculations for system design.

Key words: Laser windows and mirrors; optical figure; pulsed laser; thermal

distortion.

Introduction

In an optical train handling high intensitltes such as those generated by lasers the maximum power

levels windows and mirrors can pass are limited by the catastrophic damage threshold of the materials.

Before that level is reached, however, thermally induced optical distortion may make the optical beam

unfocusable and hence in many cases unusable. The amount of thermally induced optical distortion which

may be tolerated depends in part on the initial optical figure of the component; figure requirements

for multicomponent optical trains can be surprisingly severe even at infrared wavelengths. In an actual

optical system it is necessary to assign an error budget to each component. If a large error is allowed

for one component, it may make the tolerances on the other components impossibly tight. The practice

of calculating allowable figure distortions for individual windows or mirrors as if they were the only

component in the optical system is thus quite unrealistic. Some of the resultant wave front errors may

be removed by using adaptive optics. However, the lower the requirements made on the adaptive optical

system by the optical train, the more easily it will be able to correct for wave front distortions

arising from the laser medium and the optical path. In this paper only the allowable optical distortion

is considered. A diffraction approach is used based on the Marechal criterion for a multielement system. Optical distortion limits for individual windows using a geometrical optics approach have been

discussed previously in a classic paper by Sparks, who has also discussed fracture thresholds [l]'^. He

has shown that when only single windows are considered the fracture threshold rather than the thermal

distortion threshold usually limits window performance. However, as discussed in this paper, unless

adaptive optics are used, optical trains consisting of many components each with a practical initial

optical figure will usually have a thermal distortion threshold for high intensity beams which is lower than the fracture threshold.

The first section of this paper discusses allowable figure errors for both windows and mirrors

based on the Marechal criterion. The second, third, and fourth sections discuss thermal distortion

limits for windows and mirrors in optical trains. The analytical expressions developed there are used

in the last two sections to provide figures of merit for evaluating various window and mirror materials.

Figure Errors

More stringent tolerances are required to focus diffraction limited intensities than are required

to give diffraction limited resolution [2], The ratio of the intensity I (at the center of a focused

1. Figures in brackets indicate the literature references at the end of this paper. -11-
spot produced by an optical train which introduces an rms wave front error a) to the intensity Ig

(which would be obtained if o = 0) is given approximately by the Strehl relation [3]

where A is the wavelength. The accuracy of the Strehl relation for differing values of I/Iq and also | that of an exponential relation which reduces to the Strehl relation for large intensity ratios but , gives a closer approximation to the exact calculation for small intensity ratios, has been discussed by

Loomis [4]. In deriving eq, (1) the limiting apertures in the optical train are assumed to be uniformly

illuminated. In practice the intensity will vary across the aperture, modifying eq. (1) to some extent.

Modifications in the Strehl relation for the case of a truncated Gaussian beam profile have been con- sidered by Lowenthal [5]. When the half-width of the beam profile is much smaller than the aperture

radius, the error introduced by assuming a uniform illumination can be very large. However, if the Gaussian intensity falls to 1/e^ of its maximum value at the edge of the aperture, an intensity ratio ! of 0.8 given by eq. (1) is low by only about 6%. Marechal has suggested [3] the criterion that the | rms wave front errors not exceed X/14, giving I/Iq = 0.8, This criterion is more quantitative, but | similar to the well known Rayleigh A/4 criterion for allowable peak-to-valley wave front distortions [3]' Equation (1) is a good approximation to the exact calculation of l/lo when the Marechal criterion is

met. If it is relaxed, for example, by doubling the loss in on-axis intensity so that only 60% of the theoretical intensity is achieved rather than 80%, the allowable distortion only increases by about a factor of 1.4. The Marechal criterion is thus a good objective to strive for in a multielement optical

system. \

I The total rms wave front distortion a is the sum of contributions from the N components in the optical train. If these contributions are uncorrelated, then we can add intensities rather than amplitudes and a2 is the sum of the squares of the contributions from the individual components [6]. Thermally induced wave front errors in different components are often correlated; in some cases the wave

front errors are of different sign and correlation between components results in partial cancellation,

i^e will assume thermally induced wave front errors between components are correlated. However, unless

similar figure errors were developed by the lap on the different components or on the two sides of a polished window there will be no correlation between figuring errors in different components. In the case of windows, wave front errors caused by optical inhomogeneity in the window material are also

assumed to be uncorrelated in different components. For an equal error budget per component the resultant intensity decrease caused by figuring errors and by inhomogeneities in the bulk of the window

materials are each then simply N times the square of the respective wave front distortions per component.

The contribution from each component will be caused partly by the initial figure and optical inhomogeneity errors and partly by the thermally induced figure error resulting from absorption. Assuming

that the initial figure and optical inhomogeneity errors are uncorrelated with each other or with the thermally induced figure errors so that the squares will be additive, assigning the same figure error budget to each component and using the Marechal criterion, we then have

X=0.8 = l- (^)^ (,„2^,„2^,2) (2)

where is the rms wave front error introduced by initial figure errors in a single component, Oi» that introduced by optical inhomogeneity if any in a window component and is the thermally induced rms wave front error. If 6n is the rms variation in the value of the index of refraction across the window

and I is its thickness, then (})£ = 6ni!,. The peak to peak wave front errors introduced by figuring are approximately l/l times the rms values (an exact relation if the wave front errors are sinusoidal) and

the wave front error introduced by a mirror is twice the physical displacement tj^, of the mirror surface so that Of = T^^jJl, Substituting in eq. (2) we then obtain a relationship for the allowable

peak to peak initial figure error Tfj,, for a mirror

^/lo.

In air for windows with index of refraction n the change in optical path caused by a physical path

difference will be (n - 1) times the physical path difference. The contribution to the rms wave front distortion arising from initial peak to peak figure errors of Jc., at each of the two surfaces will be

o| = (1/2/2)2 2(n - 1)2t2„ or

"f=i^"-l)^fw (4)

assuming that the figuring errors are uncorrelated. From eq. (2) an expression relating the magnitude

of the allowable errors for a window on an equal error per component basis is then

-12-
By comparing eqs. (3) and (5) and letting 0^=0^= 0, we see that for windows whose index of refraction

is less than 2.414, the maximum figuring error per surface can be larger for a window than for a mirror.

In other words, when initial figure errors are uncorrelated, the initial figure requirements for a homogeneous window (n < 2.414) are not as stringent as those for a mirror. If we had assumed [7] that the initial figure errors on the two sides of the window were correlated, windows whose index of refraction

was less than 2.0 would have less stringent figure requirements than for mirrors, but those with higher

indices would have more stringent figure requirements. Uncorrelated initial figures appear to be the more realistic assumption in most cases. Dueweke et al. [8] have experimentally verified that initial

figure errors on different windows in one typical infrared optical train were uncorrelated and hence

that the resultant wave front error was given by the sum of the squares of the figure errors for each

window.

If the sum of the squares of the maximum allowable initial figure and homogeneity errors are restricted to one-half of the square of the thermally Induced error, the maximum allowable initial

figure error can be calculated for mirrors from eq. (3) and for windows from eq. (5). Results for homogeneous windows of ZnSe, KCl, CaF2, and SrF2 and for mirrors are shown in figure 1 for a 10-component

system. For KCl a full wave of figure error is permissible at 10.6 urn, as compared to slightly over a quarter-wave for ZnSe and for a mirror. At CO and DF wavelengths about an eighth-wave figure is required

for ZnSe and for mirrors, which is a very severe tolerance for components of appreciable size. Figure

tolerances for ZnSe and for mirrors coincide because the index of ZnSe is 2.4, the crossover value as discussed above. For KCl and both CaF2 and SrF2 the figure tolerance is about one-half to three-eighths

wave at CO and DF wavelengths. In the general case the figure tolerances on homogeneous windows are a factor of /2/(n - 1) larger than those of mirrors. Inhomogenelties in the window will tighten these

figure tolerances since under the above error budget the allowed peak to peak figure error per surface

for a window is

1 fw (n - 1) y^3^2N i 4a?. (6)

Thermal Distortion in Windows

The thermally induced wave front distortion a window can contribute without serious degradation of

the optical image can be calculated approximately from eqs. (2) and (5) for either pulsed or CW operation. Calculation of the energy in a single pulse which can be passed without violating the Marechal

criterion for the optical train is simplest since lateral thermal diffusion effects can be neglected.

In one second the thermal diffusion lengths for typical window materials are 2-3 mm, so the calculated

maximum single pulse energies represent approximately the maximum amount of energy the window can pass without significant optical distortion over periods as large as a few seconds. Even if we neglect

lateral diffusion, however, the relation between the heating of the window and thermally Induced optical

distortion is strongly dependent on the beam profile. Since the thermally Induced wave front distortion

results from differences in optical path length across the component, not from the total thermally

Induced change in optical path lengths, the beam profile is very important. If a homogeneous window

were heated uniformly, for example, the optical path length might change significantly without introducing any wave front distortion. To calculate representative wave front distortion values some model

of the beam profile must thus be made. A convenient model is a top hat beam profile which occupies

the center half of the area of the aperture. In calculating the wave front distortion, however, the changes in optical path length in all equal areas of the mirror or window are weighed equally. This

approximation will in most cases result in overestimating the resultant wave front distortion somewhat.

The effect of a similar assumption using Gaussian beam profiles has been studied by Lowenthal [5].

2 The top hat function described above is a first approximation to a Gaussian beam profile whose 1/e

points coincide with the component aperture. The power P passing through an aperture of radius b when

the beam profile is Gaussian is

P = ^^^^^ - ^o""^^^^

'o •'0 which may be approximated by a top hat beam profile with maximum intensity 1^ and radius b//2.

In actual applications the beam profile is often neither flat-topped nor Gaussian. For example. In

the near field of an unstable resonator the intensity profile is donut shaped with a minimum at the center as well as near the edge of the aperture. Asymmetric energy profiles are commonly observed.

Thus the limiting energy levels calculated assuming the top hat beam profile described above are at best

only approximate for an actual system. They provide a useful figure of merit for various window or mirror materials, but more detailed calculations involving the actual beam profile are required to establish maximum permissible power levels in an actual system.

-13-
For a window the energy absorbed per unit area of the beam is, for a temporally square pulse of incident intensity Iq and pulse length t, Iot(Sii + 2As) where Ag is the surface absorption per surface,

a the thickness of the window and S the bulk absorption coefficient. Both Bl and As << 1 and we are neglecting the window reflection loss. Let the thickness of the surface absorption layer be ^.g. Then

the increase in surface temperature Tg will be

I t A

T =-^x^ (8)

s PC H p s

where c_ is the specific heat at constant pressure of the window material and p is its density. Similarly the temperature increase T in the interior of the window, neglecting thermal diffusion, will be

I t6

(9) PC

P

The change in optical path length caused by thermal heating depends both on the window material, how it

is polished, the beam profile and the window mounting configuration. Assume that the dominant source of thermal heating is volume absorption. If the window were uniform in absorption, uniformly heated and unconstrained, the change in optical path length would be simply

However, if nonuniform heating occurs, stresses will be set up in the window. The cool window material

at the rim will constrain the expansion of the hotter window center in the plane of the window and this stress will cause the window to bulge and grow somewhat more normal to its surface than it would if thermal expansion in a radial direction were unconstrained. Making the simplifying (and incorrect)

assumption that the window is isotropic in all crystalline directions, we can calculate the resultant

axial growth caused by this constraint from Hooke's law. Consider a thin circular disk with an axisym--

metric temperature distribution, but with no temperature variation normal to the disk surface. If z is the normal coordinate and r and 9 the cylindrical coordinates in the plane of the disk, Hooke's law

including thermal expansion for the differential strain e„ in the z direction may be written

e

z aT = I

[o^ - v(a^ + a^)] (11) \

where E is Young's modulus, v Poisson's ratio, a the linear expansion coefficient and o^, aj.,and ag are i the differential stresses in the z, r>and 9 directions, respectively. Similar expressions hold for Ej. and £0, the differential strains in the r and 9 directions. If we assume the stress is entirely in the ' plane of the disk (plane stress approximation) then o^. = 0. If the disk is simply supported so that there are no external body forces then [9]

aE { — f Tr dr - — [ Tr dr ) (12) b2 Jq r2

= aE ( - T +

I Tr dr + ^ I Tr dr ) (13) b

where b is the disk radius. The axial displacement w(r) = lle_(r) is then

w(r) = allT(r)

2 f'^ 1 + V 1

^ T(S)S dC b2T(r) 'o (14)

For the "top hat" function T(r) = T for r < b//2 and T(r) = 0 for r > b//2. In the heated area the axial displacement w^ is then

wi = a£T[l + v/2] . (15)

The compressive radial stress exerted by the heated area on the unheated area causes a small compressive

stress radially and a larger tangential hoop stress which is tensile. The resultant axial displacement

^2 is thus negative in the unheated area. From eq. (14) w^ = -ailTv/2 . (16) The relative axial displacement of the heated area with respect to the unheated area is then

wj - W2 = aET(l + v) . (17) -14-

I
Another effect of thermally induced stress in a heated window is to introduce strain birefringence.

A pressure differential across the window will also produce this effect, which is quite pronounced in materials such as the alkaline earths and to a lesser extent in the alkali halides. Representing the contribution of strain birefringence by <f , we then have for t^, the maximum thermally induced change in optical path length for a window,

It d 1 ^ (2Ag + m [(n - 1)(1 + v) a + +

*J

. (18)

P The rms change in optical path length for the "top hat" beam profile is then Tr^jJl. In deriving eq.

(18) we have neglected the temperature dependence of the material parameters, a good approximation since

the allowable temperature rise of most windows is only a few degrees centigrade.

Thermal Distortion in Mirrors

In windows surface absorption occurs on both sides in approximately the same amount and bulk absorption occurs more or less uniformly so that little or no asymmetric bowing of the window occurs. However,

in mirrors absorption occurs on only one face. The thermal expansion of this face is restricted by the colder body of the mirror and a bowing occurs at the mirror surface distorting the optical figure. In addition, the heated center of the mirror expands normal to the surface distorting the optical figure

further. As a first approximation consider only the heated center of the mirror and add the expansion

and bowing terras to obtain the resultant wave front distortion.

Let the thermal expansion of the mirror center normal to its surface be Wj where

W3 = aT(z,r,t)dz . (19) -111

But the energy absorbed by the mirror per unit area is

/2

111

I (1 - R)t = PC Tdz (20)

o J p

so that

al (1 - R)t

o Wo = —

^ PC

p which is independent of the thermal conductivity of the substrate.

(21)

To calculate the bowing of the mirror surface we utilize the derivation for a thin circular shell

of diameter Jl b which is uniformly irradiated on one side with a flux of intensity Iq. The axial

deflection caused by bowing of such a shell simply supported is [10]

w, = -^(^ - r2) j^^%(z,t)z dz (22)

^ -111

where [11]

T(z,t)

21^(1 - R) (My/2 ^-z^MkT.,/ _^p/^(-)^/\-,2 V

\ /if •'0 /

(23)

and K is the thermal conductivity of the mirror. The thermal diffuslvity k is

^ = ^

(2A) P and the thermal diffusion depth is given by

= (kt)l/2 . (25)

For typical substrate metals the thermal diffusion depth in cm is approximately equal to the square root

of the pulse length in seconds. Thus even for honeycomb mirrors , cooling will be ineffective during the pulse even for pulse lengths as long as ten milliseconds. At a depth below the surface of the mirror

the temperature is seen from eq. (23) to be 80% of that at the surface and it falls rapidly at greater
depths. A simple model of the surface is then a thin slab of thickness which has been raised to the surface temperature T(0,t) and is restrained by the unheated body of the mirror. If << 1/2, the integral in eq. (22) then becomes

T(0,t)il£

T(0,t)z dz ^ . (26)

(ii/2)-!l^

Evaluating T(0,t) using eq. (23) the axial displacement of the simply supported thin shell is seen to be parabolic [12] in this approximation and is given by

(b2 - 2r2) . (27) 3I_(1 - R)at

/tt PC

p The rms displacement over the mirror surface is then

I (1 - R)at — (1 + 3X + 3x2)1/2 (28)

>/2 PC

P where X = (I/Ztt) (b/)t)2. Equation (28) has been derived on the basis of the thin shell model and hence

will become less accurate as the mirror becomes thicker relative to its diameter. To estimate the magnitude of this error a finite element analysis was performed on aluminum shells of various thick- nesses which were simply supported and had a temperature distribution which did not vary with lateral

position on the surface. The calculated maximum axial displacements were higher than the values predicted by the shell model by 15% or less for diameter to thickness ratios up to 3 and the 2:1 thickness

ratio differed by only 34%. Equation (28) should thus be usable even when h/l ~ 1. It predicts that

the surface distortion and thus the change in optical path length for a mirror caused by thermal heating

is independent of the thermal conductivity of the mirror substrate and depends only on the absorbed

flux, the mirror's linear expansion coefficient, its heat capacity per unit volume and its diameter to thickness ratio.

TTb"

r2TT rh//2

(wj + W2)2 rdrde

Energy Transmission Limits for an Optical Train

Since the figuring errors, bulk inhomogeneities and thermally induced wave front distortions are assumed to be uncorrelated but the thermal distortion between components is correlated, eq. (2) gives

mi Naf 1/2 (29)

20tt^

The thermally induced changes in optical path length can be calculated from eq. (18) for windows and will be positive if dn/dT is positive. For mirrors it will be negative since expansion of the mirror

decreases the optical path length by twice the axial movement of the mirror surface. The change in

optical path length for mirrors is twice the physical displacement of the mirror surface and hence can

be calculated from eq. (28)- Since the thermal distortion is proportional to the energy density striking each component, an accurate calculation of thermal distortion must involve an analysis of the beam

diameter and profile at the surface of each optical component. Some components will have greater energy

densities incident on them than others and the trade-offs which can be made are only revealed by de- tailed analysis of the entire system. To obtain a rough approximation of the power levels which may be

involved and to obtain an approximate figure of merit for comparing different materials assume an equal

error budget for each component and assume the "top hat" beam profile just discussed. Then o is given

by

wit r -,m rr °T

=

I (2A + BO (n - 1)(1 + v)a + + 4 -I

1 t (1 - R) — (1 + 3X + 3X^) ^ /2 (30)

^ p=l /2pc

s L dT J oq PC

where the first summation gives the contribution from the N„ windows in the optical train, and the second summation the contribution from the mirrors, where N„ + Nm = N. The peak beam intensity at a given window p is lop and at a given mirror q, loq where p and q are running indices specifying the individual mirrors and windows. The values of lop and loq for the various components will depend on the design of the optical train. If the wave front error produced by the window is positive, as it is for ZnSe for example, it will cancel some of the negative wave front error produced by the mirrors, perhaps

allowing greater powers to be transmitted by the optical train than would be possible if the window

were not present. The assumption that the allowed thermal distortion per component is 1/N times the total allowed thermal distortion will thus often be unduly conservative. It does, however, give a useful figure of merit for mirror or window performance. For mirrors the limiting energy density using

this criterion is, from eqs. (29) and (30)
PC

'l5Tr2N V

oq

2(1 - R)a[2N(l + 3X + 3X^)]^^^

(31)

and for windows

I t op

r x2 1/2 PC (n - 1)2 t2 - 4o2

p

/2N(2A^ + 0JI) [(n - 1)(1 + v)a + ^ + <).]

(32)

The limiting pulsed energy density which the component can pass is thus inversely proportional to its absorption and directly proportional to its specific heat. For mirrors it is inversely proportional

to the expansion coefficient of the mirror substrate. For windows it is Inversely proportional to

[ (n - 1) (1 + v)a + dn/dT + 4>]» which is a particularly important term when dn/dT is negative, as it is for alkali and alkaline earth halides. A pole may occur in this case, making it possible for the window

to handle very large energy densities without significant optical distortion.

Comparison of Window Materials

Equation (32) furnishes a convenient figure of merit for the optical performance of a window material. Other material properties must also be considered, however, including physical strength, type of

fracture, polishability, surface microroughness, achievable optical figure, hygroscopicity , coatability,

etc. In the intermediate infrared the alkaline earth fluorides are promising laser window materials

with low indices of refraction and high strengths. Unfortunately they do exhibit brittle fracture so that their engineering strength is significantly lower than their breaking strength. They are not hygroscopic and are nearly as easy to work as optical glass. Quarter or even eighth wave figures can be obtained routinely. One CaF2 sample examined at Michelson Laboratory had an rms roughness under 10 A, but commercially polished CaF2 samples typically have roughnesses of 50-60 A and SrF2 is slightly

higher. The microroughness determines the scattered light level in the visible region and to some extent in the infrared region and also is related to the laser damage threshold [13].

Zinc selenide, a leading candidate for windows for carbon dioxide lasers, is a soft but very brittle

material which is fairly difficult to polish and has a high index of refraction. Optical figures of one quarter-wave in the visible on 15 cm diameter windows have been reported [14,15] with microroughnesses

under 25 A rms. KCl, another leading candidate for carbon dioxide lasers, is the softest and most difficult to polish of the four materials discussed. However* quarter-wave figures have been obtained while

minimizing surface absorption [16]. Microroughnesses as low as 27 A have been obtained on KCl [17], but more typical values are in the 100-200 A range. The low index of refraction of KCl is a very important

asset for laser applications.

A comparison of experimentally observed minimum surface and bulk absorption values for various

window materials is shown in figure 2. KCl has the lowest absorption coefficient in the 5-10 ym wavelength range. The volume absorption coefficient value of 8 x 10"^ cm"-' for KCl at 10.6 ym was reported

for single crystal material [18]; the best forged polycrystalline material is 2-4 times higher. The

increase in absorption in the vicinity of 9.6 ym has been reported by several authors [19-21], its origin

is obscure but may be associated with impurity levels of less than 0.1 ppm [22]. Values of surface and

bulk absorption at CO wavelengths for KCl are those reported by Deutsch [23], and for SrF2 and CaF2 by

Newberg and Pappis [24] for cast material. Comparable values were reported for single crystal material

by Larkin et al., [25]. Volume absorption data at DF and HF wavelengths are as reported by Harrington

et al., [26]. They report surface absorption values in the 10"^ range for both SrF2 and ZnSe, lower

values for CaF2 and KCl, However, Miles [27] has reported a value of 2.5 x 10""^ for surface absorption

of SrF2 at DF wavelengths. The quality of the surface finish is usually not reported, but it is safe to assume that the surfaces were all etched. Other material properties [28] were taken from table 1.

We may conclude from figure 2 that the minimum surface absorption per surface is usually comparable

in magnitude to the minimum volume absorption coefficient in cm~^. Unless extreme care is taken in producing the bulk material and the polishing process, however, either bulk or surface absorption values

may be orders of magnitude higher than the values quoted. Both the surface absorption and the bulk

absorption coefficient tend to be lower at CO wavelengths than for either CO2 or HF/DF. The short

dashed lines indicate the intrinsic multiphonon absorption edges. KCl and ZnSe at CO2 wavelengths and CaF2 and SrF2 at CO wavelengths are thus nearly at Intrinsic levels. At the shorter wavelengths the absorption appears to be extrinsic and, as pointed out by Hass [29], is much higher than would be

expected at the HF/DF wavelengths. The surface absorption at these wavelengths may thus not continue

to track the volume absorption coefficient, as Miles' data for SrF2 indicates.

The product of the absorption term and the term giving the change in optical thickness with temperature in the denominator of eq. (32) determines to a large extent the energy density which a window can transmit without excessive thermal distortion. Values calculated from eq. (32) for a ZnSe, KCl, CaF2 or -17-
SrF2 window in a 10-component system using the material parameters given in table 1, are shown in figure 3. The surface absorption per surface shown in figure 2 was doubled to account approximately

for the minimum increase in surface absorption which can be expected because of polishing. A 2-cm

thick window was assumed with an initial figure error per surface of a quarter-wave. A figure error of this magnitude is not significant except for ZnSe, but a larger figure error could drastically reduce

the calculated energy levels. Since data exist only at the laser wavelengths, the curves connecting

the points are not significant.

Table 1. Material parameters for window materials used to calculate figure 3 as referenced in

text. Symbols are X wavelength, n index of refraction, T temperature, S bulk absorption coefficient. As surface absorption coefficient, a linear expansion coefficient, v Poisson ratio, Cp

specific heat at constant pressure, and p density.

Optical Parameters Non-Optical Parameters

L Id U C 1. J.ct JU A ( yin) dn/dT(°K ") e(cm ")

2 A

s V Cp(J/gm°K) p (gm/cm'^)

KCl 10.6 1. 454 -2.7x10"^ 8x10"^ 2.4x10"'^ 36. 8x10"^ 0.11 0.6895 1.9893

5.25 1. 470 -3.15x10"^ 1 .5x10 ^ 2xlO~^

3.8 1. 473 -3.26x10"^ 3 .12x10"'^ 2xlO~^

ZnSe 10.6 2. 40 6xl0~^ 4 -4

.1x10 11x10"^ 7. 2x10"^ 0.30 0.3 5.265

5.25 2. 43 9x10"^ 1 -4

.5x10 10-3

CaF^ 5.25 1. 3952 -7.2xlO~^ 4 -4 .2x10 4x10"^ 18. 2x10"^ 0.26 0.854 3.18

3.8 1. 4122 -7.8x10"^ 2 .67x10"'^ SrF2 5.25 1. 40 -1.19x10"^ 7 .6x10"^ 2.5x10"^ 17. 5x10"^ 0.26 0.556 4.18

3.8 1. 44 -1.19x10"^ 5 .0x10"^ 2.5x10"^

Although zinc selenide has the lowest pulse energy threshold for optical distortion of the four materials examined, it should pass over 100 J/cm^ at 10.6 ym without significant optical distortion.

When used in an optical train containing several mirrors it may pass significantly higher energy densities than this since the thermal distortion produced in the window will be largely cancelled by that produced by the mirrors. Potassium chloride windows exhibit less than l/30th the optical distortion

found in ZnSe windows at 10.6 ym and hold particular promise as internal windows in lasers where flux

densities tend to be higher than at the output window.

At the shorter wavelengths CaF2 and SrF2 may be able to operate at even higher flux levels than KCl. The advantage of the alkali halides and alkaline earth fluorides lies not so much in the low absorption

of these materials, but in the magnitude of their negative dn/dT values, which cause a pole to occur at some wavelengths. The predicted distortion threshold near this pole can depend strongly on the assumptions made in the calculations. The most extreme example is CaF2. If the Poisson ratio correction

(1 + v) in eq. (32) is neglected, the calculated pulsed energy density threshold increases by over two orders of magnitude at 5.25 ym. For SrF2 the threshold decreases by a factor of 1.6 if (1 + v) is neglected and for KCl by less than a factor of 1.2. The uncertainty in the optical constants of the material and the contribution from the strain birefringence term ({>, which has been neglected in these

calculations make the calculated threshold values near the pole very questionable. For the alkaline

earths strain birefringence, which may cause slewing as well as defocusing [30], is probably comparable

in the region of the pole to the sum of the other terms [31]. The alkaline earths are more sensitive to mechanically and thermally induced strain birefringence than is KCl, which is more sensitive than ZnSe. An analysis of thermally induced strain birefringence has been given by Flannery and Marburger [32] and more recently by Loomis and Bernal G. [33], but it is difficult to calculate the magnitude of ^ accurately using present theory.

Fracture may limit the energy density which the window can withstand even though the optical distortion is acceptable. An approximate value for the energy density at which pressed polycrystalline

KCl 2-cm thick and having a breaking strength of 4000 psi would be expected to fracture [34] is indicated by the dashed line in figure 3. In practice the fracture level would depend strongly on the beam

profile, diameter and thickness of the window and the differential pressure which it must withstand.

Fracture resulting from laser induced damage to the window becomes increasingly important as the pulse length decreases, particularly for pulse lengths shorter than a microsecond. If the energy density per pulse is to remain constant, the beam intensity must increase as the pulse length decreases.

The limiting damage mechanism for very pure single crystal or amorphous window materials is thought to be dielectric breakdown, which depends on the local electric field strength at optical frequencies and hence on the beam intensity rather than the energy density. The maximum field strength occurs at the exit surface of the window [35,36] and in the case of the best etched KCl, damage there occurs [37,38]

-18-
at a beam intensity in excess of 8 GW/cm at a pulse length of 100 ns- For polished samples the surface damage threshold is between [38] one and two GW/cm^. At longer pulse lengths damage would be

expected at this same beam intensity, but at significantly shorter pulse lengths the incomplete data

available suggests that it may increase inversely as the fourth root of the pulse length [13,39].

Extrinsic effects such as impurities, surface absorption and probably lattice irregularities can decrease the threshold values by an order of magnitude or more [38]. In the example shown in figure 3 the limiting energy density for KCl is ~10-^ J/cm^; we may conclude that for submicrosecond pulse lengths

the threshold for catastrophic damage to the KCl window. rather than that for thermal distortion, determines the maximum energy density which an actual KCl window may transmit. A similar argument can be

made for the other window materials. Surface damage thresholds of 5 to 70 MW/cm^ have been reported [AO,

41] for ZnSe at 10.6 ym. For CaF2 and SrF2 at 3.8 pm the surface damage thresholds are lower than that

of KCl by a factor of two or three [A2].

Comparison of Mirror Materials

The limiting energy density for mirrors may be determined from eq. (31) if equal density per mirror

is assumed. Results for a 10-component system are shown in figure 4 for silicon carbide, molybdenum,

copper, and aluminum mirror substrates with a 0.998 reflection coating and a diameter to thickness ratio

of three. Material parameters used in calculating data shown in figures 4-7 are given in table 2.

Table 2. Material parameters for mirror materials used to calculate figures 4-7 as referenced

in text. Symbols are X wavelength, R reflectance, p density, Cp heat capacity, a linear

expansion coefficient, K thermal conductivity and T the melting temperature of the ^ ' ' melting ° material. The values of R are optimum values measured at Michelson Laboratories on various

samples.

Optical Parameters Non-Optical Parameters

Material X(ym) R R , p(g/cm3) c (J/gm°K) a K(W/cm/°K) T (°K) coated p meitmg

Mo 10.6 0.9814 0. 998 9.01 0.2477 5.0x10 1.4 2890

8 0.9792 0. 998 5.25 0.9754 0. 998

3. 8 0.972 0. 998 Cu 10.6 0.9919 0. 998 8.993 0.3851 16.8x10"^ 4.0 1356.5

Al 10.6 0.9814 0 998 2.70 0.9024 23.2x10"^ 2.38 933.2

SiC 10.6 0.85 0 998 3.2 0.75 4.3x10"^ 1.8 2000

Coatings are often the limiting parameter, but if these are adequate silicon carbide would be predicted

to handle the maximum energy. Molybdenum is slightly (20%) lower. It can handle over twice the energy

density of copper, which itself can handle nearly twice the energy density of aluminum. An eighth wave

optical figure in the visible is assumed for each substrate material. A perfect figure would result in

less than a 3% improvement over the eighth wave mirror in energy density at 10 pm. However, at 3.8 pm

the eighth wave mirror decreases the allowable energy density by nearly 20% as compared to a mirror with

a perfect figure. The shorter wavelength also reduces the allowable thermal distortion the mirror can have and hence its energy handling capability. In the example given the allowable energy density at a wavelength of 3.8 um is less than one-third that at 10.6 urn. In figure 5 the energy density threshold

of well polished molybdenum mirrors having various initial figure errors is compared to the theoretical

performance of a hypothetical molybdenum mirror with no figure error coated with a very high reflectance coating. The importance of a high reflectance is evident. The reflectance is often strongly

influenced by the polishing technique used; polished molybdenum mirrors with twice the absorption

assumed here for the uncoated mirror are not uncommon. The distortion threshold for these mirrors

would be lower than that shown in figure 5 by a factor of two.

The increasing importance of a good optical figure with decreasing wavelength is also illustrated

in figure 5. At 10.6 um there is relatively little difference between the energy density threshold for

a perfectly figured mirror and one with eighth and quarter-wave figure errors. However, as the wavelength decreases these differences become pronounced and at 3.8 ym the Marechal criterion cannot be met

at any energy density if the figure error is as large as a quarter-wave.

The effectiveness of substrate cooling between pulses and hence the allowable energy densities for multipulse operation are influenced by substrate conductivity and thickness. However, the energy density threshold for a single pulse is not a function of the thermal conductivity of the mirror until

pulse lengths approach the tenth microsecond range and the threshold is determined by catastrophic

damage to the mirror rather than by optical distortion. The maximum energy density which the mirror

can pass without being damaged can be obtained from eq. (23) by letting z = 0, giving

-19-
(33)

where Tg is the initial temperature of the mirror and Tj^ its melting temperature. The heat of fusion

does not appear in eq. (33) since the heat taken up by melting depends on the mass of material melted,

which at threshold is very small [43]. As before we are assuming that temporally the pulse is flat- topped and of duration t. More complicated pulse shapes would require that the pulse intensity be

integrated over its duration.

For a pulse length of 100 ns the catastrophic damage threshold for a diamond turned copper mirror

would be calculated from figure 6 to be about 110 J/cm2. Flux densities of optical beams are often

averaged spatially over the beam diameter so care must be taken to use peak values rather than average

values when computing thresholds using eq. (33). For a Gaussian spatial beam profile the peak energy

density is twice the average energy density over the l/e^ diameter of the beam, the quantity usually

specified as the beam energy density. For example, the catastrophic damage threshold for a diamond

turned copper mirror in terms of conventional beam densities for a Gaussian beam profile would then be

about 55 J/cm2, a value in good agreement with experimental results for diamond turned copper mirrors.

Results of several other threshold studies of laser damage in metals [43] are also in reasonable agreement with eq. (33).

Figure 6 gives a comparison of threshold values [44] for uncoated and coated copper mirrors prepared

using various surface finishing techniques with the theoretical value predicted by eq. (33). The experimental values are a factor of two higher than those initially reported by Soileau and Wang because of

a calibration error which was discovered after the paper was published [42]. Several points should be

emphasized. First the maximum experimentally measured threshold value for an uncoated copper sample is within 6% of the theoretical value. Second, if a different preparation technique is used, damage can occur at one-fourth the theoretical threshold level. Finally, the coated copper substrates tested had

a lower damage threshold than the best uncoated samples even though the absorption of the coated samples

was lower than that of the uncoated samples by about a factor of two. The best sputtered coating was

higher than the best evaporated coating, possibly because of improved adhesion in the sputtered film.

Significant gains in damage threshold resulting from improved coating techniques may thus be possible.

Figure 7 shows the calculated threshold values of peak energy density for uncoated Cu, Mo, and Al mirrors in a 10-component system at a wavelength of 10.6 ym as a function of pulse length. Eighth wave

optical figures are assumed for each mirror and reflectances are assumed to be 0.9918 for copper

(achievable for good diamond turned surfaces), 0.9814 for aluminum, and 0.9814 for mechanically polished

molybdenum. At pulse lengths longer than 10 to 40 ns the threshold value is determined by optical

distortion and is over four times as high for molybdenum and copper as it is for aluminum mirrors. For shorter pulse lengths it is determined by the catastrophic damage threshold for the mirrors. Copper

would be expected to be a better choice than molybdenum in this region since its higher thermal conductivity and higher reflectance make its damage threshold higher than that of molybdenum by nearly a factor of four.

Conclusion

In multicomponent optical trains handling intense beams of light the limiting parameter is fre- quently the optical figure. Both thermal distortion, initial figure error, and optical homogeneity must

be considered. If there are many components in the optical train, initial figure error tolerances are typically a fraction of a visible wavelength even for optical trains used at infrared wavelengths. For materials with low indices of refraction such as the alkali halides and alkaline earth fluorides, tolerances can be significantly lower than for mirrors and high index window materials such as ZnSe. Although the initial figure errors and homogeneity errors are usually uncorrelated with each other and with thermal distortion, figure errors caused by thermal distortion typically are correlated. The

allowed thermal distortion tolerances are thus lower than if the thermally induced figure errors were

uncorrelated. An exception is when they are of opposite sign, as is the case when mirrors are used

with windows such as ZnSe which have positive values of dn/dT. On the other hand, window materials with

negative dn/dT can develop resonances in some wavelength regions which should practically eliminate

their contribution to thermal distortion.

Calculations suggest that selected window materials can handle much higher single pulse energy

densities than can mirrors. On the other hand, metal mirrors are much easier to cool than are windows,

an important consideration for multiple pulsed operation. To satisfy the Marechal criterion in a multicomponent system used in the 3-5 ym wavelength region, optical figures for mirrors may have to be

as good as an eighth wave in the visible. In the ten ym wavelength region tolerances can be

relaxed significantly. Coated molybdenum is clearly the best choice of the mirror materials considered

here for use for pulse lengths in the microsecond range or longer. Uncoated molybdenum or copper will

have a significantly lower pulsed energy density threshold than coated samples provided the catastrophic

damage threshold of the coating is high enough to avoid limiting mirror operation. Uncoated mirrors

are probably the best choice at shorter pulse lengths where catastrophic damage becomes important. At

these pulse lengths copper mirrors should perform better than mirrors of molybdenum or aluminum.

-20-
Acknowledgments

The author would like to thank Dr. Jean M. Bennett, W. R. Compton, R. G. Purcell and Dr. J. B. Fatten of Michelson Laboratory, NWC; Dr. T. G. Bergman, presently at the Navy High Energy Laser Office,

Washington, D. C; and Major J. L. Stapp and Lt. Col. R. F. Prater of the Air Force Weapons Laboratory,

Albuquerque, New Mexico, for helpful discussions.

References

[1] Sparks, M. , J. Appl. Phys. 42, 5029 (1971);

Sparks, M. and Chow, H. C, J. Appl. Phys.

45, 1510 (1974).

[2] Conrady, A. E. , Applied Optics and Optical

Design (Dover Publications, Inc., New York,

1960), pp. 139-141 and 627.

[3] Born, M. and Wolf, E. , Principles of Optics,

1st ed. (Pergamon Press, New York, 1959),

p. 468.

[4] Loomis, J. , in Proceedings of Fourth Annual

Conference on Infrared Laser Window Materials, 1974. C. R. Andrews and C. L. Strecker, eds. (Defense Advanced Research

Projects Agency, Arlington, Va. , 1975),

pp. 281-297.

[5] Lowenthal, D. D. , Appl. Opt. 13, 2126 (1974).

[6] Winsor, H. V. , in Proceedings of Third

Conference on High Power Infrared Laser

Window Materials, 1972. C. A. Pitha and

B. Bendow, eds. (AFCRL-TR-74-0085 , Special

Report 174, 1974), pp. 1069-1082.

[7] Bennett, H. E. and P. C. Archibald, in Laser Induced Damage in Optical Materials:

1974, A. J. Glass and A. H. Guenther, eds.

(NBS Spec. Publ. 414, 1974), pp. 23-29.

[8] Dueweke, P. W. , Kester, J. M. , McBroom, R. J., and Preonas, D. D., in Proceedings of Fourth

Annual Conference on Infrared Laser Window

Materials, op. cit., pp. 789-802.

[9] Boley, B. A. and Weiner, J. H. , Theory of

Thermal Stresses (John Wiley and Sons, Inc., New York, 1960), p. 290.

[10] ibid, p. 402.

[11] Carslaw, H. S. and Jaeger, J. C, Conduction

of Heat in Solids (Oxford University Press,

London, 1959), p. 75.

[12] Timoshenko, S. and Woinowsky-Krieger , S., Theory of Plates and Shells, 2nd ed. (McGrawHill Book Co., New York, 1959), p. 43.

[13] House, R. A., Bettis, J. R. , Guenther, A. H. and Austin, R. , Laser Induced Damage in Optical Materials: 1975, A. J. Glass and

A. H. Guenther, eds. (NBS Spec. Publ. 435,

1976), pp. 305-319.

[14] Austin, R. R. , in Proceedings of Fourth Annual

Conference on Laser Window Materials, op. cit.,

pp. 97-98.

[15] Braunstein, M. and Rudisill, J. E., op. cit.,

pp. 23-39.

[16] Soileau, M. J., Bennett, H. E. , Bethke, J. W.,

and Shaffer, J., in Laser Induced Damage in

Optical Materials: 1975, op. cit., pp. 20-28.-21-

[17] Shaffer, J. J., Martin, E. C, Soileau, M. J., and Bennett, H. E., in High Energy Laser

Mirrors and Windows (Semi-Annual Report No. 4, ARPA Order 2175, Naval Weapons Center, March

1974), pp. 103-110.

[18] Hass, M. , Davisson, J. W. , Rosenstock, H. B., and Babiskin, J., in Proceedings of Fourth

Annual Conference on Laser Window Materials,

op. cit., pp. 231-239.

[19] Hass, M. , Davisson, J. W. , Klein, P. H., and

Boyer, L. L., J. Appl. Phys. 45, 3959 (1974).

[20] Deutsch, T. F. , Appl. Phys. Letters 25, 109

(1974) .

[21] Stierwalt, D. L. and Hass, M. , in Proceedings

of the Fourth Annual Conference on Laser

Window Materials, op. cit., pp. 8-11.

[22] Duthler, C. J. and Sparks, M. , in Laser Induced Damage in Optical Materials: 1974,

op. cit., pp. 219-226.

[23] Deutsch, T. F. , in Proceedings of Third Con- ference on High Power Infrared Laser Window

Materials, op. cit., pp. 13-29; P. Kraa^z and

P. J. Mendoza, Proceedings of Fourth Annual

Conference on Infrared Laser Window Materials

,

op. cit., pp. 213-230.

[24] Newberg, R. and Pappis, J., in Proceedings of

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), pp. 1065-1078.

[25] Larkin, J., Klausutis, N. , Hilton, R. and Adamski, J., in Proceedings of Fifth Annual

Conference on Infrared Laser Window Materials

,

op. cit., pp. 1079-1085.

[26] Harrington, J. A., Gregory, D. A. and Otto,

W. F. , in Proceedings of Fifth Annual Con- ference on Infrared Laser Window Materials,

op cit. , pp. 871-886.

[27] Miles, P., in Proceedings of Fifth Annual

Conference on Infrared Laser Window

Materials, op. cit., pp. 7-15.

[28] Dickinson, S. K. , "Infrared Laser Window

Materials Property Data for ZnSe, KCl, NaCl,

CaF2, SrF2, BaF2." Air Force Cambridge

Research Laboratories AFCRL-TR-75-0318

,

Physical Sciences Research Papers, Ho. 635

(1975) ; C. S. Sahagian and C. A. Pitha,

"Compendium on High Power Infrared Laser Window Materials." Air Force Cambridge

Research Laboratories AFCRL-72-0170, Special

Reports, No. 135 (1972).

[29] Hass, M. , in Proceedings of Fifth Annual

Conference on Infrared Laser Window

Materials, op. cit., pp. 849-858.
[30] Skolnik, L. H, , Bendow, B., and Gianlno,

P. D., in Proceedings of Third Conference

on High Power Infrared Laser Window Materials, op. cit., pp. 967-1009.

[31] Miles, P., Raytheon Corporation (private

communication)

[32] Flannery, M. and Marburger, J., in Laser

Induced Damage in Optical Materials: 1974,

op. cit,, pp. 31-38.

[33] Loomis, J. S. and Bernal G., E. , in Laser Induced Damage in Optical Materials: 1975,

op. cit. , pp. 126-141.

[34] Sparks, M. and Duthler, C. J., J. Appl.

Phys. 44, 3038 (1973).

[35] Crisp, M. D. , Boling, N. L. , and Dube, G.

Appl. Phys. Letters 21, 364 (1972);

Crisp, M. D., in Laser Induced Damage in

Optical Materials: 1973, op. cit., pp. SOBS.

[36] Boling, N. L. , Dube, G., and Crisp, M. D.

in Laser Induced Damage in Optical Materials: 1973, op. cit., pp. 69-79.

[37] Braunstein, M. , in Proceedings of Third

Conference on High Power Infrared Laser

Window Materials, op. cit., pp. 863-890.

[38] Soileau, M. J., Bennett, H. E., Porteus,

J. 0., Temple, P. A., and Bass, M. , in Proceedings of Fifth Annual Conference on Infrared Laser Window Materials, op. cit.,

pp. 391-417.

[39] Sparks, M. , in Laser Induced Damage in

Optical Materials: 1975, op. cit., pp. 331-

345.

[40] Wang, V., Rudisill, J. E., Giuliano, C. R.

Braunstein, M. , and Braunstein, A., in Laser Induced Damage in Optical Materials:

1974, op. cit., pp. 59-65.

[41] Shatas, R. A., Smith, J. L. , Tanton, G. A., and Meyer, H. C, in Proceedings of Fourth

Annual Conference on Infrared Laser Window

Materials, op. cit., pp. 53-62.

[42] Leung, K. M. , Tang, C. C, Bass, M. , and

Soileau, M. J., "Damage Resistant Optical

Components for 2-5 um Laser Systems," this

conference.

[43] Sparks, M., J. Appl. Phys. 47, 837 (1976).

[44] Soileau, M. J. and Wang, V., Appl. Opt. 13, 1286 (1974).

Figures

1 1 1 1 1 1 \ ^

CaFj AND SrFj ^

1 1 1

,C0

1 1

ZnSe AND MIRRORS

COj ,

1 , , < 1 WAVELENGTH liimi

Figure 1, Initial optical figure required for each

component in a ten component optical

train when the thermal contribution to wave front distortion is twice the con- tribution from the initial figure error

and the Mare'chal criterion is met.

-22-
WAVENUMBER. cin~ 2.000

1

3,000

WAVELENGTH, itm Figure 2. Best reported surface and bulk absorption values for various

laser window materials. The circles, squares, and triangles

are bulk absorption values and the numerical values of Ag

are surface absorption values. Values of Ag at DF and HF wavelengths are for SrFa. All values for ZnSe were reported

by M'les [23],

Figure 3. Calculated distortion threshold for a 2-cm

thick window of CaF2, SrFj, KCl, and ZnSe

as one component of a ten component optical

train. Assumptions are an equal energy density budget per component and an initial

figure error of a quarter-wave in the visible, and a resultant wave front distortion which does not violate the

Ifere'chal criterion.

i

10''

a

> 10-

SrF, ,^j2X

/

"FORGED KCl FRACTURE

ZoSe_ _ _

WAVELENGTH, Mm

6 8 WAVELENGTH i^m)

Figure 4. Calculated distortion thresholds for SiC,

Mo, Cu, and Al mirrors with a 0.998 reflec- tance coating in a ten component optical

system. Assumptions are an equal energy density budget per component, an Initial

figure error of an eighth-wave in the visible, and a resultant wave front distortion which does not violate the Mare'chal criterion.

-23-
COATED

X/" R= 0.998

UNCOATED COPPER

68 J/cm^

DIAMOND TURNED 206 llcn?

POLISHED DISPERSION HARDENED

270 JW '.

254 J/cm^

SILVER COATED COPPER

EVAPORATED 126 J/cm^

206 J/cm^

4 6 8 10 WAVELENGTH (pm)

Figure 5. Calculated distortion threshold for polished molybdenum mirrors having

various initial figure errors compared

to the theoretical performance of a hypothetical molybdenum mirror with no figure

error and coated with a 0.998 reflectance

coating. Results are for a ten-component

system, equal error budget per component,

and a resultant wave front distortion

which does not violate the Mare'chal

criterion.

Figure 6, Damage threshold of copper for 10 o 6 ym

radiation, 600 ns pulse lengtho The theoretical value was calculated from

eq. (33), the experimental results are taken from [44].

Figure 7. Calculated threshold values of peak

energy density for uncoated Cu, Mo, and Al mirrors in a ten-component system at

a wavelength of 10.6 ym as a function of pulse length. Eighth-wave optical figures

are assumed. At pulse lengths longer than

10 to 40 ns the threshold value is de- termined by optical distortion as in previous figures. At shorter pulse

lengths it is determined by the catastrophic damage threshold for the mirrors.

n—I—I I I I 1

1—I I

I M M

_i I I III III! 1 I I I I I M I 10"° 10- PULSE LENGTH (j)

COMMEWTS OW PAPER EEMNETT

The. amtyi.vi, pKUzntzd hzxz -U a.dmitte.dly pzM^titLc. In a {>pzcJ.{^lc iyitm, -it -ci QnnnfiaJitij

poii-ibZe. to impnovt th^ pM.^omancz above, the. leveLi indicated by thJj, anatyii^, by oie o^ Kz^ocuJiing,

adaptive, optical coKAzction, and otheA ipecif,ic me.aiuAe.i. Atio, in the treatment Oj^ theAmaZ diiton.- tion ofi miAHofUi, heat conduction into the minAon. waA neglected, while ^OK palieA longeA than 10 mi,

conduction cooling can be an impofitant {^actoH.. The goal heAe MOi to obtain an analytic tool, capable

od pH.ovidU.ng a bai>ij> {^oK choice among di^^eAent mateAAjoJU> {^oA uie in miAAoHM oA windom, and ^OA eAtimatlng ia^e levels o^ opeAotion.

A gKeat deal of^ inteACit wai expieA>i,ed in the uie SiC ai a mihAOK iubifmtz mateAMxl. Thz

ipeake/L indicated that SiC iompleA teAted at the MicheLion labonato)yieM wcAe among the be.it miAAon. iuA^aceA they had cveA ieen, vxith nmi iuA{^ace KoughneAi o{^ about 5 A. The mateAial uxu pftepoAcd by

Vk. Choyke at the CentAot RcicoAch LaboiatonteA o{, Wcitinghouie. It Zi machinable in the uncuAed

ffOAm, and thui iA pnepoAed to an appn.oxA.mate {jiguAe. It ii then cuAed, and a iuA{^ace o^ SiC -c6 de- poiited on it by chemical vapoK depoiition. The ^inal iuA^ace ij> then polished to the dziiAcd ^iguAe.

The potential (^ot extending thii approach to the (^abntcation o(^ loAge miAAofii appeoAA to be itAoight-

{^omoAd,

-24-
1.2 ELECTRIC FIELDS AND POWER DISSIPATION IN WINDOWS

AT OBLIQUE INCIDENCE

Joseph H. Apfel

Optical Coating Laboratory, Inc.

Santa Rosa, California 95402

Formulas are provided for calculating the strength of the

electric fields and the power dissipation due to weak

absorption in a window illuminated by a collimated beam at

non-normal incidence. For a glass window the power

dissipation per unit volume varies approximately as the

cosine of the incident angle.

Key words: Electric fields; laser damage; oblique

incidence; standing waves

Introduction

The power of a collimated beam of radiation which is intercepted by a flat sample

surface varies as the cosine of the incident angle. One intuitively expects, therefore, that the electric field and the volumetric power dissipation in the window due

to weak absorption will vary similarly. Exact formulas are derived and shown to

predict the angular dependence which is only slightly different from cosine Oq.

Theory

A collimated plane polarized monochromatic beam of intensity Mo measured in

W/cm^ propagating in a non-absorbing media of index no is composed of sinusoidally

varying electric and magnetic fields. The peak amplitude of the electric field

measured in V/cm and the intensity are related bv the formula

Mo = .0013 noEo^ (1)

and the time average square of the electric field strength is

i7~^ = 5? Eo^ (2)

If a sample of surface area A is placed within the beam so that its normal makes an angle Go with the beam axis, the total power intercepted by the sample is Mo A cos 60 (watts) . This energy will be reflected or transmitted through the sample (assuming no absorption)

.

Assuming there is no reflection from the second surface of the sample, the

electric field within the sample of index n is given by [l]

n E^ cose = T no Eo^ cos Go (3)

where T is the conventional transmittance of the surface and 6 is the propagation

angle in the sample.

If there is absorption within the sample the volumetric power dissipation is

P = C n a E^ (W/cm^ ) , (4)

where C = .0027 for field strength in units of V/cm, and a = 4Trk/X is the

absorption coefficient (cm M. Assuming that the absorption is very weak, the

attenuation of the electric field within the sample can be ignored. Combining all the

1. Figures in brackets indicate the literature references at the end of this paper.

-25-
numbered equations above, we have

P n ^

which describes the volumetric power dissipation as a function of incidence angle,

Equation (5) merely states that power dissipation is proportional to the local

intensity

The transmittance T of a single interface is given by

4nno

where the indices must be replaced by effective indices at non-normal incidence.

Combining (6) with Snell's Law we have, for the two polarizations with no = 1.0

cos6o 4n'cos^eo

p cos 9 ~

fn^ 0050 0+/—5 :

Jq 12

(7) cos 0 0 4n cos'

S COS0

~ fcosB 0 + —2 n 1 2

which, from equation (5) are seen to equal the ratio of beam intensities within and

incident on the sample.

Numerical example

Figures 1 and 2 are profiles of the time average square of the electric field

along the normal to a glass (n = 1.51) surface as a function of incidence angle for

and s polarizations. From equation (5) we expect the volumetric power dissipation

within the glass due to weak absorption to vary as the height of the field profiles

in the glass multiplied by the index ratio (1.51).

Figure 3 is a graph of formulas (7) for n = 1.51 and no = 1.0 which shows how

nearly the volumetric power dissipation follows a cosine dependence (broken curve)

Conclusion

The electric field strength and the intensity of a refracted beam in a transparent medium vary with incidence angle according to stated formulas. For glass

windows the angular dependence can be approximated by the cosine 0o curve.

Reference

[ij J.H. Apfel, "Electric Fields in

Multilayers at Oblique Incidence,"

to be published in Applied Optics,

October 1976.

-26-
Figures

P POLARIZATION

60'

Figure 1. Time averaged square of the electric

field strength for p radiation Incident

from the left on a surface of glass

(n = 1,51). The abscissa Is metric distance along a normal to the surface.

30*

15'

Figure 2, Same as figure 1 for s polarized

radiation.

-27-
COMMENTS ON PAPER BV APFEL

Jt pointzd out that thz angatoK d&ptndznc^ o{j the. damagz thAuhotd, cu, dU>cuJ>{>zd in tkU papM.,

maij be consequence in thz anaiyiij, oi5 iuA^acz damage. txpzHAjmnntA uAing 6ho>vt docal tzngth £en4e4,

wfeeAe a KJOA^ety 0(J z^^zctivz angles ojj incldmcz aAz pneMznt.
1.3 ANALYSIS OF LASER-INDUCED THERMAL STRESSES IN LASER WINDOWS

Roger E. Rondeau

Air Force Materials Laboratory

Wright-Patterson AFB, Ohio 45433

and

Dale Ford

University of Dayton Research Institute

Dayton, Ohio 45469

Laser induced damage in the ZnSe output coupler of a 15 kilowatt

cw CO2 coaxial electric discharge laser has been investigated. An

analytical technique has been developed for predicting thermal

stresses from in depth absorption of laser energy in laser window

materials. A two-dimensional transient heat transfer code, APPLE,

(Aerotherm Prediction Procedure for Laser Effects) was used to

obtain the temperature histories of the irradiated window. The

effect of window thickness, transmitted DOwer level, absorptance

and exposure time were investigated. The stress distribution in

the circular disk was computed from radial and axial temperature

distributions obtained from the APPLE code. This analytical technique has been used to determine maximum power levels and exposure

times to prevent window damage in laser experiments.

Key words: Computer analysis; laser induced stresses; stress

analysis; thermal analysis; zinc selenide window.

1. Introduction

In the fall of 1975, the Air Force Materials Laboratory realized its goal of con- structing a 10 kilowatt laser facility capable of producing a uniform intensity beam.

VJith consultant support from the Air Force Weapons Laboratory, the "flat-top" device

was modeled after the AFWL Electric Discharge Coaxial Laser (EDCL) system which has

recently been described [l]'.. Both of the above systems are continuous wave carbon diox

ide lasers which emit a circular, uniform, temporally stable, and easily characterized

beam. Both systems are equipped with a zinc selenide output coupler - a solid, transparent window at the muzzle of the laser. To maintain the inherently high power capacity of these optimized gas laser systems and to insure a high quality output beam the

window must not only be transparent at 10.6 ym, but must absorb very little energy

at the thin film coatings. Excessive absorption of radiant energy within the window

leads to degraded beam quality, optics misalignment, and if the absorption is high

enough, to catastrophic thermally-induced fracture. For these reasons, considerable

time and effort have gone into the development of laser window materials [2,3]. Thus

far, zinc selenide fabricated by chemical vapor deposition has emerged as one of the

leading materials for this application and is used as the output coupler in both of

the above laser devices

.

On the 25th of March 1976, an AFML output coupler developed cracks and failed.

Although the cause of the failure is not known for sure, we believe moisture attacked

the multilayered thin film coatings of thorium tetraf luoride on zinc selenide causing

an increase in absorption and introducing temperature gradients. The resultant,

nonuniform temperature distribution produces stresses which, if large enough, can lead

to mechanical failure of the window.

Replacement windows have recently been obtained, which from witness plate measurements, exhibit total absorptances of less than 0.8% [4,5]. These new windows are also

much thicker than the old ones. The use of analytical techniques for computing in

depth temperature profiles and stress distributions in both the old and new laser windows is the subject of this paper.

This study was undertaken in order to find answers to the following questions:

How hot did the old window get and how large were the induced stresses? How much heat

and stress are generated in the new window? How long a cool-down time is required

between shots? What is the effect of thickness on window performance? Assuming that

with time, some degradation will occur within the window, how much of an increase in

total absorptance can be tolerated before the fracture limit of the window is reached?

The answers to these questions will not only shed light on revealing a failure

mechanism for the old window, but also provide valuable information that can be used to

prevent recurrence of wi.ndow failure. Moreover, parametric analyses can be used to

project approaching damage levels and to monitor window performance.

1. Figures in brackets indicate the literature references at the end of this paper.

-29-
2. Analytical Approach

The codes used in this analysis are shown in figure 1. The coupled codes, ACE and

ASTHMA, are part of a family of codes called APPLE ( Aerotherm ' s Prediction Procedure

for Laser Effects) , which treats the transient response of a solid subjected to radiative and convective environments [6]. APPLE evolved from a DoD-sponsored program for

developing a unified analysis of laser-heated metals and nonmetals. The procedure is

based on well-verified aerospace codes originally written for reeetry ablation studies

and extensively modified for radiant heating at flux levels of current laser sources.

Some other features and capabilities of the APPLE codes include: arbitrary axisymmetric

incident radiant flux; one- or two-dimensional solid heat conduction; temperature dependent thermophysical properties; in-depth charring and pyrolysis; multilayer, multimaterial capability; coupling to aerodynamic environment; and in-depth absorption of

radiant energy. This last feature is based on a linear absorption coefficient and has

been included in the ASTHMA code (Axi-Symmetric Transient Heating and Material Ablation) . Different absorption coefficients can be used at different depths extending the

computational capability to multilayered , partially transparent materials.

The ACE code (Aerotherm Chemical Equilibrium) is a general thermochemistry code

which performs a set of calculations of surface mass balances to determine a parametric

relationship between surface temperature, pressure and other information needed to perform an energy balance on the surface. Thus ACE output is in the form of a series of

tables which include all the surface mass transfer and chemical relationships. Computation of surface temperatures requires the ASTHMA solution of the surface energy

balance. For transient heating response, the surface energy balance is intimately

coupled to the in-depth material response and is solved simultaneously with the in-depth

code. Besides providing ablation, recession, and surface energy flux data, ASTHMA also

furnishes the data needed for the thermal analysis; that is, surface and nodal tempera-^

tures as a function of time and position.

The next step in the analysis requires using the output from ASTHMA as input for

DISC [7]. DISC treats axisymmetric heating of thin plates and computes the stress distribution in a circular disk exposed to a radial and axial temperature distribution.

Using plane stresses and the assumption that radial temperature gradients are much

greater than the axial gradients, the code computes radial and axial displacements as well as radial and circumferential stresses as a function of distance from the center.

3. Analytical Modeling

The model used in the analysis is shown in figure 2. A flat-top beam with 8.9 cm diameter enters the 11.4 cm wide window from the cavity side where, for the new window,

71% of the energy is reflected. Since the window is wider than the beam, two radiation

tables were needed, one having zero flux for the surface of the window outside the beam

Three absorption coefficients were used: for the reflective coating, the bulk material

and the anti-reflective coating at the exit face. The old (fractured) window absorption coefficients used were front to back: 5.239 , .0075, and .295 cm while the respec-j

tive new window values were .012, .0026, and .088 cm [8]. Although model thicknesses

of 0.0254 cm were held constant for the two surfaces, the overall window thickness was varied systematically. Other parameters varied include the transmitted power (5-15

kilowatts) radiation time (0-20 seconds) and total absorptance (0.5"^). Window thickness was changed from 0.635 to 2.1 cm in six steps. It should be noted that using

more realistically thin surface nodes did not change the computed surface temperatures

and served only to drastically increase the computational time.

The first step in the analysis involves a closed system, mass balance calculation

of the laser gas mixture of 13.5% nitrogen, 4.5% carbon dioxide and 82% helium (by

volume) on the cavity side of the window. This solution was obtained from the ACE code.

ACE also computed the cavity wall enthalpies for surface temperatures ranging from 25°

to 800°C and generated input tables for ASTHMA which relate the surface temperatures

to the rate of energy conduction into the material. Free convection heat transfer

coefficients were calculated for the cavity and output sides from Kreith [9]. The

ASTHMA option used in this analysis couples the film coefficient boundary condition to

the in-depth solution.

For specifying sample geometry, a 7X13 nodal scheme was drawn, sectioning one half]

of the axisymmetric flat plate. Thin nodes were used at the front and rear of the win-J

dow for better thermal definition at the surfaces. For the same reason, narrow nodes were employed at the beam edge where the temperature gradients were expected to be

large. The nodal temperatures generated by ASTHMA were used as input data for the DISC]

code. However, since the latter code calls for a 3X6 nodal temperature grid, six temperatures were taken from each of the first, fourth, and seventh row of nodes. In addi-j

tion to the temperature input, the mechanical properties of CVD (chemical vapor deposition) zinc selenide were taken from [2] for use in the stress analysis code.

-30-
4. Results

I The maximum temperature rise computed by ASTHMA for the damaged coupler is plotted

as a function of time for three transmitted power levels in figure 3. In operating the

laser facility, the majority of tests are in the 5 to 10 kW power level range for

i 5 to 10 seconds. The shaded area covers this range and shows a temperature rise in the

window of approximately 40° to 150° C. Figure 4 illustrates the nodal network and the temperature distribution across the

window at the 10 kilowatt, 10 second point of figure 3. The thermal profile was obtained by writing the 91 (7X13) nodal temperatures into their respective nodes and

connecting isothermal points. Near the center of the beam, the axial temperature graI dient is approximately 80°C. Radiating out from the center toward the edge, the tem-

:! perature drops 155° with most of the drop occurring near the edge of the beam. This

1 thermal gradient at the window surface is shown in figure 5

.

I A stress analysis was first performed on the damaged coupler using 18 of the temperature points from figure 4. For each DISC run, stresses and displacements were

I computed for 50 locations in the window: 10 radial points on each of 5 axial planes.

J Radial stresses were compressive and relatively small. On the other hand, the tangeni tial stresses were compressive only near the center and became increasingly tensile

I toward the edge of the window, reaching a maximum near the edge of the beam. Maximum

! stresses were computed at the surface plane, Z=0 , where the greatest thermal load is

I imposed. Thus, in order to obtain only the largest induced stresses, the analyses were

j performed only for the thin plane section at the surface with no axial gradients specij fied. Radial stresses at the outer radial boundary were also assumed to be zero. The

|| effect of these assumptions and simplifications will be evaluated with a much more sophisticated analysis procedure called DOASIS, a computer code for the Deformation

plastic, Orthotropic, Axisymmetric (and plane) Solution of I^nelastic Solids [lO].

As noted above, the largest computed stresses are tangential tensile stresses

which occur near the edge of the beam - and these are the stresses (in megapascals)

plotted in figure 6 as a function of time and for three output power levels. The

' superimposed box shows the ultimate tensile strength of the zinc selenide window with

I the percentages indicating the cumulative fraction of samples failing at that imposed

I' stress [2]. Based on this data, the analysis predicts that during operation at the

l! 10 kW output power level, the probability of fracture increases to 50% within 8 seconds.

It is interesting to note that window fracture did occur while operating under conditions very close to those just mentioned. Another important piece of information in

, the stress analysis plot is the 100 kilojoule curve drawn as a dotted line. Setting an operational laser output energy limit of say, 100 kilojoules, can be misleading because

1 of the power dependence shown here. Operation at 15 kilowatts for 5 seconds, for examI pie, while well below the 100 kilojoule limit is also well above the 95% failure level.

; Thermal and stress analyses were also performed on the new window and the results

i compared with those of the damaged window analysis. Assuming laser beam transmission

i' at a 10 kilowatt power level, the maximum temperature rise and stress at times between

1; 0-20 seconds were computed to be approximately only 3% of the old window values. For

I example, after 10 seconds the greatest temperature increase and tensile stress were

5.6°C and 1.7 megapascals for the new output coupler as opposed to 169° and 50.8 MPa

I for the old window.

In addition to the large optical difference, there also exists a significant physi-

,! cal difference between the two laser windows - the new window is significantly thicker.

Since thickness is a relatively easily controllable material parameter, its effect on window performance deserves attention. A parametric study was made using the high

i absorption data of the damaged window, whereby material thickness was varied from

\ .635 cm to 2.05 cm while all other material parameters and properties were left

unchanged. The results, in terms of time rates of temperature rise during 10 kilowatt

operation, are shown in figure 7. Similar studies were made at 5 and 15 kilowatts,

again indicating a heat sink effect in the windows — the thicker the window the more

heat it can dissipate. At the 10 kilowatt/10 second point the maximum temperature

] generated in the thinnest window is 32% higher than in the thickest window. The difj ferences increase with time and power. Incidentally, whereas the damaged coupler was

1 1.05 cm thick, the new window is nearly twice as thick at 2.05 cm. Therefore, neglecting any of the dramatic optical differences, the new window should be capable of

handling appreciably more heat simply on the basis of its greater bulk.

I Recognizing the fact that laser windows degrade with time, another parametric analysis was performed to study performance as a function of total absorptance. Although

the total absorptance of the new 'Output coupler is now only 0.8%, there is no doubt that

this value will eventually increase. Figure 8 is a plot of the increase in window tem-

-31-
perature as a function of total absorptance. Assuming that degradation is most likely

to occur at the reflective surface — as it did in the case of the damaged window - the

increase in absorptance is assumed to be due entirely to the increase taking place at

that surface. The arrow points to the current absorptance of the window. Based on data from the thermal analysis, the parameterization study was extended to a stress

analysis — and this is shown in figure 9. Again using the 10 kilowatt/10 second point

of reference, this study shows that even after more than a six-fold increase in window

absorptance, the maximum stress induced is approximately 19 megapascals which is still

well below the point where only 1% of the samples would be expected to fail.

5. Conclusion

The analytical procedure described above represents a convenient method of assessing damage in partially transparent laser materials and of monitoring the performance

of an installed laser window. Used in the latter sense, periodic measurements of a window's absorptance and checks against the predicted stresses may reveal progressive

or incipient damage. Thus, through the combination of thermal and stress analyses,

sufficient information is provided to predict damage thresholds, guide laser experiments, and guard against unexpected window failure.

6 . Acknowledgments

The authors are indebted to Edmund Rolinski (AFML) for encouraging and promoting

this study, to Kurt Suchsland (Aerotherm/Acurex) for his help in the use of the ASTH^4A

code and to Henry Tong (Aerotherm/Acurex) for his help in using the DISC code.

[1] Carlson, R. Lab. TR-76-

L., Air Force Weapons

115 , June (1975)

References

[6] Tong, H Lab. TR'

, et . al

-73-190 ,

, , Air Force Materials

June (1975).

[2] Wurst, J. C. and Graham, T. P. Force Materials Lab. TR-75-28,

April (1975)

Air [7] The DISC code was obtained from the

Aerotherm Division of the Acurex

Corp

[3] Dickinson, S. K., Air Force Cambridge

Research Lab. TR-75-0318, June (1975),

[8] The witness plate optical measurements were performed by U. D.R.I.

[4] The optical data for the damaged

window was obtained from University

of Dayton Research Institute.

[5] The new window was purchased from

II-VI, Inc., 207 Garden Lane,

Glenshaw, PA 15116.

[9] Kreith, F., "Principles of Heat

Transfer," International Textbook

Co. , August (1960) .

[10] Weiler, F. C, Air Force Materials

Lab. TR-75-37, Vol I, October (1975)

8. Figures

CODES USED

i rr-ACE

ik

ASTHMA DISC

I I

! i Thermal Stress \ Analysis Analysis |

:

. Figure 1. Codes used and their coupling for

' the thermal and stress analyses.

-32-
LASER

CAVITY

Figure 2, Analytical model of the coated zinc TIME (secondsi selenide.

Figure 3. Plot of maximum temperature increase

in the irradiated window as a function

of time.

Figure A, Nodal grid and temperature profile

of the window at 10 kilowatts output power and 10 seconds.

SURF. TEMP. DISTRIBUTION

O SURrflCE NODES

= 5

CCS

Figure 5. Surface temperature distribution at

10 kilowatts and 10 seconds. 2.00 3.00 T

RHDIflL DISTANCE

-33-
-3A-
MRX. STRESS VS flBS a 10

ffl r= 5 s » r= 10 s + r=i5 s X T- 20 5

COmENTS ON PAPER BV RONVEAU AMV EORV

It Mcu, polvvto-d out bg J. laA. o{, AVCO-EvsAeXt that the. tAejxtme.nt 0(( thz vilndoiM (U a thin plutu -U not iu^^iicidnt. TfU-axAjit itfieMU mint be con^idM-e.d cu, mM. ch planoA itAUie^i. i'l-indom QdnoAntly

{^cuJL due to iheoA, Mhich axial itnej^i -ci the itAongeAt componznt. The ipmkeA nmoAkzd that although

the. Vise code, aMumed only planoA itAeMeA, the Keiulti oAe being c^iecfeeci mXh a mon.t anmpletz code. The question mu nxbiied oi to u)hy theAe. lolndom {,al£ed, whe,n uilndom of, ilmllaA design at the AFWL

had ifxithitood hlghex mdJjxtlv^ levels. The ipeakeA attxtbuted the dt^exmaz to wateA abiofibed inom

the humid otmoipheAz Into the ThV^ coating. QueAtlom weAe fuvUed about the veAy high value neponted

{^01 the, coating abionptton.

-35-
1.4 INTERFEROMETRIC MEASUREMENTS OF LASER HEATED WINDOWS

Enrique Bernal G. Honeywell Corporate Research Center

Bloomlngton, Minnesota 55420

and

John S. Loomis

University of Arizona

Tucson, Arizona 85721

The optical aberrations in infrared window materials irradiated with a CO2

laser have been measured at 0.6328 pm. The contributions of anisotropic

aberrations due to polarization dependence were examined and found to be small

for several materials of interest. Techniques for the semi-automatic digital

analysis of our interf erometric data will be discussed. The results will be

applied to f igure-of-merit characterization of window performance and calculation of far-field irradiance degradation.

Model calculations of the thermal profiles in irradiated windows and the resulting optical aberrations will be reviewed. Synthetic interf erograms

generated from these calculations were used to exhibit many of the experimentally observed features and to test the digital analysis techniques mentioned

earlier

Key words: Interf erometry; laser windows; optical materials; thermal distortion.

1. Introduction

Efforts to improve infrared transmitting materials have been directed for the most part toward de- | creasing the absorption coefficient and increasing the strength of the material. The success of these 1 efforts is measured by the steady decline in absorption coefficients to nearly intrinsic levels and |, substantial increases in the fracture strength of the alkali halides and fracture energy of alkaline [ earth fluorides. The very fact that measurement techniques were available contributed significantly to r the success of the development program. Without measurements of absorption and strength, progress would

have had to be defined by qualitative factors or measurements (such as infrared spectrophotometric trans- mission) which were relevant, but not sensitive enough.

|1' We believe that the measurement problem is most difficult in the area of optical testing, especially r' laser-induced optical distortion. One approach to the problem is to measure individual parameters which

contribute to thermal distortion and combine them with calculations of optical performance such as far- ^ field irradiance distributions. This is unsatisfactory because errors of the individual terms accumulate' when they are combined. Our approach is to use calculations to identify the important combinations of

^_ parameters which affect optical performance and then try to relate these in a meaningful way to optical [ measurements which can be made in a laboratory. By measuring one or two lumped parameters which ^ characterize optical performance using a laboratory laser, one can then predict how large components, r with the same parameters, will behave in a large laser system. I

Interf erometric methods are unique among the many techniques for characterizing infrared optical

properties because they directly measure wavefront distortion, and are more sensitive than measurements 1 of far-field profiles. However, the interpretation of interf erometric results is time consuming and may

become very complicated. Furthermore, making the measurements at the wavelength of interest is difficultf: because of the lack of suitable techniques for recording interf erograms in the infrared with high enough , spatial resolution.

j In this paper we address the problem of interpretation of interf erometric tests. We use a theoreti-

cal model of laser distortion developed earlier [l] to generate synthetic interf erograms . These can be |- compared with experimental results and are shown to demonstrate good qualitative agreement. We then |, discuss some of the details of interf erogram analysis procedures and illustrate some of the preliminary , results obtained.

This work is partially supported by Defense Advance Research Projects Agency under contract

//DAHC15-73-C-0464 and by Honeywell Independent Research and Development Funds.

1. Figures in brackets indicate the literature references at the end of this paper. -36-
2. Synthetic Interf erograms

Generation of synthetic interf erograms based on an analytic model of window distortion is a useful

tool for exploring the effects observed in the interf erometry of laser-heated optical materials. Such

a model provides a common basis for comparison of the response of various materials to laser irradiation

as a function of combinations of their physical properties. Once the parameters involved have been

obtained for laboratory lasers with stnall beams and relatively low powers, the model can be used directly

to predict the performance of these materials at large apertures and high powers. Synthetic interf erograms are also useful in previewing the effects of experimental parameters such as beam shapes, interferometer bias pattern, and sample geometry. Finally, synthetic interf erograms can be used to verify

that data reduction procedures yield correct results.

Generation of a synthetic interf erogram can be divided into three separate processes. First, the temperature distribution produced by the absorption of energy from the laser must be calculated. Second,

expressions for the optical path differences (OPD) as a function of temperature and thermally-induced

stress must be found. Third, the intensity variations in the interf erogram must be calculated and displayed photographically. We shall describe each of these processes in turn.

The window is assumed to be in the form of a disc heated by a laser with only radial variations in intensity referred to the center of the window and uniform absorption of energy in the axial direction.

The temperature is then only a function of the r, z cylindrical spatial coordinates and time and is obtained by solving the heat conduction equation

where Q is the heat generation function

k is the thermal conductivity

K is the thermal diffusivity.

This equation, with the appropriate boundary conditions, has been solved by an eigenfunction series

[2] or by finite-difference techniques [3]. Both methods have produced very adequate numerical results

and are applicable to a variety of beam profiles. We shall not dwell on the details of the numerical

analysis, but make some general remarks on the nature of the solution.

It is easily shown for the sample dimensions used in the laboratory that after a few seconds the temperature distribution within the sample is almost stationary in time, but that the edge temperature

rises uniformly [3]. This corresponds to the well-known linear rise period in calorimetric measurements

of absorption. One can conclude from this observation that long irradiation times are not necessary in making interf erometric measurements. Once the edge temperature increase becomes linear with time, the temperature distribution and hence the optical distortion does not change significantly. Note also that

the heat conduction equation suggests that the temperature distribution scales as Q/k. Any measure of optical distortion will likewise scale proportionally. Finally, the heat generation term used in solving

the heat conduction equation is assumed to have a Gaussian intensity profile to simulate the laser beam

profile for a laboratory laser.

Changes in optical path arise because the index of refraction and the sample thickness depend on temperature and thermally induced stress in the sample. The thermal stresses are calculated using thinplate theory for isotropic, elastic media [A]. This approach, which has been extensively used by others,

is somewhat unsatisfactory because several window materials of interest exhibit stress anisotropy, even

though they possess a cubic structure. A general solution, however, violates our assumption of cylindrical symmetry. The results of thin-plate theory for radial and azimuthal stresses are

2.1. Temperature Rise

V^T + (Q/k) = (l/K)(9T/3t)

2.2. Optical Path Differences

a

r T (b) - T (r)J

a, T (b) + T (r) - 2T(r)

e 2 a E

where a = thermal expansion coefficient

E = Young ' s modulus

r

and T(r) r dr. -37-

r
The latter integral evaluated at the edge reduces to the average temperature rise of the window.

The change in optical path for radial and azimuthal polarizations can now be calculated. The results are then expressed as an isotropic OPD c{) and anisotropic OPD 6 given by [1].

Xjd [T(r) - T (b)]

X^d [T(r) - T (r)]

where Xj = dm/dT + (n-l)(l+v)a + h a.^ [(l-v)P^^ + (l-3v)P^2^

\ = 55 an^(l+v)(P^^-P^2)

n = index of refraction \

V = Poisson's ratio 1

and ^11' ^^12 stress-optic coefficients.

2Figure 1 shows the radial dependence of T(r), T(r), (i)(r), and 6(r) for a Gaussian intensity with ; 1/e radius = b/A, and an irradiation time of 10 seconds. The isotropic OPD is proportional to the i deviation of temperature from average temperature and resembles the original intensity distribution. i The anisotropic OPD, however, is zero at the center of the window and attains its maximum value at a i radius which is typically larger than that of the incident laser beam.

2.3. Intensity Variations

The intensity distribution in the image of the interferometer is calculated [l] for an interferomete' linearly polarized in the y-direction (vertical) . The intensity is given by

2

1 = 1 [1+cos ((!)+a))cos 6 + sin (<t)+cj) sin 6 cos 6].

o

Factors of 2Tr/X have been omitted for simplicity. toCr.G) is the bias OPD contributed by the reference \ beam of the interferometer. The bias produces reference fringes which are typically either equally ! spaced straight lines obtained by introducing a wedge into one arm of the interferometer or unequally

spaced circular fringes introduced by changing the curvature of the reference beam relative to the ; sample beam.

The effect of the window on the interf erogram can be understood as resulting first, from changes in j the OPD for the y-direction, '

if (r) =h (<l>r+*e^ "

6 (r) = S2 -*e)

=

(!) = d) + iS cos 29

which deforms the shape of the fringes and, second, from changes in polarization of the sample beam whlct reduces fringe contrast, especially along the 45 directions. When P^^ and '-^^ anisotropic OPD vanishes and 6=0. Then the azimuthal variations in the intensity distribution disappear and the fringe

deformation retains circular symmetry. I'Jhen the anisotropic OPD is not small, the deformed fringes

resemble ellipses whose eccentricity varies with radius.

A synthetic interf erogram is a pictoral representation of the intensity distribution just presented.

Figure 2 is an example of such a process. An array of printed characters was used to display the intensity distribution. The "finesse" was adjusted by only printing characters where the relative

Intensity exceeded a predetermined threshold. Contract was then enhanced by blackening the printed area

with India ink. This method has the advantages of speed and low cost. Another type of synthetic interferogram is shown in Figure 3. Here a laser beam was used to expose an array of points on a sheet of photographic film. The intensity of the laser at any point on the film is controlled to produce the appropriate exposure. The array used in Figure 2 consists of approximately 2000 x 2000 points. The

results closely approximate the ideal interf erogram, but are time-consuming and expensive to produce.

-38-
3. Actual Interf erograms

Interferometric measurements of materials heated by a CO^ laser were made using a holographic interferometer operating at 0.6328ym. The primary advantage of the holographic method is that it allows

interferometric measurements on samples that do not have optically flat surfaces because holographic

recording permits one to combine the wavefront from a heated window to that previously recorded for the unheated window. The holographic interferometer also has more flexibility in the introduction of bias

fringes.

The experimental arrangement is shown in figure A. It consists of a two-beam interferometer using a thermoplastic holographic recording material [5]. Prior to irradiation by the CO^ beam, a hologram is

I recorded on the thermoplastic recording material. The hologram can be developed in situ and it can be

erased and used for repeated tests. Once the hologram is recorded and developed, the laser is used

to heat the window material. The interference pattern between the new wavefront and a holographic

reconstruction of the old wavefront from the window is imaged on a diffuse screen where it is observed

by a video camera and recorded onto magnetic tape. Recently [6] a video data processing system has been

built which will allow us to analyze numerically the distortion detected by these interferometric tests.

4. Comparison of Real and Synthetic Interf erograms

A crucial question in judging the usefulness of the theoretical model for window distortion which has been developed is whether it can reproduce qualitatively the effects which are observed in actual interferometric tests. We have found that this is the case, and will illustrate with examples of interf ero- grams published earlier [1].

4.1. Isotropic Materials

As a working definition of an isotropic material we offer the criterion that an azimuthally uniform

fringe pattern (circular fringes) remain uniform during heating by a laser at the center of the pattern.

As shown by the synthetic interf erogram in figure 3, the magnitude and sign of the bias fringes will

charge the spacing and width of the fringes for the heated window. However, the fringes remain circular. A good example of an isotropic material is ZnSe, see figure 5. The fringe motion observed during the course of heating is proportional to the average temperature of the window, which is rising more or less linearly in time. The distortion introduced by the window is revealed by changes in the spacing of the fringes during heating.

Other materials which have been observed to be isotropic in practice are KCl and NaCl. That these

materials have some anisotropic contribution to the OPD is indicated by observing the window samples

between crossed polarizers [3]. The absorbed power needed to produce an observable change in the polarization of visible light is an order of magnitude higher than the power needed to produce significant optical distortion. Thus, while birefringence can be observed, its magnitude is much smaller than

the Isotropic distortion and can be ignored in most applications.

4.2. Anisotropic Materials

The materials in which we have observed significant anisotropic effects are BaF^, SrF2, and CaF^. Figure 6 shows synthetic interferograms which have the same isotropic OPD function. Figure 5A, in fact.

Is the same function shown in figure 1. Figure 6B has an anisotropic factor equal to 20 percent of the isotropic factor Xj- The original circular fringes are deformed into ellipses except for the inner- most fringe. For comparison interferograms of BaF^ are shown in Figure 7. Note that the innermost

fringes retain circular symmetry while the outer fringes exhibit a strong elliptical appearance.

For these materials, of course, the CO. laser was adjusted to very low power because of their high

absorption at 10.6lJm. The purpose of the CO2 laser is solely to produce the appropriate heating

mechanism in these tests.

The degree of anisotropy present in the alkaline earth fluorides compared to the alkali halldes is shown by the differences in ^^"^^2 these materials [7]. The values of P^i~Pi2 ^Cl, NaCl, and

KBr at 0.589um are reported to be approximately - 0.02 whereas the values for CaF^ and BaF^ are approximately unity, an increase of a factor of fifty. Thus it is really not surprising that anisotropic effect

are significant for the alkaline earth fluorides.

5. Interf erogram Analysis

The fringes in an interferogram represent contour levels of optical path difference (OPD) between a reference wavefront and an optical system under test. The positions of these fringes are the basic raw

data for analysis of the wavefront of the optical system. This section will describe some of the results obtained using FRINGE, a computer program for the analysis of interferometric test data, and the techniques used by that program.
For numerical analysis of wavefront information it is desirable to have as uniform a sampling of the wavefront over the pupil as practical. This is best achieved by using straight line bias fringes as in

the synthetic interf erogram in figure 3. Furthermore, it is useful to conduct the test to generate welldefined patterns without closed fringes. This is illustrated by figure 8, which shows the optical dis- P tortion for synthetic interf erograms with increasing absorbed heat. |

1' The wide dark areas shown in the bottom two interf erograms do not accurately define the contour line \ for data reduction purposes. Fringes in the top two interf erograms are much better suited for manual * or semi-automatic location of fringe peaks. Manual location of fringes involves setting a cursor at the

| desired point as the interf erogram and pressing a button to digitize the coordinates of the cursor. ! Typically 300-500 points can be located in 1-2 hours. With semi-automatic operation using a video

| digitizer [6], a minicomputer assists in locating the fringes. Approximately 1200 points can be located

in 20-30 minutes. j

Auxiliary information about the interf erogram must also be provided for the proper scaling and definition of the pupil, by locating the edges of the pupil and measuring the magnification of the image.

A reference wavefront can be subtracted from the data to improve the resolution of the measurement of | thermally induced effects. Then the analysis proceeds in two stages. The first stage is a global I fitting of the data by a polynomial series. The second is a calculation of the diffraction intensity I spread function (point spread function) which is the expected intensity profile in the far field, of ' light imaged through the system.

| 5.1. Global Fit '

The program fits a 37 term series of Zernike polynomials to the wavefront data and uses this series I expansion to represent the wavefront in later calculations. Using a global function to represent the ] distortion has the advantage of smoothing the original data and simplifying the process of averaging

several wavefronts to assess the consequences of combining several distorting elements into the optical | train. Zernike polynomials were selected for several reasons. The polynomials are orthogonal and form 1 a complete set of functions over the domain of the unit circle. Each member of the series minimizes / the rms deviation for that order. Finally, the Zernike polynomials are easily understood in terms of [ optical aberrations, so that tilt and defocus and, if necessary, coma and astigmatism can be removed I from the wavefront. I

The Zernike polynomial series is fitted to the data using the Gram-Schmidt method with a least- squares minimization. A statistical characterization of the wavefront from the original data is compared

to that obtained for the polynomiol fit. This information provides a measure of optical quality (rms

values, Strehl definition) and a judgement on the goodness-of-f it of the polynomial series (how many i terms were adequate). Other useful information includes a contour map of the wavefront after all correc

tions have been applied.

Figure 9 shows a typical contour plot from a test on BaF„. The elliptical distortion pattern is clearly visible in the contour map, but was not apparent in the original interf erogram [1], which incorporated straight line bias fringes. Subjecting a synthetic interf erogram, such as shown in figure 3, to the data reduction process provides a measure of the adequacy of the process. Typically a synthetic

interf erogram such as this can be analyzed to an absolute precision of about 1.5 percent compared to the original wavefront - if the original is smooth enough. The rms deviation of the wavefront can be

matched to the original within 0.5 percent. Problems arise when the heating beam is much smaller than

the size of the pupil and when anisotropic effects are significant. Then 37 terms are not sufficient to adequately represent the wavefront. This deficiency will be corrected by using the calculated temperature distribution itself as the basis function and fitting the parameters with a non-linear least-squares

minimization algorithm.

5.2. Far Field Analysis

Interf erometric data can be used to calculate the degradation in far field irradiance of light imaged

through the optical system under test. The complex pupil function is obtained by taking measurements of the intensity across the pupil and interf erometric measurements of the optical phase. The amplitude and phase of the light is then calculated for a 31 x 31 mesh of points on the pupil. This mesh is embedded

in the center of a 6A x 64 matrix of zeros, and the Fast Fourier Transform [FFT] algorithm is applied.

The modulus squared of the resultant matrix is the point spread function [PSF] of the pupil function,

which gives the intensity distribution in the far field.

As an example of the results of this analysis, we considered the situation where the heated window

is used in a visible light imaging system simultaneously with the incident CO laser beam. Such a situation is realistic in experiments on beam degradation because light in the visible wavelengths is more sensitive to optical distortion and is easier to photograph than is infrared imaging. Figure 10 shows

the Airy pattern produced for an undistorted beam. Figure 11 is the result obtained for a distorted

BaF- window, using the optical phase shown in figure 9. The peak intensity in each case was normalized

to the same value. The principal observation is that the relative contribution of the ring structure is greatly enhanced, but that the size of the central spot does not change significantly.

-40-
I 6. Conclusions

I

I It has been possible to generate synthetic interf erograms which display most of the characteristics

of actual interf erograms. Only two optical parameters were required (Xj. and Xa) t° describe the optical

pathlength change per degree of temperature rise once the temperature distribution is known. For isotropic materials the OPD is proportional to the deviation of temperature in the window. Knowledge

of these parameters and the geometry of a high power laser beam profile makes possible the prediction

of far field patterns for realistic systems. Measurements of Xj and X^ are expected to provide useful

information for the optical design of high power laser systems.

7. References

i [l] Loomis, J. S. and Bernal G., E. , in "Laser

I Induced Damage in Optical Materials: 1975'

,

i NBS Special Publication 435, U.S. Government

I Printing Office, p. 126 (1975).

[2] Bernal G., E. , Appl. Optics J^, 314 (1975).

[3] Loomis, J. S. and Huguley, C. A., in "Laser Induced Damage in Optical Materials: 1974", NBS Special Publication 414, U.S. Government

I Printing Office, p. 94 (1974).

1 [4] Boley, B. A. and Wiener, J. H. , "Theory of

, Thermal Stresses", Wiley, New York, p. 290

' (1960).

[5] Lee, T. C., Applied Optics, 13^, 888 (1974).

[6] Bernal G., E., Anderson, R. H., Koepke, B. G.,

Loomis, J. S., Skogman, R. A., Stokes, R. J., Wertman, D., Honeywell Semiannual Technical

Report #5, ARPA Contract //DAHC-73-C-0464, July

1976.

[7] Flannery, M. and Marburger, J, in "Fifth Annual

Conference on Infrared Laser Window Materials",

Defense Advanced Research Projects Agency,

Washington, D.C., p. 782 (1976).

8. Figures

Figure 2. Synthetic interferograms showing the effect of focal shift bias fringes.

The top picture is an unbiased

interf erogram. -3 too •3 U

-41-

BEAM

STEERING

DEVICE

Figure A. Layout of experimental apparatus.

Figure 6. Synthetic interf erograms for isotropic (A) and anistropic (B) effects. Xa = 0 for (A) and Xa = ''•2 for (B) . Xj = 1 for both cases.

Figure 7, Interferogram of laser- induced optical

distortion in single crystal barius

fluoride. Upper left photo shows

reference fringe bias.

-43-
Figure 8. Synthetic Interferograms showing

effect of changing Xj. The ref- erence fringe bias was a simple

wedge (straight line fringes).

CONTOUR STEP .10

Figure 9. OPD contours for optical distortion in barius fluoride. Contour step is frac- ' tions of He-Ne laser wavelength (.6328 ym)

Figure 10. Point spread function (PSF) showing Airy

disc for unaberrated pupil.

Figure 11. Point spread function (PSF) for optical

distortion in barium fluoride.

COmENTS ON PAPER BY BERNAL AUV lOOms

Mo du>cu^.l>-ion.

-44-
1.5 NEW MATERIALS FOR CHEMICAL LASER WINDOWS

James A. Harrington

Physics Department

University of Alabama in Huntsville

Huntsville, Alabama 35807

Many of the more common materials such as the alkaline earth fluorides, ZnSe,

and the alkali halides have been studied for use as low loss windows on DF-HF

chemical lasers yet there remain others which have received little or no attention

at these wavelengths. We have begun to investigate some of these other materials

in hopes of finding additional low absorbing materials which show potential as laser windows. Specifically, our measurements of the optical absorption in LiYF^, Yttralox, ZnS, CdTe, KBr, SrF^, and KRS-5 will be discussed and related

to the intrinsic absorption in these hosts. In all cases, the measured absorption

has been found to be greater than the intrinsic value. In addition to the ab- sorption measurements on these substances, some recent optical absorption measurements on water repellent coatings on KCl will be discussed.

Key words: Chemical lasers; infrared materials; laser windows; multiphonon

absorption; optical absorption; water repellent coatings.

1. Introduction

During the past several years, a wide variety of materials have been studied for use as low loss windows on high powered infrared lasers. At DF/HF chemical laser wavelengths, the range of potentially

low loss materials is particularily wide and the optical absorption in the most common hosts such as the alkali halides, alkaline earth fluorides, ZnSe, Si, Ge, Al^O^, and MgO has been previously discussed

[l.Z]'^. There remain, however, other promising materials which have received little or no attention

at these wavelengths. In this investigation, we report the results of our optical absorption measurements at DF and HF wavelengths on some less studied substances (Yttralox, KRS-5, ZnS, CdTe, and LiYF^)

as well as on some improved KBr and polycrystal ine SrFp. In addition to these measurements, the optical

absorption in polymeric (water repellent) coatings on KCl will be presented.

2. Experimental Procedure and Techniques

The- optical absorption was measured using standard laser calorimetric techniques [s]. In order

to minimize surface contamination, all samples were measured in a vacuum calorimeter. In the case of

SrF2, a special vacuum calorimeter was used so that the samples could be baked out under vacuum and

then measured without moving the samples (cf section 3.2). The cw DF/HF chemical laser used as a source

was constructed from our own design and delivered from 5 to 10 watts of multiline power.

Samples were supplied for measurement from a variety of laboratories engaged in state-of-the-art

materials preparation. Each sample was cleaned with spectrograde CCl^ just prior to measurement. In instances when the surface quality looked particularly bad as viewed under a Nomarski microscope, the surfaces were mechanically polished, cleaned, and then remeasured. When this was done, the absorption

was always less than the initial value. Examples of this behavior are noted in the next section.

3. Experimental Results

3.1 Semiconductors

Two semiconductors, which had previously not been measured, were studied. Single crystal CdTe

obtained from II-VI, Inc. was found to have an absorption coefficient S at DF (3.8 ym) and HF (2.8 ym)

wavelengths of: .

^ CdTe ^ ^'^''^ " 4.8x10"-^ cm"-^

e (HF) = 5.3x10"-^ cm"-^ These values are approximately an order of magnitude higher than the values reported by Deutsch [4] at C0„ and CO wavelengths. A sample of CVD ZnS was obtained from Raytheon. In the past this has been

a high loss material with a measured B(C0„)=0.15 cm' [4]. At chemical laser wavelengths, however, we

*Work supported by DARPA and ONR.

^Figures in brackets indicate the literature references at the end of this paper.

-45-
measured smaller absorption as follows: , 6 (DF) = 0.049 cm'-^

6 (HF) = 0.024 cm'-^

3.2 Raytheon Cast SrF2 Three samples (VHP 597) of the most current, polycrystal ine SrF2 material were obtained from

Raytheon. The three pieces enabled us to obtain 3 for four different lengths L ranging from 0.5 to

2 cm and thus to plot 3L vs L for separation of bulk and surface absorption contributions from the

total absorption.

The first measurements performed on the as-received samples are shown at 3.8 um in figure 1. It can easily be seen from the figure that the absorption data (•) are quite scattered and therefore no reliable surface and bulk separation is possible. A similar result, not shown, was obtained at 2.7 pm. This scatter in the data is often due to unequal preparation or contamination of the sample surfaces.

In order to minimize this problem, each sample was cleaned in trichl oroethyi ene and then mounted in

a special vacuum calorimeter for a bakeout at 200 C for several hours under vacuum. The samples were

then cooled and 6 remeasured without breaking vacuum. These remeasured absorptions, which are plotted

in figure 1 (A), yield more consistent data and thus a least squares fit to the data was made. From

the slope and intercept of this fit the following bulk (Sg) and surface (3^) absorption coefficients

are obtained: r i S„ = 5.4x10' cm"-^ -5

r _•, = 5.3x10 /surface

The bulk value of 5.4x10" cm' represents the smallest absorption of any sample measured to date at 3.8 ijm. Unfortunately, the results after bakeout at 2.7 m were again quite scattered and it was

not possible to fit the data. A summary of all data for these samples is given below in table 1.

Table 1. Optical absorption in SrF, (Raytheon VHP 597)

Sample Condition Wavelength 6-p(cm'-^) 6g(cm"^) Bj/surface

As-received DF l.SxlO''^ HF 4.7x10"'^ Vacuum bakeout DF 1.6x10"^ 5.4x10'^ 5.3x10'^

HF 4.1x10''^ The total absorption coefficient Sj is the average S obtained from the three samples.

3.3 Alkali Halides

KRS-5 is an attractive substance for a laser window because of its resistance to environmental

attack. It has received little attention, however, due to its relatively high loss at CO2 frequencies

[4]. At DF and HF wavelengths we have measured the following absorption in a sample of Harshaw KRS-5:

8 (DF) = 2.3x10"^ cm"-^ ^^^^

3 (HF) = 3.5x10"^ cm'^ Recently grown single crystal KBr was obtained from Dr. Phil Klein of NRL for our absorption

studies. This material was RAP processed [5] and represents some of the best KBr we have measured

to date. Table 2 summarizes our absorption coefficient measurements on the samples as-received and after a mechanical polish. It can be seen that polishing just prior to measurement lowers the abTable 2. Optical Absorption in KBr

Sample No . Surfaces 3j (DF) [cm'-^] By (HF) [cm'-^]

B-324 As-received 3.6x10' 4.2x10"'^ Mech. Polish 2.2x10'^* 3.2x10''^ B-326 Mech. Polish 1.3x10'* 4.1x10'*

-46-
sorption as might be expected for a hydroscopic material yet the absorption still remains well above

the intrinsic value for KBr. Further surface treatment such as chemical etching should be done to try

to minimize surface absorption.

3.4 Yttralox

Yttralox [y„0o(90%): ThO„(10%)] is an oxide with many attractive features as a potential low

loss laser window. It has the usual strength and environmental resistance typical of oxides while at the same time exhibiting lower absorption than, for example, sapphire [ij. The Yttralox obtained

for this study was supplied by General Electric [6].

The infrared absorption spectrum for Yttralox is shown in figure 2. An extrapolation of the multiphonon edge to g^.^^(DF)=2.6xlO"^cm"''' and 6^-^^(HF)=4.6xlO~^cm~''' indicates the very low intrinsic

values for this material at the chemical laser wavelengths. However, an extrinsic band is present in the as-received material. This band, unfortunately, appears directly between the DF and HF frequencies

and is responsible for the higher absorption in this region.

In order to improve this material it is necessary to reduce the extrinsic absorption band centered

near 3 pm. One method used to minimize this band is a reheating of the sample in a small amount

('^^10 ppm) of oxygen at high temperature ("v.l700°C). In reheating the sample whose absorption is shown

in figure 2, the extrinsic band completely disappeared. In this treatment, oxygen is presumably being

forced into the Yttralox and the likely OH" contaminant removed. On remeasuring 3 calorimetrically

,

the DF 3 in one sample (not shown in figure 2) decreased by a factor of 6 and the HF 3 by a factor

of 4 [l].

It would seem that the most likely contaminant would be OH" and that it is being at least

partially removed in the retreatment process described above. Sulfur containing compounds, however, are also a possible source of contamination. In some cases the starting powder is ball-milled and in some cases this step is eliminated from the processing procedure. Ball-milling can introduce sulfur

and, therefore, we have looked at different samples which are identical in processing except for the ball-milling operation. For those samples which involved the ball-milling operation, the absorption

was always less at both DF and HF wavelengths than the corresponding samples which did not include

ball-milling in their processing. Therefore, we cannot conclude that sulfur is definitely an impurity

contributing to the extrinsic band near 3 ym.

3.5 Li YF^

Li YF^ is a potentially low loss material with good environmental resistance, a low index of

refraction (1.46), and relatively high thermal conductivity (0.044'^''cm°K) . The infrared absorption of

undoped, single crystal Li YF^ obtained from Sanders Assoc., Inc. is shown in figure 3. Included

on the figure are our calorimetrically measured absorption coefficients

. . yp 3 (DF) = 6.3x10"^ cm"-^ ^ 3 (HF) = 4.1 X 10 -^cm

^ along with the much smaller extrapolated intrinsic values. Again it can be seen that the measured

absorptions lie well above the multiphonon edge indicating that the absorption is extrinsically

1 i mi ted.

3.6 Polymeric coatings

Water repellent coatings have been produced to protect hydroscopic materials such as KCl . These

coatings have, as well, to be evaluated in terms of optical absorption. To study the DF and HF ab- sorption. Research Triangle deposited a water repellent, polymer coating on one-half of a KCL substrate.

Our absorption measurements on this half-coated substrate are given in table 3. Unfortunately, the

total absorption in the uncoated half is essentially the same as the coated half, within experimental

Table 3. Polymeric coatings on KCl

Sample Section 3j(DF) [cm'-^] 3i-(HF) [cm-1

Uncoated 1.9x10"-^ 1.3x10"^

Coated 2.1x10"^ 1.6x10"^

-47-
error, so that it is impossible to extract from the data the absorption in the polymeric coating

alone. All that can be said is that the coating material may have a low absorption but reliable

coating absorptions cannot be obtained until better KCl substrate material is used.

4. Conclusions

The optical absorption in all of the materials studied has been found to be extrinsical ly limited.

In the case of some materials such as Yttralox, the extrinsic bands are visible and methods have been attempted to reduce these bands near 3 ym. Yttralox, as well as, LiYF,, and CdTe are previously little

studied materials which could be improved through further materials processing. This would be espe- cially profitable for Yttralox and Li YF- since they have appealing physical properties. Other

materials such as ZnS and KRS-5 have absorption coefficients too high to be viable alternatives for

low loss windows.

5. Acknowledgments

The author gratefully acknowledges the assistance of D.A. Gregory and W. Otto, Jr. for technical

assistance in these measurements.

6. References

[l] Harrington, J. A., Gregory, D.A., and Otto, [4]

W.F., Appl. Optics, Aug., 1976 (to be

publ ished)

[2] Harrington, J. A., Gregory, D.A. , and Otto,

W.F., Fifth Conference on Infrared Laser Window Materials, Las Vegas, NV, 1975,

p. 871. [3] Hass, M. , Davisson, J.W., Klein, P.H., and

Boyer, L.L., J. Appl. Phys. 45, 3959 (1974).

[5]

[6]

Deutsch, T.F., J. of Elect. Mat. 4, 663

(1975).

Klein, P.H., Fifth Conference on Infrared Laser Window Materials, Las Vegas, NV, 1975, p. 983. Greskovich, C. and Woods, K.N., Amer. Cer. Soc. Bull. 52, 473 (1973).

Figures

1 RAYTHEON CAST SrFj (VHP 597)

1 • AS RECEIVED AFTER VACUUM

BAKE OUT

•

•

(ig = 5.4 X 10-5 ^^-1 — •

iSg = 5.3 X 10^5/SURFACE

1

•

1

<

<5 1

1 2 SAMPLE THICKNESS, cm

Figure 1. Intrinsic absorption in Raytheon cast Sr F^

at DF laser frequencies.

-48-
5294-2 6294-3

YTTRALOX TM

Y2O3 (90%): Th02 (10%)

HF

DF

1200 1600 2000 2400 2800 3200 3600 4000 WAVE NUMBER, cm"''

Figure 2. Infrared absorption in Yttralox. Note

the extrinsic band near 3 pm and the extrinsically limited DF and HF absorption coefficients'.

10"

LiYF^

/3|^, (DF) = 4.6 X 10"'* cm"''

DF HF

1400 1800 2200 2600 3000 3400 3800

1 WAVE NUMBER, cm"

Figure 3. Infrared absorption in Li YFi,.

COmEMTS ON PAPER BV HARRINGTON

The. ip^akeA po-inte.d oat thcut tkz caJionJjn^tAA.c. anaZyi^ -Lnclude.d a coM.e.cJxon ^on. thz VKZAnaJi

Kzfile.citA.on at the iuA^iaae., and {^ofi itanding wave z^dzcti in panaJUizt iuAf,acz^, J. Jokmton of^ UPRI itaXzd that kii mza^uAz/nzyLti madz on {^bxoKldz matzAAjati at thz CO taizA wavzlzngth con^-Omzd tkz

obizAvatxan that vzAZzd 6uA{,aczi o{,tzn ikow lomzn Inf^KOAzd abi,on.ptlon than cJizjxx. tsuxiazzM, l.z,, that

thz v-Uitblz appzanancz o{^ thz ^uAjJace -c6 not a izJLlabtz guA.dz to -uti tn^njOJizd chanjxztznJj>tlcJ>

,

-49-
1.6 STUDIES OF ABSORPTION IN MID IR LASER WINDOW MATERIALS

A, Hordvlk, B, Bendow, H. G. Lipson,

L. H. Skolnik, and R. N, Brown

Rome Air Development Center

Deputy for Electronic Technology

Hanscom AFB, MA 01731

An investigation has been made of the absorption properties

of various potential laser window materials in the wavelength

ranges of the CO2 , C0> and HF lasers. Various experimental

techniques were utilized including emittance spectroscopy,

Fourier spectroscopy, laser calorimetry, and a new photoacoustic

technique which allowed both bulk and surface absorption to be

determined simultaneously. The materials investigated were

ZnSe, AI2O3, MgO, LiF, MgFa , CaFa , SvFz, and BaFa . At both the

CO and HF laser wavelengths it was found that surface

absorption was substantial in most cases and particularly so in

the HF laser region. Among the materials tested, sapphire

emerged as an excellent candidate for use with the HF laser

provided its large surface absorption can be reduced

substantially. Preliminary theoretical calculations have been

made of the intrinsic absorption in ZnSe to interpret the

measurements

Key words: Bulk absorption; CO laser; HF laser; infrared

laser window materials; multiphonon absorption; surface

absorption.

Introduction

Materials suitable for use in high power infrared laser windows have been the

subject of considerable research in the last 5-6 years [l-sj . In general,

candidate materials must possess very low optical absorption coefficients at the

laser wavelength of interest in order to avert unacceptable optical distortion and

catastrophic mechanical failure. A large number of absorption measurements have

been performed at the CO2 laser wavelengths [l-sj , but the work at the CO and HF

laser frequencies has been more limited [S-Sj , and in particular only a few

attempts have been made to distinguish between bulk and surface absorption [7,8] . We report here a number of measurements of both bulk and surface absorption at CO and HF laser wavelengths. Various experimental techniques were used including

emittance spectroscopy, Fourier spectroscopy, and laser calorimetry. In addition, a new photoacoustic method was utilized whereby both surface and bulk absorption can be measured simultaneously. Finally, to determine the intrinsic value of the

absorption coefficient in semiconductors, multiphonon calculations of the type

carried out previously for ionic crystals have been modified to provide preliminary

theoretical results for the optical absorption in ZnSe.

Experimental Procedure

The various experimental techniques utilized for measuring absorption have all

been described in the literature [^9j . But since the use of the photoacoustic

effect for measuring absorption in solids is rather new flO, 11, I2I , a short

description of this method will be given.

When a periodically interrupted light beam is incident on an absorbing medium,

heating and subsequent expansion occurs resulting in the generation of an acoustic

or elastic wave. Since the amplitude of the elastic wave is proportional to the

absorbed energy, the effect can be used to measure absorption coefficients. The

technique can be viewed as a variation of the standard laser calorimetric method,

but since an elastic wave propagates much faster than a thermal wave, one can use

ac instead of dc detection. In principle, the absorption coefficient can be

1. Figures in brackets indicate the literature references at the end of this paper

-50-
calculated from the measured amplitude of the acoustic wave, however, the calculation is difficult. Instead, each sample is calibrated at a wavelength of

known absorption. The calibration is usually done at a frequency where the

absorption is large enough to be measured accurately by transmission. Once the

calibration constant has been established at one wavelength, the number can be used

to determine the absorption coefficient at other wavelengths. The technique has

been found to be quite sensitive and should be able to measure absorption co- efficients in the 10-^ cm-^ range using laser powers of about one watt [12] .

A schematic of the experimental arrangement used in the photoacoust ic absorption measurements is shown in figure 1. It consists of a cw laser, a beam

chopper, a sample with transponder attached for measuring the amplitude of the

generated acoustic wave, a power meter, and a lock-in amplifier. The transponder

is made of a piezoelectric ceramic. An unattached transponder (see fig. 1) serves

to detect any signal generated by scattered radiation. If necessary, one can then

compensate for scatter contribution to the signal generated by the attached

transponder. Typical sample dimensions are 5 x 10 x 12.5 mm where the beam

passes through the 5 x 12.5 mm surfaces, and the transponder is attached to one

of the 10 X 12.5 mm surfaces.

With this experimental configuration it is possible to determine both surface

and bulk absorption [13] . By measuring the transducer output as a function of

distance between beam and transducer for pure bulk absorption and for pure surface

absorption one finds that the two generated curves have quite different shapes.

The functional dependence of the two curves is independent of the absorbed power

and can be found experimentally. If both surface and bulk absorption are present,

the magnitude of each can be found by curve fitting provided the signals generated

by the two types of absorption are within one to two orders of magnitude of each

other. Calibration is done at a wavelength of known surface absorption, for

instance in the opaque region.

Experiments were performed on ZnSe, MgO, AI2O3, LiF, MgFa , CaFz, SrFz, and BaF2

.

The ZnSe (CVD) was obtained from Raytheon [14] and polished by Man Labs. The MgO

sample was grown by Norton Research Corp. (Canada) Ltd. and polished by Man Labs.

All the other samples were obtained from A. Meller Co. in finished form, and the

material was of standard commercial quality. The samples were stored in a desiccator, but otherwise no particular care was taken to protect the surfaces.

Any surface contamination that might occur during mounting of transponder or during the experiment was removed with acetone.

For measurement and calibration we had available a CO2 , CO, HF, and Ar-laser.

The CO laser was tunable to a large number of discrete lines in the 5.2 to 6.5 um region. The HF laser oscillated on several wavelengths simultaneously with most

of the power being in the 2.7-2.8 ym range. Typically, laser powers of a few

hundred mW or less was used in the various experiments.

Results and Analysis

The results of bulk absorption measurements on CslFz, SrF2, and BaF2 performed

with the tunable CO laser are shown in figures 2, 3 and 4 and in table 1. The

straight, dashed line in each figure is drawn through the measured absorption

coefficient at 10.6 ym and with the slope determined by Deutsch [l5]. For CaF2 the

absorption decays exponentially with frequency as predicted by the multiphonon

theory [l5,16], although the slope is slightly lower than that measured by

Deutsch [15]. For both SrFa and BaFa the absorption coefficient clearly levels off

at the higher frequencies. Lower absorption values have been reported in

specially prepared samples [6], The higher absorption levels observed here may be

caused by impurities, dislocations and/or inclusions.

Results of surface absorption measurements are also given in table 1. CaFa

had high bulk absorption and this made it difficult to determine the surface

absorption accurately, and it was measured at only two wavelengths. Surface

absorption was found to be strongly dependent on wavelength. In the shorter wavelength regions surface losses will in some cases be larger than bulk absorption for

windows less than 1-2 cm thick. Surface losses for the materials in table 1 have

previously only been reported for the 5.2-5.3 ym region [l7] . Our results are in

general agreement with the former measurements.

-51-
Bulk absorption for MgO, MgF2 , Al203»and LiF is plotted versus frequency in

figure 5. The curves have been extrapolated down to the 1 x 10-"* cm--' level

assuming an exponential decay. Measurements at the HF laser wavelength were done

with the photoacoustic technique. For MgO, the high absorption level at 2.7 \im

is probably caused by the closeness to the bands indicated in the figure. The

absorption coefficient for AI2O3 at 2.7 pm is near the predicted intrinsic level.

Data for both bulk and surface absorption are given in table 2 for the above

materials and also for ZnSe, CaFz, SrF2> and BaF2 . Note that at 2.7 pm surface

absorption may become dominant over bulk losses for most of the materials depending

on sample thickness.

Both bulk and surface absorption are larger than expected in the 2.7 um region.

The da+'a for SrF2 , CaF2, and ZnSe are consistent with the measurement by Haas

et al, [sj'and Harrington et al.^&J. Sapphire is the only material with bulk

absorption close to the expected value. It has been suggested that the higher than

predicted absorption is due to carbon containin'r compounds which have contaminated

the materials during fabrication and polishing [3, IS].

To compare the optical performance of the various materials as an HF laser

window the figure of merit for optical distortion at 2.7 ym has been calculated for

each sample. The figure of merit has been defined as follows [l9, 20]

e +

2 3sL

-1

dn

dT

-1 ^ + a (1 + V) (n - 1) + ct (1 - v)p

12 11

CorK

(1)

Here 6 is the bulk absorption coefficient, 6 the surface absorption, L the window

thickness, n the index of refraction, T temperature, a the thermal expansion co- efficient, V Poisson's ratio, p. . the strain optic coefficient, C the heat

capacity, and K the thermal conductivity. For the short time response appropriate

to pulsed laser conditions the heat capacity is used. The thermal conductivity is

used for the equilibrium response under cw laser irradiation. It should be noted

that the stress optics term used in eq. (1) is really only applicable to

isotropic materials, but in most cases this simplification does not give a large

error. It will be assumed that L = 2 cm for all materials. The values of the

absorption coefficients used in the calculations are from table 2. The values of

various physical constants in eq. (1) have been taken from [6 and 21].

Results are listed in table 3. In the column labeled "total" both surface and bulk

absorption have been included. In the column labeled "bulk" the surface losses

have been neglected. All values have been referenced to ZnSe. CaF2 , SrF2> and BaFz

have high figure of merit since dn/dT is negative for these materials while the two

other terms in the bracket are positive resulting in some degree of cancellation.

In this case the stress-optic effect contributes significantly to the optical

distortion, and when comparing the figure of merit for the various materials, it

should be remembered that we have used an approximate stress-optic term.

Sapphire also looks like a promising candidate if the surface absorption could be

reduced substantially. In addition sapphire has mechanical, chemical, and thermal

properties which are well suited for an external window.

Theoretical Considerations

The absorption spectrum of ZnSe (CVD) in the 500-1000 cm-'' range is shown in

figure 6. The solid curves are experimental, and the dashed line is the result of

preliminary calculations. A variant of the non-interacting cell approach has been

used [22] in which full phonon spectral properties have been retained, but multicell dynamic interactions have been neglected. All k-selection rules have been

suppressed, and only linear moments are included. One multiplicative parameter

was adjusted to make the theoretical and experimental curve coincide at one frequency. The calculated rate of decrease in absorption with increasing

frequency is too rapid at the higher frequencies, but it should be noted that the

experimental curve was determined from a sample with somewhat high absorption.

Recently, we have acquired a lower absorbing sample. The result of a calorimetric

measurement at 10.6 ym (943 cm-^ on this sample has been indicated on figure 6,

and it is much closer to the calculated curve. It is also possible that by

-52-
including nonlinear moments the calculated absorption might become somewhat

enhanced.

Conclusion

A study has been made of the absorption properties of a variety of potential

window materials suitable for the mid IR region. It was found that both bulk and

surface absorption were generally higher than expected and that at the HF laser

frequency in particular, surface absorption was dominant in several materials. A preliminary attempt to calculate the absorption level in ZnSe from first

principles has yielded encouraging results which account for various trends

observed experimentally.

References

"Conference on High Power IR Laser Window Materials, 27-28

Oct. 1971", edited by C. S. Sahagian and C. A. Pitha.

AFCRL-71-0592, Spec. Rep. No. 127, (AFCRL, Bedford, Mass.).

"Conference on High Power IR Laser Window Materials, 30-31

Oct, 1 Nov. 1972", edited by

C. A, Pitha. AFCRL-TR-73-

0372, Spec. Rep. No. 162,

(AFCRL, Bedford, Mass.).

"Third Conference on High Power

IR Laser Window Materials,

12-14 Nov. 1973", edited by

C. A. Pitha and B. Bendow.

AFCRL-TR-74-0085, Spec. Rep.

No. 174, (AFCRL, Bedford,

Mass . )

.

"Proc. of the Fourth Annual

Conf. on IR Laser Window

Materials", compiled by C. R. Andrews and C. L. Strecker.

AFML-TR-75-9. (AFML, WrightPatterson AFB, Ohio)

"Proc. of the Fifth Annual

Conf. on IR Laser Windows

Materials", compiled by C. R. Andrews and C. L. Strecker.

(AFML, Wright-Patterson AFB,

Ohio)

.

A listing of many of these

measurements can be found in

"Infrared Laser Window

Materials Property Data for

ZnSe, KCl, NaCl, CaFz , SrFz

and BaFa", by S. Dickinson,

AFCRL-TR-75-0318, Phys. Sc.

Res. Paper No. 635 (AFCRL,

Bedford , Mass . )

.

J. Harrington, D. Gregory,

and W. Otto, in Ref. 5 p. 871 (1975).

M. Hass, J. A. Harrington,

D. A. Gregory, and J. W. Davisson, Appl. Phys. Lett.

28, 610 (1976).

10

§0

[la]

[if]

120J

rail

-53-

For a review see L. H. Skolnik

in "Optical Properties of

Highly Transparent Solids",

edited by S. S. Mitra and B. Bendow, p. 405, (Plenum Press,

New York, N.Y. , 1975)

.

A. Hordvik and H. Schlossberg,

J. Opt. Soc. Am. 65, 1165 (1975).

A. Hordvik and H. Schlossberg,

in [5] , p. 639 .

A. Hordvik and H. Schlossberg,

to be published in Applied Optics.

A. Hordvik, to be presented at the OSA 1976 Annual Meeting, 18-

22 Oct. 1976, Tucson, Ariz.

We are grateful to Mr. Donadio

of Raytheon for providing us with

this sample.

T. F. Deutsch, J. Phys. Chem.

Solids 34, 2091 (1973).

T. C. McGill in "Optical

Properties of Highly Transparent

Solids", edited by S. S. Mitra

and B. Bendow, p. 3 (Plenum Press,

New York, N.Y. , 1975)

.

T. F. Deutsch, [3] , p . 13.

P. H. Klein, paper presented at Infrared Laser Window Materials

Meeting, Boulder, Col., 12 July

1976.

B. Bendow and P. D. Gianino,

Appl. Opt. 12, 710 (1973).

P. Miles in [5] , p. 8.

C. S. Sahagian, C. A. Pitha,

"Compending on High Power IR Laser Window Materials", AFCRL72-0170, Spec. Report No. 135

(AFCRL, Bedford, Mass. 1972).

B. Bendow, S. P. Yukon and

S. C. Ying, Phys. Rev. BIO , 2286 (1974).
Figures and Tables

PHOTO- ACOUSTIC ABSORPTION MEASUREMENT

CW LASER

CHOPPER

REFERENCE

ATTACHED

TRANSDUCER

w-SAMPLE

iJ

UNATTACHED

TRANSDUCER

POWER

METER

LOCK- IN AMPLIFIER

Figure 3. Bulk absorption coefficient versus wavenumber for SrFa.
5 T

1 1 r

Figure 6. Bulk absorption spectrum for ZnSe. The fully drawn curves

are experimental. The dashed

\ curve is theoretical. The

point marked (+) is a calori-

, I

1 I

1 1 metric measurement at 10.6 ym.

500 TOO 900 MOO — 'A (cm-')

-55-
I

1

Table 1. Bulk and surface absorption of CaFa , SrFa and BaF2

I CaF2

Wavelength (ym) Bulk abs . ( cm- ' ) Abs. per surface , 10.6 3.60

6.546 1.5 X 10-^

6.466 1.4 X 10-^

6.330 1.2 X 10-^ 2 X 10-'

6.137 7.3 X 10-'

6.085 6.3 X 10-'

5.906 4.9 X 10-'

5.782 3.4 X 10-' 6 X 10-"* 5.700 2.2 X 10-'

5.424 1.1 X 10-'

5.257 6.7 X lO-"

SrF2

Wavelength (ym) Bulk abs. (cm-') Abs. per surface

10.6

6.466

6. 330

6. 058

5.782

5.488

5.280

BaF2

Wavelength (ym) Bulk abs. (cm-' Abs. per surface

10.6

6.466

6.330

6.058

5. 782

5.488

5.280

0.71

7.4 X 10-"* 5.7 X lO-** 3.2 X 10-** 2.1 X 10-"* 1.2 X 10-"* 9.8 X 10-=3 X 10-'

3 X 10-'

9 X 10-"* 7 X 10-"* 8 X 10-=

9 X 10-=

0.21

4.4 X 10-"

4.3 X 10-"* 3.2 X 10-"* 1.6 X 10-** 1.3 X 10-"* 1.1 X 10-**

1 X 10-'

1 X 10-'

5 X 10-"* 5 X

10-"* 6 X 10-=

5 X 10-=

-56-
Table 2. Absorption at HF las er wavelength

Material Bulk (cm-M Surface

MgO 5.2 X 10-^ 9 X 10-"

LiF 8.3 X 10-" 3 X 10-"

ZnSe 5.8 X 10-'' 1 X 10-3

BaFz 3.0 X 10-" 4 X 10-"

SrFz 2.7 X 10-" 6 X 10-"* MgF2 2.5 X 10-" 1 X 10-3

AI2O2 2 X 10-" 2 .4 X 10-3

CaFz 1.9 X 10-" 5 X 10-"

Table 3. Figure of merit at HF

CW Pulsed

Material Total Bulk Total Bulk

ZnSe 1 3.1 1 3.1

MgO 1.2 1.4 2.3 2.7

LiF 2.2 2.9 17.5 24

AI2O3 4.7 61 7.8 100

MgF2 10.6 53 55 280

CaFz 21 75 78 278

SrF2 37 117 126 420

BaF2 192 416 685 1420

All values normalized to the ZnSe figure of merit

COMMENTS ON PAPER BV HOIWVIK, BENDOl'J, LJPSON, SKOLNJK, AND BROWN

S^veMxil points ^zgoAcUng thti exptinAjntntal appoAattu uieAz cZoAl^-Lzd -in diicuiii^on. The. .ip&aktn.

mzntion&d that tho. photoacouAtic mzoMuAmmt uxus caAAA.zd out In a Kingz wheAZ the. photoacoaitic itgnaZ

mi itmply pKopoHJtioncUi to the chopptng ^Kequency [tow jjA-equencxe* ) . It uxu iuggeAt&d that bztttfi towpting might eyvitiz t^ the. acouAtic itgmt tn the. iomple mu couple.d thnaugh the. iuAAounding gat, latheA

than contacting the. tuamdacen. dOizctly to the. iomplz. The. i>pe.akeA eAtimate.d that the. ieniittvity

the. pfizAcnt method ^houZd enable the. meoiuAmtnt 0^ abio^ptionA cu> lou) a6 10~^ cm"'.

-57-
1.7 EFFECT OF TEMPERATURE AND STRESS ON THE REFRACTIVE

INDEX OF WINDOW MATERIALS*

Albert Feldman, Deane Horowitz and Roy M. Waxier

National Bureau of Standards

Washington, D. C. 20234

Windows subjected to high average power laser radiation experience a rise

in temperature due to residual absorption. The temperature distribution, which

is non-uniform, will distort the wavefront of the beam. The distortion results

both from changes of refractive index with temperature and from changes of

refractive index with stress induced by the temperature gradients. We are presently measuring dn/dT and the photoelastic constants of laser window materials

at 632.8 nm, 1.15 ym, 3.39 ym, and 10.6 ym. Two new experimental arrangements

have been constructed. The first permits measuring dn/dT and thermal expansion

from -180 °C to +200 °C. The second is a highly stable and sensitive interfero- meter for measuring photoelastic constants in the visible, the near infrared,

and the near ultraviolet. Thermal expansion data are obtained on CVD ZnSe, CaF2 and BaF2 between -100 °C and +120 °C and fitted to a third degree polynominal

in temperature. We obtain dn/dT for CVD ZnSe at 532.8 nm over the same temperature

range. The piezo-optic constants of fused silica and CVD ZnSe obtained with the new interferometer at 632.8 nm are in excellent agreement with values obtained by

other methods

Key words: BaF2 ; CaF2 ; fused silica; interferometry ; photoelasticity ; piezo-optic

constants; thermal coefficient of refractive index; thermal expansion coefficient;

ZnSe.

Introduction

When high-power radiation propogates through a laser window the residual absorption causes a temp- erature rise in the window. The temperature distribution in the window is, in general, non-uniform and

hence will distort the wavefront of the beam. This distortion arises from three sources: the direct

change of the refractive index due to a change in temperature; the change of optic path due to a change

of thickness with temperature; and the change of refractive index due to stresses induced by thermal

gradients. If the distortion is sufficiently severe, the laser window becomes unusable [1]^. In order

to predict the distortion of a laser beam wavefront from the laser energy deposited in a window, one requires certain material parameters. These parameters include absorption coefficient, refractive index

n, change of refractive index with temperature, dn/dT, the piezo-optic constants, '3^^ j » thermal expansion

coefficients, a, and elastic constants, S^^ or c^_. [1]. It is the purpose of the Optical Materials

Characterization Program at the National Bureau of Standards to measure n, dn/dT, q. ., a, and if necessary S^j [2]

In this paper we describe two new apparatuses constructed \ander this program. The first apparatus

consists of a cryogenically cooled furnace that permits us to measure a and dn/dT interferometrically

from -180 "C to +200 °C. The second apparatus consists of a polarizing interferometer that is capable

of measuring fringe shifts to within a precision of A/500 at 532.8 nm. This precision is useful for measuring the piezo-optic constants of materials at low stresses. The use of the interferometer is limited in the ultraviolet by the cutoff of calcite which is at about 300 nm, and in the infrared by

the useful range of the Soleil-Babinet compensator which is at about 1.15 ym. The Soleil-Babinet com- pensator is used for measuring fringe shifts. We present data obtained with the two new instruments. We have measured the photoelastic constants

of chemical vapor deposited (CVD) ZnSe and fused silica and found excellent agreement with previously

reported values [3,4,5]. We have measured the linear expansion coefficient of CVD ZnSe, CaF^ , and BsiF^ from -100 °C to +120 °C and the data were fitted to a third degree power expansion in temperature. The

fit for CaF^ and BaF^ agrees with available published data to within about 1% [6] . There was a slightly poorer agreement of our fit for CVD ZnSe to single crystal values of ZnSe [7] , however, the observed

difference is not necessarily significant because of the different materials measured. We have also mea

sured dn/dT of CVD ZnSe at 532.8 nm and have found a value 2% smaller than our previously reported value

[8]. We attribute this difference to our improved measurement technique; however, our value differs

significantly from Mukai et al. [9].

*Research supported in part by Defense Advanced Research Projects Agency.

^Figures in brackets indicate the literature references at the end of this paper.

-58-
For brevity we do not discuss the phenomenological theory of photoelasticity , because this theory

has been described adequately in the literature [10]

.

Apparatus and Experimental Method

The thermal expansion coefficient and dn/dT are measured by the method of Fizeau inter ferometry

.

The general method for making these measurements has been described in the literature [11]

.

To measure thermal expansion we measure as a function of temperature the shift of Fizeau fringes

between two optical flats separated by the specimen. To measure dn/dT we measure as a function of

temperature the shift of Fizeau fringes formed from reflections from the front and back surfaces of

a specimen polished plane parallel. We have constructed a compact apparatus for obtaining a and dn/dT from -180 °C to +200 °C. A schematic of the apparatus is shown in figure 1. The furnace is constructed from a cylinder of copper 37 mm in diameter by 75 mm high with walls

6 mm thick to permit rapid transfer of heat. A commercial band heater, which is clamped around the furnace, generates 175 W with an input of 120 VAC. The furnace rests at the bottom of an evacuable chamber 100 mm in diameter by 150 mm tall. Protruding from the bottom is a copper rod, 12 mm in diameter by 150 mm long, that conducts heat away

from the furnace to the liquid nitrogen reservoir. Thus, we can stabilize the temperature in the furnace at an arbitrary point by balancing the heat input from the heater with the heat leak to the liquid nitrogen.

The specimen rests within the furnace over a depression that is milled at an angle of 1° with

respect to the furnace axis in order to deflect laser beam reflections to the side. Holes are drilled

at several locations within the furnace to allow placement of thermocouples in contact with the specimen and to allow the pressure within the furnace to equalize with the pressure outside the furnace.

Two thermocouples measure the temperature at the top and at the bottom of the specimen. A copper

cover with a window allows access of laser radiation to the specimen in the furnace while maintaining

a uniform thermal environment around the specimen. All windows in the system are tilted at a 1° angle

to eliminate unwanted reflections.

We find that the system operates well when filled with a helium exchange gas of several millimeters pressure. The helium environment has several advantages over air or vacuum. During cooldown,

products in the air condense on the system optics and hence interfere with the laser beam. With

vacuum, the thermal response of the system is sluggish because of poor heat transfer between the

furnace, the heat leak, and the specimen. In addition, there is a large temperature difference between

the two thermocouples. With the helium present, the maximum temperature differential measured with the

two thermocouples across a 12 mm thick specimen is less than 1 K at a given temperature.

The procedure used for measuring dn/dT or a consists of first cooling the specimen to approximately liquid nitrogen temperature. Sufficient time is allowed for the two thermocouple readings to agree to within 1 K. The furnace is then heated very slowly until a fringe minimum is observed on a strip chart recorder monitoring the Fizeau fringe intensity. The temperature is then recorded. Subsequently, the furnace is heated rapidly and after a convenient nxmber of fringes has been observed, power to the furnace is cut back to allow the thermocouple readings to equilibrate while the temperature is slowly rising. At a fringe minimum, the temperature is recorded. The heating process is repeated until the maximum desired temperature is achieved.

The fringe counts and their corresponding temperatures are p'jnched onto computer tape and the fringe count, AN is then obtained as a power series expansion of the temperature by computer. For

thermal expansion measurements, we obtain an expression for the relative incremental thickness change

of the specimen

= A^T + A^T^ + A^T'^ (1) o where t^ is the specimen length at room temperature. At is the incremental length, and A^ , A^ , A^ are the expansion parameters. The derivative of eq. (1) with respect to T gives a. We obtain dn/dT by calculating first the incremental change of refractive index An as a function

of temperature with the formula

/ana At\ A ^ At\ -1 ...

where n^ is the room temperature refractive index and AN and ht/t^ are representations of power series

in temperature. The slope of the curve gives dn/dT, which we tabulate at a variety of temperatures.

The polarizing interferometer we have constructed for measuring piezo-optical constants is shown

in figure 2. This interferometer is based on a design by Dyson [12], which was further modified by Green [13] for measuring thermal expansion and dn/dT. Our design, which is a slight modification

of Green's, is capable of measuring fringe shifts to within a precision of A/500 at 632.8 nm. The

modification involves a change of position of the quarter wave plate within the interferometer.

-59-
The output of a helium-neon laser is propogated through a Glan-Thompson polarizer and is focused

into a Wollaston prism. The prism splits the beam into two orthogonally polarized beams. The intensities of the two beams are balanced by a rotation of the polarizer. The beams are then collimated

and made parallel to each other by passing through a second lens. The outer beam is intercepted by

the specimen. Both beams are now brought to a focus at a rear mirror after propogating through a quarter wave plate. The two beams then return through the system; the central beam retraces its path

while the outer beam propogates along the opposite side of the interferometer. Double passage through

the quarter wave plate interchanges the states of polarization of the two beams. Thus, when the two beams are rejoined at the Wollaston prism, they exit as a combined beam at an angle with respect to

the incident beam. A spatial filter and beam expander are placed in the exit beam to eliminate spurious

laser radiation. A Soleil-Babinet compensator and a second Glan-Thompson prism are then used to analyze the state of polarization of the exit beam. The output fringe patterns are observed with a silicon matrix vidicon camera and a television monitor.

This interferometer has several advantages over conventional Twyman-Green interferometers : It

is highly stable with respect to motion of system elements because both beams traverse the same optics,

hence the optic path changes in both arms tend to be equal. In addition, the close proximity of the

arms minimizes the effect of thermal currents, which are further reduced by placement of a cover over

the interferometer part of the apparatus. Moreover, the fringe shifts are analyzed with ellipsometric

techniques which have great inherent precision.

At the position of the specimen shown in figure 1, the radiation is polarized in the horizontal

plane, hence, the fringe shift per unit applied uniaxial stress, AN/AP, for an isotropic material will

depend on the piezo-optic constant '^^ "® shift the specimen to the outer return beam, where the radiation is vertically polarized, the fringe shift will depend on 'J.-^-^- The expression for the piezo-optic constant is

where q = q^^ for vertical polarization, q = q^^ horizontal polarization, \ is the wavelength of

the radiation and S _ is the elastic compliance component. In this equation t lacks a factor of two present in the equation for the Twyman-Green interferometer because the modified Dyson interferometer

is used only as a single pass instrument.

Sample preparation and the method of applying stress have been described in the literature and

so will not be discussed here [14] . The direction of stress is always vertical.

In table 1, we list the thermal expansion parameters (eq. 1) of CVD ZnSe that we have obtained

with seven runs on three specimens. The bottom row gives a statistically weighted average of the coefficients and the errors are the standard deviation of the mean. These values permit us to calculate the linear thermal expansion coefficient between -100 °C and +120 °C. These coefficients differ

from our earlier reported values [8] , however, we believe our present values are better because of

our improved experimental technique. The thermal expansion coefficients calculated with these parameters are slightly higher than values obtained by Novikova [7] , however, the type of material we

have used differs from that used by Novikova. Moreover, Zhdanova and Sergeev report that the thermal

expansion coefficient of ZnSe varies with free carrier and impurity content [15]

In table 2, we list the thermal expansion parameters we have obtained for CaF^ and BaF^ between

-100 °C and +120 °C. When these parameters are used to calculate a, we find agreement within about 1% with values published in the literature [6]

The expressions we have obtained for the thermal expansion facilitate our measurement of dn/dT

by the interferometric method. In table 3, we list the values of dn/dT we have obtained for CVD ZnSe

at 632.8 nm from -100 °C to +120 °C. The value we obtain at 25 °C is 1.06 x 10~^ k"""" which is 2% smaller than our previously reported values. We attribute this difference to an improvement of our measurement technique; however, we cannot explain the discrepancy of our value with that of Mukai

The piezo-optic constants of fused silica and CVD ZnSe have been measured at 632.8 nm with

the modified Dyson interferometer. The results obtained for fused silica, given in table 4, agree extremely well with those of Primak and Post [5] . The standard deviation we give in the table

represents the deviation from a linear least squares fit of optic path change as a function of stress. The actual accuracy is not as good because of possible systematic errors in the system, but these we believe to be less than 2%. Such systematic errors would include stress gradients.

The results we have obtained for CVD ZnSe are given in table 5. The agreement obtained with

our previously reported values [3] is remarkably good, where the errors in the table are attributed

mainly to the uncertainty in S . The data also agree, within experimental error, with the results

of Goldstein et al. [4].

(3)

o

Results

et al. [9]

-60-
Acknowledgment

We thank the Raytheon Company for the CVD ZnSe.

References

[1] Sparks, M. , J. Appl. Phys . 4£, 5029 (1971)

Jasperse, J. R. , and Gianino, P. D. , J. Appl. Phys. £3, 1686 (1972); Bendow, B.

,

and Gianino, P. D. , Applied Physics 2^, 1 (1973); Bendow, B. , Gianino, P. D.

,

Hordvik, A. , and Skolnik, L. H. , Optics

Comm. 7, 219 (1973).

[2]

[3]

[4]

[5]

Feldman, A., Malitson, I., Horowitz, D.

,

Waxier, R. M. , and Dodge, M. , in Laser Induced Damage in Optical Materials

,

edited by A. J. Glass and A. H. Guenther,

National Bureau of Standards Special

Publication 414 (1974) pp. 141. Feldman, A. , Waxier, R. M. , and Horowitz, D. , in Optical Properties

of Highly Transparent Solids, edited

by S. S. Mitra and B. Bendow (Plenum

Publishing Corp., N.Y., 1975) pp. 517. Goldstein, L. F. , Thompson, J. S.

,

Schroeder, J. B. , and Slattery, J. E.,

Applied Optics 14, 2432 (1975).

Primak , W. Phys. 30,

, and Post,

779 (1959). J . Appl

.

[6] Baily, A. C, and Yates, B. , Proc.

Phys. Soc. 9^^, 390 (1967); Sharma,

S. S. , Proc. Indian Acad. Sci . A31,

261 (1950).

[7] Novikova, S. I., Soviet Physics-Solid

State 3_, 129 (1961) [Fizika TeverdogoTela 3, 178 (1961) ]

.

[8]

[9]

[10]

[11]

[12]

[13]

[14]

[15]

Feldman, A., Malitson, I. H. , Horowitz,

D., Waixler, R. M., and Dodge, M. , in Proceedings of the 4th Annual Conference on Infrared Laser Window Materials

,

(1974) pp. 117. Mukai, H. , Krok, P., Kepple , G. , Harris,

R. , and Johnston, G. , in Proceedings of

the Fifth Conference on Infrared Laser Window Materials (1975) pp. 225. Nye, J. P., in Physical Properties of

Crystals (Oxford University Press, London,

1957) pp. 243-254. Waxier, R. M. , Cleek, G. W. , Malitson,

I. H. , Dodge, M. J., and Hahn, T. A.,

J. Res. Nat. Bur. Stand. (U.S.) 75A , 163

(1971)

.

Dyson, J., Interferometry as a Measuring

Tool (The Machinery Publishing Co., Ltd.,

Brighton, 1970).

Green, F., in Optical Instruments and

Techniques, 1969, edited by J. H. Dickson

(Oriel Press Limited, New Castle upon

Tyne, England, 1970) pp. 189-198. Feldman , A .

,

Instrum. 46, and McKean , W. J., 1588 (1975). Rev. Sci.

Zhdanova , V. V., and Sergeev, V. P., Soviet Physics-Solid State L4, 1859 (1573)

[Fizika Tverdogo Tela 14, 2153 (1972)].

Figures

SILICON MATRIX VIOICON

LOW PRESSURE

HELIUM

WINDOW

COPPER FURNACE

SPECIMEN

HEATER THERMOCOUPIES

Figure 1. Cryogenically cooled furnace for measuring thermal expansion and change of refractive index with

temperature. The specimen shown

is for dn/dT.

SOLEIL BABINET COMPENSATOR

SPATIAL FILTER AND

BEAM EXPANDER

WOLLASTON PRISM

OUARTER WAVE PLATE

] MIRROR

Figure 2. Modified Dyson interferometer for measuring plezo-optic constants.

-61-
Tables

Table 1. Tabulation of expansion parameters of CVD ZnSe for -100 °C < T < +120 °C

2 3 At/t = A T + AT + AT

Run # Specimen # do'

6

^2 do" 9 A3 (10-^^

1 1 7 .23 + 08 5 .24 + 31 -1 40 + 30

2 1 7 .35 + 03 4 .62 + 16 -1 67 + 15

3 1 7 .33 + 05 4 .80 + 17 -1 47 + 22

4 1 7 .23 + 04 5 .19 + 17 -1 26 + 17

5 2 7 .24 + 05 5 .51 + 25 -1 53 + 22

6 3 7 .28 + 04 5 .94 + 13 -1 44 + .13

7 3 7 .30 + .04 5 .27 + 15 -1 34 + 16

Weighted Average 7 .29 + 02 5 .26 + 17 -1 45 + 07 Errors are the standard

deviation of the mean.

Table 2 . Expansion parameters of CaF^ and BaF^ for -100 °C < T < +120°C

At/t = A^T + A^T^ + A3T^

CaF^ I8.4±.l 16.4±.3 -3.7±.3

BaF^ 18.1 ±.1 11.5 ±.3 -2.0 +.3
Table 3.

Temperature

dn/dT of CVD ZnSe at 632.8 nm

dn/dT (lO"^ k""*")

-100 9 . 3 - 80

— bO Q Q

- 40 lU . U

— 20 in 0

0

in A

20 XU . D

Af\ 4U in 7

60 10.9

80 11.0 100 11.1

120 11.2

Table 4. Piezo-optic constants of fused silica at 632.8 nm

Present Work Primak and Post

^11

^12

0.42 ±.002

2.70 +.006

0.44

2.71

-12 2 Units - 10 m /N

Reference [5]

3Errors represent the standard deviation of the experimental data to

a straight line least squares fit. Accuracy is believed to be

within 2%.

Table 5. Piezo-optic constants of CVD ZnSe at 632.8 nm

Present Work Previous Work

^11

^12

0.17 ±.04

-1.44 ±.04

0.17 ±.05

-1.44 ±.04

0.22 ±.05

-1.48 ±.05

Units - lo"'''^ m^/N

Reference [3] ^Reference [4]

No ducuMZon,

COmENTS on PAPER EV FELPMAW, HOROWITZ, AMD WAKLER

-63-
1.8 REFRACTIVE INDEX AND TEMPERATURE COEFFICIENT OF REFRACTIVE INDEX OF HOT-FORGED CALCIUM FLUORIDE

Marilyn J. Dodge

Optical Physics Division, Institute for Basic Standards, National Bureau of Standards

Washington, D.C. 20234

The refractive index of a sample of hot-forged CaF2 was measured from

0.25 ym to 8.0 ym by means of the minimum-deviation method on a precision spectrometer. Data were obtained near 21°C and 34°C. Each data set was fitted to a three-term Sellmeier-type dispersion equation, which permits interpolation

of refractive index as a function of wavelength within a few parts in

10 ^. Using the index values obtained at the two temperatures, the change in

index with temperature, An/AT(°C) "'" was calculated. The refractive index and

An/ATC^'C) values obtained for this specimen are compared with data previously

published.

Key words: Calcium fluoride; refractive index; temperature coefficient of refractive index.

1. Introduction

Optical distortion as a result of temperature gradients can occur in transparent or semi-transparent components of high-power laser systems at powers below that which would be required to melt or fracture the component [1-2]'''. Knowledge of the refractive index, n, and temperature coefficient of refractive index, dn/dT of a candidate material to be used in a high-power laser system is necessary

before the amount of optical distortion which might occur can be predicted. The study of these parameters is a part of an optical materials characterization program [3] which is currently in progress at

NBS.

The increased use of high-power lasers in the 2-6 ym wavelength region has resulted in a need for improved transparent component materials to be used in large, high-resolution optical systems. Ideally,

the materials should be stronger, exhibit less scatter, and have lower absorption coefficients than

the materials which are currently available [4]. Hot forging of alkali halides and alkaline- earth

fluorides is one fabrication technique being developed to achieve these goals. Alkaline- earth fluorides

are the materials of choice for use in the 2-6 ym range [5]. If the new manufacturing technique does induce changes in the optical absorption of these materials, corresponding changes in the refractive

index and dn/dT would also be expected from dispersion theory [6] and need to be determined.

John R. Fenter of the Air Force Materials Laboratory provided a large finished prism of hotforged CaF2 manufactured by Harshaw Chemical Corp.^. This specimen has a refracting angle near 60°,

with one polished face about 4 cm x 5 cm and the other polished face about 4 cm x 6.5 cm. Unlike the synthetic single crystal CaF^ which is commercially available, the new material is polycrystalline.

The index of refraction was determined by means of the minimum-deviation method using a precision

spectrometer shown schematically in figure 1[7]. From the ultraviolet to 2.0 ym, the index was determined at known emission wavelengths of mercury, cadmium, helium, and zinc. Beyond 2 ym, a glo-bar was used for the radiant-energy source, and measurements were made at known absorption bands of water,

carbon dioxide, polystyrene and 1,2-4 trichlorobenzene . A series of narrow-band interference filters

was also used between 3.5 ym and 6.5 ym. A thermocouple with a cesium iodide window was used for the detector. The scale of this spectrometer can be read to 1.0 second of arc. The accuracy of this scale permits the determination of the refractive index of good optical quality material to within

2 X 10 over a wide wavelength range.

*This work was supported in part by the Defense Advanced Research Projects Agency.

1. Figures in brackets indicate the literature references at the end of this paper.

2. The use of company and brand names 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.

-64-
The index was detertnined from 0.2483 to 8.03 ym and at average controlled room temperatures of 20.8°C and 33.6°C. Each set of experimental data was fitted to a three-term Sellmeier-type dispersion

2 2 2 2 equation [8] of the form n -1 = I[AjA /(A -X^)]. The index of refraction is represented by n, A is the wavelength of interest, the ^j's are the calculated wavelengths of maximum absorption and the A^'s

are the calculated oscillator strengths corresponding to the absorption bands. The ^j's and A^'s are

not intended to have any physical significance. Primary emphasis is given to procuring a mathematical

fiu of the measured data useful for interpolation.

2. Index Data

The constants calculated for each dispersion equation, 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: Constants for Dispersion Equations

Constant 20.8°C 33.6°C

No. of Wavelengths

Average Absolute

residual of index x 10^

0.98584551

0.05290246

4.2816899

0.07218116

0.14114719

36.465937

69

1.9

0.97066991

0.06792205

4.1634415

0.07115659

0.13670627

35.984292

47

1.9

It should be emphasized that these constants refer specifically to this sample of hot-forged CaF2, at the stated temperatures, and should be used for interpolation only within the wavelength range of the experimental data, or approximately 0.25 to 8.0 ym. The average absolute residual is an indication of the overall accuracy of the experimental data. Generally, the largest residuals were in the IR beyond

5 ym, where 35 strong atmospheric absorption bands existed between 5.0 and 7.3 ym. Because of the large size of the prism and the apparent excellent quality of the material, these bands were sharp and

the minimum was easily determined. Therefore, the index determinations are not considered to be any

less accurate than in the UV or near IR. The wavelengths of these atmospheric bands were assigned

from values in the literature [9,10]. The uncertainties in the wavelength values would be difficult

to define due to differences in the laboratory conditions and the existence of many weak bands which

could affect the wavelengths of nearby strong bands. An error of 0.1% in wavelength would result in an index residual of 9 x 10 ^ in this particular sample.

The refractive index was calculated at regular wavelength intervals for each temperature using

the appropriate fitted parameters. The results of the 20.8°C data are plotted as a function of wavelength in figure 2.

The index values obtained for this specimen are compared in figure 3 with those obtained by I. H. Malitson on single crystal CaF-[ll]. The single crystal data is represented by the "zero" line. The

^ -5 differences reach a minimum between 0.7 and 0.9 ym where the hot-forged CaF. is only -0.5 x 10

_5 ^ less than the single crystal. A maximum difference of -6.6 x 10 between the two types of CaF2

exists at 0.28 ym and 4.8 ym. The index variance at 0.28 ym is probably the result of a weak absorption

band near 0.3 ym. The Harshaw catalog [12] shows transmission runs for two grades of CaF2, the UV

grade in which this band does not appear, and the optical grade, sold for use in IR components, in which the band does exist. Assuming this band did not exist in the single crystal studied previously,

the differences in index would become more negative, reaching the maximum An, then receding as the index of the new CaF„ approaches that of the single crystal in the red region of the spectrum.

-65-
The maximum magnitude of An at A. 8 ym is not as easily explained. The absorption bands and interference filter used for index measurements between 4 and 5 um generally yield low residuals in highly transmitting materials such as Ca¥^ when fitted to the dispersion equation. The Sellmeier

equation will fail completely in an area of strong absorption in the material, but will attempt to fit

data through a region of weak absorption yielding larger residuals in the area of the band. In this

study, the residuals between 4 and 5 vm were an order of magnitude higher than the ones immediately on either side of the region where they agreed well with the overall average of 1.9 x 10 ^. The Sellmeier

fit called for higher wavelengths in this region, indicating that the experimental index values were

lower than what would be expected in a region of normal dispersion. From dispersion theory, this

could be an indication of a weak absorption band near 5 pm. There are indications of a weak absorption

band in the 3-4 ym region of ionic crystals including fluorides [13] , Most absorption studies which

have been made on the polycrystalline material have been done at the specific laser wavelengths of HF

(2.7 um) , DF (3.8 pm) and CO (5.25 ym) . To get a clear picture of the absorption characteristics in

this area it would be necessary to run an absorption curve as a function of wavelength.

i

3. Temperature Coefficient of Index

The index values calculated using the fitted parameters at the two temperatures were used to determine An/AT(°C) as a function of wavelength and are indicated in figure 4 by the solid line. ' Also shown in this figure for comparison purposes, is the An/AT curve for the single crystal material ! referenced previously. Generally the hot-forged CaF2 shows much smaller An/AT values than the single ; crystal. The maximum An/AT(°C) ^ of the new material over the wavelength range studied is -4.5 x 10 ^

i between 0.5 and 0.6 ym, or a little less than half the An/AT(°C) "'' reported for the single crystal in

the same region. The ratio is about the same at 2 ym where An/AT(°C) of the hot-forged sample is a -3.9 X 10 ^, but at 6 ym An/AT of the polycrystalline material is only about a third of that given

for the single crystal material, or a -2.2 x 10 ^. Beyond 6 ym, An/AT(°C) ''" increases to a -4.3 x 10 ^

j at 8.0 ym. The negative An/AT indicates that the effect of thermal expansion on the change in refractive index is greater than the effect of the frequency contributions. Because of this, An/AT is expected to increase as either the ultraviolet or infrared limits of transmittance are approached

[14]. The deviations from this expected behavior could be another indication of some type of absorption

in the 5 ym region. The An/AT(°C) values which were calculated from the measured data are shown as

^ata. points in figure 4. The magnitude of the values between 3 and 4 ym increase to almost 1.5 x 10 ^

higher than those calculated from the smoothed data, but then decrease to about 2 x 10 ^ less than the smoothed values at about 4.5 ym, giving an anomalous appearance. The smoothed An/AT values are considered accurate within 1 x 10 ^. Other investigators have stated that when determining An/AT by

the technique used in this study fourth decimal place accuracy in index is required to achieve a 10 percent accuracy in An/ AT [15].

4. Conclusions

This is the first detailed study of the refractive properties of hot-forged CaF2. From the standpoint of the reduced temperature coefficient of refractive index, it appears that components

fabricated from this material could produce less optical distortion then those made from synthetic

single crystal CaF2. However, since the hot-forged manufacturing technique is still in a stage of development and Improvement, and in light of the refractive index anomaly which appeared in the 4 to

5 ym region of primary Interest to laser systems designers, further studies on other samples of this

material should be considered. It should not be assumed that the refractive properties of this

sample are necessarily typical of what can be expected of future production runs of hot-forged CaF2.

5. Acknowledgments

The author expresses sincere appreciation to Warren K. Gladden for his technical assistance In this study.

6. References

[1] Sparks, M., J. Appl. Phys. 42, 5029 (1971).

[2] Loomis, J. S. and Bernal, E. G., "Laser Induced Damage in Optical Materials: 1975,"

p. 126, NBS Special Publication 435, US Govt. Printing Office, Wash. (1976). -66-

[3] Feldman, A., Malitson, I., Horowitz, D., Waxier, R. M. and Dodge, M., "Laser Induced Damage in Optical Materials: 1974,"

NBS Special Publication 414, p. 141, US Govt. Printing Office, Wash. (1974).
[4] Newberg, R. T., Readey, D. W., Newborn, H. A., and Miles, P. A., Proceedings, Fourth Annual

Conference on Infrared Laser Window Materials,

p. 445, AFML (Jan. 1975).

I [5] I.B.I.D.

[6] Jenkins, F. A. and White, H. E., Fundamentals of Optics , 3rd ed. McGraw-Hill

Book Company, Inc., 1957).

j [7] Rodney, William S. and Spindler, Robert J.,

! J. Res. Nat'l. Bur. Stds. (US), 51, 123,

! (1953).

! [8] Sutton, Loyd E. and Stavroudis, Orestes N.,

J. Opt. Soc. Am. 51, 901 (1961).

[9] Downie, A. R., Magoon, M. C, Purcell,

Thomasine and Crawford, Bryce Jr., J. Opt.

Soc. Am. 43, 941 (1953)

.

[10] Benedict, W. S., Claassen, H. H. and

Shaw, J. H., J. Res. NBS 49, 91 (1952).

[11] Malitson, I. H., Applied Optics 2^, 1103

(1963) .

[12] Harshaw Optical Crystals, The Harshaw

Chemical Co., 22, (1967).

[13] Hass, Marvin, Proceedings, Fifth Annual

Conference on Infrared Laser Window

Materials p. 850, UDRI (Feb., 1976).

[14] Krishnan, R. S., Progress In Crystal Physics

Vol. 1, Ch V, p. 146 (Viswanathan, S. at the Central Art Press; Chetput, Madras 31, 1958)

[15] I.B.I.D., p. 144.

Figure

SCANNING

DEVICE

1. Schematic diagram of the modified Gaertner pre- cision 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.

-67-
-10 10

.1

HOT-FORGED CaFg

SINGLE CRYSTAL

Malitson|1963)

WAVELENGTH.Min

J L

.5 10

Figure 3. The differences in index of hot-forged CaF2 from

single crystal CaFz at 20.8°C as a function

of wavelength (logarithmic scale) . The index of the single crystal is represented by the zero

line.

Figure 4, Temperature coefficient of index as a function of wavelength (logarithmic

scale) of CaF2. The solid line represents An/ATCO"' calculated from the fitted values at 20.8''C and 33.6°C for the hot-forged CaF and the data points

(x's) represent An/AT values calculated

from the measured index values. The

dashed line shows An/AT for single

crystal CaF2 from the literature.

2x10

-4

-8

-10 -

-12x10 ^

HOT-FORGED CaFg

--- SINGLE CRYSTAL

Malitson|1963|

10

WAVELENGTH,/iiii

COmENTS ON PAPER BV VODGE

Mo (LLiaii&ilon on thii papeA,

-68-
* 1.9 FABRICATION AND PROPERTIES OF LASER WINDOW MATERIALS

R. M. Cannon, H. K. Bowen, A. M. Glaeser,

H. J. Mayson, F. A. McClintock, W. M. Sherry,

J. B. Vander Sande and M. F. Yan ! Massachusetts Institute of Technology

Cambridge, Massachusetts 02139

In hot forged CaF2, as in forged alkali halides, fine subgrains develop and the subgrain size varies inversely with the

forging stress. Dynamic r ecr y s t a 1 li za t ion is more of a problem

than for KCl or KBr ; it results in very coarse grained regions

in which the subgrains are poorly developed. In alkali halides

aliovalent impurities exert a significant drag on grain boundaries causing large reductions in grain boundary migration rates

and significant retardation of recrystallization and grain

growth. Theoretical results for the boundary mobility and for

the conditions under which a boundary may escape from the

segregated Impurity cloud are presented.

Theory of the macroscopic fracture behavior resulting from

failure by coalescence of subcritical cracks shows the extent of

detectable precracking and the effects of biaxial stresses,

specimen size, and the distribution of microstrengths . The

ultimate strengths do not fit an extreme value distribution even

if the microscopic strengths do; this makes statistical design

more difficult and may necessitate proof testing or inspection.

Flaw strength distribution curves can be determined from acoustic

emission measurements during microhardness testing. Crack openings as small as 0.1pm have been nond e s t r uc t ive ly detected using

scattered light. Better resolution is required for stronger

materials for which crack openings as small as O.Olym must be

detected . Key words: CaF2; crack coalescence; flaw detection; flaw

distribution; forging; grain boundary mobility; subgrains.

Introduction

In recent years, alkali halides have been forged both to shape them and to produce the strengthening necessary for use in high power laser windows. The desirable fine subgrain micros tructures can be formed by forging under appropriate

conditions of temperature and stress. However, it has been found, that impurities

must be added in order to inhibit recrystallization and stabilize these microstructures [1]^ Theory has been developed for the effects of impurities on grain

boundary mobility and on the conditions under which boundaries can breakaway from

the impurity cloud resulting in much faster rates of recrystallization [2]. The first

part of this paper presents the results of similar experimental work on forging and

substructure development in CaF^ and summarizes some of the more recent results of the

analysis of impurities on boundary mobility and breakaway during recrystallization.

The second part of the paper deals with the problem of designing with brittle

ceramics in applications such as laser windows. It is often assumed, frequently

without sufficient justification, that a simple analytical form can be used todescrilie

the distribution of flaws, this simplifies the statistical design and prediction of

size effects and probability of failure. This paper presents the results of some analytical models based on failure from coalescence of subcritical cracks in which the

effects of specimen size and loading mode on the stress for initial cracking and for

final failure are investigated. Finally, methods for determination of flaw distributions are presented. A partially destructive method involves indentation tests and

acoustic emission monitoring from which complete distribution curves of flaw density

can be determined. Nondestructive methods of detection of worst flaws by light

scattering are also discussed.

* Work sponsored by ARPA and monitored by RADC/ET.

1. Figures in brackets indicate the literature references at the end of this paper.

-69-
Forging of CaF^

Undoped single crystals of CaF2 with random initial orientations have been forged'

within the temperature range of 475-1229°C. The samples were typically forged to 60%

reduction by which time they had reached a steady state condition. The temperature

dependence of the steady state flow stresses is shown in figure 1. For forging at temperatures greater than 1100°C oxide contamination was apparently a problem as manifest by a cloudy condition of the crystal and micros true tural evidence of second

phase precipitates which may have been calcium oxide. This occurred despite the

reducing atmosphere which can be expected from forging in vacuum in a graphite die.

For specimens which were hot ejected and air quenched in order to preserve the forging

micr o s t r uc t ur e cracking frequently resulted from thermal shock; this could usually be

eliminated by proper cooling. At temperatures below about 600°C there was evidence

for cracking during forging usually starting as edge cracks, although nearly crackfree

pieces could sometimes be obtained. Anderson, et al [3] reported that crack free

specimens in this temperature range can be achieved by forging with an additional

hydrostatic constraint to suppress fracture.

The microstructures generally exhibited subgrains resulting from recovery during

forging as is shown in figures 2a and 2b. At the lowest forging temperature, UlS'C,

which is 0.45T , the mic ro s t rue tur es have a cellular pattern. The boundaries themm selves are rather poorly resolved suggesting that at these temperatures the dislocations are not knit into tight subgrains, but perhaps are still better characterized as loose dislocation cell boundaries. At higher temperatures, the subgrain boundaries

are much sharper, figure 2a. Examination by transmission electron microscopy,

figures 3a and 3b, also indicated that at low temperature the subboundar ies do not ' reach an equilibrated structure as is found at higher temperatures. Within these

subgrains finer dislocation cells can often be seen as shown in figure 4. The subgrain boundaries have a low mi s o r i en t a t ion angle and the samples have a strong \ c r y s t a 1 lo gr aph i c texture. i The subgrain size varies inversely with the forging stress, figure 5, similar to i that previously reported in forged KCl and KBr [1]. The data suggest that in CaF2 a

j somewhat stronger a stress dependence may result than the daa ^ dependences typically

found after creep of metals and alkali halides.

Recrystallization !

In many of the CaF2 samples dynamic recrystallization occurred during forging, \ figure 6. Areas essentially free of subgrains or with poorly defined subgrain

boundaries were observed in samples forged at temperatures as low as 580°C or about ] half the melting point. Apparently these are sub-boundaries in which the mis- | orientation angle is much lower than in those cases in which recrystallization has not|

occurred. Similar behavior has been reported in KCl and KBr, but was not usually seen | at temperatures below 0.8T for pure specimens and at even higher temperatures for

m ] doped crystals [1,2]. In CaF2 forged at temperatures above 800°C and quenched to room temperature, the interior was frequently largely recrystallized and well developed ^ subgrain deformation structures were only found around the edges.

j This results from nucleation of new grains during forging which as they grow

produce strain free regions in which the subgrain structure is largely removed.

Sometimes these new grains grow until they impinge with other similar grains leaving

occasional high angle boundaries within the crystal, whereas in other cases they ' apparently slow down and leave regions of essentially un-recrystallized material ' between the new grains. Within these strain free grains transient creep starts again

and new substructures develop with continuing strain. The degree of development of

the new subgrains depends on the amount of strain which has occurred between

recrystallization and the end of forging.

In KCl the extent of recrystallization during forging or subsequent annealing is

very sensitive to impurities due to the effect of impurity drag on grain boundary

mobility and to the effect of impurities on nucleation [1,2]. The nucleation problem

per se is currently not well understood. Very likely it does not involve discreet

j nucleation in the classical sense, but instead may result from the occasional develop-|

ment of a subgrain boundary of higher than average misor ientation angle which has a higher mobility; it then acts as a nucleus and grows through the crystal rapidly

leading to the strain free region. Thus the breakaway of a portion of the boundary

or sub-boundary from the impurity cloud may be important in its formation as an nuclei.

Modeling of the effects of impurities on the steady state boundary mobility and

on the process of breakaway of the boundary from the impurity cloud has been done

particularly with emphasis on doped KCl [2]. Previously reported experimental work

showed that there is a much greater drag force from divalent impurities than from

-70-
monovalent impurities; divalent materials such as Ca or Sr give greater drag at the

100 ppm dopant level than do Br or Rb at concentration levels of approximately 5% [1,2].

Consequently, calculations of the impurity segregation and the effect on the boundary

mobility are based primarily on the electrostatic interaction. It has been found

theoretically that the size misfit or any other interaction energies between the

aliovalent impurities and the boundary are coupled to and modify the electrostatic

effect in an important way, particularly at temperatures near the isoelectric

temperature

.

A calculation of the theoretical mobility vs reciprocal temperatures for Sr doped KCl is shown in figure 7. Those curves with the full line represent a set of

calculations for one plausible functional dependence of the strain interaction energy

which would be typical of Sr doped KCl. Those with the dashed line represent the

results of the calculations for another plausible strain energy Interaction. For both

cases, the maximum interaction energy is presumed the same, and is calculated from the

size misfit; the difference is in the functional dependence of the interaction energy

on distance from the boundary. The results for either assumption show similar

important characteristics. There are two solutions to the steady state impurity drag

problem over much of the temperature range. At very low velocities typified by the

lowest curves shown, the impurities drag along with the boundary and the concentration

of impurity near the boundary, although somewhat less, is approximately equal to that

for a static boundary. The highest mobilities shown are those which would result for

a grain boundary which has broken away from its impurity cloud. This mobility is

typically many orders of magnitude higher than that when the boundary is moving with

its impurity cloud. The points represent experimental data obtained from discontinuous

r ecr y s t a 11 i za t ion in Sr doped KCl. Agreement is encouraging although not perfect at this time. Both the modified theory and the experimental results suggest that even at temperatures very near the isoelectric temperature, which for figure 7 is 444°C, the

additional small strain energy interactions lead to coupled electrostatic interactions.

This yields electric fields of changing sign near the boundary and distributions of the

impurity concentration near the boundary which result in greater drag than would be

predicted from purely electrostatic considerations. At temperatures well away from

the isoelectric the calculated drag is similar to that based on purely electrostatic

considerations

.

At high temperatures, boundaries can move rapidly enough even when dragging the

impurity cloud to give exper imentially observable r ec ry s t al li za t ion rates although

these rates may be much slower than those for pure samples. At low temperatures the

boundary velocity when moving in the impurity drag region is much too slow to account

for any observable grain growth. However, in KCl grain growth has been reported in

pure specimens at temperatures as low as room temperature [2,4]. In these cases, the

few ppm impurity which exist in available crystals would be sufficient to prevent

recrystallization if the boundaries were constrained to move in the low velocity

regime. The observed mobilities are frequently very high and are consistent with

those to be expected for the high velocity intrinsic regime.

The question then is under what conditions does a boundary moving at very low

velocities breakaway from its impurity cloud and move at much higher velocities? If the driving force is sufficiently high the low velocity regime will not be stable and

boundaries will all be torn away from the impurity clouds and only the high velocity

regime will be observable. As can be seen from the calculations both regimes are stable over wide ranges of conditions. The method of breakaway is thought to involve

a spontaneous growth of small velocity fluctuations along the length of a boundary.

Small fluctuations may of course occur for a boundary even if the driving force and

Impurity distribution are uniform. In the real case they will no doubt be

accentuated by non-uniform distributions of impurity and of strain energy or subboundary energy.

The results previously discussed were for a 1-dimensional problem in which the

boundary was assumed to remain planar. It can be extended by considering the

conditions for unstable growth or decay of perturbations as compared to those for

stable or spontaneous growth of perturbations of an inf initesimally fluctuating

boundary [2b, 5], as sketched schematically in figure 8. The instability results in

part from convective motion of impurities which results because the local direction

of migration is not exactly equal to the average direction for the entire boundary,

and it is further effected by the ability of impurities to diffuse preferentially

along grain boundaries. Finally it results from the fact that as the boundary

velocity increases the steady state concentration of impurities decreases and so impurity should be evaporated from the boundary. An analysis which incorporates the

first of these two features explicitly and only partially incorporates the last one

by using the non-linear drag velocity relation calculated from the steady state, has

been used to indicate the conditions under which an initially sinusoidal boundary

would grow spontaneously. It is found that for wave lengths longer than the critical

value, any perturbations would grow, but smaller wavelength perturbations would

-71-
decay . The critical wavelength is

X = 4Tr DYaCoo

^ 2 2 2 F (1+6 V

o where D is the impurity diffusivity parallel to the boundary, y is the grain boundary

energy, a is proportional to the impurity drag in the low velocity regime, is the

average dopant concentration, B is an additional parameter from the impurity drag

theory which is equal to the drift velocity of impurity atoms across the boundary,

and Vq is the nominal velocity. The critical wavelength increases with dopant level,

but decreases strongly with applied driving force, F. The behavior to be expected is shown schematically in figure 8. Similar effects

have been seen in very irregular grains in KCl and are suggested in the isolated grain

in CaF^ in figure 5. It is thought that the growth of one of the perturbations into a bulge can eventually, by lateral expansion of that bulge, lead to an entire segment of

a boundary shedding its impurity cloud and moving at high velocity.

To summarize, an analytical model has been developed to the extent that calculations of grain boundary mobility as a function of temperature or driving force can be

done for divalent doped alkali halides. These results incorporate measured diffusional activation energies and impurity vacancy interaction energies and best estimates

of the individual energies of formation of the two types of vacancies. The electrostatic calculations require no additional unspecified or arbitrary parameters. Strain

calculations are based on the ion size misfit to give the maximum interaction energy

and require an arbitrary assumption about the form of the strain function. Because of

the dominance of the electrostatic requirements near the boundary the calculated

mobilities apparently are not strongly dependent on the assumed form of the strain

energy. Limited progress has been made in predicting the conditions under which a boundary can breakaway from the impurity cloud and migrate at high rates. This is

thought, at least in some situations, to be an important part of the nucleation process

for r eery s tal 1 Izat ion

Fracture by Crack Coalescence

As the fracture in brittle ceramic materials is statistical, proper design

requires knowledge or control of the flaw distribution and of the crack dynamics in

order to design adequate lifetimes or probability of failures. Techniques which have

variously been considered include nondestructive evaluation to determine pre-existing

flaws and flaw distributions, determinations of flaw distributions from strength or other destructive tests, proof test techniques to eliminate those specimens with

particularly severe flaws and monitoring by acoustic emission in order to detect

fracture prior to the critical failure. Frequently failure statistics considerations

utilize an assumed extreme value function for the flaw or strength distributions, and

then predict the size dependence and the probability of failure for a particular loading mode. However, an analytical model for crack propogation by coalescence as well

as measurements of particular flaw distributions in materials with optical polished

surfaces indicate that such an assumption is often inappropriate.

A numerical model [6] has been used to suggest assumptions for and to test

approximate analytical solutions for the problem of calculating the effects of applied

stress state and of part size on failure stress. A hexagonal net is assumed to represent grain boundaries and grain boundary strengths have been taken to satisfy an extreme value function. The part is loaded until the first weakest segment under the

applied stress cracks. The stress distribution is recalculated to include the

modification from this crack and the loading is increased until an additional segment

cracks. This process is repeated until a critical crack is formed from coalescence

of several fractures or from fracture ahead of a particular flaw which develops into

a flaw that limits the load to a maximum.

Some typical results are shown in figures 9 and 10. The cracked segments are numbered in order of cracking. Those that cracked after a maximum load had been

attained are denoted by dashed lines. Two different orientations were considered for

the hexagonal array relative to the direction of the maximum principal component of

applied stress, Sj. In the "vertex" orientation, one of the vertices of the hexagons

is in the direction of maximum principal stress component; in the "face" orientation

one of the faces of the hexagon is in that direction.

Approximate analytical solutions for the two orientations have been developed.

They first find the density of unit cracks as d function of applied stress level and

then the probability of coalescence of these cracks is estimated. Maximum strength is

assumed to have been attained when the crack lengths and the applied stresses are high

enough to cause crack extension even with median strengths at the crack tip.

The effect of the biaxial stress state which is applied is shown in figure 11.

-72-
In this case the solid lines represent the analytic solution whereas the data points

each represent the result of five numerical calculations on the computer. The stress

for instability is considerably less sensitive to the biaxial tensile condition or to

a compressive stress for one of the principal stresses than is the stress for first

cracking. The vertex case is somewhat more sensitive to the stress state

than is the face case. The coefficient m which, for example, is taken equal to 3 is

the so-called Weibull modulus. For larger values of m the difference between the

stresses for first cracking and failure is reduced.

The effect of part size is shown in figure 12. The stress for first cracking is

very much more strongly sensitive to the part size than is the stress for final cracking. This difference is of course smaller for higher values of m. The results shown

are for embedded fracture and have neglected any surface effects on the stress around

a particular flaw. The numerical model has been extended to enable calculations for a semi- inf ini t e array in which the stress field has been modified as a result of the

free surfaces. The results are relatively insensitive to the presence of the surface

assuming that the distribution of strength near the surface is the same as that in the

interior. The strength of the individual grain boundary facets have been assumed to follow an extreme value function. As a result the size dependence shown for first

cracking is that to be expected for such a function. It can be seen however, from

figure 12jthat the size dependence for the final cracking or instability condition is

much weaker than that predicted for an extreme value function. This means that when

the first crack is not critical, designing on the assumption that the final strength

satisfies an extreme value function will not be correct in its predictions on size

dependence. Although the predictions would be desirably conservative in that larger

pieces would not be weakened as much as would be predicted, the degree of error can be seen to be large enough to be appreciable for the larger sizes of particular interest

here and would lead to considerable overdesign which is also undesirable.

Flaw Detection

For more realistic design techniques it is desirable to know the actual

distribution of strength impairing flaws to be expected within or on the surface of

a brittle material. This can be inferred from macroscopic strength tests. However, to enable reliable failure rate predictions to be made requires testing an impractical

number of samples. Alternative techniques that yield large amounts of data from

only a few samples have been developed for determining the distribution of surface

flaws [7,8]. This is appropriate for those materials in which the density and

severity of surface flaws if greater than that for volume flaws so that failure largely

originates at the surf ace, which is a frequent situation in brittle materials. The

distribution obviously depends in detail on a particular surface treatment. Such a technique involves making a series of hardness indent measurements and monitoring

the cracks formed using acoustic emission transducers. This technique increases the

sensitivity particularly for opaque materials in which detection of surface flaws is

much more difficult. In addition, a computer program has been developed for determining the actual flaw density distribution from the distribution of experimentally

observed failure loads at which cracking occurs [8]. Some typical results for flaw

densities on sheet glass surfaces before and after etching with an HF solution are shown in figure 13. In this case it can be seen that in the untreated condition a smooth extreme value distribution is completely inappropriate. The results indicate

there are two low strength peaks in addition to a rapidly rising region of high

strength. Most of these low strength flaws are removed by the HF treatment and the

flaw distribution fits a more regular distribution function.

Experiments have also been undertaken to detect the most severe flaws from light

scattering. It is expected that the intensity of scattered light from a columnated

beam will be proportional to the square of the ratio of the width or crack opening

of the flaw to the wavelength of the light. For small crack opening displacements,

the scattered intensity will be very small. Experiments have been conducted using

optical white light and glass slides with various flaws. To date, cracks as small as about O.lym have been detected by this technique. However, for most brittle ceramics

the critical flaws may be much narrower than this if they are sharp cracks. For a perfectly sharp crack the crack is virtually closed under no load, and some prior load

must be applied in order to open it. This displacement will increase with the load,

but it is desirable to apply small enough loads so significant numbers of specimens

are not destroyed in order to detect flaws. Figure 14 represents a typical calculation expected for a material such as ZnSe [9], with Kic=0.75 MN/m3/2^ in which the

crack opening displacement for an elastic crack is plotted against the applied stress

for various assumed crack lengths. The lengths are presented in terms of the final

failure stress which would be expected. It can be seen that for a sensitivity or detection limit of about O.lym that median strength specimens as strong as 45 Mpa

(6600 psi) would have to be stressed to loads, in general, greater than 1/3 of the

fracture stress in order to reach this detection limit. Consequently, it would be

-73-
highly desirable to reduce the detection limit by up to another order of magnitude fori inspection using visible light. However, the results look encouraging for the use of

a light scattering technique as a means of detecting the critical, but relatively \ rare, low strength cracks. Further work in this direction is underway. Scattered | intensities will be reduced by a factor of about 100 for materials transparent only | to longer wavelengths and in this case proof testing must be used. j

i Summary and Conclusions

Fine grained microstructures can be obtained in CaF„ by forging at the ^ ^

I appropriate stresses and temperatures. These grains are in fact subgrains resulting

from a recovery microstructur e rather than from actual recrystallization and the size

of the subgrains varies inversely with the applied stress. However, dynamic

recrystallization is a significant problem in undoped CaF2 at temperatures as low as half of the melting point. Modeling of the recrystallization process, is in progress.!

Theory for the grain boundary mobility in dlvalently doped KCl shows a strong drag i results from the electrostatic interaction of divalent impurities with the boundary.

This is consistent with experimental results which show that divalent impurities are much more effective than monovalent ones in reducing boundary mobility . The theory , further predicts that additional drag will result from those divalent impurities

with a greater size misfit or other secondary Interactions with the boundary.

Preliminary modeling has been accomplished on the problem of breakaway of the grain ' boundary from the impurity cloud. Further work is continuing pn this and other j problems relevant to the nucleatlon problem. ' Numerical and analytical modeling has been carried out on the failure of brittle

materials by the coalescence of subcritical microcracks and subsequent formation of

a macrocrack that limits the load to a maximum. These models have been used to predict the effects of applied stress state, surfaces and of part size on the failure 1 stress. A significant result is that for such materials an extreme value function is j not found to be appropriate for the final fracture behavior. Two schemes have been

investigated for detecting actual strength impairing flaws. A hardness indentacoustic emission method capable of determining the flaw density curves has been

discussed. Limited success has been achieved with detecting more severe flaws with a non-destructive light scattering technique. Further work in both of the areas is

underway

Acknowledgments

This work has been sponsored by ARPA contract F19628-75-C-0191 , under the

direction of Dr. C. M. Stickley and has been monitored by Dr. H. Posen of RADC/ET.

The continued assistence and support of both are gratefully acknowledged.

References

II] Cannon, R. M., Yan, M. F. and Bowen, [5] H. K., "Substructure Formation During

Halide Processing", in Proc. of the

Fifth Conf. on Infrared Laser Window

Materials, ed. Andrews, C. R., and

Strecker, C. L., (Feb., 1976). [6]

[2] (a)Grant, N. J., et al., "Research on Materials for High Power Laser

Windows", Final Report, ARPA Contract

F19628-75-C-0304, (Dec, 1975).

(b)Yan, M. F., "Grain Boundary

Mobility at KCl", Sc.D. Thesis, M. (1976) . I.T,

Roy, A. and Bauer, C. L., "Effect of

Impurities on the Stability of a Moving Grain Boundary", Acta Met. 23 , 957 (1975).

(a) McClintock, F. A., and Zaverl , F.,

Jr., "A Computer Model of the Mechanics and Statistics of Brittle Crack

Initiation", to be published. Int. J. of Fract.

(b) McClintock , F. A., and Mayson, H.J. "Principal Stress Effects on Brittle

Crack Statistics", ASME Applied Mech.

Conf., (June, 1976). Salt Lake City. ]

[3] Anderson, R. H., Koepke, B. G., Bernel

G.,E., "Intermediate Temperature

(0.5 to 0.7 Tmp) Forging of Alkaline

Earth Fluorides", this volume.

[4] Koepke, B. G., Anderson, R. H.,

Bernel G., E., and Stokes, R. J.,

"Room Temperature Grain Growth in

Potassium Chloride, J. Appl. Phys . , 45 , 969, (1974).

[7] Argon, A. S., "Distribution of Cracks

on Glass Surfaces", Proc. Roy Soc.

250 , 482 (1959).

[8] Matthews, J. R., McClintock, F. A.,

Shack, W. J., "Statistical Determination of Surface Flaw Density in

Brittle Materials", J. Am. Ceram. Soc. 59, 304 (1976).
References

9] McClintock, F. A., and Hahn, W.,

"Design of Br Ittle Components Using

Flaw Analysis", in ref. 1.

Figures

I0"3

0 8

Figure 1. Steady state flow stress versus temperature for forged CaF2 samples

,

-75-


-78-
Figure 8. Schematic of breakaway of a grain

boundary from Its Impurity cloud

by growth of unstable velocity

perturbations

.

-79-
-80-
1.0

UJ 0.8

0.6

0.4 -

0.2

0.0

Face

Vertex

CiFirst cracking •Instabilit y

'First cracking -

'instability

Vertex \

Face \

<!. First cracking

10 10^ 10* 10* PART SIZE N

105

Figure 12. Statistical strength as a function

of Dart size for a strength distribution with m = 3, Sl/Sq = 0. Samples of 5, giving 93.75%

confidence for the median.

?n m'm -2''"^*' ZINC SELENIDE

10 MNm 2

0 SO 10 TEST STRESS (MNm^ )

Figure 14. Predicted maximum openings of cracks

having different strengths under

different test loads for ZnSe.

LUQ

-i 102

Ll_

10.

1.0

0.1

1 T- TREATED

1 1

/fl° /

/ °o/ A

/

J

1 1 u

/ 0 /

/ 0 /

L°\ / 1 05 0.6 0.7 0.8 0.9 1.0 STRENGTH S/Smax

Figure 13. Flaw density distributions obtained

from indentation tests for sheet

glass before and after treatment with

HF solution (S^_^ - 225,000 psl) [8].

COMMEWTS OU PAPER BV

CANNON, BOWEN, GLAESER, MAVSON, UaCLTNTOCK, SHERRV, UANVERSANVE, ANV VAN

The. 4peafeeA woa qautiomd KzgoAding th& vaZuti 0^ i.on mobiLitiu mzd in tkii anaty-i-vi. A fiange. o{, vaZuu e.xi6ti In the. titeAcutuA^ {^01 Ionia mobitiXieA in cAijitaLi,, and tkz caZzixlatlon

oi the di{,^uu>ion pKoce^-i ij, complicatzd. The ipeakeA indicated that in hif> opinion, the

titen.atjafie valaeyii me/ie ULi^iaizntly KeJUiabZe to pfiovide a vaZid ba^i^ ^on. the compuJjxtion,

-81-
1.10 DOUBLE-DOPED ALKALI HALIDE LASER WINDOWS

Harold Posen, Normantas Klausutis, Jane Bruce,

Joseph A. Adamskl, Joseph R. Weiner

Solid State Sciences Division

Deputy for Electronic Technology (RADC) Hanscom AFB, MASS 01731

and

S. Andrew Kulin

Manlabs, Inc. Cambridge, MASS

We have observed grain boundary instability in alloys doped with 1.75% RbCl. Theoretical studies have indicated that doping with divalent ions can provide

some strengthening, and at the same time provide grain boundary stability. In an attempt to provide both solid-solution strengthening and grain-boundai^ stability, we have studied a series of double-doped RAP-grown KCl crystals. The primary

dopant is fixed at 1.75 mole %RbCl, while the secondary dopant is varied from

10 ppm to 250 ppm SrCl2. Polycrystalline discs were forged under constant strain

rate. The strength, texture, 10.6 f^m absorption, and grain boundary stability were studied as a function of SrClp dopant level. Our studies indicate that

double doping with a divalent ion enhances grain boundary stability without degrading other desirable window properties.

Key words: Double doped KCl laser windows; grain boundary stability; hot forging;

KCl; mechanical strength; microstructure; optical absorption; RbCl; SrCl2.

Introduction

The alkali halide materials, as a class, have for many years been considered as promising candidate

materials for high energy laser window applications. In particular, KCl has received extensive study

and has responded well to optimization of mechanical and optical parameters. Purification and growth

techniques using reactive atmosphere processing (RAP) has reduced the 10.6 //m optical absorption to the

lO"'* cm-1 range. Alloying with a few percent of RbCl coupled with hot forging to produce a polycrystalline microstructure has dramatically increased the strength of these materials without adversely affecting the optical properties.

However, one of the problems which has been observed in hot-forged Rb-doped KCl is instability of

the microstructure. Secondary recrystallization and grain growth have been found to occur in materials

processed both by us and by other laboratories. This phenomena, of materials returning to their single

crystal state causes these materials to becone weaker. They no longer have the desirable property of

high strength provided by the polycrystalline microstructure. This paper defines a new 10.6 /um laser

window material composition which appears to satisfy both the optical and mechanical requirements of

high energy laser optics coupled with increased grain boundary stability.

In this study, we explore the use of double doping of KCl in order to simultaneously enhance the

mechanical strength and microstructural stability of hot forged billets without degrading their optical

properties. A nominal conposition of 1.75 mole % RbCl-KCl was selected in order to correlate with

forgings being conducted under AFML contract with Honeywell. To this composition were added various

small amounts, on the order of parts per million, of SrCl2. The RbCl serves primarily to enhance

mechanical strength, while the divalent Sr2+ has been shown by theory [1]^ to be a much more effective

stabilizing impurity than the monovalent Rb+. Thus the ultimate objective of this study was to find

some low Sr2+ doping level which stabilized microstructure without introducing optical scattering

problems in the forged billets.

Hot Forging

All forgings were carried out in a Vacuum Hot Pressing System* using an automatic strain rate

control system capable of true-strain rate or ram rate control. This system achieves smoother strain

rate control than the manual technique used formerly. Sample deformation takes place Inside a Poco Graphite punch and die set using Graphoil as a lubricant. This technique differs from that used by some investigators in that no constraining rings or hydrostatic forces are applied to the sample during

deformation. The sample undergoes a fully unconstrained deformation without contacting the die wall.

It has been shown [2] that when a deforming billet does contact a constraint, residual strain is

*This apparatus was procured with financial support from APWL.

1. Figures in brackets indicate the literature references at the end of this paper. -82-
introduced in the finished billet in the area of contact.

All forgings were conducted in vacuum and at a true-strain rate of less than 0.01 cm/cm/min. Deformation is begun very slowly, gradually accelerated over a 20-30 minute period to the desired strain

rate, then maintained at constant strain rate until the required total deformation is achieved. The

forging stress is then removed and the sample is allowed to cool in-situ for approximately 16 hours.

This technique produces forgings free of any edge tearing, cracking, or veiling. The residual strain in

forged billets depends in part on the quality of strain rate control achieved.

All of the starting material used was grown at RADC/ET from J. T. Baker reagent grade KCl, RbCl.and

SrCl2 using the Bridgman technique with a reactive atmosphere of CClij-Argon. The resulting crystals had

a random crystallographic orientation and were forged in the growth direction. Forging blanks were

prepared from the single crystals by cutting with a wet string saw and water lapping to the desired size

and shape. Due to time limitations, these crystals were not annealed prior to cutting and forging.

Forgings were carried out on a series of KCl crystals nominally doped with 1.75 mole % RbCl and

with SrCl2 in amounts varying from 10-250 ppm. A summary of the forging data for each composition is

presented in table 1. Also included is data on the resulting grain size and microstructure . With

SrCl2 doping levels of 50 ppm or greater, complete recrystallization was not achieved and the grain

size could not be measured. Typical exanples of these microstructures are shown in figure 1.

Table 1. Summary of forging data for a series of KCl

crystals doped with 1.75 mole % RbCl and varying ppm SrCl2

Sample No. ppm

SrCl2

Forging Temp

oc Strain Rate cm/cm/min

Grain Size

;um % HeightReduction

0 300 <0.01 7.6 55-60

LQ-652 10 300 <0.01 6.2 60 LQ-666 50 300 <0.01 I* 55-60

LQ-661 100 300 <0.01 I» 55-60

LQ-665a 200 300 <0.01 I* 55-60

LQ-665b 200 330 <0.01 J** 70

*Insufficient height reduction caused inconplete recrystallization.

**Microstructure more fully developed than ]J3-665a.

These results indicate that for low SrCl2 levels of 10 ppm, the same forging conditions apply as for the Rb singly doped KCl. However. as the SrCl2 content is increased to 50 ppm or greater, it becomes necessary to increase the total deformation up to 80 per cent or higher and also increase the

forging temperature in order to achieve fully developed microstructures. Failure to produce total

recrystallization results in a decrease in strength. The underdeveloped microstructures did not reveal

themselves clearly upon polishing and etching, hence it was difficult to evaluate them accurately.

Optical Absorption

The total optical absorption at 10.6 ^m of the RAP grown Bridgman double doped single crystals

was measured using laser calorimetry. The results of these measurements, shown in table 2, list optical

absorption coefficients in the range of 2-6x10-^ cm-1. These values are similar to those measured by

Klausutis [2] and Larkin [3] on KCl doped with RbCl only. The data does not show any trend in optical

absorption with Sr content. In fact, the crystal with the highest SrCl2 content gave the best value.

These results indicate that the addition of small amounts of SrCl2 to Rb doped KCl has no appreciable

deleterious effect on the optical absorption. These measurements were on samples cut from boules prepared during early growth runs, where the quality was somewhat questionable. However, the quality of

the boules is continually improving, and measurements on samples taken from more recent boules should

yield consistently lower absorption values.

Mechanical Strength

The mechanical strength of the forged double doped KCl specimens was evaluated using four-point

bend measurements. Mechanical test bars were cut from the as-fabricated disks with the longitudinal

beam axis parallel to the radial direction of the disk. Typical dimensions were approximately

0.25x0.5x3-0 cm. Sanple surfaces were prepared by grinding followed by polishing with magnesia or

ceria powder in alcohol on billiard cloth. -83-
Table 2. Summary of 10.6 lum absorption and strength data

vs SrCl2 content

Sample No

ppm 0TLI2

(cm-1) Ultimate

Strength

(psi)**

LQ-652 10 6.4x10-^ 5840 LQ-666 50 5.5x10-'+ 4650

LQ-661 100 3850

LQ-665a 200 3020

250 2.5x10-'*

*Calorimetry data obtained from single crystals only.

**Average of several measurements, on forged bars.

Specimen loading was acconpllshed using a Manlabs Inc. constant strain rate Polanl-type tensile

tester. In these tests a specimen deflection of approximately 3.8x10-4 cm/s was employed. Maximum

stress, a, was calculated from the equation

^ = ^ (1)

bh"^ where P is the applied four-point (total) bending load, a is the moment arm, b is the specimen width

and h the specimen height. A typical load deflection curve is shown in figure 2. The curve indicates

that failure was by brittle fracture with very little plastic deformation prior to failure.

The results of the four-point bend tests are presented in table 2 in terms of ultimate strength

(osl). Each value is an average of 3-4 measurements. The highest value was obtained for the fully

recrystallized sample containing a nominal 10 ppm SrCl2. Strength values were found to decrease with

increasing SrCl2 content. It was noted earlier that the degree of recrystalllzation after forging

also decreased with increasing SrCl2 content. It follows that those specimens with underdeveloped

microstructure have lower ultimate strength.

Mlcrostructure Stability

The mlcrostructure of the forged double doped KCl was, at 10 ppm SrCl2, very fine with welldefined grains. However, as the SrCl2 content was increased to 50 ppm or more, the grains coarsened

and were not well defined.

In order to investigate the stability of the mlcrostructure, accelerated lifetime tests were performed. Samples, in the form of unstrained ends of the mechanical test bar samples, were vacuum

annealed in a globar tube furnace. The furnace was maintained at temperature and the samples were

slowly moved into the hot zone, held at temperature for one hour, and then slowly removed. Annealing

temperatures of 350, 400, and 450^0 were used. Samples were then water polish-etched on billiard cloth,

examined and photographed immediately.

The results of the annealing studies are best demonstrated by the micrographs of figures 3 , 4> and

5. At 3500c, some areas of the 10 ppm SrCl2 specimen had undergone complete secondary recrystalllzation with 50 ppm and above, no secondary recrystalllzation was observed. At 400OC, both the 10 ppm

and the 50 ppm specimens had undergone complete secondary recrystalllzation; the 100 ppm specimen

showed some grain growth, while the 200 ppm specimen remained unchanged. Finally at 450OC, the 100 ppm specimen showed partial secondary recrystalllzation, and the 200 ppm sample showed some grain

growth, but no secondary recrystalllzation.

Conclusions

These preliminary experiments have demonstrated that the addition of a few ppm of SrCl2 to KCl

doped with 1.75 mole % RbCl can yield forged billets which

1. have small grain size (less than 10 /urn) 2. have high strength when complete recrystalllzation is achieved (5840 psl)

3. have low 10.6 /um optical absorption (2-6x10-^* cm-1)

4. show greater grain boundary stability with increasing ppm SrCl2 (billets containing 50 ppn

SrCl2 or greater showed no secondary recrystalllzation or grain growth after 350°C anneal)

-84-
Crystals containing more than 10 ppm SrCl2 are now being forged, but as SrCl2 content is increased,

forging conditions must be adjusted to higher temperatures and greater % height reduction in order to achieve complete recr^stallization. However, increasing these parameters may cause inhomogeneities in

the optical path length of such specimens. So it appears that the minimum SrCl2 content which would

improve grain boundary stability without adversely affecting optical and mechanical properties is

probably in the range of 0-50 ppm SrCl2.

In conclusion, the technique of double doping of an alkali halide as described in this paper is capable of producing a material with the required optical and mechanical properties for HEL applications

and with greater microstructural stability than singly doped alkali halide materials. Such a material

should receive very serious consideration for use in 10.6 ^um high energy laser window applications.

References

[1] K. Bowen (private communication). [3J

[2] N. Klausutis et aL "High Strength Forgings of

RbCl doped KCl", Proc. of Fifth Annual Conf

.

on IR Laser Window Materials, 1-4 Dec 1975, pp 1037-1050.

Figures

Figure 1. Microstructure of forged double doped

KCl crystals illustrating full re- crystalllzatlon (left) and incomplete

recrystallization (right)

.

Figure 3. Microstructure after annealing at 350°C

for 1 hour: (left) 10 ppm SrCla - shows

some areas of complete secondary re- cyrstalllzation; (right) 50 ppm SrClg

- no secondary recrystallization.

J. Larkin et aL ."Mechanical and Optical Properties of RbCl-KCl Solid Solutions", presented

at Preparation and Properties of Electronic

Materials Conf., 9-11 Sep 197^, Boston MA,

sponsored by AIME.

a

e

Figure 2. Typical a-e curve for 4-point bend test of double doped KCl.

-85-
Figure 5. Microstructure of 200 ppm SrCla specimen as fabricated (left) and after

450° anneal (right) indicating grain

growth, but no secondary recrystalllzation.

COMMEWTS ON PAPER 8/

POSEN, KLAliSUTJS, BRUCE, AVmSKJ, OJEWER, AWP KULJN

No diicuM-Lon on thii papoA,

-86-
1.11 INTERMEDIATE TEMPERATURE (0.5 TO 0.7 T °K) FORGING OF mp

CALCIUM FLUORIDE CRYSTALS*

R. H. Anderson, B. G. Koepke, and E. Bernal G. Honeywell Corporate Research Center Bloomington, Minnesota 55420

Crack free forglngs have been successfully produced from CaF^ crystals at tem- peratures as low as 0.5 T (°K) using an isostatic forging technique. Forging

TMP o o 2 temperatures and isostatic pressures have ranged from 500 to 750 C and to 28 MN/m

respectively. Most forging has been along <111> directions. The resulting microstructures varied from highly deformed structures with barely recognizable subgrains to structures with subgrain sizes of around lOym in crystals forged at higher

temperatures. Annealing for long times at 720°C resulted in recrystallization and

a large grained polycrystalline structure. Internal cloudiness or "veiling" is observed in all <111> oriented forgings. The extent of the veiling decreases with

increasing forging temperature. The fracture energies of several forgings are compared with the fracture energies of single crystals and large grained polycrystalline material.

Key words: Annealing; fracture energy (y ); isostatic forging; kinking; microstructures

;

recrystallization temperature; veiling.

1. Introduction

Hot press forging is a technique that has been quite successful in producing infrared laser window

blanks from alkali halide single crystals [1]-'^. The forging operation introduces a fine grained microstructure that accounts for considerable strengthening at little expense to the absorption at 10,6ym.

By forging at lower temperatures (l.e.j<200°C for KCW material can be produced with acceptable

mechanical properties [2,3]. To eliminate fracturing of the billet during the lower temperature working,

isostatic forging techniques have been adopted [2,4].

To produce laser window blanks for operation in the 3-5pm range, alkaline earth fluoride crystals can be hot forged in ways identical to those used for alkali halides. The biggest difference is that the working temperatures are increased due to the differences in melting points. For instance, KC£ crystals

isostatically forged at 250 C (.5 Tmp ( K)^ show strength increases of an order of magnitude. The equivalent working temperature for CaF^ is 550 C.

Calcium fluoride crystals have been forged without constraints to large reductions at temperatures

ranging from 550°C (.5 Tmp(°K)) to lOOO^C ( . 8 Tmp (°K)) by workers at Hughes [5-7]. The material forged

at the lower temperatures exhibited fine grain sizes, but cracked during or after hot working [7]. These

workers have since found that crack free CaF^ forgings can be routinely made at 900°C (.7 Tmp°K) , but

that the grain sizes of these forgings are fairly large (i.e., .5 to 4mm) [6]. This material exhibits

rupture strength of about 69 MN/m2.

The high yield strength (I.e. 15 times that of KC£) and the limited number of slip systems available

at temperatures lower than about 300°C dictate that the predominant mode of mechanical failure in CaF2

laser windows will be brittle fracture [8,9]. This is further evidenced from the above quoted value of rupture strength which is clearly less than the extrapolated room temperature single crystal yield

strength of CaF2 of 97 MN/m2 [8, 10].

It is important, theref ore^ to characterize the fracture resistance of CaF2 laser windows and to determine if deformation processing can improve the fracture resistance. The fracture resistance of a material is given by its critical fracture energy, y . is the energy required to create a unit area

of crack during rapid fracture. Freiman et. al.^ [10] have determined yc for {lll}<110> cleavage in CaF crystals to be 0.51 J/m^. They further showed that hot worked, transparent large grain CaF (Polytran)

exhibited a fracture energy of 1.6 J/m^ [10].

In this paper we address the question of whether the fracture resistance of forged CaF2 can be

increased over that of large grained samples. To do this we have adapted an isostatic forging technique

that can produce fine grained, crack free forgings at working temperatures of 0.5 to 0.7 Tmp °K(500° to 750°C). Fracture energy measurements made to date on these forgings have, in fact, shown that a 67% increase in yc is observed when compared to large grain size polycrystalline material.

Work supported by Defense Advanced Research Projects Agency under contract DAHC15-73-C-0464

.

1. Figures in brackets indicate the literature references at the end of this paper. -87-
\

i

! 2. Experimental Procedure I 2.1 Isostatic Forging

A schematic showing the isostatic forging technique is given in Figure 1. The equipment is i essentially identical to that used to forge halides [2,4], but has been modified to operate at higher

temperatures. Crystals are forged in a closed chamber which is pressurized with helium gas to pressure^

up to 34.5 MN/m^. Pressures are maintained during forging by a water cooled 0-ring seal and spring

actuated pressure release valves. Forging is carried out with a 3450 MN/m^ hand pumped hydraulic | press. Deformation rates are monitored with a dial gauge mounted on the press. Two thermocouples I? inside the chamber are used to note the temperature. Forging temperatures up to 800 C can be reached - with this equipment. Typical heating and cooling times during a run were 3 hours to heat and 16 hours

to reach room temperature after forging. ,

Cylindrical single crystals 2.54 and 3.81 cm in diameter with an aspect ratio of unity were used as starting material. In most cases the crystals were forged along <111> at a constant true strain rate of 6x10" min under isostatic pressures of 10.3 to 27 MN/m^ . Lower pressures were used at higher forging,

temperatures to minimize chances of permanent deformation of the chamber. Each starting crystal was , carefully polished with 6 micron diamond paste to eliminate any surface imperfections that could \ initiate a crack during forging. i

1

I 2.2 Fracture Energy Measurements | Fracture energy measurements were made using the modified double cantilever beam technique of

[ Freiman et. al . [11]. A schematic of the sample and loading geometry is shown in Figure 2. Loading arms cemented to a precracked specimen apply a constant moment to each half of the specimen» thereby propagating the crack to failure. The fracture energy is given by '

AM^

c

i where A is a geometrical term, E is Young's modulus and M^ is the applied moment at failure. Fracture [ energy specimens were cut from forgings with a diamond cut-off wheel to typical dimensions of 1cm. wide,; j 1.9cm. longi and 0.1cm. thick. A groove was cut half way through the thickness of specimens cut from |i \ forgings to guide the crack. No groove was necessary to measure the fracture energy of single crystal | i specimens. Samples were cut from forgings such that the crack ran parallel to the top and bottom ; surfaces of each forging about midway through the thickness . i

The microstructures of the forged samples were revealed by polishing through No. 1 alumina and , etching for 1/2 hour in concentrated sulphuric acid. |

1 3. Results and Discussion

I

I 3.1 Forging Results and Microstructures 1

I Table 1 lists forging conditions and heat treatments of a number of calcium fluoride single crystals

isostatically forged at temperatures ranging from 515 C to 750 C. In all cas|s the crystals were forged , to 60% reduction in thickness at a constant true strain rate of 6 x 10""^ min

Purchased from Optovac and Hawshaw

-88-

i
Table 1. Forging conditions and heat treatment of Caf ^ crystals forged to 60% reduction at different temperatures at a constant true strain

-3 -1 rate of 6 x 10 mln

Sample

Forging

Temp. (°C)

Chamber

2 Pressure (MN/m )

Final

Forging

Stress (MN/m^) Heat Treatment

510

515 520

600

650

700

750

27.6

27.6

27.6

20.7

17.2

13.8

10.3

*

* 66.9

56.6

45.5

40.7

Not annealed after forging.

Annealed in situ 1 hour at 720°C. Not annealed after forging.

Annealed in situ 1 hour at 600°. Annealed in situ 2 hours at 800°C. Removed from press and annealed

24 hours at 720°C. Annealed in situ 2 hours at 800°C. Annealed in situ 2 hours at 80n°C.

Without exception, crystals Isostatically forged at temperatures greater than 500 C come out of the press crack free. We have further determined that the forged billets can have their top and bottom

surfaces polished for examination without Introducing cracks. Examination between crossed polarizers

indicates, however, that even forglngs produced at 750°C contain significant internal stresses. Attempts

to cut the forglngs with a diamond wheel for mechanical property tests always result in the Introduction

of one or more large cracks. Further cutting is then possible because the long range Internal stresses

are relieved by the fractures. Crystals forged at 500 to 600°C and subsequently polished, fractured

during room temperature storage as a result of subcrltical growth of surface flaws. Subcritlcal crack

growth in single crystal and forged CaF2 has been fully documented by Freiman et. al. [10] who showed

water to be a major cause. Crystals forged at 700 and 750°C have survived long storage periods at room

temperature. Our observations that the samples fractured during storage prompted the incorporation of the post forging anneals listed in table 1. Although the residual stresses are relieved somewhat by

these treatments, an acceptable annealing schedule has not been determined at this point.

A photograph of crystals forged along <111> to 60% reduction at 650°, 700° and 750°C is shown in Figure 3. As the forging temperature is Increased the shape of the deformed crystals becomes more

uniform. Note particularly that the serrations visible on the sides of the 650°C forging are absent in

the 750°C forging. We attribute this to the increased ease in cross slip at the higher temperatures.

Crystals forged along <111> at temperatures below about 700 C apparently deform by both slip and by

kinking. The serrations on the side of the crystals indicate regions in which deformation occurred

mainly by kinking. The vertical lines at the edges of the billet at which outward flow is impeded appear

to be kink boundaries. One observation we have made of a small fracture that propagated along a kink

boundary in a highly deformed crystal indicated that the boundary was parallel to a {llO} plane in the starting crystal. When the outward flow of material is Impeded at these boundaries the tops of <111>

crystals forged to large reductions at 650°C or below have a "petal"-llke shape.

Photomicrographs of the mlcrostructures of forged CaF2 crystals are shown in Figure 4. Figures 4a, 4b show a structure typical of a crystal forged at 500°C to 650°C. The structure appears highly deformed

with a high dislocation density, but does not contain many regions with recognizable subgralns. The

dark lines running across the structure are kink boundaries. Annealing for an hour at 720°C does not alter the appearance of this structure. Forging at 700 or 750°C plus annealing at 800°C for 2 hours produces a structure with recognizable subgralns as shown in Figure 4c. We have not forged a CaF2 crystal

at 700 or 750°C without an 800°C post forging anneal and cannot comment on whether the anneal causes

or enhances polygonizatlon. The mlcrostructure of the 650°C forging annealed 24 hours at 720°C is shoxm

in figure 4d. The structure is large grained indicating recrystallization had occurred. The grain size

of this material is 0.5mm and is typical of crystals forged at higher temperatures [6].

All <111> crystals forged at 500 to 750°C exhibited Internal cloudiness or "veiling" to some extent.

As the forging temperatures were Increased the amount of veiling decreased. An example of veiling in a crystal forged at 650°C is shown in figure 5. We have noticed a number of things about the veiled

regions that suggest they are regions of high void or microcrack density formed as a result of slip band

intersection. First, the veils have crystallographic features such as three fold S37minetry when viewed

from the top of the crystal. Second, the veiling is heaviest in regions where kinking is observed.

Finally, veiling, decreases at higher forging temperatures due to enhanced ease of cross slip.

-89-
Microcracks can form as a result of the plastic deformation of a crystalline solid by a number of I mechanisms. Two likely mechanisms are by dislocation pile-ups [12] or by dislocation intersections [13],' In the first case, the dislocations at a pile up become so close they coelesce to form a microcrack. In! the second case, glide dislocations on two intersecting slip systems react to form a dislocation on a third slip system which in turn acts as a barrier to further deformation and a microcrack forms. Phillips [8] has pointed out how dislocation reactions of this type can possibly account for both {lOO}

and {no} microcrack formation in CaF„. At temperatures above 500°C the microcracks would be expected

to round out into voids as a result or surface diffusion and be swept along on active slip bands as deformation proceeds. This accounts for the voids being frequently observed in the form of sheets or veils. Since kink boundaries are obstacles to flow, one would expect more slip intersection in these

regions

3.2 Fracture Energy Measurements

As noted in the introduction, a main goal of this study has been to determine if the fracture

resistance (as given by the fracture energy, Y ) of calcium fluoride crystals forged at intermediate

temperatures can be raised above that of large grained polycrystalline material forged at temperatures

above the recrystallization temperature.

The results of fracture energy measurements on single crystals and hot forged crystals of calcium

fluoride are given in table 2. Recall that in the hot forged samples, the crack plane is parallel

to the forged surfaces of the billet and about midway through the thickness.

Table 2. Fracture Energies, Y » of Hot Forged Calcium Fluoride Crystals

No. of Sample Tests (J/m ) Comments

{111}<110> 7 0.45 + .07 Single Crystal

{111}<110> - 0.51*

Single Crystal

Harshaw "Polytran" 1.6 Large grain polycrystal

A 10 2.53 + -03 Undeveloped microstructure

B 9 2.00 + .02 Undeveloped microstructure

C 4 1.63 + .02 Undeveloped microstructure

D t

E 9 1.56 + .03 Large grain polycrystal

F 7 2.03 + .04 lOym subgrains

G 7 2.67 + .01 20um subgrains

Measured by Freiman et al.,[10].

•f Not measured.

We note from table 2 that the single crystal fracture energy measured on the material2used in this

work compares well with the value measured by Freiman et al., [10]. The value of 1.6 J/m measured on Harshaw Polytran material appears to be about what one should expect for large grained polycrystalline

samples since our measurements of fully recrystallized forged CaF (i.e., sample E) gave a comparable

value. The fracture energies of crystals forged in the 500 to 706oc temperature range that did not recrystallize were noticeably higher. Material with both fully developed substructures (samples F and

G) and with undeveloped substructures exhibited fracture energies over 2 J/m^. By undeveloped substructures we mean that subgrains are not recognizable at magnifications used with an optical microscope.

The forging produced at 750°C had a fracture energy 67% higher than that of large grained polycrystalline material. A number of comments can be made concerning these data. First it would be premature to comment on the relation of the fracture energy to the structures introduced by intermediate temperature

forging. More data are needed to determine the effects of residual stresses, subgrain size, etc. Second, the values of Yc quoted in table 2 for the crystals forged at 500 to 700°C should be taken as
conservative. Pole figures taken on these samples have shown them to have a strong <111> texture.

Thus, on the average there is a {ill} cleavage plane oriented approximately parallel to the forged

surfaces in all regions of the forging. Since this is the crack orientation used in the fracture

energy measurements, the measured fracture energies should be a minimum. If samples are cut, on the other hand, such that the crack propagates perpendicular to the forged surface of a <111> textured

forging, the fracture energy is expected to be measurably higher. In this case {ill} cleavage planes

are, on the average, 30.5 from the propagating crack.

Finally, Freiman et al., [lO] determined y for hot pressed CaF^ (Kodak Irtran) with a grain size

of 50 to lOOym to be 3.6 J/m^. This is clearly higher than the press forged material. This hot pressed

CaFp is fabricated from powder, however, and does have some porosity.

4. Conclusions

It has been demonstrated that isostatic forging is a viable process for producing crack-free, fine

grained material from CaF^ single crystals. The forgings have been produced below the recrystallization

temperature. The fracture energy, y , has been significantly increased over single crystal or large

grained polycrystalline materials.

The birefringence observed indicates measurable internal stress. If a suitable heat treatment can

be developed to reduce this internal stress without initiating exaggerated grain growth, an additional

increase in the fracture energy, y , will probably be noted.

c The detrimental effects of "veiling" to the optical properties have not been determined. It is possible that at 3-5Mm, the effects may be minimal due to the small size of the imperfections.

5 . Acknowledgments

The continued interest and support of Dr. H. V. Winsor, Defense ARPA is gratefully acknowledged.

The assistance of W. J. Polzin, J. G. Bruce, and D. W. Woodward is gratefully acknowledged.

6. References

[l] see, for instance, papers in Second, (1972)

through Fifth, (1975) Annual Conferences on Infrared Laser Window Materials and papers in Deformation of Ceramic Materials , R. C. Bradt

and R. E. Tressler, ed., (Plenum Press, New

York, 1975).

[2] B. G. Kopeke, R. H. Anderson, and R. J. Stokes,

Deformation of Ceramic Materials , R. C. Bradt

and R. E. Tressler ed., (Plenum Press, New York,

1975) p. 497.

[3] P.

S.

F. Becher, R. W. Rice, P.

W. Freiman, ibid p. 517.

H. Klein, and

[4] B. G. Koepke, R. H. Anderson, E. Bernal G.,and

R. J. Stokes, Proc. Fourth Annual Conference

on Infrared Laser Window Materials (1974)

p. 621.

[5] R. C. Pastor, et al. , Proc. Third Annual Con- ference on Infrared Laser Window Materials

(1973) p. 509.

[6] H. V. Winston, et al., Proc. Fourth Annual

Conference on Infrared Laser Window Materials

(1974) p. 438.

[7] R. R. Turk, Deformation of Ceramic Materials , R. C. Bradt and R. E. Tressler, ed. (Plenum

Press, New York, 1975) p. 531.

[8] W. L. Phillips, Jr., J. Amer . Ceram. Soc. 44, 499 (1961).

[9] A. G. Evans, C. Roy and P. L. Pratt, Proc. Brit. Ceram. Soc, No. 6, 173 (1966).

[10] S. W. Freiman, P. F. Becher, R. W. Rice, and

K. N. Subramanian, Proc. Fifth Annual Con- ference on Infrared Laser Window Materials,

(1975) p. 519.

[11] S. W. Freiman, D. R. Mulville, and P. W. Mast,

J. Mater, Sci., 8, 1527 (1973).

[12] A. N. Stroh, Proc. Roy. Soc. A223 , 404 (1954)

[13] A. H. Cottrell, Trans. Met. Soc. Am. Inst. Mining Met. Engrs., 212, 192 (1958).

-91-
7. Figures

i i i i I

Is used as the pressurizing

medium.

CROSS SECTIOIM

Figure 2, Schematic showing loading and specimen geometry for fracture energy measurements.

'^igure 3. Photogranhs of CaF; crystals forged to 60% reduction at (from left to right in photograph)

650°, 700°, 750°C, respectively, with the single

crystal starting material in the background.

-92-
-1
Figure 5. Photograph showing veils in a CaFz crystal

forged at 650°C.

COMMENTS ON PAPER 8/ ANVERSON, KOEPKE, ANV EERMAL G.

No (Hiicuiiion on tkAj, papeA.

-94-
* 1.12 FORGING FINISHED HALIDE OPTICAL COMPONENTS

R. H. Anderson, E. Bernal G. and R. J. Stokes

Honeywell Corporate Research Center Bloomington, Minnesota 55A20

The forging of completely finished halide optical components to eliminate the necessity of shaping and polishing is being evaluated. The forging operation is being done isostatically using a closed die technique. Quartz die faces are being

used to produce the desired surface shape and finish. A synthetic oil is being

used as the lubricant while the forging is performed at 275 C.

To produce the finished component, two forging operations are being used. The

first forging is necessary to produce uniform strain distribution. This pressing

is then preshaped by water polishing before the final minor shaping operation.

These, as forged surfaces, appear superior to mechanically prepared surfaces

for the deposition of the required protective and optical coating of thallium iodide.

Optical performance results obtained with these lenses will be compared with those

obtained with conventionally polished halide and glass lenses.

Key words: Closed die; forging; isostatic; polishing; quartz; shaping.

1. Introduction

The difficulties encountered using either mechanical or chemical methods to attain optical surfaces

and specific geometries with alkali halide materials has promoted this investigation. A forging process

to produce a finished halide optical component was envisioned as the logical approach to the problem.

The method was suggested by the success achieved by injection molding of plastic optical components.

From the outset, it was realized that plastics are amorphous materials while the halides are crystalline. Due to this difference, the techniques associated with injection molding of plastics would not apply directly to the forging of finished halide optical components. In the first placej the plastic

deformation of crystalline materials is not isotropic, and second, after the forging operation the halides are subject to nonuniform elastic relaxation. This shape change is related to the crystalline

texture developed during the forging process and is a function of the elastic anisotropy of the material.

This research was initiated to determine the fundamental limitations of the forging process in producing finished optical components. It was necessary to identify what physical geometry could be

attained and what optical figure was possible.

The following sections will describe, in turn, the forging system as it has evolved to date, the forging process including material preparation and parameters, followed by some optical performance

results, and projection for the future.

2. The Forging System

All of the forging operations associated with this program are being done isostatically. A high

pressure fluid environment provides the necessary constraint to prevent cracking during deformation as well as to promote lubrication which prevents welding between the halide material and the die surfaces.

Figure 1 is a sectional view of the isostatic forging system including the closed die used for the final

shaping operation.

A closed die technique is dictated by the necessity to maintain the geometric dimensions associated

with optical components. The present closed die fixture consists of a cylindrical steel tube into which

close tolerance metal pistons are inserted. The tube maintains the axial alignments while the pistons

produce the desired surface shape and finish. The present design uses quartz inserts as die faces on

either side of the halide component to produce the final shape and surfaces.

Work supported by Defense Advanced Research Projects Agency under Contract DAHC15-73-C-0464

.

-95-
3. The Forging Process

j Potassium chloride single crystals 3.81 cm dia. x 3.81 cm high are typically used as the starting I material. All three usual orientations with the <100>, <110> and, <111> cylinder axis have been forged | to determine the particular forging problems each might pose, and also later to be able to evaluate ' optical surfaces and geometries as a function of orientation.

j The forging process as presently developed is a two stage process. In the initial preforging

step the single crystal is deformed by 60%. This amount of deformation has been found necessary to I insure uniform strain distribution throughout the bulk of the forging. Intuition might have suggested ; that the amount of deformation should be kept to a minimum to produce the final forged shape. However, | as deformation starts, discrete slip systems begin to operate producing inhomogeneous strain distri- 1 bution. The interaction of these slip systems at the surface of the crystal produce kink arrays which , entrap lubricant as deformation progresses. This orthogonal surface aberration is referred to as | "rumpling", and it is essential that it be eliminated by strain homogenlzation .

j To enhance the development of uniform strain distribution during this preforging operation, ] Teflon sheet is Inserted between the crystal faces and the metal forging faces of the isostatic chamber.

Figure 2 illustrates the different forged shapes assumed by <100> oriented crystals with and without the|

introduction of Teflon. On the left, the frictional resistance produced between the crystal and the

| metal chamber has restrained the contacting crystal faces and produced the "barreling" as well as the ^ rectangular shape. On the right, the vertical sides and circular shape indicate low surf ace^friction

| with the Teflon and more uniform strain distribution. This forging operation is done at 300 C and an ^ imposed isostatic pressure of 29.655 MN/m . This temperature prevents "veiling" within the forging. |

The preshaped blank is next placed between the quartz die surfaces which have been finished to an optical figure of A/ 10. The closed die is assembled and placed in the forging chamber. The final , forging is done at 275°C with the same isostatic pressure, 29.655 MN/m^.

The finished alkali halide forging is removed axially from the quartz surfaces to prevent relative movement between the die and the halide surfaces which would otherwise result in scratching.

j The lubricant presently being used is a synthetic called Amsoil. It has a di-ester base. Other

lubricants have been evaluated, but have proven unsatisfactory due to chemical breakdown or failure to provide the necessary lubrication. Fluids with high vapor pressures which would eliminate rinsing for lubricant removal have failed to provide the necessary lubrication. The Amsoil is removed by rinsing in xylene followed by carbon tetrachloride.

4. Optical Results

Several forgings have been made which appear to have replicated large areas of the quartz die 1 surfaces. Figure 3 is typical of the clarity exhibited by one of these forgings. | A formal evaluation of the surface contours will be made using a (LUPI) Laser Unequal Path Interferometer. However, as a preliminary evaluation, the projection lens of a lantern slide projector was removed and a variety of lenses were substituted. Using various lenses to project a standard resolutionj

target a comparison can be made. All lenses had the same geometry. Figure 4 illustrates four projections comparing glass, "state of the art" mechanically polished (Janos), early forging, and a recent

forging. It is apparent that the recent forging compares favorably with the mechanically prepared lens.

It has been noted that forged surfaces appear to be better than those of mechanically prepared

j surfaces for antiref lection coating. It is to be determined if this is a real effect.

5. Future Developments

Surface replication results are encouraging and it is hoped that the process will soon become

routine. The effects of elastic anisotropy on the final surface geometry are to be determined as a function of starting orientation. Analysis of these results should yield insight into understanding

methods whereby this problem can be overcome such as special shaping of the die surfaces related to forging orientation.

I 6. Acknowledgments

The continued interest and support of Dr. H. Windsor of Defense ARPA is gratefully acknowledged.

The assistance of W. J. Polzin and D. W. Woodward is also gratefully acknowledged.

-96-
7. Figures

I 1 I I I
MAT

T

' T F s

> «3 4 5 6

» lO 1

12 13

18 ie 20

3 ^ 25 26' 27

O .

Figure 3. Recent KCS forging Illustrating the clarity

achieved

Figure 4. A series of resolution targets to compare various projection lenses.

CLASS

JANOS

E/\PLY FOKCINC

RECENT FORGING

-98-
COMMENTS ON PAPER 8/ ANDERSON, KOEPKE,. AWP 8ERWAL G.

No diicuM-Lon on thli papeA..

-99-
1.13 Vapor Deposition and Mechanical Properties of CdTe

H. L. Tuller, K. Uematsu, J. B. Vander Sande,

E. L. Hall, and H. K. Bowen

Department of Materials Science and Engineering

Massachusetts Institute of Technology

Cambridge, Massachusetts 02139

{

The vapor deposition of CdTe from elemental cadmium

and tellurium sources was studied as a function of the

Cd/Te ratio, the super s a t ur a t ion , and the substrate

temperature. A mu 1 t isubs t r a t e arrangement was designed

to enable simultaneous acquisition of growth data on six

CdTe windows under conditions where substrate temperature

and radial positioning remained the only variables. Polycrystalline blanks 4cm^ x 1mm thick were grown at rates

of .02 - .10 mm/h with the growth rate exhibiting

sensitivity to all of the above parameters. The affects

of varying Pcd/PTe2 ratios during growth, as well as post j) growth annealing, on surface m i c r o s t r uc t ur e , grain size I and void density, as observed under S EM and IR microscopy,

willbepresented. f Shear stress-shear strain results have been obtained

from single crystal CdTe deformed in compression at 300°C.

The defect mi c r o s t r uc t u r e s associated with this deformation

schedule have been observed by transmission electron

microscopy. The macroscopic mechanical behavior will be ^ discussed in terms of the dislocation interactions observed ^ forthevariousdeformationstates. f Key words: Cadmium telluride; growth kinetics; mechanical

properties; microstructure ; s toichiometry ; transmission ,9 electron microscopy; vapor deposition. s

i

1. Growth and Characterization of CdTe Window Material. '

Previous attempts to grow large plates of CdTe for laser window applications have I resulted in materials with absorption coefficients at 10.6 m far in excess of that

| expected theoretically. This can be related to the difficulty in obtaining CdTe ' windows with desired levels of intrinsic and extrinsic defects. Because CdTe exists j over a range of compositions, around the stoichiometric composition, the concentration!

of defects in a given window will be very sensitive to the original growth conditions

(le., T v„^„ f. and Cd/Te ratio) and to the composition-cooling path followed to room temperature.

Since the major factors known to contribute to optical absorptivity include

free carrier absorption and absorption due to defects in the structure of the

material, it becomes clear that one must be able to control the levels of these

defects before one can approach the minimum theoretical absorption limit of ~-10~^cm~^

due to multiphonon absorption [I]-'-. Vapor deposition was chosen as the most

appropriate means of growing large CdTe windows with controlled composition. First

of all, vapor growth affords one the means for close control of the growth parameters

which should ultimately result in samples with more precisely controlled s to ich iome t ry, Furthermore since vapor growth can proceed at temperatures considerably below the

melting point one can expect to minimize the level of thermally generated intrinsic

defect pairs which are said to form agglomerated voids during cooling [2]. Similarly,

one also expects the limits of nons t o ich iome t ry to decrease with decreasing temperatures, thus minimizing the number of free carriers necessary to contend with.

1.1. Experimental

The vapor deposition facility constructed at MIT for the growth of 2-inch

1. Figures in brackets indicate the literature references at the end of this paper

-100-

I
diameter samples has been previously described in detail [3]. This system Includes

separate temperature controls for the sources (Cd and Te) and for the substrate. In this manner one can control independently thevapor pressures of the Cd and Te sources

as well as the deposition temperature. Mass transport is therefore controlled by the

level of the source temperatures and the volume flow rates of the carrier gases; He

gas for Te and He + 8% H2 for Cd to prevent oxygen contamination. The actual

concentration of Cd and Te vapor at the substrate and therefore the supersaturation , Q

[^Cd^e/^']-P

[P,,P,^^^/']equil

(1)

was calculated by making use of previously determined evaporation rate versus

temperature curves for cadmium and tellurium measured at fixed flow rates.

Although relatively large po ly c r y s t a 1 1 i ne windows of 2-inches in diameter and up

to 0.5 cm thick were grown with this apparatus, it was often difficult to obtain

reproducible results. Since one would ideally like to obtain information about the

affects of varying Cd/Te ratios, supersaturation, flow rates, substrate temperature,

etc. J on the deposition rate, s to ich iome t r y and quality of growth of the CdTe windows

in as short a time as possible it was decided to change from a single to a multiple

substrate system. The end plate and window at the exhaust side of the reaction

chamber was replaced by an end plate with 6 0-ring f eed-throughs and a small 2-cm

diameter window to enable observation of the reaction chamber. The end plate is

shown in figure 1. Through each of the 0-ring f eed-throughs an 8mm diameter quartz

tube, sealed at one end, is fitted. A 2 x 2cm vitreous carbon substrate is cemented

to the closed end of each of the quartz tubes with carbon cement. The temperature

of each substrate can easily be measured by sliding a thermocouple into the appropriate

quartz tube from its open end located outside of the system.

This arrangement has a number of important advantages. First of all, because of

the 0-ring feed-through arrangement, it is possible to position each of the six

substrates independently of each other along the length of the reaction tube. Thus

one is able to grow six windows simultaneously under identical growth conditions

except for the variable substrate temperature. Secondly, one is able to study the

affect of radial positioning of the substrates within the tube and finally it is

possible to quench the sample some hundreds of degrees while still within the reaction

chamber. Further modifications to this apparatus include the addition of baffles to enhance mixing of the vapors and improved temperature control of the source materials.

1.2. Results and Discussion

A number of deposition runs were made with the mu 1 t i sub s t r a t e system both as a function of ^ q^^^ ratio (1.9 < Pcd/^Te2 8.5) and of the substrate temperatures.

Table 1 lists the results for a number of these early runs. The growth rate is

observed to be sensitive to the gas composition and the substrate temperature both of

which together determine the supersaturation value, 0. Table 1. Values for the growth rate and supersaturation, Q as

a function of the Pcd/PTe2 ratio and the substrate

temperature, T^.

Run// P^^(atm) TeoCatm) P^^P^ ^^^(atm^^^) T (°C) Q GRl O" ^ (n"TM/ Cd CdTe2 s

KU-2 5.01x10"^ 1.55x10"-^ 1.97xlO~'^

KU-3 8.22x10 ^ 9.67x10 ^ 2.56x10 ^

KU-4 Source material dried up. KU-5 Deposition time too short.

746 11 60 4 16

771 5 06 4 97 780 3 65 4 79 803 1 79

750 13 45 4 11

782 4 41 3 80

785 3 87 3 27 795 2 84 3 16

806 1 97 2 11

-101
Run#

Table 1. (cont

P^^(atm)

as

}CU-6 5.48x10

KU-7 A. 12x10

KU-9 4.40x10

Values for growth rate and s up er s a t ur a t ion , Q a function of the Pcd/^Te2 '^stio and the substrate

temperature , T .

Te2 (atm)

2 . 90x10

4 . 05x10

4 . 06x10

-3

2 . 95x10

2 . 62x10

2

-4

-4

T (°C)

2 .80x10

GRIO (TMTM/h)

758 11 8 6 22 770 7 56 6 42 785 4 54 3 98

785 4 54 5 64

795 3 28 4 35

764 7 95 4 78

782 4 10 4 65

785 4 03 3 30

793 3 08 2 87

805 2 10

792 3 41 3 91

795 3 11 2 65

796 2 98 3 13

796 2 98 3 52

A simple model is proposed below to predict the growth rate under various

experimental conditions. One assumes that when a supersatured gas mixture of Cd and

Te reaches the substrate, CdTe deposits so that the vapor pressures of Cd and Te are reduced to the equilibrium values, P^j'and

C d the exhaust gas are then given by

^Cd ^^Te,

Te2

1/2

The relation between the vapors in

) 'CdTe (2)

while the initial partial pressures of Cd and Te in the reactant gas mixture are described by the relation

Q K CdTe = ^Cd ^ ^e

1/2

(3)

where Q, the supersaturation is defined as in relation ea. (1). Since each molecule of|

Te2 reacts with two molecules of Cd, one requires for mass balance that

Cd

P^'l = 2(P Cd ^ Te, ) T^2 (4)

The number of moles of CdTe deposited per unit time is given by the expression

CdTe (P Cd (6) RT where v is the total gas flow rate. One can easily convert this to an expression

for the growth rate, GR which is given by

CdTe

'cdTe

(P Cd Cd'^ GR = (v) (6)

2

TT rRT where r is the reaction tube radius, M„j„ the molecular weight of CdTe and p„.„ CdTe CdTe

the density. Figure 2 shows the calculated and observed growth rates as a function

of Q for the conditions present in run KU-6. Considering the total duration of each

run (heat up and cool down time (-10 h) and deposition period (-30 h)) the

experimental scatter is not unreasonable.

An equally important parameter to determine is the affect of gas composition on growth rate. This is particularly important if one desires to grow samples of CdTe

off s t o ic hiome t r y . To separate out the affect of changing gas compositions the

growth rate can be calculated as a function of "P , , for instance, by choosing fixed

1/2 1/2 ~4 values of Q and P„, x P„ . The parameters Q~3 and P^,P,„ -2.7x10 were chosen Cd 1 e 2 Cd Te

to enable comparison of our experimental results with the calculated values,

results of both experimental and calculated values of the growth rate vs P Cd

The

are

-102-
plotted in figure 3. The condition of P^,,^ = IP^^^ gives the maximum growth rate in

the calculated curve as might be expected. It then rapidly decreases as one goes to higher or lower Pcd- "^^^ experimental points, as obtained in table 1, fit the

predicted curves reasonably well. This indicates the assumption that the vapor

over the growing sample is in equilibrium with the sample seems to be well justified.

Since the electrical conductivity, O is directly proportional to the concentration

of free electronic carriers, it can be used as a sensitive measure of the degree of

nonstoichiometry existing within a given sample. Table 2 shows the dependence of a (measured at room temperature) on the gas composition during growth or post growth

anneal. It is clear from these results that the level of free carriers in CdTe can

be significantly reduced by equilibrating the samples with the appropriate atmospheres.

For example, for sample KU7-5, the free carrier concentration was decreased by three

orders of magnitude by annealing the sample in a cadmium rich atmosphere. It is

expected that even lower values of a can be achieved by appropriate treatment.

Table 2. Electrical conduc t ivi ty dsf3c; di3 J-LLIlCU-i-ULl fitTi/^t'T/^Ti r o y T.T t* ri UJ. gXUwLll or annealing atmosphere

Growth Annealing

Sample Temp (°C) P ^j(atm) P„_^ (atm)

1 e 2 Temp(°C) P^^(atm) (ohm cm

KU3-1 785''C 8. 22x10"^ 9 .67x10"'^ 3 -4

5x10

KU6-1 785''C 5 . 48x10"-^ 2 .90x10"^ 5 -41x10

783°C

-4

KU7-1 4 . 12x10""^ 4 . 05xlO~^ 6 7x10

KU7-2 763°C 4, 12x10""^ 4 . 05x10"^ 2 6x10"-^

KU8-3 783°C 1 2x10"-^

KU7-5 782°C 4. 12x10"^ 4 . 05x10"^ 1 5x10"-^

KU7-5-A 782"'C 4 . 12x10"^ 4 . 05x10"^ 668°C LOSxlO"-*- 1 5x10"^

KU3-5-A 782''C 8. 22xlO~^ 9 .67x10"'^ ees'c i.osxio"-"- 1 5x10"^

To ascertain the microstructures of the samples, as-grown samples were polished

and examined under an IR microscope. All samples contained large numbers of dark

and semi-transparent platelets assumed to be voids. These were observed both at the

grain boundaries as well as inside the grains. The number of voids tended to increase

when the samples were grown under near stoichiometric gas compositions of P., « 2P

L< Q 1 e 2* Figure 4 shows the significant affect that growth conditions have on both the density

and size of voids present in the samples. Generally crystals grown at very large

super-saturations tended to have the largest number of voids.

In an attempt to correlate growth conditions with the number and size of macrovoids, the as-grown surfaces were examined under a scanning electron microscope, SEM,

An example of a SEM micrograph is shown in figure 5. The samples always exhibit

faceted surfaces and pores are often present at the grain boundaries. These pores

are assumed to form the voids at the grain boundaries observed previously under the

IR microscope. The mechanism of void formation inside the grains is less clear.

High supersaturation [4], constitutional supercooling [5], and turbulance over

the substrate generally tend to contribute to void formation in specimens grown from

the vapor phase. In order to minimize some of these affects a new arrangement was made In which the substrates were protected by a length of quartz tubing. Thereby

mass transport of the vapor species was limited to diffusion through the tubes.

Preliminary runs with this arrangement show that specimens can be grown with smaller

values of supersaturation, Q. Examination of these specimens under the IR microscope show a decrease both in the concentration and size of voids.

Attempts to eliminate grown-in voids were also made. In preliminary experiments

samples were annealed at 668°C for 5 days with P^,^ ~ 0.1 atm. However, no significant

change in the level of macro voids was found under inspection with the IR microscope.

Further study is underway to aid in understanding the mechanism of void formation and

thus their possible elimination.

In conclusion the above results show that vapor deposition affords one the means

for achieving close control over a number of important parameters during growth

including composition, free carrier and void density and growth rate. Further

experiments are expected to result in specimens with low absorptivities in the

10.6)Jm range without the need for compensating donor impurities.

-103
I

I

I

2. Mechanical Properties and Dislocation Structure of | Single Crystal CdTe Deformed in Compression at 300°C.

|

II When considering the II-VI semiconductor compounds such as CdTe and ZnSe as j: candidates for low absorptivity window materials for utilization in high power laser i optical systems, it is important to insure that the materials possess sufficient f mechanical strength and fracture resistance to avoid the possibility of failure while 3 in service. In the as-received condition, these materials just barely meet the i minimum standards with regard to mechanical properties, a situation shared with most

other laser window candidate materials. The II-VI compounds possess neither the k ductility generally associated with metals nor the high hardness characteristic of 5 ceramic materials. Thus far, the solution to the problem of insufficient |) mechanical strength in these materials has been to increase the thickness of the 1 windows used. But since the ratio of the transmitted intensity to the incident |l intensity in a laser window is given by the equation | ^ = e-^^ (7)

o where 3 is the optical absorptivity and t is the window thickness, it is clear that

the vast effort which has been expended to lower 3 can be negated if the window

thickness must be increased.

The mechanical properties of materials such as CdTe were poorly understood prior

to this Investigation, and a systematic study of these properties had never been

undertaken. In addition, the mi c r o s t r u c t ur al basis for the observed mechanical

behavior had not been established. The obvious advantages of improving the mechanical

strength of laser window materials therefore led to the present study. This paper

describes the results of mechanical tests conducted in compression on single crystal

CdTe at 300°C. The goal of these experiments was to understand plastic flow in this

material and to eventually improve the room temperature mechanical properties.

2.1 Experimental

Single crystal specimens of melt grown CdTe, manufactured by Hughes Research

^ Laboratories and by II-VI Co. Inc., were prepared for mechanical testing as described i in a previous paper [6]. After preparation, the orientation of each of the samples

was determined by Laue back-reflection X-ray diffraction. The apparatus used to ^ conduct the compression tests was a modified Tinius-Olsen testing machine, which I deformed the samples at a constant strain rate. The test temperature was maintained ! using a radiant-heating furnace, with an accuracy of +5° during the test. Five | single crystal CdTe specimens of identical size and orientation were selected for the [ mechanical tests at 300°C. This temperature was chosen since considerable plasticity

was evident in the samples, and at the same time compositional stability was insured

and thermal effects on the flow properties were minimized. The samples were compressed at a constant strain rate of .002 inches per minute.

2.2 Results and Discussion

Using the test data and the orientation information obtained from the Laue

patterns, a composite shear stress T versus glide strain Y curve was constructed

for the compression of CdTe at 300°C, which is shown in figure 6. The orientation of

the compression axis (CA) of the samples is shown in the standard stereographic

triangle, and the point of termination of the mechanical tests for each of the five

specimens is indicated. The initial portion of the curve, up to approximately 25%

glide strain, can be seen to consist of three distinct stages, with a distinct

slope or work hardening rate associated with each stage. In addition, a region of

negligible work hardening exists beyond this region, and persists to very high

strains. Total shear strains as large as 137% have been achieved in this material

at 300°C. A distinct yield point was also observed although it is not represented

on this curve, occurring at a shear stress level of approximately 125 psi (.862 MPa).

As can be seen from the curve, sample 1 was strained just past yield, to 2% glide

strain. Sample 2 was strained into the first stage of the curve, with 6% glide

strain. Sample 3, with 15% glide strain, is in the second stage of the curve. Sample 4, with a glide strain of 23%, represents the early region of the third

stage. Finally, sample 5 was compressed to a large glide strain, 56%.

Additional tests at 300°C showed no effect of changing either the specimen size,

length to width ratio, or strain rate on the results shown in figure 6, within

reasonable limits. As expected, changing the sample orientation caused a considerable

effect, with the existence and extent of various stages of the curve dependent upon

the orientation. ...
The slip band structure caused by the Intersection of mobile dislocations with

the sample surface was examined for each of the compression specimens in figure 6 using a light microscope operating in a Nomarski interference mode to enhance the

contrast due to surface relief. Figure 7 shows the structure of sample 2, and this

micrograph is typical of the results obtained from samples 1 and 2 in the first

stage of the stress-strain curve. The slip lines do not show great prominence and

are generally uniform in distribution. Activity appears to be limited to one set of

slip planes.

Figure 8 shows the structure of sample 3, deformed into the second stage region

of the T-y curve. In this sample some of the slip lines show increased prominence

compared to the previous sample. Most striking is the appearance of a second

operating slip plane, and the amount of activity on this second plane seems to be

roughly equivalent to that of the primary plane. However, dislocation motion appears

to be limited to these two sets of planes, and there is no evidence of cross slip.

Figure 9 shows the results obtained from sample 5, compressed far into the third

stage of the curve. The slip lines are now greatly accentuated and have lost some of

the linearity associated with previous samples. The deformation appears to be much

less homogeneous. A total of three active slip planes were observed. The most

prominent slip lines were those corresponding to the primary plane, and there was considerable evidence for cross slip associated with this plane. However, although

cross slip was most evident on the primary plane, it was also observed on at least

one of the secondary planes. The results from sample 4 were similar, although these

features were not as pronounced in this sample.

Each of the compression sepcimens was sectioned and polished parallel to the

primary slip plane in preparation for transmission electron microscopy, and this

microscopy was performed with the electron beam normal to the primary slip plane and

the primary dislocations lying in the plane of the foil. This was done in order to insure the accuracy of any quantititive measurements and to simplify the interpretation of results. Since CdTe has the zincblende crystal structure, the slip system

is of the type {ill} <110> , similar to fee materials. The microscopy was always

performed using one of the three <220> reflections which exist in the {ill} reciprocal

lattice plane in fee as the operating satisfied reciprocal lattice vector i'g) . It

is important to note that all of the dislocations lying in the primary plane will

exhibit contrast in all three reflections. However, the type of contrast displayed

using a given g will depend upon the Burgers vector of the dislocation . If we define the dot product 'J.'K = n, then a primary dislocation with a fixed will have

n=l for two of the possible <220> reflections and n=2 for the third. These two

conditions produce markedly different contrast. The case n = 1 produces a single

Intensity minima which lies slightly displaced to one side of the dislocation core. The case n= 2 exhibits two intensity minima separated by a region which may be

as bright as background. In some cases one of the minima may be suppressed. Therefore, the two cases n = 1 and n = 2 can be distinguished in the electron micrograph.

This becomes Important since for the case n = 2 the dislocation Burgers vector lies

parallel to the operating reciprocal lattice vector. This situation does not apply

to dislocations lying in planes other than the primary slip plane. The contrast_due

to these secondary dislocations will become very weak or vanish in one of the <220>

reflections. Therefore, by examining the dislocation images in all three <220>

reflections, the following information can be determined: the density of dislocations

on the primary plane, the density on secondary planes, the distributions of co-planar

slip systems, the direction of the Burgers vector of the dislocations on the primary

plane, and the character (edge vs. screw) of these dislocations.

Figure 10 shows the dislocation structure of sample 1. The dislocation

distribution in this sample was extremely heterogeneous, with tangles such as seen

In figure 10 lying widely separated in a matrix essentially devoid of dislocations.

Figure 11 Is a typical area In sample 2. The characteristics of the dislocation

structure in these two samples were similar, and therefore they may be discussed

together. The Increase in dislocation density in sample 2 corresponding to the

increase in strain manifested itself not in a change in structural characteristlcs>

but Instead by an increase in homogeneity and a decrease in the distance between

tangles

.

Close inspection shows that in figure 10 the dislocations are displaying n=l

contrast, while in figure 11 the case n=2 is present. The structure in general

consists of two types of dislocation: long dislocations and dislocation loops of a widely varied size. Examining the dislocation Images in all three <220> reflections,

as is seen in figure 12 taken from sample 2, reveals several important results:

1) since none of the dislocations vanish, both the long dislocations and

the loops lie in the primary slip plane. ^

2) since all of the dislocations display the same contrast features as g is

changed, then is Identical for all of the dislocations and one unique

slip system exists.

-105-
3) the long dislocations tend to have a mixed character. The direction

of b is marked in figure 12. In these samples some activity on other slip planes can be seen, but it is

minimal in comparison to the primary plane. Another striking feature of the

structure, most evident in figure 11, is that the large loops are of the dipole type,

and the long dislocations tend to form a zig-zag configuration, also a type of dipole.

Several mechanisms for the formation of the dipole loops can be postulated, using

the dipole segments of the long dislocations as a starting point. One mechanism

involves the cutting of primary dislocations formed early in the deformation by

dislocations moving on secondary planes. The pinned ends of the dipole segments tend

to lie in a <110> direction, which is the trace of the intersection of two {111}

planes. However, other mechanisms could be envisioned which do not involve secondary

d i s loc a t lon^ , but instead can be constructed solely based on minimization of energy -j and properties of dislocation line segments. In any case, it seems likely that the

dipole segments and the dipole loops are related. Stereomicroscopy on these samples

also aided in the determination that the dislocation loops lie in the plane of the

foil.

Figure 13 shows the dislocation structure typical of sample 3, which represents

the second stage in the stress-strain curve. An area of this sample which has been

examined using the three possible < 22 0> r ef lec t ions is seen in figure 14. Several

changes in the dislocation structure are evident as compared with the earlier

samples. The obvious dual features of long dislocations and large dipole loops have

been replaced by a more densely tangled structure comprised of shorter dislocation

segments. These tangles are more homogeneously distributed than in the previous

samples. The tangles are beginning to lie along <110> directions in the crystal,

suggesting the later possible development of a cell structure. Most importantly,

figure 14 shows the existence of dislocations in the primary plane with each of the

three possible Burgers vectors, so that three co-planar slip systems are now in

operation. Again in this sample the dislocations predominantly lie in the primary

slip plane. Increased activity on secondary planes is obvious in figure 14, and

several short segments and many small loops can be seen to exist in these planes,

but the dislocation density on the primary plane is still much greater than on the

secondary planes. The primary dislocations also appear to have a stronger edge

character than previously.

Figure 15 shows the structure of sample 4, deformed to a point in the third

stage of the x-y curve. The scale of the microstructure has decreased significantly,

and very short dislocation segments lying in tangles predominate. Again the tangles

tend to lie along <110> directions, and the matrix regions between tangles are becoming denuded of dislocations. The structure exhibits greater homogeneity than

previous samples. Contrast experiments show that the majority of the dislocations

still lie in the primary slip plane and a fairly uniform distribution of the three

co-planar slip systems in this plane are seen. The density of secondary dislocations

continues to increase, but still does not rival the primary density.

The structure of sample 5, compressed to approximately twice the strain of samplej

4, does not vary markedly from the previous sample. Figure 16 shows a typical microstructure, which shows the same general characteristics as figure 15. In some areas

of this sample the cell structure was much more highly developed, with the formation

of dislocation cell walls with sub-boundary characteristics. Figure 17 shows one of

the regions. Figure 18 displays a portion of one of these walls taken under different

contrast conditions, and it shows that the density of secondary dislocations in

these walls is at least comparable to the primary dislocation density.

Since the image characteristics of the dislocations observed in CdTe in the

bright-field transmission electron microscopy mode suggested that these dislocations

might exist in a dissociated state, weak-beam dark-field microscopy was utilized to investigate this possibility. The weak-beam dark-field technique makes use of a weakly diffracted reflection to form the image, in contrast to the bright-field mode,

which uses the transmitted beam in the presence of one strongly diffracting

reflection, or conventional satisfied dark-field, which forms the image using a strongly diffracted beam. Two major advantages are obtained when viewing dislocation

images using weak-beam dark-field microscopy. First, the bright-field image of a dislocation, caused by electron interaction with the dislocation strain field, is

generally on the order of 200 A wide, and so fine features associated with the

dislocation on a scale smaller than this are lost. The weak-beam dark-field

dislocation Image is generally from 15-40 A in width, and so many fine features may

be resolved. Secondly, in bright-field the dislocation image is usually displaced

to one side of the dislocation core and this shift is difficult to determine. In weak-beam dark-field microscopy, the dislocation image corresponds more closely to the true core position and occurs at a calculable distance.

In materials which have a slip system of the type {lll}<110>, it is mechanistically possible and in some cases energetically favorable for the unit dislocations to -106-
dissociate into two partial dislocations with a Burgers vector of the form | <112>,

separated by a faulted region. A typical dissociation reaction for a unit dislocation

I on a (111) plane can be represented as

I f

[101] I

[112] + I

[211].

If the diffraction conditions are selected properly, both of the partial dislocations

will show contrast and will be visible in the electron image.

Weak-beam dark-field electron microscopy on the dislocations in CdTe showed

that the dislocations were indeed in many cases dissociated. In this study, the weakbeam dark-field microscopy was performed using a <220> reflection, with the corresponding <4A0> reflection satisfied in bright-field. Figure 19 shows a bright-field, weak-'

beam dark-field pair of micrographs illustrating the dissociation of a dislocation

lying in the primary slip plane. This image was taken from sample 2, and both partials

are clearly visible in the dark-field view. The observed partial separation, A__„, in

^ U D O figure 19 is 125 A. An analysis of the dislocation strain field enables the actual

separation. A, to be calculated. It can be shown that [7]

where

C = sg/g-tp (1 + 1 ) . (9)

2. 2(l-v)

Here Sg is the deviation from the perfect Bragg condition for the reflection used, "fip is the Burgers vector of the partial, and v is Poisson's ratio. Using this formula,

the actual separation is calculated to be 119 A. The dislocation line is also observed to be constricted at irregularly spaced

intervals. This type of constriction has been observed in weak-beam images of dissociated dislocations in metals, and was shown to be a real effect [8]. An explanation of the occurrence of these constrictions can be postulated in terms of the

reduction of the energy of the dissociation.

Several interesting and potentially important implications can be derived from

the results of the experiments to date. The mechanical tests have shown that CdTe

samples can undergo a large amount of plastic strain at 300°C before failure. This

immediately suggests the possibility of forging as a potential method of preparing

large window blanks. Forging large plates from material initially in the form of a cylinder would seem to be economically and mechanistically superior in terms of

problems with crystal growth.

The mechanical tests have also shown that a definite increase in strength can

be achieved by deformation at elevated temperatures. The problems of utilizing this

result to improve the room temperature properties and the effects of deformation on

the optical properties will be considered in the future.

It also seems likely that a microscopic explanation for the plastic deformation

behavior of CdTe can be developed. At the present time, it appears that the first

stage of deformation involves the activity of one slip system exclusively. The onset

of the second stage seems to involve the activation of additional slip systems,

although it is not clear at the present time whether the key role is played by co- planar systems or by systems on slip planes other than the primary. Finally, in the

third stage a number of co-planar and non-coplanar slip systems are activated, and

there is considerable evidence for cross slip and cell wall and sub-boundary formation.

In the future, additional tests at various temperatures will be performed in order to determine the key process which governs the work hardening behavior in each of these

stages, and also to isolate any thermal component of the flow processes involved.

-107-
3. References

[1] Sparks, M., Proc. Conf. on High Power

Infrared Laser Window Materials, 27,

28, Oct. 1971 , C. A. Sahaglan, C. A. Dltha, eds., AFCRL-71-0592 , Special

Report No. 127, Dec. 1971, p. 1.

[2] Shiozawa, L. R., Jost, J. M., and

Roberts, D. A., Final Technical

Report , AFML-TR-73-163 , Sept. 1973,

Gould Laboratories, Cleveland, Ohio.

[3] Bowen, H. K., Vander Sande, J. B.,

Final Technical Report , AFML-TR- 7 5-91,

Aug. 1975, M.I.T., Cambridge, Mass.

[A] Ballentyne, D. W. G., Wetwatana, S. and White, E. A. D., J. Cryst. Growth

1, 79 (1970).

[5] Reed, T. B. and LaFleur, W. J.,

Appl. Phys, Lett. 5_, 191 (1964).

[6] Hall, E. L., Nurmikko, A. V.,

Vander Sande, J. B. and Bowen, H. K.,

Proc. of the Fifth Annual Conference

on Infrared Laser Window Materials

(ARPA, Arlington, Va.) Feb. 1976 , 715.

[7] Cockayne, D. J. H., Ray, I. L. F.,

and Whelan, M. J., Phil. Mag., 20^, 1265 (1969).

[8] Ray, I. L. F. and Cockayne, D. J. H.,

Proc. R. Soc. Lond. A., 325 , 543

(1971)

4. Figures

Figure 1. Schematic of end cap with multiple

substrates.

-108-
T'igure 3. Change of growth rate with V^^ under

-4 conditions of K = 2.7 x 10 (atm ^^^) and Q = 3. Observed

growth rates are marked by (x)

.

Figure 2. Calculated and observed growth rate as a function of Q for

P , = 5.48 X

10"-^ atm, -3

P ^ = 2.90 X 10 atm and flow

Te2 rate of 600 cc/min at 25°C. Dotted line shows the growth

rate at Q-**".

(lo'atm)

-109-
-110-
-Ill-
0 0.1 0.2 0.3 0.4 0.5

Figure 6. Deformation curve for compression of CdTe single crystal

specimens at 300°C, in terms of resolved shear stress T and glide strain y. The point of termination of the mechanical test is marked for each of the five samples

used in this study.

Figure 7. Light micrograph of surface structure of sample 2, y = 0.06,
I

Figure 10. Transmission electron micrograph of the dislocation structure of sample 1, Y

= 0.02. Foil normal [111],

-113-
I

Figure 11. Transmission electron micrograph of the dislocation

structure of sample 2, y = 0.06. Foil normal [111].

Figure 12. Dislocations in sample 2 imaged under^ three different diffracting

conditions. In each case a unique <220> reflection in the (111)

reciprocal lattice plane was strongly excited, and the operating

reciprocal lattice vector, g,is marked on the micrograph. The

Burgers vector b for the dislocations is indicated on the micrograph where g.b = 2, since in this case "B is parallel to "g.

-114-

I

Figure 15. Transmission electron micrograph of the dislocation

structure of sample A, y = 0.23, Foil normal [111],

Figure 16. Transmission electron micrograph of the dislocation

structure in an area of sample 5, y = 0.56. Foil

normal [111].

-116-
Figure 17. Transmission electron micrograph showing dislocation cell

wall in sample 5. Foil normal [111],

Figure 18. Portion of dislocajtion cell wall in sample 5 imaged using

three different <220> reflections.

-117-
COMMENTS ON PAPER 8/

TULLER, UEMATSU, VAHDER 5AMPE, HALL, AWP BCWEN

Wo dcicuM-ion on thJj> papeA.

-118-
1.14 ABSORBING TELLURIUM INCLUSIONS IN POLYCRYSTALLINE CADMIUM TELLURIDE

:

ESTIMATES FOR EXTRINSIC DAMAGE THRESHOLDS

Herbert S. Bennett

National Bureau of Standards

Washington, D. C. 20234

and

Cyrus D. Cantrell

Los Alamos Scientific Laboratories

Los Alamos, New Mexico 87544

Cubic cadmixim telluride is a promising infrared material for highly trans- parent windows. However, one problem encountered in high-power laser systems is thermal extrinsic damage to the optical elements which arise from absorbing inclusions. Absorbing inclusions are impurities with physical and optical properties

which differ substantially from those of the host material. Such inclusions may

absorb an appreciable amount of the incident radiation and thereby may undergo

thermal expansion. This produces major stresses within the host. In this paper,

we give estimates of the maximum tensile stress as a function of inclusion size,

laser pulse width, and laser power for polycrystalline Te ^heres in po lycrystalline

CdTe . Our computations suggest that the heating of 0.1 )jm to 10 \im aggregates of

Te in CdTe when subjected to power densities of about 100 MVcm^ and pulse widths

of about 200 ns may produce stresses comparable to or greater than the breaking

strength of the CdTe host.

Key words: Cadmium telluride; elastic compliance tensor; elastic moduli;

extrinsic damage thresholds; high-power lasers; infrared materials; tensile

stress

.

1. Introduction

Researchers have shown considerable interest in CdTe as a highly transparent window material for

use in high-power infrared laser systems and as a sum and difference frequency generator for use in laser isotope separation processes because of its resistance to environmental influences and reasonably

good thermoelastic properties. However, the production of uniform large surface area samples which -4-1

have infrared absorption coefficients as small as 10 cm at 10.6 ym and which have negligible

concentrations of defects poses challenges for the materials scientist [1]''. Using infrared and optical microscopy, researchers reported macroscopic defects in CdTe which are

of the order of one micrometer or greater in size [1-4] . More recent examinations of the microstructural defects by transmission electron microscopy showed that many of the macroscopic defects are aggregations of platelets, precipitates, dislocations, or stacking faults [1,5]. The authors of

reference [1] found that the microstructural defects are frequently In^Te^ platelets, CdCl^ platelets,

and Te precipitates . The densities of such precipitates varied from 10"'"^/cm'^ to greater than 10'''^/cm"^ and their individual sizes varied from 0.006 pm to 0.015 ym. These authors also reported abnormally

high infrared absorption sites of varying morphologies and of sizes from about 1 ym to 6 ym. Annealing

at temperatures greater than 500 °C in cadmiiam vapors reduced substantially the sizes of several of

the absorption sites and left other absorption sites essentially unaltered after 48 hours. The latter

macroscopic absorption sites were distinct defect aggregations of dislocations, stacking faults, and

localized regions of In^Te^ or CdCl^ precipitate platelets.

In this paper, we estimate the stresses which occur near localized absorption sites in CdTe

samples. Such absorption sites as Te , In^Te^, or CdCl^ may absorb sufficient radiation from an incident

laser beam to produce deleterious stresses within the CdTe host. To illustrate representative numerical

predictions, we compute the stresses near Te absorption sites. Our computations suggest that the

2 heating of Te precipitates when subjected to 100 MW/cm power densities may produce stresses which are comparable to the fracture strength of CdTe . We do not treat the equally important In^Te^ and CdCl^

precipitates because literature searches did not provide us with the many optical and mechanical

parameters required in the theory. However, the earlier research results of one of us [6,7] showed

that the optical, thermal, and mechanical properties of the host dominate in determining the stresses

^Figures in brackets indicate the literature references at the end of this paper.

-119-
near absorbing inclusions. Hence, even though the quantitative predictions will differ among Te

In^Te^, and CdCl^ precipitates, we conclude that all such absorbing sites in the size range from .1 ym

to over 10 ym may produce, when sxibjected to laser power densities of the order of 100 MW/cm^, thermal

stresses which are comparable to or exceed the fracture strength of CdTe.

In section 2, we apply the linear-coupled-quasistatic thermal elastic theory to absorbing polycrystalline aggregates in CdTe to compute the thermal stresses.

In section 3, we introduce approximations by which we reduce the absorbing irregularly shaped

aggregate in polycrystalline CdTe to an absorbing polycrystalline sphere in an isotropic host material.

The mechanical properties of the sphere and isotropic host are derived respectively from those of Te and CdTe by averaging procedures . We also give the approximations which are necessary to uncouple the linear-thermal-elastic equations [7] . We use this theory in section 3 to formulate the procedure by

which we compute estimates for the tangential components of the stress tensor in the vicinity of absorbing Te aggregates in CdTe. The heat diffusion equation with time-dependent boundary conditions

for the temperature as a fiinction of time t and of radial distance r was solved by Laplace transforms.

Section 4 contains representative nianerical estimates for the stresses around Te aggregates in CdTe and the conclusions which these estimates support.

2. Thermal Elastic Theory

We consider an absorbing site, such as an aggregate of Te , In^Te^ , or CdCl^ precipitates, in CdTe. An infrared laser beam is incident upon the absorbing aggregate. The resulting heating in the region of the aggregate produces stresses which may become very large. To calculate such stresses, we shall summarize several concepts which together comprise thermal elastic theory [8] . The first concept is that the Newtonian equations of motion apply to any infinitesimal volume

element.

The next concept concerns the conservation of energy. The energy balance equation is as follows

Q - q. . = pC OT/3t) - TOa. ./8T) Oe. ./at) (1) ^v 1,1 e 1] 13 The quantity is the internal energy absorption per unit volume; is the i^^ component of the heat flux vector; C^ is the specific heat at constant deformation of the elastic solid in question; T

is the temperature of the infinitesimal volume element, e . . is the ij^'^ component of the small-strain

th ^-^ tensor, a. ., is the ij component of the stress tensor; and p is the mass density. Here, we denote

partial differentiation with respect to coordinate x^ by ,k and a summation by a repeated index. For an isotropic material, the heat flux becomes = -KT^, where K is the thermal conductivity.

This last equation is called the Fourier law of heat conduction. The internal energy absorption per

unit volume is related to the energy density of the local radiation by an absorption equation; that

is, Q^(r,t) = yl{r,t) where the linear absorption coefficient is Y- The third concept is Hooke's law which expresses the strain e in terms of the stress a; namely,

e . . - a. . (T-T ) = S. .,,0,, , (2) 13 13 o i]kl kl where the fourth rank tensor S is called the elastic compliance tensor, and its components S. ., , are ijkl

the compliances. When a small temperature change (T - T^) occurs uniformly throughout the material,

ct the resulting deformation is homogeneous and all components e . , which contribute to the total strain

ex

"^"^ e. , are proportional to (T - T„) ; namely, e. . = a.

(T-T„) , where T„ is the reference temperature for

ij 0 ij 1] 0 0 which the material is free of stress and where the coefficients of thermal expansion are a. .. Alterij

natively, we may use the fourth rank stiffness tensor C. ., , to express the stress ct in terms of the

n i^kl strain; namely,

CT. . = C. ., , {e, ^ - a, , (T-T ) }. (3) ij i^kl kl kl 0 In order to make these equations amenable to solution, we must simplify them. The approximations

to accomplish this simplification are given in the next section.

-120-
3. Approximations and Boundary Conditions

Because we seek only estimates for the stresses, we want to simplify the problem. In particular,

because a theory for absorbing spheres in isotropic host materials exists [6,7] , we shall reduce the

problem outlined in section 2 to the problem of an absorbing polycrystalline sphere in an isotropic

host material and therby shall apply much of the formalism presented in references [6] and [7] to our present problem.

Our literature survey provides us with complete sets of the essential values for the thermal,

elastic, and mechanical properties of Te and of CdTe and only incomplete sets of values for such

properties of In^Te^ and CdCl^. We therefore consider in the remaining discussion only the Te precipitates in CdTe [9]. However, the concepts presented below apply equally well to In^Te^ and CdCl2

precipitates

.

Several thermal , elastic , and mechanical properties of the absorbing center and isotropic host

enter the linear-uncoupled-quasi-static thermal elastic theory described in references [6] and [7]

.

Such properties include the density p, the specific heat C, the thermal conductivity K, the linear

expansion coefficient a, the Young's modulus E, the shear modulus G, the volume compressibility X' and

the Poisson ratio v

.

3.1 Crystalline and Polycrystalline Elasticity

In this subsection we shall discuss the relation between crystalline and polycrystalline elasticity.

The quantities E, G, X/ si^^^ "lay be expressed in terms of either the compliances S^^j^^ or the stiffnesses C^^j^^. In general, for crystalline solids, the quantities E, G, and v depend upon directions

relative to the crystalline axes. We assume that the polycrystalline aggregate contains many grains and is macroscopically homogeneous.

This assumption then enables us to use the methods of Hill [10] to compute reasonable values for the

elastic moduli of polycrystalline aggregates from a knowledge of the crystalline stiffnesses and

compliances. The measured moduli for polycrystalline aggregates invariably lie between the Reuss and

Voigt approximations for the moduli of polycrystalline aggregates [10, 11].

The measured values of x = X_mmand G = G then have the following limits: S (1/x ) $ d/X,,) R m V

S G i G^, where the subscripts V and R refer respectively to the Reuss and Voigt approximations. The

corresponding values of the Poisson 's ratio v and Young's modulus E for isotropic materials are obtained

from the relations

,

E = [(1/3 G ) + (x /9)]~-^ (4) and m mm,

Either the geometric means or the arithmetic means of the Reuss and Voigt values compare well

with known experimental values for many polycrystalline materials. Because the experimental values of

1/2

E, G, Xi V for polycrystalline CdTe and Te are incomplete, we calculate G = (G G ) , G = 1/2 g R V a

(1/2) {G„+G,J , x„ = KvgKv(XdX,,) and x, = (1/2) (x„+X,,) » for CdTe and Te. We then assume that G

a Kv av

(1/2) (G +G ) and x = (1/2) (x +X ) are reasonable estimates for the measured values G and x and

g a av g a m m

use these values in eqs. (4) and (5) to compute estimates for v and E av av

Table 1 contains the numerical values of G , x / E , and v for polycrystalline CdTe' and Te. av '^av av av j ^ The values for CdTe given in table 1 are reasonable whenever the CdTe host is polycrystalline.

The CdTe host may also be a single crystal for which E, G, and v are functions of directions in the

crystal. To simplify the mathematical complexities, we want to approximate the CdTe single crystal by

ein isotropic material which has properties derived from the single crystal properties. In references

[6] and [7] we find that the stress components in the isotropic host are proportional to the Young's

modulus of the host. Therefore, we may compute estimated upper and lower limits [12] for the stress components in a single crystal of CdTe by using the respective extremal values for Young's modulus in the isotropic model of references [6] and [7]

.

The coefficients of thermal expansion for CdTe are isotropic; that is, a. . = a, 6 . . . The thermal

expansion coefficient parallel to the c axis of Te, a^, is not equal to the expansion coefficient

perpendicular to the c axis of Te, a^. Upon averaging over all lattice directions, v;e obtain an effective thermal expansion coefficient for the polycrystalline Te sphere a = (2/3)a, + (l/3)a^.

c 1 3

-121-
3.2 Isotropic Thermal Elastic Theory

In the previous subsection (3.1), we have simplified the problem of an absorbing, irregularly

shaped aggregate of Te in either polycrystalline or single crystal CdTe to a problem involving a polycrystalline sphere of Te in an isotropic host which has properties derived from those of CdTe. This simplification permits us to use the isotropic thermal elastic theory in references [6] and [7]

Using the several approximations given in subsection 3.1, we reduce eqs . -{3) and (6) of this paper, respectively, to eq. (2) and eq. (1) on page 250 of reference [7] for the equations of motion and of

energy balance. We refer the reader to references [6] and [7] for the details of this theory, of the procedure by which the coupled equations are decoupled, and of the boundary conditions. We summarize

here the many assumptions which enter the calculation of the radial stress component a^^(r,t) and the tangential stress component a..(r,t)

y u (a) The absorbing site is a polycrystalline sphere of radius r^ and is always in good thermal contact with the host. The number of inclusions per unit volume is small so that they do not interact

with one another. The effects of shape and orientation to the incident radiation are neglected in the model

(b) The host material is isotropic, continuous, and of infinite extent. It initially is at an ambient temperature T^ and free from all stresses and strains. We do not consider the distribution

and nature of microcracks and optical imperfections.

(c) The isotropic-linear-thermal-elastic equations give a reasonable description of the processes which ultimately may lead to catastrophic damage when the CdTe host material is approximated by an isotropic continuum with representative properties derived from polycrystalline CdTe.

(d) We neglect the radiation of heat by the absorbing site-host interface and by the host material

close to the absorbing site. The laser beams for the examples used here contain energy fluxes much

greater than the black body energy flux. In addition, for times less than about one second and for laser pulse widths less than several microseconds , the black body energy flux is very small compared

to the energy fl\ix due to thermal conduction.

2 2

(e) The coupling term T„C. ., ,a, , (8e. ./3t) and the inertia term p(3 u./3t ) may be neglected

0 i^kl kl 1] 1 2-0

whenever three characteristic times which occur in the absorbing site-host system satisfy a set of

inequalities. These times are the following. The pulse width x of the laser beam determines in part

the rapidity of heat generation. We shall consider two values of x : x = 200 ns and x = 500 ns. The

2 2 characteristic relaxation time for temperature equilibration (thermal diffusion) is t (r /a^) =

2 T Hi

18. 6r s for CdTe. In this equation and all subsequent equations the units of r and r are centimeters.

2 2 ° The thermal diffusivity of CdTe is a^ = 0.0538 cm s and r is a radial distance. The characteristic

mechanical time required for the production of stress waves is t (r/v ) = 6.45 x 10 ^ rs, where v is m s s the speed of propagation of elastic shear waves. Boley and Weiner [8.] demonstrate that when x >> x M

and t^ >> Xj^, then the coupling and inertia terms are small compared to the other terms in the equatic

The above two inequalities for x = 200 ns then give us upper and lower limits on the radial distance

-2 -7

r; namely, 3.10 x 10 cm >> r >> 3.47 x 10 cm.

(f) The radius of the absorbing sphere is such that the temperature gradients in the sphere are small compared to those in the host material.

The equation which we evaluate is eq. (11) of reference [6]; namely,

Ct E %fl(r,t) = ^ ^

, r T rp,t) p'dp

+

,1 ^ 3 •'r h^

(1-v, )r o EC a,E,T (r,t) h 2h h h h

(6)

(l+v,)r^ U-v,)

h h

3 -1 where (C_,/r ) = a T, (r ,t)[l+{2E,x /3(l+v, )}] . The radial stress component a is not primary to 2h o c h o h e h rr

the mechanism for damage considered here.

4. Numerical Examples and Conclusions

The numerical examples which we give here are based on a model whose elastic, thermal, and optical

properties are constant independent of temperature. The values for the elastic properties are given

in table 1 and are valid for small strains. The values for the thermal properties of density, specific

heat at constant volume, thermal conductivity, and effective linear thermal expansion coefficients are given in table 2. Even though the model does not take into account the crystalline aspects, one hopes -122-
that it does give a reasonable description of the actual system. Because we seek here to avoid questions

of the value to use for the absorptance A(X,T), we use the value A(X,T) = 1 for all numerical examples

in this paper. This presents no additional problem because the temperature in the host T^{r,t) and

the stress components a^^ and a^g are all directly proportional to the absorptance A(A,T). The absorption efficiency for a sphere may be calculated by Mie scattering theory from the complex index of

refraction m = n^-in^. But the reported values of n^ in Te vary considerably at 10.6 pm from about

. lo"** to 10~^. This variation arises partly from impurities. Such values for the imaginary part of the

refractive index depend upon the direction of the electric field relative to the c axis and the number

of free carriers [13]. For a sphere of radius 10 ^ cm, Mie scattering theory predicts absorption effi-

-2 ciencies from about 10 to 0.5. In addition, the theory in reference [7] reveals that the values of

the radial and tangential stress components are directly proportional to the incident laser beam power

2 density, E . Hence, we shall use for all energies the value E = 100 MW/cm . p P

We have used the model outlined in this paper to determine how the maximum tangential stress a^g

varies as a function of the radius r^ of the spherical absorbing site for fixed power density and

pulse width of the laser beam. Figure 1 contains the dependence upon radius r^ of the maximum value of the tangential component

of stress for polycrystalline Te aggregates in polycrystalline CdTe. The maximum tensile stresses for In^Te^ and CdCl^ precipitates in CdTe are expected to be at least of the same order of magnitude as those for Te in CdTe as given in figure 1. As shown by the asymptotic expressions in reference [7] and the numerical examples in reference [6], (max) is dominated by the properties of the host and is only slightly dependent upon the absorbing site properties.

CdTe laser windows can be expected to fail when a (max) exceeds the macrocrack formation stress. Macrocrack formation is dependent upon three major factors [14] . (1) the nature of the stresses in three mutually perpendicular directions as a function of distance from the absorbing site host interface;

(2) the presence and characteristics of small percursor flaws at the interface and in the host material

near the interface; and (3) the size of the precursor flaws, which tend to be larger at the interface.

As a result of these factors, macrocracks rarely develop at the theoretical strength a (theoretical)

,

-2 but may occur at a stress as small as 10 x a (theoretical) , particularly for brittle materials such

as CdTe. A general condition by which one may compute a (theoretical) is to minimize the following

quotient Q with respect to orientation of the fracture plane; namely, Q = {square root of (the value

of Young's modulus in the direction perpendicular to the fracture plane) times (surface energy for bond rupture)} divided by {the stress normal to the fracture plane}. Calculations [14] show that

minimizing Q yields values of a (theoretical) '^^ 10 ^ x the Young's modulus. Past experience indicates

that the fracture strength which is measured a (experimental) is probably bounded by 10 "'' x Young's

modulus and 10 x Young's modulus. In reference [4], Kuhl gives measured values for the fracture

7 2 7 2 strength of CdTe which lie between 1.38 x 10 (N/m ) and 2.75 x 10 (N/m ). Referring to table 1, we

see that these measured fracture strengths are of the order of 10 times the Young's modulus used in the numerical examples of this paper. Figure 1 illustrates the range of r values for which a (max) -5 -2 ®^ exceeds (E^^IOOO) . The range of r^ values is from 10 cm to over 10 cm. Hence, we conclude that

Te, In^Te^, and CdCl^ precipitates in the size range near 10 ^ cm haje the highest probability to produce catastrophic damage in the CdTe host crystals whenever they are subjected to laser power densities

2 of the order of lOOMW/cm . Even though annealing CdTe at temperatures above 500 °C removes most of the Te precipitates , the

In Te. and CdCl_ precipitates remain and are expected to be equally as troublesome as the Te precipi-

-5 -2 tates for the size range from 10 cm to over 10 cm. In view of the above conclusions, researchers have some options. They may increase the size of

the infrared optics and reduce the power density in them; they may devise ways to reduce all damaging

precipitates to tolerable sizes ; and they may encourage the materials scientists to develop new alternative infrared materials with improved figures of merit when compared with presently available

materials such as CdTe

.

5. Acknowledgments

The authors wish to thank A. G. Evans, A. Feldman, R. A. Forman, and C. Johnson for several helpful

discussions.

-123-
6. References

[1] Magee , T. J., Peng, J., and Bean, J., Phys

Stat. Solidi A 2]_, 557 (1975).

[2] Gentile, A. L. , Kiefer , J. E., Kyle, N. R. , and Winston, H. V., Proc. Conf. High Power

Laser Window Materials, Oct. 30-Nov. 1, 1972, edited by C. A. Pitha, U.S.A.F. Cambridge Research Laboratory Technical

Report TR-73-0372, Vol. II, p. 625.

[3] Shiozawa, L. R. , Roberts, D. A., and Jost,

J. M. , ibid. , p. 639.

[4] Kuhl, G. E. , ibid., p. 607.

[5] Kroger, F. A., Chern, S. S. , and Vydyanath,

H. R. , Proc. Third Conf. on High Power Infrared Laser Window Materials, Nov. 12-14,

1973, edited by C. A. Pitha, A. Armington,

and H. Posen, U.S.A.F. Cambridge Research

Laboratory Technical Report TR-74-0085 (11) , Vol. II, p. 437.

[6] Bennett, H. S. , J. Appl. Phys. 42_, 619

(1971)

[7] Bennett, H. S. , J. Res. Natl. Bur. Stand.

(U.S.) 75A, 247 (1971); and 75A, 261 (1971).

[8] Boley, B., and Weiner, J., Theory of Thermal Stresses (John Wiley and Sons, Inc., New York,

1960) .

[9] The authors welcome data on the thermal,

elastic, and mechanical properties of In2Te3 and CdCl2.

[10] Hill, R. , Proc. Phys. Soc . London A65 , 349

(1952)

[11] Anderson, 0. L. , in Physical Acoustics:

Principles and Methods, edited by W. P. Mason

(Academic Press, New York, 1965), p. 43.

[12] Nye, J. F., Physical Properties of Crystals,

(Oxford University Press, London, 1960),

Chap. VIII.

[13] Caldwell, R. S., and Fan, H. Y. , Phys. Rev. 114 , 664 (1959)

[14] Evans, A. G., private communication, Feb. 1976.

7 . Fig\ires

©+

9

$

-5.0 -4.0 -3.0 -2.0

LoQioIro -cm-i)

Figure 1. The maximum tensile stress Ogg (max) for Te precipitates in polycrystalline CdTe as a function of radius r^ and pulse width T for

a fixed laser power density Ep = 100 MW/cm^.

The + denotes values for T - 200 ns and the © denotes values for x = 500 ns. The dashed

lines are the upper limit (E^^/IO) and the lower limit (E^^/IOOO) in MN/m^ of the estimated fracture strength. The measured

fracture strength of CdTe is very close to the lower limit (E„„/1000)

10.0

9.0

8.0

oQ

®

3

0)X

-12A-
Table 1. Computed values for the shear modulus G^^, volume

compressibility X' Young's modulus E^^, and Poisson

ratio V , for polycrystalline CdTe and Te. av

G y E V av av av

(N/m^) X lO"''^ (m^/N) X lo"''"''' (N/m^) x

10"''° (dimensionless)

CdTe 1.4 2.0 4.0 0.37

Te 1.5 5.1 3.7 0.20

Table 2. Values of the thermal properties. The quantities p, C, K, and ct are, respectively, the density, the specific heat at constant volume, the thermal conductivity, and the linear

thermal expansion coefficient. The latter for Te is given

by the relation a = a = (2/3)a + (l/3)a .

CdTe

Te

(g/cm )

5.85

6.23

C

(J/g °c)

0.222

0.164

K

(W/cm °C)

0.0699

0.0628

(1/°C) X 10

5.5

17.8

C. S. Sahagian and C. A. Pitha, Compendium on High Power Infrared Laser Window Materials, U.S.A.F. Cambridge Research Laboratory Special Report No. 135, 9 March 1972.

COmEKTS ON TAPER BV BENNETT ANV CAUTRELL

No diicuMyLon on tku> papeji.

-125-
1.15 HOT-PRESSED CdCr2S4 FOR FARADAY EFFECT ISOLATION

AT 10.6ym: PRELIMINARY DAMAGE TESTS

Stephen D. Jacobs

Laboratory for Laser Energetics

and

Kenneth J. Teegarden

Institute of Optics

University of Rochester

Rochester, New York 14627

The ferromagnetic spinel, CdCr2S^ is considered an attractive candidate

for future use in target back reflection isolation of high power infrared

lasers for fusion investigations. Used with liquid nitrogen refrigerant in

a Faraday rotator configuration, this material demonstrates isolation in excess

of 30dB and an insertion loss (due to material attenuation) of 30 percent in saturating magnetic fields of 3500 Oe. Minimal homogeneity requirements for

dc magnetic field biasing and the scalability of the hot-pressing process make

CdCr2S^ suitable for large aperture applications. Single crystal growth and hot-pressing procedures are currently being optimized to reduce extrinsic

attenuation at 10.6um. The threshold for laser induced damage in CdCr^S^ (n^g g

= 2.84) is of fundamental importance to the proposed application. Preliminary measurements have been made using a 50nsj TEA CO2 laser with a multimode output of

20 joules. The results indicate that this material has a damage threshold in excess of 2 J/cm2,

Key words: CdCr2S^; chalcogenide spinel; Faraday effect; ferromagnetic semiconductor; hot-pressed; infrared transmission; laser induced damage; optical isolation

1. Introduction

The discovery in 1965 [1]^ of a family of semiconducting ferromagnetic chalcogenide spinels

with the potential for optical transmission in the infrared [2] opened up the possibility of their

future use for optical isolation. Magneto-optical investigations of these materials progressed'

slowly at first due to a difficulty in growing large single crystals or large polycrystalline

mosaics [3]; but the application of hot-pressing techniques [4,5] and the subsequent fabrication

of samples with large cross-sections enabled optical and magneto-optical investigations to rapidly

advance. A chalcogenide spinel which received particular attention was CdCr2S^. By 1971, enough

optical and magneto-optical data had been collected for this material [6-8] to indicate that hot- pressed CdCr2S^ could provide a substantial Faraday effect accompanied by low optical absorption

in the infrared. The subsequent acquisition of hot pressed samples of various sizes enabled an investigation of optical isolation to proceed.

This paper is divided into several sections. The first provides an introduction to CdCr2S4,

reviewing the physical, optical, and magneto-optical properties which make it suitable for use

as an optical isolator at 10.6yn. Figures of merit measured for breadboard isolator configurations

are then presented, and results of a preliminary damage test with high power, short pulse radiation

are discussed.

2. Physical Properties and Structure

Single crystals of CdCr^S- form in the spinel structure with 8 molecules per unit cell. The sulfur anions are packed in a face-centered cubic arrangement with the various Cd +2+3 and Cr cations

occupying 24 of the 96 interstitial sites present in the unit cell. The compound considered here is "normal" spinel; only Cd cations are found to occupy A sites (surrounded by a tetrahedron of sulfur

1. Figures in brackets indicate the literature references at the end of this paper.

-126-
anions), with Cr cations exclusively on B sites (surrounded by an octahedral arrangement of sulfur

anions). Because of the regular ordering of ions, exchange interactions can force CdCr^S^ into a +3 ferromagnetic state via parallel alignment of spin magnetic moments of Cr moments being quenched by the crystal field) at a Curie temperature of T valence electrons (orbital

84.5 K.

It is difficult to grow large, pure, stoichiometric single crystals of CdCr^S^. The largest

single crystals (a few mm in size) have been grown using a liquid-vapor-solid method developed by

Shick and Von Neida [9,10]. This fundamental materials acquisition problem was overcome by using the hot-pressing process. The ability to produce a product with useful optical properties depends equally

upon spinel powder preparation and hot-pressing technique. Two successful methods of powder preparation [4,5] start with anhydrous chlorides or hydroxide coprecipitates . The former offers a means of working with an oxygen-free system, whereas the latter method produces a finely divided powder. Ideally

the prepared powder should have a cubic spinel structure, be stoichiometric, and free of second phase

impurities. Powders prepared from C-P grade starting compounds and ground to agglomerate sizes less

+3 than 140ym (crystalline sizes of 0.2ym) are sometimes nonstoichiometric (excess Cr ) and contain such impurities as CdS, Cr^O^ and ^^2^3' Hot-pressing is done in a vacuum of 10" torr, between 800 and

900 C, for 10-20 minutes at 50,000 psi.

The objective of this process is the densification of the powder and the preservation of the spinel structure. Densification is thought to occur via plastic flow [4]. The simultaneous application of high pressure and temperature reduce porosity and prevent decomposition. Hot-pressed crystallite grains are 1 to 2um in size, and show no preferred shape as a result of the hot-pressing process. X-ray diffraction analysis reveals the hot-pressed material to be in the cubic spinel phase, with a density greater than 99.8 percent of the theoretical value [4,5]. Measurement of lattice constants

agree with single crystal data. Electron microscopy of fractured surfaces reveals porosity at crystal

interfaces of poorly transmitting samples. Pores are not connected and frequently contain impurities.

No porosity is observed in samples of good optical transparency [4]. It is not possible to chemically

ascertain the stoichiometry of hot-pressed samples of CdCr^S^ due to their insolubility in nitric

acid; but the presence of decomposition products such as CdS or CdO on cold portions of the hot-pressing

apparatus indicate a non-stoichiometric excess of Cr^^ in some samples. It has been shown, however,

+2 +2 +1 that excess Cd or Co is far more detrimental to optical transmission than excess Cr [5].

Hot-pressed samples exhibit good thermal shock resistance, and may be cycled from room temperature to 77 K in minutes without cracking. The materials are easily ground and polished using standard

optical techniques, are black in appearance, and exhibit a hardness just under 5.0 on the Moh scale

(a value comparable to cerium oxide). Important physical data is summarized in table I.

Table 1. Physical data for h-p CdCr2S^

Property

0 Lattice parameter, A

3 Theoretical x-ray density, g/cm

Electrical resistivity

(300K) . ' ohm- cm

(77K)

Curie temperature, K H-p grain size, ym

Chemical inertness of H-p product

Hardness of H-p product, Moh

Value

10.244^

4.259

2x10^

2.86x10- 84.5

1 - 2 good^

5

Reference

[1] [12]

[12]

[9]

[4], [5]

[11]

Varies less than 0.1 percent from room temperature to 4.2 K [13]

'H-p CdCr^S. may be etched in boiling concentrated nitric

-127-
3. Optical and Magneto-Optical Properties

The index of refraction for CdCr2S^ at 10.6pni is 2.84 [7]. Typical values of attenuation

vs wavelength for an optically good sample are shown in figure 1. The infrared window for CdCr2S^ extends from 1.0 to IZ.Oym with restrahl processes occurring at 26.0 and 28.8ym. Laser calorimetry shows scattering to be unimportant at 10. Gym [8,11], and absorption coefficients less than 1.0 cm"^ have been attained with improved hot-pressing techniques. An intrinsic lower limit

to absorption has not yet been determined, and the levels depicted in figure 1 are thought to be due to residual absorption. Cr^O^ is known to cause absorption at 13.5, 16.1, 17.8, and 18.7ym,

and the presence of as little as 0.001 percent oxygen [4] is sufficient to cause the observed

structure near 13vim in figure 1. When cooled below its Curie temperature, CdCr2S^ exhibits a saturated Faraday rotation at lO.Spm of 120 degrees per cm of material. Rotation is linear with applied field and saturates at

3.5 kOe (CdCr2S^) in thin disc samples exhibiting maximum demagnetizing effects [11]. The Faraday

effect in these materials at long wavelengths is due to magnetic dipole transitions having reso- nance frequencies in the far-infrared or microwave regions; it is, therefore, essentially independent of optical absorption at 10.6ym. The absolute value of Faraday rotation as a function of wavelength is shown in figure 2.

4. Faraday Effect Optical Isolation

Isolation figures of merit, referred to as the passive extinction ratio (PER) and forward

to backward transmitted energy ratio (F/B) have been measured for 12 mm diameter discs of CdCr2S^.

The former gives the amount of extinction that would be obtained for a sample in an unactivated

state between crossed polarizers; the latter measures the amount of extinction that would be obtained

if the sample were placed between polarizers oriented with pass directions at 45 degrees, under

magnetic field biasing sufficient to produce 45 degrees rotation within the activated sample per

pass of incident (and reflected) radiation. Measurements on several CdCr2S^ samples indicate

maximum PER and F/B ratios of about 2000/1 [14]. These values are thought to be limited by (a) no prior history of thermal annealing for samples used, and (b) thermally induced strain randomly

generated in cool down by a poor sample mounting configuration. Confirmation of depolarization via

thermally induced strain birefringence was obtained by measuring the F/B ratio over a 5 mm diameter

portion of a 1 cm thick piece of hot-pressed CdCr2S^ (see figure 3). Two regions exhibiting good

isolation (F/B - 500) and one region of poor isolation (F/B - 200) are well defined. The F/B contour

could not be reproduced upon cycling the sample to room temperature and back to 77 K. Pertinent data for hot-pressed CdCr2S^ in an isolator configuration is summarized in table

2. At 10.6ym, in a saturating field of 3.5 kOe, an a-r coated CdCr2S^ isolator element 0.375 cm thick would exhibit a transmittance of 70 percent and provide from 20 to 30 dB isolation. A CdCr2S^

isolator would require liquid nitrogen coolant.

Table 2: Probable performance characteristics of h-p CdCr^S. as an optical isolator at 1.06ym.

Operating temperature, K 77 Element thickness reqd. for 45 degree rotation, cm 0.375

Attenuation coeff . , cm"^ <1 A-R coated element

transmittance, % ~70

Magnetic field reqd. for saturated

operation, kOe 3.5 Optical isolation, dB 20 to 30

Unique possibilities for isolator design are offered by this material and are discussed as follows:

(1 ) Fabrication of large diameter elements - CdCr2S^ is easily worked and polished, hard, and insoluble in most chemicals. The progress recently made in understanding the affect of nonstoichiometry upon optical

transmittance [5] has made it possible to fabricate one cm diameter CdCr2S^ discs with

-128-
uniformly low attenuation. The hot-pressing process has been well characterized [4] for this material and no new problems should be encountered in producing large discs

20 cm in diameter or greater, possessing good optical quality and uniformity.

(2) Element transmittance - Because absorption is impurity related and not intrinsic, there are no fundamental

limitations to increasing material transmittance at 10.6ym through use of higher purity

starting compounds while maintaining large values of Faraday rotation. A reduction in absorption by a factor of 5 for CdCr2S^, would increase transmittance to greater than

90 percent for AR coated isolator elements of optimum thiclness.

(3) Magnetic field requirements - Because it is ferromagnetic, this material exhibits a saturation of its magnetization

and a saturated Faraday rotation. For thin, large diameter discs exhibiting maximum

demagnetizing effects, saturation would require low fields of 3.5 kOe. The uniformity

of the applied magnetic field throughout the isolator active element volume would not

be critical, the Faraday rotation being a maximum uniform value throughout a magnetically

saturated ferromagnet.

(4) Optical isolation - Samples investigated had no history of thermal annealing. Experimental evidence also

confirmed that these samples were subjected to additional strains from constriction in cooled mounts. Yet even under such conditions, F/B ratios in excess of 3GdB were measured. The uniformity and magnitude of isolation could be expected to improve sub- stantially with annealed samples mounted in an improved manner.

(5) System design - CdCr2S^ isolator elements providing 45 degrees of rotation at 10.6ym could be self- supporting discs. Convective edge and face cooling of a CdCr2S^ element suspended in

a helium gas atmosphere at 77 K would provide a set up capable of maintaining the material

below its Curie temperature without exerting a large amount of thermal or mechanical

stress. The magnetic fields required could be produced by liquid nitrogen cooled solenoids. Modest cooling constraints and minimal magnetic field magnitude and uniformity

requirements would permit isolator systems using this material to be fabricated

and operated at low cost.

5. Laser Induced Damage Tests

A Lumonics Series 600 TEA CO2 laser, characterized by a 50ns pulse with an energy density

of approximately 0.45 J/cm^ was used to irradiate a typical disc sample of hot-pressed CdCrjS^. The

sample #16A-53, 12.7 mm in diameter and 0.585 mm thick, had a passive attenuation coefficient of 7cm"^ at 10.6ym. The damage test consisted of placing the sample behind a circular aperture and varying

the energy density incident upon its surface using a gold coated concave mirror (see figure 4). Beam size at the sample was monitored with thermofax paper. The results are indicated in table 3 and in the accompanying figures. A total of three shots were taken, the first being unfocussed, and the next two being focussed to average calculated energy

densities of 2.3 J/an^ and 6.5 J/cm^, respectively. Burn spots typical of these latter shots are

shown in figure 5. The nonuniformity of the energy density within the irradiating beam is readily

apparent in figure 5. Using visual inspection, we observed no damage for the first shot (0.45 J/cm2). The average

energy density in the second shot also failed to cause any visually observable damage except within

a 1 mm area accounting for 2 percent of the total irradiated area, where hot-spots did cause significant damage on the entrance surface only (see figure 6). Hot-spot induced damage increased to 30% of the area irradiated in shot 3 and was visually apparent on both entrance and exit surfaces

(see figures 7a, b). (Different surface areas were irradiated in shots 2 and 3 so that the damage

morphology of each shot could be separated.) Formation of an intense plasma on the entrance surface

was noted during this shot. The Nomarski optical micrographs in figures 6 and 7 indicate several types and stages of

damage. Surface cracking, peeling and lifting is apparent, as is the removal of surface material by

ablation. The pits in figures 6 and 7b are 2 and 9ym deep, respectively. Numerous scratches visible

in these two figures were present on the surface prior to testing, as can be seen in figure 8 which

shows an unirradiated portion of the sample near its edge.

It is difficult to draw a definite conclusion as to the damage threshold for hot-pressed

CdCrgS^ at 10.6ym based on the above test results. Repolishing of the sample exit surface did

reveal that most of the observed damage was surface related.

-129-
Table 3. Damage test results - 50ns pulse irradiation of H-P CdCr2S^ at 10,6)jm

Energy on Sample Area Irradiated Energy Density Power Density Damage

Shot # joules cm^ j/cm2 MW/cm2 Observed

1 0.5 1.17 0.43 8.6 none

2 1.3 0.56 2.3 46 none*

3 1.3 0.20 6.5 130 Surface+Bul k?"*"

Uncertainty: 10% 10% 40% 50%

* Hot spot induced damage was observed over 2% of the irradiated entrance surface

f Hot spot induced damage was observed over 30% of the irradiated entrance and exit surfaces

However, a fundamental limitation to further interpretation is the lack of a TEM^^ mode energy distribution in the irradiating pulse. The fraction of the average energy which is concentrated

in the hot spots of the laser pulse is not known; it is the energy in these hot spots (which may differ from the average energy density) which caused the observed damage. Microscratches on the surfaces of the tested sample also present an obstacle to interpretation of the test results. Such surface defects are known to cause electric field enhancement and surface damage at irradiation

levels below those found for defect-free surfaces. This is especially so for high index materials

[15] (n^Q g

= 2.84 for CdCr2S^j

Given the above it is still useful to pick a rough number for the damage threshold in hot- pressed CdCr2S^ and make a comparison with thresholds of other high index materials. Table 4 gives

CdCr2S^ data from shot 3 along with 10.6ym induced damage thresholds of ZeSe, GaAs, CdTe, and Ge. It would be useful to scale the data from long to short pulse lengths. One calculation predicts that

for inclusion-free dielectrics a 100-fold decrease in irradiation pulse length from 200ns to 2ns would be accompanied by a 4 to 8-fold increase in power density threshold [16]. If electrical

breakdown mechanisms hold in the same way for CdCr2S^, one would predict a damage threshold of

0.4 J/cm^ for a Ins pulse. Similar extrapolations, indicated in table 4 for GaAs, CdTe, and Ge,

bracket the above value for CdCr2S^. The data for ZnSe show this extrapolation to agree within a factor of 2 with experimental results.

Table 4. Damage thresholds of high index materials at 10.6ym

(B = Bulk, S = Surface)

Extrapolated Threshold

for Ins pulse*

Reference

2 T GW/cm or J/cm-'

15 1.5

16 3.0

19 0.8

0.4

17 0.2

18 0.1

Material Pulse Length Energy Density Power Density

7 2

(n-jQ g) (mode) ns J/cm'^ MW/cm

ZnSe Iqq 1.5(B) 1,500

(2.41) ZnSe 600-. 55(B) 90

(2.41)

°°

Ge 75„n 13(S) 173

(4.0) "° CdCr2s/ 50^^^^. 6.5(?) 130

(2.84) GaAs 100, 3(B) 30

(3.3) ?

CdTe 600qq 2(S) 3

* See text

^ Based on data from shot #3

-130-
I

Future systems requirements for high power laser amplifiers specify that optical components be capable of handling 2 J/cm2 in 1 ns [20]. From this damage test it may be argued that

I hot-pressed CdCr2S^: a) is certainly within a factor of 5 of this specification, and b) warrants

continued investigation as an isolator material for 10.6ym.

6. Acknowledgments

i The authors would like to acknowledge E. Carnall, Jr., D. Pearlman, and T.W. Martin of the

I Research Laboratories - Eastman Kodak Company for the provision of hot-pressed material, and J.

j Wilson from the Laboratory for Laser Energetics, University of Rochester, for assistance in performing

I the damage tests.

7. References

[1] Baltzer, P.K., Lehmann, H.W. and Robbins, M.,

"Insulating Ferromagnetic Spinels," Phys.

Rev. Lett . J^, 493 (1965).

[2] Harbeke, G. and Pinch, H., "Magnetoabsorption

in Single-Crystal Semiconducting Ferromagnetic Spinels," Phys. Rev. Lett . 17, 1090

(1966).

[3] Coburn, T.J., Pearlman, D., Carnall, Jr., E., Moser, F., Lee, T.H. , Lyu, S.L. and Martin,

T.W., "The Preparation and Magneto-Optical

Properties of Hot-Pressings of the Magnetic

Semiconductors Cd^C0i_^Cr2S^," AIP Conference

Proc. on Magnetism and Magnetic Materials in Denver, Colo. 740 (1972).

[4] Pearlman, D., Carnall, Jr., E. and Martin,

T.W., "The Preparation of Hot-Pressed Chalcogenide Spinels," J. Sol . State Chem . 1_, 138

(1973).

[5] Pearlman, D., Carnall, Jr., E. and Martin,

T.W. "The Preparation of Hot-Pressed Chalcogenide Spinels. II. Stoichiometry and Optical Transparency in CdCr2S^," J. Sol. State

Chem . 9, 165 (1974).

[6] Ahrenkiel, R.K., Moser, F. , Carnall, E., Martin, T., Pearlman, D., Lyu, S.L., Coburn,

T. and Lee, T.H. "Hot-Pressed CdCrgS^: An Efficient Magneto-Optic Material," Appl . Phys. Lett . 18, 171 (1971).

[71 Moser, F., Ahrenkiel, R,K., Carnall, E., Coburn, T., Lyu, S.L., Lee, T.H., Martin, T. and Pearlman, D., "Optical and Magneto-Optic

Properties of Polycrystal 1 ine CdCr^S.,"

J. Appl. Phys . 42, 1449 (1971).

^

[8] Coburn, T.J., Moser, F., Ahrenkiel, R.K. and Teegarden, K.J., "Infrared Absorption Spectrum and Faraday Rotation of Hot-Pressed

CdCr2S4," IEEE Trans. Mag.. MAG-7 . 392 (1971).

[9] Baltzer, P.K., Wojtowicz, P.J., Robbins,

M. and Lopatin, E., "Exchange Interactions

in Ferromagnetic Chromium Chalcogenide Spinels," Phys. Rev. 151 , 151 (1966).

[10] Von Neida, A.R. and Shick, L.K., "Single

Crystal Growth of Some Chalcogenide Spinels," J. Appl. Phys . 40, 1013 (1969).

[11] Jacobs, S.D., "Ferromagnetic Isolation and Modulation in the Infrared Using Hot-Pressed

CdCr2S4," Ph.D. Thesis, University of

Rochester, 1975.

[12] Lehmann, H.W, and Robbins, M. , "Electrical

Transport Properties of the Insulating Ferro- magnetic Spinels CdCr2Se^," J. Appl. Phys .

37_, 1389 (1966).

[13] Martin, G.W., Kellogg, A.T., White, R.L. and

White, R.M., "Exchange-striction in CdCr2S4

and CdCr2Se4," J. Appl. Phys . 40, 1015 (1969).

[14] Jacobs, S.D., "Faraday Rotation, Optical

Isolation, and Modulation at 10.6ym Using

Hot-Pressed CdCr2S4 and CoCr2S4," J. Elect . Mater . 4, 223 (1975).

[15] Stark, Jr., E.E. and Reichelt, W.H., "Damage

Thresholds in ZnSe, A/R Coated NaCl and Micromachined Mirrors by 10.6ym Multijoule, Nano- second Pulses," NBS Special Publication 414,

p. 53 (1974).

[16] Wang, V., Rudisill, J.E., Giuliano, C.R.,

Braunstein, M. and Braunstein, A., "Pulsed

CO2 Laser Damage in Windows, Reflectors, and Coatings," NBS Special Publication 414, p. 59 (1974).

[17] Smith, J.L., "Effects of Laser Flux in GaAs," NBS Special Publication 387 - Laser

Induced Damage in Optical Materials: 1973,

p. 103.

-131-
[18] Braunstein, A,, Wang, V., Rudisill, J. and Wada, J. Braunstein, M.,

, "Pulsed C0„ Laser Damage Studies of Windows and Window

Coatings," NBS Special Publication 387,

p. 151 (1973).

[19] Davit, J., "Damage Threshold in 10.6ym Laser Materials," NBS Special Publication 387,

p. 170 (1973).

[20] Figueira, J., LASL: private communication

i. Figures

100.0

10.0

1.0.8 "l.O ZO 2D 4.0 5.0 6.0 7.0 B.O 9.0 10.0 12.0 14.0 150

Wavelength X,/im —

Figure 1. Typical optical attenuation in hot- pressed CdCr2Si4 vs wavelength at 300 K (sources indicated in figure.

-132-
Figure 3, F/B ratio over face of cylindrical

sample of hot-pressed CdCraSa 1 cm

long by 0.635 cm in diameter, at

80 K, Measurements were made with

10,6 ym radiation from a 0.3 watt

cw CO2 laser focussed to less than

0.5 mm spot size (as measured with

thermofax paper) [11].

-I cm Aperture

LASER PULSE

Gold Surfoce Mirror fY. i 50cm.

Figure 4. Optical layout for laser induced

damage test.

0

L

SITE-SHOT 2 SITE-SHOT 3

Figure 5. Thermofax burn spots characteristic of 10,6 ym irradiating

pulse.

Figure 6, Nomarski micrograph of damage to entrance surface of h-p CdCr2Sit caused by hot spots in shot 2; 750x - micrograph spans 150 ym

of surface.

-133-

1
-134-
COmENTS ON PAPER BV JACOBS AUD TEEGARVEN

No diica&i'ion on tkLi papeA.

-135-
1.16 PULSED LASER INDUCED DAMAGE AT THE DF LASER WAVELENGTH*

M. Bass, K. M. Leung, C. Tang

Center for Laser Studies, University of Southern California

Los Angeles, California 90007

and

M. J. Soileau

Naval Weapons Center

China Lake, California 93555

A pulsed TE DF laser has been used to study the damage phenomenon In materials which are transparent to DF laser light (3. 5 - 4. 1 |am). We measured

the bulk damage threshold In several alkaline-earth fluorides, alkali -halldes, and

sapphire. The peak on axis Intensity which damaged CaF and sapphire was 25

2 GW/cm . Surface damage occurred at Intensities less than 1/10 that required

for bulk damage. This was attributed to poor surface finish quality and points

out the need for improvements in finishing optics for use with DF lasers.

Keywords: A Ikali-halldes; alkaline-earth fluorides; bulk damage; focal spot

diameter; Inclusion damage; Interference ripples; pulsed DF laser damage;

sapphire; scattering; waveform distortion.

1. Introduction

In recent years, the development of high power lasers In the Infrared region has motivated

studies of laser induced damage to infrared transmissive optical materials. Alkaline-earth fluorides

have been studied for use in optical components in laser systems operating in the spectral range from

2-5 |jm. The damage resistance of optical materials suitable for use In the 2-5 |jm region has not yet

been systematically evaluated. In this paper we present preliminary measurements of the damage

resistance of the best obtainable samples of certain alkaline-earth fluorides, alkaU-halldes,and sapphire. We also discuss the properties of these materials as they relate to high power 2-5 \jxn laser

systems. The morphology of damage to the surfaces of several samples Is described. We found that

2 sapphire has the same bulk damage threshold as that of CaF^, 25 GW/cm . Since sapphire is a mechanically strong material and is also commercially available in large pieces, it Is an attractive

material for laser windows in the 2-5 \jm region.

2. Experimental

The experimental set up and procedures are similar to those described In reference [1]'. The

basic apparatus consists of a pulsed TE DF laser, a beam attenuator, a test chamber, and an energy/

waveform monitoring system. The experimental parameters are summarized In table 1. The pulsed

Table 1, Summary of experimental parameters

Laser TE - DF

Wavelength 30 lines (3.5 -4.1 ym); 3 dominant lines @ 3. 8 |jm each separated by ~0. 038|im

Waveform 2 leading sharp peaks followed by along low tail, equivalent pulsewldth Is 75 ns Transverse mode Lowest order unstable resonator mode

Attenuation NaCl beam splitter and two ZnSe Brewster angle stacked plate polarizers

Focusing lens ZnSe, 3. 58 cm focal length, AR coated @ 3. 8 |j.m 2 Focal spot diameter 120 ^m at 1 /e in Intensity

Detection system Photon drag detectors and Tektronix 7844 oscilloscope; risetime = 2. 5 ns

* Research sponsored by the Defense Advanced Research Projects Agency, Contract No. F19628-

76-C-0162, and monitored by Air Force Cambridge Research Laboratories.

1. Figures In brackets indicate the literature references at the end of this paper.

-136-
DF laser used in this work has an unstable resonator cavity and a multiline output of ~400mJ. We have

I tried to obtain single-line operation by tuning an intracavity grating, but the energy output for the strongest single-line was not sufficient for our damage studies. Two major experimental problems were encountered in this work. Details of these problems and how we attempted to solve them in orI der to define the damage flux are discussed below.

I 2. 1 DF Laser Pulses

Due to the multiline operation, we observed the two peaked waveform from the DF laser shown in figure 1. If the laser was operated in a single-line manner, then a single peak waveform was obtained.

However, the energy output of this single-line laser was too weak for our purposes.

i In order to define the damage flux of the laser pulse we constructed an equivalent pulse having a

I single-peak waveform with its maximum equal to that of the second peak and with leading and trailing

edges drawn to average out the enclosed area under the original waveform. This equivalent waveform

is indicated by the dashed curve in figure 1. It contains 50% of the energy that was in the pulse and has

a full width at the half maximum of 75 ns. We monitored the waveforms of both the incident and transmitted pulses in the manner described

In reference [l]. The transmitted pulse was attenuated when damage occurred as shown in figure 2. Near threshold, the attenuation occurs at the peak of the second pulse while above threshold it occurs

at the peak of the first pulse. This is evidence for the importance of the peak intensity in the damage

process. It is also one rationale for constructing the equivalent pulse as described above in order to define the damage flux.

2. 2 Focal Spot Diameter

Another problem encountered in our experiment was that due to the unstable resonator cavity configuration, the transverse mode output of the DF laser was an annular ring in the near field [2]. Only

In the far field, about 10 meters away from the DF laser, would the mode consist of a near -diffraction

limited central spot plus a series of weak secondary annular rings. The experiment in this work was

located in the near field of the DF laser output because of limited laboratory space. Asa result of placing a focusing lens in the near field, the determination of the focal spot diameter was more nontrlvlal than in other circumstances.

Three different approaches were used to estimate the focal spot diameter of the DF laser beam

when focused by a 3, 58 cm focal length lens. The results of these three approaches which agree very

well with each other are summarized in table 2. In the first approach, the burn patterns produced by

Table 2. Determination of focal spot diameter for DF laser and 3. 58 cm focal length lens

1. By burn on anodized aluminum 130 |jm

2. By scanning 23 cm focal length lens focal spot and scaling according

118

to the ratio of focal lengths

3. By diameter of region of surface damage showing interference "ripples" < 150

the focused DF laser beam on an anodized aluminum block were measured. The laser energy was

varied until the size of the burn pattern became constant,but before massive disruption occurred. Using this definition of the measured focal spot diameter we obtained a value of 130 (am. In the second

approach, a small pin-hole of 35 |jm in diameter was used to scan the focus of a long focal-length lens. The focus of the 3, 8 cm focal length lens was not scanned because the focused laser beam readily damaged the pin hole. Instead, we scanned the focus of a long focal-length lens (f=23 cm) and scaled the result to the short lens according to its focal length. The expected Gaussian spatial profile of the focused beam was observed at the focus of the long focal-length lens. The result of scaling to the

2 short focal-length lens gave 118 \im for the diameter at the 1/e points of intensity. In the third approach, we measured the diameter of a surface damage site showing interference ripples [3]. These

ripples are produced when dust particles, defects or inclusions, at or near the surface scatter light

which then interferes with the incident light. The resulting interference pattern is seen in the damaged material only in the region directly exposed to the incident beam. The parts of the damage sites

showing interference ripples were found to be less than 150 \sm in diameter. This observation is Illustrated in the micrographs shown in figure 3.

3. Damage in the 2-5 |am Optical Materials

3. 1 Damage Morphology

Several types of damage morphology were observed on both the entrance and exit surfaces of

-137-
the tested samples. Figure 3 is a micrograph of a damage site located at the exit surface of a SrF^

crystal. (Nomarski illumination was used to obtain the micrographs presented in this paper.) The

central region in figure 3 contains interference ripples and is the area directly irradiated by the DF

laser pulse. The diameter of this region in which the damaging interaction took place is I 50 )_im. The extended area showing massive disruption results from the response of the medium to the violent

interaction that took place in the irradiated area. Almost all damage sites on the surfaces of the alkaline -earth fluorides show aligned cracks cor- responding to the cleavage planes of the crystals. Figure 4 shows a damage site at the entrance surface of a CaF^ crystal. A distinctive family of aligned cracks can be seen oriented in a preferred

direction within the site. However, on the same crystal, another damage site shows two sets of cracks having two different orientations as seen in figure 5. The dividing line which we thought was

a polishing scratch is actually the edge of a twin plane. On one side of the edge, the cracks are

oriented with respect to the cleavage plane of one twin, and on the others Ide they are oriented accord'

Ing to the cleavage plane of the other twin. For a MgF^ crystal, which has no well defined cleavage

plane, no oriented cracks were observed as shown in figure 6,

Although no pictures are available to show the bulk damage morphology, we observed that for most of the fluoride materials the damage site locations varied along the axis of the Incident laser

beam. This suggests that the damage mechanism In the samples that we have studied is determined

by the presence of defects or Inclusions.

3. Z Some Properties of Potentially Useful 2-5 [jm Materials

Several important physical properties of the 2-5 (am materials which we studied are listed In -4 -I table 3. In general, the alkaline-earth fluorides have absorption coefficients of ~10 cm . On the other hand, the absorption of sapphire Is two orders of magnitude higher than that of the alkaline

earth fluorides.

Table 3. Some properties of potentially useful 2-5 Jim materials

Index of Average Sample's Knoop Melting

Refraction ,. ., a Hardness Point

Absorptivity q

-1 P(cm ) A Ikallne - Earth

Fluorides

MgF 1. 355 4. 8 X

10-^ 415 1255 ^ 2 CaF^ 1. 41 ~ 3. 5 X

10-^ 120 1395

SrF^ 1. 41 ~6 X

10-^ 130 1450

BaF^ 1. 457 ~ 18. 5 X 10-4 65 1355

Alkali - Halides

LIF 1. 353 25. 5 X

10-4 102 844

NaF 1. 31 5. 2 X

10-4 60 1012

NaCl 1. 52 4. 2 X

10-4 18 801

1. 68 4 X

10-2 -1700 2040

These values are taken from references [4] and [5].

If damage In these samples Is determined by the presence of defects or inclusions, one might

expect low damage threshold to correlate with high scattering at the laser wavelength. Therefore, we measured the total scattering for these samples at four different wavelengths including 3. 39 |im

which is sufficiently close to the DF laser wavelength (~3.8 pm) to permit a comparison of scattering and damage. The results of our scattering measurements are given in table 4.

-138-
Table 4.

3 Scattering* in 2-5 Mm window materials (xl 0 )

Material Forward

= 0. 4762

Ba c kwa r d

0. 6471

Forward Backward

1. Forward1 5Backward

3. 39

B a. ckwa rd

MgF^ L, C.5 1. 68 9. 00 0. 554 0. 282 n AO A CaF^ •2 La i, d4 4.. D4 2.15 2.36 0. 960 0. 541 n c: 9 U« D 0 C. n noA BaF^ 1.41 1.47 0.455 n 1 t; 7 U • 1 D f n n A e: Q U, Uo OO SrF^ 1 QQ 1. 9 AC 1.16 1.39 0.404 0. 318 n 1 7Q

NaF 14. 1 13. 8 7.64 8.53 1. 75 2. 30 0. 184 0. 21 0 LiF 4. 14 2. 29 3. 03 0. 564 0. 0973 0. 0700

* Scattering is the fraction of the light scattered into the forward or backward Zv steradians with

respect to the total quantity of light transmitted through the sample,

3.3 Results of Damage Threshold Measurements

The bulk damage thresholds for the materials we studies are listed in table 5. Only single shot on single site testing is reported. For the interest of the readers, we also include the ratio of damage thresholds for different materials normalized to that of NaCl. All the samples were obtained

from commercial sources and were specified as having "high-power laser" finishes.

Table 5. Threshold for bulk laser irradiation induced damage (DF laser light, K = 3.8 )Jm)

Material I (GW/cm^)* R = I /I (NaCl) B B B

t MgF^ 18. 5 1.19

CaF^ 25. 5 1.65

SrF^ 28. 5 1. 84

BaF^ 27. 5 1. 77

t LiF 20. 0 1. 29

NaF >30. 0 >1. 94

NaCl 1 5. 5 1. 00

2 3 25 1.61

This table gives peak intensity on axis at the threshold for damage.

i~ These materials had nonuniform damage thresholds possibly due to inclusions.

The measured bulk damage thresholds do not correlate with the total scattering which we measured. On the basis of visual observation we conclude that most of the scattering was caused by the inadequate quality of the surface finishes. In fact, surface damage threshold for these samples was 5 to 1 0 times lower than the bulk damage threshold, and was determined by the low quality of the surface

finishes. It is for this reason that we do not list the surface damage fluxes.

4. Conclusions

The alkaline-earth fluorides tested in this work have very similar bulk damage thresholds and

the site morphology suggests that inclusions or defects are responsible for the damaging interaction.

The surface damage thresholds for these samples are much lower than the bulk damage thresholds.

This can be attributed to poor surface finish quality and leads us to recommend that improved polishing techniques be developed in order to make these materials applicable to high power laser systems.

2 We also found that sapphire has a very high bulk damage threshold, 25 GW/cm . Since sapphire is mechanically strong and readily available in large pieces, it should be considered for use in the 2-5

ym region.

5. Acknowledgements

The authors are grateful to Crystal Systems Inc. for providing one of the sapphire samples and

also thank many individuals at NWC, China Lake, CA. , for their technical assistance especially in the scattering measurements.

-139-
6. References

[l] Leung, K. M. , Bass, M. and BalbinVlUaverde, A.G.J., in Proc. 7th ASTM/

ERDA/NBS/ONR Symposium on Laser

Induced Damage in Optical Materials: 1975,

NBS Special Pub. 435, 107 (1975).

[2] Stegman, A.E., Proc. IEEE 53, 277 (1965).

[3] For example, see Temple, P. A, and

Soileau, M. J., "Resonant Defects Enhancement of the Laser Electric Field," this Proceedings.

[4] Hass, M. , Harrington, J. A., Gregory,

D.A. and Davis son, J. W. , Appl. Phys.

Lett. 28, 610 (1976).

[5] Harrington, J. A., Gregory, D.A. and Ott W. , "Infrared Absorption in Chemical

Laser Window Materials," Appl. Opt. (to be published).

IT<

a.

TIME (ns)

Figure 1. Temporal profile of a multi-line DF laser

pulse. The dashed curve represents the equivalent pulse constructed in order to deduce a damage threshold intensity.

NEAR

THRESHOLD

Figure 2. Typical oscilloscope traces for bulk damage

in SrF2 due to DF laser light. The upper

trace is the incident pulse waveform and the lower trace is the transmitted pulse waveform.

When damage occurs the transmitted pulse waveform is attenuated.

ABOVE

THRESHOLD

-140-
Figure 3. Nomarski micrographs of exit surface damage

to SrFz. The lower photograph is an en- largement of the central region of the irradiated site showing interference ripples.

-141-
Figure A. Nomarski micrographs of entrance surface

damage to CaF2 . The lower photograph is an enlargement of the damage site showing the the aligned cracks.

-1A2-
Figure 5. Nomarski micrographs of entrance surface

damage to the same CaF2 crystal as in

figure 4. The enlarged picture shows

clearly the edge of a twin plane which

divides two sets of aligned cracks.

Figure 6. Nomarski micrographs of entrance damage

to a MgG2 single crystal. No aligned

cracks are observed in this case since

MgF2 has no well defined cleavage planes.

-143-
COMMENTS ON PAPER BV BASS, LEUNG, TANG, AND S01LEAU

Pue to the. poofi quoLity 0($ the. iampZeA, no conAeJLcvtion between iccutte/Ung and damage. tlnAe^kotd could be obieAved.

-144-
2.1 NAVAL WEAPONS CENTER DIAMOND TUENED OPTICS FACILITY

D. L. Decker and R. E. Cram Michelson Laboratories, Naval Weapons Center China Lake, California 93555

The generation of optical surfaces by diamond single point turning or milling

is of much current interest. This fabrication method can be cost effective and can readily produce very unconventional surfaces and components, allowing radically new system designs. In addition, this fabrication method can reliably

produce optical components with superior characteristics including very low scatter, near intrinsic absorption and laser damage threshold. This paper will briefly

describe the requirements and existing machines for diamond single point machining

of high quality optics, and materials known to be amenable to this fabrication

method. The characteristics of a new machine at NWC, which includes an air bearing spindle and carriage ways and closed loop Interferometer control will also be

discussed. The emphasis of the NWC effort is to explore selected areas of machine

technology and to further explore the capabilities of diamond turned components.

Key words: Diamond single point turning; interferometric control; laser damage

threshold; optical absorption; optical components; optical scattering.

1. Introduction

The fabrication of high quality optical components by diamond single point turning has been demonstrated in a number of laboratories, notably the Union Carbide Y-12 Plant [1]-^ at Oak Ridge, Tennessee

and the University of California, Lawrence Livermore Laboratory (LLL) [2], Livermore, California. These

laboratories have extensively documented machine requirements and have explored machining conditions and machinability of a wide range of potentially useful materials. This work has been primarily done with

an emphasis on superprecision machined mechanical parts, not optical components per se. With due respect,

this paper summarizes these results and indicates some outstanding problems. This paper further presents

ideas and hardware approaches based heavily on this information and Naval Weapons Center experience in conventional optics fabrication and characterization. Also included are details of a diamond turning

facility being built up at NWC.

2 . Background

Table 1 is a summary of present state-of-the-art optics machining capability [3]. The surface roughness values quoted for small samples are derived from total integrated scattered light measurements

assuming the applicability of the simple scalar scattering theory [4]. The very best mechanically polished metal surfaces typically have roughness values in the range 25-40 A rms. There are, however,

significant differences between the diamond turned and mechanically polished surfaces—notably the type

and scale of lateral structure. The diamond turned surface typically has residual periodic structure

corresponding to the tool feed (typically 100 yinA'ev) and sometimes also to lead screw or drive train

irregularity, whereas only random structure is present on a mechanically polished surface.

Table 1. Physical characteristics of state-of-the-art diamond turned metal status as of 1976.

Part Figure accuracy Surface finish

Small diameter (< 6 in)

Flat (one axis) A/4 (5.7 pin P-P) < 40 1 rms (< 0,16 pin rms) Contour (two axis) A/2 to A/1 50 to 100 A rms Large diameter (80 in max) A/0.5 180 A rms

Optical absorptance of diamond turned metal surfaces is typically nearly as low as the best evaporated films and much lower than for corresponding polished surfaces [5]. In addition, laser damage

threshold values are high. Pulsed laser damage threshold values in many cases are nearly intrinsic [4]. A wide variety of metals and insulators have been successfully diamond turned as of 1976: metals including copper and several copper alloys including brass, gold, silver, aluminum, and various aluminum alloys,

lead and nickel (electroless) ; insulators including NaCl, KCl, CaF2, SrF2 , BaF2, ZnS , ZnSe, MgF2 ,and

acrylic plastics. Several criteria determine the machinability of a material—absence of significant

tool wear from chemical dissolution or from abrasive action of the workplace and low friction between

the tool and work. Excess friction is manifested primarily by the built-up edge resulting from cold

1. Figures in brackets indicate the literature references at the end of this paper. -145-
I

welding of the work material to the tip of the tool. The formation of a built-up edge severely degradej

finish quality and endangers the cutting edge of the tool. The machining action on materials like the i alkaline earth fluorides whose range of ductile behavior is limited or nonexistent is clearly much ' different than experienced on say, OFHC copper. Regardlessly , very good surfaces can be generated on

such materials [6]. It is possible to generate directly and easily components by turning which have ' nearly arbitrary surfaces of revolution. One is not limited to spherical or approximately spherical

| surfaces as In conventional pitch lap techniques. To generate an arbitrary surface of revolution, tool motion on two Independent coordinate axes is required. Most common is a facing lathe arrangement, withj two orthogonal linear slides [1,2], but other configurations have also been Implemented [1]. Independently of the details of machine design, a number of general principles can be listed. Machine qualltyJ

meaning basic machine slldeway and spindle accuracy and reproducibility of motion is paramount. A ! number of factors directly contribute: (1) the stability of the machine with time and with whatever j temperature fluctuations may be present In the machine environment, (2) machine stiffness and absence ' of excessive sources of vibration. Also essential to satisfactory diamond turning results is a quality

tool and proper workpiece composition and support. These requirements are discussed In greater detail

in the following paragraphs. '

James Bryan, of LLL and others have pointed out that vibration which is synchronous with spindle j rotation does not contribute to workpiece roughness at least In first order [7]. As5mchronous sources i are then particularly troublesome—coolant and alrconditioning pumps, carriage drive motors, etc. Since there are no suitable zero temperature coefficient materials available for all critical machine parts,

temperature control of the machine becomes essential. This problem can be ameliorated to some extent by

using Interferometric tool position determination, with temperature stabilization required for whatever

machine elements exist between tool and its reference mirror (s) and between workpiece and its reference

mlrror(s). Much effort has gone into machine temperature stabilization, with several notable successes.

The LLL approach using an extensive oil shower is particularly easy to Implement in a machine shop

environment, and provides a clean (absence of particulates), temperature stable environment [2]. However> in this approach it Is difficult to explore the effect of machining fluid (necessarily the temperature stabilizing fluid) on the optical properties of the resulting work. Several outstanding problems,

including reliable coatablllty of diamond turned surfaces remain to be solved. Other approaches to the machine temperature stabilization problem Include the use of interferometric position measurement as an element in the carriage control feedback loop with a less extensive oil shower such as the approach

which has been implemented, for example, at the Y-12 plant [8].

For materials which can be successfully diamond turned, there is minimal friction between the tool and work, minimal wear and no built-up edge. Hence, as the tool passes over the work, the tool Imparts

a surface finish which is a transfer of the finish of the tip of the tool. It is axiomatic that a very

smooth, sharp and accurately shaped tool must be used. These tool requirements are met commercially

for the most part in a satisfactory manner. Although, since machlnablllty of a given material is determined by tool compatibility, it is quite possible that materials now considered unmachlnable with

diamond tools, e.g., iron and ferrous alloys, could be amenable to precision machining with tools of different composition. The last requirement indicated above is no less important than those listed

first. It has long been appreciated in the optics fabrication community that nondlstortlng work support

is required. For example, parts are commonly waxed down to support fixtures. However, in the case of the often complicated mechanical or very unconventional optical parts which are diamond turned, support

can be a problem. A useful general solution has Involved the use of a rubber potting compound, which

since it cures from the liquid does not generate shear forces, and as a cured product has a modulus so low that no large tensile or compressive forces are exerted on the work [9]. Even so, uniform support

(distribution of holding forces) is an essential feature in the design of a work support. The problems

associated with existing machines can be summarized as follows: (1) failure of the tool to accurately

track as a consequence of way friction, stlctlon, or irregularity or noise in the case of roller way

systems, (2) large potential Abbe offset errors as a consequence of large separation between tool tip and the primary source of positioning accuracy (lead screw) , and (3) constraints which item (2) places

on the system—very tight temperature control of the entire machine, for example. These problems

become more severe as machines are made larger. Of course, there are many other difficulties which are Independent of machine design. For example, the problem of tool/material compatibility.

3. NWC Diamond Turning Facility

With these introductory remarks, this paper will now briefly describe a machine which is being

built up at NWC. The design of this machine is intended to address many of the operational difficulties

of existing optics turning machines. There is no claim that the design of this machine is unique, as

it contains elements which have been developed and used extensively on other machines. However, it does combine many features and capabilities found only separately into one Integrated system, designed

from the very beginning as an optics turning machine. This machine is intended to fill a research role:

(1) explore selected areas of machine technology, and (2) further explore the capabilities of diamond

turned optics. The initial machine configuration will permit swinging parts up to 14 inches in diameter

with a total chuck/part weight of greater than 100 lbs. The basic machine design includes a state-ofthe-art air bearing spindle, and air bearing slide ways. Figure 1 is an isometric schematic of the NWC machine, showing the arrangement of the slides and spindle. The machine base is an air suspension

-146-
I isolated black granite surface plate. This arrangement has several advantages over the most common

1 machine configuration, namely, the Moore #3 conversions. All critical components are tied thermally and

! mechanically to the base in a similar manner, with all parts at essentially the same height from the

I floor. Vertical temperature gradients are thus less important. Also, the granite surface plate represents a very flexible two dimensional optical/mechanical bench, allowing changes in components or con- figuration to take place without extensive redesign or rework. For example, the incorporation of an interferometer to measure slide position is easily accomplished, as is shown in figure 1. It is relatively easy to have the interferometer beam at tool height, thus minimizing Abbe offset errors. It is in general not possible to gauge tool/work positions right at the tool tip. The present design calls

for gauging on the back surface of the spindle faceplate through the spindle, and gauging tool position

right at the tool holder. The Moore #3 conversion unfortunately does not conveniently permit use of a gauging/ control interferometer at tool height. The carriage control loop will include a very high speed

calculator or computer which will issue rate and position commands to the dc torque motor drive chassis.

The interferometer will provide position correction via hard wired comparator. The feedback loop bandwidth including motor drive electronics will be in excess of 20 kHz3. Table 2 indicates basic specifications for the NWC machine.

Table 2. Basic specifications and features of the NWC diamond turning machine.

Spindle

Hydrostatically supported

Axial and radial runout Angular (wobble) error Axial and radial stiffness

Slides

Hydrostatically supported

Straightness (yaw) and

flatness (pitch)

Stiffness in pitch and yaw

Other features:

Flat belt spindle drive from vibration isolated variable speed motor

Traction drive dc torque motor reduction, and possibly traction final

drive to carriage

Temperature stabilizing oil passages incorporated in slides, spindle and

all critical members

Closed loop interferometer positioning and control of slide motion with

interferometer beam at tool post height

Very high speed calculator or computer in control loop

3 uin TIR max

0.1 arc s max

10^ lb /in

3 pin TIR max in any inch,

10 yin max in any 10 inches

10^ lb /in

As has been extensively documented primarily by the Y-12 laboratory, the incorporation of a slide

position interferometer will not solve all control problems. In particular, short period motion/position

errors associated with slides or slide drives cannot be removed by interferometric control. Smooth, if not accurate, drives are mandatory. The NWC approach will use a traction drive reduction from an encoded

dc torque motor to the final carriage drive, as has been suggested and experimented with by Y-12 [8]. It is quite possible the final connection to the carriage will be by traction drive also rather than the conventional lead screw drive. Hydrostatically supported slides also provide important improvements in slide position accuracy, especially for intermittent slide motion. This machine will be set up in February 1977.

4 . Summary

The NWC machine then specifically addresses the following areas of machine technology: (1) slides

and slide drives, (2) slide position/motion control, and (3) tool/workpiece position/motion errors as a consequence of Abbe offset errors. Some of the approaches suggested in this paper are admittedly relatively high risk, but such a course is appropriate for a research machine. The present goals of the NWC machine are summed up simply: to be able to machine a component requiring two axis motions to a quality

equal or better than existing machines can produce with one axis motion alone, and to be able to readily

vary machining parameters such as machining fluid to explore the effect of such changes on the ultimate

optical properties of the part. It is clear why diamond turned optical components have created so much

interest. It is the intention of the NWC effort to contribute in a significant way to this vital

endeavor

.

5. Acknowledgments

The authors of this paper acknowledge many valuable discussions with personnel at Lawrence Livermore

Laboratory, notably Jim Bryan and with James Arnold, Dick Sladky, and Del Bender of the Union Carbide

Y-12 plant. Also acknowledged is support from the U. S. Army Missile Command, and the Air Force Weapons

Laboratory. Support was also provided by Naval Weapons Center Independent Research Funds.

-147-
6. References

[1] Arnold, J. B. and Stager, P. J., "Diamond [6] Turning on Advanced Machine Tool Prototypes,"

presented at the SME Clinic on Industrial

Application of Precision Machining and Gaging, November 19-21, 1974, San Francisco,

CA. (Union Carbide, Inc., Oak Ridge, Tenn. [7] 37830, Report Y-DA-5975, 1975.)

[2] Bryan, J. B. , et al. , "A Practical Solution

to the Thermal Stability Problems in Machine [8] Tools." (Society of Manufacturing Engineers,

Dearborn, Mich. 48128, Technical Paper No. MR72-138, 1972.)

[3] Abstracted from numerous reports and publications, principally from Y-12 and LLL, and

from NWC measurements of available diamond [9] turned specimens.

[4] Decker, D. L. , Soileau, M. J., Porteus, J. 0. and Bennett, J. M. , "Surfaces, Optical and Laser Damage Characteristics of DiamondTurned Metal Mirrors," these Proceedings.

Arnold, J., Sladky, R. and Steger, P., "Machinability of Window Materials,"

presented, at the ARPA Infrared Window Meeting, 12 July 1976.

Bryan, J. B. , Clauser, R. and Holland, E., "Spindle Accuracy," Amer. Mach. ill . No. 25, December 4, 1967, pp. 149-164.

Arnold, J., et al., "Diamond Machining of Reflectors," Progress Report under AFWL Contract 75-343 and 76-114 period ending January

1976. (Union Carbide, Oak Ridge, Tenn. 37830 Y-12 Plant Document No. Y-DA-6681, February

1976.)

Jones, W. F. , "Part Support," presented at the SME Clinic on Precision Machining and Gaging, Oak Ridge, Tenn., November 14-16,

1972. (Union Carbide, Inc., Oak Ridge, Tenn.

37830, Preprint Y-DA-4841.)

[5] Bennett, H. E., Soileau, M. J. and Archibald, P. C. , in High Energy Laser Mirrors

and Windows (Semi-Annual Report No. 6, ARPA

Order 2175, Naval Weapons Center, May 1975),

pp. 53-61.

Figure

GRANITE BASE

"X" SLIDE 26CmTRAVEL

Figure 1. Isometric schematic of the NWC diamond

turning machine. Shown are X and Y hydrostatic slides, hydrostatic spindle, and laser position/control interferometer

with beam at tool height. All components

mounted on granite surface plate base.

COMMEMTS OW PAPER ZV VECKER. AMD CRAM

No (LiiciUi-ion.

-148-
2.2 10.6 pm PULSED LASER DAmGE STUDIES OF DIAMOND

TURNED KCl WINDOW SURFACES

M. J. Solleau, J. M. Bennett, J. 0. Porteus,

W. Faith and J. Jernigan

Michelson Laboratories, Naval Weapons Center China Lake, California 93555

and

T. T. Saito Lawrence Livermore Laboratory

Livermore, California 94550

Laser damage measurements at 10.6 pm are presented for diamond turned surfaces

of polycrystalline KCl. The two types of windows tested were hot-forged RbCl- and

EuCl-doped KCl. Surface damage thresholds of the two materials range from 1.8 to 2.4 GW/cm^. The optical properties of these specimens were measured after diamond

turning and compared to chemically etched surfaces on the same specimens. RMS roughness, infrared scattering, and absorption at 10.6 vm were measured. The

influence of the machining grooves on the character of the laser damage on the surfaces is discussed. The regularity of the surface defects produced by the diamond turning process offers a unique opportunity to study the effects of surface

defects on the laser damage threshold of optical surfaces.

Key words: Diamond turning; KCl laser windows; laser damage; 10.6 ym laser.

Introduction

Surface finishing problems have severely limited the usefulness of KCl for laser windows. The major

problem is to produce low scatter, low absorption, damage resistant surfaces with good optical figures.

Many techniques have been tried with mixed results [1-5]^. Standard lap polishing produces well finished surfaces having good optical figures, but the surfaces are highly scratched and have high absorption at 10.6 ym [1]. Chemical etching produces surfaces which are scratch free [2,3], laser damage

resistant [3,4], with low surface absorption [4], but with poor optical figures [1]. Various combinations of chemical /mechanical techniques have produced surfaces with good optical figures and low

absorption [1,5]. However, even with the best of such techniques, one is plagued by scratches, relatively poor damage thresholds, high cost of manufacture, and poor reproducibility in the results.

Single point diamond turning has been used to produce optical surfaces on metal mirrors. Mirrors

produced using this technique have been shown to have high damage thresholds, low absorption? and good

optical figures [6]. The purpose of this work was to determine if similar results could be obtained by

diamond turning KCl. Measurements of pulsed laser damage thresholds, optical scattering, and absorption

at 10.6 um indicate that diamond turning is an attractive alternative to mechanical polishing and

chemical etching for producing optical surfaces on KCl windows.

Experimental

The two specimens tested were doped, polycrystalline KCl. The KCl was grown (and doped during the growth process) by Harshaw, and hot-forged into polycrystalline form by the Honeywell Ceramics Center [7]. One specimen was doped with EuCl and the other with RbCl. These specimens will be referred to as KCl:EuCl and KCl: RbCl.

The samples were diamond turned on the same day at Union Carbide's Y-12 plant in Oak Ridge, Tennessee. Both were fly cut off center using a feed rate of 0.2 in/min, and a spindle speed of 1200 rpm. No lubricant was used to minimize surface contamination. Off center fly cutting was used since we have found

in cutting single crystal windows that turning the window on center resulted in a rougher surface [8]. Some machined polycrystalline samples were found to be rougher than machined single crystal specimens.

Repeated machining of the same part seemed to reduce the roughness of some crystals, but increased the

roughness of others. We were not able to get a completely clear surface; the unclear areas may be

related to the crystal orientation of the tool. Nomarski micrographs of typical areas on each specimen

tested are shown in figure 1. As seen in the figure, the fly cutting off center produced machining

grooves which were approximately straight and parallel (at least over the diameter of the laser probe

used in the damage measurements)

•

1. Figures in brackets indicate the literature references at the end of this paper. -149-
3

After diamond turning, the specimens were shipped to NWC, China Lake, California for optical characterization and pulsed CO2 laser damage testing. The specimens were wrapped in tissue and sealed in a plastic bag for shipment. Our experience has been that any physical contact with a finished KCl surface

raises the surface absorption and lowers the damage threshold. If the surfaces of the specimens had

been coated with protective layers before shipment, the results of our measurements might have been

better.

Laser absorption calorimetry was performed on each of the specimens on the same day they were

received by NWC. These measurements were repeated after the conclusion of all damage testing on the specimens. The specimens were then chemically etched to remove any surface damage and surface contamination introduced by the diamond turning [1], and the absorption measurements were again repeated.

Total integrated scatter (TIS) in the forward and backward directions was measured as a function of wavelength using the NWC Optical Evaluation Facility [1,9]. This instrument uses a Coblentz sphere to collect the hemispherical scattering from the specimen and focus it onto a detector mounted at the con- jugate point of the sphere. The TIS backscatter data for a wavelength of 6471 A were used to compute

an equivalent surface roughness for the two specimens.

Surface scans were made on both specimens using a Talystep surface profiling instrument. This

instrument, manufactured by Rank Precision Industries Ltd., Leicester, England, uses a conical diamond

stylus tip of approximately 8 um radius to record variations in the surface height. The stylus loading

is continuously variable from less than 0.5 mg to about 50 mg. A load of approximately 1 mg was used

for the traces. This was about the smallest amount possible to keep the stylus from skipping. Some flattening of surface features was observed by comparing two traces made over the same track. However,

the flattening was generally less than 40 A on features having peak to valley depths of the order of 400 X. (A previous experiment using stylus loadings of 0.6 mg and larger on a sputtered silver film

showed no indentation for loadings under 4 mg when replicas of single traces were observed in the electron microscope. This result is in agreement with the previously observed phenomenon [10] that the hardness of materials can be much larger for very small loadings.) A scan speed of 3 ym/s on the sample

surface was used, and the total distance covered by a scan was 61 ym. The lateral resolution of the stylus tip was checked by tracing over slip planes on (110) single crystal aluminum produced by laser

pulses. Structure 0.7 ym wide having a slope of about 1° was well resolved.

The output of the Talystep (voltage variations of ± 1 volt, corresponding to height variations on

the sample ranging from ± 5 ym to ± 250 X depending on the gain used) was fed into a Hewlett-Packard

Model 5480 B Signal Averager and thence into a Hewlett-Packard Model 2100A Minicomputer. The signal averager digitized the data at preset time intervals, so that an entire scan consisted of 1000 data

points. In this way surface height information could be sampled at intervals as small as a few hundredths of a ]Sm or as large as 1.5 ym. The sampling rate was chosen so that the curve formed by

connecting adjacent points was a reasonable representation of the surface, commensurate with the limiting resolution imposed by the radius of the stylus tip.

A computer program originally written to determine surface statistics from surface scan data taken

with the FECO Scanning Interferometer [11] was modified to calculate the same statistical quantities

from the digitized Talystep data. These quantities are rms height, rms slope, height and slope distribution functions, and autocovariance function. The statistical quantities could be calculated for a single scan (1000 data points) or for averages of many scans.

The Talystep was equipped with two micrometers having least counts of 2 ym which moved the sample

parallel and perpendicular to the direction of motion of the stylus tip. Thus a sample could be positioned as desired and a series of parallel, equally spaced scans could be made, for example, across a laser damage crater. This was done for a crater on the exit face of the KCl:EuCl sample.

Polarization interference photomicrographs were taken at the Y-12 Plant to estimate the peak-valley

surface roughness for the two samples. If the peak-valley values are divided by 2/2 to convert them to rms values, one obtains 177 A rms for the KCl:RbCl and 205 A rms for the KCl:EuCl. Diamond turned surface roughness measurements made with polarization interference microscopes have historically Indicated

somewhat rougher surfaces than FECO, light scattering, and stylus instrument measurements [12].

Damage testing was performed using the Naval Weapons Center's laser damage facility [4,13]. The

experiments reported in this paper were conducted on a pulsed, single-mode CO2 laser operating in the 10.6 ym wavelength region. It has reproducible temporal and spatial characteristics which are detailed

in the following discussion. The laser is sealed with Brewster windows, resulting in a linearly polarized output beam (vertical polarization direction) . Beam intensity is controlled by a rotating polarizer-]

analyzer combination, each consisting of two ZnSe plates tilted in opposite directions to minimize beam

shift, as well as by CaF2 attenuators. Following the attenuators, the energy in each pulse is measured

by a pyroelectric energy meter which is calibrated using a thermopile calorimeter. The beam enters the

test chamber through an NaCl window and is focused on the sample with a 2.54 cm diameter, 3.546 cm focal

length ZnSe meniscus lens. (Focal length given is for 10.6 ym.) A meniscus lens was used in order to minimize spherical aberrations. A HeNe laser is used for convenience in aligning the optics.

-150-
1 The temporal beam characteristics are monitored with a photon-drag detector having a 1.5 ns rise

I time. Figure 2 shows the measured intensity as a function of time, together with an analytical expres-

!' sion which describes this dependence. As can be seen in figure 2, the laser output increases linearly

^ to the peak and falls off as a steep exponential. The peak power, calculated in watts using the total

output energy and temporal pulse shape, is 1.27 x 10^ times the total output energy (in joules). From

this expression we can define an equivalent pulse width of 78.7 ns ; i.e., the total energy divided by

78.7 ns gives the peak power of the pulse. Note that the 1/e full width of the pulse is approximately

78.7 ns. The full width at half maximum (FWHM) of this pulse is approximately 60 ns.

The spatial profile of the beam as it enters the damage chamber was measured using a pinhole scanned

across the beam, and can be represented by a Gaussian distribution to within 4%. The 1/e full width of

the unfocused beam was 5.8 mm. The spot size for the 3.546 cm focal length ZnSe lens was determined by

I scaling the measured spot size using a 11.91 cm focal length lens [4] by the ratio of the focal lengths

! of the two lenses. This calculation yields a l/e^ spot diameter of 59.5 um for the 3.546 cm focal

length lens. The effective f-number of this lens is approximately 6. The correct position for the ZnSe

lens was determined using a technique previously described by Porteus et al [13]. This procedure

allows us to set the position of the specimen surface to within 10 pm (the repeatability of the translation micrometer)

.

i

' The test chamber and the analytical instrumentation used in this experiment have been described

I previously [14]. The chamber is designed for ultrahigh vacuum, including bakeout. However, the measureI ments reported in this paper were made at a base pressure better than 1 x 10~^ Torr without bakeout.

Visible changes in the specimen (surface or bulk) were monitored by observing the scattered light with a 20-power optical microscope.

^ Results

I The two specimens were examined using Nomarski microscopy to determine the qualitative nature of the surfaces and with a Twyman-Green interferometer to determine optical figure. The Nomarski micrographs

of these surfaces showed indications of tool marks from the diamond turning process. However, the appearance of the tool marks varied greatly with position on the specimens, and as can be seen in figure

1, was quite different for the two specimens. Portions of the surfaces had a "cloudy" appearance when

viewed with the unaided eye. The cloudy areas did not correlate with the areas of heavy tool marks and

may be the result of improper handling or atmospheric attack.

Twyman-Green interferograms to show the optical figure of the surfaces were taken at the Y-12 Plant

and are shown in figure 3. As can be seen in this figure, the KCl:EuCl specimen was flat to about two waves in the visible, and the KCl:RbCl specimen was flat to about six waves in the visible. In both

cases the fringes show little irregularity, indicating that the figure errors may be caused by improper

blocking procedure.

The results of the total integrated scattering (TIS) measurements at 10.6 um are summarized in figure

4, and the backscatter is plotted in figures 5 and 6 as a function of wavelength. In figure 5 the backscatter of the KClrEuCl specimen is compared to that of a mechanically polished Eu"*^ doped KCl surface.

Note that the scattering levels and wavelength dependence of the two specimens are virtually identical.

The scattering levels include bulk scatter which is quite large for small grain polycrystalline KCl such

as these specimens. The rms roughness calculated from the TIS value at 6471 X was 374 A. The plot in the lower left of figure 5 is the theoretical scatter computed from the 135 A rms roughness of this

specimen as measured by the Talystep surface profiling instrument. In figure 6 the scattering from the

machined sample is compared to that of an etched single crystal. In the visible the scattering from the machined sample is over an order of magnitude higher than that of the single crystal, but at 10.6 ym the scattering levels are virtually identical. The rms roughness of the machined surface is 326 A as calculated from the 6471 A TIS value.

The average absorption (surface plus bulk) at a wavelength of 10.6 ym was 12.6 x lO"** for the machined KCl:EuCl specimen and 20.3 x

10"** for the machined KClrRbCl specimen; average deviation from

these values were approximately ±30%. Both specimens were approximately 1 cm thick. After damage

testing was completed, the samples were etched in concentrated HCl to remove any surface damage and

surface contaminants associated with the diamond turning process, and the absorption was measured.

The results for the KCl:EuCl sample are shown in figure 7. The average absorption of this sample after

etching was approximately 5.2 x 10"^, in good agreement with absorption measurements made on similar

material etched using the same technique [1]. From the absorption measurements before and after etching

we can infer an absorption of 3.7 x 10"'* per surface, which is an estimation of the surface absorption

caused by the diamond turning process. Since the specimen experienced considerable handling in the interval between the time the diamond turning was completed and the first absorption measurement was

made, the value 3.7 x lO"'* per surface represents an upper limit for the absorption caused by the diamond turning process.

The KCl:RbCl specimen could not be etched satisfactorily using our standard technique. Each attempt

to etch the surface resulted in an increase in the total absorption of the specimen. This effect was

probably due to the tendency of the material to undergo preferential etching, thus enhancing grain

-151-
I

I

boundaries. Because of this problem we were unable to obtain an estimate of the surface absorption of • the KCl:RbCl specimen. I

In order to determine surface profiles, ten Talystep scans were made on each side of each sample, inf all cases perpendicular to the grooves. Five of the scans were made in a visually clear area and five

in an obviously grooved area. There was no large difference between the rms roughness values for the different surfaces. However, the KCl:RbCl sample was slightly smoother than the KCl:EuCl sample (difference ~ 15 A rms) . There were definite roughness differences between the clear and grooved areas on

a given surface (difference ~ 20 A rms). Roughnesses in the grooved areas averaged about 148 A rms, while those in the clear areas averaged about 128 K rms. The smoothest area measured had a roughness

of 102 A rms. On a few scans there were positive spikes, as large as 1500 A above the mean surface

level, but no correspondingly deep grooves.

Eight scans were made parallel to the grooves on a visually clear area of the exit face of the KCl:EuCl sample. The rms roughness values for these scans were nearly identical on the average with

those made perpendicular to the grooves on the same surface (134 A rms as compared to 130 A rms) . However, the character of the roughness was different and the autocovariance functions were different for

the two types of scans. In figure 8 are shown typical scans parallel and perpendicular to the grooves,

and the corresponding autocovariance functions. On the surface scans, well resolved structure as closely spaced as 0.6 um is shown, indicating that the stylus tip has at least this good resolution.

This result is in agreement with the previously mentioned measurement of structure on an aluminum surface. Interesting differences can be observed between the two autocovariance functions: (1) Both curves have initial exponential shapes, but the scan parallel to the grooves has a larger correlation

length. If both curves are extended uniformly down to zero, the crossing distances (or approximate

correlation lengths) are about 1.6 ym for the scan perpendicular to the grooves and about 3.2 ym for the scan parallel to the grooves. (2) The periodicity of the autocovariance function for the scan perpen- dicular to the grooves is more definite than for the other one, and the separation between secondary

peaks is ~ 5.5 ym, in good agreement with the value of 5.3 ym measured for the separation of the grooves

on a Nomarski micrograph similar to the one shown in figure 1. (3) The rms slope is 2.1° for the scan perpendicular to the grooves and 1.7° for the scan parallel to the grooves. Although the above mentioned

differences are small, there is evidence that roughness comes both from the grooves made by the cutting

tool and also from tool chatter as the grooves are being cut. It is also possible that the stylus tip was not tracing exactly parallel to the grooves in the one case, since the grooves could only barely be

seen during the alignment process.

After nondestructive optical characterization was completed, the laser damage thresholds of the specimens were determined at a wavelength of 10.6 ym. The single shot thresholds for the entrance and exit surfaces of each specimen were measured as a function of the angle between the electric field vector of the laser and the machining grooves; the results are summarized in figure 9. The power levels required for damage varied greatly and a "threshold" could not be determined. The power level which

caused damage in 50% of the shots was arbitrarily defined as the "damage threshold" (P5o%) • Note the large spread in power levels from the lowest power which caused damage (Pl) to the highest power for which no damage occurred (Pjj) . This large variation in threshold for a given orientation relative to the machining grooves made it very difficult to see any dependence of the threshold on orientation

angle. The entrance and exit damage thresholds for each orientation were averaged and the results are given in figure 10. The average thresholds thus computed compare quite well with previous measurements

on similar material [4]. However, as previously indicated, these surfaces had very nonuniform surface

quality, surface absorption, and damage threshold.

Typical Nomarski micrographs of entrance and exit surface damage sites are shown in figure 11. The

exit surface damage is characterized by ripples whose spacing is nearly equal to the laser wavelength

in the material. The ripples were most pronounced when the machine grooves were normal to or at large

angles to the incident laser electric field. The ripple spacing and orientation in the exit surface

damage sites are as predicted by the laser induced dipole theory [15]. However, this theory predicts

that similar features should also be seen on the front surface damage sites, and none were observed.

The reason for this apparent discrepancy is not clear and should be resolved in future work.

One of the exit damage sites in the KCl:EuCl specimen was scanned with the Talystep surface profiling Instrument. The site chosen was produced with the laser electric field perpendicular to the machine

grooves, and the scan was made parallel to the machine grooves. The scan was perpendicular to the ripples in the damage site. Three of the scans of this crater are shown in figure 12. The scan 80 ym

from the crater center is comparable to the traces in the upper part of figure 8, but plotted on a greatly reduced vertical scale. The features to note in this figure are the symmetry of the crater and

the "modulation" of the crater depth, corresponding to the ripples seen in the micrograph of the exit

surface damage crater in figure 11. At the center of the crater the amplitude modulation is about 0.1 ym in a total depth of 2.5 ym. These scans show conclusively that the observed ripple pattern is removal and/or displacement of material and not simply changes in the refractive index in the damage

region.

-152-
Discussion

The optical properties of surfaces produced by single point diamond turning of polycrystalline,

doped KCl compare favorably with surfaces on similar materials generated by other techniques. However,

the cosmetic quality of these surfaces is not as good as that which can be obtained by other techniques.

Residual machine tool marks or grooves were apparent over much of the surface of the KCl:EuCl specimen,

and neither specimen had completely clear surfaces. However, the scattering at a wavelength of 10.6 pm was as low as that from surfaces on similar materials generated by conventional techniques.

The surface absorption of the diamond turned specimens at a wavelength of 10.6 pm compared favorably

to that measured for standard commercial surfaces on KCl. However, the surface absorption was five

times higher than that which can be produced by chemical etching and approximately twice the absorption

I of the surfaces produced by the best chemical/mechanical techniques [1]. Part of the surface absorption

I measured in the diamond turned specimens may have been caused by contamination from handling and atmospheric attack. Reductions in surface absorption can be achieved with better handling techniques and

improvements in the machining process such as chemically etching the specimen prior to the diamond turning. This step would ensure that surface damage and surface contamination caused by the grinding and shaping of the window would be removed. An initial chemical etch has resulted in a significant reduction

in the surface absorption in KCl polished using abrasive polishing techniques [1].

The optical figure of the diamond turned surfaces was not as good as can be achieved by lap polishing of KCl, but was superior to the figure of surfaces on KCl produced by etching or cloth polishing.

The figure errors of the diamond turned specimens had little irregularity, indicating that they may have

been caused by an improper blocking technique, and could probably be reduced considerably by improving

the blocking technique.

The damage thresholds were quite good as compared to those of mechanically polished and chemically

etched surfaces on similar material (fig. 10). The fact that the diamond turned surfaces had higher

thresholds is probably not significant since the previous data were taken with a 200 pm spot size (1/e^

diam.) and these data were taken with a 59.5 .m spot size (l/e^ diam. ) . Thus, the higher damage threshold could be simply a spot size dependence. What is significant is that the thresholds for the diamond

turned surfaces are the same order of magnitude as the thresholds for surfaces prepared by more conventional techniques. Diamond turning of KCl is at an early stage of development and significant improvements can be expected.

Exit surface damage seems to be associated with defects, as evidenced by the ripple structure in the damage craters. The ripple structure is strongly correlated with the direction of the diamond turning

grooves for sites illuminated with the laser electric field normal to the grooves. However, we were

unable to show correlation of the thresholds with the angle between the electric field and the grooves.

This is not too surprising since little long term correlation of surface roughness was found in the surface scans made parallel to and perpendicular to the machine grooves. There was such a large variation

in the thresholds for a given groove orientation that all but a very large orientation dependence would

probably have been masked by the variations caused by surface quality variations.

Future work on diamond turned KCl surfaces should be performed on surfaces having more uniform

quality. Presumably as the diamond turning technology is improved, uniform quality surfaces will be

available. Hopefully the effects of specific surface defects and their orientation with respect to the incident laser field can then be determined.

Conclusions

Diamond turning offers an attractive alternative to abrasive polishing and chemical etching of KCl

surfaces. These surfaces are comparable in optical figure, roughness, and absorption at 10.6 pm to good

commercial finishes on KCl. The scattering at 10.6 pm and laser damage threshold of diamond turned surfaces are essentially equivalent to those of the best surfaces on KCl produced by chemical/mechanical

techniques. The results reported in this paper are extremely encouraging considering the limited

experience in diamond turning of KCl.

The nonuniformity of the surfaces tested prevented any definitive determination of the effect of machine groove orientation on the laser damage threshold. However, the presence of ripples in the exit

surface damage craters is suggestive that defects are important in the laser damage process, since such

ripples are produced by the laser interaction with surface defects. The laser damage resistance of diamond turned KCl surfaces should increase, surface uniformity should improve, and surface defects

should be reduced as more experience is gained in the machining of KCl.

-153-
Acknowledgments

This work was supported by the Defense Advanced Research Projects Agency, the Air Force Weapons

Laboratory , and Naval Weapons Center Independent Research funds. The authors are grateful to the Y-12 Division of Union Carbide, particularly to R. E. Sladky and J. B. Arnold for providing the specimens

used in these experiments. We are also grateful to P. Archibald of NWC for making the scattering measurements.

References

[1] Soileau, M. J., et al., in Laser Induced

damage in Optical Materials: 1975, A. J. Glass and A. H. Guenther, eds. (NBS Spec. Publ. 435, 1976), pp. 20-28.

[2] Davisson, J. W. , in Proceedings of Third Con- ference on High Power Infrared Laser Window

Materials, 1973, C. A. Pitha and B. Bendow,

eds. (AFCRL-TR-74-0085, Special Report 174,

1974), pp. 525-534.

[3] Allen, S. D., et al., in Laser Induced damage

in Optical Materials: 1974, A. J. Glass and

A. H. Guenther, eds. (NBS Spec. Publ. 414,

1974), pp. 66-75.

[4] Soileau, M. J., et al., in Proceedings of

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), pp. 391-417.

[5] Turk, R. , et al., in Proceedings of 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),

pp. 103-112.

[6] Saito, T. T. , in Laser Induced Damage in

Optical Materials: 1974, A. J, Glass and

A. H. Guenther, eds. (NBS Spec. Publ. 414,

1974) , p. 227.

[7] Harrison, W. B. , et al. , in Proceedings of

Fourth Annual Conference on Infrared Laser

Window Materials, 1974, C. R. Andrews and

C. L. Strecker, eds. (Defense Advanced

Research Projects Agency, Arlington, Va. , 1975) , pp. 599-610.

[8] Arnold, J. B. , et al. "Diamond Machining

of Reflectors," Report Y-DA-6681, Union

Carbide, Y-12 Plant, Oak Ridge, TN 37830.

[9] Bennett, H. E., et al., in Proceedings of

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), pp. 910-916.

[10] Whitehouse, D. J., in Characterization of

Solid Surfaces, P. F. Kane and G. B. Larrabee, eds. (Plenum Press, New York,

1974), pp. 49-73.

[11] Bennett, J. M. , "Measurement of the rms Roughness, Autocovariance Function and Other

Statistical Properties of Optical Surfaces

Using a FECO Scanning Interferometer," Appl.

Opt. 15 (November 1976).

[12] Sladky, R. E. , et al. , in Laser Induced

Damage in Optical Materials: 1975, A. J. Glass and A. H. Guenther, eds. (NBS Spec. Publ. 435, 1976), pp. 57-65.

[13] Porteus, J. 0., et al., "Focusing Method

Used in Laser Damage Experiments," Appl. Opt

15, 2024 (1976).

[14] Porteus, J. 0., et al., in Laser Induced

Damage in Optical Materials: 1975, A. J. Glass and A. H. Guenther, eds. (NBS Spec. Publ. 435, 1976), pp. 207-215.

[15] Temple, P. A., and Soileau, M. J., "Resonant Defect Enhancement of the Laser Electric

Field," these Proceedings.

-154-
KO: RbCI KG: Eua

Ka:Eua KCI:Rba

Figure 3. Twyman-Green interf erograms of the diamond turned KCl surfaces.

KCIiEuCI KChRfaCI

WAVELENGTH FORWARD SCATTER BACKSCATTER FORWARD SCATTER BACKSCATTER

0.4762 6,0 X 10"^ 3.6 X 10-2 4.0 X 10-2 3.0 X 10-2

0.G471 4.5 X 10-2 2.3 X 10-2 2.2 X 10-2 1.7 X 10-2

1.15 1.8 X 10-2 8.4 X 10-2 5.8 X 10-^ 4.8 X 10"^

3.39 1.6 X 10-^ 1.5 X 10-^ 7.0 X 10^ 4.7 X 10-*

10.6 3.5 X 10-^ 1.5 X

10-'' 5.7 X lO-'' 6.8 X 10-5

Figure A. Total integrated scatter (TIS) in the forward and back directions for various wavelengths (in ym) . The values

are the ratios of the total hemispherical scatter to the incident beam

intensity.

-155-
1 1 1 1—I—I—I—; DIAMOND TURNED KCI: RbCI O ETCHED SINGLE CRYSTAL KCI

1.0 2.0 WAVELENGTH (jim)

I I

I I L.

Figure 6. Backscatter as a function of wavelength for a diamond turned KCl:RbCl window and an etched

single crystal sample.

TOTAL ABSORPTION OF DIAMOND TURNED SPECIMEN

ABSORPTION AFTER

CHEMICAL ETCH

INFERRED

ABSORPTION

PER SURFACE

4 6 8 10 ABSORPTION (UNITS OF lO"*)

Figure 7 . Absorption measurements at a wavelength of 10.6 pm on diamond

turned KCl:EuCl before and after

etching.

SURFACE SCANS

500 I

SURFACE SCANS SCAN PARALLEL

TO GROOVES

60 urn SCAN PERPENDICULAR TO GROOVES

LENGTH p ((im)

Figure 8. Typical surface scans made parallel and perpendicular to the grooves on the exit

face of the KCl:EuCl diamond turned „ sample. The rms roughnesses are 127 A

rms parallel to the grooves and 135 A perpendicular to the grooves. The autocovar iance functions calculated

from the scan data are shown below;

the curve with the steeper initial

slope and more periodicity is for the scan perpendicular to the grooves.

Figure 9. PuD.sed damage thresholds for the diamond turned KCI samples. The values

are peak on-axis intensities. Pj^ is the lowest power which produced damage,

the highest power which did not produce

damage, and P5o% the power which pro- duced 50% of the time.

SPECIMEN SURFACE

ORIENTATION ANGLE

(DEGREES)

(OW/CIVl'l P„ (GW/CM'I

KCkEuCI EXIT 89

50

a

1.98

2.61

1.3S

1.49

1.85

1.17

Ml

2.77

2.34

ENTRANCE

1

84

32

8

2.03

3.98

2.62

1.90

1.01

1.60

1

2.16 4.35

4.09

KChRbCI EXIT 90

1

1.87 0.67 2.13

61 1.58 1.42 3.50

18 1.51 0.91 1.92

ENTRANCE 84 1.77 0.65 2.65 66 1.78 1.01 2.55

13 2.48 0.37 2.51

-156-
2.39 GW/cm' EXIT SURFACE ENTRANCE SURFACE

polished KCl. The diamond yooian turned specimens were damaged

with a 59.5 ym spot

(1/e^ diam.), and the other Figure 11. Nomarski micrographs of entrance and

specimens with a 200 ym spot exit surface damage sites on KCl:EuCl.

(1/e^ diam.). All values are peak on-axis intensities.

comEhrrs on paper bv

SOJLEAU, BENNETT, POUTEUS, FAITH, JERNTGAW, AMP 5ATT0

Wo i>honJ:-tQMm dzgH/idation. chmicoLty ztchzd iuA^aa&A hoi fceen obi&nvzd, even {^oh. pznlodf, up to

one weefe, w/ien th^ iamplt wtu mcuntalmd In a. cowtHjolltd, dty a;tynoi>phzn.ii. OtheA mvlfiomndntaJi

itabitCty teAti, kavz not be.m madz on thuz iampteAo Tko. ipzakzA mphiulzzd that the Zjuzn, b&am nAzd

in thz ^tudlzi iA)cu> ccviziuM.ii diagnoizd, and wca veAy nzanJiy Gauiilan -in pKoillz. He f,u/LthzA. cormmtzd that tkz 6uA{,acz6 tzitzd uieAz within a ^acton. Oj{ two in iu/L^aaz fioughnzM, th& tut avaiZabtz iKom indu&t/iy.

-157-
2,3 SURFACE, OPTICAL AND LASER DAMAGE CHARACTERISTICS OF DIAMOND-TURNED METAL MIRRORS

D. L. Decker, M. J. Soileau, J. 0. Porteus and J. M. Bennett Michelson Laboratories, Naval Weapons Center China Lake, California 93555

Diamond turning, in addition to its attractive manufacturing advantages,

can produce superior optical components. Low scattered light, low absorption>

and high damage threshold can be reliably realized with present technology.

This paper intercompares the surface and optical characteristics of a large

number of diamond-turned metal mirrors, including bare, electroplated, and sputtered substrates. Many diamond-turned specimens display near intrinsic

pulsed laser damage threshold values, often accompanied by slip banding.

This phenomenon has been previously observed only on atomically clean, well

ordered single crystal surfaces of aluminum. In contrast, however, some electroplated specimens fail catastrophically by delamination . Regardless of sample type, most sites fail by melting initiated by localized absorbing centers. Key words: Auger analysis; diamond-turned metal mirrors; laser damage; light

scattering; optical absorption; surface roughness.

1. Introduction

Diamond turning, in addition to its attractive manufacturing advantages, can produce superior

optical components. Low scattered light, low absorption, and high laser damage threshold can be reliably realized with present technology. This paper intercompares the surface and optical characteristics of a number of diamond-turned metal mirrors, including bare and electroplated substrates. The data presented include selected high resolution profilometer traces with autocovariance function and height and slope distributions computed from these traces. Many diamond-turned specimens display

what can be described as intrinsic laser damage behavior, often accompanied by slip banding. This

phenomenon has been previously observed only on clean, well ordered single crystal surfaces of aluminum. In contrast, however, diamond-turned electroplated surfaces can fail catastrophically by delamination. Auger studies of such a disrupted surface are presented with some surprising conclusions.

In the same section some preliminary Auger data taken during ion profiling are given.

2. Surface Characterization

The most striking features of diamond-turned surfaces are the residual machining grooves left as the tool passes over the work. It has been shown that magnitude of the surface structure can be predicted to a large degree from an idealized view of the machining process, given values of tool nose

radius, and tool feed rate [1,2]'''. Figure 1 is a Nomarski micrograph of a Lawrence Livermore Laboratory diamond-turned copper surface, denoted Cu 95. The apparent spacing between grooves in this photograph is 7 um. However, since the Nomarski microscope is slope sensitive, the actual period of this structure is 14 pm. Also visible in this micrograph are the boundaries of individual crystallites

in addition to a myriad of small "particulate" appearing structures which are often seen on diamondturned surfaces. If we examine the same surface with a diamond stylus profilometer [3], scanning per- pendicular to the groove structure, the periodicity is very obvious, together with much random excursion including relatively sharp, narrow valleys that may correspond to the grain boundaries, as shown

in figure 2. The autocovariance function, derived from an analysis of the digitized profilometer data,

readily shows the large correlation associated with a repeat distance of 14 ym and again at two and

three times this value, but the correlation for higher multiples of the fundamental is nonexistent.

Figure 3 shows the corresponding height and slope distribution functions. Both height and slope distributions can be reasonably approximated by Gaussians, in contrast to the autocovariance function,

which includes a contribution from the periodic grooves in addition to the more or less Gaussian short

range order. The values of rms height and slope are somewhat typical for a diamond-turned surface,

although significantly lower values have been measured on some specimens. Note then the rms slope is twice that associated with the residual machining grooves per se as a consequence of the occurrence of other much steeper structures.

The presence of periodic structure means that the surface will scatter light in preferential directions in the manner of a diffraction grating, although a poor one, as a consequence of lacking long

range order [2,4]. The magnitude of the total scattering from this surface is much lower in the visible than one would predict from simple scalar scattering theory and the prof ilometer-measured rms roughness. For example, in the case of Cu 95 the rms roughness inferred from measured scattered light

values in the visible is ~ 35 A, a factor of over two lower than directly measured. Since the scat- tered light intensity varies as the square of the rms roughness, the measured scattered light intensity

is a factor of over four lower than one would expect. Taking into account only the spatial frequencies

important to visible light scattering helps, but does not remove this discrepancy [4]. Most surfaces.

1. Figures in brackets indicate the literature references at the end of this paper. -153-
including mechanically polished and diamond turned, follow the wavelength dependence of the scalar scattering theory in the visible, but lie above theoretical values in the infrared, often by several orders

of magnitude (fig. A). It has been conjectured that the scattering mechanism in the infrared involves

poorly correlated, but very effective scattering centers, which have been given the name "particulates"

[5], A proper theoretical explanation of the scattering in the infrared from these surfaces remains

unresolved

.

3. Laser Damage Studies

Pulsed laser damage to these surfaces is a closely related problem. Whatever damage mechanisms

exist are tied directly to the energy deposited in, or to the electric fields at, the metal surface.

Electric field interference effects associated with the polarizability of structure on KCl [6] and metal

[7] surfaces have been observed. There is obvious influence on laser damage structure by the machining

grooves and by crystallite grain boundaries on the diamond-turned metal samples of this study. Figure 5 is a Nomarski micrograph of a laser damage site on LLL Cu 95, showing the effect of the machining

grooves in defining the upper and lower edges of the crater just at melt threshold (56 J/cm^) [8]. Note

that this crater is entirely within the boundaries of a single crystallite. Theoretical calculations

indicate an intrinsic threshold on copper of about 55 J/ cm^ for the 100 ns pulse length used in these experiments [9]. This calculation assumes room temperature values of the absorptance, thermal diffusivity, and heat capacity. This good agreement may be fortuitous as all pertinent quantities are temperature dependent, especially the absorptance. This calculation should be performed as an integration over temperature with appropriate temperature-dependent variables in the Integrand. The melt crater shown in figure 5 is very uniform by comparison to the damage crater on Cu 95 (fig. 5). The small circular

structures are melt pits initiated by localized absorbing centers in contrast to the uniform, intrinsic

absorption demonstrated in the melt crater of figure 5. The structure of figure 6 is further complicated by the influence of grain boundaries. Note the ripples associated with the nearly horizontal grain

boundary in the upper center of this photograph. The laser electric field is in the vertical direction.

The crest-to-crest spacing is 10 ym, which suggests a standing wave interference effect [7]. Also obvious in the lower center of this damage site is a patch of extensive slip banding with different orientation on either side of a slip vertex or grain boundary. Slip banding has previously been observed only

on very clean surfaces of single crystal aluminum [10]. Figure 7 is a damage site on LLL Cu 8B at 54 J/cm^. The slip bands are more or less equally spaced about 8 to 9 ym apart. The energy density of

the pulse initiating the slip banding in this case was essentially identical to that which generated

the melt pit in figure 5. Since no melting is involved in slip banding, it could conceivably occur at lower energy and may be a significant effect below catastrophic damage, especially in a repetitively

pulsed system. Figure 8 is a Nomarski micrograph of another damage site on diamond-turned LLL Cu SB, and obviously considerably above threshold for that site. The very nonuniform crater is defined in part

by melt puddles. Grain boundary influence is also obvious, but it is not clear whether we are seeing

cause or effect here. There is an apparent tendency for melt puddles to congregate on grain boundaries,

presumably a result of contaminants more or less permanently located at grain boundaries (e. g. , oxide

particles) or else swept to that location during "cleaning."

Figure 9 is a damage site on LLL Cu SB which was repolished at NWC after a damage series was performed on the initial diamond-turned surface. The polished surface displayed a quite sharply-defined

threshold value of about 36 J/cm^. The particular crater in figure 9 was produced at 59 J/cm^ and very

closely resembles the crater in the previous figure on a diamond-turned surface. Diamond-turned bare

copper surfaces can display damage characteristics which can be described as intrinsic. In the case of

the melting threshold, such a surface (if clean and free from defects) can yield threshold values in excellent agreement with simple theory. However, there are many sites on a diamond-turned bare copper

surface which do not damage either in appearance or threshold differently from a polished copper surface. Figure 10 shows a damage site on a bare copper diamond-turned sample supplied by Heliotrope, Ltd.

of England. The surface of this sample has a much different appearance in the Nomarski microscope, and

the proprietary surface appears to have been ion or chemically polished after diamond turning, displaying an extremely poor figure. It was of considerable interest to examine the damage characteristics of

this surface, since it was possibly "cleaner." This surface, however, behaved very much like the polished LLL Cu 8B surface with a reasonably sharp melt threshold at 40 J/cm^ as determined by extrinsic

absorption at localized points.

Since grain boundaries play a role in defining a relatively low extrinsic threshold, a microcrystalline surface with its necessarily smaller scale grain boundary structure might provide a significantly higher average threshold. Such a surface is provided by electroplating. However, the results

obtained to date on diamond-turned electroplated gold and silver show about the same relative spread in threshold values from site to site compared to the bare copper results. In some cases spectacularly

poor results were obtained for LLL Au 50 (see fig. 11). This coating catastrophically delaminated at

an energy density of only 37 J/cm^, 20% above the value which universally produced damage at all sites. The delamination may have been related to the negative 75 V bias on the sample relative to the faraday

cup ion/electron collector. The origin of such an effect is unknown. This delamination site was atypical, most sites being much less spectacular and more like those seen on bare copper. Figure 12 shows

a more typical melt crater on LLL Au 50 occurring at 30 J/cm^.

4. Auger Surface Analysis

The battery of characterization instruments which are incorporated into the NWC UHV damage chamber

includes an ion sputtering gun and an Auger spectrum analyzer. This paper presents some preliminary

-159-
Auger results on two sample types: diamond-turned bare copper and electroplated silver. Figure 13

j indicates the Auger spectrum obtained on the Heliotrope copper surface before and after ion-milling awayi 40 A of the material. The initial sulfur, chlorine, and carbon peaks are not present after sputtering.

Figure 14 shows the time evolution of the copper, carbon and oxygen peaks. Carbon definitely seems to be associated only with the surface. Although oxygen initially decreases, the peak-to-peak intensity

resumes its initial value (before sputtering) after sputtering ceases. A probable explanation is preferential sputtering of oxygen from the surface region [11] and subsequent diffusion of oxygen from

the bulk, perhaps along grain boundaries. A depth profile study on LLL Ag 14 shown in figure 15 indicates the presence of sulfur and oxygen contamination limited to within 20 X or so of the surface. The silver and copper intensities increase in figures 14 and 15 as surface contaminating atoms are removed

which are absorbing copper or silver Auger electrons. An interesting discovery was made on LLL Au 50: Nowhere in the area of the delamination could any evidence of copper from the substrate be found. The delamination occurred entirely within the gold electroplated coating.

5. Summary and Conclusions

Figure 16 is a summary of properties of several diamond-turned and polished samples discussed in this paper. The surface roughness values are derived from scalar scattering theory using visible light

scattering values. The actual topography measured by a diamond profilometer is much rougher than these numbers indicate with rms values in some cases, e. g., the Heliotrope sample, nearly an order of magnitude larger. In fact, the roughness values inferred from infrared scattering more nearly agree with the profilometer roughness values. The values of E^q , E^jq, and E^-j^qq are, respectively , damage thresholds in J/cm^ indicating some statistics of the damage threshold from site to site; e. g. , 0, 50, and 100 percent of the sites examined would damage at the energy density indicated. There is little difference

among the LLL diamond-turned bare copper samples in values of E^q and Ep^Q. Variation in E^^qq is probably not significant either due to so few data points even with several dozen damage sites and shot energies. One conclusion which can be clearly stated, however, is that the great majority of sites

examined showed damage at energy densities much less than the intrinsic melt threshold as a consequence

of absorbing centers and/or field enhancement due to polarizable structure. Both macroscopic grain

boundaries and perhaps some sort of particulates are involved. LLL 8B and 2D are identical except that

2D was vapor degreased with Freon as a final cleaning step. Since there are no observable damage differences in these two samples, one concludes that whatever comprises the particulate contamination is not so removed. Since values of Ep^Q lie 20% or so below the intrinsic melt threshold, a very significant Increase in operational mirror flux loading could be accomplished if the nature of the localized

extrinsic absorption on these surfaces were understood and could be eliminated. The observation of intrinsic damage at a significant fraction of sites examined on diamond-turned surfaces Is a good indication of the likelihood of improving the cleanliness and perfection of the surface overall, with a | corresponding Increase in average threshold.

6. Acknowledgments

The authors of this paper acknowledge the contributions of several coworkers at NWC: P. Archibald,

E. Ashley, W. Faith, and J. Jemlgan. We thank J. Bryan of the Lawrence Livermore Laboratory, T. Saito

of the Air Force Weapons Laboratory, and I. M. H. Pagden of Heliotrope, Ltd. for samples. This work was carried out under sponsorship of the U. S. Army Missile Command and with NWC Independent Research funds.

7. References

[1] Donaldson, R. R. , "Nomograms for Theoretical [6] Solleau, M. J., Porteus, J. 0., Faith, W. N.

Finish Calculations with Round Nose Tools," Jemlgan, J. L. and Saito, T. T. , "10.6 um presented at the Society of Manufacturing 100 ns Laser Damage Studies of Diamond-Turned

Engineers Conference, San Francisco, Ca. , KCl Window Surfaces," these Proceedings.

November 19-21, 1974. r-,n „ , „ , „ t nr, [7] Temple, P. and Solleau, M. J., "Resonant

[2] Sladky, R. E. and Dean, R. H. , in Laser Defect Enhancement of the Laser Electric

Induced damage in Optical Materials: 1975, Field," these Proceedings.

A. J. Glass and A. H. Guenther, eds. (NBS

Spec. Publ. 435, 1976), pp. 57-65. [8] The damage studies reported in this paper

were carried out in the NWC laser damage

[3] Bennett, J. M. , in High Energy Laser Mirrors chamber, at 10"^ Torr using a Lumonics CO2 and Windows (Semi-Annual Report No. 7-8, ARPA TEA laser with a 5-inch focal length ZnSe Order 2175, Naval Weapons Center, 1976), in lens. The focal spot is 200 ym in diameter

press. at the 1/e^ points on the Gaussian beam pror,, . - . r rjjn • file, and the pulse length is 100 ns.

[4] Stover, J. C. , in Laser Induced Damage %n > r o Optical Materials: 1974, A. J. Glass and [9] Bennett, H. E. , "Thermal Distortion ThreshA. H. Guenther, eds. (NBS Spec. Publ. 414, olds for Optical Trains Handling High Pulse

1974), pp. 163-168. Powers," these Proceedings.

[5] Bennett, H. E. and Archibald, P. C. , in High [10] Porteus, J. 0., Solleau, M. J., and Fountain,

Energy Laser Mirrors and Windows (Semi-Annual C. W. , "Character of Pulsed Laser Damage to Report No. 6, ARPA Order 2175, Naval Weapons Al at 10.6 ym as Inferred from Single-Crystal

Center, 1975), pp. 145-155. Targets in Vacuum," these Proceedings.

-160-

J
[11] Wehner, G. K. , In Proaeedinge of the Sixth

Annual Saanning Electron Miarosaopy Symposium, 1975. 0. Johari and I. Corvin, eds.

(IIT Research Institute, Chicago, II. 1975),

pp. 133-139.

8. Figures

Figure 4. Total Integrated backscattered light as

a fraction of the Incident beam intensity

vs wavelength for two diamond-turned

samples, and an aluminlzed supersmooth

polished fused quartz flat. Straight

lines are theoretical best fits of the aluminlzed and diamond-turned surface

scattering data.
I 1 10^

Figure 5. Nomarski micrograph of a laser damage size on LLL

Cu 95, just at melt threshold. Note the effect of the machining groves in defining

the upper and lower edges of the crater.

Figure 7. Nomarski micrograph of a damage site on LLL Cu 8B,

with very striking slip

banding. This effect was produced at a loading more

or less identical to the melt threshold, 54 J/cm^.

-162-

Figure 6. Nomarski micrograph of another

j damage crater on LLL Cu 95, , considerably above threshold j for this site. Very complicated effects are present and ; described in the text.

I

I

I (

Figure 8. Nomarski micrograph of another

damage site on LLL Cu 8B,

considerably above threshold

for that site.
Figure 9. Nomarski micrograph of a damage

site on LLL Cu 8B, after being

mechanically polished at NWC following an initial series of damage tests on the diamondturned surface.

Figure 11. Nomarski micrograph of a damage site on LLL Au 50. Delamination was atypical and may have been related to bias

conditions on sample. Additional details are provided

in the text.

I ,

Figure 10, Nomarski micrograph of a damage site on a diamondturned copper surface

supplied by Heliotrope, Ltd.

of England,

t i lOQian

Figure 12. Nomarski micrograph of a more

typical damage crater on LLL Au 50, constrasted with the

site shown in figure 11.

-163-
T 1 ELAPSED TIME IMIN.) 0 2 4 6 8 10 ,,16 18 20 M 1-0| 1 1 1 1 1 Ih 1

ed on the Heliotrope, Ltd. Figure 14. Auger electron spectrum showing time

sample before and after sputter- evolution of the copper, carbon, and etching away 40 A of material. oxygen peaks during sputter etching.

ELAPSED TIME (MIN.) 2 4

Ag (356) X 1

\ S(181)x5

\\

\^ O (503) X 50

20 40 60 80 PROFILE DEPTH ii)

Figure 15. Auger electron spectrum taken

during sputter etching of LLL

Ag 14, showing decrease of sul- fur and oxygen concentration

away from the surface.

SURFACE ROUGHNESS

V RMSI

LLL

Cu95 AFTER 'CLEANING

Ag14 Au50

0 9931 09931

0.9934

0.9884

Figure 16. Summary of optical and laser damage

characteristics for several diamondturned metal mirrors. Epo» Ej)5o, and %100 are, respectively, the energy

densities such that 0, 50, and 100%

of the sites examined would damage

at that energy. Other details are provided in the text.

COMMEMTS OM PAPER BV VECKER, SOILEAU, PORTEUS, AW BENNETT

R, Sladky o{i ORNL comrmntdd that diamond-tuAntd iuAdaae^ nxkib-Ut tiA)o icateA peAA.odZc iPiactuAZ.

Thz actual tooling giooveA ate ipaae.d at abomt 2.5 to 3.S \im [TOO to 150 micAoinchu] . Many diamondtuAnlng mackinoj^ alio gdnoAoto. a peAiodlc iuAi^ace wave oi^ about 25 to 30 pm ipaclng. The. latteA can o^ten be zltminatzd by impn.ov-ing the. machine iXielf,.

V-UcizpancieJ) ex^iit among iuA^ace chaKacteAA.zatA.onf> obtained by dl^^eA&nt mejanl>, iach oi tight

icatteAing, TECO meoMViejnentf, , NomaKf>\ii mioioicopy, and Tallyitep pn.o iiZometfiy . Each technique Lf> (>eM>iti\ie to a dl^^eAent fiange of^ ipatial {fieqaencie^ . The ipeakeA commented that they have been unabl^

to leMolve dAj>cKepancieyf> between light icattefving and TalZyitep meoiu/iementi.

R, Wood od Genvuxl Elect/Uc, Ltd. commented on the loM damage thAuhoZd o^ coppeA miAAo>Ui made with,

hand dAown coppeA KatheA than OFHC due to oxide decoration at gn/xtn boundanlcd. He expanded on tkiA

point In kii iubiequent papeA.

-164-
2.4 CHARACTER OF PULSED LASER DAMAGE TO Al AT 10.6 pm INFERRED FROM SINGLE-CRYSTAL TARGETS IN VACUUM

J. 0. Porteus, M. J. Soileau and C. W. Fountain t Michelson Laboratories, Naval Weapons Center

China Lake, California 93555

I

Pulsed laser-induced stress can produce permanent damage to metal surfaces, even in the absence of melting. Possible causes of such damage in Al are differential expansion resulting from thermal gradients and compression resulting

from vaporization. By using single-crystal targets in which plastic deformation

I can be observed in the form of slip bands, one may obtain information on the

I direction, magnitude, and thus on the origin of stress. Targets having (111),

i (001), and (110) surface plane orientations were prepared and tested under oxideI free, ultrahigh vacuum conditions, using well-characterized 100 ns , 10.6 pm

focused laser pulses. Vapor particle emission was monitored to provide an indication of vaporization pressure. At energy densities ^ 37 J/cm^ the results

indicate the dominance of evaporative compression. At energy densities near and below the threshold for melting (14.2 J/cm^) the interpretation of the observed

slip banding is less clear, apparently due to the combined effects of thermal and evaporative stresses. Key words: Laser- induced stress; plastic deformation; single-crystal aluminum;

slip bands; vaporization.

I Introduction

The orderly development of laser-hardened materials requires an understanding of basic failure modes

and their relative importance. In metals plastic deformation from pulsed laser-induced thermal stress

may be important well below the melting point [1]^. An alternative source of stress at high temperatures

is evaporative compression, i.e., the reaction pressure on the target produced by the ejection of vapor

[2]. Because of the complex characteristics of the target and its environment, and the complexity of

the laser source, it is usually difficult to distinguish between surface damage due to thermal expansion,

melting, and vaporization. By using pure, annealed single-crystal Al targets, whose plastic response to

a given stress is sensitive, predictable and easily recognized, we can make this distinction and obtain

other valuable information on laser-induced stress fields. The target surfaces are cleaned and shot in ultrahigh vacuum to avoid possible complications resulting from the presence of air or an oxide film.

Vapor particle emission is monitored on each pulse to provide an indication of vaporization pressure.

Finally, the laser has simple, reproducible spatial and temporal characteristics which permits accurate

thermal modeling.

Plastic deformation is indicated by the formation of slip bands peculiar to the crystallographic

orientation of the target and the direction of laser-induced stress. In general, planes with the densest packing, and directions with the smallest interatomic spacing offer the least resistance to slip.

In Al these are the {lll}-type planes and <110)-type directions, respectively [3]. The intersection of

the slip planes with the target surface form slip traces. If the slip direction in these planes is entirely parallel to the surface plane, i.e., along the slip traces, no slip bands appear, provided the surface is reasonably smooth. This situation has been reported for a Cu(lll) surface exposed to pulsed

1.06 ym laser radiation [4]. Slip was detected by etch pits indicating accumulations of dislocations

along slip traces. If the slip has a component normal to the target surface, slip bands appear along

the slip-trace directions. This effect, which requires no etching for its observation, is the main subject of this report.

Damage Test Facility

The general layout and operation of the damage test facility has been previously described [5,6]. The source is a CO2 TEA laser which operates single-line single-mode. The pulse envelope is a sharp

spike of approximately 100 ns duration, which decays rapidly to zero. The target lies at the focal

point of a 24 cm ZnSe lens, where the spatial profile of beam intensity as a function of the distance r from the beam axis is represented by exp [-(r/225. 5 ym)^]. The use of this lens, instead of the 12 cm

lens used in previous work [5,5], permits mounting the lens outside the ultrahigh vacuum test chamber,

where it can be removed for bakeout.

1. Figures in brackets indicate the literature references at the end of this paper. -165-
Sample Preparation

The single-crystal targets are identical to specimens previously used in low energy electron dlffri

tion experiments [7]. The samples were spark-cut, oriented, mechanically polished, and electropolished

before being placed in the test chamber. Final cleaning was performed by Ar-ion sputtering after bakeJj out. To remove residual Ar, the targets were annealed for 1 hour at 500°C; they were then allowed to f cool to room temperature. Surface cleanliness was verified by Auger analysis. To produce lightly f oxidized surfaces (less than 50 K. of oxide) the chamber was subsequently filled with bottled N2 for a J few minutes and then repumped. Surfaces produced by these procedures have the "orange peel" type of « roughness characteristic of etched surfaces, and are not of optical quality. Targets having three dlf-f ferent surface plane orientations were prepared: (111), (001), and (110).

Damage Thresholds

Thresholds for damage due to melting, as indicated by visible crater formation, were measured

Threshold energy densities were found to be 14.2 i 0.6 J/cm2 for all targets, with no significant depei

dence on orientation or light surface oxidation. Evidence of plastic deformation in the form of surfaci "haze" is always seen when the threshold for melting is approached, as discussed below. However, viewH

ing limitations of the 20X microscope used for damage monitoring would make damage thresholds based on < plastic deformation much more subjective.

It is instructive to compare this result with that obtained on a more conventional target under

equivalent conditions. We, therefore, measured the threshold for crater formation on a polished 2024 Al target from a previous study [5,5] using the 24 cm lens. The value of 8.3 + 1.3 J/cm^ is significantly

lower than that measured on the relatively clean single-crystals, probably indicating a higher surface

absorption. Embedded contaminants are apparently responsible for the localized pitting observed on thl' sample, as the threshold for cratering is approached. This, combined with the higher background scattel; ing, precluded the immediate detection of evidence for plastic deformation of the matrix although a surj face roughening can be observed with Nomarski.

Damage Above Threshold i

To interpret the observed slip banding one must be able to identify slip systems in the face-center

cubic lattice. Figure 1 shows perspective diagrams of the preferred slip planes, directions, and tracesi in relationship to the (lll)-oriented target surface. The laser beam is indicated by the heavy verticaj arrow. In the (111) target the slip systems may be conveniently classified into three types as repre sented in the figure. Slip into or out of the surface may occur in {111} planes inclined away from the beam axis, as in (a), or inclined toward the beam axis, as in (b) . The same sets of planes and direc

tions are involved in both (a) and (b), but the position of the planes with respect to the beam axis is reversed. The reversal manifests itself in the orientation of the equilateral triangle corresponding b

the slip traces, as shown in the figure. Figure 1(c) represents slip in planes intersecting the surfac plane and in directions parallel to the surface plane. This type of slip cannot be observed unless the surface is irregular, or has been etched [4].

Figure 2(a) is a Nomarski micrograph of a portion of the (111) -oriented target after exposure to tvi laser pulses, each producing an energy density of approximately 37 J/cm^. Surrounding the resulting

crater, and extending out to a distance r equal to 2-3 crater diameters are prominent slip bands exhibli ing the threefold symmetry characteristics of the (111) orientation. Figure 2(b) is a perspective dia- gram of the associated slip systems. The orientation of the triangle formed by the slip bands typifies

figure 1(b), which is reproduced here. The bands show no evidence of cross slip which would indicate

the type of slip indicated in figure 1(c). The relationship of the direction of illumination to bright

lines and shadows produced by the slip bands, both in optical (fig. 3), and in scanning electron micrographs, shows that the slip bands form terraces which are stepped toward the laser as the beam axis is approached.

Slip-banding on the (OOl)-oriented target, produced by a single 28 J/cm^ pulse, is shown in figure

4(a) and analyzed in figure 4(b). A fourfold slip band pattern surrounds the crater corresponding to the four {111} planes which make equal angles with the (001) surface. The preferred slip may be classi- fied into three types in analogy with figure 1. However, because of the fourfold symmetry we cannot distinguish between (a) and (b) . Type (b) is more probable because of its similarity to the (111) case. The bands are again found to be stepped toward the laser. This, combined with the fact that the bands

are visible in dark field micrographs, also favors the indicated interpretation.

The corresponding result on the (110) target is shown in figure 5, where the slip banding was pro-]

duced by a single 47 J/cm^ pulse. Here slip in {111} planes making a dihedral angle of 35.3° with the]

surface (110) plane supersedes slip in {111} planes perpendicular to the surface. In contrast to the

(111) and (001) targets the bands are stepped away from the laser.

-166-
Laser-Induced Stress

It is possible to use the foregoing observations to arrive at a result for the direction of the laser-induced stress. The combined results from figures 2-5 are summarized schematically in figure 6. The approximate direction of the stress component in an azimuthal plane has been inferred by applying

the following criteria to be projected slip directions shown in the figure: (1) The stress has a shear

component along the active slip planes, whose direction agrees with the observed direction of slip.

(2) Slip is preferred along planes which are subjected to the maximum resolved shear stress, i.e., which

are inclined most nearly at 45° to the direction of applied stress [8]. An exact analysis, in which

resolved shear stresses in the actual, rather than the projected slip directions are considered, should

yield qualitatively similar results.

The inferred stress in figure 6 must be associated with a thermal and/or a vaporization effect.

Radiation pressure, which may influence the distribution of molten material near the damage threshold [5,

6], is well below the critical shear stress and may be ignored. Thermal stresses produced by short

laser pulses are predominantly parallel to the surface plane [1,4]. In the azimuthal planes a compressive radial component dominates, which is not reconcilable with present results. Evaporative compression on the crater and surrounding area, resulting in a predominantly normal stress, is the alternative

mechanism. Supporting evidence for the latter mechanism comes from measurements of vapor particle

emission, which is discussed in a later section.

Damage Near Threshold

At energy densities insufficient to produce melting Nomarski micrographs reveal a surface roughening

on polycrystalline, as well as single-crystal samples. Figure 7 shows this effect on two different

areas of the Al(lll) target, for which both Nomarski and dark-field micrographs are presented. Except

for magnification, the dark field images correspond more closely to what is seen with the 20X microscope

mounted on the test chamber. Thus the energy density represented in (a) is below, and that in (b) is just above the threshold for crater (dark circle at lower right) formation. The Nomarski micrographs

show that the roughening consists of slip bands along the three <110>-type crystallographic directions.

However, the bands show a sixfold, rather than the threefold symmetry apparent in figure 2. Because of this and the difficulty in determining the slip direction here, the possible importance of thermal

stress cannot be excluded in this situation. In the Nomarski photograph at the upper right, the roughening inside the cratered area indicated by the dark field photograph shows little correlation with

crystallographic direction, confirming that the cratered area corresponds to a melt zone. The conclusion is that the roughening, which is observed with Nomarski both in single-crystal and polycrystalline

Al, is plastic deformation, possibly resulting from thermal stress.

The slip banding is faintly visible in the dark field photographs, and undoubtedly accounts for the surface haze observed in the test-chamber microscope as one approaches the threshold for cratering.

This can be seen more clearly on the (110) target, as shown in figure 8. Here the energy available for plastic deformation is concentrated in a single slip direction, providing greater sensitivity. The sequence of events is also unmistakable here. That is, slip banding occurs before the crater is formed.

Vaporization

An indication of the stress available for evaporative compression can be obtained from the intensity

and energy of vapor particles emitted by the target. Figure 9 shows schematically the detector used for

monitoring particle emission. The collector is typically biased at either + or -12.6 V, while the grid

is grounded. Typical signals developed across the 50 9. input of a Tektronix 7844 oscilloscope are

shown in figure 10. With the negative bias one observes a positive signal showing two prominent, and numerous smaller peaks. With the positive bias one observes a negative-going peak which is about ten

times as great as the largest positive going peak. The positive peaks are undoubtedly due to positive

ions. The origin of the negative-going peak is less certain, but is believed to be due to secondary

electrons generated at the grid by a large flux of high energy neutrals. The arrival time excludes an explanation of the signal in terms of photo-electrons, while plasma conduction is excluded on the basis

of the relatively small magnitude of the signal with negative bias.

The energy of the ions or neutrals can be obtained from the time of flight, provided the particle

mass is known. In figure 11 we have plotted the arrival time of the most intense ion peak from the clean targets vs the energy density of the pulse. The ion energies are given by the scale on the right,

assuming the mass is that of a single Al atom. Corresponding information for the neutrals is given in figure 12. Here the energy is found to vary more strongly with pulse energy than for the ions.

The emission density of the ions at the target can be obtained from the collected ion signal if the charge per ion is known. In figure 13 we have plotted the maximum ion signal vs the pulse energy

density. The scale on the right gives the corresponding emission density computed from the collector

geometry, assuming uniform emission with a cosine- squared angular distribution over a 451 um diameter

illuminated area and unit ionic charge. The corresponding plot for the neutrals is given in figure 14. Unfortunately, it is impossible to put a reliable emission density scale on this figure, since very

-167-
little is known about the secondary yield Y for neutral atoms. The only estimate available [9] indi- I cates a yield ~0.1 for 100-200 eV neutrals. Using this value and the fractional opaque area of the grid (~20%) we find that the neutral emission density is ~300 times the ion emission density at a pulse

energy of 37 J/cm^. Thus the reaction pressure due to vaporization is predominantly due to neutrals.

Assuming that the neutrals are accelerated to their full energy within the 100 ns duration of the laser pulse yields a value ~150 bars (10^ N/m^) for the reaction pressure. This is in satisfactory

agreement with published values measured under similar conditions with a piezoelectric gauge [2]. The

critical yield stress of our target is estimated at ^ 20 bars [10], which implies that the evaporative

mechanism is feasible at the energy densities represented in figures 2-6. Near, and below threshold

the signal from emitted vapor particles is unobservable, although in the case of neutrals this may be

associated with a rapid decrease in Y with reduced particle energy. These results, therefore, also admit

the possible significance of thermal stress near threshold.

Summary and Conclusions

In summary, we have made pulsed laser-damage measurements at 10.6 ym on clean and lightly oxidized

single-crystal Al targets in ultrahigh vacuum. We find no significant dependence of the threshold for melting on crystallographic orientation, or on the presence of a thin oxide layer. We also find that

the threshold for crater formation is almost twice as high on these surfaces than it is on polished 2024 polycrystalline targets. Microscopic analyses of the laser-damaged single-crystals reveal slip bands,

which provide detailed information on laser-induced stress fields and their effects on the target. For example, the roughening observed on polycrystalline targets below the threshold for crater formation

can now be assigned to plastic deformation. At higher intensities the slip patterns observed on the differently oriented targets permit determination of the stress direction. Monitoring of the particle

emission energy and intensity accompanying damage permits an estimate of the reaction pressure induced

by vaporization. The evidence from slip banding and vapor particle emission implies that evaporative

compression is dominant at intensities well above the threshold for crater formation, while thermal stresses may be important at lower levels.

Acknowledgments

Various aspects of this program were supported by Naval Air Systems Command, Office of Naval

Research, Army Missile Command, and Navy Independent Research Funding. The authors are grateful to Drs. H. E. Bennett and K. H. Westtnacott for helpful comments and criticism, to W. N. Faith for preparing the samples, and to J. H. Dancy for electron microscopy.

References

[1] Apollonov, V. V. et al., Sov. J. Quantum

Electron. _5, 216 (1975).

[2] McMordie, J. A. and Roberts, P. D. , J. Phys.

D: Appl. Phys. 8^, 768 (1975).

[3] Hlrth, J. P. and Lothe, J., Theory of Dislocations (McGraw-Hill, New York, 1968),

pp. 255-257.

[4] Haessner, F. and Seitz, W. , J. Materials

Sci. 6, 16 (1971).

[5] Porteus, J. 0., Soileau, M. J. and Bennett,

H. E. , in Laser Induoed Damage in Optical

Materials: 1975, A. J. Glass and A. H. Guenther, eds. (NBS Spec. Publ. 435, 1976),

pp. 207-215.

[6] Porteus, J. 0., Soileau, M. J., Bennett,

H. E. and Bass, M. , in High Energy Mirrors

and Windows (Semi-Annual Report No. 7-8, ARPA Order 2175, Naval Weapons Center,

1976) , in press.

[7] Porteus, J. 0. and Faith, W. N., Phys. Rev.

B 12, 2097 (1975)

[8] See Ref. 3, pp. 269-270.

[9] Rostagni, A., Z. Phys. 89, 55 (1934); see also Levine , L . P . , Ready , J . F . and Bemal

G. , E., IEEE J. Quantum Electron. QE-4 , 18

(1968).

[10] Lucke, K. and Lange, H. , Z. fur Metallkunde

43, 55 (1952).

-168-
Figures

•. b. c.

Figure 1. Preferred slip systems in Al(lll) target.

Slip planes are {ill}. Slip directions

are <011> in (a) and (b), and <101> in

(c). Slip traces (heavy lines) are <ClOl)>,

Figure 2. (a) Nomarski micrograph of pulsed laser damage on Al(lll) target at 37 J/cm^ (max. of 2 pulses),

showing triangular slip banding in [101], [110],

and [Oil] directions. (b) Diagram showing slip

(arrows near bottom) associated with banding in

(a) . Corresponding slip traces are indicated by

heavy lines lying in the direction indicated near

the top of the diagram. The heavy vertical arrow represents the laser beam.

/ ILLUMINATION ILLUMINATION

tal (bl Figure 3. Slip-band terraces of Al(lll) target near edge of damage crater. Risers

between terraces show as bright lines

when facing direction of illumination.

Exposure is 37,0 J/cm^ (max. of 3 pulses) in (a) and 31.6 J/cm^ (max. of

3 pulses) in (b)

,

-169-
Figure 4. (a) Nomarski micrograph of pulsed lasier damage

on Al(OOl) target at 28 J/cm^ (one pulse),

showing fourfold slip banding, (b) Perspective

diagram similar to figure 1(b), showing slip

produced in the (001) target.

M 0»)

Figure 5, (a) Nomarski micrograph of pulsed laser damage

on Al(llO) target at 47 j/cm^ (one pulse) showing slip banding in [110] direction,

(b) Analysis of slip shown in figure 3(a).

Banding along [110] directed slip traces (heavy

lines) is much more pronounced than along

traces lying in [112] and [112] directions.

Ill

I PLANES FOR VARIOUS TARGETS

(110)TARGET

DESIGNATION

(1111

(110)

Figure 6. Summary of slip-banding results at energy densities 37 j/cm^. Diagonal lines at the bottom of the figure represent the dihedral inclinations of available {ill}

slip planes to the target face in all three targets, which are identified in each

case. Active planes are designated by heavy

lines and arrows showing the projected direction of slip. The direction of the com- pressive stress component indicated by the shaded arrows is consistent with the ob- served slip, according to the criteria

given in the text. -170-
lOOMm

DARK FIELD

Figure 7

()

just above (b) the threshold for crater

formation. Corresoonding areas are shown

in Nomarskl and in dark field micrographs.

The bright spots in the dark field micrographs are dirt specks. Exposure is 8.0 J/cm^ (one pulse) in (a) and 15.3

j/cm^ (one pulse) in (b)

.

Figure 8. Damage on the Al(llO) target below

(a) and above (b) the threshold for crater formation, as seen in dark

field micrographs o Exposure is

12 o 2 J/cm^ (max, of 2 pulses) in

(a) and 16o9 J/cm^ (one pulse) in

(b) .

11101

DARK FIELD

2§<Vm

(b)

COLLECTOR (Tal

V

Figure 9. Charged particle detector, showing

relationship of Al target sample

and laser beam. Distances are on

the same relative scales.

Collector diameter is 2.5 cm.

-171-
2 E s5

20 30 40 ENERGY DENSITY IJ/cm^l

Figure 11. Arrival time of emitted ions vs pulse energy density, showing com- puted particle energies. Results

are from differently oriented,

clean targets.

Figure 13.

20 30 40 50 ENERGY DENSITY (J/cm^)

Peak signal of emitted ions vs pulse energy density, showing com- puted emission densities. Results

are from clean targets.

1 1 1 1 1 CLEAN

1 OXIDIZED

(111) d A

(Ml) a p

(IIOI o •

-

_ o V

\/

1 > 0 10 20 30 40 so 60 ENERGY DENSITY (J/cm'l

200

I 300

Figure 12. Arrival time of emitted neutral

particles vs pulse energy density,

showing computed particle energies.

Results are shown from both clean

and lightly oxidized targets of different orientation.

Figure U,

20 30 40 50 ENERGY DENSITY (J/cin*l

Peak signal due to emitted neutrals

vs pulse energy density. Results

are from both clean and oxidized

targets

C0MME.VT5 ON PAPER BV PORTEUS, SOJLEAU, AWP FOUNTAIN

It MU mphiulziA that itip planu could be gminxUizd by thvmoeZcUttic ifieMZi at poweA ttvtli,

MelZ beloM the. lt\>dU fizqwiAzd to pfiodacz local melZlng o^ th& iotjjace.

-172-
2.5 MEASUREMENTS AT 10.6 ym OF DAMAGE THRESHOLD IN GERMANIUM,

COPPER, SODIUM CHLORIDE, AND OTHER OPTICAL MATERIALS AT LEVELS UP TO 10"^° W/cm^*

J. J. Hayden and I. Liberman**

University of California

Los Alamos Scientific Laboratory

Los Alamos, NM 87545

We have measured the damage threshold levels in various optical materials useful

as mirror surfaces and windows at 10.6 ym. Typical values range from .73 J/cm^ for a 2 micro-machined aluminum mirror up to 12.6 J/cm for a micro-machined OFHC copper mirror.

Materials tested include: ZnSe, conventionally polished copper mirrors, micro-machined

aluminum and copper mirrors, uncoated and AR coated NaCl windows, polished germanium

flats and tungsten mirrors. Tests have been done in air and vacuum, at different angles

of incidence, and at different angles with respect to the plane of polarization of the incident 1 ight.

Key words: Beam quality; damage threshold; Gaussian beam; peak energy density; pulse

width 1.4 nanoseconds (FWHM).

1 . Experimental Setup

The laser system used to obtain these measurements is a mode-locked TEA double-discharge laser

system illustrated in figure 1. An oscillator with a mode-locker and Pockels cell is used to generate

a 1.4 ns (FWHM) pulse containing 5 mJ. The beam passes through a 2:1 beam expander, which utilizes a spatial filter at the focus to improve beam quality. The beam is then amplified to the 2 J level by pre- amplifiers 1 and 2. The prepulse absorption cell is an SFg cell which removes precursors of the main

pulse. Oriented polyester films (.0064 mm to .051 mm thick) were used as attenuators to provide a variable laser output. The beam then passes through a pick-off pellicle (0.0064 mm thick) which is used

to reflect a portion of the beam into a calorimeter. The beam is focused with a 30 meter radius mirror

and a spatial filter is placed at the focal spot. The expanding beam is again focused with a 5 meter

radius mirror onto the test sample.

2. Analysis

For a Gaussian beam, the energy density at a given radius can be expressed in the form

2 where I = energy density at radius r, I = peak energy density, w = spot radius at 1/e intensity. The

total energy, J^, can be expressed as

I r dr de = ^ " ^0 (2) Jo Jo

Solving for I

2J

(3)

Substituting this expression for I^ into eq. (1) we obtain

I = ^ exp (- ^) (4) «.2 TTW

A pyroelectric detector array was used at the sample position to determine the energy distribution in the beam. Figure 2 shows the output of the pyroelectric detector array. Fiquce 3 shows how the individual detector outputs match a Gaussian. The beam is a Gaussian, and the l/e2 diameter as determined

from the detector array is 0.36 cm. Since w = .18 cm, we determine from eq. (3) that _i7q'_
I

tt{.18)2

19.6 J„

0

To calibrate the pick-off pellicle a calorimeter is inserted after the 5 meter radius focusing

mirror. A shot is taken and the calibration factor of total joules in the beam per mV of pick-off

calorimeter response is calculated. Thus the peak energy density delivered to the sample is the calibration factor times the mV from the pellicle calorimeter times 19.6.

The energy densities necessary to damage copper mirrors is great enough to cause a plasma to be formed on the mirror surface. In order to make sure that this plasma did not affect the energy density]

actually arriving at the mirror surface, measurements were also made in a vacuum (pressure^ 1 torr j absolute). \ Damage threshold was determined visually. An argon-ion laser was injected collinear with the

j beam. The scattering of the argon-ion beam was observed during exposure and was observed to change

dramatically when damage occurred. Additionally, a very bright visible light source was used on the ' optical surfaces during laser light exposure. When damage occurred, the reflective character of the sample surface was observed to change. Damage threshold was arbitrarily defined as the minimum peak '< energy density at which a sample damaged in three exposures (or fewer) on the same spot.

| The angle of exposure of surfaces was varied during these experiments. In the following tables,

90° indicates the surface was normal to the laser beam. An angle indicated "in plane of incidence" means the surface was tilted in the p plane of polarization. An angle "out of plane of incidence" was tilted in the s plane of polarization.

Several spots were tested on most samples. The energy density was progressively increased on a single spot until damage was observed. Then another spot was tested at an exposure level nearly as high

as the damage threshold first observed, and exposure raised until damage occurred. Threshold values are listed in two columns. The first column "Highest J/cm2 without damage," indicates the highest exposure

level used (three shot exposures) which did not cause damage. The second column "Lowest J/cm^ with

damage," lists the lowest value of damage threshold which was observed. The actual "average" damage

threshold is approximately the arithmetic mean of the values in the two columns.

Table 1 shows the damage threshold of conventionally polished and micro-machined (diamond turned) OFHC copper mirrors.

Unexpectedly high damage thresholds were measured for OFHC copper mirrors. Micro-machined mirrors

had a slightly higher damage threshold than conventionally polished mirrors in both air and vacuum. At the 45° angles no increase was noted in damage threshold, contrary to what might be expected by increasing the exposure area by a factor of 1.414.

The most significant point to note in table 1 is that for all conditions tested, the damage threshold was 10.1 J/cm^ ± 25%. This uniformity indicates good reproducibility of surface which can be ex- pected of good quality copper mirrors.

Figure 4 shows a photomicrograph of an undamaged micro-machined OFHC copper mirror (80x). Note the machining marks and the crystal boundaries. Figure 5 shows the same mirror at a damage site (80x). Note that the grain boundaries have melted together and the surface appears to be smoother. Figure 6 shows a conventionally polished OFHC copper mirror (80x). It appears to have a much smoother surface

than the micro-machined mirror. Figure 7 shows a damage site 011 the conventionally polished OFHC mirror

The character of damage is different than the micro-machined mirror in that small "pits" are blasted out

of the surface.

3. Other Considerations

4. Results

-174-
Table 1. Damage Threshold for 1.4 ns pulse at 10.6 ym on OFHC copper mirros.

Material

OFHC Copper Mirror

Conventionally Polished

OFHC Copper Mirror

Micro-machined

OFHC Copper Mirror

Conventionally Polished

OFHC Copper Mirror

Conventionally Polished

OFHC Copper Mirror

Micro-machined

OFHC Copper Mirror

Conventionally Polished

OFHC Copper Mirror

Micro-machined

OFHC Copper Mirror

Conventionally Polished

OFHC Copper Mirror

Micro-machined

Med i urn

Air

Air

Air

Air

Vacuum

Vacuum

Vacuum

Vacuum

Vacuum

Angle

90°

90°

45° in Plane

of Incidence

45° out of Plane

of Incidence

90°

90°

45° out of Plane

of Incidence

45° out of Plane

of Incidence

45° in Plane

of Incidence

Highest J/cm

without damage

10.8

11.3

8.9

11.3

7.3

7.3

9.9

7.1

Lowest J/cm

with damage

9.3

9.7

9.8

12.6

10.5

7.4

11.0

7.6

Table 2 shows the damage threshold of NaCl windows, both AR coated and uncoated.

The damage threshold for NaCl windows which were AR coated was considerably (almost a factor of 2) higher than for uncoatea samples. All samples were very good quality single crystal NaCl with "high power laser finish." The vendor would not reveal what AR coating was used. Analysis of the coating by

an electron microprobe indicated that it was most likely a single layer CaF2 coating. Transmissions

were measured for the uncoated (T = 9^ .5%) and AR coated both surfaces (T = 98%) samples. Other poor

2 surface quality NaCl samples (uncoated) have damaged as low as 1 J/cm . Figure 8 is a photomicograpli

of damage to AR coated NaCl (320x). Notice that the damage appears to be mainly in the coating.

Figure 9 shows uncoated NaCl (200x). The damage here appears to be a fracture pattern in the NaCl crystal

.

Material

Table 2. Damage Threshold for

Medium

.4 ns pulse at 10.6 ym on NaCl.

NaCl Window

uncoated-uncoated

NaCl Window

AR - AR

NaCl Window

AR - uncoated

NaCl Window

uncoated-uncoated

NaCl Window

AR - AR

Air

Air

Air

Vacuum

Vacuum

Angle

90°

90°

90°

90°

90°

Highest J/cm

without damage

5.7

10.8

9.6

5.4

6.1

Lowest J/cm

with damage

5.7

9.9

7.7

5.4

8.2

Table 3 lists damage threshold for several other materials of interest.

-175-
The sample of copper electroplated on a SiC substrate was, without question, the best of all samples tested. Repeated shot at 11.4 J/cm^ failed to produce damage. The SiC substrate is a potentially useful substrate material for large diameter mirrors because of its high stiffness to weight

ratio.

Damage threshold for 1.4 ns pulse at 10.6 \sm for other materials.

2

Table 3.

Material Medium

Aluminum Mirror Air

Micro-machined

Germanium Polarizer Vacuum

Plates, Polished,

uncoated

ZnSe Window Air

uncoated-uncoated

ZnSe Window Air AR - AR

Copper Electroplated Air

Sic Substrate

Conventionally Polished

Tungsten Mirror Air

Conventionally Polished

Tungsten Mirror Vacuum Conventionally Polished

Angle

90°

70° in plane

of incidence

90°

90°

90°

90°

90°

Highest J/cm Lowest J/cm'' without damage with damage

4.1

1.5

1.9

11.4

1.6

0.7

4.6

1.9

1.4

11.4

2.44

2.26

5. Summary

Copper mirrors, both micro-machined and conventionally polished have been measured to have a high

damage threshold. Both types of copper mirrors are suitable for most high peak power applications

because beam quality deterioration resulting from plasma formation will generally occur before mirror

damage occurs (in air).

The peak energy density at which NaCl windows can be used without damage is improved by almost a factor of 2 by using an AR coating of CaF2.

*Work performed under the auspices of the U. S. Energy Research and Development Administration.

**;Westinghouse Research Laboratories, Pittsburgh, PA.

-176-
6. Figures

Prspulse .0064 mm

Switch- Mode Oscillator out locker

|ot^_'alamet

TSL-29 damage study facility

Figure 1. TSL-29 damage study facility.
o

Fit of Pyroelectric

Array Output to a Gaussian Curve

Figure 3. Fit of pyroelectric array output to a Gaussian

curve.

Figure 4. Photomicrograph of an undamaged micro-machined

OFHC coppor mirror (80x)

-178-
-179-
COmENTS OM PAPER BY HAVVEN ANV LI5ERMAN

In fieyisponie. to a quution conceAvUng the, chaAacteA o{, the, fiUA^aczA thz NaCl iampte^, tkz ipejikeA

commented that the iampZej, MSAe ^Kee 0($ haze, and had excellent 4uA){ace pfiopeAttei. The coat&d and

uncoated iuA{iaceJ> uieAc t/ieated idznticatly except {^on, the coating pKoceAi. The obieAved damage on coated eZemcnti a&my.i occaHAed at the ancoated iuA^ace, MhetheA It vooi the extt iuA^^ace on the zntAxncz

iuAiace. ThAJ, may be a standing wave e^^ect. The coatlngi weAc 6uJ>pecte.d to be CaFj, but the iupptieA

had not tdentL^^ied the coating mateAial. The icii-ion chcUApeA^on enteAcd a plea don. coating uendoA^ to idznti^y the mateAiali, o(^ theln. coatingi, to avo-id the nece^itty o^ deAtAuctlve anatyiAj, by the ixieA.

-180-
2.6 LASER-INDUCED DAMAGE OF MIRROR AND WINDOW MATERIALS AT 10.6 ym

R. Gibbs and R.M. Wood

The General Electric Co. Ltd. Hirst Research Centre Wembley HA9 7PP, England

Pulsed laser-induced damage thresholds (CO2 TEA laser, 60 ns FWHH) and 10.6 ym absorption measurements have been made on a wide variety of mirror and window materials. The effects due to different methods of substrate preparation and coating of metal mirrors have been studied, and as a result, mirrors of higher damage threshold

have been manufactured. Front surface damage thresholds of uncoated windows have

been measured and correlated with microscopically visible surface features. Improve- ment has been achieved by ion-beam etching of single-crystal materials. The damage

threshold of dielectric coatings, both reflecting and anti-ref lecting, has been

found to be limited by microscopically visible coating defects.

The absorption and damage measurement techniques will be discussed together

with the details of the power and energy measurement programme. The measured

results will be presented, and in the case of metal mirrors, compared with computed

values of damage threshold derived from the measured values of absorptivity.

Key words: Dielectric coatings; infrared laser windows; laser heating of metal

surfaces; laser mirrors; pulsed CO2 laser damage; surface damage; 10.6 pm absorption; 10.6 pm optical components.

1. Introduction

A programme of research was commenced in 1974 under DCVD sponsorship at the Hirst Research Centre

of The General Electric Company, with the aid of studying 10.6 pm laser-induced damage and distortion

in optical components for use with a variety of laser systems. This paper reports work on pulsed,

60 ns, laser damage threshold measurements, absorption measurements, surface improvement, and accurate

power energy measurement.

The components studied include a range of window materials, metal mirrors, and dielectric coated

substrates

.

2 . Damage Threshold Measurement

A sealed-off CO2 TEA laser [l]'^ was used in the damage studies. The advantage of using this type

of laser is its pulse to pulse repeatability. The output of this laser consists of 50 mj pulses in a

60 ns FWHH main pulse and a 0.7 \is tail. The tail, although of low power, contains two-thirds of the pulse energy (see fig. 1). The spatial intensity distribution of the pulsed laser beam is Gaussian

in the TEMqo mode, with a 1/e diameter of 4.5 mm. More recent development of this laser, developed by

RSRE, Baldock and Marconi-Elliott Avionic Systems Ltd. , Borehamwood, has concentrated on increasing the peak power/unit volume in order to make an even more compact device and to simultaneously increase the percentage of energy in the main pulse.

The damage measurement apparatus is shown diagrammatically in figure 2. The beam is focused to 100 urn, 1/e intensity diameter, using a 64 mm focal length germanium meniscus lens. The beam power may be varied by means of thin polythene attenuators. The power is monitored using a Rofin model 7443 germanium photon drag monitor in conjunction with a Tektronix 585A oscilloscope with type 82 plug-in.

Damage sites are observed in situ under xlOO magnification and the sample is scanned in the focal

plane of the lens. This methodolgy has been built up in order to study the effects of surface defects

and is analogous to the raster-scan method previously used at HRC to examine surface variation of

1.06 ym damage thresholds [2].

3. Beam Characterisation

Calibration of the photon-drag monitor was obtained by comparison with a black-painted disc

calorimeter. The black paint (3M black suede) has been shown to have a spectrally flat absorption

out to 10.6 ym by measurement of diffuse reflectivity at the National Physical Laboratory [3]. The

calorimeter was calibrated against an NPL secondary standard in the visible (0.69 ym) and near I-R

(1.06 ym).

Figures in brackets indicate the literature references at the end of this paper.

-181-
Beam profiles were measured by measuring the transmission through a set of calibrated circular

apertures. For a Gaussian beam profile, log (1-transmisslon) Is proportional to the diameter squared.

The optical distortion produced by the attenuators was shown to be negligible by measuring the focused

beam profile with and without the attenuators.

A. Absorption Measurement

A calorlmetric absorption measurement has been developed which can measure a total absorptivity

less than lO""* In either windows or mirrors at normal incidence.

The apparatus (fig. 3) consists of a matched pair of copper blocks with thermistors embedded in

them, to one of which is clamped the sample. The blocks are mounted in a thermally Insulated environ- ment such that cooling times are in excess of twenty minutes. A lOW CO2 laser is used to irradiate the sample, whose temperature rise is displayed on a pen recorder, via a dc bridge circuit and voltage

amplifier. A small correction for cooling [5] is applied (fig. A). Calibration was obtained by

substituting a black-painted brass disc, whose absorptivity is taken to be 98% [3], for the test sample. By deliberately using a large sample holder, problems commonly encountered with thermal isolation, the contacting of thermocouples to samples and thermal stability are avoided while achieving

enough sensitivity to accurately measure the smallest absorption so far encountered.

5. Damage to Metal Mirrors

The damage thresholds and absorptivities of a wide range of metal mirrors have been measured

(table 1). Visible surface features which correlated with lowered damage thresholds were observed on all polished surfaces, but not on diamond-turned surfaces. The latter were also found to have low

absorptivity compared with the conventionally polished surfaces.

Correlation between damage thresholds and absorptivity and thermal properties has been found. Theory [A] predicts that the damage threshold intensity, 1^^, is proportional to

Tm = melting temperature

To = ambient temperature

k = thermal conductivity

P = density

s = specific heat

a = 10.6 ym absorptivity

For a mirror with a thin gold coating, a is that of gold, k, p and s refer to the substrate. Tm refers

to the material with the lower melting point. If only the 'clear' areas of mirrors are considered, then

the damage threshold is related to the absorptivity of a comparable but scratch-free surface. This may

be taken as a diamond-turned surface in this context, as diamond-turned mirrors have the same damage

threshold as clear areas of polished mirrors, but with no site-to-site variation (see table 1).

Good correlation is found between experimental damage thresholds and theory for a selection of mirrors including plated ('hard') gold surfaces (fig. 5). Evaporated gold-coated mirrors have a lower

threshold than predicted, and this is thought to be related to the poor adhesion of the films.

Simplified theory predicts that the damage threshold, Tj^, is related to pulse length, t, by the relationship

and this may be used to compare results measured with different pulse length lasers.

6 . Damage to Window Materials

Front surface damage thresholds have been measured for a wide variety of window materials. These

are summarized in table 2

-182-
By using the scanning technique and simultaneously observing the focal area for signs of damage,

it was found that in all materials tested, lowered damage thresholds could be correlated with visible

surface defects, i.e., polishing scratches, pits, and absorbing surface inclusions. With practice and patience, it was found to be possible to pre-select defect-free areas on a given surface, whose damage

threshold represents the highest measured on that surface. The scanning technique allowed the lowest

defect-limited threshold to be measured accurately, ensuring that damage occurred only at the centre of the focused spot, i.e., at the measured power density.

It was observed that not all defects affected the damage threshold equally, and that those which

had the greatest effect were the sharpest discontinuities. Ion beam etching techniques were used in an attempt to smooth out such discontinuities, and met with some success in single crystals (fig. 6) raising the defect-limited damage threshold of the surface up towards its intrinsic value.

7 . Damage to Coatings

Damage thresholds of multilayer mirror coatings and anti-reflection coatings on germanium have

been measured (table 3). Results are presented on representative coatings from two suppliers.

The dominant effect in mirror coatings is due to 1-10 ym coating defects which damage at a much

lower power density than the rest of the coatings (fig. 7). The sample (A) coatings had a high defect

density and it was difficult to find a defect-free area to irradiate with the 100 ym spot used. The sample (B) coating had a much lower defect density.

Defects in single-layer AR coatings have much less effect on the damage threshold than those in mirror coatings, and the effect is of the same order produced by normal polishing imperfections in substrate finish.

8. Conclusions

Surface variations in damage threshold have been shown to correlate with microscopically visible

features, of roughly 1-10 ym dimension. These features are typically polishing scratches in windows

and metal mirrors, and. absorbing defects in dielectric coatings. The effect of coating defects has

been found to be most catastrophic in highly-reflecting mirrors, lowering the damage threshold by a factor of ten or more, whereas in A/A layers the threshold lowering factor is less than two.

Scratch effects in copper mirrors have been eliminated by the adoption of diamond-turning instead

of conventional polishing techniques and hence good correlation has been found between measured damage

thresholds and theory based on surface melting.

Plated god surfaces have been found to have significantly higher damage thresholds than

evaporated gold coatings on identical substrates, as well as having higher reflectivity and greater

durability.

9. Acknowledgment

This work, has been carried out with the support of Procurement Executive, Ministry of Defence,

sponsored by DCVD.

10. References

[1] Stark, D.S., Cross, P.H. and Foster, H., IEEE [4] Ready, J.F., J. App. Phys . , 36(2), 462 (1965).

J.Q. Elec, QE.11(9), 774 (1975).

[5] Hass, M. , Davisson, J.W., Rosenstock, H.B., and

[2] Wood, R.M., Taylor, R.T., and Rouse, R.L., Babiskin, J. App. Dpt., 14(5), 1128 (1975).

Optics & Laser Technology, 105 (June 1975).

[3] Edwards, J.G., Private Communication (1976).

-183-
11, Tables & Figures

Table 1. Damage thresholds - metal mirrors.

Description a Damage threshold MW mm~^

worst scratch -free

evaporated Au/polished Ni/C u 1.8 0.7 1. 2

plated Au/polished Ni/Cu 1.4 2.0 2. 7

plated Au/polished Ni/Cu 3.0 0.7 1.

diamond turned (d-t) Au 1.2 5. 5

d-t thick plated Au/Cu 1.0 7.

evaporated Au/d-t Cu 1.3 2. 0

plated Au/d-t Cu 1.0 8.0

polished Cu 1.1 6.0 9.0

d-t Cu 0.8 9.0

Table 2. Front surface damage thresholds - windows

Material 6 cm"^ Damage threshold worst clear area

Ge 0 ,032 0.7 4.0 etched Ge 3.0 5.0

GaAs .011 1.6 3.2 etched GaAs 0 .011 2.5 4.0 CdTe 1,0 1.5 CVD ZnSe (Raytheon) 0 .0049 3,0 7.5 CVD ZnSe (Raytheon) 0 .0049 2.8 6.2 CVD ZnSe (UK) 0.8 4.5 CVD ZnS (UK) 0 .35 0.6 6 KCl (Harshaw) 5.0 12 NaCl (Harshaw) - 15 >17

-184-
Table 3. Damage thresholds - dielectric coatings

Description

Damage threshold MW mm ^

defect clear area

Single-layer AR

ZnS on Si (A) 0.4 1.2 ZnS on Ge (B) 1.4 2.8

ZnS on Ge (B) 2.8 4.0

ThFij on ZnSe (B) 3.0 >6 R = 85%

ZnS/ThFit on Ge (B)

5 10 R » 100%

on Si (A) <0.2 1.2 ThF4/ZnS on fused Si02 (B) 0.8 3.8

Figure 1. TEA laser pulse Ctimescale

100 ns/1 div).

SCHEMATIC OF PULSED DAMAGE MEASUREMENT APPARATUS

TEA Laser

Photon Drog

Monitor

Attenuotor

Scanning

Sample

Mount

xlOO

Microscope

Disc Calorimeter

(to calibrate power

measurements)

Figure 2. Schematic of pulsed damage

Tekt^nix

3o3M

^\^ ^

measurement apparatus. CJl.O.

-185-
SCHEMATIC OF ABSORPTION APPARATUS

1

Sample

CW CO2 Loser up to low

Thermistors Figure 3. Schematic of absorption apparatusjj

I'

I

TYPICAL ABSORPTION TRACE

ZnSe:

Figure 4. Typical absorption trace. ^ (minutes)

METAL MIRROR DAMAGE THRESHOLDS:

SCRATCH-FREE AREAS 60ns PULSE LENGTH

Tkrcsheld MW mm*'

Evoporoted Au/Cu

'Au/MI/Cu C«-J%)

1 I L OS 10

(Tm-To)yyr/<xx<0-*

1-5 Figure 5. Metal mirror damage thresholds

(scratch free area).

-186-
-187-
COMMENTS ON PAPER BV GIBBS ANV WOOV

The. ipaakoA commznt^d on the. zHzct moutz/UaZ imovaZ by lon-be,am eXcking. He indicatzd that up

to 1 \im ofj imovat unpioved thz optical quaLLtg of^ the iu/i^ace, bat imovaJt of^ 1 - 10 vm incAnaizd thz

icattsAing at thz iuA^jacz. In InSz thz damagz thAzihold woi -incAzoizd by Kzmovai 0(5 up to 1 pm 0($ matzAial, bat ^anXhzA. fizjnoval ZoMZAzd thz damagz th/LZihold. In geAjmanjium and CdTz, hoMzvzA, thz damagz

thAzihold continazd to -unpfiovz with continazd matzfUaZ fizmoval ap to 10 ym ofj fLZjmovzd matztial. Thii may fae due to thz fizmoval of, dzzply Ajnbzddzd polishing matznlat.

It KUA noted that thz obieAved iuA^acz damagz th/izAhoZdi uizAz uieZl below KepoKtzd baJLk vataej,. Thz

ipzakzA cormzntzd that dae to the then iampleJ> adzd [about 5 mm], bulk damagz thKZAholdi uizAZ did^icalt

to mzaiuAe.

The ipzjakzA indicatzd that thz InO impunity izzn in InSe imh in thz ionm o^ -iabiuA^acz miciocAyitaltite^. Identi-iication uioi by x-njay ^luoxzAczncz ^pzctA-OAcopy.

-188-
2.7 ABSORPTANCE OF DIELECTRIC ENHANCED MIRRORS AT 5.3 [jlitl*

P. Kraatz, J. R. Buckmelter, and S. J. Holmes

Northrop Corporation

Northrop Research and Technology Center

Hawthorne, California 90250

The absorptance of a variety of dielectric -enhanced mirrors has been

measTored in the 5. 3 ^m region using CO laser calorimetry. The samples comprise nineteen designs fabricated from ten dielectric material combinations deposited on thirteen basic mirror types (i.e. , four substrate materials with a

total of six metallic coating combinations). Thirty-two samples were fabricated

by six vendors without duplications of any complete mirror design among any

two (or more ) vendors . Results are groUped according to dielectric material combinations and design. Effects of substrate material selection and metallic coatings (if any) are

discussed. The precision (i.e., standard deviation) of all absorptance measurements is presented. Accuracy, relative to (1-R) obtained from standard reflectometer measurements, is evaluated for selected mirrors.

Keywords: Absorptance; CO laser calorimetry; CO laser mirrors; dielectricenhanced mirrors; infrared laser mirrors; metal mirrors; reflectance.

1. Introduction

A wide array of designs and materials is available for dielectric enhancement of metal mirrors

used in lasers operating in the mid -infrared. In order to quantitatively evaluate the absorptance and

reflectance of a number of such designs for the CO laser wavelength region centered at ~5. 3 fxm, a systematic investigation has been carried out using CO laser calorimetry as the primary tool. Results

are reported here.

2. Experimental

In order to make accurate and reproducible measvirements of the absorption of mioltilayer dielectric mirrors, it is necessary to measure a quantity which is proportional to the absorption. On this basis a calorimetric technique is chosen in which one measures the temperature rise of the coated

mirror subjected to illumination from a laser.

The system configiaration for measurement of absorptance in partial or total reflectors is shown

in figure 1. In this mode of operation, the power reflected from the sample S is monitored by the cone R, referenced to a cone r having identical thermal mass. The power absorbed by the sample

is calculated from the sample temperature rise monitored by the thermocouple A referenced to a thermocouple r^ affixed to the reference cone r^. The transmitted power is monitored by a cone T,

referenced to the cone r^ of identical thermal mass. The power output of the laser is monitored by

a power meter and calibrated beam splitter. The calorimeter cones R, T, r ^, and r^ are of identical

design, thermal mass (within 1%) and mounting within the chamber so that their increases in temperature are comparable for a given energy input; heating and cooling behavior is also very similar, precluding difficulties in interpretation of the resultant curves. The raw data obtained from the system thus comprise the incident power P^ , the power reflected

from the sample surface P , the power absorbed by the sample P , and the power transmitted R A

through the sample P^ . The data can be checked for self-consistency using the relation,

P = P„ + P. + P^ (1)

I R A T

* Work supported by AFSWC Contract No. F29650-75-M-D707.

-189-
ii provided that P , P and P are of comparable magnitude. If P » P , P as in mirrors reflecting!

>99% of the incident power, P = P within the precision of power measurements using a calibrated

I aluminum cone or a ballistic thermopile (typically ±5%). The fractional power absorbed (absorptance) j of an opaque reflector (P^ = O) with negligible scatter is then ,

and the reflectance is

Hence, results can be verified by comparison with values of R and (1 -R) obtained using a reflectometer [1]^

All samples were measured in this system with the vacuum in the chamber maintained between 40

and 125 millitorr, using the multiline output of the longitudinal discharge CO cw laser. A gas mix

comprising helium, oxygen, carbon monoxide, and nitrogen was used to produce an output spectrum

centered at a wavelength of '^-5. 3 |jim with the shortest wavelength lines at ~5. 05 ^m and the longest at I ~5.7 fjim. The beam diameter is approximately 8 mm, limited by a variable intracavity iris; total power is '^IW

The beam is incident upon the mirror sample at an angle of approximately 10° ; the distance from

the mirror surface to the aluminum calorimeter cone monitoring reflected power is '^19 cm. A variable

diameter iris is placed between the mirror surface and the calorimeter cone to limit pickup of spuriou^

multiply reflected or scattered radiation. With this iris set at a diameter of 10 mm (to avoid clipping

the beam) the acceptance cone seen by the reflected power monitor has an included angle of '^3° . Alignment of the sample and reflected power cone is achieved first with a Helium-Neon laser coaxial with the CO laser and calorimeter chamber. This alignment is subsequently checked with the CO

laser beam and a fluorescent screen before pumping down the sample chamber, since alignment of reflecting optics is quite critical to absorptance measurements.

Absorptance and reflectance of a mirror is calculated from eqs . (2) and (3) using the temperature

rise of the sample and calorimeter cone as raw data, where the mass and heat capacity of each is known. The "peak method" as adapted by Saito, et al., [1] is used to determine the sample temperature!

rise, except as noted. The peak method previously described by Kraatz and Mendoza [2] for measuring transmitted power in window absorption measurements is employed in the determination of reflected power from the temperature -time trace generated by the monitor cone. The heat capacity of aluminum (calorimeter cone) and molybdenum (sample substrates ) were taken as 0.215 cal/g°K and 0.0597

cal/g°K, respectively, from the AIP Handbook [3] in the present measurements.

3. Res\ilts and Discussion

Res\xlts of the study are detailed in figures 2-7, grouped according to coating materials and design. All absorptance and reflectance values presented in the figures were obtained calorimetrically

through eqs. (2) and (3). A total of thirty-two mirror samples from six vendors, and one diamond

turned single crystal CaF^ substrate obtained from T. Saito of Lawrence Livermore Laboratories, were measured in the study. The six vendors were OCLI, Perkin-Elmer , CVI, Spectrum Systems

Coherent Radiation Laboratories, and Northrop.

Results for the ThF^/ZnS combination are listed in figure 2. It is clear that optimum reflectivity

is obtained with three or four pairs of these materials and little improvement is to be expected with

additional layers. Addition of a half wave thickness of ThF^ (presumably to reduce electric field strength and increase damage threshold) does not appreciably affect absorptance. The effect of addition of a quarter wave thickness of ZnS for the same purpose cannot be established imequivocally

from the present data. Substitution of PbF^ for ThF , results in very low reflectance, since the dif- ference in refractive indices is too low ("p^p

2 ^ 1.73,

4'

ii^nS ~ ^-^l) effective enhancement with a minimum number of pairs .

1. Figures in brackets indicate the literature references at the end of this paper,

-190-
FigTire 3 lists results obtained with another popular material combination for this wavelength

j region (ThF^/ZnSe). Inspection of the first five entries in the figure show an increase of 0. 0004 in R

or a decrease of '^-30% in absorption with addition of one pair. However, the use of seven pairs as an

all-dielectric stack on molybdenum (with poor base reflectivity) results in relatively low total reflectance

.

Effects upon absorption of addition of half waves of ThF^ or quarter-waves of ZnSe to the dielectric

stack are again unclear from the present data. Substitution of CaF^ for ThF^ in the design appears to result in decreased reflectance, as indicated by the last two entries in the figure. Similarly, reflectance of CaF or CerVit mirrors is less than that of molybdenum mirrors with identical coating designs.

I ^

I In summary, it appears that this material combination is most successful in terms of low absorptance and high reflectance, with three samples from three different vendors having absorptances of -3

I less than 1 x 10 (i.e., reflectances greater than 0.9990). This performance is unequalled by any

other material combination.

j Designs employing SiO combined with Si or Ge are listed in figure 4. The first four entries and

I last entry in the figure illustrate the importance of careful control of the thickness of the chromium

i layer used to promote adherence of the SiO to the metallic layer. Although this Cr layer has a nomij nal thickness of only 50 1 or less, it is apparent that reflectance has been impaired by it, especially in

I the last entry in the figure. Comparing this result with the identical design deposited on a silver layer

I shows the effect of the poor base reflectivity of the Cr layer. On the other hand, no real conclusion can be drawn regarding the effect of micromachined copper substrates upon absorption in mirrors

where other materials and the design is unchanged. One example exhibits an appreciable increase in absorptance, while the other shows no significant difference.

With regard to the number of pairs required for optimum reflectance, it appears that one pair is not adequate and three pairs may not be at the saturation point. Substitution of Ge for Si as the high

index layer apparently does not alter this conclusion.

Results for designs employing (Al^O^/Si) listed in figure 5 indicate that this is not a particularly

successful combination. The three-pair designs on copper and molybdenum should yield higher reflectance values if problems with deposition parameters have not occurred. High absorptance in the sapphire layers, due to entrapped water or non-stoichiometry , or low refractive index of silicon, due

to excessive defect concentrations, could explain these results.

Results for various combinations of materials with germanium are listed in figure 6. Comparing

the second and third designs indicates that a saturation of the enhancement effect occurs at about six

pairs, bearing in mind that the base reflectivity of the silver used in the second design is higher than

that of the Mo substrate. Poor consistency of results is indicated for both the first and third designs,

wherein two samples were fabricated by the same vendor (the vendors of the first and third designs are different). The most obvious explanation for these inconsistencies is poor control of optical thick- ness of the various layers, leading to an offset from the design wavelength. Addition of a half wave

ThF^ layer for greater resistance to pulsed laser damage, as shown in the last two entries in the fig- ure, does not appear to affect absorptance appreciably. It is also interesting to note that the choice of substrate (CaF^ or Mo) has little effect upon absorptance.

Results of absorptance measurements on uncoated CaF^ substrates finished by micr omachining or polishing to a low scatter finish are listed in figure 7. Unfortunately, the comparison is not as meaningful as it might be, since the bulk absorptance of the micromachined sample is not known. The temperature time trace from the calorimeter rvm on this sample indicated heavy direct scatter to the sensing thermocouple, probably as a result of the surface finish. The beam was offset from the sample center to avoid the worst areas for this. An indication of the overall accuracy of the calorimetric results, relative to standard reflectometer measxirements , is provided by the data of fig\ire 8. The reflectance of four mirrors measured

in the calorimetric study was subsequently measured in a standard reflectometer at the Michelson

Laboratory, NWC, China Lake, California, at 5.2 |xm, 5.4fxm, and 5.6 fj.m. Since the center of the CO laser spectrum employed in calorimetry is between 5.2 p.m and 5. 3 |j.m, calorimetric results are

correlated with reflectrometer data taken at 5. 2 |j.m. With the exception of the second entry in the

figure, the results are encouraging. A similar comparison of absorptance (A) with (1-R ) by Saito R

et al., [l] at 10.6 [im showed agreement within 18 - 20%, with the calorimetric results indicating the -191-
lower value for absorptance as in the present work. With respect to reflectance, the results are quite

encouraging. Three of the four calorimetr ic reflectance values are within 0.0004 or less of the reflectometer values, while the fourth deviates by 0.002.

Reasons for these discrepancies are difficult to determine. Possible problem areas include the bandwidth of the laser and reflectometer spectra, the acceptance cone of the reflectometer and calo- rimeter, and the reflective bandwidth and homogeneity of the coated sample.

4. References

[1] Saito, T. T., Callender, A. B., and [3] Gray, D. E. , Ed. American Institute of

Simmons, L. B., "Calorimeter to Physics Handbook, 3rd. Ed., McGraw-Hill,

Measure the 10.6 \xm Absorption of Metal N. Y.

Substrate Mirrors," Appl. Opt., Vol. 14, p. 721, 1975.

[2] Kraatz, P. and Mendoza, P. J., "CO Laser

Calorimetry for Surface and Coating Evaluation," Proc. Fourth Conf. on Infrared

Laser Winder Materials, p. 77, 1975.

5. Figures

CO LASER

CHART

RECORDER

POWER MHER

INTRA- BEAM

CAVITY SPLinER

APERTURE I

Figure 1. Schematic of calorimeter measurement

system for partially transmitting/

reflecting optics.

Figure 2. Dielectric enhancement designs

utilizing (ThF(,/ZnS) and (PbFs/ZnS)

SUBSTRATE COATING MATERIALS S DESIGN A R Mo (TliF^/ZnS)^ + ZnS 3. 191 + 0. 144 X 10'^ 0.9968

Mo Aq + (ThF^/ZnS)^ 1.740 + 0.048 X 10 0.9983

CerVit Aq + (ThF^/ZnS)^ 1.840 + 0.124 X 10"^ 0.9982

CaF^ Cr-Au+(ThF./ZnS)''

i

+ ThF.

4 5.954 + 0.263 x lO"^ 0.9940

Mo Cr-Au+dhF./ZnS)"

4

+ ThF.

4 1.302 + 0.046 x 10"^ 0.9987

Mo (PbF^/ZnS)^ + ZnS 2.520 t 0. 156 x 10'^ 0.9748

-192-
C 1 lOCTD ATf bUUb 1 KAI t A

Mo All A fTKC /7nd»l^

-4 0.9990

CerVit

3 Aq + (Inr^/inbet

-4 0 9983

Mo Lr-Au TUnr./inDei + ifir. 4 4 4

0, 700 ' U. LO*t A lU 0 9993

CaF^ Pr-Aii + (ThF /7n*ipl^ + ThF

4 4

1.489 + 0. 103 X 10^ 0.9985

Mo

4 An + (ThF /7n'spl rtq T u J CI

-4

6. 160 + 0. 256 X 10 0. 9994

Mo ITKC I'fnQaJ + 7n^i> Mrir./i.n3c) /.noc

4

-3

0. 9988

CaF^

1 2 U Itr ./Z.II J cl ' ILdr ^/i-li jci

4 i

+ ThF.

4

2, 191 + 0. 102 X lO"' 0.9978

Mo (IhF./ZnSe)' + (CaFJZnSe)^

4 c

+ ThF.

4 2.006 + 0.087 X lO"' 0,9980

Figure 3. Dielectric enhancement designs

utilizing (ThFi,/ZnSe) and CaF2/ZnSe)

.

Figure 4. Dielectric enhancement designs

utilizing SiO in combination

with Si and Ge.

SUBSTRATE COATING MATERIALS & DESIGN A R

Cu' Cr + Ag + Cr + (SiO/Si)' (9.619j^0.121lx 10 0.9904

Mo Cr + Ag + Cr + (SiO/Si)' (9.752 j^O. 309) x 10 0.9902

Cu' Cr +Au +Cr + (SiO/SIl' (1.526 + 0.0455) x lO'^ 0.984?

Mo Cr + Au t Cr + (SiO/Si)' (5.293 _|_0. 107 )x 10"^0. 9947

Mo Ag + (SIO/SIl' (1.391 + 0.066) X 10"^0. 9986

(1.966 + 0.059) X lO'^ 0.9980

Mo Ag + (SiO/Ge)^ (1.395 + 0.042) X lO"^ 0.9986

Mo Cr + (SIO/SI)^ (8.811 + 0. 325) X 10'^ 0.9912

1. MICROMACHINED COPPER SUBSTRATE FURNISHED BY T. SAITO AND LAWRENCE LIVERMORE.

SUBSTRATE COATING MATERIALS & DESIGN A R

Cu Cr + (AV^Oj/SII^ 2.000 + 0.0525 x 10'^ 0.9800

Mo Cr + (AlijOj/Sl)^ 2.405 + 0. 102 x 10'^ 0.9760

Mo (AV^Oj/SI)^ + SI 5.504 + 0. 175 x 10"^ 0.9945

1. MICROMACHINED COPPER SUBSTRATE FURNISHED BY T. SAITO AND LAWRENCE LIVERMORE.

Figure 5, Dielectric enhancement designs

utilizing AI2O3 and Si.

Figure 6, Dielectric enhancement designs

utilizing germanium in combination with various materials.

SUBSTRATE COATING MATERIALS & DESIGN A R

Mo (ZnS/Gel^ + Ge (9.753 + 0.4531 x 10 0.9025

(5.773 + 0. 158) X lo"^ 0.9942

Mo Ag + (ZnS/Gel" (9.550 + 0.500) X 10'* 0.9990

Mo (ZnS/Ge)^ (6.951 + 0.358) X lO''' 0.9993

(4.948 + 0. 121) X 10'^ 0.9950

CaF^ Cr-Au + (ThF^/Ge)' + ThF^ (1.690 j_0. 053) X 10'^ 0.9983

Mo Cr-Au + (ThF./Ge)^ + ThF.

4 4

(1.374 + 0.066) X 10'^ 0.9986

-193-
SUBSTRATE COATING MATERIALS & DESIGN A

CaF^^

UNCOATED (MICROMACHINED)

UNCOATED (NRTC POLISH)

(4. 358 ± 0. 203) X 10'"

(4. 050 ± 0.260) X lO'''

L MICROMACHINED SINGLE CRYSTAL CaF^ SUBSTRATE

FURNISHED BY T. SAITO AND LAWRENCE LIVERMORE.

L = 0.947 cm = 4.60 X lO''' cm'^ (SURFACE AND BULK).

Figure 7. Absorptance results for uncoated

single crystal CaFa substrates.

2. SINGLE CRYSTAL CaF^ SUBSTRATE POLISHED AT NRTC.

L

= 0.715 cm = 5.664 X 10 "'^ cm'^ (SURFACE AND BULK).

P = 3.0 X lO''' cm"^ (BULK), p

= 9. 5 x lO"^ PER SURFACE.

MIRROR DESIGN CALORIMETER REFLECTOMETER^ (A55.2 urn) AA

I%1 AR^(%l A «C "r

Mo + Ag + (ThF./ZnSel"

4

6. 16 X lo" 0.9994 0.9991 9.0 X 10 -31.5 + ,03

Mo + Ag + ISiO/Si)' 1.39 X lO"' 0.9986 0.9970 3.0 X 10 -53,7 + .16

Mo + Ag + ISiO/Gel' 1,40 X lO"^ 0.9986 0.9982 1.8 X lO'' -22.2 + ,04

Mo t Ag + (ZnS/Gel" 9.55 X lO'" 0.9990 0.9991 9.0 X lO'" * 6,

-,01

Figure 8. Comparison of techniques

measurement

REFLECTOMETER MEASUREMENTS PERFORMED AT MICHELSON LABORATORY, NWC, CHINA LAKE, CALIFORNIA,

. ., A-11-Rd)

b. AA ,. n «

comEm-s ON paper sv kraati, buckmelter, amp holmes

Wo cLLiaaii'Lon on tkii pap&A.

-194-
2.8 OPTICAL PROPERTIES OF >aRRORS PREPARED BY ULTRACLEAN dc SPUTTER DEPOSITION

P. A. Temple, D. K. Burge, and J. M. Bennett Michelson Laboratories, Naval Weapons Center

China Lake, California 93555

Sputter deposition is a method for producing mirror surfaces. However,

the sputtering gas is generally incorporated in the metal film. This investigation was undertaken to determine the amount of gas trapped and the effect

this gas had on film optical properties. Thin film mirrors of aluminum,

silver, copper, and gold were prepared by dc sputter deposition under ultraclean conditions using argon or xenon as the sputtering gas. The sputtering

gas entrapment was measured by a flash desorption technique. Film reflectance was correlated with preparation conditions and gas entrapment. In general, the film reflectance was reduced by the presence of sputtering gas. The gas trapped was a function of the metal being sputtered, the sputtering

gas type, and the sputtering gas pressure. For example, the gas content of silver films could be varied over nearly two orders of magnitude. In some

cases, films with reflectances as high as the best UHV evaporated films were

prepared.

Key words: Aluminum; copper; flash desorption spectroscopy; gold; reflectance; silver; sputter deposition.

Introduction

Although It is an accepted method for preparing thin films, particularly in the microelectronics

industry, little has been done with sputtering in making high quality optical films for mirror coatings.

We wish to report on the method of preparation and on the properties of high quality optical thin films

for copper, gold, silver, and aluminum deposited by dc diode sputtering under ultraclean conditions.

Films were prepared using either xenon or argon as a sputter gas. There is evidence that a fairly high

atomic percentage of the sputtering gas is trapped in the films prepared by dc diode sputtering [1] . Therefore, in this report an effort will be made to correlate film gas content with film reflectance.

The sputtering gas content was determined by a flash desorption technique.

Description of Sputtering System

The dc sputtering source (fig. 1) consisted of a dc diode sputtering gun especially designed for

use in an UHV system by Sputtered Films, Inc. [2]. This gun used 12.7 cm diameter annular ring targets

surrounding an insulated center anode and employing a magnetic field to increase the path length of the sputter gas ions and confine them to the target region. The sputter target was water cooled. A sub- strate holder designed to expose any one of five sets of substrates was positioned 15 cm above the upward-looking sputter gun. Each set of substrates consisted of one 3.86 cm diameter and two 0.95 cm diameter polished fused quartz substrates. The larger substrate was used for reflectance and scattering measurements while the smaller substrates were used for electron microscopy. A Sloan Technology

Corporation Model DTM-4 deposition rate monitor [3] was located adjacent to the rotatable substrate

holder and allowed for continuous monitoring of film thickness. A shutter was placed just above the sputter source and made it possible to adjust the plasma current prior to actual deposition.

This apparatus was placed in an all-stainless-steel vacuum system pumped with a 500 l/s

Varian Vaclon pump and was capable of a base pressure in the 10"^^ Torr range. A residual gas analyzer

(RGA) was attached to the chamber, so that the residual gas in the system could be monitored prior to admitting the sputtering gas. Care was taken to minimize the amount of water vapor and hydrogen present in the system by initially rough-pumping the system using a Cast Manufacturing Corp. oilless reciprocating pump [4] followed by two backfillings with dry nitrogen (boiloff from a liquid nitrogen tank)

and pumpings. The system was then baked for two to four hours while being pumped sequentially by three

Varian VacSorb pumps. During this initial baking and pumping, no hot filaments were present in the

system, and the ion pump was not turned on. It is well known that hot tungsten causer the formation of carbon monoxide from hydrocarbons and water, with the subsequent release of hydrogen [5]. The entire system was further baked, with the ion pump on, for at least 24 hours at approximately

200°C and allowed to cool for another 24 hours. Prior to the admission of the sputtering gas, titanium

was evaporated from a sublimation gun to produce a fresh gettering surface. Since the ion pump could

not be operated in the pressure range required for sputter deposition, a liquid nitrogen trap was used

instead. It was cooled for one-half hour prior to turning off the ion pump. With the ion pump shut

off, but the liquid nitrogen trap operating, the chamber pressure would rise very slowly to about

1. Figures in brackets indicate the literature references at the end of this paper.

-195-
1 X 10~^ Torr by the end of a 30-iiiinute period. Generally, sputter deposition required less than one- half hour, and if begun immediately after the ion pump was turned off, it was assumed that the residual

gas pressure remained below 1 X 10 5 Torr. RGA analysis showed that this remaining gas consisted

mainly of helium, hydrogen, and argon while the active gases were unchanged from the pump-on amounts and evidently were either being gettered on the fresh titanium surface or condensed in the cold trap. Previous experience with the growth of silver films under various evaporation conditions indicated the importance of having low residual amounts of oxygen and water vapor present during deposition [6]. Since some of the films produced during this investigation were grown over an extended period and had

exceptionally good optical properties, the quantities of active gases were apparently being kept at veryl low levels. Research-grade argon and xenon (impurities less than 5 ppm for argon and 50 ppm for xenon) were usedj for the sputtering and were admitted through a Varian leak valve. The minimum pressure at which the plasma arc would form was 2-3 mTorr as monitored with a Varian Millitorr Gauge. Unlike most sputtering

systems which use continuously flowing gas, a fixed quantity of gas was admitted to the sputtering cham^' ber and then the gas source was valved off. When necessary to replace gas that had been implanted in ] the target or trapped in the film, fresh gas was leaked in to return the chamber to the desired pressurej

The loss rate of gas was noted and used as a rough measure of the relative gas entrapment by the various]

films prepared. However, since the substrate represented only a small percentage of the total area i' available for trapping, this method gave only a very rough indication of gas entrapment.

The arc power, supplied by a Hitran Corporation Model DCS-3000L power supply, could be varied from

190 to 2370 watts, and gas pressures were in the 2 to 40 mTorr range. Deposition rates were found to be|

nearly linear with power. For example, the deposition rates for aluminum at 5 mTorr argon pressure were; 9.5 (A/s)/kW. Only at very low power settings did this relationship change. At the lowest gas pressures, the mean free path of the metal atoms was comparable to the source-to-substrate distance, so that metal atoms primarily traveled in straight lines from the target to the substrate. At higher pressures

the mean free path was reduced and metal atoms impinged on the substrate from all forward directions.

This led to reduced film thicknesses at the edges of the sample because of shadowing by the sample

holder. In particular, difficulty was experienced with the Sloan thickness monitor, and in some cases, actual film thicknesses in unshadowed areas on the sample were as much as 30% greater than those

indicated by the thickness monitor. While the holder for the quartz crystal in the monitor was modified

to minimize this effect, it was necessary to also use a calibration appropriate to the pressure involved

Actual film thicknesses were determined interf erometrically [7].

Results

Aluminum, copper, gold, and silver films were prepared under different conditions by varying the sputtering rate, gas type and gas pressure. Normal incidence reflectance [7,8] was measured from 0.26

to 20 urn, and film roughness was determined from scattered light measurements [7,9]. Reported below are data on the spectral reflectance of films produced by choosing the parameters listed above such that the highest deposition rates were obtained for the two sputtering gases used. Table 1 shows the conditions

under which these films were produced and the reflectance at a wavelength of 10 um.

Aluminum

Figure 2 shows the reflectance of aluminum films prepared using argon or xenon as a sputtering gas. Also shown is the reflectance of an evaporated film deposited under UHV conditions. It is evident from

this figure that films of optical quality equal to that of UHV evaporated films may be produced by rapid

deposition using argon as a sputtering gas.

Copper

Figure 3 shows the reflectance of copper films produced by sputter deposition and UHV evaporative

deposition. Deposition by both argon and xenon sputtering produced films of reflectance slightly higher

than those produced by UHV evaporative deposition.

Gold

Figure 4 shows the reflectance of gold films produced by sputter deposition along with the reflectance of an UHV evaporated gold film. In the case of gold, the films produced by xenon sputtering were

of the same quality as those produced by evaporation. The argon-produced films were approximately 1% lower in reflectance throughout the spectrum.

Silver

Figure 5 shows the reflectance of silver films produced by sputter deposition along with the reflectance of a UHV evaporated silver film. The xenon sputtered film shows a reflectance nearly as high as UHV evaporated silver, while the argon sputtered film has a lower reflectance everywhere, with the j visible reflectance falling particularly low.

-196-
Gas Entrapment

Several authors [10] have studied argon trapping rates occurring during sputter deposition and have

found that the trapping rate varies widely, depending upon the material being sputtered. Lee and Oblas

have proposed a model which describes a mechanism for the generation of fast neutral gas atoms (by re- flection from the sputtering target) which can travel to the substrate and embed themselves in the film

as it is being formed. However, they suggest the existence of a threshold kinetic energy for the atoms

below which they will not stick in the film. For example, this threshold kinetic energy for silver is

i 110 eV [1]. Lee and Oblas predicted in their model that the percentage of trapped gas will increase

when the target material has high atomic mass, the sputtering gas has low atomic mass, and the sputter

gas pressure is low. Their data verify the dependence on target mass. We have found that the depend-

'li ence on gas atomic mass does not follow such a simple model, but that the dependence on gas pressure is

j as predicted by the model.

I Figure 6 shows the atomic percentage of gas trapped in films sputtered under the conditions given

in table 1. As predicted and demonstrated [1], sputtering gas content does decrease with decreasing

,' target atomic mass. However, since xenon is so much more massive than argon, one might expect that the

J atomic percentage of xenon trapped in the films would be well below the atomic percent of argon trapped

i in films prepared under similar conditions. As figure 5 shows, the atomic percent of xenon present in

I gold is lower than the atomic percent of argon in gold. However, the silver films showed essentially

! identical atomic percentages of argon and xenon, and the copper films showed more xenon than argon.

I These results are not wholly consistent with the model of Lee and Oblas which would anticipate a lower

I xenon content for all metals.

I A further consequence of the Lee-Oblas model is that films made with high sputtering gas pressure

I would have less trapped gas than those films made under lower gas pressure. Figure 6 also shows the

I argon content of silver films deposited at 3, 20, and 40 mTorr pressures. The amount of argon trapped

using 40 mTorr argon pressure is almost two orders of magnitude less than when deposited with 3 mTorr of argon pressure. As suggested by Lee and Oblas, high kinetic energy neutral argon atoms are reflected

from the target and impinge upon the substrate and growing films. The number of collisions suffered

between the target and the growing film will determine the kinetic energy of gas atoms upon reaching the

film. In general, the higher the kinetic energy, the greater the probability of capture. The presence

of gas will increase the number of collisions suffered by gas atoms and therefore reduce their kinetic

I energy. The results shown in figure 6 are consistent with this view,

i

I Reflectance versus Gas Content Figure 7 shows the reflectance of two silver films produced at 3 mTorr and 40 mTorr argon pressure.

It would appear from these data that the films with the least amount of argon entrapment produce the highest reflectance films.

In the case of gold, the argon-produced film contained a higher percentage of gas than did the xenon

produced film. Figure 4 shows the high gas content film again having lower reflectance.

Figure 3 shows the reflectance of copper films produced by both argon and xenon deposition as having

the same reflectance. Figure 6 indicates that both films contained approximately the same atomic percent of sputtering gas. At this time insufficient data exist to determine the relationship between gas content and reflectance of aluminum. However, data reported elsewhere [11] do indicate that those aluminum films which

were deposited at a low argon pressure exhibited the highest reflectance. Deposition at low pressure

generally results in a higher percentage of gas entrapment, and it would be expected that aluminum

would follow this trend. Figure 6 indicates that the argon content of aluminum films is in the ppm

range, and any modification of optical properties due to argon content is not expected. A more likely

explanation of the lower reflectance of xenon sputtered films (fig. 2) and of high pressure argon sputtered films [11] relates to the rate of deposition. In all cases the reflectance of aluminum films

is lower for slowly deposited films and higher for the rapidly deposited films. Aluminum is highly

I reactive, and it is likely that impurities present in the system—particularly oxygen containing com- pounds—may be adding impurities to the films.

Figure 5 shows the reflectance of silver films sputtered using both argon and xenon. Figure 6 shows that these two films contain essentially identical atomic percentages of gas. The reduced reflectance of argon sputtered films and the unaffected reflectance of xenon sputtered films seem to indicate

' that xenon does not affect the reflectance of silver in the same way that argon does. It is not clear

at this time the reasons for this occurrence.

Conclusions

We have prepared thin films of aluminum, copper, gold, and silver by dc sputter deposition which

I have a reflectance as high as, or in some cases higher than, the reflectance of UHV evaporated films.

' These films were deposited using argon and xenon as sputter gases. The resultant sputtering gas content

of all films was determined by a flash desorption technique. The measured gas content was compared to the (relative) content predicted by a gas entrapment model suggested from Lee and Oblas. Our results

I were substantially in agreement with this model.

-197-
The reflectance of these films was correlated with sputter gas content. In the case of gold and

silver the reflectance of films with high gas content was less than the reflectance of films with low

gas content. This was most striking in a study of silver deposited with argon as the sputter gas. It would appear, however, that other effects such as impurities incorporated into aluminum are present and may play a stronger role in the final reflectance than does sputter gas incorporation.

Table 1. Sputter Deposition Conditions and Measured Reflectance at 10 pm.

METAL GAS ARC VOLTAGE CURRENT PRESSURE RATE REFLECTANCE

(kV) (A) (mTorr) (A/s) (at 10 um)

Aluminum^ Argon 0 710 3.00 3.0 25.0 0 988 Aluminum Xenon 1 000 0. 35 5 .

3.5 0 984 Aluminum (UHV evaporated) 0 988

Copper Argon 1 000 1. 00 2. 0 21 .

0 994 Copper Xenon 1 000 0.45 5.7 8.5 0 992 Copper (UHV evaporated) 0 992

c Gold Argon 1 000 1.00 8.0 30.0 0 981

Gold Xenon 1 000 0.10 5.7 5.0 0 992 Gold (UHV evaporated) 0 992

Silver"^ Argon 1 000 1.50 3.0 67.0 0 .993 Silver Xenon 1 000 0.45 5.7 22.0 0 994 Silver (UHV evaporated) 0 995

Silver^ Argon 1 000 1.50 3.0 67.0 0 993 Silver Argon 0 500 2.00 40.0 28.5 0 994

See figure 2. See figure 3. See figure 4. See figure 5. See figure 7.

Acknowledgments

This work was supported by ERDA, ARPA, AFWL and NAVWPNCEN Independent Research funds. One of the authors (P. A. Temple) was a NRC-N^iJC Postdoctoral Fellow during the period this work was done and wishes to acknowledge the support of the NRC-NWC postdoctoral program. The authors are grateful to Mr. Richard Smith, Mrs. Verla Walker, and Ms. Jane Williams for their I invaluable assistance in preparing the artwork and manuscript for publication. 1

Appendix

The apparatus used to determine the sputter gas content of sputter-deposited films is shown in figure 8. A linear feedthrough held three 0.95 cm diameter quartz substrates on which films had been

deposited. The feedthrough allowed the selection of any vertical position on the three substrates.

The entire chamber was mounted on a translation stage which allowed for horizontal motion. I A RGA was used to monitor the argon or xenon content of the small vacuum chamber (the total volume'

was 1 X 10~^ m^). The RGA was set at mass 40 for argon and mass 129 for xenon. Pulsed, 10.6 vm laser

radiation entered the chamber through a NaCl window and was focused on the film. Sufficient laser

energy was available to totally evaporate the metal in a small circular spot on the film. The typical

spot size was 0.9 mm in diameter, and all films tested were 2000 A thick. The evaporated spots were

subsequently photographed, and the area of each spot was determined using a planimeter.

Prior to the laser evaporation, the ion pump attached to the chamber was valved off. The laser was

then triggered and the increase in RGA current caused by the released gas was recorded on an x-y recorder. The RGA was used in the faraday cup mode whenever possible. The RGA sensitivity was determined by]

measuring the response of the RGA when a known pressure of argon or xenon was present. The argon and xenon pressures were measured using a Varian nude ion gauge and the calibration adjustment for argon

and xenon published by Varian. These numbers are considered to be accurate to 20% which limits the accuracy of the flash desorption apparatus to no better than 20%. The calibration procedure was carried out while the RGA head was attached to the sputter chamber (fig. 1).

-198-
References

[1] Lee, W. W. Y. and Oblas, D. , J. Appl. Phys.

46, 1728 (1975),

[2] Sputtered Films, Inc., 725 E. Yanonali St., Santa Barbara, Cal. 93103.

[3] Sloan Technology Corp., Santa Barbara, Ca.

[4] Cast Manufacturing Corp., P. 0. Box 97, Benton Harbor, Mi. 49022.

[5] Tharp, L. N. and Schelbner, E. J., J. Appl.

Phys. 38, 3320 (1967).

[6] O'Handley, R. C. et al. , Surf. Sci. 50, 407

(1975).

[7] Bennett, H. E. and Bennett, J. M. , "Precision

Measurements in Thin Film Optics," in Physios

of Thin Films, G. Hass and R. E. Thun, Eds. (Academic Press, New York, 1967), Vol. 4, pp. 1-96.

[8] Bennett, H. E. and Koehler, W. F. , J. Opt.

Soc. Am. 50, 1 (1960).

[9] Bennett, H. E. and Porteus, J. 0., J. Opt.

Soc. Am. 51, 123 (1961).

[10] See Ref. 1, references.

[11] Temple, P. A. et al. , in High Energy Laser

Mirrors and Windows (Semi-Annual Report No. 7-8, ARPA Order 2175, Naval Weapons Center,

\ NWC TP 5845, 1976), pp. 35-53.

Figures

Figure 1. Photograph of the sputter chamber showing

the central anode (held in place by three

cap screws) surrounded by the cathode

(target) made of the material which is to be sputtered. Outside of the target is a shield which contains magnets which cause

ions to spiral toward the cathode. Above

the sputter gun is a substrate holder

which is designed to expose any set of three substrates simultaneously.

Adjacent to the substrate holder is a quartz crystal deposition monitor.

-199-
Figure 3. The spectral reflectance of sputter

deposited copper. The conditions

under which the films were deposited

are shown in table 1. The reflectance of UHV evaporated copper is shown for comparison.

-a a.

UHV EVAPORATED

SPUTTERED lAr) SPUTTERED (Xe)

1.0 2.0 WAVELENGTH, iim

UHV EVAPORATED

SPUTTERED (Ar) SPUTTERED (Xa)

1.0 2.0 WAVELENGTH, )jm

Figure 4. The spectral reflectance of sputter

deposited gold. The conditions

under which the films were deposited

are shown in table 1. The reflectance

of UHV evaporated gold is shown for comparison.

-200-
— UHV EVAPORATED

o SPUTTERED (Ar) o SPUTTERED (X*)

"oia OB t.O 2.0 WAVELENGTH, (im

10.0

Figure 5. The spectral reflectance of sputter

deposited silver. The conditions

under which the films were deposited

are shown in table 1. The reflectance

of UHV evaporated silver is shown for

20.0 comparison

.

Figure 6. The atomic percentage of trapped sputtering gas

In thin films of aluminum, copper, silver, and

gold. Two sputtering gases, argon and xenon, were used. Also shoim is the argon content in silver films when the deposition was made at three different pressures. The gas content was

determined using a flash desorptlon technique.

The absolute accuracy is approximately 25% while the precision is at least 10%.

10-2

10-'

~r I

Au0

0

AgSapm

Cu0

20 ym

4oym ARGON

O XENON

. Al

1 1 100 200 ATOMIC MASS

: 0.M

1

1 1 I 1

?

8 8 b i

A

/ ^

/ ^

jo UHV EVAPORATED

0 3 mTorr Ar PRESSURE

1 ^

o 40 mTorr Ar PRESSURE

1

1 1 on 0.B 1.0 2.0 WAVELENGTH, (im

10.0

Figure 7. The spectral reflectance of silver films

sputter-deposited using argon at 3 mTorr

and at AO mTorr pressures. Also shown is the spectral reflectance of UHV evaporated silver. The conditions under which

the films were deposited are shown in

table 1.

-201-
Figure 8 . A photograph of the flash desorption

apparatus used to determine the atomic percentage of sputtering gas present in thin

sputter deposited films. The entire top

half of the apparatus is mounted on a translation stage which allows for horizontal

motion of the laser beam across the sample, A NaCl window is visible in the foreground.

COmENTS ON PAPER BV TEMPLE, SURGE, AMP EENNETT

The, ipexikeM. nottd that iX. uxu, hand to cxuifiy out aoyi&Aj,te.nt pa>iimeXfu.c itudiu 0){ the. iputteJi

ipfLocddii rfue to thz iviteAjd.zpzndzn.ce. 0)5 thz voAiableA, 6uch cm, iputtzA gcu>, dzpoittion fiatz, and hnpuJuJuj IzMzt, It iMOA obizAvzd that oAgon ipwttexing tzd to highzA dzpoi-ction natzi than xznon, {^ofi zxamplz. Aluminum {^Ibni, which ofiz highly Kzactivz, zxhtb-itzd much bzttzA quaZity when IcUd dom

quickly. Uilng oKgon 6puttz>ilng , but maintaining a ilou) dzpoiltxon natz, yleZdzd M iUbni o^ pool

quality.

-202-
il 3.1 Materials for High-Power Window and Mirror Coatings and Multilayer-Dielectric Reflectors*

M. Sparks

Xonics, Incorporated, Santa Monica, California 90401

Antireflection coatings and reflectors having lower values of absorptance than are

presently available are needed for high-power, and even some low-power, applications.

Intrinsic values of absorptance of antireflection coatings, multilayer-dielectric reflectors,

and metallic-reflection-enhancing coatings are orders of magnitude lower than current ex- perimental values Aexp • A major factor limiting the values of Agxp that a given coating

material usually has orders of magnitude greater absorption when deposited as a film than when grown as a bulk crystal. The most likely source of this extra film absorptanc_e Af _is contamination of the film. Numerous contaminants include water, HCO3, CIO3, NO3, OH , and CN . Contamination can occur before or during deposition, upon exposure to the atmosphere (where absorption by porous films and surface adsorption are important), or during

use or storage. At 10. 6jim only two molecular layers of water or a packing density of 99. 95%

are formally required to give Af = 10"*, which is a typical desired value. High-packing-density films should be vacuum vapor deposited using ultraclean deposition conditions including

thorough baking of the entire system, high-purity single-crystal starting materials, special care in evaporation, state-of-the-art substrate preparation, and ultrahigh vacuum. Spectroscopic and calorimetric measurements of Af (co) on very thick films and on normal-thickness

films, both deposited on attenuated-total-reflection plates, should be made. Material- selection

guidelines, including a bulk absorption coefficient less than 0.5 cm"-^ and a value of the index

of refraction Ln the required range, are developed and used to select the following 10.6(im

candidate materials: ThF4, NaF, BaF2, SrF2, NaCl, KCl, KGaF4, AS2S3, As2Se3, ZnS, ZnSe,

and Tll. Distinguishing absorption on the coating surface from that in the bulk of the coatings

by utilizing the nearly zero electric field on the surfaces of certain coatings is proposed. In- clusion damage is expected at ~1-10 ]/cmr for nanosecond pulses or ~10 to several hundred

J/cm for microsecond pulses. Intrinsic damage is expected by heating at ~100 J/cm^ for a

lOjis pulse and Af = 10"^, or by such nonlinear processes as electron -avalanche breakdown

at ~10 J/cm^ (i.e. I 2; 10 GW per cm^) for a 10 ns pulse. Strongly absorbing l-/im-radius

inclusions spaced 175 jim apart in the coating give rise to absorptance Af 2: 10"^. Thermally

induced stresses in a 2 mm-diameter detached film segment may be sufficient to cause additional detachment and a runaway process.

Key words: Attenuated total reflection spectroscopy; dielectric coatings; infrared

lasers; laser damage; thin films.

1. Introduction

Deposition of thin dielectric films is of great interest for (1) antireflection coatings, where the reflectance is

to be reduced, and for (2) multilayer-dielectric reflectors, and (3) metallic coatings, where the reflectance is to be

increased. Some high-power systems of current interest cannot be operated without antireflection coatings on the windows. The best uncoated metallic mirrors have high intrinsic values of normal incidence absorptance, ranging

from approximately one-half percent at 10. 6fim, to approximately two percent at 500 nm, to approximately 10 peri cent from 400 nm to 100 nm (12. 5 eV). Much lower values of absorptance are needed in some high-power systems.

I By contrast, multilayer-dielectric reflectors (and window and mirror coatings) have much lower intrinsic values of

-8 absorptance, probably less than 10 in many cases, for all wavelengths from the vacuum ultraviolet through the -4

infrared to ~10fxm. Since the lowest current experimental values of absorptance are a few times 10 for window

and mirror coatings and multilayer-dielectric mirrors, it is possible that technical improvements in film deposition techniques could result in lower absorptance for all three of the film devices.

In view of the current difficulties in obtaining satisfactory coatings for high-power infrared windows and mirI rors, a study of the problems of these coatings was made [ IJ'l The present paper is a summary of the results,

* This research was supported by the Defense Advanced Research Projects Agency

^Figures Ln brackets indicate the literature references at the end of this paper. -203-
most of which also apply to multilayer-dielectric reflectors and to wavelength regions other than the infrared. It i emphasized that the study was limited to a near-term program of obtaining coatings in a research-laboratory en- vironment in the shortest period of time for the lowest cost. Fundamental investigations and manufacturing of field-use components were not explicitly included in the study, although it is necessary to keep the ultimate use in|

mind even during the research-laboratory stage of the investigation. i

2. High Absorptance of Films

j

In addition to the already stringent requirements of coatings for low-power use, the higji-power coatings I must have low optical absorptance. Required values for the film absorptance per surface A- have not been estab- j -4 I lished for any of the major systems of interest. A typical desired value is A^ = 10 , which is chosen to make the: absorptance of two coated surfaces equal to that of the window for the typical case of a 2 cm-thick window with ab-i -4 -4 sorption coefficient jSj^ = 10 , The value of the film absorption coefficient corresponding to A^ = 10 and a

typical thickness of 2 ^im is |3f = 0. 5 cm ^. Since much greater values of absorption coefficient can be tolerated '

in the film (~0. 5 cm-1-4-1 ) than in the bulk (~ 10 cm ), it would appear that obtaining materials for films should be

j much easier than for windows. However, the absorption coefficient of a given material usually is much greater

1 when deposited as a thin film than when grown as a bulk crystal. As a typical example, a material with a bulk value

of /3j^ = 10 cm may have a film value of j3j = 10 cm . This much greater absorptance in film form than in form is one of the most serious coatings problems, thou^ by no means the only one.

In general the source of the extra absorptance in films is not known. It is suggested that contamination of the film is by far the most likely candidate for explaining this extra absorptance. Water is the most important contamy

inant for the 2 to 15 pan wavelength region, but there are many others. The contamination can be in the bulk of the ji film, adsorbed on the surface of the film, or in pores in the film. The contamination can occur during deposition,

J from the residual gas for example, after deposition, by surface adsorption or by absorption into the pores for ex- ample, or before deposition, by substrate contamination for example.

j As seen in figure la, liquid water absorbs strongly from 2 through 15 ^m. The absorption coefficient at ' 3 -1 10.6|im is ~10 cm , and the values at the other prominent laser frequencies are shown by the vertical lines. In ] passing notice that this top figure illustrates the invalidity of the myth that the frequency must be near an absorption line in order to have great absorptance. From ^6. 5 to 10. 5 ^rni there is very little structure in the absorption ! 3-1

spectrum, but the absorption coefficient has the very large value of 10 cm . Hie broad absorption from ~11 to

| 15fmi results from the hydrogen-hydrogen interactions in the liquid water, and this absorption is missing in the i water-vapor spectrum, which is shown in figure lb , In fact, careful measurements [ 3 ] indicate that the absorption coefficient of water vapor at 10. 6 ^m may be orders of magnitude smaller than that of liquid water,

3-1 -4 For adsorbed water, only Inm is required formally (using j3 = 10 cm for liquid water) to give 10 ab- i sorptance. Pores in films are known to readily absorb water from the atmosphere on removal from the vacuum- j -4 -4 i deposition chamber, and a fraction of 5 X 10 of pores filled with water formally gives an absorptance of 10 ,

|

At 10. 6 (im, molecules containing 0^ are particularly offensive since they usually absorb near 10. 6 ^im. There

are many known molecules that are strongly absorbing in the infrared. Deutsch [4] observed molecular absorption on sample surfaces in attenuated-total-reflection studies. Duthler [5] showed that molecular-ion absorption was

^ an important source of bulk absorption, pointed out the relation to matrix-isolation spectroscopy studies, and cata-

logued strongly absorbing molecular ions, Phil BClein [6] has argued that molecules containing carbon probably cause a wide absorption band extending from ~2 to ~5;im. <

There are absorption bands of molecules and molecular ions extending throughout the range from 2 to 12 ym i as seen in figure 2. These and other film contaminants include the following:

-204-

i
• Water

• Other molecules and molecular ions such as CHO3, CIO3, NO2, CO2, CO, N2O, DHO, SO4, and CH4

• Other hydrocarbons

• Cleaning materials

• Rinses

• Residues from plastic containers

• Macroscopic inclusions

• Particles spattered from the starting material

• Dirt, dust, pollen

• Residual gas contamination

• Diffused ions or molecules

• Fingerprints.

It is not surprising that film contamination is important since current film-deposition processes are simply- very dirty methods of growing crystals. The deposition process should be cleaned up by pushing the process to

the current ultimate limit. It is known that some films that cannot be deposited under ordinary deposition conditions can be deposited successfully with sufficient care including long baking of the complete system at high temperatures. Reduction of film contamination by the ultraclean deposition conditions described below is important in obtaining strong adhesion of the film to the substrate and in reducing film stress as well as in lowering the film absorptance. The ultraclean deposition conditions include use of state-of-the-art substrates, experimental deter- mination of the optimum substrate temperature and careful subsequent control of this temperature, baking of the

entire system at the highest practical temperature for long periods of time, use of spattering filters, use of highpurity single-crystal starting materials, and use of state-of-the-art ultrahigh vacuum techniques. At the present

stage of the program, in which the goal is to obtain satisfactory research-laboratory films, it is important to have

all these conditions met absolutely perfectly. Once satisfactory coatings are obtained it is hoped that some of the stringent deposition conditions may be relaxed. Indeed, it may not be currently feasible to manufacture coatings

for the large optical elements needed in some systems unless some of these conditions are relaxed.

Only vapor vacuum deposition should be used in the near-term program. Other deposition techniques may

be interesting, promising, and important, but not for the near-term program.

3. Suggested Measurements

Just as for windows, the measurement of small values of absorptance is a problem. Measurements of ab-

-4 sorptance values in the range of 10 , and as a function of frequency, are needed. The frequency dependence is of course one of the most useful clues in determining the source of absorptance. Recall that by normal transmittance

-2 spectroscopy, measurements of values of absorptance of ~ 10 can be made. For the films of usual thickness,

-4 say 2 fim thick, this is not sufficient since values of 10 are of interest. For films that can be deposited as very

thick layers, say 200 |jm thick, normal spectroscopic measurements could be useful. Calorimetric measurements

-4 are the most useful for measuring small values of absorptance, and it is believed that absorptance values of 10 can be measured with confidence. However, the complete frequency dependence cannot be determined since there is not a continuous range of wavelengths at which sufficiently high power is available.

Attenutated-total-reflection (ATR) spectroscopy is another important method. The film is deposited on the surfaces of an ATR plate, which is a substrate constructed such that the beam makes a number of total internal

reflections in traversing the plate. Since the beam samples the surface at each of approximately 50 total internal

reflections in a typical plate, 50 times smaller values of surface absorptance can be measured. Thus, a film ab-

-4 sorptance of ~2 x 10 can be measured. The frequency dependence can also be determined since this is a spectroscopic measurement. The main difficulty is in interpreting the experimental results, and in particular in obtaining the numerical value of the absorptance.

-205-
Figure 3a is an illustration of the liquid-water absorption spectrum for a water-contaminated thorium-tetraij

fluoride film deposited on a zinc-selenide ATR plate. This spectrum leaves little doubt that there is water contamiii nation in this sample. Figure 3b illustrates one of the difficulties in extracting the value of film absorptance from p the curves and in interpreting the ATR spectrum results in general. Here the transmittance increased when the 3 film was deposited. Thus, even a rough estimate of the absorptance cannot be obtained. a

An important factor that has not been emphasized in previous ATR measurements of film absorptance is thati the angle of the ATR plate should be adjusted for the particular film -substrate combination, rather than using suchj a fixed angle as 45 degrees for all measurements. A simple calculation illustrates that for barium fluoride the inde: of refraction is very close to the value at which total internal reflection occurs for a 45 degree plate. In fact at a 1 wavelength value near 5 y,m the index of barium fluoride drops to the value at which total internal reflection is no \ longer possible, and some leakage out of the ATR plate is expected. It seems likely that the reduction in transmit-} tance from approximately 4 to 8 in figure 3b is due to this lack of total internal reflection.

|

In general the total internal reflection can occur either at the surface of the film or at the interface of the film and the substrate. The angle of the ATR plate should be chosen such that the total internal reflection is at the film surface in order to give the greatest electric field and the greatest absorptance in the film. The conditions on 1 the angle, which are easy to derive and are discussed elsewhere, [1] are as follows: For the case of n^ > n^ ,

' where n, is the index of refraction of the film material and n is the index of refraction of the substrate, total in-

f s

' ternal reflection is possible only at the film surface. In this case the ATR plate angle 6 must satisfy the inequality! 6 > sin ^

( 1/n ), wdiich is the general condition for total internal reflection (for arbitrary values of n and n- ). S S 1

j For the case of ^ , total internal reflection is possible at either the film surface or at the substrate -film , interface. As mentioned above, in order to have the maximum absorptance in the film the total internal reflection should occur at the film surface. This requires that 6 satisfies the double inequality sin (l/n^) <6 < sin (n^/n^).

There is a possibility of distinguishing between absorption on the surface of the film , at the film -substrate ! interface, or in the bulk of the film. The reason is that at the surface of a metal the electric field is very small,

| as illustrated schematically in figure 4. For a dielectric coating that is one-half of an optical wavelength thick, the ! electric field is also nearly equal to zero on the outside surface of the coating. For a quarter-optical-wavelength-

] thick dielectric coating the electric field has a large value at the surface, and for a dielectric coating deposited on I a dielectric substrate the electric field is not equal to zero at either the interface or the surface. Thus, by measuring the absorptance under various conditions it is possible in principle to distinguish between absorption at the

interface, absorption at the surface of the film, and absorption in the bulk of the film. In particular, distinguishing

optical absorption by adsorbed layers from that by other sources should be possible. Either ATR or calorimetric 1 measurements could be used. \

This measurement technique is included in the category of measurements wiiich are interesting, but not for

j the near-term program. There are many promising methods of measuring absorptance including ellipsometry, | emissivity, surface acoustical-wave-type measurements, low-temperature measurements, alpha phones, and a number of others. Even the simplest sounding of these methods has turned out in the past to be technically diffi- |

cult, and the development of new measurement techniques should not be included in the near-term program.

r 4. Material Selection Guidelines and Suggested Materials

-4 Since there are no coatings available with A^ = 10 for any of the infrared frequencies of interest and only a small handful of coating materials have been investigated, it is important to select new candidates with the highest '

probability of success. For window materials, values of the total power through a window under various conditions i were convenient figures of merit [8,9]. The dependence P(j3, a, C, K, • • • ) of the power P, calculated by making

simplifying assumptions, on such parameters as the absorption coefficient /3, material strength a, heat capacity C, < and thermal conductivity K clearly shows the relative importance of these parameters. For example, diamond is a -206- •

\

A
good material in some applications because its great strength and thermal conductivity more than compensate for

ji its high 10. 6fim absorption coefficient. Coatings on the other hand must satisfy a number of independent require- ments. For example, coatings must have good adhesion to the substrate, homogeneity, stability in the atmosphere,

and low absorptance, reflectance, and scattering. Failure to achieve any one of these properties makes the coating

unacceptable, no matter how good the other properties are. An absorptance of 10 would not compensate for the

inability to adhere to the substrate. Thus the figures of merit are replaced by a set of guidelines for selecting candidate materials:

• The value of the bulk absorption coefficient jSj., must be less than a required value such as 0. 5 cm"'^. (Large measured values of film absorption coefficient do not preclude the use of a material since

a major goal of the program is to reduce the values of film absorptance.

)

• The value of the index of refraction nj- must be within the required range. In multilayer coatings, at least one low-index and one high-index material is required.

• The thermal-expansion-mismatch contribution to the film stress must be sufficiently small to prevent

failure. Other information on stress should be used when available. • Nonhygroscopic materials are desirable, although it may be possible that hygroscopic materials can

be used as an inside layer of a multilayer stack.

• High-purity, single -crystal starting materials must be available. The single-crystal requirement

may be relaxed later in the program.

• Materials must be nonradioactive and nontoxic for some applications. • The hardness of the outside materials should be great for coatings that must withstand abrasion,

• Previous experience with the materials in question should be considered, but caution is required to avoid misleading negative results. • The following properties are desirable, but are difficult to predict for new materials that have not been previously deposited successfully in thin-film form:

film packing density

stability (water or other chemical attack, thermal or photo decomposition)

deposition compatibility in multilayer coatings

adhesion to substrate stoichiometry in thin-film form

no adverse diffusion effects at interface homogeneity

.

By using these guidelines, the following best candidates were chosen for 10. 6fim use:

• Na(F,Cl)[10]

• KCl

• (Ba,Sr)F2

• KGaFj and other Cryolite-type materials

• As2 (Sg, Seg

)

• Zn(S, Se) • Til.

The fluorides and chlorides are low- refractive-index materials, and the remaining five materials at the bottom of

the list are high- index materials. These candidates are discussed, and a list of other good candidate materials

and a preliminary discussion of materials for use at 2 to 6^^m are given elsewhere [ 1] •

Of the unacceptable and unlikely candidates for 10.6jitm coatings, the following are important: Materials

containing the light elements H, He, Li, Be, C, N, and O . The light masses are expected to give rise to a funda- mental resonance too near 10. 6/im, making jSj^ > 0.5 cm . Note that the vast number of materials containing oxygen and carbon are expected to be unsatisfactory. Many oxides are known to have 0.5 cm''^ at 10. 6jiim. Although it is not certain that some oxides and carbon-containing materials will not have a sufficiently low absorption coefficient, the generally hig^ absorption at 10. 6ym of these materials make them less likely candidates.

-207-
The absorption coefficients of the fluorides are marginal. Fluorides that are known to have too great absorp

tance at 10. 6 ^m include LiF, (Ca, Mg)I^, -^IFs* NaAlF4 (cryolite). Potassium fluoride is more hygroscopic

than KCl or KBr. Lead fluoride would have to be deposited with greater packing density than that which has been

obtained previously (0. 9). Thallium fluoride is a less desirable candidate since thallium is toxic. The rare-earth

fluorides (Ce, La, Nd, Pr, Yb) F3 probably will not have < 0. 5 cm ^, although this is not certain. Other unlikely and unacceptable materials are listed elsewhere [ 1 ]

1

5. Protective Coatings

Protective coatings for such hygroscopic window and coating materials as alkali halides have been discussed

in the literature for some time [ 11 ], In addition to the prevention of wafer-vapor penetration , it may become ex- tremely important to develop a thin coating to reduce adsorption of water and other contaminants on the surface of optical components. It was shown [1] that information in the literature suggests the following results: (a) It is not

likely that polycrystallLne films can protect large areas of alkali halides from moisture attack [ 11 ]; (b) It is possible, though not certain, that polycrystalline films can be sufficiently dense and sufficiently hydrophobic to allow

-4 = 10 to be attained in carefully prepared films [ 11]; (c) Very thin glassy films were found not to protect

alkali halides [ 11]; (d) Thick glassy films , say l/im thick on well polished substrates, did protect alkali halides. However, on long exposure to humidity, moisture attack that started at the edges of the sample eventually covered

the entire substrate under the coating [ 11 ] ; (e) Polymer films will be considered further in order to determine

the possibility of their providing sufficient moisture protection in spite of the known great moisture vapor trans- mission rates and to determine the possibility of reducing their normally great optical absorption to acceptably Iom values, (f) The possibility of using very thin polymer films as hydrophobic outside layers over antireflection coat

ings to reduce adsorption on the surface will also be investigated, (g) Priority of future research on such amorphous semiconductors as germanium is low since the likelihood of obtaining /Sj less than 0.5 cm ^ is small.

The main concern about poljrmer coatings in the past was that of obtaining pin-hole-free coatings in order to prevent moisture attack of the underlying antireflection coating or substrate. This is indeed important since it is known that moisture will attack underlying substrates through pin holes in polymer films. Unfortunately, in addition to this technical problem there are two fundamental properties of polymer films that possibly could prevent

their use as moisture-protection films.

First, the rate at which moisture vapor permeates through a polymer is one of the routinely measured properties of polymers. Formally using the measured values of this moisture-vapor transmission rate throug^i unsupported (i. e. , not deposited on substrates) polymer films suggests that pin-hole-free polymer films would not be

useful in preventing water from penetrating through the polymer to a substrate or an antireflection coating. However, the moisture-vapor transmission rate could be drastically different for a polymer deposited on a solid from

that of an unsupported polymer film. The results from the literature and from workers in the field are conflicting,

in some cases suggesting that moisture -vapor protection is possible and in other cases suggesting that it is not. We suspect that inhibition of the moisture-vapor transmission at the polymer- substrate interface is important if polymers do indeed provide sufficient moisture -vapor protection.

In considering the problem of moisture-vapor transmission through polymer coatings, it must be recalled

that these coatings are extremely thin and that only two monolayers of water transmitted through the film could cause unacceptably great optical absorptance. According to available data, absorption of water by plastic films

(as opposed to water-vapor transmission through the films) probably will be tolerable at 2.8,3.8,5.3, and 10.6/im.

The second fundamental problem with polymer coatings is that the published infrared spectra of all polymers

-4

that we have found to date show values of absorptance that are too great to allow A^ = 10 for either antireflection coatings or for very thin protective coatings. Professor M. Shen and coworkers at the University of California

at Berkeley have deposited polyethane coatings by a plasma deposition method which is believed to give pin-hole-

-208-
free coatings. [ 12] Measurements of the absorption coefficient of these coatings made at the Hughes Research

Laboratory gave values of jS^ = 3-7 cm"^. Shen and coworkers plan to reduce the contamination of the film in the hopes of reducing this value of the film absorption coefficient.

6. Stresses

The stress in a thin film is believed to consist of three components, one from the mismatch of the thermal

expansion coefficient of the film and the substrate, one from contaminants in the film , and one from an intrinsic contribution. The contribution from the mismatch of the thermal expansion coefficient is well understood, and

very simple calculations give the observed values of this component of stress. For any proposed film -substrate combination this thermal-expansion mismatch component of stress should be calculated in order to determine if the film can withstand this stress. Examples are given In the complete report [1], Unfortunately, the thermalexpansion mismatch contribution to the stress is often less than that of the other two contributions.

Film contamination typically leads to compressive stresses. Campbell [13] states that, "In impure films in

\(4ilch oxygen and other materials are incorporated into the structure during growth, the resultant stresses are usually sufficiently higji to overshadow any of the "pure film" effects. The stresses are then usually compressive, at least in the initial stages of growth and provided that the reacting species migrate through growing film to the interface. Compressive stresses are observed in thermally grown, anodized, and certain sputtered and evaporated

films. If the ion migration takes place from the interface to the film surface, tensile stresses will be observed,"

Of great Importance for the power of stress are the packing density and the water vapor absorption. During exposure to atmosphere very often a stress relief takes place.

The Intrinsic contribution to the stress in a film is not well understood. It is not possible to predict in ad- vance what the stress will be In a new film material on a given substrate. However, for film materials that have

been previously deposited it is possible to predict with a fair degree of success what the stress will be for a given

film- substrate combination or for a particular multilayer coating on a given substrate.

7. Laser Heating and Damage

There are three damage or failure regimes of Interest in general. The first is damage or optical distortion resulting from heating by ordinary linear absorption in the coating. The second is the absorption by macroscopic

imperfections such as inclusions. It is well known that Inclusions are extremely important in laser damage under

short-pulse operation. It has also been pointed out [14] that inclusions can increase the average heating In addition to causing the well known damage. The absorption can, in general, be linear or nonlinear. The third is the

Intrinsic nonlinear damage mechanism such as electron-avalanche breakdown.

The results of the heating and damage studies are very briefly as follows: -4

• For 2 Af = Ajj = 2 X 10 , the film and bulk contributions to the spatial average of the window temperature, vfiiich determines the thermally Induced optical distortion, are equal and have a typical value Tfot = l.OK, which is sufficient to cause thermally Induced opticalrdistortlon failure in some

cases.

• In a typical system with repeated short pulses, only the time-average heating need be considered

(apart from inclusion damage, nonlinear effects and possible more strmgent conditions of future systems).

2 • The linear absorption (below the damage threshold) by strongly absorbing (CTabs = tta ) one-_. micro -radius inclusions with an average spacing of 175 jum gives rise to absorptance Af = lO' . 2 • Inclusion damage is expected to occur at energy densities Itp a: 1-10 J/cm for nanosecond pulses or at Itp af 10 to several hundred joules per square centimeter for microsecond pulses. In principle, values of Itp < IJ/crcr are possible for nanosecond through microsecond pulses. • Intrinsic damage of coatings is expected to occur at I tp as 10^ J/cm^ for a pulse of duration tp = 10|Lis and Af = lO"^ as a result of linear heating of the coating, or at Itp as 100 J/cm^ (or

I as 10 GW/cm2) for a 10 ns pulse of ~1 to 10j;im radiation as a result of electron-avalanche

breakdown or perhaps another nonlinear process. -209-
• The calculated value of 25 K temperature rise for a detached film segment of diameter D = 2 mm

under continuous illumination at 10^ W/cm^ and Aj - 10"^ is surprisingly small.

• The temperature is proportional to for a detached segment.

3 • The thermally induced stress corresponding to T = 25 K has a typical value of 20 MPa ( 3 X 10 psi), ^ which may be sufficient to cause further detachment, then further temperature increase since T~D , and possibly a runaway condition resulting in damage.

2

In pulsed operation, for short pulse durations tp « Tjj, Vi^ere s Cjlf/4K has a typical value of 0,4jxs,

the detachment causes little additional temperature rise since thermal diffusion into the substrate already was neg-l

ligible. For values of energy density near the threshold for failure (of the undetached areas of the film), the size

of the detached area probably will be increased, and various types of fracture both in and out of the plane of the film could cause failure, especially in subsequent pulses.

2 -2 For very long pulse durations tp » Tq, where as C(D/2) /4K has a typical value of 2.5 X 10 s for detached diameter D = 1mm, the above steady-state results are approached. For intermediate pulse durations

Tjj < tp < Tq, the detachment causes a greater temperature rise Ts: IAtp/Ci,f than the corresponding value T = IAtp/C£2;=0» ^^"^ ^z=0 ~ (lTKtp/4C)'''^^, for unin^eded thermal diffusion into the substrate.

8. Acknowledgments

|||

In preparing the report of v^ich the present paper is a summary, comments were solicited from a representative sample of active workers in the field of coatings for high-power infrared optics. 1 am indebted to a number

of people for a great amount of assistance, and I deeply appreciate the time and effort they have contributed. At the beginning of the program the current status of coatings of high-power infrared materials was discussed with the following investigators: Dr. R. R. Austin, Dr. H. E. Bennett, Dr. E.BemalG., Mr. A.Braunstein, Mr. M.Braunstein,-! Dr. T. F. Deutsch, Dr. T. M. Donovan, Dr. C. J. Duthler, Dr. S. Geller, Dr. A. J. Glass, Dr. M. Hass, Dr. A. R.

Hilton, Dr. T. D. Holstein, Dr. J. Kershenstein, Dr. G. E. Kuhl, Dr. J. R. Kurdock, Dr. J. Lehman, Dr. J. H. Marburger, Dr. D. Milam, Dr. P. A. Miles, Dr. T, Norwood, Dr. H. Posen, Dr. G. Ruse, Mr. C. S. Sahagian, Dr. M. Shen, Maj. J. L. Stapp, Dr. C. M. Stickley, Dr. L. Van Uitert, Dr. B. L. Weigand, Dr. R. W. White, and Capt.

H. V. Winsor.

I want to thank the following for their comments on a preliminary draft of the report [1 ] : Dr. R. R. Austin, Dr. E. Bernal G. , Mr. A. Braunstein, Dr. T. M. Donovan, Dr. M. Hass, Dr. L. P. Mott, and Dr. H. Posen. Dr. Guy

Ruse, University of Colorado, stressed the importance of thorough baking of the entire system and pointed out that

this was essential in attaining stable films of certain compounds.

9. References

[1] Sparks, M. , Xonics, Inc. Sixth Technical Report, [8] Horrigan, F. , Klein, C. , Rudko, R., and Wilson, D., Contract DAHC15-73-C-0127, December (1975). Microwaves 8, 68 (1969).

[2] Sadtler Standard Infrared Prism Spectra, Midget [9] Sparks, M., J. Appl. Phys. 42 , 5029 (1971). Edition (Sadtler Research Labs, Philadelphia, Pa., nm c u u i m j ^ c jQCQv » F » » [-jQ"! Such symbols as Na(F, CI) denote the set of materials NaF and NaCl.

[3] CRC Handbook of Lasers with Selected Data on Op- rin v n a rTM- c tj ci n ^oo /imnv

tical Technology, ed. R.J. Pressley (Chemical Ru6- ^^^^ ^^^g' P- A.. Him Solid Films 6. 423 (1970). ber Co., Cleveland, Ohio, 1971). [12] Reis, T. A. , Hirtsuka, S. , Bell, A. T. , and Shen, M.

,

[4] Deutsch, T. F. , J. Electron. Matls. 4, 663 (1975). proceedings (to be published).

[5] Duthler,C.J..J.AppLPhys. 45, 2668 (1974); Proceed- ^^^^ Campbell, D.S. in Handbook of T^in film Technolings of 4th Annual ConferenSF on Infrared Laser ^

I- Ma^ssel and R.Glang (McGraw-HiU,

Window Materials, Tucson, Ariz. , Nov. 18-20, 1974 ^ovY, 1970). (January 1975). [14] Duthler, C.J. and Sparks, M. , J. Appl. Phys. 44,

[6] Klein, P. , this proceedings (to be published). (1973).

[7] Willingham, C. , Bua, D. , Statz, H. , and Horrigan, F.

,

Raytheon Research Division Final Technical Report, Contract DAAH01-74-C-0719, August (1975). -210-
2 3 4 5 6 7 8 9 10 11 12 13 14 15 Wdvelengtti (/xm)

(a) Liquid Water Specttwm

cin"l 4000 1

1 200 5000 3000 2000 1500 1300 1100 1000 900 800 700

Wavelength (/im)

(b) Vapor Water Spectrum

Figure 1. Transmission spectra of (a) liquid water and of (b) water vapor [2],

-211-
2-

3-

OlT (2.75 >im, strong); CO, , , -Uquld 1^20(2. 95 pm, ^=2.9x 10 cm"^) ~ \ CH. and C-H stretching bands of effectively all organic surface contami- ~ nants (3. 43 and 3. 51 itm)

4-

5-

\yCO2

liquid HjO (4. 70>im, 0 = 540 cm"^

)

CN"(4. SOjim) - BO2(5.07>im)

• HF laser

SH (3.86 fxm)

DF laser

CO laser

6-

CO2 (5. 90 fim)

liquid K^O (6. 04 Jim, 2. 9 X 10^ cm"^

)

5 HDO

7-L Nl^"*" (7. 12 pun)

"

I NO2 (7.52 and7.75|um); NO2

8-

HCOg (8.21^ni, i8 2! 10^ cm'^)

- SO^" (8.70j;im)

9- - NOg (9.42fjm); O3

10- _ HCO3 (10.3^01)

r } CO2 (weak) CIO3 , NaClOg

11-

12'

- CrC^" (11.2 Jim)

_ NOg (11.9|im)

_ NO2 (12. 4 Mm)

^C02 laser

Figure 2. Positions of absorption peaks of some possible coating contaminants.

-212-
4000 3S00 3000 2500 2000 1S0O 1600 1400 1200 tOOO WAVENUMBER (CM ^

600 400 250

TbF4 OH ZnSe

ZnSe ON BaF2

Figure 3. Attenuated-total-reflection spectra

[7], (a) This ATR spectrum for a

ThFt, coating deposited on a ZnSe substrate clearly shows the spectrum

of contaminating water, (b) This ATR

spectrum of ZnSe on BaF2 illustrates

the difficulty of interpreting ATR- spectrum results (an apparent in- crease in transmittance when the coating was deposited) and illustrates a decrease in transmittance

with increasing wavelength probably

resulting from a lack of total internal reflection.

Dielectric

Figure A. Schematic illustration of coatings used to distinguish between absorption

at the film surface, in the bulk of the film, and

at the film-substrate interface.

comEm's ON paper bv sparks

Thz question was fioLitd abovut zvZdmci. oj} mtdfi contarru.ncvtion 'in tkin fi-Linn,, ca a ^iincXZon depoiUxon conditiom. HaZ. B&nmtt th& Hich^Lion labonaJjOfLLj Indicoutzd that by a4e o{, eZtipiomeMy,^

one can d&te.cJ: a i^ingle, monolayM. o{, H2O on f^-Ltmi dom to a vapoh. pKUivJie. o^ 10'^ toM. oi H2O. Tkii

monotayeA 4eem6 to be tn zquuLLbnlim, and a idcond layeA dooA not ZmmndMLtzty ioHm. He LndU.c.atzd thaX

tha, obi,eAvatA.on aj, in iabitantlaZ agn.zmznt uiith the. BAunaaeA-Ewimtt-TeMeA thzoKy.*

UokKaJ) BnxxuYUiteAn oi HuQheA, Ru&oAch Labonatofiy Indicatad that in a itudy o^ ThTk {^ibra, canxizd oat at HagheA by Vavz luccaAo, H2O ab6on.ption wa6 det^ctzd antui, uLOia-htgh vaauum de.poi'ition ttchniquU

iMeAe. a6ed. It iaxu> aQiae-d that aaZoHtmetAtc on. ATR. rmaAuAmenti axe, much moKz ie.M>itivz than iptctAoicopy ^ofL detecting weafe impuxity abioKption,

C. B. W-LZLLngham Oj$ Raytheon K&pontzd ob-izAvabZe wateA ab&onptton in the. InS iibni detected by

ATR. He aLio commented that the pfieAence o{^ impafutieA in the coating ptoce64 can tead to attefWitLon&

0)5 the iibn itAuctane, neMjJlting In pofLouUi ^ibni,.

* Handbook 0^ Phy-iici, 2nd Edition, E. U. Condon and H. Odiihaw, Ed6. IMcGfLOW'HUl, Mew Vofik, New VoAk, 7967) pp. 5-96 to 5-715.

-213-
3.2 ALTERNATE MATERIALS FOR INFRARED WINDOW COATINGS

A. D. Baer, T. M. Donovan, A. K. Green and G. Turner Michelson Laboratories, Naval Weapons Center

China Lake, California 93555

The results from studies of a number of new materials (NaF, SrF2, and As2Se3)

to be used in antiref lectance coatings for KCl and NaCl windows are reported.

The preparation and measured properties of thin films of these materials are reported. The properties studied include absorption at 10.6 ym, scattering, structure, index of refraction, and damage threshold. NaF is a promising material in that it has a relatively low absorption and high damage threshold and can be used

as a single-layer antireflectance coating on NaCl . The properties of NaF films are good enough that this material can be used as a component of 10.6 ym coatings.

SrF2 films are higher absorbing, but less easily damaged than BaF2 films, As2Se3

films are more easily damaged than AS2S3 films.

Key words: Alkali halides ; antiref lectance coating; chalcogenide glasses;

infrared optical properties; laser damage; optical absorption; optical scattering;

thin-film coating.

Introduction

Laser systems utilize thin-film coatings to create high- and low-reflectance surfaces. At present,

the performance of optical coatings in a system limits the power-handling capability of that system.

Improvement in coating performance has been sought by investigating design [l-3p, thin deposition [4-6], and the properties of new materials [7]. It is important to identify new materials with properties

which can be used in optical coatings because it may be impossible to fabricate a viable coating with

standard materials. The work reported here includes a study of thin films for some materials which have

not been studied thoroughly heretofore, with special emphasis on the performance of these materials as composites of 10.6 ym antiref lectance (AR) coatings for NaCl and KCl.

Thin films of only a few of many promising materials have been studied carefully [8]. An earlier

study was performed on Ge, MgF2, AS2S3, BaF2,and ZnS ; results indicated that AS2S3 and BaF2 were the best choices for multilayer coatings at 10.6 ym wavelengths based on the protective and power-handling

capabilities [9]. More recent work with 10.6 ym AR coatings has stressed the use of ThFi^ , BaF2 , ZnS,

ZnSe, AS2S3 and Til [10-14]. Thin films of the lanthanide fluorides have been studied very recently [7 Properties of NaF, SrF2, and As2Se3 are discussed and compared in this paper with the properties of BaFj

ThFi^, and AS2S3 thin films.

Many of the properties of NaF are consistent with its use as an optical coating at wavelengths of 10.6 ym. Its index of refraction at 10.6 ym is 1.23, very close to the square root of the index of re- fraction for NaCl [15,16]. Consequently, a single layer of NaF, a quarter-wave thick, would almost

totally eliminate reflections of 10.6 ym light from a NaCl surface. This is the simplest possible de- sign for a broad-band AR coating. Moreover, NaF appears to perform well with high-intensity pulses of light; a study of the performance of NaF with 1.06 ym light in 30 ps pulses showed that the damage

threshold of NaF is exceptionally high, and the nonlinear index of refraction is exceptionally low [17]. One possible disadvantage of NaF is its relatively high solubility in water; however, the solubility of NaF though large is only .12 of NaCl's [16], and NaF films can be used in applications where the humidity of the environment can be controlled.

Low water solubility is an important advantage of both As2Se3 and SrF2. The solubility of SrF2 is

0. 1 that of BaF2 and 3 x lO""* that of NaCl [15,16] while As2Se3 is insoluble in water [18]. Moreover,

As2Se3 and SrF2 also seem promising because similar materials (AS2S3 and BaF2) have been used successfully as composites of AR coatings at wavelengths of 10.6 ym [9]. The indices of refraction for these materials are such that a zero-reflectance coating on NaCl or KCl must consist of two or more layers.

In order to further verify the usefulness of these materials, low-absorbing three-layer coatings [1-3]

composed of AS2S3 and ThFi^, and of As2Se3 and NaF were designed, fabricated, and tested. An As2Se3/NaF/As2Se3 with 10.6 ym absorptance of .06% is reported below. This absorptance is lower than

that of an As2S3/ThF4/As2S3 coating (1,2%), because NaF is lower absorbing than ThF^.

Experimental

Thin films of NaF, SrF2 , BaF2, ThF^ , As2S3,and As2Se3 were deposited on NaCl and KCl windows, then

evaluated with calorimetric and transmission measurements. Transmission electron microscopy, total

int egrated scatte r, and laser damage measurements were performed on selected films. The measurements

1. Figures in brackets indicate the literature references at the end of this paper. -214-
and sample preparation were standardized as much as possible to facilitate comparison of the performance of the various materials.

Single-crystal NaCl and KCl windows, 38.6 mm in diameter, were obtained from Harshaw Chemical Co. These windows were etched for three minutes in concentrated HCl prior to deposition. The absorptances

of these windows after the etch, but prior to deposition, were 3-8 x 10"^ for the KCl windows and 7-15 X

10"'* for the NaCl. The [100] direction of the crystalline lattice in the windows was found to diverge

at least 7° from the direction normal to the face of the window. All samples were handled and stored in

the same way.

Depositions were performed in two different trapped, diffusion-pumped vacuum systems and an ionpumped system according to the conditions outlined in table 1. The evaporators were loaded with materials from the following sources: pure AS2S3 and As2Se3 provided by Cerac, and single crystals of NaF,

SrF2,and BaF2 prepared by Harshaw Chemical Co. As table 1 shows, different sources and conditions were

employed for different films. In order to compare pairs of materials, some AS2S3 and As2Se3 films and

some BaF2 and SrF2 films were deposited in the same systems under identical conditions. The properties

of these paired films are those discussed in the following section. NaF was prepared in all three deposition systems with equivalent performance for films prepared in two of the systems, but with high

absorptance for films prepared in the third system. Water absorption bands were visible in the trans- mittance traces of the highly absorbing films. Measurements on NaF films reported here were performed

on those samples for which water absorption bands were not visible in the transmission traces.

Spectral dependence of transmission was measured using a Perkin-Elmer 180 spectrophotometer, a two-beam instrument. All but one of the transmittance measurements reported here are absolute. An

uncoated KCl window was placed in the reference beam when the transmittance of the As2Se3 film deposited

on a KCl window was measured. For this measurement, 100% transmittance is defined as the transmittance

of an uncoated window.

Table 1. Deposition conditions for thin films.

Material Source Substrate

Temperature

Base Pressure

^Pa)

Deposition

Pressure

(Pa)

Deposition

Rate

(nm/s)

AS2S3 Quartz Flask with

Tantalloy Element,

75°C

E Beam

c 10-^ -4

7 X 10 0.5-2

^2^^3 Quartz Flask with

Tantalloy Element

,

75°C

E Beam

7 X 10-6 7 X 10-^ 0.5

ThF.

4 Mo Boat 150 A 150°C 7 X 10"^ -4

1 X 10 0.5-5

NaF E-gun, Mo Boat,

and Bn Crucible

75°C 7 X 10"^ 11 X

10"'^ 0.1

SrF^ C-Crucible 3A0°C 1 X 10"^ -4

7 X 10 0.1

BaF2 C-Crucible 150°C

(25°-340°C)

1 X 10"^ -4

7 X 10 0.3-3

Absorptances of the windows were measured calorimetrically before and after coatings were applied.

This was done by measuring the rate of temperature rise due to power absorbed with a 60 watt beam of light in a 10.6 um laser [10,14,19]. The beam, 6 mm in diameter, was shown through the center of the window. Absorptance measurements were performed three times, the window being rotated by 120° about

its axis between measurements; the absorptance of the sample was taken as the average of these measurements and the uncertainty of the absorptance was defined as the standard deviation of the measurements.

The spread in the three values of absorptance, 1-8 x lO"'* with typical values of 4 x 10"'*, appears to be

due to nonuniformity in the windows. These uncertainties can amount to as much as 80% of the value

of low absorptance coatings and are the primary limitation of the accuracy of the absorptance measurement. Consequently, the 1-2% corrections associated with the non-ideal heat flow in the 38.6 x 6 mm

windows has been neglected [20]. In estimating the absorption constant from absorptance, interference

effects have been included [10,21]. These estimates are accurate to 10% when not limited by substrate

nonuniformity . When the standard deviation due to nonuniformity is significant, it will be given

explicitly.
Laser damage measurements were performed using 100 ns pulses from a CO2 TEA laser focused to a 200 iiu spot as has been discussed elsewhere [22,23], These data are included to illustrate their cor- relation with optical properties of the films. With the exception of NaF films where a range of damag^

thresholds are given, the laser damage and absorptance values reported below were measured on the same [ films. r

Properties of Single Layers 1

Transmittance spectra of windows coated with thin films of NaF and As2Se3 are presented in figures

and 2. The clean fringe patterns displayed by these curves show that the films are not heavily contam-' inated with impurities. The transmittance curves peak at the 100% line across the infrared band, indicating the films absorb less than 5 x 10~^ from 2.5 to 12 ym. The flatness of the NaF transmittance

curve around wavelengths of 10.6 ym illustrates the broadband AR action for a single layer of NaF. Indices of refraction of BaF2 and SrF2 are close enough to those of KCl and NaCl that strong fringes

are not present. Some of our BaF2 and Srli'2 films showed water absorption bands. All films of these

two materials were deposited in one particular deposition system. It may be that all films of these

materials are significantly contaminated with water, even though structure indicating water absorption

is not present on the transmittance spectra. Water contamination could be the source of the high

absorption constant found here for BaF2 and SrF2 thin films.

The absorption constants of most films studied here were substantially higher than constants for bulk samples of the same material. Absorption constants at 10.6 ym have been reported to lie between

0.5 and 1.5 cm~^ for bulk samples of AS2S3, NaF, and SrF2. While the absorption constants shown in table 2 for the AS2S3 are close to' bulk values, the constants for the NaF and SrF2 films are far higher

than the bulk values. The extra absorption in all poly crystalline filmd including NaF may be due to contamination of the films or may be associated with the interfaces between crystallites in the layers;

in either case, it may be possible to reduce absorption in the films by optimizing deposition parameter

Even so, the absorption constant of the NaF films is almost low enough that coatings with absorptances less than 5 x 10"** can be produced. Coatings with such low absorptances will be possible if films with absorption constants less than roughly 2 cm"'^ can be deposited; that is, the absorption constants in the NaF films must be reduced only a factor of three, in order to obtain a film with com- pletely acceptable performance.

Table 2. Absorption constants and damage thresholds. Measured with 10.6 ym laser radiation.

The range given for the threshold of NaF films represents damage thresholds for several films.

FILM/WINDOW INDEX 6(cm"l) (J/cm2)

NaF/NaCl 1.23 6.0 + 0.4 38 + 7 ThF4/KCl 1.35 10-12 SrF2/KCl 1.36 25 194 BaF2/NaCl 1.38 15 76

As2S3/NaCl 2.37 1.2 + 0.6 38

As 2 Se 3 /KCl 2.72 1.1 + 0.4 12

The damage thresholds of most films listed in table 2 are high enough that the films could be

used in practical coatings. Thresholds of all the films except As2Se3 are comparable in size to the thresholds found for the best polished surfaces of windows: KCl windows, 73 J/cm^ and NaCl windows,

110 J/cm^ [23] . The damage thresholds of SrF2 and BaF2 films are actually the same or larger than

the polished surfaces, although thresholds of films of the other materials are lower and should be

increased if possible.

The shape of the damage sites in the AS2S3 films indicates a way in which the damage thresholds

might be increased. At each damage site one or more damaged areas were present which were centered

about small pits, the same size and density of ym-sized defects in the films. Apparently the defects nucleated the damage. It was difficult to determine the shape of the defects through a microscope; however, similar defects found in annealed AS2S3 films were large enough to show facets characteristic of crystalline structure. Crystallites of AS1JS3 have been reported in AS2S3 films grown by

physical vapor deposition [26]. The pm-slzed defects in our films may be of similar origin, but

whatever the nature of the defects, their mmber and size is a strong function of deposition conditions

and surface preparation of the window. Consequently, there is a good chance that a way can be found to produce films with fewer defects and which have a higher damage threshold. A second interesting aspect of the data listed in table 2 is that the damage thresholds are not Inversely correlated with linear optical absorption. If the films were damaged because of thermal

effects associated with linearly absorbed power, one would expect that a high absorptance would produce

-216-
I a low damage threshold. This is exactly opposite to the trend indicated by the data in table 2; high

i| damage thresholds were found for films with high absorption constants. We interpret this as showing

that pulse-damage thresholds are not related to linear absorption. Rather, the critical parameter

is the magnitude of the electric field in the material as found by Bettis and coworkers in their 1.06 ym

damage studies [24,25]; the damage thresholds at 10.6 um do tend to be inversely correlated with index

of refraction as suggested by these authors

.

Optical scattering is another physical process which could limit the performance of coatings.

This is a very real possibility for materials such as NaF which usually form polycrystalline thin

films. Interfaces between the crystallites cause optical scattering. NaF films, 2.16 ym thick, were

deposited 9n unoriented single-crystal windows and on windows oriented with their faces normal to the

[100] direction. The performance of films deposited on unoriented windows is emphasized here; however,

we did find that films deposited on oriented windows were not polycrystalline while those deposited on unoriented windows were polycrystalline with crystallite sizes on the order of 250 nm. These polycrystalline films scattered enough visible light that they had a hazy blue cast, but apparently the crystallites are small enough that the infrared light is not significantly scattered. Measurements of the

total Integrated scatter of NaF films deposited at two different temperatures are listed in figure 3. About 10""* of the incident beam was scattered by the films deposited at 75''C. Consequently, for applications at wavelengths longer than 3 um optical scattering does not appear to be a serious performance

limitation of polycrystalline films.

Antireflectance Coatings

Two- and three-layer antiref lectance coatings composed of the films studied here were deposited on

KCl windows and the absorptance and transmittance of each window was measured. The measured absorptances are listed in figure A along with that of a single, quarter-wave thick layer of NaF and the calculated absorptance of the films. The calculations were performed assuming that the absorption constants

of the layers in the multilayer coatings were equal to those measured for the same materials deposited

as single layers [2]. Except for the As2S3/ThFi^/As2S3 coating, the calculated and measured absorptances

are essentially equal. This shows that the films can be deposited in multilayer configurations with the same absorptive properties as in single layer configurations. The absorptance of a single NaF layer can

be seen to be somewhat high. The advantage of the three-layer configurations is obvious; the actual

absorptance of the three-layer coating is half that of the two-layer coating, and the theoretical

absorptance is less than one-third that of the two-layer coating. The absorptance of the coating composed of As2Se3 and NaF is lowest because of the lower absorption constants of these materials.

Transmittance spectra measured on the two three-layer coatings are shown in figures 5 and 6. The

clean shapes of these curves show that contamination of these coatings is low. By comparing these

figures with figure 1, the bandwidth of the AR action of the three-layer coatings can be seen to be less

than that of a single, quarter-wave layer of NaF.

Conclusions

Both SrF2 and NaF show promise for applications as components of multilayer coatings for use with

infrared lasers. SrF2 films have a very high damage threshold and an absorption constant which can be

reduced. NaF films function well as a single-layer AR coating on NaCl and have the damage threshold and

absorption constant almost good enough; deposition conditions for improved NaF films may well be

discovered. Optical scattering of the polycrystalline films is not an important limitation on performance of the films in the far Infrared. The damage threshold of As2Se3 is quite low; unless a way is found to deposit As2Se3 films with a substantially higher damage threshold, AS2S3 is a better material

for high- index layers in multilayer coatings.

Acknowledgments

The authors gratefully acknowledge the technical assistance and advice of P. C. Archibald, E.J. Ashley, C. L. Barker, D. L. Burdlck, J. H. Dancy.and M. J. Soileau as well as the suggestion that NaF

films be studied by W. Reichelt. This work was supported by the Defense Advanced Research Projects

Agency, the Energy Research and Development Administration, and the Naval Weapons Center independent

research f6nds.

-217-
References

[1] Braunstein, M. et al. in 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. 135.

[2] Baer, A. D., "Design of Three-Layer Antireflectance Coatings," these Proceedings.

[3] Starling, J. et al. in Proceedings of the Fifth Annual Conference on Infrared Laser

Window Materials: 1975, op, cit., p. 144.

[4] Golubovic, A. et al. in Laser Induced Damage

in Optical Materials: 1974, A. J. Glass and

A. H. Guenther, eds. (NBS Spec. Publ. 414,

1974) , p. 76.

[5] Golubovic, A. et al. in Laser Induced Damage

in Optical Materials: 1975, A. J. Glass and

A. H. Guenther, eds. (NBS Spec. Publ. 435,

1975) , p. 236.

[6] Baer, A. D. et al. in Laser Induced Damage in

Optical Materials: 1975, op. cit., p. 244.

[7] Golubovic, A. et al. in Proceedings of the Fifth Annual Conference on Infrared Laser Window Materials: 1975, op. cit., p. 168.

[8] Black, P. W. and J. Wales, Infrared Physics 8_, 209 (1968).

[9] Young, P. A., Thin Solid Films 6^, 423 (1970).

[10] Willingham, C. B. et al^ in Proceedings of

the Fifth Annual Conference on Infrared Laser

Window Materials: 1975, op. cit., p. 356.

[11] Braunstein, A. I. et al. in Laser Induced

Damage in Optical Materials: 197S, A. J. Glass and A. H. Guenther, eds. (NBS Spec.

Publ. 387, 1973) , p. 151.

[12] Chaffin, J. H. and R. A. Skogman in Proceedings of the Fifth Annual Conference on Infrared Laser Window Materials: 1975, op. cit., p. 155.

[13] Donovan, T. M. and A. D. Baer in Proceedings

of the Fifth Annual Conference on Infrared j Laser Window Materials: 1975, op. cit., ! p. 291.

I

[14] Gibbs, W. E. K. and A. W. Butterfield, Appl. ; Opt. 14_, 3043 (1975) .

'

[15] Dickinson, S. K. in Infrared Laser Window \ Materials Property Data for ZnSe, KCl, NaCl, ^ CaF^, SrF2 and BaF2 (Air Force Cambridge I Research Lab, Bedford, Mass., AFCRL-TR-75- i 0318, 1975) . j

[16] Moses, A. J. in Handbook of Materials Prop-

| erties. Vol. 1 (Plenum Press, New York, ' 1971).

[17] Smith, W. L. et al. in Laser Induced Damage

in Optical Materials: 1975, op. cit.,

p. 321.

I [18] Handbook of Chemistry and Physics, R. C. , Weast, ed. (Chemical Rubber Co. , Cleveland, \ Ohio, 1969), p. B-83.

| [19] Loomis, J. S. in Laser Division Digest (Air Force Special Weapons Center, Kirtland AFB , , N. Mex., TR-LRD-72-1, 1972), p. 29. I

[20] Bernal, Enrique G., Appl. Opt. 14_, 314 | (1975). i

[21] Baer, A. D. in High Energy Laser Mirrors and \ Windows (Naval Weapons Center, China Lake, I Ca., NWC-TP-5845, 1976), p. 120.

j

I [22] Soileau, M. J. et al. in Proceedings of the ' Fifth Annual Conference on Infrared Laser ' Window Materials: 1975, op. cit., p. 391.

i [23] Soileau, M. J. et al., "Laser Damage Studies

j of Low Index Coating Materials at 10.6 ym,"

these Proceedings.

[24] Bettis, J. R. et al. in Laser Induced Damage

in Optical Mateidals: 1975, op. cit.,

p. 289.

[25] Bettis, J. R. , et al. in Laser Induced Damage in Optical Materials: 1974, op. cit.,

p. 214.

[26] Braunstein, M. et al. in Laser Window Surface Finishing and Coating Technology

(Hughes Research Laboratories, Malibu, Ca.

AFCRL-TR-74-0425, 1974), pp. 15-19.

-218-
NiCC WINDOW

(BOTH SIDES COATED. 0.25 WAVES AT 10.6;iml

8 9 10 12 15 20 30 50 200 WAVELENGTH, iim

Figure 1. Absolute transmittance measured on a NaCl window coated with a 2.16 ym

thick layer of NaF on both sides.

Figure 2. Relative reflectance of a KCl window

coated on one side with a 2,24 ym

thick layer of As2Se3, The transmittance

is ratioed to that of an uncoated KCl

window.

KCe WINDOW

(1 SIDE COATED. 0.5 WAVES AT 10.6fim) m

I. -I

-t- f

if -art- I-'- : 4 Li

r TT

r Ht —

r

I I 1 I 1 1 5 6 7 8 9 10 12 15 20 30 SO 200 WAVELENGTH. im\

WAVELENGTH, T, °C

(DEPOSITION! FORWARD

3.39 3.39 RT

75°

5.49 X 10-*

1.06 X 10-*

10.6

10.6 RT

75°

5.28 X 10-3

1.33 X 10"*

Figure 3. Total integrated scatter measured

on KCl windows coated with a 2.16 ym

thick layer of NaF, Films were deposited at room temperature (RT) and

75°C.

LAVERS MATERIALS ABSORPTANCE, 10-3 (MEASURED) ABSORPTANCE. 10'^ (CALCULATED)

Calculated and measured absorptance 1 NaF 1.8 of 10.6 ym light for various types 2 AijSj/ThF, 2.5 2.5 of antireflectance coatings. The 3 A.2S3A-hF4/Af2S3 1.2 0.7 multilayer coatings were deposited

3 Ai2S«3/NaF/Ai2Sa3 0.6 0.6 on KCl windows and the NaF coating was deoosited on an NaCl window.

-219-
KCC WINDOW

(BOTH SIDES COATEDI

100

80

US

.1. r /-

WAVELENQTH, im

Figure 5. Absolute transmlttance measured on a KCl

window coated on both sides with an

AszSs/ThFit/AsaSa coating.

(ONE SIDE COATED)

Figure 6. Absolute transmlttance measured on a KCl

window coated on one side with an A82Se3/NaF/As2Se3 coating.

comEms ON paper sv baer, vonovm, gheen, m tuhner

It McUt pointed oiU that low Intzmity abioKption m&ciiuA.mznti n&zd not cofLn.t6pond to thz abioKption

which cAixiu dcmiQz at high inteniity. NonlimoA abioiption pKoczMU my pJtay an impoAtant lotz.

-220-
3.3 DESIGN OF THREE-LAYER ANTIREFLECTANCE COATINGS

A. D. Baer Michelson Laboratories, Naval Weapons Center China Lake, California 93555

Design constraints are less stringent for three-layer coatings than for two-layer coatings. Mathematical relationships which govern the design of threelayer antiref lectance coatings are presented. These new relationships are analogous to those governing the design of two-layer coatings; however, a continuous

range of solutions exists. Three-layer solutions are represented by closed curves

in coating-thickness space, while two-layer solutions are represented by two discrete points. Armed with a knowledge of the performance of similar designs

with similar materials, one can select the three-layer solution with lowest absorptance (within 10%) . Three-layer coatings offer greater design flexibility than

two- layer coatings not only because of the wider-range of solutions, but also

because of a wider choice of viable materials. Three-layer antireflectance coatings can be formed entirely of low-index materials (BaF2, NaF) , or with the low

index material next to the window (AS2S3, NaF).

Key words: Abscrptance; antireflectance coating; bandwidth; coating design;

multilayer coating.

1. Introduction

Three-layer antiref lectance coatings have been receiving attention recently because, at infrared

wavelengths, the coatings have lower absorptances than do other antiref lectance coating [1-3]^. For

example, consider coatings composed of NaF and As2Se3 thin films. The calculated absorptance is 0.2%

for the best two-layer coating, but only 0.05% for the best three-layer coating. The absorptance of

the three-layer coating is lower because the third layer makes it possible to partially compensate for

the high NaF absorption [1,2]. As illustrated by this example, design flexibility is the primary

advantage of three- layer coatings, although the nature of the flexibility is not well understood. The

present work aims to clarify three-layer design by detailing the properties of solutions of the threelayer design problem, and by showing how the performance of the various three-layer coatings can be

systematically investigated.

Three-layer antireflecting coatings have been designed using several approaches. One approach sets

the thicknesses of the layers equal to a quarter wavelength, and determines conditions on the indices of refraction which must be satisfied for the reflectance to be zero [4,5]. It is usually not possible to find a set of materials for which these conditions are satisfied exactly, so that in practice the reflec

tance of the coatings will not be zero, although it may be quite small. This method of design is at a disadvantage if only a few viable coating materials are available, and if coatings with extremely low

loss must be produced. Coatings with lower loss can be obtained if the thicknesses of the layers are

not restricted a priori. Design without such restrictions has been done using two approaches: a computer optimization method [1,2], and the Thetford method [5,6]. The computer optimization method determines layer thicknesses for which loss in the coatings is minimized; this method has the advantage that

a minimum-loss design can be determined even if a zero reflectance solution does not exist. The Thetfor

method determines layer thicknesses for zero reflectance by utilizing vector calculations, an approximat

method, and a systematic trial-and-error approach. A third method, used here, is based on closed form

equations not previously published, which give solutions for zero reflectance at one wavelength; the

advantage of this method is that a single calculation gives a layer thickness, and that all solutions

which exist can be determined and examined.

2. Design Equations

The following development of three-layer design parallels the textbook developments of two-layer

design [4,5]. The materials are assumed to be lossless, and two equations are derived which must be

true if the reflectance of the coating is to be zero. A zero-reflectance requirement sets two conditions because both the real and imaginary components of the Fresnel coefficient must vanish. The two conditions are expressed in the form of two equations which determine families of solutions; solutions

with layer thicknesses differing by a half-wavelength belong to the same family. Layer thicknesses for

1. Figures in brackets indicate the literature references at the end of this paper. -221-
r

The optical thickness is

each family of solutions are determined by a three-step calculation which utilizes the two equations.

The following s)rnibols will be used in the design equations:

i =

nj index of refraction of layer j r Fresnel reflection coefficient

rjj^ Fresnel coefficient of isolated interface

6 phase of Fresnel coefficient

X wavelength in free space

tj thickness of layer j (meters)

Xj optical thickness of layer j (wavelengths)

The Fresnel coefficient for an isolated interface is [5]

jk = - V- (i>j

}

X. = n.t./X. (2)1

The various media associated with the three- layer configuration will be numbered as shown in figure 1. The two equations to be derived will relate X2 and X^ (or equivalently t2 and tj) in terms of X3 and

the rjj^ for the three interfaces shown in figure 1.

These equations follow directly from Smith's theorem [5-8]. This theorem is derived by asking what,

conditions must be satisfied in order for the transmission of a multilayered structure to be unity. The! structure is divided conceptually into two parts and the Fresnel coefficients for reflections from each

part are examined. Reflectance will be zero only if the magnitudes and phases of the two coefficients

are related as shown in figure 2. Derivation of design equations will consist of determining r^ and r^,

and then using the relationships in figure 2 to establish the equations.

In invoking Smith's theorem we are already asstiming that the various materials are lossless. If this is a bad assumption, the thicknesses we calculate will be wrong, and the reflectance of the coating!

will not be close to zero when the materials have their normal amount of absorption. A coating of particular design can be tested by calculating the reflectance of the coating, putting in reasonable values;

for the absorption constants. This has been done for several designs using standard computer programs;

the reflectance at the chosen wavelength was less than 10~^ for these designs. For the low loss materials used in these designs (absorption constant less than 10 cm"-'), design errors due to the assumptionj

of losslessness are completely negligible. Losses will be neglected throughout the development of the design equations, and added back in later when performance is calculated.

In applying Smith's theorem, we will conceptually divide the three-layer coating at its center

layer. The structure remaining on either side consists of a single thin film separating two media.

The formula for the Fresnel coefficient of this kind of structure is well known [4,5], and in the nota- tion used here gives

T21 + rioexp(-4TrXii)

r = (3) a 1 + r2irioexp(-4TTXji) r23 + r3L,exp(-4TrX3l) r = . (4) b 1 + r23r34exp(-4TTX3i)

Smith's theorem states that the magnitudes of r^ and must be equal. If one equates the square of the magnitude of these coefficients as given by eqs. (3) and (4), and then solves for Xj , he finds

Xi = i arc cos

4tt

-21 + 1^10 - i^bl^ ^ 4Ao^'

(5)

The phase constraint shown in figure 2 is a simplification of the requirement.

which reduces to

cos(6 + 6^ - 4ttX,) = 1 (6) a D

^

X = (6 + 6^ + 2Trm)

2 4tt a b m = 0, ± 1, ± 2, . . . (7) -222-
Equations (5) and (7) are the design equations which make it possible to compute the thicknesses of the layers in a three-layer antireflectance coating.

Before attempting to apply these equations, visualize the solutions plotted in three-dimensional

space. The thickness of each layer is represented by a point. There are a series of solutions, the points falling on a line in layer-thickness space. Consequently, a range of solutions exists. In applying these equations, we will have to deal with the half-wavelength ambiguity of sinusoidal functions,

and the sign ambiguity of an arc cos function, and the range of solutions. In order to select a single

solution on a line, we will begin by picking a value for X3.

The three steps in the calculation of layer thicknesses are as follows: (1) Select a value for X3.

(2) Use this value of X3 and eq. (4) to compute r^j, then substitute the magnitude of r\, into eq. (5) and compute the two possible values of Xi. (3) Select one of the values of X^ and compute r^ using

eq. (3), then compute X2 using the phases of r^ and r^ and eq. (7). In performing these calculations,

make the X's lie between 0 and 0.5. This can be done by adding or subtracting 0.5 to the values. By

keeping the X's between 0 and 0.5 we are selecting two lines of solutions, and in step (3) we select one

of these two lines. The recipe given here will produce all solutions, (Xj , X2, X3) , with layer thick- nesses between 0 and 0.5. Because of the sinusoidal ambiguity, solution space has the same sort of symmetry as a crystal with translational periodicity of 0.5 wavelength. Consequently, solutions for values of X larger than 0.5 can be found by increasing the approximate X's an appropriate number of half

wavelengths.

Half wavelength manipulation of solutions is possible because of a well-known property of lossless

thin films: The reflectance of a stack of thin films does not change when the thickness of one of the films in the stack is changed by exactly half a wavelength. In the present case this effect manifests

itself by the way Xi, X2, and X3 are involved in eqs. (4), (5), and (7). As far as these equations are concerned, (X + 0.5) is the same as X. This is true because the equations are based on the reflectance

of thin films of lossless materials at one wavelength.

If one considers absorption and spectral dependence in addition to the reflectance at one wavelength,

then the solutions are no longer equivalent. Clearly, increasing the thickness of a layer by half a wavelength increases its absorption, and this increase will be at least (gX/2n), where g is the absorption

constant of the material [9]. Solutions with the lowest absorptance must belong to the set of solutions

with the thinnest layers, those with X between 0 and 0.5. Only these solutions will be considered in the remaining sections. Even without absorption, the spectral dependence of different thickness is different.

This follows from eq. (2) which shows that a particular change in wavelength produces a change in X which

is proportional to thickness. Reflectance is determined by the X's. Therefore, because (dX/dX) is smallest for the thinnest layers, coatings with the thinnest layers have the largest bandwidth.

3. High-Low-High Coatings

In this section solutions are presented for coatings with the indices of refraction of the layers

ordered higher, lower, and higher than that of the window. Figure 3 shows an example of this class of coating; it is typical of the three-layer coatings reported which have been studied for use on infrared

windows. Although the equations derived in section 2 are quite general, only examples of coatings for which the outer and innermost layers are composed of the same materials will be presented here. Solutions for these examples are expected to be very similar to solutions for coatings composed of three

different materials.

The graph in figure 3 is a plot of layer thicknesses for various solutions. This plot was created

by selecting a value for X3 and then calculating Xi and X2 using the recipe of section 2. Both Xx and

X2 are plotted on the same graph as a function of X3, X2 being represented by the closed curve in the center of the graph, and X^ being represented by the curves at the top and the bottom of the graph. For each choice of X3 there are two choices for the pair, (Xj , X2); the dashed lines in figure 3 represent one set of values, and the solid lines the other set of values. As an example of the use of these

curves, we will find a solution for a coating with X3 equal to 0.3, selecting the solution represented

by the dashed lines, the values of the dashed curves at 0.3 gives X2 equal to 0.04, and Xi equal to 0.13. The graph in figure 3 represents every possible solution for which the X's are positive, but less than

0.5.

Since two-layer coatings are just special cases of three-layer coatings with a zero thickness for one layer, the two-layer solutions must be included with those plotted in figure 3. Examining figure 3, one finds that the Xj curve crosses zero at two different values of X3. It happens that the X2 curve is single valued at this value of X3, so that there are exactly two points (X2, X3) for which Xj is equal

to zero. These points are represented by triangles in figure 3, and are the solutions to the two-layer

problem.

The solutions for a different high-low-high coating are shown in figure 4. Even though these

solutions are for a different wavelength and different materials, the shapes of curves in figure 4 are strikingly similar to those of the curves shown in figure 3. There are two reasons for the similarity:

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First, by dealing with X instead of t, we have eliminated all wavelength effects except changes in the index of refraction; and second, high-low-high coatings have similar solutions. Since solutions for ' high-low-high antireflectance coatings are so similar, one can obtain a good general understanding of I the solutions and their performance by examining only a few examples.

| Performance can be readily examined by using thickness of a layer to label a particular solution, | Specification of the dashed or solid lines and specification of X3 completely specifies the solution.

In the performance studies which follow, X3 will be used to label solutions, and a particular line of ' solutions will be examined. !

I

I The performance studies will be formulated as design problems. The first design problem is to ' select the solution with the lowest absorptance. If the typical coating materials are to be used (the | low- index materials are most highly absorbing) , an almost optimum coating can be selected using figure ^ The absorption constant of ThFi^ is six times that of AS2S3 so that the solution with the lowest absorpJ

tance must be one in which the ThFi, is thin. According to figure 4, X2 is thinnest for solution 0.32 j and this solution should have low absorptance, although it may not have the lowest possible absorptance

because of absorption in the AS2S3, and interference effects. The importance of these factors can be I seen by examining the actual absorptance of the coating. Absorptances of the solutions represented by ' the dashed curves in figure A were calculated and plotted in figure 5. This was an exact calculation

absorptance, using realistic absorption constants for the materials. As shown in figure 5, the absorptance of solution 0.29 is lowest, although that of solution 0.32 is not much higher. Evidently the thickness of the highly absorbing middle layer is the factor vrtiich is most important in determining the

absorptance of the coating.

In order to guess which solution has the lowest absorptance, one must know in a qualitative way ho*' the absorptance depends on X3. This dependence is determined by the refractive and absorptive properties of the materials. For the materials discussed above, the dependence of absorptance can be inves- [ tigated by examining the dependence of X2 on X3, because X2 is closely related to the absorptance. In figures 3 and A, X2 depends on X3 in much the same way. With increasing X3 , X2 decreases and reaches a! broad minimum between 0.3 and O.A. Moreover, the equations of section 2 show that the dependence of X2

is determined by the relative sizes of the indices of refraction. This shows that the dependence of X2 will be similar to that shown in figures 3 and A for any high-low-high coating, and if the low-index

layer is highly absorbing, the dependence of the absorptance will be very similar to that shown in figure 5. The absorptance will reach a minimum for values of X3 near 0.3, and will be close to minimum)

for a wide range of values of X3 around the minimxim. In particular, if X3 is chosen to be 0.3, the absorptance will be close to minimum. Given this relationship, one does not have to perform an extensive calculation in order to identify the solution with the lowest absorptance. For all practical

purposes, solution 0.3 will have the lowest absorptance. Of course this applies only to the absorptance?

of high-low-high coatings in which the absorption constant of the middle layer is substantially higher

than those of the other layers.

The second design problem considered here is to select the solution with minimum absorptance, but

| which also has certain minimum bandwidth. Bandwidth is defined here in terms of loss due to reflectance|

and absorption in the coating. The spectral dependence of the loss of solution 0.3, dashed line of figure A, is plotted in figure 6. The curve labeled "reflectance" includes only losses due to reflections while that labeled "total" includes loss due to absorption as well as reflection. Bandwidth will

be defined as the width of the spectral region for which the total loss is less than 0.5%. A solution

is to be found for which the bandwidth is larger than 1 pm.

The bandwidth of various dashed-line solutions from figure 3 were calculated using existing compute

programs and plotted in figure 7. All effects, absorption and interference, are included in these calcu

lations. Bandwidth is plotted as a function of X3. As can be seen in figure 7, the bandwidth is greate' than 1 ym for X3 less than 0.095 and greater than 0.48. The optimum solution is the one for which X3 falls in these ranges and has the lowest absorptance. According to figure 5, solution 0.095 has the lowest absorptance, 0.15%. The bandwidths calculated for the solid-line solutions are much narrower

than those of the dashed-line solutions. As argued previously, any solution with X greater than 0.5 haa

a narrower bandwidth than that of a thinner, half-wave-equivalent solution. Therefore. solution 0.095 isj the answer to the second problem.

The two design problems just discussed illustrate how performance can be systematically optimized

using X3 as a label. A second advantage to the present approach is that all three-layer solutions for which reflectance is zero are displayed. This makes it possible to determine the existence of solutions[

to various problems. For example, suppose that given the materials in figure A, one were attempting to design a symmetrical three-layer combination with a Herpin equivalent index such that the combination

would serve as an antiref lectance coating for the window [A]. The critical requirement is that Xj and ' X3 be equal. Since there is no solution in figure A for which Xj is equal to X3 , no perfect symmetrical

antiref lection coating exists. As another example, suppose that one were attempting to design a threelayer coating such that the reflectance is zero at two different wavelengths. Figure 3 and A show that

the solutions have only one degree of freedom. Since the requirement of zero reflectance at a second

^ wavelength sets two additional constraints, the problem is overconstrained and there is no general

-22A-
solution. Solutions exist only for particular choices of the Indices of refraction. This analysis

suggests that there are no solutions for which the reflectance Is exactly equal to zero In either of

these examples. These problems are best approached using some sort of computer program which determines

solutions for minimum lose, given certain constraints.

4. Existence of Solutions

The existence of solutions help determine which mathematical approach will be most fruitful, and

which materials can be used. In this section, degrees of freedom and constraints are discussed. Conditions on the Index of refraction for existence of solutions will be set forth for three-layer coatings

In which the outer and Inner layers have the same Index of refraction. These existence conditions are compared with those for two-layer antlreflectance coatings composed of the same materials. With some materials combinations. It proves possible to form a three-layer antlreflectance coating, but not a twolayer antlreflectance coating. Solutions for a coating with one of these extra materials combinations

are presented and discussed.

Each layer of a coating has two degrees of freedom, one degree for the Index and one degree for the

thickness of the layer. As discussed previously, the requirement that reflectance vanish at some wavelength eliminates two degrees of freedom. A one-layer antlreflectance coating has two degrees of freedom

and two constraints, leaving zero degrees of freedom. A solution exists only for a particular Index and

thickness. A two-layer antlreflectance coating has four degrees of freedom and two constraints, leaving

two degrees of freedom. If the thicknesses of the layers In the coating are not fixed, then solutions

exist for pairs of Indices of refraction x«hlch range over two-dimensional areas of layer-Index space. This Is utilized In the Schuster plot, discussed below. In which the two Indices of refraction corresponding to a two-layer coating are plotted In order to determine whether a solution exists [4,5]. If, on the other hand, one fixes the Indices of refraction by choosing materials, then there are zero degrees

of freedom and the solutions are represented by points In layer-thickness space. A three-layer antlreflectance coating has six degrees of freedom and two constraints, leaving four degrees of freedom. Even If the Indices of refraction are fixed by choosing the materials, one degree of freedom remains and

the solutions are lines In layer-thickness space. If the layer thicknesses are not fixed, then solutions exist over three-dimensional regions of layer-Index space. In what follows, our attention will be

limited to only those three-layer coatings for which the Inner and outermost layers have the same index

of refraction. In this case one degree of freedom has been eliminated as the existence of solutions to this three-layer problem can be determined using a two-dimensional plot directly analogous to the

Schuster plot.

In order for a solution to exist. It must be possible to meet the requirements of Smith's theorem.

These requirements are related to the Indices of refraction by way of the various rj;^ which appear In

eqs. (3) and (4). Since the thickness of the middle layer can always be chosen so that eq. (7) is satisfied, the critical requirement is

IrJ - IrJ . (8)

According to eqs. (3) and (4), the magnitudes of and r^, vary periodically with Xi and X3, respectively.

Each Fresnel coefficient assumes a range of values with the maximum and minimum value determined by the

indices of refraction. If it is ever going to be possible to make eq. (8) true, then these ranges must

overlap; that is, eqs. (9) and (10) must hold.

I'^almax^ I'^blmm

I'^blmax ^ I'^almin '

When the magnitudes of r^ and r^, are maximum or minimum, they are real functions of the indices of refraction. Equations (9) and (10) can be reduced to a collection of simpler Inequalities involving the

indices of refraction. The derivation is lengthy and will not be reproduced here. Rather a graphical

summary of the results will be presented.

Two-layer coatings and the class of three-layer coatings considered here are composed of two materials. Each set of materials is represented by a point on the graph of figure 8, with the ordinate

given by the index of the material in the innermost layer, and the abscissa given by the index of the

other material. Various lines divide the graph into regions. Most of these lines apply to both threeand two-layer coatings, but one line applies to only three-layer coatings. Existence of a solution

depends on whether the point representing the coating falls into an appropriate region. When applied to two-layer coatings, this type of graph is the Schuster plot [5]. Two-layer coatings do not exist in the

region marked with a question mark, but we have not determined whether three-layer coatings exist in

this region. The inequalities required by eqs. (9) and (10) reproduce all the lines which apply to the

two-layer coatings, but a third line, given by

n /n (11)

with the n's defined as shown in figure 8. This line and the horizontal dashed line define new regions

in which three-layer solutions exist, but two-layer solutions do not exist. New combinations of viable

-225-
coating materials fall into one of these regions; that is, practical three-layer coatings can be formed

from materials that cannot be used to form two-layer coatings.

| Coatings composed of various practical infrared coating materials are represented by points on ^ figure 8. The standard high-low-high coatings are represented by the three horizontal lines at values

| of no corresponding to ZnS, AS2S3, and As2Se3. Points on these lines represent specific low-index materials. There are new combinations for which Uq must be less than the index of the window. Any high- oij low-index material is suitable for the middle layer as long as its index is a bit larger than that of Points falling on the line in the new region of figure 8 represents the combination of a variety of low-|

index materials with NaF. These low-medium-low combinations represent a new class of coating with new ! combinations of materials, and a different form of solution.

j Solutions for one of the low-medium-low coatings are plotted in figure 9. The dependence of the thickness of the middle layer on X3 is much the same as that of the solutions shown in figures 3 and 4 On the other hand, the dependence of Xi on X3 is quite different for the two types of coating. The "outer layer" curve in figure 9 never crosses the axis so that there is no solution for which the thicfc ness of any layer vanishes. As advertised, there is no two-layer solution for this combination of coat-j

ing materials. !

I The preceding discussion points out that additional materials combinations are possible with three- layer coatings and offer a guide to determining which particular combinations will work. Low-medium-loHj combinations may prove useful at infrared wavelengths because the damage threshold of low-index is higher than that of high-index materials. The combinations may also prove useful at ultraviolet wavelengths where transparent, high-index materials do not exist.

Acknowledgments I

The author gratefully acknowledges the contribution of Dennis Burge and Joe Haberman, who employed

their computer programs to calculate absorption, reflection and transmission of various optical coatingsj

This work was supported by the Defense Advanced Research Projects Agency, the Energy Research and

Development Administration, and the Naval Weapons Center.

References

[1] Braunstein, M. , et al. , "Laser Window Surface

Finishing and Coating Science," Semi-Annual

Technical Report 2, AFCRL-TR-76-0048, Hughes

Research Laboratories (1976)

[2] Braunstein, M. , et al. , in Proceedings of

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), pp. 135-42.

[3] Baer, A. D. , et al. , "Alternate Materials

for Window Coatings," these Proceedings.

[4] Cox, J. T. and Hass, G. , "Antireflectance

Coatings for Optical and Infrared Materials,"

Physios of Thin Films, Vol. 2, G. Hass and

R. E. Thun, eds. (Academic Press, New York,

1964), p. 239.

[5] Macleod, H. A., Thin Film Optical Filters

(American Publishing Co., Inc., New York,

1969).

[6] Thetford, A., Optica Acta 16, 37 (1969).

[7] Smith, S. D., J. Opt. Soc. Am. 48, 43 (1958)

[8] Smith's theorem is nowhere articulated in i precisely the form used here, but is used in i all the references cited. Following Macleod,! we ascribe the theorem to Smith. It can be

| derived by writing down the expression for i transmission in the lossless case, and deter-|

mining the conditions which r^ and r^, must satisfy if the transmission is unity.

[9] Baer, A. D. , "Absorption in Optical Coatings

Interference Effects," in High Energy Laser

Mirrors and Windows (Semi-Annual Report No. 7-8, ARPA Order 2175, Naval Weapons Center, NWC TP 5845, 1976), pp. 128-35. I

i

-226-
7 . Figure^

0 1 2 3 4 AIR

AIR XI X2 X3 WINDOW

Figure 1. Numerical labels of

the media associated

with a three-layer

coating. These labels

are used in subscripts

to denote layers or interfaces.

TRANSMISSION WILL BE 100%

ir AND ONLY IF

(6. + *b)

AND MEDIA ARE LOSSLESS

Figure 2. Mathematical conditions which must be

satisfied for a coating to have no

loss due to reflection or absorption

of light.

ANTIREFLECTANCE COATINGS (3.8fiin)

0.1 0.2 0.3 0.4 LAYER III THICKNESS (WAVELENGTHS)

A^S.3

Figure 3. Antlreflectance solutions for an AszSea/NaF/AsaSes coating deposited

on a CaF2 window. The wavelength of zero reflectance is 3.8 ym, and the indices of refraction were assumed

to be 2.72 for AsaSes, 1.32 for NaF,

and 1.41 for CaFa.

ANTIREFLECTANCE COATINGS (10.6|im)

Figure 4. Antlref lectance solutions for an

As2S3/ThFi,/As2S3 coating deposited

on a KCl window. The wavelength of zero reflectance is 10.6 ]im, and

the indices of refraction were

assumed to be 2.37 for AS2S3, 1.35 for ThFu, and 1.46 for KCl.

z

I-

(9Z

uj

-I 0.4

lu>

<

» 0.3

<

- 0.1

ai 0.2 0.3 0.4 LAYER III THICKNESS (WAVELENGTHS)

III A.,S,

KO

-227-
_1_ "0 0.1 0.2 0.3 0.4 0.6 OPTICAL THICKNESS OF INNERMOST LAYER

(WAVELENGTHS)

Figure 5. Calculated loss of the solutions

plotted in figure 4. Reflectance

losses were less than 10"^. The

absorption constants were assumed

to be 1.7 cm"' for AS2S3 and

10 cm-^ for ThFi,.

•a 0.3 p

REFLECTANCE

10.6 WAVELENGTH, tm

(Figure 6. Spectral dependence of the total loss and the reflective

loss of solution 0.3, dashed

line, of figure 4. Trans- mission bandwidth is defined

as shown.

—1

: 1 BANDWIDTH (0.5%/SURFACE)

Aj^Sa/ThFj/AljSj ON KCI

TOTAL LOSS

0.1 0.2 0.3 0.4 OPTICAL THICKNESS OF INNERMOST LAYER (WAVELENGTHS)

Figure 7. Transmission bandwidth of the solutions represented

by the dashed lines in

figure 4.

WINDOW

KCI

1.0 vs. Z.O 3.0

„ (LAYER. INDEX)

Figure 8. Plot for determining the existence of solutions to the three-layer problem

for various combinations of coating

materials. The window is KCI. The index of refraction of the outer and inner layers is n^; the middle layer,

xi^, and the window, n„. Solutions

exist when a point falls in one of the unshaded areas.

-228-
ANTIREFLECTANCE COATINGS (10.6(im|

Figure 9. Antiref lectance solutions for a NaF/ThFi, /NaF

coating deposited on a KCl window. The wavelength of zero reflectance is 10.6 ym and the indices of refraction were assumed to be 1.32

for NaF, 1.35 for ThF.,, and 1.46 for KCl.

COHMENTS ON PAPER EV BAER

No diicuM-ion on tlvu, papeA.

-229-
3.4 CHARACTERISTICS OF PLASMA POLYMERIZED ETHANE

FOR LASER WINDOW COATING* !

}

\ T. A. Reis, H. Hiratsuka, A. T. Bell, and M. Shen Ij Department of Chemical Engineering

University of California I Berkeley, California 94720 I

Thin film coatings were produced by passing ethane through a radiofrequency glow discharge in a tubular plasma polymerization

reactor. Infrared absorption characteristics of the plasma

polymerized ethane (PPE) depends on the reactor conditions. Absorptions are large for C-H stretching (3.5 ym) and bending (7.0 ym)

frequency regions. However, outside of these regions the absorptions are low. Under appropriate polymerization conditions, the

absorption coefficient at 10.6 ym for PPE film on polished, finished

KCl crystal is 4-7 cm ^. The refractive index for PPE is 1.51 (at

sodium D line) , which is higher than that of KCl. Thus the use of

multilayer coating for good antireflection is indicated. Moisture

resistance was tested by exposing the coated crystals at room temperature and 84% relative humidity for 100 hours.

Key words: Laser window coating; plasma polymerized.

1. Introduction

In the search for optical coatings for high energy infrared laser windows, most

of the attention has been directed toward the inorganic materials. The requirements

for such a coating are that it has low absorption characteristics in the 10.6 ym and

3-5 ym regions, is antiref lective, and moisture resistant. Most organic polymers havei

not been considered viable candidates for such coatings because of their generally

high absorptions in the infrared region, and the difficulty of depositing very thin

^ and adherent films on the alkali halide substrate. However, it has recently been i pointed out that by the use of plasma polymerization such difficulties can be cir- ^ 1

* cumvented [1] . By passing an organic vapor through a cold plasma generated by a i radiofrequency glow discharge, thin, adherent, and pinhole free films can be readily 1 deposited on a variety of substrates. These plasma polymerized materials are generally highly crosslinked amorphous networks. By carefully controlling the molecular

structure, it is possible to produce coatings that have low absorptions in much of f the infrared region. The purpose of this paper is to present some of our most recent

results in the studies of plasma polymerization of simple hydrocarbons and of the

properties of the resulting films. \

2. Experimental

Tubular flow plasma polymerization reactors were used in this work. The design

of a glass tubular flow reactor has already been described previously [2]. We have now constructed another tubular flow reactor out of Lucite. Although most of the

data here were collected from the Lucite reactor, the glass reactor was also used.

Plasma polymers of similar structure and properties can be prepared by either reactor

using the appropriate reaction parameters.

Figure 1 shows that the Lucite tubular flow plasma reactor consists of a rec- tangular channel, which is the reaction chamber, surrounded by a protective cylinder

and held in place end plates. Two parallel plate electrodes are inserted inside

the reaction chamber with a fixed separation of 3 cm. Sample substrates are placed

on a slide-in-plate which can be easily introduced into and removed from the bottom

electrode. Both electrodes can be cooled by circulating fluids. The discharge is

at 13.56 MHz. CP grade ethane gas was purchased from the Matheson Gas Company and

* Work supported by the Defense Advanced Research Projects Agency, monitored by

the Air Force Cambridge Research Laboratory.

1. Figures in brackets indicate the literature references at the end of this

paper.

-230-
w^s used as received. Before use the reactor effluents from air and inert gas discharges were analyzed by mass spectrometry. The composition was found to be nearly

identical to that of the glass reactor, indicating the essential inertness of Lucite

to the plasma conditions.

Mechanically polished crystals of potassium chloride were purchased from Harshaw

Chemical Company. No further treatments were applied before use. The KCl crystals

were placed on the bottom electrode in the reactor where the plasma polymerized film

wa^ deposited. Deposition rates were determined by weighing methods [3] . Film

thickness was estimated by assuming the density of the polymer to be unity. Infrared

spectroscopic analysis of the coated crystal was conducted with a Perkin-Elmer 467

spectrophotometer

.

Humidity-resistance testing was carried out by exposing the crystals to the

constant humidity atmosphere produced over an aqueous salt solution. After 100 hours,

crystal surfaces were examined by optical microscopy to determine the effects of the

exposure

.

Refractive indices were determined on an Abbe refractometer by depositing the

plasma polymer on a cut glass ( n = 1.518). 1-Bromonaphthalene was used as matching

liquid, which has a refractive index of 1.658.

3. Results and Discussion

3.1. Plasma Polymerization of Ethane

The form of the plasma-polymerized materials can be a film, an oil, or a powder.

The general form of the polymer is related to the polymerization rate and whether the

reaction occurs in the gas phase or on a surface. A powder generally results from

a rapid gas phase reaction, while a film is generally the result of a slower surface

reaction [3]. Thus, prior to any routine film depositions for any application, it

must be determined under what reaction conditions that a good film can be produced

for a particular reactor configuration. These conditions can best be summarized on plots known as characteristic maps [2-4]. The usual system variables that are chosen

in the development of the characteristic map are the monomer flow rate and the total

system pressure with the power as the parameter.

The characteristic map was constructed by depositing the polymer on an aluminum

foil substrate attached to the slide-in-plate . The polymer coated foil was examined

visually and tactilely after the deposition of polymer in order to qualitatively characterize the form of the polymer, and to determine the amount of the foil covered by

the polymer. Characteristic maps for the Lucite tubular reactor at two different

powers using pure ethane are shown in figure 2. Increasing the power from 200 watts

to 300 watts shifts the boundaries and increases the regions somewhat, but the basic

form of the characteristic map remains unchanged.

After the characteristic map was constructed for this particular reactor, the

reaction conditions for the production of a good film could be determined, and the

deposition rate of the film as a function of the various system variables could be

explored. The variables of interest are the monomer flow rate, the total system

pressure, and the deposition power.

The effect of the monomer flow rate on the deposition rate is shown in figure 3. The existence of a maximum in such a plot is characteristic of plasma polymerization

[2]. The deposition rate of polymer as a function of the total system pressure is

shown in figure 4. The corresponding weight of film deposited is also shown in the

same figure. Film as well as oil and powder form part of the polymer deposited above

a pressure of about 1.5 torr. This is the form of polymer observed on the characteristic map at lower flow rates and above that pressure (see fig. 2) . The effect of

the deposition power on the deposition rate was investigated from a threshold power

of about 50 watts to about 450 watts, which is near the maximum output of the RF

generator. From figure 5 it can be seen that even with reaction conditions that

produce a maximum deposition rate, an 800% increase in the deposition power results

in only about a 35% increase in the deposition rate. The corresponding weight of

film deposited is also shown in the same figure, revealing that the corresponding

increase is about 40%.

-231-
3.2. Moisture Resisteince Studies

c The plasma-polymerized ethane films deposited on KCl crystal surfaces were tested

for moisture resistance by exposing the film coated surface to a controlled humidity

environment for 100 hours. The coated and uncoated crystal surfaces at various con- ditions of exposure were examined and photographed at 256X magnification using a standard transmission binocular microscope. The photographs that were taken are i shown in figure 6 (a) -(d). Figure 6a shows the surface of a new, uncoated, and ' unexposed crystal that may be used as a comparison standard. The crystal was me- chanically polished, but not etched by HCl [5-7], thus surface scratches are numerous. The surface on which plasma polymerized ethane (~ 1 ym thick) has been deposited is seen to be very similar to the original surface (fig. 6b) . Exposure of a film coated

crystal to room humidity (55-60% RH) for 100 hours does not appear to deteriorate the ^ film surface as shown in figure 6c. A definite film deterioration is evident when a film of about the same thickness (~ 1 \m) is exposed to 84% RH in the constant humidity chamber for the same length of time as shown in figure 6d.

Comparison of figures 6a and 6d shows that most of the deteriorations of the

crystal occur at the scratches and imperfections. It has been demonstrated in a previous electron microscopic study [8,9] that while more gentle variations on the

surface of substrate can be evenly coated by a plasma polymerized film, sharp edges

and small holes ceinnot. Thus the surface imperfections on these unpolished crystals

are in fact not protected by the plasma polymerized film, thereby resulting in the

observed deteriorations. Studies of plasma polymer coated crystals that have been

finished are now underway.

3.3. Infrared Absorptions of Plasma Polymerized Ethane

Figure 7 shows an example of the infrared spectrum of plasma polymerized ethane.

The strong absorptions at 3.4 ym, 6.9 pm^and 7.3 ym are, respectively, due to C-H

^ stretching, C-H bending in CH2, oind C-H bending in CH^. Ideally if the polymer

j consists of completely saturated aliphatic chains, then these characteristic bands

would be the only absorbing regions. The absorptions in the region of 7.5 ym to

12 ym, which covers the 10.6 ym wavelength, may be ascribed to two types of non-

| aliphatic, non-hydrocarbon structures in the polymer. The first type may have re- sulted from molecular rearrangement in the plasma. In a recent paper we have demonstrated that plasma polymerized materials produced from unsaturated hydrocarbon

monomers also contain double bonds and aromatic groups not originally present in the monomer [10]. These functional groups strongly absorb infrared radiation. To wit

C=C stretching frequencies are in the region of 5.0 - 6.2 ym, out of plane C-H deformations in unsaturated hydrocarbons are between 10-11 ym, and C-H vibrations of

mono-substituted phenyl groups are near 14 ym. The second type of structures may be

attributed to the oxygenated groups. It has been shown that initiation of plasma

polymerizations in a radiofrequency discharge is dominated by the free radicals, some

of which are trapped in the polymer matrix and cannot readily terminate. These radicals are long-lived and are easily oxidized to form hydroxyl, carbonyl, and other oxygenated fiinctional groups, which also contribute strongly to the infrared absorption

[11]. For example, the 0-H stretching is at 3.0 ym and the carbonyl stretching at

5.9 ym. In addition, increased cibsorption in the 8 - 10 ym region can be expected

as a result of 0-H deformation and C=0 stretching vibration in alcohols and alHyl

ketones.

In order to minimize these undesirable side effects, plasma conditions (slower

rate of deposition) are used to minimize molecular rearrangement and radical trapping.

A completely linear hydrocarbon polymer would have a chemical structure of-^CH2-J-»

with a H/C ratio of 2.0. Increased crosslinking and unsaturation would decrease this

ratio. Figure 8 shows that in fact the H/C ratio is quite low at high deposition

rates, but increases rapidly with decreasing deposition rate to approach 2.0. In

addition, the reduced rate of polymer deposition also favors the recombination of

radicals. The annihilated radicals will render the polymer less susceptible to oxidation. Figure 9 shows that absorption due to the oxygenated group C=0 (at 5.9 ym)

continually decrease with decreasing rate of depositon, and at low rates the absorption at 5.9 ym falls below the limits of detectcibility of the spectrometer. Both

these factors combine to reduce the absorption at 10.6 ym (fig. 9).

The best PPE film was shown in figure 9 to have an absorbance of about 0.5%

per ym thickness at 10.6 ym. These results were, however, obtained on the infrared

spectrometer using polished, but unfinished crystal substrates. A more sensitive and

-232-
elegant technique to detect low absorptions is microcalorimetry . PPE was deposited

in this laboratory on the surface of a finished KCl crystal kindly supplied by Dr.

M. Braunstein of Hughes Research Laboratory and then was returned to his laboratory

for microcalorimetric measurements. Preliminary results indicate an absorption co-

-1 -4

efficient (3) of 4 - 7 cm , and an absorption index (k) of 3.5 - 6 x 10 for our

-3 -4

PPE film. By comparison we note that k = 2 x 10 for SrF2, 5 x 10 for BaF2 and

1 X 10~^ for ThF^. These results are gratifying in indicating that plasma polymerized ethane film is in fact a viable candidate for optical laser window coatings.

Further improvement in lowering the infrared absorption may still be possible

using the pulsed plasma technique. It is hoped that radical concentration may be

further decreased through recombination during the pulsed periods when the plasma is

turned off, thereby resulting in simpler polymer structures. As we have just discussed, reduction in the number of unsaturated and oxygenated groups will lower the

absorption in infrared radiation.

3.4. Antireflection Characteristics

The reflectance (R) of a KCl substrate coated with PPE can be computed using the

following equation [12 J

:

, ,o ^1^+ 2r,r^ cos 2<1) ^ R=|r|2 = ^__i- 1 (1)

1+r,isr + 2r,r cos 24), Is 1

where r^^ and r^ are the Fresnel coefficients of the coating and the substrate, respectively:

^1 = ("o-^l'/^^o"""!'

^s = ('^l-'^s^/^^l+^s^

and n^, n^, and n^ are the refractive indices of the air, coating and substrate, d^

is the thickness of the coating and ^ is the wavelength. By using the Abbe refractometer, the refractive index (n^^) of plasma polymerized ethane was found to be 1.51

at sodium D line. For X = 10.6 ym and n = 1.49 (KCl) , the reflectance of plasma

s polymerized ethane coated potassium chloride is calculated as a function of (n^^d^^) , the results of which are shown in figure 10. It is not surprising that PPE actually

increases the reflectance of KCl, since in this case the condition of n^<n^

'^'^s' is not satisfied. To achieve low reflectance, PPE must be used as a coating for

substrates with higher refractive indices. Figure 11 shows that n^ should be arovind

2 in order for PPE to act as an antireflection coating.

An alternative consideration is to use multilayer coatings [13,14]. For a two

layer coating, the reflectance is (for n^^d^^ = t^2^2^ ["i-^] :

R =

2 Sj^+acos2(t) 2+bcos 2(j) 2 ^2)

2 S2+acos2<j) 2+bcos 2c|) 2 where

S, = ri2H.r22H-r3^r,2r22r32.2r^r^(l-r22)

S2 = H-r^2^2'-^^2'^sH'^l'-2^1^s(l-^2')

-233-
a = 2r^r2(l+r^')+2r2r^(H-r^2j

b = 4r^r^

^2 = (ni-n2)/(n^+n2)

= ("2-''s>/<"2+^s^

*1 " *2 ^TM2*^2'^^'

Subscript 1 refers to the coating exposed to the air and 2 to that adjacent to the

substrate s. The reflectance changes with i> as follows:

dR (n,-l) (S„-S,)

=J ± ^—±

^ ^

(3)

d(2(})2) (n2+ng) (n-j^+1) (S2+acos2(t)2+bcos 2(t)2)

where

^2"^1 " (1-^1^) (1-^2^^ (l-r3^) >

The sign of this slope is determined by the discriminator J:

J = (n2-ng) (cos2())2 -Q) sin 2(J)2 (4)

where

Q

=

2 2 2 (n,-n,) '1 "2' (n» ^"2 -n, n^ ) (5) "1 s

"2—

T (nj^+n2) (n2 -n^ ) (n^^ -1)

We can now apply the preceding results to our plasma polymerized coatings on KCL for any fixed wavelength. In the case of PPTFE/PPE/KCl , n^^ = 1.38, n2 = 1.51,

n^ = 1.49, therefore, Q = 1.615. The value of the refractive index for plasma

polymerized tetrafluoroethylene (PPTFE) was taken from the work of Hollahan, et al.,

[15], The dotted line in figure 10 shows that a minimum reflectance of 0.012 can be

expected theoretically at quarter wavelength. However, for the case of PPE/PPTFE/KCl

(where the order of the layers is reversed), n^^ = 1.51, n2 = 1.38, and thus Q = 0.352

Now it can be seen in figure 10 (broken line) that minimum reflectance occurs at a much thinner layer of coating (~X/12) . These results indicate that it is possible

to obtain low reflectance in double layer coatings much less than the quarter wavelength thickness. In addition, it is apparent that the parameter Q is very useful

in designing coatings for optical applications.

4 . Acknowledgments

We are indebted to Drs. M. Braunstein, S. D. Allen, and J. E. Rudisill of Hughes

Research Laboratories, Malibu, California for the microcalorimetric measurements.

-23A-
5. References

[1] Tibbitt,J. M. , Bell, A. T. and

Shen, M. , Proc. Fifth Annual Conference on High Power Infrared Laser

Window Materials, Air Force Cambridge Research Labs., 1975, p. 205.

[2] Kobayashi, H. , Bell, A. T. and

Shen, M. , J. Macromol. Sci,- Chem.

,

AlO , 123 (1976)

.

[3] Kobayashi, H. , Bell, A. T. and

Shen, M. , J. Appl. Polymer Sci.,

17, 885 (1973)

.

[4] Kobayashi, H. , Shen, M. , and

Bell, A. T. , J. Macromol. Sci.

Chem. , A8, 1345 (1974)

.

[5] Davission, J. W. , Proc. 2nd Conf.

on High Power Infrared Laser

Window Materials , Air Force

Systems Command, 1972, p. 525.

[6] Allen, S. D. , Braunstein, M.

,

Giuliano, C. and Wang, V., Proc.

Symp, Damage in Laser Materials,

1974.

[7] Turk, B. R. , Pastor, R, C. , Timper,

A. J. , Braunstein, M. and Heussner,

G. K. , Proc. Fifth Annual Conf. on High Power Infrared Laser Window

Materials, Air Force Cambridge

Research Laboratories, 1974, p. 103.

[8] Niinomi, M. , Kobayashi, H. , Bell,

A. T. and Shen, M. , J. Appl. Phys.

44, 4317 (1973)

.

[9] Niinomi, M. , Kobayashi, H, , Bell,

A. T. and Shen, M. , J. Appl.

Polymer Sci., 1£, 2199 (1974).

[10] Tibbitt, J. M. , Shen, M. and Bell,

A. T. , J. Macromol, Sci.- Chem.,

in press.

[11] Tibbitt, J. M. , Bell, A. T. and

Shen, M. , J. Macromol. Sci.- Chem.,

AlO, 151 (1976)

.

[12] Cox, J. T. and Hass, G. , in G. Hass

And R. E. Thun, eds. , Physics of

Thin Films, Vol. 2, Academic Press,

New York, 1964, p. 239.

[13] Braunstein, M. , Braunstein, A. I. and Rudisill, J. E. , Proc. 2nd Conf. on High Power Infrared Laser Window

Materials, Air Force Systems

Command, 1975, p. 777.

[14] Loomis, J. S., Proc. 2nd Conf. on High Power Infrared Laser Window

Materials, Air Force Systems

Command, 1972, p. 799.

[15] Hollahan, J. R. , Wydeven and Johnson

C. C. , Appl. Opt., 13, 1844 (1974).

6. Figures

Figure 1. Schematic diagram of the tubular plasma polymerization reactor.

-235-
200 W

, Pump Limil

20 —1

'

1 ' 1 ^ 1

40 60 80 100 120 Monomer Flow Rale (STPcc/min)

Figure 2. Characteristic map for plasma polymerization of ethane in the Lucite

tubular plasma polymerization reactor.

too

1000

X 500

E

•se

Q)O

cr. co

55o

a.

0) o

T I

I lyi '

I

1 1

1

I M I

LFR, 100% Conversion

5 10 50 100 Monomer Flow Rate (STP cc/min)

Figure 3. Deposition rate of ethane as a function of monomer flow rate.

Ethane

Power = 200 W

F = 10 STP cc /min

Time = 30 min

Decreosing Coveroge • Film Film, Oil, Powder

Figure A. Deposition rate of ethane as a function of pressure.

^ 160 30 o

. 140 e

-f 120

20 S ^ 100

O.

ft) Q 1 80

«

ft> o 60

(r 10

E 1 40

iZ S 20

ft) o

4 5 6

Pressure ( Torr

Ettione P = 1.0 Torr

F = 10 STP cc /min

Time = 30 min

100 200

r40

-30

20

10 :!

300 400 SOO

Power ( Walts

Figure 5. Deposition rate of etnane as a function of power.

Figure 6. Photomicrographs of (a) uncoated KCl

crystal unexposed to humidity; (b) plasma

polymerized ethane coated KCl crystal un- exposed to humidity; (c) plasma polymerized ethane coated KCl crystal exposed

to 55% humidity for 100 hours; (d) plasma

polymerized ethane coated KCl crystal ex- posed to 84% humidity for 100 hours (coat-|

ing thickness 'v 1 ym)

-236-
Frequency (cm"') 4000 3200 2400 1800 1400 1000 800 600

inni '

' 1 1 1 1 1 1 1 1 1 1 ill I I I I I II II

I

Figure 7. Infrared spectrum of plasma

polymerized ethane.

o

X

Figure 8. Hydrogen/carbon ratio as a function of

deposition rate of plasma polymerized

ethane.

1.7

1.6

1.5

1.4

Plasma Polymerized Ethane

Deposition Rate (mg/cm h ) x I0 20C

25 50 75 100

Deposition Rote (mg/cm^-h ) i 10 ' 125

I Figure 9. Infrared absorbance at 10.6 um as a function of deposition rate for plasma

polymerized ethane.

-237-
1

0.08

0:03

0.02 -

0.01 -

T—I—I—

X; .. (PPTFE/F>PE)/KCI

PPTFE/KO'v;.

J I L J L

X/8

n,d, =n2d2

X/4

0.14

0.12

0.10

0.08

K 0.06

0.04

0.02

1.5 2.0

d, (ym)

Figure 10. Calculated reflectance of single layer

plasma polymerized ethane coated KCl

(solid line) , single layer plasma

polymerized coated tetrafluoroethylene KCL (chain line) , double layer

PPTFE/PPE/KCl (dotted line) and PPT/PPTFE/KCl (broken line) as a function of optical thickness.

Figure 11. Reflectance of plasma polymerized ethane coated crystals

of various refractive indices

as a function of coating

thickness.

COMMEWrS OM PAPER BV REI5

It imi iuggutzd that to te^t ^oa the. abimcz

the. abiznce. zZzcjUwlytxc condaation u)hm the. {^ilm

HIVfiJSUKA, BELL, AMP SHEW

p-in-holej) In {^Ztmi, om ihoiM not fieJiij onZy on

-Us -umeAJ>eA In a. {hLtd, fattt ihouZd tut tkz i-Llmi,

-238-
3.5 EPITAXY AND OPTICAL SCATTERING IN Till FILMS*

R . A . Skogman

Honeywell Corporate Research Center

Bloomington, Minnesota 55420

Thallium iodide has been identified as a protective high index coating

material for potassium chloride laser windows. T£l has a high refractive index

and an optical absorption of less than 1 cm"'^ at 10.6 \m. The protective

quality of Till lies in its insolubility and remarkable adhesion to potassium

chloride. This adhesion is due, in part, to the epitaxial growth of Til on KC£. THI microstructures resulting from different epitaxial growth habits are responsible for the optical scattering observed in films grown on certain KCJl orientations.

We have found that the alignment of the T8.I b^ axis parallel to the KCJ. <110>

direction nearest the substrate surface determines the epitaxial growth habit. On

the precise KCS, (100) orientation, for example, two KCJl <110> directions lie in

the surface. This causes the Til to nucleate in two orientations rotated 90° from

one another. This unusual microstructure results in scattering by the diffraction

of light passing from one biaxially birefringent orientation to another.

This model has been demonstrated for all KC£ orientations and is well supported

by x-ray data and polarized light microscopy. A practical solution to this problem

has been found which routinely yields visibly clear T2.I coatings.

Key words: Biaxial birefringence; epitaxial microstructure; optical scattering,

1. Introduction

TS-I has been demonstrated as an excellent protective high index coating material for KCl!. laser windows.

Humidity resistance of Til is excellent and films adhere extremely well to KCJl substrates. T£.I films have absorptions of less than 1cm ^ at 10.6lJm and coatings of Til have withstood 9 seconds of

80 kW cm ^ of CO2 radiation without damage [1]I Anti-reflection coatings prepared with TJlI have met

reflectivity requirements. Till is a biaxially birefringent material, but the effects are negligible at

10.6 V'm[2]. Cloudiness in certain Til films, though not considered a factor at infrared wavelengths,

is of concern because of its unpredictable occurrence. An investigation of the causes of cloudiness was

performed along with the investigation of the material's other properties.

The results of this investigation fall into two categories. The first category includes the cloudiness caused by the usual deposition process variables, i.e., substrate cleanliness, source conteuninatlon.

etc. The second category consists of cloudiness caused by an interesting phenomenon unique to birefringent materials capable of epitaxial growth.

Cloudiness due to process variables will not be discussed here. It should be stated, however, that

state-of-the art thin film evaporation procedures and newly developed KC£. substrate preparation

techniques will routinely yield clear T?I films. The exception to this statement is the subject of this paper

.

2. Optical Scattering in Epitaxial Till

It has been determined that certain KCJl orientations produce cloudy Till films independent of the

cloudiness mechanisms related to process variables.

Unique microstructures developed during the epitaxial growth of birefringent Till on certain KCil orientations are responsible for this cloudiness. Figure 1 shows a KCJl substrate coated with Til. The

cloudy areas in the T£l correspond to different KCJl subgraln orientations.

Figure 2 shows a microstructure typical of a cloudy Till film of this type. The boundaries are visible only between crossed polarizers suggesting that they are very narrow and by themselves are not

the source of scattering.

In order to determine the KC£ orientations responsible for this peculiar microstructure, single

crystals of KC£ were shaped and polished to expose a wide range of orientations to the condensing Til.

*Work supported by Defense Advanced Research Projects Agency under contract DAHC15-73-C-0464.

1. Figures in brackets indicate the literature references at the end of this paper. -239-
Figure 3 is a photo of two such crystals. The cylindrical crystal contains all orientations normal to

j the KC«. <100> axis which coincides with the axis of the cylinder. The other crystal in the figure is a| plano-convex shaped lens coated on the convex side. The KC£ <100> axis here is parallel to the axis of the lens.

Cloudiness in films on these crystals occurs in specific crystallographic regions. The lens-shaped

crystal, for example, is cloudy along the two KCX. <100> directions projected from the KC£ <100> pole.

These cloudy regions contain the mlcrostructure mentioned previously and divide the lens into four

quadrants

Figure A is a macrograph of the entire lens as seen between crossed polarizers. With the KCX. <100>

directions parallel to the polarizer axes only the microstructural boundaries are extinguished. As theP crystal is rotated with respect to the polarizers the entire film is extinguished when the KCJl <110>

directions become parallel to the polarizer axes. This implies that the T£l contains, at most, two orientations, both of which have their optic axes parallel to the two KCA <110> directions. The fact

that boundaries appear means that there are indeed two TUl orientations.

X-ray Laue photos shown in figure 5 taken on both sides and in the middle of the cloudy region

along the KCJ, <100> direction confirm the existence of the two Till orientations. The Laue spots from j either side of the cloudy area are mirror images of those from the opposite side implying a 90 t rotation between the TiLl orientations. The Laue photo taken in the center (cloudy) area contains both ' sets of spots which means that both orientations occur in the cloudy area.

j Scattering results as low incident light is refracted between these T&I grains whose optic axes are ! rotated 90 from one another . The axes in this case are aligned slow to fast and fast to slow: a ; situation of maximum refraction. This scattering causes the films to appear cloudy to the eye. At j 10.6)Jm, of course, the birefringence is small and since the beam is normal to the film no scattering

will occur.

3. The Epitaxial Habit of Til on KC£ } The discrete spots on the Laue photos are sufficient evidence of the epitaxial growth of Til on KCR.' Further x-ray analysis details this epitaxy and provides a model for the observed microstructures

Figure 6 Includes the results of x-ray analysis. In every case the TJlI b axis is aligned parallel

o

to a KCJl <110> direction. This suggests the importance of the KCH <110> direction that was noted under

polarized light. Observation of orientations away from the principle orientations reveals that the Till b axis aligns itself with the KCii. <110> direction nearest the substrate surface. Recalling the lens sRaped crystal we realize that this alignment is the cause of the two orientations and their intersection along the <100> projection on the lens. This intersection has both KCil <110> directions

equally near the surface. On exact KCil (100), which includes cleaved KCil, both <110> directions are in

the surface plane and films on these surfaces contain the duplex mlcrostructure and are cloudy.

Figure 7 is a model of the mlcrostructure seen in the lens-shaped crystal. The Till b directions

o are parallel to the KCil <110> direction nearest the surface and there is a mixture of the two T*.I orientations along the KC£ <100> directions.

On KCA (111) which has three <110> directions, three Till orientations are observed. Since the <110> directions here are 120 from one another, only one orientation of TUl can be extinguished between

crossed polarizers at a time. Films on KCil (111), as observed, can never be completely extinguished in

polarized light. These films, as would be expected, scatter less than films on KCil (100) which contain

the orthogonally oriented grains.

Finally, since there is only one KCil <H0> direction in the KC£. (110) plane, Tfcl grown on that surface is of a single orientation and does not scatter light.

To complete the study of Till epitaxy on KCH we examined the lattices involved in the epitaxy. Since

both TAI and KCSi are ionic materials cation-anion alignment must be considered. Figures 8, 9, and 10 are scale drawings of the KCfc (100), (110), and (111) lattice planes, respectively. Figures 11 and 12 are

scale drawings of the T£I (004) and (200) planes, respectively . Examination of the alignments of planes

(as determined by x-ray analysis) shown in figures 13, 14, and 15 reveals an excellent ionic interaction

in all cases. A lattice mismatch of only 3Z between the T£l bo and the KCfc d (110) helps account for

the alignment of the Till b and KCil <110> directions on all KCil orientations.

o Having established that epitaxial Ti.1 microstructures on certain KCil orientations are the source of

cloudiness in some Till films two solutions become apparent. One solution, of course, is to use only

KCfc orientations which yield clear films i.e. .without the peculiar microstructures. This means using

orientations on or near the KCil (110) or orientations near, but not on the KCil (100). A more practical

solution, however, is to elimltaate the epitaxial growth entirely.

We have found that when Till is condensed in its cubic (3) phase on KCfc and allowed to transform to -240-
Its stable orthorhombic (a) phase a clear, polycrystalline , randomly oriented film develops independent

of substrate orientation.

The condensation of the 3 phase is accomplished by holding the substrate at 140°C or above during

the deposition. The phase change occurs during substrate cooling with the a grain size being dependent

on the condensation temperature and cooling rate.

At elevated substrate temperatures and low deposition rates, however, T£l will not condense on the

KCJi. Use of higher evaporation rates solves this problem. By using a graphite Knutsen style evaporation source we have achieved more uniform heating of the T£l and are able to successfully deposit films

I at rates over 30008 per minute.

Films prepared in this manner at an arbitrary rate of 200o8/min are not cloudy, have low absorptions, and have withstood the laser damage tests mentioned previously.

This fabrication technique is especially compatible with the subsequent high temperature deposition

of low index coatings

.

4. Conclusion

This investigation has furthered the fundamental understanding of Til whose qualities as a protective laser window coating have already been exhibited.

5. Acknowledgments

I gratefully acknowledge C. I. Knudson for the excellent x-ray analysis and E. Bernal G. and

R. J. Stokes for their valuable discussions and encouragement.

6. References

[1] Detrio, J. A.; Petty, R. D. ; Ohmer, M. C. and

Swenson, 0. F., from "AR Coated KC£ Damage at 10.6ym" presented at 8th Laser Damage Symposium.

[2] Chaff in, J. H. and Skogman, R. A., Proc. of 5th Annual Conf . on Infrared Laser Window Materials.

1976, p. 156.

7. Figures

Figure 1. Polycrystalline KC£ coated

with T£I.

-241-
Figure 2. Microstructure typical of cloudy T<ll.

Figure 3. Cylindrical and lens shaped

single crystal KCJl coated

with Til.

Figure 5. X-ray Laue photos of area on both sides

cloudy zone (left and right) and ±n^ the cloudy zone (center)

Figure 4. Lens shaped KC£ coated with

T£l with KCJl <100> parallel

to crossed polarizers.

-242-
X - RAY OBSERVATIONS

KC£ SUBSTRATE Til FILM OOo") Dome,

(110) 12001 TEXTURE AND T£l b. PARALLEL TO KCIL|110|.

Figure 7. Model of TUl orientations on KCJ, (100) lens-shaped crystal.

Figure 9. KCJl (110) plane.

-2A3-
Figure 10. KCS, (111) plane.

1X1 b.-

Figure 11. T£l (004) plane.

Figure 12. Til (200) plane.

Figure 13. Til (004) on KCH (100).

-244-
Figure 14. T£I (004) on KCJl (111). Figure 15. Til (200) on KCH (110).

coMmrrs on paper by skogman

Tfie ApeedteA noted that no dzgmdatijon oi Til ^iJbn& uxu, ^een upon AevemZ day& expo&uAe. to 100%

fieZjatxve. hmixUty,

-245-
3.6 THIN-FILM COATING EVALUATION BY ATTENUATED TOTAL REFLECTION

R. T. Holm and E, D. Pallk

Naval Research Laboratory

Washington, D. C. 20375

Attenuated total reflection (ATR) is a particularly

attractive technique for the determination of absorption

constants of thin-film coatings because a wide spectral

range can be studied in a relatively simple way. Calculations of ATR spectra have been made for typical thinfilm laser-window coatings, e.g., ZnSe on BaF2 and ThF^

on ZnSe. With the aid of such calculations, it is possible to establish the optimum ATR trapezoid geometry,

angle of Incidence, and film thickness for particular

film-substrate configurations to maximize the sensitivity

of the experiment to the absorption constant. ATR has potential applications in thin-film characterization

for large-window coatings, as examination of an ATR

sample coated at the same time would enable rapid quantitative evaluation of resulting coatings

Key words: Absorption; antireflection coatings; attenuated total reflection; laser windows; spectroscopic.

1. Introduction

One of the important properties of an antireflection coating that must be

determined is its absorption constant. This can be accomplished either calorimetrically or spectroscopically . Although calorimetric techniques are very

sensitive, they are not readily amenable to the measurement of absorption bands.

Standard reflection and transmission experiments can be used to determine absorption bands, but suffer from lack of sensitivity, especially for thin coatings and

low absorption. One technique that is both sensitive and spectroscopic is atten- uated total reflection (ATR).

2. Discussion

We have made a series of detailed calculations to elucidate the general

features of ATR as applied to antireflection coatings and also to determine

the optimum conditions for doing the measurements. The ATR technique is shown schematically in figure 1(a). A trapezoid is made from the laser-window

material. Thin films of the antireflection-coating material of thickness d are evaporated on both sides of the trapezoid. Light is multlp,ly reflected

down the trapezoid at an angle of incidence 6 which results in total reflection.

If there is no absorption in the film, the total reflection is unity. When

absorption is present in the film, the total reflection is less than unity, or, in other words, is "attenuated." The difference from unity is a direct measure

of the absorption in the film. With e and d fixed, the frequency of the light

can be varied to measure the absorption bands in the film. The ATR technique

is more sensitive than standard ref lectance/transmittance measurements because

the light undergoes absorption each time it is reflected as it transverses down

the trapezoid. The ATR technique has been used by a group at Raytheon who

studied a variety of film-window combinations with 6 = 45° [2]. We have calculated the reflectance R for a single reflection as a function

of e, d, and film absorption constant a for both s- and p-polarlzed light. These

calculations are based on Wolter's reflectance equations for M layers [3] and,

as such, will be applicable to multilayer coatings. We consider here only one

layer. In the calculations we assume that the window material is nonabsorbing

and that the refractive indices of the window and film are constant. The overN all reflectance for N reflections is R . An expanded view of the window- film-air interface region is shown in figures 1(b) and (c) . TWo different window-film configurations are considered:

a high index film (HIF) on a low index window (LIW) and the reverse, a low index

film (LIF) on a high index window (HIW)

1. Figures in brackets Indicate the literature references at the end of the paper. -246-
For the case of a HIF on a LIW, we have chosen ZnSe (n = 2.435) on BaF2

(n = 1.458), as indicated in figure 1(b). Total reflection cannot occur at the window-film interface, but only at the film-air interface. The critical angle

9 is 24.2° which corresponds to an angle of incidence of 43.3°. Thus, for 6 > c 43.3° total reflection occurs, giving rise to an evanescent wave in the air contiguous with the film. Since the total reflection takes place at the film-air interface, the light propagates back and forth across the film, sampling the whole film.

Whenever there is loss in the film, interference effects arise because the reflectance is less than unity.

In figure 2 the reflectances and for s- and p-polarization as a function of the angle of incidence 9 are shown for ZnSe on BaF„ for a film thickness

-I

^ d = 2 ym at a frequency o) = 2632 cm (3.8 vim) with a film intensity absorption

constant a = 3 cm At 9 = 43.3°, both and R^ increase very rapidly to about

0.999, rather than 1.0 because a is nonzero. As 9 increases beyond 43.3°, both

R and R exhibit a minimum due to interference effects in the film and this minis p mum is related to 9 and d. For maximum sensitivity, an ATR measurement should be

made with the polarization having the lower reflectance and at an angle of incidence corresponding to a minimum of that polarization. In this case it would be

s - polarization at 9 = 70°. It must be kept in mind, however, that the total number of reflections in the is reduced as the angle of incidence is increased.

To demonstrate the spectroscopic features of ATR, we put into the film a fre- quency dependent, Lorentzian absorption band centered at 3.8 ym with a peak absorption of 10 cm"''' and a full width at half maximum of 200 cm"''', as shown in figure 3(a).

In figures 3(b) and (c) we show the reflectances as a function of frequency for pand s-polarization, respectively, for angles of incidence of 50 and 70° with d 2 ym. Also given on the ordinate is R^^ , corresponding to fifty reflections. The minimum

of Rg at 70° is about 0.98 giving an Rg^^ of less than 0.4. The very weak fringes

and the asymmetry of the absorption lines are caused by interference effects. A

molecular absorption band with a peak absorption of 0.1 cm

''' would correspond to a minimum Rg^^ at 70° of 0.99, which should be measurable.

The second case, a LIF on a HIW, is exemplified by ThF^ (n = 1.45) on ZnSe

(n = 2.435), as shown in figure 1(c). In this case, total reflection can occur at either interface, depending on the angle of incidence. The critical angle for the film-air interface is 9^^ = 43.6°, corresponding to 9 = 24.2°. The critical angle

for the window-film interface is 9 = 9 „ = 36.5°. For 9 between 24.2 and 36.5° total

c? reflection occurs at the film-air interface, and the ATR situation is identical to that discussed in connection with ZnSe on BaF2. For 9 greater than 36.5° total reflection occurs at the window-film interface. Now the evanescent wave associated with total

reflection exists in the film, with the depth of penetration into the film decreasing

as 9 increases . With this dependence of penetration depth on angle of incidence , the absorption in the film can, in principle, be probed as a function of distance from the window-film interface.

The reflectance as a function of 9 for ThF^ on ZnSe with d = 2 ym, a = 3 cm

^

and u = 2632 cm

''' is shown in figure 4 for s- and p-polarization. At 9 = 24.2°, and R abruptly increase to about 0.999. As 9 increases, R^ exhibits a strong interference

minimum at 9 = 31.5° and R^ a weaker minimum at a slightly larger value of 9. When 6 goes beyond 36.5°, R^ and R^ both increase rather quickly.

Figures 5(b) and (c) show the reflectances as a function of frequency when the film absorption is given by the Lorentzian absorption band of figure 5(a) for p- and s-polarization, respectively, for 9 = 31.5 and 45 wllth d = 2 ym. At 9 = 31.5 the minimum value of R is almost 0.97, resulting in an R of less than 0.3.

^ ^ o The reason why the reflectance at 31.5° is so much less than at 45 is explained

by figure 6, which shows the reflectance as a function of the film thickness for 31.5 -247-
and 45° for p-polarization with a = 3 cm and oi = 2632 cm . At 45° the penetration

depth of the evanescent wave into the film is about 0.8 \m. As the film thickness

increases , the reflectance at 45" saturates at d ^- 2 ym. Any further increase in

d beyond 2 um is not "seen" by the evanescent wave. At 31. 5**, however, the llg^t

samples the whole film, no matter how thick it is. Thus, the reflectance decreases

as the film thickness increases, asymptotically approaching the value of obtained

when the film thickness is semi-infinite. The interference fringes in the 31.5** reflectance are readily evident.

3. Conclusion

In order to evaluate antireflection coatings while developing growth procedures,

a simple and routine technique to spectroscopically measure absorption bands in the

coating is needed. As we have shown, attenuated total reflection appears ideal for

this purpose for absorption constants of approximately 1 cm or greater. The ultimate

sensitivity of an ATR experiment is controlled by the film thickness, the polarization

and angle of incidence of the light, and the total number of reflections in the trapezoid. ATR apparatus is available commercially and fits many infrared spectrometers.

4 . Acknowledgments

The authors thank M. Hass of the Naval Research Laboratory for helpful discussions

and suggestions.

References

1 Harrick, N. J. , Internal Reflection

Spectroscopy (Interscience, New York,

1967).

3 Wolter, H. , in Handbuch der Physik . S. Flugge, ed. (Springer - Verlag,

Berlin, 1956), Vol. 24, p. 461.

Willingham, C. B. , Bua, D., Statz,

H. and Harrigan, F., "Laser Window

Studies", Final Technical Report

under Contract DAAH01-74-C-0719,

Raytheon Research Division, Ualtham,

Mass. (August 1975), and Deutsch,

T. F., J. Elect. Mater. 4, 663(1975),

-248-
6. Figures

Figure 1. (a) Schematic illustration of ATR technique showing

light transversing down a trapezoid of the window

material with antireflection coatings of thickness d at an angle of incidence 6. Expanded view of encircled

region in (a) showing details of the light propagation for (b) a high index film (ZnSe) on a low index * window (BaF2) and (c) a low index film (ThF^) on a high index window (ZnSe)

.

0.99999

0.9999

0.999

o

coo

0)

•+-v

0.99

0.9-

Figure 2. Calculated reflectance R as a function of the angle of incidence 6 for ZnSe on BaF2 with

CO = 2632 cm~^, a = 3 cm~^ and d = 2 ym for s- and p- polarization.

1 1 1 1 1 1 1 1

/t

^-pol. /_£

\ /

\ / s-pol.

1 1 1 1 " 20 40 60 80

9 (degree)

-249-
Figure 3. Frequency dependent, Lorentzlan absorption band for

the film used in the calculation of (b) and (c).

Calculated reflectance R as a function of frequency

(jj for ZnSe on BaF2 for (b) p-polarization and (c) s-polarization for d = 2 ym and 6 = 50 and 70°. The ordinate in (b) and (c) also shows R^",

corresponding to fifty reflections.

0.99999

0.9999 —

0.999 —

^ 0.99 —

o

Figure A. Calculated reflectance R as function of the angle of incidence 6 for ThF2 on ZnSe with

U) = 2632 cm"', a = 3 cm"' and d = 2 ym for

s- and p-polarization. 20 40 60

9 (degree)

80

-250-
10 8

Wavelength Xifitn) 6 5 4 3j5 3

1000 2000 3000

Frequency ui (cm"')

2.5

4000

Figure 5. (a) Frequency dependent, Lorentzlan absorption band

used in the calculations of (b) and (c) for the film.

Calculated reflectance R as a function of frequency oj for ThFn on ZnSe for (b) p-polarization and (c) s- polarization for d = 2 ym and 6 = 50 and 70°. The

ordinate in (b) and (c) also shows R^", corresponding

to fifty reflections.

-251-
COmEMTS ON PAPER BV HOLM kW PMIK

A4 a dmonitA-cutLon the, izn^-ctLv-ity ATR techyiiqao^i, , the. ipe.a.keJL rmvttiomd that the.y had

defected a SO A tayeA Oj^ coAbon at a St02 - tnteAi^atz by ATR. He MOi not iuA.e Mhat the eiie.cX 0|5 icotte/u^ng MoaZd be on ATR me.aian.ementi . It did not appexxA that the zxte.n6A.on o{, the ATR mzoiuAeimnt

to maJbtiZcLyeA {jAJmi ok tnhomogemoii& (,lljmi hoi been (^uZly analyzed, biit In the e.vaneJ>ce.Yvt wave coie,

-iX doeA ieem poaZbZe to pnobe muIXlLayeA thin {^Ajjni> a^ a duncJtLon od depth.

-252-
4.1 ZnSe SPUTTERED COATINGS*

David A. Walsh

John A. Detrio

University of Dayton

Research Institute Dayton, Ohio 45469

The sputter deposition of ZnSe for laser window coating preparation has

been studied using substrates of ZnSe and KCl. Sputtering targets made from CVD

ZnSe and commercially hot pressed material were also compared. Self films of ZnSe on ZnSe were studied for absorption versus thickness using films of X/4,

3X/4, and 5X/4 at 10.6 j^m. Calorimetric absorption measurements showed the HP target material produced coatings with 3 to 6 times greater absorption. Auger

analysis indicated the presence of sulfur in the HP films,but none in films made

with the CVD target. Both types of coatings appeared to be non- stoichiometric

.

Film and substrate inhomogeneity prevented reliable measurements of the film

absorption coefficient. ZnSe on KCI coatings were investigated with Auger

Spectroscopy in order to distinguish between target material effects and the influ- ence of surface preparation methods. As-polished material showed AI and oxygen

present at the interface which indicate a residue of polishing compound. The

residue was readily removed by etching in concentrated HCl. Damage measure- ments were also made on the ZnSe coatings.

Key words: Absorption damage levels; CVD; coatings; hot pressed;

sput tered

.

1 . Introduction

This work represents the continuation of efforts to investigate the properties of sputter deposited

ZnSe as a potential laser window coating. Previous work was directed at the modification of sputtering

equipment to allow convenient optical monitoring of the film thickness [ 1 ] , and attempts to obtain

uniform coating thickness over practical dimensions [23. The deposition of ZnSe on KCl as a test vehicle for evaluating the potential for depositing compact, pinhole-free , and tightly adherent coatings was reported previously [3]. During this latter study the effect of sputtering process variables

(voltage, current, spacing, argon pressure) was further investigated. Mechanical strength problems

associated with the thermal expansion mismatch between coating and substrate greatly reduced the survival of thick coatings of ZnSe on KCl. However, our experience indicated that the sputtered films were probably superior to thermally evaporated films with respect to environmental durability and we have continiied to investigate this coating process for application to other substrates and for KCl

coating designs which do not require a thick first layer of ZnSe. In this paper we report some of our

initial attempts to understand the source of the 10.6 fixn absorption in sputtered ZnSe films and the

role of sample surface treatnifent in the production of low absorbing coatings.

2. Experimental

Two ZnSe targets have been used in the current investigation. One is composed of hot-pressed

(HP) Kodak Irtran IV material; the other of chemical vapor deposited (CVD) Raytheon ZnSe. Previous

evaluations have shown that there is a trend toward lower absorption in the coatings deposited from

the CVD material, however, these differences were not defined well enough to make reliable judgements regarding the optimization of the individual coating p>arameters involved. The present work

has inc],uded samples for Auger analysis of the film/ substrate combinations and more sophisticated

polishing methods were employed to allow more precise optical characterization.

* Work supi>orted by Air Force Materials Laboratory Contract F33615-76-C-5093.

1. Figures in brackets indicate the literature references at the end of the paper.

-253-
2.1. Auger Analysis of Sputtered ZnSe ? To evaluate the relative purity of coatings deposited from the two target materials, four sets

6 mm square by 1 mm thick substrates were prepared from commercial ZnSe and KCl bulk material.'

For KCl, a freshly polished surface was generated by rough lapping with 9 1-tm SiC on a glass flat • followed by pad polishing with 3 fim and 1 j/m aluminas. The ZnSe samples were prepared from

optically polished specimens. These surfaces were used as-received from the polishing vendor.

J The finished surfaces were covered with a layer of Spawr "Opticoat" [4] to protect them during

subsequent wire saw cutting and dicing operatings. Two of the KCl sample sets were then etched for

'

2 min in concentrated HCl to permit a comparison with the polished-only surface. As shown in table 1, two types of rinsing following the etch were evaluated to determine whether denatured ethanoj:

would produce a cleaner surface than the 2-propanol customarily used. Some workers have employeq; an ethanol rinse prior to the 2-propanol since the KCl is slightly soluble in ethanol which prevents I the recrystallization of dissolved KCl on the etched surface [5]. Samples were dried in a warm air { stream during a slow withdrawal from the final 2-propanol bath. Both the polished-only and polished^

etched specimens were stored in an evacuated dessicator prior to coating. Final cleaning of the ZnSe

samples consisted of washing with a cotton swab in a mixture of "Micro" [6] detergent and deionized

water followed by rinsing in running deionized water and immersion in three successive rinses of

reagent grade 2-propanol with warm air drying. Samples were then dessicator- stored until coated.

Table 1. Preparation and coating results: CVD vs hot-pressed sputtering targets

Runs: 196 through 199

Variables: Target Materials, Argon Purity I Sample Coding: !

i KCl: Pol = Polished only i Eth = 2 min HCl etch + rinse in Ethanol + 3 rinses in 2-propanol

2-prop = As above with 2-propanol rinsing (3x) only

ZnSe Zn = Polished + detergen + 3 rinses in 2-propanol

Sputtering Conditions: lOOOV (260 W), 8 ym, 2" spacing

Thickness measurements for ZnSe coatings:

Run ZnSe Argon Coating

No. Target Treatment Th'k R

196 CVD None 7,000

197 H/P " 9,100

198 CVD Purif. 9,500

199 H/P " 10,200

Target materials were cleaned prior to film coating by pre- sputtering for 30 minutes onto a dummy substrate platen. Pre- sputtering conditions were the same as those used for the experimentalj

films. These were held constant in all of the runs with R F voltage at lOOOV (260 watts), argon

pressure at 8 \m and target to substrate spacing of two inches. The depositions were duplicated

from each of the targets using in one case the grade 5 argon as-received and in the second case the same argon further purified by passing it through a dessicant drying column followed by a G.E. "GoGetter" catalytic Titanium gas purifier.

The results of Auger analysis of the sputtered ZnSe target materials on the ZnSe substrates are shown in figure 1. Sulfur contamination in the amount of approximately 1 At % is detected in the coatings deposited from the hot-pressed target. No contaminants were detected in the films deposited

from the CVD material. Compositional shifts at the film /substrate interface are indicated for the hot-pressed target coatings where the sulfur concentration drops to a background level and there is an

apparent shift to the Se-rich condition within the substrate. This is probably due to the normalization

of the curves since the substrates are also CVD ZnSe. This Se-rich indication may also be seen in the film deposited from the CVD target in the plasma of as-received argon, while the gettered argon

and CVD target produce a film which varies in intensitytbut remains stoichiometric.

-25A- / .

ilri

Kl(
The presence of sulfur is also seen in figures 2 and 3, the Auger traces of films deposited from

the hot-pressed ZnSe target onto the KCl substrates. Scans of CVD ZnSe coatings on similar substrates again indicate that this target material is free of sulfur contamination.

Additional contamination is indicated by the detection of Al and oxygen as the coating is penetrated and the substrate surface is approached. These signals are absent from the films deposited

on the etched substrates. Since the intensity of both Al and oxygen reach a maximum intensity just

below the interface, it is apparent that these peaks are due to imbedded Al^O^ polishing abrasive.

This is confirmed by their absence at the interface of the etched KCl where the polished surface has

been dissolved by the concentrated HCl.

In previous work, it was found that the sputtering rate of the more dense CVD target is appreciably higher than the lower density hot-pressed target. This has been observed again during this study as shown in table 2, The difference in sputtering rates would imply a more efficient energy

transfer between the argon ions and the target surface.

Table 2. Sputtering rate of target materials vs argon treatment*

Sputtering Rate

R/min

Argon

Treatment Hot- Pressed - CVD

None 376 422

Gettered 352 420

*A11 other conditions held constant at lOOOV

RF, 8 urn Ar pressure, 2" spacing 250 watts.

It was also observed that the film tends to be Se-rich at the interface. Changes in the sputtering

rates and variations in Ar pressures during the runs will be carried out to investigate if this is actually occurring or whether it may be due to the Auger measuring procedure.

Finally, it was concluded that the gettering of the 99.999% pure Argon used in the sputtering

procedures results in no significant changes in the films which could be detected by Auger

spectroscopy

.

2.2. Absorption of ZnSe Target Materials

Two experiments were designed to measure the differences in absorption in films deposited

from the hot-pressed and CVD ZnSe targets. In the first, several previously coated 1" ZnSe speci- mens were repolished in our laboratory. These were then coated with films of equal thicknesses

from the hot-pressed and CVD targets with and without argon gettering and the coating absorption

evaluated at 10,6 ym. In the second experiment, ^ vs coating thickness tests were carried out on

similarly re- cycled window specimens in a quadrant format to provide uncoated, X/4, 3X/4, and 5X/4

thick segments for 10.6 \m calorimetry.

a. Absorption of Sputtered ZnSe Films of the Same Thickness

The 1" witness plate samples were rough ground to identical thicknesses then polished and

figured on a pitch lap to a flatness of 3 fringes. Surface finish on one of the samples from the block was determined to be 60 R by T.I.S. evaluation.

Samples were cleaned after deblocking by immersion in trichloroethylene to remove residual wax followed by a detergent washing and 2-propanol rinsing using the same procedures as for the Auger samples. Sputtering conditions were held the same in these runs as in the coatings for Auger

evaluation.

Results are shown in table 3. It was found that for botli CVD sputtered films, the absorption is much less than in similarly deposited coatings from the hot-pressed target. This higher absorption

correlates with the sulfur contamination detected by Auger analysis.

As in the case of the specimens prepared for Auger analysis, efforts to further purify the argon gas by gettering produced no significant influence on the coating performance.

-255-
Table 3. Absorption of ZnSe coatings on ZnSe substrates.

Coating Coating Absorption

Target

H/P

CVD

Argon

as rec'd

gettered

as rec'd

gettered

Rate

K/min

380

385

491

488

Thickness

R

10,500

10,000

10,500

10,000

B X 10'

c4.04

2.60

.68

.97

B cm

c28.4

26.0

6.5

9.7

b. Absorption vs Thickness of Sputtered ZnSe Coatings

In this evaluation absorption versus coating thickness for both the CVD and hot-pressed targets was investigated. Samples of l|^" diameter were repolished following the same procedure outlined

above. The same sputtering conditions were used to permit a direct comparison with the previous

results. Optical thicknesses of X/4, 3X/4, and 5X/4 were sputtered onto three 90*^ quadrants of

each sample with the fourth quadrant left uncoated as a control.

The absorption of each quadrant was measured calorimetrically at 10.6 fitn and the results are shown in table 4. Absorption, in general, is again seen to be lower for the CVD target material than f for the hot-pressed, however, the measured values for the X/4 coating is much higher than in the previous evaluation and an inconsistency can be noted in the case of the X/4 coatings with an apparent

reversal in relative absorption to what would be expected: the hot- pressed material showing a much

lower 0 and the CVD film much higher. The cause of this result is presently unclear. Calculations

of refractive index based on observation of the thickness monitor interference fringe peaks at 6328 R. also indicates an anomalous value for the X/4 coating. Measurement of a witness plate coated in the same run indicates that the proper physical thickness Jjad been deposited. Measurement of the scattering of HeNe laser radiation at the same absorption site tends to exclude the possibility that an

inclusion within the film has produced this reading.

Table 4. Summary of sputtered ZnSe coating absorption vs thickness

Hot-Pressed Target

Thickness

Run No.

206-1

206-2

206-3

Optical Calc. R Obs . R

Absorption

X/4

3X/4

5X/4

11 .050

33, 150

55,250

11,000

36,600

55,200

B X 10

c 1.08

5.65

5.71

B cm

c9.8

15.4

10.3

calc

2,44

2.39

2.54

Scatter

0.053

0.097

0.066

CVD Target

Run No.

207-1

207-2

207-3

X/4

3X/4

5X/4

Thickness

Optical Calc. R Obs. R

11 ,050

33, 150

55,250

11,050

34,000

55,300

Absorption

B X 10

c3.54

4.23

3.53

B cm

c32.2

12.4

6.4

-1

calc

2. 18 2.48

2.48

Scatter

0.053

0.052

0.069

calc

= qX/2je

2.40

Focused CO^ Laser Damage Tests The damage resistance of these films to focused CO^ laser radiation was measured. The details of the experiment are described elsewhere in these proceedings [7], The results of the tests are somewhat surprising as shown in figures 4 and 5 which show the sequence of 3 s laser irradiations at various sites on the samples. The hot-pressed materials produced a coating which had some very tough spots which survived repeated irradiation while the CVD material coatings in general had lower damage levels. The CVD coatings were more uniformly damage resistant,howeveri

-256-
In summary, the CVD coatings had a higher threshold and were more homogeneous with respect to

I damage while the HP material was capable of with standing high power densities in selected areas.

In general, the samples developed a residual strain at powers substantially below (30-50%) the level

of catastrophic damage. Because these coatings were quite thick they suffered crazing and spontaneous destruction over much of their surface when first removed from desiccator storage just prior

I to irradiation. The damage results are subject to considerable uncertainty due merely to the variation

in coating coverage. The power density corresponding to the average level of irradiation which pro2 2 duced catastrophic damage is 63 +_ 32 kW/cm for the HP target coatings and 51 11 kW/cm for the CVD material.

d. Water-Immersion Test: ZnSe on CaF^

A test for the environmental stability of ZnSe coatings on CaF^ substrates has been developed

by Major D. O'Brien of AFML/LPO for evaluation of e-gun deposited films . By immersing the coated sample in deionized water and observing the elapsed time required to delaminate the coating,

a qualitative measure of durability has been obtained. For most of these coatings, survival times

have been fairly short; on the order of seconds up to a maximum of one to two minutes.

To compare sputtered ZnSe coatings to those fabricated by e-gun evaporation, coatings were

deposited onto two pad-polished CaF^ samples comparable to those used in the previous absorption

work. Sputtering was done with the CVD target under the standard conditions of lOOOV RF, 8 ^m Ar

pressure and 2" target to substrate spacing. Two thicknesses were deposited. One of these was

7800 R which approximated the thickness of those being produced by Maj . O'Brien. The other was

29,000 R to observe the effect of this testing scheme on a thicker coating. A 1/8" stripe across the sample face was masked during film deposition to allow for coating thickness determination.

Following 5 minutes immersion of the two samples in deionized water at 25°C the thicker film was observed to be developing a very fine crazing pattern about 1/32" wide at the edges of the masked

area. The thinner film was unaffected at 5 minutes, but by 15 minutes it too had developed a similar

crazing adjacent the masked area. At the end of 45 minutes, the crazing had spread to approximately

1/8" wide into the coating around the masked area and 1/32" around the outer edge of the thick coating

sample. The thinner sample had developed a crazing only about 1/32" wide in the masked area during

this same time interval. At the end of 75 minutes the test was terminated, but the samples were

allowed to remain immersed overnight to observe the effect of this long exposure. At this elasped

time the thicker film had crazed over about 50% of its area; the thin coating remaining unchanged

with crazing limited to about 1/32" at the masked stripe. At a total immersion time of 17 hours,

the thicker coating was completely crazed. The thinner film was crazed over about 60% of its area. While not originally intended as part of the evaluation, it was observed that a portion of the coated specimens survived intact throughout the entire 17 hours. The samples used in the test were

approximately equal halves of a 51 mm diameter by 10 mm thick KCl disc which had been fractured

during prior handling; the fractured surface planes being at an angle of «»45° to the polished faces.

During the coating procedures these areas also received a layer of ZnSe deposited at this oblique

angle. It was in these portions of the samples that both of the sputtered coatings were unaffected by

the immersion procedure. No special effort had been made to prepare these surfaces prior to deposition. Immersion in the detergent and water solution and a 2-propanol rinsing being the only

cleaning they received.

This severe moisture resistance evaluation indicates that the integrity and bonding of the sputtered ZnSe is definitely superior to e-gun deposited coatings when applied to polished surfaces.

Where the latter were completely crazed in minutes, the sputter deposited films remained intact for

a much longer time and the crazing progressed at a slow rate from an area of exposed substrate

surface where the solvent was able to penetrate under and lift the coating. It is possible that had

the polished face received a continuous deposition over its entire surface the test results may have

been more favorable. This implication is supported by the apparent durability of the coating sputtered onto the fractured surface.

It is of interest to note that in spite of the critical thermal shock characteristics of CaF^

material, the temperature changes which are inherent in the sputtering process incurred no damage

to the substrates used in this work.

-257-
3 . References

[1] Walsh, D.A., Pukite, G.U., Fenter.J.R.,

Rev. Sci. Instr., 47, 8 (1976)

[2] Johnston, G.T., Walsh, D.A. , Harris, R.

J., and Detrio, J.A. , UDRI-TR-75-23

30 June 1975

[3] Walsh, D.A. and Bertke, R.V., Proceedings of the Fifth Laser Window

Conference, p. 139, 1976

[4] Trademark, Spawr Optical Research, Inc. Corona, California

[5] Turk, Roger, Priviate communication,

Feb. 1976

[6] Trademark, International Products Corp.,

Trenton, New Jersey

[7] Detrio, J.A. and Petty, R. D., Laser

Induced Damage in Optical Materials; 1976

to be published.

-258-
A. Figures

AUGER ANALYSIS : ZnSe on ZnSe

ARGON TARGET MATERIAL

AS REC'D

,i .2; 20;'

15

UJ>

< 10

_jUJ

cc 5

0

HP CVD

GETTERED

0.2 0.4 0.6 0.8 DEPTH - ym

0 02 0.4 0.6 0.8 LO I. DEPTH - ym

Figure 1. Auger spectra of ZnSe coatings on ZnSe substrates.

-259-
AUGER ANALYSIS ZnSe on KC/ ARGON GETTERED

TARGET MATERIAL TREATMENT

POLISHED ONLY

POLISHED AND

HCJ ETCH

25 zg

520

(ESl5 zo

"10

111 >

!5 5

_iui

IE 0

25 zo

fe20

uj 15 2 - PROPANOL ^

RINSE 8

iij >

S 5

25 zo

K

§15 z

o

" 10

UJ>§5

UJ

a: 0

ETHANOL

AND

2- PREPANOL

RINSE

HP CVD

S(><5) U\,Ji^

\ ^

\

ZrUAVse

S(x5) / W

TV m:/

// VVse — T

\f ^

1 V*

0.2 0.4 0.6 0.8 1.0 12 DEPTH- \im0.2 04 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 DEPTH -ym

Figure 2. Auger spectra of ZnSe coatings on KCl substrates: argon gas as-received.

-260-
AUGER ANALYSIS ZnSe on KCA ARGON AS REC'D

Figure 3. Auger spectra of ZnSe coatings on KCl substrates: gettered argon

gas.

-261-
O IRRAD. DAMAGE

1000

900

800

700

'H 600

cd» 500 >—

a:u 400 O

°- 300

200

100 ABCDEFABCDEFABCD I n IE

LOCATION

E F A 8 C D

Figure A. Irradiation sequence - ZnSe (HP) on KCl substrate.

IRRAD DAMAGE
COUUEHTS ON PAPER SV WALSH AMP VETRW

It uxii pointed out that in gtmAot, ZnSe cocutLngi on Caf 2 iubitxatu did not ituivlve. \JZAy long, bat

in ont caA& uheAz the. coated iuAiace am ^itihty cleaved KatheA than potUhed, the coating ieeme.d to

iuKvive. inde-iiniXeZy

,

-263-
4.2 LASER DAMAGE STUDIES OF LOW INDEX COATING MATERIALS AT 10.6 ym

M. J. Solleau, A. D. Baer, J. 0. Porteus , J. L. Jemigan,

W. N. Faith, T. M. Donovan, and A. K. Green Michelson Laboratories, Naval Weapons Center

China Lake, California 93555

The 10.6 ym laser damage thresholds were measured for a variety of promising low index coating materials. Coatings deposited on KCl and NaCl windows

were studied. Measurements on NaF, BaF2, and SrF2 indicate that all these

materials have thresholds in excess of 1 GW/cm^ when properly deposited. A

comparison of the bulk and thin film threshold of these materials will be

given. Our results indicate that NaF is an attractive alternative to ThV^^ since it has reasonably low absorption, a high damage threshold, and can be used as a single layer ant ire flection coating on NaCl. The primary mechanism

of failure in the coatings tested was found to be coating delamlnatlon

associated with defects on the substrate surfaces.

Key words: Coatings; laser damage; laser windows; 10.6 ym lasers.

Introduction

Windows in most laser systems require antireflection (AR) coatings to reduce Fresnel losses and feedback to the laser's oscillator-amplifier chain. Infrared systems utilizing NaCl or KCl windows

have the additional requirement that the exposed window surfaces be protected against attack from the atmosphere. As an illustrative example consider a CO2 laser fusion system that uses an oscillator and five amplifier stages with two NaCl windows on each amplifier. Antireflection coatings on these windowj^

could increase total system output by a factor of two (assuming 20 surfaces with 4% reflection loss per'

surface for uncoated windows) while minimizing the problems of isolating the oscillator from back re- flections and protecting the windows from atmospheric moisture attacks.

In order to realize such improvements in systems performance one must have low index coating materials to use as single layer or components of multilayer AR coating designs. Such materials must have

reasonably low absorption at the optical frequency of use, must be compatible with the window substrate

and must be able to survive the extreme flux from the laser. In this paper pulsed laser damage studies

on low index coating materials are presented. Materials studied were NaF, BaF2j and SrF2. These mate- i. rials were chosen because they all have reasonably good transmission at 10.6 ym (6 < 1 cm~^ in bulk

form) and have excellent transmission in the 3-5 ym region [1,2]^. NaF is of special interest since iti index of refraction is almost exactly equal to the square root of the index of NaCl at 10.6 ym. NaF

can therefore be used as a single-layer (quarter wave) AR coating for NaCl at 10.6 ym.

Experimental

Films of NaF, SrF2, and BaF2 were deposited on NaCl or KCl substrates. All films were an optical

thickness of one quarter wave at 10.6 ym. The films were deposited in a standard oil-pumped vacuum sys- tem in the manner described by Baer et al. [2], The 10.6 ym absorption of the films was determined by

laser calorimetry [3,4]. The total absorption of the uncoated windows was measured before coating

deposition, and the total absorption of the window /coating combination was measured after coating depO'

sition. The differences in the two absorption values were attributed to the particular coatings

jj involved. I Damage testing was performed using the Naval Weapons Center's laser damage facility [5]. The expetf

iments reported in this paper were conducted on a pulsed, single-mode CO2 laser operating in the 10.6 \si region. It has reproducible temporal and spatial characteristics which are detailed in the following

discussion. Beam intensity is controlled by a rotating polarizer-analyzer combination, each consisting

of two ZnSe plates tilted in opposite directions to minimize beam shift, as well as by CaF2 attenuators After the attenuators, the energy in each pulse is measured by a pyroelectric energy meter which is calibrated using a thermopile calorimeter. The beam enters the test chamber through a NaCl window and

Is focused on the sample with either a 5.08 cm diameter, 11.91 cm focal length or a 2.54 cm diameter,

3.546 cm focal length ZnSe meniscus lens. The meniscus lens was used in order to minimize spherical

aberrations. Focal lengths given are for 10.6 ym. A HeNe laser is used for convenience in aligning th< optics.

1. Figures in brackets indicate the literature references at the end of this paper.

-264-
The temporal beam characteristics are monitored with a photon-drag detector having a 1.5 ns rise

time. Figure 1 shows the measured intensity as a function of time, together with an analytical expression which describes this dependence. As can be seen in figure 1 the laser output increases linearly

to the peak and falls off as a steep exponential. The peak power, calculated in watts using the total

output energy, and temporal pulse shape, is 1.27 x 10^ times the total output energy (in joules). From

this expression we can define an equivalent pulse width of 78.7 ns ; i.e., the total energy divided by

78.7 ns gives the peak power of the pulse. In previous work [5,6] we described this pulse as being

approximately 100 ns wide and having peak power 1.27 times the power calculated by dividing the total

output energy by 100 ns. The peak power calculated using either technique is the same. However, the

78.7 ns width is more meaningful since it and the total energy yield the peak power directly. Note that

the 1/e full width of the pulse is approximately 78.7 ns. The full width at half maximum (FWHM) of this

pulse is approximately 60 ns. The spatial profile of the beam as it enters the damage chamber was measured using a pinhole scanned

across the beam and can be represented by a Gaussian distribution to within 4%. The 1/e full width of

the unfocused beam was 5.8 mm. When focused by the 11.91 cm focal length ZnSe lens the spatial profile,

as measured using a scanning slit, can be represented by exp\-2{v/100)^^ where v is the half width of

the Gaussian in ym. Thus, the l/e^ spot diameter for this lens Is 200 ym. The focused beam width was

measured as a function of the ZnSe focusing lens position. From this measurement the depth of field of

this lens was determined to be approximately 3 mm [5,6]. The approximate effective /-number of the

lens, i.e., the focal length divided by the 1/e beam width, is ~ 20 and helps to explain the large depth

of focus which was observed. The spot size for the 3.546 ym focal depth was determined by scaling the measured spot size of the 11.91 cm focal length lens as the ratio of focal lengths to the two lenses.

This calculation yields a l/e"^ spot diameter of 59.5 ym for the 3.546 cm focal length lens. The effective /-number of this lens is approximately 6. The correct focal position for the lens was determined using a technique previously described by

Porteus et al. [7]. This procedure allows us to determine the focal position of the specimen surface

to within 10 ym (the repeatability of the translation micrometer).

The test chamber and the analytical instrumentation used in this experiment have been described

previously [6]. The chamber is designed for ultrahigh vacuum, including bakeout. However, these measurements were made at a base pressure below 1 x 10"^ Torr without bakeout. Visible changes in the specimen (surface or bulk) were monitored by observing the scattered light with a 20-power optical

microscope.

Results

Bulk thresholds for single crystal NaF, BaF2» and SrF2 were measured to determine if intrinsic

10.6 ym damage limits would prohibit the use of these materials as coatings for NaCl or KCl windows.

The single crystal materials were acquired from the Harshaw Chemical Company. The 3.546 cm focal length

lens was used in this part of the experiment to minimize the problem of surface damage. The results of these measurements are summarized in figure 2. As can be seen in this figure all of the bulk values are

quite high. The values given are for the peak on-axis intensity at the focal plain of the lens (approximately 0.5 cm into the bulk of the 1.0 cm thick specimens) and includes the effects of attenuation of

the beam by these weakly absorbing materials. The high number for the BaF2 threshold may be due to the

fact that it is an n-on-one number and the other numbers are for single shot damage. The one-on-one

(single shot) damage threshold for the BaF2 could not be determined because of the very low surface

damage threshold for this specimen. By preconditioning the surface (successively higher energies after

initial illumination applied below the single-shot threshold) we were able to transmit sufficient

energy into the material to cause bulk damage without damaging the front surface.

The bulk threshold for Harshaw Chemical Co. 's single crystal NaCl was determined using the same ex- perimental arrangement described above (the same lens, focusing procedure, detector calibration, and

data reduction techniques). The threshold for breakdown thusly determined was 1.92 x 10^ V/cm, which

is quite close to values of 2.02 x 10^ V/cm measured in similar material in our laboratory with the

same laser, the 11.91 cm focal length lens [6], and the value of 1.95 x 10^ V/cm reported by

Yablonovitch [8]. The NaCl breakdown threshold from this work and the breakdown thresholds for the BaF2, SrF2, and

NaF are compared In figure 3. Note that the measured thresholds for the proposed coating materials are

all significantly higher than that of NaCl. The ratios of field strengths given in figure 3 agree

quite well with values obtained at other wavelength^ [9,10] and in the case of NaF, with the ratio of

the dc thresholds [11]. The relatively large discrepancy (~14%) between the 10.6 ym and 3.8 ym ratios

for BaF2 is probably due to the fact that the 10.6 ym threshold for BaF2 was determined for the n-onone case and ratloed to the one-on-one threshold for NaCl. The implication of these results is that coatings of NaF, BaF2> and SrF2 on NaCl will not be limited

by intrinsic processes. Films of one quarter-wave optical thickness (for X = 10.6 ym) were deposited

on substrates of NaCl and KCl in the manner described by Baer [2] and the total 10.6 ym of the films

determined using laser calorimetry. The results of the absorption measurements are given in figure 4. The absorption values given are the fraction of the total incident power absorbed by the films. The

-265-
substrates on which these films were deposited all had total absorption < 10"^ each at 10.6 ym prior to coating deposition. Therefore, the film absorption values computed by subtracting the initial substratef absorption from the total absorption of the substrate/coating combination should be reasonably accurate. The accuracy of each absorption measurement is ± 10%; thus, ± 20% would be the accuracy of the differ- m ence if the coating and substrate absorption were equal. Since the substrate absorption was always less? than the coating absorption the uncertainty of the numbers in figute 3 should be less than ± 20% and

greater than ± 10%.

These films were damage tested, and the results are summarized in figure 5. As can be seen in these

data, the thresholds for NaF, BaF2, and SrF2 are all significantly greater than values measured for ThFij

| by other workers [12]. Then, from a pulsed laser damage standpoint, these materials offer a significantlj

advantage over ThFi| films. The numbers given in figure 5 are peak-on-axis intensities (single shot)

' which produced damage 50% of the time. The NaF data were taken with the 11.91 cm focal length lens ^ spot diameter equal to 200 ym) , and the BaF2 and SrF2 data were taken with the 3.5A6 cm focal

' length lens spot diameter equal to 59.5 ym) .

Discussion

Simple one- dimensional heat flow calculations indicate that the temperature of the films tested in- creased approximately 1000°, 640°, and 170°C for the SrF2, BaF2, and NaF, respectively , for power densities

equal to the damage thresholds . Temperature increases were calculated using room ten^jerature thermal

properties. Since the heat capacity of these materials increases with temperature, these temperature

rises represent an upper limit, assuming the average absorption of the films are as given in figure 4. From this we conclude that the films did not fail due to thermal evaporation or linear absorption in the I films.

Micrographs of the damage sites and comparisons with uncoated window surface damage data suggest

that the coatings failed due to substrate surface limitations. Figure 6 is a Nomarski micrograph of a typical damage site on the SrF2-coated specimen. Failure appears to have been by massive or explosive

fracturing of the KCl substrate which was chemically etched [4] prior to coating deposition. Previous

work indicated that the one-on-one threshold of etched KCl surfaces is 67% [6] of the bulk value reported by Yablonovitch [8]. This measured threshold is very close (1.13 MV/cm compared to 0.97 MV/cm) to the expected value for an uncoated etched surface. From this evidence we conclude that the SrF2/KCl

combination failed due to dielectric breakdown of the KCl surface.

Figure 7 is a Nomarski micrograph of a typical damage site for BaF2 on a NaCl substrate. The Isiser spot size was 59.5 ym diameter). The laser was linearly polarized with the electric field vector

in the vertical direction. The damage threshold given in figure 5 for this coating is 1.22 times that measured for an uncoated NaCl window with a similar surface finish [6], a standard commercial polish

from Harshaw Chemical Co. Several defects can be seen in the damaged area in figure 7. Close inspection of this micrograph reveals a ripple pattern associated with the coating delamination or removal

and similarly with the large defect that is evident. The ripples are due to resonant effects for polarizable elements (defects) on the surface [13]. The presence of these defects and the associated

ripple patterns, combined with the nearness of the threshold to values measured for similar uncoated

surfaces, strongly suggests that damage to this film was initiated by defects on the NaCl surface.

Figure 8 is a Nomarski micrograph of a typical damage site on NaF-coated NaCl substrate. The laser

spot size was 200 ym (.1/e^ diameter). The laser was linearly polarized with the electric field vector

in the vertical direction. Note the defects in the damage area and their associated ripple patterns.

The illuminated area contained many defects, and the specimen damaged at this point with an incident power level approximately 40% below the power level which produced damage 50% of the time in this

specimen.

The average threshold for NaF on NaCl was 77% of the value measured for another commercially polished surface (Harshaw Chemical Co.) using the same laser, focusing lens and diagnostic facilities [6]. This decrease may be due to the introduction of defects in the coating process (either on the surface

or in the coating). However, it is equally likely that this 23% difference is due to variations in surface quality from specimen to specimen.

The effects of surface defects on laser damage to surfaces and coatings are illustrated in figure 9. Note that the coating "separation" line has a set of ripples which contour it. These ripples are in the coating material and are separated by one wavelength in the NaF coating. The ripples on the uncoated surface are on spacings of one wavelength in NaCl. The ripples in the coated area and the uncoated area are physical variations on the surface and not just density or index variations. The micrograph

in figure 9 was taken after the area was silvered. The only effect of the silver coating was to enhance

the visibility of the ripples. The micrographs in figures 6, 7, and 8 are of unsilvered surfaces.

-266-
It is not clear whether the ripples associated with the coating separation line are "cause" or "effect." The separation line itself could be a source for the ripples or it may represent the point

where the intensity of the interference between the incident beam and the dipole field from a surface

defect has decreased to the point that sufficient energy for coating removal is not available. Since

there seems to be a reasonable correlation with the ripples in the uncoated area, the latter case seems

more likely. A NaCl window was coated on each side with a quarter-wave optical thickness of NaF. Since the

square root of the index for NaCl is almost exactly equal to the index of NaF, this window was effectively AR-coated on both sides. The window was placed in the damage chamber, and the entrance and exit

surface (or coating) damage threshold was measured. The two thresholds differed by about 15%. The window was then turned over so that the side that was the exit was now the entrance surface and vice versa.

The result of this experiment was that a given side (i.e., coating plus surface) had the same threshold

for the entrance and exit surface case. This is as expected for an AR coating since the coating/window

system is symmetric with respect to electric field strengths and total transmitted energy; this result

is summarized in figure 10.

Conclusions

NaF, SrF2, and BaF2 offer attractive alternatives to ThFi^ as low index coatings for windows in the

10.6 ym range. NaF is particularly attractive since it can be used as a single layer AR coating at 10.6 ym. The bulk damage mechanism for these materials is Intrinsic dielectric breakdown, and the breakdown thresholds are considerably above those of the NaCl and KCl windows on which coatings are to be deposited. Thus, coatings of these materials should not be limited by intrinsic properties.

Coatings of NaF, SrF2, and BaF2, deposited using state-of-the-art techniques in conventional vacuum

systems, exhibit absorption of a factor of 10 to 200 times that of the bulk single-crystal material.

However, this relatively high absorption does not limit the use of these materials for low average, high

peak power pulsed applications. In order to be useful in systems with high average power outputs this

extrinsic absorption in these materials in thin film form must be reduced.

The peak power handling capability at 10.6 ym for thin films of NaF, SrF2, and BaF2 on NaCl and KCl

is limited by the substrate surfaces. Surface defects interact with the incident beam to produce optical

interference in the substrate surface and/or in the film. Evidence for such interference is seen as permanent damage to the surfaces and coatings that have periodic structure with spacing of one wavelength in the material. NaF is an excellent material for use as an AR coating for NaCl windows in pulsed laser applications.

Since it is a square root of the index match for NaCl, AR coatings can be formed using a single quarterwave layer. For an AR-coated window the entrance and exit surface damage thresholds are equal. Such a window/coating system is symmetric with respect to electric field intensity and power transmission.

Acknowledgments

This work was supported by ERDA, ARPA, and NAVWPNCEN Independent Research funds. The authors are grateful to Mr. Richard Smith, Mrs. Verla Walker, and Ms. Jane Williams for their invaluable assistance

in preparing the artwork and manuscript for publication.

References

[1] Sparks, M. , "Coatings for High-Power Infrared [9] Bass, M. , et al. , "Pulsed Laser-Induced DamOptics," these Proceedings. age at the DF Wavelengths," these Proceedings.

[2] Baer, A. D. , et al. , "Alternative Materials for [10] Fradin, D. W. , et al. , Appl. Opt. 12, 700 Laser Window Coatings," these Proceedings. (1973).

[3] Loomis, J., Appl. Opt. 12, 877 (1973). [11] Von Hippie, A., J. Appl. Phys. 8, 815 (1932).

[4] Soileau, M. J., et al. , in Laser-Induoed Damage [12] Wang, V., et al. , in Laser-Induced Damage in

in Optical Materials: 1975, A. J. Glass and Optical Materials: 1974, A. J. Glass and

A. H. Guenther, eds. (NBS Spec. Publ. A35, A. H. Guenther, eds . (NBS Spec. Publ. 414,

1976), pp. 20-28. 1974), pp. 59-65.

[5] Porteus, J. 0., et al. , op. ait., pp. 207-15. [13] Temple, P. A. and Soileau, M. J., "Resonant

r,, „ .-, T ^ 1 J T, J- ^ Ti- j-j-i Defect Enhancement of the Laser Electric

[6] Soileau, M. J. , et al. , in Froceedzngs of Ftfth ^^^^^ „ ^^^^^ Proceedines Annual Conference on Infrared Laser W%ndow ' ° Materials: 1975, C. R. Andrews and C. L. Strecker, eds. (Defense Advanced Research Projects Agency, Arlington, Va. , 1976), pp. 391-417.

[7] Porteus, J. 0., et al. , "Focusing Method Used

in Laser Damage Experiments," Appl. Opt., in press

.

[8] Yablonovitch, E. , App^ "hys. Lett. 19, 495

(1971).

-267-
1

1^ t/58 0 < t < 58 NS

1-0.0215(1-581 CO £• WHERE 1 - 0.0132 PULSE ENERGY

200 400

TIME, t (ns)

Figure 1. Temporal profile of laser output. The peak power in this

pulse is 1.27 x 10^ times the total energy in the pulse.

The 1/e full width of the pulse is approximately 78.7 ns. The FWHM pulse width is approximately 60 ns.

Figure 2. 10.6 ym bulk thresholds single crystal NaF, SrFz, and

BaF2. The powers given above are peak-on-axis intensities. The high value for the BaFa threshold is probably due to the fact that it is an n-on-one number,

and the others are for single-shot damage.

SrF, 26.1 GW/cm^

BaF, * 33.3 GW/cm'

NaF 30.3 GW/cm'

*n ON ONE THRESHOLD

HAT/OS Of DAMAGE FIELDS IN NaF. SrF2 AND BaF2 TO NaCI

10.6 iim 3.8 iim 1.06 dc

NaF 1.57 > 1.51 1.64 1.60

SrFj 1.39 1.41

BaF, 1.55 1.36

3. Ratios of damage fields in NaF, SrF2

and BaF2 to NaCl. The 10.6 Um ratios were all obtained with the same ex- perimental setup. The 3.8 ym, 1.06 \im, and dc values were obtained from other

work [10,11].

NaF 1.5 X 10-'

Figure 4. 10.6 Vm absorption for quarter-wave

coatings of NaF, BaF2, and SrF2. The

NaF and BaF2 films were deposited on

NaCl, and the SrF2 was deposited on

KCl. 4.9 X 10-'

-268-
ThF^ ON KCI*

0.130 GW/cm'

BaFj ON NaCI

1.65 GW/cm^

NaF ON NaCI

1.07 GW/cm^

SrFj ON KCI

4.24 GW/cm^

•V. Wang, atal.. NBS SPECIAL PUBLICATION

414 (1974), P. 59-65.

Figure 5. Summary of 10.6 Mm damage data for low index coat- ing materials. The power densities given above

are the peak-on-axis intensities which produced

damage 50% of the time. The NaF data were taken

with the 11.91 cm focal length ZnSe lens (1/e^

spot diameter equals 200 pm) , and the BaF2 and

SrF2 data were taken with the 3.546 cm focal

length ZnSe lens (l/e^ spot diameter equals

59.5 ym).

Figure 6. 10.6 ym laser damage to a SrF2 on KCI (exit

surface). This site is typical of all damage

to the SrFz film. Failure appears to have been

initiated at the KCI surface.

100 tim

5.34 GW/cm'

lOO^m

Figure 7. 10.6 ym laser damage to a BaF2 coating on NaCI

(exit surface) . This site is typical of all damage

to the BaF2 film. Failure appears to be initiated

at defects on the NaCI surface. The laser is polarized in the vertical direction in this picture.

The line in upper right represents coating

delamination; a large defect can be seen in lower

right corner.

-269-
0.63 GW/m'

lOO^m

100 ^im n 1.49

X 122

lO.e^jm 10 6ym

Figure 8. Exit surface damage to NaF-coated NaCl, Note ripple structure associated with

surface defects. The laser spot 1/e^

diameter was 200 ym.

Figure 9. Effects of surface defects on window coating damage (exit

surface). Note that the ripple

pattern is associated with the coating separation line. The laser spot 1/e^ diameter was

200 ym. As in all the micrographs presented here, the Incident laser electric field

Is in the vertical direction.

NiF COATING No. 1

EXIT SURFACE

ENTRANCE SURFACE

NiF COATING No. 2

760 MW/cm'

7*0 MW/cm^

EXIT SURFACE 872 MW/cm'

ENTRANCE SURFACE 870 MW/cm^

Figure 10, Entrance and exit surface damage on AR-coated

NaCl. Threshold values given are peak-on-axls

intensities. The 1/e^ spot diameter was 200

ym. Coatings No, 1 and No. 2 refer to AR

coatings of NaF on either side of a NaCl

window.

COmEMTS OM PAPER BV SOILEM, BAER, FORTEUS, JEmOAM, FAITH, DONOVM, MV GREEN

Vojmagz in theAz txpojUmzrvUi mcu, InvoAicLbly a(icompanA.e.d by thz QjnLi,{>ion of, a {^Ituk oi tight, IncUcaJxve. Oj5 pinj,ma donmation. Uzax thuojihold, the. tight midilon wa4 lui Int^me.. Thz dz^zcti

obieAvzd In thts itudy weAe aJboayi on the. iubit/uxte, and weAe attntbuted to the iuA^acz potLiking

pnozeM. The ipeakeA Indicated that the homogenetty o{) the ^tuoAtde {^itmi had not bzzn inveAttgated

thoKoughly.

-270-
A. 3 THE USE OF RARE EARTH FLUORIDES AS

10.6 tim ANTIREFLECTIVE COATINGS

A. Golubovlc, J. Fitzgerald, I, Herman, J. Bruce

R. Bradbury, J. J. Comer, W. S. Ewing and H. G. Lipson

Deputy for Electronic Technology (RADC)

Solid State Sciences Division

Hanscom AFB, MA 01731

The rare earth fluorides have been evaluated as possible antiref lective

laser window coatings at 10.6 ym. Emphasis was placed on SmFs, EuFs, and

GdFa as a single layer coating for ZnSe and as the low index material in a 2-layer design with ZnSe on KCl substrates. The chemistry, structure and

texture of the coatings were examined by several techniques including

electron and optical microscopy, x-ray diffraction, Auger spectroscopy , and

optical spectroscopy. Pulsed laser damage studies at 10.6 pm were performed

to evaluate the relative merits of these coatings. As expected, the damage

threshold for the single layer fluoride coatings as ZnSe were found to exceed

the threshold of the 2-layer designs on KCl substrates.

Key words: Laser antiref lective coatings; materials purification requirements;

pulsed laser damage; rare earth fluorides; zinc selenide coatings.

1. Introduction

In a previous paper [1]^ we reported on the optical, mechanical and structural

properties of several rare earth fluorides, namely LaF^, NdF3, CeFg, and GdPo, for use

as antireflective coatings at 10.6 /urn. There it was noted that these materials might

serve as alternatives to ThFij as low Index coating materials for HEL application. It

is the purpose of this paper to report on a continuation of that study and to present

the results of recent investigations on several more rare earth fluorides namely SmPj,

EUP3, GdF3,and PrF3.

For this investigation we have considered the fluorides as antiref lective coatings

for use on ZnSe as a single layer A/4 design and for the alkali halldes as a doublelayer coating together with ZnSe as the high index first layer. These designs represent practical applications for these low index materials and provide a realistic

means for evaluating their performance in HEL applications. Pulsed laser damage thres*

hold is used as a diagnostic tool in the overall evaluation of these coatings in addition to the structural and chemical analysis by various techniques.

2. Experimental

The potassium chloride substrates used In this investigation were flOO) oriented

Czochralski grown crystals obtained from Optovac, Inc. as 2.5 cm diameter x 1.25 cm thick rough polished blanks. Prior to final polishing, each substrate was checked for

proper orientation by x-ray diffraction. The mechanical-chemical polishing procedures

used for these substrates has been described in an earlier paper [1] and Is similar to

that described by NHL [2]. The substrates used in this investigation were subject to

pitting during mechanical polishing and were Judged inferior to those used In previous

investigations. When etched, a slight mosiac structure was observed for these crystals . The ZnSe substrates were fabricated from Raytheon CVD material obtained from

Dr. J. Pappis. Substrate blanks were cut 2.5x2.5x0.300 cm by Manlabs, Inc. and then

polished and etched according to the method described by Hughes [3].

The two layer antiref lective coating designs for KCl were based on ZnSe as the

high index (n = 2.42) coating together with the low index fluorides. Coating designs

were computer optimized for the individual fluoride coatings based on experimentally

measured indicesfrom single layer fluoride coatings on ZnSe substrates. The coating

design for SmF3 (n = 1.59) is given in figure 1 together with the theoretical and ex- perimentally measured spectral response for this coating pair. All of the two layer

designs have the some broad band response at 10.6 /um. For the two layer coatings on KCl the ZnSe first layers were deposited by both

thermal and R P sputter deposition techniques. The R P . sputter deposited samples

1. Figures in brackets indicate the literature references at the end of this paper.

-271-
were produced from a pressed powder ZnSe target In a Randex turbo pumped system In an ultrapure argon atmosphere. R F power levels between 50 and 250 watts were used

to determine optimum deposition parameters for these coatings which are given in

table 1. The samples were ion etched prior to deposition for 30 seconds at 50 watts

power for improved film adherence. The sputter system did n,ot have provision for

in-situ thickness monitoring, but careful control of pressure and R F power provided

accurate, reproducible calibration of rate. Thus, film thickness was controlled by

time of deposition at a calibrated rate. The final thickness for each coating was determined by measurement of the interference maxima in the spectral scan between 1 and 15 /"m. The thermally deposited ZnSe coatings were produced in a VEECO VE-400 medium highii! vacuum system. Prior to deposition each sample was glow discharge cleaned for a period of 10 minutes in research grade argon gas at a pressure of 150 ym. The starting^

material for these depositions was scrap Raytheon CVD ZnSe remaining from substrate

preparation. Early depositions used a quartz crucible source, however, the films pro- - duced with this crucible material showed unidentifiable lines in powder x-ray analysis 4 For later depositions AI2O3 crucibles were used eliminating this contamination. For ?, these depositions, rate and thickness control were maintained by an Inficon XMS-3 ; quartz crystal monitor which allowed rate control to ±0.1 nm/s. In addition, substrate temperature was controlled by a quartz iodine lamp behind the substrate. The

deposition parameters for the thermally deposited ZnSe coatings are shown in table 1. All of the rare earth fluoride coatings reported in this investigation were thermally deposited in either medium high vacuum or in special cases ultra high

vacuum. The medium high vacuum depositions were performed in a LN2 trapped NRC-801

bell Jar system equipped with an Inficon quartz crystal monitor and quartz iodine

heating lamp. Substrate temperatures were continuously monitored during deposition by r a chromel-alumel thermocouple in direct contact with the substrate fixturlng. For

these depositions no in-situ cleaning was performed, however, prior to deposition the

substrates were heated to 373°K for at least one hour to eliminate water adsorption.

The ultrahigh vacuum depositions were performed in a General Electric all stainless steel ion pumped system equipped with quartz iodine heaters, a quartz crystal

monitor and a UTI 100 c precision mass analyzer. No provision was made for in situ

cleaning in this system, however, a 525°K bakeout of the entire system insured

elimination of adsorbed water from the substrate prior to deposition.

The starting materials for the fluoride depositions were either Cerac/pure

(99.9^) or Ventron optical grade. No detectlble difference were observed from these

different starting materials. Each starting material was powder x-ray analyzed to

determine rare earth oxide content. In the case of PrF3, PrOF was detected as a major

contaminant of the starting material. In those cases where powder (-325 mesh) starting material was used, it was necessary to vacuum press the material to prevent

spatter as the source was heated.

The deposition parameters for each of the fluoride materials is summarized in

table 2 together with the measured indices for the coatings. For the EuFo deposition,

the parameters given are for the cubic difluoride EuF2. The preliminary depositions

of EUF3 were found to be erratic, leading to a two stage deposition beginning with the

trlfluoride and finishing with the difluoride. The initial stages of these depositions were characterized by rapid pressure and deposition rate fluctuations. The

coatings produced from this stage were found to be unstable and very highly stressed.

The second stage of these depositions was controlled and required additional source power to establish rate. Coatings produced from starting material after taken com- pletely through stage one, or using already sublimed material were found to be stable,

clear and adhered well to all substrate combinations. The trlfluoride and difluoride

structures were identified by x-ray and reflection electron diffraction of stage one and stage two material.

3. Characterization

The structure and chemistry of the coatings studied in this investigation were characterized by a variety of techniques. X-ray and electron diffraction were used to determine the crystallinlty , preferred orientation and composition of the coatings.

Powder x-ray dif fractrometry was used to determine the gross composition of the starting material. X-ray diffractrometry scans of the coated samples were used to determine preferred orientation where possible. Reflection and glancing angle electron

diffraction patterns were also used to establish structure and preferred orientation

of the coatings prior to removal from substrates. Reflection electron diffraction

patterns were also made for several films at the substrate-film Interface by

-272-
floatation of the films from the substrates. Identification was made of the various

structures by comparison with the ASTM Powder Diffraction Pile. For the ZnSe coatings.

Auger spectroscopy was used to determine the relative stochiometry of the films by

profiling techniques. Surface topology was studied by Nomarski interference contrast

microscopy as well as both scanning and transmission electron microscopy. Spectral

scans from 1-15 were made of each coated sample as well as laser calorimetric

absorption measurements at 10.6 pm. Finally, pulsed laser damage measurements at 10.6

fim were used to evaluate and compare the various coatings and coating parameters. The

results of these various analyses are summarized below.

The sputtered ZnSe coatings on KCl were identified as cubic with a strong preferred orientation of the (lll| planes parallel to the substrate surface. This same preferred orientation was observed on glass monitor slides and is the same as reported

earlier [4] for ZnS sputter deposited coatings. Figure 2 shows an electron micrograph

of a surface replica of a typical sputtered ZnSe coating together with a reflection

electron diffraction pattern of the surface. The mlcrocrystals visible in this micrograph are believed due to dust falling from the pressed ZnSe sputtering target during

deposition. Finally, the dark artifacts which appear In the coating are believed to be non stochlometric inclusions of ZnSe, however, no free zinc or selenium was detectable

by powder x-ray diffraction techniques.

The thermally deposited ZnSe was also found to be cubic with a preferred (111)

orientation as determined by x-ray diffraction. Figure 2 shows an electron micrograph of a surface replica of the thermally deposited ZnSe together with a reflection

electron micrograph of this surface. The diffuse electron diffraction pattern suggests

a somewhat less ordered surface than the sputtered ZnSe coating. The overall smoothness of this coating is superior to the sputtered coating surface with only small

micro crystals interupting the texture.

The quality of the thermally deposited coatings was strongly dependent on deposition rate. High rate depositions produced highly stressed films which crazed immediately when removed from the deposition chamber. In addition, the high rate films

tended to have a deep orange color suggesting decomposition, however, no deviation

from stochiometry could be detected by Auger spectroscopy measurements of these films.

The sputtered ZnSe coatings were more adherent than the thermally deposited films.

None of the thermally deposited films survived the high humidity of our laboratories

without crazing. However, the sputtered films of ZnSe were not masked and the coating

extended over the edges of the substrate as well as the surface, which allowed all but

the most highly stressed films to adhere to the substrate without crazing. All of the

two layer coatings reported on here had sputtered ZnSe first layers.

The surface textures for the four fluorides deposited on ZnSe are shown in the

electron micrographs of figure 3. The hexagonal structured SmFg and PrF^ are smoother

with fewer surface features than the cubic EuF2 or the orthorhombic GdF3. X-ray

diffraction measurements showed preferred orientation for all of the fluorides on both

ZnSe and KCl substrates. For the hexagonal structures this preferred orientation was easily identifiable as (100). However, the orientation of the GdP3 and the EuF2 could

not easily be determined from the limited x-ray data available.

The transmission spectral scans for 1 pm films of each of the fluorides deposited

on KCl are shown In figure 4. The absorption peaks at 3 and 6 pm seen in all of these

coatings is presumed to be due to OH. It is of note that hexagonal structures show

greater absorption at these wavelengths than either the cubic or orthorhombic materials. In an attempt to eliminate these absorption bands, two experiments were performed for LaF^ to reduce the OH in the deposited films: 1) a codeposition experiment

involving a simultaneous deposition of Ca metal during the fluoride deposition, and

2) deposition in ultra high vacuum with known low H2O and OH residual gas pressures.

For experiment 1, a second filament deposition arrangement was Installed in the NRC

medium high vacuum system with a pyrex glass shield around the filament. This allowed

a continuous evaporation of Ca metal without contaminating the system. A few minutes

before depositing the fluoride coating, the Ca source was brought to deposition temperature and a low rate deposition was established. During the fluoride deposition

the Ca deposition remained continuous providing a continuously replenished getter for

OH and other getterable materials. This procedure reduced the 3 and 6 ^m OH bands in

LaFg by 40% using a normal deposition as a reference. The second experiment Involved

deposition of LaF^ in well characterized ultra high vacuum using the ion pumped system

described above. During the deposition the residual gases present In the system were continuously monitored between mass 2 and mass 70. At no time did the residual gas

pressure of H2O exceed 7x10-8 pascal during the LaF-^ deposition. The effect of the

ultra high vacuum on this deposition was dramatic, with an 86% reduction of the 3 and

-273-
6 /um absorption bands in this material. The transmission spectral scans resulting

from these experiments is shown In figure 5 together with the spectra for LaPo from a,' medium high vacuum deposition. Ultra high vacuum depositions were also tried for

SmF3 and CeF3, however no effect was seen for these materials. It was also determine'

that the fluorides with less OH absorption had greater indices of refraction. This

effect has been reported elsewhere [5]-

10.6 lum laser calorlmetry measurements were made for each of the fluorides deposited on KCl. The results of these measurements are summarized in table 3 and are expressed in terms of the ratio of power absorbed by the coating to the power trans- i mitted. These absorption measurements are relatively high yielding film absorption Ij values between 100 and 200 cm~l. The Fourier spectral scans of these materials (see , figure 6) suggest residual contamination of the substrate as the cause of this high

absorption.

Pulsed laser damage measurements at 10.6 Mm were used for the final evaluation o; these coatings for HEL application. The experimental arrangement used Is Identical tc the one described In an earlier paper [4] and is shown diagramltically in figure 7- The characteristics of the TEA CO2 laser used are given in table 4. The Incident

energy density was controlled by placing calibrated CaF2 absorbers in the beam betweer:

the laser and the GaAs focussing lens. For all of these measurements a I80 ym spot

size was used. Ten selected sites on each coating were irradiated at each energy , density (one on one firings are reported here). Damage was evaluated by Nomarskl

microscopy and statistically analyzed to determine the damage threshold for each

coating. In most cases a minimum of two samples of each coating design were analyzed.

The results of these measurements are given in table 5, and as expected, the damage

thresholds were the highest for the single layer coatings on ZnSe. The damage

threshold for the sputtered ZnSe coating on KCl is particularly interesting. The

threshold of 20 J/cm2 reported here is comparable to the threshold of thermally deposited ZnSe on KCl and Indicates that for this material sputter deposition is a viable

alternative to thermal deposition.

The damage thresholds for the fluorides reported here are lower than the value

measured for CeF^ coatings (35 J/cm^) reported in an earlier paper [4]. The double

layer damage thresholds are all similar and indicate that interfaclal damage between

coatings is the limiting factor for this design. Of particular Interest Is the damage

threshold for EuF2 on ZnSe which was the lowest threshold reported for the fluoride

single layers. This is surprising since the absorption coefficient at 10.6 ^m was the,

lowest for the fluorides reported and additionally this material was cubic (and thus

Isotropic) with only small 3 and 6 fj-ra absorption bands.

4. Discussion

The rare earth fluorides under consideration are the compounds of the form XF3

where X is a metal anion with atomic number between 57 and 71- Three of the fluorides

of this series are also known to have stable dl-fluorides namely EuFp, SmF2 and YtF2.

The europium difluorlde has been reported on here. These fluorides are all low index

materials with useful transmission between the near UV and the Infrared to a least

15 y^- The stable crystal structures of the rare earth fluorides reported are either

hexagonal or orthorhombic in the trl fluoride form and cubic (fluorlte structure) in

j the difluorlde form. The hexagonal fluorides tend to be susceptible to water absorp- j tlon when deposited as thin films. This may be due to a fairly open structure for 1 these films, and the results of the ultra-high vacuum deposition for LaF^ seem to sup-,

port this hypothesis since for at least this material water absorption was partially i eliminated. A final test of the absorption mechanism will require In situ measurements of the 3 and 6 Mm absorption bands for these films.

The structural stability of these films has not been treated here. There is

evidence that the fluorides are structurally unstable when measured over long time

periods [6]. Thin films of those fluorides which are known to have both hexagonal

and orthorhombic structures have been found to be of the stable hexagonal form. Obviously, more work Is needed in this area. The starting materials used in this investigation were of the highest comercial

purity available. The heavy metal Impurity levels have not been established for these

materials and for most of them only x-ray diffraction has been used to evaluate

presence of contamination. The chemical nature of the rare earths makes it difficult

to prepare ultra pure starting material and further work is needed In this area. It

Is clear from the work on ThF/j [3] that ultra pure starting materials are necessary

for" the optimization of the fluorides for use as HEL coatings.

-274-
References

[1] Golubovlc, A. at al Proceedings of [4]

the Fifth Annual Conference on Infrared Laser Window Materials, 1976, C. R. Andrews and C. L. Strecher,

Editors. [5]

[2] Davlsson, J. W, Proceedings on High

Power Infrared Laser Window Materials, 1972, C. A. Pltha, Editor. [6]

[3] Bijaunsteln, M. , Braunstein, A. I.,

Zuccaro, D. and Hart, R. P., Proceeding of the Fourth Annual Conference on Infrared Laser Window

Materials, 197^, C. R. Andrews and

C. L. Strecher, Editors.

Ewlng, W. et al ., Proceedings of the

Fifth Annual Conference on Infrared

Laser Window Materials, 1976, C. R. Andrews and C. L. Strecher, Editors.

Bananova, L. P. and Grigor'Eva, G. N. Izvestha Sibirskogo Otdeleni'a Akad.

Nauk SSSR 2, 115 (1962) . Holmes, S. J. and Kraatz, P., AFML

Technical Report No. AFML-TR-75-188

,

January 1976.

Table 1. Deposition parameters for ZnSe on KCl

sputtered coatings.

Tables and Figures

Table 2. Deposition parameters for fluoride coatings.

Sputter Etch P = 50 watts - 50 s

Deposition P = 100 hatts - 1 hour

Argon pressure 1.3 x 10'^ torr

Thermal Deposition

Glow discharge cleaning - Argon Gas - 10 minutes

Deposition; Rate = 2A°/S T; = 70°C

Source: AL2O3 crucIble

Coat i ng Substrate TjCOC) Rate

ShF, KCl 150° 0.5A°/S

(Cerac/Pure)

N = 1.59 - 1.60

ZnSe 150° 0.5 - 1.0A°/S

(KCl) ZnSe/ 100° 0.5fl°/S

EUF3 (EuFj) KCl 150° G.5fl°/S (Cerac/Pure) ZnSe 120°-150° 0.5 - 1.0A°/S

N = 1.59 - 1.60

(n = 1.55 - 1.56) (KCl) ZnSe/ 100° 0.5A°/S

GdFj KCl 150° 0.5 - 1.0A°/S

(Cerac/Pure) ZnSe 150° 0.5 - 1.0A°/S

N = 1.56

(KCl) ZnSe/ 90° - 120° 0.5A°/S

Table 3. 10.6 \im absorption data for single layer

coatings on ZnSe.

Coating Thickness Pabs/P(,

(watts) (ym) (coating) PULSE ENERGY REPETITION RATE

G0F3 2.50 1.75 0.052 SPATIAL MODE

SMF3 2,55 1.40 0.054 MODE DIAMETER LONGITUDINAL MODES

EuFj (EUF2) 5.00 2.19 0.032 PULSE WIDTH

Table A. Characteristics of TEA CO2 laser pulses.

55 MILLIJOULES

1 PPS

TEMoo, GAUSSIAN TO I/Io i 0,01

5,5 Mn MANY

100 ns FWHM

Table 5. 10.6 \im pulsed laser

damage for reported

fluoride coatings.

coating substrate threshold (J/cn2)

EuFj KCL

(KCl) Z»Se/ 6.5

ZnSe ll.S

GdFj KCl W

(KCl) ZnSe/ 6.5

ZnSe 18

KCl 10.5

(KCl) ZnSe/ e.o

ZnSe 20

PrFj (KCl) ZnSe/ 7.0

ZhSe KCl 20,0

-275-
10 n

KAVELEi'lGTH, ' \m

Figure 1. Spectral response of ZnSe/XFa antireflective coated KCl.

Figure 2. Electron micrographs comparing texture of sputtered and evaporated ZnSe on KCl,

Figure 3. Electron micrographs comparing texture of evaporated fluorides on ZnSe.

-276-
100

s 100

£ 100

in

100 ^

OPTICAL TRANSMISSION XF^ ON KCl (1 pm)

(80)

prFgOg.g)

(80)

SmFjOQ.g)

(80)

EuF3(EuF2)(99.9)

(80)

GdF3(99.9)

1 1 1 6 10 WAVELENGTH pm

15

Figure 4. Optical transmission of XF3 on KCl.

100

100

S 100

'

(SO) 100%

10"'' TORR, Ca CODEPOSniON

(30) 10"^° TORR, UHV m

1 3 6 10

WAVELEIIGTH pn

15

Figure 5. Transmission spectra of LaFa vs deposition vacuum Integrity,

t 30

CDI

70 p

1 60

HAVELEflGTH . A h—

SmFj on ZnSe

EuF? ON ZnSe — • - 6DF7 ON ZnSe

JO, 20,

Figure 6. Fourier spectral response of fluoride

films.

-277-
C«r ATTtNUATORS 10 cm LENS DAMME

SAHPLC

Figure 7. Experimental set-up for pulsed CO2 laser

damage experiments.

COMMENTS OhI PAPER 8/

GOLUBOnC, flTIGERALV, 8ERMAN, BRUCE, BRAPBURV, COMER, EWIWG, AW LIPSON

No dLicaii-Lon on thu> papeA,

-278-
4.4 PERFORMANCE OF Ge„ ,cSe„ ^^/ks^S^ ANTIREFLECTION

0.45 0.55 2 3 COATINGS AT 3.8, 5.3, and 10.6 ym

John F. Lewis and Melvin C. Ohmer

Air Force Materials Laboratory

Wright-Patterson AFB, OH 45433

Transparent vitreous films of Ge^Se^ ^ have been grown by conventional

evaporation techniques whose refractive index can be varied from 2.5 to 3.1 by

varying x from .35 to .5. For x=.45 it is possible to construct a two-layer

double-quarter wave AR coating where Ge ^^Se is the layer nearest the substrate and AS2S2 is the outer layer. In a cooperative program, AR coatings

of this type were synthesized at the Materials Research Laboratories, Victoria,

Australia, and evaluated at the Air Force Materials Laboratory. AR coatings

[ I were deposited on Eu doped KCl (10,6 \im) , BaF2 (5.3 ym) , and CaF2 (3.8 ym) . The

coatings were characterized with regard to absorption, transmission, and bandwidth. For the KCl sample, the cw CO2 damage threshold was determined. The absorption per surface for all wavelengths was less than .1%. Absorption bands

at the shorter wavelengths were not visible in the spectrophotometer data.

Key words: Absorption; antiref lection coatings; germanium selenide; infrared

laser windows.

INTRODUCTION

The infrared properties of Ge^Se^^ ^ alloy films have been investigated extensively by Butterfield

[11 and Gibbs and Butterfield [2]. Films containing up to 50% germanium can be produced by evaporation

from carbon boats and the film composition is identical to that of the initial alloy. The index of refraction of these films may be varied from 2.5 to 3.1 by varying x from .35 to .5. The films show no evidence of crystallinity and have no absorption bands between 2.0 and 15 ym. The extinction coefficient

[2] for GeSe films at 10.6 ym is "of the order of 1.4 x 10"^^ corresponding to a beta of 16 cm"-'-. The

beta value is comparable to that of good ThF^ at 10.6ym. The films are reported to be physically robust

and to adhere strongly to many substrates. Therefore, Ge^Se^ ^ films are promising as components of

antireflection (AR) coatings for infrared lasers.

In an exchange program, AR coatings were deposited by the Materials Research Laboratories [3] on

CaF^, BaF2, and Eu doped KCl substrates supplied by AFML for, respectively, 3. 8, 5.3 and 10.6 ym. All

substrates were polycrystalline. The coatings were in the half aperture format and on one side only.

The coated substrates were returned to AFML and characterized with regard to absorption, peak transmission, bandwidth, physical thickness, and transmision in the visible. For the KCl sample, the cw CO2

damage threshold was determined.

AR DESIGN

The AR design was the two-layer double-quarter type where Ge ^^Se is the layer nearest the substrate and AS2S2 is the outer layer. The indices nearly satisfy the relationship n^=n^/rP' where n^^ is the layer nearest the substrate and n^ is the substrate index. The design indices [4] for 10.6, 5.3

and 3,8 ym were, respectively, for AS2S3 2.376, 2.395, 2.397, and for Ge^^3Se 2.866, 2.886, 2.896. The

Calculated reflectivity for all three wavelengths is less than .001.

1. Figures in brackets indicate the llterlature references at the end of this paper. -279-
ABSORPTION

The absorption was determined by vacuum laser calorlmetry. The laser beam was incident on the coated surface. The absorption values at 5.3 ym and 10.6 ym were obtained from the University of Dayt<i

Research Institute (UDRI) and at 3.8 ym from the University of Alabama at Huntsville. The results of these measurements are summarized in table 1. The substrates were nominally 1 cm thick. The absorption per surface was .1% or better at all three wavelenths . At 10.6 ym, this is the best result ever attained in a two layer coating on KCl. The value of the AR coating absorption at 3.8 ym indicates that the materials incorporated into the design are not plagued by water absorption bands. While the coating absorption at 5.3 ym is very low there are harder coatings designs [5] with lower absorptions.

4 LASER DAMAGE THRESHOLD AT 10.6 ym

A Sylvania 971 CO2 one kilowatt laser was utilized to determine the damage threshold of the coated

KCl sample. The beam was focused to a diameter of .8mm and the power varied from 100-900 watts. The coating displayed a conditioning effect. Without conditioning, the coating would fail at 300 watts

corresponding to a respectable threshold value of 60 kW/cm^. With conditioning, a level of 500 watts was observed. The threshold is defined in terms of the average power (the total power divided by

2 the area of the beam at the 1/e point). However, irreversible strain was introduced in the coatings at

a level of 100 watts. Since only one sample was available these damage measurements should be con- sidered as preliminary.

SPECTRAL TRANSMISSION

The spectral transmission of each AR coating was obtained on a Perkin-Elmer 621 spectrophotometer.

The transmission vs wavelength for the 10.6 AR coating is shown in figure 1. The experimental peak

transmission for the 10.6, 5.3 and 3.8 ym coating occurred at, respectively, 10.6, 5.5, and 4.1 ym. Th bandwidth, the wavelength region over which the transmission is within .5% of the peak value, was .2, .37 and .3 ym for .respectively, the 10.6, 5.3, and 3.8 AR coatings. The coating thicknesses were controlli

by a 1.55 ym laser optical thickness monitor. For the 10.6 ym coating the required thickness, compensatll for dispersion, corresponded to an integral number of fringes (seven). Therefore, it was easier to accurately control the thickness for the 10.6 AR coating. The total physical thicknesses of the two layer coatings were determined on a Sloan Dektak and are listed in table 2. The theoretical thickness | was determined from the AR design. The experimental peak transmission and the experimental thickness were used to calculate the expected theoretical spectral response for the 3.8 AR coating. The reduced

| optical thickness for each coating at 3.8 ym was .27 instead of .25 and the peak transmission was 4.1 ]im^ An 8% thickness error in each coating thickness shifts the transmission peak from 3.8 to 4.1 ym. |

j OPTICAL DENSITY j

It is very desirable that infrared laser windows be transparent in the visible for ease of align-

' ment. The optical density (CD = logj^Ql^/I) of AR coatings was determined at 6328A. The OD was 1.0, 1 1.4 and 2.9 for, respectively, the 3.8, 5.3 and the 10.6 AR coatings. These OD's correspond to 10%, 4% and .1% transmission. The 10.6 coating appears opaque and metallic, and the other coatings appeared 1 reddish. The coatings showed no stress induced birefringence even though they were deposited on | polycrystalline substrates.

CONCLUSIONS

The Ge ^^S ^^2^3 ^ coating absorption was the lowest ever measured for a two layer AR coating

on potassium chloride at 10.6 ym and its damage threshold is acceptable. Many coating materials con- sidered for application at 2-6 ym have water absorption bands at 2.9 and 6.1 ym. Ge ^^Se 55-AS2S3 AR

coatings do not have that problem resulting in a very low coating absorption at 3.8. On the negative

side the 10.6 ym coating is not transparent in the visible. It would be interesting to investigate two layer AR coatings which utilize appropriate GeSe alloys for each layer.

ACKNOWLEDGMENTS

The authors would like to thank William C. Schoonover of the Air Force Avionics Laboratory for arranging the substrate and sample exchanges.

-280-
REFERENCES

[1] A. W. Butterfield, Thin Solid Films, T3, 191 (1974).

[2] W. E. K. Gibbs and A. W. Butterfield,

Applied Optics, 14, 3043 (1975).

[3] Materials Research Laboratories, Cordite

Avenue, Maribyrnong, Victoria, P.O. Box 50, Ascot Vale, Vic, 3032.

[4] A. D. McLachlan, Private Communication.

[5] Melvin C. Ohmer, AFML-TR-76-103.

Substrate

Table 1. Absorption of Anti-Reflection Coatings Determined By Calorimetry

Wavelength A ^10

o A tIOo -4

% per

surface

KCl

BaFj

CaF„

10.6

5.3

3.8

8.9

11.9

51.3

19.5

19.0

56

.11%

.07%

.05%

Table 2. Comparison of Theoretical and Measured Coating Thickness

Substrate Theoretical

Thickness

Experimental

Thickness

% Error

CaF^

BaF„

KCl

7,244 A

10,124

20,400

7,875 A

10,500

21,060

8.7

3.7

3.2

-281-
Figure

COmENTS ON PAPER BY LEWIS AWP OHMER

Wo di6cjLU>'!,Zon on tkci papeA,

-282-
4.5 AR-COATED KCl DAMAGE AT 10. 6 |jxn*

John A. Detrio and Roger D. Petty

University of Dayton Research Institute Dayton, Ohio 45469

and

Melvin C. Ohmer and Orven F, Swensen

Air Force Materials Laboratory

Dayton, Ohio 45433

Antireflection coatings on KCl windows for high power CO^ lasers

have been studied using focused radiation to establish relative damage

thresholds. Single layer coatings composed of Til, multilayer designs employing Til and either KCl or ThF^ were capable of withstanding 9 seconds

irradiation at an average power density of nearly 130 kW cm ^. Two definitions

of the onset of mechanical damage were used; physical degradation including

thermally induced fracture, and the first appearance of permanent residual

strain as observed in a plane polariscope. Nomarski microscopic observations were also utilized to search for subtle signs of damage. Data were taken to detect any "conditioning" effects due to below- threshold irradiation. Conditioning was found to be negligible in all coatings studied except possibly one. No

special handling precautions were taken and therefore these values represent

the inclusion of ordinary laboratory environmental degradation. Correlations

between damage levels and factors such as absorption were investigated.

Keywords: Alkali halides; antireflection coatings; coatings; KCl; laser damage.

1, Introduction

In order for alkali halide materials such as KCl to be practical windows for high power infrared

lasers, it is essential that they be antireflection (AR) coated both for the control of reflected energy

and for environmental protection. Several promising AR coating designs employing Til on KCl have

been evaluated. The coatings have low absorption at 10. 6 yxn and low scatter also. In order to assess

the power handling capability of these coatings they were tested in the focused beam from a 1 kW CO^

laser. Since this study represents an initial survey of such materials, the damage tests were struc- tured so that the effect of "conditioning" and subtle forms of damage could be investigated following

the tests. The paucity of samples leads to a restricted definition of damage thresholds which does not permit any statistically sound comparison or evaluations,

2. Experimental

The basic experimental apparatus is that described in previous CW laser damage work con- ducted at our laboratories [ l]^. The test plan employed in this study represented a considerable

compromise in obtaining data on the various parameters which might be used to characterize the

laser- induced damage of AR coatings. The parameter of greatest interest is the so-called "damage

2 threshold" (kW/cm ). Radiation densities above this threshold always destroy the coating. Because

the damage can be initiated by defects in any practical window it is also of interest to obtain damage

data as a function of area, either by using a variable beam size or by multi-point sampling. Certain

other systematic aspects of the damage testing must be \mderstood and controlled in order to obtain

meaningful damage data. These effects include the influence of focused radiation, temporal effects in both irradiation and storage times, and the existence of "conditioning" [ 2] effects in which prior ir- radiation below threshold tends to improve the material' s damage resistance. The physical format of

* This work was supported by the Air Force Materials Laboratory, Wright- Patterson Air Force

Base under contract no. F 336 1 5- 76- C- 5092.

1. Figures in brackets indicate the literature references at the end of this paper.

-283-
the coatings (full aperture both sides, half aperture one side, quadrant coated one side, etc.)» and a desire to search for the existence of subtle damage indicators or precursors has greatly complicated

the testing. The influence of coating design (thickness and order in multilayer designs) and the

influence of coating deposition parameters and correlations between nondestructive measurements

such as absorption and damage levels are very desirable.

Our approach to these varied requirements has been to divide the 1-1/2" diameter circular

samples into four quadrants to conform to the quadrantly coated sample format. The irradiation site location is shown in figure 1. Six locations are generally available for irradiation in each of the four

quadrants. When the sample fractures or otherwise degrades, there is a chance that the damage may

extend into another site and thereby reduce the total number of test locations available. To minimize

the potential impact of this problem, the sample is flooded with a stream of dry nitrogen from below

and an exhaust system is set up over the sample to collect any debris or smoke and draw it upward.

Depending on the coating format, either of the upper quadrants I or II was tested first. A series of exposures starting at 100 W, the lowest power available, were made with increments in power up to the maximum available- -about 1 kW. If the sample is damaged, the second location, B was irradiated

at a power 20-30% below that which produced damage at the first position, A. li damage is observed

a further reduction in power below the damage threshold is made and a new location is tested. If no damage occurs, the power is increased at suitable increments (50- 300 watts) until damage is again

observed. If no damage is observed the remaining locations are each irradiated at different power

levels below threshold to permit post- test investigations of more subtle forms of degradation. With

this strategy, conditioning, damage threshold, and subtle forms of damage can be simultaneously

searched for. Not all sample formats permitted this same strategy to be employed with equal success nor did all of the samples cooperate in the tests by maintaining their physical integrity after the first damage occurred.

The samples were irradiated for times of 9, 3, or 1 second required to obtain damage within

the power levels available. Generally, 9 seconds is considered sufficient for these tests based on peak temperature and stress considerations and the spacing between the irradiation sites. Longer

times will tend to raise the average temperatures without significantly changing the local temperature

and stress gradients. For focused damage tests, we therefore elected to utilize the longest times

consistent with the test requirements.

The spot sizes and powers were measured before or after a given exposure. The power is read with a CRL Model 213 power meter which has been checked against a total capture ballistic calorimeter. The spot size measurements were made using burn patterns in plastic blocks. The

spot size was found to be somewhat irregular with a 0. 075 cm ± 0. 012 cm diameter, full width at half maximum (FWHM) measured at power levels from 100 to 500 watts. The spot size did not appear to be sensitive to the incident power, but the depth of penetration was linear with incident power only up

to 500 watts (see figure 2). The spot size (FWHM) used in reporting these data is 0. 08 cm and has a standard deviation of about ± 20%. The spot size data are probably the weakest part of a good power

density measurement and therefore only average powers are given. The average power will be de2 2

fined as the total power divided by the area of the beam at the 1/e point. The 1/e diameter is "VZ In 2 times the FWHM or 0. 094 cm. The experiments were conducted in much the same way as our previous KCl window damage

tests with all observations being made through polariscopes. The value of the polariscopes was

greatly diminished in the case of Til coatings where natural birefringence dominated the observed

behavior over any induced strain effects. Photographs were made before and after each exposure using 35 mm film (KODAK TRI-X) and

a dark field linear polariscope. The exposure is 1/2 second for the entire field with a strobe supplying light for the polariscopic observations. This arrangement gave the most consistent results with

respect to exposure times through the telephoto lens system. The aperture was set at f/11 to insure

sufficient depth of field. Following development, the film rolls were contact printed for ease of study. The samples were also photographed in polarized light before and after completion of the

entire test. The damage threshold is defined as the lowest power at which damage was observed. This

definition is not consistent with our previous definition which would be the power half-way between

the level at which damage is observed and the next lowest level at which no damage occurs. Suffi- cient data do not exist for these samples to use Loomis' criteria f 3] for damage threshold.

-284-
3. Data

The data for damage threshold, as defined above is summarized in table 1 including the first appearance of residual strain and the detection of physical degradation. The number of damage sites on each sample or quadrant are also given in table 1. The photographs of the samples taken after

completion of the testing are shown in figure 3 and also serve to summarize the strain and visible

damage. In those cases in which the coatings are on both sides of the sample, the data may include

information on the damage to each side. In general, the incident side is the primary side being tested.

In several cases the first side tested had a higher damage threshold; therefore, the exit side damage

defined the damage limit at a given location.

The exposure data for the various coatings tested are summarized graphically in figures 4 through 7. The sequence of "shots" is shown by the open squares and circles for radiation incident on "Side 1" and "Side 2", respectively. If only one side was irradiated, the coated side is designated as "Side 1". To avoid the repetitious appearance of the damage test sequence graphs, only

representative data are shown. Additional data concerning the thin films are listed in tables 2-4. A Nomarski microscope was utilized to search for subtle forms of surface damage or deformation. No useful information was gathered by this technique since the appearance of strain in the

polariscopic photographs is the most subtle form of degradation observed.

4. Discussion of Results

Earlier studies [ 1, 2] have shown a quadratic relationship between damage and absorption for

alkali halide materials. The damage resistance of these materials is dominated by their surface

absorption [ ij and it is reasonable to examine these data for such a relationship. The absorption

data shown in figure 8 were taken in our laboratory. The curve does not show a clear trend since the coating absorption data are not equally distributed along the abscissa nor did the sariiples all suffer

damage. There is a general trend for highly absorbing samples to damage more easily. Too few

data exist in this study to provide a sound basis for any empirical or theoretical relationship between

damage and absorption. In most cases these absorption data have been in excellent agreement with

Table 1. Coated KCl 10. 6 |jLm data summary.

Damage I^evel and No. of Damage Sites Sample

No, LocationQuad Side Coating DesignSub/Cj/C^/Cj

Coating

Format

Strain kW/cm^ (No.)

Decomposition

kW/cm^ (No.)

Absorption

(percent)

8A5 I & II KCI/TII Half Aperture >144 (0) >144 (0) 8C6 I & II KCI/TII Half Aperture > 91 (0) > 91 (0) 8B5 I KCI/TII/KCI/TII Full 57 (13) >129 (0) 0.07

8B5 129 (3) 0.07 8A7 I KCl/TlI/ThF^/TU

4 Quadrant >136 (0) >136 (0) 0. 20 8A7 II KCI/TII >129 (0) >129 (0) 0. 14 8A7 IV KCl/ThF^

4

29 (4) 115 (1) 0. 12 8B2 II KCl/TlI/ThF^/TlI

4 Quadrant 86 (1) 0. 21 &B2 III KCl/ Til 126 (2) o; 18 8B2 I KCl/ThF^

4

57* (3) 0. 20 8B6 I KCI/TII Half >1I5 (0) >115 (0) 0.03

8A6 I KCI/TII HaH >137 (0) >137 (0) 0.03

8B4 KC1/ ZnSe / KCl/ ZnSe > 86 (1) 129^ (2) 0.09 8B4 43 (3) 0.09

8D2 I KCI/As^Sj/KCl/As^Sj Full < 14 (3) 3.0 8D2 II < 14 (3) 3.0 8C2 KCl/TlI/AgCl/TlI Full Both Sides 21. 5 (11) 0. 28 8C2 2 < 14 (13) 0. 28

a Sample fractured on initial exposure

b Showed conditioning. -285-
Table 2. Tll coating design data.

Sample^ Materials Thickness

(Micrometers)

Refractive

Index

Coating

Absorption

(Percent)

8A5 Tlx 1. 0 3 (X /4) 2. 4 0. 1 8A6 Tll 1. 10 (\/4) 2. 70 0. 09

8B6 Tlx 1. 08 {\/4) 2. 41 0. 03

8C6 Tlx 1.77 (\/2) 2. 47 0. 03

a Substrate designations are, A and C EurKCl; B and D Rb:KCl.

Table 3. Two- layer coatings design data.

Sample Materials Thicknes s (Micrometers)

Refractive

Index

Coating

Absorption

8A7-I TlX/ThF,

4

2/1, 2 (AR) 0. 20 8A7-II Tlx 2. 57 - 2. 43 0. 14 8A7-IV ^\ 1. 30 - 1. 35 0. 12 SB 2- II TlI/ThF^ 2/1. 2 (AR) 0. 21 SB 2- III Tlx 2. 55 - 2. 51 0. 18 8B2-I ThF/

4

1. 33 - 1. 35 0. 20

a This lay er failed the " Scotch tape" test.

Table 4. Three- layer coatings design data.

Sample Materials Thickness

(Micrometers)

Reflectance

Coating

Absorption

(Percent

8B5 TlI/KCl/TlX (0. 88/0. 81/0. 48)^ 5. 5% R 0. 07

8B4 ZnSe/KCl/TlX (0.88/0.81/0.48)^ 7. 0% R 0. 09

8C2 TlX/AgCl/TlX (1. 0/1. 25/0. 12)^ 4. 5% R 0. 28

a These are full aperture coatings and the designed thicknesses are shown.

that obtained by the supplier. We have observed an increase in the absorption of uncoated samples

exposed to the laboratory environment. This atmospheric degradation will probably adversely affect the data for samples that are partially coated or coated only on one side. The coating damage observations will be discussed in three categories of coatings: Tll in single layers, TlI/ThF^ two- layer designs, and three-layer designs.

The Tlx coatings of both \/4 and \/2 thickness were very damage resistant to focused CO^

radiation. The absorption is also low in these coatings as shown in table 2. The film' s microstructure and cpmposition were investigated with x-ray techniques and light microscopy. The films are shown at low magnification in figure 9 which serves to illustrate the wide variations in the apparent

structure of the Tll film- -yet all were quite damage resistant. X-ray diffraction studies showed that the KCl substrates for samples 8A5 and 8C6 were polycrystalline but highly textured with the (220)

approximately parallel to the surface. The coating was also identified as Tll with an orthorhombic

structure. No evidence was found of CoCl or NaCl type lattice forms or of the iodine rich Tl^I^ or

Tll phases.

-286-
The TlI/ThF^ two- layer coatings were observed to damage when the ThF^ coated quadrant was

irradiated. The two-layer AR design quadrant was destroyed by a "Scotch tape" test of adhesion

prior to the laser damage tests. Thus, only the Til layer was available for study. Previous experi- ence in our laboratories with ThF^ coatings has shown that the high 10. 6 \xm absorption of these

films is related to the presence of oxygen. Consequently Auger spectroscopy was used to study the

chemical composition as a function of thickness through the ThF^ film The in-depth profile plots for

samples 8B2 and 8A7 are shown in figure 10. Although the fluorine deficiency seen in these graphs is partially an artifact of the sputter- etch procedure used to profile the coating in depth the changes in oxygen content are real. The chemistry of the two samples is not the same, nor is the chemistry of each quadrant the same. Sample 8B2 showed an increase in oxygen content at the ThF^-KCl interface,

while the same quadrant of 8A7 showed no similar increase at the coating substrate interface. Sample

8A7 was also profiled in the two- layer region in which iodine contamination was found in the ThF^.

The unexpected presence of copper at the ThF^-TlI interface was not found elsewhere.

The three- layer AR designs were not as damage resistant as the other coatings reported upon

here. At this time, no chemical studies have been conducted on these multilayer samples. With such

a limited sample population and with little information on the status of attempts to optimize the threelayer coatings for low absorption it is not prudent to draw conclusions concerning the relative merits

of these designs with respect to their damage resistance.

5. Acknowledgments

The Auger spectra were taken by Messrs. Tom Wittberg and James Hoenigman and interpreted with the help of Dr. William Moddeman of the University of Dayton. The x-ray analysis and

microstructural evaluations were contributed by Dr. Richard Harmer also of the University.

It is a pleasure to acknowledge the assistance of Ms. Marile McMahon and Mr. Douglass Sauer

in obtaining these damage data, Ms, Mary Wendeln for computational support and data reduction, and

Mr. Donald Stevison of AFML/LPE for the use of the 1 kW laser.

The samples were all prepared by Honeywell under contracts with the Air Force Materials

Laboratory, Wright- Patterson Air Force Base, Ohio.

References

[1] Detrio, J., Petty, R. , Fox, J., Larger, P. and Fenter, J. , Fifth Conference on Infra- red Laser Window Materials. Compiled by

C. Robert Andrews and Charles L. Strecker,

389-390 (1976).

[2] Loomis, J. S. and Saito, T. T. , "Third

Conference on High Power Infrared Window

Materials, " Vol. 3, Air Force Cambridge

Research Labs, AFCRL- TR-0084(IID (1973). -

[ 3] Huguley, C.A. and Loomis, J. S. , Proc.

7th NBS, ASTM, ONR, ERDA Symposium

on Laser Induced Damage in Optical

Materials, NBS Spec. Pub. 435, 187-201

(1975)

7, Figures

Figure 1. Sample irradiation site format. The quadrants and

site locations are shown for the normal orientation

of the sample in its mount with quadrants I and II at the top, and radiation incident upon the front

of the sample.

-287-
T^l/ThF4/T^I

1 00 200 300 400 500 600 700 800 900

POWER (WATTS)

Figure 2. Depth of pentratlon and power of CO2 radiation near the focus of a 1 m

focal length mirror. All points were

irradiated for the same time interval — 0.1 s.

884

ZnSc/KC//ZnSe

8C2

T/I/AqCi/T/1

Figure 3. Composite polariscopic photograph of the coatings made upon completion of the tests.

1000 900 800 ^ 700

I

600 a 500

iij O 400

Q. 300

200

100

TII HALF APERTURE ONE SIDE ON KCl IRRAO DAMAGE

I I I

I I I I I I I I

I I I I I I I CDEFABCDEFABCDEF

m

LOCATION

rz

Figure 4. Damage test sequence for sample 8B6 which is representative of the TII

on KCl coatings studied.

-288-
1000 900

800 _700

tn^600 isoo

tr5 400 o

"^300

200

100

T/l/ThF<i QUADRANT COATING ONE SIDE ON KC^

ThF4 T/I/ThF4

IRRAD DAMAGE

8 C 0 E F D E F Am

LOCATION CDEFABCDEF

Figure 5. Damage test sequence for TIl/ThFi,

quadrantly coated sample 8B2 showing

the effect of sample damage on the subsequent test procedure which

deviated significantly from our initial plan.

T/I/KC//T/I FULL APERATURE BOTH

SIDES ON KC/

1000 900

800 700

: 600

Q- 300

200

100 Figure 6. Damage test sequence for TII/KCl/TII

coated KCl sample 8B5.

IRRAD DAMAGED

C 0 E F A

II LOCATION

C D E F A

jsz:

li I /Ag C//T/r FULL APERTURE BOTH SIDES ON KC^

O SIDE I IRRAD

SIDE I DAMAGE

O SIDE 2 IRRAD • SIDE 2 DAMAGE

1000 900

400

300

200

100

-J—I—I—I I I I I I

I I I ABODE _l I I I I I I I I

I I I FAB

n

E F A B C D m

LOCATION

E F A B C D

Figure 7. Damage test sequence for TIl/AgCl/TIl

coated KCl showing the Influence of irradiation from different sides.

-289-
150

140

130

120

-I 10

CVJe

'100-

90- o 80

X

CO 70

cr

I- 60

hJ

'i 50h g 40

30-

20-

10-

0.5 1.0 1.5 20 2.5 COATING ABS( PERCENT)

I 3.0

I Figure 8, Measured damage threshold versus coatingj

absorption measured for the samples in I this study. The data show all samples

tested. The circled data represent \ samples for which damage was observed.

Each quadrant of the quadrantly coatings ^ are plotted separately. The correlation

between the two quantities exists, but

its functional dependence cannot be

deduced from these data.

-290-
AUGER IN-DEPTH PROFILE OF ThF4/I/l COATED KC/

Figure 10. Auger profile scans of the TII/ThFt, coatings showing the presence of oxygen

In the ThFit.

COMMEWTS ON PAPER BV PETRIO, VETTV, OHMER, ANV SlUENSEN

Vnnrnxmnt fLU^Ldual iVLolnA wzfiz obizn.\}(i.d In many icmplu at -ULlumination tuvdU weJUL bzZow the. thAgyikoZd iofL catcU)tn.opkic damage, Thut obieAvationi, weAe mode by rmam o{, a potaAjjsCopt.

-291-
4.6 SPECTRAL DEPENDENCE OF DAMAGE RESISTANCE OF REFRACTORY OXIDE OPTICAL COATINGS*

Brian E. Newnam and Dennis H. Gill

University of California

Los Alamos Scientific Laboratory

Los Alamos, NM 87545

The laser damage thresholds of three refractory oxide material coatings

(Ti02> Zr02,and Hf02) and Si02 were measured at three wavelengths (355, 532, and

1064 nm). For each wavelength, quarter-wave thick films were deposited on fused silica substrates by three manufacturers using electron-gun evaporation. Samples were irradiated with the primary, doubled and tripled frequency of a mode-locked

Nd:YAG laser with pulse duration 20-30 ps. An increase of the threshold electric

field from 1064 to 532 nm was measured, which was consistent with that predicted

for electron-avalanche breakdown. Reduced thresholds at 355 nm indicated

dominance of two-photon absorption.

Key words: Damage thresholds; electron avalanche; picosecond pulses; refractory

oxides; standing-wave fields; thin films; two-photon absorption.

1. Introduction

The damage resistance of optical thin film components has proven to be a major limitation on the

[ peak intensity attainable for laser fusion experiments at 1064 nm. The desire to conduct future fusioi* experiments at visible and ultraviolet wavelengths has prompted the anxious suspicion that optical

coatings may become an even more limiting factor. This is due to the probable onset of multiphoton abs sorption as a mechanism leading to early damage at short wavelengths and high intensity. In addition,

laser isotope separation has posed severe endurance requirements for multilayer reflections for sub- microsecond pulses in the ultraviolet.

Previous laser damage experiments at 1064 and 694 nm have provided a useful, if not satisfactoril

complete, characterization of various thin film materials. However, no reports of controlled tests be low 694 nm have been found by these authors. The present experiments at 355, 532, and 1064 nm were thu? strongly motivated.

2. Test specimens

The refractory oxides Ti02, Zr02> and Hf02 were chosen for evaluation because they have been sue- , cessfully used as the hard, high-refractive-index components of multilayer stacks for 694 and 1064 nm Also, Si02 films, often used as a hard, low-refractive-index component, were selected for evaluation.

Single-layer films of Ti02, Zr02' and Si02 were evaporated onto fused silica substrates (Optosil I) by

two commercial vendors who represented the state-of-the-art of electron-gun technology. The Hf02 filmi: were deposited on Ultrasil substrates by electron-gun evaporation by P. Baumeister and 0. Arnon at the University of Rochester. Film thicknesses of one quarter-wave (QW) at 355, 532. and 1064 nm were obtained. Spectral transmission curves for these films are shown in figure 1.

Hf02 was considered of special interest because of its short wavelength cutoff at 230 nm. Al- though the HfOp films we tested showed absorption at 260 nm (unbaked), heating to 400° C in air would

1 have shifted the edge to 230 nm[l]. There are a few reports of rf sputtering[2] and electron-gun deposition [3] of this material, but its damage resistance has not been published previously.

3. Experimental procedure

The experimental apparatus and techniques were similar to those described previously[4]. A sche- matic of the apparatus is shown in figure 2. For these experiments KDP-I and KDP-II frequency doublinc and tripling crystals were used with overall energy efficiencies of A5% and 20%, respectively. The samples were placed near the focus of a 50 cm focal-length, fused silica lens for the 355 nm tests. The spot size radius (at Ig/^^) incident on the lens was approximately 1.2 mm. A 100 cm lens was used for the 532 and 1064 nm tests where the incident radii were 1.5 mm and 2 mm, respectively.

* Work supported by the Energy Research and Development Administration.

1. Figures in brackets indicate the literature references at the end of this paper. -292-
The laser pulse profile was measured on every shot (see fig. 3) by a linear Si diode array

(RL512C) with resolution of 25 ym manufactured by Reticon Corporation. The spot-size radius was

0.12 mm for both 355 and 532 nm and 0.22 mm for 1.064 nm. This device was suitable for use at all three test wavelengths due to the adequate sensitivity of silicon from the UV to 1.1 ym. The ex- perimental parameters are listed in table 1.

Table 1. Experimental parameters

A. Wavelength = 1064 nm, 532 nm, 355 nm

B. Pulsewidth = 30 ps, 20 ps, 17 ps

C. Spot-size radius = W = .12 - .22 mm

D. Single shot per irradiated site

E. 40 shots (average) per threshold

F. Normal incidence

For laser irradiation at 1064 and 532 nm, laser-induced-scatter (LIS) and spark detection were

used simultaneously to monitor film disruption. Irradiation at 355 nm, however, caused fluorescense in

all the materials studied, so that our standard method of detecting spark emission at 420 nm could not

be used here.

As shown in figure 4, the damage threshold is evaluated as a range of values rather than a single

number. The range indicates the variability of the damage resistance over the sample surface and is not related to the accuracy of the measurements. The upper end of the threshold range was defined as

the maximum pulse energy density for which neither visible film disruption nor spark radiation of a minimum value was detected. The lower end was defined as the minimum energy density for which damage

was detected either by the LIS method or by the occurrence of a spark radiation with at least the pre- scribed energy.

4. Spot-size tests

j Before measuring thresholds versus wavelength, preliminary tests were conducted to determine an

! approximate range of spot-size radii. A large irradiation spot-size was desired to sufficiently aver-

!i| age over film densities, being mindful of the limited laser energy available at 355 nm. By irradiating

a Ti02 film with 30-ps pulses at 1064 nm, we found that the damage threshold did not change significantly for radii ranging from 70 to 500 ym. This evidence suggested that the separation of defects most easily damaged by 30-ps pulses was very much smaller than 70 ym, possibly even less

than one ym.

, 5. Spectral test results

A list of the damage thresholds measured in this study is given in table 2. The results for

1 1064 nm are typical of commercially-available coatings, with the exception of the low value for one of

!t the Ti02 samples. In addition, the threshold energy densities for each film material for a selected

I manufacturer are plotted versus wavelength in figure 5. Between the wavelengths of 1064 nm and 532 nm,

it may be seen that the damage thresholds of Ti02 and Zr02 increase, whereas that of Hf02 is rather

, constant and that of SiOg decreases. The thresholds for all four materials decrease rapidly between

532 and 355 nm.

Now, representing the results in terms of incident energy density does not give the complete pic-

• ture for thin films. It has been established that the standing-wave (SW) electric field must be taken

I into account when evaluating damage resistance[4,5]. In particular, the fields in a QW thick film of

I HfOg at 355 nm are quite different for 532 and 1064 nm irradiation (see fig. 6). The same is true for

j TiOg (fig. 7) and Zr02. Of course, minimal electric-field variations occur in Si02 films on fused

I silica substrates. Moreover, the damage resistance may be dependent on the pulsewidths which varied

3 for each laser wavelength.

A more absolute measure of damage resistance is the rms electric field E at breakdown which most

likely occurs at the location of the SW maximum in the film[6]. The threshold field in a thin film is computed using the relation

^0

p

E = (377 Iq) 1/2

-293-
I/) C\J o CO o n o 00 o e cn LT) «d- CM CO ro CJ 00

OJ CO CO CO 00 CO CO

+j E 1 1 1 1 1 1 1 1 1 1 1 1 1 u 1

o CO 00 CO ro ir> CO o IT) O 00 CO CJ 00 3 CO

CJ ro Ln

r-. o

CM LO

CO l£> «* I—

1— O 00 o ro r—

1— ^ .—

ro I— ro

r— o in

CM ro ro I—

ro r~-. CM roLO CJ

in vo

o4-» IDO o

Lf) ro i£> in

•a- cvi lO CO o lf>

CO CO CJ CO ^ ro ro ro ro i£) LO in in in o

in in in <:j- in in in ID ro ro ro OCJ C>J CM CO

ro ro ro CO

in m in m

-294-
where 377 ohms is the free space impedence, Iq is the incident peak intensity (GW/cm^) and |E/E"^Q|p

is the peak electric field normalized to the incident field.

Figure 8 shows the spectral dependence of the threshold electric field in MV/cm. For each material the threshold field increases from 1064 nm to 532 nm before falling from 532 nm to 355 nm.

6. Discussion

The increase of the threshold field from the near infrared through the visible region would be unexpected if absorption processes initiated laser damage. However, for an electron avalanche, the qualitative expression for the frequency dependence of the breakdown field is[7]

E (c) = {UJ r^J-^E (2)

where is a relaxation time determined principally by phonon collisions and is a characteristic of

each material

,

When oj is comparable to l/x^ then frequency dispersion should be noticed. This is just

what is seen in figure 8. A similar increase in thresholds between 1064 and 694 nm was reported by

Bass and Barrett[8] for surface damage of several crystals. Smith, et al.[9], have also reported a qualitative increase in the bulk thresholds at 532 nm over that at 1064 nm for several alkali halide

crystals and fused quartz irradiated by 30-ps pulses. For wavelengths longer than approximately 532 nm,

it is not possible from the present experiments to say at what wavelength the maxima of the thresholds

are reached. However, it is apparent that linear or multiphoton absorption are not the dominant pro- cesses in the visible region from 1064 to 532 nm. The rapid decrease that was measured below 532 nm

does suggest that multiphoton absorption is dominant in the ultraviolet region. A comparison of the absorption edge energies of the films with the multiphoton laser energies (see table 3) suggests that

resonant absorption by a single photon at 355 nm is the damaging mechanism in Ti025 and that two-photon

absorption is very possible at that wavelength for Zr02, Hf02, and SiO^ for these short (~ 17 ps)

pul ses.

Table 3. Comparison of multiphoton energies and film properties

Laser Wavelengths and Photon Energies

0.355

0.532

1.064

2.82

1.88

0.94

Material

TiOo

ZrO,

HfO.

SiO,

Bandgap Energy (ym

2.9

4.1

4.4

~ 5.6

1

Resonant Absorption by

one photon at 355 nm

two photons at 532 nm

two photons at 355 nm three photons at 532 nm

two photons at 355 nm three photons at 532 nm

two photons at 355 nm three photons at 532 nm

The threshold fields for 532 and 355 nm plotted in figure 8 were measured with shorter laser

pulsewidths (-21 and -17 ps, respectively) than the nominal 30 ps of the 1064 nm fundamental. It is of interest to consider the possible pulsewidth influence on the results. Bettis, et al . [10], have

recognized a t"^''^^ dependence of the threshold electric field in the data reported by several researchers in crystals and glasses. This is consistent with the avalanche breakdown mechanism, and it should

be noted, is the same as for metal surfaces.

-295-
A pulsewidth effect on the threshold of thin films has also been reported by a few workers.

The spark thresholds of a Zr02 film measured between 10 and 35 ns at 694 nm by Newnam and DeShazer[ll]

fit the t"^^^ dependence, but the lower LIS thresholds measured simultaneously showed a much weaker

pulsewidth dependence. The thresholds at 694 nm for several multilayer reflectors reported by Bliss,

et al . [12], also exhibited a pulsewidth dependence roughly comparable to t"^''^.

-1 /4

To normalize the present results all to 30 ps using a x law the data in figure 8 would be multiplied by 0.95 and 0.87 at 532 and 355 nm, respectively. However, the effect of two-photon ab- sorption at 355 nm would be less at 30 ps than 17 ps which might affect the above factor. Finally,

the presence of film defects may well override any pulsewidth dependence, especially at subnanosecond

pulses. Obviously, further experiments versus pulsewidth would be necessary for satisfactory clarification of this matter.

7. Conclusions

The electric fields at the damage thresholds of Ti02, Zr02, Hf02, and Si02 have been determined

to increase between 1064 and 532 nm for 20-30 ps pulses, which is consistent with an avalanche breakdown process. The sharp reduction in thresholds at 355 nm indicates the probable presence of two- photon absorption except for Ti02, for which resonant linear absorption is identified.

8 . Acknowledgments

The authors are grateful to John McLeod for numerous helpful discussions, and to George Faulkner

for help in taking and analyzing the data.

9. References

[1] Baumeister, P., Univ. of Rochester, private [8] Bass, M. and Barrett, H. H., Appl . Opt. 12 , communication (1976). 690 (1973).

[2] Motovilou, 0. A., Optics and Spec. 22, 537 [9] Smith, W. L., Bechtel , J. H. and Bloembergen,

(1967). N., NBS Spec. Pub. 435, op. cit., 321 (1975).

[3] Icenogle, W., Spectra Physics Corp., private [10] Bettis, J. R., House, R. A. and Guenther, A. communication (1976). H., presented at the 1976 Symposium on Optical Materials for High Power Lasers,

[4] Newnam, B. E., Gill, D. H. and Faulkner, op. cit.

G. E., Laser Induced Damage in Optical

Materials: 1975 , NBS Spec. Pub. 435, 254 [11] Newnam, B. E. and DeShazer, L. G., Laser

(1975). ~ Induced Damage in Optical Materials: 1972 , NBS Spec. Pub. 372, 123 (1972T:

[5] Boling, N. L., Crisp, M. D. and Dube, G.,

Appl. Opt. 12, 650 (1973). [12] Bliss, E. S. and Milam, D., Laser Induced

Damage in. Optical Materials: 1972 , NBS Spec.

[6] Apfel, J. H., Matteucci, J. S., Newnam, B. E. Pub. 372, 108 (1972X1 and Gill, D. H., presented at 1976 Symposium

on Optical Materials for High Power Lasers,

Boulder, CO., July 13-15, 1976.

[7] Bloembergen, N., IEEE J. Quant. ELECT,

QE-10 , 375 (1974).

-296-
10. Figures

1

1 i I I I ] I I 200 300 400 500 600 800 1000 WAVELENGTH (nm)

Figure 1. Spectral transmission curves for test samples. The peak values for Hf02

and TiOz are low due to a wedge in the samples

.

Figure 2. Experimental setup for laser damage

threshold measurements

.

-297-
o

J3

•H T3U

a 3 O to V n) •H (U 4J 0

o

<u U r-l

4J O

3 *-l o a-
+ = DAMAGE

• = NO DAMAGE

THRESHOLD RANGE

H 3.5-4.5 J/cm^

Figure 4. Scatter plot of laser damage data showing how the quoted threshold range is determined.

Figure 5. Threshold energy densities versus wave- length for each of the film materials

tested. 200 400 600 800

WAVELENGTH (nm)

1000 1200

-299-
1.0

0.8

0.6

0,4

0.2

0"-

AIR

-> NORMAL INCIDENCE

Figure 7. Standing wave electric field pattern In

a single layer of Ti02, A/4 thick at

532 nm, used at two different wavelengths:

532 and 1064 nm.

200 400 600 800 1000 WAVELENGTH (nm)

1200

Figure 8. Threshold electric fields versus wavelength for each of the film

materials tested.

COmENTS ON PAPER 8/ WEWWAM ANV GILL

Th& ip&akeA mphcu>i.ze.d that the. abioKption meMuH.ejnmts> weAz tak&n on the. thin f^lbn iampZeJ> thmieJtveA, He fimankexi thxt on H^Q ^-ilmi, thoiz wtth hlgheA. tinejxM. abioKption ihouo&d a higheA. damage

thueihold, LmUcatcng that two-photon abioKption, fiatheA than Ujizoji woi caiiiiing the obieAved damage.

When oAked about iipaxk detection, he pointed oat that aZZ the iompte^ {itaotieAced vohen itluminated at

0.335 ym, io that ipaAk detection o^ pla&ma Mm not poiiibte ^on. the UV damage itadieA, He itated that

itnce no ipot i-ize dependence couLd be ob^eAved, ettheA no indLiidtoM on. MideZy dtit/iibuted veAy imaZt

incZuyitom uieAe KeApoMiJble ^oK the damage. The moKphotogy 0|J the damage at ihont iMveZength hcu> not

been in\jeAtigated in thii mnk, but otheA^ have Kepofited no dif^^eAence between dhoht mue length damagz

morphology and that ieen at 1.06 ym.

-300-
A. 7 THE ROLE OF ELECTRIC FIELD STRENGTH IN LASER

DAMAGE OF DIELECTRIC MULTILAYERS

Joseph H. Apfel and John S. Matteucci

Optical Coating Laboratory, Inc.

Santa Rosa, California 95402

Brian E. Newnam and Dennis H. Gill

University of California

Los Alamos Scientific Laboratory

Los Alamos, New Mexico 87 545

The intensity of the local electric field within a multilayer

illuminated by a laser beam is determined by the vector addition of

forward and reverse flowing waves as a result of interference. The

profile of the electric field intensity will therefore depend upon

the multilayer design and can have a peak value which is more or less than the peak field of the incident beam. We have examined

four multilayer designs, each composed of approximately equal numbers

of high and low index films arranged so that the electric field profiles are significantly different. Laser damage thresholds for

these coatings were compared with calculated electric field strength

profiles

.

For electron-gun evaporated titania/silica coatings damaged by

30 picosecond pulses of 1.064ym radiation the damage threshold is

dictated by electric field intensity in the titania layers.

Key words: Dielectric films; electric fields} laser damage;

optical coatings; standing-waves.

Introduction

Precision determination of laser damage thresholds is possible, yet among workers

there is still uncertainty regarding the damage mechanisms. Several workers have

reported experiments which support the expected relationship between damage and local

electric field strength [l-3] ^

. However, in multilayer coatings, the variations of

material thresholds due to processing by different or common manufacturers may be

masking the field strength dependence. In these studies four multilayer designs with

widely different electric field enhancement factors were prepared simultaneously in one coating operation. The results of laser damage threshold experiments are then compared

with calculated field strengths.

Description of the Experiments

Figure 1 describes the multilayer designs which are composed of eight or nine

layers of titanium dioxide and silicon dioxide deposited onto fused silica substrates.

Design A is a simple quarter-wave stack of nine layers beginning and ending with high

index (TiOa) films. In Design B the centermost layer of the quarter-wave stack was

omitted resulting in an eight- layer design with a central, half-wave optically thick,

low index (SiOz) layer. In Designs C and D the fourth or sixth layer was omitted in

similar fashion.

The spectral plot of figure 1 shows that the peak transmittance of Designs B, C,

and D is not precisely centered at 1.064ym. From the scan it can be inferred that the

optical thicknesses of the individual layers vary by less than one percent and that

both coating materials have an extinction coefficient (imaginary part of the complex

refractive index) of less than 0.001.

1. Figures in brackets indicate the literature references at the end of this paper.

-301-
For laser radiation incident on the coated surface of the substrate the four

designs are specified as A, B, C, and D (see fig. 1) . For laser radiation incident

from the reverse direction, i.e., incident upon the multilayer from the substrate side,

the designs are specified A', B', C, and D' . When the laser beam which is incident

on the substrate at 8 passes into the fused silica it is refracted to an incident

angle of 5.51 . For the^reverse (primed) exposures the incidence angle onto the multi

layers is therefore 5.51 . Figure 2 and figure 3 show the computed profiles of the time averaged square of

electric field strength (E^ ) for each of the designs []4] . In figure 3 where

radiation is incident upon the multilayer from the reverse or siibstrate side (primed)

the incident field intensity has been adjusted to represent the same beam intensity

used for the other cases (unprimed) . Thus, the profiles represent the field in the

various designs with equal beam energy densities incident on the samples. In these

figures the high and low index layers are distinguished by thickness. Since the

optical thickness of all layers is equal, the high index layers have smaller physical

thicknesses

The coatings were prepared in a multiple spindle coating machine equipped with

externally actuated masks which permit the omission of any one or more layers from

substrates mounted in each spindle rack. Thus, all four designs were prepared in a single run and have minimum differences attributable to process variation.

The four 38mm diameter by 7mm thick fused silica substrates were cleaned by

gentle scrubbing in a hot solution of detergent in deionized water, rinsed with

deionized water and then immersed into the vapor of an alcohol dryer. Electron beam

heated evaporation sources with copper rotary hearths were used for deposition. The

titanium dioxide films were deposited at 80A/min and the silicon dioxide at ITsX/min

onto substrates held at 225 C. The completed coatings were exposed to 30ps pulses of 1.064pm radiation as des- | cribed previously QlJ . Each sample was tested with radiation polarized in the s plane

and incident on the coated side of the substrate, and from the reverse direction at ar. angle of 8°.

Results

Table I and figure 4 include the measured laser damage threshold ranges. The top

end of the threshold range was defined as the maximum pulse energy density for which

neither visible damage nor spark radiation of a minimum value was detected. The lower

end was defined as the minimum pulse energy density for which damage was detected

either by the LIS method or by the occurrence of a spark with at least the prescribed

radiant energy. An average of 68 laser shots was made for each range determination;

the number for each is shown in figure 4. Table I. Damage thresholds of Ti02/Si02 multilayer designs.

Design Incident From Air Incident From Substrate*

A 4.8-7.4 J/cm^ 3.6 - 4.4 J/cm^

B 3.5 - 4.5 2.9 - 3.3

C 4.4 - 5.6 1.58- 1.85

D 1.46- 1.68 2.1 - 2.8

*Pulse energy densities incident upon the multilayer

from within the fused silica suibstrate.

-302-
Discussion of results

We have attempted to fit the experimental threshold range determinations to

computed values for three models of electric field dependence. It is assumed that the

damage will correlate with field strength in either the high (Ti02) or low (SiOa)

index layers. Although we have plotted damage threshold as a function of E^, these

experiments do not allow us to identify the power n in the hypothesized e" dependence

of damage. With the large spot-size used for irradiation, these experiments may show

that damage threshold at a particular damage site is reached when E reaches a certain

level

.

iVbdel 1 assumes that damage occurs in either material when the local field exceeds

a threshold value. This is the simplest model and probably the most applicable. If

damage results from homogeneous absorption or from pure avalanche breakdown, this

model should apply. The predicted thresholds for the two materials in the eight

configurations will be proportional to the reciprocals of the peak E^ values

(see figs. 2,3, and 5). Figure 6 (Model 1) shows the result of a least squares fit^ of

the mean value of the experimental ranges to the predicted thresholds for both high

(TiOa) and low (SiOa) index failure.

High index failure is the best fit as indicated by a standard deviation^ of 0.4 3 compared to the next lowest value of 0.60 for Model 2 (see below) . The slope of the

best fit line for high index failure corresponds to an RMS electric field of 7.9MV/cm.

In Model 2 it is assumed that damage occurs when the volume integral of the time

average squared field over one layer equals the threshold value. This integral is proportional to the area on the E^ plots (see figs. 2,3, and 5) . The model would apply

if damage results when the total homogeneous absorption in a layer reaches threshold

value. Since the layers have equal thickness, the value of the integral is proportional to the average value of E^ . Figure 7 is a plot of the least squares fit of

experimental data to Model 2. High index failure is again a better fit with the data.

Model 3 is similar to Model 2 except that the total integral of E^ is taken over

all layers of the same material throughout the multilayer. The fit between experiment

and model shown in figure 8 is not very good.

Failure due to peak electric field in the high index layers is indicated by these

results. The fact that Model 2 is close to Model 1 is consistent with the shape of

the profiles (see figs. 2,3, and 5) because the area under each layer profile is

approximately proportional to the peak height of the profile.

Sample B (Designs B and B') are critical to the separation of failure due to high

or low index materials. In the other three samples the peak fields are equal for both

materials so the experiments will not distinguish which fails. Only when data from

Sample B are included can we distinguish the materials

.

Figure 9 shows the electric field profiles for the eight designs with incident

intensities adjusted to equal the upper and lower ends of the experimental threshold

ranges. On each profile small horizontal lines have been added to indicate the best

fit solution of Model 1. These lines correspond to an internal RMS electric field of

7 . 9MV/cm

.

If we assume that thresholds are correlated with the peak electric field in the

high index (Ti02) layers, then it is interesting to speculate about the discrepancies

in the fit of Model 1. The fit is improved if the threshold data from Sample A are omitted. Table II shows how the peak field locations are distributed throughout

2. The least squares fit was made by minimizing the square of the deviations of the mean threshold values from a straight line through the origin.

3. The standard deviation is here defined as the square root of the mean squared

deviation

.

-303-
various designs. Notice that Sample A (Designs A and A') have peak fields in the Is' outermost and innermost layers, whereas Samples B, C, and D have peaks confined to thec four central layers.

Table II. Identification of layer in which the peak ' electric field occurs for both materials. ' Designs

A D C D A

B

C

' D' 1 1 H H I 2 L L L L

3 H H H H H

4 L L 0 L L 0

i 5 H 0 H H 0 H H l 6 L L L 0 L L 0 , 7 H H H H H

8 L L L

1 L

9 H H 0 indicates an omitted layer *Count layers from substrate

On the assumption that the outer and innermost pairs of layers may have different

thresholds than the others, Model 1' is presented (see fig. 10). Here the experimental threshold ranges are fit only to Samples B, C, and D. The comparable data for Designs A and A' are shown, but they were not used in determining the least squares fit The standard deviation, calculated for the six data points, is 0.24 for the high index failure and shows improvement compared to 0.43 for Model 1. The slope of the best fit! line for high index failure in Model 1* corresponds to an RMS electric field of

8.3MV/cm. I In Model 1' (fig. 10) the experimental threshold is below the predicted value for

high index failure in Design A'. Perhaps this indicates that the first layer adjacent]

to the substrate is induced to lower threshold by a phenomenon not considered in this } analysis. A simple explanation of this discrepancy is not likely to also cover the case of Design B' in which the threshold range is slightly higher than prediction.

The peak fields in the low index (SiOa) layers numbered 4 and 6 of Sample B are very high when damage occurs. According to the computation, the peak time average

square of field in the low index layers is 2.3 times the peak in the high index layers

therefore, the central low index layers of Sample B are experiencing an RMS field of

12MV/cm at the threshold of damage.

In their damage studies of sputtered single Ti02 layers. Gill, Newnam, Hartman,

and Coleman [5"| found a threshold range of 8-11.3 J/cm^ for 30 ps pulses. The

RMS field within the film, computed from an electric field profile, was 6.9 to 8.3MV/cm. Smith, Bechtel, and Bloembergen [6] found an RMS breakdown field strength

of 11.7MV/cm for fused silica window material.

Conclus ions

We have investigated the laser damage for several multilayer designs produced

under identical conditions. For the materials studied in these experiments, there is

a definite correlation between the peak field in the weak component and pulsed laser

induced damage thresholds. The results indicate that the peak pulse energy density at

threshold produces an RMS electric field about 8MV/cm in the high index titanium

dioxide layers of each design. This compares with the field in the most damage resistant films previously reported. The field in the low index silicon dioxide layers

varies between 8 and 12MV/cm and is felt not to induce failure.

-304-
i

There is an indication that the first high index layer coated on the fused silica

substrate may have a lowered threshold. This could be due to coating process variahe I tion or to substrate induced defects

.

Acknowledgments

The authors are grateful for the skilled technical assistance of George Faulkner

(LASL) in conducting the damage experiments and William Klapp (OCLI) in preparing the

coatings

.

References

Ci] [4]

[2]

[3]

B.E. Newnam, D.H. Gill, and

G. Faulkner, "Influence of Standing

Wave Fields on the Laser Damage

Resistance of Dielectric Films",

NBS Special Publication No. 435, 254

(1976) . N.L. Boling, M.D. Crisp, and G. Dub6, [p]

"Laser Induced Surface Damage",

Appl. Opt, 12, 650 (1973).

B.E. Newnam and D.H. Gill, "Laser

Damage Resistance and Standing-Wave

Fields in Dielectric Coatings",

J. Opt. Soc. Am. 66, 166 (1976).

J.H. Apfel, "Electric Fields in

Multilayers at Oblique Incidence", to

be published in Applied Optics, Oct.

1976.

D.H. Gill, B.E. Newnam, J.S. Hartman,

and W.J. Coleman, "Super-High Damage

Thresholds for RF-Sputtered TiOa

Films", J. Opt. Soc. Am. 66,78 (1976).

W.L. Smith, J.H. Bechtel, and

N. Bloembergen, "Picosecond Breakdown

Studies", NBS Special Publication

No. 435, 321, (1976)

.

Figures

IH 1 L

|H 1

B

Ih I L

|H 1 FUeCD SILICA auesnuTE

DESIGN k, A NINe LAYER QUARTER WAVE STACK TROM WHICH lOCNTIFlEO LAYERS WERE OMITTED TO FORM

DESIGNS B.C. AND O.

1.0

WAVELINSTH - ]Jm

Figure 1. Schematic description and spectral

transmlttance plots of the four multilayer designs.

-305-
Figure 2. Electric field profiles for the four

designs for s plane radiation Incident at 8° from the air side. The

abscissa is physical thickness

measured normal to the layers and the ordinate is the computed value of the time average of the square of the electric field. Each profile is shown for an optical half-wave

distance in the incident medium and , for a short distance into the sub- | strate where there is no standing

wave. The field intensity of the incident beam is Indicated by the arrow on each plot. The dotted lines connect the location of omitted

layers with sample A.

C D B' H' C

DE5IBN5

Figure 4. Experimental laser damage thresholds

for the four multilayer designs. Data from table 1 for the samples

irradiated from substrate side

(primed) have been corrected for first surface reflection loss before inclusion in this plot. The

number of laser shots Is given in each test.

Figure .3, Similar to figure 2 except the radiation is Incident on each design at 5.51° from the sybstrate side. The incident field intensity

has been adjusted for first surface reflection and for substrate refractive index so that nrofiles can be compared with flRure 2 for equal laser pulse energy density.

AIR HL HLHL HLH 1. 45

Figure 5. Graphical description of the models used

for prediction of damage thresholds

attributable to high index layers.

Model 1 uses the peak value, Model 2 used the peak volume Integral represented by indicated shading, and Model 3 uses the total volume integral of E^. -306-
10 HIGH INDEX LRYER5

0.H3

LDW INDEX LHYER5

1.19

I 2 3 H 12

MDDEL I PRDPDRTIONRL TD: [e^peaJ'

Figure 6. Model 1. Best fit of experimental re- sults to prediction If threshold is propor_tional to the reciprocal of the peak In the multilayers. The

numbers are standard deviations of the plotted lines relative to the mean values of the threshold ranges.

Figure 7, Model 2. Best fit of experimental

results to prediction if threshold is proportional to the reciprocal of the maximum Integrated time averaged

square of electric field over one

layerv. The numbers are the standard

deviations of the plotted lines re- lative to the mean values of the threshold ranges.

u

HIGH INDEX LHYER5

I /

I/I

LDW INDEX LRYERB

I .21

I /I

MODEL 2 FRDPDRTIDNBL TD^ [( jE*«il)pEAK]

HIGH INDEX LRYER5

I .S7

LDW INDEX LRYER5

I .75

/

l/i

/

1/

/

MODEL 3 PRDPDRTIDNRL 70= [s|E*clt]

2 3 4

Figure 8. Model 3. Best fit of exoerimental re- sults to prediction if threshold is proportional to the reciprocal of the sum over all layers of similar index

of the integrated time averaged square of the electric field. The

numbers are the standard deviations of the plotted lines relative to the mean

values of the threshold ranges. -307-
jUio/Mi

^ iC ^ S ^ S

s B si B si a B

I I I I I I I — B O to j- N B

guio/r

01 -a B « S S

U U

X a 0) w

« U

.e -o

•4J <H

(0

en 0) 9 U

o- a) « u

4J o to3

•c n

01 4J B

eg o

cd "oB

09 (fl 0) •H 0) U "O

•rt -rt « ca c

U Id • 4J -H B C eg u

•H ^

« il c u

0) M-i 0> a • •H u r» o c

C U U-l •H -D O

•H

o

01

•H ts a Ti

0) tj . u

s u

o o

a) 00 -H x 01 -a

4J eo c

c -H

^1 CO o >d a

n eo -H

0) (9 iH

iH B

•H (B iH

M-i « <d O 4J u u c a 01 o

m N

•O CO -H

r-< i-l u

0) o

•H TJ JC

<M 01

0) . o

•H o

0) rH <« J5 H O H

U QQ

0)

t-l 9

00

-308-
e

u

S H u

IEX

LJ 2

a:

a: 0

HIGH INDEX LAYERS

0.2H

/

LOW INDEX LHYER5

I .22

/

2 3 H I 2 3 H MDDEL I' PRDPDRTIDNHL TD^ [e*peak]''

Figure 10. Model 1'. Best fit of experimental results

to prediction if threshold is pr£portional

to the reciprocal of the peak

in the multilayers. Only samples B, C, and D were used in establishing the best fit,

otherwise this is similar to Model 1.

COMMEMTS ON PAPER BY APFEL, MATTEUCCI, NEWNAM, AWf GILL

Th^eAhold valaeA which wqaq. h-zpontzd In thii papeA weAe hiqheJi than thoie fiepohte-d In thz pfizvloai, papeA., ThU, uiau, the. lUixJU: Ajnpn.ovmznX^ in thz {jabfu.catlon pn.oc.iiMi> a6e.d In making thut i-ilmi. The. iptakeA mphcLiizzd that due to thz coating design, aJ: moa not poatble to nxcdcd the. anticipated damage

thxe^hotd in the toixs index mateAiat wiXhout exceeding iX {^iut in the high index mateJuxiZ. Coni>equently,

damage MOi aJbmyi expected to occua in the latteA material.

.

Morphological examination hoA not yet been caxfuied out on thej>e iomple^ iince otheA. teJ>ti ate planned ioK them and mofiphological investigation being uMiaZly deitAuctive. It iaku, pointed out that

even i^ poit-moltem tests ihow that damage is localized in the vicinity o^ inclusions , that does not

iu^i^ice to isolate the mechanism of, damage, 6ince all damage mechanismi aAe enhanced neoA an inclusion

ofi impeAiection.

It ms iuAtheA. emphasized that light icattening mensuAements on multi-layen. dieZectAics axe an unfieJUjable. test o^ damage leveJis. T.^ the. icatteAing occuAS in the low index mateAiat, ^OK example, that

might not af^f^zct the damage threshold. Obviously, in multi-layeA {yilmi,, icatteAing and damage lesalts

can only be cofiAelated i^ coAtied out at the -iome wavelength and angle o{i incidence, as the ^iim de- signed {^oK, to cAeate the same standing wave patteAn. ImpeA^ections at Kegions o/j zeA.o ^ield witt not lead to icatteAing incAeases,

-309-
4.8 Preparation Techniques and Hydroxyl Concentration

vs. Surface Damage Threshold*

R. A. House II, J. R. Bettis, and A. H. Guenther

Air Force Weapons Laboratory

Kirtland AFB , NM 87117

A previous study on pulsed laser-induced damage on fused silica and other optical

materials indicated that the damage threshold E (in V/cm) depended upon the RMS surface

roughness a (in A) as follows: e/o" ^ const. Application of this relationship to pre- viously measured thresholds affords a measure of the efficacies of various surface

preparation techniques in improving surface damage thresholds, allowing a quantitative

comparison. We have, in addition, observed an apparent correlation between thin film damage

threshold and hydroxyl concentration in fused silica substrates. Depending on the particular film materials, the results imply that hydroxyl ions on the surface of silica substrates

can be either beneficial or deleterious.

Key words: Damage threshold; etching; flame polishing; fused silica; hydroxyl; ion polishing; polishing; roughness; surface damage; thin films; threshold.

1. Introduction

In 1975, House et al. [1] **, reported preliminary results of an experimental study on pulsed laserinduced damage to optical surfaces. The experiments were accomplished at a wavelength of 1.06 micrometers and a pulse length of 40 ns (full width at ^alf maximum) using a TEMqq beam focused by a 50 cm

lens to an effective spot size of 147 pm at the 1/e points. A major result for fused silica was the following relationship:

const

o where E (in V/cm) is the macroscopic, threshold electric field and a (in A) is the RMS surface roughness. The constant depends upon the surface preparation technique used; hence, comparison of the quantities E /a (termed normalized thresholds) affords a way to directly and quantitatively relate

various preparation techniques. This results since the normalized threshold explicitly accounts for

the gross effect of roughness and separates that effect from the effect of the preparation technique.

The roughngss/threshold relationship has been found to hold for roughness values from about 10 A to about 350 A. In addition, data on samples of fused silica overcoated with either MgF2 or Si02 show

correlations with the transmittance of the substrate at 2.2 and 1.4 micrometers and in the ultraviolet.

These correlations appear consistent with the interpretation that the hydroxyl concentration of the substrate affects the damage resistance of the film by oxidizing it - promoting the formation of MgO

or Si02 rather than MgF2 or SiO.

2. Data

2.1 Surface Preparation Techniques

Samples of fused silica were prepared to various final values of surface roughness by the following procedures: conventional polishing with barnesite (CB) ; CB plus etching in dilute nitric acid;

CB plus half-wave overcoat of either Si02 or MgF2; CB plus flame polishing; conventional rouge polishing followed by ion polishing using H"*", Ar"*", and Xe"*", and bowl-feed polishing with rouge. Details on

the preparation techniques and experimental procedures are given elsewhere [1].

The figure presents a graphical summary of the data, using a log-log plot of breakdown electric

field versus surface roughness. The types of surface preparation are identified by the symbols indicated; and the least-squares, best-fit lines are also depicted. The datum on the bulk threshold value

of fused silica determined by Fradin [2] has also been plotted, assuming a hypothetical surface roughness equal to the average interatomic spacing of fused silica. The approximate spreads of breakdown

values exhibited by the ion polished and bowl-feed polished samples are shown as hatched-in areas. The 100 A surface roughness limit of Bloembergen [3] has also been identified to explicitly indicate

that much of the present data fall below that limit.

*Based on portions of dissertations submitted by the first two authors in partial fulfillment of the requirements for the PhD at the Air Force Institute of Technology.

**Figures in brackets indicate literature references at the end of the paper. -310-
A comparison of the effectiveness of each technique In Improving damage threshold is presented

in table 1. The first column lists the particular techniques, T-j^, used in the experiment. The

average < e/o" of the normalized threshold values was computed for each data set, and the results

are shown as E£ in the second column. These averages have an accuracy of + 18% of the listed values,

resulting from the measured accuracies of the field values and the roughness values. Column three

shows a normalized comparison of the various techniques to the conventional polishing technique.

Table 1. Technique vs Average Normalized Threshold

Technique (MV/cm)

Ti : 1 Conventional 5 .18 1.00

To

:

Etching 5 .47 1.06

T3 = Ion Polish (H+) 6 .82 1.32 Ion Polish (Xe"*") 8 .08 1.56

T5:

-4-, Ion Polish (Ar^) 10 .61 2.05

^6 = Flame Polish 10 .97 2.12

^7 = Si02 Coat 4 .06 0.78

%= Bowl-Feed 3 .29 0.64

Tg: MgF2 Coat 2 .40 0.46

(Data taken from [1])

Note : T^, T^, Tg, T^, Tg barneslte polished

Note : T3, T^, T5, Tg rouge polished, absorbing

Contamination of the samples T^, T^, T^ has obviously

been reduced by subsequent ion polishing.

It is evident that techniques 3, 4, 5, and 6 resulted in significantly improved thresholds, that techniques 7, 8, and 9 resulted in significantly reduced thresholds, and that technique 2 resulted in a possible improvement. If we exclude the probably erratic data point at In a = 4.4 in the figure, then

E2 becomes 6.27 and E2/Ej^ becomes 1.21 and technique 2 can then be said to have improved the damage

resistance significantly.

Since nitric acid is not believed to attack fused silica [4], the improvement due to etching is attributed to the removal of surface contamination. The effect of contamination due to rouge [5] is very evident in the result for bowl-feed polishing. This result leads to a significant conclusion

regarding the ion polished samples. Although all the ion polished samples had previously been rouge

polished, it is clear from table 1 that the ion technique effectively removed the deleterious effect

of the rouge. In addition, it is to be noted that it was the intermediate-mass ion (rather than the light or heavy ions) which gave the greatest improvement for the ion polishing procedure.* As readily

observed, the flame polishing technique yielded the highest thresholds. It seems clear, that while

the flame technique indeed smoothed the surfaces, a subtle change in the character of the surfaces

must also have occurred. The evidence is the great difference in slope for these samples as compared

to the other samples and the fact that the fit of the data to a straight line was five times better

than for any of the other techniques. Not surprisingly, the overcoated samples exhibited reduced

thresholds. However, it was surprising that the MgF2 overcoated samples not only had lower thresholds

than the Si02 overcoated samples, but also these thresholds were lower than those of the rougecontaminated, bowl-feed polished samples. On theoretical grounds, Bettis et al. [6],have predicted that

the damage threshold of a material should be proportional to the following quantity:

N

where N is the atomic number density and n is the index of refraction. The ratio of this quantity for MgF2 to that for Si02 is 1.6, so that the MgF2 overcoated samples should be expected to have higher

thresholds than the Si02 overcoated samples. Although the explanation for the lowered thresholds was

not definitely attributable to film stress or film scattering, the MgF2 films were observed to have

an index inhomogeneity of about 7%. In addition, as pointed out below, there is a likelihood that the MgF2 films were not atomically pure and were partly composed of MgO.

*Note: Four other materials - BX-7 , Cer-Vit, ED-2, and ED-4 - were also subjected to ion polishing

and bowl-feed polishing. In each case, the ion polished samples had higher thresholds. Ion energies

ranged from 20-30 KV at close to grazing incidence.

-311-
2 . 2 Hydroxyl Concentration

A series of thln-fllm samples of MgF2 and SIO^ on fused silica was prepared by electron beam . deposition with the angle of Incidence of the coating material as the variable. These data are presented in table 2. As can be seen, there is no obvious correlation of breakdown threshold with angle

of incidence. Taking into account the measured values of roughness (which were nearly the same for ' all the samples of each data set) did not explain the variations of threshold. However, comparison

the thresholds with the transmission of the substrates at 2.2 and 1.4 micrometers and in the ultraviolet did result in a definite correlation. Table 2 shows the comparison of threshold with the sub- , strate transmission at 1.4 micrometers. The correlation coefficient for the Si02 samples is -0.96,

and the correlation coefficient for the MgF2 samples is +0.80. The handling history of the samples i was traced. It was found that the fused silica substrates had not all been supplied to the coating ! company by the same vendor. In fact, the percent transmission at 1.4 and 2.23 micrometers was cor- i related with the values of hydroxyl concentration quoted in vendors' literature [7]. Since the cor- relations of threshold with transmission are high, it is believed that the present data imply that the thresholds of MgF2 and Si02 films fabricated by electron beam deposition do not greatly depend upon

the deposition angle of incidence. The signs of the correlation coefficients indicate that the presence of hydroxyl is beneficial in the fabrication of S±Oy films and deleterious to the fabrlcatioi

of MgF2 films. It is hypothesized then, when depositing S162 (n = 1.45) films, there is a tendency t< produce some SiO (n 'v, 2.0). Since SiO has a higher index, to a first approximation, it has a lower

damage threshold than Si02. Thus Increasing the oxidation of SI to SIO2 by the presence of hydroxyl

ion is beneficial, and more damage resistant films will result. On the other hand, if hydroxyl ions

in the substrate leads to the production of some MgO when depositing MgF2 films, a higher index

material than the desired MgF2 will be produced leading to a less damage resistant film. That this

competition between SiO/Si02 and MgF2/MgO exists is well known [8].

Table 2. Threshold vs 1.4 micrometer transmission

Film E(MV/cm) 1.4ym Trans. Inc . Angle

Si02 1.24 83% 25° Si02 1.16 85 20.3

Si02 0.813 92.5 42

Si02 0.681 93 29.3

Si02 0.564 94 18.6

MgF2 0.61 95 20.3 MgF2 0.50 85 18.6

MgF2 0.50 85 25 MgF2 0.50 86 29.3 MgF2 0.50 86 42 (Data taken from [6]) Note: Decreasing transmission associated with

increasing hydroxyl concentration.

3. Conclusions

The roughness normalization of threshold which appears to be valid for a wide range of substrate

and film materials [1, 6], provides a very useful method for comparing the relative merits of various

surface preparation techniques in affecting surface damage threshold. The utility of the normalized

threshold is that the gross effect of roughness is accounted for and separated from the gross effects

of the preparation techniques themselves. As an aside, the most striking example of this that we have

so far observed is the effect of ultrasonic cleaning [1]. Rouge polished fused silica was damage

tested before and after a long period of ultrasonic cleaning. Identical thresholds were observed.

However, when the fact that the roughness Increased by a factor of five was taken into account (the

final normalized threshold being more than twice the initial normalized threshold) , it became clear

that ultrasonic cleaning is very effective in removing contamination. This conclusion was not at all evident from the basic threshold data, because the effect was completely masked by the roughness change

We believe that the threshold/roughness relationship is applicable for materials other than fused

silica. Its usefulness over a wide range of film and substrate materials was clearly demonstrated

previously [6]

As for coating failure mechanisms, we believe that we have observed a strong indication that the chemical composition of the substrate (hydroxyl in fused silica in the present case) can significantly

alter the damage threshold by affecting the character of the deposited film.

-312-
4 . Acknowledgments

The authors gratefully acknowledge the assistance of R. Austin and J. Kurdock of the Perkin-Elmer

Corp. for measurements pertaining to the hydroxyl ion concentration.

5. References

[1] House, R. A., Bettis, J. R. , Guenther, A. H.

,

and Austin, R. , NBS Special Publication 435

(Glass, A. J. and Guenther, A. H. , Eds.),

U.S. Government Printing Office, Wash, D.C. (1976), p. 305; also House, R. A., Dissertation (1975), AF Institute of Technology,

WPAFB, OH.

[2] Fradin, D. W. and Bass, M. , Appl. Phys. Lett.

22, H, 157 (1973).

[3] Bloembergen, N., Appl. Opt. 12, #4, 661

(1975)

[4] Ringlien, J. A., et. al. , Appl. Phys. Lett.

25, #10, 598 (1974).

[5] Boling, N. L. , Dub^, G. , and Crisp, M. D. in NBS Special Publication 387 , (Glass, A.J. and Guenther, A. H., Eds.), U.S. Government

Printing Office, Wash, D.C. (1973), p. 69.

[6] Bettis, J. R., House, R. A., Guenther, A. H.

,

and Austin, R. , NBS Special Publication 435 , (Glass, A. J. and Guenther, A. H., Eds.),

U. S. Government Printing Office, Wash, D.C.

(1976), p. 289; also Bettis, J. R. , Dissertation (1975), AF Institute of Technology,

WPAFB, OH.

[7] Austin, R. private communication.

[8] Ritter, E. in Physics of Thin Films . (G. Hass,

M. H. Froncombe, and R. W. Hoffman, Eds.),

Vol. 8, p. 6, Academic Press, New York, NY

(1975).

Figure

1.5 -

1.0 -

0.5 -

12 3 4

RMS Surface Roughness

Figure 1, Threshold vs roughness for various surface preparation

techniques.

-313-
COmENTS ON PAPER BV HOUSE, BETTJS, AMP GUENTHER

No di6caii'ion on thJj> papeA.

-31A-
4.9 LASER INDUCED DAMAGE TO THIN FILMS INMERSED IN LIQUIDS

A. Balbin-Villaverde

Institute de Fisica

Universidade Estadual de Campinas

Campinas S.P. 13100 - Brasil

L. G. DeShazer*

Center for Laser Studies

University of Southern California

Los Angeles, California 90007

A high power ruby laser system, operating in a single longitudinal and trans- verse mode was used to measured the damage threshold of a series of quarter-wave

and half-wave single coatings, as well as for multilayer reflectors and antiref lection "vee" coatings, inmersed in different liquids: water, carbon disulfide, nitrobencene, ethanol, methanol, acetone, and diiodometane . Coatings of TiO^, MgF^, Si02,

Zr02,and ZnS on substrates of glass were studied. Liquids were chosen covering a wide range of their physical properties. A correlation is intended between the damage threshold of the coatings and liquid properties. It is found that the damage

threshold for all the films increases steadily with the low frequency dielectric

constant of the liquids. For multilayer reflectors is also observed a clear

dependence of the damage threshold on the liquid index of refraction, through the modification of the electric field inside the films.

Key words: High power ruby laser damage; multilayer reflectors; single layer

coatings; spark detection; thin films inmersed in liquids.

1. Introduction

Although up to the present time, the main effort was done on the damage study of solids, surface

and bulk, caused by irradiation with high power lasers, several thorough studies of laser induced damage in thin films, were also undertaken. Among those, we can mention the work of A.F. Turner [ij , who

measured the damage threshold of single-double- and multi-layer coatings, using a high power ruby

laser system, but because of the multimode operation of his laser, the thresholds reported by him might

be considered as giving only relative values of the damage resistence of the different coatings tested.

R. R. Austin and A.H. Guenther [2,3] determined the resistence of a set of AR coatings to irradiation

by a high power neodymium laser. B.E. Newnam and L.G. DeShazer [a] reported a thorough and reliable

study of the laser induced damage in thin films irradiated by a single mode ruby laser. That study

comprised the measurement of damage thresholds for single-layer coatings of various thicknesses and

materials, as well as antiref lection "vee-coat" and multi-layer reflectors, being many of the films

tested also in the reverse orientation.

In the present work is intended a different approach to the problem of the laser induced damage in thin films. D.W. Gregg and S.J. Thomas [s] have previously reported that inmersing two 99% dielectric

mirrors (one "soft": zinc sulf ide/thorium oxifluoride, and the other "hard": silicon oxide/titanium

oxide) in nitrobenzene, their resistence to laser induced damage was greatly improved. They found that

at the laser intensities for which the mirrors damaged in one or five shots in air, there was litlle

damage in them, even for more than 50 laser shots, when they were inmersed in nitrobenzene. In that experiment the two mirrors under study were used as the back reflectos of the laser optical cavity.

Based in those results a sytematic study of this effect was undertaken. In our experiment the coatings were placed outside of the laser cavity, and in order to achieve the necessary laser

intensities to damage them, a lens was used. A variety of coatings were tested, inmersed in liquids

* Present address: Office of Naval Research, 223 Old Marylbone Road, London NWl 5th, England.

^Figures in brackets indicate the literature references at the end of this paper. -315-
with different physical properties, in an attempt to get a better understanding of this effect, and consequently, of the damage mechanism. |i

2. Test-samples.

The effect of liquid inmersion on the damage threshold was investigated for several single-films

made of different materials, with thicknesses ranging from a quarter-wave up to three-quarter-wave as well as for multilayer reflectors, each made with various combinations of two different films materials.

Most of the samples were tested in two ways: the first one with the liquid in front of the film

facing the laser beam (which will be called forward orientation), whereas in the second way the sample

was turned around with its substrate side towards the laser beam (reverse orientation). Likewise, a "vee-coat" sample was tested, this consisted of a bilayer coatingwith the layer in contact with the substrate made of a material of high index of refraction (of the order of 2 or higher)j and the other

layer of a lew index material (about 1.4 or 1.5), with the thicknesses of both layers being designed to give a minimum of reflectance in air.

This thorough study of the damage threshold behaviour was done in order to have an extended basis

for comparison between possible thin-film damage mechanisms. In table 1 and 2, all the cases investigated are enumerated, the first of these two tables is reserved for single-layer samples, whereas the second one is for multilayer reflectors and one "vee-coat". The nomenclature used specified the structure of the coatings when there are more than one layer, but made only with two different materials

is as follows: for each layer of a quarter-wave thickness is used a capital letter H, if the material

has the highest index of refraction or L, if it has the lowest one, raised to a symbolic power equal

to the actual thickness of the layer expressed in units of a quarter-wave. The letter G stands for the substrate (BCS-2 glass), and A for the air. When some combination of layers is repeated itself, this is indicated through the use of the corresponding exponent. Letters are writen on a consecutive order

starting from the substrate and ending with the air.

The substrate for all the samples is made of BCS-2 borosilicate glass, with an index of refraction

of 1.513 at the wavelength of 694. 3nm. The coatings were supplied by Optical Coatings Laboratory Inc. (O.C.L.I.), except for four of them which were made by Ray Owens at the University of Southern California. The index of refraction at a wavelength of 694. 3nm for each material we used is as follows:

Ti02-'2.28, ZnS- 2.32, Zr02- 1.975, MgF2- 1-38, Si02- 1.456, and for the sample 19: Ti02- 2.35.

3. Liquid Selection for Inmersion Test

Eight liquids were tested in our experiment of laser induced damage in inmersed coatings. They were

chosen according to the two following criteria:

1) wide range of refraction index values; because the electric field squared inside a film depends on

the index of refraction of the liquid in which it is inmersed, we tested liquids covering the maximum

available range of refractive indeceses, from water and methanol with n= 1.33 up to diiodomethane for which n= 1.72. Also a test in air ( n=l ) was included for the sake of completeness.

2) wide range of dielectric constant: because a preliminary test showed some dependence of the laser induced damaged threshold on the dielectric constant of the liquids, we selected them covering the whole available range of values of K, from air with K=l up to formamide with KsiOO.

Some of the most relevant physical properties of the liquids are listed in table 3. They are: [6,7,8,9j : 3 a) density (g/cm ) b) dielectric constant K, average value between low frequencies (audio) and radio-wave frequencies

(400 MHz) . The dielectric constant drops monotonously from dc up to about 500 MHz, depending on the liquid; for higher frequencies resonances begin to appear.

c) refraction index corresponding to the Ha line of the hidrogen spectrum at 656. 3nm.

d) viscosity n ( x 10~3 Pa.s)

e) heat of vaporization A H (KJ/g)

f) specific heat at constant pressure c (W/cm°K)

2 ^

g) thermal diffusivity D ( cm /ms).

. . . 2 Transmissivities were measured for all these liquids, at a laser power density of 2 GW/cm , with the same experimental arrangement used for the laser induced damage threshold measurements. Benzene,

nitrobenzene, and carbon disulfide (which are very strong Raman and Brloullln actives) and diiodomethane,

showed a loss of the order of 4%. For the other liquids the losses were negligible.

-316-
Table 1. List of tested thin films: single-layer

# Material Thickness Orientation Supplier

1,2 TiO, qw forw. ,rev OCLI(0121)

3,4 Ti02 hw forw. , rev OCLI(0123)

5,6 TiO, tqw forw. ,rev 0CLI(013 7) 7,8 ZrOj qw forw. ,rev. OCLI(0129)

9,10

2 hw forw. , rev. OCLI(0131)

11,12 MgF^ qw forw. ,rev. OCLI(0133)

13,14 MgF

2 hw forw. ,rev. OCLI(0135)

15 2hS qw forw. use

16,17 ZnS hw forw. ,rev. use

18 ZnS tqw forw. use

qw: quarter-wave; hw half-wave tqw: three-quarter-wave ; forw. : forwards; rev. : reverse.

Table 2. List of tested thin films: multi-layers*

# Materials #of layers Structure Supplier

19 Ti02/Si02 20 C-(HL)^HL^A OCLI(0102)

20 Zr02/Si02 29 G(HL)^^HA 0eLI(0105)

21 Zr02/MgF2 25 G(HL)^^HA 0CLI(0107)

22 Ti02/Si02 2 GH°-2V-32a 0CLI(0141)

23 ZnS/ThF^ 9 G(HL)^HA use

*Samples were tested forwards

.

Table 3. Physical properties of the liquids

Liquid K n AH P c D

P

I Ethanol 1.36 25 i:i9 0.85 0 791 2.40 0.89

' Water 1.33^ 80 1.0 2.26 0 998 4.18 1.45

1 Methanol 1.32^ 33 0.58 1.10 0 826 2.51 0.96 Benzene 1.49^ 2.3 0.65 0.40 0 879 1.7 0.88

Formamide 1.44^ 100 3.36 1 133 2.31 earbon

Disulfide 1.62 2.6 0.37 0.35 1 26 0.97 1.31 Nitrobenzene 1.54^ 35 2.01 0,33 1 20 1.46 0.93

Diiodomethane I.7I3 5.2 3

*Measured at 20 °C

-317-

i
!

I

4. Experimental apparatus [

t A schematic diagram of the experimental arrangement is shown in figure 1. The ruby laser oscil- ! lator operates in a single longitudinal and transverse mode TEM^^. The transverse mode selection is achieved through the use of a intracavity aperture of 1 mm of diameter (Fresnel number of 0.4), whereas

for the longitudinal mode selection we used an etalon as the output mirror. The shape of the output beat

is monitored by making a spatial scan with a pinhole. The laser beam is Gaussian with a measured spot- | size radius of 0.060 cm, at a distance of 100 cm from the aperture of the laser oscillator with a beam;

divergence of 1.3 mrad. A sample of the output beam, after to be delayed for about 30 ns , is displayed 1 in the same oscilloscope than the transmitted beam for normalizing purposes. The photodetector system Ij comprises a ITT F-4000 fast biplanar photodiode and a 519 Tektronik oscilloscope. A typical FWHM value ^ for the pulse is 12 ns , Energy is measured using a RCA 7102 photomultiplier with an integrating circuit and a 555 Tektronix oscilloscope. The energy system is calibrated against a 102 thermopile from Hadron.

In order to achieve the neccesary power density to damage the samples the beam is focused with a ; lens of 10 cm focal length placed at 100 cm from the laser output aperture. J

The cell for the thin-films damage studies is made with the sample as one of the windows,

whereas the other one is an optical flat BCS-2 borosilicate glass, with a thickness of 6.2 mm. The windows are separeted by an 0-ring teflon spacer, this material has the advantage to be resistent to the action of organic liquids. The spacer is made as thin as possible in order to reduce the losses by

liquid absorption and stimulated scattering processes- The best thickness was found to be 3 mm. The cell

is positioned non-normal to the beam to avoid the problem of the backscattered light.

The spark produced when the sample is hitted by the laser beam with an intensity above some threshold value was photo-electronically detected by using a 6199 RCA photomultiplier and recorded by a 555 Tektronix oscilloscope. The light emitted by the spark, corresponding to the black body emission at several thousands of degrees kelvln, has its peak in the visible region of the spectrum towards the violet. At the same time, the light emitted by the flash-tube and some small amount of ruby laser radiation, both scattered by the various optical components also reach the photodetector, giving

signals of the same order or even greater than the corresponding to the spark emission. To overcome this problem we used a 6199 RCA photomultiplier with a peak of sensitivity at 400nm,a narrow-band uv filter j and a high-frequency-pass RC filter in the output of the photodetector. |

The duration of the spark emission pulses is of the order of 100 or 200 ns []l0,llj for laser

irradiated transperent solids, and thin-films, while on the other hand, the duration of the flash-lamp

emission is about 1 ms. This difference in the frequency of both pulses of three or four orders of magnitude allows to the filter to cut-down the low frequency signal from the flash-lamp emission by a factor of 20, whereas only affect the spark signal by 20%. The spark detector is located off-axis of the laser and facing the back of the cell, in order to avoid the difference in the spark detection sensitivii

ty, when this is observed through a layer of liquid.

5. Experimental Procedure.

The damage threshold was measured for each sample in air and when it was inmersed in different

liquids. Between liquids the cell was rinsed with ethanol several times to avoid contaminations. We

chose this liquid because of its solubility for all the liquids tested in our experiment. To keep the

same all the experimental conditions the damage threshold in air was determinated with the sample

mounted in the cell without any liquid. The effective thickness of the cell spacer, corrected for the index of refraction effect was: 3 mm for air, 2.4 mm for the liquid with the lowest index of refraction (n=1.33)> and only 1.8 mm for diiodemethane (n=1.74). To overcome that source of error the cell was moved 1 cm far away from the lens, whenever a liquid was inserted into the cell. By turning

around the cell the damage threshold of the thin-film samples were also measured with the laser beam

irradiating them from the substrate side. In the last case, it is not neccesary a correction in the cell position when this is filled with different liquids.

For samples 1 through 14, their damage threshold was identified with the corresponding threshold

for the spark detection. The same was done for the samples 18 to 23. The signal fron the spark detector

was sent to one channel of a 555 Tektronik dual-beam oscilloscope, while on the other hand, the signal

from the laser energy detector was displayed on the other channel. With this technique it is possible to measure for each laser shot its energy and the corresponding spark intensity. For each liquid between

10 and 20 shots were required to find the damage energy threshold. That was assumed to be the midpoint

between the maximum energy for which there was a non-detectable spark and the minimum energy for which

there was still a tiny spark. That energy interval was taken as the uncertainty associated with the measurement

By moving the cell parallel to the lens we used different spots over the coating surface for each laser shot, measuring in that way the one-to-one damage threshold. The laser beam was

-318-
attenuated by means of a set of CUCI2 cells. The distance lens-sample was kept constant throughout the whole set of liquids for each particular sample, in that way the effect of the beam spot-size on the damage threshold was avoided [l2]

.

For samples 15 through 18 another damage threshold criterion was applied. There was a low power

cw He-Ne laser beam superimposed to the ruby laser beam, irradiating boths the same spot on the sample

surface. When some damage was produced in the coating, the scattering for the cw laser increased over

its normal value, being that very small change detected visually. The same criterion as for the

I previous spark method was used to define the threshold, in this case will be called laser-inducedscatterer (LIS) threshold.

6. Experimental Results

Damage thresholds for the coatings inmersed in different liquids, relative to the corresponding

value in air, are plotted in figures 2 and 13for all the samples tested as a function of the refractive

index of the liquids. Open circles are used in those figures for the data corresponding to the liquids:

formamide, water, and nitrobenzene. The first two liquids have the highest values of the low-frequency

dielectric constant, K. The third one has a special interest, although having only a moderate value of

K, it is the only liquid tested previously by D.W. Gregg and S.J. Thomas [5] and it will be used for later comparison. We also take into account the difference in the reflection losses in the window when

the cell is filled with a liquid. The curves drawn in thoses figures only intend to visualize the apparent trend of the damage threshold.

The damage threshold in air for each sample is given in the figures mentioned above. They are expressed as the peak energy density (J/cm ) and the corresponding peak power density (GW/cm )

.

It also includes the spot-size radius of the beam at the position of the sample cu^ . The laser

pulsewidth fluctuated from 10 to 12 ns for different shots. Because in our study we are only

interested in relative damage thresholds, the laser intensities are calculated aproximately by using

only Dickson equations for the focusing of a Gaussian beam, without taking into account the effect of

the truncation of the beam which is only severe near the focus of the lens

.

The spark threshold criterion was applied for almost all the coatings because of the high initial

scattering of the He-Ne laser by the films and by the liquids; that makes the LIS threshold measurement

very difficult and inaccurate. Only for ZnS quarter- and half-wave films, with an initial very low laser

scattering, the LIS threshold criterion was preferred to the spark method due to the fact that in this case the LIS threshold is between three and four times lower than the corresponding spark threshold. For

a three-quarter-wave ZnS coating, being both thresholds very close to each other both were measured,

that allows us to make a comparison of the effect of the liquid inmersion on each case.

The experimental results can be summarized in the following way:

1) There is definite dependence of the damage threshold on the index of refraction of the liquids,

that relation depends on the film material , thickness and orientation of the samples. In some

cases the threshold undergoes one hundread percent change, going from air to diiodomethane

,

whereas in some other cases the effect is almost null.

2) The damage threshold when the film is inmersed in a liquid with a high value of the dielectric

constant K (open circles in figures 2 to 13) is higher than it corresponds from its index of refraction, increasing that difference with the value of K. Nevertheless, this effect is in average, only of the order of 20% for formamide which has the highest value of K. The effect is observed for all the films we tested regardless of the material ,thickness and orientation of the samples. It is also found when the LIS criterion is applied.

7. Discussion of the experimenal results

In order to relate the observed differences in the behaviour of the costings when they are inmersed in liquids, it is neccesary to calculate in each case the real laser intensity that produces

the damage, i.e., to take into account the interference effects in the coating. We calculate for each

sample the electric field squared (EFS) at every point in the liquid, film_,and substrate £l3] . Because

the incident laser intensity is measured in air, all the EFS values inside the sample should be normalized to the corresponding values for air, which will be called [e^]^. The index of refraction

of the substrate is taken equal to 1.51, and furthermore, reflection losses were taken into account in

the calculations

.

Relative laser intensities are propertional to the quantity

n.(E/E^|2 _3j^9_
2 where n and [eI are the index of refraction and the EFS, respectively, in each medium. This last quantity unlike the EFS, is not continous across the medium boundaries. Relative laser intensities in each medium are given for all the samples in table 4, corresponding to the liquid refractive indeces

of: n^= 1.33, n^= 1.50, and 1.72. For comparison, the inverse of the corresponding measured damage

thresholds is included in the table. The possible correlations we found between damage threshold and

laser intensity are summarized as follows:

1) Multilayer reflectors, the laser intensity in the liquid is independent of its refractive index

and the changes in the damage thresholds are in agreement, within experimental errors, with the corresponding changes in the laser intensity in the coating for Ti02/Si02, Zr02/Si025 and Zr02/MgF2. For the ZnS/ThF^ reflector is observed the same trend, but the effect is reduced.

2) Ti02/Si02 "vee-coating" , in this case the laser intensity in the coating does not depend on the refractive index of the liquids and the damage thresholds can be correlated with the laser intensities in the liquid at its interface with the coating where there is a minimum of the standing wave pattern.

3) Half-wave-layer coatings, three of them Ti02, Zr02> and ZnS present the same trend, whereas for MgF2 is the opposite one. In the coating the laser intensity is independent of the liquid in

which it is inmersed. For the three coatings mentioned in first place, the change in the threshold can be correlated with the corresponding change in the laser intensity, averaged in the|

liquid, whereas for the last film of MgF2 is with the laser intensity at the liquid-coating

interface. Notice that for the MgF2 film the laser intensity in the coating has a maximum,

whereas for the other three films presents a mininum. Let us add that in ZnS is observed the same|

change in damage threshold than for Ti02 ( boths have almost the same index of refraction),

although for the first film a LIS threshold criterion was used.

4) Half-wave-layer coating (reverse), all the samples show the same trend, even for ZnS for which the[ LIS criterion was applied; nevertheless, the intensity of the effect is different in each case. The best possible correlation, although poor, is with the laser intensities at the center of the

| film. Notice that for ZnS, Ti02» and ZtQ^, there is a minimum of the laser intensity in that

point, while for MgF2 the intensity is a minimum or maximum depending of the index of refraction of| the liquid. We can not find any correlation with the average intensity in the film. In these coatings the laser intensity in the liquid is the same for any liquid.

5) MgF2 quarter-wave-layer, a fairly good agreement is found between damage threshold and the laser

intensity in the liquid at its interface with the coating. In the film the laser intensity is almost independent of the liquid.

6) MgF2 quarter-wave-layer (reverse), there is a good agreement with the laser intensities at the substrate-film interface and/or in the liquid. Notice that for the position mentioned in first

place, there is a peak of the intensity, while the average intensity in the film does not depend

on the liquid.

7) ZnS and Ti02 quarter-wave coatings, boths show almost the same standing-wave pattern; nevertheless, they behave oppositely to each other, with regard to the damage threshold. In the first

case we found a correlation with the peak laser intensity in the liquid, but for the second film

there is not a simple correlation, probably because of the simultaneous changes of the laser intensity in the liquid (at the film-liquid interface) and in the film.

8) Ti02 one-quarter and three-quarter layers (reverse), we found the same correlation for both

with the laser intensity at the film-liquid interface (liquid-side) and in the liquid too. In

this case the average laser intensity in the film is independent of the liquid.

9) Zr02 quarter-wave coating (forwards and reverse), for the two orientations we found an anomalous

behaviour, the damage threshold presented a minimum for some value of the liquid index or refraction between n=l and n=1.33,but the position of that minimum can not be determined

precisely because there is not any liquid with a value of n between 1 and 1.33. The existence o^, that minimum was confirmed by repeting the measurements several times. By discounting that effect, it is possible to correlate the damage threshold with the peak laser intensity in the

film. The laser intensity in the coating is almost independent of the liquid. That is also true for the average laser intensity in the film for the reverse orientation of the sample. In that last case, the best correlation is with the laser intensity in the film at the film-liquid

interface.

10) Ti02 and ZnS three-quarter-wave coatings, the measured damage thresholds are almost independent

of the refractive index of the liquids, being the observed differences probably due to the -320-
inherent fluctuations in the damage threshold measurements. That can also explain the slight differences observed for the ZnS when the two threshold criteria are used. No straight correlations can be found with either the EFS or laser intensity distributions.

Table 4. Comparison between the laser intensity and the inverse of the damage threshold for liquid inmersed coatings.

"l

RELATIVE LASER INTENSITY /INVERSE

DAMAGE THRESHOLD

miARTFR AND THREF -QUARTER T.AYERS OF Ti02 AND ZnS

0.90 0 .84 0 76 1.58 1 .89 2 31 L-F interface (F-side), film, F-G interface and

1.20 1 .26 1 33 Ti02 ( X /4) 1.07 1 .12 1 78 ZnS ( X /4) (LIS threshold) 0.86 0 .74 0 59 Ti02 ( 3X/4) 0 .89 0 92 ZnS ( 3X/4) (Spark threshold) 1 .06 1 10 ZnS ( 3X/4) (LIS threshold) ____ 1 .03 0 95

QUARTER AND THREE-QUARTER LAYERS OF Ti02 (REVERSE)

0.87 0 .81 0 74 1.53 1 .81 2 22 0.97 0 .97 0 97 0.87 0 .81 0 .74 0.87 0 .81 0 74 1.15 1 .21 1 28 Ti02 ( X/4) 1.08 1 .10 1 33 Ti02 (3X/4) 1 .22 1 35

HALF-WAVE LAYER COATINGS

0.81 0 .72 0 62 1.43 1 .63 1 87 1.08 1 .09 1 08

1.08 1 .09 1 08 0.88 0 .85 0 77 Zr02 0.78 0 .73 0 56 MgF2 1.48 1 .78 2.28

0.90 0 .85 0 74

-321-
Table 4. Continued

HALF-WAVE LAYER COATINGS (REVERSE)

1.57 1 .81 0.70 0.60

1.57 1.81 0.70 0.60

1.04 1.04 1.04

1.32 1.66 1.24 1.36

1.42 2.09 1.49 1.72

QUARTER-WAVE LAYER OF ZrQ^

0 80 0.71

1 80 2.16

1 21 1.24

1.02 0.75

QUARTER-WAVE LAYER OF MgF^

0 70 0.59

1 57 1.78

1 05 1.03

1 50 2.18

QUARTER-WAVE LAYER OF ZrO^ (REVERSE)

0 77 0.68

1 .74 2.09

0 97 0.97

0 .77 0.68

1 .15 1.19 ZrO^ 1.21 0 .99 0.90

QUARTER-WAVE LAYER OF MgF2 (REVERSE)

0.67 0.57

1 .51 1.71 0.96 0.96

0 67 0.57

1 .51 1.71 MgF 1.31 1 .50 2.27

-322-
Table A. Continued

MULTI-LAYER REFLECTORS

1 04 1.04 H-Low interfaces (and at the center of the half-wave

1 56 1.80

1 34 1.64

2 2

1 55 2.11

1 62 2.21

1 12 1.24

4

TiO^/SiO^ VEE-COATING

2 2

0 67 0.55

2

1 .50 1.67 L-SiO^ interface (SiO^-side), SiO^ film, Si02-Ti02

interface, Ti02 film, Ti02-G interface, and substra1 00 0.96

1 .62 2.26

F: filia; L: liquid; G: BCS-2 glass substrate; H: high index material; Low: low index material

Liquid index of refraction: n^^ = 1.33 ~ ^2 = 1.50 ~ ~ 1-72

8. Discussion of laser induced damage mechanisms.

A comparison between the physical properties of the liquids listed in table 3, and the damage

threshold of the coatings inmersed in those liquids shows no correlation at all for none of the properties, but with the low frequency dielectric constant. It is observed that when the films are inmersed in a liquid with a high dielectric constant, like water and formamide (and at less extent,

nitrobenzene), their damage threshold data are not well fitted by the relative threshold vs index

of refraction experimental curves, showing always higher values of the threshold. That effect is also

observed when the films are tested in the reverse orientation, as well as when the LIS criterion is used. A possible explanation of this effect of the laser induced damage dependence on the dielectric

constant of the liquid can be as follows: there is a potential barrier at the film-air interface whose

high controls the electron protoemission of the film surface when this is irradiated with the laser. The

electrons emitted from the film surface, by absorbing energy from the laser beam, produced a "hot plasma"

(with temperatures of the order of thousands of degrees, well above the melting point of any of the coatings). The exact form of that potential barrier is not known in general [l42 , being also afffected

by the presence of inclusions, cracks, and pores on the film surface.

In spite of the fact that the exact shape of the potential barrier can not be calculated, we know

that when a film is put in contact with a liquid, the height of the barrier at the new liquid-film

interface should increase. That is due to the fact the positive image charges left in the film surface

when the electrons are emitted, induce negative charges in the polarizable liquid at its interface with

the film. Therefore, a new contribution is added to the interface barrier so its height increases, and acccording to that the electron photo-emission is reduced. That is in good qualitative agreement with

the observed increase in damage threshold.

Let us also recall that the peak frequency in the Fourier transform of the laser pulse (about 10 ns) is of the order of 10^ Hz, and consequently, this also should be the characteristic frequency for

the induced charges. That supports the correlation we found with the dielectric constant of the liquids

Q at low frequencies (up to about 5x10 Hz) . The fact that the effect is observed also when the LIS

criterion is used, means that the decrease in the electron photoemission not only affects the spark

detectivity (reducing the size and/or the temperature of the spark plasma)» but also acts on the intrinsic damage threshold of the films.

No definite correlations can be found for all the samples between the changes in the damage

-323-
threshold for different liquids and the corresponding change in laser intensity (or equivalently for

the energy density of the laser). For some of the films the laser intensity has a different dependence \' on n (liquid index of refraction) in the liquid than in the film, and consequently, in those cases the lack of a correlation could be due to the combined changes in both regions. Only in those cases for which the standing-wave pattern is independent of n, either in the liquid or in the film, some degree ofi correlation can be found, like in the following cases: I' For all the multilayer reflectors the laser intensity in the film controls the damage threshold, | whereas for the bilayer "vee-coating" is the laser intensity in the liquid at the liquid-film interface,

which fixes the threshold. The same is also true for MgF^ quarter-wave coating in both orientations,

and for MgF^ half-wave layer in the forwards orientation. For some other samples their damage

thresholds are also related to the laser intensities in the liquid, not just at the interface^ but to some average value in the liquid.

Let us also mention the fact that when the LIS criterion is used a similar behaviour is observed

for the damage threshold as a function of the liquid index of refraction. All the facts previously

discussed (including the dependence on the low frequency dielectric constant of the liquids) give

us a strong evidence that the photo-electrons emitted by the film through the production of a "hot plasma", play an important role in the damage of the coatings, as well as the intensity of the beam

in the liquid, by controling the formation of this plasma (which grows in size and temperature by

absorbing energy from the laser beam) . It is observed from the oscillograms of the transmitted beam, jj that the on set of the spark is accompanied by an absorption of the laser beam reaching a complete !' cutoff of the transmission for the laser intensities above damage threshold.

The possibility should be excluded of having only an "apparent change" in the damage threshold \' when different liquids are used due to the differences in the spark detectivity (by changing the size

and/or the temperature of the spark), since a simmilar behaviour is observed for the LIS thresholds.

Furthermore, as it was mentioned previously, the absorption of the laser beam by the liquids before 1 damage takes place is negligible for all the liquids used in our experiment.

There is not evidence against the fact that some other mechanisms can also contribute simultaneously to the coating damage process. It is only found that when the conditions are the same for the ! mechanisms, the damage threshold is controlled by the photoemission of electrons from the films and the 1 subsequent growing of the spark. Among those damage mechanisms we can mention; the electron avalamche

j induced by the intense optical field in the medium [l5,16,172; the heating of the films by linear

absorption of the laser beam [l] and the induced stresses [2,3]. B.E. Newman discusses the possibility

of the simultaneous action of those three different mechanisms of damage in thin films with their | relative important depending of the coating material and the thickness [is]

D.W. Fradin and M. Bass [19] and N. Bloembergen [20] point out the important role played by the t surface imperfections (micropores, cracks, inclusions) in the damage of solids. Simmilar discussion i for the inclusion effect in thin film damage was carried on by L.C. DeShazer et al. [2l] . In our case we[ have that the liquids, with their different values of n, should modify the degree of the optical

electric field enhancement produced by the imperfections, and consequently, we are in presence of another mechanism by which the liquids can influence the damage threshold.

Finaly, a comparison between the results obtained in our experiment with the ones reported by D.W. Gregg and S.J. Thomas [s] shows a total disagreement, nevertheless, it has to be taken into account

that different experimental procedures were followed in both cases. They used the dielectric mirror

samples as the back mirror of the laser cavity inmersed in nitrobenzene, which also was used as the solvent for the Q-Switch dye. Another significant difference is that they do not measure the one-to-one

damage thresholds, but the relative resistance of the samples to the laser induced damage with and

without the liquid, after firing the laser many times.

9. Acknowledgements

The authors are grateful to Ray Owens for making some of the samples. We also thank B.E. Newman for providing the computer program for the determination of the standing-wave pattern in the bilayer "vee- coating" .

10. References

[l] A.F. Turner, in Laser Induced Damage in Optical Materials: 1971 , NBS Special Pub. 356,

119 (1971).

[2] R.R. Austin and A.H. Guenther, in Laser Induced

Damage in Optical Materials: 1971 , NBS Special

Pub. 356, 137 (1974).

-324-

[3] R.R. Austin and A.H. Guenther, Appl. Opt. 1_1, 695 (1972).

[4] B.E. Newnam and L.G. DeShazer, in Laser

Induced Damage in Optical Materials: 1972 , NBS Special Pub. 372, 123 (1972).
D.W. Gregg and S.J. Thomas, Appl. Phys . Lett.

8, 316 (1966).

R.C. Weast, editor, Handbook of Chemistry and Physics, 53rd ed . (Chemical Rubber Company,

Cleveland, 1972).

C. J. West and C. Hull, editors, International

Critical Tables of Numerical Data, Physics , Chemistry and Technology (7 vol., MacGrow-Hill

,

New York, 1933).

A. A. Maryott and E.R. Smith, editors. Table of Dielectric Constants of Pure Liquids , NBS Circular 514 (1951).

D. E. Gray, editor, American Institute of Physics

Handbook, 2nd ed . (McGrow-Hill , New York, 1963).

H, Dupont, A. Donzel and J. Earns t, Appl. Phys.

Lett. 11, 271 (1967).

L.D. Khazov, A.N. Shestov and G.P. Tikhomirov,

Soc. Phys. Tech. Phys. 13, 1112 (1969).

B. E. Newnam and L.G. DeShazer, IEEE J. Quantum

Electron. QE-8 , 611 (1972).

1^133 O.S. Heavens, Optical Properties of Thin

Solid Films (Dover Publications, New York,

1965) .

[14] T.E. Fisher, J. Vac. Sci . Tech. 9, 860 (1972)

[lf\ B.S. Sharma and K.E. Rieckoff, Can. J. Phys.

45, 3781 (1967).

[16] B.S. Sharma and K.E. Rieckoff, Can J. Phys.

48, 1178 (1970).

[17] M. Bass and H.H. Barrett, in Laser Induced

Damage in Optical Materials: 1971 , NBS Special Pub. 356, 76 (1971).

[is] B.E. Newnam, Dissertation, University of Southern California (1973)

[19] D.W. Fradin and M. Bass, Appl. Phys. Lett.,

22, 157 (1973).

[26] N. Bloembergen, Appl. Opt. _12. 661 (1973).

[21] L.G. DeShazer, B.E. Newnam and K.M. Leung,

Appl. Phys. Lett. 23, 607 (1973).

11. Figures

0-©

Figure 1, Schematic diagram of the setup for measurement of the damage threshold of liquid Inmersed coatings:

(1) single-mode ruby laser, (2) beam-splitter,

(3) optical delay-line, (4) energy detector: RCA 7102 PM, (5) Tektronix 555 oscilloscope,

(6) RC high-frequency-pass filter, (7) sparkdetector: RCA 6199 PM, (8) focusing lens,

(9) glass window, (10) liquid, (11) coating sample,

(12) substrate, (13) ITT F-4000 PD, (14) Tektronix

519 oscilloscope.

-325-
TiOp-\/4-forw

>

i=0.4 <

UJc:0.2r

Wo "0.06 3 mm

Po > 3.7GW/crrf

Co • 50J/cm'

1.0 1.4 1.6 1.8 INDEX

1.2

g 1.0 o

I 0.8 X

>

<

TlOg -"X/4- rev.

1 1 -,

-

-i ^.5^ ?

1

-

Wo • 0. 066mm

Po • 1.5 GVJ/cnf

eo 20J/cm^

] 1 r

1 OF

1.0 1.2 1.4 REFRACTION

1.6 1.8

Figure 2. Relative spark threshold for a quarter-wave layer of Ti02 vs index of refraction of the liquid,

(a) forward orientation, and (b) reverse orientation.

Figure 3. Relative spark threshold for a half-wave layer of Ti02 vs index

of refraction of the liquid,

(a) forward orientation, and

(b) reverse orientation.

2.0

0.8

Ti 0, - \/2 - f orw

• O.I07mm

1.16 W/cm*

l4J/cnf

1.0 1.2 1.4 1.6 1.8 INDEX

1.2

0 1.0 _io

1 0.8

ccX

H

T] Og-VZ - rev.

0.6 >

:0.4

q: 0.2

1 1 1 I

0.099 mm

- p. l.6GW/cm*

2IJ/cri^

t r r 1

OF

1.0 1.2 REFRACTION

1.4

(b) 1.8

Ti09-3>4X-forw

i.8n r I 1

Wo

P.

0.068 mm

l.6GW/cm

• 2 2J/cm*

1.2 1.4 1.6 1.8 INDEX

1.2 Ti02-3/4X- rev.

r r

\

l.5GW/cm

2 I J/cm

OF

1.0 1.2 1.4 REFRACTION

1.6

Figure 4. Relative spark threshold for a three-quarter-wave layer of Ti02 vs index of refraction

of the liquid, (a) forward

orientation, and (b) reverse

orientation.

-326-
ZrOo - \/4-forw

0.097mm

1.2 1.4

(a)

l.8rrZrOg -"X/4- rev.

1.6

1 1

1

• 0.087mm

P. 15GW/cm*

e • 20J/cm*

1.6 1.8 INDEX OF

°-^1.0 1.2 REFRACTION

1.4 1.6 1.8

Figure 5. Relative spark threshold for a quarter-wave layer of Zr02 versus

index of refraction of the liquid,

(a) forward orientation, and (b) reverse orientation.

Figure 6. Relative spark threshold for a half-wave layer of Zr02 vs index

of refraction of the liquid,

(a) forward orientation, and (b) reverse orientation.

2.0 ZrOg - X/2-forw

a: 1.0

0.8

1 1 W," 0.082mm

P. • l3GW/trt^

1.0 1.2 1.4

(a)

Zr O2 -\/2 - rev. 0.2n 1 \ 1

9 1.0

Wo- 0.092mm

P. l.2eW^m

I 7J/cm*

1.6 1.8 INDEX OF

^ 1.0 1.2 REFRACTION

1.4

(b)

1.8

MgFa -^/4-forw MgFa - '>v/4- rev.

Figure 7. Relative spark threshold for a quarter-wave layer of MgF2 vs index of refraction of the liquid,

(a) forward orientation, and (b) reverse orientation.

-327-
1.2 MgFg - \/2 -forw

3j6GW/crn

48 J/cm

1.2 1.4

(a)

1.6 1.8 INDEX

1.2 MgF2 - X/2- rev.

Po • 2.3GWA:m

Co • 31 J/crr?

OF

" 1.0 1.2 1.4 REFRACTION

1.6

(b) 1.8

Figure 8. Relative spark threshold for a half-wave layer of MgFa vs index

of refraction of the liquid,

(a) forward orientation, and

(b) reverse orientation.

Figure 9. Relative threshold for a singlelayer of ZnS, forward orientation

vs index of refraction of the liquid, (a) LIS threshold for a quarter-wave, (b) spark threshold for a three-quarter-wave,

and (c) LIS threshold for a threshold-quarter-wave.

2.0

3 1.8 o

1 1.6 I

ccl.O

0.8

ZnS -\/4- forw

Wo = 0.1 4mm

P„ - 05 6W/tnf

1.0 L2

(a)

1.4 1.6 L8 INDEX OF

1.4

o 1.2 _io

(t>) T 1 1 1 ZnS- 3/4 Vt forw

Wo» 0.14mm 1 P„-0.44GW/fcn?

1.2

1.0

0.8

-ZrS-3/4 \-forwWo» Q 1 22 mm

"fco^SGW^

eo= 7.8J/tm*

1.0 12 REFRACTION

1.4 1.6 1.8

ZnS -\/2- f orw

2X)n 1^ , ,

0.8

0.14mm

05eOW/cn^

IX) I, 2 1.4

(a) .6 1.8 INDEX

ZnS - \/2 -r ev.

i.2n \ 1 1

0.4-

Q:0.2h

W.

P.e.

0.1 15 mm

0.82 6WAan

11 J/bn?

OF

1.0 1.2 REFRACTION

Ij4 1.6 1.8

Figure 10. Relative LIS threshold for a half-wave layer of ZnS vs index of refraction of the liquid, (a) forward orientation, and (b) reverse orientation.

-328-
TiOj/SiOj

0.044mm

4.5GW/cm

t

• 61 J/cm

1.4

(a)

1.6 1.8 INDEX OF

O2/S iOg

1.0 1.2 REFRACTION

1.4 1.6

(b)

Figure 11. Relative spark threshold for Ti02/Si02 coatings

vs Index of refraction of the liquid, forward

orientation, (a) 20-layers reflector, and (b) bi-layer "vee-coatlng,"

Figure 12. Relative spark threshold for multi-layer reflectors vs index of refraction of the liquid, forward orientation, (a) 29-layers Zr02/S102

reflector, and (b) 25-layers Zr02/MgF2 reflector.

-329-
ZnS/ThR

_JO

JLCOUJ

trX

I-

>

I-<

_JUJ

1.0

1.8

0.6

0.4

0.2

1

-

1 1

•'I

i-

= O.I46mm

Po = 0.68GW/cm

_

= 9.2 J/cnr?

1

1 1 1 1.0 1.2 1.4 1.6 1.8 INDEX OF REFRACTION

Figure 13. Relative spark threshold for a 9-layer ZnS/ThFi, reflector.

COmENTS ON PAPER 8/ BALBlN-VniAVBWE ANV PeSHAZER

Wo cUiCLU,6-ion on this papfi.

-330-
4. in DIELECTRIC COATINGS ON METAL SUBSTRATES*

S. S. Glares, P. Baker and D. Milam Lawrence Livermore Laboratory

Livermore, California 94550

Large aperture, beryllium substrate based mirrors have been used to focus

high intensity pulsed laser beams. Finished surfaces have high reflectivity,

low wavefront distortion and high laser damage thresholds. This paper describes

the development of a series of metallic coatings, surface finishing techniques

and dielectric overcoatings to meet specified performance requirements. Beryllium

substrates were coated with copper, diamond machined to within 5 microinches to

final contour, nickel plated and abrasively figured to final contour. Bond

strengths for several bonding processes will be presented. Dielectric overcoatings were deposited on finished multimetalic substrates to increase both reflectivity and the damage thresholds. Coatings were deposited using both high and

low temperature processes which induce varying stresses in the finished coating

substrate system. Data will be presented to show the evolution of wavefront

distortion, reflectivity, and damage thresholds throughout the many steps involved

in fabrication.

Key words: Laser damage; laser fiision; metal mirrors; reflecting optics.

1 . Introduction

The reflectors described in this paper were designed for use with the ARGUS laser. The system

shown in figure 1 consists of a pair of nested ellipsoidal reflectors [1]\ coupled with an aspheric

doublet refracting element, which provides uniform illumination of the specified laser beam profile.

There are three distinct, but interrelated sections of this reflector design; the substrate choice,

mul ti -metal 1 ic coating adhesion, and dielectric overcoating choice.

Figure 1. Argus optics parameters.

*Work performed under the auspices of the U.S. Energy Research and Development Administration under

contract No. W-7405-Eng-48.

[.Figures in brackets indicate the literature references at the end of this paper. -331-
2. Substrate Choice

The mirror substrate material choices were bk-7, Cervit and beryllium. There are four primary

properties which determine the final choice since each of the four effect the substrate design [2] at different times during processing and final use.

The first property affecting substrate choice is strength and micro-yield strength. Macroscopic

strength affects the shape of the substrate. Micro-yield strength describes local substrate strength

Here beryllium substrates are seven times stronger than those of BK-7 or Cervit thus giving superior

rigidity for the same geometric dimensions as seen in table 1. The second property of interest is the modulus of elasticity which is the magnitude of stiffness. Again from table 2, beryllium is four

times stiffer than BK-7 and three times stiffer than Cervit.

Due to the required high aspect ratio required in the geometry of the reflecting portion, both strength and stiffness necessitate thicker peripheral sections for BK-7 and Cervit than for beryllium

substrates.

The third property affecting substrate choice is thermal diffusivity (K/pC). A certain amount

of the laser pulse energy is absorbed in the coating and transmitted to the substrate. An excellent

diffusivity such as that of beryllium, allows for rapid thermal equilibrium.

The fourth design property is thermal distortion which would be experienced in the final step

of mirror processing, the dielectric overcoating. This overcoating subjects the substrate to temp- eratures of 200-300°C. BK-7 experiences nearly six times the distortion of Cervit while beryllium

has only twice the distortion. The amount of distortion seen by beryllium substrates can be further

reduced by coating both sides and by substrate stabilization where the substrate has been previously

subjected to numerous cycles at these temperatures such that all creep has been accelerated prior

to final lapping.

The logical choice based on the forementioned properties is beryllium [3], [4], [5], but even

this material is subject to further choices. The desired high strength and low creep with relatively low cost and delivery time was met using ultra pure beryllium particles further shattered by

impact with a beryllium target in an inert gas atmosphere. This material is subjected to a number

of proprietary elevated temperature pressing and sintering operations to give the highest density

with a maximum isotropy. The amount of beryllium oxide in the chosen material directly determines

the strength and indirectly the amount of long term creep, hence long term dimensional stability.

A number of different vendors processes will meet the forementioned criteria^ but hot isostatic

pressed beryllium was the least expensive and was available in shorter delivery times than some of

the more exotic, but slightly superior processes.

Another factor which enters into the substrate choice is economics. Figuring a Cervit or BK-7 substrate is projected to be more expensive than diamond turning with final light lapping of the

nickel coated beryllium substrate system.

Table 1. Mirror substrate design parameters.

MATERIAL

p

3 (gm/cm )

BK-7 2.200

CERVIT 2.500

BERYLLIUM

(KB! HP-20) 1.852

MODULUS OF ELASTICITY

(lO^N/cm^)

7.00

9.23

28.00

MYS MICRO YIELD

STRENGTH

1 .500

1.500

3. 000-10. OOC

UTS ULTIMATE TENSILE

STRENGTH

(psi)

K THERMAL

CONDUCT

(Cal/cm- s C)

0.0033

0.0040

55.200 0.3800

SPECIFIC

HEAl (Cal/gm C)

0.188

0.217

0.450

COEF OF THERMAL

EXPANS

{10"^/C)

0.55

0.10

12.40

THEKllAL DIFFUSIVITY

K/pC

0.008

0.008

0.460

THERMAL DISTORTION

a/D ^

(x 10^)

69.0

14.0

J7.0

(C)

1 .500

800

1,300

-332-
3. Multi-Metallic Coating

The proposed processing of the mirrors is detailed in table 2. In order to insure that this

processing would yield the required diffraction-limited, high-damage threshold mirrors, a test program was instituted to investigate the variable parameters. Two inch diameter beryllium sub- strates were cut from KBI HP-20. The substrates were plated with best combination of metallic

coating found in another sequence of ring shear tests shown in table 3. The copper plated samples were diamond turned after which interferograms were taken. The test samples were then nickel

plated and another set of interferograms and capacitance noncontact measurement of figure.

Polishing was by normal mechanical lapping on a conventional machine. The lap material was a soft pitch and beeswax compound. The abrasive was successive grades of synthetic Dupont diamond,

the final grade being 0.1 ym size powder. The vehicle was distilled water with a small amount of

liquid soap added as a wetting agent. The final surface was obtained by running speeds at the minimum range for the machine, 2-5 rpm on the rotational speed and 1-3 rpm on the reciprocal

speed. The slow speeds cause less disturbance to the contacted lap surfaces.

Another set of interferograms was taken after polishing and then the specimens were sent to three different vendors. The processing up through dielectric coating is shown with its step by

step interaction with substrate figure, in figure 2. A sample set of interferogram records is shown in figure 3. Since movement in either side indicates induced stress, interferograms of

both sides of the sample are shown.

Table 2. Be substrate processing.

1. CUT ROUGH BLANKS 2mm OVER EACH

DIMENSION EXCEPT 3mm OVER THE BEST FIT RADIUS ON THE ELLIPTICAL CONTOUR, OUT OF

HP-20 MATERIAL.

2. VACUUM ANNEAL: HEAT AT LESS THAN

150°C/h UNTIL 900°C IS REACHED, HOLD FOR

1 h, COOL TO AMBIENT AT NOT MORE THAN

25°C/h

,

3. ROUGH MACHINE UNTIL SURFACE IS 0.46mm OVER FINISH CONTOUR.

4. REPEAT VACUUM ANNEAL.

5. MACHINE IN CUTS NOT EXCEEDING 0.254,0127 AND 0.051mm IN THAT SEQUENCE UNTIL SURFACE IS 0.025mm BELOW THE FINISH CONTOUR.

6. REPEAT VACUUM ANNEAL.

7. CHEM MILL UNTIL ALL SURFACES ARE 0.152mm BELOW THE FINISH CONTOUR AND RINSE.

8. BAKE FOR 6 HOURS AT 100°C IN A CHAMBER

EVACUATED BELOW IQ-^mm Hg.

9. AFTER MASKING, IMMERSE IN TURCO 4215, RINSE, IMMERSE IN BOILING DIST. H^O FOR

5-10 min, ACTIVATE IN COMBINED SOLUTION OF

97g/l AMMONIUM SULFATE AND 51 ml/1 SULFURIC

ACID AT ROOM TEMP FOR 60 s, RINSE, IMMERSE IN A ZINCATE SOLUTION FOR 5 min, AT 80°C, RINSE, STRIP ZINCATE WITH NITRIC ACID AT ROOM TEMP, RINSE, REPEAT ACTIVATION THROUGH ZINCATE IMMERSION, APPLY SPECIAL COPPER STRIKE COATING AT 40.6°C FOR 5 min, RINSE, COPPER PLATE IN COPPER PYROPHOSPHATE

TO A THICKNESS OF 0.300mm, RINSE, REMOVE

MASKING.

10. REMACHINE HOLES AND TURN DOWN NON- FUNCTIONAL SURFACES UNTIL REQUIRED

DIMENSION.

11. Y-12 R-O MACHINE TO SINGLE POINT DIAMOND

TURN CONCAVE SURFACES OR FLAT ON

PRIMARY REFLECTOR UNTIL FINAL CONTOUR

IS ACHIEVED.

12. STABILIZE: COOL AT 2.3''C/mln TO -70°C AND

HOLD FOR 15 min, HEAT AT SAME RATE UP TO ROOM TEMP, HEAT AT NOT MORE THAN 150°C/h UP TO 200°C AND HOLD FOR 15 min, COOL AT

50°C/h DOWN TO ROOM TEMP AND HOLD 15 min. REPEAT COMPLETE CYCLE ONCE MORE.

13. FINAL DIAMOND TURN SURFACES TO 0.013mm BELOW FINAL CONTOUR.

14. INSPECT FIGURE FOR ELLIPSOIDAL MIRRORS;

FIGURE, REFLECTIVITY AND WAVEFRONT

DISTORTION FOR FLATS.

15. MASK, IMMERSE IN 60g/l ENTHONE ENBOND S-61 AT82°C FOR 1-5 min, RINSE, IMMERSE IN 60g/l ENPLATE AD-480 FOR UP TO 1 min AT ROOM

TEMP, RINSE, ACTIVATE USING 30ml/l ENPLATE

440 AT ROOM TEMP FOR UP TO 30 s, RINSE, PLATE WITH SHIPLEY'S NICULOY 22 AT 90.5"C AT

A RATE OF 0.013 TO 0.018mm/h UNTIL 0.025mm PLATED ON, RINSE.

16. REMACHINE HOLES.

17. REPEAT STABILIZATION CYCLE.

18. LAP ELECTROLESS NICKEL TO REQUIRED FIGURE AND WAVEFRONT DISTORTION.

19. INSPECT FIGURE, WAVEFRONT DISTORTION AND

SPOT SIZE.

20. DIELECTRIC OVERCOAT CONCAVE SURFACE FOR

99% REFLECTIVITY AT 1.064 pm DAMAGE THRES- HOLD GREATER THAN 25GVV/cm2 FOR 100-150ps PULSES.

-333-
30

20

.70

1

1

1 Rough data — average of each 4 disks

"

\

\ 900 C anneal

1 1! 825 C anneal

t 1 "

\ ' >

\ '

'

1 II

\\ 1

1

1 / I ;

\ 1 / 1 / \i /

I / \' ' \

,'

" V'A //

. \ 1 \ \ \

V //\\ V//

\ V • r \

1 1 1 1

< 5

= 15 E ^ III!

Operations

c — oj = o -goo a ^ >- a. Q 5

Figure 2. Non-contact Interferometer results

on HP-20 Be 2" disks.

Side A after Ni coating

Side A after polish

Side B after Ni coating

Side B after polishing

Figure 3. Interferograms during processing of a sample.

-334-
Table 3, Plating bond strengths.

PLATINP BCriD STRENGTHS DOUBLE DOUBLE

ZINCATE ZINCATE

SINGLE DOUBLE Cu STRIKF- Cu STRIKE ZINCATE ZIKCATE Cu SAMPLE Cu STRl KE M. CT P 1 1^ t IM 1 o I n 1 r. t STABILIZED STABILIZED

NO. LOC. Cu Cu AT 100° AT 325°C

1 'RF 29,400 p$i 4,400 psi 21,700 psi 25,900 psi 2 'RF "3,700 1,600 15,300 27,600

3 'RF 17,600 2,000 15,500 4 'RF 16,600 1,700 5 *RF 15,200 1,300 6 'RF 15.100

7 *RF 11,900 8«RF 11,300 9 'RF 22.500 DOUBLE DOUBLE DOUBLE ZINCATE DOUBLE ZINCKTE ZINCATE Ni STRIKE ZINCATE C STHIKE

Cu STRIKE Cu Cu STRIKE Hcl DIP Cu (Nr 2.-) Cu Cu

1 LLL 2,389 ps: 18,875 psi 11.011 ps; 2 LLL '• 502 27,6-4 12,991 3 LLL 1,805 33,518 11,161 4 LLL 2,004 28,557 in.654

(Ni 22 + NiCI) 5 LLL 1,863 6LLI 6,054 7 LLL 4,040 8 LLL 10,595

(415 Ni) 9 LLL 8.564

10 LLL 4.390

11 LLL 6.839

12 LLL 3,341

•ROCKY FLATS PLANT OF NORTH AMERICAN ROCKWELL

-335-
4. Dielectric Overcoating

A number of different dielectric coatings were deposited on the successful substrate system

candidate. Additional coating runs were made at a number of temperatures since potential stress introduction was suspected at the elevated temperatures. Since the samples consisted of many

coatings of dielectrics, metals and beryllium, there was good reason to expect the coefficients

of thermal expansion to vary such as to induce stress hence change the final figure. Interferograms were taken after coating to determine whether stress induced figure changes had occurred.

Table 4 shows the relationahip of the number of coatings on each sample, type of coating, application temperature, induced wavefront distortion, reflectivity, surface quality, and damage

threshold.

Table 4. Dielectric coating parameters.

REFLECTIVITY SURFACE

{%) AT QUALITY SAMPLES 1.06 /um !SCRATCri/DIG) LAYERb

1 99.9 9

2 99.8 9 3 >99.0 20 4 >99.0 20

5 >99.0 12 6 >99.0 12 7 >99.0 12 8 99.6 80-50

9 99.7 80-50

Id 99.9 80-50

11 99.7 80-30

12 99.6 80-50

• AUGER ANALYSIS OF TOP COA i !NG AUGER ANALYSIS OF TOP TWO COATINGS

COATING DAMAGE

APPLICATION WAVEFRONT THRESH L TEMP DISTORTION (J/cm^ FOR COATING rc) X AT 1.06 fjm 12 5 ps Th (IO3)/ 350 0.10 7.0+ 1.5 !h(l03)/ 300 0.05 6.5 + 1.5 ZrOj + SiOj 275 ± 25 0.75 4.-: ± 0.5 ZrOj + SiO^ 275 ± 25 0.05 3.6 ± 0.5 TiOj + SiO^** 275 ± 25 0 6.5 + 1.5 TiOj + SiOj 275 + 25 0 5.0 ± 1.0

TiOj, + SiOj 175 + 25 0 1.4 + 0.4 ZrO^ + SiOj** 300 0 3.6 + 0.5 ZrOj + SiO^** 300 0.05 3.1 ± 0.5 TiOj + SiOj** 300 0 3.3 ± 0.5 ZrOj + SiOj** 200 0 2.3 I 0.5 TIOj + SiOj" 200 0 1.75 ±0.5

5. Dielectric Coating Damage Testing

Damage thresholds were measured using linearly polarized 1064 nm pulses 125 + 25 ps in duration at nearly normal incidence. Peak energy density for each pulse was determined by

reconstructing the spatial profile of the beam from a photograph, and normalizing the profile

to agree with the energy contained in the pulse. A typical beam profile is shown in figure 4,

with the associated isointensity contour plot. The circle scribed on the contour plot indicates

the portion of the profile used in the normalization. On-axis integrated energy density can be determined to within + 5-7%.

Energy density (J/cm ) Isointensity contour plot

Figure 4. Laser pulse utilized In damage testing.

-336-
6. Summary

From the data collected in table 4, it becomes apparent that the higher coating temperature

induces distortion into the coated substrate. The coating with neglible distortion and next to highest damage threshold was found in sampe #5. This sample was a 12 layer stack alternating

Ti02 and Si02 at 275°C temperatures resulting in no induced distortion and a damage threshold

of 6.5 J/cm2.

7. Conclusion

The beryllium substrate system with all of its complicated processing did prove to be about

50% less expensive than the comparable Cervlt or BK-7 substrate system after coating.

The multi -metal lie coatings applied to the substrate did provide bond strengths nearly equivalent to the beryllium substrate strength itself.

The dielectric overcoating developed for this substrate does provide a distortion free coating

with as high a laser damage threshold as any glass substrate coated system.

8. .Acknowledgments

The authors wish to acknowledge the extensive support of J. L. Emmett and H. G. Ahlstrom.

The mul ti -metal 1 ic coating work was done by W. Kelly and E. Childs. The dielectric coatings were developed in conjunction with Coherent Radiation, OCLI and Spectra Physics.

9 . References

[1] S. S. Glaros and A. J. Glass, "Compound

Ellipsoidal Focusing System for Fusion

Lasers", Sixth Symposium on Engineering

Problems of Fusion Research, San Diego,

Nov. 1975.

[2] R. R. Altenhof, "The Design and Manufacturing of Large Beryllium Optics",

SPIE 19th Technical Symposium, San Diego,

Aug. 1975.

[3] T. T. Saito and J. R. Kurdock, "Diamond

Turning and Polishing of Infrared Optical

Components", LLL document UCRL-77392,

Oct. 1975.

[4] M. D. Guadagnoli and T. T. Saito,

"Beryllium and its Optical Properties",

App. Opt. 75.

[5] P. C. Baker, J. B. Sonderman and T. T. "Finishing of Precision Generated Metal Optical Components", SPIE 19th Technical

Symposium, San Diego, Aug. 75.

COMMENTS ON PAPER BY GLAROS, BAKER, AWP MILAM

No <ic4ca4.4-c.on on tku, papeA,

-337-
5.1 Spot Size and Pulse Duration Dependence of Laser-Induced Damage

j( J. R. Bettis*, R. A. House II, and A. H. Guenther '\ Air Force Weapons Laboratory |l

Kirtland AFB, NM 87117 I

The threshold electric field for laser-induced damage in transparent dielectrics is Ij shown to follow an inverse relation with the fourth root of the pulse duration. Preliminary analytical studies suggest the process involves the free-free plasma heating rate. | The relationship is demonstrated for pulse durations ranging from a few picoseconds to |! several tens of nanoseconds. Furthermore, from considerations of total energy deposition,

it is proposed that the threshold electric field follows an inverse relation with the i square root of the damaging spot size. The results are compared to measured values with

acceptable agreement. I,

Key words: Frequency dependence; laser damage; spot size dependence; time dependence. i

1. Introduction I

;i It has been proposed that there are three main processes involved in the pulsed laser-induced

damage of optical components. Although we primarily consider in this paper pulsed laser effects,

thermal cw-like damage is not completely ruled out. The three principal steps as given by Milam [1]^ are: (1) creation of an initial low-density free electron concentration, (2) buildup of this electron

concentration until a density of 10^^ to lO-'-^ cm"-^ is reached, (3) absorption of the remainder of the laser pulse by the resulting plasma. The details of the first two processes as presented by the authors in the 1975 Damage Sjnnposium [2] give the material-to-material variation of the threshold for laser-induced damage. In the present paper we will show that the details of step (3) explain the spot

size and pulse duration dependence of the threshold for observed laser- induced damage. This paper proposes an explanation of the pulse duration dependence which is an alternative to that explanation

presented by proponents of the purely electron avalanche model of laser-induced damage [3].

It is essentially universally agreed that laser- induced damage will not occur unless a critical

number density of free electrons are created. In gaseous dielectric breakdown the accepted mechanism

for reaching a number density of lO-'-^ to 10-^^ cm~3 ±s cascade or avalanche ionization. This is a process whereby a free electron is accelerated by the applied field until it reaches ionizing energy

and collisionally ionizes a neutral or ion. The process repeats until a critical number density of free electrons is reached. Such a process is possible although unlikely [4] for laser frequency electric fields if the free electrons undergo collisions which momentarily align the momentum of the electron with the oscillating field. Under these avalanche conditions the growth of ionization would

follow

N^^ = exp [a(E)t] (1)

where and N^^ are the initial and critical free electron densities respectively, a (E) is the field

dependent ionization coefficient of the material and t is the time. Thus for decreasing pulse duration, a (E) must increase in order to maintain the required critical electron density. Since a (E) is proportional to the electric field, one would expect an increase in the threshold optical electric

field for decreasing pulse duration. Yablonovitch and Bloembergen [3] have used d c breakdown of dielectrics to obtain appropriate ionization rates which predict about an order of magnitude increase

in threshold field for a 10 ps pulse compared to a 10 ns pulse. Because the observed increase [5] is only about half this prediction we sought an alternate explanation of the observed phenomena.

2. The Model

We propose an explanation of the observed pulse duration dependence on the basis of the absorption

of the laser pulse by the resulting electronic plasma. The absorption of the laser pulse and the subsequent heating of the plasma melts, spalls, crazes and otherwise photo-degrades the surface of the surrounding dielectric. Although the model proposed herein was developed for surface damage we will

subsequently attempt to show that both surface and bulk damage are adequately described by the sug- gested model. A crucial point to note is that it is the heating of the plasma which ultimately leads

to permanent or catastrophic damage. The assumption is made that the total heating of the plasma

must remain constant for a given material to damage as the pulse duration and spot size are varied.

*Based on portions of a Ph.D. Dissertation submitted to the Air Force Institute of Technology,

Wright-Patterson AFB, Ohio.

1. Figures in brackets indicate the literature references at the end of this paper. -338-
Laser-Plasma Heating. In a plasma (kT > leV) the absorption of the laser beam by the plasma

Is dominated by the "free-free" or Inverse Bremsstrahlung process [6]. Consider a plasma of thickness

dx with laser power density, Incident. To a first approximation, by Beer's law, the power density

at the exit of the plasma is '

S = exp (-Kdx) (2)

where K is the absorption coefficient. The absorbed energy is

= AS Adt = [S^ - exp (-Kdx)] Adt (3)

with A the area of the laser beam and dt a differential element of time. Expanding the exponential for small Kdx values yields for the heating rate,

dt dt "o ^ <^> - 4^ = S [1-(1-Kd^ ...)].at "

Neglecting higher order terms gives

g«AS^Kdx. (5)

A given material will damage when the total heat input is sufficient to cause localized melting. Thus,

the total heating is a constant, or

t 4? t A S Kdx = constant . (6) p dt p o ^ '

The absorption coefficient for inverse Bremsstrahlung as given by Johnston et al. [6] Is

_ 3.08 X 10"^ Z Ng^ In A (u) 1 (7)

0.2 (kT^)3/2 (1 - .p2/,2)l/2

where kT^ is in eV, A (w) > V^/(Wp

'^m^n^

• Here Zq^ Is the ionic charge, T^ is the electron temperature. Is the thermal velocity of the electrons, is the electron number density (we have assumed

singly ionized species such that N^Ng = Ng^), and Pmin is the minimum impact parameter for electronion collisions (Pjjj^^ maximum of Zq|/kT or (1i/mkT)l/2] , and is the plasma frequency.

David et al. [7] has shown that both Ng and Tg obeyed a power law dependence with laser flux. Fauqulgnon and Floux [8] calculated and experimentally verified that Tg s2/3, while Boland

et al. [9] showed that a laser-induced plasma expanding from the surface of a dielectric obeyed an adiabatlc expansion law. They experimentally verified that

Tg = const Ng2/3 . (g)

Then

Ne'

^e

Thus for a given material at a fixed laser frequency and constant spot size equations (6) and (9) can

be used to write

t = const (recall S ^ E^.h (10) p th

or

E^. = const/t (11) tn p Likewise, for a given material at a fixed laser frequency and constant pulse duration one obtains

2 A S = const (12)

-339-
or

E^j^ = const/ /d (13)

where d is the spot diameter.

Equations (11) and (13) indicate inverse relations between the threshold field and both the square! root of the beam diameter and the fourth root of the pulse duration. These dependencies will be com- pared to several existing observations.

Both equations (11) and (13) are expected to exhibit ranges of applicability. For pulse durations less than tunneling times ('^10" '^ s) equation (11) will probably be invalid. This cannot now

be checked since the shortest pulse duration data available are for pulses of the order 10~^^ s» For pulse durations greater than typical electron recombination times (a few microseconds) the added

loss mechanism of recombination will begin to affect the inverse pulse duration dependence. An additional loss mechanism will be thermal diffusion. The time needed for heat to diffuse out of a spot

with diameter d is

= d^/4D . (14)

2 Where D is the material diffusion coefficient which has values between 1 and. 10 cm / s. For pulse

durations much less than the thermal diffusion time one can neglect this loss which puts an applicability constraint on pulse duration and spot diameter d of

t /d^ < 1/4D (15)

For a 150ym diameter spot equation (15) implies that the pulse duration dependence should hold for pulses shorter than about 5.6 to 56ijs depending on the value of D.

For large diameter spots or for long pulse durations, the relative field calculated from

equations (11) and (13) will become too small to create a critical number density. Coupling this with

appropriate loss mechanisms implies a leveling off of the dependencies given by (11) and (13)

3. Comparison with Experiments

As an indication of the validity of equations (11) and (13) we present data collated from various

laser damage investigations. Figures 1 and 2 are results collected by DeShazer et al.,[10]. In figure 1 the intensity necessary to damage the surface of Ti02 crystals is plotted versus reciprocal

spot diameter. In figure 2 the relative threshold intensity for X/A ZnS films is plotted against

reciprocal spot diameter. The limited amount of data precludes one from choosing between the model

expounded here and the explanation given by DeShazer [10] based on a statistical concentration of surface defects. Figure 3 presents the results of Wang et al., [11] in which seven different dielectric reflectors were damaged as a function of spot size. The fit to the predicted 1/d dependence

is again no worse than the defect model relationship presented by Wang [11]. It should be noted here

that Newnam [12] has recently presented results which showed no spot size dependence for picosecond

pulsed laser- induced damage of thin films.

In 1970, Nesterov et al.,[13] showed that for the surface of sapphire the spot size dependence

followed E(.jj l/dO.67. With this correction the pulse length dependence can be separated. In figure 4 we plot threshold field versus pulse duration for the data collected by Nesterov et al. For pulses from 15 ns to 1.8 ys the relationship is E^-j^ t~0'251. we next consider the alkali halide

work of Fradin [5] and Smith [14]. Using the same laser system and focussing arrangements they

damaged several alkali halides while varying the pulse duration from 10.3 ns down to 15 ps. For

NaCl, five pulse durations; 10.3 hs, 4.7 ns, 300 ps, 30 ps, and 15 ps were used. This' data

is presented in figure 5 where a variation of E^j^ ,--0.25/ obtained with an experimental fit of + 8.5%. Four other alkali halides were damaged at 4.7 ns [5] and 30 ps [14] with similar results.

In figure 6 we compare the results presented in figure 5 with those obtained by Penzkofer [15], Bass [16], and Allen [17]. The spot size dependence given by equation (13) has been accounted for

in this graph. For pulse durations varying from 6 ps to 311 ns the relation is again remarkably

securp. The 311 ns duration data point was determined from the pulse shape as recorded by Allen

et al., by neglecting a long tail which has a much lower peak intensity than the initial portion of the

pulse. As can be seen from the figure, laboratory-to-laboratory variation in techniques and reporting

procedures may still make direct comparisons tenuous.

Finally in table I we present the data of figure 6 in tabular form along with selected other

results. From equations , (11) and (13) the expression /d t^' E^h should be constant for a given

material. The first three NaCl results obey this relationship quite well. Notice that the NaCl

results of Smith and Fradin are quite close although a factor of three lower in value than the other

-340-
NaCl results. The ZnSe films tested by Wang et al.,[18] and Bettis et al., [2] are also amenable

to this treatment. Finally the KCl results of Smith and Fradin agree with /d t-*-' E(.j^ = constant,

whereas the results of Bass and Allen do not agree. The suggestion has been made that this negative

result Is due to material variations in the KCl.

Table 1. Spot Size and Pulse Duration Variation of Thresholds

Source Material F"''"^!, t.-^''"^ E^h (MV/cm) /d t^''* E^, Length Diameter tn ^ ' th

Penzkofer [15] NaCl 6 ps 60 pm 28 7 0 344 1.06

Bass [16] NaCl 92 ns 107 1 83 0 33 10.6

Allen [17] NaCl 311 ns 73 1 63 0 33 10.6

NaCl 1 s ps 18 12 4 0 103 1 06 Smith [14] NaCl 30 ps 18 7 34 0 073 1.06

Fradin [5] NaCl 300 ps 18 4 7 0 083 1.06 Fradin [5] NaCl 4.7 ns 18 2 3 0 08 1.06

Fradin [5] NaCl 10.3 ns 18 2 1 0 09 1.06

Wang [18] X/4 ZnSe 311 ns 73 0 122 4 38 pc 10"^ 10.6

Bettis [2] X/2 ZnSe 40 ns 147 0 14 4 27 X 10"^ 1.06

Smith [lA] KCl 30 ps 18 5 86 0 058 1.06

Fradin [5] KCl 4.7 ns 18 1 31 0 046 1.06 Bass [16] Kci 92 ns 107 0 9 0 162 10.6

Allen [17] KCl 311 ns 73 2 05 0 413 10.6

5. Conclusions

J— -I 11. A wide variety of data correlates well with the expression /d t E^.^^ = constant. Further

comparisons are difficult due to the lack of complete data. Extension of these results as well as the refractive index and surface roughness results to CW operation is an intriguing possibility. The

characterization of these variables may make a search for a frequency dependence a reliable venture.

6. Acknowledgments

The authors would like to express their sincere appreciation to Dr. Alex Glass of Lawrence Livermore Laboratory for critical and constructive discussions.

7. References

[1] Milam, David, elsewhere in these proceedings.

[2] Bettis, J. R., R. A. House, A. H. Guenther,

and R. Austin, "The Importance of Refractive

Index, Number Density, and Surface Roughness

in the Laser-Induced Damage of Thin Films

and Bare Surfaces," Laser Induced Damage

in Optical Materials: 1975 (A. J. Glass and

A. H. Guenther, Editors), NBS Spec. Pub. 435,

p. 289 (1975).

[3] Fradin, D. W. , N. Bloembergen, and J. P. Letellier, "Dependence of Laser-Induced

Breakdown Field Strength on Pulse Duration,"

Appl. Phys. Lett. 22. PP- 635-7 (1973).

[4] Sparks, M. , "Current Status of ElectronAvalanche-Breakdown Theories," Laser Induced

Damage in Optical Materials: 1975 (A. J. Glass and A. H. Guenther, Editors), NBS Spec. Pub. 435, p. 331 (1975).

[6] Johnston, Tudor Wyatt, and John M. Dawson,

The Physics of Fluids , Vol. 16, No. 5, p. 722 (May 1973).

[7] David, C, P. V. Avizonis, H. Weichel,

C. Bruce, and K. D. Pyatt, "Density and

Temperature of a Laser Induced Plasma,"

IEEE J. of Quant. Elect., QE-2 , no. 9 (Sep. 1966).

[8] Fauquignon, C. and F. Floux, "Hydrodynamic

Behavior of Solid Deuterium Under Laser Heating," The Physics of Fluids , vol. 13, no. 2, p. 386 (Feb. 1970).

[9] Boland, B. C, F. E. Irons, and R. W. P. McWhirter, A Spectroscopic Study of the

Plasma Generated by a Laser From Polyethylene, U. K. Atomic Energy Authority,

Abingdon Berkshire (1968)

.

[5] Fradin, D. W. , Laser Induced Damage in Solids , Harvard University, Cambridge, Mass. (1973). -341-
[10] DeShazer, L. G., B. E. Newnam, and K. E. Leung, "The Role of Coating Defects in Laser-Induced Damage to Thin Films,"

Laser Induced Damage in Optical Materials ; 1973 (A. J. Glass and A. H. Guenther,

Editors), NBS Spec. Pub. 387, p. 114 (1973).

[11] Wang, v., C. R. Giuliano, and B. Garcia,

"Single and Multilongitudinal Mode Damage

in Multilayer Reflectors at 10.6 um as a Function of Spot Size and Pulse Duration,"

Laser Induced Damage in Optical Materials:

1975 (A. J. Glass and A. H. Guenther,

Editors), NBS Spec. Pub. 435, p. 216 (1975).

[12] Newnam, B. E., Elsewhere in these proceedings.

[13] Nesterov, L. A., A. A. Poplavskii, I. A. Fersman, and L. D. Khazov, "Variation of Destruction Threshold of a Transparent

Dielectric with Laser Pulse Length," Soviet

Phys.-Tech. Phys . , Vol 15, No. 3, p. 505

(Sep. 1973).

[15] Penzkofer, A., Optics Comm, Vol. 11,

p. 265 (1975).

[16] Bass, M. , and K, M. Leung, "The Dependence

of the Pulsed 10.6 \im Laser Damage

Threshold on the Manner in Which a Sample

is Irradiated," IEEE J, of Quant. Elect.,

p. 82 (Feb 1976).

[17] Allen, S. D. , M. Braunstein, C. Giuliano,

and V. Wang, "Pulsed CO2 Laser Damage

Studies of RAP Grown KCl," Laser Induced

Damage in Optical Materials: 1974 (A. J. Glass and A. H. Guenther, Editors), NBS

Spec. Pub. 414, p. 66 (1974).

[18] Wang, v., J. E. Rudisill, C. R. Giuliano,

M. Braunstein, and A. Braunstein, "Pulsed

CO2 Laser Damage in Windows, Reflectors, and Coatings," Laser Induced Damage in Optical

Materials: 1974 (A. J. Glass and A. H. Guenther, Editors), NBS Spec. Pub. 414,

p. 59 (1974).

[14] Smith, W. L. , J. H. Bechtel, and N. Bloembergen, "Picosecond Breakdown Studies:

Threshold and Nonlinear Refractive Index Measurements and Damage Morphology," Laser Induced Damage in Optical Materials: 1975

(A. J. Glass and A. H. Guenther, Editors),

NBS Spec. Pub. 435, p. 321 (1975).

8. Figures

Figure 1. Threshold damage intensity versus reciprocal spot

size for bare TiO?.

-342-
l/d {fim'')

Figure 2. Threshold damage intensity versus reciprocal spot

size for quarter-wave ZnS,

1200

-343-
I0-" lO-'O 10'' 10"' PULSE LENGTH (S)

Figure 5. Threshold field versus pulse duration for NaCl.

10'' 10'' PULSE DURATION (si

Figure 6. Threshold field versus pulse duration for NaCl. -344-
COmEMTS ON PAPER BY 8ETTTS, HOUSE, ANV GUENTHER

It Li di^^Zcult to dOKfizJiaute. xeAult^ obtcu.n&d by (Li^zfLunt txp&AJjn<int^>u> on di^fieAznt -iampteyi Ion (U^^eAent pui^e l&ngtki on i>pot ilzu, i^nco. thz mcut&AMxL!, may exhibit fuidicatly (UlizKdnt damagz

thAUhotdi and may even be damagAjig du& to dA.{i{^2M.znt caiuzA {^^om one. iompZo, to anotkeA, It mu6 po-inttd oat that hdcut di^^va^on, vuh^ch wa4 not txpttcltly txexiutad ^in tki!> anajLy6-u>, may be. -unpofLtant at longeA.

pu£&& dujuitiom.

NOTE ADDED IN PROOF

Since the preparation of this manuscript covering material presented at the Symposium, it has come

to our attention through a more complete re-searching of the published literature that several other

individuals have also Investigated pulse length and spot size scaling of laser Induced damage thresholds. The reported observations being accomplished primarily in the Soviet Union. It is Indeed gratifying to note that there is excellent agreement with the proposed scaling relationships presented in our

paper proper. Unfortunately, no detailed theoretical derivations of the scaling relationships was

proposed in the earlier Soviet reports.

Verification of temporal scaling for damage thresholds in W/cm^ (Q^ ) as follows,

1

= (ti/t2)^ (A)

t2 tl

for pulses t^^ between 9 x 10"^^ s and 10~^s is reported for the surfaces of several optical glasses [a]*. In addition, scaling of damage thresholds in W/cm has been reported to follow a

Q ^ ^ (B)

relationships for diameters (d) between 9 ym and 140 pm in bulk breakdown experiments [b] as well as between 190 ym and 10,000 ym at the surfaces of dielectrics.

If these relationships hold up under the test of time and repeated investigations, they will form a useful mechanism in which to correlate various reported damage measurements. They, together with

already proposed scaling expressions for refractive index and surface roughness, afford the laser designer a powerful tool for selecting candidate optical materials. What is still lacking, however, is an appropriate scaling relation for the frequency dependence of laser induced damage. A simple

frequency dependent relationship may be difficult to achieve due to non-linear behavior at high photonfluxes, as well as the presence of resonances and other frequency specific properties of individual

materials.

References

[a] Buzhinskii, I. M, and Pozdnyakov, A. E. , [c] Buzhlnskii, I. M. , Pozdnyakov, A. Ye., and

Sov. J. Quant. Electron., _5, No. 7, (1975) Ushakov, S. A., Sov. J. Opt. Tech., 40, (1973) 835-836. 335-336.

[b] Nesterov, L. A., Poplavskli, A. A.,

Fersman, I. A., and Khazov, L. D., Sov. Phys.

Tech. Phys., 15, No. 3, (1970) 505-507.

*Letters in brackets indicate the literature references at the end of this paper. -345-
5.2 RE-EXAMINATION OF LASER INDUCED BREAKDOWN IN THE ALKALI HALIDES AT 10. 6^m * !

C.C. Tang, K. M. Leung.and M. Bass

Center for Laser Studies, University of Southern California

Los Angeles, California 90007

Since the 10. 6 )jm bulk laser damage thresholds of alkali halides have been

shown to depend on the experimental procedures used, we have remeasured the damage flux for these materials. Multiple shot per site thresholds were found to be higher than single shot per site thresholds for samples containing a large density of defects. By irradiating with two different spot sizes and observing an increase in damage threshold with decreasing spot size, a correlation between

damage thresholds and the distribution of defects was determined. The single

shot per site bulk breakdown electric field for NaCl using a 59 |jm spot diameter was 1.8 MV/cm. Measurements made on conventional and RAP grown samples

showed that both types of material preparation techniques are not yet able to produce samples with reproducible damage thresholds. Some, but not all, RAP

grown samples are harder to damage than conventionally prepared materials.

Key words: Alkali halide crystals; laser induced damage; RAP grown materials.

1. Introduction

The laser induced bulk damage thresholds of IP. materials have been shown to depend upon the manner in which the samples are irradiated [l]^. In reference [l], it is reported that sites in a sample which have been pretreated by low laser intensities will have higher damage thresholds than unconditioned sites. Other investigators [2] have also observed this effect. Since the damage thresholds of alkali halides have been used to identify avalanche breakdown as the intrinsic damage mechanism [3], the results of references [l] and [2] indicate the need for remeasuring the damage properties of these materials. This requires re-examining the procedure of irradiation used by various

researchers in measuring breakdown thresholds. In this paper, we will discuss the variations in damage thresholds between different samples of the same material. We present data which show

that the dependence of the damage threshold on the method of irradiation is related to the presence of defects and inclusions in the medium. To determine the exact role of defects in laser induced breakdown of materials, a study was also made of 1) the relationship between the damage threshold and the size of the irradiated focal volume, and 2) the damage thresholds of recent conventional and RAP

grown samples as compared to previous samples of the same materials.

2. Experimental

The experimental set up used has been described in detail earlier [4]. High speed photon drag

detectors were used for comparing the Incident laser pulse waveform with that transmitted through

the sample. The occurrence of damage was visually determined during the experiment by the occurrence of a spark or by the appearance of residual damage following Irradiation. An optical microscope was used later to examine the morphology of each Irradiated site. Physical damage was

accompanied by an attenuation of the transmitted pulse. The damage threshold was defined as the Incident flux at which 50% of the Irradiated sites damaged and Its value was determined from the average of the results for 10 sites. The Important parameters of the TEA CO^ laser system are summarized in table 1. The laser was operated in the fundamental TEM^^ transverse mode in order to provide a well characterized

beam spatial profile. This was focused into the bulk of the samples with lenses resulting in focal

spot diameters of 59 Mm or 89 M.m. In our experiments, the diameter is defined as the diameter at

2 which the intensity falls to 1 /e of its peak value. Consistent with the notation used In the literature.

* Research sponsored by Defense Advanced Research Projects Agency, Contract No. F19628-76-

C-0162, monitored by Air Force Cambridge Research Labs.

1. Figures in brackets indicate the literature references at the end of this paper,

-3A6-
Table 1. Summary of experimental parameters

Laser CO^ TEA

Wavelength 10. 6 (jm

Waveform 92 ns gain switched pulse with self mode locked spikes < 2. 5 ns duration

cLIlU O 11 o LXlLt:;! ^Lllo C o j^a. ^ Lllg Transverse mode

00

Attenuation ZnSe beam splitter and two Brewster angle stacked plate polarizers

Focusing lens Ge, 2. 54 cm and 3. 8 cm focal lengths, abberation free

Focal spot diameter 2 59 (jm and 89 \Jin at 1/e in intensity

Detection system Photon drag detectors and Tektronix 7844 oscilloscope. Risetime = 2. 5 ns

this definition corresponds to the diameter at which the electric field falls into 1/e of its peak value.

Since damage only occurs in the first half of the gain-switched CO^ laser pulse as evidenced by observation of the transmitted waveform [4], the long tail of the pulse, containing 50% of the total energy, ^ 2 does not contribute to the breakdown mechanism of the material. The average energy flux £(J/cm ) is given by

a

= nwp

where £ is the total energy contained in the entire pulse. The average power density P(W/cm ) and

2 the peak on-axis power density P^CW/cm ) are then related by

Pq = 2P = 2 S /r

where T is the full width at half maximum of the gain-switched part of the TEA laser pulse.

3. Comparison of N on 1 and 1 on 1 Damage Thresholds

An N on 1 measurement of the laser damage threshold has been defined as one in which a site is initially irradiated at a low intensity and the flux is increased by a small amount (5% in these experiments) on successive pulses until damage occurs [l]. This was the procedure used by Allen et al. [2] in reporting damage thresholds for RAP grown alkali halides. A more conventional threshold

measurement is obtained in a 1 on 1 experiment where each site is irradiated only once and the intensity which produces damage 50% of the time is sought [1,3,4].

Our data for the N on 1 and 1 on 1 thresholds of several materials is summarized in table 2. Table 2, Comparison of N on 1 and 1 on 1 laser damage thresholds

Material 10.6 |Jm Damage Threshold in GW/cm Spot Size 2W^ = 89fim

1 on 1 N on 1 Wshold

Wshold <^ NaCl

(conventional) 9.83 9.89 1. 00

NaCl

(RAP) <. 38 < . 38

KCl

(conventional) .87 1.13 1.3

KCl

(RAP) < . 38 < . 38

KBr

(conventional) < . 38 < . 38

KBr

(RAP) 7. 23 15. 35 2. 12 NaF

(conventional)

16.49 17. 07 1.04

KI

(conventional)

1. 07 1. 97 1.84

We note that the differences are not due to the difference in the definitions of the N on 1 and 1 on 1 -347-
thresholds because by increasing the incident flux by only 10% in a 1 on 1 experiment, we could produce damage on all sites. Thus, a difference greater than 10% is indicative of a conditioning effect due to the low intensity pulses in an N on 1 test. Table 2 shows that the N on 1 experiment gives noticeably greater thresholds in the RAP processed KBr and the conventionally grown KI. The difference is less in KCl and in NaCl and NaF the two thresholds are the same. A few of the samples had very low thresholds due to high concentrations

of inclusions and so in table 2 their damage fluxes are given as upper bounds. The presence of inclusions or defects in certain samples was detected by the observation that the positions of the damage

sites varied randomly along the incident beam direction. In particular, this was seen in the RAP

processed NaCl and KCl samples used in the present study. We note that as a result these samples

had lower damage thresholds than conventionally prepared samples of NaCl and KCl. The reverse

is true for the samples of KBr studied in this work.

4. Comparison of Materials

Upon comparing the thresholds of these materials with other samples investigated by Bass and

Leung [l], a marked increase in damage thresholds for the conventionally grown NaCl and KCl sam2 pies was noted. The thresholds which they obtained for conventional NaCl and KCl were 6.10 GW/ cm

2 2 2 and .65 GW/cm , respectively,as compared to 9.83 GW/cm and 0.87 GW/cm obtained herein. The

2 previous RAP grown KCl had a threshold of 4.60 GW/cm which is significantly higher than that of the sample studied in this work. This indicates that RAP grown materials are not necessarily always

better than conventional materials. Clearly, they could be more damage resistant than conventional

materials if they are inclusion and defect free. A comparison of the breakdown electric fields of the recent samples with those obtained by

Yablonovitch [3] at 10. 6 |-im is shown in table 3. The electric fields are normalized to that of NaCl.

Table 3. Relat ive breakdown fields at 10.6 Lim ^

- normalized to E,, „, NaCl

Material Current Results Previously Published Results

NaCl 1 1 KCl . 573 .713

KBr . 309 .482

NaF 1. 33 1. 64 (@1. 06 Mm) [5] KI . 440 .27

In the present work and for our particular sample of NaCl, the breakdown electric field was found to be (1.8 i 0. 18) MV/cm, Our experimental conditions were nearly identical to those of Yablonovitch

who obtained (1. 95 i . 2) MV/cm for the breakdown field of NaCl at 10.6 |jm [3], We can see that the two results agree within the experimental uncertainty of the measurements. Table 3 shows that there is general agreement in the breakdown field ratios for KCl and NaF. For the other samples,

however, the ratios differ widely. Since the agreement of the results for NaCl indicates adequate

experimental accuracy, we conclude that the discrepancies in the electric field ratios for the same

materials are due to sample variations. This suggests that the damage threshold for any material

should be obtained from the average of the value obtained for several samples.

5. Dependence of Damage Thresholds on Spot Size

DeShazer, Newnam,and Leung [6] have previously reported that the damage thresholds of thin

optical coatings correlate strongly with the size and distribution of defects. Since the probability of

hitting a randomly distributed defect is greater when a large cross -sectional area is irradiated than

when a smaller area is irradiated, and since materials containing defects damage more easily than

defect free materials, it is to be expected that large diameter beams, on the average, damage

materials more easily than small diameter beams. The present work extends the results of [6] to bulk damage in transparent materials.

In order to investigate the role of defects in solids, the power density thresholds for damage to the materials shown in table 4 were determined using two lenses (their focal lengths were 25 and

38,0mm) yielding focal spot diameters of 59 |jm and 89 (Jm. The thresholds for NaCl, NaF.and RAP

KBr did not depend noticeably on the focal spot diameter. For other samples, such as RAP NaCl

and RAP KCl, the damage thresholds using the different lenses differed by an order of magnitude.

Due to the smallness of the RAP KBr sample, only preliminary data consisting of a single site could

be obtained using a 59 Mm spot size. The presence of defects in RAP KCl and NaCl was verified by

-3A8-
Table 4, Comparison of damage thresholds for different spot sizes

Material 2 10.6 |jm Damage Threshold in GW/cm

2W„ = 59 |Jm

0 2Wp = 89 lam

NaCl 13. 21 9.83

NaCl - RAP 3. 30 < . 38

KCl 2. 92 0. 87

KCl - RAP 3. 21 <.38

KBr 0. 88 < . 38

KBr - RAP 7. 83* 7. 23

NaF 19. 02 16. 50 KI 1. 91 1. 07

* preliminary data

morphological studies. The samples showing no dependence on focal spot diameter were either

densely filled with small inclusions or relatively inclusion free and damaging due to an intrinsic

process.

6. Conclusion

Our results indicate that the damage threshold depends on the manner of irradiation as shown by

the N on 1 and 1 on 1 experiments. This effect appears to be caused by the presence of inclusions or

defects in the materials. When inclusions or defects are present the damage threshold is dependent

upon the particular sample studied and the volume of the site irradiated. A thorough study of a tnaterial must necessarily include measurements of several samples. Various focal spot sizes

should be used with the intrinsic breakdown limit being approached for very small spot sizes.

7. Acknowledgments

The authors acknowledge the assistance of G.J. Benedict in this work as well as the Hughes

Research Lab and the Naval Research Lab for supplying the RAP grown samples. Valuable conversations were held with M.J. Soileau who also arranged for the finishing of the RAP grown NaCl at the Naval Weapons Center.

8. References

[1] Bass, M. , and Leung, K. M. , IEEE J. of [4] Quantum Electron, QE-I2, 82 (1976),

[2] Allen, S. D. , Braunstein, M. , Giuliano, C. , and Wangj V. , NBS Special Publication No, [5] 414, 66 (1974),

[3] Yablonovitch, E, , Appl, Phys, Lett, 19., [6] 495 (1971).

Leung, K. M. , Bass, M. , and BalbinVillaverde, A.G.J., NBS Special Publication No. 435, 107 (1975).

Fradin, D. W. , Yablonovitch, E. , and Bass,

M. , Appl. Optics 1_2, 700 (1973).

DeShazer, L.G., Newnam, B.E. , and

Leung, K.M. , NBS Special Publication No.

387, 114 (1973).

COMME^frS ON PAPER BV TANG, LEUNG, ANV BASS

It Mou> pointed oat that In ondzn. to veAt^y thz pn-zdiction^ of, the. itattitiaat modeZ, It -li n&ceAioAy to be abtz to detect thxuhold damage., i.e.., damage oicuAJving at a -dtngle defect. In actual,

fact, a duMtntbution of defecti, mth voAylng thfie^holdA, , wWL occua. One muAt fold thti dlitntbution

and the LthminatLon d2j>tntbutlon Into the anatyiti.

The qaeAtion uia6 fwui&ed a& to how foK beZoiM the noHmaZ damage thxeAhold one muAt loweA the tttum- Ination level tn ofideA to achieve a .specif-Led tlfe-time of a component. No convenient luZe of thumb

appejivUi to be available. It woA emphasized that queyStiom of thu, iont oAe paAamount fan. the iystem

deMignen. and that this kind of infofmation -c6 uAgently needed by the opticA community.

It Moi funthen. emphasized that when fieAuZti of diffeAent inveAtigations ax.e neponted on. compared,

the spot size, puZ&e dunation, peak intensity, focusing geometAy, and St/ieht natio achieved in the

expeJtiment afie all fiequiAed to chafiactexize the peak fieZd.

-349-
5.3 CAN A MODEL WHICH DESCRIBES GAS BREAKDOWN

ALSO DESCRIBE LASER DAMAGE TO THE BULK AND SURFACES OF SOLID DIELECTRICS

David Milam

Lawrence Livermore Laboratory

Livermore, California 94550

By assuming that laser-induced breakdown in highly transparent solids

is a three-step process consisting of preionization, ionization growth, and

absorption, it is possible to explain "explosive" threshold damage, typical

surface damage morphology, the presence or absence of damage statistics,

and the distribution of breakdown times. Special constraints on the pre- ionization stage necessary to explain these phenomena are discussed.

Key words: Absorption; avalanche ionization; damage morphology; damage

statistics; distribution of damage times; photo- ionization.

1. Introduction

Laser-induced gas breakdown experiments were recently performed [l^as a test of the luckyelectron hypothesis [2]. While these experiments were adequate to demonstrate the problems associated

with that hypothesis, the data could not be readily explained by existing gas-breakdown theory. The intent of this paper is to suggest an alternate interpretation for those particular results, and to show that the same interpretation suffices to explain many results obtained in other laser damage

experiments

The proposed damage mechanism is valid only for highly transparent materials, and appears to be most functional as an explanation for results obtained at 1.06 \im and 0.69 pm. It is more

difficult, but possible, to apply the mechanism to results obtainea at 10.6 ym.

It is emphasized that the identification of highly transparent media can be done experimentally

by using square-waveform pulses to induce damage. If a spatially homogeneoiis absorption is responsible

for the damage, a well defined time of damage will be associated with a given intensity. The variation

with intensity of the time required to induce damage indicates whether the absorption is linear or nonlinear. Induced effects such as free-carrier absorption can be identified by recording the waveform of the pulse transmitted through the sample. It is necessary in all cases, particularly in studies of dielectric films, to properly account for thermal diffusion [3] and effects due to standing- wave electric fields [4].

2. Review and Interpretation of Recent Gas Breakdown Data

Laser-induced gas breakdown is widely believed [5] to be highly analogus to microwave breakdown,

consisting of three steps - preionization, ionization growth, and absorption. The first two steps are most important in determining whether or not breakdown will occur in a given set of circumstances.

The second step, ionization growth, is believed to occur by impact ionization, and can only proceed

if initial starting charge is supplied. The number of initial electrons will grow exponentially at a rate which depends linearly on the intensity and the ionization potential, and inversely on pressure

and pulse duration [6].

Preionization can be externally supplied, or induced by heating particulates or by photoionization. Several experiments [7-10] have indicated that thermal mechanisms are the most important

sources of seed electrons in 10.6 pm induced breakdown. Sources at 0.69 ym or 1.06 \im are not as well understood, but direct photoionization is' the most probable source.

Several primary features of laser-induced gas breakdown are illustrated in figure 1, which gives

the threshold for breakdown as a function of pressure. The data of Gill and Dougal [11] have been extended to lower pressures as indicated by the dashed portion of the curve. The low pressure regime

has been reported by Krasyuk et. al . [12]. For moderate pressures, the breakdown threshold increases

as the pressure decreases. At a lower pressure, which depends on pulse duration, direct photo-ionization becomes dominant over cascade ionization, and the threshold ceases to rise as the pressure is further reduced. This is because the photoionization rate is only weakly dependent on pressure [6]. The ionization rate is, however, highly nonlinear in intensity.

Consider now the application of these concepts to explain the experiments [1] used to test the .lucky-electron theory [2]. These experiments consisted of measuring the distribution of breakdown

times for Argon volumes irradiated by square-waveform ruby laser pulses'. Data at two pressures, 20 psi and 500 psi, are shown in figure 2, where the fraction of volumes N(t)/No which have not yet suffered

breakdown is record ed as a function of time. At 20 psi, breakdown was deterministic, always occurring

1. Figures in brackets indicate the literature references at the end of this paper. -350-
at the same time for a given input intensity, to within experimental error. At 500 psi , breakdown

times were distributed throughout the laser pulse. The transition from deterministic to statistical

behavior was readily mapped by studying intermediate pressures [1].

The statistical characteristics observed at the higher pressures could not be readily described

by current gas breakdown theory, which assumes the existence of well defined rates for all processes.

It was argued [1], however, that this behavior could be predicted if one allowed statistics in the start-up of the avalanche growth. At low pressures, ample photoionization was induced by the large

fields necessary to cause ionization growth, and starting statistics were masked. At higher pressures,

breakdown fields were lower, and photoionizations rare, so that statistics were observed.

Such an explanation does not depart significantly from the current understanding of laser damage.

It keeps intact all debates concerning the number and origin of starting electrons, and the mechanisms

by which statistics could be generated in the cascade process itself.

It will be demonstrated below that it is advantageous to assume that a major portion of the statistical nature of breakdown can be attributed to the preionization stage. Since even the smallest

focal volumes studied contained > 10^^ atoms, photoionization of the basic gas itself would have

occurred at a well defined average rate even when that rate was small. Statistics are possible in photoionization only when a small number of atoms are irradiated. It will be asserted that the pre- ionization at high pressures was due to photoionization of impurity ions present at a sufficiently

low density that only a few such ions were in the focal volume. Note that the photoionization rate

is greater when atoms are colliding than for individual atoms; such colliding atoms might serve as impurities.

3. Basic Damage Hypothesis

By analogy to the gas breakdown studies, the following hypothesis is suggested. Damage in solids

for which absorption is not a problem is a sequence of the following three steps:

Preionization - This occurs by photoionization of isolated impurity ions for bulk damage, for surface damage induced by pulses < 200 ps in duration, and for most experiments with X < 0.69 ym. Heating of particulates to obtain starting electrons will be important for surface damage by long

pulses > 20 ns in duration, or in damage induced by wavelengths > 3-5 ym. The probalJility for direct

photoionization is assumed to depend nonlinearly on intensity.

Ionization growth - By processes such as cascade ionization or vertical transitions which are described by probabilities which are linear in intensity.

Absorption - Which occurs when the electron population exceeds a critical density of about

10^^-10^" cm"". Linear absorption will always be present to some extent, and may become dominant for breakdown induced by very long duration pulses or by wavelengths near absorption maxima in the material.

4. Data Which Supports the Hypothesis

The Occurrence of Explosive Threshold Damage

When special techniques were used to prepare surfaces, it was frequently noted that the damage

threshold on some sites increased greatly, and that damage when induced was explosive in nature. It was impossible to induce an arbitrarily small damage. This was observed in ion-beam polished sapphire

[13] super-polished surfaces [14], leached ED-2 glass surfaces [15], ectched KCl [16], and in flamepolished surfaces [17,18].

It is important to note that such damage cannot be explained by any deterministic mechanism

such as thermal absorption. Furthermore, the preionization/ionization growth/absorption sequence

cannot explain explosive threshold damage if both growth stages are well represented by rates. In all these cases, the time required to induce damage is precisely calculable for a given laser pulse, and damage can be induced at an arbitrary chosen time, early or late, by adjusting the pulse intensity.

All reported explosive threshold damage occurred for pulses > 5 ns in duration, induced at 10^ - 10^° W/cm^. Explosive damage has not been reported for damage by pulses < 200 ps in duration,

which occurs at 10^^ - 10^^ W/cm^. Instead, it has frequently been noted that short-pulse damage

is very deterministic in nature. Abritrarily small damage could be induced in dielectric films [19-21],

in the bulk of dielectrics [22] and on dielectric surfaces which had been polished [20], leached [20], or flame-polished [20]. It is important to note that such determinism in short pulse damage at 1.06 ym

or 0.69 ym is not due to linear absorption. Absorbing filter glass, with absorption coefficients of

100-500 cm"' have about the same damage threshold at 1.06 ym 100 ps as the best BK-7 glass. Only metal surfaces have been observed to undergo shot-pulse damage due to linear absorption.

-351-
The presence or absence of explosive threshold damage in solids appears to be closely analogous to the presence or absence of statistics in gas breakdown. When in gas breakdown the pressure is reduced,

the breakdown field increases, photoionization becomes plentiful, and statistics disappear. In solids, reducing pulse duration increases breakdown fields and explosive thresholds disappear. The analogy suggests that explosive threshold damage occurred because a required trigger, an initial

electron, was missing. Cleaning the surfaces by special techniques had removed the impurities and inclusions which were most readily ionized. The field required to ionize the remaining material

exceeded the field required to produce ionization growth, and the growth was explosive once starting

charge was obtained. Explosive threshold damage is not observed with short pulses because many atoms are subject to ionization by fields of 10^^ W/cm^, and starting charge is always provided.

It is important to note that the ionization growth stage appears to be dominant for short-pulse

damage, and that the rate of growth depends lineary on intensity. Defects which only serve to furnish starting electrons will have almost no effect on short-pulse damage. Enhancement of the field

at mechanical flaws will be important only as an aid to ionization growth. By contrast, damage

induced by pulses > 5 ns in duration can be dominated by either preionization or ionization growth.

Due to possible nonl inearities in the preionization, field enhancement, surface preparation, and impurity concentrations may be very important in setting long-pulse thresholds.

Consider now the application of the hypothesis as an explanation of a variety of experimental

results.

5. Applications of the Hypothesis

5.1 Explanation of Surface Damage Morphology

5.1.1 Density of Damage Micropits vs. Pulse Duration

Threshold damage at 0.69 pm in thin films of refractory oxides produced randomly distributed

micropits at three pulse durations [23]. The results are summarized in table 1.

Table 1. Density of Damage Micropits vs. Laser Pulse Duration

Pulse duration Cps) Pit density (cm"^) Avg. Pit diameter (ym) Avg. Damage Threshold (W/cm^)

20 10' 0.1-0.3 10^ 1400 10= « 1.0 10^° 20000 10"* 3-5 10'^

The data is in reasonable agreement with calculations [24] which predicted that asorbing inclusions of one preferred size would be most readily heated by pulses of a given duration. This explanation was adopted [23], but inclusions in the sizes and spatial densities required to satisfy the hypothesis have not been located by any passive technique. An alternate explanation is suggested by the hypothesis of section 3. The pits are all due to micro-avalanches which do not connect into one common failure of sites irradiated at the threshold level. The pit size increases as the pulse duration increases due to longer "burn" times after pit formation at the temporal peak of the pulse, when

seed ionization is produced. The spatial density of pits increases as the pulse duration decreases

since more entities are available as sources of photo-induced starting charge in the larger breakdown

fields associated with reduced pulse durations. An interesting test could be conducted by terminating

long pulses [> 20 ns) at various times after the temporal peak to determine whether a relationship

between "burn" time and pit size could be observed.

5.1.2 Relationship Between Surface Artifacts and Laser Damage

Flaws such as mounds or pits which are observable by microscopy on dielectric films have been identified as the cause of laser damage by Bradbury [25], who used the simple procedure of taking

microphotographs before and after damage to ascertain tfiat artifacts were weaker than the surrounding

film. His experiments were done at 1.06 ym using pulses > 50 ns in duration with highly modulated

waveforms. The same result has been inferred by deShazer et. al . [26] from measurements of the damage

threshold as a function of irradiated spot size.

There is a fundamental problem with this assertion - the density of pits produced by laser

damage depends on pulse duration [23] but the density of observable artifacts is clearly a constant

for any given sample. There are additional problems with the technique described by deShazer et.al.,

which claims that measurements of the damage threshold as a function of focal spot size will yield

(1) a damage threshold for the defects, (2) the spatial density of defects, and (3) the intrinsic

damage threshold of the defect-free material. Only the defect damage threshold is gotten with any

certainty. This comes from threshold measurements on large areas whtch always contain defects. The defect density and intrinsic threshold are to be obtained by fitting the data to a theoretical experssion for the spot size dependence of threshold. The derivation of that expression is itself in error; -352-
it is for cylindrical beams, not beams of Gaussian spatial profile as asserted. Even when the corrected equation [27] is used there remains a fundamental ambiguity. As smaller beams are used,

the probability of avoiding the irradiation of defects increases, and larger thresholds are measured

on some sites. The maximum threshold observable on very small sites is possibly the intrinsic damage

threshold. It could as well be the damage threshold for a second class of defects.

The model for damage given in section 3 suggests that artifacts may be important in setting

damage thresholds for long pulses, in agreement with the observations of Bradbury [25]. Field enhancement at such artifacts will greatly aid preionization. The model also suggests that artifacts will

be much less important for short-pulse damage, for which the linear electron growth process is dominant.

5.1.3 Cone-Shaped Damage to Polished Surfaces

Threshold damage on ED-2 glass surfaces polished with abrasives containing iron produced per- fectly smooth, round microcraters [15]. These were explained [20] as a result of thermal expansion of

absorbing particulates buried under the glass surface by cold-flow during polishing. It is equally

plausible to assume that the craters are caused by microavalanches seeded by charge obtained from

ionization or heating of the iron compounds, a process which would tend to produce identical craters

by plasma absorption during the last portion of the pulse. Identical craters produced by thermal expansion of particulates requires identical particulates, which seems less likely.

5.1.4 Isolated Damage Micropits in the Bulk

Smith et. al . [22] have observed that threshold bulk damage induced by 30 ps 1.064 urn pulses

results in isolated damages which they attribute to microavalanches formed about available electrons.

This assertion was a valuable input to formulation of the hypothesis being advanced here, and is in keeping with the predictions of the hypothesis. The density of the pits, up to 10^ cm"^, results

from the fact that many entities can be ionized by the large breakdown fields, and the sharpness of threshold follows from the assured supply of starting charge.

5.2 Damage Statistics

Statistics in laser damage have been observed in many experiments since they were first systematically studied by Bass and Barrett [2]. Statistics have been observed for breakdown on polished sur- faces [2,29,30], in the bulk [29,31,32], in dielectric films [26,29], and in gases [1]. In all these

instances, pulse durations exceeded 5 ns.

Statistics have not been reported for damage induced by pulses < 200 ps in duration.

These statistics follow from the hypothesis in section 3 as a close parallel to the occurrence

of explosive threshold damage. For long pulses, at low field strengths, starting charge may not

be reliably produced and statistics can be observed. For short pulses, in high fields, starting

charge is always adequate.

It is probable that some statistical aspects of damage are due to the presence or absence of

particulates. Thermal damage to inclusions could cause either temporal or spatial statistics [29].

There are only a few instances in which such particulates have been identified, and even when they

are known to be present it is difficult to determine that they produce damage by some direct thermal process. The damage process near absorbing particulates should depend on the pulse duration. For

long pulses, adquate time is available to allow thermal expansion or melting. With short pulses,

electrons from the heated particle may lead to avalanche breakdown before simple thermal expansion

is effective in destroying the sample. Some indirect support for this hypothesis is obtained by the behavior of the pole pieces in laser- triggered spark gaps. In gaps triggered by focusing 50 kW 20 ns

ruby laser pulses onto one pole piece, the starting electrons are generated thermally. The pole

piece erodes rapidly even on poles fitted with tungsten target areas [18]. The erosion creates

thermal craters which grow progressively deeper with repeated firings. Gaos triggered

by 100 ps 1.064 ym pulses require much less input laser energy and do not show the thermal

cratering [18]. This suggests that nonlinear photoemission from metals may be more important in the very short pulse regime than thermal absorption and expansion. Care should be exercized in assuming

that platinum or iron particulates will be a major cause of short pulse damage, even though they

are known to be serious problems in long pulse damage [15,23].

5.3 Distribution of Breakdown Times

For nonabsorbing media, the experimental observations of the distributions of breakdown times

are well summarized by the following:

With 5-20 ns Gaussian waveform pulses, threshold bulk breakdown is restricted to times near the temporal peak of the pulse [31]. Threshold breakdown in the bulk could be induced at any time during

square-waveform ruby laser pulses [1,29]. In contrast, laser-induced gas breakdown could be induced

-353-
at any time during either Gaussian or square-waveform pulses [1,18]. In the last two observations,

statistics were frequently seen, so that only a mean breakdown time could be associated with each

input intensity. Nevertheless, the observations demonstrate that the strict exclusion of late breakdowns holds only for bulk breakdown of solids induced by ~ Gaussian pulses.

Attempts have been made to calculate breakdown times by means of rate equations [34], as is done in gas breakdown. As expected, the calculation involving time-constant rates for electron

production resulted in the prediction that a critical electron population could be generated at a specific time by a laser pulse of specific waveform and intensity. Breakdowns could be made to occur arbitrarily late in the pulse by adjusting pulse intensity. The authors complaint [34] that such late breakdowns have not been observed because of poor experimental control is unconvincing. Thousands of such breakdowns have been recorded. If late breakdowns were easily generated, they would

have been observed by simple chance, particularly if experimental fluctuations were a problem.

The hypothesis advanced in section 3 suggests that the systematics in breakdown times are due to the dominance of preionization in the described experiments. With Gaussian waveform pulses of 5-20 ns, at threshold, preionization is restricted to the intense temporal peak of the pulse. The liberated

electrons must be used immeidately or they are lost as seed charges by rapid recombination. Seed charge, and breakdown, can be produced at any time during a square-waveform pulse. Seed charge is produced most copiously at the peak of the pulse in gas breakdown, but the seed charges survive

and remain available for production of late breakdowns.

6. Summary

A hypothesis for the breakdown in solids has been constructed by analogy to gas breakdown. The hypothesis does not differ markedly from existing schemes [2, 35], which generally assume that avalanche

ionization is the failure mechanism in transparent materials. Rather, it is suggested that more

attention must be given to the method by which starting charge is obtained.

The hypothesis was used to explain a wide variety of existing experimental data, and to suggest

several experimental tests. Hopefully, a number of these tests can be conducted in the near future

to refine, or if necessary, repudiate the hypothesis in an expedient manner.

7. Acknowledgments

The author gratefully acknowledges extensive and helpful conversations with M. J, Weber and

A. J. Glass of Lawrence Livermore Laboratory, R. H. Picard and R. A. Bradbury of Rome Air Development

Center [Hanscom Field, Mass.), N. Boling of Owens-Illinois Corporation, and M. Bass of the Center

for Laser Studies, University of Southern California. He is further grateful for the continuing

technical assistance of G. E. Murphy, and for the assistance of Patricia Page and Audrey Heine in preparation of the manuscript.

8. References

[1] Milam, D., Bradbury, R.A. , and Picard, R.H., Laser- Induced Damage in Optical Materials : 1975 , N.B.S. Spec. Pub. 435, pp.347, (1976).

[2] Bass, M., and Barrett, H.H., IEEE J. Quant.

Electron., QE-8 , 338 (1972).

[3] Picard, R.H., Milam, D., Bradbury, R.A.,

and Fan, J.C.C., Laser-Induced Damage in Optical Materials : 1975 , N.B.S. Spec. Pub. 435, pp.272, (1976).

[4] Newnam, B.E., Gill, D. H., and Faulkner, G., Laser-Induced Damage in Optical Materials : 1975 . pp. 254, (1976).

[5] Smith, D.C., Overview of Laser Radiation

Induced Gas Breakdown, presented at First

DOD Conference on High Energy Laser Technology, San Diego, CA., Oct. 3, 1974.

[6] Ready, J.F., Effects of High Power Laser Radiation , Chapter 5, Academic Press, New York, (1971).

[7] Canavan, G.H., and Nielson, P.E., Appl . Phys.

Lett. 22, 409 (1973).

[8] Lencioni, D.E., Appl. Phys. Lett., 23, 12 (1973).

[9] Brown, R.T., and Smith, D.C., Appl. Phys.

Lett. , 22, 245 (1973).

[10] Smith, D.C., Berger, P. J., Brown, R.T., and Fowler, M.C., Investigation of Gas Brea kdown with 10.6 Micron Wavelength

Radiation , United Aircraft Research Laboratory

Report L921 104-6, (1972), Unpublished.

[11] Gill, D.H., and Dougal , A. A., Phys. Rev. Lett.

1^, 845 (1965).

[12] Krasyuk, I.K,, Pashinin, P.P., and Prokhorov,

A.M., Soviet Physics J.E.T.P. 31_, 860 (1970).

[13] Giuliano, C.R., Laser-Induced Damage in Optical Materials : 1972, N.B.S. Spec. Pub. 372, pp. 55, (1972).

-354-
[14] Fradin, D.W., and Bass, M. , Appl , Phys. Lett.,

22, 157 (1973).

[15] Boling, N.L., Ringlien, J. A., and Dub§, G., Laser-Induced Damage in Optical Materials : 1974, N.B.S. Spec. Pub. 414, pp.119 (1974).

[16] Allen, S.D., Braunstein, M. , Giuliano, C.R.,

and Want, V., Laser- Induced Damage in Optical

Materials : 1974, N.B.S. Spec. Pub. 4T4,

pp. 66 (1974).

[17] Bettis, J.R., House, R.A., Buenter, A.H.,

and Austin, R., Laser-Induced Damage in Optical Materials : 1975, N.B.S. Spec. Pub. 435, pp.289 (1976).

[18] Milam, D., unpublished.

[19] Bliss, E.S., Milam, D. , and Bradbury, R.A.,

Appl. Opt., 12, 602 (1973).

[20] Milam, D., Laser-Induced Damage at 1064 nm

125 ps , submitted to Appl. Opt. for publication.

[21] Newnam, B.E., private discussions.

[22] Smith, W.L., Bechtel , J.H., and Bloembergen,

N . , Laser-Induced Damage in Optical Material

s

: 1975 , N.B.S. Spec. Pub. 435, pp. 321 (1972).

[23] Milam, D., Bradbury, R.A., and Bass, M.

,

Appl. Phys. Lett. 23, 654 (1973).

[24] Hopper, R.W. and Uhlman, D. R., J. Appl.

Phys., 41, 4023 (1970).

[25] Bradbury, R.A., unpublished.

[26] DeShazer, L.G., Newnam, B.E., and Leung, K.M

Appl. Phys. Lett. 23. 607 (1973).

[27] Picard, R.A., Milam, D., and Bradbury, R. A. Statistical Analysis of Defect - Caused Damage

in Thin Films , submitted for publication

to Appl . Opt,

[28] Duthler, C.J., Appl. Phys. Lett., 24, 5 (1974).

[29] Milam, D., Bradbury, R.A., Picard, R.H., and

Bass, M. , Laser- Induced Damage in Optical

Materials: 1974.

[30] Wang, V., Giuliano, C.R., Allen, S.D., and Pastor, R.C., Laser-Induced Damage in Optical Materials : 1975 , N.B.S. Spec.

Pub. 435, pp. 118 (1976).

[31] Fradin, D. W. , Yablonovitch, E., and Bass,

M,, Appl. Opt., 12^, 700 (1973).

[32] Fradin, D.W., and Bua, D.P., Appl. Phys.

Lett. , 24_, 555 (1974).

[33] To the author's best knowledge, the existence of platinum inclusions in glass was first demonstrated by R. F. Woodcock of

American Optical Company.

[34] Kelly, P., Braunlich, P., and Schmid, A.,

Appl. Phy. Lett., 26, 223 (1975).

[35] Bloembergen, N., IEEE J. Quant. Elect.,

QE-IQ , 375 (1974).

9 , Figures

LASER-INDUCED BREAKDOWN IN ARGON

10"

10''

10'

1 10» -

1 1 1 1 1 N

\

N

X

1 1 1 1 1 10 100

Pressure, atm

1000 10,000

Figure 1. Breakdown threshold of Argon as a function of pressure. The data of Gill and Dougal [11] has been ex- trapolated using the pressure

dependence found for avalanche

ionization [6], The low pressure

region has been studied by

Krasyuk, et al. [12], and shows

the pressure dependence expected

for dominance by direct photoionization.

-355-
Figure 2. Distribution of breakdown times as a function of intensity. Argon volumes of about 10~^cm^ were Irradiated by square- waveform ruby laser pulses [1],

COmEHTS ON PAVER BY MILAM

The. iptakM. mphai>'izzd that although both phototontzation and tht avalanche pfwceJii ate manCie^tty

nontlnzoA., thz ^omeA depzndi on a much kigheA poMM. 0(5 the. Intensity and io aj> {,aA mofie dramatic In

itl, nonZLnexinxty. The queition of, looking fon. polaJiization dependence wo* brought up itnce the 6am&

eneAgy dejK>-uty conjie^pondi to a louoeA electnxc {^eZd value at coicvJLan poljvUzatxon than at IcneoA

potcuvizatyion. In angon goA, {^on example, higheA thAe^holdi, MeAe obieAved uAtng cOlcuIoa poljVu.zaZion.

B. Hewnam of LASL tndtcattd that he had not obieAved dtftf^eAent thAe^holdi ion. thin fttm damage at

30 p6, except (,on. IkOz, In Mhich two-photon abionption woi expected to be opeAative. No pola/Uzatton

&iiect had been obieAved -in 17 nA put&eA in damage expeAimenti catAied out at Hughes Re^eaxch

Labon.atofiieA on iolid mateAial6.

S. Newnam alio nepofvted that he had 6een explosive damage at 30 pi in one cvicumitance u)hen highdamage thxe^hold ffilmi uieAe deposited on glass substAotes and damage occuHfied at the iubsthate exit

SuA-iace. This obseAvation seems to contnadtct Hilam's hypotheAls.

-356-
5.4 LASER DAMAGE TO SEMICONDUCTOR MATERIALS FROM 10.6 Um CW CO LASER RADIATION*

S. K. Gulati and W. W. Grannemann

Department of Electrical Engineering and Computer Science

The University of New Mexico

Albuquerque, New Mexico 87131

Damage thresholds for Si, Ge, GaAs , GaAsP, and GaP have been found experimentally.

2 All the damage thresholds are of the order of KW/cm for an irradiation time of 0.15

to 0.18 second. The damaged devices of Si and Ge show 30 to 50 percent decreases in carrier mobility at lower temperatures, thereby showing lattice damage. It is found

that the experimental damage thresholds, surface and lattice damage can all be explained

by applying the theory of parametric instability of the type formulated by DuBois and

Goldman. The same theory is also applied to explain CW CO^ damage thresholds for KCl

windows as experimentally observed by Loomis and Huguly.

Key Words: Carrier mobility; damping rates; laser damage of semiconductors; parametric

plasma instability; photon induced excitation.

Introduction

Damage thresholds for different semiconductors using pulsed laser radiation have been determined

experimentally by several authors [1,2,3,4]^. Different authors use different criteria for the occurrence of damage. However, the damage thresholds are all of the order of tens of MW/cm^, which

are about three orders higher than the damage thresholds in the present cw damage work. An attempt to explain the damage thresholds of semiconductors for pulsed lasers was recently make by Shatas , et al.

[5] , in which they used the theory of parametric instability as formulated by Gerstein and Tzoar [6]

.

These damage thresholds depend critically on carrier mobility and band gap. However, no such critical

dependence is observed in the present work for cw CO^ laser damage. Shatas, et al. in the same paper

also applied the theory of modified two stream instability following Kaw and Dawson [7] in order to ex- plain the observed damage thresholds. The damage thresholds in the present work are too low to be ex- plained by any of these theories or the lucky electron theory of avalanche. Drude absorption is ruled

out because the damage occurs at the same low power density in intrinsic GaAs. Simple Joule heating is insufficient to explain the observed damage. But the thresholds calculated by using the theory of parametric instability of DuBois and Goldman [8] are in a reasonable agreement with the experimental damage

thresholds. The same theory also predicts correct damage thresholds for KCl windows as observed experimentally by Loomis and Huguly [9] .

Experimental

The semiconductor chips which were irradiated were in the shape of a square of .25 cm x .25 cm. The cw CO2 laser beam used was Gaussian in space with a diameter of 12 mm. A salt lens of 25 cm focal

length was used to concentrate the beam. The sample was displaced from the focus to radiate it uniformly. The experimental arrangement is shown in figure 1. Mobility, resistivity, and carrier concentration

measurements before and after damage were made over a temperature range of 20 °K to 300 °K using a CS202 cryogenic refrigerator and applying Van der Pauw's [10] method. This apparatus is shown in figure

2.

Results

The damage thresholds were obtained by radiating the semiconductor until damage was visible on the surface. Different semiconductors with their solid-state parameters and experimental damage thresholds

are listed in table 1.

The changes in mobility, resistivity, and carrier concentration in Ge and Si due to laser damage

are shown in figures 3, 3a, 4, 5, and 5a, respectively. All of these show significant lattice damage.

*Work supported by Air Force Office of Scientific Research iinder Contract No. F44620-75-C-0077.

1. Figures in brackets indicate the literature references at the end of this paper. -357-
Table 1. Different semiconductors with their solid-state parameters and experimental damage

thresholds

Semiconductor

Mobility

cm2(V-s)-l

Carrier

Concentration

(cm )

Band Gap

(ev)

Damage Threshold

_2 in W cm

Ge (n-type)

Si (n-type)

Si (p-type) GaAs (n-type) GaAsP (n-type) GaP (n-type)

2300 1200

300 3500

2000

110

4 X 10

6 X 10

3 X 10

1 X 10

2 X 10

1 X 10

0.79

1.11

1.11 1.60

2.00

2.4

8.5 X 10

4 1.1 X 10

4 2.2 X 10

6 X

10'^

3.5 X

10"^

1.33 X 10'

Explanation of Damage Thresholds

The observed damage thresholds can be explained by using the theory of parametric instability of

the type formulated by DuBois and Goldman [8]. This kind of instability can be excited in a system if the energy is fed into the solid-state plasma at a rate faster than the system can dissipate it.

The intense laser beam of frequency u (2.83 x

10^'^ Hz in this case) interacts nonlinearly with th^

° 2 2 2 1/2 electron plasma, that has a resonance frequency given by w^^ = (to^ + 3k Se ) (where to^ is the Langm

frequency of the electron plasma and Se is the thermal velocity of the electron) and pumps the ion plasm^

at the beat frequency (u If 0) and

o

u are very nearly equal, then (oo R O

is small and is characteristic of the ion acoustic frequency. Similarly, the laser beam can interact with the ion acousj

tic wave and pump the electron plasma. If this type of instability has a positive growth rate, then it can cause anomalous absorption and heating of the sample [11] with subsequent lattice damage.

The calculation of the damage thresholds is based on finding the nonlinear susceptibility in terms

of the laser beam parameters (E , k ) and electron plasma parameters (n, T, u )

o e It has been shown that

(k,a)) (k,uj) -

Li (kVk^)e^ (k,w-a) ) D L o

(1)

->• NL where e (k,(i)) is the linear longitudinal dielectric constant of the electron plasma and e (k,co) is the

L nonlinear dielectric constant under the action of the intense laser beam. Here

nckT

(2)

in which is the usual Langmuir frequency of the plasma and to^ is the frequency of the incident laser

n is the electron concentration of the plasma,

the Debye-Huckel shielding length.

field and I its intensity

o Also, k^ is the inverse of

The nonlinear dielectric constant has two parts. The real part, when equated to zero, would give

the resonance frequency of the plasma and is always very close to oj . The imaginary part gives the damp'

ing rate (y /'^p' NL

P

IM (k,u ) 1j J-i — - A P {< -1

(k,oo^ - 1^) L o

(3)

where -358-
and is ordinary Landau damping. In the case of a solid-state plasma, it can be approximated as ^ - £. = -i— (5)

0) OJ 0) T p p p c

where x is the collision time of the carriers with the lattice phonons.

c

2 Im (—^ 1 e (k, 0) -0) ) can be estimated by ( — | ( — | ^2 / L ^ ' L o \ y . / \ Y J

where y./u. and 'J^/'^^ give the damping rates for acoustic and optical phonons, respectively.

Hence

NL

(Jj CO P P

This damping rate is negative (i.e., we have the growth of instability) if

p

or writing in terms of power threshold

4nckT

\ LP

18 19 Now OJ = to and for this to be true the plasma concentration should be of the order of 10 to 10

-3 ° P

cm ' but the starting carrier concentrations in our case are much less than this. Therefore, we propose a new excitation mechanism which alleviates enough electrons from the valence band to the conduction band via a continuous distribution of surface states within the band gap. This will be discussed

at the end

.

In a solid-state plasma Drude absorption causes the electron temperature to be higher than the ion

bed temperature. Therefore, one can approximate [12]

1/2 1/2

(I)

-1 -2

yj^/o)^ usually lies between 10 and 10 [13] depending upon the semiconductor. This mode is more

readily damped in Si and Ge than in GaAs , GaAsP, GaP and salt windows.

Using appropriate values of the damping rates, and equation (6), we have calculated the damage

thresholds for different semiconductors. The calculated values are compared with the experimental

values in table 2.

The damping ratios used in the calculation are only approximate. These ratios are difficult to estimate from theories as well as from experiments.

-359-
Table 2, Calculated values are compared with the experimental values.

Material

Calculated

Damage Thresholds

_2 in W cm

Experimental

Damage Thresholds

_2

in W cm

Si (n-type) 7.2 X lo"* 4 1.1 X 10 Si (p-type) 9.8 X lo'* 2.2 X lo'* Ge 4.08 X lo'* 8.5 X 10^ GaAs 7.14 X 10^ 6 X 10^ GaAsP 5.7 X

10'^ 3.5 X 10^ Gap

4

8 X 10 1.33 X lO'^ KCl

4 5 10 ^ 10 lo'^ ^ 10^ [9]

Photon Induced Excitation

It is well known that the width of the band gap for a semiconductor is much less near the surface

as compared to within the bulk, e.g., the band gap for GaAs near the surface is 0.30 eV to 0.36 eV and

1.6 eV within the bulk [5]. Also within the band gap, there is a continuous distribution of surface

12 14 -2 -1 states having a density of about 10 to 10 cm (eV) . The lifetimes for these states are 3 to 4 orders longer as compared to oj^^ for CO^ laser. The electrons in the states near the Fermi level in an n-type semiconductor are almost resonantly excited to the conduction band. The emptied states are filled again by the electrons excited by the photons from the states lying below the Fermi level. This

streaming process can produce electron concentrations of 10^^ to lO"*"^ cm ^ within milliseconds or less. The quantum yields and transition probabilities for such processes have been calculated by Kane [14] and

experimentally verified by Gobeli and Allen [15] . The quantum yield has been shown to be of the order

(hv-E where n = 1 to 5/2, E is the separation between two surface states and hv is the photon

s s energy. This high quantum yield makes the proposed mechanism highly probable.

Conclusion

Damage thresholds of different semiconductors for CW CO^ laser radiation have been found experimentally. It appears that the theory of parametric instability in solid-state plasmas explains the re- sults satisfactorily. The same theory also seems to explain the observed damage thresholds for KCl

laser windows

Acknowledgments

We gratefully acknowledge very useful discussions with Dr. DuBois of the Los Alamos Scientific

Laboratory and Drs . House and Bettis and Major J. L. Stapp of the Kirtland Air Force Weapons Laboratory.

Our special thanks go to Mr. C. Huguly also of Kirtland Air Force Base for help in the experimental

work.

-360-
References

[1] D. C. Emmony, R. P. Howson and L. J. Willis,

Appl. Phys. Lett. 23, 598 (1973).

[2] M. Birnbaum, J. Appl. Phys. 36, 657 (1965).

(3] M. Bertolotti, L. Stagni, G. Vitali, S. L. Muzii, J. Appl. Phys. 42, 5843 (1971).

[4] J. L. Smith, N. B. S. - SP 414 Report, 200

(1974)

.

[5] R. A. Shatas, J. D. Stettler, L. M. Narducci,

S. S. Mitra and H. C. Meyer, N. B. S. - SP 414 Report, 200 (1974).

[6] I. I. Gerstein and N. Tzoar, Phy. Rev. Lett.

27, 1650 (1971)

.

[7] P. K. Kaw and J. M. Dawson, Phys. Fluids 12^, 2586 (1969)

.

[8] D. F. DuBois and M. V. Goldman, Phys. Rev. Lett. 14, 544 (1965)

.

[9] J. Loomis and C. Huguly, N. B. S. - SP 414 Report, 94 (1974)

.

[10] L. J. Van der Pauw, Phillips Res. Reports 13^ (1958)

.

[11] J. Dawson and C. Oberman, Phys. Fluids 5, 517

(1962)

.

[12] D. F. Dubois in Statistical Physics of Charged

Particle Systems, Eds. R. Kubo and T. Kihara,

Syokabu and W. A. Benjamin, Inc. (1968)

.

[13] N. Tzoar, Phys. Rev. 164 , 518 (1967).

[14] E. 0. Kane, Phys. Rev. 127 , 131 (1962).

[15] G. W. Gobeli and F. G. Allen, Phys. Rev. 127 , 141 (1962).

Ha-Ue USER

Figures

OSCILLOSCOPE

Figure 1. Laser setup.
30 90 150 210 240 300 TCK)

Figure 3. Electron mobility versus temperature for Oe before

and after laser damage. The sample was radiated at 8.3 X 10^ W/cm^, The original electron concentration

was '\^10'^ at room temperature.

I05

to

O

lo'

25 75 125 175 225 275 325 TCK)

Figure 3(a). Electron mobility versus temperature for Si before and

after laser damage. The sample was radiated at 0.96 X lO"* W/cm^. The original carrier concentration

was 'V'lO'^ cm"' at room temperature.

-362-
T i'K)

Tigure 4. Conductivity versus temperature for Ge before and after laser damage for the same sample parameters as In figure 3.

-363-
'e

BEFORE RADIATION

J L J L J L J L

30 90 ISO 210 270 330 390

T CK)

Figure 5. Carrier concentration versus temperature for Si before and

after laser damage for the sample parameters as in

figure 3(a).

5X10

Figure 5(a). Magnified view of figure 5 to show the significant charge carrier removal due

to laser damage.

BEFORE RADIATION

150 210 TCK)

330,

COmENTS ON PAPER BV GULATl kHV GUMMEUMN

S^veAot mmbfii oi the. aadimce. nottd the. appcuie-nt (LL6cAzpa.ncy between the utinated dzMity

iuAia.ce. i>tateM [lO' 10' m~ ) and the appajient deyii-ity eZ&atfLoyii geneAot&d {W^" an' ' The. ipeakeA oAgued that due to the Long tif^eXAjme ztectAom tn the conduction band, the cafOvieA concentAjatxon couZd qiow to the izquAJied vatae duAtng the ixueA pwLie. Tki6 muZd fiequAAe the contxnuoa6

excitation o^ eZecXAom ^fiom the valence band to iu/L^^ace itateA and thence to the condujction band

duJving the peJuxd o^ -illumination.

-364-
5.5 PHOTOTRANSPORT DAMAGE OF OPTICAL MATERIALS

Shew M. Wong

Rocky Flats Plant, Atomics International Division, Rockwell International and

David L. Olson Rocky Flats Plant, Rockwell International and Colorado School of Mines

Golden, Colorado 80401

Described are well-established electrotransport phenomena which may serve as analogies for studies of laser-induced damage in optical materials through phototransport. Phototransport or photomigration relates to the segregation of ionic species or in vacancies in the optical materials caused by a competitive momentum transfer from the scattered photons to the specific ionic species of the optical

material. Such microstructural changes in the material are expected to cause severe increases in localized absorption and enhance laser-induced damage.

Keywords: Absorption; electrotransport; ionic segregation; laser; momentum transfer; phototransport; scattering; vacancies.

Introduction

Transport of specific atomic alloy components in both solid and liquid electrical conductors caused by a high-electrical direct current density has been well established [1,2].* Much of this research effort has been performed to increase the service life of

integrated circuits based on thin-film technology. Electrotransport in liquid alloys has had the interest of those investigators in- volved in purification technology, as well as those interested in exploring the electronic behavior of alloys. The concepts which have been established for the understanding of electrotransport and the resulting damage to electrical conductors should provide direct analogies to a phototransport phenomenon in optical materials which experience high-photon density. Phototransport or photomigration will be defined as the segregation of ionic species or vacancies in an optical material caused by a competitive momentum

transfer from the scattered photons to the specific ionic species which are adjacent to the ionic vacancies. Given is a review of some

electrotransport concepts based on momentum transfer and electronic scattering, and predictions are made as to the nature of

phototransport damage in optical materials.

Electrotransport

Epstein and Paskin [3] have suggested a model for electrotransport behavior in liquid alloys which should be applicable to an

ionic segregation phenomenon in optical materials caused by a high photon density or phototransport. Epstein and Paskin [3] assumed that the atomic alloy component which receives the largest momentum transfer from the scattered conduction electrons should be the atomic species that moves toward the anode. The atomic component of the alloy which receives the smallest momentum transfer would be expected to fall behind in this competitive process and thus concentrate at the cathode.

Epstein and Dickey [4] using the concept that the largest momentum transfer from the scattered conduction electrons should

be with the alloy component that has the largest electronic scattering cross section, formulated the mechanics necessary to calculate the apparent electronic-scattering cross section of each of the alloy components as a function of the Fermi wave vector for the conduction electrons. This approach has been extended [5,6] and experimentally verified [7, 8, 9] for a number of liquid alloys. Figure 1 shows the calculated electronic back-scattering cross section (S) for the sodium-rubidium (Na-Rb) alloy system [8]

.

Notice that the Rb atom has the largest back-scattering cross section in Na-rich alloys, but with increasing Rb content there is a

reversal and the sodium (Na) atom has the largest electronic back-scattering cross section. This reversal would suggest a change in which the alloy component rubidium or sodium (Rb or Na) goes to the anode as a function of alloy content. Figure 2 shows the

results of measured electrotransport behavior of this Na-Rb alloy system. The drag coefficient is the quantity which measures the

efficiency of the electrons in dragging with them a specific atomic-alloy species toward the anode. Notice that at the Na-rich side of the diagram, the drag coefficient for Rb is positive indicating that Rb is migrating toward the anode. In the Rb-rich side of the diagram, the drag coefficient for the Rb atom is found to be negative, which indicates that Rb atoms are migrating toward the cathode and confirming the predicted reversal behavior. The electrotransport of vacancies must also be considered in solid electrical conductors. With high direct-current density, the metal atoms migrate toward the anode, while the vacancies migrate in the opposite direction. In an infinitely long and uniform

conductor, electrotransport would not be expected to be destructive, but in situations where there is a divergence of the vacancy

flux, localized high-vacancy concentration and void-formation results. Grain boundaries and surfaces are subject to a major

*Figures in brackets indicate the literature reference at the end of the oaoer. -365-
nonequilibrium segregation of vacancies and void formation. Such defects have been found to enhance the failure of the

metallization layers on integrated circuits.

Phototransport

One of the possible photon-scattering sites for optical materials is with the electronic defect structure associated with

vacancies in nonstoichiometric optical materials. Figure 3 illustrates a cation sublattice in which two different types of cations (M** and N'^'^), positioned around a cation vacancy, are in competition for successful migration into this adjoining vacancy. It is expected that the electronic defect region of these vacancies is sufficient in cross section to have photoelectric interactions which

will scatter photons and transfer momentum. The ionic species, associated with this electronic defect which contributes the most

to overall scattering phenomena, would be expected to receive the largest fraction of the transferred momentum, and the jump of

this ionic species with the largest scattering cross section is expected to be the most probable.

The scattering phenomenon and the resulting momentum transfer can be analyzed with wave mechanics in the same manner

as demonstrated for the electrotransport phenomenon [3-6] . With the determination of the proper interaction potential, the phase

shifts for the scattered photon wave vectors can be calculated. Phase-shift data would allow calculation of the photon-scattering

cross section as a function of the incident wavelength. With the photon-scattering cross section calculated for the specific ion- vacancy combinations of the optical material as a function of the wavelength of the incident radiation, predictions of the resulting mass-transport behavior can be made.

It will be assumed that the N** ion adjacent to the vacancy has a larger photon-scattering cross section, Sfj, than does the M** ion, Syi, and will be transported in the direction of the photons in this competitive transfer process. This will leave the surface region of the optical material deficient in 'N** ions and in excess of vacancies as illustrated in £ igure 4. If the phototransport

results in large deviations from stable phase compositions of the optical material of interest, it is possible that the surface region,

which is a subject to a large divergence in fluxes, may experience the precipitation of a second phase. Such localized microstructural changes will affect the optical behavior of the material. Also excessive transport of ions from the surface regions can produce a high vacancy concentration, such as is experienced in the failure mode of metalUzation on integrated circuits. Such localized high vacancy

concentrations near the surface may result in void formation which would certainly influence the optical behavior of the material. Consider the specific example of LiNbOs as the optical material. The equilibrium phase diagram [10] for the LijO-NbOs

system is shown in figure 5. The Nb"^* ion adjacent to the cation vacancy is expected, with its more complex electronic structure, to have the largest photon-scattering cross section and thus contribute more to the photon scattering process than the Li*' and O"^ ions. Further,it would be expected that the Nb*' ion concentration near the surface of this optical material would decrease under high incident photon density. From the equilibrium phase diagram (fig. 5), it can be seen that excessive depletion of Nb*'

ions may result in the precipitation of a new phase (Li3Nb04 ) and thus cause major changes in optical behavior of the surface

material. With the assumption that the photon-scattering cross section is a function of the number of electrons associated with each of

the ions, a prediction will be made as to which materials are most likely to receive phototransport damage. This assumption, known

to be acceptable at the limit of small wavelengths, as elemental X-ray scattering, is cubic-power function of the number of electrons

associated with each atomic species. Tables I, II, and III list possible optical materials with the number of electrons associated with

each ion. For this competitive mass-transport process, materials with ions that have approximately the same number of electrons

(table I) will not be subject to phototransport damage, but materials with large differences in the number of electrons (table III) for each ion are expected to be susceptible to phototransport damage. These predictions (tables I, II, and III) suggest that specific

salts (NaF, KCl, AIF3 , MgF2 , SiF4), and magnesium-aluminum siUcates (spinel, enstatite, cordierite, sapphirine, and forsterite) , should not experience phototransport damage. These calculations do raise concerns based on the experience with electrotransport, that phototransport damage would be expected in LiF, NaCl, TiOj , AgCl, LiNbOs , LilOs.and other materials with large differences

in the number of electrons associated with each ion of the optical material.

-366-
Table I. Calculated predictions showing which optical materials should not experience phototransport damage.

Electrons per Ion

Materials Cations Anions

NaF 10 10 MgFj 10 10 AlFj 10 10

SiF4 10 10 KQ 18 18 CaQi 18 18 RbBr 36 36

SiOj 10 10 AI2O3 10 10 MgO 10 10 Magnesium-Aluminum Silicates: MgAl2 04 (spinel) 10, 10 10 Mg2Si04 (forsterite) 10,10 10 MgSiOj (enstatite) 10, 10 10 Mg2Al4SisO,8 (cordierite) 10,10,10 10 Mg4Al,oSi2023 (sapphirine) 10, 10, 10 10

Table II. Calculated predictions showing which optical materials may experience minor phototransport damage.

Electrons per Ion

Material Cations Anions

CdTe 46 53 GaAs 29 36 ZnSe 28 35 AgBr 46 36

-367-
Table III. Calculated predictions showing which optical materials should experience major phototransport damage.

Electrons per Ion

Material Cations Anions

T iF 1 n

T in z lo

T iHf L ^0

DC\J 1 c\

B2 U3 Z lU

JNaci lU lo Kbr Jo 11) Aga 46 18 TiOj 18 10 ZrOi 36 10 CaFj 19 10

lilOs 2,46 10

Li>fb03 2,36 10 CaCOs (calcite) 18,2 10

Acknowledgments

The authors acknowledge and appreciate support for the work performed under the U. S. Energy Research and Development

Administration, Contract E(29-2)-3533.

References

[ 1 ] Rigney, D. A., Electromigration in Metallic Systems,' Charge Transfer/Electronic Structure of Alloys, Ed.

L. H. Bennett and R. H.Willers, AIME N.Y., N.Y.,

87, 126(1974).

[2] d'Heurle, F. M., Proc. IEEE 59, 1409 (1971).

[3] Epstein, S. G. and Paskin, A., Phys. Letter, 24A, 309

(1967).

[4] Epstein, S. G. and Dickey, J. M., Phys. Rev. Bl, 2442

(1970).

[5] Olson, D. L., Blough, J. L., and Rigney, D. A., Scripta Met. 4, 1023 (1970).

. [6] Olson, D. L., Blough, J. L., and Rigney, D. A., Acta

Met. 20,305 (1972).

[7] Blough, J. L., Olson, D. L., and Rigney, D. A., Mat.

Sci. Eng. 11,73 (1973).

[8] Lakshmanan, T. S., Olson, D. L., and Rigney, D. A., Scripta Met. 5, 1099 (1971).

[9] Lakshmanan, T. S. and Rigney D. A., Mat. Sci. Eng. 12,285 (1973).

[10] Phase Diagrams for Ceramists—1 9 75 Supplement,

Ed. Levin, E. M. and McMurdie, H. P., Amer. Cer. Society, Columbus, Ohio, 86 (1975).

-368-
Figure 2. The electrotransport drag co- efficient for rubidium as a function

of the rubidium content in sodiumrubidium alloys [8],

DRAG COEFFICIENT

VS. At % Rb Na-Rb

_l_ 10 20 30 40 50 60

ATOMIC 7o Rb

70 80 90 100

Figure 3. Photon scattering at the electronic

defect structure associated with

vacancies in nonstoichiometric

optical material. Electronic defect structure is illustrated with cation

CONDITION S^++> S„++ -369-

sublattice.
OPTICAL MATERIAL

DISTANCE FROM SURFACE

figure A. The- Ion and vacancy concentration near the surface of the optical material damaged by a photQtransport process.

COmENTS ON PAPER BY WONG ANV OLSON

It wan potntzd out that thz momentum t/iankleA. {^nom a. photon to an atom -ii a maximum whm th& photon

/U fieAonantJty abiofiba.d oK fiz^tzctzd ^fwm the. atom and that tkz momzntim tAoni^zA Li j'uit given by

hv/c OK 2 hv/c in that cxi&e., Thti i^ the. bai-U oi one pKopoitot {^on i^eA Ziotopz sepoJuvtion, The mo- mentum tAon&ien. ^ hmaJUi and tncAeme^ with decAeoAing mvetength. The ipeaken. ieJit that the e^iect

might be much mole &i.gviifficjant at x~Aay la^eA uiaveZengthi , The e^^^&ct -ci expected to be tempeAatiine

dependent a& wetL and one ma6t examine uiheXheA the momentum obtained ^Kom the photond -ci ia^ii-cient to

oveAcjome the nandomizing e^ecX o^ the lattLce phonon coUUJiioni. It iA expected that the poituZated

eiiect, ii it occuAS, wiZZ be a cimuLative e^ect leading to a net i>egAegation and defect accwmuZation

oveA a ieAA.&& o^ &hoti> u)hLch In tarn couZd tejod to a higheA pncbabitity 0){ damage, Ratz6, being con- centAotion def)endent, wiJUi change duAtng the cooie o£ prolonged iAAadiation.

-370-
5.6 Resonant Defect Enhancement of the Laser Electric Field

P. A. Tenple and M. J. Soileau

Michelson Laboratoties , Naval Weapons Center China Lake, California 93555

This paper describes a physical model for laser damage induced by surface

defects. The model predicts that: (a) a polarizable defect will give rise to an electric field which can interfere with the laser field, (b) the interference pattern will be fixed in space along the direction of the laser beam

polarization and with a spacing equal to the wavelength of the laser beam in

the material, and (c) there is a strong defect size effect causing a significant Increase in the field strength when the defect size is an odd number of

half wavelengths. Experimental evidence supporting this model is presented.

Key words: Damage theory; laser damage; 10.6 ym laser.

Introduction

It has been observed that the laser damage thresholds of optical surfaces and thin films are reduced by defects [1,2]^. This is to be expected since the electric fields ate higher within defects

[3]. A topological feature seen on laser-damaged surfaces is a wavelike set of ripples (figs. 1, 2) [4-6]. For the case of metals or semiconductors, the ripple spacing is X, the free space laser wavelength. In the case of dielectrics, the spacing is X' = X/n, where n is the index of refraction of the material. These ripples are associated with surface defects, but the ripples occur away from the defect.

We wish to suggest an explanation for the ripple patterns. The patterns discussed in this paper

were produced when the material was irradiated by a normally incident, linearly polarized, single-mode

TEMqo laser pulse having a wavelength of 10.6 pm. The laser pulse width was approximately 60 ns full

width at half maximum (FWHM). The ripples, such as those shown in figures 1 and 2, extend along the

polarization direction (that is , the E vector is normal to the "waves" of the pattern) and are found to be associated with surface defects. While defects are generally recognizable as scratches, voids, etc.,

in the context of this paper a defect is any region of permittivity £2 which Is different from the bulk

permittivity e^.

Damage ripples are generated by a spatial modulation of the electric field intensity over the surface of the sample. The damage which occurs is a physical displacement and/or removal of material from

the surface [7,8]. The spatial modulation in the electric field Intensity will be shown to arise from

the coherent interference of the incident laser field with the nonradiative longitudinal electric fields

generated by the induced polarization charges present on the "boundaries" of defects within, or on the

surface of the specimen. It will be shown that there is a definite size effect which causes an enhancement of the polarization-induced electric field and therefore , an Increase in the modulation Intensity

when the defect diameter (or size) is an odd number of half wavelengths in the material. Finally, it is suggested that damage sites, once initiated as above, will act as new sources of electric field which

then add in phase with the original defect field, resulting in a bootstrapping of damage ripple sites

away from the original defect.

Electric Fields Caused by Defects

Figure 3 shows the surface polarization charge a present when two dielectric materials of different

germittlvlty are placed in a uniform electric field. If the electric field Just within the defect is

E^-« and the field is E{ just outside the defect, then from the requirement that the normal components of

the displacement vector be continuous across the boundary, it can be shown that the surface polarization

charge density is

a - e^fi -(Ej^ - (1)

where n is a unit vector normal to the defect and pointing away from it. In general the defect surface

polarization charge will "follow" the incident (laser) electric field, and the magnitude of the polarization chacge will be determined by the permittivities of the defect and host materials of the laser

frequency.

A polarizable defect, for exaiq>le a spherical particle, will generally reradlate energy with the

familiar dlpole radiation patteim when driven by a time-varying electromagnetic field. The maxlmu^

dipole radiation intensity is in the plane normal to the induced dlpole axis. The dlpole radiation intensity Is zero on the line containing the induced dlpole because the magnetic field is always zero on

1. Figures in brackets indicate the literature references at the end of this paper. -371-
this S5nnmetry axis, resulting in a zero Poynting vector. The electric field caused by the induced po- larization surface charges, however, is not zero on this axis [9]. It will be shown that it is this

time-varying "electrostatic" field which gives rise to surface ripples. The electrostatic fields

resulting from induced surface charges will be calciilated for two cases: a sphere, and a cylinder oriented with its axis normal to the incident laser polarization direction as well as normal to the laser

radiation propagation direction. These two cases are simple models for pits or voids and scratches.

In both cases we will only discuss the electric fields along the line which is parallel to the incident

laser polarization direction and which passes througih the center of the sphere or intersects, and is normal to, the axis of the cylinder. Along this "symmetry line" the induced electrostatic field is parallel to the laser field.

The defect electric field is time-varying as

E(r,t) = E(r) sin 2ir(vt + A). (2)

E(r), the maximum static defect field, is the electric field of the induced polarization charges gener- ated by the peak electric field of the incident laser beam.

Sphere

The electrostatic field along the symmetry line of the sphere caused by the induced polarization

charges is given by [10]

^^^^phere = 2a3[(c2 - z^)Kz2 + 2e,)](E,/K^) (3)

where a is the radius of the sphere, the permittivity of the surrounding medium, the peak electric field of the laser beam in the defect-free surrounding medium, £2 the permittivity of the defect

material, and x the distance from the center of the sphere to the field point. For example, for a spherical void in KCl of radius 2.5 pm, the static fields at distances of 10 um and 30 um from the void

on the symmetry line are

E(10 ym) ^ = -6.7 x 10~^ Ej; E(30 pm) ^ = -2.5 x lO"'* E, sphere ' sphere ^ (4) and are parallel to the laser field. For a metallic defect we can assume that ^2 ^1' then becomes a function of only a and x. For a particle of 2.5 pm radius the static fields are

E(10 um) , = 3.1 x

10"^ Ei; E(30 ym) . = 5.8 x lO""* Ei. (5) sphere ' sphere '

Cylinder

The electrostatic field along the symmetry line (actually symmetry sheet) is given by [11]

E(x)^y^ = a2[(e2 - £i)/(e2 + £i)](Ei/x2) (6)

where all the symbols have the same meanings as for the sphere. For a cylindrical void of radius

2.5 ym in KCl, the electric fields at distances of 10 ym and 30 ym from the cylinder are

E(10 ym) , = -2.2 x

10~^ Ei ; E(30 ym) , = -2.5 x lO"^ E, , (7) cyi cyi

again parallel to the incident laser field.

Interference with the Laser Field

The nonradiative electric fields expressed in eqs. (3-7) are time-varying with the frequency v of the driving (laser) field. The net electric field intensity seen at any time and position along the symmetry line discussed previously is given by

E(x,t) = E(x) . , ^ sin 2Trv(t - x/c') + E, sin 2TT(vt + 6) (8) detect ' where c' is the phase velocity of the electrostatic field of the induced polarization charges, and x

is the position along the symmetry line from the defect to the field point; Ej sin 2ir(vt + 6) is the electric field of the laser beam. Any time- and position-independent phase shift which may be present

is given by 6

At the time when Ej sin 2Tr(vt +6) = E^, the field given by eq. (8) has the following characteristics (fig. 4a): Points A and C are positions of constructive interference, while points B and D are positions of destructive interference. One-half period later (fig. 4b), the positions of constructive

interference, such as A and C, will again be positions of constructive interference. but with both the -372-
laser electric field and the defect electric field being reversed in direction. Thus, when the defect

field is added to the laser field, the resulting net field will exhibit maximal values spaced X' apart

along the symmetry line (points of constructive interference) and intermediate points with a field

slightly lower than the laser field (points of destructive interference). The regions of constructive

interference are fixed in space. The exact radial positions of the maximal points in the net electric

field will depend on any time- and position-independent phase lags which exist between the induced po- larization charges and the laser beam field, and on a 180° phase shift that may be present depending on which is larger, or £2 (see eqs. 3,6). Point defects will exhibit fan-shaped ripple patterns when

produced by linearly polarized light (fig. 2). Line defects will generate lines of ripples (fig. 1) which are parallel to the original defect.

The modulation in the electric field intensity resulting from the electrostatic field of a defect

is on the order of ±2% adjacent to the defects (eqs. 4,7). It will now be shown that defects of certain

sizes will result in a much larger modulation of the net field, particularly at points far from the

defect.

Enhancement Caused by Propagation Delays

The instantaneous polarization charges generated in the above examples are distributed along the surfaces of the defect such that positive charge is along one side of the defect and negative charge Is along the opposite side of the defect. The electric fields which are generated by these two sheets of charge and seen at some time t at a point P along the symmetry line are the result of the states of charge on the two sheets at two different earlier times. When the displacement of the two sheets of

charge on the defect is on the order of one-half the wavelength of the incident laser radiation, a con- structive interference condition will exist which will enhance the net defect-generated field as seen

along the symmetry line.

Consider the field strengths Ej and E2 generated by two time-varying point charges, qi and q2 at some time t (fig. 5). At a point P on the symmetry line, the fields seen at time t are

Ei(P,t) = /° 2 sin 2-nv it - ]

E2(P,t) = — sin 2Trvft - ^^^^1

4Tre(x+d)2 I c J

Case A

d/c' = nT , (or d = nX'), where n is an integer and t is the period of oscillation.

In this case Ei -E2 and we have the usual near field "static" electric field due to a dipole.

Case B d/c' = nt/Z, (or d = nX'/2), where n is an odd integer.

In this case Ej +E2 as seen at point P, and the net electric field from the two point charges appears

to be due to two charges of like sign . This case shows constructive interference, or resonance, with a resulting sinusoidally time-varying electric field which is the sum of two nearly identical electric

fields rather than the difference of these two fields. As discussed above, this electric field will

bear a fixed phase relation to the driving laser beam field independent of d and will give rise to a spatially modulated electric field on the symmetry line with maximal points spaced X' apart (eq. 9).

While the spherical and cylindrical defects do not give rise to the idealized dipole configuration used

above, they do have well-separated charge distributions. The electric field for case B (resonant) will

now be calculated.

Spherical Case, Resonant Condition

In the case of the sphere, charge is distributed on each hemisphere with charge density

a = 3e^[(e2 - ei)/(e2 + Zej)] Ej cos 9 (10)

where 6 is the polar angle. The total charge on one hemisphere is

q = 3e^ iTa2[(e2 - £i)/(e2 + 2ei)] Ej . (11)

If we assume one-half the total charge on each hemisphere contributes to the constructive effect caused

by propagation delay considerations , then we can approximate the field in the constructive case by con- sidering +q/2 to be located as point charges on opposite sides of the sphere and on the symmetry line. A further simplification is to assume this net charge of q is located at the center of the sphere

-373-
rather than on opposite sides of the sphere. The field will then appear as the simple point charge

field,

E(x) = q/4Treix2 = [ 3e^a2/4e ] [ - ei)/(e2 + 2ei)] Ei (12) ~2 — 3 which falls off as x rather than as x as in the static dipole case.

The fields for a spherical void in KCl having a radius a of 2.5 um and distances x of 10 ym and

30 ym are

E(10 ym) , _ = -4.7 X

10~^ Ei ; E(30 ym) , = -5.2 x lO"'* Ei , (13) sphere sphere

The field for a distance of 30 ym is significantly larger than the static field calciilated for an identical distance and radius (eq. 4). For a metallic particle of similar size in KCl, the field can be

calculated by assuming that £2 » in eq. (12). Then E(x) becomes independent of £2, E(10 ym) = 3.2 X 10~2 El and E(30 ym) = 3.6 x 10~^ E^. Again the field for a distance of 30 ym is significantly larger

than the static field calculated for comparable conditions (eq. 5).

Cylindrical Case, Resonant Condition

In the case of the cylinder the polarization charge is distributed on each half cylinder with

charge density,

a = 2e^[(e2 - ei)/(e2 + ^l)] cos 6 . (14)

The total charge per unit length on each half cylinder is,

T = 4ae^[(e2 - ei)/(e2 + ei)] Ej . (15)

As in the resonant case for the sphere, we assume that the apparent charge density seen at P(x) is equivalent to that of a wire on the axis of the cylinder having charge per unit length t. Then the field is

E(x) = T/2Treix = [2ae^/Tre ix] [ (£2 - ei)/(e2 + ^l)] (16)

- 1 -2 which falls off as x rather than as x as in the static cylinder case.

The fields for a particle radius a of 2.5 ym and distances x of 10 ym and 30 ym are

E(10 m)^^^ = -2.7 X

10~^ El ; E(30 Vm) = -9.0 x lO"^ Ei (17)

which for E(30 ym) is three times larger than the static field for a defect of the same dimension

(eq. 7).

The strength of the additional fields discussed above are on the order of 2% of the incident field.

If the incident laser beam intensity is sufficient to cause damage, then a ±2% modulation on the net electric field in the vicinity of the defect may well cause more damage in the regions of constructive

interference and less damage in the regions of destructive interference. It will be shown in the next section that any damage which does occur will be spaced so as to cause additional fields of proper phase

to add constructively with the original defect field. In this way, once initiated the damage pattern

will tend to propagate in bootstrap fashion across the surface of the sample, with each successive dam- age site causing increased coherent scatter both back toward, and away from, the original defect.

Interference From Damaged Areas

Figures 1 and 2 show the appearance of typical ripple patterns as seen on the exit surfaces of KCl

windows. Profilometry indicates that the patterns are actually undulations of the surface and not variations of the refractive index in the material [8]. A schematic diagram of this type of surface is shown in figure 6. In this figure the spacing between undulations is X', the wavelength of the laser

beam in the medium. While the damaged areas are shown as depressions, the only requirement is that they

have a lower refractive index than the bulk material. When this lower refractive index appears, the sign of the polarization surface charge on each undulation corresponds to that shown in the figure. The charge distribution in each damage site is similar to the distribution in the original defect. However,

each new "defect" is displaced an integral number of wavelengths from the original defect. Equation (9) indicates that the field at a point P and distance x from a defect is in phase with the field from

another defect located x + nA' from the same point P, where n is an integer. Therefore, all the electric fields from all defects contribute in phase, resulting in a runaway situation once damage is initiated. The pattern finally dies away as the laser spot intensity falls off in the skirts of the beam. -374-
The pattern, once initiated, will scatter both forward causing more damage ripples, and back

causing increased damage to the already existing pattern. This scattering results in a larger net field

for the constructive interference regions while the intermediate regions of destructive interference

receive an ever decreasing net field. As the laser radiation continues, the pattern of ripples is therefore enhanced rather than washed out. Ripples having a peak-to-valley height of 0,1 ym have been

observed in the bottom of a 2.5 \im deep damage crater in KCl [8].

Discussion

This paper presents a model for the interaction of a laser beam field with defects on optical surfaces. The model predicts that a nonradiative longitudinal electric field is generated by the laserinduced polarization charges at defect sites. The coherent interference of the induced field and the

incident laser field results in a spatial modulation of the electric field intensity over the sample

surface. This modulation of field intensity results in the ripple-like structure seen in laser-damaged

surfaces. The sizes and orientations of the defects are found to be important. Maximum interaction

with the defect occurs when the defect is an odd number of half wavelengths in width (or diameter) , and

in the case of a scratch, when the defect is normal to the polarization direction of the incident laser

beam. Once generated, "ripples" on the surface become new defects of exactly the right spacing and orientation needed for maximum interaction with the incident laser beam. Thus, the ripples become self re- inforcing and this bootstrap effect results in the production of more ripples. Near threshold, surface

damage will be dominated by defects. Even very small defects become important because of the unstable

condition produced by the induced electric field. All of these effects have been observed experimentally, and the details of two such observations are discussed in the following paragraphs.

Figure 1 shows the exit surface of a KCl window which was damaged by a single 60 ns pulse (FWHM) of

polarized radiation having a wavelength of 10.6 um. The damage pattern of ripples, spaced 7.5 um apart, was propagated outward from a scratch. Notice that the scratch was enlarged and has the size and ap- pearance of one of the damage ripples, indicating that the ripples have scattered the electric field

back to the original defect. Once the damage pattern was established, the original defect was incorporated into the pattern as one more damage ripple. While other scratches exist on the window surface,

the low polarizability of a defect whose axis is parallel to the laser polarization direction kept those

scratches from generating damage patterns.

For a spherical defect (fig. 2) the pattern is fan-shaped. The orientation of ripples in this figure indicates that the longitudinal electrostatic field from the defect contributes to the damage and

that the radiative electric field does not.

The phenomena discussed in this paper have direct bearing on the fabrication of optical components

for laser use. Scratches normal to the laser field are to be avoided. Sharp surface discontinuities

are more of a problem than are smooth ones. One should not use polishing or grinding particles of sizes near odd half-integral multiples of the laser wavelengths, and defects of these' widths should be

avoided. Surfaces produced by single point diamond turning have defects which are relatively welloriented and have a relatively well-defined spacing which can be controlled by varying the machine

parameters, such as spindle speed, feed rate, and tool-cutting angle. Deep chemical etches which enhance

grain boundaries should be avoided, but light etches which tend to smooth defect edges could be very beneficial in raising surface damage thresholds. In summary, by controlling the shape, size, and orientation of defects on optical surfaces one can raise the damage threshold even if the total number of

defects is not decreased.

Acknowledgments

This work was supported by the Energy Research and Development Administration, the Defense Advanced

Research Projects Agency, the Air Force Weapons Laboratory, and Naval Weapons Center Independent Research

Funds. The authors wish to acknowledge many helpful discussions with D. L. Decker concerning polarizable defects in optical materials.

-375-
References

[1] For example see A. J. Glass and A. H. Guenther, in Laser Induced Damage 'in Optioal

Materials: 1974, A. J. Glass and A. H. Guenther, eds. (NBS Spec. Publ. AlA, 1974),

pp. vii-xx.

[2] De Shazer, L. G. , et al. , in Laser Induced

Damage in Optical Materials: 1974, A. J. Glass and A. H. Guenther, eds. (NBS Spec. Publ. 414, 1974), pp. 214-218.

[3] Bloembergen, N. , Appl. Opt. 12, 661 (1973).

[4] Willis, L. J., and Emmony, D. C., Optics and Laser Technol. 111 (1975).

Figures

[6] Bass, M. , et al. , "Pulsed Laser-Induced Damage at the DF Wavelengths," these Proceedings.

[7] Solleau, M. J., et al. , "Laser Damage Studies

of Low Index Coating Materials at 10.6 um/'

these Proceedings.

[8] Soileau, M. J., et al. , "10.6 um Pulsed Laser

Damage Studies of Diamond Turned KCl Window

Surfaces," these Proceedings.

[9] Jackson, J. D. , Claesiaal Electrodynamics

(John Wiley & Sons, New York, 1962), p. 272.

[10] Stratton, J. A., Electromagnetic Theory

(McGraw-Hill, New York, 1941), p. 204^205.

Morse, P. M. and Feshbach, H. , Methods of The- oretical Physics (McGraw-Hill, New York,

1953), p. 1184.

Figure 1. Nomarski micrograph of the exit surface

of a KCl window which has been damaged

by a single pulse of 10.6 pm laser radiation. The polarization direction is indicated by the arrow in the lower

right comer of the figure. The spacing

of the ripples is 7.5 ym. The upper arrow Indicates the scratch which

initiated the damage pattern.

[5] Soileau, M. J., et al. , in Proceedings of

Fifth Annual Conference on Infrared Laser [11] Window Materials: 1975, C. R. Andrews and

C. L. Strecker, eds. (Defense Advanced Re- search Projects Agency, Arlington, Va. , 1976),

pp. 391-417.

-376-
figure 2. Nomarski micrograph of the exit surface of a KCl window which has been damaged by a single

pulse of 10.6 pm laser radiation. The polarization direction is indicated by the arrow. The damage was initiated by a particle defect

and the soacing of the ripples is 7,5 ym.

Figure 3. Schematic diagram showing the electric fields

El and E2, the permittivities Ei and e2» unit

normal n, and surface charge density a for two dielectrics in contact. Equation (1) gives the

relationship between these variables.

U)

4. Electric field near a surface defect showing

the spatial modulation in electric field intensity introduced by the defect, (a) The

electric field is shown at time t, the time

when the electric field of the laser beam

is a maximum, Ei . (b) The electric field

at time t + t/2, showing that points A and

C, which were points of constructive interference at time t are also points of con- structive interference one-half period later. Points B and D are points of destructive

interference. The spacing A-C is X', the laser wavelength in the medium.
Figure 5. Spatial arrangement of two point charges

qi and q2 separated by a distance d.

' d The field point P is located directly

above the two charges, a distance x from

the top charge, eq. (9).

Figure 6. Schematic diagram of the original

defect, such as a scratch, and the ripples caused by the enhanced

electric field caused by the defect. The signs of the surface

polarization charge produced by

the laser beam electric field are indicated.

ORIGINAL DEFECT

COMMENTS ON PAPER BV TEMPLE AHD SOILEAU

Tt IMCU) anticipatzd that CAAcaZoAly poZa/iizzd tight uiouZd Zzad to a di^^eM-^nt kind o{ patteAn,

peAkapi cAJicalanJiy {>ymrmtAA.caZ, but. the. amty&-ii, had not been ca/uiizd out. Thz ipeakeA cautionek that

thz pnccQj>-!> can pfwpagoitz th/wagh thz mcLteAxal iZncz. thz fiipptz pattzAn &zt up by a icattzAzt Izadi to

IocmI. -indzx voAyiationi [duz to thz unipzcA^i-izd damagz pKoczdi) which In tu/m Zzadi to ^uAthzt

icattzting.

-378-
APPENDIX I PARTICIPANTS

AA

c. M. Allred

R. H. Anderson

J. P. Anthes

J. H. Apfel

L. J. Aplet

BA. D. Baer

A. Balbin-Villaverde

N. Barnes

M. Bass

H. E. Bennett

E. Bernal G.

J. R. Bettis

A. D. Blackburn

N. Boling

W. T. Boord

G. H. Boultbee

H. S. Boyne

M. Braunstein

R. E. Brocklehurst

R. R. Brown

J. Bruce

B. Burdick

M. W. Burnham

C

R. M. Cannon, Jr.

G. E. Chamberlain

J. Y. Chang

A. R. Cook

V. Costich

F. K. Crosher

D

B. L. Danielson

G. W. Day

D. L. Decker

L. Decker

J. A. Detrio

M. J. Dodge

C. J. Duthler

E

F. L. Ebright

D. F. Edwards

R. Esposito

W. S. Ewing

F

A. Feldman

J. R. Fenter

D. G. Fischer

A. M. Frank

D. L. Franzen

W. G. D. Frederick

W, J. Fredericks

Electromagnetics Division, National Bureau of Standards, Boulder, CO 80302

Honeywell Corp. Res. Ctr., 10701 Lyndale Ave. So., Bloomington, MN 55420

Sandia Labs, Laser Dev. Div. , Albuquerque, NM 87115

Optical Coating Lab., Inc., P. 0. Box 1599, Santa Rosa, CA 95402

Hughes Aircraft Co., M.S. E119, Centinela & Teale Sts., Culver City, CA 90230

Naval Weapons Center, Research, Code 6018, China Lake, CA 93555

Institute De Fisica-Universidade Estadual De Campinas, Campinas, Sao Paulo,

Brazil 13100 Los Alamos Scientific Labs., M/S 564, P. 0. Box 1663, Los Alamos, NM 87545

U. of So. California, Center for Laser Studies, University Park, Los Angeles,

CA 90007

Michelson Lab., Naval Weapons Center, Code 6018, China Lake, CA 93555

Honeywell Corp. Res. Center, Materials Sc. Dept., 10701 Lyndale Ave. So.,

Bloomington, MN 55420

U. S. Air Force, AFWL/LRE, Kirtland Air Force Base, NM 87117

U. S. Air Force, AFWL/LRE, Kirtland Air Force Base, NM 87117 Owens-Illinois, Inc., 1700 No. Westwood, Toledo, OH 43666

Honeywell Systems & Res. Ctr., R2340, 2600 Rideway Pkwy., Minneapolis, MN 55413

Optical Coating Lab., 2789 Giffen Ave., Santa Rosa, CA 95404

Electromagnetics Division, National Bureau of Standards, Boulder, CO 80302

Hughes Res. Labs., Chem. Physics Dept., 3011 So. Mallbu Canyon Rd.,

Malibu, CA 90265

Wright-Patterson AFB, AF Materials Lab., AFML/LP, Dayton, OH 45433

Boeing Co., Res. & Eng. Div., MS 87/75, P. 0. Box 3999, Seattle, WA 98006 Hanscom Air Force Base, RADC/ETSO, Stop 30, Bedford, MA 01731

United Technologies Res. Ctr-73, Silver Lane, East Hartford, CT 06108

Rockwell International, Rocky Flats Plant, Box 464, Golden, CO 80401

Mass. Inst, of Technology, Dept. Mat. Sc. & Eng., Cambridge, MA 02137

Electromagnetics Division, National Bureau of Standards, Boulder, CO 80302 Grumman Aerospace Corp., Res. Dept., Plant 26, Bethpage, NY 11714 Electromagnetics Division, National Bureau of Standards, Boulder, CO 80302

Design Optics, 155 Moffett Park Blvd., Sunnyvale, CA 94086

Optical Coating Lab., Inc., Adv. Prod. Dev., P. 0. Box 1599, Santa Rosa, CA 95403

Electromagnetics Division, National Bureau of Standards, Boulder, CO 80302

Electromagnetics Division, National Bureau of Standards, Boulder, CO 80302

Naval Weapons Center, Code 6018, China Lake, CA 43555

Lockheed Res. Labs., Dept. 52-56, Bldg. 201, 3251 Hanover St., Palo Alto, CA 94304

U. of Dayton, Res. Inst., 300 College Park, Dayton, OH 45469 National Bureau of Standards, Div. 232.03, A251 Physics Bldg.,

Washington, D. C. 20234

Xonics Inc., 6849 Hayvenhurst Ave., Van Nuys, CA 91406

Science Applications, Inc., 201 S.W. Monterey Rd., Stuart, FL 33494 Los Alamos Scientific Lab., MS/564, Los Alamos, NM 87545

International Laser Systems (Kirtland AFB), 729 Sagebrush Trail,

Albuquerque, NM 87123 Hanscom Air Force Base, RADC/ETSO-30, Bedford, MA 01731

National Bureau of Standards, A259, Materials Bldg., Washington, D. C. 20234

Wright-Patterson AFB, AF Materials Lab., AFML/LPO, Dayton, OH 45433

Coherent Radiation, 3210 Porter Dr., Palo Alto, CA 94304 Lawrence Livermore Lab., L-384 ME/Optics, P. 0. Box 808, Livermore, CA 94550

Electromagnetics Division, National Bureau of Standards, Boulder, CO 80302

Wright-Patterson AFB, AF Materials Lab., AFML/LPO, Dayton, OH 45433

Oregon State University, Chem. Dept., Corvallis, OR 97331

-379-
G

D. E. Gilbert

D. H. Gill

A. J. Glass

A. Glassman

G. A. Graves

P. M. Gruzensky

A. H. Guenther

S. K. Gulati

H

R, J. Harrach

J. A. Harrington

J. Hayden

K. Hebert

D. Heiman

R. Hellwarth

G. L. Hennessey

L. M. Hob rock

R. T. Holm

S. J. Holmes

A. Hordvik

R. A. House II

J. E. Howard

L. Huff

C. A. Huguley

G. J. Hutcheson

T. V. Hynes

IW. Icenogle

J

S. D. Jacobs

W. Q. Jeffers

R. J. Jelly

G. T. Johnston

K

M. A. Kahan

T. Kardos

C. A. Klein

P. H. Klein

A. Klugman

B . Koepke

T. Kohane

P. Kraatz

L

H. P. LeBodo

H. A. Lee

K. M. Leung

P. W. Levy

D. H. Liebenberg

R. M. F. Llnford

H. G. Lipson

J. S. Loomis

J. D. Lytle

Research Triangle Institute, P. 0, Box 12194, Research Triangle Park, NC 27709 Los Alamos Scientific Lab., MS/552, Los Alamos, NM 87544 Lawrence Livermore Lab., L-547, P. 0. Box 808, Livermore, CA 94505

Spectra Physics, 1250 W. Middlefield Rd., Mountain View, CA 94042

U. of Dayton Research Inst., Rm 162, 300 College Pk., Dayton, OH 45469 Electromagnetics Division, National Bureau of Standards, Boulder, CO 80302 Air Force Weapons Lab/CA, Kirtland AFB., NM 87117

U. of New Mexico, Dept. of Elec. Eng. & Computer Sci., Albuquerque, NM 87131

Lawrence Livermore Lab., L-71, P. 0. Box 808, Livermore, CA 94550

U. of Alabama in Huntsville, Physics Dept., Huntsville, AL 35807 Los Alamos Scientific Lab., MS-548, Los Alamos, NM 87545 Design Optics, 155 Moffett Park Dr., Sunnyvale, CA 94086

U. Southern California, Physics Dept., University Park, CA 90007

U. Southern California, Physics Dept., University Park, CA 90007 Valtec Corp., Optical Coatings Div., 36 Pope Rd., Holliston, MA 01746

Hughes Aircraft Co., Optics Dept., Centinela & Teale Sts., Culver City, CA 90230

Naval Research Lab., Code 5233, Washington, D. C. 20375 Northrop Research & Technology Center, 3401 W. Broadway, Hawthorne, CA 90250 Hanscom Air Force Base, RADC/ETSL, Bedford, MA 01731

Air Force Weapons Lab/LRE, Kirtland AFB, NM 87117

U. of Wisconsin, Nuclear Eng. Dept., Madison, WI 53706

U. of Dayton Res. Inst., 300 College Park Ave., Dayton, OH 45469 Air Force Weapons Lab/LRE, Kirtland AFB, NM 87111

U. S. Army Missile Command, DRSMI/QRT, Bldg. 4500, Redstone Arsenal, AL 35809

U. S. Army Materials & Mechanics Res. Ctr., Arsenal. St., Watertown, MA 02173

Spectra Physics, 1250 W. Middlefield Rd., Mountain View, CA 94042

U. of Rochester, Lab for Laser Energetics, Hopeman Hall, River Station,

Rochester, NY 14627 Helios, Inc., P. 0. Box 2190, Boulder, CO 80302 E-A Industrial Corp., 4500 No. Shallowford Rd., Chamblee, GA 30341

U. of Dayton Res. Inst., 300 College Park Ave., Dayton, OH 45469

ITEK Corp., Optical Systems Div., Dept. 1-550/MS/K-346, 10 Maguire Rd,

Lexington, MA 02173 Broomer Res. Lab., 23 Sheer Plaza, Plainview, NY 11803

Raytheon Res., 28 Seyon St., Waltham, MA 02154

Naval Research Lab., Code 5222, Washington, D. C. 20375 Northrop Research & Tech. Center, 374/61, 3401 W. Broadway, Hawthorne, CA 94304 Honeywell Corp. Res. Center, 10701 Lyndale Ave., So., Bloomington, MN 55420

Raytheon Res. Div., 28 Seyon St., Waltham, MA 02154

Northrop Res. & Tech. Center, 374/61, 3401 W. Broadway, Hawthorne, CA 90250

Laboratoire National d'Essais, Laser Div., 1 rue G-Boissler, Paris 75015, France

Owens-Illinois, Inc., Box 1035, Toledo, OH 43666

U. of So. California, Center for Laser Studies, 206 APL, University Park, Los Angeles, CA 90007 Brookhaven Nat. Lab., Physics Dept., 510-B, Upton, NY 11973

Los Alamos Scientific Lab., MS/530, P. 0. Box 1663, Los Alamos, NM 87545 McDonnell Douglas Corp., E-457, P. 0. Box 516, St. Louis, MO 63166 Hanscom Air Force Base, RADC/ESD, ETSS, Bedford, MA 01731

U. of Arizona, Optical Sciences Center, Tuscon, AZ 85721 Lockheed Res. Labs., Dept. 52-56, 3251 Hanover St., Palo Alto, CA 94304

-380-
M

R. W. MacPherson

S. Manlief

D. Milam

D. Morelli

D. L. Mullen

N

B . E . Newnam

D. B, Nichols

W. P. Norris

0

M. C. Ohmer

D. L. Olson

D. R. Olson

R. M. Ono

p

R. H. Picard

R. W. Pierce

P. Pirooz

B. Piatt

R. 0. Pohl

J, 0. Porteus

R

W, L. Ramer

S. Refermat

W. H. Reichelt

R. E. Rondeau

T. Roszhart

J. Rowe

A. M. Rubenstein

J. E. Rudisill

P. M. Rushworth

S

T. T. Salto

A. A. Sanders

R. E. Seddon

M. Shen

D. H. Sherman

A. R. Shimkunas

R. A. Skogman

R. E. Sladky

R. L. Smith

M. J. Soileau

M. Sparks

W. Spawr

J. L. Stanford

J. L. Stapp

J. Starling

T. G. Stoebe

R. Strandlund

C. L. Strecker

P. R. Stuart

0. F. Swenson

H. F. Swift

Defence Res. Estab. Valcartier, Electro-Optics Div. , P. 0. Box 880,

Courcellette, Quebec, GOA IRO

Mass. Inst, of Technology, Lincoln Lab., Box 73, Lexington, MA 02173 Lawrence Livermore Lab., P. 0. Box 808, Livermore, CA 94505

Optical Coating Lab., P. 0. Box 1599, Santa Rosa, CA 95403

U. of Dayton Res. Inst., Kirtland AFB, AFWL/LRE, Albuquerque, NM 87117

Los Alamos Scientific Lab., MS/552, P. 0. Box 1663, Los Alamos, NM 87545

Boeing Aerospace Co., MS/88-46, Box 3999, Seattle, WA 98055

Naval Weapons Center, Code-6056, China Lake, CA 93555

Wright-Patterson AFB, AF Materials Lab, Dayton, OH 45433

Colorado School of Mines, Dept. of Met. Eng., Hill Hall, Golden, CO 80421

Perkin-Elmer, MS/217, Main Avenue, Norwalk, CT 06856

Rocketdyne Div., Rocketwell Int., 521-197, 6633 Canoga Ave., Canoga Park, CA 91304

Rome Air Dev. Center, RADC/ETSL, MS/30, Hanscom AFB, Bedford, MA 01731

Naval Weapons Center, Code-4051, China Lake, CA 93555

Owens-Illinois Co., Corp. Dev. Dept., P. 0. Box 1035, Toledo, OH 43666

U. of Dayton Res. Inst., Kirtland AFB, AFWL/LRE, NM 87117 Cornell U., Physics Dept., Clark Hall, Ithaca, NY 14850

Naval Weapons Center, Code 6017, China Lake, CA 93555

Rockwell Int'l., Dev. & Proj . Eng., P. 0. Box 464, Golden, CO 80401

U, of Rochester, Lab. for Laser Energetics, Hopeman 110, River Campus,

Rochester, NY 14618

Los Alamos Scientific Lab., MS/532, Los Alamos, NM 87545

Wright-Patterson AFB, AF Materials Lab., AFML/LPJ, Dayton, OH 45433 TRW Systems, Inc., One Space Park, Redondo Beach, CA 90278

Northrop Res. Tech. Center, 3401 W. Broadway, Hawthorne, CA 90250

Inst, for Defense Analyses Res. Staff, Sci. & Tech. Div., 400 Army Navy Dr.,

Arlington, VA 22209 Hughes Res. Labs., 3011 Malibu Canyon Rd., Malibu, CA 90265

Martin-Marietta Corp-Orlando, MP/276, Box 5837, Orlando, FL 32808

Lawrence Livermore Lab., L-140, P. 0. Box 808, Livermore, CA 94550

Electromagnetics Division, National Burieau of Standards, Boulder, CO 80302

Optical Coating Lab., Inc., Technical Div., P. 0. Box 1599, Santa Rosa, CA 95401

U. of California, Chemical Eng., Berkeley, CA 94720

II-VI, Inc., 207 Garden Lane, Glenshaw, PA 15116

Optical Coating Lab., Inc., Research, P. 0. Box 1599, Santa Rosa, CA 95402

Honeywell Corp. Res. Ctr., 10701 Lyndale Ave. So., Bloomington, MN 55420

Union Carbide Corp., Y-12 Plant, Bldg. 9998, Oak Ridge, TN 37830

Electromagnetics Division, National Bureau of Standards, Boulder, CO 80302

Naval Weapons Center, Code 6018, China Lake, CA 93555

Xonlcs, Inc., 1333 Ocean Ave., Santa Monica, CA 90401

Spawr Optical Res., Inc., 1521 Pomona Rd., Corona, CA 91720

Naval Weapons Center, Code 6018, China Lake, CA 93555

U.S. Air Force, 2115 Carter Hill Rd., Mongtomery, Al 36617 Honeywell, Ceramics Ctr/V-1864, 1885 Douglas Dr., No., Golden Valley, NY 55422

U. of Washington, Dept. of Mining, Metallurgy & Ceramic Engr.,

Roberts Hall, FB-10, Seattle, WA 98195

Optical Coating Lab., Inc., P. 0. Box 1599, Santa Rosa, CA 95401

Air Force Materials Lab., WPAFB, LPO, Dayton, OH 54533

National Physical Lab., Dov. of Mechanical & Optical Metrology, Teddington,

Middlesex, England AF Materials Lab., AFML/LPO, WPAFB, Dayton, OH 45433

U. of Dayton Res. Inst., Dayton, OH 45469

-381-
T

C. Tang

P. A. Temple

H. L. Tuller

W

V. Wang

R. Watt

J. A. Weiss

C. B. Willlngham

R. Windebank

I. M. Winer

H. V. Wlnsor

R. M. Wolff

S. M. Wong

R. M. Wood

Y

S. Yee

Z

J. L. Zar

R. W. Zimmerer

D. Zuccaro

U. of So. California, APL 211, Ctr. for Laser Studies, University Park, Los Angeles, CA 90007 Naval Weapons Center, Code 6018, China Lake, CA 93555

Mass. Inst, of Technology, Dept. Materials Sci., Cambridge, MA 02139

Hughes Res. Lab., Opto Electronics Dept., 30-25, 3011 Malibu Canyon Rd.,

Malibu, CA 90265 L.A.S.L., P. 0. Box 1663, Los Alamos, NM 87545 Energy Res. & Development Admin., Div. of Laser Fusion, Washington, D. C. 20545 Raytheon Co., Research Div., 28 Seyon St., Waltham, MA 02154 Laser Optics, Inc., Mill Plain Rd., Danbury, CT 06810

Kirtland AFB, AFWL/ILS, Box 9316, Int'l. Airport, Albuquerque, NM 87117 DARPA/MSO, 1400 Wilson Blvd., Arlington, VA 22209 Valtec, Western Div., Box 2311, Newport Beach, CA 92663

Rockwell Int'l., Atomic Energy Div., Rocky Flat Plant, Bldg. 779, P. 0. Box 464,

Golden, CO 80401

Hirst Res. Centre, General Electric Co., Ltd., East Land, Wembley, Middlesex,

England

U. of Washington, Elec. Engr., Dept. FT- 10, Seattle, WA 98105

AVCO Everett Research Lab., Inc., Everett, MA 01845

Scientech, Inc., 5649 Arapahoe Ave., Boulder, CO 80303

Hughes Res. Labs., 3011 Malibu Canyon Rd., Malibu, CA 90265

-382-
NBi-1 14A (REV. 7-73)

U.S. DEPT. OF COMM. PUBLICATION OR REPORT NO. 2. Gov't Accession BIBLIOGRAPHIC DATA No. SHEET Special Publication 462

3. Recipient's Accession No.

4. TITLE AND SUBTITLE

Laser Induced Damage in Optical Materials: 1976

5. Publication Date December 1976

6. Performing Organization Code

7. AUTHOR(S)

Edited by Alexander J. Glass and Arthur H. Guenther

8. Performing Organ. Report No.

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

10. Project/Task/Work Unit No.

11. Contract/Grant No.

12. Sponsoring Organization Name and Complete Address (Street, City, State, ZIP)

electromagnetics Division, National Bureau of Standards, Boulder, CO

toerican Society for Testing & Materials, Phila., PA 19103

)ept. of the Navy, Office of Naval Research, Arlington, VA 22217

Snergy Research & Development Admin., Washington, D. C. 20234

)efense Advanced Research Project Agency, Washington, D. C. 20234

13. Type of Report & Period

14. Sponsoring Agency Code

15. SUPPLEMENTARY NOTES

Library of Congress Card Catalog Number: 76-600074

16. ABSTRACT (A 200-word or less factual summary ol most si^ificant information. If document includes a significant

bibliography or literature survey, mention it here.) The Eighth Annual Sympositim On Optical Materials

or High Power Lasers, the Boulder Damage Symposium, was hosted by the National Bureau of

Itandards in Boulder, Colorado, from July 13th to 15th. The Symposium was held under the

luspices of ASTM Committee F-1, Subcommittee on Laser Standards, with the joint sponsorsh|Lp

)f NBS, the Defense Advanced Research Project Agency, the Energy Research and Development

Administration, and the Office of Naval Research.

About 160 scientists attended the Symposium, including representatives of the United

kingdom, France, Canada, and Brazil. The Symposium was divided into five half-day session

:oncerning Bulk Material Properties and Thermal Behavior, Mirrors and Surfaces, Thin Film

Properties, Thin Film Damage, and Scaling Laws and Fundamental Mechanisms.

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

:owards new frontiers and new developments. Particular emphasis was given to new material

:or use at 10.6 ym in mirror substrates, windows, and coatings. New techniques in film

deposition, and advanced in diamond turning of optics were described. The scaling of dama

>

:hresholds with pulse duration, focal area, and wavelength are 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 Ninth Annual SymposiuiL

:s scheduled for October 4-6, 1977, at the National Bureau of Standards, Boulder, Colorado

17. KEY lyORDS (six to twelve entries; alphabetical order; capitalize only the first letter of the first key word unless a proper name; separated by semicolons)

Avalanche ionization; infrared windows; laser damage; metal mirrors; optical

fabrication; optical materials; thin films.

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environment.

• Color plans

for the home.

• Using color to dramatize or to hide. • Color and illumination. • Experimenting with color. This new basic guide can

serve as your handbook in helping you make decisions

about how to use color in your

life and make it work for you. Order Color in Our Daily Lives

prepaid for $1.70 from the

Superintendent of Documents,

U.S. Government Printing

Office, Washington, D.C. 20402.

Use SD Catalog No. C1 3.53:6.
NBS TECHNICAL PUBLICATIONS

PERIODICALS JOURNAL OF RESEARCH reports National Bureau

of Standards research and development in physics,

mathematics, and chemistry. It is published in two

sections, available separately:

• Physics and Chemistry (Section A)

Papers of interest primarily to scientists working in these fields. This section covers a broad range of physical and chemical research, with major emphasis on

standards of physical measurement, fundamental constants, and properties of matter. Issued six times a year.

A.nnual subscription: Domestic, $17.00; Foreign, $21.25.

• Mathematical Sciences (Section B)

Studies and compilations designed mainly for the mathematician and theoretical physicist. Topics in mathematical statistics, theory of experiment design, numerical

analysis, theoretical physics and chemistry, logical design and programming of computers and computer systems. Short numerical tables. Issued quarterly. Annual

subscription: Domestic, $9.00; Foreign, $11.25.

DIMENSIONS/NBS (formerly Technical News Bulletin) —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 performance.

In addition, it reports the results of Bureau programs

in measurement standards and techniques, properties of matter and materials, engineering standards and services, instrumentation, and automatic data processing.

Annual subscription: Domestic, $9.45; Foreign, $11.85.

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 coordinated 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 (JPCRD) 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 infor- mation 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 proce- dures 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 rec- ognized 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. 20Jt02. Order following NBS publications—NBSIR's and FIPS

from the National Technical Information Services,

Spyingfield, Va. 22161. Federal Information Processing Standards Publications

(FIPS PUBS)—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 stand- ards 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

bibliographies are issued periodically by the Bureau:

Cryogenic Data Center Current Awareness Service. A

literature survey issued biweekly. Annual subscription: Domestic, $20.00; Foreign, $25.00.

Liquified Natural Gas. A literature survey issued quarterly. Annual subscription: $20.00.

Superconducting Devices and Materials. A literature survey issued quarterly. Annual subscription: $20.00.

Send subscription orders and remittances for the pre- ceding bibliographic services to National Bureau of Standards, Cryogenic Data Center (275.02) Boulder,

Colorado 80302.
U.S. DEPARTMENT OF COMMERCE

IMational Bureau of Standards

Washington, D.C. 20234

OFFICIAL BUSINESS POSTAGE AND FEES PAID

U.S. DEPARTMENT OF COMMERCE COM-215

Penalty for Private Use, $300