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NBS SPECIAL PUBLICATION 568

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

National Bureau of Standards

Library, £-01 Admin. Bldg.

OCT 1 1981

19108*

Laser Induced Damage

in Optical Materials:

1979

7

BOULDER DAMAGE SYMPOSIUM

STP 726

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NATIONAL BUREAU OF STANDARDS

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National Bureau of Standards Laser Induced Damage u ^"

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In Optical Materials: 1979

Proceedings of a Symposium Sponsored by:

National Bureau of Standards

American Society for Testing and Materials

Office of Naval Research

Department of Energy

Defense Advanced Research Project Agency

October 30-31, 1979

NBS, Boulder, Colorado 80303

Edited by:

Harold E. Bennett

Naval Weapons Center

China Lake, California 93555

Alexander J. Glass

Lawrence Livermore Laboratories

Livermore, California 94550

Arthur H. Guenther

Air Force Weapons Laboratory

Kirtland Air Force Base, New Mexico 971 17

Brian E. Newnam

Los Alamos Scientific Laboratory

Los Alamos, New Mexico 87545

BOULDER DAMAGE SYMPOSIUM

U.S. DEPARTMENT OF COMMERCE, Philip M. Klutznick, Secretary

Luther H. Hodges, Jr., Deputy Secretary

Jordan J. Baruch, Assistant Secretary for Productivity, Technology and Innovation

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director

Issued July 1980
Library of Congress Catalog Card Number: 80-600100

National Bureau of Standards Special Publication 568

Nat. Bur. Stand. (U.S.), Spec. Publ. 568, 530 pages (July 1980) CODEN: XNBSAV

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WASHINGTON: 1980

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

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20402
Foreword

The Proceedings contain the papers presented at the Tenth Anniversary Symposium on Optical Materials for High Power Lasers held at the National Bureau of Standards in Boulder,

Colorado, on October 30-31, 1979. 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 Department of Energy. The

Symposium was attended by about 150 scientists from the United States, the United Kingdom,

France, Canada, West Germany and Denmark. It was divided into sessions devoted to the

following topics: Transparent Optical Materials and the Measurement of Their Properties,

Mirrors and Surfaces, Thin Film Characteristics, Thin Film Damage, Considerations for High Power Systems, and finally Theory and Breakdown. The Symposium Co-Chairmen were Dr. Harold

E. Bennett of the Naval Weapons Center, Dr. Alexander J. Glass of the Lawrence Livermore

Laboratory, Dr. Arthur H. Guenther of the Air Force Weapons Laboratory, and Dr. Brian E. Newnam of the Los Alamos Scientific Laboratory. They 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 Twelfth

Annual Symposium on this topic will be held in Boulder, Colorado, from September 30 to October 1, 1980. A concerted effort will be made to ensure closer liaison between the

practitioners of high peak power and the high average power community.

The principal topics to be considered as contributed papers in 1980 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 to 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 and repetitively pulsed damage problems is anticipated. Fabrication and test procedures will continue to advance, particularly in the diamond-turned 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, and particularly relative to our plans for the Twelfth

Annual Symposium.

H.E. Bennett, A.J. Glass,

A.H. Guenther and B.E. Newnam

Co-Chairmen

<

iii
DISCLAIMER

Certain papers contributed to this publication have been prepared by other than NBS authors. These papers have not been reviewed or edited by NBS; therefore, the National Bureau of Standards accepts no responsibility for comments or recommendations contained

therei n.

Certain commercial equipment, instruments, and materials are identified in this publication in order to explain the experimental procedure adequately. Such identification in no way implies approval, recommendation, or endorsement by the National Bureau of Standards,

nor does it imply that the equipment, instruments, or materials identified are necessarily

the best available for the purpose.
CONTENTS

Page

Foreword iii

H. E. Bennett, A. J. Glass, A. H. Guenther, and B. E. Newnam

Symposium Welcome v A. H. Guenther

Remarks to Eleventh Annual Symposium Optical Materials for High Power Lasers viii

H. V. Winsor

Introduction 1 H. E. Bennett, A. J. Glass, A. H. Guenther and B. E. Newnam

Principal Conclusions 2

Materials and Measurements 3

Mirrors and Surfaces 4

Systems 5

Thin Film Characterization and Design 5

Thin Film Damage 6

Summary of Papers 8

Transparent Optical Materials and the Measurement of their Properties 8

Mirrors and Surfaces 14

Thin Film Characteristics 18

Thin Film Damage 24

Considerations for High Power Systems 27

Theory and Breakdown 29

Recommendations 32

Materials and Measurements

Press Forging and Optical Properties of Lithium Fluoride 39

J. F. Ready and H. Vora

Optical Properties of Forged CaF

? 47

A. K. Hopkins, R. H. Anderson; J. F. Ready, J. M. Bennett,

P. C. Archibald and D. K. Burge

Mechanical and Optical Properties of Forged NaCl 65

G. A. Graves, J. A. Detrio, D. McCullum and D. A. Dempsey

Optical, Thermal, and Mechanical Measurements on C0

? Laser-Irradiated ZnSe 73

J. A. Detrio, J. A. Fox and J. H. O'Hare

Electronic, Nuclear and Total Nonlinear Indices of Liquids 91

D. C. Brown, J. M. Rinefierd, S. D. Jacobs and J. A. Abate

V
Wavelength Modulation Spectroscopy of Highly Transparent Solids 99

R. Braunstein, R. K. Kim and M. Braunstein

Synchrotron Radiation Studies of Beryllium Fluoride Glass , 119

R. T. Williams, D. J. Nagel , P. H. Klein and M. J. Weber

The Role of Fe in Laser-Induced Damage in Ultrapure KBr 125

D. F. Edwards, B. E. Newnam and W. J. Fredericks

Properties and Fabrication of Crystalline Fluoride Materials for High Power Laser Appl ications 127

T. M. Pollak, R. C. Folweiler, E. P. Chicklis, J. W. Baer,

A. Linz and D. Gabbe

Laser Damage in Yttrium Orthophosphate 137

R. Allen, L. Esterowitz, P. H. Klein, V. 0. Nicolai, and W. K. Zwicker

Compressive Failure in Sapphire under C0

? Laser Heating 141

P. A. Miles, J. Gallagher and R. L. GSntilman

Failure Criteria for Laser Window Materials 151

J. A. Detrio, G. A. Graves and J. M. Wimmer

Surface and Mirrors

Thermomechanical Stress Degradation of Metal Mirror Surfaces under Pulsed Laser Irradiation 159

H. M. Musal, Jr.

Defect-Damage-Resitant Copper Mirrors 175

J. 0. Porteus, D. L. Decker, D. J. Grandjean, S. C. Seitel and W. N. Faith

Laser-Damage Resistant Copper Surfaces with High Reflectivity after Oxidation 187

M. Oron, L. G. Svendsen and G. Sorensen

Laser-Induced Breakdown of Diamond-Machined Window Surfaces 195

M. J. Soileau, J. 0. Porteus and D. L. Decker

Optical and Surface Physical Characteristics of Diamond-Machined Infrared

Window Materials 199

D. L. Decker, D. J. Grandjean, J. M. Bennett

Short-Pulse C0

? -Laser Damage Studies of NaCl and KC1 Windows 209

B. E. Newnam, A. V. Nowak and D. H. Gill

COp-Laser Polishing of Fused Silica Surfaces for Increased Laser Damage

Resistance at 1.06 ym 229

P. A. Temple, D. Milam and W. H. Lowdermilk

Thin Film Characterization

The Relative Importance of Interface and Volume Absorption by Water in Evaporated Films 237

T. M. Donovan, P. A. Temple, Shiu-Chin Wu, and T. A. Tombrello

Characterization of Small Absorptions in Optical Coatings 247

W. N. Hansen, L. Pearson, G. Hansen, and W. J. Anderson

Surface Microanalysis Techniques for Characterization of Thin Films 257

T. W. Humpherys, R. L. Lusk and K. C. Jungling

vi
Pyroelectric Measurement of Absorption in Oxide Layers and Correlation

to Damage Threshold 269

H. Kiister and J. Ebert

High-Temperature Optical Characterization of Thin Film Reflector and

Absorber Layers 281

M. R. Jacobson and R. D. Lamoreaux

CVD Molybdenum Thin Films for High Power Laser Mirrors 287

G. E. Carver and B. 0. Seraphin

Photoacoustic Experimental Studies on an AR Coated Laser Window and Some

Related Theoretical Calculations 293

N. C. Fernelius

Calculations Using a Two-Layer Rosencwaig-Gersho Photoacoustic Spectroscopy

Theory Applied to an Anti-Reflective Coated Laser Window 301

N. C. Fernelius

Pre-Pulse Identification of Localized Laser Damage Sites in Thin Films

Using Photoacoustic Spectroscopy 313

R. P. Freese and K. J. Teegarden

Examination of Laser Damage Sites of Transparent Surfaces and Films Using

Total Internal Reflection Microscopy 333

P. A. Temple

Thin Film Damage

Parameters Affecting Damage Threshold in Thin Film Infrared Chalcogenide

Glass for Applications in Laser Optical Switching Systems 343

W. E. Kienzle and N. I. Marzwell

Improvements in Clarity and Environmental Stability of TLI-Containing

Anti reflective Coatings 355

W. T. Boord, P. P. Chow, W. B. Harrison and J. E. Starling

Reactively Sputtered Optical Coatings for Use at 1064 nm 359

W. T. Pawlewicz, R. Busch, D. D. Hays, P. M. Martin and N. Laegreid

Recent Damage Results on Sil ica/Litania Reflectors at 1 Micron 377

C. K. Carniglia, J. H. Apfel , T. H. Allen, T. A. Tuttle,

W. H. Lowdermilk, D. Milam and F. Rainer

Damage to Coatings and Surfaces by 1.06 ym Pulses 391

W. H. Lowdermilk, D. Milam and F. Rainer

Pulsed Damage Thresholds of Fluoride and Oxide Thin Films from 0.26 ym to

1 . 06 ym 405

T. W. Walker, A. H. Guenther, C. G. Fry and P. Nielson

Laser-Induced Damage Measurements with 266-nm Pulses 417

T. F. Deaton and W. L. Smith

Systems

High Energy Laser Optical Train Performance: A Reassessment 425

C. A. Klein

Analysis of an Imperfectly Coated Conical Element for High Energy

Laser Resonators 439

W. P. Latham, Jr.

vii
Theory and Breakdown

Nonlinear Absorption in Direct-Gap Crystals 445

A. Vaidyanathan, T. W. Walker, A. H. Guenther, S. S. Mitra, L. M. Narducci

Competing Mechanisms in Laser-Induced Damage 457

A. Vaidyanathan, T. W. Walker, and A. H. Guenther

Theory of Electron-Avalanche Breakdown in Solids 467

M. Sparks, T. Holstein, R. Warren, D. L. Mills,

A. A. Maradudin, L. J. Sham, E. Loh, Jr., and F. King

Impurity Breakdown Model in Thin Films 479

T. W. Walker, A. Vaidyanathan, A. H. Guenther and P. Nielsen

Frequency and Focal Volume Dependence of Laser- Induced Breakdown in Wide Band Gap Insulators 497

M. J. Soileau, M. Bass, and P. H. Klein

SBS Influence on Laser Damage of Optical Materials 519

P. Balkevicius, E. Kosenko, J. Lukosius, E. Maldutis

Computer Simulation of Damage Morphology 529

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

APPENDIX 1. Participants 531

viii
SYMPOSIUM WELCOME

Arthur H. Guenther

Air Force Weapons Laboratory

Kirtland AFB, New Mexico 87117

Last year at the Tenth Annual Boulder Damage Symposium, Alex Glass expressed his

hope that it might be the last. We made a vain attempt to call a close to these annual

gatherings because of two reasons. First, we thought that after ten years we should

have been able to solve the problem of laser damage and move on to new challenges,

but as we enter our second decade the continued attendance and interest accorded

these annual sojourns to Boulder indicate that laser damage is still a subject of import

to a large, varied and growing community. The second and perhaps subconsciously greater

reason for our predisposition to suggest a cessation was a result of the pressures of

other commitments for both Alex and myself. However, since you the participants would

have no talk of concluding this series we did the next best thing, which some of you

may have noticed from the published program, that is, enlisting the services and

aid of two additional individuals well known for their many contributions in the

laser damage area. They are from the two largest concerned organization in this technology, namely DOD and DOE, whose interests span the field from high average power,

generally CW, to high peak power, presently single pulse interaction regimes. Undoubtedly

somewhere in the future these diverse extremes will meet in the shorter wavelength, high

power, rep rate laser arena. These individuals, two gluttons for punishment, I mean

pillars of perseverance, are old timer Hal Bennett from the Michelson Laboratory at the

Naval Weapons Center and Brian Newnam, one of a more recent vintage, young enough to have accomplished his doctoral research in laser damage, from the Los Alamos Scientific

Laboratory. Hopefully, this selection suggests a certain objectivity on Alex's and my

part, since we asked individuals from the enemy's camp--I mean sister laboratories.

Both being government organizations, we hope to be able to rely upon a certain donation

of necessary resources, primarily in the secretarial and communication areas which are

essential in the operation of any meeting such as this one.

This now enlarged steering committee would like to formally welcome you to the Tenth

Anniversary Damage Symposium. We are pleased to see many familiar faces, and again

would like to specifically welcome those of you who are new to these deliberations.

We would especially like to welcome our foreign participants who have through their

attendance added a truly international flavor to this meeting. However, we are disappointed that other anticipated contributors from abroad could not come and present

their latest work. However, it is anticipated that in some cases their manuscripts

will be included in the conference proceedings.

I would be remiss if I did not mention that due to a reorganization here at the

National Bureau of Standards we have a new team responsible for the sponsorship and

operation of the meeting proper as well as publication of the proceedings. Bob Kamper,

Chief, Electromagnetic Technology Division, has graciously offered the services of Aaron Sanders, Group Leader, Optical Electronic Metrology Group, and his able

assistant, Mrs. Norma Lear, two individuals of long standing interest and association

with this meeting, to coordinate the Bureau's involvement. We welcome the opportunity

to work with two such cooperative and capable individuals, as we say farewell and

many thanks to Hal Boyne and his staff who so admirably aided us in the past.

Concerning this year's meeting, you will note from the program that an acceptable

basis for observed scaling relations of laser damage thresholds with a variety of

parameters may be just around the corner. I believe this results from several factors.

In the first place, this year there are many more than the usual number of papers

which can be characterized as theoretical, and, in the main, they are supported by

carefully obtained experimental data. In the second place, these more analytical reports

with their adjunct data are taken on real world samples, and thus are of immediate

interest to anyone interested in the response of optical elements in practical high

i x
power laser environments. Finally, the samples utilized in these tests (with the

aid and input of material scientists) have been characterized better and more

completely. In the future, we expect that the real world response will be unraveled

even further as use of sophisticated and sensitive characterization methods become more generally available. However, any quantum jumps in damage resistance I feel will come from improvements in materials purity and more importantly in fabrication and

deposition procedures, such as in the areas of polishing, the introduction of chemical

vapor deposited materials and proper background gases in the deposition of thin films

or through continued progress in the area of forging of large size optical elements.

Certainly any improvements in damage threshold arising from developments in these areas

can only be realized in real applications through concurrent advancements in avoiding

environmental degradations introduced in the utilization of these elements in the

comparatively adverse environments associated with operating systems. On the other

hand, quantum jumps in our understanding of the damage mechanism will undoubtedly

come in part from work presently being performed at shorter wavelengths, with the

impact of rep-rated studies still largely in the future.

Thus we see the narrowing of our more fundamental interests to the arena of material

response and scaling primarily in those domains where the elements will be used and not necessarily as keys to delineating the intrinsic response of pure dielectric or metallic

elements to high power coherent optical radiation. The intrinsic behavior of the interaction scenario will have to wait, if it ever comes, as one may never be able to convince

everyone that they have intrinisc breakdown data upon which to benchmark their theory,

and thus I tend to agree with Prof. Manenkov that there may never have been intrinsic

material breakdown and that all threshold data is impurity or imperfection dominated.

However, there is still plenty of good physics and chemistry in the laser damage field,

as we will see during this symposium.

We would like to second John Detrio's invitation to the participants of this

meeting to aid in the establishment of a separate ASTM Committee on Lasers and Optics.

After all, this meeting and the reason for our being here was initiated and supplied

by the ASTM over a decade ago.

Recalling those early stages of the ASTM Committee's activities in this symposium,

it is with much sorrow and regret that we note the passing of one of the early mainstays

of the ASTM's activities and this conference. He was a man dedicated to the highest

professional and individual qualities, and as such has set a high standard for us to emulate, Allan Carlson, President of Cleveland Crystal, passed from the scene

this August. Allan was one of the earliest members of the ASTM Subcommittee on Laser Standards and chaired the group on crystals. He participated in this meeting

many times and contributed significantly to the development of crystalline materials

for large aperture, high power lasers. Both his personal qualities and his technical

contributions will be sorely missed.

In closing, I would like to acknowledge the continuing financial support of the

Defense Advanced Research Projects Agency, the Office of Naval Research, the National Bureau of Standards, and the Department of Energy, and particularly to John Detrio and

the American Society of Test and Materials for professional sponsorship. I am sure

I speak for Drs . Bennett, Glass and Newnam in wishing you a warm welcome to this our Tenth Anniversary Damage Symposium.
BOULDER DAMAGE SYMPOSIUM

In commemoration of our tenth anniversary meeting and in expectation of its

continuance, the organizers of this meeting decided that it would be appropriate to establish a logo which could be used to highlight the symposium and its future

correspondence and publications. Thus a contest was held during this meeting, with

the ground rules that the logo must be simple, and easily recognizable as representing

the subject of the symposium. After a difficult time of evaluating over 30 entries, the

organizers of the conference selected the logo you see decorating this our tenth

anniversary proceedings. It was submitted by Major J. R. Bettis (USAF) of the US Naval

Academy. Jerry is well known to the participants of this conference for his many

technical presentations and stimulating demeanor. He now has made a contribution of

another sort, one we are sure is as important and lasting.

xi
it
REMARKS TO ELEVENTH ANNUAL SYMPOSIUM

OPTICAL MATERIALS FOR HIGH POWER LASERS (SOMHPL)

Major Harry V. Winsor, USAF

Defense Advanced Research Projects Agency, Materials Sciences office

1400 Wilson Boulevard

Arlington, Virginia 22209

Welcome to this year's "Boulder Damage Symposium" (BDS). Our topic

of laser damage to mirrors and window materials has expanded until it now encompasses the making and using of all sorts of optical components

for high power (mostly pulsed) laser systems. Somehow, however, SOMHPL

is much harder to say than BDS!

DARPA remains interested in the progress shown at this meeting as well as at closed topical meetings and it is my pleasure to give a summary of the reasons for our continued interest.

Laser systems for space defense and various other military applications

have been one of DARPA' s primary thrusts for more than 10 years. The success of these programs depends on having suitable components to handle and direct the laser beam so that it performs its intended

function. Most high power laser components require thin film coatings

for reflection control, so a large amount of our research has been

devoted to achieving improved thin film behavior. Several specific thin

film efforts are worthy of note here:

Hydrogen Fluoride lasers emit light at around 2.8 um, which lies in

the center of the hydrogen band for most materials containing hydrogen.

Removing all hydrogen from a thin film requires exacting care and specific measures during film deposition. This is because the tolerable amount

of hydrogen may go below the equivalent of two monolayers of water!

This must be achieved with components which can be handled in a real

world containing humidity, fuel vapors, and other hydrocarbon vapors.

If we go into space, we must add molecular and atomic hydrogen bombardment. Recent thin film research reported at this meeting and elsewhere

promises to make substantial improvements in space tolerant films for HF

lasers

Films are important to UV lasers as well. Here, since no CW UV laser of high power has been built, the research problems center about

how one develops the ability to withstand repetitive pulses of laser

energy as high as 10 Joules/cm^/pulse . While the pheonomena of single

pulse damage have been investigated for many years, almost no work has

been done on the durability of repetitive pulsed coatings. I suspect

there are many surprises in store for thin film designers when this work

is performed - in fact several phenomena have been observed at the

present state of our researches:

One appears to be thermal fatigue of the mirror's metal substrate under

a loading less than 10% of the single shot damage threshold. Nominally

identical sample, with somewhat higher absorption, did not exhibit this

effect. It would appear that absorption measurements must be refined to measure either transient (bleachable) absorption, or the location of

absorption within the film to give us diagnostics suitable for making

orderly progress in overcoming these effects.

xiii
The other is that films on metal substrates may exhibit two thresholds: one at initiation of damage and another for propagation of the damage.

With large components, it is possible that isolated sites may exhibit

initiation at fluxes well below a more uniform propagation threshold,

allowing the component to be used with isolated damage sites without

further concern. Under other conditions, the propagation threshold may

be well below the desired operational loading, necessitating perfect

control of the initiation threshold. The field of high power thin films

is now mature enough that these distinctions must be taken into account

in most further research and development.

With these problems, and the desire to exploit IR, UV and Visible

lasers for military and civilian applications, a new era of more basic

research appears to be upon us. DARPA will be steering its research

toward these more fundamental topics as soon as the rather more applied

tasks have been accomplished.

Passing on to the topic of mirrors, it is apparent that the problem

of selecting the correct substrate material has not yet been reduced to

a simple procedure, and it may never be. The heat exchange function

must not set requirements that affect the optical requirements the

mirror must meet. This is ever harder to accomplish as the system

designers seek to raise the power density in their beam trains. New methods of removing heat rapidly and requiring lower coolant pump power

are needed. Lower pumping pressures are especially sought, since the

current generation of mirrors must be polished under pressure - not an easy operation. The next generation may have to be polished under flow

conditions - an even harder proposition - unless better cooling methods

are devised.

Another example of the incompatibilities encountered in mirrors is the choice of thermal conductivity for the substrate. Higher thermal

conductivity is a virtue, as long as the thermal expansion and elastic

modulus are not compensatingly worse. The substrate however will experience

no thermal distortion if its thermal expansion coefficient is zero. This is obtainable - but it requires a much lower thermal conductivity,

which lowers the pulse damage threshold of the thin film, and consequently

the fatigue limit power threshold.

Still another example of conflicting requirements that do not appear until the component is assembled and characterized is due to the

metallurgical condition of the substrate. Molybdenum suitable for high

performance heat exchangers does not appear to be polishable to UV tolerances. The problems of making mirrors is becoming a multidiscipiinary

problem of first order - a fact that was remarked upon last year, but is now a far more immediate concern.

Windows have not received much attention at DARPA over the last

year, but may still be required for many proposed laser systems. Developments

that may help when windows are addressed again include the successful

forging of IR lenses to figure from KBr blanks. This technology appears

exciting for its promise of reducing polishing lead time significantly.

Current trials show that every lens produced meets IR specs, and it appears that control to visible tolerances can be obtained with several

years more research.

xiv
Fiber optics and integrated optics may well need to borrow from the

many technologies, tricks and theories developed under laser damage and

power handling component research, since power densities in optical

fibers are approaching component damage levels. Specific near term problems will concern the insertion of maximum power at 1.45 um into

silicon waveguides. These problems arise when trying to use the fibers

for great distances without introducing repeaters.

In all the above efforts, and in general, it is apparent that the cost of the optics must be reduced, and that the reliability and ability

to design to desired performance must increase. This is the task ahead

of us over the next several years as this field matures. I know that

this conference will continue to play an important role in improving

optical components technology and I hope you enjoy it as much as I will!

XV

Laser Induced Damage in Optical Materials

Tenth Anniversay ASTM Symposium

October 30-31, 1979

Abstract

The Tenth Anniversary Symposium on Optical Materials for High Power Lasers

(Boulder Damage Symposium) was held at the National Bureau of Standards in Boulder, Colorado, 30-31 October 1979. The Symposium was held under the

auspices of ASTM Committee F-l , Subcommittee on Laser Standards, with the

joint sponsorship of NBS, the Defense Advanced Research Project Agency, the

Department of Energy, and the Office of Naval Research. About 150 scientists attended the Symposium, including representatives of the United

Kingdom, France, Canada, Japan, West Germany and Denmark. The Symposium was divided into sessions concerning Transparent Optical Materials and the

Measurement of Their Properties, Mirrors and Surfaces, Thin Film Characteristics, Thin Film Damage, Considerations for High Power Systems, and finally

Theory and Breakdown. As in previous years, the emphasis of the papers

presented at the Symposium was directed toward new frontiers and new developments. Particular emphasis was given to materials for high power

apparatus. The wavelength range of prime interest was from 10.6 ym to the uv region. Highlights included surface characterization, thin film sub- strate boundaries, and advances in fundamental laser-matter threshold

interactions and mechanisms. The scaling of damage thresholds with pulse

duration, focal area, and wavelength was discussed in detail. Harold E. Bennett of the Naval Weapons Center, Alexander J. Glass of the Lawrence

Livermore Laboratory, Arthur H. Guenther of the Air Force Weapons Laboratory, and Brian E. Newnam of the Los Alamos Scientific Laboratory were co- chairmen of the Symposium. The Twelfth Annual Symposium is scheduled for

30 September - 1 October 1980 at the National Bureau of Standards, Boulder,

Colorado.

Key Words: Laser damage; laser interaction; optical components; optical

fabrication; optical materials and properties; thin film

coatings.

1. Introduction

The Tenth Anniversary Symposium on Optical Materials for High Power Lasers (Boulder

Damage Symposium) was held, as in previous years, at the National Bureau of Standards in Boulder, Colorado, 30-31 October 1979. The Symposium was held under the auspices of the

ASTM Committee F-l, Subcommittee on Laser Standards, with the joint sponsorship of NBS, the

Defense Advanced Research Projects Agency, the Department of Energy, and the Office of

Naval Research. Working sessions of the Committee F-l Subcommittee on Lasers were held on Monday, 29 October. About 150 scientists attended the Symposium, including representatives

of the United Kingdom, France, Canada, Japan, West Germany and Denmark. The Symposium was divided into sessions concerning Transparent Optical Materials and the Measurement of Their

Properties, Mirrors and Surfaces, Thin Film Characteristics, Thin Film Damage, Considerations for High Power Systems, and finally Theory and Breakdown. As in previous years,

several poster sessions were held. The general consensus of those presenting poster papers

and those viewing them was highly favorable. In all, over forty technical presentations
were made. Harold E. Bennett of the Naval Weapons Center, Alexander J. Glass of the Law- rence Livermore Laboratory, Arthur H. Guenther of the Air Force Weapons Laboratory, and

Brian E. Newnam of the Los Alamos Scientific Laboratory, were co-chairmen of the Symposium.

At this, our Tenth Anniversary Symposium, a logo was selected to highlight this series of

meetings as we embark on our second decade of damage studies. The purpose of these symposia is to exchange information about optical materials for high power lasers. The authors

will welcome comments and criticism from all interested readers relevant to this purpose

and particularly relative to our plans for the Twelfth Annual Symposium, scheduled for

30 September - 1 October 1980, at the National Bureau of Standards, Boulder, Colorado.

2. Principal Conclusions

The field of laser-induced damage in optical material continues to attract considerable interest. The specific focus of the field, however, changes as problems are solved

and new applications emerge. It is obvious after a decade of research in the response of

optical materials to high power laser radiation and the impact it has had on laser system

design that a reassessment needs to be made. In simpler terms, somewhat simple problems

have been overcome primarily through the applications of obvious solutions such as highpurity starting materials, high-precision fabrication and the identification of damage

initiators through the use of existing instrumental methods. However, now that the cream

has been skimmed, we are left with the requirement to go back to first principles and make

long-range plans to allow one to optimize optical material and system performance. As an indication of this philosophy, Major Harry Windsor, Defense Advanced Research Projects

Agency, indicated that that agency was encouraging more fundamental work on material and

material/optical interactions than formerly was the case. Studies will undoubtedly become more experimental in attempting to identify salient properties and features of the fundamental laser material interaction scenario and thus less applied. This is not to mean that

applied technology will not continue but the emphasis will most certainly change. It is anticipated that the directly applied work will concentrate on real world components as opposed to studying intrinsic damage mechanisms in an attempt to reduce costs and improve

component reliability. As far as the U.S. Government is concerned, increased emphasis will

be given to shorter wavelength systems, particularly those which can be repetitively pulsed,

an area in which there is scant data to date. There is no question that as one approaches

the uv, novel polishing procedures, more stringent coating tolerances, etc., will be

required. At the Symposium, the presented papers concentrated on materials and measurement of

their properties, mirrors and optical surfaces, thin films and damage sensitivity, systems

considerations and, as usual, theory and breakdown phenomena. As expected, the majority of

papers again concerned the Achilles heel of thin films. It was there, together with new revealing theoretical descriptions of the interaction process, that the greatest advances were reported.
2.1 Materials and Measurements

In the area of materials and measurements, continuing attention is being given to the area of press forging as a technique for the low cost manufacture of large, low-cost trans- parent elements. The use of hot isostatic pressing (HIP) techniques is proving successful

in removing veils within the material. The optical performance of forged calcium fluoride

for infrared windows was not markedly improved by the HIP process, but elimination of veils

led to potential improvement in the short-pulse damage threshold over windows which contain

veils. Press forging of lithium fluoride and calcium fluoride has shown increased resistance to humidity and, in the case of lithium fluoride, the highest surface damage threshold of any ultraviolet crystal studied to date. In an additional study, two-stage forging

of lithium fluoride was found necessary to avoid veiling in that material. This process

increased the micro yield strength by a factor of 10 while producing little, if any, in- crease in scattering.

Several new candidate laser materials were discussed including yttrium orthophosphate

(YPO^) proposed as a substitute for calcite. Beryllium fluoride (QeF^) glasses as a low

index but hygroscopic material was proposed as a host for laser glasses. Indications from

reflectivity measurements were that its band gap is similar to that of lithium fluoride.

The lowest energy reflectance peak was located at 12.8 eV. This material should be usable

to 150 nm. Other materials discussed included YLF and KYF which have shown potential to be grown to large-diameter crystals. Continued interest in chalcogenide glasses was also

evident. In an interesting study it was noticed there was an increase in absorption measured by a reflectivity change just prior to catastrophic damage which could serve as a nondestructive prethreshold test for these materials.

Two papers concerned themselves with the performance of windows at 10.6 urn. In one

case, absorbed C0

2 laser radiation in a ZnSe window produced the expected thermal lensing;

however, the minimum focal spot size was not changed over that manifest under low-power

conditions. The other work of note concerned damage in alkali halides. It was revealed

that the electric field at which damage occurred on the surfaces of state-of-the-art crystals of NaCl and KC1 was only one-half of the bulk damage value. There was considerable

agreement with previously reported Russian work that adsorbed water is the main initiator

in the surface damage process at 10.6 ym. Freshly cleaved NaCl samples exhibited an initial

damage threshold more than double that of polished surfaces. However, after less than one

week, only a 50 percent advantage was measured, presumably due to readsorption of moisture.

Improvements in materials require the development of more sensitive techniques to measure properties of import and thus be able to assess progress. Two novel spectroscopic

applications were discussed which will play a major role in the development of optical

materials as well as become an adjunct to damage threshold experiments. The first method

concerns the use of photo-acoustic spectroscopy to identify damage-sensitive areas from

increased localized absorption both within the bulk of materials and for thin films, as well. In this latter case one is required to determine both surface (film) and volume

(substrate) terms. The Bennett-Forman and modified two layer Rosencwaig-Gersho theories

had nearly identical results and were in good agreement with experimental data. The other

3
method concerned wavelength modulation spectroscopy. This technique was also shown to be a very sensitive method to identify impurities on the surface or within a sample. Reproducibility can be as high as a part in 10" . One is able to identify particular constituents

and the type of chemical bonding. It also appears to be a powerful technique to study the

effect of adsorbed materials on surfaces; however, it has not yet been determined how

quantitative this technique will be.

In a somewhat related paper the total nonlinear indices, including electronic and

nuclear contributions, for 25 liquids useful as index matching fluids, were calculated.

-13 -11 Values ranged from 10 esu for acetic acid to 10 for CS

2

. The nonlinear index was

seen to decrease as the Abbe value CYj) increased.

2.2 Mirrors and Surfaces

In the area of mirrors and surfaces, diamond turning has lead to great improvement in the fabrication of precise metallic optics. Its extension to dielectrics such as SrFe and

CaF

2

, however, appears to offer no real advantage in damage resistance (at the HF and DF laser wavelengths) over normal polishing. The advantage comes in the speed of fabrication

and is thus a positive economic factor. However, a totally different surface finishing

technique, laser annealing of glasses, has resulted in improved damage thresholds. This is

a very exciting observation but not totally unexpected since flame polishing, acid etching

or any other process which tends to remove impurities as a method to improve surface damage

resistance is well-known. More work needs to be done in this area, such as the demonstration of an ability to maintain a surface figure of high tolerance. It could well be a useful adjunct to diamond machining.

As an indication that we have progressed in the understanding of laser interaction

with materials, particularly in the case of metals, several papers dealt with both the

theory of repetitive interaction of laser pulses with copper and experimental verification

of the theoretical predictions. The onset of laser-induced slip of copper was shown to occur at fluence levels of a few joules to tens of joules per square centimeter depending

on the pulse length. Damage was shown to be cumulative and could result in coating degradation and increased scattering for mirrors under continued laser irradiation. Theoretical predictions were shown to be in essential agreement with experimental results

obtained over very small areas. If one continued to pulse until melt damage, the experimentally measured threshold agreed with a first principles calculation to better than one percent. It was observed that slip threshold was inversely proportional to the melt threshold. Micromachining is a technique that can produce nearly undistorted surfaces exhibiting

low slip and high melt thresholds. In related work on copper mirrors, the ion implantation

of aluminum followed by oxidation in air produced a stable oxide layer which was believed

to be a double layer of CuO on the outside and probably CuAlO,, between the substrate and

outer layer. This surface was very resistant to further oxidation and maintained a very

high reflectivity in excess of 98 percent at 1.06 urn.

4
2.3 Systems

Papers discussing engineering details relating to system design and performance were

heard. One interesting presentation concerned the selection of components in high average

power laser systems. These systems normally employ cooled mirrors and uncooled windows.

Using an exponential Strehl relation as an indicator of the figure of system performance,

expressions were developed to describe distortion in high power systems. It was concluded

that the windows frequently compensate for the distortions normally introduced by cooled

mirrors. Perhaps of more utility is the proposed application of Weibull statistics, which

is a distribution based upon critical flaws, to characterize the brittle fracture of IR windows. The Weibull distribution is appropriate to these low ductility components and

allows scaling from small to large samples. This should be an important engineering tool

in systems design. Its application is an indicator of the growing maturity of the field.

2.4 Thin Film Characterization and Design

As is the usual case, the greatest number of presentations at the Symposium concerned

thin films, both their characterization and damage threshold. This is not surprising

considering that the damage threshold of thin films is routinely lower than surface damage

thresholds and lower still than bulk breakdown limits. However, from this year's Symposium

it is obvious to note the increased sophistication with which this area is being attacked,

and real improvement both in performance and understanding of films that has been made. Of

considerable importance is the careful correlation of optical and physical properties with

process variables. In this regard, a correlation of these process variables with grain

size in Ti^ films was heard. It was shown that there was a ratio of 8 to 1 in damage

threshold when the grain size changed from 20 to 600 K. The smaller size grain film cor- responds to a glassy coating exhibiting a lower refractive index film. Similar careful

work was performed on two other promising new coating materials, tantalum oxide and niobium

tantalium pentoxide. Another new material discussed at the conference was alpha-silicon

hydride, an amorphous silicon hydride which has a refractive index of about 3. These films were all produced by rf sputtering.

In some cases, improvement in damage threshold can be achieved through design. One

design trick is the use of overcoats. Although the particular mechanism which leads to an improvement is not definitely established, the use of SiO^ overcoats on high reflectivity

coatings leads, on the average, to a 50 percent improvement in the pulsed laser damage

threshold. Reproducibility of improvement is reported to be about 30 percent.

Even before we have arrived at a satisfactory understanding and development of thin

film technology, very stringent coating requirements are being established. Not the least

of these are stringent tolerances and uniformities in phase sensitive optical coating for

use in cylindrical unstable resonators. These types of annular resonators are very sen- sitive to polarization and one must design phase-shifting coatings to maintain proper

polarization such that one can operate in low-order models. The very idea that people are considering these stringent coating specifications is an indication of our progress in the

field. However, it is still a very tough problem area.

5
Central to progress in thin films is the development of techniques to characterize

films not only from an optical standpoint but from a chemical and structural standpoint as well. This year several papers dealt with various aspects of this concern. The high

temperature properties of molybdenum reflectors was correlated with the substrate structure. Chemical vapor disposition of molybdenum was used to produce very high reflectivity,

better than super polished or sputtered molybdenum, achieving almost 99 percent reflectivity at 10 urn. By annealing at temperatures between 700 and 1000° C, the molybdenum film

was converted to a body centered cubic form, resulting in the very high reflectivity with

relatively large grain sizes.

Reflection spectroscopy was also used as a tool to identify contaminants in evaporated

films of ThF^, ZnS, ZnSe, and Si 0 . The presence of the contaminant could be assigned to either the bulk or the surface of the material with a sensitivity as good or better than

calorimetric procedures. The obvious advantage of this technique is that measurements can be made over a broad wavelength range. Two other techniques were useful in the mapping of

damage sensitive areas. These were spatially resolved photoacoustic spectroscopy and a new microscopic technique called TIRM which stands for total internal reflection microscopy.

The latter may be used in conjunction with a Nomarsky microscope. Greatly improved sen- sitivity in locating sites potentially susceptible to damage have been demonstrated by both

techniques.

The identification of water within materials at certain wavelengths continues to be an area of considerable interest. We can not only determine the presence of water but determine its location, whether within a film layer, at an interface between layers or between

the film and substrate. This has proven to be very important when dealing with coatings at the HF wavelengths. The presence of water in components used in the HF lasers is without

question the prime factor leading to a decreased damage resistance at that wavelength

compared to operation at the DF wavelength. Anytime a new material is proposed, such as NaF, one of the first procedures will be to assess its affinity for water when used at specific wavelengths.

We continue to see an increase in the growth of the use of advanced instrumental

techniques such as SEMS, SIMS, AES, ESCA (XPS) to analyze impurities and defects in thin

films. Most recently these techniques have been employed to study ThF^ films and locate

the usual impurities of Th0F

2 and Tl^.

2.5 Thin Film Damage

This years Symposium was highlighted by the reporting of large quantities of damage

thresholds for thin films. The data was of high quality and directly applicable not only

to revealing the mysteries of thin film damage but also as useful indicators in the selection of thin film materials for high power laser applications. The data spanned pulse

lengths from picoseconds to milliseconds from the ultraviolet to the infrared. Several

efforts were pointed towards the frequency dependence of damage in a variety of thin film

materials. The data reported at the meeting was used in several instances to support the

selection of a particular damage mechanism, most notably the impurity dominated model as

6
opposed to avalanche or multi-photon processes. This does not mean that these later pro- cesses are ruled out as playing a role in the resultant morphology of catastrophic damage.

Oxide and fluorides were highlighted in the results and important differences in the re- sponses of these classes of materials were identified.

Concerning pulse length scaling, none was observed in one report for pulses ranging

between 100 and 700 ps at 266 nm. However, the same authors and others at the meeting

1 II often have found the normal t ' scaling at longer nanosecond pulse lengths. Damage threshold scaling with film thickness was used to support the impurity model. Attempts to scale

damage with film absorption met with mixed results; however, when the absorption was high,

there was correlation between damage sensitivity and film absorption. But, on high quality

films where absorption was low, variations of the damage threshold with film absorption

were not readily evident. Some interesting results on the damage sensitivity of films

subjected to 800 ns, 1 urn laser pulses were reported. At these relatively long pulse

lengths correlation between film absorption and damage threshold was also reported. The

ratio of the temperature at damage to the melting temperature of the substrate material was nearly a constant. There was not much variation in the damage threshold at these relatively long pulse lengths since most of the materials tested were refractory oxides.

To the trick of using overcoats to improve damage thresholds must now be added the use

of chemically graded index surfaces. In this process, material is leached from a glass to reduce the index in a controlled manner. This obviates, in many applications, the need for

the application of damage-prone anti reflection coatings. Damage thresholds have been

2 reported as high as 30-50 J/cm for 1 ns pulses at 1.06 urn. Achieved reflectivities ranged

from 1/10 to 2/10 of a percent. These high damage values are obtained only after the

2 material is subjected to initial damage pulses at the 8-12 J/cm range. Normally, AR

2 coatings under the same conditions fail at about 4 J/cm . These graded index surfaces are porous; thus we will have to await full impact of the potentially larger scattering and

handling sensitivity which should be characteristic of this type of surface.

Historically, the session devoted to theory and breakdown phenomena has been among the most agitated. In comparison, this year the participants were more attentive, primarily

for two reasons. First, some of the presentations were not very controversial and secondly,

theories were supported by careful experimental data as opposed to the conjecture previously

heard when there was inadequate data available.

In the first category, the individual theories due to Keldysh, Braunstein, and Basov

were compared based on their ability to accurately predict multi-photon absorption coefficients. Keldysh appeared to be the preferred description in cases of import to this

meeting. An ancillary paper dealt with a comparison of multi photon and avalanche processes

in damage. In that report, it was concluded that much more work needs to be done before

one can adequately describe the breakdown processes. Furthermore, one needs to understand

the interdependence of the two breakdown theories. An improvement in the description of

avalanche breakdown was also heard. This year the inclusion of the Holstein theory for

phonon-assisted absorption was included. The key feature of that theory, as opposed to the

intermediate collision model, is that the electron can absorb essentially all the photon

7
energy in a single event rather than in many small increments as for example in a series of

scattering processes. This approach is probably appropriate between 1 and 10 urn, but at shorter wavelengths a further evaluation is necessary.

Another interesting paper dealt with the subject of stimulated Brillouin scattering

(SBS) and its use as a limiter of nonlinear processes such as self-focusing within materials.

Methods of preventing feedback from SBS were also discussed.

The two most important analytical revelations at this meeting concerned the frequency

dependence of thin film damage based on an impurity model and a quantitative assessment of

focal volume effects in relation to recently reported Soviet work. Concerning the thin

film issue, early work on the role that metallic impurities play in damage of bulk dielectrics has been extended to dielectric impurities in optical thin films. Impurities were assumed to be no larger than the thickness of the films. Based upon this restriction, Mie

scattering was involved to describe the absorption of the impurities invoked. From studies

of damage to a variety of materials, general agreement was obtained with the predictions of

the theory. Interestingly, the theory predicts that as one progresses to thinner films the

damage resistance increases. This may offer great benefit for ultraviolet coatings. In the other prime theoretical work, a resolution of the difference between small focal spot

size and large focal spot size data for bulk damage was presented. Generally there is very

little spot-size dependence for large beams while there is a spot size effect for small

beams. This difference manifests itself in a different frequency scaling as well. This

data now adequately describes some heretofore confusing results reported by Soviet workers.

3. Summary of Papers

The subject matter of the Tenth Anniversary (Eleventh) Symposium covered six broad

areas of interest to the high power laser community. These are (1) Transparent Optical

Materials and the Measurement of Their Properties, (2) Mirrors and Surfaces, (3) Thin Film

Characteristics, (4) Thin Film Damage, (5) Considerations for High Power Systems, and

finally (6) Theory and Breakdown. These Conference Proceedings are organized accordingly.

In this Section, a concise summary of each paper is provided. Closely related papers are discussed together, whenever possible. The interested reader is referred to the complete

manuscript of any paper for further details. Our intention here is to provide the reader

with an overview of the Symposium and to identify the topics of current interest, and the

authors and sponsoring organization of the papers. Each topical area is introduced with a brief statement of the underlying problems and the status of understanding within the area of interest as well as to highlight this year's presented papers.

3.1 Transparent Optical Materials and the Measurement of Their Properties

As is the usual case, this year's Symposium led off with papers dealing with trans- parent optical materials, the measurement of their properties, and the correlation of these

properties with failure. The materials discussed were those most useful at the extremes of

the high power frequency regimes, namely the near ultraviolet and the infrared at 10.6 urn.
The first set of papers reported the continuing advances being made in the press-forging of

optical materials, namely LiF, CaF,,, and NaCl. Another paper concerned itself with the

accurate prediction of the physical response of ZnSe windows subjected to high power CCL

lasers.

Until this year, LiF has not been seriously considered as a practical window material

for uv lasers because of its low yield strength and low fracture energy. To overcome these

objections, J.F. Ready and H. Vora of the Honeywell Corporate Material Sciences Center have

produced polycrystall ine LiF windows using a two-stage forging process. Use of the twostep process was found necessary to avoid veiling in the bulk material at total thickness

reductions of up to 75 percent. Depending on the percent reduction and forging temperature, the optical inhomogeneity of the forged windows was greater than the single-crystal

starting material by a factor of between 30 to 750 percent, but the absolute values re- mained in the 10"^ range. Although the visible absorption of one forged window did increase by a factor of two between 458 and 514 nm, the laser damage resistance for short pulses was not affected.

Measurements (at the Lawrence Livermore Laboratory) at 1064 with FWHM 1-ns pulses yielded

2 rear surface and bulk thresholds of 8 and ^ 20 J/cm , respectively, which were the same as for single-crystalline windows. Likewise, at 266 nm with 700-ps pulses, front-surface

2 thresholds of both forged and single-crystal samples were 14 J/cm . Their studies of the microyield behavior, i.e., plastic deformation at a small fraction of the conventional yield strength, showed that the threshold for LiF single crystals

can be increased by an order, of magnitude by press forging. Finally, no increase in either

grain size or optical scattering was observed when forged specimens were exposed to ambient

conditions for one year or to elevated humidity (94% at 39° C) for a period of five days.

One of the problems resulting from forging of crystalline windows to increase their

mechanical strength is the production of optical scattering sites in the bulk material,

commonly referred to as veils. Elimination of these veils from CaF2 was the object of a study conducted by A.K. Hopkins of the Air Force Materials Laboratory, R.H. Anderson and

J.F. Ready of Honeywell and J.M. Bennett, P.C. Archibald and O.K. Burge of the Naval

Weapons Center. First, they determined that the veils did not contain entrapped gases.

This is consistent with the conclusions of previous electron micrograph examinations at Honeywell where it was shown that veils are microvoids caused by the coalescence of vacancies. With the object of collapsing these postulated voids, they heated previously forged

samples of CaF

2 to 750° C (below the 800° C recrystall ization temperature) in argon within

an autoclave for a period of ten hours, a process known as hot isostatic pressing (HIP).

This HIP process did significantly reduce the veil content without any increase in the

grain size (10 urn). Interestingly, pits were observed on previously smooth surfaces as if

the veils migrated to the surface from the bulk.

HIP-processing appeared to slightly increase the optical absorption at 2.7 and 3.8 urn. Optical inhomogeneity also increased but remained in the range of 1 to 3 parts in 10^. The

scattering in the visible was reduced by a factor of eight and it became more spatially

uniform. However, at 3.39 urn, reduction of scattering was not generally observed.
Furthermore, the spatial variation of the scattering was not appreciably affected. No obvious correlation was found between the high-scattering areas in the visible and infrared

either before or after HIP-processing. They concluded that the optical performance of

forged CaF2» as an infrared window, was not obviously improved by the HIP process, but

further, systematic parameter variations need to be explored before a final judgment can be

rendered. For use in short-pulse laser applications, elimination of veils within the bulk can only increase the damage threshold since microvoids are sites of enhanced electric field.

Such evidence was observed in alkali halides at 10.6 ym, as reported by Newnam and co- workers at Los Alamos in a later paper. Polycrystall ine forged NaCl is being increasingly specified as the window material of

choice for high-power infrared lasers. The results of an experimental investigation on the

homogeneity of the mechanical, thermal and optical properties throughout a given forging were described by G.A. Graves, J. A. Detrio, D. McCullum and D.A. Dempsey of the University

of Dayton Research Institute. The test object of their study was a 32-cm diameter Polytran

(Harshaw) hot-forged disc from which more than 100 specimens were cut from prescribed

locations in three planes. Overall, the properties were found to be uniform throughout the

forging within the experimental limits of the measurement techniques employed.

Values of the average yield and ultimate mechanical strengths of 70 uniaxial flexural

test bars were 15 ± 3 MPa and 23 ± 6 MPa, respectively. The only significant variation

observed was between samples from different azimuthal locations. For the biaxial mechanical strength, measured in a concentric ring structure, the average yield and ultimate

strengths were 18 ± 2 and 26+4 MPa, respectively.

The total absorption (bulk + surfaces) was measured calorimetrical ly at 10.6, 9.27,

5.3, 3.8, 2.7 and 1.3 ym on samples with surfaces polished by Harshaw and some of which were subsequently treated by an HCl-etch. At each wavelength, the variation in the absorption coefficients 6 had a standard deviation of ± 10 percent among the samples with an exception at 3.8 urn, where B was homogeneous to only ± 40 percent. The magnitudes of 8 for

"polished only" samples at these wavelengths were approximately 3, 2, 0.3, 0.5, 2 and

-3 -1

1 x 10 cm . The etch-treated samples had a moderately reduced absorption, but at 3.8 ym

a large (nearly 50%) reduction was observed.

-6 The average coefficient of thermal expansion for several samples was 42.8 ± 0.9 X 10 /°

and the average value of the specific heat was 0.2 cal/gm/°C over the temperature range

of -65 to 200° C. Finally, the microstructure was determined to be uniform with an average

2 grain size of 34 urn, and the average Knoop hardness for four samples was 20 kgm/mm for a 50 gm load.

The object of a study, conducted by J. A. Detrio, J. A. Fox and J.M. 0'Hare of the

University of Dayton Research Institute, was to assess the accuracy of prediction of the

optical, thermal and mechanical responses of ZnSe windows subjected to high power C0

2 laser

radiation. In particular, the temperature distribution, optical distortion and strain were measured for three ZnSe specimens with absorption coefficients between 0.006 and 0.020 cm ^.

When the transmitted beam was focused, the focal plane was observed to shift toward the

10
focusing mirror but without any significant change in the minimum beam diameter. The

largest shift was associated with the largest window absorption. However, the value of the

spot size was not accurately predicted by the thermal model. For laser powers of 230 and

705 W and beam diameters of 2.1 and 4.2 mm, the predicted and measured temperature rises at the edge of the windows were in good agreement. The measured radial distribution of tem- perature rise was lower than that predicted (60 to 70% at the center) however. The least

agreement of the thermal model was in predicting the strain in the window. This was a result of the considerable uncertainty (30 to 100%) in the measured values due to the

gauges themselves.

Another diverse group of papers dealt more with the measurement of optical properties

for materials important in high power laser systems. These included the determination of

non-linear indices of refraction for several candidate index matching fluids, and the use

-5 -1 of wavelength modulation spectroscopy to measure very small (^ 10 cm ) absorption coefficients in the infrared. This latter technique offers potentially greater detectabil ity

for adsorbed as well as included impurities. Finally, the use of synchrotron radiation to measure the photo-electron spectra in the valence band region of low index materials such

as BeF£ glasses was reported.

Nonlinear phase distortion leading to self-focusing is an especially critical issue

whenever liquids are used for either cooling or index-matching the optical components of

high-power lasers. For 25 liquids, D.C. Brown, J.M. Rinefierd, S.D. Jacobs and J. A. Abate,

of the University of Rochester, computed the electronic, nuclear and total nonlinear re- fractive indices. For the electronic portion, they used the equation of Boling, Glass and

Owyoijng which requires measurements of only the linear index at the sodium "d" lines and

the Abbe value y.. For nine of the liquids, they computed the nuclear portion and the

T total nonlinear index n 9 using the values of the Born-Oppenheimer coefficient B n as preT -13 -11 viously reported by Hellwarth. Values ranged from 10 esu for acetic acid to 10 esu

for carbon disulphide. For these nine liquids a good correlation of decreasing n-y with

increasing was observed.

The capability of wavelength modulation spectroscopy (WMS) to measure absorption

-5 -1 coefficients a at levels of 10 cm for a 1-cm sample length in the infrared from 2.5 to 12.0 ym was described by R. Braunstein and R.K. Kim of the University of California at Los Angeles and M. Braunstein of Hughes Research Laboratories. Previously, they had developed

this technique for use in the ultraviolet and visible regions. WMS is a measure of the

energy derivative of the absorption coefficient from which the absolute value of a is obtained by integrating the observed derivative spectra. The constant of integration can

be determined by a supplementary laser calorimetric measurement at a fixed single wavelength.

The power of this technique was illustrated by absorption measurements of KBr and KC1 crystals in laboratory air and in dry with a calorimetric measurement at 3.8 urn (DF

laser) as the reference. From the details of the observed structure, identification of

volume and surface impurities was possible. The spectral distribution of the various KBr and KC1 samples all had the same dominant features with slight differences attributed to various surface contaminants and differences in crystal linity and growth procedures.

11
Continuing the search and evaluation of new materials for high-power laser systems,

R.T. Williams, D.J. Nagel and P.H. Klein of the Naval Research Laboratory and M.J. Weber of

Lawrence Livermore Laboratory reported on their recent synchrotron radiation studies of

pure beryllium fluoride (BeF^) glass. BeF

2 glasses are of special interest because of

their very low nonlinear refractive indices. Due to its low linear index (e.g., n = 1.275

at 587.6 nm), BeF

2 may also be a useful multilayer coating material if its highly hygroscopic properties can be shielded.

Measurements in ultra-high vacuum included the reflectance in the range 9 eV to 120 eV and the photoelectron spectra in the valence-band region. A lowest-energy reflectance peak was found at 12.8 eV, implying an exciton resonance at 13.1 eV, similar to LiF. The photoelectron spectra for the bulk revealed a valence band which is roughly symmetrical and

about 5.8-eV wide (FWHM), with a tail of states extending out to 7 eV. These features are consistent with previous measurements that the practical transmission limit is 150 nm

(8.3 eV).

The final group of papers in this topical area dealt primarily with crystalline and

glassy materials ranging from the role iron impurities play in damage of infrared materials

to stress failure analysis in sapphire, and a variety of ir materials. Between the papers,

other reports detailed characteristics for LiYF^, KYF and YPO^ crystals. The first two materials can act as hosts for rare-earths, while the phosphate was assessed for its potential use as a polarizer.

To reach the intrinsic damage threshold limit, a material must be free of deleterious

defects such as impurities and dislocations. D.F. Edwards and B.E. Newnam of Los Alamos

Scientific Laboratory, and W.J. Fredericks of Oregon State University (OSU) reported on their efforts to isolate the significant impurities in KBr crystals that limit the laser

damage threshold. Seven high purity samples of KBr were grown at OSU from starting material

purified by selective ion filters. The concentrations of OH, 0

2

, halogen ions and heavymetal ions were restricted to less than 100 ppb and there were very few dislocations be- cause of carefully controlled growth and annealing conditions. The two major impurities,

Rb

+ and Fe +

, were in concentrations ranging from 5 to 9 ppm and 0.030 to 0.150 ppm, respectively. From light scattering measurements with an Argon laser of the unshifted Rayleigh

peak (intrinsic + extrinsic contributions) relative to the shifted Brillouin peaks (in2+ trinsic contributions, only), correlation was found with the Fe concentration, but none was found for the Rb

+ concentration. Laser damage measurements at 1.06 ym (27 ns pulsewidth, 90 ym spot-size radius) obtained bulk thresholds ranging from 0.4 to 1.1 MV/cm.

Consistently higher thresholds were obtained for the crystals exposed to an oxygen atmo- sphere (during growth) below the melting point. Correlation of the scattering measurements

and the damage thresholds must await repeat of the scattering measurements in which surface

scattering contributions are eliminated by index matching in a liquid.

T.M. Pollak, R.C. Folweiler, E.P. Chick! is and J.W. Baer of Sanders Associates and

A. Linz and D. Gabbe of Massachusetts Institute of Technology reviewed the optical properties and crystal -growth procedures for two fluoride-crystal families which exhibit a natural

rare-earth site, in particular Li YF- , (YLF) and KYF. These materials are characterized by

12
a low nonlinear refractive index (0.6 X 10" esu) and short wavelength cutoffs of 120 nm and less than 200 nm, respectively. Over twenty laser transitions have been demonstrated

from 300 nm to 3.9 ym for doped YLF, and Tm:YLF is currently being assessed for use as a storage laser at 453 nm for laser fusion applications. In addition, Tb-doped YLF and KYF have a large Verdet constant, e.g., ^ 0.4 min/Oe-cm at 633 nm, making them useful as Fara- day rotator materials.

A top-seeded solution growth technique, related to that of Czochralski, was described

which has been optimized to allow growth of 10 cm diameter YLF with a low density of scat- tering sites. Technical problems were also addressed in scaling to a 40-cm diameter.

Yttrium orthophosphate YPO^ (also referred to as xenotime) was evaluated as a potential

substitute for calcite polarizers in a paper presented by R. Allen, L. Esterowitz and

P.H. Klein of the Naval Research Laboratory, V.0. Nicolai of the Office of Naval Research

and U.K. Zwicker of Philips Laboratories. YPO^ is mechanically harder than calcite and

insoluble in water. However, it's birefringence of + 0.107 at 589 nm is only 62 percent of

that of CaC0

3

. The fact that YP0

4 can be grown synthetically in the laboratory and its

relatively high damage resistance are compensating factors in its favor. Crystals, measuring about 15 X 3 X 0.7 mm, were grown at Philips Laboratories by solution of yttrium oxide

in excess lead pyrophosphate which was slowly cooled. The region of transparency (80% or higher transmittance) was determined to extend from 0.5 to 4.5 ym with major absorption

bands at 2000 and 3000 cm"

1

. The largest faces, being optically smooth, were suitable for measuring the surface

damage resistance. When irradiated with 17-ns (FWHM) pulses at 1.06 ym focused to a 0.25 mm

(FWHM) spot, no damage was observed unless accompanied by luminous discharge at the surface.

A threshold of 2.2 - 2.4 GW/cm was obtained for both single-shot (1 - on - 1 ) and multipleshot (N - on - 1 ) irradiation. Identical measurements on calcite obtained a slightly lower

p value of 2.0 - 2.4 GW/cm . Further work will be directed toward growing larger crystals

for practical application.

In predicting the likelihood of mechanical failure of optical components exposed to high-power laser irradiation, attention has usually been directed to the limits set by

fracture under thermally-induced tensile stresses at the cold edges of centrally-heated

windows. Compressive stress, however, may cause earlier failure via plastic flow if sufficiently high temperatures are reached. As reported by P. Miles, J. Gallagher and

R. Gentilman of Raytheon Missile Systems Division, such conditions occurred in their ex- periments with C02~laser irradiated sapphire crystals. Directing a 1.3-cm diameter beam of

relatively uniform intensity on a 5-cm diameter sapphire window, they obtained heat de2 4 2 position rates of between 10 to 10 W/cm for exposure times between 0.5 and 10 seconds.

Due to the high absorption coefficient of sapphire at 10.6 ym, the laser heating was con- fined to the first few micrometers at the front surface. Two distinct effects, light

scattering and then a bright incandescence, preceded the surface fracture that occurred on further heating or cool-down. The initiation time t of both effects was describable by the

2 equation H t = constant, where H is the average laser intensity and the constant is proportional to the square of the temperature rise. These observations indicated the occurrence

13
of plastic deformation of the heated region under the action of compressive and shear

stresses followed by tensile failure on cool -down. To explain these phenomena, a detailed thermo-mechanical analysis was carried out

(using a temperature-dependent thermal conductivity) to predict the temperature and stress conditions throughout the disc samples as a function of time, flux level and flux distribution. Compressive stresses in excess of 200,000 psi were calculated as having caused the

observed fracture. The overall conclusion was that compressive failure is likely to pre- cede tensile fracture in most experiments where partially heated discs are used.

Infrared laser window materials of current interest, such as ZnSe, CaF£ and the alkali

halides, exhibit brittle fracture when heated by intense laser radiation. The mechanical

failure criteria and statistical techniques applicable in brittle materials were discussed

by J. A. Detrio and G.A. Graves of the University of Dayton Research Institute, and J.M.

Wi miner of the AiResearch Mfg. Co. The authors centered their discussion on the three

salient characteristics describing failure of brittle materials: (1) lack of appreciable

ductility prior to fracture, (2) initiation by flaws, and (3) statistical occurrence of

failure. Fracture by tensile stress was cited as dominant, and the Weibull mathematical

function was shown to be useful to describe the distribution of critical flaws. The Weibull

distribution also explicitly allows scaling from small test specimens to larger window

sizes.

Flaws may be introduced in the manufacture, handling and in use of laser windows. An interesting example of flaws generated during use is the heating of absorbing inclusions

which are not relevant to the mechanical strength until irradiated by the laser. Likewise,

absorption by dust particles on the surface can be the fatal flaw, causing localized stress. Other factors which can cause early failure of brittle windows are slow crack growth caused

by a corrosive agent such as water, residual stresses introduced during the fabrication

process, cyclic loading and vibratory loads and mounting stresses. To improve the reliability of a window in actual performance, it was suggested that an array of windows be

proof-tested to screen those parts containing critical flaws.

3.2 Mirrors and Surfaces

The papers dealing with mirrors and surfaces were divided between failure to metal

mirrors and dielectric surfaces. The first paper dealing with metal mirrors was concerned

with the response of each element when subjected to repetitive laser pulses. Papers along

these lines have been eagerly awaited by the Symposium participants. Other papers on metal

mirrors were centered on a first principal calculation of an ultimate melt failure criteria

and experimental verification, while the last paper dealt with the treatment of Cu surfaces

by Al implantation as a means of stabilizing high reflectivity by avoiding environmental

degradation.

Intergranular and intragranular slip in metal surfaces exposed to high-intensity,

pulsed laser radiation has been reported experimentally and explained qualitatively by

Porteus, et. al , at a previous symposium. Now H.M. Musal , Jr., of Lockheed Palo Alto

Research Laboratory has developed a quantitative theory predicting the flux level at which

14
slip will be observed with optical absorptance, thermal conductivity, density, specific

heat, modulus of elasticity, Poisson ratio, yield stress, coefficient of thermal expansion

and laser pulse fluence and duration as parameters. Numerical values for slip threshold

calculated for copper using this theory are in reasonably good agreement with previously

reported experimental results. One result of this new work is that the temperature rise at which plastic yield will occur is very low, being approximately 20° K for pure copper under

short-pulse, large spot size laser irradiation. Repetitive-pulse irradiation, where each

pulse raises the temperature by more than 40° K , is expected to cause progressive surface

degradation via slip bands, intergranular slip and fatigue cracking. A mechanism for

increased optical scattering and coating degradation caused by localized delamination or cracking of the film under continued laser irradiation is suggested by this mechanism and

should be investigated further. Laser damage thresholds observed experimentally are usually significantly lower than

those predicted theoretically from the material properties of the irradiated samples. Some

of this disagreement can be attributed to experimental uncertainty, but much of it arises

because the surface conditions of the samples differs from that assumed in the calculations.

A paper by J.O. Porteus, D.L. Decker, D.J. Grandjean, S.C. Seitel and W.N. Faith of Michelson Laboratory, Naval Weapons Center, reported the results of a study of the effect of

surface conditions on damage threshold in copper. They obtained excellent agreement with

theory when using samples with the highest quality surface conditions. They reported that

the measured peak damage threshold for laser-induced melting of pure single-crystal copper

2 prepared in ultrahigh vacuum was 76.6 ± 8.6 J/cm for a nominally 100 ns pulse length.

This value agrees almost exactly with a first-principles calculation of the melt threshold

of copper for this pulse shape. The quoted uncertainty in the measurement was 10 percent.

This agreement between theory and experiment is very reassuring since in typical damage

experiments agreement between theory and experiment to within a factor of 2 is generally

considered good.

Porteus, et al., further considered the effect of diamond turning and mechanical

polishing on the damage threshold of copper. They found, as expected, that the surface

distortion introduced by finishing reduced the melt threshold by as much as 20 percent even

for these carefully finished surfaces. However, the threshold for laser-induced slip,

2 2 which was only 16 J/cm for the single-crystal sample, increased to 44 J/cm for the sample

with the 20 percent decrease in melt threshold and was nearly inversely proportional to melt threshold. This behavior is expected on theoretical grounds as a result of the

crystalline disorder at the surface produced by optical finishing. Center-turned diamondmachined samples exhibited the least surface disorder, approaching the single-crystal

sample in both melt and slip thresholds. Laser-induced pitting also exhibited a low damage

threshold in commercially available copper, although it was several times that for laserinduced slip. Under optimum conditions polishing can increase the threshold for pitting,

probably by "healing over" small defects with disordered material. A more reliable method

for raising the slip and pitting threshold, however, is vapor deposition of the copper. No slip was observed for these samples and the pitting and melting thresholds coincided.

IF
Silicon carbide substrates were used for the particular samples tested and the reduced melt

threshold observed (50 J/cm ) was attributed to their poor conductivity. The melt threshold

increased with increasing thickness of copper, and calculations indicate that if higher

conductance substrates were used, a significant improvement in damage threshold would have

been observed. Even with silicon carbide substrates, these vapor-deposited copper samples

showed the highest threshold for initial damage of any tested.

The development of environmentally stable copper mirrors for pulsed-laser applications

was reported by M. Oron, L.G. Svendsen and G. Sorensen of the Institute for Physics, University of Aarhus, Denmark. Fresh copper mirrors have good reflectivity and high damage

thresholds but oxidation lowers their damage resistance. By ion implanting electropol ished

17 2 copper surfaces with aluminum at energies of 60-80 keV at dosages of 1-2 x 10 ions/cm

and then annealing in air for several hours at 200° C for several days at 100° C, they

produced double layers, the outer one consisting of CuO, the inner one probably CuA10

2 as determined by secondary-ion mass spectroscopy and Rutherford backscattering. The reflectance of these ion-implanted surfaces at a wavelength of 1.06 um remained at about 0.98,

whereas the reflectivity of copper surfaces which were not implanted dropped to 0.50-0.60

after a heavy tarnish layer was formed by oxidation in air at 200° C for 100-200 minutes.

The ion-implanted samples, although initially grey in color, regained their red copper

color after annealing, whereas the area which was not implanted was tarnished. Significant

protection against oxidation was thus furnished by the ion implantation process followed by

surface oxidation. Laser annealing offers another avenue for modifying metal -surface

properties of ion-implanted samples and should be explored further.

On the dielectric surface front (and rear), a report on extending diamond turning to materials such as CaFp, MgF

2 and SrF2 was heard. However, as might be expected, adsorbed

water played a role in controlling their utility at HF frequencies. A following paper

dealt with the assessment of diamond-turned dielectric surface topography and scattering

behavior. The last two papers also were concerned with surface preparation procedures, one with cleaning, etching, etc., of NaCl and KC1 surfaces and the last with the use of a high

power C0

2 laser to "treat" fused silica surfaces. These last two papers quantified the

damage resistance of these new surface treatments. Undoubtedly more will be heard of these

innovations.

Inasmuch as diamond turning of metal optics has led to an improvement in the damage

resistance over polished elements, M.J. Soileau, J.0. Porteus and D.L. Decker, of the Naval

Weapons Center, reported on a detailed study of the efficacy of diamond turning dielectrics

such as CaF

2

, MgF

2 and SrF

2 for use as windows at 2.7 and 3.8 um. Damage testing was done

at these wavelengths using ^ 55-ym spot diameters at the e"^ points employing a singlemode pulsed laser of ^ 100 ns FWHM. Unlike earlier studies on diamond-turned KC1 , a relatively soft material where an improvement over polished surfaces was manifest, diamond

turned surfaces of SrF

2

, CaF

2 and polycrystalline MgF

2 exhibited damage thresholds of the same order as polished surfaces of these materials at HF/DF wavelengths. Failure for CaF

2 and SrF

2 appeared to be an electron avalanche initiated by defects and impurities for both

classes of surface finishing. MgF

?

, on the other hand, apparently fails through the

16
absorption by defects present throughout the material. Lower thresholds at HF frequencies

and a larger n-on-1 effect at this wavelength indicated that absorbed water has a significant effect on surface damage thresholds. The best damage thresholds were usually obtained

for CaF

2 with polycrystall ine MgF2 very, very poor by comparison.

In a continuation of their efforts to assess the suitability of diamond-turning for

the generation of optical-tolerance surfaces of dielectrics and semiconductors, D.L. Decker,

D.J. Grandjean and J.M. Bennett studied the optical and surface characteristics of diamondturned Ge, CaF^, MgF

2

, SrF

2

, KC1 and GaAs. The surfaces were generated on a new machine at their organization, the Naval Weapons Center, and were characterized by an analysis of

scattered and absorbed light, profilometry and various microscopic techniques. As an indicator of surface quality, the TIS (total integrated scatter) at two different wavelengths

(0.647 and 3.39 ym) were determined and compared to high-quality, conventionally polished

material. For MgF

2 , CaF2 and SrF2 at 3.39 ym the TIS of polished and and diamond-turned

surfaces was 0.006 and 0.01, 0.00004 and 0.00003 - 0.004, 0.0001 and 0.0001 - 0.0006,

respectively, indicative of quite good surfaces. It was noted that much of the residual

scattering in the diamond-turned samples arose from turning debris and that improved cleaning procedures are needed. The damage resistance of these samples was assessed in another

work at the Symposium.

In a very careful study, B.E. Newnam, A.V. Nowak and D.H. Gill of the Los Alamos

Scientific Laboratory evaluated the short-pulse (1.7 ns), 10.6 ym damage to the surfaces

and bulk of NaCl and KC1 window materials. As the Soviets have previously reported in longer pulse width experiments, the limiting factor in surface thresholds is adsorbed

water. In particular, Kovalev and Faizullov had determined that a freshly HCl-etched NaCl

surface, while initially exhibiting a marked improvement in damage resistance, readsorbed a water layer in under 20 minutes with a concurrent loss in damage resistance. This behavior

was also extended to cleaved surfaces in this study. The damage threshold of NaCl and KC1 is of considerable import to LASL since 8.5-cm thick windows up to 48 cm in diameter are

used in their C0

2 laser-fusion program and represent not only a design constraint to their

systems but are as well a considerable capital investment.

The laser used for damage testing operated at 10.59 ym (P20) in the TEM

Q0

mode. Pulse

widths could be varied from 1 to 65 ns (FWHM). Experiments were conducted over 0.15 to 1.5-mm radius spot sizes in air at 600 Torr. Usually 40-60 sites were interrogated in a 1-on-l irradiation sequence. Criteria for damage were permanent disruption or increased

2 2 light scatter. The rear surface damage threshold for NaCl was 7 J/cm compared to 4 J/cm

for KC1.

Some other noteworthy conclusions from this work were (1) no spot-size dependence was evident for beam radii from 0.14 to 1.5 mm; (2) a freshly cleaved NaCl surface exhibited a

2 front-surface damage threshold of 17.6 ± 0.6 J/cm and after 10 days the resistance dropped

p to 13.1 ± 1.5 J/cm , (3) for recently grown high-quality samples the ratio of bulk-toexit surface thresholds (MV/cm) was almost two, while for the bulk-to-front surface the

ratio was somewhat less; and (4) the scaling of electric field breakdown proposed by Bettis,

House, Guenther and Austin of atomic density/(n -1) was fit exactly, while the pulse width

17
dependence for the exit-surface damage from 1.7 to 65 ns followed t

1 rather than the

1 12 proposed t ' scaling.

Any polishing or finishing procedure which cleans up surfaces, i.e., removes con- taminates or heals geometrical defects, leads to improved damage resistance. Such techniques as acid-etching, flame-polishing or low energy ion-planing at angles near grazing

incidence have all demonstrated this feature. Unfortunately, these techniques are difficult to control for the maintenance of surface figure and are expensive and time consuming. Now another polishing technique has been added. Taking a lead from the laser annealing of

semiconductors, P. A. Temple, of the Naval Weapons Center and D. Milam and W.H. Lowdermilk,

of the Lawrence Livermore Laboratories, have made a preliminary investigation of the efficacy of C0

2 -laser polishing of fused silica surfaces for improving the damage resistance of

such elements to pulsed 1.06 ym laser radiation. Preliminary results are most encouraging.

Their analysis shows that under controlled conditions of appropriate scanning rates the

absorbed 10.6 urn CO2 radiation causes the removal of a uniform layer of fused silica and a probable refusing of surface and subsurface regions, leading to a very low scatter, smooth

surface. Whereas normally polished Si 0g surfaces damage at levels from 10 to 20 J/cm

using a 1 ns, 1.06 ym laser, corresponding CO,, laser polished Suprasi'l II fused quartz

surfaces damaged anywhere from 1 1/2 to 2 1/2 times that level with many less initiating

sites evident.

A physical inspection indicated that the laser polished surfaces were depressed below

the mean surface level and that these regions were under a tensile stress. These observations lead to the obvious question -- can one retain optical figure when employing CO2

laser polishing?

3.3 Thin Film Characteristics

As might be expected, the majority of papers presented at the Symposium were concerned

with thin films, the most damage-sensitive class of optical materials frequently employed

in high power laser systems. Because of the number of papers devoted to optical coatings, we have separated them into sections on thin film characterization and thin film damage

studies. The properties of coatings continues to be of crucial interest to the high power

laser community. Coating absorption is typically orders of magnitude larger than would be

predicted from the absorption coefficients of the coating materials in bulk form, and this

year's Symposium contained several papers shedding new light on this puzzling situation.

Water in the film volume or at interfaces has been quantitatively identified as the source

of most of the absorption at HF wavelengths and several powerful experimental techniques

are reported for analyzing its presence. Happily, they all give results which are essen- tially in agreement and provide a satisfying experimental redundancy.

There is relatively little data in the literature concerning the behavior of films at elevated temperatures, and two papers addressed that question. The deposition of superior

films of molybdenum, a high temperature substrate material, was also reported. The application of Photoacoustic Spectroscopy (PAS) to thin-film absorption, its spatial variation

and the possible relation to laser damage was discussed in three papers. In the final

18
paper in this section, a novel experimental technique for detecting surface defects and

laser-induced surface damage with greatly increased sensitivity was reported.

A theoretical analysis by Sparks at a previous Symposium concluded that contaminants

such as water were the principal cause of absorption in multilayer films. Experimental

verification of this conclusion and a technique for experimentally separating surface and

bulk absorption in single layer films has been presented at previous symposia. Now

T.M. Donovan and P. A. Temple of Michelson Laboratory, Naval Weapons Center, and S.C. Wu and

T.A. Tombrello of the California Institute of Technology have reported similar results for

two-layer films and have applied a novel nuclear resonant technique to the problem which

may be applicable to multilayers. They have shown that the hydrogen concentrations determined using this technique correlate very well with both bulk and surface absorption as determined using the calorimetric technique on single line HF measurements at six wavelengths between 2.7 and 2.9 ym and with the spectral absorption of water at these wavelengths.

The nuclear reaction + > ^0 + ^He + y, which is very sensitive to the accelerating

1

9

voltage of F, was used to profile through the films and detect both the position and

concentration of H present. Comparing these results with the calorimetric results and with

the calculated absorption of water gives a redundant experiment leading conclusively to the

identification of most of the H being found as ^0. Measurements on various kinds of films

led them to conclude that, in NaF and ThF^ films, the water is uniformly distributed through

tlvj film, whereas in As2Se2 and ZnS, it occurs primarily at interfaces. They succeeded in reducing the water content of ThF^ films by a factor of 50 by encapsulating with ZnS. SiO

x and A^O-j films showed both significant volume and interface absorption. As in the other

cases tested, the absorption values obtained using the nuclear resonance and calorimetric

techniques were in good agreement.

Key to reducing absorption by contaminants in multilayer dielectric films is an understanding of what the contaminant is and where it is located in the coating. Internal and

external reflectance spectroscopy provides a powerful tool for characterizing contaminants.

Its application to optical thin films was discussed by W.N. Hansen, L. Pearson and G. Hansen

of Utah State University and W.J. Anderson of the United States Air Force Academy. To illustrate the technique, evaporated films of ThF^ (some capped with 2 ym thick layers of ZnS and ZnSe) and single layers of ZnS, ZnSe and SiO

x were analyzed. The coatings were deposited on both silver-coated microscope slides and on ZnSe and Ge internal reflection

plates. In some cases a thin 100 A Si0

x layer was deposited between the silver and the ZnS

and ZnSe coatings to demonstrate the sensitivity of the technique in detecting or ignoring

surface layers.

By changing the angle of incidence and polarization ratio of the incident light for

both external and internal reflection, the high field intensity in the standing wave could

be positioned at an interface or within the bulk of the film. Multiple reflections also

enhanced the signal-to-background ratio. The films, bulk or interface absorption could

thus be enhanced thousands or millions of times as compared to transmission or reflection

measurements at normal incidence, and a position in the film could be selected. Hansen

has written a comprehensive paper in the Journal of the Optical Society describing this

procedure, and the present paper applies these results to optical films of current interest.

19
A principal result was that all ThF^ films contained large amounts of water in the bulk

of the film. Even encapsulated films exhibited this behavior, and great care must be taken

to prevent its occurrence. In a paper by Donovan, et al . , in which an entirely different

technique for monitoring water content in films was used, the same conclusion was reached.

Donovan, however, reported some encapsulated ThF^ films with relatively low water content,

so encapsulation may be effective. It is clearly difficult. Hansen, et al., make the point

that reflection spectroscopy is a very convenient method for easily monitoring the success

or failure of encapsulation. In the case of ZnS and ZnSe films, water was not found in significant quantities in the films themselves, but existed primarily at the film/substrate

interface. These results are again in agreement with Donovan's result and also with those

of Temple, et al . , reported at last year's Conference. The films also contained a small

amount of hydrocarbon contaminant at the film/substrate interface. The silicon monoxide

films contained some water, much less than ThF^ and much more than ZnS or ZnSe, and the

stoichiometry of the SiO

x films could be determined conveniently by this method. Finally,

the 100 8 thick SiO film could be either readily observed or not unobserved depending on the experimental conditions used, illustrating a versatile selectivity and sensitivity for

this analytical procedure.

An analysis of some typical multilayer coatings containg ZnS and ThF^ using Scanning

Auger Microscopy (often called Auger Electron Spectroscopy or AES), X-ray Photoelectron

Spectroscopy (XPS), (often called Electron Spectroscopy of Chemical Analysis (ESCA)), and

Secondary-Ion Mass Spectroscopy (SIMS) was presented by T.W. Humpherys, R.L. Lusk and

K.C. Jungling of the Air Force Weapons Laboratory. Each film in this study was approximately 2000 8 thick. SIMS data was taken during Ar sputtering with XPS and AES data taken

between sputtering sequences to afford coating profiles. Vacuum was in the low 10"^ Torr

range. Several prominent dust particles were found on the otherwise featureless surface and were sputter-profiled. Sputtering into the ZnS film revealed additional defect sites, each

enclosed by a halo. The point defect and halo region remained intact as the ThF^ film was penetrated. A substantial amount of oxygen was found in the ThF^ film outside the halo area, suggesting the presence of water. Studies using other techniques support this con- clusion. In the halo area, AES indicated a fluorine deficiency and oxygen enhancement.

Also, Zn and Na signatures were found as well as S. It was proposed that point defects in the ThF^ film were composed of, or are a trap for, ZnS. A SIMS comparison on Balzers ThF^

and RAP grown ThF^ revealed more of the impurities Na, Al , Li and K in the RAP-grown film

than in the commercial evaporant material. Similar analyses were made on the starting

materials with similar results.

Films in storage were also analysed and found to deteriorate as a function of time.

Defect centers in this case appeared to be pores with a halo pattern similar to that of a snowflake. Water may enter through such pores. Not all films studied exhibited numerous

impurity sites and halos. None of the films studied, however, were completely free from

point defects or impurity sites. Surface analysis techniques, such as AES, XFS and SIMS, are thus extremely useful and are fast becoming an integral characterization technique

during the development and analysis of thin-film coatings. Continued use of such analysis
techniques should provide a significant insight into the performance exhibited by many thinfilm coatings.

Most measurements of absorption in multilayer dielectric films are made at room tem- perature. However, laser-induced heating may occur prior to damage, and the high temperature absorption of the films as well as that at room temperature is of importance in high

average power applications. H. Ku'ster and J. Ebert, of the Institut fur Angewandte Physik,

Universitat Hannover, have devised a technique for measuring, to a first approximation, this

high-temperature absorption at 1.06 ym by measuring the temperature of the coating for known

laser fluences using an optical pyrometer. The emittance of the samples as a function of

temperature was measured independently using an electric oven. They found that film behavior can be fit to an exponential increase in absorption with temperature. Using this

model, they have determined the absorption coefficients of A^O^, BeO, MgO, HfC^, ZrC^,

^2^3' Ce^2» TiOg and SiC^ films about 5000 A thick, deposited on Suprasil I substrates.

They also compared the damage threshold for a long-pulse Nd^

+ glass laser having a pulse

width of 800 ns, with the observed high temperature absorption coefficients. Several

interesting results emerged. The damage threshold, to a good approximation, was inversely

proportional to the absorption coefficient at 600° C for all materials tested. By contrast,

the same plot made using ambient temperature absorption coefficients, the agreement between

theory and experiment was very poor. As an additional test, ZrO^ layers having different

absorption coefficients at 600° C were damage tested. Again the inverse relationship be- tween damage threshold and absorption coefficient held. Damage occurred to all coatings at surface temperatures of 625° C ± 55°, regardless of the melting temperatures of the coating

materials, which varied from 1840° C to 2700° C. The authors concluded that the surface

temperature at which damage occurred was probably determined more by the relatively low

melting point of the substrate material than by the melting point of these coating materials.

Similar behavior has been observed for electrically pulsed, coated filaments, where the

coating material is ejected at a temperature corresponding to the filament rather than to any thermal property of the coating material itself. There was some correlation between the

damage threshold of the oxide coatings tested and band edge for ultraviolet absorption,

although S^ and BeO had anomalously high damage thresholds and A^O^ had an anomalously

low threshold. Nevertheless, the probability is that the shorter the wavelength of the

ultraviolet absorption edge the higher the damage threshold will be at a wavelength of

1.06 ym. This is not too surprising since shorter cut-off wavelengths generally correspond

to lower refractive indices.

Most reflectance and transmittance measurements are made at room temperature. An instrument development reported by M.R. Jacobson and R.D. Lamoreaux of the Optical Sciences

Center, University of Arizona, makes it possible to measure reflectance on mirrors at tem- peratures up to 700° C and the transmittance of windows heated to 500° C. The sample

chamber is evacuated to about 5 X 10"^ Torr to prevent sample degradation during testing.

This double-beam instrument affords relative reflectance or absolute transmittance and can

be used for measurement between 0.38 and 15 ym. An unheated aluminum standard is employed

in the relative reflectance mode of operation. Heaters are used on the edges of the

21
transmittance samples and on the back of the reflectance samples. Temperature is monitored

using thermocouples cemented to the heating blocks. A Leiss sodium chloride double-pass

double-prism monochromator equipped with a stepper motor disperses the light, which is detected using a thermocouple or a combination lead sulfide-silicon photodiode detector.

The output of the instrument is computerized for rapid data analysis. Scanning from 0.38 to 15 ym takes about 20 minutes, and contains 340 data points. No estimate of accuracy was

given.

The most common mirror material used for high-power laser applications is molybdenum.

Frequently, in infrared applications molybdenum mirrors are used uncoated, so a high infrared reflectance is desirable and may lead to an increased damage threshold. G.E. Carver

and B.O. Seraphin of the Optical Sciences Center, University of Arizona, have demonstrated

that molybdenum deposited by one of two chemical vapor-deposition processes on fused silica

substrates and subsequently annealed at 750 - 1000 °C can have a reflectance at a wavelength

of 10 ym of 0.987, which is 0.005 higher than the highest value reported for mechanically

polished molybdenum. The reflectance is stable under ambient conditions to within 1 percent over the measurement period of 45 days. Total integrated scatter for the best of these

_3

films was 1.2 X 10 at a wavelength of 0.63 ym, which corresponds to an rms roughness of

18 8 before and after annealing. The two CVD processes are the pyrolysis of Mo(C0)

6 and

the reduction of MoClg. Grain sizes obtained by the two processes were similar and ranged

from 0.03 to 0.5 ym in diameter.

Photoacoustic spectroscopy (PAS) offers considerable promise as an alternate evaluation

technique for thin-film coated optical samples. The sensitivity of PAS is comparable to that of calorimetry. However, interpretation of the experimental results in terms of a theoretical model is not nearly as well understood for PAS as for calorimetry. In a pair of

papers, Nils C. Fernelius of the University of Dayton Research Insitute, compared the ex- perimental results for both amplitude and phase measurements using PAS on an anti -reflection

coated ZnSe laser window with calorimetric results and with the theoretical predictions of

the Bennett-Forman (BF) theory and a modified Rosencwaig-Gersho (RG) theory. Unlike the BF theory, the unmodified RG theory does not allow for surface absorption. Even though analysis

suggested that 93 percent of the absorption in this particular sample arose from bulk absorption and only 7 percent from surface absorption, the unmodified RG theory predicts only a 0.04° change in phase angle for frequency changes over a large frequency range, 1 Hz to 10 kHz, as compared to an observed 7° change over a smaller frequency range, 100 Hz to

-1 04

1 kHz. Furthermore, the frequency dependence, which was experimentally varied as f , -1 5 is predicted by the simple RG theory to vary as f . The BF theory can be reduced to the product of a term containing the dimensions of the

cell, type of gas, etc., and a term containing the sum of contributions from surface and

bulk absorption. The experimental phase-angle variation of 7° between 100 Hz and 1 kHz was obtained for a ratio between surface and bulk absorption of only 0.03. Utilizing the cal-

-3 -1 orimetric results of an effective absorption coefficient $aff = 7.07 x 10 cm , one pre-

-3 -T diets the bulk absorption coefficient to be 6,6 x 10 cm and the surface absorption to be

2 x 10"^ cm"^. The frequency dependence observed for this choice of values was approximately

f -1.0

22
The RG theory was extended by Fernelius to cover a double layer structure. The results

using this modified theory were in good agreement with the BF theory and with the experimental results both in amplitude and in phase angle. Additional experimental studies applying these theoretical results would be most desirable.

R.P. Freeze and K.J. Teegarden of the Institute of Optics, University of Rochester

reported its use as a scanning technique for identifying potential laser damage sites on substrate surfaces and films exposed to high-intensity laser irradiation. Scan rates of

less than 400 ns per resolution point were demonstrated on germanium films deposited on germanium substrates. However, since the surface to be examined had to be encapsulated in a cell which fit tightly to the surface, the technique at present is restricted to an analysis

of laboratory-sized specimens. Good agreement is reported between average absorptance

results obtained using PAS and those using calorimetry for absorptance values of ^ 1 percent.

Signal-to-noise ratios of ^ 50:1 were demonstrated for PAS. Thus samples having absorptance

values 10 times smaller could presumably be accurately measured as well. Damage from pulsed

C0

2 laser irradiation all correlated with PAS structure. However, there was little relationship between the size of the damage site and the height of the PAS peak and many more

PAS peaks were recorded than resulting damage sites. A clearer understanding of the re- lationship between absorption as determined by PAS and damage is clearly dictated.

There is a history in the Damage Symposium of the development of increasingly sensitive

techniques for detecting laser damage. Now P. A. Temple of Michelson Laboratory, Naval

Weapons Center has reported a new technique which is significantly more sensitive than

Nomarski microscopy, the interferometric technique now nearly universally used for determining the presence of laser damage. Temple has called the new technique total internal

reflection microscopy (TIRM). If the substrate is transparent, an intense visible laser

having as much as 2 W of power is coupled into the substrate through a prism and matching

index fluid such that the beam is incident internally on the surface to be inspected at slightly more than the critical angle, i.e., total internal reflection. If the surface is perfect, no light escapes. If there are imperfections, however, they scatter and light

escapes and can be viewed with an external microscope. This microscope can be a Nomarski

microscope, so that the two techniques are complementary. If a polarized light is used, the

standing wave field near the surface is nearly an order of magnitude higher for internal

reflection than it is for the external reflection case used in dark-field microscopy.

Temple developed two instruments utilizing TIRM, one for in situ measurements of laser

damage and another utilizing Nomarski and TIRM together. Using these instruments he was able to show that details of the damage process invisible using Nomarski were readily observable using TIRM. However, there was little correlation between scattering sites seen before laser damage occurred and the location of subsequent damage pits; and thus unfortunately, it cannot be used as a screening instrument.

23
3.4 Thin Film Damage

Equal in importance to film characterization is resistance to laser damage. The Symposium has fortunately been a gathering place for the reporting and collecting of damage

susceptibilities. In many cases, the documentation of this data ensures that the testing

limits are specified. The Symposium organizers have tried to ensure that efforts to identi

fy, develop and assess the utility of new materials is reported whether or not the effort

was successful. Along these lines, papers on both the continual development of thin film

materials such as TK.I and novel materials, such as chalcogenide glasses were heard. In addition, reports of numerous damage surveys and parametric studies were presented. They

will undoubtedly serve as a benchmark for future progress and for any proposed analytical

description of laser induced damage to thin film. For many years, Honeywell has been perfecting the use of TU as a coating material for

the infrared. At this year's Symposium T.W. Boord, P.P. Chow, W.B. Harrison and J.E. Starling, all of Honeywell , reported on improvements in the clarity and environmental stability

of anti-reflective coatings containing T£I, achieved through the use of a thin Srf^ film to mask the texture of polycrystall ine substrates. The best masking layer thickness was deter

mined to be ^ 200 8. On the other concern, the environmental stability of 3-layer AR coatings was improved by substituting either NaF or PbF^ for the usually employed hygroscopic KCl layer. However, the deposited films containing the fluorides exhibited much

higher absorption. It is hoped that improvements in deposition procedures will alleviate

this latter problem.

Chalcogenide glasses are being considered as thin-film optical switches in the infrared. Obviously their ability to withstand intense laser irradiation will determine their

ultimate suitability for use in high power lasers. To this end W.E. Kienzle of Science

Applications, Inc., and N.I. Marzwell of Rockwell International performed a comprehensive

study of deposition parameters affecting the damage threshold of the chalcogenide glass

material Tl2SeAs

2 Te3 in film thicknesses from 0.6 to 9.9 urn. Changes in the refractive

index and absorption coefficient were deduced from measured reflectance- and transmittancespectrophotometer curves. The deposition parameters varied were sputtering pressure, sputtering voltage, azimuthal target/source geometry and substrate-target separation. Damage

testing was accomplished by using a 2 to 4 ms pulsed CO2 laser emitting up to 0.16 J/pulse

focused to a spot diameter of 0.4 mm. The chalcogenide studied in this research effort was an amorphous semiconductor which

when irradiated by infrared light had a rate of energy absorption that increased markedly

with an increase in temperature. This effect led to a drastic change in light transmission

resulting from free-carrier absorption. Its onset corresponds to the switching rate of the

device which can vary from nanoseconds to microseconds depending on the incident flux and

film thickness.

The largest group of papers in this section deals with the very important areas of (1)

the correlation of deposition conditions with resultant characteristics such as grain size

and damage susceptibility, (2) the utility of overcoatings to improve damage resistance in multilayer stacks designed for use as high reflectors, and (3) three papers which together

24
represent a major reliable and very useful contribution to this thin film damage data base.

These latter papers not only shed light on the role of defects, the limits of correlation of

damage sensitivity with film absorption, but most importantly, frequency dependence of

damage and its interrelation with both the mechanical and optical properties such as film

thickness and refractive index. In many cases, data were also recorded as a function of

pulse length, albeit only over a limited range.

It is well known that processing variables and deposition techniques are prime factors

governing thin-film character and performance. Furthermore, some materials can only be

deposited with high quality by a specific method. To this end, W.T. Pawlewicz, R. Busch,

D.D. Hays, P.M. Martin and N. Laegreid of the Battel le-Pacific Northwest Laboratory, assessed

the potential of reactively sputtered optical coatings for use at 1.064 ym. Specific materials studied were TiC^, In-j g Sng ^^(ITO), ^^2^S' I^O^, amorphous aSi:H and SiC^, with

the latter several emphasizing the need for high-index coatings. A principal feature of the

research on TiC^, the material on which most of the work was done, was the demonstration

that precise control of the crystalline phase and grain size are important factors governing

both the optical properties and damage sensitivity. Glassy coatings of this material were determined to be the most damage resistant.

All coatings were rf-sputtered in an Ar/C^ atmosphere with the exception of the amorphous alloy a-Si:H which was deposited in Ar/h^. Principal deposition parameters affecting

the resulting material properties were substrate temperature (20° to 700° C), oxygen or hydrogen partial pressure (0 to 10 Pa) and rf-induced dc bias voltage applied to the sub- strate (0 to 500 V dc). Films were characterized as to crystalline structure, phase com- position and grain size. Optical properties were assessed spectrophotometrically and the

optical constants were deduced over the wavelength range 0.2 to 2.5 ym. Damage thresholds were reported at this meeting in another paper by the group at Lawrence Livermore Laboratory.

This work is both comprehensive and of a nature necessary to improve our understanding

of the film preparation -- film properties -- damage sensitivity sequence. More such work

is necessary. Some of the principal results of this research are the controlled variation

of Ti O2 coating properties, with microstructure which can be varied from the glassy state to 60-nm grain size, and the demonstrated ability to control phase composition from 60 percent

anatase/40 percent rutile to 100 percent rutile. The relationship between grain size and

refractive index has been established for rutile coatings. Finally, the relatively high

damage resistance and low attenuation have been demonstrated for reactively sputtered,

transparent coatings of IT0. Recent damage results on sil ica/titania reflectors were revealed by C.K. Carniglia,

J.H. Apfel, T.H. Allen and T.A. Tuttle of the Optical Coating Laboratory, and W.H. Lowdermilk, D. Milam and R. Rainer of the Lawrence Livermore Laboratory. As is the interest of

these authors, tests were accomplished using 1 ns laser pulses at 1.06 ym. Samples tested were limited to variations of 15-layer sil ica/titania high-reflectance coatings. They

determined that a halfwave silica overcoat increases the average damage threshold by about

50 percent, with the standard deviation of damage threshold for a series of coatings of

about 30 percent of the average value. These deviations were determined to be due to pointto-point and coating run-to-run variations. By shifting film boundaries relative to the

25
field, they found (like others at the Symposium) that neither the average field nor the peak

field in the outer silica or titania layers correlated with damage, but as might be expected

it was the field at the first or outermost sil ica/titania boundary that showed a correlation. It is well known that absorption and impurities frequently are concentrated at interfaces. In addition, for these high reflectors damage did correlate inversely with absorption levels, when absorptance was > 100 ppm. Absorption in the films was controlled by

varying the oxygen pressure during deposition of the titania layers. Extensive morphological examination clearly indicated the uniform or "bulk" absorption damage characteristic

of high absorptance films vis-a-vis the point damage associated with low absorptance films

with isolated impurities. The absorption is expected to play an even greater role for

coated components under rep-rated conditions.

One group vitally interested in pulsed laser-induced damage to optical components is the Lawrence Livermore Laboratory. This year W.H. Lowdermilk, D. Milam and F. Rainer, of

that organization, reported on a comprehensive set of damage tests performed on both optical

coatings and surfaces to aid in establishing damage thresholds for different substrate

materials, surface preparations, coating materials and designs. This study was performed as

an aid in investigating the causes of damage and identifying promising materials and processes. Not surprisingly, they found that defects (not yet identified) dominated all sur- face and film thresholds. They are responsible for the relatively large absorption coef3 -1 -3 -1 ficient characteristic of films (1-10 cm ) and surfaces (10 to 10 cm ) compared to bulk

-4 -3 -1 values (10 to 10 cm ). Polishing compounds are an apparent source of absorbing surface

layer impurities. Along these lines, concerning absorption and its effect on damage sen-

-5 -2 sitivity, a series of high reflectivity coatings where absorption varied from 10 to 10

-4 were fabricated. Generally, coatings with absorption greater than 10 had damage thresholds which decreased with increasing absorption, while coatings whose absorption was less

-4 than 10 were uncorrected with absorption. Of course, what is important is where the

absorption is and if it coincides with a high field in the standing wave, i.e., probably not

the absolute value of absorption. Many conclusions which were drawn from this comprehensive

study will have to await the test of time and attention of others in the field for corroboration. Some of the conclusions are not in complete accord with other observations presented

at this series of meetings.

Hard data on the damage resistance of optical thin films is always of interest, par- ticularly when the tests cover a wide range of parameters and are performed on carefully

characterized samples. This year T.W. Walker, A.H. Guenther and C.G. Fry of the Air Force Weapons Laboratory and P. Nielsen of the Air Force Institute of Technology subjected CaF

2

, MgF

2

, ThF

4

, MgO, A1

2 0

3

, T.^, Si0

2

, Hf0

2 and Zr02 films of various thickness (1 A, 1/2 A, 1/4 A, 1/6 A and 1/8 A at 1.06 urn) to 5- and 15-ns duration laser pulses (t ) of 1.06 urn,

r 0.53 ym, 0.35 ym, and 0.27 ym to assess their damage resistance. It was found that the

pulse length dependence of breakdown fit a square root of t curve for most all materials.

The only exceptions were some oxides at 1.06 ym. Interestingly, fluoride and oxide films were more damage resistant in thinner coatings which did not correlate with changes in the E field inside the films. Breakdown thresholds decreased with decreasing wavelength and

26
increasing absorption coefficient although the correlation was not linear.

Morphological investigation indicated a gradual but marked change from large isolated

pits at 1.06 ym to a more uniformly disturbed region at 0.27 ym. This trend, together with

the pulse length and film thickness dependence, were discussed in detail later by the same

principal author, where impurity, avalanche and multiphoton approaches to the damage process

were considered. The wealth of data reported in this paper should be very useful as benchmarks against which parametric trends predicted by various theories can be compared. All films were prepared under similar conditions, on commonly polished substrates and finally,

tested in the same laboratory on the same laser under essentially identical conditions.

In yet another paper, indicative of the growing interest in short wavelength lasers,

T.F. Deaton and W.L. Smith of the Lawrence Livermore Laboratory discussed a comprehensive

series of laser-induced damage tests at 266 nm using pulses of ^ 150 ps and 1.0 ns. In this

survey, 30 different samples were tested which included dielectric and metallic reflectors,

anti -reflection films, an extensive series of fluoride and oxide films and twelve bare

surfaces of common substrate and crystalline materials. Some interesting damage values

2 reported in J/cm were, for bare surfaces at ^ 100 ps: fused silica, 3.0 ± 1.0; BK-7,

2.9 ± 0.8 and KDP, > 1.7 ± 0.3, while at ^ 700 ps the damage levels were 10.0 ± 3.0,

1.9 ± 0.7, 6.5 ± 1.4 for the same materials respectively. At 700 ps, the damage level for

Li F was 14.4 ± 2.5. All results however, were indicative (to a large extent) of surface

properties rather than bulk behavior. At the short pulse lengths used in this study there

was very little discernable difference in the damage threshold of the eight halfwave fluorides and oxides tested, save the very poor performance of Y^O^ films. As a result of this

clustering no attempt was made at correlating thresholds and bulk material properties. An average pulse length dependence of /t was observed for the bare surfaces and some dielectric

reflectors.

3.5 Considerations for High Power Systems

Of course the end product of this Conference is to lead to improved systems as opposed

to an individual element, as in performance or altruistic research pursuits. Thus the

interrelation, correlation and infusion of the more fundamental and somewhat esoteric

studies into the real world of improving large, often complex high power lasers is the

benchmark upon which the Conference must be measured. Two very interesting papers were

presented this year relating ultimate system performance to fundamental characteristics of

optical elements. One dealt primarily with thermal distortion, while the other was con- cerned with the polarization sensitivity of thin films and its influence on resonator

performance.

C.A. Klein of Raytheon presented a summary of the argument relating the wavefront

distortion produced by various components in an optical train to the resulting fluence which

can be deposited on a target in the far field. He adopted the exponential expression for

the generalized Strehl relation which, for reductions in focused intensity of more than 0.6,

gives a better approximation to the exact degradation of target intensity than the more commonly used parabolic expression. Using the exponential relation, he developed compact

27
expressions for contributions from both cooled mirrors and an exit window to the wavefront

distortion and concurrent decrease in peak focused intensity on target. The analysis was conveniently expressed for use by the systems designer. In calculating mirror distortion he assumed that the mirror is cooled and comes to equilibrium in a time short compared to the

length of the laser pulse. Thus there is no time-dependent term in the expression for

mirror distortion, and uncooled mirrors are not considered. In the case of window distortion, cooling and lateral heat conduction during irradiation were neglected, a good approximation in most applications, and the single-pulse distortion was calculated in terms of

pulse duration. Time dependence is thereby included in the window distortion. If window

materials are taken singly, the figure of merit for CaF^ is nearly four orders of magnitude

higher than ZnSe for comparable window thicknesses, and KC1 is over one order of magnitude

higher than ZnSe. However, Klein emphasized that ZnSe has a positive distortion coefficient

and hence will cancel some of the distortion introduced by the mirror train. On the other

hand, the distortion coefficient of KC1 is negative. Thus, for pulse lengths of a critical

length, a ZnSe window will compensate to first order for thermally-induced mirror aberrations. For longer pulse lengths window distortion dominates, ultimately rendering the beam

unfocusable. The author concludes that "the presence of a positively lensing window, how- ever, always results in a substantial enhancement of the energy-delivery capability of HEL systems." It is not clear that this conclusion is justified except in special cases. Unstable resonators are frequently used in high power laser systems. Their mode structure can be calculated using scalar field theory provided the polarization state is un- changed during a round trip through the resonator. However, if the resonator contains an odd number of conical elements, a 90° phase shift is introduced during the round trip and

compensation must be made to stabilize the optimum resonator mode structure. In the case of

the half -symmetric unstable resonator with a coated rear cone, W.P. Latham, Jr., of the Air

Force Weapons Laboratory, pointed out that this polarization effect can be removed by using

a coating on the conical element which introduces a 90° phase shift upon reflection.

If the coating is imperfect, the I = 2 mode will be present as well as the desired

1=0 mode. If there is substantial power in the l = 2 mode, the power-method calculation

technique does not converge, but if only a small percentage of power is in this mode it can

be employed to establish tolerances on the coating design. The resulting coating tolerances

are extremely tight. To maintain most of the power in the 1=0 mode the phase shift of the

coating should be 90° ±5°. A narrow band, 90° phase-shifting coating design has been

reported using quarterwave multilayer dielectric stacks, but Latham indicated that if it is designed for a wavelength of 3.8 urn, the ± 5° phase shift will be exceeded at wavelengths

more than 0.2 urn on either side of 3.8 urn. DF lasers may have significant power levels over

a broader wavelength range. A broader band, 20-layer coating employing ZnS and ThF^ was designed to meet this restraint. However, appropriate deposition conditions and damage

resistance with these close-tolerance coating designs are still to be demonstrated.

28
3.6 Theory and Breakdown

The utility of much that has gone before not only applies in the near term to improved

system performance, but more importantly for longer range application, forms the basis for

an understanding of the interaction of intense coherent optical radiation with matter. The

session dealing with Theory and Breakdown always leads to the greatest and often most heated

participation of the attendees. This year's was no exception. Very basic papers on a detailed formalism describing multiphoton absorption and avalanche breakdown were heard, as well as an extended and more comprehensive model for wavelength-sensitive, impurity-initialed

breakdown on optical thin films. That paper was followed by reporting of very carefully

performed experimental measurements to elucidate the focal volume and frequency dependence

of bulk breakdown, which led to an unraveling of data presented at previous symposiums in this series. Finally, a paper dealing with the influence of stimulated Brillouin scattering

on the damage process in solid state lasers was presented, followed by the last paper

dealing with an attempt to computer-model the resulting morphology of laser-induced damage

in the bulk of optical materials. An inspection of the growing number of analytical papers

presented at the Symposium will attest to the maturity of the field of laser-induced damage.

These theoretical expositions generally attempt to correlate with the expanding data base of

carefully performed experiments in which there is not only wide acceptance, but high confidence. In addition, we now see the theorists suggesting determinative validation experiments. This synergestic interaction is most rewarding.

As time progresses, interest increases in short-wavelength high- power lasers whether

pulsed, rep-rated or continuous wave. While one generally feels that thermal distortion in windows due to absorption can be handled semi-quantitatively, calculation of the absorption

itself is another matter. This latter subject was addressed in a pair of papers dealing

with nonlinear absorption in direct gap crystals and the competition between avalanche

(impact) and multiphoton ionization by A. Vaidyanathan , T.W. Walker and A.H. Guenther of the

Air Force Weapons Laboratory, together with S.S. Mitra of the University of Rhode Island and

L.M. Narducci of Drexel University. In the first paper, they performed a critical analysis

of available theoretical descriptions for nonlinear absorption processes. They specifically

treated the Keldysh, Braunstein and Basov formulations. It was shown that the Keldysh

treatment is very closely related to Braunstein's formula for all owed-al lowed transitions,

while the Basov formula corresponds to Braunstein's allowed-forbidden case. Concerning this

latter case, there are several errors in the original Basov treatment which have been

identified and corrected. The inclusion of non-parabolic energy bonds and excitonic intermediate state in the Braunstein treatment leads to much improved agreement with reported

measurements. In general, the allowed-forbidden formulas lead to gross underestimates of

the two-photon absorption coefficients, while both the Keldysh and Braunstein allowedallowed treatements afford better agreement.

A point of further interest is that the Keldysh formula offers a sufficiently accurate

description of the multiphoton absorption process even when the photon multiplicity is small. These just stated conclusions are drawn from a comparison of available experimental

results and calculations based upon the improved theoretical descriptions. Materials

29
considered were several direct gap crystals and semiconductors including NaCl , KC1 , RbCl

,

KBr and Rbl, plus CdS, ZnSe, GaAs, InP, CdTe and InSb.

In the companion paper on competing mechanisms in pulsed laser-induced damage, the

workers at the Air Force Weapons Laboratory computed the damage thresholds for several

alkali halides by means of separate avalanche and improved multiphoton models as well as by

18 3

a combination of the two. The damage criterion was the production of 10 e/cm in the

conduction band. Calculations were performed for both picosecond and nanosecond pulse

durations for wavelengths ranging from 0.355 urn to 1.064 um. Neither mechanism studied can adquately explain all the observed features such as the reported scaling relations in laserinduced damage. However, the avalanche mechanism treatment offers better agreement with

measured values. The multiphoton treatment gives opto-electric field strengths substantially higher when it is used alone. However, when photon multiplicity is £4, the damage

fields predicted by the latter model can be smaller than those calculated for avalanche

multiplication, and are in better agreement with experimental results. The authors con- cluded that since neither treatment adequately explains all the appropriate features of

damage, such as frequency and pulse length dependence, it is better to combine them. This was done in a simple manner with improved results. They were careful to point out that a more careful meshing of these two approaches is necessary with appropriate attention given

to their interdependency, such as including the effect of multiphoton transitions on the

avalanche growth-rate constant. There has not been an adequately comprehensive model of electron avalanche breakdown in dielectric solids, capable of explaining all the observed features of pulsed laser-induced

breakdown. However, until recently there has not been an adequate data base to discriminate

among competing theories. A. A. Manenkov and his co-workers at the Lebedev Physical Insti- tute reported such a set of data at the 1978 Symposium giving breakdown thresholds for a variety of alkali halides as a function of band gap, laser wavelength, and most important,

temperature. In a paper presented at the 1979 Symposium, Marshall Sparks, T. Holstein,

R. Warren, D.L. Mills, A. A. Maradudin, L.J. Shem, E. Loh, Jr., and F. King, all of the

Pacific Sierra Research Corporation, presented a theoretical model of avalanche breakdown

which predicts results in good agreement with Manenkov's experimental results.

The model of Sparks, et al., represents a significant advance over the theory of the

Lebedev group in that it models electron-phonon collision processes more realistically. In particular, the dependence of the rates at which electrons lose energy to the lattice, and

at which the electron current relaxes, are taken to be energy-dependent. The energy dependence of these rates were calculated on a detailed basis which took explicit account of

Umklapp processes. Fairly uncomplicated results for theoretical expressions were obtained

from a simplified model of the growth of the electron energy distribution in the presence of

the optical field. A clear difference was found between the temperature dependence at low

frequencies, and at high frequencies, where the distinction lies in the comparison of the

optical frequency to the electron current relaxation rate. These predictions generally

agree with Manenkov's observations. The relative values of the breakdown thresholds for

LiF, NaF, NaCl, KC1 , and KBr are in excellent agreement with Manenkov's measurements, and

30
additionally the absolute damage threshold values for these materials at 293° K and 1.06 ym

are in general agreement with the observations.

Using Hopper and Uhlmann's classical paper describing metallic impurity initiated

breakdown in glass hosts as a jumping-off point, T.W. Walker, A. Vaidynathan and

A.H. Guenther of the Air Force Weapons Laboratory, and P. Nielsen of the Air Force Institute

of Technology, extended the early model to dielectric impurities as optical thin films with

great success. The success of the improved model was judged by its ability to predict the

parametric-variation of thin film damage as a function of film thickness, pulse length and

frequency, extracted from a comprehensive set of experimental damage assessments reported

earlier at the Symposium.

In studying this problem, the authors happened upon an exact solution of the thermal

equations by Goldenberg and Tranter. There, it is not assumed that the radial distribution

within the impurity particle is constant, an important feature when dealing with dielectric

impurities, since the thermal conductivity of the impurity is small. A new feature in this proposed model is the inclusion of a Mie absorption coefficient

for the impurity particle. For non-metallic impurities, the model predicts a damage threshold dependent upon film thickness, pulse length and frequency all in essential agreement

with experimental results on fluoride films of calcium, thorium and magnesium. This arises

from the fact that the model predicts that different sized impurities are the most damage

sensitive at different pulse lengths and frequencies, the pulse length variation of damage

1 12 goes almost t . However, the exact description better fits the data of Milam, et al .

,

(presented at an earlier conference in this series) for the pulse-length dependence of

damage to fused silica surfaces.

There has been a continuing controversy arising from conflicting reports on the subject

of the scaling relationships governing the variation of damage thresholds with spot size and

frequency. Nowhere has there been more disagreement than when dealing with wide band gap materials in the infrared. In a major effort to resolve this issue, M.J. Soileau of the

Naval Weapons Center, M. Bass of the University of Southern California and P.H. Klein of the

Naval Research Laboratory, studied the frequency and volume dependence of laser-induced

damage in the alkali halides NaCl and KC1 prepared by several procedures. The experimental

parameters were varied throughout the investigation in pulse widths (31 - 140 ns), wavelength (10.6, 3.8, 2.7 and 1.06 ym) and spot size or focal volume. Failure was assumed to

be multiphoton assisted (impurity) electron avalanche. A scaling distinction was made when

-9 -3 describing focal volumes -- small being volumes < 1/Ne [y 3 - 5 X 10 cm ), where WQ is the free electron density. Under this condition frequency dependance of breakdown is determined by multiphoton ionization of impurities. On the other hand, for large focal

-4 -3 volumes, (>,10 cm ) it was postulated that the free electrons initially present in the

crystal initiate breakdown and the frequency dependence is determined in this case by the

electron-phonon relaxation time. This leads to the conclusion that for large focal volumes

breakdown fields increase with increasing laser frequency, and the opposite behavior for

small focal volumes. It is noteworthy to point out that the authors did not find a pulsewidth dependence for the relatively long pulsewidths employed in this study. Furthermore,

31
as a result of their effort one should not attempt to infer (the nonlinear index) from

laser-induced breakdown intensities as has been attempted and reported several times in the

past.

An area not discussed at these Symposiums for many years was the subject of a paper by

P. Balkevcius, E. Kosenko, J. Lukosius and E. Maldutis, of the Institute of Physics of the

Academy of Science of Lithuania. They experimentally investigated the influence of stimulated Brillouin scattering (SBS) on the threshold and morphological character of bulk laserinduced damage. Uusing a 0.53 ym laser of either 10 or 20 ns pulse duration, they varied

the spectral purity of the incident laser from < 0.2 to 2 cm~^ employing both single and

multi frequency operation. Samples tested were K-8 optical glass and fused silica. From their studies it was concluded that sufficient upstream SBS conversion can prevent damage by

limiting power in the focal volume and furthermore, that SBS can also modify greatly the -

damage threshold of optical surfaces. Careful measurements of the redistribution of incident laser light into SBS allowed them to understand the difference between narrow- spectrum (large SBS) and wide-spectrum (low SBS) damage morphologically. For example, widespectrum laser radiation having the same power as narrow spectrum interactions always produced long, thin tracks due to self-focusing, evidently a result of the narrow-spectrum

irradiation being converted to SBS. Therefore, the exact relation between self-focusing and

SBS can be different depending on the coherence length. When it is small, self-focusing

suppresses SBS; when it is large, self-compression causes SBS which then scatters out the

radiant energy, limiting the tendency of the beam to self focus. Last year at the Tenth Boulder Damage Symposium, P. Kelly and D. Ritchie of the

National Research Council of Canada and A. Schmid and P. Braunlich of Washington State

University presented a preliminary account of the use of a "particle in cell" (PIC) computer

code to simulate laser induced damage morphology. This year they reported on an improved

version (PULSE) which now incorporates the flexibility to accomodate variations in flux,

wavelength, material or temperature dependance of the dielectric function and electronic

excitation of the solid (avalanche, multi photon, etc.). An important addition treats the

conversion of energy in the phonon field to excited electron and phonon distributions which

can increase the lattice temperature. However, many additional features of the real -world

laser damage scenario await inclusion, such as impurities and defects as well as adequate

treatment of additional nonl inearities beyond multiphoton generation. A computer simulation

of the spatial -temporal behavior of a fast, just-damaging pulse focused into an optical

material was illustrated in the form of a 16-mm movie. If anyone is interested in further

details of this work, contact the authors directly.

4. Recommendations

A survey of the papers given at this year's Symposium reveals that the question of

laser damage and related material properties has become a much more mature technology in the

eleven years since the Symposia were initiated. The number of questions yet unanswered is at least as large as those under discussion earlier, but now they tend to be more quantitative, often subtler and frequently more closely related to laser performance in actual

3?
devices than in earlier years. The theories are less global and qualitative and tend to be susceptible to quantitative verification. Interaction between theorists and experimentalists tends to be closer and a high standard of experimental work is now demanded. Many

of the sources of systematic error in earlier measurements are now well documented and those

careless enough to repeat them do so at their peril. New experimental techniques are still being developed and their use is welcomed. This

year saw the introduction of two novel techniques. One was total internal reflection microscopy (TIRM) which has a sensitivity to surface defects or imperfections well over an order

of magnitude better than differential interference contrast (Nomarski) microscopy, previously regarded as the most sensitive practical technique employing optical microscopy. The

second is quantitative wavelength modulation spectroscopy, which is believed to have a sensitivity comparable to calorimetery for detecting absorption. Both these techniques

deserve to be exploited commercially and TIRM now is. Photoacoustic spectroscopy is a third

relatively new experimental technique and its use in identifying potential laser damage

sites as well as in making sensitive and quantitative absorption measurements, which was discussed in this Symposium, will surely receive further attention. The importance of and

paucity of information on damage properties at temperatures above or below ambient was emphasized and will be pursued. Finally, the use of electron spectroscopic techniques such

as AES, ESCA and SIMS for evaluating the absorption and damage characteristics of thin films

and surfaces was re-emphasized and is expected to become a necessary tool in all wellequipped laboratories investigating such effects.

Extension of laser damage and component measurements into the ultraviolet region was a feature of this Symposium. Clearly much more work needs to be done in this area. The

physics of the damage process changes as the interband edge is approached and as the scale

of defects, which may be important, decreases. Clearly, optical scattering and the possible

relation between scattering level and laser damage threshold becomes even more important.

Veils in window materials, which are reported to have little effect on the infrared damage

threshold of alkaline earths, may become important in the visible and ultraviolet. The

degradation (e.g., via color center formation) of some materials exposed to ultraviolet

radiation and implications on damage threshold is a fertile field for further work. Considerable uncertainty exists concerning the optical properties of films at ultraviolet

wavelengths and better data needs to be generated. Their nonlinear absorption coefficients,

in particular, need to be explored. The possibility that BeF^ films might be made less

hygroscopic could, if demonstrated, have far-reaching results in ultraviolet coating design

and should be explored. Also the polishing of surfaces for ultraviolet use is a challenging

question and may significantly limit the usefulness of ultraviolet systems if presently

ongoing investigations are not successful. Laser polishing and its apparent beneficial

effect on laser damage thresholds needs to be pursued aggressively.

An increased emphasis on scale-up processes and an understanding of how to develop

components for use in actual systems was evident in this Symposium. The use of the Weibull

distribution and proof testing of large components susceptible to brittle fracture was clearly outlined and will be increasingly used by systems engineers in the future. Scale33
up of materials such as CaF

2

, YP0

4

, YLF and KYF for use in actual devices will be pursued as will the use of strengthened LiF. Stress calculations, the spatial distribution of tem- perature and the resulting tensile or compressive stress on large brittle window samples

will hopefully be used by laser engineers to prevent catastrophic failure due to system

design errors. The system requirements for multilayer films used in novel laser cavity

designs such as the half -symmetric unstable resonator provide further evidence of the demand

for continued development of multilayer film technology. On the other hand, simple laser

systems may benefit by the possibility of oxidation-resistant, bare copper mirrors raised by

the technique of ion implantation of aluminum and subsequent annealing. This technique

merits further study.

The problem of the sensitivity of thin films to laser damage is of continuing importance. Several redundant techniques for separating and measuring the magnitude of surface

and bulk absorption in thin films were reported at this Symposium. They include a resonant

nuclear technique for determining the presence of H, and thus H

2 0 or 0H~ or a hydrocarbon,

measurement of H using SIMS, measurement of localized absorption using internal and external

reflection spectroscopy and adiabatic calorimetry. The location and magnitude of surface

and bulk absorption in multilayer as well as single-layer films can now be investigated

using a combination of these techniques. How to reduce the level of water and other con- taminants is the next problem. In theoretical treatments of thin film damage threshold it

is important to use values derived from these thin film measurements rather than intrinsic

material measurements when predicting film performance.

Our quantitative understanding of laser damage continues to grow satisfyingly. The

demonstration that the laser damage threshold predicted for copper using a sophisticated

temperature-dependent theory agrees to within experimental uncertainty (10%) with that

determined experimentally on samples whose surface structure is nearly undistorted is most

satisfying. The damage threshold decreased as the surface disorder increased, again in agreement with theoretical expectations. It would be most desirable to qualify this de- crease, however, in terms of measurable surface parameters so that a predictive capability

can exist. Also such measurements should be carried out on other metals to understand the

generality and limitations of this nascent theory.

Another theoretical triumph was the demonstration that laser-induced slip, which occurs

at much lower levels than melting, can be predicted quantitatively by theory. Such slip may

increase scattered light, degrade coating performance and reduce damage threshold. Its continued development may in fact precipitate catastrophic damage in laser components.

These possibilities have not, however, been demonstrated and more experimental and theoretical work employing large spot sizes should be done to determine the implications of

this theory.

Since real devices, particularly those operating in the ultraviolet, are frequently

repetitively pulsed, there is a growing need for damage thresholds under repetitive pulse

conditions. Also, very long quasi-cw pulse lengths (10-20 us) need to be investigated.

Damage in low-absorption films or surfaces frequently occurs at localized defect sites and

the identification of these sites is thus a crucial problem. Precatastrophic damage

34
measurements and the full use of the analytical techniques now at our disposal are thus

essential. Mie absorption in films and the consequent sharp resonances possible as a function of wavelength or defect size and shape needs to be evaluated analytically using

adequate experimental data, most of which at present is unavailable. The possibility of

non-uniformities in impurity, distribution, grain boundary effects, and results of microscans of the sample for localized impurities should also be investigated.

Over the many years of this Symposium, numerous and often conflicting reports of spotsize effects have been heard. On several occasions, scaling laws have been proposed, justified primarily using selected data from the proceedings of this meeting and the Soviet

literature. However, in many instances no effect has been seen. This is an area which

demands attention since individuals are, in some instances, unwarrantingly relying on un- verified scaling relations. A careful assessment of spot-size effects should be undertaken

to determine and bound any range of application. Of course, other experimental parameters

such as pulse length, sample properties, etc., should be held constant while varying the

spot size over a sufficiently large range of diameters. Then other experimental conditions

can be altered to bound the parameter space and identify any interdependency. To this area

of concern, we should add the need for hard experimental data on repetitively irradiated

samples. One must not rely on single exposure failure thresholds as absolute indicators of

component failure thresholds when they are intended for use in repetitively pulsed systems.

Thus, the scaling of damage thresholds from single event levels to a range of repetitively

pulsed conditions is needed as well.

A very satisfying theoretical explanation of size and wavelength in damage thresholds

based on an avalanche model and self-defocusing was reported at this Symposium. It should

be pursued and the experimental results on which it is based should be extended to shorter

wavelengths. Multiphoton effects will become increasingly important as the wavelength

decreases, and a complete theory must include them. Although to a first approximation the two effects may be additive, they are doubtless synergistic. The additive approximation may

be a poor one and should be investigated. In developing the formalism required for treating

the interdependency of multiphoton and avalanche mechanisms, the Soviet literature should be

consulted. Too little acquaintance with it is found in nearly all aspects of laser damage

phenomena reported in the West. The same may be said of the potential contribution which

could be made by other scientific disciplines to the laser damage field. For example, more chemists should be involved, not only for the insight which they can furnish in surface-

| related matters such as adsorption, but to help explain why, for example, fluorides behave

so much differently from refractory oxides when used in thin films.

Laser damage is a fertile area for a multidisciplinarian approach and furnishes a continuing number of challenges for both scientific and engineering efforts of the highest

order. It is to be anticipated that continued progress in this area during the next year

will make next year's Symposium as productive as this year's has been.

35
5. Acknowledgment

The editors would like to acknowledge the invaluable assistance of Mr. Aaron A. Sanders,

Ms. Norma Lear, Ms. Shir Lee Brubaker, 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 the 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.

36
6. Bibliography

1. A.J. Glass and A.H. Guenther, Editors, "Damage in Laser Glass," ASTM Special Technical

Publication 469, ASTM, Philadelphia, PA (1969).

2. A.J. Glass and A.H. Guenther, Editors, "Damage in Laser Materials," NBS Special Publication 341 (1970).

3. N. Bloembergen, "Fundamentals of Damage in Laser Glass," National Materials Advisory

Board Publication NMAB-271 , National Academy of Sciences (1970).

4. N. Bloembergen, "High Power Infrared Laser Windows," National Materials Advisory Board

Publication NMAB-292, National Academy of Sciences (1972).

5. A.J. Glass and A.H. Guenther, Editors, "Damage in Laser Materials: 1971," NBS Special

Publication 356 (1971).

6. A.J. Glass and A.H. Guenther, Editors, "Laser Induced Damage of Optical Materials:

1972, " NBS Special Publication 372 (1972).

7. A.J. Glass and A.H. Guenther, "Laser Induced Damage of Optical Elements, A Status

Report," Applied Optics 12, 637-649 (1973).

8. A.J. Glass and A.H. Guenther, Editors, "Laser Induced Damage in Optical Materials:

1973, " NBS Special Publication 387 (1973).

9. A.J. Glass and A.H. Guenther, "Laser Induced Damage in Optical Materials, 1973: A Conference Report," Applied Optics 14, 74-88 (1974).

10. A.J. Glass and A.H. Guenther, Editors, "Laser Induced Damage in Optical Material:

1974, " NBS Special Publication 414 (1974).

11. A.J. Glass and A.H. Guenther, "Laser Induced Damage in Optical Materials: 6th ASTM

Symposium," Applied Optics 14, 698-715 (1975).

12. A.J. Glass and A.H. Guenther, Editors, "Laser Induced Damage in Optical Materials:

1975, " NBS Special Publication 435 (1975).

13. A.J. Glass and A.H. Guenther, "Laser Induced Damage in Optical Materials: 7th ASTM

Symposium," Applied Optics 15., No. 6, 1510-1529 (1976).

14. A.J. Glass and A.H. Guenther, Editors, "Laser Induced Damage in Optical Materials:

1976, " NBS Special Publication 462 (1976).

15. A.J. Glass and A.H. Guenther, "Laser Induced Damage in Optical Materials: 8th ASTM

Symposium," Applied Optics 16, No. 5, 1214-1231 (1977).

16. A.J. Glass and A.H. Guenther, Editors, "Laser Induced Damage in Optical Materials:

1977, " NBS Special Publication 509 (1977).

17. A.J. Glass and A.H. Guenther, "Laser Induced Damage in Optical Materials: 9th ASTM

Symposium," Applied Optics 17. No. 15, 2386-2411 (1978).

18. A.J. Glass and A.H. Guenther, Editors, "Laser Induced Damage in Optical Materials:

1978, " NBS Special Publication 541 (1978).

19. A.J. Glass and A.H. Guenther, "Laser Induced Damage in Optical Materials; 10th ASTM

Symposium," Applied Optics 18, No. 13, 2212-2129 (1979).

37
If! I

ft

It

il

li

J)I

litii
PRESS FORGING AND OPTICAL PROPERTIES OF LITHIUM FLUORIDE*

J. F. Ready and H. Vora Honeywell Corporate Material Sciences Center

10701 Lyndale Avenue South

Bloomington, Minnesota 55420

Lithium fluoride is an important candidate material for windows on high-power,

short-pulse ultraviolet and visible lasers. Lithium fluoride crystals have been

press forged in one step over the temperature range 300-600°C to obtain finegrained polycrystal 1 ine material with improved mechanical properties. The

deformation that can be given to a lithium fluoride crystal during forging is limited by the formation of internal cloudiness (veiling) with the deformation

limit increasing with increasing forging temperature from about 40% at 400 C to 65% at 600 C. To suppress veiling, lithium fluoride crystals were forged in two steps over the temperature range 300-600°C, to total deformations of 69-76%,

with intermediate annealing at 700 C. This technique yields a material which

has lower scattering with more homogeneous microstructure than that obtained in one-step forging. The paper will describe the results of characterization of

various optical and mechanical properties of single-crystal and forged lithium

fluoride, including scattering, optical homogeneity, residual absorption, damage

thresholds, environmental stability, and thresholds for microyield.

Key words: Fluorides; forging; laser windows; lithium fluoride; mechanical

properties; optical properties; ultraviolet transmission.

1. Introduction

Improved materials for the laser-assisted, thermonuclear-fusion program are needed for

optical components to be used in the beam, including windows, lenses and isolators. The

materials will be exposed to high-power, short-duration laser pulses, possibly at short

wavelengths in the blue or ultraviolet portions of the spectrum. These materials have a number of stringent requirements, including high transmission at ultraviolet wavelengths,

resistance to damage by the high-power laser beam, low scattering losses, a low value of

! the nonlinear index of refraction, and high mechanical strength, at least for some applications such as windows for gaseous systems operating at pressures of many atmospheres.

Lithium fluoride (Li F ) possesses a combination of optical properties which is of interest

for window application on such lasers. These properties include a large band gap for transmission in the visible and ultraviolet, low refractive index for high damage threshold, and

low nonlinear refractive index which minimizes beam break up and self focussing.

Pure Li F single crystals, while they show excellent transmission in the visible and

ultraviolet, are characterized by low yield strength and low fracture energy. Also, the

diameter of the largest commercially available Li F crystals is limited. The objective of

our program is to overcome these shortcomings of Li F crystals without degrading their good

optical properties. For this purpose, we have investigated the response of Li F single

crystals to press forging, an approach that has been used successfully in the past to improve mechanical properties and increase the diameter of single crystals of various alkali

I halides and alkaline earth fluorides [1,2] J We have characterized the effects of press

forging on the optical and mechanical properties of LiF. In an earlier paper, it was shown

that press forging increases the fracture energy and the compressive yield strength of LiF

I single crystals by more than an order of magnitude [3].

* Work supported by the U.S. Department of Energy under Contract #ED-78- C-08-1 555.

^ Figures in brackets indicate the literature references at the end of this paper.

d

39
2. Press Forging and Veiling

All Li F crystals were forged isostatical ly in the presence of a helium atmosphere at 13.8 MN/nr in a system that has been described elsewhere [3]. The crystals were forged

using a displacement rate of 0.013 cm/min for the top ram. Most of the crystals used for

forging were of <100> orientation and of dimensions 2.54 cm in height and 3.8 cm in diameter, although crystals of different aspect ratios and orientations were occasionally

employed, as described below.

Several Li F crystals were forged initially over a temperature range 300-600°C to deformations of up to 80%. Deformation is defined as the percentage reduction of the original height of the starting material. These forged samples were polished and examined

visually using an optical microscope in transmission for the presence of internal

defects that might have been introduced by press forging. These examinations indicated

that excessive deformation at any temperature introduces veils (or internal cloudiness)

which act as scattering centers and look like voids or partially-healed cracks in an optical microscope. The observed limit for deformation without veiling was about 40 percent

at 300°C. This limit increased with increasing forging temperature to about 65 percent at 600 C (Figure 1). Metal lographic examinations indicated that the employment of forging

temperature of 600°C or higher for large-scale deformation would be undesirable because it produces a coarse-grained material possessing non-uniform grain size.

A few crystals of <110> and <111> orientations were forged to determine the effect of

initial orientation on veiling in press-forged LiF. After these samples were forged to 50%

deformation at 500°C, veils were observed in the billet of <111> orientation, but not in that of <110> orientation. A sample of <110> orientation forged at 600°C to 69% reduction

in height was observed to be nearly free of veils. All press-forged LiF samples of <110>

orientation were elliptical in cross section with the length of the minor diagonal equal to the length of the diameter of the starting single crystal.

In order to suppress veiling in samples forged to large reductions and to produce a material possessing more uniform microstructure , several LiF crystals were forged in two

steps. The forging billet is first given X% reduction at some temperature T] (step 1) and

later given an additional Y% reduction at some temperature T2 (step 2). Between steps, the

forging billet is annealed at some temperature T3 > T. and T2 to induce recovery. The

billets for various two-step forgings were obtained by cutting Harshaw crystals (3.8 cm

dia. x 2.54 cm ht.) of <100> orientation either along the (100) plane into two sections of

equal heights or along the (010) and (001) planes into identical quarters. In one case, sample H-32, the forging billet was an uncut Harshaw crystal.

All two-step forgings were examined visually for the presence of veils and were classified as veiled or unveiled. The results of this examination and the forging parameters of

various two-step forgings are summarized in table 1. The data in table 1 indicate that the

two-step forging approach permits a large scale deformation (amounting to 69-75% reduction

in height) of LiF without introducing veils. Samples classified in table 1 as unveiled may

contain a very few internal defects which are difficult to detect either visually or microscopically. Metal 1 ographic examinations of two-step forgings have revealed the presence of

fine, and fairly uniform, grains of size in the range 15-30ym.

3. Optical Properties

3.1. Scattering Profiles

We have measured scatterinq profiles of several single-crystal and press-forged LiF

samples with a laser flying-spot scanner, which uses a helium-neon laser as a light source

and galvanometer deflectors, and a photo diode positioned such that it detects scattered

light at 90° from the forward direction. Figure 2 illustrates the scattering profile

obtained by scanning over a 5 mm x 5 mm area of a sample of forged LiF which contained veils

because of excessive deformation.

Scattering profiles of two-step forgings which appeared unveiled in a visual examination indicate the presence of a few scattering centers, which are not seen in the scattering

4n
profiles of single crystals or single crystals forged to low reductions. Since such scat- tering centers are fine and their density low, it is felt that a fine adjustment of the

two-step forging parameters would enable us to prevent veils in Li F crystals press forged

to a total reduction of ^ 75%.

3.2. Optical Homogeneity

Measurements of the optical inhomogeneity of several Li F crystals at 633 nm have been

made, both before and after forging, using the technique described by Bennett et al . [4].

The optical inhomogeneity is defined by An/n, where An is the variation in the index of

refraction n of the sample averaged over its thickness. A description of the experimental

procedures employed in the present work has appeared elsewhere [3]. Table 2 summarizes the

measured optical inhomogeneities of various Li F samples, along with their forging parameters. The data indicate that the Harshaw Li F single crystals possess very good optical

homogeneity (Ari/n in the range 7 x 10"? - 3 x 10"6), and that the Li F single crystals can

be press forged without seriously degrading their optical homogeneities. In the worst

case, sample H - <110> - 2, the observed increase in An/n is by about a factor of eight,

but in four out of the remaining six- samples, H - <111> - 4, H-28 Q-2 , H-32 and H-36

Q-4, the observed increase in AiVn is less than a factor of two. Samples press forged in two steps can retain an optical inhomogeneity in the range of a few parts in 106.

3.3. Optical Absorption

We are currently investigating the residual ultraviolet and visible absorption in LiF

using absorption cal orimetry with a line-tunable argon ion laser. The laser is a SpectraPhysics Model 171-19 which operates single line at nine wavelengths in the visible (454.5-

528 nm) at power levels in the range 0.7-6.5 watts, and multiline in the ultraviolet at a power level of 2 watts which is distributed ^ 45%, 45% and 10% among three lines of wavelengths 364, 351 and 339 nm, respectively. Most of our measurements were made at three

wavelengths in the visible, 514.5, 488 and 457.9 nm, and using multiline operation in the

ultraviolet. Some single-line data were obtained in the ultraviolet using an external prism.

The laser calorimetric data that have been obtained are summarized in Figure 3. The data

indicate that the forging of sample H-31 at 525°C to 61% reduction raises its absorption

by approximately a factor of two. Note that the residual absorption in the ultraviolet and

visible in sample H-41 , and in the ultraviolet in sample H-42, is a factor of four or more

lower than the absorption of an Optovac LiF crystal reported by Harrington et. al . [5].

3.4. Damage Thresholds

Dr. D. Milam of the Lawrence Livermore Laboratory has measured bulk and rear-surface

damage thresholds of both single-crystal and press-forged LiF for 1-ns, 1064-nm laser

pulses. The samples used in this work were forged at Honeywell and polished at either

Lawrence Livermore Laboratory or at Honeywell Ceramics Center. Table 3 summarizes the

data obtained by Dr. Milam. Within the limits of the errors, the rear-surface damage

threshold is not significantly changed by the forging process. Also the polishing procedure used does not seem to change the surface threshold significantly. The bulk damage

thresholds remain high after forging, in fact higher than the values for the single crystals. The bulk damage threshold for one single-crystal sample does seem anomolously low.

Front-surface damage thresholds of single-crystal and press-forged LiF for 700-ps,

266-nm laser pulses have been measured by Drs. T. F. Deaton and W. L. Smith, Lawrence

Livermore Laboratory. Their data are given in table 4. Both samples used in this study were polished at Honeywell. Under these irradiation conditions the relatively

high value of damage threshold does not appear to be reduced by the forging operation.

4. Environmental Stability

Press-forged crystals of many pure alkali halides may undergo grain growth or secondary recrystal 1 ization when aged at room temperature for a long period of time [6]. These

processes are accelerated if the relative humidity of the surrounding atmosphere is high.

Such samples often show internal cloudiness, which is not present in the as-forged crystals.

Studies of the microstructures and scattering profiles of press-forged LiF crystals before

and after exposure to a relative humidity of 94% at 39°C for periods up to five days show

41
no increase in either grain size or optical scattering due to the exposure. Similarly,

periodic examinations of the mi crostructures of several press-forged Li F crystals exposed

to ambient conditions of room temperature and relative humidity for periods up to one year

have not shown a significant increase in grain size. Thus press forged Li F is stable under

reasonable environmental conditions.

5. Threshold for Microyield

Strengths of alkali halides are usually measured in compression at offset strains of

0.1 or 0.2%, or in 3- or 4-point bending. In bending experiments, one measures the pro- portional limit which corresponds to the stress at which the stress-strain curve deviates

from linearity. Our measurements of the conventional yield strengths indicate that the

press forging increases the strength of Li F crystals to values in the range 20.7-55.2 MN/m

(3000-8000 psi). For comparison the starting single crystals have values around 2.4 MN/m2 (350 psi).

The fact that many materials can undergo plastic deformation at a small fraction of

their conventional yield strengths is of concern in precision design. We have studied the

microyield behavior of LiF. The samples for these studies were mechanically polished bars ^ 2.5 mm x 6.5 mm in cross section and 2.5 cm or more in length. These samples were subjected to a series of load-unload cycles in 4-point bending with a distance of 1.91 cm between the supporting pins and 0.79 cm between the loading pins. The load on the sample

was increased incrementally in successive cycles. After each cycle the permanent strain was measured by a bonded wire resistance strain gage which was attached to the sample face

subjected to maximum tensile stresses. Figure 4 illustrates the microyield behavior of one single-crystal (H-31 ) and two press-forged (H-31 and H-42) LiF samples. The measured

thresholds for microyield were 1.37 MN/m2 (200 psi) for the single-crystal sample and

10.3 MN/m2 (1500 psi) and 13.8 MN/m2 (2000 psi) for the forged samples H-31 and H-42,

respectively. A conventional proportional limit of 55.1 MN/m2 was measured for a sample

prepared from forging H-42. These results indicate that press forging also significantly

increases the microyield strength of LiF.

6. Summary

Studies of the response of LiF crystals to press forging indicate that they are sus- ceptible to veiling at large deformation. Using a two-step forging approach, a significant

reduction in the degree of veiling has been achieved in crystals forged to reductions of

70-75%. Forged LiF crystals have been shown to possess stable mi crostructure which is not changed by aging in ordinary ambient environments or after exposure to an atmosphere of

high relative humidity. Studies of the microyield behavior indicate that the threshold for

microyield of LiF single crystals can be increased by an order of magnitude by press forging. Results of optical characterization indicate that the LiF single crystals can be press forged without seriously degrading their optical homogeneity, transmission in the

ultraviolet and visible, and thresholds for damage by short laser pulses of 266 nm and

1004 nm wavelength.

Table 1 . Forging parameters for various two-step forgings.

% Reduction (Forging Temp., °C) Fo rg i n g Step 1 Step 2 Overall Reduction Comment

H-27 (S* II) 45 (500) 45 (500) 70 Unveiled

H-28 (q* I) 55 (600) 45 (500) 76 Veiled

H-28 (q H) 43 (400) 56 (500) 75 Unveiled

H-32 42 (400) 52 (525) 73 Veiled

H-34 (q I) 28 (300) 58 (500) 69 Unveiled

H-34 (q ID 28 (300) 62 (525) 72 Unveiled

H-34 (q in) 72 (525) Veiled

Section (One-Step

Quarter Fo rging

)

All two-step forgings were annealed at 700 C for 1-4 hrs. between steps.

42
Table 2.

Sample

H-<110>-2

H-<110>-3

H-<110>-4

H-28 Q-l

H-28 Q-2

H-32

H-36 Q-4

Optical inhomogeneity , before and after forging, of various Li F samples.

Forging Parameters Inhomogeneity (An/n)

(% Reduction - Forging Temperature )

50%-500°C

69%-600°C

50%-500°C

55%-600°C

43%-400 C 42%-400°C

37%-350°C

500°C

45%

56%

52%

30%

600 C 500° C 525°C

360°C + 50%

Before Forging After Forgi nq

7.1 x 10"^ 5.4 x 10"!? 1.5 x 10~° 8.1 x 10"°

1.3 x 10"° 1.7 x io~?

1.7 x 10"?

1.7 x 10

7.0 x 10

"J 2.6 x 10~£

1.2 x

10~J? 2.3 x10'J? 3.0 x 10"° 5.5 x 10"°

Table 3. Damage thresholds for 1-ns, 1064 nm laser pulses for single-crystal

and press-forged LiF.

9 Damage Threshold (J/cmr)

Sample Pol ish Rear Surface Bui

k

Single Crystal Lawrence Livermore Laboratory 10 ± 1.0 24 ± 4 Press Forged Lawrence Livermore Laboratory 8 ± 1.5 > 24 Single Crystal Honeywell 7.3 ±0.7 9 ± 1.3 Press Forged Honeywell 8.5 ±1.4 ^20

Table 4. Damage thresholds for 700-ps, 266-nm laser pulses for single-crystal

and press-forged LiF.

Sample Polish Front Surface Damage Threshold (J/cm )

Single-Crystal

Press-Forged

Honeywel

1

Honeywel

1

14.3 ± 2.5

14.4 ± 2.5

Acknowledgments

We would like to thank Drs. D. Milam, T. F. Deaton and W. L. Smith of Lawrence

Livermore Laboratory for performing the measurements of laser damage threshold. We also

acknowledge the capable technical assistance of Clark Olson, Dick Brinda, Mark Paquette and

Don Parkin in performing the press forging and experimental measurements. We also thank

Joe Starling of the Honeywell Ceramics Center who polished some of the samples for laser

damage testing.

References

[1] Bernal G. , E., Koepke, B. G., Stokes,

R. J., and Anderson, R. H., in Proc. Conf. High Power Infrared Laser Window

Materials (October 30-November 1, 1972),

Bedford, Mass., Carl A. Pitha, Ed.,

p. 413 (1973). Air Force Cambridge

Research Laboratories, AFCRL-TR-73-0372.

[2] Anderson, R. H., Skogman, R. A., and

Vora, H., Opt. Eng. 18, #6 (1979).

1978 (NBS Special Publication 541),

Glass, A. J. and Guenther, A. H., Eds.,

p. 59 (1978).

[4] Bennett, H. E., Bethke, J. W., and

Norris, W. P., in Proceedings of the

High Power Laser Optical Components and

Component Materials Meeting, 3-4 October

1977, Boulder, Colorado, Harris, J. S. and Strecker, C. L., eds., p. 328.

[3] Ready, J. F., Vora, H., Skogman, R. A.,

Leung, K. M., and Bernal G. , E., in Laser Induced Damage in Optical Materials:

43
[5] Harrington, J. A., Bobbs, B. L . , [6] Koepke, B. G., Anderson, R. H.,

Braunstein, M. , Braunstein, R. , Kim, Bernal G. , E . , and Stokes, R. J., J. R. Y., and Stearns, R. , in Laser Appl . Phys. 45, 969 (1974).

Induced Damage in Optical Materials:

1977 (MBS Special Publication 509),

Glass, A. J., and Guenther, A. H., eds.

p. 127 (1977).

Figures

I 1 UNVEILED

V77\ VEILED

DEFORMATION 1%)

•Figure 1. Excessive deformation at any

temperature introduces veils in LiF. This figures shows the

approximate deformation limits at various forging temperatures.

44
Figure 2. Scattering profile of a 5 mm x

5 mm area of a veiled LiF sample.

45
WAVELENGTH, nm

339 351 364 457.9 489 514.5

o

:

1 1 1

•

1

^multiline

1 1

0 single line

H 31 (52$C-6I%) •

H 31 (SINGLE CRYSTAL) — o •

•

H 41 (SINGLE CRYSTAL) - - A r H 42 (SINGLE CRYSTAL)--

8 • « o c

1 I

1 1

1 38 36 3.4 3.2 30 28 26 2.4 PHOTON ENERGY, «V

Figure 3. Residual ultraviolet and visible

absorption in various Li F samples.

0 100 200 300 400 500

PLASTIC STRAIN , 10

Figure 4. Microyield behavior of singlecrystal and press-forged LiF.

ae
OPTICAL PROPERTIES OF FORGED CaF

2

A. K. Hopkins

Air Force Materials Laboratory

Wright-Patterson AFB, Ohio 45433

and

R. H. Anderson, J. F. Ready

Honeywell Corporate Material Sciences Center

Bloomington, Minnesota 55420

and

J. M. Bennett, P. C. Archibald and D. K. Burge

Michelson Laboratory, Physics Division

Naval Weapons Center, China Lake, California 93555

To satisfy the dimensional requirements for large CaF 2 optical components

while reducing the sensitivity of this material to thermal and mechanical

damage, some form of deformation processing is likely. Strengthened, fine

grain polycrystall ine CaF 2 can be routinely obtained by isostatic forging.

When the deformation of the single crystal exceeds 50%, "veiling" appears

within the bulk of the forging and increases with increasing deformation.

To assess the effects of forging and the resultant "veiling" on the

optical properties of CaF2 , forgings of (111>, (100) and <113> orientations were prepared. Scattering measurements were made in the visible and at 3.39 pm. Then these forgings were isostatically hot pressed (HIP) in an effort to eliminate the veiling. The scattering levels in the visible and at 3.39 ym were

remeasured. Homogeneity measurements were obtained using two different

methods for single crystal, forged and forged plus HIP CaF 2 . Photographs of

the stress birefringence in the same three conditions were also taken. These

optical data are presented and discussed.

Key words: Calcium fluoride; hot isostatic forging; hot isostatic pressing; laser windows;

optical absorption; optical homogeneity; optical scattering; stress birefringence.

1 . Introduction

Calcium fluoride is a leading candidate for application as a laser window because it exhibits low infrared absorption, high mechanical strength, respectable thermal conductivity, low thermal expansion and small dn/dT. The major limitation to its use, however, is the susceptibility to surface flaws in the form of scratches, digs and chips. Because of

this flaw sensitivity, CaF 2 does not obey a Petch relationship of fracture strength

inversely proportional to the square root of the grain size. Previous studies have demonstrated an improvement of fracture surface energy of about an order of magnitude as the

grain size is reduced from that of a single crystal to values of 10 ym [l] 1

. Data from

various sources indicate a surface energy dependence inversely proportional to grain size

to the one-fifth power. Qualitatively, single crystals of CaF 2 that have undergone press

forging are observed to be less sensitive to thermal and mechanical shock.

In order to satisfy the dimensional requirements of large laser optical components,

there are two leading methods of manufacture: large single-crystal growth and fusion

casting. The former, by definition, yields enormous grain sizes, and the latter produces

equiaxed grains averaging 1 to 2 cm. Thus, the material produced by the fusion casting

process has a flaw sensitivity, if related to the grain size, not much different than that

of single-crystal material. Thermo-mechanical processing of this large grain material -

1 Figures in brackets indicate the literature references at the end of this paper.

19

47
either single crystal or fusion cast - is required to minimize the grain size and maximize

the fracture surface energy. Hot press forging of the CaF2 at temperatures below that at which recrystall ization occurs is an ideal method of grain refinement. Mechanical con- straint on the free periphery of the workpiece during forging is desired to reduce tensile

stresses below the level at which cracking would occur. The constraint mechanism chosen

for this study is pressurized inert gas (helium) in the forging chamber [2].

When the deformation exceeds 50% true strain, optical scattering sites occur in the

bulk of the forged sample. In some starting crystal orientations (e.g., (100), (113)),

these sites, called "veils," occur in a random fashion and cause a uniform clouding in the

forging. Other orientations (e.g., (111>) produce a striated veiling as if the phenomenon were occurring on slip planes within the bulk. Electron micrograph examinations suggest

that veiling is caused by microvoids or void sheets formed by the coalescence of vacancies

which were produced by dislocation intersections and annihilations as a consequence of

plastic deformation [2]. Since these veils cause scattering of light in the visible spectrum, studies have been pursued to determine the effect of the veils at the wavelengths of

intended use, 2.7 and 3.8 ym, and to establish methods to solve this problem.

2. Experimental Program/Results

Since the veils were postulated to be microvoids caused by the coalescence of vacancies, a method was sought which would collapse or compact the voids and render the bulk

material transparent at all wavelengths of interest. In the metal -working industry, voids

within forged or cast products have been successfully eliminated through the process of

isostatically pressing the workpiece at elevated temperatures, otherwise known as hot

isostatic pressing (HIP). The workpiece is placed in an autoclave and raised to a tempera ture approximating the recrystal 1 ization temperature, simultaneously imposing inert gas at high pressure on the piece; these conditions are maintained until the voids are collapsed,

perhaps several hours in duration. After this autoclave processing, the piece is cooled,

removed from the autoclave and examined by any of several techniques to determine its

residual porosity. A series of these experiments was conducted on several samples of

single-crystal CaF 2 that had been hot isotatic forged and in which a large number of veils were present. The following describes the experimental program and the results.

2.1 Hot Isostatic Forging

A number of single crystals were purchased from Optovac Corp. in a variety of crystal

lographic orientations: <100>, (111), <112> and (113). For this study, Honeywell thermomechanically processed crystals of selected orientation for grain refinement according to the following schedules:

<100>, <111>, (113): At 750°C with a helium gas pressure of 13.8 MPa, forge at 250 ym/min for 30% true strain; anneal at pressure and temperature for 6 hrs

(annealing at each step under identical conditions); forge an additional 15% at 190 ym/min; anneal for 6 hrs; forge an additional 10% at 63 ym/min; anneal for

6 hrs; forge an additional 5% at 25 ym/min; anneal for 6 hrs; slow cool to room temperature at 25°C/hr.

<113>: At 750°C with a helium gas pressure of 13.8 MPa, forge at 250 ym/min

for 60% true strain with no annealing and rapid cool-down to room temperature.

The two forging schedules were pursued to determine the effect of strain rate, anneal

ing and cooling on the production of veils. For all practical purposes, the high strain

rate, single-step forging with no anneal and rapid cool-down did not magnify the veil

production nor content.

2.2 Hot Isostatic Pressing

In order to determine if veils contained entrapped gases, a small chordal slice was

taken from the <113> sample in figure 1, wrapped in Ta foil, inserted in a furnace and

heated at atmospheric pressure to 750°C. The test philosophy was that if the sample con- tained entrapped gas, the veils would greatly expand upon heating at atmospheric pressure

4R
and either fracture the specimen or remain as enlarged optical scattering sites. Postheat treatment examination revealed no substantial change in veil content, size or fractures

of the specimen.

A second piece of the same (113) sample (the segment to the right in figure 1) was wrapped in Ta foil, placed in an autoclave and heated to 750°C in an argon atmosphere at a pressure of 138 MPa for a period of three hours. Upon rapid cooling to ambient conditions,

the piece was found to have significantly reduced veiling, but the veils were not totally

el iminated.

A third set of experiments was run in which a sample of (111) orientation (the piece

to the left in figure 2) was wrapped in Ta foil, placed in an autoclave and heated to 750°C in an argon atmosphere at 276 MPa for a period of three hours. Virtually total

elimination of the veils resulted.

Encouraged by these results, a set of experiments was designed in which quantitative

reductions of the veil content could be evaluated. Samples of three orientations of CaF 2 <100>, (111) and <113) were forged by Honeywell. The (100) and the (111) samples were

forged at the sequential strain rates with intermediate anneals, as discussed in section

2.1. The <113> sample was forged at the higher strain rate with no annealing or slow cooldown, as discussed in section 2.1. Pre-HIP-processed optical absorption measurements were

made at the Air Force Materials Laboratory and scattering measurements made at the Naval

Weapons Center. A HIP-processing run was conducted at Battelle Memorial Institute using

their autoclave capable of accepting workpieces up to 30.5 cm in diameter, a typical laser

window size. This autoclave is limited in pressure to 207 MPa. The samples were wrapped in Ta foil and heated to 750°C in an argon atmosphere at 207 MPa for a period of 10 hours.

Again veiling was virtually eliminated. Post-HIP-processed optical absorption, scattering,

homogeneity and stress birefringence measurements were made. The results of these measurements are discussed below.

Two other results of the HIP-process experiments are noteworthy. The first is with

regard to the appearance of pits on the specimen surface after HIP-processing. Before

being placed in the autoclave, the samples were optically polished. After HIP-processing,

examination of the samples revealed many pits, a number of which were replicas of the

striated veils, as noted in figure 3. This pitting led to the postulation that veils

migrated to the surface from the bulk. Thus, the bulk material is seen to collapse upon

itself.

The other result of the HIP-processing is that the grain size did not change, remaining approximately 10 ym. This is not surprising in that the HIP temperature is below the

recrystall ization temperature (800°C for highly mechanically strained CaF 2 ).

2.3 Optical Absorption Measurements

Three samples of CaF 2 ((100) , (111) and (113)) were examined for their optical absorption before and after HIP-processing. A 10-watt HF/DF chemical laser was used to provide

the requisite energy at 2.7 and 3.8 ym with the absorption determined in an adiabatic

calorimeter. The results are shown in table 1.

Table 1. Absorption measurements in forged/HIP-processed CaF 2

. Sample Before HIP After HIPA After HIPB orien- g (cm -1

) £ (cm -1

) 3 (cm" 1) tation 2.7 ym 3.8 ym 2.7 ym 3.8 ym 2.7 ym 3.8 ym

(100) 2 97 X

10" 4

1 91 x 10" 4 7.80 X 10" 4 4 40 X 10" 4 6.67 x 10" 4 2.62 x 10" 4

(111) 1 74 X 10" 4 0 827x 10" 4 4.07 X 10" 41 44 X 10" 4 * *

(113) 3 95 X 10" 4 2 42 x 10' 4 8.65 X

10" 4 6 57 X lO"

4 5.99 x lO"

4 2.38 x 10" 4

Specimen fractured, data inconclusive.

The last two sets of columns, labeled "After HIPa" and "After HIPn", represent two different

methods of specimen polishing. Honeywell polished the samples before HIP-processing using

49
various grit sizes of diamond powder (down to 0.3 um) in a kerosene solvent. After HI P- processing, the University of Dayton Research Institute repolished the samples before making

the second set of absorption measurements. They used alumina grit (down to 0.3 um) in a water solvent. The data after this polish are represented by After HIP/\. To eliminate this

variable, Honeywell again polished the samples using diamond/kerosene. The third set of

data representing this final polish is shown as After HlPg.

2.4 Homogeneity

In order to determine the effects of the forging and HIP-processing on the homogeneity

of the CaF 2 , the optical homogeneity was measured using two different methods. The first

method was an interferometric technique developed by Bennett and co-workers [4] in which

a sample with flat, parallel sides is inserted between two optical flats having high

reflectance coatings. An index matching fluid is used to eliminate the effect of the sample surfaces. Fizeau fringes are formed between the coated surfaces, and the fringe pattern

is detected with a TV camera and displayed on a TV monitor screen. Photographs of the

interference fringes are then digitized and the digitized interferograms converted to con- tours of optical path difference through the use of the computer program FRINGE developed

at the Optical Sciences Center, University of Arizona.

Figure 4 shows a typical fringe pattern obtained in the interferometer. The circular

fringes are ghosts caused by reflection of the laser light from the faceplate of the vidicon.

The straight fringes near the edges are the Fizeau fringes outside the sample. The

slightly curved fringes near the center are the fringes obtained through the thickness of

the sample. The curvature represents the inhomogeneity of the sample.

The inhomogeneity, expressed as the variation An in the index of refraction from its

average value, is given by the equation [4]:

An

A a ZT b (1)

where X, the wavelength of the helium-neon laser used as a light source in the interferometer, is 633 nanometers, a is the peak-to-valley irregularity in the fringes, b is the

separation between the fringes, and T is the sample thickness. Results are shown in table 2,

which gives the inhomogeneity in terms of the root-mean-square value of the variation of the

index of refraction. Results are presented for three different crystal lographic orientations for the starting material.

Table 2. Inhomogeneity of CaF2-

(All values are An in units of TO" 5 .)

Sample

(100) Orientation

Single crystal

Forged

Forged and HIP-processed

Inter- ferometric

(rms value)

0.5

0.6

2.3

Stress

birefringence

("typical")

0.2

1.1 1.0

(111) Orientation

Single crystal

Forged

Forged and HIP-processed

(113) Orientation

Single crystal

Forged

Forged and HIP-processed

0.3

0.5

1.8

0.2

0.3

0.6

0.2

2.0

1 .0

0.2

2.0

3.0

50
The lowest values of the inhomogeneity are the values for the single-crystal material.

Forging increases the inhomogeneity, and forging and HIP-processing increase the inhomogeneity still further. However, the inhomogeneity for forged and HIP-processed material is still in the range of a few parts in 10 6

.

The second method of determining inhomogeneity involved measurements of stress birefringence. These measurements were carried out using a circular polariscope. The circular

polariscope allows quantitative measurement of the inhomogeneity to be made by determining

the angle of rotation of a final polarizer used to extinguish the transmission [5]. This

is essentially a point-by-point measurement made manually at a number of different positions

on the sample.

Figures 5-7 show transmission patterns in the circular polariscope caused by stress birefringence in the CaF 2 samples. These samples are oriented in the <100> direction. The

photographs were taken with the polarizers crossed so that the light transmitted is a rough measure of the amount of stress-induced birefringence. Figure 5 shows a single crystal

sample, figure 6 a forged sample, and figure 7 a sample which was forged and HIP-processed.

The pattern of stress becomes progressively more complex in this series of three photographs.

Results are presented in table 2 for typical values of inhomogeneity caused by stress birefringence. These values are a subjective interpretation of what represents a "typical

value" for a sample, rather than representing a rigorous mathematical characterization such

as a root mean square value. The values do, however, compare reasonably well with the

values of inhomogeneity obtained from the interferometric measurement. The inhomogeneity

caused by stress birefringence tends to be lowest for the single-crystal material and high- est for the forged and HIP-processed material. The order of magnitude is also comparable

to that obtained in the interferometric measurement. The implication, therefore, is that

the inhomogeneity is due mainly to the presence of stress-induced birefringence in the

samples, and that the stress-induced birefringence increases as the material is forged and

HIP-processed. Although forging and HIP-processing increase the inhomogeneity, it still

remains in the low 10"° range.

2.5 Optical Scattering Measurements

Bulk forward scattering measurements were made on the Optical Evaluation Facility [6]

by placing the CaF 2 sample in a cell containing a liquid whose refractive index matched

that of the sample and measuring the total light scattered out of the transmitted beam, as

a function of position on the sample. This method of measurement eliminated the effect of

scattering from the surfaces of the sample, which otherwise might mask the bulk scattering.

Scattering from the outer surfaces of the CaF 2 windows on the cell was determined by measuring the forward scattering with the sample removed and the cell filled with the index

matching fluid. The orientation of the sample in the cell was recorded, and maps were made

of the total bulk forward scattering as a function of position on the sample. It was hoped

to be able to correlate areas having high scattering levels with the veils in the as-forged

material. Secondly, by measuring the scattering as a function of position on the sample

before and after the HIP-processing, it was hoped to determine whether the high scatter

points changed by the processing. Unfortunately, neither of these objectives was achieved.

It was not possible to correlate either the bulk forward scattering in the visible spectral

region (6471 A) or in the infrared (3.39 um) with observable veils in the samples. Secondly,

since the identifying index marks on the samples were removed during the repolishing after

HIP-processing, it was not possible to compare the spatial location of the scattering in the

two cases. However, by rotating the contour plots for both the visible and infrared scattering, it appears that the high scattering areas do not correlate. More will be said about

this result later.

The results of the bulk forward scattering measurements are given in table 3 and are shown in figures 8-15. Table 3 lists the average values of the scattering in the visible

and infrared spectral regions for the as-forged samples with veils and for the same samples

after HIP-processing. Sample <111> developed a crack during the repolishing, and hence only

a few measurements were made after HIP-processing. Enough data were taken on the other two samples to enable contour maps and three-dimensional scattering plots to be made. In the

maps and plots the contribution of the scattering from the cell was not subtracted from the

51
scattering measurements because it was quite uniform and did not vary from point to point. For the as-forged samples the scattering contribution from the cell is less than one-fourth

of the total scattering (cell + sample) at 6471 A and about one-third of the total scattering at 3.39 urn. After HIP-processing the contribution from the cell is about one-third of

the total scattering at 6471 A. In the infrared it is about one-third of the total scattering for samples <100> and <111> but almost equal to the total scattering for sample (113>.

This latter sample had by far the lowest average infrared scattering but had scattering

comparable to that of the other samples in the visible.

Table 3. Bulk forward scattering in CaF 2

. Sample Forged, with veils After HIP-processing

ui i eri- tation 6471 K 3.39 urn 6471 K 3.39 ym

< 1 00> 2 1 1+0. 54xl0- 2 4.07±1 .77x10"^ 3.60±0. 86xl0- 3 5 3512.12x10-^

(111) 1 23±0. 56xl0- 2 4.7411.79x10"^ 2.78±0. 69xl0- 3 6 73±2.80xl0" 1+e

(113) 2 34±0. 53x10-2 6.75+1 .64x10"^ 3.16±0. 49xl0- 3 2 94±2. 19x10"^

Cell only 3 04±0. 84xl0 -31 .53+0.40X10" 4

1

29x10-3 2.18X10' 4

Data corrected for scattering from cell

C100> 1

81x10-2 2.54X10- 4 2. 31x10-3 3.1 7xl0 _t+

<111> 0. 93x10-2 3.21x10-^ 1

49x10-3 4.55X10- 4

(113) 2. 04x10-2 5.22X10" 1* 1. 87xl0- 3 0.76xl0 _l+

Averages of 817 points in concentric circles; radial increment 0.050 inch, maximum

, radius 0.800 inch.

Averages of 631 points in concentric circles; radial increment 0.050 inch, maximum

radius 0.700 inch.

^Averages of 35 points on an 0.500 inch radius circle.

Averages of 469 points in concentric circles; radial increment 0.050 inch, maximum

radius 0.600 inch.

By comparing the as-forged and HIP-processed scattering values in table 3, it is seen that the HIP-process reduced the visible scattering by about a factor of eight on the

average, but did very little to the infrared scattering. The average infrared scattering

level actually increased for samples < 1 00> and (111) .

The contour plots in figures 8-11 show primarily the variation of the scattering level at different places on the sample. (The orientation of the sample was the same for the

pair of plots in each figure.) The following features are evident in the plots: (1) For sample (100) there is a large spatial variation in the visible scattering, while in the

infrared the scattering level is more uniform, although isolated high spots are present;

there is very little correlation between the high scattering points in the visible and in the infrared. (2) After HIP-processing the visible scattering is much more uniform over

the sample, but the spatial variation in the infrared scattering is essentially unchanged;

the high scatter regions in the visible and infrared again show very little correlation.

(3) The spatial variation in the visible scattering for sample <113> before HIP-processing

is less than for sample (100); the gradations seem to be more like undulations over the

surface rather than like point defects; there appears to be no correlation between the

spatial patterns in the visible and infrared scattering. (4) After HIP-processing sample

(113) has extremely small spatial variation in the scattering both in the visible and infrared spectral regions. This effect is especially pronounced in the visible where the contour

interval in figure 11 is half that in figure 10. The few isolated high scattering areas in the infrared scattering do not correlate with high areas of visible scatter or with high areas in the infrared scatter before HIP-processing. This latter conclusion can be drawn

by rotating one contour plot relative to the other since the orientation of the second plot

is not known relative to the first.

52
Figures 12-15 give a three-dimensional representation of the same scattering values

that were shown in the contour plots. In this representation the position on the sample

surface is shown in the x-y plane, while the magnitude of the scattering is shown in the z direction. (The orientation of the sample is the same for the pair of plots in each figure.)

In the after-HIP-processing plots, the scales for the visible and infrared differ by a fac- tor of 10, while for the as-forged plots the scales differ by a factor of 40. The feature

most clearly shown in the three-dimensional plots is the spiky nature of the infrared scat- tering as compared to the visible scattering.

3.0 Conclusions

Qualitatively, higher pressures and longer times for the HIP-process progressively

reduce the veil content. Only one set of optical measurements was made on the HIP-processed

material and that at 207 MPa and 10 hours. Regarding the optical absorption, HIP-processing

appears to slightly increase the absorption at 2.7 and 3.8 um. The optical inhomogeneity

increases after HIP-processing but remains in the range of 1 to 3 parts in 10 6

. The main

contribution to optical inhomogeneity is stress-induced birefringence.

The general conclusions which can be drawn from the scattering measurements are as follows: (1) There is no obvious correlation between the veils in the as-forged material

and either the visible or infrared scattering. (2) In the visible the HIP-processing

reduces the scattering by about a factor of eight on the average and reduces the spatial

variation of the scattering. (3) In the infrared the HIP-processing does not affect the

scattering (except for sample <113>) and does not affect the spatial variation of the scattering. (4) There is no obvious correlation between high-scattering areas in the visible

and infrared either before or after HIP-processing.

For applications such as infrared windows, it is not clear that HIP-processing

improves the performance of the material. Further work is needed to understand the interrelations between visible and infrared scattering, absorption, and homogeneity. Systematic

variation of HIP-processing parameters and their effects on these optical properties is required. This should better define the importance of HIP-processing for improving the

performance of infrared window materials.

References

[1] Anderson, R. , Skogman, R. , Ready, J.,

and Bennett, J., "Bulk Optical Properties of Fine Grained Forged Calcium

Fluoride," in Laser Induced Damage in

Optical Materials: 1978 , A. J. Glass

and A. H. Guenther, eds., National Bureau of Standards, Washington, DC,

1978 (NBS Spec. Publ. 541), pp. 70-77.

[2] Anderson, R. , Koepke, B. , and Bernal G.

,

E. , "Intermediate Temperature (0.5 to 0.7 Tmn °K) Forging of Calcium Fluoride

Crystals," in Laser Induced Damage in Optical Materials: 1976 , A. J. Glass

and A. H. Guenther, eds., National Bureau of Standards, Washington, DC,

1976 (NBS Spec. Publ. 462), pp. 87-94.

[4] Bennett, H. E. , Bethke, J. W., and

Norris, W. P., "Optical Homogeneity

Measurement Technique for Laser Windows,"

i n Proceedings of the High Power Laser

Optical Components and Component Mater - ials Meeting , J. S. Harris and C. L. Strecker, eds., DARPA, 1977, pp. 327-338.

[5] Dally, J. W., and Riley, W. F. , Experi - mental Stress Analysis , McGraw-Hill Book

Co., New York (1965).

[6] Bennett, H. E., and Stanford, J. L.

,

"Structure-Related Optical Characteristics of Thin Metallic Films in the

Visible and Ultraviolet," J. Res. Natl.

Bur. Stands. 80A, 643 (1976).

[3] Bernal G., E. et al_, A Study of Polycrystal 1 ine Hal ides for High Power

Laser Applications , Final Technical

Report on Contract DAHC-15-73-C-0464,

Honeywell Inc. Corporate Material

Sciences Center (May 1978).

53
(.113) Forged/HIP.

Figure 2. (Ill) Forged/HIP.

Figure 3. Surface pit replication of veils. 500X magnification.

The scratches remain from the initial polishing.

Figure 4. Fizeau fringes in single-crystal CaF

^4
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63

MECHANICAL AND OPTICAL PROPERTIES OF FORGED NaCl*

George A. Graves, John A. Detrio,

Dale McCullum, and David A. Dempsey

University of Dayton Research Institute

Dayton, Ohio 45469

The mechanical and infrared laser absorption properties were determined

on test specimens cut from a 32-cm diameter forging of Polytran Nacl

produced by Harshaw. The homogeneity of the mechanical and optical properties

statistically showed a significant dependence of the ultimate strength on azimuthal position in the forging. The absorption of the specimens at CO2,

CO, HF/DF, and Nd:YAG laser wavelengths were measured and no significant

dependence on specimen location was observed. The absorption variations

were less than 10 percent for all of the laser wavelengths studied except

at 3.8 ym (40 percent). Other properties examined include specific heat,

thermal expansion, and hardness. Strength measurements were made with

both uniaxial flexural bars and biaxial disc test specimens.

Keywords: Hardness; homogeneity; infrared laser absorption properties;

mechanical properties; specific heat; thermal expansion

1 . Introduction

Increasing interest in polycrystalline NaCl material for use as high energy laser

windows provided the initiative for this investigation. Although a number of studies

have been performed on the mechanical and optical properties of polycrystalline NaCl,

no comprehensive investigation has been accomplished on a single forging that included

information on both sets of properties. Therefore, a designed experiment was initiated

that would provide sufficient information to determine the homogeneity of mechanical and

optical properties within a single forging of NaCl material.

(S Specimens to conduct the investigation were obtained from a 32-cm diameter Polytran

NaCl hot forged disk, prepared by Harshaw Chemical Company, Solon, Ohio. Ten optical (0), two long optical (L0), 72 uniaxial flexural test (FT), and 20 biaxial flexural test (BF)

specimens were cut from the forged disk in accordance with the sectioning diagram shown

in Figure 1. The specimens were prepared by Harshaw. The specifications for specimens

preparation are given in Table 1 . The cutting diagram only shows the layout for the top

of the forging, whereas the disk was actually sectioned into three slabs of material. In order to make the most economical use of the hot forged material the 0 specimens, due to their greater thickness, were all cut from the top layer and the BF disks were taken from

the middle and bottom layers. The L0 specimens' thickness was that of the original

forging. The FT specimens were divided equally between the top, middle, and bottom layers

(24 each).

2. Test Procedures

Specimen Handling

The specimens were received in sealed plastic bags containing dessicant. They were

immediately inspected and placed in vacuum dessicators until just prior to testing.

* This work is supported by the Air Force Materials Laboratory, Wright-Patterson Air

Force Base, Ohio 45433.
Table 1. Specifications for specimens from 32-cm diameter Polytran® NaCl

1. "0" specimens: 10 each; diameter 38.1 ±0.5 mm; thickness - 8.9 mm (+1.3, -0.00 inch)

with size to be maximized as total material thickness permits;

polish - "absorption coefficient sample" polish with flatness A/10 at 10.6ym parallelism of less than 30 arc seconds, and scratch/dig better than 60/40.

2. "BF" specimens: 20 each; diameter - 38.1 ±0.5 mm; thickness - 2.5 mm

(+0.5, -0.0 mm); polish - same as "0" specimens.

3. "L0"specimens : 2 each; length - 102 ± 1 mm; width - 25.4 % 1.3 mm: thickness - 25.4 mm (+0.0, -6.4 mm) with size to be maximized as total material

thickness permits; polish, long surfaces - best defect-free surface possible so as to minimize mechanical damage site, parallelism of 0.1 mm or better, 0.8 mm approximate radius on all edges; polish, end faces - "absorption coefficient sample";

polish as "0" specimens above.

4. "FT" specimens: 72 each; length - 57.2 ± 1.3 mm. width - 10.2 ± 0.5 mm; thickness 5.1 ± 0.5 mm; polish - same as "L0" specimens, long surfaces.

Uniaxial Flexural Tests

Flexural strengths were obtained by testing prismatic beam specimens in four-point

bending using kinematical ly designed fixtures. A universal testing machine was used to load the specimens at a rate of 8.3 x 10"° m/s at 23°C. An LVDT deflectometer was used

to obtain center-point specimen deflection as a function of applied load.

Biaxial Flexure

A concentric ring configuration was used to obtain biaxial flexural strengths. The

outer load ring had a diameter of 32 mm and the inner load ring a diameter of 16 mm. The

specimens were loaded at a rate of 8.3 x 10~6 m/s at 23°C.

Hardness

Room temperature hardness measurements were performed at 50 and 100 gm loads using

a Knoop diamond indentor.

Coefficient of Thermal Expansion

A quartz tube dilatometer was employed for thermal expansion measurements over a temperature range of -65 to 200°C. The thermal expansion detected by an LVDT unit and

recorded simultaneously with temperature on an X-Y recorder.

Specific Heat

A differential scanning calorimeter was used to determine specific heat. The

reference sample was aluminum oxide.

Thermal Diffusivity

The thermal diffusivity was measured by a flash technique developed by Parker, et a!.[l]l. A Xenon flash lamp was used as the heat source. A 13 by 13 by 3 mm specimen,

coated on one side with graphite powder, was used.

Figures in brackets indicate the literature references at the end of this paper.

66
The temperature rise on the back surface of the specimen is sensed by an E-type thermocouple and the temperature rise recorded on an X-Y recorder.

Optical Measurements

The as-received surface finish was provided by Harshaw as specified.

The specimens were prepared for absorption measurements according to the following

procedures

1. A beaker was filled with 300 ml of concentrated HC1

.

2. Two beakers were filled with 300 ml of 2-propanol . One beaker was placed directly

beneath a heat gun.

3. The specimens were etched in the HC1 solution for two minutes at approximately

22°C with gentle agitation.

4. The specimens were rinsed for twenty (20) seconds in the first 2-propanol rinse.

5. Rinse for five (5) seconds in the second 2-propanol rinse and removed slowly

so that they dried as it is withdrawn.

6. The chemicals were changed after every three (3) specimens. The heat gun was on only during the final rinse cycle. Extended operation might raise the temperature of the

chemicals and change the etching process.

The laser parameters used throughout the study were held as constant as practical for

each laser used. The laser parameters were: power (nominal) - 10W, spot size - 4 mm, irradiation time - 250s, and cooling time - 1000s.

The effective absorption coefficient, Be

, was defined by:

S

e

= S + 2a s /£ (1

)

where B is the bulk absorption coefficient (cm"''), a

s is the single surface absorption

coefficient (fractional power absorbed), and i is the specimen thickness (cm). The

effective absorption was measured, for low loss materials, by the interpretation of the

laser-induced heating and subsequent cooling of the irradiated specimen. The equation

used was:

S e

= (mC

p

/Jc) • 2n/(n 2 +l) • (AT/At) Net • (1/P T ) (2)

The parameters are defined in Table 2.

Table 2. Parameters used to obtain the absorption coefficient

from calorimetric absorption data for NaCl specimens.

Parameter Value Mass

m specimen mass

J g-^C" 1 Cp specific heat capacity 0.847

SL thickness 0.89 cm

Pj laser power M0 kU

C C sec" 1 (AT/At)N~t net heating rate

n refraction index rnCO21 .490 10.6 ym

1 .499 9.27 ym

CO 1 .518 5.3 ym HF/DF 1 .522

1 .525 3.8 ym

2 . 7 ym

Nd : YAG 1 .510 1 .3 ym
3. Discussion of Results

Mechanical Tests

The yield and ultimate strengths were determined for 70 uniaxial flexural test (FT)

bars. Two of the specimens were too short to allow for proper testing. The average yield

and average ultimate strength for the three layers are shown in Table 3.

Table 3. Mechanical strength results for Polytran^

hot forged NaCl

Layer

Top

MPa (ksi)

Middle

MPa (ksi)

Bottom

MPa (ksi)

Yield Strength

Standard Dev.

Ultimate Strength

Standard Dev.

14.3 (2.1)

3.6 (0.5)

23.3 (3.4)

6.0 (0.9)

15.0 (2.2)

2.9 (0.4)

23.8 (3.5)

5.5 (0.8)

15.5 (2.3)

2.8 (0.4)

22.9 (3.3)

5.3 (0.8)

Four groups of flexural test specimens were selected to permit statistical analysis

of the ultimate and yield strength measurements for effects due to azimuthal position at 90°, 180°, 225°, and 315° (Figure 1), layer (top, middle, and bottom), and radial position

(inside and outside)

An analysis of variance for this factorial experiment showed that the only factor

which produced a significant effect at the 95 percent level of confidence was that of

azimuthal position. The results were identical for both ultimate and yield strength.

The average strength was significantly greater for specimens along the 90° and 180°

positions than along the 225° and 315° positions. There was no significant difference

between the 90° and 180° positions nor was there any difference between the 225° and 315°

positions. No significant differences were found due to layers, radial position, or joint effects.

The results of the biaxial tests are shown in Table 4. The biaxial strength measurements were analyzed to test for differences between the middle and bottom layers. Since

the biaxial were obtained from the two layers at identical locations, the measurements

were treated as paired comparisons and the average differences between the strengths at each location were tested to determine if it was significantly different from zero. No significant differences were found at the 95 percent level of confidence for either the

ultimate or yield strengths.

Optical Tests

For convenience the results are discussed in order of decreasing wavelength. The

overall results are summarized in Figure 2 which shows the absorption at the laser wavelengths. Both the as-received and freshly etched data are illustrated; the error bars are the standard deviation of the measurements.

The objective of the optical evaluation portion of this study was to assess the absorption homogeneity of the NaCl forging. This assessment was made by measuring the

calorimetric absorption at various laser wavelengths and at different locations on the

forging. The variations in absorption within one specimen were measured by sampling the center and three points on a 0.95 cm diameter circle; the variation within the forging

was measured by taking samples from various locations across the forging. The data are then analyzed to provide the homogeneity--defined as the standard deviation of the absorption measurements.

68
Table 4. Biaxial mechanical strength results for Polytran®

hot forged NaCl .

Standard Deviation

Yield Strength

MPa (ksi)

Ultimate Strength

MPa (ksi)

Middle

Standard Deviation

Bottom Standard Deviation

18.1 (2.6)

1.55 (0.23)

18.1 (2.6)

2.2 (0.32)

28.0 (4.1)

2.2 (0.32)

25.4 (3.7)

3.9 (0.57)

The influence of surface treatment on the measured absorption was studied by comparing

the effect of results for specimens in the as-received condition, immediately after

etching, and after etching and storage.

CO2 Absorption:

The absorption was significantly higher at 10.6 ym than at 9.27 um, and the etched

specimens are significantly lower in absorption than the as-received specimens at both

wavelengths. There appeared to be a significant "interaction" or correlation between

the surface treatment and the absorption at 9.27 ym. The conclusion was that there is significant surface absorption at 9.27 ym. The 10.6 ym absorption was predominantly

bulk absorption or the surface absorption was not influenced by etching in HC1

.

The variation of absorption within a given specimen was within +10 percent. The

variations within the entire forging were also +10 percent. A slight systematic difference was observed in the absorption measured at the center of a specimen and that

measured on the 0.95 cm radius. This bias was a result of the experimental conditions

and not variations in the specimen. One specimen showed an anomalously high absorption

at both wavelengths. The cause of the increased absorption was not determined.

The CO laser absorption at 5.3 ym was approximately a tenth of that measured at CO2

wavelengths. The measured values were approximately 0.003 to 0.004 cm~l with a standard

deviation of +10 percent. The as-received specimens were only slightly higher in absorption than the etched specimens.

HF/DF Absorption:

The DF (3.8 ym) absorption was significantly lower than the HF (2.7 ym) values in all cases. The variations over the forging were not statistically significant. The 2.7 ym

data gave a homogeneity of +10 percent, but the 3.8 ym data were homogeneous to only

+40 percent.

Etching significantly improved the absorption of the specimens with a greater (nearly

50 percent) reduction observed at 3.8 ym. The 2.7 ym (HF) averages were not significantly

reduced by the etching procedure. The implication was that either the bulk absorption

was dominant at 2.7 ym or that the etching procedure was ineffectual at 2.7 ym. The

dominant role of atmospheric water vapor contaminants in producing HF absorption was frequently cited as producing surface absorption. Therefore, the aqueous solutions used

to etch the specimens may not remove the water.

I Absorption:

The absorption measured at 1.3 ym (iodine laser operation. 1.319 ym approximated by

1.315 ym Nd:YAG (laser) was between 0.0006 cm"! anc| 0.0009 cm~l and was not strongly

dependent upon the surface treatment. Two specimens had a very high absorption that could

not be readily explained. The homogeneity of the forging was between 8 and 14 percent

(excluding the high absorbers).

6Q
Thermal Properties

Thermal Expansion:

The coefficient of thermal expansion was determined using specimens taken from

several different areas of the forging. The average coefficient of thermal expansion

obtained for the forging was 42.8 x 10~6°cH (S.D. = 0.86 x 10"°) over the temperature

range of -65°C to 200°C.

Specific Heat:

Specific heat measurements were conducted on two specimens for the forging from

62°C (335°K) to 207°C (485°K). C p values were calculated at 5°C temperature intervals

resulting in an average specific neat value of 0.2 cal/gm/°C over the temperature range

for both specimens

Microstructure and Mi crohardness

The microstructure of the forged material was uniform throughout the forging. A typical microstructure is shown in Figure 3. The average grain size was 34 urn. The

average Knoop nardness value obtained from four randomly selected specimens using a 50 gram load was 20.1 kg/mnr.

4. Conclusions

With the exception of ultimate and yield flexural strength variations relative to azimuthal position the mechanical, optical, and thermal properties were uniform through- out the bulk of the Harshaw Polytran® NaCl hot forged material, within the limits of

the experimental design.

5. Acknowledgements

The support of the Air Force Materials Laboratory, Wright-Patterson Air Force Base,

Ohio is gratefully acknowledged. The contractual effort concerning the mechanical

characterization of infrared window materials is monitored by Mr. Charles Strecker. The

optical characterization of laser window materials has been monitored by Dr. G. Edward

Kuhl and Dr. Walter Knecht.

6. References

[1] Parker, W. J., et al . , "Flash Method of Determining Thermal Diffusivity, Heat Capacity, and Thermal Conductivity," J. Appl . Phys., 32(9), pp. 1679-1687 (1961).

70
7. Figures

No CI (HARSHAW POLYTRAN FORGING)

AS RECEIVED

WAVELENGTH ( ^.m)

Figure 2. Absorption coefficients at various wavelengths for as-received

and etched specimens taken from Polytran® NaCl forging.

71
M I CR OSTRUCTURE

72
OPTICAL, THERMAL, AND MECHANICAL MEASUREMENTS

ON C02 LASER-IRRADIATED ZnSe

J. A. Detrio, J. A. Fox, and J.M. O'Hare

University of Dayton Research Institute

Dayton, Ohio 45469

The laser power dependence of the temperature distribution, optical

distortion, and strain have been measured for specimens of ZnSe with absorption values between 0.006 and 0.020. For a well behaved Gaussian beam

profile we observe a thermal lensing effect which changes the location of

the beam waist but which does not significantly increase the minimum beam

diameter. The predicted and measured temperature rise agree for laser

powers of 230 and 705 watts and beam diameters of 2.1 mm and 4.2 mm, respectively. The identification of factors which influence the departure

of predicted from measured response are discussed. The precision of the

model is also examined.

Key words: Absorption; calculated performance; C02 laser; holographic

interferometry ; measured performance; optical path difference; temperature

profile; thermal distortion; thermal lensing; ZnSe

1. Introduction

The goal of this work is to assess the accuracy of the prediction of the optical,

thermal, and mechanical responses of ZnSe to C0

2 laser radiation. The accuracy of the

model in predicting the performance will depend upon the accuracy of the input data and

upon the adequacy of the theoretical model.

The validity of the prediction model is tested by comparing the measured response

to the predicted response. The optical performance includes the thermally induced optical path differences, 0PD, and the changes in the far field irradiance (focal plane spot

size) produced by the thermally distorted window. The thermal response of the window

includes the temperature distribution within the window and the time rate of rise of the

edge temperature.

The scope of this work includes three different specimens of CVD ZnSe with different measured absorption and the use of two different C0

2 lasers. Thus the influence

of variation in absorption and the influence of different laser power and beam diameters

can be assessed. The study is limited to a single material and to the use of only one computational model.

In a previous paper [l] 1 the effects of laser irradiation on ZnSe at a single

power level (230 W) was assessed. The present study extends the measurements to higher

power levels (705 W) and includes some refinements in techniques. Because reference 1 may not be readily accessible to the reader some of the relevant details will be repeated

here for completeness.

Figures in brackets indicate the literature references at the end of this paper.

73
2. Calculational Model

The primary response of a window to laser radiation is thermal; as energy is

absorbed the temperature rises. The results of this temperature rise are thermally

induced stresses and thermally induced distortion. The primary quantity of interest

is therefore the distribution of temperature within the window as a function of time,

T(r,z,t).

The temperature distribution is the solution to the heat conduction equation:

pC

p (3T/8t) = kV2 T + Q(r,z,t).

The heat input Q(r,z,t) (W cm-3) is assumed to be proportional to the laser beam intensity

profile, I(r,t) (W cm

-2

) and the absorption, Be (cm_1

).

The absorption can be distributed between the surface, a

s (fractional absorption)

or the linear absorption coefficient, Btcm- 1 ). This model assumes a homogeneous absorber

and neglects scattering. The boundary conditions that apply include heat loss from the

surfaces

F(r,t) = k(3T

s /3p) + hT$ = a

s I(r,t)

where (3T

s /3p) are directional derivatives normal to the surface, h is the heat transfer

coefficient for the surface at temperature T . The surface absorption is explicitly

included in these boundary conditions.

The total energy absorbed by the specimen is given by:

E a

= W + 2a

s

) P Q

t

p

= 3e Pt

p where t is the irradiation time and P Q

is the total laser power obtained by integrating

the incident intensity distribution.

The solution of the time dependent heat equation for a Gaussian beam profile and

the given boundary conditions is obtained by the use of a computer program "THERM" written by Dr. John Loomis [2] and modified by Dr. J.M. O'Hare [3]. The program is described in UDR-TR-78-76 dated August 1978.

The computations yield the following output:

T

e

(t) Edge temperature as a function of time

T(r,t) Average temperature as a function

radial position versus time

of

a

r (r,t) Radial stress versus position and time

a

t (r,t) Tangential stress versus position and time

*(r,t) OPD versus position and time

Kr.z) Intensity distribution at axial location z

74
The materials parameters used in these calculations are summarized in table 1.

Table 1. Properties of CVD ZnSe employed in calculations

Mechanical

Poisson's Ratio v 0.28

Young's Modulus (psi) E 9.75xl0 5

ThermalHeat capacity (j/gK) C

p 0.355

Thermal

conductivity (W/cmK) k 0.17

Thermal

expansion (K_1

) a 7.8xl0- 6

Physical

Density (g cm

-3

) p 5.27

Optical

Refractive Index n 2.41

Thermooptic

Coefficient (K- 1 ) dn/dT 6. 17x10- 5

Elasto-optic p n -0.08

Coefficients p J2 0.02

3. Experimental Details

Three specimens of Raytheon's Chemical Vapor Deposited (CVD) ZnSe with different

absorption values were selected for study. The specimens were polished by PTR Optics of

Waltham, MA. The specimens are 38 mm in diameter, approximately 4 mm thick with a 1 degree wedge angle. The surfaces are flat to A/10 at 6328$ with a 50/20 scratch/dig

specification.

These specimens were selected because they had absorption values that were significantly different but with the same visual appearance. The absorption values have been

measured at various times during the course of this work. The absorption data are sum- marized in table 2.

Two C02 lasers were used in these studies: A Coherent Radiation Model 41 with 4.2 mm

Gaussian beam at 230 W and a Sylvania Model 971 with 9.6 mm Gaussian beam at 705 W. The

nominal power levels used were selected to give the most stable laser output and to give

a 3:1 power ratio.

The experimental layout is shown in figure 1. This is the same basic arrangement

used in Ref. 1. The laser power is measured with a CRL Model 213, or Model 201 power

meter with the output recorded on one channel of a strip chart recorder. A typical strip

chart trace is shown in figure 2.

The edge temperature is monitored with a copper-constantan thermocouple and recorded

on one channel of the four-channel chart recorder. For the 705 W studies an aluminum heat

sink is placed between the specimen and the thermocouple to minimize the effect of

scattered radiation. If this thermal mass is not used, the scattering is absorbed by the

thermocouple and produces a larger temperature rise in the specimen at the point of contact. The A£ block reduces the transient heating effect by distributing the scattering

over a larger thermal mass.

75
Table 2. Absorption coefficients, 6g for CVD ZnSe

at different laser powers, times, and with

different strain gauge attachment conditions

Average laser power (watts)

10^20 230 705

1105 3.3 6.5 7.2 5.43 8.5 4.0 3.7 2.8

1246 10.3 10.4 14.0 11.7 18.0 12.1 13.1 11.4

1244 18.7 19.6 25.3 23.5 28.0 24.0 25.2 19.4

Date 5/77 5/78 7/79 10/78 5/78

a 3/79 5/79 7/79

No. of

Gauges 0 1 2 1 1 2 2 2

a Incident power only was measured. For this situation

6e

= (mC

p

/A) (AT/At) Net

/P

i where P. is the incident laser power.

A holographic interferometer is used to measure optical path differences. The interferograms are obtained by photographing the specimen image, as viewed through the reference

hologram. The 35 mm photographs are made at selected times during irradiation while the

entire interaction is recorded on video tape. Each 35 mm frame is photographically

enlarged and printed, then the fringe positions are plotted with the aid of a Tektronics

graphics tablet.

The $(r,t) function of position is obtained at HeNe wavelengths. This function is interpreted to give the corresponding temperature distribution T(r,t). The interferograms

are analyzed to measure the rms deviation of the wavefront from a plane or a spherical

reference surface, the Strehl ratio, and the effective focal length of the best fit

quadratic surface.

The beam diameter is measured by a moving knife edge technique. The method is based

on the fact that the integral of a Gaussian intensity distribution leads to an error

function of the form

1 - 2T = erf(/2"y

o /p),

where T is the fraction of the total beam power passing the knife edge, y

Q is the position

of the knife edge and p is the beam radius. The transmittance is measured every ten microinches and we use a graphical solution which transforms this function to a straight line

when T is plotted against yQ

(if the beam is Gaussian). The beam diameter is the distance

1/e points in the distribution (13.5% and 86.5%).

A procedural change over the work of reference 1 has provided more accurate information on the presence of thermal lensing. In the earlier work the beam diameter was only

measured at the primary focus; now we measure the diameter as a function of position along

the mirror axis. This procedure allows us to separate defocusing effects from distortion.

The earlier work used two Micromeasurements (MeM) Type EA-06-062AQ-350 gauges

installed on opposite sides of each specimen; one gauge oriented to measure the radial

strain component, the other to measure the tangential strain. An improved T-rosette

gauge from MeM; Type EP-08-062TH-120 was installed for the high power measurements. These

latter gauges represent a better match to the ZnSe materials expansion coefficient.

76
Two strain amplifiers were used—a custom-made differential amplifier described in

reference 1 and a Vishay Model P 350 from M=M. The gauges were connected in a bridge

circuit using the radial gauge as one input and the tangential gauge as the reference.

This configuration provides the algebraic difference of the signals (sum) while subtracting the thermal effects. This latter point is essential because of the heating

effect of scattered radiation. The measured strain e is related to the stress according

to:

The calibration of the gauges and amplifiers were verified using a beam with known loads.

The response data will be discussed in three categories: thermal, optical, and

mechanical. The specimen characterization data will be presented first. The customary

equation for obtaining the effective absorption from the thermal rise data is:

which relates the absorbed power to the materials parameters and the power circulating

within the specimen. Pj is the transmitted power and the factor 2n/(n 2 +l) relates the

transmitted power to the power within the sample.

The temperature measurements consist of edge temperature versus time and the tem- perature profile derived from the holographic interferometry. The validity of edge

temperature measurements made with the M heat sink was verified by comparing measurements made with and without the heat sink. The measured values of temperature rise

differed by less than 4%.

The thermal rise data for the three specimens when irradiated by each laser are

shown in figure 3. The calculated thermal rise curves are also shown for comparison.

The thermal profile data are shown in figure 4. The temperature difference is related to the fringe distortion, according to prior interferometric measurements [4],

as 1.713°C per fringe at 6328$. The refractive index of ZnSe at the HeNe wavelength

is 2.59. The calculated temperature distribution and the 230 W laser beam profile are

also shown in figure 4.

The optical response data are derived from the holographic interferograms and from

the beam diameter measurements. The holograms are reproduced in figure 5. Since the OPD

versus position is correlated with the thermal profile of figure 4 a separate presentation

is unnecessary. The spot size data are of special interest. In figure 6 the beam

diameter measurements are shown as a function of position for the high power measurements.

The measured strain is shown in figure 2 for a typical data trace on the 4-channel

recorder. The strain data represent the practical state-of-the-art for resistance

strain gauge techniques. This problem is compounded by the thermal contributions. The

data are summarized in table 3 which includes selected data for both kinds of strain

gauges. The calculated values are also given in the table. The results are independent

of the gauges or amplifiers used. The new gauges provided a smaller standard deviation

in the measurements. The uncertainty in these measurements is between 30% and 100%.

er

- e

t = [d+v)/E](a

r

- o

t

).

4. Data and Results

Net

77
Table 3. Comparison of measured and calculated

"ostrain for ZnSe discs irradiated with a C0

2 las of 230 W and 705 W power for one minute

Specimen Absorption 120ft Gauge 350ft Gauge

No.

3e (cm- 1

) 705 W 705 W 230 W Meas

Calc. MeasCalc. Meas.

a Calc

1105 0.0065 5.6 7.4 6.3 6.7 2.5 2.3

1246 0.0104 11 .6 11 .8 9.9 10.7 3.2 3.7

1244 0.0191 29.6 22.4 40.8 19.6 4.5 6.8

Reference [1]

5. Discussion and Correlations

Two kinds of comparisons can be made: one to assess the consistency of the input

data (materials parameters), and the second to assess the validity of the predicted

response. Graphical presentations of these comparisons are especially helpful and are

used repeatedly; the measured response is plotted as a function of the measured absorption or the laser power to ascertain the consistency of the input data, or the measured

reponse is plotted against the calculated response to test the accuracy of the predicted

response.

5.1. Optical

In figure 7 the fitted spot size data from figure 6 are superimposed at the axial

position corresponding to the minimum beam waist to show that the spot size is not significantly altered by the thermal lensing. The distance to the minimum spot size' (effective focal length) is plotted as a function of the effective absorption in figure £ The fit to a quadratic dependence of this function is shown.

The diffraction limited spot size for a Gaussian beam is given by

, 4 Af

tT "D~

•

In these experiments the focal length f is approximately 524 mm, the wavelength, X, is 10.6 urn and the beam diameter D is 4.2 mm and 9.6 mm for the 230 W and 705 W laser,

respectively. The corresponding minimum spot sizes are 1.6 mm and 0.71 mm. The per- formance calculation assumes diffraction limited optics; however, the measured spot sizes

with no specimen in place are approximately twice the diffraction limit. For this reason

we compare the measured change in spot size to differences between the calculated spot

size and the diffraction limited spot size. The data for measured spot size versus absorption are shown in figure 9.

The calculation of the focal plane response assumes that the window is located at or near the focusing element. In the experimental configuration the window is approximately 220 mm from the mirror.

5.2. Thermal

The calculated and measured edge temperatures observed after one minute of irradiation are plotted as a function of the measured absorption in figure 10. Both the 230 W and 705 W data are shown. The calculated values which are based on the low power measurements of absorption are well behaved; a linear fit to these quantities intercepts the

origin and correlates well with the measured absorption. The high power data are con- sistently overestimated while the low power data are generally underestimated by the

calculation. The low absorption specimen showed the greatest variability.

7R
The peak center-to-edge OPD is shown as a function of absorption for two sets of

measurements made at 230 W in figure 11. The earlier data show excellent correlation,

but some discrepancies in the later date indicate the spread in such measurements.

The history of the specimen absorption values is found in table 2. The influence

of strain gauge mounting is evident by the increased absorption. The mechanism which

explains the increased absorption is the absorption of scattering by the strain gauge

which then heats the specimen. This internally scattered radiation would normally

escape from the specimen if the gauges were not attached.

5.3. Mechanical

The strain measurements are clearly the most difficult and uncertain of the

response characterizations made. The difficulties are related to the very small signals

available and to the effect of temperature changes produced by the heating of the specimen

and the absorption of scattering by the gauge. Other sources of error are damage to the

gauges and leads, the effect of humidity and aging on the gauge response, and differences

in the quality of the bond between the gauge and the specimen.

The measured and calculated strains are compared in figure 12 which uses selected

data. The uncertainty in these measurements is between 30% and 100%. The data were

selected for cause; in most cases a contact was damaged which rendered the data unusable;

in some cases the data for one specimen were suspect without any identifiable cause.

5.4. Validity

The overall validity of the model is assessed by its ability to predict the edge

temperature. The results are shown in figure 13. Only one data point deviates significantly from the ideal correlation; that for the lowest absorbing specimen (number

1105). The difficulty probably lies with the measured absorption which fluctuates with

the number of strain gauges attached and with the condition of the surface. This

difficulty is expected since good absorption measurements are required and they are difficult to make for low absorption specimens.

The comparison of spot size measurements to the predictions of the model is shown

in figure 14. The two slopes correspond to the two different beam diameters at the

different power levels. The correlation is linear but not ideal. It appears that the

model is sensitive to the laser beam diameter or mode structure with the small diameter

results being underestimated by the theory, while the high power data are overestimated.

6. Summary

The response of ZnSe to an axi symmetric C0

2 laser beam is adequately predicted by

the analytical mathematical model used for these studies. The model is valid for laser

powers up to 700 W. The agreement between the predicted and measured response depends

upon the materials or laser parameters involved and upon the techniques used to measure

the response. The results are summarized in table 4.

In general the response correlates well with the effective absorption. The thermal

response provides the best agreement. The optical characterizations are well correlated,

but the quantitative agreement suffers from an inability to match the experimental con- ditions to the theory. The agreement between predicted and measured strain is the least

precise of the response characterizations. Further strain measurements are being made

to refine these data.

79
Table 4. Summary of the comparison of the response

of C0

2 laser irradiated ZnSe to the

performance predicted by the thermal model

Response

Thermal

T

e

(t)

T(r,t)

Optical

*(r,t)

Kr,z)

Mechanical

a

r (r

Q

,t)-a

t (r

o

,t)

Correlation

with absorption

Excel lent

Good

Good

Excel lent

Good

Agreement with

prediction

(approx. %)

±15%

-20%

-20% Not matched to theory.

±30%-100%

7. Acknowledgments

This work is supported by the Air Force Materials Laboratory, Wright-Patterson Ai Force Base, Ohio 45433. The authors are pleased to acknowledge the assistance of their

colleagues at the University of Dayton. The fringe data were analyzed with the aid of

computer program written by Mr. Richard Harris. The graphical method for measuring the

beam diameter was provided by Mr. D.D. Preonas. The strain gauges were installed by

Mr. Dale Grant.

8. References

[1] Detrio, J. A., Petty, R.D., O'Hare,

J.M., and Fox, J. A., published in

Proceedings of the High Power Laser

Optical Components Meeting, 14-15

Sept 1978, Part 1, June 1979. Compiled by James L. Stanford, NWC TP

6111 Part 1, pp. 148-157.

[2] Loomis, John S., "Computer Program for

Calculating Thermal, Mechanical, and

Optical Properties of Discs Heated by

Laser Radiation," July 1977.

[3] O'Hare, J. M., UDR-TR-78-76

,

21 August 1978.

[4] Harris, R.J., Johnston, G.T., Kepple,

G.A. , Krok, P.C. and Mukai , H.

,

Appl. Optics, V. 16, #2, 1977.

9. Figures

pn
ansmitted

1

till

/

JlCfJ

Focal

Spot

Oc

lusion

Loser

Power

On

Beam

--—

DumpTioow 1 \

o

> r j % 1 ° ° ¥ ° k "° 4 ,

2, ce °- Y r- -3

^ a Q3 CD, -«

uj e »

-^r « ^ "N ^

CO J

\ ~% ° \ ^ w

1 o \

1 <M 1

/ v o

! ^

O

a>

CZ +-> C CD <e to

-CZcj CU >>

1

'

CD (/) 3

a> Q Q

JZ E CD

-i-> O CZ CJ CO M- +-> o CU i

-CZ 3

at -t-> E

•r— Cl LTi E CD rd -£Z TJ X 3 CD

ai "O

CD i- (J o

=t o

^ CU

s- CM ^ CD

CD <^ CD T3 03

<_O re

cr, u 4-J CD 03

Ll_ 5-

Eo on 1 +J

sz CD T3 S- 03 o 03 4- C (/) CD 4- 03 Z3 QCD +-> X CD

-l-J CD -l-> 03 CD

c: 1/1 S- 3 CD 0) +-> ro 3 E

cz CL I/) O

CD 03 CJ CD Q-O 10

cr. S- E ro ro

c: CD -cz CD CSJ -M

ft! CL CL CU c o

S- E 03 3 JZ CD CD

CD S- 1/1 h- -SZ E cn-o

03 +J cn re 4-> CD CZ CD o CD cn

CD E • i_ C CZ CD

03 en o cr> O 03 o +J

+-> TJ .e CD C 4- s_

<z CD rsj s- -i-j CO

CD S_ "O 03 E 4— CD i/i O CD iyi CD O E +-> s_ S_ O -Q

s- +-> r- 3 03 Cl

CD +-> +J O c cn 4-J CL CL o T- CD i/i X CD (/) 1/1 E 4- E 3

LU E Z5 C E

CD V o (/) ^ ^ 03 CD O 4-> </) 3 CD CJ 3 =t 3 +-> en > o O c/i -O CZ ro 4- CL fD CD ai CD C C

E S- c (/) 3 O

3 03 >, CD 3 •M CL

CT> CD s_ S- i/l o CZ LO E

-£Z +-> 4—

>

co x: CD O O

U_ +-> 1/1 CU 1— 1/1 E u

PI
82
TEMPERATURE DIFFERENCE vs. RADIAL POSITION

SAMPLE 1105 TOTAL POWER IS 230 WATTS

w 2 0

Q.s

MEASURED TEMPERATURE

CALCULATED TEMPERATURE

10 6UM BEAM PROFILE

SAMPLE 1246

1

i i

-10 -05 0 05 10

SAMPLE 1244

-05 0 05

RADIAL POSITION(CM)

Figure 4. Thermal profile measurements and predictions are compared for the three ZnSe specimens at 230 W. The laser beam profile is also shown. Some of the differences

between the measured and calculated temperature profiles are due to the choice of the reference background fringe pattern. Some of the differences are due to artifacts in the ntting process when there is an unequal number of fringes on each side of the maximum distortion.

83

i
>-<

>-O

at

a<oz

(X3Q<5

inO

CD

Oza

— CN I

O. uj ~: u < z OC QJ O £ O S ox

R4
No Sample

#1211

100 200 300 400 500 600

AXIAL POSITION (mm I

Figure 6. The moving knife edge measurements of the beam diameter near the focus for

the case of the three ZnSe specimens and

the case with no window in place are shown for the region near the focus.

5 Or

LEGENO

- No Sample

Sample 1105

- Sample 1246

- Sample 1244

-100 0 0 100 0

AXIAL POSITION (mm)

Figure 7. The beam diameter versus axial

position near the focus for the four cases plotted in Figure 6 are superimposed with

the fitted curves adjusted along the z- axis to align all of the minimum. The

significance of this presentation is that the minimum spot size is unchanged

by the windows; however, the focal position

changes.

85
- 100 r

E

Figure 8. The distance from the focusing

mirror to the position of the minimum spot

size is plotted as a function of the

specimen absorption. The continuous curve

is a quadratic fit to the experimental

data. The fit is excellent and the

quadratic dependence is expected.

ABSORPTION Bt (cm I - 10 Figure 9. The change in the measured and

predicted spot size for the three specimens

is plotted as a function of absorption.

This presentation illustrates the difficulty of predicting the minimum spot

diameter for the non-ideal optical system

used in the experiment.

86
ABSORPTION 0e ( * IO" 3 cmH

)

Figure 11. The peak optical path differences for several measurements at 230 W are summarized. The primary purpose of showing

these data is to illustrate the experimental error that can occur. Part of the

variability is due to the factor discussed

in the caption to Figure 4.

87
30

20

T. GAUGES

LINEAR GAUGES

230 WATTS

705 WATTS

20 40

MEASURED STRAIN fit (cm/fcm) -r 10""

Figure 13. A comparison of the measured

and calculated peak edge temperature rise

for both 230 W and 705 W experiments is shown. These data summarize the most, successful application of the model. Only

the lowest absorption specimen deviated

from the predicted value and most of this

deviation can be attributed to the

increased absorption due to the strain

gauges

Figure 12. A comparison of the measured

and calculated strain for selected strain

data at 230 W and 705 W. The data shown were selected because no obvious cause for

rejecting the values could be identified

such as a damaged lead or amplifier malfunction. The accuracy of the measurement

was therefore difficult to assess; based

on all of our data the uncertainty is between 30% and 100%.

MEASURED(°C)
750 Warts o 250 Watts

0

-0.10 0 O.K) MEASUREOtmm)

0.20

Figure 14. A comparison of the measured

and calculated change in spot size. The

data do not lie on a single line because

of the difference in the beam diameter

and the lasers used at 230 W and 705 W.

89

ELECTRONIC, NUCLEAR AND TOTAL NONLINEAR INDICES OF LIQUIDS

David C. Brown, J. M. Rinefierd, S. D. Jacobs, and J. A. Abate

Laboratory for Laser Energetics

University of Rochester

Rochester, New York 14627

We have extended the work previously reported to this conference on the

nonlinear indices of a wide variety of liquids. Based upon measurement of linear

indices and calculation of the Abbe value, we obtained the electronic contribution to the total nonlinear index. Using measurements tabulated recently by

Hellwarth we are then able to calculate the nuclear and total nonlinear indices.

Agreement between the total nonlinear index and measurements reported in the

literature will be discussed as will the correlation with Abbe" value and linear

index.

Key words: Abbe value; coolant; index-matching liquids; nonlinear refractive index;

self-focusing.

1 . Introduction

It is well known that the principle mechanism limiting the achievable peak power from

Nd: Glass lasers is small scale self focusing, arising from the existence of a cubic non- linear index, a subject which has been thoroughly addressed in the literature [l,2]i a major development in understanding such effects was the work of Boling, Glass and Owyoung,

[3], which for the first time allowed one to calculate the electronic part of the nonlinear

index nf for a wide variety of transparent solids using only the linear index at the location of the Na "d" lines n^ and the Abbe" value

i both easily measurable quantities. With

few exceptions the agreement between calculated values and experimental measurements has

been excellent; the reader is encouraged to see the recent excellent article by Weber,

Milam, and Smith [4]. An assessment of the nuclear contribution n^ to the total nonlinear

index n| in fluoroberyllate glasses has been made recently by Weber, Cline, Smith, Milam,

Heiman, and Hellwarth [5].

It has been realized for some time by a number of workers that the same formalism used

to calculate n| for solids should be equally applicable to liquids. Preliminary measurements and calculations of n| were presented at the 1978 Conference on Laser Induced Damage

in Optical Materials [6]. Interest in the use of such liquids for applications in high

peak power Nd: Glass laser systems has heightened in recent years. For example, there is interest in using a liquid to cool the front face of an active-mirror amplifier, in index

matching and cooling various zero axial -gradient high-repetition-rate amplifiers, and in index matchinq for larqe aperture Pockels cell devices.

We present below measurements and calculations of n^ for a number of liquids.

It will be seen that by combining that information with a recent tabulation of nucleer

parameters by Hellwarth [7], it is possible to find the nuclear n£ and, thus, the total non- linear index nl as well. We will discuss the dependence of the total nonlinear index on various optical parameters as well as implications for high peak power system design.

2. Theoretical Background

One can show that the total nonlinear index n

z is related to the third order susceptibility xSiJj through [3]

figures in brackets indicate the literature references at the end of this paper.

91
n

2

= Re CxJm(-(*>»w.w.-u)] (1)

where w is the laser angular frequency, n. the linear index, and Re denotes the real part.

The total index may be written as

n = n d + n

2 <E>

2

= n d + YI (2)

where E is the electric field, < > denotes the time average, y the nonlinear coefficient,

and I the intensity. <E> 2 is related to I in electrostatic units through

1 - 4^ <E>2 (3)

where c is the speed of light. Since the value of n

2 is usually quoted in electrostatic

units (esu), y is given by

(4.19 x lo"

3 )n 9 V- * (4,

Two major effects account for the third order nonlinearity in solids and liquids, namely

the electronic part n^ due to nonlinear distortion of the electron clouds around the nu- cleus, and the nuclear part n£ which is due to the nuclei as they attempt to minimize the

total matter-field interaction. Both mechanisms are extremely fast (<10" 12 sec.) and therefore operative for picosecond duration pulses. Then we write the total nonlinear index

nj as

nj = n| + n£ (5)

We consider here only the nonlinear index for linear polarization. It has been shown that

the nonlinear index for circularly polarized light is less than that for linear polarization [7], and in glass is lower by a factor of 1.44 [8].

To calculate n^ for a solid or liquid one may use the theory developed in Reference

[3] to show that n| is given by

e « n d- P("d + 2 > 2 ...

L v

d {1.517 + [(nj + 2)(n

d + 1 )vd /6nd ]}

Wd

where n<j and v

d are the values of the linear index and Abbe

- values at the location of the

Na ""d" lines, and K a normalization constant determined by fitting to experimental data.

Although eq. (6) is limited since it ignores the dispersion or frequency dependence of n|,

it has nevertheless been extremely useful in finding nf for solids at 1.06y based only

upon measurements of n^ and yj. The Abbe* value, a measure of the reciprocal dispersion

is formal ly given by

where n,j, np and are the linear indices measured respectively at the corresponding wavelengths \. = 5876A, Xp = 486lA\ and A

c = 6563A\ A low value of nf can thus be obtained by

using a material with a low n d and large v<j.

For linear polarization, Hellwarth [7] has shown that it is possible to write n^ as

where a, A

Q

, and B0 are Born-Oppenheimer coefficients which describe the electronic (a) and

nuclear (A0,B 0

) contributions. Then from eq. (5) we see that

and

d 4 ' ^

(

fl o + V < ,0 >

92
Formal expressions for A0 and B 0

, available from Raman scattering data, may be found in the

article by Hellwarth [7].

It is. also useful to separate the total third-order susceptability x{j?| into electronic (x||^|) and nuclear parts (x

n wj),

T(3) _ e(3) , n(3) nu

xini " xnii + xnn

and to write n^ and n" as

and

4 - f ^xftj) 02)

"2 = fMxj{«) (13)

For liquids [7], it can be shown that A

Q

= - \ B

Q

, so may be written

In what follows below, we will also calculate for liquids the fraction f

e of the total non- linear index which is electronic in origin, given by

f

e

= n^/nj (15)

3. Measurements

In order to calculate n^ from eqs. (6) and (7), we have measured n<j, np, and ru for

all of the liquids shown in Table I where n^ and are tabulated. The measurements were

performed using a Hilger-Chance refractometer with a tunable dye laser source. To calculate

n| it is necessary to determine the value of the normalization constant K in eq. (6).

Normally, one would obtain it by a least squares fit to experimental data, however, little

data is available at present. Instead, we have set the value of K by normalizing to the

well known values for benzene. In particular, we have used the values determined by

to find nS average value

[n| = 4.82 xio-13 esu) one then obtains K = 101.5. This value for K along with n^ and

enabled us to calculate n^ for all the liquids shown in Table 1. n| varies from

0.40 xio-13 esu for FC-1 04 to 13.44 xio-13

esu for 1-bromonapthalene. The efficacy of

Levenson and Bloembergen [9] and Owyoung [10] for Xjfn anc' used ecl- 02) lo una

4.62 xio-13

esu and nf = 5.02 xio-13 esu respectively. Taking the averagi

this procedure is unknown at present due to the paucity of experimental data on x?f n

or

n|. A more precise value for K could be obtained when such data becomes available.

Using eq. (9), we have also calculated the electronic coefficient a as shown in Table

I. For nine of the liquids listed there, we show data taken from a comoilation by

Hellwarth [7] for (a + B 0

). Thus we can then find B Q and from eqs. (14) and (5) the values

of n" and nT, with the results shown in Table I. It should be noted that for three of the

liquids (cyclohexane, acetic acid, CCliJ B 0

< 0. As pointed out by Hellwarth [7], theoretically B

0

> 0 always, hence the data here should be treated as only preliminary, a re- flection of uncertainties in the normalization procedure mentioned above, and in the

measurement of (a + B 0

). It is likely that as more precise values of n| and B 0 are obtained from measurements this situation will be resolved. It is obvious that at present

it is impossible to assign error bars to the obtained values of n|, n" or nL

The lowest value of obtained is for H 20 (1.36 xio-13 esu), not unlike that of

typical optical glasses, while the largest value is for CS 2 (9.67 xl0~ 13 esu); the fraction of electronic nonlinear index (f

e

) varies from 0.12 for CS 2 to 0.61 for H 2 0.

In figure 1 we show the calculated total nonlinear index n

2 as a function of n^ for

93
the nine liquids in Table 1. Recently Veduta and Kirsanov [11] published data (nT) obtained

from a number of liquids, including benzene, toluene, water, acetone, and ethanol . Normalizing the calculated value of benzene to that obtained in reference [11], we found the value

of n| shown in Figure 1 for comparison to the calculated ones. It can be seen that even with the uncertainties mentioned above the agreement is good for toluene, hLO, and acetone. More importantly, o.ne may observe in Figure 1 the trend that as the linear index n. in- creases, so does n

2

, in agreement with earlier work on solids. The trend would be even more pronounced if the cause of the negative values of B for CC1 4 , cyclohexane, and acetic

acid can be found. In Figure 2, we show the calculated nT as a function of v and again

one observes as in solid material s, the trend that as v. increases (low dispersion) one obtains low values of nj.

In Figure 3 we show n

2 and n" as a function of n . in an attempt to view their scaling

behavior. While it is generally true that a larger n e is associated with a larger n

2

, it is impossible at present to draw any firm conclusions; again it is evident that the

values for CCl^, cyclohexane, and acetic acid are affected by the negative values of B . The study of scaling is important since if it can be shown that the nuclear part scale? with

the electronic contribution it is possible to describe the total nonlinear index by an equation such as (6) with a suitably adjusted constant, K, as has been done in the case of solid

transparent optical materials [3].

Finally, while we have not explicitly considered the effect here, it is necessary to examine the absorption spectra or energy level structure of each liquid to insure that the

calculated nonlinear index is not enhanced by a two photon resonance. It is well known that

if the optical material is operating at a laser frequency w and possesses a bandgap energy,

Eg, then the condition

2fiw > E B (16)

leads to the onset of two-photon absorption as well as an enhanced non-linear index near the

frequency oj.

4. Conclusions

We have applied the Boling, Glass, and Owyoung [3] theory to the calculation of n

2 for

a variety of interesting liquids. By using the tabulated values of B [7], n

n and nj may

also be found, nj may be comparable to optical glass in some cases, and as large as - xlO

greater in others. One consequence of this is that in calculating the nonlinear phase distortion or "B integral" for a high power beam, formally given by [1]

_K_

An.

L njl(z)dz (17)

where K is a constant, relatively small thicknesses of liquid {y mm) will have^the same nonlinear phase distortion as 1cm) of optical glass for high index or low Abbe value

1

i quids.

The correlation of total nonlinear index with n. and v, previously observed in solid

optical materials has also been observed here and agrees werl with available measurements

of nj.

The question of scaling n

2 with n2 will await accurate measurements of n

2 and B

Q

for

1 iquids.

5. Acknowledgments

The authors acknowledge conversations with M. Weber, N. Boling and A. Glass, all of

whom knowingly or unknowingly contributed to this work. The authors would also like to thank all of the many people who aided in this work.

94
This work was partially supported by the following sponsors: Exxon Research and

Engineering Company, General Electric Company, Northeast Utilities, Empire State Electric

Energy Research Corporation, and New York State Energy Research and Development Authority.

References

[1] Laser Program Annual Report,

UCRL-50021-73, Lawrence

Livermore Laboratory [1973].

[2] Laser Program Annual Report,

UCRL-50021-74, Lawrence

Livermore Laboratory [1974],

[3] N. L. Boling, A. J. Glass,

A. Owyoung, IEEE Jour. Quant.

Elect. QE-14, 601 [1978], and

UCRL-75628 [1974].

[4] M. J. Weber, D. Milam, W. L. Smith,

Optical Engineering, 1_7, 463 [1978],

[6]

[7]

[8]

[9]

[5] M. J. Weber, C. F. Cline, W. L. Smith, [10]

D. Milam, D. Heiman, R. W. Hellwarth,

UCRL-80434 [1977].

Dl]

J. M. Rinefierd, S. D. Jacobs, D. 'C. Brown, J. A. Abate, 0. Lewis, H. Appelbaum, Laser Induced Damage in Optical Materials: 1978, NBS Special

Publication 541

R. W. Hellwarth, Prog. Quant. Elect.,

5, 1 [1977].

S. D. Jacobs, Laboratory for Laser Energetics, University of Rochester,

unpublished results.

M. D Levenson and N. Bloembergen,

Phys. Rev. B 10, 4447 [1974].

A. Owyoung, Optics Comm. H>,

266 [1976].

A. P. Veduta, and B. P. Kirsanov,

Soviet Physics JETP, 27, 736 [1968].

Tab le 1. Summary of Data and Calculations

t (T+B„ Bo

n T „2e LIQUIDS n„. cd n2

(T "2 "2

(ilO" ,3e»u) (i10- ,3e»u) (x10- ,3esu) ( x10- 13esu) (xl0- ,3esu) («10"' 2esu) "2

a-Chloronapthalene 1.631 22 11.72 4.06 36.0 31.94 82.03 9375 0.13 CSj 1.624 22 11.50 3.96 37.0 33.04 85.22 9.672 0.12 Benzene 1.502 31 4.82 1.54 6.0 4.46 12.44 1.726 0.28 Toluene 1.495 31 4.71 1.49 90 7.51 21.04 2.575 0.18 CC1 4 1.458 24 6.09 1.88 0.5 -1.38 -3.96 0.213

Cyclohexane 1.424 53 1.67 0.50 0.1 -0.40 -1.18 0.049 Acetic Acid 1.369 46 1.68 0.49 0.3 -0.19 -0.58 0.110 Acetone 1.356 51 1.37 0.39 1.0 0.61 1.88 0.325 0.42 Water 1.334 67 0.83 0.23 0.4 0.17 0.53 0.136 0.61 1-Bromonapthalene 1.656 21 13.44 4.72

Trtcreayl Phosphate 1.556 33 5.19 1.71 ZnBr2 1.555 35 4.74 1.56 ZnC1 2 1.536 41 3.54 1.15 DC-710 1.533 33 4.84 1.57 C5O40 1.511 39 3.53 1.13

CS610 1.510 36 3.96 1.27 crXylene 1.503 31 4.84 1.54 Dimethyl Sulfoxide 1.479 40 3.07 0.96 Glycerol 1.474 59 1.69 0.53

1,2 Dlchloroethane 1.442 49 2.00 0.61

Ethylene Glycol 1.432 62 1.36 0.41 Ethanol 1.359 60 1.09 0.31 D20 1.327 55 1.08 030

Methanol 1.326 54 1.11 0.31 FC-104 1.273 91 0.40 0.11

95
CALCULATED NONLINEAR REFRACTIVE INDEX

n](x10- 12esu)

I mil

o

o

uJ water

• o acetone

ethanol • acetic acid

• cyclohexane

• CCI 4

< = 2 & s %

c 3 c«

S S. c gee o a a 0» toluene benzene mo „ylene

• cs2 • achloronapthalene

Figure 1. Calculated versus n

d

Q6
CALCULATED NONLINEAR REFRACTIVE INDEX

nZ(xlO-Kesu)

J ' L-LLLLU J ' I I I ml

o

o

. .1. 11 jjij .... orchloronapthalene m CS,

CCI,

benzene • • toluene

• acetic acid

• acetone • cyclohexane

• water

oo . O

Figure 2. Calculated n versus Abbe value

Q7
CALCULATED n|( (x 10~TMesu)

I I I I 1 I I I I I I 1 I I I 1 I I 1_

U> —

OD

§

m

>w —

c — 3J O

m

o

o

KJ

— — to o oo o

_i i i

' '

C» water

eo acetone --• acetic acid

—• cyclohexane

•• CCI4

•

11 11

toluene -O benzerve

CS2 , achloronapthalene

.^,(x4.06) -D(x3.91)

Figure 3» Calculated n

2 and versus

98
WAVELENGTH MODULATION SPECTROSCOPY OF HIGHLY TRANSPARENT SOLIDS*

Rubin Braunstein, Ryu K. Kim

Department of Physics

University of California

Los Angeles, California 90024

and

Morris Braunstein

Hughes Research Laboratories

Malibu, California 90265

We have used wavelength modulation spectroscopy to measure the absorption in

highly transparent solids at levels of 10" 5 cm"

1 in the spectral region from 2 to 12.0 urn. For the first time, it is now possible to obtain the continuous spectral

distribution at this level of absorption where previously the only measurements have

been laser calorimetric measurements at a few discrete wavelengths. We have used

wavelength modulation techniques in conjunction with laser calorimetry to study

absorption in the ultraviolet and visible regions. In the present work, a new wavelength-modulated spectrometer system was constructed so as to encompass the

dominant HF, DF, CO, and C0

2 laser wavelengths. The richness of structure

observed throughout this spectral region provides a possible identification of

surfaces and volume contaminants as well as intrinsic absorption processes in solids. Results on KBr and other alkali halides are presented that demonstrate

the versatility and power of this technique.

Key words: Extrinsic infrared absorption; volume and surface impurities; wavelength modulation spectroscopy; KBr; KC1 ; laser windows.

1. Introduction

The requirements for low-loss optical components for high-power laser, fiber-optic,

and other light-guiding systems have stimulated the development of material growth and sur- face preparation techniques to yield materials with absorption levels on the order of

-5 -1 10 cm . Interest in a particular spectral region of transparency has varied depending

on the proposed applications of particular laser wavelengths. These material requirements

have created a need for instrumentation to measure low-level absorption over an extended

spectral region. A variety of sensitive techniques have been developed to measure absorption at discrete wavelengths of existing laser lines [1,2,3]. 1 Many of these methods

employ a high-power laser in some calorimetric configuration and can realize sensitivities

-2 -5 -1 that vary from 10 to 10 cm . To ascertain intrinsic and extrinsic absorption mechanisms as well as to identify specific impurities, it is clear that it would be desirable

to perform measurements over an extended spectral range.

Previously we have used wavelength-modulation techniques in conjunction with laser

calorimetry to study absorption in the ultraviolet and visible regions [4]. At present,

we report on the operation of a newly developed infrared wavelength modulation system

operating between 2.5 to 12.0 ym that can measure the absorption in highly transparent

*Supported by the Air Force Office of Scientific Research.

1. Figures in brackets indicate the literature references at the end of this paper.

99
-5 -1 solids at levels of 10 cm .for a 1-cm path length. Since our system encompasses the

dominant HF, DF, CO, and C0£ laser wavelengths, the spectral distribution of absorption

bands in this technologically important region can be observed. Measurements have been

performed on a variety of alkali halides and semiconductors in the study of intrinsic and

extrinsic absorption processes. The results on KBr and KC1 are presented in some detail

since an extensive literature exists concerning laser calorimetric measurements [2,3] on these technologically important infrared laser window materials. For the first time, it

has been possible to obtain the continuous spectral distribution of the absorption at levels

-5 -1 of 10 cm and so provide a possible identification of surface and volume contaminants.

2. Experimental Technique

We have previously described the use of wavelength-modulation techniques to obtain the

derivative and the subsequent integration of the absorption spectra in the visible and

ultraviolet regions in laser-window materials [4]. The derivative spectra were obtained

by sweeping the output wavelength of a monochromator across the exit slit by vibrating the

output diagonal mirror at a depth of modulation that can be continuously controlled. Since

the wavelength modulation measures not only the derivative of the transmission of the

sample, but the derivative of the background as well, it is necessary to subtract this

background to obtain the actual derivative of the absorption of the sample. In the visibleultraviolet system [4], the modulated beam was directed alternately between the sample and

a reference channel; since a single detector was used, it was necessary to use gating

circuits that allowed independent measurement of the sample and a reference channel. The

resulting spectra were normalized for the background variation by a servo-system. Although

such an analog servo-system can be used to normalize background using photo-multiplier

detectors, it is difficult to employ with infrared photoconducti ve detectors due to the

dark current of these detectors.

To extend wavelength-modulation techniques into the infrared, we have modified a grating monochromator by oscillating an output diagonal mirror similar to the system employed

in the visible-ultraviolet system [5]. This method of modulation is equally good for any

spectral region of the monochromator. However, the system to subtract the background is different. This wavelength-modulated monochromator is used in conjunction with on-line

digital techniques employing a PDP 11/20 computer. A sample-in sample-out procedure is used, the wavelength and sample position are under stepping-motor control, and provision is made for multiple scans for signal averaging. The digital data of background intensity,

derivative of the background intensity, and the transmitted intensity through the sample

and its derivative are independently recorded and are used in the on-line computation of

the derivative of the spectral absorption. The wavelength-modulation technique yields essen- tially the energy derivative of the absorption coefficient. The absolute value of the

absorption coefficient is obtained by integrating the observed derivative spectra, with

the constant of integration supplied by a direct absorption measurement in the same apparatus at wavelengths where the absorption can be measured directly with good precision.

Alternatively, the constant of integration can be supplied by a laser calorimetric measure- ment at a fixed wavelength.

Figure 1 shows a block diagram of the system implementing the above operation. The

detector employed in the present work was a liquid-nitrogen-cooled PbSnTe detector and the source was a globar. This system can be used in the ultraviolet, visible, or infrared regions

with appropriate changes of source, grating, and detector [6].

3. Experimental Results

A detailed study was made on a series of KBr and KC1 samples selected for their low

level absorption as indicated by laser calorimeter measurements at HF, DF, CO, and CO2 laser

wavelengths [2,3]. These crystals were obtained from various sources (Hughes Research

Laboratories, the Naval Research Laboratory, and Oregon State University). The material

was selectively ion filtered and reactive gas treated for purification prior to crystal

growth. The samples were surface ground, polished with a slurry of Linde A in ethanol or

propanol , and then dried under a heat lamp; this is the standard procedure for polishing

alkali halides. Measurements were performed with the samples in the laboratory atmosphere

100
I

as well as in a dry atmosphere. The data were obtained initially in the form of the

logarithmic derivative of the transmitted intensity with respect to energy with and without

the sample. The energy derivative of the absorption coefficient was subsequently obtained

! by determining the constants of proportional ity of the instrument by a computer correlation

procedure to uncorrelate the background derivative [6]. The change in the absorption coefficient AK in cm"* was then determined by a standard trapezoidal integration. Although the

derivative data contain all the physical content of the spectral distribution of the absorption, the data in this report are presented in integrated form to facilitate comparison with

the literature.

The absorption spectra of a typical KBr sample obtained with a conventional double-beam

spectrometer are shown in figure 2. As can be seen, virtually no absorption structure is

I present above the noise level of the instrument, confirming the relative purity of the

sample.

Figure 3 shows the integrated derivative spectra of a KBr sample taken with the sample

in the laboratory atmosphere. The richness of the observed spectra should be noted in coni trast to the featureless spectra shown in figure 2 for the same sample. The right hand

ordinate in figure 3 indicates the absorption coefficient at 3.8 )jm as inferred from a laser

calorimeter measurement with a DF laser; the left hand ordinate indicates the relative change

of absorption obtained by integrating the wavelength modulation derivative data. The depth

of modulation of the monochromator frequency used to obtain these data was 10 cm" . The

zero of the AK cm"* wavelength modulation result is registered with the absorption calibra-

-4 -1 tion point of 0.4 x 10 cm obtained by laser calorimetry. Therefore, to obtain the

actual absorption coefficient at a particular wavelength, one merely adds or subtracts the

-4 -1 appropriate AK value at a given frequency to the 0.4 x 10 cm value. This type of representation of the data allows us to display the fine structure excursions in absorption above

and below the calorimetric point. A point of caution isnecessary in comparing a calorimeter measurement to our wavelength modulation results. Most calorimetric measurements are

•made with multi-line lasers, and, as our data show, the laser lines can straddle absorption

lines. Successive runs on this sample reveal that the structure shown in figure 3 is -6 -

1

reproducible within a mean deviation of AK ^10" cm" . Consequently, the noise level of

the sample is at levels of the width of the drafting lines. The data points were taken at

5 cm"* frequency intervals. The sample thickness was 4 cm.

When this sample is placed in a dry atmosphere, a continuous change in the spectral

distribution of the absorption is observed until the spectra stabilize after the sample has

been in this gaseous ambient for an hour. The dominant features of the spectra of this

sample when in the laboratory atmosphere displayed in figure 3 are: a band near 2.5 ym with

fine structure, multiple structure between 3 to 4 ym with fine structure, a sharp strong

band at 4.2 ym, a band at 5 ym. and multiple structures between 6 to 12 ym. The data shown

in figure 4 are for this same sample in a dry atmosphere. Although there is a distinct

change in the spectra in fi-gure 4 compared to figure 3, some of the original prominent features can still be recognized. The sharp peak at 4.2 ym is greatly reduced, and the band

at 5 ym is gone, while some of the original structure between 5.8 and 12.0 ym is still

present; however, a valley develops around 9 ym.

The absorption spectra for another KBr sample are shown in figures 5 and 6, taken in the laboratory and a dry ambient, respectively. It is possible to recognize the general

spectral features in figure 5 that were seen in figure 3 despite the different relative

intensities of the bands. Again, when the sample is in a dry N„ atmosphere, as shown in figure 6, some of the features of figure 5 remain. It is interesting to note that both of

these samples in dry N~ show similar spectra (see figs. 4 and 6); again, a valley develops

at 4.8 ym in both samples in this ambient.

101
Figures 7 and 8 display the spectra in a laboratory atmosphere for two other KBr samples.

Although the details of the structure are different in these samples, there is a great deal

of similarity with the general features in figures 3 and 5. However, it should be noted

that the band observed at 4.8 ym in figures 3, 5, and 7 is absent in figure 8.

1

The spectral distribution of the absorption of all samples of KBr have the same domi- nant features. However, one can distinguish slight differences in the spectra from sample

to sample, despite the fact that the same preparation techniques were used in the growth

of these samples. Thus, our measurements can reveal as yet unknown subtle differences in

growth conditions.

The integrated derivative spectra of KC1 in the laboratory and dry N

2 atmosphere are plji shown in figures 9 and 10, respectively. Again for this substance, we observe changes in

spectra as a function of gaseous ambient with some of the prominent features remaining in

both spectra.

4. Discussion and Analysis of Data

In general, for the spectral region from 2.5 to 12 un, the samples of KBr in this work

show structure at 2.5 urn, multiple structures between 3 and 4.0 urn, a sharp peak at 4.2 ym,

a band centered at 4.8 ym, multiple structures between 6 and 8 un, a valley near 9 ym, and

-5 -1

a peak at 10.6 ym at levels of absorption coefficient of 10 cm . Since the relative

magnitudes of these structures differ in different samples , the indications are that these

bands have an extrinsic origin. Although the spectra differ in detail, the clustering of

the spectra lines in similar spectral regimes indicates a common origin of the spectra. The

magnitudes and line widths of some of the bands vary when the samples are in a dry atmos- phere compared to a laboratory ambient, indicating that a portion of the absorption is due

to surface physical adsorption. The structures that persist even when the samples are flushed in dry may be due to surface chemi -adsorbed species or volume impurities. The

above measurements were made on 1- to 4-cm-thick samples. The surface character of some of these bands was confirmed by performing similar measurments on 1-mm-thick samples. Absorp-

-5 -1 tion coefficients on the order of 10 cm were observed on thick and thin samples with

slight differences in the spectral distribution. Due to the complex superposition of bands,

it was difficult to unambiguously distinguish which bands would scale with thickness and

which would identify volume absorption. The 4.2-ym band was observed with the same intensity

for all thicknesses, confirming a surface origin. The continuous variation of the spectral

-5 -1 features with thickness of sample that remain at 10 cm levels indicates that at this level

of absorption we have a combination of volume and surface absorption or there is an inhomogeneous distribution of volume impurities.

Correlation-type charts have been published that represent a tabulation of molecularion vibratonal frequencies as a means of identifying possible surface and volume impurities

in alkali-halide laser window materials [7,8]. If one considers a frequency overlay of

possible impurities that can be present in concentrations of 0.1 ppm, one finds that a quasicontinuum absorption would be expected throughout the 2.6- to 12.0-ym region due to the overlap of the Lorentzian tails of the various absorption bands. It has been felt that a heterogeneous distribution of a conglomeration of chemical compositions which can be deposited

on the surface from the environment would produce a uniform absorption throughout the infrared spectral region [8]. However, the distinct absorption bands observed in this work indi- cate the possibility of specifically identifying dominant absorption centers.

Rather than considering the multiplicity of possible impurities that are displayed in

the vibrational correlation charts [7,8], it is instructive to consider: (1) the "historical" two principal impurities, 0H~ and CO^

2

, which have been involved in the infrared absorption in the alka-halides [9,10,11]; (2) the C-H bands [12]; and (3) the surface carbonates [13]. The dominant bands for these groups are shown in figure 11.

102
The region between 2 and 4 ym reveals several structures, while previous measurements

by laser calorimetry have indicated a peak near 2.7 ym (see fig. 12, reproduced from the

literature [14]), which has usually been associated with the stretching frequency of the

"isolated" 0H~ radicals [14]. However, laser calorimetry measurements are performed with

multi-line lasers at a few discrete wavelengths and so can only infer a broad band with the

great possibility of missing peaks or valleys.

In the present work, a prominent peak is observed at 3850 cm"*, which is shifted from

the peak that is observed at 3610 cm"

1 in deliberately doped KBr [15]. It is not clear that

such a large shift could occur due to different crystallographic locations of OH" in the

lattice. The magnitude and line width of this peak shifts from sample to sample; in addition,

placing a sample in a dry environment results in spectral changes that indicate that part

of this band may be due to surface adsorption. However, there is a shoulder in this line

(as can be seen in figs. 3, 5, 7, and 8) near 3700 cm"

1 that persists in dry ambient and

can be associated with adsorbed H^O.

The region between 3 and 4.0 ym shows a superposition of structures that could be attributed to various C-H bonds (see fig. 11). The origin of these groups can result from surface

contaminants due to the alcohol that is used to disperse the grinding compounds used in the

surface polishing of the crystals. However, some of these bands may be due to volume absorption since some of the spectra scale with sample thickness. Volume absorption due to C-H

bands can be due to carbonaceous fractions, which can result from organic materials such

as water-soluble alcohols that decompose on melting and are incorporated in the crystals.

A classical example of physical adsorption taking place on a surface of KBr is seen

at 4.2 ym. This strong narrow band with varying intensity is observed at 2358 cm"

1 on

all samples of KBr. This is close to the gas-phase value of 2349 cm"

1 for the (X^ vibration.

Since this band is easily suppressed by flushing with dry U^, it is readily identified

as due to a physically adsorbed species. It is interesting to note that the (#2 adsorption

varies from sample to sample for the laboratory ambient, as can be seen from figures 3,

5, 7, and 8; it is not unreasonable to expect physical adsorption to depend on surface

variations or impurities in the substrate.

The band at 4.8 ym is most likely due to surface-adsorbed "liquid" water as the absorption in this region is markedly reduced in a dry ambient; some of the additional structure in the 2.7 ym region can be due to the virational structure of water as it is also

reduced by dry flushing. The absence of the 4.8 ym band in figure 8, even when the

sample is in a laboratory ambient, indicates that H2O adsorption depends on the condition

of the substrate.

The bands between 5.5 to 8 ym are the most persistent of all structures seen in all the KBr samples. Measurements on thin samples still reveal these structures, which lends

evidence that these are due to surface chemical adsorption. The positions of these bands

are consistent with the vibrational frequencies of various surface carbonate complexes [13].

It is interesting to note that our data reveal in general a peak in absorption at 10.6 y.n and a valley around 9 ym. It is possible to identify the 10.6 ym peak as due to the C-C vibration. It is our interpretation that laser calorimetry data indicate a possible

minimum in absorption near the (#2 laser line at 9.27 ym [16]. Our wavelength modulation

data definitely establish this valley. The absorption between 5.5 to 8 ym seems to be

due to surface carbonates. However, the absence of absorption at 1070 cm"

1 signals that

the species are in the form of carboxylates. Therefore, we can infer the presence of chemisorbed "carbonates" not only by the presence of absorption in the 5.5 to 8 ym region but

also by the absence of absorption at 9,27 ym!

103
If one examines the data for KC1 shown in figures 9 and 10 in the light of the

discussion of the identification of the bands seen in KBr, similar features can be discerned

It is interesting to note that the physically adsorbed C0

2 band is markedly suppressed

in KC1 relative to KBr. The structures in the 6- to 8-ym region are still present together

with the tendency for a valley at 9 ym. However, the valley at 5 ym in KBr that opens

up in a dry N

2 ambient due to the possible desorption of^O is not seen in KC1 . The wavelength modulation results confirm the similarity of the extrinsic absorption in KC1 and

KBr, which has been formerly inferred from laser calorimetric data [2,3].

5. Conclusion

A powerful new method of infrared optical derivative spectroscopy was developed that

-5 -1

is capable of measuring the absorption coefficients at levels of 10 cm for a 1-cm path

length in the spectral region from 2.5 to 12.0 ym. This instrumentation has enabled us, for the first time, to measure the complete spectral distribution of low-level absorption

in highly transparent materials. The richness of the observed structure enabled us to identify volume and surface impurities in KBr and KC1 . The spectral distribution of the

absorption of all samples of KBr and KC1 have the same dominant features. However, we can distinguish slight differences in the spectra from sample to sample, despite the similarity in crystal growth; thus our measurements can reveal as yet unknown subtle differences

in material synthesis.

6. Acknowledgments

The authors wish to thank Michael Burd for aid in the computer programs, James A. Harrington for laser calorimetric measurements, and Lloyd DeVaux for fabricating the exellent PbSnTe detector without which this work could not have been performed. We add our appreciation to Bradly Bobbs and Donald Deal for several helpful discussions.

References

[1] Deutsch, T.F., Laser window materials — an overview, J. Electron. Mater.

4, 663 (1975).

[2] Hass, M. , and Bendow, B., Residual

absorption in infrared materials,

Appl. Opt. 16, 2882 (1977).

[3] Mitra, S.S., and Bendow, B., Residual

absorption in infrared materials,

Appl. Opt. 16, 2882 (1977).

[4] Harrington, J. A., Bobbs, B.L,

Braunstein, M. , Kim, R.K. , Stearns,

R. , and Braunstein, R. , Ultravioletvisible absorption in highly trans- parent solids by laser calorimetry

and wavelength modulation spectroscopy, Appl. Opt. J7, 1541 (1978).

[5] Welkowsky, M. , and Braunstein, R., A double beam single detector wavelength modulation spectrometer, Rev. Sci, Instrum. 43, 399 (1972).

[6] To be published.

[7] Duthler, C.J., Extrinsic absorption

in 10.6 m-laser-window materials

due to molecular-ion impurities,

J. Appl. Phys. 45, 2668 (1974).

[8] Flannery, M. , and Sparks, M. , Extrinsic absorption in infrared laserwindow materials due to moleuclarDamage in Optical Materials; 1977,

4-6 October 1977, Boulder, Colo.,

ed. by A.J. Glass and A.H. Guenther,

NBS Special Publication 509, U.S. Government Printing Office, Wash.,

D.C. 1977, p. 5.

[9] Wedding, B., and Klein, M.V., Infrared absorption of the hydroxy 1 ion in

alkali halide crystals, Phys. Rev. 177 , 1274 (1969).

[10] Lipson, H.G., Larkin, J.J., Bendow,

B., and Mitra, S.S., Molecularimpurity absorption in KC1 for infrared laser windows, J. Electron Mater.

4, 1 (1975).

[11] Harrington, J. A., Gregory, D.A., and

Otto, Jr., W.F., Infrared absorption

in chemical laser window materials,

Appl. Opt. 15, 1953 (1976).

[12] Pal i k, E.D., Gibson, J.W., Hass, M. Braunstein, M. , and Garcia, B., Infra

red characterization of surfaces and

coatings by internal-reflection spectroscopy, Appl. Opt. 17, 1776 (1978).

104
Hair, M.L. Infrared Spectroscopy

in Surface Chemistry (Marcel Dekker, New York, 1967).

Hass, M., Harrington, J. A., Gregory,

D.A., and Davisson, J.W., Infrared

absorption limits of HF and DF laser

windows, Appl . Phys. Lett. 28, 610

(1976).

[15] Handler, P., and Aspnes, D.E.,

Electric-field modulationof the

vibrational absorption of OH" in

KBr, Phys. Rev. Lett. U_, 1095 (1966).

[16] Rowe, J. M. , and Harrington, J. A.,

Extrinsic absorption in KC1 and KBr at C09 laser frequencies, J. Appl.

Phys. 47, 4926 (1976).

105
PDP 11

ADC

LIGHT

SOURCE

LOCK-IN I AT t-\

CHOPPER

AT

h

MULTIPLEX

SAMPLE CHAMBER

|WITH STEPPING MOTOR|

FOR SAMPLE AND

BACKGROUND

LOCK-IN II ATf2

OSCILLATOR f2

MONOCHROMATOR

WITH MODULATION

ATf2

STEPPING MOTOR

DRIVER

DETECTOR

MICROPROCESSOR

Figure 1. Infrared wavelength modulation system.

1P6*
ULI O

z

<

0.2

COCCo 0.4

CO

CQ 0.6

< 1.0 :

1 5 :

WAVELENGTH,

5 6 7 8 9 10 11 12 15 20 40

BACKGROUND

J u 4000 3500 3000 2500 2000 1600 1200

FREQUENCY, CM"1 800 400 200

Figure 2. Absorbance of KBr measured with a conventional double-beam monochromator

.

107
• 170-14 WAVELENGTH,

2.5 3 4 5 6 7 89 1012

o _4 LJ I I I I L_U I I I I i I * 4300 3700 3100 2500 1900 1300 700

< FREQUENCY, CM"1

Figure 3. Wavelength modulation spectra of KBr in the laboratory atmosphere.

108
9170-13 z

o

6

4

2

0

-2

-4

-6

inO

7-10

H -12

<

2.5

WAVELENGTH.

4 5 6 7 8 9 1012

I II I I

I I I I I —

KBr — 3

DRY N2 ATM

3.8 fj.

, t , ,

I -

cc 0.4 P

4300 3700 3100 2500 1900

FREQUENCY, CM"1 1300 700

Figure 4. Wavelength modulation spectra of the sample of KBr in figure 3 in a dry N

? atmosphere.

109
9170-12 WAVELENGTH, M m

2.5 3 4 5 6 7 8 91012

g 4300 3700 3100 2500 1900 1300 700

< FREQUENCY, CM"1

Figure 5. Wavelength modulation spectra of KBr in the laboratory atmosphere.

110
8170-11 WAVELENGTH, /xm

2.5 3 4 5 6 7 8 91012

* 4300 3700 3100 2500 1900 1300 700

FREQUENCY, CM"1

Figure 6. Wavelength modulation spectra of the sample of KBr in figure 5 in a dry N„ atmosphere.

Ill
9170-1S z

o

D

Q©

1

21

1r= o

z

yy

IT)O

10 -

8

6-

4

2

0 IXI - >

1-2

-4

-6

2.5 3

WAVELENGTH, /im

4 5 6 7 8 1012

k

"8

o

* 4300

<

KBr-9

LAB ATM

4.48

>

DC

LU

DCo

—J<

o

3700 3100 2500 1900

FREQUENCY, CM"1 1300 700

Figure 7. Wavelength modulation spectra of KBr in the laboratory atmosphere.

112
9170-18 WAVELENGTH, /xm

FREQUENCY, CM"1

Figure 8. Wavelength modulation spectra of KBr in the laboratory atmosphere.

113
9170-17

Figure 9. Wavelength modulation spectra of KC1 in the laboratory atmosphere.

114
2.5 3

WAVELENGTH,

4 5 6 7 8 1012

4300 3700 3100 2500 1900 1300 700

FREQUENCY, CM"1

Figure 10. Wavelength modulation spectra of the same sample in figure 9 in a dry N

9 atmosphere.

115
WAVELENGTH, M m

Figure 11. Correlation chart of bond frequencies of volume and surface

impurities for KBr and KC1

116
0 1 2 3 4 5 6

WAVELENGTH, ^im

Figure 12. Absorption ofKCl and BaF? by laser calorimetry [14].

This paper was subject to extensive discussion. The speaker expressed concern about the

baseline data provided by multi-mode laser calorimetry. Among the concerns regarding laser

calorimetric data is the problem of data obtained with multi-mode lasers in which the mode

structure of the laser can straddle the particular line features in the absorption spectrum

of the sample. In response to a question concerning the application of this technique to coated surfaces^ the speaker indicated that it had been applied to silicon samples on which

an oxide layer had developed. There was rather lengthy discussion on the problem of the

adsorption and desorption of surface contaminants. The speaker commented that the detectability of surface contamination by this method showed the importance of carrying out measurements on samples in vacuum with careful surface preparation. It was observed that the

physi-adsorption depended on the techniques of sample preparation. Adsorption and desorption

of atmospheric water was observed in the 2. 8 and 4-6 micrometer regions.

117
1 :»i

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hi
SYNCHROTRON RADIATION STUDIES OF BERYLLIUM FLUORIDE GLASS

R. T. WT I I Tarns, D. J. Nagel, and P. H. Klein

Naval Research Laboratory

Washington, DC 20375

and

M. J. Weber

Lawrence LIvermore Laboratory

LIvermore, CA 94550

We report measurements of the reflectance of pure BeF2 glasses In the

range 9 eV to 120 eV, as well as photoe I ectron spectra In the valence band

region. Based on these data, which should be regarded as preliminary, the

lowest-energy reflectance peak Is at 12.8 eV, Implying an exclton resonance

at about 13.1 eV, In close correspondence to LIF. Photoelectron spectra

for the bulk glass show a valence band which Is roughly symmetrical and

about 5.8 eV wide (FWHM), with a tall of states extending Into the gap.

val^ravMel?"

raSO" anCe: Ph°t0el «+ TM

spectra; refiectance; synchrotron

I . I ntroductlon

Beryllium fluoride glasses are of current Interest as possible materials for high

power laser systems because of their very low nonlinear refractive Indices [I] . In addition the linear refractive Index of BeF2 Is extremely small, (e.g., n=l.275 at 587. 6nm),

so that It may be attractive for use In multilayer coatings for the ultraviolet. Photoemlsslon spectra of evaporated films of BeF2 have Indicated that the valence level binding

energy (center of the F~2p peak relative to the vacuum energy) Is 17.2 eV, compared to

12.8 eV In LIF [2]. These data suggest that the Intrinsic band gap of pure BeF2 should be

comparable to or greater than that of LIF. In further support of this expectation, figure

I shows empirical trends of the valence exclton energy In the alkali fluorides and alkaline

earth fluorides.

Aside from Interest In the possibility of using BeF2 for vacuum ultraviolet optical components, knowledge of the Intrinsic band gap and fundamental electronic structure

Is useful In predicting the wavelength dependence of multlphoton absorption coefficients,

In understanding the source of near-uv linear absorption, and In obtaining a more complete

spectrum of the refractive Index. Absorption measurements on presently available bulk

BeF2 glasses have Indicated a practical transmission limit of 150 nm [I], but until the

present work no determination of the Intrinsic band gap or far-uv properties had been made.

We have measured reflectance of pure BeF2 glasses In the range 9 eV to 120 eV, as well

as photoelectron spectra of the bulk glass In the valence band region. Ultraviolet

reflectivity and photoemlsslon are useful measurements for Indicating the ultimately

achievable transmission edge since they sample very high densities of states, assumed

Intrinsic In nature. On the other hand, both methods probe only the near-surface region,

and so the hygroscopic nature of BeF2 presents a problem. We have used an argon sputter

etching gun to clean the samples In ultra-high vacuum prior to measurement.

2. Experiment

Glass samples were prepared at Lawrence LIvermore Laboratory by melting pure BeF2

In graphite crucibles. A typical Impurity analysis Is shown In table I. Polished pieces

about 0.5 mm thick and I cm^ In area were prepared. Since pure BeF2 Is hygroscopic,

preparation and subsequent handling were conducted In a dry environment.

119
The measurements of UV reflectance and photoemlsslon were made using radiation from

the Synchrotron Ultraviolet Radiation Facility of the National Bureau of Standards

(Galthersburg) . The UHV experiment chamber and related measurement apparatus have been

used In previous NRL investigations of the reflectance, fluorescence, and photoemisslon

of alkaline earth fluorides [3] and other materials. The monochromator used In this

work is a toroidal grating Instrument providing useable flux from roughly 10 to 135 nm with

a 0.6 nm bandpass chosen for thilis work. The monochromator Is not optimized for high

flux in the 8-15 eV range which is of greatest interest In the present work, nor for

resolution at the highest photon energies, so that there are uncertainties on either extreme

of the spectrum. Continuing work Is addressing these spectral ranges more thoroughly.

Reflectance was measured using an aluminum photocathode which could be moved to Intercept the Incident or reflected beams. Photoelectron spectra were measured using a double-pass cylindrical mirror analyzer (PHI Model 15-255-G). Since these were bulk

Insulator samples, It was necessary to provide charge neutralization when making photoelectron measurements. This was done by means of a filament serving as a flooding source

of low-energy electrons. The pressure In the experiment chamber was typically 5 x I0~9 torr in these measurements.

3. Results and Discussion

The reflectance of sample number B8 1 5 (pure BeF2) In the energy range 9 to 120 eV (roughly 135 to 10 nm) is shown In figure 2. The maximum reflectance In this figure Is about 10$. This is lower than one expects based on other alkaline earth fluorides.

Since specular reflectance Is measured In this experiment, we assume surface roughness may

be a significant factor, but have not yet characterized the diffuse scattering experimentally. Sample B8I5 appears fairly smooth to the eye, but sample P-7 I shows obvious roughness. The reflectance of sample P-7 I exhibits essentially the same features as B8I5,

though at a lower absolute value (about 5% maximum). For the time being, absolute reflectivities should be regarded as tentative, but the spectral features can be discussed. We

have marked locations of spectral features in figure 2 which are attributable to transitions

originating in the F~2p, F~2s, and Be+_r ls levels.

In figure 3, we show the low-energy portion of the reflectance spectrum on an expanded

scale, and suggest 13.1 eV as the energy of a resonance (assumed to be the valence exclton)

responsible for the first reflectance peak. When we have more complete data, a determination of the optical constants will be made to locate the absorption resonance more

precisely. For the time being, we assume that the exclton resonance Is located slightly

to +he high-energy side of the first reflectance peak. The sources of the reflectance

peaks at 15 and 17.5 eV are not known at present.

Figure 4 shows a photoelectron energy distribution curve corresponding to the

valence band of BeF2 sample number B8I5 (after sputter cleaning) for an Incident photon

energy of 50 eV. The filament used as a "neutralizing" source of electrons was In fact

operated at a current sufficient to charge the sample to a negative potential. Hence, the

observed photoelectron energies extend above 50 eV. For constant photon energy and flux

of the Incident radiation, the energy scale should be rigid (i.e. relative energies are meaningful), although floating In the sense that absolute energies relative to vacuum are

not known. In order to establish an estimated scale of binding energy (est. E^), we have

assigned the center of the valence peak the same binding energy as was found for thin

evaporated films of BeF2 by Poole et. al. [2]. The peak In figure 4 Is more symmetrical

than that found in the thin film work, however. There Is an apparent tall of valence

states out to about 7 eV on the binding energy scale. This Is roughly consistent with the

observation that the practical transmission limit Is 150 nm (8.3 eV)

4. Acknowledgment

The support of DARPA, DOE, and ONR is gratefully acknowledged. We wish to thank the

NBS Synchrotron Ultraviolet Radiation Facility for providing beam time and technical

assistance.

120
[1] Weber, M. J., ClJne, C. F.,

Smith, W. L., Milam, D., Helman, D. , and

Hel Iwarth, R. W., Measurements of the

electronic and nuclear contributions to the nonlinear refractive index of beryllium

fluoride glasses, Appl. Phys. Lett. 32,

403 (1978).

[2] Poole, R. T., Szajman, J.,

Leckey, R. C. G., Jenkln, J.G., and

Llesegang, J., Electronic structure of the

alkaline-earth fluorides studied by

photoelectron spectroscopy. Phys. Rev. BI2 , 5872 (1975); Poole, R. T. Jenkln, J. G.,

Llesegang, J., and Leckey, R. C. G.,

Electronic band structure of the alkali

ha I Ides. I. Experimental parameters, Phys.

Rev. BM_, 5179 (1975).

[3] Williams, R. T., Nagel. D. J., and

Kabler, M. N., Reflectance, fluorescence

yield, and photoelectron spectroscopy of

CaF2 In the 10 to 100 eV range, J. de

Physique (to be published).

Table I. Impurity analysis of sample SS-1643: pure BeF2 melted In graphite.

K 5000 ppm (by weight)

Ca 300

A I 200

Mg 30

Si 30

B 6 Mn 4 Cr 3 Sr 2

Elements not detected:

Na <300 ppm

Fe, Nb, Sb, Zr < 30

Ba, Cd, Pb, Ti, Zn < 10 Bi, Co, Cu, Mo, Ni, Sn,

V < 3 Ag < I

121
3 I 1 1 1 1 1 3 II 19 37 55

4 12 20 38 56

ALKALI /ALKALINE EARTH

ATOMIC NUMBER

Figure I. Empirical trends of the valence

exclton peak energy In alkali fluorides

and alkaline earth fluorides.

Figure 2. Reflectance of BeF2 glass with

probable atomic-level origin of features

noted.

122
z

<o

UJ_lu_

UJ

—1— 1 -—i

1 1 1 1

1

f\ BeF2 glass

/

/ A B8I5

. /

13.1

T eV \

exciton ?

I I I I I I I l 10 20 30

PHOTON ENERGY (eV)

40

Figure 3. Reflectance of BeF2 glass

showing suggested position of the lowest

exciton resonance.

o

CD

(/) o

o

<

o

o

o

cc

I-o

UJ

_JUJ

BeF2

hi/ = 50 eV

(negative bias)

RESOLUTION

H

17.2 14 \ 10 6

-H 1 1 H

1 1 1

1—1 1-

est. Eb

l 1 1 «H 40 50 60

KINETIC ENERGY (eV)

Figure 4. Energy distribution curve of

photoelectrons excited from BeF2 glass

by 50 eV photons, under conditions of

negative sample charging by an electron

flood source (filament). The binding

energy scale is estimated by comparison

to Ref. 2.

123
cr

in

rai

or

to

tlx

obi

rel

PPt

bou

n

in

ar

pub

hi

tie

i
THE ROLE OF Fe IN LASER-INDUCED DAMAGE IN ULTRAPURE KBr

D. F. Edwards and B. E. Newnam

Los Alamos Scientific Laboratory

Los Alamos, NM 87545

and

W. J. Fredericks

Oregon State University

Corvallis, OR 97331

ABSTRACT

The laser-damage threshold has been measured for several samples of ultrapure KBr

crystals. Induced neutron activation analysis and thermal light scattering measurements

2+ indicate the major impurity to be Fe with concentrations in the range .04-. 095 ppm and

radii in the 17-20 X range. Other impurities either do not contribute to the scattering

or are in concentrations 100 ppb or less. The measured damage threshold values were found

to have the functional dependence on particle radius and pulse width predicted by the

theory of Bennett."'" Oxygen treatment to the KBr during the crystal growth process is

observed to effect the damage threshold values. This suggests a damage mechanism that is

related to a species derived from carbon. As an example, carbon present in the sample in

ppb concentrations might be converted to CO which is covalently bound to the Fe. In this

bound state the Fe no longer contributes to the laser damage.

NOTE: The authors withdrew this paper from publication in these proceedings

in order to repeat the scattering measurements without the complication of

surface-scattering contributions. When completed, these results will be

published separately in the appropriate technical literature.

The speaker commented that no correlation was observed between the observed laser damage

threshold and the iron concentration in the samples. He also remarked that he was aware of

the difference in the damage thresholds observed in this work from those reported by Manerikov,

but oould not explain the difference. It was obvious that the source of the material used in

the two experiments was different.

"""H. S. Bennett, J. Res. NBS 75A (4), 261 (1971).

125

PROPERTIES AND FABRICATION OF CRYSTALLINE FLUORIDE MATERIALS FOR HIGH POWER LASER APPLICATIONS

T.M. Pollak, R.C. Folweiler, E.P. Chick! is , and J.W. Baer

Sanders Associates, Inc. Nashua, NH 03061

and

A. Linz and D. Gabbe

Massachusetts Institute of Technology

Cambridge, MA

Crystalline fluoride materials exhibit certain desirable properties for ap- plications in advanced laser technology. Fluorides are characterized by excellent

ultraviolet transmission, low values of nonlinear index of refraction ( r\2) and

high optical damage resistance. Crystal growth temperatures of fluorides are relatively low; and scaling of growth to large diameters has been achieved,

demonstrating fabrication capabilities of large apertures of high optical quality

material

.

This paper describes the fabrication and properties of two fluoride crystal

families which exhibit a natural rare earth site. Two crystalline fluorides,

Yttrium Lithium Fluoride (YLF), or LiYF^, and Potassium Yttrium Fluoride (KYF), or KY^F.q, have specific desirable properties for high power laser applications.

The optical performance of materials is very dependent upon the fabrication

processes. Advanced material processing techniques have been developed, including feed purification and crystal growth techniques, which have been em- ployed to fabricate fluoride crystals exhibiting excellent transmission, low

scattering, and high resistance to laser damage. In addition, recent crystal

growth developments have scaled the crystal growth of YLF to 100 mm diameter.

Growth parameters were optimized to maintain proper crystal-melt interface to minimize the formation of defects. In addition, technical problems were addressed in scaling the crystal growth of YLF to 40 cm diameter.

Key words:

Advanced laser materials; Crystal growth; Crystalline fluorides; Faraday rotator; High powe

laser materials; Optical damage threshold; Top-seeded solution technique; Two photon absorp

tion; Uniaxial crystal; Yttrium lithium fluoride

1. Introduction

The general materials requirements for advanced laser applications include high peak

power and, in some cases, high average power propagation, together with material availability in large apertures of excellent optical quality. Crystalline fluoride materials

exhibit certain properties inherently suited to these requirements. Furthermore, certain

classes of these materials exhibit favorable spectroscopic, magneto-optic, or laser properties for specific component applications in advanced laser technology.

Optical materials for high power lasers are required for both passive components and

active components. Passive components include lenses and windows; active components are

laser materials and faraday rotators. The common requirements of all these components for

high power propagation are related to the following specific material properties:

127
a. Nonlinear index of refraction (n^): Low values of n,, (the change in refractive

index at high intensities) minimize the tendencies for sel f- focusing and beam

breakup.

b. Optical transparency: Transparency at the operating wavelength and its harmonics is important to minimize two photon absorption and associated damage

effects

c. Material purity: Lack of inclusions which can become damage sites is essential for high power applications. These extrinsic defects can be a result of

impurities in feed material or growth atmosphere.

In addition to the above properties, active components require appropriate spectroscopic properties for specific applications. Generally, these properties are obtained by

including a dopant (active) ion in the crystal structure. Among the specific applications

of interest are laser materials and faraday rotators. For such applications the dopant ion

with the appropriate properties are trivalent rare earth ions. A host material with a natural lattice site for such ions is desirable to accommodate a wide range of dopant ion

concentrations.

2. Crystalline Fluorides

Crystalline fluoride materials exhibit low values of n^; transparency over large spectral ranges, and high optical damage thresholds. In addition, growth temperatures of

fluorides are relatively low, enabling growth of crystals with minimal induced thermal stress. Crucible solubility at the growth temperatures is undetectable and will not lead

to inclusions or defect centers. Generally, fabrication techniques of fluorides are well

advanced and are capable of producing large diameter single crystals displaying high op- tical quality.

Two crystalline fluorides, YLF (LiYF^) and KYF (KY^F^), have specific desirable pro- perties for high power laser applications. YLF has been noted as a very unique laser host. More YLF laser transitions have been documented than in any other host. KYF, although

still under development, displays excellent potential as a laser host and as passive ele- ments due to its isotropic nature. The cubic structure of KYF allows the potential use of

hot forging techniques to expand aperture size. In addition, the scalability of crystal

growth for these materials has been demonstrated in recent programs at Sanders. As a re- sult, YLF and KYF have excellent potential for use as active and passive elements for high

power lasers.

3. Properties of YLF and KYF

The properties of the crystal families of YLF and KYF are desirable with respect to the established criteria for optical elements, passive and active. Tables 1 and 2 summarize

some of the properties of these materials. The calculated for YLF and the estimated n

2

fo*~ KYF are very low. YLF also exhibits one of the shortest ultraviolet cutoff wavelengths

of the high power compatible materials. In addition, laser damage resistance appears very

high. At 1.06ym,YLF displays the highest threshold damage of all neodymium laser

hosts [1] . Additionally, recent measurements by Liu, et al [2], reported that the two photon absorption coefficient (6) for YLF was undetectable at 266 nm. These data all con- clude that YLF will transmit high peak power, far into the UV wavelengths with minimum loss.

Figures in brackets indicate the literature references at the end of this paper.

128
Table 1, Properties of YLF and KYF.

STRUCTURE

UV EDGE

N

2 (ESU)

k(W/CM-K)

GROWTH TEMPERATURE

(C)

APERTURES GROWN

g

' (k- 442 NM)

(V1

) ^

TpA (CM/MW)

LiYF

4(YLF) TETRAGONAL (SCHEELITE)

120 NM

0.6 x

10"' 3

0.06

820

10 CM

-3.0 x I0~6 i-POLARIZATION

-0.8 x 10"*" POLARIZATION

6*

KY

3

F ]0(KYF)

CUBIC

< 200 NM

-13 0.6 x 10 (EST) M. WEBER LAWRENCE LIVERMORE LABORATORY

0.03

1030

3 CM

J. RINEFRIERD, UNIV OF ROCHESTER <4 x 10 X = 266 NM

*P. Liu, R. Yen, N. Bloembergen, "TWO PHOTON ABSORPTION COEFFICIENTS IN U.V. WINDOW AND COATING MATERIALS.

Table 2. Properties of KTb^F^Q- and Li TbF

4

.

CRYSTALLINE STRUCTURE

KTb

3

F 10 CUBIC (FLUORITE)

LiTbF,

TETRAGONAL (SCHEELITE)

OPTICAL

INDEX OF REFRACTION

X = 633 nm

NONLINEAR INDEX n

2 (10-13

eso)

- VERDET CONSTANT

V (MIN/Oe-cm) X=633 nm T = 295° K

RELATIVE FIGURE OF MERIT

nV/n

2 X = 633 nm

*Marvin Weber, Lawrence Livermore Laboratory

** Where Hoya FR-5 is 1

1.50

~ 0.7

0.40

4.3

n = 1.473

e

n = 1.502

o-0.7

0.425

4.9

129
As a uniaxial crystal, YLF has two distinct optical directions. When oriented with its

"C" axis parallel to incoming radiation, the YLF acts as an isotropic window. With its "C" axis in the plane of the window, YLF displays a polarization effect.

Unlike YLF, KYF exhibits a cubic structure which is advantageous for several applications. The UV transmission of KYF, although not accurately known, is expected to be similar

to YLF. The KYF is presently under development and its performance will be further elevated.

Both YLF and KYF allow free substitution of rare earth ions (RE

+3

) and little (if any)

host interaction with the active ion. The established criteria for laser materials has

indicated that both YLF and KYF are excellent candidate materials for high power laser ap- plication.

4. Crystal Growth

Sanders employs the top-seeded solution growth (TSSG) technique as described by Linz,

et al [3], for growth of YLF and other fluorides. The TSSG method is a modification of the

Czochralski growth technique. Various fluorides have been grown employing TSSG, including

YLF, KYF, KTb

3

F 1Q

, and Ho : ErF^

The Czochralski technique, or modification of this technique, displays inherent advantages over other growth techniques in the fabrication of high optical quality crystals.

Top-seeded techniques allow growth of crystals unrestrained by crucible walls, unlike many

"bottom seeded" techniques; i.e., Bridgmen. The absence of physical contact can eliminate

defects and residual stress caused by the restraint. The mixing performed by the rotation

of the crystal in the top-seeded methods enhances impurity and dopant distribution within

the melt and the crystal. This factor is especially critical with dopants with distribution

coefficients much less than one; i.e., neodymium. Single crystal materials grown by these

techniques typically display wavefront homogeneity and low scatter.

The concept of crystal growth with the TSSG technique can be illustrated employing the

phase diagram of the Li F - Y system, figure 1. The compound LiYF^ is i ncongruently melting, meaning that upon solidification of the stoichiometric composition, a phase transformation would occur at a temperature of about 819°C. To avoid this destructive transformation,

excess Li F "flux" is added to the melt to form a "solution". The resulting growth temperature is lowered below the temperature of the phase transformation.

The design of the crystal growth module illustrated in figure 2 is specifically to grow

optical quality fluorides. The furnace uses commercial vacuum components for basic con- struction and seals. This provides a system that can effectively prevent any interchange

between the internal and external atmospheres for an extended period. The furnace is com- posed of two units connected by an isolation valve, allowing manipulation of the seed rod

without disturbing the lower working chamber. System control is maintained with a digital

process computer receiving inputs from thermocouples and an optical sensor. This system

detects a bright spot reflected from the meniscus between crystal and melt, using a He-Ne

laser indicating the position of the crystal edge. This signal is processed and a correction is made to the system proportional to the diameter of the crystal. This looping process allows fine, continuous control of the growth process for periods over 200 hours

unavailable through manual operation.

Materials for application as optical components in high power lasers must not only

display uniformity, but must be free of impurity ions to avoid absorption leading to damage

or scattering from inclusions. Sanders has developed advanced material handling techniques

to purify and maintain purity levels in fluorides for crystal growth. Raw materials are purchased in the highest commercial purity, and processed through wet chemical, zone re- fining, and hydrof1 uori nati ng processes. Purification methods are used which are applicable to removal of a broad spectrum of suspected impurities from commercial alkali, alkaliearth, and rare earth compounds. When the input feed is an oxide, carbonate, or water

130
soluble compounds, purification involves aqueous solution chemistry followed by synthesis of

a solid purified fluoride. Starting with a commercial fluoride on the other hand, zone rec ining is the purification method applied. The final step is rigorous hydrofluori nation to

-ernove remaining anionic impurities.

Crystal growth atmosphere is as critical to the complete process as any components.

Since fluorides easily convert to oxides at the growth temperature, a reducing or inert at- nosphere must be employed during growth. Growth in a vacuum is not possible due to the

-ypical high vapor pressures of fluorides. We employ high purity argon as a growth atmosjhere. Argon will maintain melt purity as well as being noncorrosive to furnace elements.

This fluoride crystal growth facility produces optically uniform, low scatter crystals.

\ photograph of Nd:YLF laser material in wide usage is shown in figure 3. Crystal growth

of boules of Nd:YLF occur in periods of 200 hours or longer. The automatic growth facility

naintains fine control of growth parameters throughout the entire process. As a result,

laser materials are routinely fabricated displaying adequate properties for high power laser

appl i cations

.

5. Scaling of Crystal Growth

Scaling of the crystal growth process has been achieved in recent development work.

Specific materials grown were RE:YLF, however, the process is adaptable to other fluorides,

the initial advancement in the growth scaling program has produced single crystals of YLF

to a diameter of 10 cm. Figure 4 illustrates a crystal of Er:YLF during the growth process.

The approach to growth scaling was to expand the previously described fabrication techniques. The TSS growth technique was applied by increasing the crucible diameter to 20.5

km, figure 5, at its mid-height. Sanders has observed with fluorides along with other investigators of both silicon [4] and gadolinum-gallium-garnet [5] that a maximum crystal

diameter/crucible diameter ratio of 0.5 to 0.6 is suitable to maintain uniform circulation

oatterns in the melt. Proper melt circulation is required to keep a homogeneous melt com- oosition, and prevention of defect formation as a result of impurity segregation at the

:rystal interface.

Parameters utilized for the growth of smaller crystals were employed as the baseline

for parametric development. Special attention was paid to the thermal gradient achieved,

and its effects on the crystal-melt interface. Growth rate and crystal rotation rate

•vere used as compensating factors to achieve stability throughout the entire process,

figure 6 displays two 6.5 cm boules of Tm:YLF and a 10 cm single crystal boule of Er:YLF.

Initial evaluation of the Tm:YLF crystals indicated that they are of good optical

quality. Polariscopic examination of polished discs did not reveal any low angle grain

boundaries. Preliminary interferometry indicated that the crystals have two or three

vaves (633 nm) of distortion in a 2 cm thick, 6.5 cm disc, but that most of this is in

(Surface irregularities. Polishing is in progress to improve the surface figure and evaluate the bulk homogeneity.

6. Applications for YLF and KYF

YLF is a versatile material and is being used in a variety of systems. Over twenty

transitions have been demonstrated to date in this host, ranging from 300 nm [6] to 3.9

'am [7]. Nd:YLF operating at 1.047 ym or 1.053 ym is a good spectral match for the advanced

laser glasses and is used as an oscillator in laser fusion systems [8, 9].

Tm:YLF, pumped by an XeF laser, and emitting at 453 nm [10], is an efficient storage

'laser and is being developed for laser fusion applications. CerYLF [6] is a tunable UV laser whose range extends from 300 nm to 350 nm.

The terbium analogs of both YLF and KYF display excellent magneto-optical rotary power;

i.e., high verdet constants, and low values of n^. The combination of these two properties

results in the highest figures of merit of all known faraday rotator materials.

131
Because of its transparency, low n,,, and its high optical damage threshold, YLF has

great potential as high powered laser windows. As its UV cutoff is 120 nm, it may be

suited to many of the new UV and VUV lasers being developed. As a uniaxial crystal, YLF

exhibits birefringent properties; potential applications include wave plates and polarizers.

At the present time both the measurement of the properties and the technology of crystal growth of YLF and KYF are progressing. Growth scaling will continue towards the goal

of 40 cm diameter. This effort will press the limits of top-seeded solution growth technology and involve evaluation of other crystal growth techniques.

Acknowledgment

The authors would like to thank the Department of Energy, Office of Laser Confinement

Fusion, for the financial support of the crystal growth scaling development work.

References

[1] Birnbaum, M. , DeShazer, L., Engineering

design of repetitively Q-switched solid

state lasers for precision range in ap- plications, Contract NASA-23698.

[2] Liu, P., Bloembergen, N., Two-photon

absorption coefficients in UV window and

coating materials, Appl . Opt., ^8, 7, 1015 (1979).

[3] Linz, A., Belruss, V., Kalnajs, J., and

Folweiler, R.C., Top-seeded growth of

oxide crystals from non-stoichiometric

melts, Mat. Sci. Bull., 6 (1971).

[4] Wilson, R., Varian Associates (private

communication)

[5] Brandle, CD., J. of Appl. Phys., 49 (3),

1855 (1978).

[6] Ehrlich, D.J., Moulton, P.F., Osgood,

R.M., Jr., Tunable UV solid state Ce:YLF laser at 325 and 309 nm, Technical Digest IEEE/0SA Topical Meeting

on Excimers, Charleston, SC, Sept 11-13,

1979.

[7] Esterowitz, L., Echard, R.C., Allen,

R.E., Long wavelength stimulated

emission via cascade laser action

in Ho: YLF, Appl. Phys. Lett., 35 (3),

236 (1979).

[8] Zimmerman, J., et al., Proc. Topical

Meeting on Inertial Confinement Fusion

San Diego, Feb, 1978.

[9] S.E. Stokewski, et al., Proc. Topical

Meeting on Inertial Confinement Fusion

San Diego, Feb, 1978.

[10] Baer, J.W., Knights, M.G., Chicklis,

E.P. and Jenssen, H.P., XeF pumped Tm:YLF an excimer excited storage

laser, Technical Digest IEE/0SA

Topical Meeting on excimers, Charleston,

SC, Sept 11-13, 1979.

132
1200

Liquid YF3 hexagonal -^^^^

1 100

Liquid 1000 o Liquid + o 900 — _f YF3 orthorhombi

a>3 800

LiF* liquid

a

£ 700 * liquid 695° 600 LiYF« YF3 orthorhombic

LiF *LiYF4 500

400 1 1 1 1 1 1 1

1148°

1052°

819°

LiF 10 20 30 40 50 60 70 80 90 YF3 YF3 (mole%)

Figure 1. Phase diagram of LiF-YF

Figure 2. Crystal growth module.

133
4

~i i i i

|—rr—i—i—i—

BOULE 522

l.St Nd:LitF^

Figure 3. Single crystal Nd:YLF laser

material

V

Figure 4. process

Er:YLF crystal during the growth

134
Figure 5. Crucible and furnace interior

for growth scaling experiments.

Figure 6. Single crystals of rare earth

doped YLF from growth scaling experiments.

135

LASER DAMAGE IN YTTRIUM ORTHOPHOSPHATE*

R. Allen, L. Esterowitz, and P.H. Klein

Naval Research Laboratory

Washington, DC 20375

V.O. Nicolai

Office of Naval Research

Arlington, VA 22217

and

W.K. Zwicker

Philips Laboratories

Briarcliff Manor, NY 10510

Preliminary laser-damage experiments on single-crystal blades of yttrium

orthophosphate (xenotime, YPO^) have been performed. Crystals, measuring

about 15 X 3 X 0.7 mm were grown at Philips Laboratories by slow cooling of

a flux containing lead pyrophosphate (Pb2P20y). Optical absorption is small

from about 300 nm to about 3200 nm, approximating the transmission of calcite. A 1.06-jJm Nd:glass laser with Q-switched pulse duration of 17 ns was used for

damage measurements. The laser beam (0.025 cm FWHM) was focussed on the large

face of the YPO, crystals. Visible damage was always observed at front and

, 2 back crystal surfaces following exposure to pulses averaging 3.6 GW/cm or more. No visible damage was found in YPO, at power densities less than 2.2-2.4

2 GW/cm . Calcite crystals showed damage at somewhat smaller power densities.

Key words: Absorption spectra, calcite, flux growth, laser damage, neodymium: glass

,

polarizer, pyrophosphate, yttrium orthophosphate, yttrium phosphate, xenotime.

1. Introduction

Its optical and mechanical properties combine to make mineral calcite the most popular polarizer material for use with laser systems. Optical-grade calcite has not been grown in the laboratory. As applications for lasers increase in number,

optical-grade calcite becomes ever scarcer. The need for a replacement material with

equivalent properties becomes correspondingly pressing.

Pertinent properties of calcite are compared with those of some other polarizers

in Table 1. Soda-niter has an even greater birefringence than calcite, can readily

be grown in the laboratory, but is too soft to polish readily and is extremely soluble

in water. It has never been seriously suggested for widespread use with lasers.

On the other hand, zircon has been proposed [1]^ as a laser polarizer. Its chief

drawback is its relatively small birefringence, about one-fourth that of calcite.

Yttrium orthophosphate is twice as birefringent as zircon, harder than calcite,

and insoluble in water. We report here some results of growth, spectroscopic, and

laser-damage studies which give YPO^ promise as a laboratory-grown, laser-tolerant

polarizer material.

*Work supported in part by the Office of Naval Research.

Figures in brackets indicate the literature references at the end of this paper.

137
Table 1. Properties of some polarizer materials.

Polarizer Structure

At 589 nm:

n -n

e o

Moh'

Hardness Solubility

Calcite, CaC0

3 Soda-niter, NaNO" Zircon, ZrSiO,

Xenotime, YPO,

Hexagonal 1.4865 -0.1720 3. Hexagonal 1.2261 -0.2513 1.5-2.0

Tetragonal 1.968 +0.045 7.5 Tetragonal 1.827 +0.107 4-5

Low High

Low

Low

2. Crystal Growth

Yttrium orthophosphate was prepared by solution of yttrium oxide in excess lead

pyrophosphate, and slow cooling of the melt. (Pyrophosphate conversion of yttrium

oxide to the orthophosphate is well known, potassium pyrophosphate having been employed

nearly a century ago [2].) A mixture containing 14 mole per cent of yttrium oxide

and 86 m/o of lead pyrophosphate (Pb^2^y^ was heated in a 100-ml covered platinum

crucible at a rate of 300 C/h. When the upper surface of the melt reached 1400 C, heating was discontinued. The temperature was held constant for 2-6 h, with the bottom

of the crucible 80 C cooler than the surface of the melt. The surface temperature

was then lowered to 970 C at a rate of 4 C/h, maintaining the 80-C vertical gradient

throughout the five-day cooling period. The crucible was then furnace-cooled to 100 C.

Crystals were removed from the flux by soaking in hot water. Blade-like specimens

measuring 15 X 3 X 0.7 mm were common. The optic axis was found to lie parallel to the 3-mm dimension. The largest faces were optically smooth, and could thus be used

without polishing for spectroscopic and damage experiments.

3. Absorption Spectroscopy

Ultraviolet, visible, and infrared absorption spectra were determined with a 0.7-mm-thick YPO^ crystal. Surfaces were as grown. Surface contaminants were removed

by rinsing with isopropyl alcohol just prior to measurement. Spectra were recorded

in the range 210-2000 nm on a Cary Model 17 spectrophotometer. From 2.5 to 8.0 micrometers, spectra were recorded on a Perkin Elmer Model 283B spectrophotometer. All

spectra were obtained with unpolarized light.

As shown in Fig. 1, the region of greatest transparency extends from 800 to 3000 nm. There are two prominent absorption^bands , near 2000 cm * and 3000 cm . These appear to be overtones of the 1000-cm fundamental absorption, ascribed to P-0 vibrations in the phosphate ion [3]

4. Laser Damage

Q-switched pulses from a 100-mJ Nd:glass laser were used for damage studies.

Pulse length was 17 ns (FWHM) . The beam was focussed to a 0.025-cm (FWHM) spot at the front surface of the sample by placing the sample 15 cm behind a plano-convex

lens whose focal length was 18 cm. Polarization was parallel to the long crystal

dimension for some measurements, perpendicular for others.

No damage was observed in YPO^ unless accompanied by electrical breakdown of

the air at the front surface. Damage was difficult to detect without a microscope.

Only the front surface was damaged at low power densities, but both front and back

surfaces were pitted at high power densities. For a given power density, occurrence

of two-sided damage was somewhat dependent on polarization. With polarization parallel

to the long (15 mm) dimension, two-sided damage took place about 90% of the time.

With polarization parallel to the short (3 mm) dimension - the optic axis - two-sided

damage was found only about 75% of the time. No difference was detected between results

of N-on-1 and 1-on-l studies.

138
For comparison, two types of calcite specimen were also subjected to damage. One

roup consisted of cleaved pieces of about the same thickness as the YPO^ specimens. The

urfaces of these had visible steps on them, which sometimes could not be avoided by the

earn. The other group was a series of thicker slices, cut from a calcite polarizer assemly. These were then polished on a wax lap with 0 . 03-micrometer aluminum oxide (Linde A),

tsing triacetin as a lubricant. Results are shown in Table 2.

Table 2. Damage with 17-ns pulses of 1 . 06-micrometer radiation.

Crystal

Surface

Thickness, mm

Power-density,

GW/cm

Under 2.0

2.1-2.4

2.5-3.0

3.1-3.5

Over 3.6

YPO. CaCO. CaC0o 4 3 3 As-grown Cleaved Polished

0.7 0.6-0.7 1.4

Occurrence of damage

Never

Never

Occasional

Frequent

Certain

Frequent

Certain

Certain

Certain

Certain

Rare

Frequent

Certain

Certain

Certain

As the table shows, yttrium orthophosphate crystals studied here appear to withstand more intense irradiation than do calcite specimens. Repeated measurements, on larger crystals of YPO^ and on optically-finished calcite, will be needed to establish

this conclusion. Work in progress, whose aim is preparation of larger YPO^ crystals,

aay make it possible to perform the experiment.

5. Acknowledgement

We are grateful to R.C. Eckardt for supplying the calcite polarizer which was dissected for these measurements.

6. References

[1] Nicolai, V.0. , Light polarizing prism

of zircon, U.S. Patent 3,700,308,

issued October 24, 1972.

[2] Duboin, A., On some double phosphates

of yttrium and of potassium or sodium,

Compt. rend. 107, 622 (1888).

[3] Nakamoto, K. , Infrared and Raman

Spectra of Inorganic and Coordination Compounds, 3rd edition,

John Wiley and Sons, New York,

1978, p. 173.

139
3 * 5 MAVELENGTH in hicwketem

Figure 1. Absorption 8pectrum of yttrium orthophosphate.

The speaker indicated that yttrium orthovanadate should also be an attractive material, but

that it has shown a tendency to hydrolize, whereas yttrium orthophosphate has not.

140
COMPRESSIVE FAILURE IN SAPPHIRE UNDER CO LASER HEATING

P. A. Miles and J. Gallagher

Raytheon Missile Systems Division

Bedford, MA 01730

and

R. L. Gentilman

Raytheon Research Division

Waltham, MA 02154

Irreversible changes have been observed in sapphire crystals subjected to surface heating by C0

2 laser irradiation at levels above 300 watts /cm. They

are interpreted as due to plastic flow under compressive stress at temperatures

above 900°C. The recognition of possible compressive failures in refractory

oxides is of importance in defining laser tolerance levels in high power optics,

in the design of laser heating experiments to assess the thermal shock resistance

of materials, and possibly in the field of laser assisted machining of ceramics.

A detailed thermo-mechanical analysis has been carried out to predict the

temperature and stress conditions throughout disk samples as a function of time,

heat flux level and flux distribution. These calculations show that compressive

stresses in excess of 200,000 psi were generated in our experiments. They also

show that compressive failure is likely to precede tensile fracture in most experiments where partially heated disks are used.

Key words: Sapphire; plastic flow, compressive failure; laser damage; thermomechanical

analysis

.

1. Introduction

The mechanical failure of optical components exposed to high power laser radiation has

been discussed and analyzed repeatedly since the first observations of distortion and frac- ture of NaCl windows in CO„ lasers. Attention has usually focused on the limits set by

fracture under thermally induced tensile stresses at the cold edges of centrally heated

windows. At the other extreme, the use of highly focused laser beams for the melting,

evaporation or sublimation of a wide range of metals and other materials , the most recent example of which is the laser annealing ion implanted semiconductors , ^ '

' can lead to the

complete change in the physical state of the system without any evidence of tensile failure

per se. These contrasting behaviors are explainable first in terms of the relatively low

level of tensile stresses in the cold region surrounding very small heated regions, and

second because of the lower ductility found in materials used as optically transmissive components. Although plastic flow in NaCl laser windows is well known, * it comes as a surprise to find conditions wherein laser heating over a relatively large area of a nominally

brittle material such as sapphire can cause a 'failure' by induced ductility. A second ex- ample is the irreversible change observed in local heating of another ostensibly brittle

material, CaF . ^ In retrospect, the explanation is quite straightforward - heat deposition

at the material surface produces both high compressive and shear stresses in a region

whose temperature is well above the ambient , and whose mechanical properties are different

from those measured at room temperature. The purpose of this paper is simply to document

our experiments on sapphire, and to set out a first order analysis of the temperature and

stress fields developed in these tests.

This work is supported by the Air Force Materials Laboratory, Wright-Patterson AFB , OH

45433 under Contract No. F33615-77-C-5280.

141
2. Experimental

The original purpose of these experiments was to demonstrate the superior resistance

of sapphire to thermally induced stresses, as compared to other materials such as MgF , that are transparent in the mid-infrared range out to 6 p wavelengths. To accomplish

this, 2 inch diameter disks of sapphire, 1/8 in. thick, were exposed to a 10.6 ym CO

laser beam. The laser intensity was relatively uniform over a circle of 0.5 in. diam., centrally located on the disk (Fig. 1). Due to the very high absorption coefficient of sapphire

at a wavelength of 10.6 ym, laser heating was confined to the first few microns at the„surface. Heat deposition rates at the center varied between 100 watts/cm-2 and 10 kW/cm for exposure times between 0.5 sec. and 10 sees. The appearance of the sample was recorded

both by a video camera and videotape with a 16.6 msec frame time, as well as a high speed

movie camera with a frame time of 10 msecs. In addition, a microphone was placed near the

sample disk to record the sounds during or preceeding the point of fracture.

/

-|-«N«*_ Laser flux

! profile

*

0.5'

@J23

t

Figure 1. Sample and Laser heating

geometry.

The results outlined in this paper are confined to the effects observed at input flux

levels below 3 kW/cm 2 . In this regime, two distinct effects were observed prior to the

fracture and overall distintegration of the sample disk. The first of these was the appear- ance of a faint haze at the center of the heated region (Fig. 2a) that proved to be scatter- ing of the light coming from the movie spot lights and indicative, therefore, of a change in

the structure of the sapphire. The second effect was the appearance of a bright incandes- cence at beam center (Fig. 2b).

In contrast to these first two effects, the times-to-fracture showed the wide statisti- cal variation expected from an effect that depends on the presence of preexisting structure

defects. However, the average time-to-fracture did not follow the locus originally expected

for tensile failure in a material whose thermo-mechanical properties were those as measured

at room temperature, but, instead, lasted for relatively longer times at the higher incident

flux levels. This result implies that the actual levels of tensile stress developed in the unheated region of the disk are lower than those predicted by a model that assumes a purely

elastic behavior of sapphire at all temperatures.

Examination of the TV tapes showed that the initiation times for each of the first two

effects was very consistent from one sample to another, and that as a function of the aver- age laser intensity H (w/cm 2 ), each initiation time x followed closely a relationship of the

form (Fig. 3)

H

2 t = const.

Since the appearance of incandescence is itself an indication that a specific surface

temperature has been reached, it is reasonable to associate each of the two lines with the

expression for the temperature rise (AT) after a time, t, in a surface-heated dielectric of

density , p , thermal conductivity , K , and specific heat , C , namely

(AT) 2 mh

TrKpC or H

2 x = j KpC (AT) 2

142
In fact, for the temperature changes experienced in these experiments, the value of K is reduced significantly from its value at room temperature (Fig. 4). In such cases, while

steady state temperature profiles in locally heated regions can be treated analytically ,

dynamic analyses require the numerical computations summarized in the next section.

Attempts were made to obtain samples that showed the initial scatter change, for later

examination. This was done by shutting off the laser immediately after the appearance of scatter - an act that usually resulted in the immediate fracturing of the central region

(Fig. 5). One such sample remained intact and showed obvious optical distortion in the

central region and increased transmission under crossed polarizers before it, too, fractured.

The evidence all points to the plastic deformation of the heated region under the action of

compressive and shear stresses, followed by a tensile failure on cooldown.

Samples were all cut from L|nde Czochral ski -grown ingots with faces parallel either to the crystal basal plane (0001), a rhombohedral plane (1012) or a (1123) plane.

The onset of incandescence showed no dependence on sample type but the onset of scatter did so (Table 1). Whether these effects, which imply a temperature and stress change of

12%, were ones of visual perception only, or represented a distinct change in the plastic

limits because of differences in resolved shear stress about the various crystal planes is uncertain. As it turns out, twinning under stress has been observed extensively in

sapphire over a wide range of temperatures, but usually above 1000°C, ~ and can take

place about both the basal and rhombohedral planes.

2 Table 1. Average onset times for scatter and incandescence at 650 W/cm

Sample Plane Surface Scatter Incandescence

(1012) 0.93 sec. 1.3 sec. (1123) 0.67 1.3

(0001) 0.87 1.3

3. Analysis

The calculations of the temperature and stress fields in a sapphire disk with localized

surface heating were carried out by computer in two stages using finite element techniques

and modified codes AMGO 65 and AMGO 54, ^ respectively. First, a nonlinear heat flow

analysis was performed which took into account the change in thermal conductivity over the

range 0°C to 1700°C. Three specific values of input heat flux, namely 100 W/cm 2, 300 W/

cm^, and 1000 W/cm^ were assumed for each such case. The resultant temperature profiles were then used to calculate the corresponding stress field in a perfectly elastic material

whose (aE) value was constant, and set equal to an appropriate average value for sapphire

at temperatures between 0°C and 1000°C , namely aE = 580 psi/°C. In fact, the elastic

constants for sapphire depend on crystal orientation, and furthermore the most significant

demonstration of these experiments is that its stress-strain curve must be nonlinear at high

temperatures. Thus our calculations provide only an estimate of the stress levels at which

plastic flow occurs , and , necessarily , give an overestimate of the tensile stresses existing near the point of fracture.

In order to simulate our specific experiment wherein a 2 inch disk was heated over

the central 0.5 inch diam. , the first calculations used this precise geometry, but with the

simplification that the level of heat flux was held constant over the heated circle. Two

other calculations that simulated surface heating over circles of 1.0 and 1.5 inch diam. were also made to explore the variation of stress field with heating geometry. The results

of all three calculations are summarized briefly in figures 6-15.

The nonlinear aspects of the sample heating are most obvious for the highest heat

flux, 1000 W/cm . Figure 6 shows the temperature generated on-axis in the top (irradiated) surface for this case. The upward deviation of the calculations from the line corresponding to the relationship

* Indices based on hexagonal unit cell with c/a = 2.73.

143
AT = /TTKpC

4t

derived from the linear theory is clearly seen over the range 0°C-1000°C and at times up to 0.5 second. By contrast, the downward curvature of the line at times beyond 0.5 second

is a result of lateral diffusion of heat out of the region directly heated by the laser beam.

In the temperature regions up to 1000°C , however, there is no signficant dependence on

laser spot size between 0.5 and 1.5 inch.

The resultant stress fields are exemplified by the profiles of hoop stress (Figs. 7-12)

at two specified times, namely 0.1 sec and 1 sec for each heating geometry and plotted for

each of three parallel planes in the sample, the top surface (curves labelled 3, 40), the

central plane (3, 30), and the back surface (3, 21). At all times, the maximum compressive

stress is found on-axis in the top surface. The maximum shear stress also occurs at this

point and is very close to one-half of the maximum compressive stress at all times. The

stress on-axis in the central plane is weakly tensile at short times , changing to an intermediate compressive stress when significant heat diffusion through the sample thickness has

taken place.

The maximum tensile stress occurs, as expected, as a hoop stress in the top surface

at a point outside the circle heated directly by the laser. The radial profiles of stress

clearly show the effects of lateral heat diffusion at t = 1 sec. For the 0.5 in. spot size,

the magnitude of the tensile stress at the edge of the sample is some 35% of the tensile stress at the maximum, but these two stresses are virtually equal for the 1.5 inch spot at

t = 1 sec. Indeed, the original choice of a 0.5 in. spot size in our experiments was made

for this very reason, to minimize the chance that tensile fracture would be initiated by

edge defects before failure could occur near the circle of maximum tensile stress.

It is instructive to plot out the time dependence of the maximum compressive and

tensile stress levels (Fig. 13). At times up to 1 sec the former grows almost exactly as

°c <v /T , following, roughly, the actual temperature of the heated region. The tensile maximum, however, is expected to be proportional to the total heat input at short

times, and therefore to vary linearly with time at times less than 10" sec, varying more

slowly at later times as a the result of heat diffusion. By inference, the ratio of compressive /tensile stress maxima increases as the laser heating time decreases, so that tensile stress should be of little concern in short pulse heating, especially for small spot sizes.

This latter point is brought out by comparing these curves with the compressive /tensile

ratios for the other two cases (Figs. 14, 15). As the spot size increases, the compressive

maximum decreases progressively, while the tensile maximum rises.

To return to our experiment,, the computer models can be used to estimate the temperatures reached with 1000 W /cm heating at which the scatter effect and the incandes- cence were seen. Adjusting for the fact that the actual laser flux on-axis is 25% higher

than the average flux indicated in figure 3 gives values of 900°C and 1200°C , respectively,

for these two effects. The levels of compressive and shear stress at the onset of scatter are estimated to be near, 200,000 psi and 100,000 psi, respectively.

The conclusions to be drawn from these efforts are more salutary than profound

inasmuch as they are typical of a variety of experiments wherein intense heating is applied

to small samples in an attempt to simulate the thermally induced tensile stresses experienced

in domes, plates and cylinders exposed to conditions such as aerodynamic heating, jet

exhausts and the like. Stress fields depend on geometry, size, and boundary conditions,

and are not easily scaled. Small scale experiments, in general, require greater temperature differences in order to generate critical levels of stress. In turn, these elevated

temperatures can lead to mechanical properties of the test piece that differ significantly

from those observed in more normal operating regimes. As a consequence, conclusions

drawn from small spot laser experiments as to the thermal shock resistance of materials are highly suspect. Experiments of this type can be useful only if the temperature dependence

of mechanical properties are accurately taken into account.

4. Conclusion

144
A possible way out from this dilemma is the use of a partially inverted temperature

nd stress distribution in flat plates produced by applying laser heating to an outer annulus,

gaving the center of the disk unheated. This ensures that the edge of the disk is always

i compression and further, that critical levels of tensile stress will be reached at the center

f the disk before the edge is heated sufficiently to induce a failure in shear.

Horrigan, F. A., et al. , Final

Report, Contract DA-AH 01-67-

1589, Sept. 1968.

Sparks, M. , J. Appl. Phys. 42,

5029 (1971).

Sparks, M. , J. Appl. Phys. 44,

4137 (1973).

Sparks, M. , and Chow, H. C,

J. Appl. Phys. 45, 1510 (1974).

Miles, P. A., Infrared Laser

Window Materials Meeting, Boulder,

Colorado, 1976, p. 27.

Kachurian, G. A., et al. , Sov.

Phys. Semicond. 10, 1128 (1976).

Gat, A. et al. , Appl. Phys. Lett.

33, 775 (1978).

References

[8]

[9]

[10]

[11]

[12]

[13]

[14]

Detrio, J. A., and Petty, R. D.,

Laser Induced Damage in Optical

Materials, NBS Spec. Publ. 541

(1978), p. 78.

Lax, M., J. Appl. Phys. 48, 3919

(1977) .

Lax, M., Appl. Phys. Lett. 33, 785

(1978) .

Kronberg, M. L., Acta. Metal 5,

507 (1957).

Conrad, H., J. Am. Ceram. Soc.

48, 195 (1965).

Heuer, A. A., Phil. Mag. 13, 379

(1966).

Brisbane, J. J., Tech. Rpt. S-198.

US Army Missile Command Contract

DAAH01-68-C-0891.

Figure 2. Initial scatter and incandescence observed in laser

heated sapphire. (A) scatter, (B) incandescence.

145
Figure 3. Loci of thresholds for scatter and incandescence and for the average

time to fracture for laser heated sapphire

disks

TEMPERATURE ( K)

Figure 4. Temperature dependence of the

thermal conductivity of sapphire.
Figure 7. Hoop stress profiles for 0.5

in. diam. laser spot at t = 0.01 sec,

stress scale factor 10.

Figure 8. Hoop stress profiles for 0.5

in. diam. laser spot at t = 1 sec, stress

scale factor 10^.

I

147
•3-H00P STRESS lit J- I- 21. !• M. !• «*.

Figure 9. Hoop stress profiles for 1.0

in. diam. laser spot at t = 0.01 sec,

stress scale factor 10.

83-H00P STRESS

PBN -79-1140 axis

83-H00P STRESS

LASER HEATING

Figure 10. Hoop stress profiles for 1.0

in. diam. laser spot at t = 1 sec. , stress

scale factor 10^.

Figure 11. Hoop stress profiles for 1.5

in. diam. laser spot at t = 0.01 sec,

stress scale factor 10.

-1696. —

«3-H00P STRESS

p. 38

Figure 12. Hoop stress profiles for 1.5

in. diam. laser spot at t - 1 sec. , stress scale factor 10^.

14R
Figure 13. Time dependence of compressive and tensile stress maxima for a

laser spot of 0.5 in. diam.

10=

10"

10'

Model 2" diam 3mm thick sapphire

heal load 1000 W/cm2 over 1.0" diam. circle

Compressive

Figure 14. Time dependence of compressive and tensile stress maxima for

a laser spot of 1.0 in. diam.

Figure 15. Time dependence of compressive and tensile stress maxima for a laser spot of 1.5 in. diam.

The speaker indicated that the sapphire used in these experiments was commercially -obtained

Linde sapphire, and that although the observed incandescence was not sensitive to the

orientation of the crystal samples, the fracture observed did depend on the crystal orientation. He remarked that zero-degree cuts were stronger than 60-degree cuts and the fracture

was observed in rhombohedral planes. He indicated that the observed scattering seemed to

come from induced variations in crystal structure on or near the sample surface.

149

FAILURE CRITERIA FOR LASER WINDOW MATERIALS

John A. Detrio, George A. Graves

University of Dayton Research Institute

Dayton, Ohio 45469

and James M. Wimmer

Ai Research Mfg. Co. Phoenix, Arizona

The brittle mechanical behavior of laser window materials must be

considered in the establishment of failure criteria for selecting candidate materials and in the design of window components. The statistical

techniques employed in brittle materials design and analysis are reviewed.

The influence of the limited plasticity observed for the alkali halides,

which exhibit fracture failure, is also discussed.

Key words: Brittle failure; failure criteria; flaws; fracture; Kj

c ; laser windows; Weibull; yield' stress

1. Introduction

The selection of a laser window material for a specific application usually results in

the consideration of trade-offs between optical and mechanical properties [l,2].l To date,

the two material properties usually considered in the trade-off are the thermally-induced

optical distortion coefficient and the ultimate strength. The optical distortion, which is dominated by the temperature dependence of the index of refraction (dn/dT) or stress optics

effects, has been explored in detail elsewhere [2-4]. The purpose of the present paper is to examine the mechanical failure criteria for window materials in greater detail since all of the infrared laser window materials of current interest exhibit brittle fracture when

subjected to tensile stresses.

Completely brittle materials are characterized by the lack of any appreciable ductility prior to catastrophic failure by fracturing. The failure of brittle materials is controlled by the presence of defects or flaws, the size of which determine the "strength"

of a window. Since these defects are the result of a chance occurrence in use, growth, or fabrication, the strength is a statistical property of the finished part.

These three elements; the absence of yielding, flaw sensitivity, and the statistical

nature of failure require a different approach to the interpretation of the strength of a brittle window material. Specifically, the failure criteria must be stated in statistical

terms, i.e., in terms of the reliability required of the window. One of the most important

consequences of this statistical nature of failure is that the prediction of survival of

full-sized components from data obtained on small test specimens requires an appropriate

theoretical model

.

2. Failure Criteria

The emphasis of this work is on the failure criteria for brittle materials and is therefore especially relevant to ZnSe and CaF 2 which are completely brittle at room tem- perature. The type of failure observed in alkali halides, will be briefly discussed first

and then a criteria for brittle materials will be presented.

Figures in brackets indicate the literature references at the end of this paper.

151
The alkali halides frequently fracture upon cooling following laser irradiation [5].

This phenomenon is generally thought to occur by the following mechanism [6]. When the

relatively small central portion of the window is heated by absorption of the laser radiation, this material expands against the cooler outer region which constrains the expansion,

resulting in localized compressive stresses. The compressive forces produce shear stresses

that exceed the yield strength of the material and this yielding is the primary failure

mechanism. The failure is detected on cooldown because the plastically deformed region

cannot relax to its original configuration resulting in tensile forces which produce

fracture.

The ultimate failure of KCil and NaC£, even with the limited ductility, is of a brittle

nature. The fracture process requires flaws or cracks, and tensile forces which tend to open the cracks. Based on experimental observation, the best failure criterion for the

alkali halides is the critical resolved shear stress, Tys. This stress is related to the

yield stress, t , found in simple tension or compression experiments according to

y~*

Tys

= V2

3. Brittle Fracture

Materials that fracture without exhibiting plasticity are defined as brittle. The

failure of these materials is assumed to initiate at preexisting flaws and the "weakest

link", i.e. the largest flaw, defines the failure stress. As a result, specimens prepared from the same lot of material demonstrate a distribution in fracture stress which is best described in a statistical fashion.

According to the Griffith theory [7] the flaw size is related to the fracture stress according to

o = \

[(Ey/a)] 1 /

2 (1)

where, Young's modulus, E, and the fracture surface energy, y> are the material parameters

which describe the fracture process. The flaw size, a, is a material fabrication or handling related quantity while Y is a geometrical factor related to the flaw type and

location. An example of the relationship between stress and critical flaw size is shown

in figure 1.

Flaws may have a variety of orientations relative to the applied stress but for

brittle material those stresses which tend to open or lengthen the crack are of primary

importance. Therefore, a flaw whose crack plane is normal to a tensile stress field is

in the most favorable position for initiating fracture.

The driving forces acting at the crack tip and tending to propagate the crack can be described in terms of the stress intensity factor, K. Three stress intensity factors are required to describe the driving forces for a crack at an arbitrary angle, 90°>6>0°, to a tensile stress. Kj describes simple Mode 1 crack opening, while Kjj and Kjjj describe the two shear modes of crack propagation. In brittle materials, fracture always propagates

normal to the tensile stress, i.e. by Mode I crack opening although Mode II and III may

influence the fracture initiation site.

The relationship between Kj and the normal stress is

Kj = Yaa 1 / 2

. (2)

When the stress intensity reaches the critical value (Kj

c

), then fracture occurs. The

critical stress intensity factor, Kj

c

, is a material parameter and describes a material's

resistance to fracture initiation. Comparing eqs. (1) and (2) it is evident that Kj

c can

152
be described in terms of the elastic modulus and fracture surface energy. Equation (2) can

be used to determine Kj experimentally by one of a number of fracture mechanics tests.

In general, these tests determine the stress required to propagate a macrocrack of known

size and geometry for which a value of Y is available. For the case of a notch placed perpendicular to the length across the tensile surface of a 4-point flexural test bar,Y is given by [8]

Y = 1.99 - 2.47(a/h) + 12.97(a/h) 2 - 23.17(a/h) 3 + 24.8(a/h)"

I (3)

where a = notch depth

h = specimen thickness.

This type of fracture toughness specimen is referred to as a single edge notched beam (SENB)

specimen

.

4. Failure Predictions

The statistical properties of the failure of brittle materials under load can be

described by a mathematical model due to Weibull. The distribution function is general in describing a wide range of material responses, but it was derived on the assumption of a random distribution of flaws with a distribution of sizes. The material is assumed to fail

when the stress in the weakest region (worst flaw) reaches the critical value.

Because of the distribution of flaws, large specimens are more likely to contain a large flaw than small specimens. The Weibull model provides a procedure for scaling between

parts of different sizes. A form of the Weibull model which applies to isotropic materials

under uniaxial loads is described by the following equation. This equation predicts the

distribution of strength as controlled by flaws within the volume of the material.

F(n) = 1-exp {- / [(a

n -a

u )/a

0

] m dV} . (4)

The parameters have the following definitions: F(n) is the cumulative fraction of failures

observed at the applied stress a^. is the "zero strength" location parameter which is the stress level below which there will be no failures. a0 is a scale parameter, m is the

shape parameter which conveys the dependence of the distribution function on the flaw

population. This latter parameter is also referred to as the flaw density exponent. The

reliability, R equals 1 - F. An analogous expression can be written for surface flaws by

replacing the volume integral with an area integral. Figure 2 shows some typical failure

versus stress data for several brittle window materials.

Equation (4) explicitly describes the scaling rule through the volume integral. The

quantities a , o and m are material parameters. In practice the parameters do not scale

in the same manner as predicted by parameters obtained from small test specimens. This

lack of predictability results from the relief of localized residual stress during the

cutting and polishing of small test specimens that may remain in a large piece of material

used to fabricate full size windows.

Figure 3 illustrates the fundamental differences between scaling the response of

ductile materials (metals) and brittle materials. Both kinds of materials are described

by a Weibull distribution. The difference between the two kinds of material is in the

value of m, the shape parameter. Metals generally have values of 40 or more while a typical brittle material may have a value of 5. The example shown assumes equal mean

strength values for small volume test specimens. Note that upon scaling the volume by two decades (volume of 100) the strength of the ductile component is only slightly

degraded while the brittle material will, on the average, fail at less than half the

applied stress.

153
In the window materials of primary interest fracture has been observed to usually

initiate at surface flaws [9]. Two principal stresses can exist at the surface and,

unlike the uniaxial stress distribution assumed in the elementary Weibull case, the total

reliability will depend in general upon the effect of both stresses.

One approach to calculating the reliability in the case of biaxial stresses is to assume that each principal stress acts independently. The reliability can then be taken

as the product of the reliability associated with each principal stress. In the case of

a varying stress distribution the surface area can be divided into elements over which the

principal normal stresses are essentially constant. The reliability of the ith element is then given by

R. = R(o a ) R(a 2 )

where Oj and a 2 are the principal stresses. The total reliability of the surface is then

given by

R . . t = RiRo...R....R

total 1 * i n

Since the independent stress assumption may be nonconservati ve in some cases, alternate

approaches [10] to reliability predictions may be preferable.

5. Flaws

There are at least three sources of flaws that may influence the survival of a laser

window. They may be introduced in manufacturing, in handling, and in use. Manufacturing

flaws can be present on the surface or in the bulk of the material. Surface flaws usually

originate during grinding and polishing steps while volume flaws consist of such defects

as inclusions or porosity and are related to the initial processing of the material. The

manufacturing flaws are accounted for in the survival prediction if the witness specimens

are properly selected and prepared. The major assumption (which should be verified) is that the same flaw population controls the strength of both the test specimens and. the

optical component. The flaws introduced by improper handling should be guarded against by

the establishment of good handling procedures, special fixtures, and installing an awareness of the nature of brittle fracture in those responsible for handling the optical

component. Handling flaws can be accounted for in the survival prediction if the window

is inspected and proof-tested periodically. Flaws that are introduced in use, especially

during irradiation represent a special problem because of their unpredictability. The

flaws generated in use can result from the heating of intrinsic defects such as small

absorbing inclusions which are not relevant to the mechanical strength when not irradiated

or they can result from extrinsic defects such as dust motes which land on the window and

are subsequently irradiated. The irradiated defect acts as a local stress rise and may

initiate a larger crack. The influence of a proof-test on survival probability is illustrated in figure 4.

6. Stresses

The stresses which result in fracture of a brittle material are always tensile in nature although, as noted above, the alkali halides can be damaged by compressive forces.

The stresses of primary interest are the result of thermal gradients introduced into the

window by the absorption of the laser radiation. The thermal gradients can exist on two scales; localized which we define to be on the order of the size of a critical flaw or macroscopic in which the stresses are distributed over a large area or volume of material.

In large windows localized stresses can be responsible for the formation of cracks or for

the initiation of crack propagation although the distance traveled is small due to the

limited extent of the stress field. The macroscopic stresses provide the energy required

for fracture propagation and the ultimate catastropic failure of the window. The stresses

on the two scales may work together to fracture the window.

154
The overall temperature rise in the center of a sample creates thermally induced stresses while a heated defect acts as a stress riser and crack nucleation site. Ring

fractures are frequently associated with surface contaminants on brittle materials. In those cases in which the strain energy from overall heating is large, the ring fractures

will nucleate cracks which propagate across the sample. In some cases, the heated defect

will nucleate a crack which is kept closed by thermally induced compressive stresses, but

which propagates when the specimen goes into a state of tensile stress upon cooldown

following the irradiation. The tensile stresses may develop either through rapid surface

cooling which is unlikely in a typical window configuration which does not involve an active cooling system; or, more likely, the tensile stresses develop when the surface has

been permanently deformed by compressive stresses which produce yielding while the beam

is on. Once permanent deformation has occurred, the material cannot relax to its former

dimensions without generating large tensile forces.

The observation of yielding due to compressive stress is not inconsistent with the

brittle nature of the material. Under a tensile load fracture would occur before the stress reaches the value required for yielding while under a compressive load, fracture

is suppressed and yielding may occur. The amount of plasticity is restricted by several

factors. A single crystal or large-grained material requires that an active crystallographic slip system be favorably oriented in the stress field. A polycrystal 1 ine material requires that slip be transmitted from grain to grain; this can occur only if the

material has five active slip systems.

7. Other Factors

The calculation of the reliability of a brittle component is complicated by several

additional factors. The most troublesome problem is slow crack growth which generally

takes place by a stress corrosion mechanism. It is believed that this mechanism is responsible for the sudden failure of windows which have previously performed without any

detectable mechanical problems [11]. The corrosive agent for many window materials is water or water vapor. In most practical situations the corrosive agent will tend to con- centrate at the crack tip where the stresses are greatest. Since both stress and the

corrosive medium must be present, slow crack growth usually occurs only under load.

Residual stresses introduced during forming or fabrication processes also tend to complicate reliability predictions because they are usually not well characterized and

are not necessarily obtained from an analysis of the failures of small test specimens.

The useful application of the scaling law therefore requires that the flaw distribution

and material itself be the same in both the small test specimens and the large component.

The effect of cyclic loading and vibratory loads may be a problem in certain window

applications and environments. Vibratory loads of low amplitude do not necessarily

represent a serious problem in laser windows but the total load, static stress plus peak

cyclic stress must be considered in establishing the reliability of a window.

Mounting stresses are also of practical importance in the use of brittle materials.

Care should be used to avoid transmitting stresses from structural members to the

windows and a rigid bezel or mounting frame should be used to minimize the transmission

of flexural deflections to the window. A thick resilient mounting gasket is also

desirable to minimize the transmission of environmental stresses. The procedures used to fasten the window in place should be carefully tested to minimize the generation of

mounting stresses by the application of unequal torque or bending. Stress risers should

be carefully avoided as should any type of localized stress at window-to -metal contacts

which can result in Hertzian-type damage.

155
8. Acknowledgments

The support of the Air Force Materials Laboratory, Wright-Patterson Air Force Base,

Ohio is gratefully acknowledged. The contractual effort concerning the mechanical

characterization of infrared window materials is monitored by Mr. Charles Strecker. The

optical characterization of laser window materials has been monitored by Dr. G. Edward

Kuhl and Dr. Walter Knecht. The authors especially wish to thank Dr. John Wurst of UDRI for many informative and helpful discussions.

9. References

[1] Sparks, M. , J . Appl . Phys . 42, 5029

(1971).

[2] Sparks, M. , and Cottis, M. , J . Appl,

44, 787 (1973).

Phys.

[3] Klein, C.A. (1977) .

[4] Klein, C.A. (1978) .

Infrared Phys. 17, 343

Infrared Phys. 18, 251

[5] Klein, C.A., This Conference.

[6] Loomis, J.S., Fourth Conference on High

Power Infrared Laser Window Materials

1974.

[7] Griffith, A. A. , Phil. Trans. Roy. Soc (London) A221 , 163 (1920).

[8] Brown, W.F., Jr., and Srawley, J.E.,

"Plane Strain Crack Toughness Testing

of High Strength Metallic Materials,"

ASTM Special Tech. Publ . , No. 410,

ASTM, Philadelphia, PA, 1966.

[9] Freiman, S.W., Mecholsky, J.J., Jr.,

Rice, R.W., J. Am. Ceramic Soc. 58,

406 (1975).

[10] Batdorf, S.B. and Heinisch, H.L., Jr.

J. Amer. Ceramic Soc. 6U 355 (1978).

[11] Arnett, C.H., Optical Spectra, May

1979, Letter to the editor.

CRITICAL FLAW DEPTH (OOOI inches)

Figure 1. The relationship between Mode I tensile stresses and the critical flaw size

is shown for ZnS. The graph is generated by

the use of experimental Kj

c data in con- junction with eq. (2)

156
STRESS (ksil 5 10 15 20 25 30

-I 1 1

1 "

1 '

25 50 75 100 125 150 175 200

STRESS (MPo)

Figure 2. The results of fitting experimental data obtained from small coupon bend

specimen testing are shown for CVD ZnSe,

fusion cast CaF 2 and SrF2. The fluorides

have significantly higher mean strengths

(stress at 50% probability of failure) but

a larger variation in failure stress.

APPLIED STRESS KSI

Figure 3. The effect of increasing the

volume by a factor of 100 is shown for parts

made with materials having equal mean strengths for small test coupons but different

m parameters (standard deviation). The significant point being made is that in scaling

the volume by a factor of 100 the mean strength

of the brittle materials, which frequently have

small m values, is greatly degraded.

157
PROOF

STRESS

PROOF TEST

SURVIVORS

RETESTED

TO FAILURE DISTRIBUTION

CURVE OF PROOF TEST

SURVIVORS IF THERE

IS PROOF TEST DAMAGE

IMPOSED STRESS

Figure 4. Brittle material can be prooftested to eliminate those parts containing

critical flaws. At the expense of sacrificing some of the parts the reliability of the

survivors can be improved significantly.

The stress level at which the proof-test is carried out should be determined by consideration of the required reliability and the

slow crack growth properties of the material

in the use environment. Proof-testing is not addressed in this paper but it is an impor- tant practical consideration in the use of

brittle materials.

158
THERMOMECHANICAL STRESS DEGRADATION

OF METAL MIRROR SURFACES UNDER PULSED LASER IRRADIATION*

Henry M. Musal, Jr. Lockheed Palo Alto Research Laboratory

Palo Alto, California 94304

Progressive degradation of the optical surface of metal mirrors in pulsed

laser applications can result from cumulative plastic deformation under thermomechanical stress caused by relatively small temperature excursions during

irradiation. An analysis of the transient stress-strain behavior of the near- surface region shows that elastic response is maintained below a limiting surface

temperature rise (AT^) , which depends on the yield stress, modulus of elasticity,

Poisson's ratio, and coefficient of thermal expansion of the metal. This temperature rise defines the threshold for first-pulse plastic yield at the surface. An elastic-plastic model of metal behavior shows that the increment of plastic strain

accumulated during each successive laser pulse will be proportional to the temperature rise above 2 AT^ . This plastic strain will appear at the surface as slip

bands and intergranular slip that progressively degrades the optical quality of

the surface. The surface temperature rise of metal mirrors under pulsed laser

irradiation depends on both the metal optical and thermal properties as well as the laser pulse parameters. The peak temperature excursion produced by a constantflux (rectangular) pulse is expressed in terms of the optical absorptance, pulse

fluence, pulse duration, thermal conductivity, density, and specific heat. Using

this relationship, the pulse fluence threshold (F ) for first-pulse plastic yield

y

is derived. The threshold for multipulse accumulation of plastic deformation is

2 F . Numerical values of these thresholds are calculated for copper under C0„

y 2 laser irradiation. The appearance of well-defined slip bands on the surface of

copper mirrors after irradiation by a mode-locked CO^ laser pulse burst has been

experimentally demonstrated. The theoretical quantification of thermomechanical

stress degradation presented here is used to calculate the expected behavior of

copper under the reported experimental conditions, and these results are correlated with the available experimental data.

.ey words: Cu mirrors; damage thresholds; laser damage; laser- induced stress-strain;

lastic deformation; surface degradation.

1. Introduction

The expanding application of high output power/energy repetitively pulsed lasers, and th

necessity for compact beam control optics, places severe demands on optical components with

espect to flux/fluence handling capability. The service life of these components is an

.mportant design consideration. Degradation of optical performance under operational condiions for which no direct test data are available must be predicted, using experimentally

'alidated theoretical analyses and/or physically appropriate extrapolations of existing data,

'he thermomechanical stress degradation of metal mirror surfaces under pulsed laser irradiaj.ion is considered here, with specific emphasis on the threshold for onset of cumulative

iffects.

Surface damage of metal mirrors in pulsed laser applications is directly related to the

iransient surface temperature rise caused by the small (but unavoidable) fraction of the inci

lent laser radiation that is absorbed. Thermal expansion of the metal, resulting from this

:emperature rise, causes distortion (strain) in the near-surface region and thereby induces

itress. If this induced stress exceeds the yield stress of the metal, plastic (irreversable)

itrain will occur. This plastic strain will appear at the metal surface in the form of slip

.ines and bands, which introduce surface roughness that can degrade the optical quality of

rWork supported by Lockheed Palo Alto Research Laboratory Independent Research funds.

159
the surface by increasing nonspecular scatter and/or absorptance. In repetitivey-pul sed

operation, plastic strain effects can become cumulative, resulting in progressive degradation to unacceptably high levels of scattering and/or absorption, and ultimately leading to fatigue failure (cracking) of the metal at the surface.

Many complicated physical phenomena and analytic difficulties are involved in the problem of transient inelastic thermomechanical effects. Only a simple model of these phenomena

and a first-order analysis, with no pretense of comprehensive treatment, is presented here.

However, this model and analysis do provide a basis for specification of operational limits,

or optimization of materials selection and fabrication, to avoid premature metal mirror

failure due to excessive plastic strain accumulation.

2. Thermomechanical stress and strain

2.1. Physical description

Consider the physical response of the near-surface heated region of a metal under short

pulse large-spot laser irradiation (figure 1) . As a result of the rapidly rising temperature, the metal tends to expand but is restrained by inertial forces (stresses). In the

direction normal to the free surface, the stress relaxes to near zero very fast via a small

outward displacement of the free surface. On the other hand, displacement (strain) in direc

tions parallel to the free surface is inhibited by the inertia of the surrounding material.

Very large compressive stresses can be induced in these directions. They will exceed the

elastic limit of the metal if the temperature rise and coefficient of thermal expansion are

large enough. In this situation the metal suffers plastic compressive deformation. This is a permanent strain that will remain even after the metal has cooled down to the initial

ambient temperature. As a result of this compressive plastic strain, tensile stresses are generated during cool-down. If the compressional plastic strain was large enough, the'se tensile stresses will also exceed the elastic limit of the metal and tensile plastic yield

will occur. Thus, during temperature cycling due to repeated pulse irradiation, both com- pressive and tensile plastic strain can accumulate. This strain is manifest as surface

roughness and intrusion-extrusion hillocks where slip planes and bands intersect the free

surface.

To quantitatively bound the response regime under consideration here, it is useful to define several characteristic dimensions. Dynamic stress relaxation is limited by the speed

of propagation of acoustic waves, hence a measure of the time required for stress relaxation

to occur over some distance is simply the time required for an acoustic wave to traverse

this distance. The acoustic wave speed in most metals is approximately 5 x 10^ cm/s. Hence

transient stress effects propagate over a distance of approximately 1 cm in 2 jxs, or 5 y.m in

1 ns . Put another way, regions having dimensions of the order of microns, or centimeters,

have transient stress relaxation times of the order of nanoseconds, or microseconds, respectively. The thermal penetration depth (section 3) is a measure of the thickness of the near

surface region that is substantially heated during pulsed laser irradiation. It can be

shown that in most metals acoustic waves can travel across distances greater than the thermal penetration depth given times in- excess of approximately 15 ps . Hence, it can reasonably be assumed that transient stress relaxation will exist normally through the near-surfac

heated region at times substantially greater than 15 ps . However, stress relaxation in

directions parallel to the free surface is determined by the size of the structure, or the

heated spot. For transverse dimensions of a few centimeters, the transverse relaxation time

will be of the order of a few microseconds, as pointed out previously. Short-pulse largespot irradiation signifies that the heated region has transverse dimensions substantially

larger than both the thermal penetration depth and the transverse stress relaxation distance

for the pulse duration under consideration.

The analysis presented here is confined to the short-pulse large-spot interaction

regime, where normal stress relaxation exists through the heated near-surface region of a metal, but there is inertial confinement of the metal in directions parallel to the free

surface. It is applicable to pulsed laser irradiation conditions ranging from a 1-ns duration pulse applied over at least a 10-fxm diameter spot up to a 10-/is duration pulse applied

over at least a 10-cm diameter spot, if only the response during heating is considered. If the response during cooling after the pulse terminates is to be included, the required spot

sizes must be increased by a factor of approximately 25 in order to remain within the

160
transverse inertially confined regime. If the spot-size criterion is not met, the trans- verse inertial confinement approximation will not be applicable. Although similar physical

behavior will still occur, the quantitative relationships derived here will not be valid.

2.2. Elastic response

The transient mechanical response of materials under rapid heating conditions, in a number of specialized circumstances, has been previously investigated. For example,

Mura [1]"*" considered both a semi-infinite block and a semi- infinitely long cylinder where

the plane surfaces were subjected to an instantaneous temperature change. Boley and Tolins

[2] considered a half-space whose free surface was subjected to instantaneous changes of

stress, strain, or temperature. White [3] analyzed a semi- infinite body whose surface

abosrbs harmonically varying power density. Zaker [4] considered both a half-space and a finite slab where the power deposition was a step-function in time and exponential in distance from one surface, assuming negligible heat conduction. Bushnell and McCloskey [5]

considered heat addition at the interface between two semi- infinite regions, neglecting heat

conduction. Duley [6] has summarized and referenced more recent work involving laser irradiation of material surfaces where surface vaporization, plasma pressure, and normal stress waves are emphasized. None of these analyses directly addresses the specific situation of

interest here. However, they do support the earlier assertation that the normal stress in

the near-surface region relaxes very quickly to the free surface value.

The transient stress-strain response of a plane metal surface under short-pulse large-spot

laser irradiation is based on the analysis of a metal half-space subjected to a timevarying temperature rise that is a spatial function of only the distance below the free sur- face. The basic dynamical thermoelastic response equations follow from the Duhamel-Neumann

law and the equation of motion [7, 8]. With the imposition of the transverse inertial con- finement constraint and the boundary condition of zero normal stress at the free surface

itself, the governing equations are:

(1 e m T

N

(X

' t} =

(1 + v) £ N

(X ' t} " aAT (X > t} (1)

(1 - 2 v) t

t (X. t) = n , „v - e.T (X, t) - a AT (X, t) (2) E T

VA ' ' (1 + v) N

d TV. (X, t) d

2 u (X, t) ax dt*

d u (X, t) S <*> t) = -\~x

(4)

where E, v, a, and p are the modulus of elasticity (Young's modulus), Poisson's ratio,

coefficient of thermal expansion, and density of the metal, (X, t) , (X, t), and

Ujj (X, t) are the normal (in the direction X into the metal) stress, strain, and displacement, and T (X, t) is the transverse (in both orthogonal directions parallel to the free

surface) stress. The four variables (normal stress, strain, and displacement and transverse

stress) are all functions of time (t) and distance (X) below the free surface, as is the

driving temperature rise AT (X, t) . This system of partial differential equations can be

manipulated so as to give a single nonhomogeneous wave equation for any one of the variables.

The remaining variables can then be expressed in terms of this primary variable. Detailed

solution of the wave equation for the primary variable can be complicated, as can be seen

from the typical specific cases considered in references [1] through [5]. Fortunately, it

is unnecessary to do this if the behavior of only the near-surface region of the metal is of concern.

^Figures in brackets indicate the literature references at the end of this paper.

161
Within the near-surface region of the metal, where the normal stress is relieved rapidly compared to temporal changes in the temperature, it is approximately correct to set

T (X, t) equal to zero. With this approximation, the governing equations reduce to

£ n

(x> t} s a - p) a AT (x

' t} (5)

T T

(X, t) = -

(1 I v) a AT (X, t) (6)

af \

(x

> t} ' a^O 01 AT (x ' c) (7)

Equation (7) can be integrated to give

X—00 ^ (o, t) s - {]_- a f

AT < x > t) dX < 8 > x=o

where the boundary condition u^ (X, t) 0 as X -— <x> has been used. The negative algebraic

signs in eqs. (6, 8) indicate a compressive stress and a displacement of the free surface in

the negative X-direction (expansion of the metal), respectively, for a temperature increase.

Thus, eqs. (5, 6, 8) provide adequate description of the strain, stress, and free surface

displacement for an arbitrary temperature rise variation in space and time within the near- surface region.

2.3. Plastic yield threshold

In the near-surface region of the metal, the dominant stresses (t ) are parallel to the

free surface, and the normal stress (t ) is negligible. These stresses are the principal

stresses. The stresses parallel to the free surface are compressive during temperature

increases, and vanish when the temperature returns to its initial value as long as the magnitudes of these stresses remain small (elastic response) . Equation (6) shows that the

magnitudes of the transverse principal stresses are proportional to the temperature rise,

coefficient of thermal expansion, and Young's modulus.

For large values of the principal stresses, the metal will yield plastically. That is,

during the initial part of the temperature increase the metal will be subject to compressive

stress within its elastic range, but upon reaching its yield stress it will suffer plastic

compression. This will take the form of "extrusions" of material out of the plane of the

free surface (figure 1). The maximum shear stresses occur along planes inclined at 45° to the free surface, hence plastic slip will take place along favorably oriented crystal planes

or grain boundaries nearest to these maximum shear stress planes. The extrusion "steps"

will therefore have sides oriented at approximately 45° to the original free surface. Now,

upon cooling, the metal will contract elastically at first. If the compressive plastic yielcll') was small, elastic behavior will continue all the way down to the initial temperature, but

the metal will be left in a state of tension in the near-surface region that compensates for

the plastic (irreversible) compressive strain that occurred during heating. If the compressive plastic strain was large, this tensile stress may also exceed the yield stress, thus

causing tensile plastic yielding during cooling down to the initial temperature. The mor- phology of the plastic slip in this case will be similar to that during heating, except the

extrusion sides will be slanted oppositely relative to the original free surface.

The Mises-Hencky criterion [9] defines the onset of plastic yield as the condition

where the octahedral shear stress exceeds the critical value J2 Y/3, where Y is the tensile

yield stress of the material. For the specific configuration considered here, the octa- hedral shear stress is just n/2|t^,|/3; therefore, the yield threshold is defined by

|t^,| = Y. The surface temperature rise at which plastic yield will first occur at the

free surface follows directly from eq. (6) , and is

162
At or Q - y) Y

y Eg;

Thus, the threshold for plastic deformation is expressed in terms of the surface temperature

rise.

As an example, consider the surface temperature rise threshold for plastic deformation

of a copper mirror surface. For pure copper, v = 0.345, a = 1.67 X 10 K , and 112 7 E = 1.23 x 10 N/m (1.79 x 10 psi) are well-established numerical values [10].

Equation (9) then becomes AT = 3.18 x 10 ^ Y, where AT is in degrees K and Y is in units

2 y y of N/m . Selection of an appropriate numerical value for Y is not trivial because the yield

stress depends critically on the heat-treatment and work-hardening histories of the particular sample. However, experimental studies [11 to 17] of the fatigue failure of pure copper

under alternating stress-strain conditions consistently show that a fatigue life greater

than 10 cycles is achieved if the maximum equivalent tensile stress is kept below + 6.2 x

7 2 10 N/m . Using this value as the yield stress gives AT^ = 20 K.

2.4. Elastic-plastic model

To quantify the inelastic behavior of the near-surface region of a metal under pulsed

laser irradiation, it is necessary to model the mechanical response of the metal in terms of

elastic and plastic modes of deformation. Although this response can be very complex, for

the purpose of this discussion a simple linear-elastic/perfectly-plastic model is chosen to represent the dominant features of the behavior. Figure 2 illustrates the stress-strain

diagram for this model. It is convenient to couch the discussion here in terms of strain as

Sthe principal independent variable. Thus, in the linear elastic regime AOA' , stress increases linearly as strain is imposed. The behavior is perfectly reversible for excursions

between points A and A'. However, if a strain greater than the yield strain is imposed, as illustrated by excursion OAB, the stress increases to the yield stress and then remains constant during plastic contraction from A to B. Now, upon returning to its original dimensions

(zero strain) the material follows the path BC (parallel to AOA'), and is left with a re- sidual stress OC. Subsequent cycling along the path CB occurs elastically with no further

plastic strain.

It is clear from the diagram that if the plastic strain AB is greater than the elastic

^yield strain, then the 'return path must also include plastic strain, as shown in OABDFGE.

In this path plastic strain GE occurs during the return to zero strain. Note that strains

A.BDF and GE are contractions and elongations, respectively, and usually do not occur along

the same slip planes. Hence, although no net strain remains at point E, there has been an ^accumulation of plastic strains, and the material is in a tensile stressed condition.

The stress-strain behavior at a metal surface under pulsed laser irradiation is given

; ;by eqs. (1) through (4) for the linear elastic regime, under the constraint that the net strain in directions parallel to the surface is zero. This constraint imposed the condition

a AT (X, t) + -| [(1 - v) T

T

(x, t) - vr^ (X, t)j (10)

where the first term is the transverse thermal strain €^ (X, t) caused by the temperature

rise and the second term is the resultant transverse mechanical strain £ (X, t) induced by

the inertial constraining forces. It has been shown that in the near-surface region, the

^normal stress is negligible (it is actually zero at the free surface itself). Thus, to good

approximation,

£ TM

(X ' t} *

(1 I ~ T

T CX ' fc) ~ " aAT (X ' t}

This relationship exists for each of the orthogonal transverse directions, and is consistent

with eq. (6) . Equation (11) describes the stress-strain relationship within the metal near

163
Si

fltuvi the surface in the linear elastic regime. This regime ends, and the plastic regime begins,

when the octahedral shear stress reaches the value J2 Y/3. This occurs when | T | = Y. The required temperature rise (AT^) has already been given in eq. (9), and the concomitant

strain magnitude is (1 - v) Y/E. These considerations are illustrated in the stress-strain

diagram shown in figure 2. Note that either mechanical strain or temperature rise can be

considered to be the independent variable in this situation, because emw - -a AT in both

TM

the elastic and plastic regimes.

Now consider the effects of cyclic temperature excursions caused by repeatedly pulsed

laser irradiation. If the maximum temperature rise (AT ) is less than AT , the material max y at the surface will experience a maximum strain less than the yield strain and the stress- strain path will lie along line OA in figure 2, which represents perfectly reversible elastic behavior. For values of AT between AT and 2AI , the stress-strain path will be max y y similar to OABC in figure 2, with plastic yield AB on the first pulse and elastic behavior

on all subsequent pulses with equal or smaller values of AT . If AT is greater than max max

2AT , the stress-strain path on the first pulse will be OABDFGE, as shown in figure 2. Plastic strain will occur during heating (ABDF) and during cooling (GE) . In subsequent pulses

with the same value of AT , the path EDFGE will be followed, again producing plastic strain

during both heating and cooling on each cycle. The increment of plastic deformation that is produced by each pulse after the first is apparent from figure 2. Point D is located at twice the yield strain (-2 ce AT ) and point F is located at a strain of -o;AT , hence the

y max magnitude of the contraction strain increment (A e) along the path segment DF is

A e = ex (AT - 2 AT ) (12) max y

Jslse

ifres

'tis

The same magnitude increment is accumulated along path segment GE, but in elongation. The same plastic deformations will accumulate in both orthogonal transverse directions.

1

When a metal is subjected to alternating strains in excess of the yield strain, such as described above, the plastic deformation tends to occur along closely spaced slip planes

that form slip bands. These bands widen across the surface and form more prominent extru- sion and intrusion steps as plastic deformation accumulates. Thus, roughening of the surface

is related to plastic strain accumulation. Eventually fatigue cracks occur at the slip

bands. Thus, the fatigue life of a metal under cyclic deformation also depends on the total

accumulated plastic strain.

3. Metal surface heating

The temperature rise of a metal surface under pulsed laser irradiation depends on both

the optical and thermal properties of the metal as well as the laser pulse parameters [18].

Absorbed laser radiation is deposited (converted to heat) within the near-surface region

(electromagnetic skin depth) of the. metal. The skin depth in metals at temperatures between

normal ambient and melting,' for radiation within the ultraviolet to near- infrared wavelength > range, lies between 10 and 100 nm. This heat is rapidly transported deeper into the metal

by thermal conduction. An approximate measure of the depth d to which thermal conduction

1/2 will transport heat energy during a time interval t is given by d = 2 (K t) , where K is

2 the thermal diffusivity of the metal. For most metals, K is of the order of 1 cm /s. Thus,

the thermal penetration depth is larger than the skin depth for irradiation periods longer

than 25 ps. Consequently, in thermal analyses of metals under pulsed laser irradiation,

absorbed laser radiation is well represented as a surface heat source when the pulse duration is at least an order of magnitude larger than 25 ps. In this regime, the temperature

rise is dominantly controlled by the thermal conduction mechanism. If the optical absorptance of the metal surface and the thermal properties of the bulk metal are independent of

temperature (which is a reasonable approximation for small temperature changes) , and the

irradiated spot size is large (an order of magnitude or more larger than the thermal penetration depth for the pulse duration under consideration) with a spatially uniform illumination, then the surface temperature rise AT (0, t) can be expressed in terms of a

164
onvolution integral of the form

AT (0, t) = ^ rry / f W, /2

dT, (13)

Or k p C) 1" (t -

here k, p, and C are the thermal conductivity, density, and specific heat of the metal, A

s the optical absorptance, and f (t) is the time-varying incident radiation flux at the

urface.

For a rectangular (constant-flux f) pulse of time-duration T , the maximum temperature

P xcursion (AT ), which occurs at the end of the pulse, is max

1/2

2 A f 1 . . _ AT = V-tjj = (14)

Or k P C)

1/2

(7T k p C T

p

) 1/2

here F = f T^ is the pulse fluence. For a parabolic-shaped pulse, the maximum temperature

I 3 .1/2 2 A f T 1/2 4/3 \3/2

2 A F

ere now f is the peak flux, T is the FWHM pulse duration, and F = 2 -Jl f T /3 is the

P P

lulse fluence. These expressions, in conjunction with the expression for AT^, can be used to xpress the plastic yield threshold in terms of the pulse parameters. Thus, the pulse

uence (F ) at which the plastic yield threshold is reached is

Or k p C T ) 1/2

(tt k p C T ) 1/2

F =

^ AT = — — 2 (16) y 2 A

a

y 2 A a E

K±0)

or a rectangular pulse. The threshold for continued accumulation of plastic strain is 2 F .

As an example, the threshold for inelastic behavior of a high-quality copper mirror sur-

— 1/2 4 2 l/2 ace under rectangular pulse C0„ laser irradiation is F T = 1.1 x 10 J/cm -s , 2

7y P2 here the yield stress has been assumed to be 6.2 x 10 N/m (ATy

= 20 K) . This relationhip is shown in figure 3, where both the threshold fluence for initial (first-pulse) plasic yield (F ) and cyclic accumulation of plastic strain (2 F ) are shown as functions of

he pulse duration.

The absorbed laser radiation resides within the metal as heat, which is evidenced by the

emperature rise of the metal. At any time t, the energy input per unit surface area (F

a^ g

)

/

Fabs Ct) = A f (,,) dTJ C17)

his is equal to the heat added to the metal, which is

165
F abs (t) = J

p C AT (X, t) dX (18)

X=0

If the thermal capacity (p C) is independent of temperature and spatial position, it follows

that

X—oo 77= t J

AT (X, t) dX - ~ j

A f (77) drj (19) x=o 17=0

from which the elastic free surface displacement can be expressed as

J?=t

0?) drj (20)

T?=0

Bennett [19] has presented a similar expression that does not include the (1 + v) / (1 - v) factor which arises due. to the transverse inertial constraint that has been imposed here.

The numerical value of this factor is approximately 2 for most metals. Because the elastic

response regime extends over only a small temperature excursion, the optical absorptance

will be essentially constant and can be removed from the integral in eq. (20). The integral

is then simply the incident fluence up to the time of concern. For a high-quality copper

mirror surface under CO laser irradiation, the predicted free surface displacement will be

2 approximately 0.6 nm per incident J/cm in the elastic response regime.

4. Theoretical-experimental correlation

Direct correlation of theoretical predictions based on the analysis presented here with

presently available experimental data is difficult for several reasons. Early evidence [20]

of plastic yield under pulsed laser irradiation was obtained under small-spot conditions,

where transverse heat conduction and stress relaxation effects would be active. Furthermore

the incident fluence levels were high enough to cause surface melting. However, plastic

yield effects were observed outside of the melt zone, as would be expected. Later studies

[21] of plastic deformation caused by pulsed laser- induced thermal stress were not significantly influenced by transverse heat conduction. However, the fluence levels were above the

melt threshold, transverse stress relaxation was not negligible, and a complex temporal

pulse shape (mode-locked pulse-burst) was employed. Here plastic yield effects were seen both within and around the melt zone. Recent characterization [22] of the thresholds for

inelastic effects still are not clearly in the large-spot regime with respect to transverse

stress relaxation, and were obtained with a mode-locked pulse-burst. The first condition

allows the transverse stress to relax somewhat during the pulse, which has the effect of

raising the temperature and pulse fluence thresholds. The second experimental condition

(pulse shape) makes it difficult to properly assign a simple pulse duration to the energy

input so that correlation with the thresholds illustrated in figure 3 can be made. Nevertheless, a correlation of some of these latest experimental results with theoretical predictions has been made.

The irradiation temporal profile in the experiments of Porteus et al. [22] was approximated by a sequence of individual pulses of 10-ns FWHM, spaced 20-ns apart, and contained

within a 100-ns duration envelope, as shown in figure 4. Both the individual pulses and

the envelope were given parabolic shapes for convenience. Also shown in the figure is the

single-pulse equivalent of the pulse train (same total fluence and overall duration; parabolic profile with a 71-ns FWHM) . The surface temperature rise responses to irradiation by

both the pulse-burst and the equivalent single-pulse are illustrated in figure 5, where the

heat input has been treated as a surface source, the absorptance was assumed to be independent of surface temperature, and the thermal response was considered to be heat-conduction

dominated. For copper, under CO laser irradiation, the scale factor between the surface

166
*|3

mperature rise and the incident fluence was 7.2°K at the temperature peak per J/cm incint on the surface in the pulse-burst irradiation condition. It can be seen that use of

e equivalent single pulse predicts an erroneously lower peak temperature. Thus, using the

lue of 20°K for AT^, the predicted threshold fluence for plastic yield is 2.8 J/cm for the

Ise-burst. It should be remembered that fluence values above this are required to produce

y plastic strain. The experimentally measured fluence at which plastic strain effects

Ire observable on the surfaces of single-crystal and bulk (polycrystalline) copper were 9

2 d 18 J/cm . The polycrystalline samples were either optically polished or diamond- turned

,

d therefore would be expected to have higher initial yield stresses as a result of the

Id-working (strain-hardening) effect of these surface finishing processes. Hence, the

her pulse fluence thresholds for these samples are not unexpected.

The predicted transverse stress-strain history of an annealed copper surface undergoing

2 e temperature rise history depicted in figure 5, at an incident fluence level of 9 J/cm

nich produces a predicted peak temperature rise of 65°K) , is traced out in figure 6 ssuming linearly—elastic/psrf ectly-plastic behavior in the inertially confined regime),

ring the first temperature rise OA, and subsequent fall AA' , the metal responds elastically

the compressive stress. However, on the second temperature rise A'B the compressive plas-

: yield point Y is exceeded, and an increment of plastic strain YB is accumulated. Cooling

Dm B to B' generates an elastic response, and subsequent heating to C causes elastic com- ession B'B plus inelastic compression BC. Cooling to C' again generates an elastic

sponse. Heating from C

1 to D causes elastic compression C'C plus inelastic compression

Cooling from D to D' and subsequent heating to E generate only elastic strains. The

nal cool-down from E generates elastic response to Y' (which occurs at approximately

0 ns) followed by tensile inelastic strain from Y' to the end point at ambient temperature.

• v^. -4 -4

e total compressive plastic strain is 7.3 x 10 , the tensile plastic strain is 3.9 x 10 , -3

that the total accumulated inelastic strain is approximately 1 x 10 for the entire

;le. Whether this is the minimum observable inelastic strain (measurable threshold contion) is a moot point at this time.

5. Discussion

The governing equations that describe the transient thermomechanical stress-strain

lavior of metal surfaces under short-pulse large-spot laser irradiation have been set

rth. It has been shown that detailed solution of these equations can be avoided if only

2 near-surface region is of interest. This is where the transverse stresses and strains

2 largest and, therefore, is the most important region with respect to surface damage.

In the near-surface region, the induced stresses and strains (both inelastic and plas-

;) have been related to the temperature rise caused by absorption of laser radiation,

Lng a linear-elastic/perfectly-plastic model of material response. The surface temperare rise thresholds at which the metal surface begins to yield plastically during heating,

1 at which plastic yield also occurs during cooling, have been quantified in terms of this

3el. Cyclic temperature excursions that exceed the second of the above-mentioned thresh2s, such as produced under repeatedly pulsed laser operation, lead to an accumulation of

astic strain. This will be evidenced by the appearance of slip lines and bands at the

:al surface that should be directly correlated to the total accumulated slip [17]. Ulti-

:ely, fatigue crack failure of the metal may occur. Numerical results indicate that the

aperature rise threshold for inelastic behavior is quite small, and therefore the short-

|[se laser fluence at which inelastic effects begin to appear is also small.

These predictions apply to the metal at the surface. Because the temperature excursions

smaller deeper within the metal, the strains and resultant stresses will also be smaller

plastic deformation will cease at some distance below the surface. The thickness of the

ild zone will depend on the maximum surface temperature and the laser pulse duration. If

oinimum thickness yield zone is required to produce observable effects at the surface,

an for very short duration pulses a higher surface temperature rise (requiring a larger

Lse fluence) than predicted here will be necessary.

167
Some of the detailed results presented here will be altered in the case of metals that

display significant work, hardening as a result of plastic deformation, and/or exhibit strainrate sensitivity of the yield stress at very high strain rates. Work hardening may be

the result of prior mechanical treatment, such as machining and polishing, or may be the

direct result of accumulated plastic strain during repeated laser pulse irradiation. This

raises the yield stress of the metal. It should also be pointed out that plastic deformation increases the electrical resistivity of metals, which will be evidenced by an increase in the optical absorptance [18]. The principal effect of both increased yield

stress and strain-rate sensitivity is to raise the temperature rise and pulse fluence thresholds for plastic yield and cyclic accumulation of plastic strain, while increased absorptance lowers the fluence thresholds for these effects.

The modeling and analysis of thermomechanical stress degradation presented here is also

applicable to mirror coating failure caused by plastic yield of the underlying metal substrate, when an appropriate adjustment is made for energy actually absorbed by the substrate

Even tightly bound natural oxide layers are subject to this kind of "failure". Compressive

plastic yield of the metal substrate usually causes localized delamination of the coating,

and tensile plastic yield usually causes cracks in the coating at the yield sites.

There is a need for more experimental data on the threshold for thermomechanical

stress degradation and the relationship of progressive plastic strain accumulation to increased surface roughness, nonspecular scattering, and optical absorptance in the shortpulse large-spot regime. As pointed out earlier, observation of plastic strain effects

requires that the thresholds be exceeded by an as-yet-undetermined increment. Sensitive

measurements of plastic yield are needed. Baxter [23] has used ultraviolet-enhanced exo- election emission microscopy in this role. The onset of exoelection emission may be a good

indicator of the onset of plastic slip. Correlation of the time-resolved emission current

with the expected surface temperature rise history could be used to validate the model and

analysis of progressive accumulation of plastic strain. Concomitant measurement of surface

roughness, nonspecular scattering, and optical absorptance changes with progressive accumu- lation of plastic strain are also needed to complete the thermomechanical degradation picture.

6. Conclusions

Several specific conclusions may be drawn from the results presented here.

• The surface temperature rise at which a metal surface under short-pulse large-spot

laser irradiation will begin to yield plastically due to thermomechanical stress is relatively small. This plastic yield threshold temperature rise (AT ) is of the

0 order of 20 K for pure copper.

-1/2

• A simple scaling relationship, namely F^T^ = constant, expresses the plastic

yield threshold in terms of pulse parameters. For a high-quality copper mirror

irradiated by a rectangular (constant-flux) CC^ laser pulse, the numerical value of

the constant is approximately 1 x 10^ J/cm^-s"*" ^ . This scaling relationship is

shown in figure 3.

• Repetitive pulse irradiation will cause accumulation of plastic strain on each suc- cessive pulse if 2 ATy (or, equivalently, 2 F^) is exceeded during the pulse. The

increment of plastic strain accumulated during a pulsed heating-cooling cycle is

proportional to the temperature rise in excess of 2 AT^, and consists of both con- traction and extension in both of two orthogonal directions along the surface. It will be manifest as progressive surface degradation in the form of slip bands, intergranular slip, and fatigue cracks.

These specific conclusions are necessarily based on the simple model for elastic-plastic

response of a metal used here, and consequently are subject to modification as a result of more realistic evaluation that incorporates the complicating details that have been omitted

in this model. However, it is expected that the basic behavior and effects will be the

same.

168
7. References

[1] Mura, T., Thermal strains and stresses

in transient state, Proceedings of the

2nd Japan National Congress for Applied

Mechanics, pp. 9-13 (1952).

[2] Boley, B. A., and Tolins, I. S., Transient coupled thermoelastic boundary

value problems in the half-space,

Journal of Applied Mechanics, 29 , pp. 637 - 645 (1962)

[3] White, R. M. , Generation of elastic waves by transient surface heating,

Journal of Applied Physics, 34 , pp. 3559 - 3567 (1963).

[4] Zaker, T. A., Stress waves generated by

heat addition in an elastic solid,

Journal of Applied Mechanics, 32 , pp. 143 - 150 (1965)

[5] Bushnell, J. C, and McCloskey, D. J.,

Thermoelastic stress production in

solids, Journal of Applied Physics, 39 , pp. 5541 - 5546 (1968)

[6] Duley, W. W. , CC> 2 Lasers: Effects and

Applications, Academic Press, pp. 341 —

351 (1976).

[7] Sokolnikoff, I. S., Mathematical Theory

of Elasticity, McGraw-Hill, pp. 358 —

362 (1956).

[8] Shames, I. H. , Mechanics of Deformable

Solids, Prentice-Hall, pp. 126 - 128

(1964)

[9] Ibid, pp. 100 —102.

[10] Gschneidner, Jr., K. A., Physical properties and interrelationships of metals

and semimetallic elements, Solid State

Physics, _16, Academic Press, pp. 275 —

426 (1964).

[11] Wadsworth, N. J., Work hardening of

copper crystals under cyclic straining,

ACTA Metallurgica, 11, pp. 663 - 673

(1963)

[12] Wood, W. A., Cousland, S. McK. , and

Sargant, K. R. , Systematic microstructural changes peculiar to fatigue

deformation, ACTA Metallurgica, 11 , pp. 643 - 652 (1963)

[13] Laufer, E. E. , and Roberts, W. N .

,

Dislocation structures in fatigued

copper single crystals, Philosophical

Magazine, 10, pp. 883 - 885 (1964).

[14] Laufer, E. E. , and Roberts, W. N .

,

Dislocations and persistent slip bands

in fatigued copper, Philosophical

Magazine, 14, pp. 65 - 78 (1966).

[15] Lukas, P., Klesnil, M. , Krejci, J.,

and Rys, P., Substructure of persistent

slip bands in cyclically deformed

copper, Physica Status Solidi, 15 , pp. 71 - 82 (1966)

[16] Lukas, P., Klesnil, M. , and Krejci, J.,

Dislocations and persistent slip bands

in copper single crystals fatigued at low stress amplitude, Physica Status

Solidi, 27, pp. 545 - 558 (1968).

[17] Roberts, W. N., Persistent slip bands

in fatigued copper, Philosophical

Magazine, 20, pp. 675 - 686 (1969).

[18] Musal, Jr., H. M. , Influence of laser

pulse shape on metal mirror damage,

Proceedings of the International Conference on Lasers '78, pp. 267 — 273

(1978)

[19] Bennett, H. E., Thermal distortion

thresholds for optical trains handling

high pulse powers, Laser Induced

Damage in Optical Materials: 1976,

NBS Special Publication 462, pp. 11 -

24 (1976).

[20] Haessner, F., and Seitz, W. , Laserinduced dislocation structures in copper single crystals, Journal of

Materials Science, 6^, pp. 16 — 18

(1971).

[21] Porteus, J. 0., Soileau, M. J., and

Fountain, C. W. , Slip banding in Al

single crystals produced by 10.6 ^im laser pulses, Applied Physics Letters,

29, pp. 156 - 158 (1976)

[22] Porteus, J. 0., Fountain, C. W.,

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

Bennett, H. E., Pulsed-laser stress phenomena on highly reflecting metal

and alloy surfaces, Laser Induced Image

in Optical Materials: 1977, NBS

Special Publication 509, pp. 204 — 214

(1977).

[23] Baxter, W. J., Laser scanner for exo- electron display and measurement of

fatigue damage, Journal of Testing and

Evaluation, _5, pp. 243 - 250 (1977).

169
THERMAL DISTORTION THERMOMECHAN I CAL DEGRADATION

"INSTANTANEOUS"

DISTORTED SURFACE

INCIDENT

RADIATION

FLUX f. (t) inc

F. (t) « inc nc

(t)dt

ORIGINAL

SURFACE

"INERTI AL" CONFINEMENT

(STRAIN = 0) ELASTIC STRAIN DISTORTION

(EXPANSION) = (Xt)

d(t) 7§ AF. (t) PC Inc

PLASTIC DEFORMATION (YIELD)

WHEN INDUCED STRESS > YIELD STRESS

HEATING

(COMPRESSION)

COOLING

(TENSION)

INITIAL PLASTIC YIELD THRESHOLD

(STRESS YIELD STRESS) AT -

y

2 _Y_

3 aE

Figure 1. Metal surface thermal distortion and thermomechanical degradation

STRESS (t t)

(COMPRESSION)

Figure 2. Linear-elastic/perfectly plastic stress-strain diagram for metal surface

under pulsed laser irradiation.

170
171
Figure 5. Copper surface temperature rise under C0„ laser irradiation

172
DC B Y

Figure 6. Stress-strain response of copper surface to pulse-burst temperature rise history

173

DEFECT- DAMAGE- RESISTANT COPPER MIRRORS

J. 0. Porteus, D. L. Decker, D. J. Grandjean, S. C. Seitel, and W. N. Faith*

Michel son Laboratory, Physics Division

Naval Weapons Center, China Lake, California 93555

The calculated threshold for 10.6-ym laser-induced melting of intrinsic

crystalline copper is found to be in excellent agreement with the latest

experimental value when the 100-nsec pulse shape and temperature-dependent

material properties are properly modeled. Melt thresholds of diamond-turned

copper mirrors recently produced and tested at the Naval Weapons Center are found to be very nearly intrinsic. However, laser-induced pitting related to surface defects often causes failure at a considerably lower level. Selection

of starting material and technique of surface finishing are shown to be impor- tant factors influencing laser damage related to surface defects and disorder.

A well-defined inverse relationship between the thresholds for laser-induced

slip and melting is demonstrated and provides a good indication of surface

disorder on polished, as well as diamond-turned mirrors. Mirrors prepared by

vapor deposition of copper on polished silicon carbide are found to be remarkably free of defect and visible (thermal) damage but have lower melt thresholds than good bulk copper mirrors. Thermal modeling indicates that the

premature melting can be avoided by improving the thermal conductivity of the

substrate.

ley words: Cu mirrors; crystalline disorder; diamond turning; laser-induced slip; polishng; ripples; Si C substrates; surface defects; thermal diffusivity; vapor-deposited mirrors

Introduction

The threshold for 10.6-ym, 100-nsec pulsed laser-induced melting of intrinsic crystaline Cu is well established, both experimentally and theoretically [1.2] 1

. Melt thresholds

f good diamond-turned Cu mirrors are very close to the intrinsic value [3]. However,

aser-induced pitting often causes spatially selective damage at considerably lower levels,

itting is believed to result from random bulk defects which may trap optically absorbing

ontaminants when exposed at the mirror surface. Such defects can also reduce damage

hresholds of dielectric mirrors prepared on metal substrates. The roles of material

election and surface finishing with respect to laser damage are not completely understood,

n this paper we examine these variables in relation to damage associated with defects and

rystalline disorder. Three different varieties of oxygen-free, high-conductivity (OFHC)

u are intercompared in terms of damage morphology, multi threshold damage characteristics

|2], and pit formation frequency. On diamond-turned surfaces the dependences of melt and

lip [4] thresholds on the type of cut and top-rake angle are examined. The well-defined

elationship between these two thresholds, as governed by surface disorder, is demonstrated

polished mirror is evaluated for comparison with the diamond-turned samples. A mirror

repared by vapor deposition of Cu on a polished SiC substrate is also evaluated and found

o be remarkably free of laser-induced pits and visible slip. However, the melt threshold

s substantially below that of intrinsic Cu as a result of the less effective heat dissipation in the SiC.

Experimental Apparatus and Procedure

Measurements are made in a previously reported laser damage facility (fig. 1) where

omprehensive target monitoring and careful beam characterization are key features [5].

he experiments were performed in vacuum in order to simplify the discussion of basic

henomena. The ultrahigh vacuum (UHV) test chamber was equipped with the following essenial monitoring apparatus for the present work: (1) 20-power microscope; (2) Faraday cup

Work supported by the Air Force Weapons Laboratory and by Navy Independent Research funds

Figures in brackets indicate the literature references at the end of this paper.

175
with grounded entrance grid (10 mA/sr detection sensitivity at 35 degrees off the laser

beam axis) for sensing of sample-emitted ions and neutrals having sufficient energy to produce secondary electrons; and (3) Auger analyzer with electron imager which is sensitive

to work-function variations ~ 0.1 eV over the target surface and to changes in surface

roughness. The 10.6-ym laser beam is focused on the sample by a ZnSe lens having a focal

length of 24 cm. This produces a Gaussian spatial intensity distribution 350 ym in diameter

at 1/e 2 of the maximum. Temporally, each pulse consists of a train of mode-locked spikes

which form an envelope of 100 nsec nominal duration. Thresholds quoted here are peak

thresholds, i.e., they refer to the energy density of the damaging pulse (fluence) at the

point of maximum spatial intensity. The Lumonics C02 TEA laser used here is capable of

delivering a maximum fluence of 300 J/cm2 on the axis of the focused beam. The fluence is inferred from the measured pulse energy on the basis of pinhole scans of the focused beam.

In the present work, a standard sample was tested on a daily basis to maintain relative

calibration uncertainties to within an estimated ±5%.

Multithreshold measurement and analysis procedures have been reported in detail [6].

Briefly, the sample to be analyzed is exposed to one pulse at each of approximately 50 sites spaced at 1-mm intervals along its surface. Pulse energies, which are measured for

each pulse, are uniformly distributed over a range which includes the thresholds for each

of the damage effects of interest. The occurrence of light emission (flash) and pit formation are noted by an observer as they occur, while damage features which are more difficult

to identify, e.g., slip, melting, and cratering, are categorized with the aid of more sophisticated optical microscopy, i.e., dark field and Nomarski, after the sample has been

removed from the test chamber. The data are reduced by entering the peak energy density

and a code representing observed damage features for each site into a computer and performing a statistical analysis. This gives an average threshold for each effect, together with

its standard deviation, the latter being a measure of the threshold uniformity over the

surface. Results are presented in the form of a bar chart called a damage profile. Examples are given below.

Intrinsic Damage

In addition to melting, slip bands due to thermal stress [4] appear in intrinsic Cu at about one-fifth of the energy density required to produce melting. Pitting rarely occurs below the melt threshold. The process of laser-induced melting in intrinsic crystalline Cu

is straightforward for pulse lengths ^ 100 nsec. Table 1 compares measured thresholds with fc)

those calculated from thermal diffusion theory. The experimental value 76.6 ± 8.6 J/cm2 wasp]|j

obtained on a sample prepared in UHV by the best techniques available for producing a clean, well-ordered, defect-free surface, although without an optical finish [3]. The

present value is higher than that previously reported by a factor of 1.69 as a result of

corrections for systematic errors in our earlier beam scan measurements [2]. The first

calculated value in table 1 for a 100-nsec square pulse was obtained earlier this year by

Sparks and Loh [1] by solving the heat flow equation exactly. We were able to reproduce

their result within 2% using a finite difference solution [7], which permits greater flexibility in modeling the actual pulse shape and the temperature-dependent material properties

Replacing the square pulse with a more representative pulse having a linear rise and expo-

| nential decay increases the calculated melt threshold from 53.2 to 67.2 J/cm2

. Including

the temperature dependence of the specific heat, thermal conductivity and density, and the

nonlinearity of the temperature-dependent optical absorption further raises the calculated

melt threshold to 76.1 J/cm2

, in excellent agreement with experiment. This also verifies

j calculated value of the 74 J/cm2 which we reported two years ago [4].

Material Dependence of Damage

Three varieties of bulk Cu were used in this study, all 0FHC: (1) AVC0, a highdensity forged Cu produced specifically for AVC0 Corporation to their specifications; (2)

Naval Weapons Center (NWC), a commercial grade purchased from Ducommun Metals and found to have good characteristics for diamond turning; and (3) Y-12, commercial grade Cu from a reliable supplier commonly used by the Union Carbide Oak Ridge Y-12 diamond-turning

facility.

176
Table 1. Comparison of measured and calculated

melt thresholds for intrinsic bulk copper.

Experimental value (Porteus and Decker, 1979) 76.6 ± 8.6

Calculated values

Absorption linear with T; other properties constant

1. Square pulse

Exact solution (Sparks and Loh, 1979) 52.2

Finite difference solution (FDS) 53.2

2. Exponential pulse (FDS) 67.2

T

2 Absorption term; T-dependent properties

Exponential pulse (FDS) 76.1

One 1 . 5-inch-diameter sample of each material was prepared in the NWC diamond-turning

facility [8]. Samples 1.5 inches in diameter were individually turned to center using the same tool under identical conditions. A constant spindle speed of 500 rev/min was used.

Cleaning was done immediately after turning using a freon degreaser and a series of ultrasonic rinses in clean acetone. Samples were then blown dry with nitrogen. Just before

testing, the surfaces were wet with acetone and dried by drawing lens tissue over them.

Figure 2 compares micrographs of laser damage morphology on the three materials. The

energy in each case was sufficient to produce slip bands, melting, intergranular slip [4],

•(and well -developed pitting, accompanied by emission of ions and visible light. Large pits

[often occur along grain boundaries, particularly on the NWC and Y-12 samples. This is less

apparent in the AVCO Cu, although the overall density of small pits is higher than in the

other two materials. Also, laser-induced ripple patterns [9] are generally more developed

on the AVCO Cu.

Figures 3-5 compare damage profiles of the three materials. There are no striking

^differences, and variations are generally within the reproducibility of results on a given

sample. Melt thresholds are very close to intrinsic in all three cases. On the AVCO

"jsample the standard deviation for light emission extends somewhat below that for pits,

jwhich is consistent with plasma formation at pits too small to resolve with the 20-power

pi cro scope. Also, the intergranular slip threshold is lowest on the AVCO Cu. This may

simply that the grain boundaries are less effectively pinned as a result of a lower concentration of defects along grain boundaries.

The pit threshold gives a good indication of the onset of pitting where the mean distance between pits is not substantially larger than the 0.35-mm focal spot diameter. However > the pit threshold is not a reliable indicator of low density pitting, which may occur at lower fluence levels. This may be a serious limitation for large beam applications. To obtain information on low-fluence pit density we conducted a comparative pit

frequency study, the results of which are given in table 2. Each of a large number of test

sites, given in the first row of the table, was shot once at a fixed fluence. The percentage of sites on which pitting was observed is given in the second row. The fixed fluence

:was chosen as 67% of the threshold for ion emission on each of the three materials. The

:ion threshold provides a convenient reference, eliminating the necessity for removing the

sample from the test chamber for microscopy or for source calibration with a standard

sample. Ion emission is somewhat less sensitive to pitting than light emission and is therefore a more appropriate reference.

The results of this study indicate that the pit frequency is substantially higher on

the AVCO sample, indicating a higher density of defect sites. On the other hand, this

sample exhibited no visible defects at any test site before laser damage, while four were

observed on both the NWC and Y-12 samples, as indicated in the last row of table 2. Our interpretation is that the special processing of the AVCO Cu has reduced defect concentration at individual sites, but has produced a larger number and more uniform distribution of

defect sites. The above-mentioned prominence of ripple patterns, the smaller average pit

size, and the lower intragranular slip threshold are consistent with this interpretation.

177
Table 2. Pitting frequency compared on three varieties

of Cu at 67% of the ion emission threshold.

fly rn 1 " 1 L

Sites tested 316 203 192

* Percent pitted 25.4 12.0 6.1 ** Visibly defective sites 0 4 4

**At 67 percent of ion threshold

Before laser damage.

Influence of Diamond Turning

To determine the influence of surface finishing on laser damage, comparative studies

similar to the above on samples which were diamond turned under different conditions and on

a polished sample were made. Diamond-turned samples which were individually turned to center were compared with others which were flycut in pairs. Flycutting differs from

center turning in that the cut is interrupted as the tool passes between the two samples.

The cutting (surface) speed was approximately 1200 in./min. Flycut samples were prepared

at top-rake angles of +5, -5, and -10 degrees in order to investigate this variable. All center-turned samples, including those discussed in the preceding section, were turned with

a top-rake angle of -5 degrees. The same diamond tool set at a side-rake angle of 0 degrees

was used in all cases. Cleaning procedures were as described above.

The melt thresholds measured on the diamond-turned samples are presented without

standard deviations in figure 6. It is clear that center turning yields higher thresholds,

other conditions being equal. Among the flycut samples the -10 degree rake angle produces

somewhat better results than the more positive angles. Figure 7 presents the same information for slip thresholds. Notice that the trends in slip thresholds are generally opposed

to the trends in melt thresholds exhibited in figure 6. This is a consequence of the

inverse relationship between slip and melting, which is caused by surface disorder. Here

(fig. 7) lower slip thresholds indicate better surface order, so that conclusions regarding

surface quality agree with those from figure 6. Evidently, the interrupted cut, higher

cutting speed, or a combination of both tend to degrade surface quality. Possibly, a large

negative top-rake angle reduces the shock of the interrupted cut, thus reducing disorder in the samples flycut at -10 degrees.

The inverse relationship between the thresholds for slip and melting becomes more

apparent when the data are plotted in terms of these variables, as demonstrated in figure

8. In addition to the data from the diamond-turned samples, data points corresponding to intrinsic crystalline Cu and polished AVC0 Cu have been included. A data point from an earlier, relatively poor-quality, diamond-turned sample is also included at the far right.

The curve was drawn merely to indicate the data trend, which is consistent with our understanding of crystalline disorder effects. Surface disorder introduced by diamond machining,

or polishing, increases the number of dislocations. This in turn tends to inhibit slip and

raise the slip threshold. However, additional dislocations also tend to increase the

scattering of conduction electrons, thereby raising the surface absorption and reducing the

melt threshold. Points at the upper left, such as that for UHV-prepared Cu, therefore

indicate better surface order than points at the lower right, such as those for Cu which

has been polished or diamond turned under less favorable conditions. The slope of the data

trend is about -0.6, indicating that the slip threshold is roughly a factor of two more sensitive than the melt threshold to the state of surface order. The fact that some of the

diamond-turned samples, particularly the center- turned samples, are so close to intrinsic

in terms of effective surface order is quite remarkable and indicates that diamond turning

can be accomplished with a minimum of cold working.

Unfortunately, a study of low-fluence pit frequency in relation to diamond turning

variables was precluded by time limitations. In order to give some indication of this

relationship, we show in figure 9 the minimum fluence at which pitting was observed in the

ordinary 50-site multi threshold damage test on each of the various diamond-turned samples.

178
As a group, the flycut samples show more pitting at low fluences. However, the cutting

method and rake angle have a less consistent influence on pitting than on uniform melting

and si ip.

Influence of Polishing

In order to determine the influence of polishing on defect damage, a pit frequency

study was performed on a sample of AVCO-pol ished AVCO Cu. Results are compared with those

from diamond-turned AVCO Cu (cf table 2) in table 3. A substantial gradation in pit frequency was observed on the polished surface, as indicated by results from two different

areas given in the first two columns of the table. In area A no advantage over diamond

turning was observed. In fact, the pit density is somewhat higher than on the diamondturned sample. In area B, however, the pit frequency is only half that shown in the last

column. This tends to substantiate the view [2] that under optimum conditions the disordered layer generated by polishing can effectively "heal" small defects present in the

starting material. The polished sample exhibited no visible defects at any of the test

sites before laser damage.

Table 3. Pitting frequency on two areas of AVCO-pol ished Cu compared with that on NWC-diamond-turned Cu at 67% of the ion

emission threshold. Both samples are of AVCO Cu.

Polished Center

Area A Area B turned

Sites tested 78 78 316

Percent pitted

** Visibly defective sites

35.9 12.8 25.4

0 0 0

**At 67 percent of ion threshold

Before laser damage.

The increased surface disorder introduced by polishing is quite evident when one compares the resulting damage profile (fig. 10) with that of a diamond-turned surface of

the same material (fig. 3). The substantially higher slip threshold and lower melt threshold on the polished sample are in accordance with increased disorder, as discussed in connection with figure 8. The large standard deviations for pits and light emission

reflect the large variability in pit density indicated in table 3.

Vapor-Deposited Cu

As an alternative to bulk Cu, we have also evaluated mirrors formed by vapor deposition of Cu on polished silicon carbide substrates [10]. These samples were prepared by the

Westinghouse Research and Development Center. The absence of pitting and laser-induced

slip in the damage morphology is remarkable (fig. 11). This is substantiated by damage

profile data (fig. 12), which also shows the excellent uniformity of these samples. Defect

damage resulting in spatially selective pitting is indistinguishable from uniform melting

as indicated by the equivalent thresholds for melting and pits. The lack of slip effects

is believed to be a result of small crystallite size, which provides numerous channels for

stress relaxation. A difficulty with these mirrors is the relatively low threshold for

melting, which is well below the 76 J/cm2 value for intrinsic bulk Cu, and varies with the

thickness of the copper layer. Using the thermal modeling methods described in connection

with table 1, it is concluded that the substrate thermal conductivity, which is substantially below that of Cu, is mainly responsible [10]. The possibility of raising the sub- strate conductivity by using improved Si C or alternative materials offers much promise for

this type of mirror.

179
Concl usions

A number of important conclusions can be made as a result of this study. First of

all, the theoretical intrinsic crystalline Cu melt threshold is in excellent agreement with

latest experimental results when the 100-nsec pulse shape and temperature dependence of

material properties are properly taken into account. We have also shown that special

processing of the starting material can have an important influence on the size and density

of defects which lead to pitting. Such processing has little apparent influence on surface

crystalline order, which is modified mainly by the method of surface finishing. The effective state of surface order on bulk (coarse-grained) mirrors can be determined quite accu- rately from a well-defined inverse relationship which exists between the thresholds for

slip and for melting. Based on this relationship, center-turned bulk Cu can be remarkably

close to intrinsic in terms of its nonselective damage characteristics. Flycutting is less

desirable, but the adverse effect on surface order can be moderated by using a large negative top-rake angle (-10 degrees). Under optimum conditions, polishing can reduce pitting,

apparently by "healing" small defects with disordered material. However, results can vary

widely, even on the same sample, and the melt threshold is invariably degraded. Vapordeposited Cu mirrors on polished silicon carbide substrates show remarkable resistance to pitting and slip damage, but melt thresholds are relatively low. Calculations indicate the

possibility of significant improvement by use of substrate materials with better heat

transfer characteristics.

References

[1] Sparks, M. , and Loh, Jr., E. , J. Opt. Soc. Am. 59, 847 (1979).

[2] Porteus, J. 0., and Decker, D. L., in Laser Induced Damage in Optical Materials (Academic Press), to be published.

[3] Porteus, J. 0., Decker, D. L. , Jernigan,

J. L., Faith, W. N. , and Bass, M. , IEEE

J. Quantum Electron. QE-14 , 776 (1978).

[4] Porteus, J. 0., Fountain, C. W. , Jernigan, J. L. , Faith, W. N. , and Bennett,

H. E., NBS Spec. Publ . 509, 204 (1977).

[5] Porteus, J. 0., Soileau, M. J., and

Bennett, H. E. , NBS Spec. Publ. 435,

207 (1975).

[6] Porteus, J. 0., Jernigan, J. L. , and

Faith, W. N., NBS Spec. Publ. 509,

507 (1977).

[7] Gaski, J. D. , Fink, L. C. , and Ishimoto,

I., "Systems Improved Numerical Differencing Analyzer," TRW Systems Report

#11027-6003-R0-00.

[8] Decker, D. L., and Grandjean, D. J., NBS Spec. Publ. 541, 122 (1978).

[9] Temple, P. A., and Soileau, M. J., NBS Spec. Publ. 462, 371 (1976).

[10] Porteus, J. 0., Choyke, W. J., and Hoff- mann, R. A., Appl . Opt. (in press).

180
PHOTON DRAG WINDOW ION COLLECTOR

DETECTORS

Figure 1. Laser damage facility. Nh-Mg are

bare Cu beam turning mirrors. Sj and S 2 are ZnSe Brewster plates used for controlling incident fluence. Si and M^-Mg may be rotated as a unit about the beam axis, while S 2 remains

fixed. D is a beam dump. S 3 is a NaCl beam

spl itter.

VARIETIES Of COPPER CENTER—TURNED AT MWC

AVCO mwc y-12

1 I

Figure 2. Damage morphology compared on bulk Cu mirrors diamond turned to center. Three varieties of OFHC Cu are represented. Laser fluence

was sufficient to produce uniform melting

accompanied by flash and ion emission in each

case.

181
CENTER-TURNED AVCO BULK COPPER

SLIP BANDS

INTERGRAN. SLIP

MELTING

PITS

CRATERS

ION EMISSION

LIGHT EMISSION

WRK. FN. CHANGE

0 20 40 00 SO 100 THRESHOLD (J/cm2 AT 100 m)

Figure 3. Damage profile for AVCO Cu, diamond

turned to center. A shaded bar for each damage

characteristic indicates the corresponding

threshold averaged over approximately 50 damage

sites on a single, representative sample.

Hatched blocks represent corresponding standard

deviations, which mainly indicate nonuniformity

of the sample surface.

CENTER—TURNED NWC BULK COPPER

SLIP BANDS

INTERGRAN. SLIP

MELTING

PITS

CRATERS

ION EMISSION

LIGHT EMISSION

WRK. FN. CHANGE

_l I I l_ 20 40 60 M

THRESHOLD (J/cm2 AT 100 ml

100

Figure 4. Damage profile for NWC Cu, diamond

turned to center.

182
CENTER—TURNED Y-12 BULK COPPER

SLIP BANDS

INTERGRAN. SLIP

MELTING

PITS

CRATERS

ION EMISSION

LIGHT EMISSION

WRK. FN. CHANGE

20 40 60 M

THRESHOLD U/cm* AT 100 ml

100

Figure 5. Damage profile for Y-12 Cu, diamond

turned to center.

i 1

— INTRINSIC

o

i

AO CENTER TURNED A FLYCUT

-W -6 0 6 RAKE ANGLE (DEGREES)10 16

Figure 6. Melt threshold vs. top-rake angle for

diamond-turned bulk Cu. Each data point represents a different sample. In addition to the

type of cut, the point symbols also show the

type of material as indicated in figure 9.

183
AO CENTER TURNED A FLYCUT

-6 0 6 RAKE ANGLE (DEGREES)

10

Figure 7. Slip threshold vs. top-rake angle for

diamond-turned bulk Cu. Point symbolism corresponds to the preceding figure.

80

75

70 Q

-JO

£ 66

6t>h

LU5 AO CENTER-TURNED A FLYCUT a POLISHED O UHV—PREPARED SINGLE-CRYSTAL

10 20 30

SLIP THRESHOLD (J/cm*)

50

Figure 8. Melt vs. slip thresholds for diamondturned, polished and intrinsic Cu samples.

Point symbolism corresponds to the preceding

figure for the diamond-turned samples. The data

point at the far right is from an earlier batch

of Y-12 Cu which was center turned with a tool

in relatively poor condition.

184
100

O

I I I 1 1

_ INTRINSIC

- A

- A

A

-

- - A

A AVCO COFFER A A NWC COPPER A O Y-12 COPPER

A

I I -16 -10 -6 0 6 10 16 RAKE ANGLE (DEGREES)

Figure 9. Minimum fluence for pit formation on diamond-turned samples. Data is based on an ordinary 50-site multithreshold damage test in each case. Light and dark symbols represent

center-turned and flycut samples, respectively,

as in the preceding three figures.

POLISHED AVCO BULK COPPER

SLIP BANDS

INTERGRAN. SLIP

MELTING

PITS

CRATERS

ION EMISSION

LIGHT EMISSION

WRK. FN. CHANGE

0 20 40 00 SO 100 THRESHOLD (J/cm* AT 100 m)

Figure 10. Damage profile for AVCO Cu polished

by AVCO. Surface disorder compared to a diamondturned surface of the same material (fig. 3) is high, as indicated by thresholds for slip and

melting. See also figure 8.

185
DIAMOND—TURNED. OFHC VAKM-DEPOMTEO ON t»C

100pm

Figure 11. Damage morphology of vapor-deposited

Cu mirror (right) compared with that of a diamondturned bulk OFHC Cu mirror (left). Slip bands,

intergranular slip faults and pits, which can be seen on the left, are absent from the vapordeposited sample.

WESTINGHOUSE Cu/Cr/SiC (1.01 nm OF Cu)

MELTING

PITS

PERFORATION

ION EMISSION

LIGHT EMISSION

20 40 60 SO THRESHOLD (J/cm2 AT 100 i»)

100

Figure 12. Damage profile of a vapor-deposited

Cu mirror having a 1.01-ym thickness of Cu. The

excellent uniformity is indicated by the extremely

small standard deviations of the thresholds for

melting and pits.

186
LASER-DAMAGE RESISTANT COPPER SURFACES WITH HIGH REFLECTIVITY AFTER OXIDATION

M. Oron , L.G. Svendsen, and G. Sorensen

Institute of Physics, Iniversity of Aarhus

DK-8000 Aarhus C, Denmark

Ion implantation of aluminum into copper has been used to produce mirror

surfaces with improved properties with respect to pulsed-laser damage. Upon oxidation of implanted samples, they show a much higher reflectivity of the laser

pulse compared to non-implanted samples. The oxidation has been followed by Rutherford backscattering technique and nuclear reactions, and the improved surface

properties are explained by the presence of compound oxide layer beneath the sur- face, which shows greater stability against decomposition than the pure metal

oxide. This novel coating technique, comprising ion implantation and oxidation,

may be advantageous in improving surface properties such as reflectivity of metal

mirrors.

Key words: Laser damage; ion implantation; corrosion inhibition; mirror surfaces.

1 . Introduction

Development of damage-resistant mirrors for high-power pulsed lasers is of crucial importance. As far as metal mirrors are concerned, Porteus et al . [T]' have performed an evaluation for high-power applications. They found that oxidation preferentially lowers the

threshold for plasma formation, which produces transient optical degradation in addition to permanent damage. Goldstein et al. [2] have studied pulsed-laser damage to uncoated metallic reflectors and indicate a ranking of material with copper being the optimum one followed

by silver and gold. However, copper as well as other metals will oxidize, which lowers the

threshold for laser damage, and the reflectivity may thus deteriorate due to thermal decomposition of the oxide layer. By alloying, it is possible to inhibit the oxidation of metals,

and in recent years, ion implantation has been used as a tool for surface alloying [3].

Copper possesses excellent thermal conductivity and good reflectivity. Oxidation of

copper has been the subject of several papers, e.g., [4], and recently, ion accelerators

have been applied to surface alloying with the purpose of obtaining corrosion inhibition

[5,6]. It has been reported that ion implantation of aluminum results in a build-up of a protective layer consisting of a mixture of oxides of aluminum and copper located somewhat

beneath the surface [7]. In the present study, ion implantation of aluminum into copper

has been used as a tool to inhibit oxidation and thus improve the surface properties with

respect to reflection of high-power laser pulses. A detailed study of the oxidation of copper influenced by various dopants (Ti, Cr, etc.) and their ability to build up protective

surface layers will be published in detail elsewhere.

As far as metal mirrors for high-power lasers are concerned, there is a need for a suitable surface coating, which inhibits oxidation and, if possible, creates a thermally

stable oxide in the surface layers. Although ion implantation is an expensive coating

technology, the application may be justified when the production cost of laser mirrors is considered.

Ion implantation offers several advantages as a coating technique, and it should be

emphasized that this technique does not introduce a definite boundary layer as evaporation

Permanent address: Soreq Nuclear Research Centre, Yavne, Israel.

1. Figures in brackets indicate the literature references at the end of this paper.

187
techniques. The thermal conductivity is supposed to remain good in the surface layers;

hence the coated layers are not burned off due to the laser power. Even though the surface

may become oxidized, the formation of a thermally stable surface oxide with a mixed compo- i sition might also improve the properties of the metal with respect to laser damage.

It has been the scope of the present study by a combination of ion implantation of

aluminum into copper and subsequent surface oxidation to create a coating in a copper-mirror

surface, which consists of a mixture of various oxides of aluminum and copper, with a good stability against thermal decomposition. Aluminum was chosen as a dopant because it can be considered a getter material in view of the reaction 2Al+3Cu 2 0-*6Cu+Al 2 0 3

. According

to the literature, A1 2 0 3 is more passivating than Cu 2 0, but it should also be possible to synthesize mixed oxides such as CuA10 2 and CuAl^ with good thermal and passivating pro- perties. The deterioration of an oxidized copper surface is particularly crucial due to the low decomposition temperatures of copper oxides. Thus an improvement of the thermal

stability of copper-metal surfaces might expand the application to mirror purposes.

2. Experimental procedure

The first part of the experimental procedure concerns ion implantation of aluminum into

copper and a study of the oxidation processes, while the second part deals with a study of

the damage to the metal-oxide surfaces made by a high-power laser.

Pure copper was electropolished prior to ion implantation, which was performed at 60-80-keV ion energy, utilizing the isotope separator at the Institute of Physics, University of Aarhus. Ion beams of Al + were produced by the carbon-tetrachloride method, where

an oxidized aluminum foil was placed in a heated quartz tube, through which there was a flow of CC1,,. Ion currents of 10-30 yA were usually obtained. To make the implantations

homogeneous, the ion beam was swept electronically. Doses between 10 17 and 2xl0 17 ions/cm 2

, which correspond to an alloying percent of 10-20% if equal sputtering of the alloying com- pounds is assumed, were used. Concerning the penetration of the energetic aluminum ions

into copper, the projected range can be theoretically estimated, as shown by Lindhard et

al. [8]. For 60-keV Al + in copper, the theoretically estimated projected range corresponds

to 28 ug/cm 2

, which is equal to 310 A. However, implantation of high doses usually leads

to a larger penetration with a rather constant dopant distribution in the surface. In this case, the range profile of the implanted aluminum was measured by comparing Rutherford

backscattering (RBS) spectra with similar spectra from pure copper, using a 3-MeV He+ beam,

which was scattered 150° by the sample into a solid-state detector. The aluminum-implanted

region was found to extend ^1 300 A from the surface, which is considerably more than the

theoretically estimated projected range of 310 A [8] that, assuming a certain straggling,

could account for a penetration of ^400 A. Further, it was found that the alloying percentage was rather constant within the total penetration depth.

The copper samples were then oxidized by isochronal annealing in atmospheric air. In

( the present communication, only results obtained at 200°C or below have been included,

while high- temperature oxidation temperatures will be published elsewhere. The oxide

thicknesses obtained at a time scale of hours at 200°C or days at 100°C are comparable with

oxide thicknesses obtained by a long-term oxidation at room temperature or at temperatures

slightly above room temperature. In the present study, the oxidation of metals and the distribution of dopants were traced using nuclear-reaction techniques ( 16 0(d,p) 17 0) and Rutherford backscattering (RBS), and these two techniques can map the elemental distribution

in the surface layer [9]. The aluminum implantations covered an area of 4x4 mm, which ap- peared more grey than the non-implanted regions. After oxidation, it was easy to observe

that aluminum implantation did indeed form a considerable protection against oxidation.

After some time of oxidation, the grey, implanted area regained its red copper colour,

while the non-implanted area was tarnished.

Reflectivity measurements are very sensitive to the surface treatment. In the present , study, the effect of ion implantation was found by performing measurements of laser-pulse

reflection from the particular samples implanted and from non-implanted areas. A conventional

technique was used. The neodynium-glass laser (wavelength 1.06 urn) was Q-switched with a Pockels cell, having a variable pulse length from 10 ys (free-running mode) to 50-100 ns (Q-switched). The laser power in the pulse could be varied up to 5 J with an exposed area

of M mm

2

. The signal from the two photodiodes was visualized on a storage scope, and thus \\

188
the ratio between the incoming and reflected laser pulses was evaluated. For the long pulse

length, the values of reflectivity could be put on an absolute scale via a measurement from

a silver mirror of known reflectivity. The laser-pulse power was measured calorimetrically.

3. Experimental results

It is possible from an analysis of Rutherford-backscattering spectra to obtain information on the elemental distribution on the surface, and a detailed description on the analytical technique will be published elsewhere [9]. In the case of copper, the backscattering yield will be qualitatively lowered by the presence of lighter elements such as oxygen

and aluminum. Figure 1 shows three stages of oxidation at 200°C of a copper sample which,

previous to oxidation, was implanted with a dose of 2xl0 17 aluminum ions/cm 2

. The RBS spectra for unoxidized samples are shown for comparison. In figure 1, the dip in the curves

shows that as the oxidation proceeds, a copper-deficient layer beneath the surface is created. This is in contradiction to what is known from the oxidation of pure copper,

where the surface composition is almost CuO, while deeper layers to a larger degree consist

of Cu 2 0. The presence in the implanted samples of a copper-deficient zone therefore indicates the presence of A1 2 0 3 . In Table 1 are listed the measured oxide thicknesses in atoms

Table 1 Oxidation,

days at 200OC Copper sample Aluminum-implanted copper sample

(2xl0 17 ions/cm 2

) CompoThickness of 0 layer sition,

(xio 18 Cu at/cm 2

) average

Composition,

surface

0 compo- 0 compoThickness of 0 layer sition, sition,

(xio 18 Cu at/cm 2

) average surface

5 3.0 Cu 2 0 CuO 0.7 CuO

10 3.1 Cu 2 0 CuO 0.7 CuO

15 3.2 Cu 2 0 CuO 0.5 CU0

] r

of copper and average as well as surface composition of the oxide layers. The implantation

of aluminum to a dose of 2xl0 17 ions/cm2 inhibits oxidation by a factor of 4-6. It should

be noted that after implantation, the aluminum concentration was almost constant, but after

oxidation, the aluminum was concentrated at a depth of ^400-600 A beneath the surface.

Hence diffusion processes may play a role in the formation of the aluminum-rich passivatinq

layer. . 3

The samples used for reflectivity measurements were implanted at 80 keV with 10 17 ions/cm2 of aluminum. In the region without the amplifier, the laser pulse was in the

0.1-0.2 Joule/pulse region. With the amplifier, reflectivity of laser pulses in the 1 Joule/pulse region was measured.

Experiments with a laser pulse of 0.2 Joule were performed in aluminum-implanted

samples, which had been oxidized for 24 h at 100°C and 150°C, respectively. At the low tem- perature, the reflectivity of the implanted and non-implanted areas was almost identical.

However, at 150°C, a marked difference was noticed. While the reflectivity for the aluminum

area was measured to be 98%, the non-implanted region showed a decrease in reflectivity of

66%. A number of aluminum-implanted samples were oxidized at 200°C for different periods

prior to the reflectivity measurements (laser pulse 0.2 Joule), and the results are shown

in figure 2. Since the reflectivity may vary from sample to sample due to the polishing

procedure, figure 2 shows the decrease in reflectivity for individual samples of the non- implanted area normalized to the implanted area. It is observed that thicker oxide layers

exhibit a considerable decrease of the reflectivity in the cases where aluminum was not implanted prior to the oxidation process.

For a laser pulse of the order of 1 Joule/pulse over an area of <3 mm diameter, the

reflectivity measurements based on the pulse itself appear to be more complex. One reason

may be the presence of aluminum impurity atoms in the surface plasma created. A copper

sample was implanted with 10 17 Al/cm 2 and oxidized at 100°C for 7 days. A laser pulse of

1 Joule/pulse will melt and decompose the surface layer. Figure 3 shows SEM pictures (mag189
nification 2000) from such oxidized copper samples in the center of the laser spot and in the border region. While the surface layer appears to have been totally melted in the center, the picture looks quite different in the border region of the laser spot. Here doughnut-shaped craters are found, which may be taken as an evidence of thermal-spike melting

combined with gas evolution from the oxide decomposition. Now the samples were used as re- flector of 1.2, 1.5, and 1.0 Joule on the non-implanted and 1.6, 1.7, and 1.6 Joule on the

implanted area. Although the total laser power in this case is 25% higher for the implanted

area, the visible damage is much less, as shown in figure 4, where the total laser spot with

a magnification of 40 in a scanning electron microscope is displayed. When looking at a magnification of 450 (fig. 5), it is observed that also the character of the damage is different. The non-implanted area appears to have been totally melted on the surface, while the

implanted area has characteristic doughnut-shaped structures, indicating that there has been

local melting around spikes in the laser profile.

4. Discussion

The experimental findings of the present study indicate that by ion implantation of

aluminum into copper, it is possible to improve the reflectivity of polished surfaces that

have been oxidized in air at a temperature of 200°C or less. At these low temperatures, the

RBS supports the idea of formation of a protective aluminum-oxide layer beneath the surface.

Whether this layer consists of CuA10 2 or mixtures of the respective oxides cannot be verified at present. However, the experiments show that after oxidation, aluminum is present

in the outer surface layer (fig. 1) only to a small extent. This finding is also supported

by preliminary SIMS measurements using the CAMECA probe at A.E.R.E., Harwell, and a com- puter unfolding of the RBS spectra [9]. By such an unfolding procedure, it is found that

a copper sample, after aluminum implantation (2xl0 17 at 80 keV) and oxidation for two hours

at 300°C, shows a two-layer structure consisting of a ^2.7xl0 17 CuO mol/cm 2 layer on top of

a 1 .5-1 . 9x1

1

CuA10 2 mol/cm 2 layer. The presence of such a two-layer oxide could explain

the experimental observation that high-power laser pulses develop deeper craters in non- implanted areas. When a laser pulse is reflected from a metal surface, the power and the

wavelength is of crucial importance because most surfaces are covered with oxide layers,

where the power is absorbed. In the case of copper oxides, these are not thermally stable,

and before melting of the metal, the oxides are decomposed. The present study shows that

for oxides containing aluminum in the surface layer, the reflectivity is decreased much

less for powers in the region of a few tenths of a Joule per pulse. When the samples were

oxidized for some hours at 200°C prior to the reflection of the laser pulse (fig. 2), it

should be noted that the largest improvement is observed for thick oxide layers. For thin

oxide layers, only a slight improvement is observed because the laser pulse cleans the

surface without further damage. It is also observed that for the same reason, the reflectivity of the non-implanted area after some pulses does not change compared to the implanted area. For the lowest reflectivity measured, the implanted areas showed a reflectivity roughly a factor of two better than did the non-implanted areas.

Reflection of laser pulses in the region above 1 Joule per pulse appears to be a com- plicated process. Thus Zavecz and Saifi [10] have studied the reflectivity from copper

single crystals and found a drop in reflectivity to values much lower than can be ascribed

to dielectric-like behaviours of the molten zone. As shown in figure 5, the ion implantation of aluminum reduces the melting of the surface; hence it must be expected that also at very high high power levels, the reflectivity of metal surfaces may be increased by ion

implantation.

In the present study, the feasibility of producing a thermally stable surface coating

has been demonstrated, but it should be noted that such a compound oxide might also change

the hardness of the surface, and possibly a metal surface of copper and other metals could

be hardened by ion implantation, oxidation, and laser annealing. Thus the combination of

ion implantation, surface oxidation, and laser annealing might open up a number of possible

new applications in the micro-metallurgy of solid metal surfaces in order to improve also

other metal-surface properties than the reflectivity.

190
[1 ] Porteus, J.O., Decker, D.L., Hernigan,

J.L., Faith, W.N., and Bass, M. , IEEE J. Quant. Electronics U, 776 (1978)

[2] Goldstein, I., Bua, D., and Horrigan,

F.A., in: Laser-induced damage in optical

materials, NBS special publication 462 , 41 (1976)

[3] Dearnaley, G., in: Applications of ion

beams to metals, eds. S.T. Picraux et al., (Plenum Press, 1974) p. 63

[4] Ronquist, A., and Fishmeister, H.F., J. Inst. Met. 89, 65 (1960)

[5] Naquib, H.M., Kriegler, R.J., Davies,

J. A., and Mitchell, J.B., J.Vac.Sci .Tech.

13, 396 (1976)

References

[6]

[7]

[8]

t9]

[10]

Rickards, J., and Dearnaley, G., in: Application of ion beams to metals, eds. S.T. Picraux et al . (Plenum Press,

1974) p. 101

Svendsen, L.G., Corrosion science, in

press (1979)

Lindhard, J., Scharff, M., and Schiott,

H.E. , K.Dan.Vidensk.Selsk.Mat.Fys.Medd.

33, No 14 (1963)

Borgesen, P., and Svendsen, L.G., to be

published

Zavecz, T.E., and Saifi, M.A., Appl

Phys.Lett. 26, 165 (1975)

6. Figures

107 -IS* OXIDATION OF IMPLANTED Cu MIRROR

• Al

r Detector oCu

\ ° 0

< 2.5 MeV H

+ ^ Cu-sample

UJ

UJQ

-i 1

1 1 1 rENERGY OF BACKSCATTERED PARTICLES

Figure 1. The backscattering yield of 3-MeV

He+ backscattered 145° from Al-implanted Cu (2xl0 17 ions/cm 2 at 60 keV). The samples have

been oxidized at 100°C for (a) 5 days, (b) 10 days, and (c) 15 days. Solid curves represent

oxidized samples and dashed curves non-oxidized samples (shown for comparison).

1

191
60 -

"i r 0

10

17 ions /cm At* - Cu

implanted at 70 keV

Laser power -0.2 J pr. pulse

0 50 100 150 200 250 OXIDATION TIMES OF Cu MIRROR SURFACES AT 200"C (minutes)

Figure 2. The change in reflectivity of an oxidized Cu area normalized to an oxidized

Al-implanted area (10 17 ions/cm 2 at 70 keV).

The framed areas show the range in reflectivity change caused by the number of pulses

(^0.2 J), which is indicated by the figure in

the frame.

Figure 3. Scanning electron-microscope pic- ture (magnification 2000) of a Cu surface ex- posed to a laser pulse of 1 J after oxidation

in atmospheric air for 7 days at 100°C. Lefthand part shows center of laser spot. Righthand part shows the border of the laser spot.

192
Figure 4. Scanning electron-microscope picture (magnification 45) of laser spot caused

by three pulses (each ^1.5 J). Prior to laser

exposure, the sample was oxidized for 7 days

at 100°C. Left-hand part shows non-implanted

area. Right-hand part shows implanted area

(10 17 ions/cm 2 Al )

Figure 5. Larger magnification of figure 3. Left-hand part shows oxidized, non-implanted

area. Right-hand part shows implanted area

(10 17 ions/cm 2 A1+).

The speaker indicated that samples were exposed for several minutes to ion currents of up to

30 microamperes.

193
II
LASER-INDUCED BREAKDOWN OF DIAMOND-MACHINED WINDOW SURFACES*

M. J. Soileau, J. 0. Porteus, and D. L. Decker

Michelson Laboratory, Physics Division

Naval Weapons Center, China Lake, California 93555

Previous work on diamond-turned KC1 at 10.6 ym has shown that this technique has significant promise for improving the resistance to laser damage over

that of polished surfaces. This work examines the particular characteristics

of turned surfaces on CaF 2 and SrF 2 which influence the pulsed laser damage

thresholds at 2.7 and 3.8 ym. We consider (1) polarization dependence due to the periodic nature of the surface roughness of turned surfaces, (2) damage to the material produced by the turning process, and (3) residual contaminants

introduced by machining. The laser source is an HF-DF TE laser which produces

plane-polarized, single-mode 120- and 90-nsec pulses at 2.7- and 3.8-ym wavelengths, respectively. The spatial distribution of intensity on target is Gaussian to the 1/e2 points. Detailed morphology is obtained from Nomarski and

dark-field optical microscopy. The effect of laser conditioning is examined by

comparing thresholds and surface characteristics on areas of the surface

exposed to nondamaging laser pulses (n-on-1 effect). The so-called n-on-1

effect was found to be wavelength selective with a much larger effect seen at 2.7 than at 3.8 ym. This result provides information on machining damage and

may have important implications for laser processing as a possible means of

improving the performance of machined optical surfaces.

Keywords: Diamond turning; infrared windows; laser breakdown; polishing; surface damage.

Surfaces prepared by precision diamond machining are compared with surfaces prepared

by conventional polishing. The results indicate that the diamond-machined surfaces have as high or higher failure resistance under high fluence loading as surfaces prepared using

conventional techniques. Failure of both types of surfaces is strongly dependent upon

surface structure and surface contamination. Laser conditioning experiments indicate that

absorbed surface water is a significant contributor to surface failure under high fluence

loading at 2. 7 ym.

Diamond machining has been shown to be an effective means of producing high quality

optical surfaces on metal mirrors [I] 1

. The technique has been applied to relatively soft

dielectrics such as KC1 and NaCl with promising results [2]. In this work, diamond-machined

surfaces were produced on single-crystal SrF 2 and CaF 2 and polycrystal 1 ine MgF 2 (Irtran II).

Disc-shaped specimens, which had previously been polished using conventional techniques,

were turned on-center with a single-point diamond tool [3]. Part of the polished surface

was left intact for comparison with the machined surfaces. The tightly focused output of a pulsed HF/DF laser was used to probe the surfaces. The critical laser parameters are given

in table 1. Further details of the laser facility are given elsewhere [4].

Table 1. Laser parameters.

Laser

Pulse energy

(J)

Wavelength

(ym)

Pulse width

(nsec)

e _z Focal

spot diam.

(ym) HF 0.2 2.7 125 53 DF 0.2 3.8 95 57

Work supported by the Office of Naval Research

1 Figures in brackets indicate the literature references at the end of this paper.

195
The thresholds for visible spark, increased visible scatter and/or truncation of the

beam transmitted through the sample, were measured in air for both the diamond-machined and

conventionally polished surfaces. Changes in the surface were monitored with a 20-power

microscope. The threshold for a particular effect was defined as that flux which produced

the effect in 50% of the sites when irradiated with a given laser intensity. Unless otherwise noted, each site was irradiated only once.

The results of the threshold measurements are summarized in table 2. The failure of

single-crystal surfaces was manifested by a visible spark, increased visible scatter, and

abrupt truncation of the beam transmitted through the sample. Failure near threshold

occurred at the peak of the pulse. This type of failure is suggestive of an avalanche

breakdown process. The failure of the polycrystall ine MgF 2 surfaces was characteristic of

thermal processes, i.e., near threshold the visible scatter increased, but there was no visible spark or truncation of the transmitted beam.

Table 2. Summary of 1-on-l damage thresholds.

Specimens Machined

low scatter

Machined

high scatter

Pol ished

HF thresholds (10 9 W/cm2

)

CaF 2 7.04 4.4 6.75

SrF2 -4 2.86 3.86 6.74

SrF2 -2 6.84 3.95 4.68

MgF2 (poly) 0.32 0.34

DF thresholds (10 9 W/cm2

)

CaF2 10.6 7.99 9.37

SrF2 -4 5.22 3.97 12.0

SrF2 -2 8.20 6.34

MgF 2 (poly) 0.51 0.47

The most significant features of the results given in table 2 are: (a) Thresholds for

the diamond-machined surfaces are of the same order as those of the polished surfaces; (b)

thresholds for the polycrystal 1 ine MgF 2 are an order of magnitude lower than those of the

single-crystal materials, (c) for all the surfaces studied, the HF (2.7 urn) thresholds are lower than the DF (3.8 um) thresholds, and (d) thresholds are dependent on the surface

quality of the materials.

The single-crystal samples showed alternating azimuthal regions of high and low visible

scatter which were apparently determined by the relationship of the cutting direction to the crystal lographic axes [3J. Thresholds were generally found to be 50% lower in the high

scatter regions.

The lower thresholds at 2.7 ym as compared to 3.8 ym is suggestive that absorbed

surface water is contributing to the failure of both the polished and diamond-machined

surfaces. More evidence for the influence of surface water is seen in the results of N on experiments given in table 3. The so-called N-on-1 thresholds were determined by first

irradiating a site five times at a flux equal to 80% of the damage threshold for single

irradiations and then increasing the flux until damage results. Note that for all cases where an N-on-1 effect is seen, the effect (an increase in damage threshold) is greater at

2.7 ym than at 3.8 ym. These results suggest that selectively absorbing contaminants,

e.g., water, are being removed from the surface. The peak of the water absorption band

corresponds to the strongest HF line (2.7 ym), and thus absorbed surface water is a likely

contributor to surface damage at HF frequencies.

196
Table 3. N-on-1 effect (percent increase)

Machined Machined Pol i shed Polished

HF DF HF DF

CaF 2 54 40 47 lb SrF 2 -4 0 SrF 2 -2 21 13 0 MgF 2 (poly) 21 14 0

The effect of the orientation of the residual machining grooves with respect to the

laser electric field was investigated. The threshold was measured with the linearly

polarized laser field parallel to and normal to the machining grooves. Polarizable defects

normal to the incident field should enhance the coupling of the laser energy to the surface

and lower the threshold [5]. The threshold was approximately 10% lower for the case of the

field normal to the grooves than for the field parallel to the grooves. This decrease in threshold is as predicted by theory and is of little consequence for these low index materials. However, for high index materials such as GaAs, Si, ZnSe and Ge, the effect could be

significant. Figure 1 illustrates the effects of the interference of the induced polarization field and the incident laser field for the case of the incident field normal to the

residual machining grooves.

In summary, diamond-machined surfaces on SrF 2 , CaF2 , and polycrystal 1 ine MgF 2 have

HF/DF laser damage thresholds of the same order as polished surfaces on these materials.

The mechanism of failure for single-crystal CaF2 and SrF 2 appears to be electron avalanche

breakdown initiated by defects and impurities for both diamond-machined and polished surfaces. The mechanism of failure for polycrystal 1 ine MgF 2 appears to be absorption by

defects present throughout the material. The lower thresholds at HF frequencies compared

to those at DF frequencies and the larger N-on-1 effect at HF frequencies indicated that

absorbed water has a significant effect on the surface damage thresholds.

References

[1] The following review article summarizes

the work on diamond-turned metal surfaces and is a good guide to work in this

area: Porteus, J. 0., in Laser Induced

Damage in Optical Materials (Academic

Press), to be published.

[3] Decker, D. L. , et al 562, 293 (1979). NBS Spec. Publ

[2~> Soileau, M. J., et al. 462, 149 (1976). NBS Spec. Publ

[4] Van Stryland, E. W. , et al Publ. 509, 118 (1977). NBS Spec.

[5] Temple, P. Spec. Publ

A. , and Soileau, M. 462, 371 (1976).NBS

100 itm

Figure 1. "Laser-Induced. . .Window Surfaces", Soileau et al

197
I

II
OPTICAL AND SURFACE PHYSICAL CHARACTERISTICS OF DIAMOND-MACHINED INFRARED WINDOW MATERIALS

D. L. Decker, D. J. Grandjean, and J. M. Bennett Michelson Laboratories, Physics Division

Naval Weapons Center, China Lake, California 93555

This paper presents the results of the diamond turning of surfaces of a wide range of single-crystal and polycrystall ine infrared window materials, including Ge, CaF2 , MgF2 , SrF2» KC1 , and GaAs. These surfaces were obtained from an advanced air-bearing, diamondturning machine recently brought to an operational state at the Naval Weapons Center. The

surface characterization performed includes a wide range of optical and surface physical

examination including an analysis of scattered and absorbed light, and a surface microtopographic examination by diamond profilometry, phase contrast interference (Nomarski),

and scanning electron microscopy. These data are compared with earlier results and are related to the characteristics of the Naval Weapons Center diamond-turning machine and to the machining parameters. A companion paper at this conference discusses the laser damage

resistance of these surfaces with implications for their use in laser applications.

Key words: Diamond single-point machining; dielectrics; infrared optical components; laser

damage; optical properties.

Introduction

Diamond single-point turning or milling as a manufacturing method for precision optics has been

demonstrated at a number of laboratories, notably the Energy Research and Development Administration

complexes at Livermore, Calif., and at Oak Ridge, Tenn. [l^]. 1 The advantages of this technology over

conventional methods include significant cost savings in many cases but, more importantly, the ability

to readily generate aspheric surfaces which would be difficult or impossible otherwise. The materials

which are amenable to diamond single-point machining are not limited to metals such as aluminum and

copper and many of their alloys, but also include an impressive list of infrared window materials,

useful as domes, windows, and lenses (see table 1). Much recent interest in diamond single-point

machining is a direct result of the superior optical components for high energy laser apolication that

can reliably be produced by this method. However, there are many low power laser or imaging optics

applications for which the method offers significant system performance improvement over what can be

done with conventional components [3].

This paper presents surface physical and optical characterization results for dielectric surfaces

machined on the Naval Weapons Center (NWC) diamond-turning machine. The optical fabrication criteria

outlined in table 2 are given as a reminder of the finishing requirements for a component. This paper

is primarily concerned with surface microtopography and the directly related topic of scattered light

[4,5]. Most of the previously reported work has been of a qualitative sort, and has not been followed

up by detailed optical characterization or optimization of results [6]. This paper presents for the

f\rst time data relating to the mechanisms of material removal and the relationship of these mechanisms

to the crystalline orientation of the work surface to the machining geometry.

Work supported by Naval Weapons Center Independent Research Funds,

figures in brackets indicate the literature references at the end of this paper.

199
Table 1. List of dielectric materials which

have been successfully diamond single-point

machined as of 1979.

CaF2 NaCl ZnS MgF 2 NaF ZnSe

SrF 2 Si LiNb0 3 KC1 Ge GaAs acrylic plastics

Table 2. Summary of the fabrication

requirements which apply to most

optical components.

Optical figure

Surface absorption

Scattered light

Damage threshold

Machining Technique and the Material Removal Process

The basic diamond-machining process discussed here is that of single-point turning or milling of a surface in the fashion of ordinary machine shop fabrication but using a very precise machine and special diamond tools (fig. 1). The diamonds used are of gem quality and are mounted and polished to a supersharp, highly accurate, radiused cutting edge. The overall view in figure 2 shows the tool and spindle slides of the NWC machine which are arranged in a T-base configuration [7].

In the case of a metal workpiece, material is removed by the tool in machining by shearing a chip

from the surface. The idealized surface profile in a diametral plane perpendicular to the turned work

surface is shown in figure 3. The tool nose radius r is typically 1/8 inch, and the tool feedrate is

in the range of 10" 1* inches per spindle revolution. The theoretical peak-to-valley roughness in this case is 10~ 8 inches. In actual practice, as the feedrate is made smaller the surface finish does

improve as predicted; it approaches an asymptotic roughness controlled by material imperfection and

irregularity in the machining process. The latter effects include very high frequency (10 6 Hz) chip

vibration/surface interaction effects which are possibly excited by chip/tool friction [7,8]. Tools

and feedrates typically used to machine dielectric surfaces are similar to those used to machine metal surfaces. In contrast with metal machining, there is strong tool/surface speed dependence to the resulting dielectric surface characteristics.

Material removal from most dielectrics examined in this paper occurs apparently as a consequence

of brittle fracture. There is no evidence of plastic deformation in these cases, either on the resulting surface or on the machining debris. This statement is based upon scanning electron microscopy of

the machined surfaces and machining debris. No subsurface examination has been made. A pitting effect

occurs which is very sensitive to tool geometry and to crystallographic orientation. The location of

"clear" and "cloudy" areas on diamond-turned surfaces of several materials is shown in figure 4. The visual effect of a cloudy area is a consequence of light scattering from microscopic pits. When turned

on center, single-crystal materials develop a clear/cloudy pattern which will usually have the rotational symmetry of the crystallographic plane exposed by machining. The [331] axis in a cubic lattice

is tipped only approximately 20° away from the [111] axis which, of course, has threefold rotational

symmetry. Samples (a) and (b) of CaF 2 have virtually the same orientation, with sample (b) showing the expected symmetry, but (a) shows curious skewed twofold symmetry. The origin of the effect discovered

in sample (a) is unknown, and this anomaly was observed only in this case out of several dozen samples

of different materials and a variety of orientations which were turned in this study.

It is useful to examine these surfaces microscopically both with a light microscope and by electron

microscopy at much higher magnification. Figures 5 and 6 are Nomarski interference contrast micrographs

of an Irtran MgF 2 sample and single-crystal CaF 2 sample (b). In all cases, the samples were originally

polished and were subsequently diamond turned, leaving a central polished spot for direct comparison.

As shown in figure 5, the quality of the Irtran surface is substantially limited by voids remaining

from processing of the material both in the polished and diamond-turned areas. In the case of singlecrystal CaF 2 (fig- 6), the clear diamond-turned area is indistinguishable from the polished area. The scanning electron micrographs of figure 7 compare CaF 2 (b) polished and diamond-turned clear areas at

200
much higher magnification. The polished surface is defined by a superposition of scratches from the polishing grains. Most of the clear diamond-turned area is an extremely smooth, almost featureless,

surface. The localized pitted areas often show the effects of multiple tool passes and a very interesting repetitive nature with a spatial period which varies from slightly less than 1 urn down to 0.5 um

(fig. 7). This variation is seen to occur even over a very small area and shows no correlation with

position on the sample. Figure 8 shows the localized nature of the cloudy areas and a high magnification view of the pits in a cloudy area. There is no qualitative shape or size difference of the pits

occurring in clear or cloudy areas —only concentration differences. The machined surface of a singlecrystal Ge sample shown in figure 9 shows structure which is very similar to that displayed by CaF2 in figures 7 and 8. Again, the difference in clear and cloudy areas is primarily in the pitting concentration. Often a decrease in spatial period can be correlated with a decrease in pit volume in a sequence of pits. The pit shape is, of course, strongly suggestive of an indentation mark made by the

tool tip. However, the relative motion between the surface being turned and the tool tip is a factor

of at least 40 smaller than the typical 1-um-deep pit. A probable explanation is that the pits are produced by spalling as a result of a localized tensile load at the surface which exceeds the material

strength following the release of the compressive loading at the cutting edge as the tool passes over

i the surface (fig. 10). The repetitive nature of the spall pit formation comes from a constructive

interaction of the tool loading and a standing acoustic wave in the material which has been excited by

the tool. The effect is similar to chatter often observed in an ordinary machine tool, except that it

is. not a machine structure resonant effect but an effect in the material itself. The effect is related

to that observed in diamond machining of metal surfaces [7,8]. The standing wave may or may not be a consequence of a resonant effect. However, the temporal frequency of the material "chatter" is approximately 10 6 Hz which is on the order of the resonant frequency of a mass on a spring consisting of a spall fragment connected into the lattice of the work material. In this simple model, the resonant

frequency would scale as the square root of the reciprocal of the volume of the spall pit. This relationship is at least qualitatively observed in the decrease in spall pit size and accompanying decrease

in spacing. The spall pit sequence on the left of the superposition shown in figure 7 is a clear

example of this effect. A resonant explanation thus seems likely.

In contrast to these brittle materials, the material removal mechanism for KC1 is much different.

There is no evidence of the pitting behavior either inside or outside of cloudy areas. Material is removed by shearing and plastic deformation much as in metal machining. The surfaces shown in figure 11 demonstrate that material removal tends to be very nonuniform and can result in a very ragged and rough

surface. Other rather ductile dielectric materials such as GaAs have been observed to machine in a similar manner, with a chip flowing from the tool rather than fine dust as in the case of CaF2

. Clear

and cloudy areas are due to subtle differences in surface texture.

As the cutting radius increases at constant rotational rate during machining, it has been observed

in a number of cases that the spall tendency precipitously can decrease beyond a specific radius

(surface speed). This may simply be due to the lack of constructive interaction of the tool and the resonant wave in the material. If so, the tendency to spall should occur in annular bands, depending

upon surface speed. This effect has also been observed. Another possible explanation is that spall

formation is dependent upon strain rate and hence surface speed. This explanation seems much less

likely, however. The location of the greatest concentration of spall pits (the cloudy spoke areas on

a single-crystal sample) is a direct consequence of the orientation of planes of easy cleavage in the

! material with respect to the tool. Due to the geometry of the tool, specifically its flat top, the

normal surface loading force of the tool acts in a plane over the contact width of the tool. When this

plane is parallel (or nearly so) to a plane of easy cleavage, the tendency to spall is greatly increased

The tendency towards spall formation can be decreased by two known approaches: (1) higher or

lower surface speeds and (2) large negative tool rake angle. By using a tool with a very large negative

rake in the range of 20 to 45° very uniform surfaces have been turned on several dielectric materials

including Ge and CaF 2 . However, the success with both surface speed variation and tool geometry are

very dependent upon the cutting edge characteristics of the tool being used. Edge sharpness and orientation and perhaps even the diamond bulk imperfection are involved in the resonant chatter effect. In machining ductile dielectric materials, the use of a lubricant such as triacetin is a definite advantage

A tool used near zero top rake and with low surface speeds, i.e., less than 300 in./min, has given the

best results. Again, results depend very strongly on the condition of the tool edge.

201
Optical and Surface Physical Characterization

A diamond stylus profilometer has been used to examine the topography of the several diamondturned surfaces reported here. The results shown in figures 12 through 15 were obtained from a Talystep

profilometer, with a 1-um-radius conical stylus. This instrument yields surface profile data along the track of the stylus with a lateral resolution on the order of a micrometer for the smoother, low slope

surfaces examined here and with a height resolution of 3 A. The output of the stylus transducer ampli- 9 fier is digitized and subsequently processed to yield distribution functions for height, slope, and

correlation [9]. In figures 12 and 13, unprocessed scans of polished and diamond-turned single-crystal

CaF2 are compared for two different scan lengths, 15 and 600 urn. The diamond-turned clear areas on the CaF2 sample of figures 12 and 13 are a factor of three smoother than the corresponding polished surface

on the same sample and have much less steeply sloped structure as well. The Irtran MgF2 surface scans shown in figures 14 and 15 indicate a much rougher surface, the surface being substantially determined

by voids still present in the hot pressed material. The diamond-turned surface is uniform and slightly i rougher than the polished surface.

The surface profiles of the polished and clear diamond-turned areas thus are very similar in distribution of height and slope. Consequently, the light scattering from the two surface types will

be similar. The magnitude of the total integrated scattered (TIS) light compared to that light specu- larly reflected, AR/R, can be related to the rms roughness 6

AR/R = (4tt6/X)

2

,

where X is the wavelength of the incident light [10]. Table 3 gives values of TIS for Irtran MgF 2 , CaF 2 , and SrF2 for light in the visible (0.647 um) and in the near infrared (3.39 urn). There is little

difference in the visible light scatter from the clear diamond-turned polished areas, with the cloudy

diamond-turned areas on CaF2 and SrF 2 scattering a factor of five more light than the clear areas. In the near infrared the differences between polished and clear diamond-turned scattered light are even Smaller, with still a factor of roughly five difference between clear and cloudy. From an examination

of the CaF2 diamond-turned surfaces shown in figures 13 and 14, it is obvious that much debris from

machining is present even after "cleaning." It is certainly true that much of the scattered light

originates from these particles and that more complete cleaning would materially reduce the TIS, espe- cially in the infrared. The presence of machining debris also has a negative impact upon the laser

damage resistance of the machined surface, especially for short pulse lengths where dielectric breakdown

Is a predominant damage mechanism [11]. However, in a companion paper at this conference [12], no significant difference in laser damage thresholds for the polished and diamond-turned surfaces described

in this paper were discovered for 100-usec pulses of 2.7- and 3.8-um wavelength laser radiation.

Table 3. Values of TIS at two different wavelengths, 0.647 and 3.39 um. for three different materials which were diamond single-point machined

in this study. Included is a comparison of polished and diamond-turned

"clear" and "cloudy" areas on the same sample.

Material

Wavelength, ym

Diamond turned

Pol ished Cloudy Clear

0.647 .02 .03 .03 3.39 .006 .01 .01

0.647 .0007 .04 .001 3.39 . 00004 .004 .00003

0.647 .0008 .02 .004 3.39 .0001 .0006 .0001

MgF 2 (Irtran)

CaF?

SrF ;

202
Concl usion

The diamond stylus prof ilometer results from diamond-turned dielectric surfaces are statistically

not much different than surfaces produced by conventional grinding and polishing by loose abrasive. As

a consequence, the TIS in the visible from a diamond-turned surface can be quite comparable to that of

a high quality polished surface and, in some cases, even lower. For the materials examined in this

paper, Irtran MgF 2 , SrF 2 , and CaF2 , the diamond-turned, surface-scattered light at 3.39-vim wavelength is on the order of that from the corresponding polished surfaces. As was shown, the diamond-turned areas

are covered with a significant concentration of debris from turning, even after "cleaning." Much of the measured scattered light certainly originates from these particles. More elaborate cleaning must be accomplished. The scattering from the "cloudy" diamond-turned surfaces is much larger as a consequence

of a much larger concentration of localized spall pitting. A full understanding of the latter effect and

its elimination are important to the fully successful application of diamond turning to infrared optics.

Some progress in this regard is reported in this paper. The Irtran MgF 2 results are very interesting

and encouraging since this polycrystall ine material is often used for low-cost infrared optics. There

are no preferential cloudy diamond-turned areas on this surface, and the diamond-turned surface scatters

only slightly more light than the polished area. It is thus demonstrated that under some conditions,

diamond-turned dielectric surfaces can be quite comparable in optical quality to corresponding polished

surfaces. The advantages that diamond single-point machining brings to the production of infrared optical components, high throughput, and ready generation of aspheric surfaces promise to revolutionize

infrared optics production.

Acknowledgment

The authors of this paper acknowledge the contributions of many co-workers at NWC, notably P. C. Archibald for scattering measurements and Cliff Fountain for X-ray orientation of samples.

References

[1] Arnold, J. B.j and Steger, P. J., "Diamond

Turning on Advanced Machine Tool Prototypes"

(SME Clinic on Industrial Application of

Precision Machining and Gaging, 19-21 November 1974, San Francisco, Calif.)

Report Y-DA-5975 (Union Carbide, Inc., Oak

Ridge, Tenn., 1975).

[2] Bryan, J. B. , et al . , "A Practical Solution

to the Thermal Stability Problems in Machine

Tools," Technical Paper MR72-138 (Society

of Manufacturing Engineers, Dearborn,

Mich., 1972).

[3] Decker, D. L. , Soileau, M. J., Porteus, J. 0.

,

and Bennett, J. M. , NBS Spec. Publ . 462, 153 (1976).

[4] Decker, D. L., Bennett, J. M. , Soileau, M. J., Porteus, J. 0., and Bennett, H. E. , Opt.

Engr. 17, 160 (Mar/Apr 1978).

[5] Decker, D. L. , and Bennett, J. M., SPIE

Optical Coatings II, 140, 32 (1978).

[6] Arnold, J. B., Morris, T. 0., Sladky, R. E.

,

and Steger, P. J., SPIE Advances in Precision Machining of Optics 93, 96 (1976).

[7] Decker, D. L., and Grandjean, D. J., NBS

Spec. Publ. 541, 122 (1978).

[8] Decker, D. L., Presented in a paper read before the Society of Manufacturing Engi- neers, Precision Machining Workshop, 27-29

June 1978, Williamsburg, Va.

[9] Elson, J. M. , and Bennett, J. M. , J. Opt.

Soc. Am. 69, 31 (1979).

[10] Bennett, H. E. , and Porteus, J. 0., J. Opt.

Soc. Am. 5T_, 123 (1961 ).

[11] Temple, P. A., and Soileau, M. J., NBS

Spec. Publ. 462, 371 (1976).

[12] Porteus, J. 0., Decker, D. L. , Grandjean,

D. J., Seitel, S. C, and Faith, W. N.

,

"Defect-Damage-Resistant Copper Mirrors,"

this conference.

The speaker indicated that although several different machining fluids had been used in these

experiments, no obvious effect on pitting or spalling had been observed. The dependence of

the surface finish on the sample temperature was not examined. He further indicated that

only preliminary studies of optical glasses were done in the course of this investigation.

Initial results appear promising. The main effort on the diamond turning of optical glasses

is being carried out at the Lawrence Livermore Laboratory. Sample preparation for the work

reported here was carried out by mechanical polishing. The samples were initially machined

to a depth of 4-5, 000 angstroms. To remove the polishing layer, the final finished cut removed another 6-700 angstroms.

203
WORK PIECE ON SPINDLE

Figure 1(a). A schematic of the singlepoint tool machining process showing a part mounted on a rotating spindle being

worked by a tool which is mounted on a slide mechanism capable of moving the

tool over a controlled path in two dimensions.

PLANE MIRROR ON INSIDE OF CHUCK

Figure 1(b). Isometric schematic of the

NWC two-axis diamond-turning machine

showing the placement of the spindle and

tool slides in a T-base configuration.

Also shown is a simple interferometer

layout for measuring the primary motion

of the slides.

Figure 2. Photograph of the

NWC machine showing the tool

and spindle slides, the tool

post assembly including precision height adjuster, chip/

coolant guard, and spindle

housing. A 6- inch -diameter

copper flat is being

machined.

204
am.« - f/2r OR RMS VALUES:

6 =» 5 //5~ rmi vp»' '

°<rm. "

ASSUMING THAT f/r « 1 Figure 3. Ideal finish achieved

by machining with a tool of nose

radius r and a feedrate f. Both

peak-to-valley (pv) and rms values are given.

CALCIUM FLUORIDE

(1101

[116]

Figure 4. Schematic showing

the locations of "clear" and

cloudy structures resulting

from diamond single-point

machining relative to crystal lographic directions for

several different materials and

orientations.

[331] [116]

POTASSIUM CHLORIDE

[001]

[010] [010]

[110]

[110]

POLISHED Figure 5. Nomarski phase

contrast interference

micrograph of a polished

Irtran surface which has

been partially diamond

machined leaving a polished

area for comparison.

1000 itm DIAMOND TURNED

205
POLISHED POLISHED

1000 tim DIAMOND TURNED CLOUDY"

1000 urn DIAMOND TURNED "CLEAR"

Figuro -6. Nomarski phase con- trast interference micrograph

of a polished single-crystal

CaFp surface which has been

partially diamond machined

leaving a polished area for

comparison. On the left

side is a representative

"cloudy" area, and on the

right is a "clear" area.

Figure 7. Scanning electron micrograph of the same single-crystal CaF2 surface shown in figure 6. On the left side is a polished area, and on the right is an area selected from the

clear diamond-machined portion showing localized pitting and some debris from turning.

5 Mm 5 *" m

POLISHED AREA CLEAR AREA

i 1

1 1 1000 fim 5 litn CLEAR AND CLOUDY AREAS CLOUDY AREA

Figure 8. Scanning electron micrograph of the same single-crystal CaF 2 surface shown in figures 6 and 7. On the left side is a low

magnification view of the localized "cloudy"

spoke-shaped area as it is seen near the

boundary between the polished and diamondturned portions. On the right, at much higher

magnification, is shown a representative

"cloudy" area.

Figure 9. Scanning electron micrograph of a machined surface of a single-crystal Ge sample. The normal to sample surface is in the [100] direction. A low magnification view

on the left shows one of the four spokes

evident on this surface; the right shows the

nature of the pitting which is prevalent in the cloudy spoke area.

500 ftm 5/jm

206
Figure 10. The mechanism of spall pit

formation in brittle dielectrics is shown in these drawings. If the tensile

strength of the material is exceeded

when the compressive stress at the tip

of the tool is released, a fragment

will spall from the surface. The

compressive stress is the resultant of

a normal loading force and a frictional

component schematically illustrated in the left-hand drawing.

Figure 11. Scanning electron micrograph of a machined single-crystal KC1 surface. The

surface normal is in the [1,0,0] direction.

On the left is shown a polished surface which

has been contacted by the tool in several

discrete, nonoverlapping passes; on the right

is a surface produced by successive overlapping passes.

i 50 iim 50 nm

207
POLISHED

85 X RMS ROUGHNESS

8

SCAN PARALLEL TO

GROOVES. CLEAR AREA

Figure 13. Unprocessed Talystep

surface scan of 600-ym length on the CaF„ sample previously shown

in figures 6 through 8 and 12. Both polished and diamond-turned

"clear" areas are shown.

600 nm

SLOPE

Figure 14. Unprocessed Talystep

surface scan of length 15 ym on the polycrystal 1 ine MgF£ previously shown in figure 5. Both

polished and diamond-turned areas are shown. o<8

SCAN PARALLEL

TO GROOVES

15 (Jm

POLISHED

43 £ RMS ROUGHNESS

WITHOUT PITS

DIAMOND TURNED

60 I RMS ROUGHNESS

WITHOUT PITS

1

SCAN PARALLEL

TO GROOVES

Figure 15. Unprocessed Talystep

surface scan of length 600 ym on the polycrystall ine MgF2 previously shown in figures 5 and

14. Both polished and diamondturned areas are shown.

600 »m

20R
SHORT-PULSE CO2-LASER DAMAGE STUDIES

OF NaCl AND KC1 WINDOWS

Brian E. Newnam, Andrew V. Nowak, and Dennis H. Gill Los Alamos Scientific Laboratory

Los Alamos, New Mexico 87545

The damage resistance of bare surfaces and the bulk interior of NaCl

and KC1 windows was measured with a short-pulse CO2 laser at 10.6 ym. Parametric studies with 1.7-ns pulses indicated that adsorbed water was probably the limiting agent on surface thresholds in agreement with previous studies at long pulsewidths. Rear-surface thresholds up to 7 J/cm 2 were measured for polished NaCl windows, whereas KC1 surfaces damaged at approximately 60% of this level. The breakdown electric-field thresholds

of exit surfaces were only 50% of the value of the bulk materials. The

pulsewidth dependence of surface damage from 1 to 65 ns, in terms of incident laser fluence, increased as t

1 ' 3

.

ey words: Alkali halides; CO2 laser; KC1 : laser damage; laser fusion; NaCl; nanosecond

ulse; surface damage.

1 . Introduction

Large-diameter NaCl windows are used extensively in the CO2 amplifiers and target

hambers of the Helios (10 kJ) and Antares (100 kJ, projected) fusion lasers at Los Alamos

cientific Laboratory (LASL). Since these large windows (46-cm diameter, 8.5-cm thick)

epresent a sizeable portion of the cost of building and maintaining these lasers, it is

imperative that they have maximum optical damage resistance. In the experimental studies

0 be reported herein, we have examined some of the material and laser parameters that

ffect the surface and bulk damage of NaCl (and KC1 ) for short ns) pulses at 10.6 ym.

1 At the 1978 Laser Damage Symposium, Kovalev and Faizullov [1] presented substantial

xperimental evidence that adsorbed H2O is the limiting agent on the front-surface (f-s)

amage thresholds of NaCl windows. They supported the hypothesis that this surface layer

f water strongly absorbs the C0 2 radiation (10 3 cm

-1 at 10.6 ym [2]), producing a rapidly

xpanding vapor front which, when a sufficiently low density is reached, allows electricield breakdown as described by the familiar Paschen curve. Their experiments were carried

ut with TEA C02 laser pulses having a high leading peak of 100-200 ns duration (^150-ns

isetime) and a long tail (1-15 ys) containing the majority of the pulse energy. Also,

heir studies were on the entrance surface. Since pulsewidths of interest to laser fusion

Ire much shorter, %1 ns, we conducted a series of short-pulse experiments on exit surfaces

the most susceptible to early failure) to determine if adsorbed water is also the primary

gent of surface damage for these conditions. These are described in section 5.

Since surface damage precedes bulk damage in NaCl windows, we have also made a careful comparison of the bulk and surface thresholds of crystals from four prominent manuacturers to quantify the difference and learn how much potential improvement may be

ossible over presently-used polishing methods. These results and a discussion are given

n section 6.

As early as 1974, Stark and Reichelt [3] compared the damage resistance of Harshaw

'olytran KC1 and NaCl window surfaces using 1-ns pulses. They measured a fluence ratio of

i.5 in favor of NaCl. In the past five years, however, significant advances in the techlology of crystal growth and surface polishing processes have occurred which suggested a ew and more extensive comparison of KC1 and NaCl thresholds. These results appear in

ection 7.

Figures in brackets indicate the literature references at the end of this paper.

209
Finally, in section 8 we present our results for the pulsewidth dependence of exitsurface damage for NaCl and KC1 windows.

2. Description of Test Specimens

Test specimens of NaCl and KC1 were supplied by four prominent sources of highquality ir crystals: Harshaw Chemical Company in Solon, Ohio (10 NaCl, 3 KC1 ) , Naval

Research Laboratory (NRL) in Washington, D.C. (1 NaCl, 1 KC1), Hughes Research Laboratories (HRL) in Malibu, California (1 NaCl, 1 KC1 ) , and William Fredericks at Oregon State

University (OSU) in Corvallis (2 NaCl). The details of these specimens are listed in

table 11. To obtain a sufficiently long path length for conducting bulk-damage tests in

the thin Harshaw windows, (without interference from concurrent front-surface damage) the

perimeter of a few Harshaw windows were also polished flat. Also, two of the Harshaw

windows were later cleaved in air to allow a direct comparison of cleaved and polished

surface thresholds.

Suppl ier

Harshaw

HRL

Table 1. Description of NaCl and KC1 test specimens.

Growth technique Crystalline state Surface state

Stockbarger s i ng 1 e and Kyropoulos

Czochralski

+ CCU RAP single NaCl

forged/polyxtal-KCl

pol ished with

0.3 ym Linde A

polished

0.3 ym Linde A

HC1 etch-KCl

Thickness

10 mm

and 32 mm

KC1 : 9 mm

NaCl : 34 mm

NRL

OSU

Czochral ski + CCU RAP

ion exchange

single

single

pol ished

0.3 ym Linde A

cleaved

KC1 : 55 mm

NaCl : 53 mm

33 & 23 mm

3. Experimental Apparatus and Procedure

Laser pulses of 1.7 ns to 65 ns (FWHM) at 10.59 ym (P-20) were emitted from a gainswitched C0 2 oscillator/double-pass amplifier in LASL's Gigawatt Test Facility. This

laser provided a single-longitudinal mode, single-line, and gaussian spatial -prof il

output every 20 seconds which was ideal for these single-shot-per-site (1-on-l) tests. The short pulses were sliced out of the 65-ns oscillator emission by use of a double

Pockel cell/spark gap arrangement. As illustrated in figure 1, the temporal shape of

these short pulses was asymmetric (300-ps risetime, 1.5-ns falltime, 10-90% levels). A fairly large laser-beam spot size, 1-mm radius at the 1/e 2 level, was used on the majority

of the surface threshold tests. Samples were placed ahead of the focus of a 1-m F.L. plano-convex lens for these tests. For use in comparison of bulk and surface thresholds,

a spot-size radius of 0.14 mm was obtained at the focus of a 19-cm F.L. lens. The spatial

profile, measured periodically by a pinhole scan, approximated a gaussian dependence. The

energy, temporal and spatial diagnostics, indicated in figure 2, have been previously

described [4], and a summary of the laser test parameters is given in table 2.

Table 2.

Wavel ength:

Pul sewidths:

Spot-size radius:

CO2 laser damage test parameters.

P(20), 10.59 ym, single-mode TEM

1 to 65 ns (FWHM)

0.15 to 1.5 mm

00

1-on-l irradiation: 40 to 60 sites

Test environment: Air at 600 torr (2200 meters alt.)

The criterion for surface damage was permanent disruption (pitting or increased light

scatter) visible under bright, white-light illumination with a 40X stereo microscope.

Isolated microbubbles occurred at the threshold of bulk damage. Generally, microsparks

preceded the occurrence of visible damage by a small margin.

210
Measurements of the exit-surface and bulk-thresholds were satisfactorily conducted in

Ijjii r rather than in the vacuum environment chosen by Faizullov and Kovalev [1] for most of

pheir front-surface tests. Front- surface tests of high-threshold materials in air are

hSften plagued by air breakdown at the pulse peak which effectively prevents energy in the

tail of the pulse from reaching the surface. We determined that in measuring exit- surface

thresholds, the interference of air sparks was not significant when viewing with the white

Ijight/ microscope method. Previously, Hayden and Liberman [5] had found the exit-surface

thresholds of a NaCl window in vacuum and air to be essentially the same, 5.7 and 5.4

J/cm 2

, respectively.

The transmitted temporal -pulse termination technique of detecting damage was not considered sufficiently sensitive in these tests with large millimeter-size beams. The area

pf the damage sites (microns in diameter) was judged much too small to block a significant

fraction of the pulse energy.

Exit-surface rather than f-s damage was of primary interest in these studies since,

r or low-index materials like NaCl, the former almost always occurs first. This derives

|

: rom the in-phase addition of the incident and reflected electric vectors at the rear

jurface as indicated in figure 3. However, for normal incidence of a coherent laser beam

jipon a window having parallel surfaces, the ratio of the front- to-rear-surface thresholds

tin MV/cm) is dependent on the thickness and can range from 1 to n [6]. Therefore, in

hese tests the windows were oriented sufficiently off normal (several degrees) to prelude multiple-beam interference effects.

4. Damage Morphology

Threshold damage on the exit surfaces was in the form of one or a few shallow microjMts, of the order of several microns diameter. Invariably, a separate microspark was ssociated with each damage site, but sparks did occur at lower laser fluences with no

[liscernible damage. On some surfaces, the pits were elliptically shaped with the long

jjxes parallel to polish sleeks.

Evidence of threshold damage in the bulk was in the form of one or a few tiny bubbles

emaining after an incandescent microspark(s ) . High-magnification studies of these bulk

lites were not completed upon this writing.

5. Influence of Adsorbed Water

In addition to schlieren photographs to establish the existence of an expanding vapor

ront at an irradiated NaCl surface, Kovalev and Faizullov [1] supported their assertion

hat adsorbed water determines the surface damage threshold with the following evidence:

1) As the laser spot size was varied over the range 0.12 to 1.12 mm radius, the f-s

threshold of NaCl in vacuum was constant at 36 MV/cm 2 (2.2 J/cm 2 ). The tests were conducted at 10.6 ym with a 150-ns pulse risetime. This result was theoretically predicted by their eq. 5 for spot sizes larger than the thickness of

an adsorbed water film uniformly absorbing over the surface.

| 2) The f-s threshold increased with pressure of the ambient gas. This was expected

because the shock wave velocity of an expanding water vapor front is reduced at higher ambient pressures, and there is an increased frequency of electron collisions with neutral particles. Thus, for full development of an electron

avalanche in the rarefied region, a higher electric field is required.

3) After an initial dramatic rise in threshold from 2.2 to 13.5 J/cm 2 by treating

an NaCl surface with an HCl-acid etch, the threshold dropped, upon subsequent

remeasurement at 20 minutes, to a value of 6.5 J/cm 2. This value appeared to be

a steady-state level as determined from additional periodic measurements up to 3 months. Readsorption of an H 2 0 layer within 20 minutes was indicated.

4) Kovalev and Faizullov [1,7] observed large increases in the f-s threshold of

NaCl when pre-irradiated at subthreshold fluences (N-on-1 effect). Removal of

volatile absorbing contaminants, such as water, was indicated.

211
In view of these important results conducted with long pulses, the present study with

1.7-ns C02 pulses was motivated. To detect the possible influence of adsorbed H 2 0 on NaCl

surface thresholds the following tests were conducted:

1. threshold versus spot size,

2. threshold of a cleaved surface versus time of ambient exposure, and

3. threshold versus total water content.

5.1. Threshold versus Spot Size

Figure 4 exhibits the exit-surface threshold measurements of one NaCl window for

spot-size radii from 0.14 to 1.5 mm. The vertical bars for each point represent the

threshold variation across the sample surface. Therefore, the solid line drawn through

the points could as well have been drawn horizontally, i.e., no spot-size dependence was evident. Similar measurements on two other windows also revealed no definite dependence.

5.2. Threshold of a Cleaved Surface

The f-s threshold of an NaCl window cleaved in air (relative humidity of 35%) was determined within one hour to be 17.6 ± 0.6 J/cm 2

. After ten days exposure, the threshold

had dropped to 13.1 ± 1.5 J/cm2

, a decline of 26%.

These tests were performed on the front surface, instead of the rear surface as were

all other of our tests, because rear-surface tests were not possible at these high-fluence

levels. When the cleaved surface was oriented as the rear surface, the polished (front)

surface damaged first, thereby shielding the rear surface of interest. Using the polished

samples available, it was not possible to obtain satisfactory cleaved surfaces for both

front and rear.

5.3. Threshold versus Total Water Content

The presence of OH" ions in NaCl is revealed by the characteristic spectral absorption peak at 187 nm, near the uv band edge [8,9]. As seen in figure 5, there was a wide

range of OH"concentrations (bulk + surface) among the test specimens grown by the several

techniques. OSU sample No. 1-65 was purposely grown with a slight water concentration.

The 187-nm absorbance of the Harshaw sample, No. 1-48-14 greatly exceeded that of the

other samples supplied by Harshaw. In table 2, we compare the absorbance with the exitsurface threshold. It is appropriate to compare the results for the cleaved and polished

windows separately. The cleaved window with greater OH" concentration did have a lower

threshold, but the polished Harshaw sample with very high_absorbance actually had a slightly higher threshold than the HRL sample with low OH" content.

Table 2. NaCl exit-surface threshold versus absorbance at 187 nm.

Sample Absorbance at 187 nm

a Exit threshold

J/cm* MV/cm

OSU - ion exchange

4-65 cleaved 0.10 7.0 1.19

HRL - RAP & forged

Polished 0.10 5.3 1.04

OSU - ion exchange

1-65 cleaved 0.30 5.3 1 .05

Harshaw - Stockbarger

1-48-14, heel, polished 1000 6.1 1.12

w = 1.0 mm, At = 1.7 ns (FWHM)

a See table 1 for sample thicknesses.

212
5.4. Discussion

The absence of a spot-size dependence and the decline of the surface threshold with

time for our 1.7-ns pulses are similar to the results obtained by Kovalev and Faizullov

[1,7]. Correlation of the total OH" ion content (bulk + surfaces) was not as conclusive,

however. Direct correlation of the surface absorption at 187 nm with surface threshold

would have been better. Unfortunately, surface absorption measurements of these relatively thin samples was not accomplished. However, the one clearly valid comparison of

ithe surface thresholds of the two cleaved OSU crystals was consistent with absorption in a jsurface layer of water. Thus, the results of our three tests do support the hypothesis

Ithat surface damage of NaCl, for 1-ns CO2 pulses, results from OH" absorption.

Our observation that microsparks were associated with the tiny damage sites was not unexpected. At surface defects (scratches, pits or included polishing residue) the local

electric field is enhanced, resulting in early breakdown most likely in the expanding

vapor. The presence of the localized plasmas causes total retro-reflection and a maximum

standing-wave field of 2.0 at the exit surface, as described by Boling et al . [10]. Early

damage is a natural consequence.

Self- focusing, as a contributor to early breakdown of the exit surfaces, was also

assessed. As reviewed in the Appendix, the magnitude of this effect in the above tests

and those yet to be described was negligible.

6. Surface versus Bulk Thresholds

In table 3, the bulk and surface thresholds of NaCl crystals from four sources are

compared. In computing the listed values, the enhancement of the electric field at the

2xit surface (recall figure 3) was taken into account. It is remarkable that the same

^alue, 2.1 MV/cm, was obtained for the bulk threshold for each of the first three samples

listed. The ratio of exit-surface to bulk thresholds for these was also about the same,

3.5. In terms of incident laser fluence (J/cm2 ), exit surfaces polished by standard

nethods survived approximately 20% as much as the bulk (25% for front surfaces).

Table 3. Comparison of bulk and exit-surface thresholds of NaCl single crystals.

Sample Bui k Surface Ratio

MV/cm MV/cm

tershaw 6-8-16 2.1 ± 0.2 1.03 0.49

Stockbarger

IRL - RAP 2.1 ± 0.1 1.15 ± 0.10 0.55

B-334

IRL - RAP 2.1 ± 0.1 1.07 ± 0.03 0.51

7B-175

•SU - ion exchange 1.02 ±0.03 1.20 ±0.11 1.17

20-2-13, 4-65 Cleaved, 14 years ago (1965)

= 0.14 mm

t = 1.7 ns (FWHM)

= 10.6 um

The bulk threshold of the OSU crystal was much lower than anticipated, expecially

ince its concentration of impurities was very low [11]. Tiny scattering sites were

isible in the bulk by use of a He-Ne laser (forward scattering, mainly). As this crystal

as grown in an atmosphere of argon, it has been suggested (unconfirmed as yet) that the

cattering sites were bubbles of argon gas [11]. For a spherical void in NaCl (n = 1.494

t 10.6 ym) an electric-field enhancement factor of 1.23 is predicted from Bloembergen's

213
analysis [12]. The remainder of the difference may be accounted for by breakdown in argon

gas at some unknown pressure rather than in bulk NaCl . The threshold of this cleaved sur- face was unexpectedly high especially considering that the crystal was grown, cleaved and

cleaned in 1965, 14 years ago!

There are several explanations why thresholds of surfaces are lower than in the bulk.

Firstly, the E-field enhancement at the exit-surface results in earlier damage there than

in either bulk or front surface. Secondly, the surface roughness produced by polishing

results in enhanced electric fields and increased absorption. (At 1.06 urn, House et al

[13] found that fused silica surface- thresholds (MV/cm) were inversely proportional to the

square root of the rms roughness.) Thirdly, polishing physically disrupts the crystalline

order of a surface layer. Fourthly, polishing compounds and pitch are embedded and cleaning methods cannot be 100% effective. Fifthly, adsorbed gases, e.g., H 2 0, absorb the

laser energy. Adsorption is augmented by surface roughness and embedded impurities.

To quantify the deleterious effect of polishing on the damage threshold, we tested

two samples cleaved in air. The results, given in table 4, provide a comparison of f-s

thresholds of adjacent polished and cleaved surface areas. First, we note that the roughness of the cleaved surfaces (^50 A) was one-quarter that of the polished surfaces as measured with a prof il ometer. Therefore, it is not too surprising that the cleaved sur- faces had much higher thresholds, by factors of 2.3 and 2.0, respectively in terms of

laser fluence. The fact that these values were only 75 to 80% of the bulk electric-field

thresholds (2.1 MV/cm) indicated that the finite roughness (50 A) may have been a limiting

factor. Sufficient moisture adsorbed during the one-hour test period may have also

depressed the damage resistance. The reduced threshold after ten days exposure to ambient

conditions was already discussed in section 5.2.

Table 4. Entrance surface thresholds of cleaved and polished NaCl (single-crystal).

Surface

Polished, * 200 A RMS

Linde A - 0. 3 ym

1 year storage

Cleaved, % 50 A RMS

Threshold

J /cm z

.4 ± 1.0

,0 ± 0.4

MV/cm

1 .09 1.13

Ratio of surface to bulk

0.52

0.54

Tested within 1 hour A 19.4 ± 2.3

in 35% RH B 17.6 ± 0.6

1 .66

1 .58 0.79

0.75

After 10 days

in 35-40% RH

13.1 ± 1.5 1.37 0.65

w

At

0. 75 mm

1.7 ns (FWHM;

Sample A: Harshaw K-24-20 (Kyropoulos)

Sample B: Harshaw 6-1-16, Heel No. 1 (Stockbarger

)

The f-s thresholds given in table 4 may be compared with their exit-surface thresholds to obtain the ratio. For Sample A, these were 8.4/6.8 or 1.24 and for Sample B, these were 9.0/6.1 or 1.48. These compare well with the theoretical value of 1.44.

Addressing the problem of surface contamination by polishing compounds and pitch, we subjected two polished surfaces to Auger analysis. To our surprise, no evidence of Al was seen in the surface polished with Linde A (A1 2 0 3 ). nor was carbon found in the surface

polished with diamond paste! Examination with a more sensitive technique is suggested as the null result was unexpected. Previously, Turk et al . [14] had presented definite

evidence of removal of an absorbing layer on a polished KC1 surface removable by use of an etchant (see figure 6). Presumably, the absorbing impurities were the Linde A, pitch and

water.

214
We measured no difference in the surface thresholds of Harshaw Polytran NaCl (forged,

polycrystall ine) and Harshaw single-crystal material. However, no measurement of the bulk

threshold of Polytran was made.

7. KC1 versus NaCl

7.1. Bulk and Surface Measurements

The bulk thresholds of both NaCl and KC1 , measured with relatively large beams

w = 0.14 mm radius) and 1.7-ns pulsewidth, are presented in table 5. Included for com- wison are the data of Soileau et al. [15] and Fradin et al . [16] both of whom tested

Sarshaw crystals with very small spot-sizes and longer pulseforms. Although the value for

^aCl is consistently the same, 2.1 MV/cm, the thresholds for KC1 varied more widely.

Since Harshaw has not concentrated on the growth of KC1 windows for high-power, short3ulse use to the extent that they have for NaCl [17], we believe that the NRL sample of

<C1 better represents the presently attainable damage resistance for that material.

Table 5. Comparison of bulk thresholds of single crystal NaCl and KC1

Test Bulk Thresholds, MV/cm

Source Conditions NaCl KC1 Ratio

4RL - RAP a 2.1 ± 0.1 1.66 ±0.09 0.79

larshaw - Stockbarger a 2.1 ± 0.2 0.76 ± 0.01 0.37

Harshaw - Stockbarger b 2.1 1.45 0.69

c 1.95 1.39 0.71

At = 1.7 ns (FWHM), w = 0.14 mm. At = 60 - 105 ns (FWHM), partially mode-locked train, w = 0.036 mm; data of Soileau

et al. [15].

' At = 200 ns (FWHM), w = 25 ym; data of Fradin et al . [16].

As the exit-surfaces present the real limitation to high-power laser applications, we

ixtended this comparison to polished surfaces of both crystals. The results, listed in

.able 6, continue to show NaCl as having the advantage. Using the mean values for the

ets of samples measured, a f 1 uence ratio of 0.63 was obtained. The corresponding field

atio is then 0.79, which is the same as that derived from the hiqhest bulk values in

:able 5.

Table 6. Comparison of exit-surface thresholds of polished NaCl and KC1

Single-crystal NaCl

Source J /cm*

Harshaw 6.0 ± 0.5a

NRL 6.2 ± 1 .0

HRL 5.3 ± 0.4

KC1 NaCl w = 1.0 mm, At = 1.7 ns (FWHM), X = 10.6 ym, 1-on-l

a Mean value of eight samples.

^Mean value of three samples.

cMean value of ten samples.

KC1 Ratio

J7c¥2

3.6 ± 0.6 0.60

4.1 ± 0.8 0.66

3.7 ± 0.4 0.70

(forged)

Mb 0.63

6.0C

215
7.2. Discussion

Based on arguments involving medium polarizabil ity and available electrons (in pure,

defect-free crystals) to contribute to an electron avalanche, one would expect that the

damage threshold (MV/cm) would scale proportionately to atomic density divided by n

2 - 1. Such a derivation was obtained by Bettis et al . [18]. The values of (At. Density)/(n 2 -l

)

for KC1 and NaCl at 10.59 ym are 0.288 and 2.362. The ratio of these values is 0.80,

which is the same as that obtained from the experimental measurements. Using the data

generated by Fradin et al. [16], we constructed a plot of E t

. (MV/cm) versus the above

expression as seen in figure 7. An exact linear relationship is not expected since some

of the physical properties of the crystals (impurities, defects, mechanical strength,

melting point) are not included in the derivation of the abscissa. However, the general

trend is obvious. Such a dependence has been observed for crystals and thin film materials at other wavelengths, e.g., by Bettis et al. [18] at 1.06 ym and by Newnam and Gill

[19] at 266 nm.

Using their recent theory of electron avalanche breakdown, Sparks et al . [20] have

computed bulk thresholds for KC1 and NaCl at 10.6 ym to be 0.63 and 0.8 MV/cm, respectively. These computed values are obviously too low in light of experimental values

presented here, but their ratio, 0.8, is in agreement with our measurements.

8. Pulsewidth Dependence

The exit-surface thresholds of several samples were measured with 65 ± 3-ns pulses as well as the short, 1.7-ns pulsewidth used as the standard in this test program. The

smooth temporal structure of both pulseforms is seen in figure 1. Due to a difference in

the beam divergence of the laser in the long-pulse mode, a larger spot-size radius (1.4 mm) was inadvertantly used. However, as demonstrated earlier in figure 4, the thresholds were

not spot-size dependent, so a direct comparison with the 1.7-ns results (1.0-mm spot size) was deemed appropriate. The results for three samples are plotted in figure 8. Consistently, a straight line drawn between the data points exhibited a t 1/3 fluence dependence.

For a few other samples, not plotted here, a slightly less-rapid increase with pulsewidth

was measured. Since tiny microsparks were generally associated with damage at isolated

surface sites, it is not understood why the anticipated t 1/2 dependence (generally asso- ciated with diffusion processes such as plasmas) was not observed. Possible explanations

include the rate of vapor expansion or the susceptibility of surface defects.

9. Summary

This study of the extrinsic surface and bulk thresholds of NaCl and KC1 windows with

millimeter-diameter, T.7-ns C0 2 laser pulses at 10.59 ym has yielded some very significant

results.

First, exit-surface damage, which occurs at 70% the fluence level of entrance-surface

damage, was examined. Thresholds up to 7 J/cm 2 were measured for polished surfaces. In contrast, cleaved surfaces with one-quarter as much surface roughness damaged at twice the

level of polished surfaces, but remeasurement after ten days found the advantage to have

dropped to 50%. No spot-size dependence was observed for beam diameters from 0.3 to 3 mm. Correlation of the damage susceptibility with the total absorption (bulk + surfaces) at the OH" ion absorption band at 187 nm was not totally conclusive. Overall, however, the

results of these experiments support the hypothesis [1,7] that surface damage results from

OH" absorption, coupled with surface roughness.

That there is much room for improvement of the damage resistance of both NaCl and KC1 surfaces was determined from comparative measurements of the bulk thresholds. Exit sur- faces, with present state-of-the-art polish, damaged at only 50% of the electric-field

threshold of the bulk, or at only a 20% level in terms of incident laser fluence (J/cm 2 ). For ideal surfaces, the thresholds at the exit could be as high as 70% of the bulk level

(J/cm 2 ). Further work on surface preparation should be the first priority.

A comparison of the bulk and surface thresholds of KC1 and NaCl produced by three

vendors obtained a field ratio (MV/cm) of 80% in favor of NaCl. This is in agreement with

theoretical predictions based on the electron-avalanche mechanism. Finally, a pulsewidth

216
dependence of t

1 /3 was measured for a few samples of both materials from 1.7 to 65 ns, which was less than the anticipated t 1/2

.

10. Acknowledgments

The authors are grateful for the cooperation of the following persons for supplying

the test specimens: David Krus and Ontario Nestor of Harshaw, Roger Turk and James Harrington of HRL, William Fredericks of OSU and Philipp Klein of NRL. Additionally, one of the autnors (BN) is indebted to the above and Guy Bastien and

Virgil Straughan of Harshaw for thorough discussions about crystal -growth technology, window shaping and surface polishing. Thanks go to Vi to Lazazzera and Linda Martinez of

LASL for polishing assistance and Robert Springer for Auger surface analysis.

Leon Sonntag and John Meier provided invaluable assistance in carrying out the damage

tests.

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University, personal communication,

1979.

Bloembergen, N., "Role of Cracks,

Pores, and Absorbing Inclusions on Laser Induced Damage Threshold at Surfaces of Transparent Dielectrics," Appl. Opt. 12, 661 (1973).

House, R. A. , II , Bettis, J. R.

,

and Guenther, A. H., "Surface

Roughness and Laser Damage Threshold," IEEE J. Quantum Electron.

QE-13 , 361 (1977).

217
[14] Turk, R. R. , Pastor, R. C, Timper,

A. J., Braunstein, M., and

Heussner, G. K. , "Optical Finishing

of KC1 Windows to Minimize Absorption

in the Infrared," in Proc. of the

Fifth Conf. on High Power Infrared

Laser Window Materials-1 975,

(C. R. Andrews and C. L. Strecker,

eds.), p. 103, DARPA, (1976).

[15] Soileau, M. J., Bass, M., and Van Stryland, E. W., "Frequency

Dependence of Breakdown Fields in

Single-Crystal NaCl and KC1," in

Laser Induced Damage in Optical

Materials: 1978 , op. cit., p. 309.

[16] Fradin, D. W., Yablonovitch, E., and

Bass, M. , "Comparison of Laser

Induced Bulk Damage in Al kal i-Halides

at 10.6, 1.06, and 0.69 Microns,"

in Laser Induced Damage in Optical

Materials: 1972 , (A. J. Glass and A.

H. Guenther, eds.). p. 27. NBS Spec.

Pub. 372, U. S. Gov't Printing Off.,

Washington, D.C., (1972); also Appl

Opt. 12, 700 (1973).

[17] Krus, D. J., Harshaw Chemical Co.,

private communication, 1979.

[18] Bettis, J. R., House, R. A.,

Guenther, A. H., and Austin, R.,

"The Importance of Refractive Index,

Number Density, and Surface Roughness

in the Laser-Induced Damage of Thin

Films and Bare Surfaces," in Laser Induced Damage in Optical Materials:

1975, (A. J. Glass and A. H. Guenther,

idsT), p. 289. NBS Spec. Pub. 435,

U. S. Gov't Printing Off., Washington,

D.C., (1976).

[19] Newnam, B. E., and Gill, D. H.,

"Ultraviolet Damage Resistance of

Laser Coatings," in Laser Induced

Damage in Optical Materials: 1978,

op. cit. , p. 190.

[20] Sparks, M. , Holstein, T., Warren, R.,

Mills, D. L., Maradudin, A. A.,

Sham, L. J., Loh, E. , Jr., and King, F.

,

"Theory of Electron-Avalanche Breakdown in Solids," in Laser Induced

Damage in Optical Materials: 1979,

(H. E. Bennett, A. J. Glass,

A. H. Guenther, and B. E. Newnam,

eds.), NBS Spec. Pub. 586, (J. S. Gov't Printing Off., Washington,

D.C., (1980).

[21] Wang, C. C, and Baardsen, E. L.,

"Study of Optical Third-Harmonic

Generation in Reflection," Phys. Rev. 185, 1079 (1969), and Phys. Rev. Bl

,

2827 (1970). ~~

[22] Maker, P. D., and Terhune, R. W.,

"Study of Optical Effects Due to an Induced Polarization Third Order in

the Electric Field Strength," Phys.

Rev. 137, A801 (1965).

[23] Smith, W. L., Bechtel , J. H., and

Bloembergen, N., "Picosecond Breakdown Studies: Threshold and Nonlinear

Refractive Index Measurements and

Damage Morphology," in Laser Induced

Damage in Optical Materials: 1975,

op. cit., p. 321.

[24] Watkins, D. E., Phipps, Jr., C. R.,

and Thomas, S. J., "Ellipse Rotation

in Germanium," in Diqest of Technical

Papers, 1979 IEEE/0SA Conf. on Laser

Eng. and Applications, p. 3T*- ( 1 979)

[25] Akhmanov, S. A., Sukhorukov, A. P.,

and Khokhlov, R. V., "Self-Focusing

and Diffraction of Light in a Non- linear Medium," Sov. Phys.-Usp. 10,

609 (1968).

[26] Kerr, E. L. , "Transient and SteadyState Electrostrictive Laser Beam

Trapping," IEEE J. Quantum Electron.

QE-6 , 616 (1970).

[27] Dawes, E. L., and Marburger, J. H.,

"Computer Studies in Self-Focusing,"

Phys. Rev. 179, 862 (1969).

[281 Marburger, J. H., "Self-Focusing

Theory," Prog. Quant. Electr. 4, 35 (1975).

[29] Fradin, D. W., "The Measurement of

Self-Focusing Parameters Using Intrinsic

Optical Damage," IEEE J. Quantum

Electron. QE-9 , 954 (1973).

218
Appendix

Measurement of the extrinsic damage thresholds of NaCl and KC1 with large laser beams

[w = 0.14 mm. for the bulk; 0.14 to 1.5 mm for surfaces) required substantial input powers.

\t these power levels, intensification of the laser radiation by self-focusing had to be

;arefully evaluated. Our analysis of the magnitude of self-focusing in the present tests

included contributions from both fast (electronic) and transient (electrostrictive) nonlinear mechanisms. For the laser parameters and sample dimensions used, the computed

lonlinear intensification was of negligible magnitude as we now review.

For a gaussian beam, the critical power for maintaining a constant shape is given by

;he familiar expression

/here n = nQ + n2E , and where n£ is a composite of nonlinear indices including n£

e and

i^ s

, representing the relatively prompt electronic and time-dependent electrostrictive

laterial responses. For NaCl and KC1 the values of n2 used here were 4.6 and 3.7 x 10~13

:su, respectively. These values are the average of three sets of measurements in the

'isible and at 1.06 ym [21,22,23]. In a recent measurement at 10.6 ym of ellipse rotation

n NaCl, Watkins et al. [24] also determined that n2

e was less than 6 x 10"13 esu, which

ras the limit of their experimental sensitivity. An expression for n2

s was derived by

hkmanov et al . [25]

'here p is the density and v is the radial sound velocity. At 10.6 ym, the values for

aCl are 8.3, 9.4, and 9.8 x 10"1 " esu along the [100], [110], and [111] directions,

espectively. For KC1 , these values are 8.2, 11.1, and 12.6 x 10"1? esu. The selfocusing arising from electrostriction is incomplete until the compressive sound wave has

rans versed across the beam diameter.

For the electrostrictive mechanism, Kerr [26] determined two temporal regimes for the

iritical power P^, which for a paraboloidal temporal pulse shape, are

(1)

n 2s

= n 0 (Pdn

0

/ d P) / 4tt Pv

2 (2)

P Is

= 0.943 P

1

5 T >1, (steady state) (3)

nd

P ]s

= 0.943 P^t2

, t < 1, (transient) (4)

ith w

Q being the spot size at the entrance of the medium for a collimated or gently

ocused beam. For the bulk tests with 1.7-ns pulses and w = 0.14 mm, the value of x was

.645 which, from eq. (4) yielded a power of 24 GW for P ls for both NaCl and KC1 along the

219
[100] direction. This is a factor of 1000 larger than P, (23 and 29 MW, respectively).

indicating a negligible role for electrostriction in these tests. For the surface threshold tests at the same pulsewidth, electrostriction had even less influence. It had only

1% as large an effect as the electronic mechanism for the 65-ns surface tests.

From exact solutions of the nonlinear wave equation, Dawes and Marburger [27,28]

suggested the use of another critical power, P~, which gives an axial singularity at infinity for an initially plane wave. is 3.72 times larger than P-j , and for NaCl and

KC1 becomes 86 and 107 MW, respectively. To calculate the increase of the axial

intensity due to self-focusing, we used formulas derived by Dawes and Marburger:

= [(1 - z/R)

2 + 4(1 - P/P 9 )z

2 /k2w4 J

-1 for P < 0.25 P 9 , (6)

1(0)

do d

and

IM =

[! _ z ( 1/z + 1/R) ] -w/2 for p > i.5 P ( 7 ) KO) f ~ 2

where z is the path length within the medium, R is the focal length of the incident phase

front within the front surface of the medium (R = -w

Q (dw/dz)~\ R > 0 for a converging

beam), w

Q

is the spot-size radius (1/e

2 level), is the self-focusing length and a is a numerically-derived exponent. For laser powers intermediate to the above, the graphic

solutions of Dawes and Marburger were used.

In the bulk tests of NaCl, the laser power levels did not exceed 0.06 P

2 at which

self-focusing produced a 0.5% intensification. In the exit-surface tests of NaCl, the

ratio of P/P^ was 0.67, or less, using a spot size of 1.0 mm. The resultant intensification was computed to be less than 1% (2%) for the 1-cm (5-cm) thick samples. Comparable,

but smaller values were computed for KC1 which has a smaller n^-

Finally, we mention that Fradin et al. [29] and Smith et al . [23] have used a different formula for computing the intensification at the focal plane during bulk damage

measurements:

= (1 - P/P-,)"

1 for P < 0.9 P n

. (8)

K0)

1 1

This formula predicts much greater intensification than is calculated from the above

eq. (6) of Dawes and Marburger which describes their exact numerical solutions.

220
1.7-NS PULSE

0,5 NS PER DIVISION

Figure 1. Oscillograms of CO2 laser

pulseforms.

221
LASER PRECISION METER

200/im dia. APERTURE

ZnSe LENS, f = lm

OSCILLOSCOPE

LASL-5GHZ

PYROELECTRICZ

OR PHOTON

DRAG

DETECTOR

/ - , He Ne

CaF2 ZnSe EYE SAMPLE

FILTERS LENS

f=lm

GET

LOST

BOX

SCIENTECH CALORIMETER

C02 LASER DAMAGE TEST FACILITY

igure 2. Schematic of C0 2 laser damage facility and associated diagnostics.

222
AIR SUBSTRATE AIR

2

E (REAR) 2n

E (FRONT)" |(n+l) +(n-l)exp2ikd|

Figure 3. Electric-field vectors of a linearly-polarized laser beam at front

and rear surfaces of a transparent

dielectric.

223
OJE

LU 0,1 0.5 1.0 1.5

LASER SPOT-SIZE RADIUS (mm)

Figure 4. Exit-surface damage threshold of a single-crystal NaCl versus C0 2 -laser

spot-size radius.

224
ABSORBANCE vs X FOR NaCtt

1 0.00 1—i—i—i—i—i—i—i—i—i—i—i—

~i—

! OSU 1-65 I

I SINGLE

' CRYSTAL HARSHAW

\ SINGLE CRYSTAL

\l-48-l4, HEEL

\\

\

OSU

4-65

SINGLE CRYSTAL ^

HRL-RAP' FORGED

7B-I75

OH" ion BAND

_L 140 160 180 200 220 240 260 280

WAVELENGTH (nm)

Figure 5. Ultraviolet absorbance for

several NaCl windows with the OH" ion

band at 187 nm indicated. See table 1 for sample thicknesses.

225
15

0

1 1 1 1 1 1

PITCH POLISH, LINDE A, I, 2 - PROPANEDIOL FORGED RAP KCI 4 500 PPMA SR* !

. ONE cm THICK ETCHANT — 1, 2 -PROPANEDIOL

2 rr,,n HCI ETCH -

1 1 1 1 1 1 0 2 4 6 8 D 12 14 THICKNESS REMOVED, ^

Figure 6. KCI absorption at 10.6 ym

versus thickness of polished surface

removed. After Turk et al . [14].

o

x

00

UJ

rrx

1.6

1.4

1.2

1.0

0.8 o

ocu

lj

_j

LlI UJ>

UJ

1 1

1

1

I 1

1 i

1

1

1

1

1

/ «KF _

NaCH*

KC4» /

KBr/NaBr

Nal/ ^RbC4

Kiy *RbBr /Rbl x= 10.6/i.m

/ 1 I , 1 , 1 ,

I

i

200 ns

I i 1 i 1 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 ATOMIC DENSITY/ (n'-l), IO"cm

Figure 7. Bulk electric-field breakdown

thresholds at 10.6 ym versus atomic

density/(n 2 - 1 ) for alkali halide

crystals relative to NaCl (M.95 MV/cm)

Data from Fradin et al . [16].

226
E 30

q 25-

O

X

CO

LU

(ZX

hLU

o

CO

I tX

LU

20 h

15

10

5

0

1

1

1

1

1

1

1

1

1 1

NaCft- HARSHAW

1 1

i

|

1

1

- - MnPO HRI (RAP) — — KC4-HRL (RAP/FORGED)

£(v^2^^^^^^-

-* "

" "

10.6 /-im

5— CU =

1 1 1 1 1 1 1 1 1

1.0, 1.4mm

1 1 1

I 4 6 8 10 20 30 40 60 80

PULSEWIDTH (ns)

Figure 8. Exit-surface damage threshold versus C0 2 laser pulsewidth for NaCl and KC1

nhe discussion of this paper centered around the question of whether or not the correlation

between damage threshold and surface absorption was actually determined. The speaker inlicated that a 20 to 50 percent increase in the observed damage threshold was generated by

re-irradiation of the surface at a power level lower than the damage threshold. The implication was that by this pre-irradiation process, adsorbed gases, water vapor and other conaminants were expelled from the surface. It was remarked that great variation could be seen

from sample to sample in alkali halide materials, and that this might obscure any fundamental

roperties of individual materials. The speaker indicated that the samples used in this work

*ere supplied specifically for damage studies, and represented the best samples available rom commercial suppliers. The surface absorption measurements used in this work were made

•y the supplier and thus it was not possible for the speaker to verify the measured values.

'Tie consensus of the discussion appeared to be that the correlation between surface absorption and the observed damage thresholds was implied by the results of this work but not nequivocally established.

221

CCL-LASER POLISHING OF FUSED SILICA SURFACES FOR INCREASED

LASER DAMAGE RESISTANCE AT 1.06 ym

P. A. Temple

Michelson Laboratory, Physics Division

Naval Weapons Center, China Lake, California 93555

and

David Milam and W. Howard Lowdermilk

Lawrence Livermore Laboratories

Livermore, California 94550

We have prepared bare fused silica surfaces by subjecting the mechanically

polished surface to a rastered cw C0 2 laser beam. Analysis shows that this

processing causes (a) removal of a uniform layer of fused silica and (b) a probable re-fusing or healing of existing subsurface fractures. The fused

silica removal rate is found to be a function of the laser intensity and scan

rate. These surfaces are seen to have very low scatter and to be very smooth.

In addition, they have exhibited entrance surface damage thresholds at 1.06 ym

and 1 nsec, which are substantially above those seen on the mechanically

polished surface. When damage does occur, it tends to be at a few isolated

points rather than the general uniform damage seen on the mechanically polished

part. In addition to the damage results, we will discuss an observational

technique used for viewing these surfaces which employs dark-field illumination.

Key words: Fused silica; laser damage; surface finishing; surface polishing.

We have prepared fused silica surfaces by subjecting the mechanically polished surface

to a cw C0 2 laser beam treatment. These surfaces are found to exhibit increased laser

damage resistance to 1.06 ym, 1 nsec pulses. In addition to a description of the damage

results, we will discuss some of the other physical properties of surfaces prepared in this

way.

All the samples used in this set of measurements were Suprasil II, polished by Zygo

Corporation. They were quite smooth with a Talystep rms roughness of under 10 A. Figure 1 shows the setup used to treat these mechanically polished surfaces with a C0 2 laser beam.

Since the wavelength of C02 radiation is much longer than the wavelength of the absorption

edge of fused silica, C0 2 radiation is highly absorbed by fused silica. The result is intense local heating of the fused silica surface. Thus far only unfocused C0 2 radiation

from a Coherent Radiation Model 41 laser has been used. By using a beam power of approximately 75 watts and a scan rate of ~ 5 mm/sec, we have succeeded in achieving the desired

result of modifying the fused silica properties without seriously degrading the figure of

the surface. At this power and scan rate, the region under irradiation appears as a bright

reddish-white spot about 2 to 3 mm in diameter. At higher laser powers or slower scan

rates, sublimed Si02 is seen to come off the irradiated region and is redeposited as a white powder on the cooler parts of the sample just above the irradiated region. By

scanning in a left-to-right motion and by beginning the scans on the lower part of the

sample and making each successive scan 0.5 mm above the previous scan, this fine white

powder is reevaporated, and the finished part does not have Si0 2 powder on the laserpolished portion.

Introduction

Surface Preparation

Independent Research funds and Department of Energy.

229
Surface Inspection

The techniques used to inspect the surfaces prepared in this study have been Nomarski

microscopy and total internal reflection microscopy (TIRM) [1.2] 1

. The latter technique

has been described elsewhere and in a poster paper at this meeting [3]. In TIRM the transparent part is illuminated from within with a laser beam (fig. 2) at an angle just greater

than the critical angle. Since total reflection occurs, no light escapes except where

there has been some perturbation of the surface. The image seen through the microscope,

then, is one of a dark background with bright regions where the surface has been disturbed.

Damage Results

Six samples prepared by C02 -laser polishing have been damage tested at Lawrence Liver- more Laboratory. These have all been front surface tests at a wavelength of 1.06 urn and a pulse length of 1 nsec. Damage is defined as the appearance of pits as seen in Nomarski

microscopy. In general, the results have indicated that these surfaces exhibit a higher

resistance to damage than do conventionally polished fused silica. This is evidenced by no pits appearing at energy density levels which would normally cause damage on a conventional

part and fewer individual pits when damage finally does occur at higher energy density

levels. Table 1 is a list of the results from two samples. Keeping in mind that typical

polishing samples have a damage threshold which ranges from 10 to 20 J/cm2

, the laserpolished samples show a very high resistance to damage.

Table 1. Fused silica entrance surface damage.

Sample J/cm2 Comments

Zl-23 30.6

40.7

55.5

No light, 1 site

Flash, 3 sites

Flash, 3 sites

Zl-21

Same

site

33.4

23.0

40.1

( 29.0

38.0

I 42.3

Flash, no damage

Flash, minor damage

Flash, minor damage

Flash, no damage

Flash, no damage

Flash, damage

Of particular interest are the three shots on one site where the first two shots caused no damage.

Damage Morphology

Figure 3 is a set of three photographs taken of a conventionally polished fused silica

surface which has been damaged on the entrance surface by a 15 J/cm2 pulse. The typical

dished-out damage pits are seen in the Nomarski photograph, while the TIRM photograph shows

a great deal of structure unrelated to the occurrence of damage. The simultaneous photograph in the center shows that most of the damage pits do indeed have a small bright spot

in the center, but certainly damage is more easily detected by Nomarski inspection.

Figure 4 shows Nomarski and TIRM photographs of a damage site on laser-polished fused

silica. Here, while much damage is visible in the TIRM photo, only one pit is visible in the Nomarski photograph. As will be pointed out below, the surface was virtually featureless prior to damage. The pit, in addition, does not have the usual conical shape as seen

on conventionally polished parts. Rather, it is quite sharp-edged. While it seems quite

certain that the damage resistance of laser-polished fused silica is substantially better

than conventionally polished fused silica, it is possible that damage may have taken place

which went undetected by Nomarski microscopy but which would have been visible in TIRM if

that technique had been visible during the damage tests.

1 Figures in brackets indicate the literature references at the end of this paper.

230
Physical Properties of Laser-Polished Fused Silica

Shown in figure 5 are two TIRM views of a fused silica surface as seen before and

i

; ter laser polishing. From these photos it is seen there are residual marks on the mechan-

• ;ally polished part due to the mechanical polishing process, and that these are no longer

?en in the laser-polished surface. As indicated in the figure, this particular part was

litially very smooth, and, at least in the short run, laser polishing did not degrade this

loothness. Shown in figure 6 are two TIRM views of a fused silica surface similar to that

. ji figure 5 but after one-half-hour etch in 5% HF acid at room temperature. The absence of

iy linear features in the laser-polished surface indicates that, at least on the surface,

has "forgotten" its past experience of being mechanically polished. While not shown,

lese two surfaces reveal similar information in Nomarski microscopy. That is, the mechan-

- rally polished surface is quite highly decorated with scratches, while the laser-polished

jrface is quite uniform in appearance.

Figure 7 shows two Zygo interferograms of a fused silica part which has been laser

0 )1 i shed by scanning at successively slower rates on widely separated regions. The figure

1 h the left is the result of interference between the polished surface and a standard flat,

i this case, changes in the interference pattern are exclusively due to changes in the

jjrface profile. This figure indicates that the regions which have been laser polished are ^pressed below the mean surface. We have verified that this is so by Talystep measurements.

|i the case of the part shown in figure 7, the pass which resulted in a featureless appearice in TIRM had a measured maximum depression of 1500 A. All passes with less depression

jtill showed the marks characteristic of polishing, while those C0 2 scans which were made

jt a slower rate showed ever wider featureless regions.

The photo on the right of figure 7 was made by placing a second fused silica blank in ontact with the laser-polished surface of the first sample. The inner region was filled

ith a fluid of index identical to that of fused silica. The existence of the fringe

jistortion coincident with the laser-polished scan lines indicates that the index of the

aterial in the laser-polished region has increased. This is possible since the specific

ravity of Si02 can range over a value of from 2.2 to 2.66, and the index ranges over a blue of 1.46 to 1.55, depending upon the crystalline state. Fused silica is, in addition,

fairly open lattice material, and it may be collapsing to a more dense form under laser

rradiation.

Mechanical tests on the surfaces of laser-polished parts indicate that these surfaces

jre under tensile stress. Surface tensile stress is expected to result from this type of

apid heating and cooling of the surface of a cool substrate. However, tensile stress

ould also result if the surface was indeed going to a more dense phase, as is suggested by

ihe data in figure 7.

Damage Resistance

; CQ2 -laser-pol ished parts appear, by etching as shown in figure 6, to have fewer defects

!§sociated with mechanical damage from previous processing. This may be the reason for the

is increased damage resistance of laser-polished parts. Even in the case of laser-polished

|arts, however, it is quite possible that deeper, unhealed regions are the sites of subseuent damage, if the part is subjected to a sufficiently high photon flux during damage

(esti ng . Figure 8 shows a damage site which is suggestive of this. Here, a laser-polished

!egion has been damaged. Both the TIRM and Nomarski photographs show a linear feature and

I amage along this feature.

Most damage sites do not reveal any particular linear pattern. In the usual mechanii ally polished surface there is literally a mat of scratches and fractures, and it is not

jurprising to see damage occurring in an essentially random fashion. Laser-polished parts,

n the other hand, appear to have a much reduced surface microcrack density, and the laser

;e flux required to cause damage to these deeper and less numerous defects is much higher, as

Ihown in table 1

Real Optics

The question of making useful optical components from C0 2 -laser-pol ished parts will

epend upon one's ability to retain optical figure during C02 -laser polishing. This is not

231
unlike the situation which exists for flame-polished or acid-etched surfaces, both of which

can show higher damage resistance than the mechanically polished surface. In the case of

C0 2 -laser polishing, however, the situation is more easily controlled. In addition, it

appears as though a sufficient C0 2 polish to improve the damage threshold may not disrupt

the figure too badly.

In this paper we have presented a method for the preparation of fused silica surfaces

which results in increased surface damage resistance to 1.06 ym radiation. These surfaces are prepared by intense heating of the fused silica surface by an incident cw C02 laser

beam. The resulting surface is essentially featureless, as seen in dark-field microscopy.

Evidence was shown to suggest that many of the microscopic fractures present after mechanical polishing are removed by this polishing technique. The simultaneous removal of

mechanical defects and increase in damage threshold was taken as evidence that surface

fractures play a role in lowering the damage threshold of polished glass surfaces. Finally, we have noted that in its present state C0 2 -laser polishing results in a degraded surface

figure, but that it appears as though surface figure could be retained with careful C02

- laser processing.

Conclusion

References

[1] Temple, P. A., "An Improved Dark-FieldLike Surface Inspection Technique Using

Total Internal Reflection," presented

at Conference on Optics '79, 23-25 May

1979, Los Alamos, New Mexico.

[3] Temple, P. A., "Examination of Laser Damage Sites on Transparent Surfaces

and Films Using Total Internal Reflection Microscopy," this conference.

[2] Temple, P. A., "Total Internal Reflection Microscopy: A Surface Inspection

Technique," submitted to Appl. Opt.

232
Y - DIMENSION

FUSED SILICA SAMPLE

CO, LASER LASER BEAM 1^

X - DIMENSION -aX - SCAN MOTOR

Figure 1. C0 2 -laser-pol ishing apparatus. The

laser is an unfocused Coherent Radiation Model

41 C02 laser. The sample is 3 meters from the

laser. The sample is moved left and right by a variable speed motor and up and down by hand.

The sample was unheated and in air during

pol ishing.

X7

MATCHING FLUID

MICROSCOPE

SAMPLE

BABINET—SOLEIL

Figure 2. The total internal reflection microscopy (TIRM) used to inspect highly polished

transparent samples. A polarized laser beam is coupled into the sample by a prism. The angle

of incidence at the sample top surface is just

beyond the critical angle. The sample is gripped by a microscope slide manipulator, and,

since the sample is supported on an oil film on the prism, it can easily be moved about for

inspection of the entire upper surface. The

microscope used for the photographs in other

figures was a Zeiss Nomarski microscope with the

Nomarski illuminator turned off.

233
NOMARSKI NOMARSKI + TRIM

I I 100 Mm

TIRM

Figure 3. Three photos of the same region of a laser-damaged, conventionally polished piece of

fused silica. The view on the left is a Nomarski

photo showing several pits caused by a 15 J/cm2

, 1.06 ym wavelength, 1 nsec pulse length exposure.

The photo on the right is a TIRM view. Since

the part is mechanically polished, it has many

features visible in this view which are not related to the damage shown in the left photo.

The center view shows the Nomarski and TIRM illuminations simultaneously. It can be seen

here that few of the TIRM features are actually

identified with damage sites. Most of these

features are mechanical damage due to the

polishing process.

Figure 4. A Nomarski and TIRM view of the same damage site on a laser-polished surface. The

laser-polished site was featureless before

damage, and any features seen in the TIRM view

are the result of damage, either as actual pits

or as debris. Note that the damage is not easily detected on the Nomarski photograph.

234
OUTSIDE C02 POLISHED REGION INSIDE C02 POLISHED REGION ROUGHNESS 3.3 A rms ROUGHNESS 2.7 A rms

100 urn

Figure 5. A piece of fused silica before and

after laser polish. This part is unusually

featureless even as a mechanically polished

part. The laser-polished part is totally

featureless in both TIRM and Nomarski illumination.

OUTSIDE C02 POLISHED REGION INSIDE C02 POLISHED REGION AFTER 1/2 HR 5% HF ETCH

I I 100 pm

Figure 6. Two regions of the same fused silica

sample as seen in TIRM after one-half hour of

etching in 5% HF acid. The linear features,

evident in the mechanically polished part, are totally lacking in the laser-polished part.

235
Figure 7. Two Zygo interferograms of a fused

silica part which has been laser polished by six

passes, evenly spaced, over the surface of the

part. The scan rates were successively slower

for scans nearer the right. The photo on the

left demonstrates that there has been a reduction in the height of the surface where the

surface was scanned. Talystep measurements

verified this to be the case. The photo on the

right shows that the material in the laserpolished region is of different optical density

than the rest of the sample as a result of laser

polishing in that region.

r

..*,".

* «

• *« -

•

•

•

100

Figure 8. A Nomarski and TIRM view of a damaged

region on a previously featureless laser-polished

part. Of particular interest is the linear

feature along which damage occurred. This is suggestive of that linear feature, probably a scratch introduced much earlier in the mechanical

polish, being the nucleation site for damage.

The speaker indicated that the scan rates used in this work ranged from 1/I0th to I oentimet

per second. He indicated that further evidence to support the contention that the surface c the sample was densified during the flam-polishing process was provided by samples that were scribed before surface treatment. In some cases, whole sections of these samples detached I spallation after the flam-polishing process, indicating the presence of very strong tension

forces which would be consistent with the model of increased density.

236
THE RELATIVE IMPORTANCE OF INTERFACE AND VOLUME

ABSORPTION BY WATER IN EVAPORATED FILMS*

T. M. Donovan and P. A. Temple

Michelson Laboratory, Physics Division

Naval Weapons Center, China Lake, California 93555

and

Shiu-Chin Wu and T. A. Tombrello

California Institute of Technology, Pasadena, California 91125

Previously, a precision adiabatic calorimeter has been used to make single

line HF calorimetry measurements of surface, interface, and bulk absorption on wedge-shaped films. In this paper we correlate hydrogen concentrations and

profiles measured using a nuclear resonant technique with these previous calorimetric measurements. Results show, for example, that H 2 0 is uniformly distributed in materials such as NaF and ThF 4 but occurs at the film-substrate interface in materials such as As 2 Se 3 and ZnS. The relative effectiveness of

encapsulation of NaF and ThF^ by ZnS will be discussed. Good agreement is found

between measured absorption and that calculated from measured hydrogen profiles.

Key words: Absorption; calorimetry; coatings; encapsulation; hydrogen; thin films; water;

water absorption.

Introduction

In a program designed to develop multilayer coatings with minimal absorption for use with HF lasers, contamination by water was expected to be a major source of residual absorption at the laser operating wavelength, 2.8 microns. Further, as absorption in multilayer

coatings is not always the same as would be predicted from measurements on single layers

[l] 1

, contamination at surfaces and interfaces was expected to be a major source of residual absorption. In the effort to minimize this absorption by improving or optimizing deposition techniques, it was important to be able to measure not only the total coating

absorption but the absorption as it occurs spatially within the multilayer design.

In previous papers [2,3] a single laser line calorimetric measurement technique was

described from which values of surface bulk and interface absorption could be obtained

directly from measurements on wedge-shaped films. In this paper we report the results of

applying a nuclear resonant technique [4], developed for measuring hydrogen content and

spatial profiles in solid samples, to measuring hydrogen content and distribution in 1- and

2-layer optical coatings. Values of absorption that we obtain, assuming the hydrogen is present as H 2 0, agree in detail, both as content and distribution, with values measured by

calorimetry, verifying that water contamination is responsible for residual absorption in these coatings at 2.8 microns. Results are presented for NaF, ThFi+ , As 2 Se 3 , ZnS, SiOx

, and

A1 2 0 3 single-layer films. Finally, the reduction of H 2 0 in ThF^ by encapsulation with ZnS

is discussed.

Absorption in NaF Films

Single HF laser line absorption of NaF as measured by Temple [2,3] are listed in table

1, along with the absorption coefficient of bulk NaF at 2.73 microns [5] and the absorption

of H 2 0 [6] in this spectral region. Two conclusions are immediately apparent from this

data: First, NaF films have about four orders of magnitude more absorption than bulk NaF

in this spectral range, and second, the ratios of absorption of the NaF film and H 2 0 from

3J |?| "5 Work sponsored by the Materials Division of the Defense Advanced Research Projects

Agency.

1 Figures in brackets indicate the literature references at the end of this paper.
2.91 to 2.73 microns are identical. This ratio of absorption is shown more clearly in figure 1, where the H 2 0 absorption data have been normalized to the NaF data at 2.91 microns.!

These results show that the predominant absorption in the NaF film is due to H 2 0 contamination.

Table 1. Absorption of thin film and single crystal NaF and H 2 0.

a {\im) iNa r \ \ i i in j \ cm ) INd r \ DU 1 Is )\ cin ) H D 1 rm- 1

n 2 U ^CITl j

2.73 2.5 4.3 x 10 _Lf

1 ,472

2.76 5.5 3,050

2.79 9.6 4,684

2.83 14.4 7,104

2.87 17.6 9,851

2.91 19.4 11 ,660

In order to determine the spatial distribution of absorptance and, therefore, the

distribution of H 2 0 in the film, measurements were made as a function of film thickness at both 2.73 and 2.91 microns [2,3]. The pronounced thickness dependence of absorption as shown in figure 2, particularly at 2.91 microns, along with the zero thickness extrapolation of the absorptance to approximately the uncoated substrate absorptance, indicates that

the water contamination occurs throughout the film with some interface contribution present.

To verify that the excess absorption in the NaF film is due to H 2 0 and that the H 2 0 is uniformly distributed in the film, measurements were made of the H content and distribution

in the films using the resonant nuclear reaction 19 F +

l W * 16 0 +

k He + y, as described by

Leich and Tombrello [4]. Hydrogen profiles measured for NaF films deposited in a helium

cryopumped UHV system, an ion-pumped UHV system, and an oil -diffusion pumped system are

shown in figures 3(a) and (b). The average H content in all cases is about 150 ppm, and

the ratio of H atoms to NaF molecules is about 0.005. The distribution of H is uniform

with some buildup at the film-substrate interface. Assuming the H is present as H 2 0 (secondary negative ion mass spectroscopy (table 2) measurements confirm that H is present

primarily as H and OH ions), we estimate a quantity equivalent to a 25 A H 2 0 to be uniformly distributed in a hypothetical 10 000 A NaF film. Calculating the absorptance from

the expression A F ad, where a is the absorption coefficient of H 2 0 and d = 25 A, gives A

= 2.9 x 10~ 3 and A = 3.6 x ]0~ k at 2.91 and 2.73 microns, respectively. These values compare

reasonably well with values of 2.2 x 10~ 3 and 2 x 10 _1+ taken from the data of figure 2 at • j a = 1 micron for the above wavelengths. Thus, the nuclear resonance data confirm the

calorimetric results that H 2 0 contamination is responsible for the anomalously high absorption in NaF films, and that the H 2 0 is uniformly distributed in these layers. The nuclear

technique also provides a quantitative measure of the H (H 2 0) content of the films.

Absorption in As 2 Se 3 Films

The reported absorption coefficient of As 2 Se 3 and As 2 $3 films is lower than NaF [7] at 2.8 microns. Measurements on wedge-shaped films of As 2 Se 3 [2,3], as shown in figure 5,

confirm the low film absorption of reference [7] and indicate there is little or no thickness dependence of the absorptance either at 2.73 or 2.87 microns. There is, however, a significant surface contribution at these wavelengths, as indicated by the difference

between the absorptance extrapolated to zero film thickness and the measured substrate

absorptance. Hydrogen profiles of AS2S3 (As 2 Se 3 ) films show a H content of about 10 ppm or f less within the film volume and significant quantities, about 200 ppm, at the fi lm-substrate i interface. Calculation of absorptance with a 500 A* interface region and 0.02 H 2 0 to As 2 S 3

\l molecular ratio yields an absorptance of ~ 1 x 10" 3

. The measured surface contribution at 2.87 microns, from figure 5, is 0.7 x 10 -3

. Again, the agreement is good, indicating the

interface absorption observed calorimetrical ly in As 2 Se 3 is due to H 2 0 contamination at the

film-substrate interface.

238
Table 2. Secondary ion mass spectroscopy (SIMS) analysis

of NaF films prepared in ultrahigh vacuum and an oildiffusion pumped system.

-Ions UHV Oildif fusion

+ Ions UHV

Oil - diffusion

H 74 63 H 216 117

C 2 4 F 7 5 CH 5 6 Na 1000 1000

3 1 71

1 JJ / U OH 33 29 NaO 0.5 0.5

F 1000 1000 SiO 3 2 Na 1 1 Na 2 7 9 C2 H2 1 4 Cr .0.5 0.5

Si 0.5 0.5 Si 2

1 1 CI 3 3 Na 2 F 2 2

F 2 0.5 0.5 Si 2 F 0.5 0.5

H Contamination in ThF^ and ZnS and the Effect of Encapsulation

The nuclear resonant. technique was used to measure H profiles in ThF^ and ZnS and then

to determine the added effect of encapsulating the ThF 4 layer by ZnS by sequential deposiI tion of the two layers without breaking vacuum. These results are compared with calorimetric measurement of absorptance of quarter-wave pairs of alternating layers of ZnS and

ThF 4 in two configurations: (1) high index layer ZnS on the outside and (2) ThF 4 as the

outside layer.

A H profile of ThF 4 is shown in figure 6. Hydrogen is uniformly distributed throughout the film as in the case of NaF; however, the overall contamination level is higher,

i.e., about 1000 ppm. If a freshly deposited 2000 & film of JhF k is coated immediately

I without interruption of the vacuum by a 2000 K film of ZnS, the result shown in figure 7 is obtained. The H content of the ThF 4 is reduced from 1000 ppm to less than 20 ppm, a reduction of about a factor of fifty in H content. The H content of the ZnS is about 10

ppm.

An independent experiment [8] in which two multilayer coatings consisting of five

pairs of alternating quarter-wave optical layers of ZnS and ThF 4 were deposited simultaneously by the Optical Coating Laboratory, Inc., Santa Rosa, California, using an appropriate

shuttering arrangement in the following configurations:

substrate (LH) 5 and substrate (HL) 5

where H = ZnS and L = ThFi+. The single line HF laser absorptance of these coatings was

6 x 1

-Lf and 2 x 10" 2 at 2.87 microns, respectively. The factor of thirty-five decrease in absorptance resulting from exchanging ZnS for the ThF^ outer layer is in good agreement

with the decrease predicted by direct measurement of H contamination of ThF^ and encapsulated ThF^. These data suggest that ThF^ has a highly porous structure and H 2 0 is absorbed

by the film on exposure to air. This is different behavior than we observed in NaF films

where the air-exposed film has similar H content as the encapsulated ThF 4 film. The H profile of ZnS as a single layer (fig. 8) shows a H content of about 10 ppm in the volume

of the film and a film-substrate concentration of 200 ppm. The interface contribution seen

in the single-layer profile is not observed at the ZnS-ThF 4 interface, suggesting that it results from incomplete cleaning (chemical and vacuum bakeout) of the fused quartz substrate

prior to deposition of the ZnS layer. Deposition on freshly deposited ThF 4 eliminates this

contamination layer, at least at the ZnS-ThF^ interface.

239
Absorption of A1 2 0 3 and SiO

x Films

A1 2 0 3 and SiOx have been evaluated using both single line HF calorimetry and with the

H profiling technique. The absorption results shown in table 3 for A1 2 0 3 indicate a bulk

or volume absorption of 39 cm

-1 with a surface absorptance of 2 x 10" 3 and a film-substrate

absorptance of 4 x 10" 3

. The measured H profile shown in figure 9, where H peaks of 250

and 500 ppm occur at the surface and film-substrate interfaces, respectively, and about 100

ppm H occur in the film volume. Calculated values of absorptance assuming 500 A* surface

and interface layers are 0.6 and 1.25 x 10" 3 for the surface and film-substrate, respectively. The calculated bulk film absorption coefficient is 50 cm

-1

. These numbers are also

in good agreement with measured values, considering the absorption and H concentration

measurements were made on different films.

Table 3. Volume and interface absorption at 2.87 microns

of several thin film coatings.

Material B (cm"M Materlal e

F [cm

> (air

A (x

film)

10 *> (filmA substrate)

(x m

NaF 17.5

Si0

x 10.0 25 0

A1 2 0 3 39.0 21 40

As 2 Se 3 0.5 0 5

The single line calorimetric measurements for Si0x are also listed in table 3 and show

a surface absorptance of 2.5 x 10" 3 with a bulk absorption coefficient of 10 cm

-1

. There

is no suggestion of absorption at the film-substrate interface. The H profile for Si0x shown in figure 10 shows a similar result with 300 ppm H at the surface and about 50 ppm

within the film. The reason for the clean substrate-film interface region is not understood.

Concl usions

The results presented in this paper show that H 2 0 is a major source of contamination

and hence absorption at 2.8 microns in a variety of coating materials. With hydroscopic

materials such as ThF^, as-deposited films absorb large quantities of H 2 0 when the coating

is exposed to air. ZnS and As 2 Se contain < 10 ppm. H 2 0 in the volume of the films are effective encapsulants capable of reducing the H 2 0 in these ThF 4 films to about 20 ppm. In materials with less affinity for H 2 0 than ThF 4 , i.e., ZnS, As 2 Se 3 , A1 2 0 3

, and Si0x , H 2 0 contamination occurs predominantly at air-film or film-substrate interfaces. Because of

the variety of H profiles and one-to-one correspondence between H profile and absorptance

profile as determined by calorimetry [2,3], we conclude that the H profiling technique is an excellent method for the identification of H 2 0 contamination and its spatial distribution in optical thin films and multilayers and complements the calorimetric technique with

the capability of profiling multilayer structures. Both techniques will be valuable in assessing the effectiveness of improved deposition techniques designed to minimize the

effect of H 2 0, particularly at interface regions by application of in situ desorption

techniques.

References

[1] Holmes, S. , Final Report on Contract

No. N00123-78-C-0627, Naval Weapons

Center, China Lake, Calif. 93555.

[2] Temple, P. A., Decker, D. L., Donovan,

T. M. , and Bethke, J. W. , NBS Spec.

Publ. 541, 37 (1978).

[3] Temple, P. A,, Appl . Phys. Lett. 34,

677 (1979).

[4] Leich, D. A., and Tombrello, T. A.,

"A Technique for Measuring Hydrogen

Concentration Versus Depth in Solid

Samples," Nucl . Instrum. Methods 108 , 67 (1973).

240
[5] Harrington, J. A., et al . , Proceedings

Laser Window Conference, 1974 (AFML,

Wright Patterson Air Force Base,

Ohio, 1975).

[6] Rusk, A. N. , Williams, D. , and Querry,

M. R., J. Opt. Soc. Am. 61_, 895 (1971 ).

[7] Harrington, J. A., Rudi sill, J. E.

,

and Braunstein, M. , Appl . Opt. 1_7, 2798

(1978).

[8] Beauchamp, W. T. , Final Report on Contract No. N60530-78-C-0252, Naval

Weapons Center, China Lake, Calif. 93555.

>

IU

cc

H,0

/

/ /

/a

2.7 2.8

Figure 1. Spectral absorption in relative

units of NaF film compared to H 2 0. H 2 0 absorption data normalized to NaF data at

2.91 microns.

Figure 2. Thickness dependence of absorptance of NaF film for the two wavelengths,

2.73 and 2.91 microns.

241
0 0.2 0.4 0.6 DEPTH IN FILM (pm)

Figure 3(a). Hydrogen distribution of NaF film deposited in cryopumped ultrahigh

vacuum system and measured using nuclear

reaction 19 F +

J H + 16 0 + 4 He + y.

z

I

g

<

K

0.005

0

1

OSTD VAC

1 1 1 1 1 1 1 1 1

UHV

0 D

O

O 0 D

0 o

o

o ° °o

o o

O

1

1 1 1 1 1 1 1 1 U.Z U.4 U.I DEPTH INTO FILM (jim)

Figure 3(b). Comparison of hydrogen

distributions in NaF films deposited in

ultrahigh and standard vacuum.

242
14

1 1 „ _ i i a - 0.4 cm _

1 r K7- X2.87

12

10 o

X

8 < V

or - 0 cm-1

6

4

2

o

i

O

I I

X2.73

1

——o_

0 0.25 0.50 0.75 1.0 THICKNESS Ifiml

Figure 4. Thickness dependence of absorptance of As 2 Se 3 film measured at 2.71 and

2.93 microns show low volume absorption

and large substrate-film interface absorptance.

-* 1—

0 0.2 0.4 DEPTH IN FILM U<ml

0.6

I I

Figure 5. Hydrogen profile of As 2 S 3 film showing large film-substrate interface contamination.

243
- 1000

0.2 0.4 DEPTH IN FILM (pm)

Figure 6. Hydrogen profile data of JhFu film.

k

0.02

0.01

0.2 0.4 DEPTH IN FILM («m)

Figure 7. Hydrogen profile data of ZnS film showing large film-substrate inter- face absorptance.

244
0 06

0.04

0.2 0.4 DEPTH IN FILM 0*m)

Figure 8. Hydrogen profile of ~ 2000 ft ThF 4 film overcoated with ~ 2000 A of

ZnS. H content of the ThF 4 is greatly

reduced over single-layer film (Fig. 7).

Film-substrate interface H of ZnS is greatly reduced over single-layer film

(Fig. 8).

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.1 DEPTH IN FILM (urn)

Figure 9. Hydrogen profile data of A1 2 0 3 film showing large air-film and filmsubstrate concentrations.

245
0.2 0.4 0.6 0.8 DEPTH IN FILM M

- 300

200.

Figure 10. Hydrogen profile data of SiOx film showing large surface concentration.

The speaker indicated that methods to avoid the presence of an interface contamination laye were under investigation. So far it had been determined that vacuum bakeout was not effective in removing an adsorption layer at the interfacial surface. It was pointed out in

discussion that the absence of a water layer at the interface in the presence of materials

like thorium fluoride was probably due to the fact that materials like thorium fluoride

absorb water in bulk, and therefore water at the interface is redistributed within the

coating layer.

246
CHARACTERIZATION OF SMALL ABSORPTIONS IN OPTICAL COATINGS

Wilford N. Hansen, Lee Pearson, and Galen Hansen

Utah State University

Logan, Utah 84322

and

Wayne J. Anderson

Physics Department

United States Air Force Academy, Colorado 80840

Probably the main causes of laser damage in thin films used

as optical coatings are impurities and non-stoichiometry , especially at interfaces. This paper considers the problems of

identifying absorbing species, locating them in the optical

structure and measuring the amount of absorbing species present.

The importance of an absorbing species at a given location in the structure is directly proportional to the relative electromagnetic field squared at that location. This fact is used

both to avoid absorption when it isn't wanted and to enhance

absorption when it is wanted, such as during analysis. Versatile

and sensive spectroscopic procedures for locating and characterizing small absorptions are discussed. Our approach makes use of

Poynting's theorem, viz., that the rate of conversion of electromagnetic to joule energy at any point in a medium is proportional

to <F.2> at that point.

Emphasis in this paper is on thorium fluoride, zinc sulfide,

zinc selenide, and silicon monoxide films on silver, germanium,

and zinc selenide substrates. Effects of<E2

^> field placement

and intensity are illustrated. Significant absorbing foreign

species were found in all films tested.

Key words: Internal reflectance spectroscopy, absorption; coatings;

zinc sulfide; thorium fluoride; zinc selenide; silicon monoxide.

1. Introduction

At the present state of art, laser damage thresholds usually are not determined by the

intrinsic bulk properties of the optical materials involved. Impurities, especially at interfaces, are still a main cause of damage because of their absorption bands at working

laser frequencies. It is therefore important to identify these impurities, find their

location in the multilayer optical system, and determine the amounts present. We show

how this can be achieved using infrared reflection spectroscopy, and illustrate the pro- cedures by a study of several important optical coatings. We also show how the stoichiometry of certain optical coatings can be studied by similar techniques coupled with a transformation to obtain the optical constants of the coating.

In this study it was found that large absorption may occur even in a perfectly

intrinsic specular silver substrate. This will happen at certain frequencies where the

average electromagnetic electric field squared, <.EO , just inside the silver surface goes through a maximum. While this intrinsic phenomenon may or may not be important as a direct source of laser damage to mirrors, depending on circumstances, its rationalization

is always essential to the understanding of infrared spectra of optical coatings on metals.

This effect is large even for silver in the infrared. For other metals it will generally

be much larger.

247
2. Experimental Procedures

In this study emphasis is on characterizing small absorptions. Therefore various

techniques were used to enhance the sensitivity of the infrared absorption methods used.

Multiple reflection spectroscopy, internal and external, was the choice. In addition to many beam encounters in series, angle settinqs and polarization choice were used to enhance

sensitivity, by making <E2 > large in the region of interest. For external reflection, two microscope slides on which were deposited the base metal and coating(s) were placed facing

each other to permit multiple reflections in series. For internal reflection, a plate of

high index material was used, either zinc selenide or germanium. The films of interest

were deposited on both faces of the plate. Attachments such as those available from Harrick

Scientific Corporation, Ossining NY, can be used to advantage. We used PE 180 and PE 621 spectrophotometers

Detailed procedures and methods of analysis of the reflection spectra can be found in reference [1]. The large (E^> was placed at the air-film or substrate-film interface as desired and alternately it was arranged for <E^> to be very large in the film center and

small at the two interfaces. This latter case is illustrated by figure 1. The beam "sees"

a particular region in proportion to <e2"> in that region. Thus we are able to locate the

position of the offending species. The profile of <E2> depends on angle of incidence, and

polarization as well as film optical thickness.

3. Results and Discussion

3.1 ThF

4 Films

ThF4 films deposited by the AFWL, Kirtland AFB, NM, were investigated by internal and

external reflection spectroscopy. Some films were deposited on silver. Others were deposited on germanium and zinc selenide internal reflection plates. The substrate was sometimes heated to 150° during deposition. A few of the films were capped with a 2y layer

of ZnS and some with ZnSe while still in vacuo . The films were then transported by mail

and were exposed to the atmosphere for at least two days and up to one month in some cases before measurements were made.

Our spectra showed that all of the ThF4 films including the capped ones contained

large amounts of water which was found to be distributed throughout the films, and typically

amounted to a liquid water equivalent of from 500 to 1000 & in a 2.4y ThF4 film. The capped

films had almost as much moisture as the uncapped films. The reason for this is not known.

Surely ThF4 can be deposited water free. And surely it can be kept anhydrous by capping.

It may require special procedures, however, and needs to be investigated further while

making use of these sensitive procedures to measure the moisture. It was further attempted

to displace the moisture from the films by heating to about 107°C, in vacuo, while at the same time taking spectra. Only a few percent of the moisture was lost. It is suagested

that a thermogravimetri c analysis be run on hydrated TI1F4 to hi oh temperatures to document

the temperature( s ) of water loss.

Hydrocarbon contamination was also found. It amounted to only a few monolayers at most, however, and was always at the external ThF4/air interface. Water and hydrocarbons

accounted for all of contaminant spectra seen except for a 100 A layer of Si 0 which was deliberately placed in some of the samples under the ThF^ to test the <E2)profile. In all cases it showed up exactly when it should, and was invisible when the expected \E2)> profile

"said" i t should be.

Examples of ThF4 spectra are shown in figures 2-5. Photocopies of the actual spectra

are used here because otherwise important detail is lost. Note that "transmittance" on the standard form paper used should be replaced by ( 1/

0

) xl 00 . Figure 2 is dominated by

water absorption in the regions of 3y, 6.1y, and beyond 10y. ThF4 also has a broad intense

intrinsic absorption band with maximum about 25y but beginning around lOy. In figure 3 there is an additional Si 0 band at about 9y. The atmospheric CO2 bands at about 4.2y

should be ignored. Figure 4 shows interference minima at 770 and 2850 cm~l. These are

due to absorption in the silver itself and are large here because of the use of J_ polarization and large angle of incidence. As discussed below, the frequency position of these

minima can be used to determine optical thickness.

248
3 . 2 ZnS and ZnSe Fi 1ms

In the case of ZnS and ZnSe films small amounts of both water and hydrocarbon were

found, amounting to a few monolayers each. They were both found to be concentrated at the

substrate/film interface. Evidently they remained in spite of careful cleaning procedures.

Again, the 100 A SiO layer deliberately placed under the coating in some samples, showed

up exactly as expected.

Examples of spectra are shown in figures 6 and 7. These multiple internal reflection

spectra are extremely sensitive, making a single monolayer easily visible. Water bands in the 3y region and the 6-7y region are clearly visible. More than usual detail is seen in the water structure, especially in the case of ZnS, figure 7. Hydrocarbon in the 3.5u

region is clearly visible in both figures. The C02 dips near 4.2y again should be ignored.

Absorption beyond lOy is due to the Ge plate itself and is of no interest here.

3.3 SiO Films

In figure 8 are shown spectra of a 0.5y SiO layer on silver. Water and intrinsic SiO

bands are seen. The first interference minimum for intrinsic absorption in the silver

substrate is off scale to the left. The water contamination is much less than in the case

of ThF4, but much more than for the zinc compounds.

The complex dielectric constant, e(X), or complex refractive index, n(X), can be calculated throughout the intrinsic SiO band from the spectra of figure 8. This can be done in several ways. It can be done by inverting _L and || reflection equations at wavelengths throughout the absorption band, or it can be done by transformation, [4], using

either the J_ or the || spectrum. The fact that the absolute position of the "no absorption"

background is not at 100% doesn't preclude high accuracy. The trick is to compare the

reflectance with what it would be, in the same spectrum, if the absorption band were

el ectromagneti cal ly bleached. Details are discussed in reference [1]. It is interesting

to note the enormous difference in spectral shape and intensity for the two polarizations.

These differences are rationalized completely when the <E^) values inside the film as a function of wavelength are considered.

3.4 Intrinsic Absorption in Film Covered Metal

Consider the external reflection spectra of figures 9, 10, 11. The dominant feature

is the absorption in the silver itself, even though the silver is behaving nearly as ideal

intrinsic silver should. And this is the infrared region where silver is one of the best

reflectors in existence, with ns3+i50 at 1500 cm"'. Clearly, absorption in the substrate

metal must be accounted for in rationalizing reflection spectra. The shapes of these

interference spectra change with angle of incidence, polarization and film thickness.

The absorption of the 100 A SiO layer is clearly visible in the unpolarized spectrum

of figure 9, and is enhanced in the || polarization spectrum of figure 10. It is com- pletely absent in the J_ polarization spectrum of figure 11. The reason for this absence

is that sE2") at the SiO layer position is typically three orders of magnitude less for

J_ polarization than for || polarization at 45°. On the other hand, the <E?) values just

inside the silver are comparable for the two polarizations.

The intrinsic absorption of zinc sulfide itself is seen best in figure 10. The well

known absorption caused by the imaginary part of e, where eH;Ree+i I me, is shown at about

270 cm"l. The band at 340 cm

_ l is less well known, and is due to the energy loss function,

Im(-l/e), and logitudinal excitation, [2]. It is completely absent at normal incidence

and forJ_ polarization at any anale of incidence. These points are illustrated by figure

11.

The interference phenomena can be rationalized with relative ease if certain points

are realized. The phase change for reflection at the dielectric/silver interface is approximately either 180° or 0° for these long wavelengths, depending on polarization.

From equation 5a of reference [3] it can readily be shown that extrema in reflectance for

249
perpendicular polarization occur at

h/A = n/4? (extrema)

~2 ? ? 1/2 whe^e h/A is the film thickness/wavelength ratio, n is an integer, and E, = {r\f-r\\ sin 9] )

- where f stands for film 1, for the incident phase and 9] is the angle of incidence. Odd n give the minima in reflectance, i.e.,

h/A = (2n+l)4£; (reflectance minima)

and

h/A = (2n)/4? (reflectance maxima)

The minima are much sharper than the maxima, so they are the obvious choice for experimental

determination of film optical thickness. Only one minimum is needed. When needed, cor- rections can be added to the above equations when the phase changes aren't exactly 0 or 180°.

4. Conclusions and Recommendations

One objective of this study has been to test some innovative procedures to see how

effectively and efficiently they work. They have worked very well. Information obtained

has been copious and to the point. The time, effort, and cost has been minimal compared

to other projects for getting similar data, with which we have had experience. We believe

that these procedures can with care, be used routinely in the laboratory.

All of the films studied were found to contain important absorbing species as con- taminants. These species were identified and located. It was also shown that the amount

present could be determined from the spectra.

Thorium fluoride films all contained large amounts of water in spite of modest efforts

to avoid it. From our results it is apparent that avoidance of water in thorium fluoride

films will require elaborate procedures. Undoubtedly, it can be done with proper procedures

during preparation and with enough impervious overcoating to prevent contamination subsequently. It may not be worth the effort, however, and it is strongly suspected that the

films would always be easy targets for moisture degradation. If the fluoride films are

used, this work shows how to monitor the moisture content with ease and accuracy.

The zinc sulfide and selenide films were contaminated with water and hydrocarbon, but were otherwise clean. These contaminants were found to be present at the solid/solid interface, indicating a preparation problem. Surely they can be eliminated with feedback from

spectroscopic testing. The amount of contamination was small, but large enough to be important if located in a large <E^> position.

Silicon monoxide contained moisture - much less than thorium fluoride, and much more

than zinc sulfide or selenide. It was shown that such films can be characterized by taking

polarized spectra. The procedures used are directly applicable to the characterization of

silicon monoxide and silicon dioxide films. In fact, silicon oxide films of various

stoichiometry could be studied conveniently by this method and their optical constants determined easily throughout the fundamental absorption region.

It is suggested that the procedures illustrated by the present study be used routinely

in optical coating evaluation. It is hoped that this study has served to illustrate how

this can be done. More extensive studies should include high temperature studies. Any

absorption will surely change at high temperatures where laser damage actually occurs. Absorption bands tend to broaden. Even the intrinsic bands of the film may broaden in some cases enough to cause laser damage due to intrinsic absorption. This should be investigated systematically. It is recommended that broad band spectroscopy be used, like the

spectra shown here from 2.5 to 50y. It is important to have qualitative as well as quantitative information.

5. Acknowledgements

The support of this work by the Air Force Weapons Lab, Kirtland AFB, New Mexico,

under contract F29601 -79-R-0081 is gratefully acknowledged. Part of the work was done

250
at the Naval Weapons Center, China Lake, CA, with help from NWC personnel. Special thanks

go to Drs. Hal and Jean Bennett for their valuable discussions and assistance.

References

[1] Hansen, W.N., Reflection Spectroscopy

of Optical Coatings, J. Opt. Soc. Am. 69, 264 (1979).

[2] Kittel, C, Introduction to Solid State

Physics , 5th ed., John Wiley and Sons,

New York, 1976. pp. 328f.

[3] Hansen, W.N., "Electric Fields Produced

by the Propagation of Plane Coherent

Electromagnetic Radiation in a Stratified Medium", J. Opt. Soc. Am., 58_ 380

(1968).

[4] Hansen, W.N., Abdou, W.A., "Analysis of

Solid-Liquid Interphase Spectra via

Causal Transformation," J. Phys. (Paris) 38 C5-207 (1977).

2 2 + Figure 1. <E >j_/<E >l] for internal reflection in

a germanium-film-air system. Phase one is Ge with n, = 4.0; phase two is a film, h/\ = 1.3,

n 2

= 2!0; for phase three, n

3

= 1.0; 8, = 29.5°. Note that the fields are large at the film center.

251
Figure 2. External reflection spectrum of 2.23y ThF

4 film on silver, sample 4-A,

12.5 reflections at e, = 45°; PE-621 spectrophotometer.

WAVELENGTH (MICRONS)

2.5 3 4 5 6 7 8 9 10 12 15

4000 3500 3000 2500 2000 1800 1600 1400 1200 1000 800 600

| WAVENUMBER (CM 1 )

Figure 3. External reflection spectrum of 2.23p ThF^ films on silver plus 100$ Si

at silver/ThF

4 interface; sample 4-B, 12.5 reflections at 45°; PE-180

spectrophotometer.

252
WAVELENGTH (MICRONS)

6 7

4000

I

3500 3000 2500 2000 1800 1600 1400 1200 1000 800 WAVENUMBER 'CM')

Figure 4. External reflection spectrum of 0.5y ThF^ on silver with 2p ZnSe cap;

10 reflections at 9-j = 67°, J_ polarization, PE-180 spectrophotometer.

WAVELENGTH MICRONS,

6 7

4000 3500 3000 2500 2000 1800 1600 1400 1200 1000 800 600 WAVENUMBER (CM')

Figure 5. ThF^ on silver, 0.2yAg/0.01ySiO/2.23yThF

4

, external reflection, ]_ polarization using PE-180; top curve, 9-j = 45 ; bottom curve e-j = 65°; bottom short curve is 10X scale expansion showing hydrocarbon bands

in 65° spectrum.

253
WAVELENGTH (MICRONS; -6 7

4000

I 3500 3000 2000 1800 1600 1400 1200 1000 800 600 400 WAVENUMBER (CM')

Figure 6. Internal reflection spectrum of ZnSe on Ge plate; 40 reflections at 9, = 30°;

| | polarization; PE-180.

WAVELENGTH (MICRONS)

2.5 3 4 5 6 7 8 9 10 12 15 20 30 50 2

Figure 7. Internal reflection spectra of 0.5y " c film on Ge plate; top spectrum,

e, = 45°; middle spectrum, e-, = 30°, bottom spectrum e, = 82°; PE-180.

254
WAVELENGTH (MICRONS)

2.5 3 4 5 6 7 8 9 10 12 15

4000 3500 3000 2500 2000 1800 1600 1400 1200 1000 800 600

I WAVENUMBER (CM')

Figure 8. External reflection spectra of 0.5p Sio on silver; 13 reflections at 6, = 45°;

top is || polarization, bottom is J_ polarization; PE-180. 1 OX expansion is for top, |

| polarization.

Figure 9. External reflection spectrum of ZnS on silver, sample 2-B; 12.5 reflections at 45°;

PE-621.

255
Figure 10. As in figure 9 but using || polarization, PE-180.
SURFACE MICROANALYSIS TECHNIQUES FOR CHARACTERIZATION OF THIN FILMS

T. W. Humpherys, R. L. Lusk, and K. C. Jungling

Air Force Weapons Laboratory

Kirtland AFB, NM 87117

With the advent of lower wavelength systems, increased sophistication of

functional requirements, and higher incident radiation levels, more stringent

demands have been placed on coating depositions with complimentary detailed

chemical and physical characterizations being required. Scanning Auger

Microscopy, X-ray Photoel ectron Spectroscopy, and Secondary Ion Mass Spectroscopy techniques have been applied to thin film coatings to detect and

identify compositional and structural irregularities. Impurity sites in ThF,

and ZnS films, principle causes of poor coating performance, have been

isolated and analyzed. Point maps, line scans, and depth profiles are shown

to provide remarkable clarity with submicron resolution. Abnormal oxygen

content in ThF^ films indicates the presence of water and may eliminate its use as a coating material for HF systems. Although these surface analysis

methods are destructive by nature, the validity of their use in analyzing

thin film coatings is demonstrated and establishes a precedent for coating

development programs.

Key words: Auger analysis; dielectric coatings, optical coatings; surface

analysis; thin film characterization; thin film impurities.

1 . Introduction

Auger Electron Spectroscopy (AES), Secondary Ion Mass Spectroscopy (SIMS), and X-ray

Photoel ectron Spectroscopy (XPS, often identified as ESCA) surface analysis techniques have

been utilized to investigate structure, defects, and contamination in multiple layer thin

film coatings. Schematic representations of these various techniques are shown in figure 1 with some typical performance parameters for each method [1]. The destructive nature of

these techniques and the general lack of spatial resolution are considered problem areas, especially when analyzing films of submicron thickness. The results of a study to determine the validity of these types of analyses are presented with particular emphasis

on analysis beam probe induced damage.

2. Experimental Approach

The experimental approach was to analyze the various layers of a multistack thin film

coating as each of the layers was sputtered through using argon ions. Special thin films were fabricated to exercise various capabilities of the analysis techniques. The results

presented are from ZnS and ThF- designs of which a typical example is illustrated in

' 0 figure 2. Each layer was approximately 2000 A thick in order to avoid excessive sputter

times when profiling. The number of layers represents a typical functional coating design

for an enhanced reflector including the base metal reflector and a chromium binder.

Silicon wafers were utilized as substrates in order to provide a conducting medium which

could also be cleaved resulting in a clean, uniform break in the coating for edge profiles.

The samples were Ar+ sputtered with SIMS data recorded during the sputtering process. AES

and XPS data would be taken between sputtering sequences to produce profiles of the

coatings. Analysis was conducted, when feasible, in the same vacuum chamber with a base

pressure in the low 10~ 10 Torr range. A PHI Model 590 Auger system was used for most of

the analysis presented in this paper with some sputtering accomplished in the PHI Model 550

ESCA system.

257
3. Results and Discussion

A specimen of the design specified in figure 2 was analyzed at various depths while

profiling through the first layer of ZnS and into the ThF^ layer. The secondary electron

microscope (SEM) photographs in figure 3 display the various stages of the profile.

Several prominent dust particles are evident on the otherwise featureless surface which

provide a reference for the remaining photographs. After sputtering into the ZnS layer,

several point defects or impurity sites appear. Further sputtering down to approximately

o 1500 A exposes additional impurity sites, all of which are enclosed by a halo region.

Each of these disk shapped areas house one of the impurity sites. This point defect and

halo region remain intact even into the ThF

4 layer. Insulating properties of ThF

4 reduce,

to some extent, the resolution of the SEM photographs. Sputtering of this specimen was terminated at this time in order to perform more detailed analysis. The final photograph

in this set is a Nomarski micrograph of a typical area in the sputtered crater which

illustrates that both the defects and corresDondina halos are al<;n visually observable.

The micrograph indicates possible heiqht differences between halo regions and the

surrounding area. Height variations could be a consequence of different or biased

sputtering rates. The geometrical patterns of the halos suggest that this anomaly is attri buteabl e to the presence of the impurity sites.

A close-up SEM photograph of a single defect region, figure 4, depicts a typical area that was analyzed in greater detail incorporating various microanalysis techniques.

The contrasting horizontal line extending through the halo region was electron beam

induced during AES line scans. Although the halo itself may be an artifact of argon ion

bombardment during the sputtering process, a similar consequence is apparent in the Auger

analyzed areas which have been subjected to the electron beam. In addition, the surface

is also disturbed, to a much lesser degree, however, during a typical SEM photograph

acquisition sequence in which the incident electrons are substantially less energetic

than the Auger probe electrons.

A three point (spatial) Auger analysis comprises figures 5 through 7. The survey

in figure 5 was obtained when constraining the probe beam to an area outside of the halo

region of figure 4 but remaining within the sputtered crater. Dominant amounts of Th and

F are as expected since sputtering was terminated in the ThF^ layer. In addition to these elements, however, there is a substantial amount of 0 present. The large amount of

oxygen present, at least in comparison with other films such as ZnS, ZnSe, Ag, Cr, CeF,

and A^O-j, indicates substantial water content in the ThF^ film. This concurs with

corresponding SIMS analysis in which both H and 0 are observable and also supports the

conclusions of an independent study of several ThF^ films in which spectral data confirms

the presence of water throughout each of the ThF^ layers [3]. The AES survey on the halo area, presented in figure 6, indicates this region to be F deficient and 0 enriched. Th signatures are relatively unchanged. The flux of incident argon ions during the

sputtering process collide and interact with the ThF^ compounds driving off some of the

loosely bound flourine atoms. This dissociation and the abundance of oxygen readily

allows Th0F2 or Tl^ type of compounds to form. In all ThF^ layers analyzed, AES and

especially XPS data taken on a newly exposed surface from sputtering indicate that the

oxygen concentration increases as a function of time. This implies adsorption of water

from the chamber onto the ThF^ where it reacts to form thorium oxide compounds.

The submicron Auger probe beam was then positioned on the point defect with the

resulting AES survey shown in figure 7. Again fluorine is deficient and oxygen dominates

the energy spectrum. Additional differences occur in the 150 and 1000 EV energy ranges

when compared to the survey on the halo. Both Zn and Na signatures are located near 1000 EV [2]. Variations in the low energy range suggest the presence of significant

amounts of sulfur. Since the film is made of ZnS, it is proposed that the point defect

is composed of or is a trap for ZnS.

258
Auger images, or spatial elemental maps, also provide useful information and verify

many of the conclusions stated above. An F elemental map, figure 8, illustrates the lack

of F in the halo region. The mapping technique displays four levels of elemental

concentrations ranging from black, representing nondetectabl e amounts, to bright white,

implying areas of high concentration, a surface AES survey was conducted in the large

rectangular region below the halo. The pronounced absence of fluorine in this area dramatically exhibits the effect of the incident electron beam during Auger analysis.

In comparison these regions are oxygen rich as shown in figure 9. The much more massive

thorium atoms are much less mobile than the fluorine atoms and are not effected by the

electron beam, nor is there any detectable difference in the halo region as seen in the

Th elemental map in figure 10.

An additional technique involves a spatial line scan searching for predetermined

elements. A scan was made across the halo region, through the defect point while

monitoring the energy windows characteristic of the signatures of 0 and F. As expected,

the 0 content increases, figure 11, as the beam transverses the halo while the F concentration decreases. The amounts detected radically change as the defect center

is reached. This sequence is reversed as the probe beam moves beyond the halo. This is the process which produced the horizontal line in figure 4. Although the halo is most

likely caused by the sputtering and analysis techniques, it is apparent that detection

of point defects is enhanced through these processes.

Line scans across the edge of a film exposed after the silicon chip is cleaved are

also possible provided the thickness of the thin film layers is greater than the

resolving power of the analysis beam. This does occur for most films designed for

operation in the infrared. To obtain an edge scan on the coating designed for and

utilized in this study, the exposed film edge created upon sputtering during a SIMS

profile was line scanned from the bottom of the crater, which was in the silicon

substrate, outwards across the film boundaries to the outer surface consisting of the

top ZnS layer. Figures 12 and 13 depict, respectively, a schematic representation of

a sputtered crater and the various elemental line scans of the film. Contaminants can

be followed in this manner also. Definite boundaries are evident indicating the

compositional structure of the thin film stack. With a high resolution probe, point

areas within layers can then be analyzed in much greater detail yielding stoichiometry

of each of the layers in the film stack.

SIMS analysis was conducted on separate thin film stacks, one made with RAP processed ThF. and the other with Balzers ThF^. Both were E-beam deposited. Each film

was sputtered through the first layer of ZnS and well into the first ThF^ layer where

SIMS data was recorded. The results are presented in figure 14. The RAP film contains

more of the impurities Na, A£ , Li, and K. Similar comparisons were made on the bulk

materials with the same conclusions. Independent analysis [4] on similarly coated

films support, in general, the conclusions presented here. The exact history and

handling procedures of these materials is not known and may have contributed to the

observed contaminants.

Impurity sites observed in this study are not unique to a particular vender,

supplier of basic coating materials, deposition techniques, or deposition facilities.

Under a space radiation program conducted by NWC [5], several coatings of ZnS and ThF^

were found to experience similar point defects with corresponding halos. Other films in

a typical storage environment were found to deteriorate as a function of time. The

latter degradation is probably from a different mechanism since the defect centers

appear to be pores and the halo is not disk shaped but follows more of a structured

pattern similar to a snowflake. Water vapor enters the film through the pores and

interacts with the underlying ThF^ layers. Nomarski photographs of some of these

specimens comprise figure 15. The common denominator at this point appears to be ThF^.

The orgin of the impurity sites, however, could result from a number of other sources,

such as improper substrate cleaning and preparation, inadequate deposition processes,

259
or contaminated coating materials.

3. Conclusions

Not all films analyzed exhibited numerous impurity sites with corresponding halos. None of the films studied, however, were totally free from point defects or impurity

sites. It was obvious that any defect or impurity became readily apparent during the

analysis process. Although the exact physical and chemical interactions are not yet

well understood, the methods discussed enhance the detection of minute irregularities

which degrade optical and mechanical properties of thin film coatings. Surface analysis

techniques are fast becoming an integral part of the development and analysis of thin

film coatings. Refined procedures and techniques for this type of usage should provide

tremendous insight into understanding the poor performance of many thin film coatings.

4. Acknowledgments

The authors are indebted to Dr. T. Wiggins and Dr. A. Taylor for their assistance

in gathering and interpreting much of the data. We also extend our gratitude to R. Espisito for many helpful discussions and for providing many of the specimens.

5. References

[1] Swingle, R. S., and W. M. Riggs, ESCA,

in Critical Reviews in Analytical Chem.,

5, 267 (1975).

[2] Davis, L. E., et. al . , Hanbook of Auger

Electron Spectroscopy, 2nd Ed., Physical

Electronics Industries, Inc., 1977.

[3] Hansen, W. N., et. al . , Characterization

of Small Absorptions in Optical Coatings,

this conference.

[4] Nelson, A. J., private communications.

[5] Donovan, T. M., private communications.

260
TYPE ANALYSIS

SPUTTERED AREA

ANALYSIS AREA

INCIDENT BEAM

ELEMENTS ANALYZED

ESCA

7X8 m

2X2 m

X-RAYS

Ll UP

ixi m

0.2J1M

ELECTRONS

Li up

SIMS

1X2 m

200 JUM

IONS

1 -300 AMU

SCHEMATIC OF THE VARIOUS SURFACE ANALYSIS TECHNIQUES EMPLOYED

Figure 1. Schematic representation of ESCA,

Auger, and SIMS microanalysis techniques with

typical characteristic parameters.

2000 A ZnS

2000 A ThF„

2000 A Ag

C 3 PAIR

V / 2 V V V 100 AC, V / / / / /\

Si substrate

THIN FILM DESIGN FOR SURFACE ANALYSIS

Figure 2. Typical thin film design

specifically fabricated for this study.

261
Figure 3. SEM and Nomarski photographs at various depths during profiling through the

outer ZnS layer and into the first ThF^ layer

of a three pair stack.

Figure 4. SEM photograph of a typical point

defect and halo region.

262
nes

1 subvey sr. i.m %wj/r> m

1 1 I i i t

1 r

i | \ \ { -j—iM&> 2 OUT SIDE HALO WED

1 i A

0 : F

2M 4M Ut M4 tUI I2U 14M MM IMS 2tM

Figure 5. Auger survey outside of the halo

region within the sputtered crater.

AES SURVEY SF = 1,67

INSIDE HALO OFF OF DEFECT CENTER

F

i,

T"

0

Th

1

i i i J I I I 1 iL 0 m W 1200 1600 2000

KINETIC ENERGY, EV

Figure 6. Auger survey inside the halo.

263
AES SURVEY 5F = 1.09

ON DEFECT IN ESCA CRATER

s

, 1 /

If

F Z.-;? 'th

Th

0

Ih

i i i i 0 WO 800 1200 1600 2000

KINETIC ENERGY, EV

Figure 7. Auger survey on the point defect.

Figure 8. Flourine Auger map of halo region.

264
Figure 9. Oxygen Auger map of halo region.

Figure 10. Thorium Auger map of halo region.

265
AES LINE SCAN SF = 2,92

OXYGEN

Figure 11. Oxygen and flourine line scans through halo region and across the point

defect

/ / / >\

/ / / / /\ /\ \ \ N -

zn

: y y x y xiK\ A x x x 2^_\

SCHEMATIC REPRESENTATION OF ION SPUTTER ETCHING

Figure 12. Schematic of a typical sputtered

crater in a three pair ZnS/ThF

4 coating.

266
Figure 13. Elemental Auger line scans across the edge

of SIMS profile crater depicting the various coating

multilayer constituents.

267
I SI CA

RAP THF,

20 30 40 50 60

ATOMIC MASS UNITS

SIMS DATA OF VARIOUS THF4 FILMS

70 SO 90 100

Figure 14. Comparison of Balzers and RAP

ThF, films by SIMS analysis.

Figure 15. Nomarski micrographs of four

coatings, two after space radiation (top)

and two after storage (bottom).

268
PYROELECTRIC MEASUREMENT OF ABSORPTION IN OXIDE LAYERS

AND CORRELATION TO DAMAGE THRESHOLD

H. Kiister and J. Ebert

Institut fur Angewandte Physik, Universitat Hannover

Welfengarten 1, 3000 Hannover, Germany

The absorption of several oxide layers used in HP

laser optics was measured by a fast pyroelectric detector in the range from 600 to 2000 °C. The layers of

Al 2 03, BeO, MgO, Hf02, ZrO^i, Nd203, Ce02, Ti02 and Si02"

thickness about 0.5/um - were electron gun evaporated on Suprasil I substrates.

Damage thresholds were obtained with 800 ns - pulses from an unstable resonator type Nd3+-glass laser

with a Gaussian farfield intensity profile. The occurence of damage is determined by electronic registration

of laser induced scattering.

The results were analyzed to determine relations

with layer absorption, absorption edge and thermal con- ductivity.

Key words: absorption edge; damage temperature; damage

threshold; microsecond pulses; temperature depending

absorption; unstable resonator.

1 . Introduction

In general absorption is determined at ambient temperatures in a calorimeter using a cw laser. The temperature rise of the sample occurring in

this kind of measurement is within the range of some degrees. In damage

threshold experiments much higher temperatures may occur, some people re- ported even a melting of oxide materials, which means temperatures well

above 1800 K. We have conducted our investigations to clarify the behaviour

of temperature and absorption in damage experiments.

Assuming an uniform rate of heat deposition in the film by the laser

beam, cooling only by the substrate, no radial heat loss and thermal constants for film and substrate approximately equal, then the equation for

temperature rise of a thin absorbing layer on a thick substrate is

AT = aP d S ( ! ) ~\ju c p k

AT temperature rise [k] _^ d layer thickness [cm]

ot absorption coefficient [cm ] c specific heat [ws/gKl

P power density [w/cm2] p mass density Lg/cm^J

t time of irradiation [s] k thermal conductivity [W/cmK]

In figure 1 the solid line indicates the calculated square-root-dependence of temperature rise for a constant absorption of 100 cm~l. That curve

does not fit either the measured time dependence of the temperature nor the

calorimetrically measured absorption coefficients, which are about 10 cm~l

for Zr02~ layers at 1.06 um. To fit both we have to regard the temperature

dependence of the absorption coefficient. In semiconductors and insulators

the electric conductivity increases exponentially with temperature. The conductivity again is directly proportional to the optical absorption [l] , so

that the electric conductivity as well as the optical absorption increase

with temperature as

269
B T-TA

(2) a (T) = a (T ) • e

ambient temperature [kJ B material constant [k]

The absorption coefficient versus temperature of a Zr0

2 sample is showi

in figure 2. There is a fairly good agreement between the measured and

calculated points. The exponential dependence of the absorption on the

temperature seems to be a correct assumption. Now it is possible to explain

a sudden layer destruction because each increase of temperature conse- quently results in an increase of absorption, which in turn results in an explosive heating with plasma formation and cracking, melting and vaporizin

of the material.

2. Experimental Procedure

2.1. Lasersystem

Damage thresholds are measured at 1 .06 jum using an acousto-optically

Q-switched Nd-glass laser (fig. 3) . The unstable resonator produces high

output energy with a far field intensity which is fairly Gaussian without

any intracavity aperture.

Table 1 . Laser system data

Laser Nd -glass, \= 1.06 jum

Output 8 J max. Pulseform max. 15 pulses 800 ns FWHM, nearly Gaussian temporal profile

Interpulse regular 50 us intervalls by acusto-optical Q-switch

duration

Laser rod Schott LG 630,diam. 12mm, length 220mm, cutted in Brewster angle

" head gold plated double ellipse

Resonator unstable, both mirrors high reflecting, output mirror coated on elliptic area 6/8 mm diameter

The laser beam is directed through a system of attenuators (fig. 4)

and further onto a beamsplitter for energy detection. The detector produce:

a signal related to the energy incident on the sample and is adjusted by a calibrated joulemeter. The time resolved energy signal is stored in a transient recorder. The laser beam is further guided through a dichroic

beamsplitter and finally through the focussing lens onto the sample. The

spot diameter at the points of 1 /e2intensity is 0.5 mm. For damage

detection a He-Ne laser beam is transmitted through the irradiated site.

The resolution in transmission is better than 0.1 % and the transmitted

signal is stored too.

2.2. Temperature measurement

The temperature radiation of the layer is collected by the focussing

lens (fig. 5), the spectral range around 850 nm is reflected by the

dichroic splitter. The outer part of the radiation cone is collected and

directed through a filter system and an aperture of 0.5 mm diameter on a silicon diode. It is necessary to use the spectral range of 850 nm for

detection because this is the range of maximum sensitivity of Si-diodes.

Special filters are used to suppress laser radiation as well as shorter

wavelength. In addition the inner part of the radiation cone is shielded

to stop the laser radiation reflected and scattered under small angles.

I

270
The signal is converted by an analogous computer into a voltage

corresponding to a temperature in centigrades. The temperature scale has

been adjusted by a calibrated tungsten tape lamp assuming an emissive

power of the layers £ = 0.15. For that fixed £ the reading error is within

+ 1% over the entire range. But the different materials have slightly

different emissive powers, which have been measured in the temperature range

up to 1100° C using an electric oven. The change of e between 0.1 and 0.3

in turn induces a relative temperature change at the lower temperature

limit of -9% and +19% respectively. If £ is known, the accuracy of the

pyrometrically determined temperature is better than + 5%. The temperature

radiation of the bare substrates has been taken into account. Only temperatures above 5 80 C are displayed because of the sensitivity limit and the

signal noise.

The corresponding signals of energy, temperature and damage versus

time are shown in figure 6. The pulse power is calculated from the energy

steps, absorption coefficients from temperature steps. Damage is detected

by the change in the transmission signal, or from the occurence of plasma

which are quite well correlated.

2.3. Evaporation process

The layers are deposited in a high vacuum evaporation plant using

electron beam technique in a reactive atmosphere (figure 7). The thickness

is optically monitored during the processes and adjusted to 4 A/4 at 1 .06 urn. The layer absorption at ambient temperatures is measured in a laser calorimeter similar to that described by Allenet at. (2).

Table 2. Evaporation process data

Substrate

Substrate cleaning

Reactive evaporation

Activated reactive

evaporation

Electron gun

Vendors of targets

Layer thickness

Suprasil I, 25mm diam., 1mm thick

vapour decreasing in C^Cl^F^, lens tissue

rub with isopropanol, draw off solidified collodium

oxygen into electron beam, partial press. 2-10~^mbar,

substrate temperature 210 C , evaporation rate ca.58/s

for high refractive materials:

ionized oxygen on substrates

6kV, rotating targets 60 mm 0

Balzers and Cerac

nd = 1.06 um = 4\/4 at 1.06 urn

3. Results

From equation (1) the presumption arises that damage threshold is

inversely proportional to absorption, if damage temperature of a substance

is constant. Taking the absorption coefficients from the calorimeter

measurement at ambient temperatures the relation does not apply at all.

The threshold values as well as the absorption coefficients reported here

correspond to layer temperatures of about 600°C and fit the proportionality

satisfactory.

In figure 8 the damage threshold of Zr0

2 -layers is shown depending on

the absorption coefficient. There is a fair correspondence between the

measured values and the relation proposed before:

D- CC -r^o = const. = 25 GW/cm3 DUU

The constant is obviously a measure for the power absorbed per volume

which causes destruction under the conditions in question. In this experiment we have changed the oxygen pressure to generate different absorptions,

but in further investigations with diverse materials we used equal eva- poration parameters (table 2) . Doing this we are in some cases far away

271
from the best parameter combinations regarding minimum absorption and

highest damage thresholds, but this is the objective method compairing materials. The absorption coefficients then scale the decomposition propability of the different materials.

In figure 9 the damage threshold of different coating substances

depicted against the absorption coefficient at 600° C performs the re- quired relation. Material properties like thermal conductivity do not seem to effect the damage threshold because BeO and SiO~ for instance show

no strong deviations, although BeO has a thermal conductivity 200 times

higher than Si0 2> The damaging temperatures and the melting points seem to

have more influence on the damage thresholds. Some materials with re- latively low melting points (Ti0„, Ce0

2

) show a lower damage threshold than

high melting materials (ThO_ , MgO)

Table 3. Average damage temperature of oxide layers on Suprasil I Coating material Ti0

2 Ta

2 0

5 Ce02 A1

2 03 Nd2 03 BeO Zr0

2 Damage temp. (°C)

Melting point ( C)

Dam. temp. (K) as part

of melt, point (K)

.^590

1840

0.41

^620

1880

0.41

^6 80

1950

0.43

«s 600

2015

0.38

?s 600

2270

0.34

^6 50

2530

0.33

<^640

2 700

0.33

The insulator type characteristics of the low melting materials are changed to semiconductor type at relative low temperature. So we have been

looking for the part of the damage temperature to the melting point. As it can be seen from the values in table 3 there is no typical behaviour perceptible. Probably the damage of the layers strongly depends on the re- latively low melting point of the substrate material Si0

2 and its vanishingly small expansion coefficient.

There is a remarkable point: changing the evaporation parameters for

lowest absorption does not essentially change the product D-ct as you can

see for Zr0

2 in figure 8. The tendency is always the same: points lying

below the curve D-ct = const remain below it and so do the points

situated above, because the damage temperature too does not change.

In addition to the material properties considered above the absorption

edge at shorter wave lengths is correlated to damage threshold. Assuming,

that the laser wave length is far away from the wave length of the absorption line the theory predicts the absorption a to be proportional to the

resonant wave length \ | . This can not be derived by the measurements

shown in figure 10. The discrepancy to \4 occurs because we did not measure the wave lengths of the center of the absorption line but a longer

one where the transmission of a 0.5 um thick layer drops to 50%. A second reason for the divergence is the constant evaporation parameter which has

not been the optimum for each material regarding the absorption coefficient.

The solid line in figure 10 indicates a possible relation to our measurements. We do not expect the indicated relation to be generally valid

but we hope that there might be some derivable expression.

4. Summary

We tried to get some information about the connection of thin film

absorption and laser induced damage. The main results have been achieved

by using a fast pyrometer for the measurement of the layer temperature.

The findings may be summarized as follows:

272
a. Absorption coefficients show a strong temperature dependence

according to an exponential function.

b. Absorption coefficients near the destruction temperature are inversely proportional to damage thresholds.

c. The average damage temperature for oxide layers on fused silica

substrates is far below the melting points.

5. References

1] Seitz, F. , The Modern Theory of [2] Allen, T.H., Apfel, H. a. CarSolids, New York 1940, p. 192 niglia, C.K. "Laser Induced Damage in Optical Material", 1978,

p. 33, NBS Special Publication

541

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Per a number of different samples, the damage temperature appeared to be about 600 degrees

centigrade. Damage was assumed to arise from an absorbing layer at the interface between the

oxide layer and the substrate. This was confirmed by the fact that in many experiments the

oxide layer was disrupted. All data were taken at one pulse length, 800 nanoseconds. At

th-us pulse length it was pointed out that standing-wave effects would no longer be important due to the effect of thermal diffusivity during the pulse duration.

279
k
HIGH-TEMPERATURE OPTICAL CHARACTERIZATION OF THIN FILM REFLECTOR AND ABSORBER LAYERS

M. R. Jacobson and R. D. Lamoreaux

Optical Sciences Center

University of Arizona

Tucson, Arizona 85721

We have developed a High Temperature Spectrophotometer (HTS) capable of

measuring the temperature-dependent, reversible changes in reflectance and trans- mittance of thin films. The effective spectral range of the instrument - 0.38

to 15 microns - covers 98% of the solar Air Mass 2 spectrum, as well as 94% of

the radiation from a 500 C blackbody. A tungsten-halogen, or globar source, pro- vides two beams which are recombined before leaving the vacuum chamber. A Leiss

sodium chloride monochromator disperses the radiation, which is detected by

either a silicon ora thermocouple detector, depending on wavelength range. A microcomputer controlled sample-and-hold data acquisition system compares the

two beams, corrects for background, and initiates data analysis. To minimize

irreversible chemical changes in the films, a vacuum on the order of 5 x 10"°

Torr is maintained by a diffusion pump in the central chamber. Cartridge heating assemblies raise the sample substrate temperatures to 700 C in reflectance

and 500 C in transmittance; chromel-alumel thermocouples embedded and cemented

into the substrates monitor temperatures.

Key words: High temperature; infrared; optical instrumentation; reflectometers; spectrophotometers.

1. Introduction

Several developing energy and military technologies require optics that perform at elevated temperatures. The photothermal conversion of solar energy, for example, demands

surfaces that both absorb well over the solar spectrum and reflect well in the thermal infrared. If such selective absorbers can maintain their optical performance at high tem- peratures, they will contribute to a higher thermodynamic efficiency for the total system.

Our group at the University of Arizona's Optical Sciences Center has been developing hightemperature stable selective surfaces for the past several years under NSF-RANN, ERDA, and

DOE funding. Measurements of optical properties at operating temperatures are crucial to the evaluation of these coatings. To this end, we have developed a High Temperature Spectrophometer (HTS) to measure reflectance and transmittance of coating samples over a spectral range of 0.38 to 15 microns. We shall describe the instrument in terms of its four

basic systems: optical, environmental, electronic, and data analysis.

2. Optics

Shown from left to right in Figure 1 are the source optics, the vacuum chamber optics,

and the monochromator optics; only one of the two beams is traced for clarity. The sources,

a 500 W tungsten-halogen lamp and a diesel engine igniter globar, are mounted on a carousel

for convenient interchange. The carousel structure also supports a cylindrical tube which

acts as a lamphouse and a fan which cools the sources. A third sector is left vacant for

an additional source. Pairs of spherical and flat mirrors transfer the beams to foci within the transmittance cell. Before entering the cell, the beams are alternately passed by

a single chopper with one open 90 degree sector. The chopper is driven by an 0-ring belt

drive powered by a stepper motor, the speed of which is controlled by a pulse train from

the electronics.

When used in the transmission mode, the beams enter through the twin sodium chloride

windows of the transmittance cell, which is mounted on the large vacuum cavity. A substrate coated with the thin film to be measured is placed in the sample beam, while a nominally identical substrate is held in the adjacent reference beam to cancel substrate ef281
fects. In the reflectance mode, the entire transmittance cell can be replaced with a simple

flange containing another pair of sodium chloride windows. Next, spherical and flat mirrors

relay the beams to the reflectance cells on either side of the vacuum chamber. The sample

reflectance cell, at the top of Figure 1, clamps the sample firmly between the heater block

and the cell structure, by means of opposed stainless steel wedges. The unheated reference

reflectance cell holds the aluminum reference mirror tightly, as well. Beyond these cells,

the final sets of optics direct the two beams to the beam recombiner, which operates as illustrated in the inset in Figure 1. A set of microscope slides was beveled; they were

glued together so that the slopes alternated. When aluminized, the recombiner interleaves

the sample and reference beams into a single beam, which leaves the vacuum chamber through

another sodium chloride window.

A Leiss Sodium Chloride Double-Prism, Double-Pass Monochromater follows. An entrance

slit admits the beam from the vacuum chamber, after which it is dispersed by the first of

two Littrow-mounted prisms. An intermediate slit blocks stray light and admits the beam

to the second set of dispersing optics. Then, the beam exits the monochromator and is directed to the detectors by a flat, followed by a spherical , mirror. The spherical mirror

can be rotated slightly to focus on either of two detectors, a two-color silicon photodiode-lead sulfide detector from Infrared Industries for the visible and near-infrared; or

a thermocouple detector from Reeder for the mid-infrared.

3. Environmental

The samples must be heated to temperatures of interest and maintained at high vacuum. Furthermore, both temperatures and vacua must be monitored. Finally, cooling arrangements

around the hot sample cells must be made to prevent undesirable effects on adjacent components.

Samples are heated by Watlow cartridge heaters inserted into closely machined copper

blocks. The block in the sample reflectance cell contains a pair of 250 W, 1/2-inch di- ameter heaters. This block directly heats the back surface of the sample to about 700 C. A chromel -al umel , or K, thermocouple monitors the block temperature; a second K thermocouple is cemented into a hole in the corner of the sample itself. The transmittance cell

carries two copper blocks, one for the sample and one for the reference beam. Each con- tains eight 3/8-inch diameter cartridge heaters dissipating a total of 1,000 W to heat the

sample to about 500 C at its center; they are arranged around a circular hole in the block

which admits the light beam. Heating of the transmittance samples is more difficult, be- cause only the edges can be heated by conduction. Each transmittance sample is pressed

against its block over the circular hole by a stainless steel holder. Again, K thermocou- J pies are cemented to the blocks, while pairs of thermocouples, one in a corner and one near

the center, monitor the samples' temperatures. All thermocouple wires are led out of the vacuum chamber via high-temperature junction strips, made of Macor machinable ceramic and

chromel /al umel 1 ugs , to vacuum thermocouple feedthroughs . Outside the instrument, a twelve- | position thermocouple switch allows rapid access to the numerous temperatures under observation. All copper heating blocks are nickel -plated to prevent oxidation of the copper. A temperature controller unit can be incorporated into the system, as well.

A cooled copper baffle surrounds the sample reflectance cell, while the transmittance

cell is a double-walled stainless vessel with feedthroughs for water circulation. A recirculating water chiller provides coolant in both cases.

_6 jj A two-inch Edwards diffusion pump evacuates the main chamber to levels of around 10 Torr. To improve speed and prevent degradation of optics by pump oil, a chilled baffle is included above the pump. The usual thermocouple gauging for roughing pump vacua and hot

filament ionization gauging for diffusion pump vacua are provided. An argon bleed line is used to prevent condensation in the chamber and oxidation of the hot sample after a run. To provide easy access to the samples and optics, the entire top of the vacuum chamber can

be removed.

282
4. Electronic

In Figure 2, a simplified schematic shows the operation of the sampl e-and-hold electronics. Signals from the silicon photodiode circuit of the two-color detector and the

thermocouple detector for wavelengths shorter and longer than one micron, respectively,

enter preampliers located adjacent to the detectors. Then, longer cables relay the signals to the sample-and-hold electronics. A switch and one stage of amplification, not

shown, precede the gate circuitry. The gates have inputs from both the LED/phototransistor pairs at the chopper, and a crystal clock. The former select the proper chopper phase

from the three possibilities; while the clock defines the actual time period during which

data is taken. Figure 3 depicts the phase synchronization system. The LED/phototransistor pairs, of which one is drawn, report when the open sector of the chopper blade admits

light into the vacuum chamber. Pulses 250 ms apart are generated, as illustrated at the

bottom of Figure 2. These pulses also set the crystal clock, which then opens the appropri

ate gate for 102 ms, thereby waiting for the slower thermocouple detector to reach a constant value. The second gate sends the integrated signal on to an A/D converter. The A,

B, and C phases refer to the sample, black chopper blade, and reference phases.

The A/D converter sends its digital output on to the 6800 computer and also drives a four and one-half digit selectable display. When the computer has recorded the data at a particular wavelength, it commands the stepper motor at the monochromator to advance to the next wavelength. As we shall see in the next section, the microcomputer provides us with efficient and flexible data analysis.

5. Data Analysis

Our preliminary analysis routine uses the microcomputer to remove various sources of

background noise and to plot the spectral reflectances. Later versions will calculate solar

absorptances and thermal emittances at various temperatures, and may automatically allow

the system to integrate for longer periods at wavelengths where the signals are weak. The

operation is similar for both reflectance and transmittance modes. Four basic sources of

error or noise are present: offsets in different channels of the sample-and-hold electronics; offsets in the preamplifiers; thermal emission; and stray light. The first problem

can be eliminated by reading the output from each channel when both beams are blocked at

the beginning of a run. The second and third are not modulated by the chopper, and will

fall out when the sample and reference beams are compared by the microcomputer after each

cycle. Stray light, however, can be generated by the sample and reference beams, and must

be quantified and subtracted. This can be done by a means of standard samples. First, a run is made with first surface aluminum reflectors in both beams; this measures the re- sponse of the system to the maximum anticipated signal. Then glass samples with blackened

first surfaces are introduced into both beams. These two measurements each provide observed signals for sample and reference beams, which consist of the actually reflected component and a stray light component. The stray light is assumed to be the same in each case

the reflectance of the blackened glass is known to be on the order of 1% of aluminum, depending on wavelength. This reduces the problem to two equations in two unknowns - which

are the corrected aluminum reflectance and the stray light. When these unknowns are computed at each wavelength for sample and reference beams, we are ready to proceed. Then,

corrected values of sample and reference aluminum mirrors are available at each wavelength

for sample measurements.

At this point, the sample itself is introduced, and runs are made at ambient and higher temperatures. As before, the sample is always compared to the aluminum reference, and

the dark phase values are subtracted from both. The adjusted values calculated above and

stored in the computer correct the measured values at each spectral point, yielding the

measured, corrected reflectance factor. Finally, reflectance values for aged aluminum are

used to change the reflectance factor to absolute reflectance. [1]

The procedure is basically the same for transmittance measurements, except that the

sample reflectance cell contains an aluminum mirror and is not heated. Approximately 340,

points are taken while covering the 0.38 to 15 micrometer range (or alternately " vm range ) these scans take about 20 minutes each. Data are plotted by a striD chart recorder and are

stored in the microcomputer.

v 283
6. Data and Research Plans

An example of data previously taken on the instrument follows in Figure 4. A 1.75

micron thick film of silicon was fabricated by chemical vapor deposition on a silver reflec- tor and heated to 500 C in the HTS. The absorption edge shifts to longer wavelengths due

to phonon-assisted transitions, while the interference fringes deepen due to increased free

carrier absorption. The first effect increases solar absorption, while the second raises

the thermal re-emission of the thin film stack. In this case, as shown in Figure 4, the

reversible thermal effect causes the use of room temperature reflectance data, to produce

an error of as much as 10% in the solar absorptance of a sample at elevated temperature.

We are planning to use the HTS at the earliest opportunity to explore the high-temperature reflectance properties of both highly reflective and black molybdenum films. We also plan to observe the transmittance properties of carbon-alloyed amorphous silicon, which

is stabilized against crystallization to temperatures of around 1000 C.

7. Acknowledgments

This work has been supported under the following contracts: "Chemical Vapor Deposition of Amorphous Silicon for Photothermal Solar Energy Converters," Department of Energy,

Division of Materials Sciences, #ER-78-S-02-4899; "Chemical Vapor Deposition of Refractory

Metal Reflectors for Spectrally Selective Solar Absorbers," Department of Energy, Advanced

Technology Branch, #EY-75-S-04-3709; and "High Temperature Reflectance and Transmittance

Measurements of Thin Films," Joint Services Optical Program, Air Force Office of Scientific Research, #F49620-77-C-01 38.

6. References

[1] H.E. Bennett, J. M. Bennett, E.J. Ashley, J0SA, 52, 11, 1245 (1962).

[1

284

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285
PHASE SYNCHRONIZATION SYSTEM IN UTS Fig. 3

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Figure 4. Change in reflectance of a 1.75

micron thickness silicon-on-silver selective surface due to heating from 20 to 500C

in the HTS.

286
C V D MOLYBDENUM THIN FILMS

FOR HIGH POWER LASER MIRRORS

G.E. Carver & B.O. Seraphin

Optical Sciences Center

University of Arizona

Tucson, Arizona 85721

Molybdenum thin films have been prepared by chemical vapor deposition (CVD)

from both molybdenum carbonyl (Mo(C0) c ) and molybdenum pentachloride (MoCl c ). b b The crystal structure, grain size and composition of the films have been determined and related to their infrared reflectance. Post-deposition annealing raises

the absolute reflectance of films deposited from the carbonyl to 98.732 at ten micrometers. The fact that these films are prepared without a polishing step

may explain why their reflectance is 0.535 higher than that of super smooth polished bulk molybdenum. The higher reflectance and absence of a polish-induced

damage layer could increase the damage threshold of uncoated CVD molybdenum mirrors. Films deposited from the chloride have largely reproduced the optical

properties of those prepared from the carbonyl, with grain sizes ranging from

0.03 to 0.5 micrometer. Polished bulk molybdenum substrates could be further

smoothed by first overcoating them with, and subsequently polishing of, a several micron thick layer of fine grained CVD molybdenum.

Key words: Chemical vapor deposition; composition; crystal structure; grain size; molybdenum thin films; reflectance.

1. Introduction

High thermal conductivity, a low thermal expansion coefficient, and the relatively

high damage threshold of molybdenum (Mo) suggest the use of Mo as a high power laser mirror LI, 2.1. The fabrication of low-scatter Mo surfaces by mechanical polishing is, however,

difficult. Molybdenum thin films, on the other hand, can be much smoother if deposited

on an appropriate substrate due to their small grain size than the polished bulk. Thin

films of Mo generally have, however, anomalously low infrared reflectance _3 3 ^1- Attempts

to smooth bulk Mo by first overcoating with, and then polishing of, sputtered Mo thin films

result in reduced damage thresholds [5J. This loss in stability has been related to the

porosity of the sputtered films and interfacial problems between the film and the bulk [1].

Many applications dictate the use of reflectance enhancement coatings which also reduce

the damage threshold of Mo-based mirrors. This paper reports on the production of Mo thin

films which have a higher infrared reflectance than the reflectance of even super-smooth

bulk Mo. We also report on the chemical vapor deposition of fine grained films which may

prove useful in additional smoothing of bulk Mo substrates.

Refractory metal films prepared by CVD for use in photothermal solar energy conversion show promise through both high temperature (500 C) stability and high infrared re- flectance [6]. Since the requirements are similar, our previouswork in this area thus na- turally extends itself into the high energy laser mirror field L7],

2. The CVD Process

Chemical Vapor Deposition occurs on substrates held above the threshold temperature

of the pyrolytic decomposition or gas phase reduction of the reactant gas. As opposed to most PVD techniques, CVD films are therefore screened against failure at temperatures below this threshold and are at thermal equilibrium with the substrate during growth, resulting in temperature stable films of high packing density. If impurities and/or imperfections

can be avoided, Mo films with very high infrared reflectance can be deposited [7j8], In addition, CVD can coat complex shapes under one atmosphere without the need for ultra high

1. Figure in brackets indicates the literature references at the end of this paper.

287
vacuum. [9]

Two methods were used in this work to form Mo thin films: the pyrolysis of Mo(CO)

and the reduction of MoCl c. The two reactions are as follows:

Details on these two processes can be found elsewhere. [7,8].

All films reported here were deposited on fused silica substrates under one atmosphere of

Argon carrier gas and were approximately 0.2 micrometer thick.

A metallographic analysis must be carried out to explain why Mo thin films have different reflectance from that of supersmooth bulk Mo. We have therefore determined the

composition of CVD Mo by Auger spectroscopy and electron microprobe techniques, the crystal structure by x-ray diffraction, and the grain size by electron microscopy.

Reaction (1) does not go to completion at 300 C. Higher temperatures result in powdery deposits if the reaction occurs under a pressure of one atmosphere. Films deposited

from the carbonyl at 300 C therefore contain considerable amounts of carbon and oxygen

and are only 54 + 14 atomic % Mo. These films also exhibit an fcc-MoOC phase in grains

which are less than 0.03 micrometer in diameter. As a result, the reflectance of films

from the carbonyl is only 87% of the reflectance of Al at 10 micrometers. As shown in figures 1 and 2, post-deposition annealling increases the reflectance. The figures show

that either 30 minutes at 750 C or five minutes at 1,000 C increases the reflectance of

the Mo films up to the reflectance of Al . The fully annealled films contain as little

as one atomic % carbon and two atomic % oxygen. The anneal also converts the films to the bcc-Mo phase and enlarges the grains to a diameter of 0.5 micrometer. Changes in composition, structure and grain size during anneal are summarized in figure 3. Even though these fully annealled films contain up to four atomic % Cr and two atomic % W,

they still have an absolute reflectance of 97.4% at 10 micrometers. We have previously

reported that this value is only 0.7% lower than the reflectance of supersmooth bulk

Mo [7]. Since then, higher reflectance values have been attained on a new set of films

deposited from purer Mo(C0)g. After anneal, the new films contain about one-half the

amount of Cr and W of the previous set, and have an absolute reflectance of 98.73% at

10 micrometers, now surpassing by 0.5% that of super-smooth bulk Mo. All of these absolute reflectance values were measured to within I 0.1%, and were provided to us along

with the super-smooth bulk Mo data, by Dr. J.M. Bennett of the Michelson Laboratory at China Lake, California.

The adsorption of oxygen or carbon monoxide on a Mo surface can reduce the reflectance

[10]. Since our films, in transit to China Lake, California, were exposed to air for several days before the reflectance was measured, we may assume that the initial reflectance

is even higher than the value measured after such exposure. The most reflective sample

was remeasured twice after a period of several weeks to insure that the reflectance was

not decreasing further in time. The results in Table 1 indicate that the reflectance

is stable to within 0.1% over the observation period of 45 days.

5

2 Mo+10 HC1

(1)

(2)

3. Optical and Metallographic Characterization

Of CVD Molybdenum

TABLE 1

Absolute reflectance, R at 10 micrometers,

versus time for annealled CVD Mo.

Deposition day zero

R = 98.73 6 days

28R
TABLE 1, (Continued)

R = 98.55 - 23 days

R = 98.64 - 45 days

The total integrated scatter (T.I.S.) from the most reflective film is 1.19 t 0.59

x 10" ^ for a wavelength of 0.6471 micrometer. This relates to an RMS roughness of 0.0018

micrometer while standard fused silica substrates have an RMS roughness of 0.0013 micrometer [11], indicating that the CVD film apparently is conformed to the quartz substrate.

In addition, the as-deposited films do not roughen during anneal as evidenced by similar

T.I.S. values of films before and after the anneal step.

Films deposited by reaction (2) at 750 C have similar optical and metallographic

properties as the fully annealled films from the carbonyl reaction. Reaction (2) can produce, however, body centered cubic (bcc) deposits of equivalently pure Mo which have fine

grains (0.030 micrometer) by depositing at 500 C. Presently, thin sputtered films of Mo

are used to smooth bulk Mo substrates, but result in reduced damage thresholds of such

mirrors. Even though CVD Mo must be deposited at high temperatures, a small grain size

can result if the substrate temperature is low enough. If fine grained CVD deposits were

formed on bulk Mo, the damage threshold may be higher than that of bulk Mo coated with

sputtered Mo for two reasons. First, the CVD Mo is likely to be less porous than the sputtered Mo due to the deposition of the CVD film at thermal equilibrium with the substrate.

Second, contaminants at the bulk-film interface can be partially removed by reaction with

hydrogen at the temperature of deposition.

4. Discussion

In comparing them to the pure bulk phase, the anomalously low reflectance of refractory metal films was originally blamed on impurities and/or structural modifications. Im- provements in the purity and structure of Mo films have been accomplished since the sputtered film of figure 4 was deposited in ultra-high vacuum two years ago [4]. Biasing during sputtering [12] and heating the substrates during evaporation [13] have increased the

reflectance of PVD films to near bulk values. To our knowledge, however, only annealled

CVD films have actually eclipsed the reflectance of super-smooth bulk Mo. This fact may

be due to the absence of a polishing step during the preparation of our films. This hypothesis is supported by three facts. First, the literature acknowledges that mechanical

polishing induces a damage layer in smoothed semiconductor and refractory metal surfaces

[14,15]. Second, Saito observed increasing specular reflectance of polished bulk Mo with

decreasing roughness until a certain RMS value was reached whereupon the reflectance began

decreasing with decreasing roughness [2]. Third, polished bulk Mo which contains hardening

agents (TZM) has exhibited a slightly higher reflectance than pure bulk Mo [1]. The hardened material may have a less extensive polish induced damage layer.

CVD Mo films deposited on an appropriate substrate without additional layers will become useful as mirrors for high-power lasers. Their high reflectance can possibly be

raised even further by the following measures. First, CVD Mo could be isolated from gases

which may be adsorbed at the surface. We have coated CVD Mo immediately after anneal with

a CVD SigN^ layer and improved their infrared reflectance [7]. Second, a substrate with

a thermal expan-sion coefficient closer to Mo than fused silica, such as silicon carbide,

may reduce elastic deformation upon cooling. Finally, residual amounts of Cr and W in our

films could be further reduced through pre-purif ication of the reactant materials. Although not yet determined, CVD Mo should have a higher damage threshold than super-smooth

bulk Mo, since the increased reflectance reduces the residual absorptance. Reducing the

surface damage by the absence of a need to polish will help further. Support for this ex- pectation is provided by the fact that diamond-turned copper has a higher threshold than

mechanically polished copper [5].

289
5. Conclusions

Mo thin films have been deposited by chemical vapor deposition from both Mo(CO),. and

o MoClg. Post-deposition annealling has increased the reflectance of films from the carbonyl

to 98.73% at 10 micrometer. The reflectance of these films is 0.5% higher than super-smoot

bulk Mo. Films from the chloride have exhibited grain sizes ranging from 0.03 to 0.5 micro

meter.

6. Acknowledgements

The authors wish to thank David Allred of the Optical Sciences Center, University of

Arizona, for the electron microprobe measurements; Jean Bennett of the Michelson Laboratory, for the absolute reflectance and total integrated scatter measurements, plus the

super-smooth bulk Mo data; Lou Demer of the University of Arizona for the electron microscopy; Tilak Raj of the University of Minnesota for the Auger spectroscopy determinations;

and the entire Solar Energy Group of the Optical Sciences Center for assistance throughout this work.

This work has been supported by the Department of Energy under S. E.R.I. Subcontract

#XH-9-8217-l

7. References

[1] S.M. Wong, G. Krauss and J.M. Bennett, "Optical and Metallurgical Characterization

of Molybdenum Laser Mirrors," in Laser Induced Damage in Optical Materials : A. J. Glass

& A.H. Guenther, Eds., N.B.S., Washington, D.C. (1978) (NBS Spec. Publ . 541 ) ,1 32-1 63.

[2] T.T. Saito, "10.6 pm Absorption in Molybdenum Mirrors," in proceedings of ASME/AIAA

Thermophysics and Heat Transfer Conference, Boston, Ma.: National Technical Information Service, Springfield, Va. (1974).

[3] H.S. Gurev, B.0. Seraphin, "Thin Film Reflectors of Improved Thermal Stability," in Proceedings of the High Power Laser Optical Components and Component Materials Meeting, Boulder, CO. (October, 1977) J.S. Harris & C.L.Strecker, Eds., Defense Adv. Re- search Projects Agncy, Arlington, Va.

[4] P. A. Temple, D.K. Burge, E.J. Ashley, Navel Weapons Center Annual Report No. 8 ,(1977)

(unpublished) p. 18.

[5] J.O.Perteus, C.W. Fountain, J.L.Jernigan, W.N. Faith, H.E.Bennett, "Pul sed-Laser Stress Phenomena on Highly Reflecting Metal and Alloy Surfaces," in Laser Induced Damage in Optical Materials : (1977) A.J. Glass & A.H. Guenther, Eds., N.B.S., Washington, D.C. (NBS Spec. Publ. 509) , pp. 204-214.

[6] B.0. Seraphin, J. Vac. Sci. Tech. 16_, (1979) p. 193.

[7] G.E.Carver, B. 0. Seraphin , Appl . Phys. Lett. 34 (1979) p. 279.

[8] G.E. Carver, "Thin Solid Films" 63, (1979) p. 169.

[9] W.M. Feist, S.R. Steele, D.W. Readey, "The Preparation of Films by Chemical Vapor Deposition," in Physics of Thin Films, Vol . 5 (1969) G. Hass & R.E. Thun, Eds., Academic

Press, New York, pp. 237-322.

[10] J. Anderson, G.W. Rubloff, P.J. Stiles, "Solid State Communications," U (1973) p. 480.

[11] H.E. Bennett, "Optical Engineering," 17_ (1978) p. 480.

[12] T. Raj, G.K. Wehner, "Composition Profiling of Solar Coating Materials," in proceedings, Second Annual Conf. on Absorber Surfaces for Solar Receivers, Boulder, Co (1979!

Solar Energy Research Inst., Golden, CO, pp. 163-170.

290
J.E. Nestell, Jr.,R.W. Christy, J. Vac. Sci. Techno! . 15, (1978) p. 366.

T.M.Donovan, B.O.Seraphin, J. Elec. Chem. Soc . 109, (1962) p. 877.

J.H. Weaver, D.W. Lynch, C.G. Olson, Phys. Rev. B 7, (1973) p. 4311.

zcz 500 7co sec :cc

~irc*f—j'. ^ in: 'u 5 --<--«

FIG. 1 . Relative reflectance, at 10 micrometers of CVD molybdenum films plotted against

the temperature of a 5-min. anneal.

35-

5 C 5 2C 25 2C

Durst; w of A/w«l at 75C*C (Minwtn)

FIG. 2. Relative reflectance, at 10 micrometers of CVD molybdenum films plotted against

the duration of anneal at 750 C.

-
ANNEAL OF CVD MOLYBDENUM

-i r

* •

90 -

Brtasui 3M *

" *** J \\

400 500 600 700 800 900 Tmpvohra o( Aimal for S mlfwtwPC)

tec

FIG. 3. Relative reflectance of Fig.l with

changes of oxygen and carbon content, crystal structure, and grain size during anneal

of a CVD Mo film.

ABS REFLECTANCE (%) 99

98

97-

96-

9b-

/ 2 ~o-°

1 CVD Mo

2 Bulk Mo

3 Mo sputtered in uhv

5 K> WAVELENGTH ( um I

FIG. 4. The absolute reflectance in the

wavelength range 3-15 micrometers of (curve

1) annealled CVD Mo:(curve 2) Super-smooth

bulk Mo: (curve 3) Mo sputtered in ultrahigh vacuum.

292
PHOTOACOUSTIC EXPERIMENTAL STUDIES ON AN AR COATED LASER WINDOW AND SOME RELATED THEORETICAL CALCULATIONS

Nils C. Fernelius

University of Dayton Research Institute

Dayton, Ohio 45469

Experimental photoacoustic spectroscopy (PAS) amplitude and phase

results on an AR coated ZnSe laser window were obtained at 10.6 ym as a function of chopping frequency. The PAS signal amplitude had an f"n frequency dependence with n = 1.04. The PAS phase angle varied by 20° between

1000 Hz and 50 Hz; 7° between 1000 Hz and 100 Hz. Theoretical calculations related to the situation were performed using the Bennett-Forman and Rosencwaig-Gersho PAS theories. The

Bennett-Forman theory agreed with the PAS amplitude results for

r

= ^surface^bulk — cm * ^

^° P nase cnan 9 e was obtained for r = 0.03 cm. The homogeneous sample Rosencwaig-Gersho theory did not fit either amplitude

or phase angle data, yielding n = 1.5 and a phase angle of 45.0° which varied

less than 0.04° over the frequency range measured.

Key words: Bulk optical absorption coefficient; optoacoustic spectroscopy;

photoacoustic spectroscopy; surface optical absorption coefficient;

ZnSe

1. Introduction

Photoacoustic spectroscopy (PAS) [1-9]'' is a recently revived technique which has

promise for obtaining optical absorption coefficients from certain classes of materials

which do not yield usable spectra by more conventional techniques. In addition to spectra

obtainable by varying wavelength, information is obtainable by varying chopping frequency.

In certain frequency ranges [10-11], the PAS amplitude changes from a f _n dependence with

n = 3/2 for predominantly bulk optical absorption to n = 1 for predominantly surface optical absorption. The PAS phase angle changes from a 45° phase lag for bulk absorption to

a 90° phase lag for surface absorption. While the phase angle change is more pronounced

in theory, due to the lack of knowledge of the actual phase zero one is forced to work

with phase angle differences or other schemes.

2. Experimental

A Coherent Radiation Model 40A C02 laser set at 10.6 ym and with power output in the

20 to 25 watt range was used as the light source. The light was chopped with a Princeton

Applied Research (PARC) Model 192 Variable Frequency Light Chopper. The signal from a GenRad 1961 -Electret Detector microphone was detected by a PARC 5204 Lock-In Analyzer. The

photoacoustic cell was arranged in a manner similar to that described elsewhere [11-12].

Thus the sample consisted of the entrance and exit windows of the cell.

The experimental results are given in figures 1 and 2. Laser rate calorimetry on the

two windows yielded an average for the samples of 0.00707 cm

-1

. The PAS amplitude

results in figure 1 fitted an f -n dependence with n = 1.04. The PAS phase angle results

Figures in brackets indicate the literature references at the end of this paper.

293
yielded a phase angle difference of 20° between 50 Hz and 1 kHz and a 7° difference between

100 Hz and 1 kHz.

3. Theoretical Calculations

The chopping frequency dependence of the PAS signal was calculated using the BennettForman [11], Rosencwaig-Gersho [10] and two-layer Rosencwaig-Gersho [13] theories. A more detailed account of this work can be found elsewhere [14]. Complex number FORTRAN IV com- puter programs were written to calculate PAS amplitude and phase for each theory. Representative thermal parameters which were inserted in these programs for ZnSe and other

materials were taken from laser calorimetry work. Varying values for the optical absorption

coefficient, in the range obtained from laser calorimetry experiments, were tried in the

program to bracket real world experiments. For the ZnSe substrate Bg = 0.008, 0.004 and

0.001 cm

-1 were used in all programs. The results of the Bennett-Forman and RosencwaigGersho theories will be presented here. The results of the two-layer Rosencwaig-Gersho

theory will be given in the companion paper.

3.1 Bennett-Forman Theory Calculations

The Bennett-Forman program calculates eq. (30) in ref. 11. After some calculations

had been made, it was realized that the signal calculated was a product of terms dependent

on the cell gas and dimensions times a term containing all the optical absorption coefficient dependence. This latter term can be written as

B B [r+(l-j)/ic

s /(2u) p

s Cs )] =es +e

B (l-2)/K

s /(2Wp

s C

s

) where B R is the bulk optical absorption coefficient in cm- 1

, oj/2tt is the chopping frequency,

r is the surface to bulk optical absorption ratio r = B

s /Bg, B

s is the sample surface

absorption (dimensionless) , k is the sample thermal conductivity, P the sample density and

C the sample heat capacity. Thus all PAS amplitude plots have their scale set by Bg. The

PAS phase angle values are solely determined by r and the sample thermal parameters.

A representative amplitude plot is given in figure 3. From this we see that an f~ n dependence with n = 1.00 for 50 Hz and higher is obtained for r > 0.05 cm. Since we measured n = 1.04 perhaps the lower limit should be extended a bTt. PAS phase angle plots

are given in figure 4. The curves near r = 0.05 cm appear to fit the data best. To avoid the problem of finding the zero phase setting, values of the phase angle difference

between fi and f2 with fj = 1000 and 500 Hz and f2 = 100 and 50 Hz are plotted in figure 5. No value of r yielded a 20° phase change between 50 Hz and 1000 Hz. Roark et at. [15]

noted in their work that the PAS phase angle changes more rapidly at low frequencies

than expected by theory and less rapidly at high frequencies. Perhaps a better number to consider is the phase angle difference between 100 Hz and 1000 Hz. Using A<f> = qoo Hz" ^IOOHz' t 'ie ex P er '' ITienta ^ value of 7° is obtained at r = 0.03 cm and 0.00015 cm. The value

near r = 0.03 cm agrees with the amplitude comparisons. Using this value and the average

laser calorimetry result of Beff

= 0.00707 cm

-1 in Bg = 3eff/( l+2r/£) where I is the

window thickness, we get Bg = 0.0066 cm" 1 and B

s = 0.0002. Thus about 93% of B ff

is due

to Bg. Yet the small amount of B

s is enough to make the PAS amplitude dependence to be near to that expected for surface absorption.

3.2 Rosencwaig-Gersho Theory Calculations

The Rosencwaig-Gersho program calculates the combination of eqs. (17) and (21) in ref. 10. The PAS amplitude results are shown in figure 6. All B value plots yielded

n = 1.5 in S = A f

n

. The PAS phase angle calculated between 1 Hz and 10 kHz varied

only between 45.04° and 45.00° in all cases. Thus a predominantly bulk-like absorption

is predicted which is in direct conflict with experimental results.

294
The Rosencwaig-Gersho theory assumes a completely homogenous sample. To take into account the effects of a surface layer representing the AR coating a two-layer RosencwaigGersho theory was developed [13]. Calculations based on it are discussed in the following

paper.

4. Conclusion

Photoacoustic chopping frequency variation studies on AR coated ZnSe windows at

10.6 ym yielded PAS signal amplitudes and phase angle information. The signal amplitude

had an f" n dependence with n = 1.04. The phase angle variation between 50 Hz and 1000 Hz was 20°; between 100 Hz and 1000 Hz was 7°.

Theoretical calculations using the Bennett-Forman theory indicated that r = $ surface

/

f?bulk = 0.03 cm seemed to fit the data best. Using this value with laser calorimetry

results of an average = 0.00707 cm

-1 for these samples, a value of BB

= 0.0066 cm -1

and 6

S = 0.0002 was obtained.

The homogenous sample Rosencwaig-Gersho theory predicted n = 1.5 for the amplitudes

and almost no phase angle changes. Thus a two-layer Rosencwaig-Gersho theory was developed

5. Acknowledgments

This work was supported by the Air Force Materials Laboratory, Wright-Patterson Air

Force Base, Ohio.

6. References

[1] Bell, A.G., Am. J. Science 3rd Series

20, 305 (1880); Phil. Mag. Series 5,

11, 510 (1881).

[2] Rosencwaig, A., Physics Today 28_ (9)

(Sept. 1975).

[3] Rosencwaig, A., Analytical Chemistry

47 (6) 592A (May 1975).

[4] Rosencwaig, A., Rev. Sci . Inst. 48,

1133 (1977).

[5] King, A. A. and Kirkbright, G.F.,

Laboratory Practice 25 (6) 377 (1976).

[6] Munroe, D.M. and Reichard, H.S., Am. Laboratory 9 (2) 119 (Feb 1977)

[7] Harshbarger, W.R. and Robin, M.B.,

Accts. Chem. Res. 6, 329 (1973).

[8] "Optoacoustic Spectroscopy and

Detection", edited by Yon-Han Pao (Academic Press, NY, 1977).

[9] Rosencwaig, A., p. 207ff in "Advances

in Electronics and Electron Physics,

Vol. 46" edited by L. Marton

(Academic Press, NY, 1978).

[10] Rosencwaig, A. and Gersho, A., J. Appl

Physics, 47, 64 (1976).

[11] Bennett, H.S. and Forman, R.A., J. Appl. Phys. 48, 1432 (1977) and

references therein.

[12] Fernelius, N.C., Appl. Optics, 18,

1784 (1979).

[13] Fernelius, N.C., J. Appl. Phys., 50,

(Oct. 1979).

[14] Fernelius, N.C., submitted to J. Appl.

Phys.

[15] Roark, J.C., Palmer, R.A., and

Hutchison, J.S., Chem. Phys. Lett. 6J3, 112 (1978).

295
SOLID ZnSe WINDOWS

COHERENT RADIATION MODEL 40A

COz LASER (3) 25 Wattt, 10 6fim PARC 192 LIGHT CHOPPER

ITHACO 167 PRE -AMP, PARC 5204 LOCK -IN FLAT MODE, INPUT » I, OUTPUT x I PRE - FILTER r 8 I sec

40 50 70 100 200 300 500 1000 CHOPPING FREQUENCY IN Hi

60

50

40

30

20

SOLID ZnSe WINDOWS COHERENT RADIATION MODEL 40A

C02 LASER® 25 Watts, 106 Mm

PARC 192 CHOPPER, ITHACO 167 PRE-AMP, PARC 5204 LOCK- IN FLAT MODE, INPUT x I.OUTPUTx I t - I sec

1

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1 1 1 1 1 1

- X

© 6 0 £> o QpO Qo^

1 1 I 1 1 I > 1 • 1 40 50 70 100 200 500 1000 CHOPPING FREQUENCY IN Hz 2000

Figure 1. Photoacoustic signal amplitude

versus chopping frequency on ZnSe laser

windows using a 10.6 ym CO2 laser source at

25 watts.

Figure 2. Photoacoustic phase angle versus chopping frequency on ZnSe laser windows

using a 10.6 ym C02 laser source at 25 watts.

296
BENNETT-FORMAN THEORY

I 2 3 45 7 10 2030 50 100 200 500 1000

CHOPPING FREQUENCY IN Hz

Figure 3. Theoretical PAS signal amplitude versus chopping frequency

for a 0.76 cm thick ZnSe sample with 3^ ^. = 0.008 cm

- "' for

various 6 surface /ebu -| k ratios using the Bennet-Forman theory.

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299
ROSENCWAIG-GERSHO THEORY

_ 3 0.76 cm THICK ZnSe - AIR BACKING

10 r 1—1—1 1 1 1 1 1

1—1

1—1—1 1 1 1

1

1

1—1 1—1—i nil.

CHOPPING FREQUENCY IN Hz

Figure 6. PAS signal amplitude versus chopping frequency calculated using the

Rosencwaig-Gersho theory on a 0.76 cm thick ZnSe sample with air backing.

m

300

HI
CALCULATIONS USING A TWO-LAYER

ROSENCWAIG-GERSHO PHOTOACOUSTIC SPECTROSCOPY

THEORY APPLIED TO AN ANTI -REFLECTIVE COATED LASER WINDOW

Nils C. Fernelius

University of Dayton Research Institute

Dayton, Ohio 45469

The ZnSe laser window studied experimentally had an AR coating con- sisting of a ZnS outer layer 0.32 jjm thick with a ThF_ inner layer 0.99 ym

thick. Laser calorimetry experiments at 10.6 ym on other samples indicated that ThF_ films have an appreciable optical absorption of "-10 cm

-1

.

Calculations were performed using a two-layer modification of the

Rosencwaig-Gersho photoacoustic spectroscopy theory. A 7

C phase change

between 1000 Hz and 100 Hz was obtained for a coating bulk absorption of

3

C = 2.7 cm

-1

. Plots of this result along with the best fit using the

Bennett-Forman theory in the previous abstract give similar amplitude

and phase results. Examples of results for various E c and coating thickness results are given.

Keywords: Bulk optical absorption coefficient; optoacoustic spectroscopy;

photoacoustic spectroscopy; surface optical absorption coefficient; ThF^;

ZnSe

1 . Introduction

Calculations using the Rosencwaig-Gersho [l] 1 photoacoustic spectroscopy (PAS) theory

assumed the window to be pure ZnSe. Experimental results on an AR coated window disagreed

significantly with the calculated results. As a first approximation to the real world,

the AR coating is treated as a single layer with optical and thermal properties different

from the substrate. A two-layer Rosencwaig-Gersho theory [2] was developed to handle this

case. Mandelis ei at. [3] have made a similar derivation. A schematic diagram of the

PAS cell is shown in figure 1.

The ZnSe laser windows studied experimentally had an AR coating consisting of a ZnS outer layer 0.32 urn thick with a ThFj, inner layer 0.99 un thick. Conventional laser

calorimetry experiments on samples with single coatings on ZnSe substrates indicate that

at 10.6 pm some ThF^ coatings have a bulk optical absorption coefficient, 6g ^ 10 cm

-1

, and some ZnS coatings have 5g ~- 15 cm

-1 [4]. If these values are applicable to our

samples, then the ThF. layer would have an absorbance of Bg£ "v 0.001 and the ZnS layer

would have Sgl ~- 0.0005. Laser rate calorimetry on our samples found an average

5 e ff of 0.00707 cm -1

. Assuming this is all bulk absorption in the substrate, then the

substrate absorbance would be Bg£ 0.007(0.76) "- 0.005. This shows that using the

quoted 5g values the absorbance of the coatings would be an appreciable fraction of

the substrate, at least 30%.

Figures in brackets indicate the literature references at the end of this paper.

301
While a three layer sample treatment would be the proper way to treat this problem,

the expressions for a two layer sample are already so complex that it was decided to use

a two layer model as a first approximation to the experimental situation. The equations

in the two-layer model are already about three times longer than the homogeneous

Rosencwaig-Gersho results which were rather complicated. The absorbance of the ThF4 layer

is about twice that of the ZnS layer. The thermal conductivity of ZnS is 0.167 W/cmK

which is about three times greater than the best estimate for that of ThF^. Thus as

a first approximation we neglect the absorbance of the ZnS layer and treat it as a thermal short. The layers are considerably thinner than the light wavelength so multiple

reflection and interference effects do not occur.

2. Calculations

A complex number FORTRAN IV computer program was written to calculate equations (48)

and (49) of reference 2. The outer layer was assumed to be ThF4 on a ZnSe substrate 0.76

cm thick. In order to determine how various parameters affected the results, a variety of

values were used. Calculations were made with substrate 8 = 0.008, 0.004 and 0.001 cm -1

. Results were obtained for thicknesses of the ThF4 layer of h = 0.1, 0.2, 0.5, 1 and 2 urn and for bulk optical absorption coefficients of 8C

= 20, 10, 5 and 1 cm

-1

.

Examples of how these parameters affect amplitude and phase angle trends are given in figures 2-5. In figure 2 the outer ThF^ layer is taken to be 1 urn thick. Plots are shown

for the coating optical absorption coefficient of 8

C = 1, 5, 10 and 20 cm

-1 for the three

substrate 8 values. When 8

c

= 1 cm -1 the differences in the substrate 8 values are readily

apparent at low chopping frequencies. As 8

c becomes larger, the coating absorption dominates the signal. The phase angle plots show almost bulk like absorption at low chopping

frequencies for 8

C = 1 cm- 1 and 6 = 0.008 cm

-1

. Again as 8C increases the phase lag

approaches 90° over a larger range of the frequencies. Figure 3 shows similar results for

h = 0.5 vim. In figure 4, h = 0.1 ym which is thin enough to exhibit considerable sub- strate effects even for 6

C

= 20 cm- 1

. Figure 5 keeps the coating optical absorption at

3

C = 5 cm- 1 and varies the coating thickness. In this case for h = 0.1 ym strong sub- strate effects are seen while for h = 2 ym the coating provides most of the signal. Thus

we see that for low 3C and small h values, the system resembles the substrate. For large

B

c and h, the coating provides most of the signal.

Since h = 1 ym corresponds to the ThF^ thickness in the coating, a number of calculations were made using that value and varying 3 . From these results a plot of

Acf> = ^ioOOHz " ^100Hz was mac' e versus See figure 6. The experimental value of A<|> = 7°

was obtained at 3

C

= 3 cm- 1 for 3 = 0.008 cm

-1

; 3

C

= 1.5 cm

-1 for 3 = 0.004 cm-1 and at

3

C

= 0.38 cm- 1 for 3 = 0.001 cm" 1

.

If we assume that there is no surface or interface absorption, i.e. that all the

optical absorption is due to the bulk ThF4 layer and the bulk ZnSe substrate, then we

can compare with the average 8 ^ = 0.00707 cnr 1 measured by laser rate calorimetry. From

the results in the previous paragraph we estimate the best fit to be 3 = 0.007 cm- 1 and

3

C

- 2.7 cm

-1

. Using these values we get 3 ff

= 3 + 28

c

h/2. = 0.0077 cm

-1 which is in good

agreement with experimental results. Since 3

c

h is not providing the dominant contribution in this region, we might take its value to be 2.7 cm

-1 and subtract that from the

measured 8 ^ to estimate 8 more precisely, i.e. 8 = 0.0071 - 0.0007 = 0.0064 cm- 1

.

m

302

HI
This compares with the best estimate of Bg for ZnSe using the Bennett-Forman theory in the

previous paper which gave Bg = 0.0066 cm

-1 .

Fitting the PAS amplitude dependence to f-n for h = 1 ym, B

c = 5 cm" 1 and

B

= 0.008 cm" 1 ' a value of n ~ 1.04 was obtained which is in exact agreement with the

experimental value. Further details of this work have been given elsewhere [5].

3. Conclusion

A two-layer sample modification was made of the Rosencwaig-Gersho photoacoustic

spectroscopy theory. Calculations using it were applied to the model of a ThF^ coating

on a ZnSe substrate made to approximate some AR coated windows measured experimentally

by laser rate calorimetry and by photoacoustic chopping frequency variations. In addition some calculations were made to see how changing certain parameters affected

the PAS amplitude and phase angles. For A<j> = i*-] QOOHz" *100Hz

= ^° ' t ^ie '3est va ^ ues seemed to be B = 2.7 cm" 1 and B

= 0.0064 cm

-1

. This gave an n^l.04 in the f -n amplitude

dependence. Plots of these values along with the experimental results and the best fits

using the Bennett-Forman theory in the previous paper are given in figures 7 and 8. The

arbitrary amplitude and phase angle in the theory were chosen to agree with experiment at

100 Hz. The amplitude plots, figure 7, seem to be in excellent agreement below 700 Hz. Above 700 Hz the tail of a Helmholtz resonance in the cell [6] around 2 kHz distorts the

experimental data. The two theories provide similar PAS phase angle results, as seen in figure 8, but both theories differ somewhat from the experimental results. These differences have also been noted by other researchers [7].

4. Acknowledgments

This work was supported by the Air Force Materials Laboratory, Wright-Patterson Air

Force Base, Ohio 45433.

5. References

[1] Rosencwaig, A. and Gersho, A., J. Appl. Physics, 47, 64 (1976).

[2] Fernelius, N.C., J. Appl. Phys., 50

(Oct. 1979).

[3] Mandelis, A., Teng, Y.C., and

Royce, B.S.A. , J. Appl . Phys.

[4] Miles, P. A., Hopson, J.E. and

Beauchamp, pp. 258-270 in Proa, of the

High Power Laser Optical Components

and Component Materials Meeting, 1977

,

editors J.S. Harris and C.L. Strecker.

[5] Fernelius, N.C., submitted to J. Appl.

Phys.

[6] Fernelius, N.C., Appl. Optics, 18 , 1784 (1979).

[7] Roark, J.C., Palmer, R.A. and Hutchison, Chem. Phys. Lett. 60, 112 (1978).

303
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on coated ZnSe windows.

310
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311

Robert P. Frees e and Kenneth J. Teegarden

University of Rochester, Institute of Optics

Rochester, New York 14627

Scanning photoacoustic spectroscopy techniques were used to monitor optical absorption and inhomogeneity in thin films and

substrates. Samples examined were 25 mm square by 2.5 mm thick

germanium substrates which were partially coated with a thin film

of germanium. Inter-sample thin film thicknesses varied from

.65 um to 1.23 pm. Raster scans made at 10.6 ym using a 75 mW

CO2 laser indicate that thin film and substrate inhomogeneities

as well as optical absorption may be nondestructively measured

at rates in excess of 400 msec per resolution point. Average

absorptance results are in good agreement with those obtained by

laser calorimetry and transmission techniques. Chopping frequency

studies indicate that some of the inhomogeneities are subsurface.

Interference effects detected photoacoustically are used to compute

the skew angle between the front and back surfaces. When the sample

is exposed to a 40 nsec (FWHM) CO2 laser pulse focused to a spot

size of 3.5 mm, gross laser damage is found to occur where the

interference effects were ohotoacoustically detected. An excellent

correlation is also found to exist between the location of structure

in the pre-damage photoacoustic mapping of the coated optic, and

the location of isolated, low damage threshold "hot spots" found

after the high power pulse irradiation.

Key -'tC r cs: Absorption; Ge; laser damage; optoacoustic spectroscopy;

photoacoustic spectroscopy; thin filrs.

1. Introduction

Studies conducted on thin films indicate that, for long (>nsec) pulses, laser

induced damage is extrinsic in nature and may be attributed to cracks, pores, defects,

inpurities, or other localized inhomogeneities [1,2]1. However, the origin, identity,

and control of these inhomogeneities is not well understood. One major hindrance

is the lack of suitable nondestructive diagnostics which could detect localized thin

film and substrate inhomogeneities and aid in their identification.

Recently, photoacoustic spectroscopy (PAS) has been shown to be a valuable tool

in the investigation of highly transparent solids :3] . Kerr [4] and others [5] have

demonstrated that absorptances on the order of 10~* to 10"5 could be detected using PAS. Hong, et. al . [6], have recently shown that both surface and subsurface structure could

be monitored with photoacoustic spectroscopy. Others [7] have used PAS to distinguish

surface from bulk absorption in laser window materials.

Work supported by the U. S. Department of Energy under Grant EY-76-S-02-4056*000

Figures in brackets indicate the literature references at the end of this paper.

- 1 -
In this work, we apply photoacoustic techniques to the study of absorption,

inhomogeneity, and laser damage in thin films and substrates. Two dimensional raster scans were obtained from several coated germanium flats and indicate that thin film and substrate

inhomogeneities as well as absorption and interference effects may be nondestructi vely

monitored using this method. Laser damage threshold measurements were also performed on these samples using a 40 nsec (FWHM) 3.5 mm diameter CO2 laser. Results to date suggest

that a strong relationship may exist between the shape of the photoacoustic signal and

the local damage threshold.

2. Generation of the Photoacoustic Effect

The photoacoustic effect refers to the generation of sound through the interaction of

light with matter. Observation of the effect is typically accomplished by placing a sample in a specially designed (acoustically resonant or non-resonant) enclosed cell and

illuminating the sample with amplitude modulated radiation. Light which is absorbed by the

sample and converted into heat will create an intermittent flow of heat into the

surrounding buffer gas, normally air. The resultant temperature fluctuation of the gas gives rise to a pressure fluctuation, i.e. the photoacoustic signal, whose phase and

amplitude may be detected with a sensitive microphone and lock-in amplifier. Since the

acoustic signal is directly related to the amount of heat transferred to the buffer gas, detection of the photoacoustic effect allows one to nondestructi vely extract information

regarding the physical, thermal, and optical properties of the sample.

An alternative method of photoacoustic detection frequently employed involves the

measurement of the elastic waves generated in the sample itself. In this method, a transducer is bonded directly to the sample, thus eliminating the need for a photoacoustic

cell

Several quantitative theoretical analyses of the photoacoustic effect have been

recently published [8,9,10]. Most models treat the problem as a one dimensional heat flow

analysis in the various sample, gas, and window regions of the cell. In general, these

treatments are lengthy and will not be reproduced here. In the interest of giving the

reader physical insight into the capabilities of the photoacoustic method, however, we present some of the more important results obtained from these investigations.

For a highly transparent (g£«l), thermally thick (d<<2.) sample, the photoacoustic

signal ,S, is related to the following parameters: 6, the optical absorption coefficient

(cm-1

); d, the thermal diffusion length (cm); P, the average incident light power (W);

and V, the irradiated volume of the sample. z is the physical thickness of the sample (cm).

In the case under consideration, it should be noted that, neglecting nonlinear absorption

and saturation effects, S scales linearly with both e and P. Also, V^Axd , where A is the

cross-sectional area of the incident light beam (cm2). The thermal diffusion length is defined by d=( K/-rrfpC )"* , where K is the thermal conductivity of the sample ( J/cm sec °C),

f is the modulation frequency (Hz), p is the physical density of the sample (g/cm3),and

C is the specific heat (J/g °C). It should be noted that d essentially defines the

sampling depth of the photoacoustic signal and may be varied by changing the modulation

frequency of the light source.

These results may be exploited to nondestructi vely study both optical absorption and

inhomogeneities in bulk and thin film materials. For example, if one mounted a perfectly

homogeneous sample in an ideal photoacoustic cell (nonabsorbing gas; nonabsorbing,

homogeneous window; nonabsorbing cell walls) and recorded the photoacoustic signal as a function of position, one would expect the magnitude of the signal to be everywhere

constant. However, if the sample contained any spatially varying inhomogeneities within the

diffusion length of the material, the signal would be altered. In particular, if the

inhomogeneity were a scratch, void, pore, or other refractive index variation, the

irradiated volume and possibly other photoacoustic parameters could be significantly

314
changed. Absorption or impurity sites would affect the amount of light absorbed.

Various other physical, thermal, or optical defects (such as density) may also modify

an important photoacoustic parameter. The net effect of any one or combination of these

inhomogeneities would be, in general, to alter both the phase and magnitude of the

photoacoustic signal. The spatial location of the defect could be found using focused

beam scanning techniques. Its depth penetration might be found by changing the modulation

frequency (and thus changing d). Impurities could be distinguished from pores using

spectral analysis. Also, thin film effects might be distinguished from bulk effects

by comparing the thin film photoacoustic signal to a substrate reference, or by scanning

the substrate before and after thin film deposition.

3. Samples

The samples investigated in this study consisted of 25 mm square by 2.5 mm thick

optical grade germanium sugstrates obtained from Eagle Pitcher Industries. The samples

were polished flat to within a/80 at 10.6 pm over the whole surface. The substrates

were then cleaned with acetone and ethanol , and partially coated with a thin film of

germanium at the Institute of Optics. Thin film deposition was accomplished using a resistance source in a conventional vacuum chamber. Thin film thicknesses were monitored

during deposition using a calibrated Inficon crystal monitor. Sample to sample coatings

varied from .65 ;_m to 1.23 urn. Care was taken to preserve clean and dust free surfaces at

all times. However, it should be mentioned that polishing, cleaning, and thin film

deposition procedures have not been necessarily optimized for these studies, and have

been included for the sake of completeness. One should also note that the germanium on germanium samples were chosen primarily for demonstration and theoretical purposes.

Nevertheless, these samples may be thought of as representative of substrate and thin

film laser window materials encountered in infrared applications.

4. Photoacoustic Scanning Apparatus

A block diagram of the photoacoustic scanning apparatus is shown in figure 1. The light source consisted of an axial flow COg laser operating in a single longitudinal

mode (SLM) at 10.6 yam capable of providing 250 mW in the TEMqo mode. Amplitude

modulation of the source was provided by a variable speed chopper, whose frequency

could be varied from 10 Hz to 2 KHz. After passing through a NaCl beamsplitter, the

beam was focused onto the mounted photoacoustic sample with the aid of an f/2 19 cm

focal length Al coated front surface mirror. Registration of the CO2 laser beam was accomplished using a collinear HeNe alignment laser. CO2 laser power fluctuations were

monitored with a Molectron P-3 pyroelectric detector.

A diagram of the photoacoustic cell, which was optimized for these experiments, is shown in figure 2. The interior dimensions of the cell were 12.7 mm in diameter by

1.4 mm thick. The microphone chamber and cell were connected by an 11 mm by .9 mm diameter channel. The total enclosed volume was approximately 240 mm3. The cell itself

was formed by clamping a Harshaw polycrystal 1 ine NaCl window and the sample against

gaskets. Alternatively, the window and sample were sometimes sealed onto the cell with

optical wax when a greater degree of permanence was desired. Interference effects

between the window and sample were avoided by a 5° angle of tilt machined into the cell.

Spurious absorption effects from the cell reported by others [11] were reduced by

minimizing the cell wall thickness and by choosing the highly reflective metal

aluminum for the cell material. Absorption effects from the NaCl were minimized by

placing a brand new window on the cell immediately prior to each scan. Also, both the

NaCl window and the cell were kept a few degrees above ambient room temperature to reduce the possibility of hygroscopic degradation. Optimization of the photoacoustic

effect was accomplished mainly by minimizing the total cell volume. Air was normally

used as a buffer gas

315
The photoacoustic signal was detected by means of a B&K 4144 2.54 cm condenser

microphone. A FET preamplifier provided an electrical gain of 10, thus establishing

an overall sensitivity of 50 mV/pbar. The electrical signal was fed into an Ithaco

Dynatrac 391A lock-in amplifier capable of tracking reference frequency fluctuations.

The photoacoustic signal was normalized to laser power fluctuations by means of the

Molectron pyroelectric detector, reference Dynatrac 391 lock-in amplifier, and ratiometer.

The output of the ratiometer was displayed on a Honeywell chart recorder. The entire

photoacoustic detection system was calibrated using a carbon black opaque sample and

displayed a responsivity of 110 mV/W at a modulation frequency of 250 Hz.

Two dimensional raster scans were made using a synchronous motor driven scanner mounted on an Aerotech microprocesser controlled stepper motor translator. The

photoacoustic cell was mounted on top of both scanners so that the sample moved while

the laser beam remained stationary. In this manner, the angle of incidence of the laser

upon the sample could be monitored and kept constant during the complete scan. Finally,

both scanners and photoacoustic cell were placed on another Aerotech translator so that

the mounted photoacoustic sample could be translated 5 cm to the high power laser damage

apparatus without loss of registration. Scanning rates, which were determined mainly by

signal to noise considerations, varied from 30 msec to 3 sec per resolution point.

5. CO2 Laser Damage Facility

A block diagram of the high power laser damage apparatus is shown in figure 3. A Tachisto TAC II Model 215A TEA laser, capable of suppling 500 mJ of 10.6 pm radiation in

a 40 nsec (FWHM) gain switched TEM00 pulse was employed as the high power laser source. The output of the TEA laser was monitored using several single shot diagnostics. The

total pulse energy was measured by directing the Fresnel rear surface reflection of

a wedged ZnSe beamsplitter into a calibrated Scientech Model 380-102 calorimeter.

Alignment of the front surface reflection into a Rofin 7400 photon drag detecter

(600 psec risetime) enabled the detection of the temporal profile. A permanent record of

this profile could be made by photographing the trace off a Tektronix 7704A 250 Mhz

oscilloscope, which was connected to the photon drag detector by means of a 50 ft coaxial cable. Spatial profile of the beam was checked using either green liquid

crystals, type 107 Poloroid film, and/or Thermofax paper. While the temporal profile

of the laser varied somewhat from pulse to pulse, the total energy per pulse was found

to be reproducible to + 3%. The e"^ points of the TEM0 o gaussian were reproducible to

+ 5% in the short term and agreed well with calculated gaussian beam raytraces.

Alignment and registration of the CO2 beam were accomplished by means of a collinear

HeNe laser beam. Germanium samples reported in this paper were generally tested while

still mounted in the photoacoustic cell. The irradiance of the high power CO2 laser

was varied by altering the distance between the sample and an f/10 760 mm focal length

gold coated front surface mirror. The focused beam diameters (e _ 2 points) in the tests reported here were 3.5 +.2 mm.

6. Experimental Results

6.1 Photoacoustic Raster Scans

Two dimensional raster scans have been made on four germanium on germanium samples. Two representative rasters from these scans are shown in figure 4 and figure 5, where

the average incident CO2 laser power was 75 mW, the modulation frequency was 250 Hz,

the thermal diffusion length was 213 pm, the lock-in time constant was 400 msec, and the

thin film thickness was 1.23 pm. In figure 4, the scanning laser beam was defocussed by

.5mm, while figure 5 shows the same scan with the beam brought to a 130 pm focus on the

front surface of the sample. Raster scans such as those shown in figures 4 and 5 were

highly reproducible providing one scanned along the identical path.

316
Comparison of the photoacoustic signals obtained from the thin film and substrate

sides of the sample reveals that, on the average, the thin film signal is far greater than

that of the substrate. Similiar results have been observed in all samples tested thus far.

The increase in the average signal may be attributed to the increased absorption

coefficient of the thin film in relationship with that of the bulk material. Indeed,

knowing the response of the photoacoustic cell and neglecting any window or cell

absorptions, it is possible to measure the thin film and substrate absorptances (within

the diffusion length of the material) from the expressions:

\ tf = Atfs - As (6.1-1)

Atfs = s tfs / RoP (1-Rtfs) t 6 - 1 " 2 )

A

s = S

s / R0

P (1-RS ) (6.1-3)

where A is the absorptance, R is the Fresnel reflection coefficient for the first

surface, R

0 is the responsivity of the photoacoustic cell, and the subscripts

tf,tfs,and s stand for thin film alone, thin film and substrate, and substrate alone,

respectively. A comparison of the photoacoustically determined absorptances and the

values measured prior to the scan using calorimetric and transmission methods is shown in Table 1.

Table 1,

Location

Thin film "best"

Thin film "ave" Substrate alone

Absorptance Measurements

PAS Calorimetry / Transmission

.010 + .001

.013 + .001

.007 + .001

.013 + .001

.008 + .002

The good agreement between the photoacoustic and measured absorptances provides

support for our initial assumption that cell wall and window contributions could be

neglected. Uncertainty in the average photoacoustic measurements is due mainly to uncertainty in the cell responsivity, R0

. It should be mentioned that the ambient

noise signal in this particular set of raster scans was + 1 yV yielding a signal to noise ratio of approximately 50:1. We also found that the photoacoustic signal was a linear function of power from 30 mW to 120 mW. This indicates that our initial assumption

neglecting nonlinear absorption was also justified.

Comparison of figures 4 and 5 demonstrate the improvement in resolution as the

scanning laser spot size is reduced. Some of the photoacoustic peaks could be related

to obvious surface flaws or scratches. However, many of the peaks occured in locations

where no surface feature could be detected, even under the high magnification of a Nomarski or optical microscope. Such features may be related to localized inhomogeneities

in the thin film or substrate. Indeed, chopping frequency studies performed on these

features reveals that some of the inhomogeneities may be subsurface, as is shewn in

figure 6. For the top set of data points in figure 6 the photoacoustic signal varied as

f- - 69. This frequency dependence is similiar to results obtained by others [12] for

transparent media using a focused beam. In the lower set of data points, the signal

varied as f--73 q

- n almost an identical manner to the top set of points. However, at a frequency of approximately 400 Hz, where d=180 \im, the signal suddenly decreased. It remained low for frequencies greater than 400 Hz. We interpret the latter result as

an indication of a subsurface inhomogeneity. In the first case, the defect was surface

related and must have been at a depth of less than approximately 150 pm. In the second

case, the photoacoustic signal dropped to a very low level for diffusion depths less

than 180 ym, indicating that the inhomogeneity must have been located at a depth greater

317
r

than 180 ym. It should be noted that the vast majority of inhomogeneities investigated

turned out to be of the surface variety. It should also be mentioned that, as a whole,

there were far more photoacoustic features on the thin film side of the sample. Since

one would expect roughly the same number of substrate defects regardless of position, it seems likely that the additional structure is due to thin film (or thin film interface)

inhomogeneities.

The raster lines were scanned in a direction perpendicular to the thin filmsubstrate boundary line. A series of points obtained from these mappings was used

to generate two scans in a direction parellel to the thin film-substrate boundary line.

Results obtained in this manner for sample #4 are shown in figure 7. Inspection of this

graph shows two periodic functions, one on the thin film side and one on the substrate

side. While the amplitude of the thin film side is generally higher than its substrate

counterpart, the two plots appear to be in phase with one another. Although this result

was initially surprising, we believe that it may be attributed to interference effects

between the internal incident beam and the Fresnel reflections off of the rear surface.

The resultant constructive or destructive electric fields near the front surface give rise

to increased (or decreased) absorption, which is detected photoacoustically. A diagram

of the physical situation is presented in figure 8. The distance between successive

maxima or minima is dictated by the skew angle between the front and rear surfaces, and is

a result of the optical thickness of the sample increasing by A/2. Using the average

value of the spatial frequency, we find the photoacoustically measured angle of wedge to

be .039° + .003°. We also can predict that the wedge should only be in one direction,

since the periodic fluctuations were detected in this direction only. A measured wedge

angle of .042° + .003° using retroreflection techniques confirmed both these results.

It is also interesting to note that, using this analysis, the two periodic functions

should be approximately in phase, since the additional optical thickness provided by the

thin film is within 4% of exactly A/2 at 10.6 ym.

6.2 Laser Damage Results

:::

Sample #4 was left mounted in the photoacoustic cell and translated a known amount over to the laser damage apparatus using the Aerotech translator. The thin film side

was then exposed to a CO2 laser pulse containing a total of 183 mJ in a 3.5 mm diameter

(e-2) beam. Plasma formation was noted during the irradiation. A sketch of the resultant

damage is shown in figure 9. Although the laser was definately in the TEM00 mode, two large (2.7mm by .8mm) elliptical damage sites were found. At these locations, the thin fi was often blown completely away or had partially seperated from the substrate. The positi

of these gross damage sites coincided within half a raster width of the interference

maxima previously discussed, as is diagrammed in figure 10. Similar results were obtaine

in additional damage tests at different locations. This seems to conclusively indicate

that the gross damage was mainly a result of the increased local electric field near the

front surface. No laser damage was found to occur on the exit surface of the sample.

In addition to the gross damage, we found a number of localized isolated damage sites

whose immediate neighborhood seemed to be unaffected by the laser pulse. The sizes of the "hot spots" varied from about 5 ym to approximately 65 ym (counting the cracks sometimes

found in the thin film surrounding the site). In some cases, the surrounding thin film

appeared to be left intact, while in other cases, the film was cracked and slightly

raised from the substrate. The depths of the pits formed at these locations varied from

1 ym to 15 ym.

While the origin of these sites in not clear, an excellent correlation was found to exist between the position of these "hot spots" and the location of structure or peaks

in the pre-pulse photoacoustic mapping of the sample. Of the 29 isolated damage sites

greater than 5 ym in size, all were found to correlate with photoacoustic structure

to within the spot size of the scanning laser. An excellent example is provided by raster scan #30, shown in figure 11, which represents an area located roughly 300 ym from

the edge of the nearest gross damage site. All of the thin film peaks have been labeled

318
Iphabetically and a physical description of any surface features seen at these locations

ider a Nomarski microscope is given. Nomarski micrographs of some of the corresponding

jmage sites are presented in figure 12 along with a micrograph of part of the thin film

n'ch was not irradiated with the high power laser. The latter micrograph may be taken

being representative of the thin film surface before damage, and shows scratches

n'ch may have been associated with the polishing process.

It should be noted that while an excellent correlation is obtained between the

>cation of photoacoustic peaks and isolated damage sites, there appears to be little

jlationship between the size of the damage site and the height of the peak. Also,

lere are more photoacoustic peaks than isolated damage sites, possibly indicating that

itential damage sites may not have received sufficient irradiation, or that a localized

feet which may have been detected photoacoustically has a threshold comparable to that

the material as a whole.

7. Conclusions

Scanning photoacoustic spectroscopy techniques have been used to monitor optical

•sorption and inhomogeneity in thin films and substrates at 10.6 ym. Average absorptance

suits obtained photoacoustically are in good agreement with those obtained by

lorimetric and transmission methods. Chopping frequency studies indicate that some

the photoacoustically detected inhomogeneities may be subsurface. Interference

fects have also been detected with this method. An excellent correlation has been

und to exist between the location of structure in the pre-damage photoacoustic mapping

the coated germanium sample and the location of isolated low damage threshold "hot

ots" found after the coated optic was exposed to a 40 nsec CO2 laser pulse.

8. Acknowledgements

The authors would like to gratefully acknowledge the assistance of Herb Graff for

lishing the samples and measuring their wedge angles, David Lees for coating the

mples, and Luigi Colombo and Pam Freese for assistance with the Nomarski micrographs.

5. References

Bloembergen, N., Role of cracks , pores

,

and absorbing inclusions on laser

induced damage thresholds at surfaces

of transparent dielectrics,

Appl. Opt. 12, 661 (1973).

Guenther, A.H. and Glass, A.J., Laser induced damage in optical

materials: tenth ASTM symposium,

Appl. Opt. 18, 2112 (1979).

Hordvik, A. and Schlossberg, H.,

Photoacoustic technique for determining optical absorption coefficents

in solids, App. Opt. 16, 101 (1977).

Kerr, E.L.,The alphaphone-a method for

measuring thin film absorption at laser wavelengths, Appl. Opt. _12, 2520 (1973).

[5]

[6]

rast

McDavid, J.M., Lee, K.L., Yee, S.S,

and Afromowitz, M.A. , Photoacoustic

determination of the optical

absorptance of highly transparent

solids, J. App. Phys. 49, 6112 (1978),

Wong, Y.H., Thomas, R.L.,and

Hawkins, G.F., Surface and subsurface

structure of solids by laser

photoacoustic spectroscopy,

Appl. Phys. Lett. 32, 538 (1978).

Hcrdvik, A. and Skolnik, L., Photoacoustic measurements of surface and

bulk absorption in HF/DF laser

window materials, Appl. Opt. 16,

2919 (1977).

[8]. Rosencwaig, A., Theoretical aspects

of photoacoustic spectroscopy,

J. Appl. Phys. 49, 2905 (1978).

[7]

319
[ 9]. Bennett, H.S. and Forman, R.A.,

Absorption coefficients of highly

transparent solids: photoacoustic

theory for cylindrical con- figurations, Appl . Opt. 15_, 1313,

(1976).

[10]. McDonald, F.A., Photoacoustic

determination of small optical

absorption coefficients:

extended theory, Appl. Opt. 18_, 1363 (1979).

[11]. McClelland, J.F. and Kniseley, R.N. Photoacoustic spectroscopy with

condensed samples, Appl. Opt. 15_, 2658 (1976).

[12]. Fernelius, N., The use of photoacoustic spectroscopy to study

laser window materials and thin

film coatings, Proc. High Power

Laser Optical Comp. and Comp.

Matls, NBS , Boulder, Colorado

(1977).

320
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THIN FILM ABSORPTION 8 INHOMOGENEITIES

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SUBSTRATE

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POSITION (mm)

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324
THIN FILM ABSORPTION 8 INHOMOGENEITIES

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325
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INHOMOGENEITY - SUBSURFACE

INHOMOGENEITY

"l I 1 Ml 1 II 1 1 1 III 50 100 500 1000

CHOPPING FREQUENCY Hz

Figure 6. Photoacoustic signal vrs. chopping frequency for surface and

subsurface inhomogeneities

326

328
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331
Figure 12. Nomarski micrographs of some of the isolated damage sites:

a). PAS peak c; b) PAS peak g; c) PAS peak k; d) non-irradiated thin film

surface.

332
EXAMINATION OF LASER DAMAGE SITES OF TRANSPARENT SURFACES AND

FILMS USING TOTAL INTERNAL REFLECTION MICROSCOPY*

P. A. Temple

Michelson Laboratory, Physics Division

Naval Weapons Center, China Lake, California 93555

This paper describes total internal reflection microscopy (TIRM), a microscopic inspection technique useful in examining the laser-damaged surfaces of

transparent samples. In this technique, the surface is illuminated from within

the sample with a well-coll imated polarized laser beam at an angle of incidence

just greater than the critical angle. Since total reflection occurs, the

illuminated region appears to be dark when viewed from outside the sample

except for scatter caused by surface defects such as laser damage sites. We will describe two instruments used for TIRM inspection of samples. Finally,

various damaged regions will be shown as they appear under TIRM inspection and

under Nomarski microscopy. It will be seen that the two techniques are complementary, and that, on highly polished surfaces, TIRM shows somewhat more detail

than does Nomarski microscopy. All of the damage sites shown will be the

result of 1.06-um, 1-nsec irradiation of fused silica or BK-7 surfaces.

Key words: Fused silica; laser damage; optical microscopy; total internal reflection.

This paper describes a method of illuminating the region at and near the surface of a

j transparent sample or of a film on a transparent substrate [1-3] 1

. The technique, total

internal reflection microscopy (TIRM), has been useful in the inspection of polished glass

surfaces and thin films both before and after laser damage has occurred. Also described

are two microscopic setups, one of which makes use of a Zeiss Nomarski microscope for

sequential viewing of the sample surface by either TIRM or Nomarski microscopy. The other

is an in situ microscope for TIRM viewing of a part before and after laser damage testing

has occurred. Finally, several photos will show the appearance of laser damaged surfaces

as seen in TIRM and Nomarski microscopy.

The physical setup is shown in figure 1. In this technique the surface is illuminated

from within the sample with a wel 1 -col 1 imated polarized laser beam at an angle of incidence

equal to or slightly greater than the critical angle [4] 2

. In this configuration, since

total reflection occurs, the only light which reaches the microscope objective is light

which has been scattered into the z-direction by surface irregularities or inclusions; the

field of view is dark in the absence of these perturbing sites.

In the study discussed in this paper, only s-polarized light was used. In this case

there is an enhancement of the electric field at the surface for angles of incidence near

the critical angle. Figure 2 shows the value of <E 2>/<ES> at the surface for internal

illumination, as used in TIRM, and for external illumination, as used in the usual darkfield microscopy. As can be seen, at the critical angle nearly an order of magnitude

enhancement exists for internal illumination over external illumination.

Work supported by Navy Independent Research and Department of Energy funds.

1 Figures in brackets indicate the literature references at the end of this paper.

2 This configuration has been used by Parks' group (Ref. [4]). However, they have primarily monitored the transmitted beam intensity for changes induced by a disrupted surface

caused by laser-induced damage. Their observation of the externally scattered light

appears to have been fairly casual.

Introduction

TIRM Setup

333
The illuminated area is viewed from above with a microscope. Only that light which is scattered into the small cone about the normal or z-direction is collected by the microscope

objective. Two microscopes have been used in the study reported in this paper. The first,

shown in figure 3, is a Zeiss Nomarski microscope. With the Nomarski illuminator turned

off and the TIRM laser turned on, it is possible to use the Zeiss microscope as a simple

magnifier. This is true even though a Nomarski objective is employed. Using this setup it

is possible to view a surface under Nomarski conditions, under TIRM conditions, or under

both types of illumination simultaneously. Figure 4 is such a set of photographs. Figure

4 shows a laser-damaged BK-7 entrance surface which had previously been mechanically

polished. The laser used to illuminate the TIRM photographs was a 2-watt argon-ion laser.

It can be seen that many features associated with mechanical damage to the surface from the

polishing process are visible in the TIRM photographs but not in the Nomarski view. The

laser damage sites are most easily seen as shallow pits in the Nomarski photograph. The

Nomarski and TIRM view shows that there is a small sharp feature at the bottom of these

shallow pits.

Figure 4 is an example of the complementary nature of these two microscopies. Nomarski

microscopy shows slope changes very well. The pits seen in figure 4 are, in fact, very

subtle features. There must be enough surface area involved, however, to show a contrasting

region against the bright background. On the other hand, the features seen in the TIRM views may be very small in extent. The only requirement is that enough light is scattered

to be seen against the dark background. Other measurements have verified that features a few hundred angstroms in extent are visible in TIRM.

Figures 5-7 show three views of a second TIRM microscope. This microscope is built

around a Leitz laboratory instrument and employs a 3-mW HeNe laser for illumination. This

low illumination power necessitated quite long exposure times but otherwise was satisfactory.

Figures 6 and 7 show the instrument as used for microscopic viewing of the sample and as used for in situ damage testing. In the latter case, all the optics are removed from the

beam path and the high power beam was incident from either the left or right for exit or entrance surface testing of the sample. With the microscope axis coaxial with the high

power laser axis, it was possible to view a surface before and after damage testing.

Figure 5 is a detailed view of the sample stage of the microscope. Unlike the setup

shown in figure 1, here the HeNe laser beam was coupled into the sample through a small

prism. After the beam entered the sample, it was totally reflected off the bottom of the

sample and then off the top of the sample, where it was viewed with the microscope. While

provision existed for coupling the beam out of the sample with a second prism, it was not found to be necessary. The setup shown in figure 5, then, had the advantage of a clear

aperture through the sample to allow for in situ laser damage testing. The sample was mounted in such a way that the coupling prism was held stationary while the sample could be

moved for viewing and testing of various regions on the sample surface.

Observations

A companion paper given at this conference [3] shows several TIRM photographs of C02

- laser polished and laser-damaged surfaces. It is suggested that this paper be consulted

for the examples presented there. In addition to these photos, several other examples will

be shown in the remainder of this paper.

Figure 8 shows two TIRM views of the same region of a fused silica sample taken before

and after it was damaged by a laser pulse. This surface was prepared by ion milling a previously mechanically polished surface. The "undamaged" picture was taken with slightly

lower magnification. These photos were not taken with the apparatus shown in figures 5-7.

Figure 8 shows that all the damage occurred along the previously present scratches. It is characteristic of ion-milled surfaces for damage to occur along defect features made visible

by the ion-milling process. Figure 9 is a pair of views of the same sample shown in figure

8. Shown in figure 9 is the scratch seen in figure 8, but in addition, a damage pit is shown. This pitted region was not photographed prior to damage, and it is not known if any

features were visible at that site before damage occurred. The Nomarski view clearly shows

the decoration brought out by ion milling along the scratch. However, it is not clear from

the Nomarski photograph where subsequent laser damage has occurred. The increased scatter

at damage sites, as seen in the TIRM view in this figure and in figure 8, makes it clear

334
where damage has in fact occurred. The single damage site in the upper right-hand portion

of the photograph is unusual in that a large halo of spots occurred which is not evident in the Nomarski view. There are, however, two pits visible at this site in the Nomarski view

of this surface.

Figure 10 shows two views of a BK-7 surface, before and after laser damage. These

photos were taken using the microscope shown in figures 5-7. The exposure times are one minute in the "before" view and one-half second in the "after" view. This was an exit

surface damage site, and the damage pits are very effective scatter sites in the case of

exit surface damage. This figure is representative of several photographs taken in an effort to search for a correlation between features seen before damage and subsequent

damage sites.

Figure 11 consists of two views of a 0. 35-ym- thick Si0 2 film on a fused silica substrate. This film had a multitude of scattering sites more or less uniformly distributed

throughout. This can be seen in the "before" view. The "after" view was taken with

somewhat less exposure so that the bright spots in this view are damage sites, while the

dim spots correspond to the sites seen in the "before" view. A careful inspection of these

two photos shows that there is little correlation between the scattering sites seen before

damage and the location of subsequent damage pits.

Conclusions

In this paper we have described a sensitive technique for microscopic viewing of

surfaces before and after laser damage has occurred. This technique, called total internal

reflection microscopy (TIRM), is complementary to Nomarski microscopy. Two microscopes

employing two different illumination beam coupling techniques were shown. The first

employed a Zeiss Nomarski microscope and allowed simultaneous viewing of the sample by

either Nomarski or TIRM illumination. The second microscope described was an in situ

microscope which allows viewing of a sample surface before and immediately after laserdamage testing. Finally, several photographs were shown which illustrated the nature of

the microscopy made available by the TIRM technique.

References

[1] Temple, P. A., "An Improved Dark-Field- [3] Temple, P. A., David Milam, and W. Howard

Like Surface Inspection Technique Using Lowdermilk, "C02 -Laser Polishing of Fused

Total Internal Reflection," presented Silica Surfaces for Increased Laser Damat Conference on Optics '79, 23-25 May age Resistance at 1.06 urn," this confer1979, Los Alamos, New Mexico. ence.

[2] Temple, P. A., "Total Internal Reflec- [4] Alyassini, N., and J. H. Parks, NBS Spec,

tion Microscopy: A Surface Inspection Publ . 435, 284 (1975).

Technique," submitted to Appl . Opt.

Figures

335
A

MICROSCOPE

V7

MATCHING FLUID

BABINET—SOLEIL

Figure 1. The physical setup used to illuminate transparent surfaces for microscopic examination. Not shown is a microscope slide manipulator used to grasp the

sample and to slide it about on the oilcovered prism face. The laser used was an argon-ion laser.

Figure 2. A fixture designed to implement

the setup shown in figure 1. This fixture

is shown mounted on the sample stage of a Nomarski microscope. The microscope slide

manipulator, which is not shown in figure

1, can be seen grasping a 1-1/2-inchdiameter, 1/4-inch-thick sample.

336
ANGLE OF INCIDENCE (DEGREES) Figure 3. The value of <E 2 >/<E 2

> for

internal and external illumination using s- polarized light for various angles of

incidence. The maximum value for

<E 2 >/<E 2

> is 4 and occurs at the critical

angle for internal illumination.

NOMARSKI NOMARSKI + TRIM TIRM

I I IOOmiti Figure 4. Three photos of the same region

of a laser-damaged, conventionally polished

piece of fused silica. The view on the

left is a Nomarski photo showing several

pits caused by a 15-J/cm2

, 1.06-ym wavelength, 1-nsec pulse length exposure. The

photo on the right is a TIRM view. Since

the part is mechanically polished, it has

many features visible in this view which

are not related to the damage shown in the

left photo. The center view shows the

Nomarski and TIRM illuminations simultaneously. It can be seen here that few of

the TIRM features are actually identified

with damage sites. Most of these features are mechanical damage due to the polishing

process.

337
Figure 5. The sample holder and beam

coupling prism for the in situ TIRM microscopy. In the center of the figure can be seen the sample with the coupling prism

which is held in place with a small metal

clip. The prism is optically attached to the sample by matching fluid. The sample

is movable below the prism for viewing

various regions of the sample.

Figure 6. The TIRM in situ microscope as configured for TIRM viewing and photography.

In this figure can be seen the 3-mW HeNe

laser, the microscope viewing optics, the

sample holding stage, and the polaroid

camera. The base has adjustable feet for

positioning the instrument in the high

intensity laser beam.

338
Figure 7. The TIRM microscope shown in figures 5 and 6 configured for laser-damage

testing. The high intensity laser beam

passes through the sample and through the

empty space in the objective turret of the

microscope.

UNDAMAGED DAMAGED

1UU /im

Figure 8. Two TIRM views of the same region of an ion-milled fused silica sur- face taken before and after it was damaged

by a laser pulse. The "undamaged" picture

was taken with slightly lower magnification. Note that all the damage occurred

along the previously present scratch.

339
NOMARSKI

1 I 100 tim

TIRM

Figure 9. A Nomarski and a TIRM view of

the same surface as shown in figure 8. The

Nomarski view clearly shows the decoration

brought out by ion milling. The single

damage site in the upper-right-hand side of

the TIRM view has a large halo around it. This halo cannot be seen in the Nomarski

photograph.

BEFORE

100 iim

Figure 10. Two views of a mechanically

polished BK-7 surface, showing its appear- ance under TIRM before and after laser

damage. The exposure times are one minute

in the undamage case and one-half second in the damaged case. This was an exit surface

damage site, and the damage pits are very

effective scatter sites in the case of exit

surface damage.

340
BEFORE AFTER

100 (im

Figure 11. Two views of a 0. 35-um-thick

Si02 film on a fused silica substrate.

This film had a multitude of scattering

sites prior to damage. The "after" view

was taken with less exposure time, and the

bright spots in this photo are damage

sites, while the original scattering sites

appear as dim spots. Very little correlation exists between the original film

scattering sites and the subsequent damage

sites.

341

PARAMETERS AFFECTING DAMAGE THRESHOLD IN THIN FILM INFRARED CHALCOGENIDE GLASS FOR APPLICATIONS IN LASER OPTICAL SWITCHING SYSTEMS

W. E. Kienzle

Science Applications, Inc. * El Segundo, CA 90250

and

N. I. Marzwell

Rockwell International

Rocketdyne Division

Canoga Park, CA 91304

The study of process control of film deposition can be important in

determining the threshold of film damage due to a laser pulse. This paper

reports the optical properties of the infrared chalcogenide glass material

T^SeAs^Te as process parameters are varied. Several RF sputtered

chalcogenide glass films of thickness varying from 0.6wn to 8.9ym were deposited

on germanium and sapphire. The index of refraction and absorption coefficient

were determined from the measured reflectance and transmittance curves for

these samples. Further, these properties were measured for two different

sputtering pressures, two sputtering voltages, twelve azimuthal target

positions and three different substrate to target distances. The damage due

to switching was assessed by subjecting different switches to CO laser

pulses with FWHM of 2 to 4 msec and energies up to 0.16 Joules which were

focused on target spot diameter of 0.4 mm on the chalcogenide optical switch.

Key words: Absorption coefficient; Chalcogenide material; Deposition

j parameters; Index of refraction

1. Introduction

J 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.

Chalcogenide materials have been studied for use in optical components in laser systems

operating in the spectral range from 2 to 10.6um, RF payloads, thermal control and memory

applications. In this paper we present preliminary measurements of the optical parameters

that affect the damage threshold of a chalcogenide thin film. We discuss possible damage

mechanisms and address the properties of these materials as they relate to their chemical

composition and fabrication techniques.

Ovshinsky [l] identified the potential of chalcogenide systems for these types of

applications. Subsequently, Bishop [2] fabricated and tested optical switches composed

of amorphous glass material from the family (Tl Se)^ (As^Te ) ^. Aerojet Electrosystems

Co. [3] has developed material and fabrication processes and characterized the performance

of several types of switches, including the family (Tl

2 Se)x

(As Te^)^ . A typical

chalcogenide switch is shown schematically in the upper right of figure 1, and consists

of a thin (M-25um) layer of switch material deposited on a wafer of substrate material

(thickness M-2mmj with one other wafer over this configuration to complete the switch

configuration. Typical switch response is also shown in figure 1 where the initially

transmitting switch rapidly becomes opaque when irradiated by a high intensity laser pulse.

2. Chalcogenide Characteristics

Chalcogenide glasses could be obtained from different combinations which have to be

differentiated from oxide glasses whose composition includes sulfur, selenium and

tellurium. It is those elements that act as glass formation inducers for other elements

* Work performed at Aerojet Electro Systems Co., Azusa, CA

343
such as thallium, antimony, bismuth. . .etc. , for chalcogenide format ion. The chalcogenide

glasses could be made in a wide variety of physical, electrical and optical properties. A study of the electrical properties shows that the group of high-conductivity chalcogenides are typical electronic semiconductors with a hole conduction mechanism. The

ability to crystallize is higher in chalcogenide glasses than in oxide glasses and it

depends upon the composition of the various systems to a very large extent. Upon

crystallization the electrical conductivity generally increases by as much as a factor of

10 . The change in conductivity is connected with the long-range order and tne different role impurities play in a glass and in a crystal. Zone purification of chalcogenide leads to a strong decrease in the electrical conductivity when it is in the

crystal state while the electrical conductivity does not change when the chalcogenide is in the glass state. Chalcogenides also offer a smooth dependence of the absorption band

edge upon change in composition. A similar smooth dependence of the absorption bands is seen also with composition. A chalcogenide glass alloy generally shows a smooth transition from the type of absorption of one component to the other component constituting the

alloy. The laws established for crystalline substances can apparently be extended to vitreous substances. In certain cases transition from the glassy state to the crystal state goes through a gradual change from the short-range to the long range. The long

wavelength cut off for the solid is usually determined by the lattice type absorption of

the constituent elements. The beginning of transmission occurs at a wavelength corres- ponding to the energy of its forbidden gap (absorption edge)

The member of the chalcogenide compositional family (Tl^Se)^ (As^Te^)^ discussed

in this paper is Tl^Se As^Te,. This material is an amorphous semiconductor, which when

irradiated by infrared radiation has a rate of energy absorption that increases with the

temperature of the material. This runaway absorption results in a transmission change

in the material. This transmission change is a result of free carrier optical absorption

of the incident laser wavelength which causes thermal runaway, and subsequent opacity at the laser wavelength. As thermal runaway occurs, the material passes through a glass

transition temperature at 353K and an order-disorder phenomena at 475K. Divitrification

of the glass does not appear to occur during the switching action. Further description

of the materials electrical, optical and mechanical behavior may be found in the list of

references [4-6] .

3. Thermal Irradiation Analysis

As part of a study to characterize the damage behavior of Tl^SeAs Te^ in a passive

optical switch configuration, a three-dimensional transient model of tne absorption of

laser energy and its subsequent dissipation in the switch material was developed [3] . Passive means that a sufficiently large irradiance itself causes the switch to become

opaque. Model predictions above 500K can only be taken as indicative of certain general

trends since thermophysical data above 500K are not available.

7 2 The model assumes that below laser irradiance levels of 10 W/cm the mechanisms

of absorption and dissipation are thermal in nature. The configuration adopted here

consists of three stacked disks of material. The center disk is the chalcogenide glass, one outside disk is a cover plate, and the other is the supporting substrate. Laser

radiation is incident upon the cover plate. Reflections at the outer faces of the model

switch were not considered since it was assumed that anti-reflection coatings would be

used in the actual switch. Furthermore, internal reflections at the support chalcogenide interface were assumed negligible. The circumference of the layers is assumed to be held at the ambient temperature of the system.

The model requires knowledge of the absorption coefficient, heat capacity, thermal

conductivity, and density of all the materials used. The power absorption per unit area within a thickness "d" of a material is assumed to satisfy

P = P exp (-ad) (1)

344
where P^ is the irradiance entering the volume. Internal energy of the glass, heat

conduction, and radiation at the interfaces are the three mechanisms considered to dissipate the energy. Because of the exponential form of equation (1), an analytical solution

of the heat equation is not possible and the method of finite differences was used to model the thermal transfer.

In discussing the results, it is useful to define the switching time, which is the

time from first incidence of a laser beam until the switch power transmission has dropped

to 1/e of its initial value. For the present chalcogenide materials used, the switch time

is primarily the time (I) to heat the chalcogenide glass from the system ambient temperature to the temperature at which the temperature-dependent absorption coefficient begins

to dominate. The additional time (II) for the temperature-dependent absorption coefficient to reach its maximum value is negligible by comparison.

If conduction and radiation effects are small, which is typically the case at the

initial ambient temperatures, heating of the glass at a constant rate depends primarily

on the heat capacity and absorption coefficient of the glass. The result is a linear

increase in temperature during the time (I). As absorption proceeds, the temperature is

highest at the middle of the chalcogenide film because of heat conduction from the chalcogenide film to the adjoining support materials. After the chalcogenide glass switches and

becomes opaque, the highest temperature is at its front face because then absorption is so strong that it occurs all at the front face, figure 2. When a temperature of 475K is reached, the temperature-dependent absorption coefficient begins to dominate the total

absorption coefficient resulting in a rapid increase in temperature and a corresponding

rapid drop in transmission. Thus, below 475K, the switch is transmitting, and above 475K,

it is opaque.

One of the problems of trying to match model performance with the performance of

actual laboratory switches is the difficulty of accurately characterizing the thermophysical properties of the chalcogenide glass used. Consequently, it is preferable to describe trends rather than absolute magnitudes. The model has been very useful in understanding switch operation and what might be the optimum thermophysical values to be

sought in the materials engineering of chalcogenide glasses.

Requirements for a variety of switch applications have been established as shown in

figure 3 for one particular application. The thermal analysis model discussed above was used to calculate switch times for a chalcogenide switch with a thickness of 5um for

different values of the chalcogenide absorption coefficient. Previous results y] have

indicated that switch damage may occur above fluences of 60 J/cm for pulses several

milliseconds long. That threshold corresponds to a required switch time of approximately

2(10"^) sec as indicated by the horizontal bar in figure 3. Since the bulk chalcogenide

material has an absorption coefficient of 15 cm"-'-, it was clear that higher absorption

coefficients would be necessary to provide the proper switch time. It was therefore

necessary to determine the effect of various sputtering deposition parameters on chalcogenide film absorption coefficient.

4. Sample Preparation

Although a variety of techniques for depositing chalcogenide switch material have

been examined as indicated in table 2, to date, the best process for fabricating chalcogenide switches appears to be the sputter technique. With this technique, high quality

films of the proper composition can be fabricated. Previous efforts have identified a set

of deposition parameters that are critical to film uniformity, compositional consistency,

minimized film defects, and optimized deposition rate for different substrates. During

the sample preparation the intent was to vary the set of deposition parameters that would

possibly induce structural variations, and possible compositional variations as well. Parameters affecting structure are RF power and the target-to-substrate geometry. The

parameters affecting compositional variations are the gas flow rate and gas pressure.

345
The Tl^SeAs^e target was synthesized from the elements, all of which had a purity

of six 9*s. The quartz tube was chemically cleaned and baked in a furnace at 400°C for

at least 4 hours. Special effort was made to flush the quartz container with argon and remove air and moisture before sealing. The temperature was raised at the rate of 20°C

each eleven minutes until it reached 550 C. The rocking furnace containing the quartz

is then activated and the temperature rate of rise is increased gradually until the

material has time to move completely from one end of the tube to the other. After twenty

hours, the ampoule is quenched in air.

The sapphire and germanium substrates, 2mm and 1mm thick, respectively, were placed

under the sputtering target. The argon bias voltage was varied between 0.6 and 1.4 kv. The distance between target arid substrate was kept at two inches while the maximum sample

temperature allowed was slightly above room temperature. An RF power at 50 watts forward

with zero reflected resulted in deposition rates of 80K. Prior to the deposition,

samples were sputter etched clean. Once deposition was complete, the samples were maintained in a nitrogen dry box until the reflection and transmission measurements were performed. The particular sputtering conditions for each sample are shown in table 1. Aximuthal position refers to clockwise locations which are a fixed distance from a line

drawn through the center of and normal to the target surface.

The switch time is defined in this document as the time for the transmitted pulse to reach i/e of its peak value. ^ Integration under transmitted response curves indicates that

the switch requires from 10~ to 10" 4 Joule/cm2 to be activated. Results obtained indi- cate that submicron chalcogenide switching material undergo optical switching in nano- seconds to microseconds when exposed to intensities 104 -105 watts/cm2

. The switch time

varies approximately directly with switch thickness and inversely with incident intensity.

Lowering the switch temperature generally increases the switch time, but the chalcogenide

material does not show appreciable change until 170°. All thermally activated switching

material including the chalcogenide material exhibits a certain degree of leakage due to the heat loss at the edge of the irradiated spot. This phenomena is more pronounced for

Gaussian and non-uniform beams. Preliminary results show that a chalcogenide switch has

an attenuation of 10 with a Gaussian beam.

A Beckman IR4240 spectrophotometer was used to make reflection measurements on the

film side, and in some cases transmission measurements from 2-12um. All films measured

were several quarter waves thick and homogeneous enough to produce interference effects.

A typical resultant reflectance curve is illustrated in figure 4. Since the reflectance

maximum occurs at an odd integral number of quarter wavelengths in optical thickness, a graph of integral value versus wave-number will be a straight line with slope given by

four times the optical thickness. For the measurements reported here this non-linearity

corresponds to a two percent change in index of refraction and is not critical to the con- clusions. The results for the samples measured are listed in table 2.

Absorption data were also deduced for those samples where transmission measurements

existed. For any assembly of thin films on a transparent substrate, the following

relation is valid [8]

5. Results of Chalcogenide Switch Response Time

6. Results of Optical Data Analysis

T

1-R Real Part (BC*)

( 2 )

where for a single film

346
and

6p =

27rnF d

F - i2TrkF d

F (4) X X

Furthermore, if k is small with respect to n, the above expressions may be combined near

the turning points to yield

1 n„ n \

n

s n?)

«

F =\—7Z — J (5)

where the following definitions apply:

R = Reflectance of film and substrate

T = Transmission of film and substrate

^n+l = Admittance of the film

= Characteristic matrix of the assembly

X = Wavelength in free space

n

F = Refractive index of the film n

s = Refractive index of the substrate

N

F = np-ikp = the complex index of refraction of the film

dp = Film thickness

The absorption coefficients which were determined as described above are listed in

table 1. Based on the results tabulated in table 3 and on the analysis of the reflection

data, the following observations could be made: The index of refraction is basically

constant from 20 to 10 urn to the accuracy of these measurements. An increase in sputtering pressure appears to produce a small decrease in the index of refraction.

There appears to be some dependence on substrate position as the number six position

consistently has a lower index of refraction than the other positions. The refractive

index of films on A^O^ is lower than that on germanium (3.8 compared to 4.15). This is

related to the equipment and the experimental set-up. Changing the voltage appeared to have no effect on the refractive index except when the voltage is reduced to 0.6KV. There

seems to be little correlation of the index of refraction with thickness. The absorption

coefficient appears to depend on the type of substrate as the value is much lower on Al-O^ than on germanium. The absorption coefficient seems to be independent of the

voltage or sample position while an increase in sputtering pressure appears to increase

the absorption coefficient as shown in table 2.

7. Results of Bidirectional Reflectance Measurements

For optical components and detector vulnerability applications, the scattering characteristics of the optical switch element is an important parameter. Measurement of the

bidirectional reflectance distribution function (B.R.D.F.) and forward scattering distribution measurement does provide the necessary information. Scattering measurements

versus temperature and angle of illumination remain to be evaluated. Until now we have

measured the scattering for uncoated germanium substrate. The same substrate was re- measured after 9.9 um of the T£

2 SeAs 2Te

2 was deposited by sputtering. The results

shown in Table 3 indicates no considerable scattering due to 9.9 um of chalcogenide

on top of Ge substrate.

347
Table 3: I-R Measurements

Material -6- BRDF/mm

Ge 1.5° <6.10 x

10" 5

Ge coated with o -3

n, , • j 1.5 3. 20 x 10 Chalcogenide

(9.9um)

8. Switch Operation vs Laser Damage Effects

One of the important parameters for determining the performance of the chalcogenide

was the power density versus optical transmission. The spatial distribution of the laser

beam near the focal point of the input lens and the depth of focus of the lens were care- fully measured. The beam as measured with a 0.001 inch slit had a diameter of 37um and a depth of focus of approximately 0.2mm. For such a large depth of focus, the power density

was uniform over a large enough region in space to make positioning of the switch less

critical than in very short focus systems. The beam entering the focusing lens was checked out for maximum circular shape and minimum ellipticity and the power incident on the lens was typically 2.7 to 3.0 watts over the several days of the tests.

The data of relative transmission for the 7 micron test sample as a function of

power density at three temperatures are shown in figures 5, 6, and 7. The relative trans- mission is the ratio of the signal amplitudes transmitted by the switch and incident on it

normalized by the ratio of the detector outputs in the absence of the film and substrates

but including all other elements in the optical train. The normalized signal amplitude

ratio is only an approximation to the absolute transmission because of the unknown amounts

of signal lost due to overfilling of lenses and beam splitters and losses due to scattering.

The relative transmission does not start at 1 for low power densities because of re- flection losses from the two uncoated surfaces of germanium and the background absorption

in the chalcogenide. These predict a relative transmission near 0.25 independent of

thermal effects. Values significantly above or below this for low power densities may be

due to surface irregularities or localized differences in film thickness. Movements of

the switch through the laser spot showed adjacent areas of high and low transmission. For

the test shown as figure 7, the sequence of trials starting with (1) was selected to have

a low relative transmission for a small power density. It appears that something about

the film must have changed as the power density was increased. From the collection of

relative transmission curves, it is evident that absorption is a nonlinear function of

power density probably through heating of the film as expected. The film of amorphous

glass appeared to have a factor of 2 or 3 increase in absorption per decade increase in

power density.

Some estimates of the damage threshold for the chalcogenide can be obtained from the

relative transmission for increases in power density. The relative transmission con- tinued to decrease with increasing power density until multiple pulses were applied at the highest power levels when the transmission increased, suggesting a change in the

switch. The pulse lengths for the tests were approximately 6 milliseconds corresponding

to an energy density of 6 joules/cm^ for no change in relative transmission for repeated

trials. The highest power levels corresponded to an energy density of 500 J/cm2 where

the switch showed the minimum transmission for several repeated pulses until it increased

to a value between its previous value and the low power value. When the power level was reduced and the film was allowed to cool, the film appeared to heal itself and return to transmission at its previous lower power value. The self repair was probably due to a revitrification of the glass after being heated.

The chalcogenide film coating on the germanium substrate was found to survive laser

irradiation and temperature cycling to 170 K with no apparent cracking, peeling, or separation of the glass material from the substrate. This was a positive contrast to one

348
sample in a previous test which separated and cracked. In each case, no damage to the

substrate was detected.

Observation of the sample after testing revealed a regular pattern on the film

surface that may have been related to variations in film thickness or structure. Figures

8 and 9 are typical views of the surface.

Presently, it is widely accepted that free electrons are the cause of laser induced

damage to intrinsically transparent dielectrics in a self focusing free situation and in

absence of absorbing inclusions. Although there is not broad agreement [9] on the process

by which the free electron density increases in the presence of an intense laser field, it has been suggested [10] that the free electron density must reach 10^-10^/ cm^ in order

to cause damage to the chalcogenide by heating due to the free electron absorption.

Such a density of free electrons will induce a detectable decrease in the refractive

index of the chalcogenide film in addition to making it lossy. The decrease in specular

reflectivity An = 10" 5 - 10"^ was measured for a BSC-2 glass which was subjected to a damaging Q-switched ruby laser pulsefn] , It was shown that the specular reflectivity

decreases occured 2.4 nanoseconds prior to the onset of catastrophic damage and was caused

by a laser induced increase in the free electron density with a measured average buildup

time of 1.6 nanoseconds. The resulting free electron absorption can transfer a damaging

amount of energy from the laser pulse to the sample causing at least melting of the

irradiated surface site, as seen in this paper. Auger analysis revealed that for long

pulse duration the material undergoes a chemical decomposition where the arsenic and

selenium are sublimated from the laser exposed spot.

It appears that a viable switch, in terms of the requirements in figure 3, can be

achieved with the sputtering techniques described above. It is also clear that variation

of the sputtering deposition parameters will permit some control over the refractive

index and absorption coefficient and that even more control over switch characteristics

may be gained through a combination of process control and material selection.

Testing a 7 micron sample of chalcogenide on germanium at 299 K, 232 K, and 173 K revealed that the relative transmission is dependent on power density such as increasing

power density produces a decrease in relative transmission. Switching was observed at 173 K. Further experimental work is needed to resolve questions of orientation in the

laser beam and the relationship between surface appearance and performance.

9. Conclusion

349
ACKNOWLEDGEMENT

The authors would like to express their appreciation for

the contribution of Ralph Somers of Aerojet Electro Systems

Co., Azusa, CA. for his experimental and technical support.

References

£l] Ovshinsky, S.R., and Fritzsche, H.,

Amorphous semiconductors for

switching, memory and imaging

applications, IEEE Transactions on Electron Devices, Vol. ED-20,

No. 2, Feb. 73.

[2] Stvom, U., and Bishop, S.A.,

Infrared optical switch-model

and experiment, DOD Conference on Laser Hardened Material and

Structures; NASA Ames Research

Center, July 7-9, 1975.

[3] Marzwell, N., Somers, R., Dong, W.,

Kienzle, W., and Jefrey, J.,

Infrared Chalcogenide glass for

applications in laser optical

switching systems, High Power

Laser Optical Components

Meeting, Lowry Air Force Base,

Sept. 15, 1978.

[4] Mitchell, D.L, , Taylor, P.C., and

Bishop, S.G., Free carrier optical

absorption in vitreous Ti SeAsTe^,

Solid State Communications, Vol. 9. pp. 1833-1837, 1971.

[5] Taylor, P.C., Bishop, S.G., and

Mitchell, D.L., Temperature Dependence of local order in the layertype liquid semiconductors As-Se,

and Tl SeAs

?Te , Phys. Rev. Letters,

27, NoT 7, p. 414, August 1977.

[6] Bishop, S.G., Taylor, P.C.,

Mitchell, D.L. and Slack, L.H., Far infrared and microwave conductivity

spectrum of semiconducting Rl AsSe^Te

glass, Journal of Non-Crystalline

Solids 5, R351-359, 1971.

[7} Somers, R., Aerojet Electro-systems

Co., private communications,

August, 1978.

[8] Macleod, H.A., Thin Film Optical

Filters, American Elsevier Publishing

Co. Inc., New York, 1969

[9] Bass, M., and H.H. Barrett,

"Avalanche breakdown and the

probabilistic nature of laser

induced damage" IEEE J. QE 8_, 338 (1972).

[lO] Bloembergen, N., "Laser-induced

electric breakdown in solids", IEEE

J. QE 1_0, 375 (1974).

[ll] Alyassini, N., and Parks, J. , "Measurement of free electron density

at the onset of laser induced surface

damage in BSC-2. Laser Damage

Conference 1977.

350
Table 1. The Optical Properties of Tl ?AsSe ? Te^ for Various Sputtering Conditions

Substrate

Material

Sputtering

Vol tage

(kv)

Sputteri ng Pressure

(u)

Azimuthal

Location

Film

Thickness F Absorption Coefficient (cm~*)

3um 5um 7ym lym

A1203 BL BL BL 5.6 3.81 35 181 CO CO

Al 2O3 BL BL BL 0.6 3.71 571 459 CO CO

Al 2O3 BL BL BL 9.9 3.83 423 700 CO CO

A1 20 3 BL BL BL 9. 9 3.91 797 1,343 CO CO

A1 20 3 BL BL BL 9.9 3.65 240 371 CO CO Ge BL BL BL 5.6 3.42

Ge BL 2.5 3 2.44 4.61

Ge BL 2.5 6 2.44 4.02 2,574 2,262 2,229 2,183

Ge BL 2.5 9 2.44 4.56 2,035 2 ,005 1 ,985 1,852

Ge BL 2.5 12 2.44 4.61

Ge BL 5.0 3 2.07 3.74

Ge BL 5.0 6 2.07 3.66 3,920 4,401 3,964 3,296

Ge BL 5.0 9 2,07 3.86 3,602 3,164 2,956 2,593

Ge BL 5.0 12 2.07 3.86

Ge BL BL 3 2.52 4.96

Ge 1.4 BL 6 2.52 4.06 2,033 1,440 1,061 1,496

Ge 1.4 BL 9 2.52 4.33 2,498 2,244 2,201 2,138

Ge 1.4 BL 12 2.52 4.25

Ge 0.6 BL 3 2.94 2.46

CO = Wavelength exceeds the transmission cut off in Al ^0 BL = Baseline parameters.

Table 2. Several deposition techniques have been

considered for the chalcogenide material

Deposition Technique Assessment

Chemical Vapor Deposition Chemical reaction Involved, anticipate

sto1ch1ometry problems

Sputtering Technique has been used successfully

Evaporation Attempts to date have not been used

successfully

Oxidation Not applicable

Reduction Not applicable

Glow Discharge Primarily for organic materials,

sto1ch1ometry considerations

Hot Press

(Heat material above the

softening temperature and

control thickness

mechanically)

Uniformity problems, not applicable

for thinner switches (^5i»)

351
23

0

TRANSMITTED

LASER THRUPUT

- SWITCH MATERIAL

2 3 4 TIME(MILLISECONDS)

Figure 1. Typical switch configuration

and response when irradiated

by a high intensity laser beam.

AXIAl POSIIION [H

ml

Figure 2. Axial temperature profile at switching time and later.

Figure 3. Acceptable switch times can be

achieved by material process control

of the absorption coefficient.

352
REFRACTIVE INDEX:

ABSORPTION COEFFICIENT NEAR THE TURNING POINTS:

ABSORPTION COEFFICIENT =

I M=i)

ifn

s /nf +

nf/n

s)\ T /

Figure 4. The technique used to determine the refractive index and

the absorption coefficient.

LOWER TRANSMISSION ^REGION

Figure 5. Power Density vs. Relative

Transmission for a 7-mm

Chalcogenide on Germanium

(232°K).

POVEA DENSITY, WATTS/CM*

353
1.00

POWEK DENSITY, UATTS/C"2

Figure 6. Power Density vs. Relative

Transmission for a 7-mm

Chalcogenide on Germanium

(168°K to 173°K).

,5 POWR DENSITY, UHTTS/CN 2

Figure 7. Power Density vs. Relative

Transmission for a 7-mm

Chalcogenide on Germanium

(299°K).

354
IMPROVEMENTS IN CLARITY AND ENVIRONMENTAL

STABILITY OF TLI-CONTAINING ANTIREFLECTIVE COATINGS

W.T. Boord, P.P. Chow

Honeywell Systems and Research Center

Minneapolis, Minnesota 55413

and

W.B. Harrison, J.E. Starling

Honeywell Ceramics Center

Golden Valley, Minnesota 55422

Clarity of Til films in visible light was improved by masking the textures of polycrystalline KC1 substrates with thin SrF^ films. Also, the use of more environmentally

durable fluoride films as replacements for KC1 in the T11-KC1-T11 10.6 micron antireflective coating system was investigated.

Key words: Antiref lective coating; deposition technique; environmental stability;

masking layer; potassium chloride window; scattering; thallium iodide

1. Introduction

This KC£ window coating study consists of two parts. The first is a study of the

ability of strontium fluoride to mask the texture of KCa substrates in order to produce

clear coatings. The second is a study of film absorptions produced by lead fluoride and

sodium fluoride, which are more environmentally durable materials being evaluated as potential replacements for KC1 in the TU-KC1-TU 10.6 ym antireflective coating system.

2. Substrate Preparation

Polycrystal 1 ine Eu-doped KC£, produced by hot forging, was used for the 3.81 cm diameter substrates in this work. Rough shaping of substrates was carried out with 600

grit silicon carbide abrasive. Intermediate polishing was then performed on a Pellon TAN-W

pad using 0.3 alumina carried in kerosene-type liquid. This process removed

approximately 100 microns of material from each face. Substrates were then etched in concentrated hydrochloric acid for 60 seconds to remove about 10 microns of material from

each face. A final mechanical polish was then done on a beeswax lap with 0.3 micron

alumina carried in triacetin.

3. Masking Layer Study

Four KC£ substrates were used for the masking layer study. Single crystal strontium

fluoride, obtained from Optovac, was evaporated from a Radak source. We found that because

;of higher efficiency of the Radak source the vacuum system did not heat up much during

deposition. As shown in figure 1 each polished substrate surface was divided into

quadrants, three of which were coated with strontium fluoride films of nominal thicknesses

of 200, 400, and 600 A. Substrates were then overcoated with thallium iodide of varying

thickness.

Clarity was characterized by measuring the transmission of the 6328 A He-Ne laser

line through each" quadrant. Table 1 shows the transmission data. It is seen that transmission was most improved by the 200 A thick masking layers. The strontium fluoride layer

modified the effect of the KC2. substrate on the growth of 111 films. It was noted that the

films on the unmasked portion of the substrate tended to grow in orientations determined

iby the texture of the KCi substrate, resulting in visible striations. As shown in figure 2,

ithis striation was not observed on the masked portion of the sample, indicating that

uniform growth of TU was promoted by the presence of the masking layer.

Work is supported by AFWL, contract No. F3361 5-77-C-5038, Dr. G. Edward Kuhl project monitor.

I

i 355
4. Alternate Materials

Another task of this coating study was to investigate materials which might replace . KCx. in a three-layer AR coating design. Viable replacements would need to have low

absorption coefficients and be less hygroscopic than KC*.. Lead fluoride and sodium

fluoride were chosen for this study. These films were deposited, using single crystal

starting materials, over TU coated KQ substrates in the pattern shown in figure 3. Lead

fluoride was evaporated from a Radak fixture with an alumina crucible. Uniform appearing

films were deposited at 600 A/min. Sodium flurodie was evapoarted using a chemically inert

BeO crucible. Molten NaF tended to wet the wall of the crucible and run up and over the

side of the crucible. By using a clean, new crucible, a NaF film was deposited by carefully

adjusting the deposition rate. Both lead fluoride and sodium fluoride are environmentally more stable than potassium

chloride, but our absorption measurements indicated that their absorption coefficients are substantially higher. We believe this is partially due to the fluorides reacting

chemically with crucible materials causing impurities to be incorporated into the films.

The PbF2 reacted with and damaged the alumina crucibles after two or three depositions.

Refinement of the present deposition technique is likely to overcome this problem.

Table 1. SrF

? masking layer results

Sample # Til Thickness

(pm)

6

s 0 3 SrF^ Thickness (A) 0 200 400 600

Transmission, 5 I 0.53 0.55 0.54 0.52

15

6

0 79 SrF^ Thickness (A) 0 200 650 850

Transmission, / i 0.48 0.51 0.49 0.49

10

6

1 29 SrF,, Thickness (A) 0 200 400 600

Transmission, "/ 0.34 0.43 0.42 0.42

14

2

1. 79 SrF,, Thickness (A) 0 200 400 600

Transmission, 7< 0.17 0.21 0.21 0.21

356
Figure 1 . SrF

2 quadrant

coating for masking study

Figure 2. Unmasked (upper-right)

quadrant shows striation

REACT! VELY SPUTTERED OPTICAL COATINGS FOR USE AT 1064 NM*

W.T. Pawlewicz, R. Busch, D.D. Hays, P.M. Martin and N. Laegreid

Pacific Northwest Laboratory

Richland, Washington 99352

Optical coatings of Ti0

2 , In

1 gSnQ

^ (ITO), Ta

20

5 , Nb> 2 05

, a-Si:H and SiO

? have been prepared by reactive sputtering for use

2 with lasers operating near 1064 nm. Damage thresholds of 7-10 J/cm

2 for Ti0

2 and 5-6 J/cm for transparent-conductive ITO were measured

at Lawrence Livermore Laboratory for X/2 coatings on fused silica

irradiated with 1 nsec laser pulses. Sputtered Ta

2 05 , Nb,,0 5 and a-Si:H

also appear to be promising high-index coating materials, but have not yet been damage tested. The reactive sputtering process is shown to allow precise control of coating crystalline phase and grain size. For

Ti0

2 both optical properties and damage threshold have been related

to these materials properties, and glassy coatings are shown to be optimal.

Key words: Glassy structure; laser damage resistance; materials

properties; oxide coatings; reactive sputtering; transparentconductive coatings.

1. Introduction

Optical materials in thin film coating form have long been the most damage-sensitive elements in high power laser systems. Attempts to maximize

coating damage resistance have traditionally involved its relationship to such deposition conditions as substrate temperature, deposition rate, partial

pressure of reactive gas, etc. Although this approach is usually effective

in optimizing coating performance, it relegates coating technology to the status of an art rather than a science. That is, the fundamental differences

between an optimized coating and an unoptimized coating are never explored.

The question "what makes a good coating good?" is not addressed. Thus experience

gained in optimizing one coating material is of little assistance in optimizing

a second. Further, knowing how a coating was made, rather than knowing the

physical characteristics of the coating, is of little help in understanding

the thresholds and mechanisms of laser-induced damage to that coating.

This paper reports results to date for a more fundamental approach to the optimization of optical coatings for high energy laser applications.

Specifically, it deals with oxide optical coatings prepared for use at 1064

nm in neodymi urn: glass lasers. The approach is to study the dependence of

damage threshold on the basic materials properties of a coating: crystalline

structure and/or phase composition, grain structure (size, shape and orientation)

and stoichiometry. Most of the work is being done for Ti0

2

. Other materials

for which preliminary results are available are In-j gSn^ ^0g (ITO), Ta^,

Nb

2 0

5 and Si0

2

. Amorphous Si : H alloys (a-Si:H) are also introduced as potential very-high-index coatings for use at 1064 nm. All of the coatings

are made by rf diode sputtering which permits excellent reproducibility and

precise control of coating materials properties through appropriate selection

of deposition conditions. This materials-oriented approach has resulted in systematic optimization of the damage threshold of Ti0

2 coatings, determination

*Work supported by the US Department of Energy, Offices of Inertial Fusion

and Basic Energy Sciences.

359
of some key materials properties for high damage thresholds, and some potentially

useful information for understanding damage mechanisms. Information gained

with TiO^ is now being extended to other metal oxides as well as novel materials

such as a-Si :H.

2. Apparatus and Experimental Details

All oxide coatings are made by rf-diode reactive sputtering in an Ar/0

2 atmosphere. Figure 1 is a simplified schematic of a typical system used for

this work. The oxides of Ti , Ta and Nb are made from metal plate targets.

ITO coatings use a pressed-powder oxide target. Silica coatings are made

with a Suprasil target. Si : H coatings are made by sputtering a vacuum-cast

Si target in Ar/h^ atmospheres. Substrates are fused silica discs. Both

substrate and target electrodes are specially designed for operation over a wide range of gas pressures. A power-splitting network is used to distribute

rf power to both the target for deposition and to the substrate for bias

sputtering. The diode systems are mounted on conventional oil-pumped vacuum

systems. Sputtering gas is mixed and pressures are measured with a capacitance

manometer. Substrate temperature is measured with a thermocouple in the

metal substrate holder. A quartz crystal thickness monitor is sometimes used

to deposit coatings with a desired thickness. Principal deposition parameters

affecting the materials properties are substrate temperature (20° to 700°C),

oxygen or hydrogen partial pressure (0 to 10 Pa), and the rf-induced dc bias

voltage applied to the substrate (0 to -500 V dc).

Crystalline structure, phase composition and grain size are determined

for each coating using conventional x-ray diffractometer [1,2] and transmission

electron microscopy (TEM) techniques. Optical properties are measured spectrophotometrically over the wavelength range from 200 to 2500 nm, and optical

constants are deduced using the method of Manifacier, et al [3]. Laser

damage thresholds are made by Lowdermilk, Milam, et al at Lawrence Livermore

Laboratory, Livermore, CA (LLL) [4].

3. Results

3.1 Control of TiO,, Materials Properties

For Ti 0^ the important materials properties are grain size, phase com- position (there are two crystalline phases, anatase and rutile) and stoichiometry

In this work phase compositions ranging from 40% rutile/60% anatase to 100%

rutile are reproducibly obtained. Phase composition is influenced only by

substrate temperature and 0^ partial pressure, as shown by the phase composition

diagram of figure 2. Intermediate substrate temperatures of roughly 200 to 500° C result in mixed-phase coatings with rutile contents given by the numbers

in the circles. The rutile content increases with the 0~ pressure. Rutile

coatings are obtained for lower temperatures and higher temperatures. O2

partial pressures less than 1.2 Pa always result in Ti metal coatings.

The occurrence of the rutile phase over a wide range of temperatures

when high O2 pressures are used permits deposition of coatings with grain

sizes ranging from 10 to 60 nm through appropriate selection of the substrate

temperature. For mixed-phase coatings, grain sizes are restricted to 10 to 30 nm since mixed-phase coatings are obtained only over a narrower temperature

range.

Figure 3 shows the influence of substrate temperature on the average

grain size of a rutile coating. Grain sizes near 10 nm are obtained for

substrate temperatures between 20 and 250°C. Above 250°C grain size increases

with temperature to about 30 nm near 600°C. Larger grain sizes can be obtained

360
at higher target power levels by capitalizing on plasma heating of the substrate,

as shown in the inset for a nominal substrate temperature of 450°C. Grain

2 size rises with power density to 60 nm near 8 W/cm . At still higher power

levels grain size decreases due to inhibition of lateral grain growth by high

deposition rates. The data in this figure were obtained both by x-ray diffraction

and TEM techniques. Agreement between the two techniques is good for larger

grain sizes and fair for the smaller.

Coatings with a predominatly glassy structure, or a grain size smaller

than 10 nm, are obtained by applying 5 to 10% of the rf power to the substrate.

With rf substrate bias, glassy coatings are obtained regardless of the substrate

temperature used. Thus by using substrate bias, substrate temperature and

target power in the right combination, mi crostructure can be controlled from

essentially glassy to 60 nm grain size.

3.2 Ti0

2 Optical Properties and Laser Damage Thresholds

Phase composition is found to have a small but significant influence on coating refractive index. Figure 4 shows the index at 1000 nm plotted as a function of rutile content. Since grain sizes in mixed-phase coatings are restricted to 10-30 nm, the data point shown for 100% rutile is the average

index observed for these grain sizes only. Only a small variation of the

index with phase composition is observed, due to the closeness of the refractive

indices of rutile and anatase. The apparent variation of the index is only

3%, which is approximately equal to the scatter in the data points.

Phase composition does not appear to influence damage resistance.

Table 1 summarizes damage thresholds for coatings having a grain size of 10

to 15 nm. Little difference is apparent between the two groups. There is also no obvious dependence of threshold on rutile content of the mixed-phase

coatings.

Grain size, on the other hand, has been found to have a very significant

influence on both the refractive index and the damage resistance of Ti0

2 coatings. Figure 5 shows the influence of grain size on the refractive index

at 1000 nm for 100% rutile coatings. For large grain sizes the ~ 2.5 index

of bulk material is obtained. With decreasing grain size, the index drops to values as low as 2.0 for glassy coatings. The discovery of this relationship

explains the wide range of index values reported in the literature for Ti0

2 coatings. Knowing this relationship, it is possible to preselect the refractive

index of a coating for a particular application by choosing deposition conditions

that give you small, intermediate, or large grain size. Alternatively, by

measuring the refractive index of a coating one can determine the grain size.

The influence of grain size on the laser damage resistance of Ti0

2 coatings is shown in figure 6. There are two key results. First, damage

resistance increases with decreasing grain size. Second, damage thresholds

2 of 7 to 10 J/cm are obtained for glassy coatings. It is also interesting to note that damage threshold increased consistently with the amount of bias

2 applied to the substrate. Thresholds of 7 to 10 J/cm are encouraging for

single layers. They are very close to the values needed for the NOVA laser

and are twice as high as the Ti0

2 coatings presently used in the laser if the

coatings are tested as single layers on fused silica substrates. Anti reflection

(AR) stacks of Si0

2 and e-beam evaporated Ti0

2 coatings exhibit twice the

threshold of single layer Ti0

2 coatings. The same improvement in threshold

for sputtered AR stacks may be possible.

361
The most outstanding difference between the glassy coatings and the

polycrystall ine coatings is a much lower surface roughness. Figure 7 shows

both Nomarski and SEM surface micrographs for a glassy coating and coatings

with grain sizes of 40 and 86 nm. The surface roughness scales with the

grain size. Because of light scattering from the rough surfaces of polycrystall

coatings, the glassy coatings are of higher optical quality. The smoothness

of the glassy coatings (figure 7) and their lower refractive index (figure 5)

may be responsible for their higher damage resistance. A third possibility

is that the glassy coatings, which are all bias sputtered, contain fewer

impurities due to preferential resputtering of loosely bonded species from

the substrate as the coating grows [5].

Recently, the first TiO^/SiOg AR stacks were made using a glassy Ti

2 layer. The 2-layer design of 0.0833 X Ti0

2 /0. 333 X Si0

2 illustrated in figure 8 was used with layer indices of 2.10 and 1.45, respectively. This

stack can be reproducibily made with reflectance < 0.1% near 1064 nm, as shown in the figure. The stacks were made both with and without a 0.5 X SiO^

underlayer between the stack and the substrate. Measurements are now in progress at LLL.

3.3 Other High-Index Coating Materials

In addition to Ti0

2

, the oxides of Ta and Nb are being studied. Excellent

glassy coatings of Ta and Nb pentoxides can be made by reactive sputtering.

It is found that glassy coatings do not scatter light like their polycrystall ine

counterparts. Shown in figure 9 are the transmission spectra and refractive

indices near 1000 nm wavelength as compared to the glassy TiOg • The coatings

are smooth, the band edges are sharp and higher in energy than Ti0

2

, there is no absorption or scattering in the visible or near infrared, and the refractive

indexes are close to that of Ti0

2

- The index of Nb^O^ is 5% higher than

Ti O2 » while Ta

20^ is 5% lower.

The very-high-index amorphous Si:H alloys are also being investigated.

These alloys are potentially important for solar photovoltaic applications

and may be useful for 1.06 ujti polarizors. By depositing Si in an amorphous

or glassy form and alloying it with hydrogen the optical band edge can be

shifted from its crystalline value near 1.1 eV (1100 nm) all the way out to 2 eV. The refractive index drops to about 3.0 from its crystalline value of

3.45, but it is still so high as to make it a very attractive high index

material. Damage testing of a-Si:H is planned soon.

3.4 Transparent Conductive IT0 Coatings

Transparent conductive IT0 coatings with high damage thresholds are also

being prepared by reactive sputtering. These coatings can be used as the

electrodes on electro-optic devices used in high energy lasers to suppress

spurious laser radiation or to prevent retro-reflections from damaging the

? laser. Damage thresholds of 5.5 to 6 J/cm were obtained for coatings with

2 sheet resistances of 500 and 17,500 ohm/cm . These high threshold values are very impressive and may make possible significant improvement in the fabrication

of electro-optic devices for HEL applications [6].

The sputtered coatings were found to have lowest beam attenuation due to scattering and absorption when compared with coatings from four commercial

sources. Measurement results are summarized in table 2. These attenuation

measurements were made by the Laboratory for Laser Energetics (LLE) at the

University of Rochester [7]. The attentuation was found to increase with

decreasing sheet resistance. For comparable resistances, the sputtered

coatings show lowest attenuation.

362
Controlled variation of the materials properties of Ti 0£ coatings has

been demonstrated with the reactive sputtering process. Microstructure can be varied from glassy to 60 nr. grain size. Phase composition can be varied

from 60% anatase/40% rutile to 100% rutile. All the sputtered coatings are stoichiometric.

The relationship between refractive index and grain size has been established

for rutile coatings. The index can now be preselected from 2.0 to 2.5 at 1000 nm, and 2.2 to 2.7 at 550 nm. The relationship explains the wide range

of index values reported for T^ in the literature. The variation of the

index with phase composition has been determined to be small.

2 -ig r zz-:Zi t^res^olds of 7-1C J on" r a.e :se r obtained for -' : - coatings

prepared by rf reactive sputtering. Correlation of damage resistance with

grain size indicates glassy coatings nave highest resistance to laser-induced

damage. Phase compositions between 40c rutile/60% anatase and 100% rutile

result in the same damage thresholds. Two-layer TiOg/SiOg AR stacks with

less than 0.1% reflection loss at 1064 nm have been made, but damage testing

results are not yet available.

Glassy coatings of TagOj., Nb^O^ and Si:H have also been made by the same reactive stuttering techniques and r ave teen ooti'ized for ootical applications.

Damage testing has not yet been done. These coatings have indices of 2.0,

2.2 and 3.0 at 1064 nm, respectively, and could be s^oerior s_:sr t-tes -"or Ti02

. 2 Finally, nign darage resistance of 5-6 J/cm" and low attenuation have

:ee r demonstrated for reacti = -s: utterec , y: r ::a''^:-:: r :.c:

; ,5 ::ar>gs :f

ITO. The coatings appear prorising for electro-optic device applications.

The authors gratefully acknowledge W.H. Lowder—ilk and D. Milam of LLL for measurement of all damage thresholds reported in this paper, and

S.D. Jacobs of LLE for providing t

n e ITO attenuation data of table 2. They

also thank I.B. Mann, E.L. McDonald and H.E. Kjarmo of PNL for technical

assistance with coating preparation and characterization and H.R. Gardner for

reviewing this manuscript.

Acknowl edgments

References

[1] Hall , W.H. , Proc. Phys. Soc. (London) A62_, 741 (1 949).

[5] Maissel, L.I., and Glang, R.

,

Handbook of Thin Film Technology,

McGraw-Hill (New York (1970)

x Barrett, C.S., Structure of Chapter 4. Metals, McGraw-Hill (New

York 1952) 425-428.

X Manifacier, J.C., Gasiot, J.,

and Fillard, J. P., J. Physics E: Scient. Instr. 9_, 1302 (1 976).

[6] Pawlewicz, W.T., Mann, I.B.,

Lowdermilk, W.H., and Milam, D.,

Appl. Phys. Letters 34, 196 (1979).

[7] Jacobs, S.D., private communication.

Milam, D., Appl. Opt. 1_6, 1204

(1977).

363
Table 1. Laser damage thresholds of TiC^ optical coatings with the same grain size but different phase compositions. All coatings are single layers with half-wavelength thickness at 1064 nm (~ 230 nm) on fused silica substrates

Coating/

Substrate

54/S1-354

57/S1-358

85/S2-365

55/S1-356

68/S3-365

56/S1-357

Phase

Composition

100% R

(a)

65% R/35% A

60% R/40% A

85% R/15% A

Grain

Size (nm)

(a)

11

15

10

10/12

10/10

12/12

Damage Threshold

(J/cm

2

) (b)

5.2 ± 0.8

3.9 ± 0.6

£ 2.5

4.1 ± 0.7

2.3 ± 0.5 < 2.0

(a) X-Ray Diffraction.

(b) Measured at LLL, 1 nsec pulses, 2 mm beam diameter.

364
Table 2. Laser damage thresholds and beam attenuation values for transparentconductive coatings of various sheet resistance

Sheet

Resistance % Attenuation

Damage

Threshol

Coating Materi al at 1064 nm^ (J/cm )

v 25/S2-360 PNL Sputtered I TO 17,500 0.9 6.0 ± 0.5

23/S2-352 Sputtered I TO 500 2.8 5.5 ± 0.5

20 Sputtered ITO 19 7.0

(c) --

OCLI Evaporated ITO 12 3.5 ± 0.5

LCU-4 Evaporated 10 100 4.4 2.5 ± 0.5

LCH-4 Evaporated 10 20 ± 10 18.2 --

LC-4 Evaporated 10 90 7.6

CC-1 Evaporated 10 100 41 .7 _ _

LCH-4 Evaporated 10 200 6.2 --

LLE 481 occ Evaporated ITO 15 14.4

LLE 461 Corning Fi red -- 15 - 100 39.7 --

LLE 468A PPG Sputtered 10 5 48.3

LLE 468B Sputtered 10 20 25.5

LLE 468C Sputtered 10 100 9.6

LLE 468D Sputtered 10 300 8.0 --

LLE 468E Sputtered 10 500 14.1

LLE 469A Fi red TO 20 37.9

LLE 469B Fi red TO 80 20.9

LLE 469C Fi red TO 180 14.1

Jk~) Measured at LLE, University of Rochester.

(b) Measured at LLL, 1 nsec pulses, 2 mm beam diameter.

(c) Measured at PNL.

365
ABS. CAP

MANOMETER

TARGET

SHUTTER

SUBSTRATE

QUARTZ CRYSTAL

THK. MONITOR VACUUM

Figure 1. Features of rf diode reactive sputtering system used to deposit

oxide coatings.

366
o

CM

(Do) auruvuadiAiai aivaisans

Figure 2. Phase composition diagram for "MO- coatings constructed from pertinent

sputtering condition data. Diagram can be used to preselect the

anatase/rutile composition of an optical coating.

367
POWER DENSITY

0 100 200 300 400 500 600 700

SUBSTRATE TEMPERATURE (°C)

Figure 3. Influence of substrate temperature on the grain size of rutile

coatings. Inset shows grain size enhancement by plasma heating

during deposition.

368
<

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AVE THK = 230 NM

o

o

oo

o

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10 20 30 40 50 60 70 80 90 100

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Figure 4. Influence of phase composition on the refractive index of TiO

coatings with intermediate grain sizes.

369
2.7

1.9 I I I I I I L.

10 20 30 40 50 60

AVERAGE GRAIN SIZE (NM)

Figure 5. Influence of grain size on the refractive index of Ti0

2 rutile

coatings.

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NOMARSKI (800X) SEM (20,000X)

TI02 #80 GLASSY

Ti02 #72 40 NM GRAIN SIZE

TiO, #77 86 NM GRAIN SIZE

Figure 7. Surface topography of glassy and polycrystal 1 ine Ti0

2 coatings

measured by Nomarski microscopy and SEM.

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374
t was pointed out that the distinction among the Braunstein, Basov and Keldysh formulae must

3 made not only in terms of allowed and forbidden transitions, but also in terms of the

nalusion of interband and intraband transitions, both in the valence band and in the conuction band. A complete theoretical treatment of two-photon absorption must include both

iter- and intraband transitions, both in the valence and conduction bands. It was also

ointed out in discussion that the calculation of absolute quantities in two-photon absorpion is very difficult, but that it might be preferable to calculate differential quantities

uch as the variation of two-photon absorption with the orientation of the incident light or

s a function of other experimental variables. These differential quantities would be

^enable to experimental test and would provide a more convincing test of theory than absoj.te calculations.

I

375
I 1i:If
RECENT DAMAGE RESULTS ON SILICA/TITANIA

REFLECTORS AT 1 MICRON

C. K. Carniglia, Joseph H. Apfel, T. H. Allen and Trudy A. Tuttle

Optical Coating Laboratory, Inc.,

Santa Rosa, California 95402

and

W. H. Lowdermilk, D. Milam and F. Rainer

Lawrence Livermore Laboratory

University of California

Livermore, California 94550

The results of several laser damage experiments involving 15 layer silica/titania high reflectors (HR) are reported. Damage testing was done by Lawrence Livermore

Laboratory using 1 ns laser pulses at 1.06ym. The addition of a halfwave silica overcoat leads to a 50% improvement in HR damage threshold. An experiment to modify the

electric field intensity distribution within the coating

indicates that the determining factor is neither the peak

field within the titania layers, nor the average field in

the titania layers, but that the field at the titania/

silica interface is important. An attempt to correlate

damage threshold with absorption met with partial success.

For absorptance values above 100 ppm, there is a strong

correlation between damage threshold and absorption.

However, for absorptance below 100 ppm, it appears that

damage threshold is not related to bulk absorption in the

coatings. Optical microscopy supports this conclusion.

Key words: Absorption; calorimetry; damage; electric

field; high energy laser; laser damage; reflectors;

thin film.

High reflector (HR) coatings usually have a higher damage threshold than

other types of coatings commonly found in high energy laser systems, such as antiref lection (AR) coatings and polarizing beam splitters. The rationale

for further investigation of the damage characteristics of HR's is that such

a study provides direct information about the damage resistance of the coating and coating materials without the complicating factors arising from the

substrate on which the coating is deposited. In a typical HR design, the

laser energy is concentrated in the outermost layers of the coating, with

very little energy penetrating to the substrate interface. Thus, the observed laser damage should occur in the outer layers, and should reflect the

properties of the coating materials.

In contrast, the interface between an AR coating and its substrate re- ceives the full energy of the laser pulse. This interface is a natural weak

link in the coating system. It can be affected by the polishing process, the

surface finish, the cleaning process, and the residual surface contamination

before and during the coating process. It has been demonstrated previously

r i

1 that laser damage to AR coatings tends to nucleate at this interface LI J • The resulting damage thresholds are significantly lower for AR's than for HR's.

1. Figures in brackets indicate the literature references at the end of the

paper
Thus, investigations which use AR coatings or halfwave layers to study the

effects of variations in coating materials or process are more susceptible to

variations in substrate preparation than are studies utilizing HR's.

The laser damage results reported here are for high reflectors consisting

of 15 alternate layers of titania and silica, vacuum evaporated onto BK7-PH3

substrates. Each layer has an optical thickness equal to one quarter of the j 1.06ym laser wavelength. Assuming refractive indices of 1.45 for silica and

2.2 for titania, the calculated reflectance for a 15 layer HR is 99.6%. The j distribution of the time average of the electric field intensity within such

a coating is shown in figure 1. The peak field within the coating occurs at

the first titania/silica boundary and has a value of 0.82 relative to the incident beam. Note that the field maxima decrease with distance from the incident medium to a value of 0.002 at the coating/substrate interface.

1 The results of three different studies are reported here: (1) the effect

of the addition of a halfwave silica overcoat to the basic 15 layer HR, (2)

the effect of radical changes of the electric field distribution within the

coating, and (3) the effect of large changes in the absorption of the titania

layers. In addition, micrographs of typical damage sites are presented illustrating features of the damage morphologies.

i The damage testing was done at Lawrence Livermore Laboratory. A 1.06um

laser and a pulse duration of 1 ns were used for all tests reported here. The

parts were inspected and photographed using an optical microscope both before

and after being irradiated by the test laser pulse. Each site was irradiated

with only one pulse (one-on-one). A visible change in the surface of the part

after irradiation by the laser indicated the occurence of damage. The accuracy of the damage threshold determination for a given part is ± 10%.

1. Halfwave Silica Overcoat

A variation on the 15 layer high reflector is the addition of a silica

layer having a halfwave optical thickness. The addition of such an overcoat

has no effect on the reflectance at the laser wavelength, nor does it affect

the electric field distribution within the HR. The usual reason for the over- coat is that environmental durability is improved. This improvement carries over into the realm of laser damage as the following data indicate.

Figure 2 is a histogram summarizing the damage threshold measurements of

29 separate HR's: 17 without overcoat (indicated by horizontal shading) and 12

with overcoat (indicated by vertical shading) . These reflectors repesent 15

separate coating runs carried out over a span of approximately one year. The

average damage threshold of the overcoated parts is 14.4 joules/sq. cm, com- pared with 8.8 joules/sq. cm for the nonovercoated parts. The improvement

due to the overcoat is greater than 60% . Note that for 8 of the overcoated parts (representing 5 coating runs)

,

there is a nonovercoated part, which was made in the same run, consisting of

the same high reflector without the overcoat. This was accomplished by means

of a masking system, which covered one or more of the parts for that portion

of the run during which the halfwave overcoat was being deposited. For this

subset of the data, the comparison between overcoated and nonovercoated

parts is less sensitive to run-to-run variations in the coating process. For

these 8 pairs of parts, the damage threshold with overcoat is 13 joules/sq. cm, compared to 9 . 4 joules/sq. cm without - a 40% improvement.

The data in figure 2 provide an indication of the effect of part-to-part \[ (or coating run-to-run) variation on the damage threshold. The standard deviation of the data is 3.9 joules/sq. cm for the overcoated HR's and 2.8

joules/sq. cm for the nonovercoated HR's, in each case about 30% of the average value. It is fair to say that ± 30% is an indication of the variation of i the damage levels to be expected when testing a series of HR coatings. This

378
i

result is useful for interpreting the results of damage experiments. It also

indicates the advisability of testing several parts from several different

coating runs to establish statistical reliability.

On the other hand, it should be pointed out that the three overcoated

samples which damaged at levels above 19 joules/sq. cm represent the highest

observed threshold values for coatings made by vacuum evaporation. This represents an achievable damage threshold level for titania/silica coatings at

1.06iam.

An additional conclusion that can be drawn from these damage data is

that the silica layers themselves are not the weak link in the coating. This

conclusion arises from the fact that the peak electric field intensity in the

silica overcoat has a value of 1.9 relative to the maximum of 0.82 in the

rest of the HR coating. Thus, the silica overcoat withstands over three

times the energy as the silica layers in the nonovercoated parts at threshold.

If the field within the silica layers were the determining factor, the overcoated parts would have a significantly lower damage threhsold.

The damage morphologies shown in section 4 indicate that there are pronounced differences between the overcoated and nonovercoated parts. The un- answered question relative to these data is why the overcoat improves the

damage threshold even though the electric field distributions within the HR

coatings (and in many cases the HR coatings themselves) are essentially

identical. A possible explanation is that the silica overcoat protects the

titania from degrading due to interaction with moisture or the atmosphere.

Another possibility is that the silica layer adds mechanical strength to the

coating

2. Shifted Stack Center

If the wavelength of the incident radiation is shifted away from the design wavelength of the HR, the field distribution changes from that given in

figure 1. As shown by Sparks and Flannery [2], it is possible to shift the

majority of the laser energy into either the silica or the titania layers by

an appropriate shift of the wavelength. If one of the coating materials is more absorbing than the other, there will be a corresponding change in absorptance with wavelength. The same effect can be achieved by shifting the

design wavelength of the reflector relative to the laser wavelength. This

approach is more suitable for laser damage experiments since the laser wavelength is fixed.

Three HR' s were prepared simultaneously during a coating run. The

thicknesses of the layers were controlled by partial masks, which the parts

passed behind intermittently during evaporation. By this technique, HR's

with center wavelengths of 1.19ym, 1.07um, and 0.92ym were obtained. Figure

3 indicates that for the HR centered at 1.19ym, the electric field within

the coating resulting from incident radiation at 1.06um is greater in the

silica layers, while figure 4 demonstrates that for the 0.92]jm reflector,

the field is greater in the titania layers. The field distribution in the

1.07vim stack is essentially the same as shown in figure 1. Table 1 compares

the peak fields, the average fields, and the fields at the interfaces for

the first few layers in these three designs.

Table 1 lists the reflectances of the three stacks. Note that as the

stack center shifts away from the laser wavelength, the reflectance decreases.

Thus, the reflectors centered at 0.92 and 1.19)jm would not be practical for

a laser system. It is possible to shift the peak fields in the outer layers

without substantially decreasing the reflectance [3], but for the purposes

of the experiment, the shifted stack centers is a more suitable approach.

379
Table 1. Comparison of reflectors with shifted stack centers

Center Wavelength (um) 0.92 1.07 1.19

Reflectance at 1.06ym

0.92

0.918 0.996 0.966

Peak Field within layers *

1st (titania)

2nd (silica)

3rd (titania)

4th (silica)

1.28

0.93

1.02

0 .76

0 .82

0 .82

0 . 36

0.36

0.91

1.38

0.61

0 .90

Average Field within layers *

1st (titania)

2nd (silica)

3rd (titania)

4th (silica)

1.09

0.27

0.86

0 .22

0.41

0.41

0.18

0.18

0.29

1.03

0 .19

0.67

Field at Interfaces *

1st (air titania)

2nd ( titania/silica)

3rd (silica/titania)

4th (titania/silica)

5th (silica/titania)

Calculated Absorptance ^ (ppm)

§ Measured Absorptance (ppm)

2 Damage Threshold (j/cm )

0.62

0.93

0.47

0.76

0.33

3700

3700

9.0

0 .00

0.82

0.00

0.36

0.00

720

510

8.9

0.38

0.91

0.25

0.61

0.15

950

960

8.0

*Layers and interfaces are counted starting from the incident

medium. The values of the field represent the time averaged

square of the electric field relative to an incident beam of

unit intensity.

+ Calculated absorptance assumes that the silica layers are non- absorbing. A value of k = 0.00032 for titania is chosen to

agree with the measured absorptance in the 0.92ym reflector.

'Measured on fused silica witnesses with the OCLI 1.06Mm laser

calorimeter

380
The values for peak and average electric fields in table 1 quantify the

differences between the coatings which are apparent from figures 1, 3, and 4.

The peak field in the first titania layer of the 0.92ym stack is 40% greater

than in either of the other two stacks. The peak field in the first silica

layer (second layer of the stack) of the 1.19um reflector is more than 40%

greater than in either of the other two reflectors. The differences in terms

of average fields within the layers is even more pronounced, being on the order of 3 to 1 between the various coatings.

For each reflector the largest value of the field at an interface occurs

at the second interface (between the outer titania layer and the adjacent

silica layer) . The value of the field at this interface is nearly the same

for each of the reflectors. In fact, the central wavelengths were chosen to

produce the largest differences between the fields within the layers while

maintaining the field at the second interface as constant as possible.

The absorptance of each reflector was calculated assuming that all of

the absorption takes place in the titania layers. This is a measure of the

average field summed over the titania layers. The 0.92ym reflector has nearly four times the absorption of the 1.19um reflector and 5 times that of the

1.07ym reflector. For each stack the calculated absorptance assumes a value

i of k = 0.00032, which was chosen to agree with the measured value for the absorptance of the 0.92ym stack.

The absorptance was measured on the OCLI laser calorimeter, which has been

previously described in detail [4]. The calorimeter utilizes a 1.06ym laser

and has a sensitivity of a few parts per million (ppm) in absorptance. The

absorptance was measured on 1" fused silica witness parts, which were made at

the same time as the reflectors themselves. The excellent agreement between

J the calculated and measured values of absorptance at 1.19ym and the good

agreement at 1.07ym provide very strong evidence that the measured absorptance is due to bulk absorption within the titania layers rather than to absorption concentrated at the interfaces between layers.

From the above measured and calculated data, one concludes that if absorption in the titania or average fields within the layers were the cause

of damage, there should have been dramatic differences between the damage

thresholds of these three parts. If the peak fields were the cause of damage,

i the differences should have been less dramatic but still measurable. As indicated in table 1, the measured damage thresholds for all three parts were

essentially the same, between 8 and 9 joules/sq. cm, in agreement with the

average of 8.8 joules/sq. cm reported in section 1 for nonovercoated

reflectors. The only conclusion that can be drawn is that the field at the

I outer titania/silica interface plays a more important role in determining the

; damage threshold than either the average or peak fields in the coatings.

3. Absorption of Titania Layers

During the vacuum evaporation of titania layers, oxygen is usually introduced into the coating chamber to produce optical coatings with the de-

' sired characteristics. Too little oxygen results in highly absorbing

| coatings, while too much oxygen yields less durable or "softer" coatings. An

intermediate value for the oxygen pressure is usually arrived at by experience

For this portion of the HR study, a series of reflectors was made using

t various values of oxygen pressure during the evaporation of the titania

layers. This resulted in a set of HR's having a wide range of absorptance

values. The purpose was to investigate the relationship between absorptance

and damage threshold.

-5 -4

The range of oxygen pressures used was 0.5 x 10 to 3.0 x 10 torr.

At the lowest pressure, the measured absorption was in excess of 1% and the

381
parts were noticably dark. For pressures above 3.0 x 10 torr, the coatings were soft, consistently damaging after 20 rubs with an eraser. Six oxygen

pressures, differing from each other by a factor of approximately 1.5, were

used. The actual values of oxygen pressure are given across the horizontal

scale of figure 5. At each oxygen pressure, two HR's were made, one with and one without a halfwave silica overcoat. (Two of each type were made at

-4

1.5 x 10 torr and the damage data for this pressure presented below is an average of the results for two parts.)

The absorptance of each reflector was measured with the 1.06 laser calorimeter [4]. The absorptance data are represented in figure 5 by the "+" symbol, and the left scale gives the log of the absorptance. The solid

curve connects the data for the overcoated parts. The dashed curve connects

the data for the nonovercoated parts. The absorptance decreases monotonically

with oxygen pressure from a maximum value above 0.01 at the lowest pressure

to a minimum of 0.000014 (0.0014% or 14 ppm) at the highest pressure. The

absorptances of the overcoated and nonovercoated parts are nearly equal at

each pressure. Note the large range of absorptance values (over 3 orders

of magnitude) for the small range of oxygen pressure (less than a full order

of magnitude)

The corresponding damage thresholds are indicated by circles and are referenced to the right hand scale. There is a clear upward trend to the

damage data as a function of oxygen pressure, indicating, in general, that

higher absorption leads to lower damage threshold. On the other hand, all of

the damage thresholds of the overcoated reflectors for pressures above

-4

1 x 10 are within one standard deviation of the average value of 14.4

joules/sq. cm reported in section 1. Similarly, except for the last point,

the thresholds for the nonovercoated parts level off at between 8 and 9 -4

joules/sq. cm for pressures above 10 . Thus, the strongest conclusion that

-4 can be drawn with certainty is that for absorptances above 10 , the damage

threshold decreases.

If one accepts that the correlation between damage and absorption ex- tends beyond this limited range, then the question arises: Why does a variation in absorptance of three orders of magnitude result in a variation in

damage threshold of less than one order of magnitude? The above data suggest that linear absorption by itself is not a sufficient damage mechanism

to explain the observed damage thresholds. The damage morphologies discussed in the next section support this conclusion.

4. Damage Morphologies

An inspection of the morphologies of the damage sites using optical microscopy reveals several interesting differences between HR's of various

types. The micrographs shown in figures 6-10 were all taken with a reflected Nomarski interference attachment. Table 2 summarizes the relevant data

for the HR coatings shown in the micrographs: the presence or absence of

halfwave silica overcoat, the measured absorptance in parts per million,

the measured damage threshold for the part, and the actual laser energy

for the site shown in the micrograph.

Figure 6 is a micrograph of a typical damage site on a highly absorbing HR. This part was made using an oxygen pressure of 5 x 10 5 torr during the evaporation of the titania layers (see figure 5 and section 3) . The

large damage crater has a diameter of about 1 mm and reflects the approximate size and shape of the laser beam used for damage testing. The removal

of the coating from a large region of the part, which is apparent from the

micrograph, is typical of what one would expect if bulk absorption were the cause of damage.

382
Table 2. Data for reflectors shown in Nomarski micrographs.

(See figures 6-10)

Figure Overcoat Absorptance

(ppm)

6 No 13, 000

7 No 39

8 No 39

9 Yes 65

10 Yes 12

Damage Threshold Energy for site shown

( j/cm2) (j/cm

3.2 5.9

13.5 18.7

13.5 18.7

8.6 19 . 9

12 .5 26.4

Figures 7 and 8 show typical damage sites on a non ercoated HR having a more normal (low) value of absorption. In this case damage occurs in isolated sites which are 20 to 60ym in diameter. Thus, they are much smaller than

the diameter of the laser beam. The damage sites are round and fairly well

defined and appear blue in reflected light. Scanning electron microscope

(SEM) pictures of the sites shown in figure 7 do not indicate that any of the

layers of the coating have been removed. Rather the top surface of the coating has a "burned" or recrystallized appearance. Some of the damage sites

appear to have small holes in the center. The second site from the left in

figure 7 is such a site. It is shown at higher magnification in figure 8. The hole has a diameter of about 2ym and SEM micrographs show that it extends

down several layers into the coating.

The damage morphologies for HR's with overcoat are much different, as figure 9 illustrates. This is a heavily damaged region, which demonstrates

the two damage morphologies commonly found on overcoated HR's: Small "pits"

ranging in diameter from 2ym to lOym, and large ill-defined spots with diameters from 20ym to more than lOOym. The pits often form on defects which

were visible as point scattering sites on the coating prior to the damage

test. The damage threshold for the pits is generally lower than for the

larger spots. SEM photographs of the pits show them as small craters typical

of a thermal fracture of the type predicted by Duthler and Sparks [5]. Figure

9 is of a site with a large number of such pits. On a damage site nearer to

threshold only a few of these pits are likely to be present.

The larger damage spots on overcoated HR's are visible as dark fuzzy

round areas in figure 9. These spots are generally very difficult to see with the optical microscope. They tend to have a bluish color in reflected

light. With the aid of Nomarski interference contrast, a faint ring of color

can be seen around the central blue spot if one adjusts the Nomarski to be

away from the null position. The site shown in figure 9 was investigated using SEM, but no visible features were observed at the positions of the large

damage spots. However, if a thin layer of silver is added to the damaged

part, a distinct pattern can be observed in reflected Nomarski. This layer

i of silver prevents light from penetrating the coating, and thus, experiencing

the interference effects of the multilayer structure. The resulting Nomarski

micrographs reveal the details of the morphology of the top surface of the

coating. Figure 10 shows several such silvered damage spots from a site different from that shown in figure 9. The shading is due to the adjustment of

the Nomarski attachment and indicates that the spot is depressed in the center and raised at the edges. (Compare the shading of the scratch.) Usually

f
no apparent break in the surface of the coating is apparent, although some sites do show a small break at the center of the spot.

The isolated nature of the damage sites in both the overcoated and non- overcoated HR's with low absorption indicates that bulk absorption is not the

major cause of damage. Rather the damage is nucleating at isolated points.

These points may contain impurities or some other type of local inhomogeneity

of the coating. The circular nature of the damage indicates that the nucleation of the sites must be small compared to the final site size. Otherwise,

one would expect more irregularly shaped sites, characteristic of the shape

of the nucleating region.

The small pits usually observed on overcoated parts, and occasionally on nonovercoated parts, are apparently related to defects in the coating, which

cause an absorption of energy and lead to a Duthler-Sparks type of failure.

The larger spots seem to be caused by a different mechanism. A possible ex- planation for the spots on the nonovercoated HR's might be that the titania

is heated, perhaps by a plasma, to a temperature high enough to cause re- crystallization of the surface. The blue color might be due either to scattering by the recrystallized region or to an oxygen deficiency in the titania.

With the overcoated parts, the heat generated by such a plasma would have to

penetrate the overcoat to cause the recrystallization of the titania, thus

requiring a higher incident energy. The profile of the surface observed for

the overcoated parts could be due to: 1) a change in volume of the titania

layer under the overcoat, 2) a delamination of the overcoat from the titania

layer, or 3) a softening of the overcoat itself.

There remains the question of the cause of the nucleation of damage.

Except for the small pit type of damage, there appears to be no correlation of

the damage with observable defects in the coating. Presumably, the nucleation

sites are small compared to a wavelength of light and will be difficult to

detect

5. Conclusions

The major conclusions of this study are:

(1) The standard deviation of the damage threshold measurement, due to partto-part and coating-run-to-run variations, on a series of otherwise identical

HR's, is about 30% of the average threshold value.

(2) A halfwave silica overcoat increases the average damage threshold of a series of HR's by about 50%.

(3) Neither the average field nor the peak field in the outer silica or titania layers correlates well with damage threshold. The field at the first

titania/silica interface is the only electric field parameter that correlates

with the damage thresholds.

(4) Damage threshold increases as absorption within the titania layers

decreases. However, the results indicate that other factors play a role.

(5) Damage morphologies on highly absorbing parts indicate that bulk absorption plays a role in the damage mechanism. For parts with low absorption,

the isolated nature of the damage sites indicates that bulk absorption is not

a major factor.

384
6. References

[l] Carniglia, C. K . , Apfel J. H.,

Carrier, G. B. and Milam, D.,

NBS Spec. Publ. 541, 218 (1978).

[2] Sparks, M. and Flannery, M.

,

J. Opt. Soc. Am. 69,933 (1979).

[3]

[4] Allen, T. H., Apfel, J. H. and

Carniglia, C. K . , NBS Spec.

Publ. 541, 33 (1978)

[5] Duthler, C. J. and Sparks, M.

,

NBS Spec. Publ. 414, 219 (1974).

Apfel, J. H., Appl. Opt. 16

1880, (1977).

Figure 1. Time averaged square of the electric

field strength for 1.064pm radiation

incident from the left on a 1.064)jm

silica/ti tania quarterwave stack. The arrows indicate the electric field intensities of the incident, reflected

and transmitted waves. The position of

the layers is indicated below the field

plot. The titania layers are shaded.

385
Figure 2. Histogram of damage data on 15 layer HR

coatings with and without a halfwave

silica overcoat.

Figure 3. Similar to figure 1 for a quarterwave

stack centered at 1.19um.

386

Figure 6. Nomarski micrograph of a damage site on

a HR with high absorption.

I lOOyra

Figure 7. Nomarski micrograph of damage sites on a non- overcoated HR illustrating circular damage morphology.

388
-j lOym

Figure 8. Nomarski micrograph of one of the damage spots of

figure 7 showing a small hole in the center.

Figure 9. Nomarski micrograph of a highly damaged region

on an overcoated HR, illustrating two damage morphologies:

small pits and large blue (dark) spots.

389
lOOym

Figure 10. Nomarski micrograph of a damage site on an overcoated HR, which has been silvered after damage.

Each damage site corresponds to a blue spot observed

before silvering. The sites are raised at the edges

and depressed in the center.

390
DAMAGE TO COATINGS AND SURFACES BY 1.06 urn PULSES

W. H. Lowdermilk, D. Milam and F. Rainer

Lawrence Livermore Laboratory

University of California

P.O. Box 5508

Livermore, CA 94550

Abstract

Damage thresholds for 1 ns, 1.06 ym laser pulses were measured for bare,

polished surfaces and thin-film coatings prepared by several companies and

laboratories. We discuss the origin and morphology of damage, review the

experiments and summarize conclusions regarding the influence of substrate

material, polishing compounds and processes, coating materials and deposition

methods and the effects of absorption, electric-field strength, undercoats,

overcoats and surface etching techniques.

Key Words: absorption; electric-field strength; film materials; impurities;

1 ns; 1.06 pm damage; overcoat; polished surfaces; pulsewidth dependence;

thin-film coatings.

1. Introduction

Laser damage to uncoated (bare) and coated optical component surfaces determines the

ouput and reliability of high-energy, high-power Ndrglass lasers. We have performed a variety of experiments in collaboration with other laboratories to improve damage

thresholds and determine empirical scaling laws required for laser design.

Accomplishment of these goals requires (1) accurate and reliable measurement of damage

thresholds for different substrate materials, surface preparations, and coating

materials and designs, (2) determination of the causes of damage and (3) identification

and development of promising materials and processes.

The experiments reviewed here were coordinated by Lawrence Livermore Laboratory;

designed and performed in collaboration with Optical Coating Laboratory, Inc. (0CLI),

the Naval Weapons Center (NWC) and the Chemistry and Materials Science Department of

LLL. Damage thresholds were measured at LLL using apparatus and methods described in detail elsewhere(l )

2. Cause of Damage

Near-threshold laser damage to bare surfaces and coatings of transparent, dielectric

materials occurs as small, isolated pits which are randomly distributed over the

irradiated surface. Damage must therefore arise from particulate inclusions, physical

defects or chemical impurities. One mechanism for damage is absorption of laser energy

by these sites, which heats a small volume of material to the melting temperature or to the point of thermal-stress fracture. A second mechanism is creation of an absorbing

plasma by electron avalanche initiated by the free, or easily-liberated electrons.

Measurement of absorption coefficients for surfaces and thin films recently became

possible using laser cal or imetry(2) capable of detecting absorptions of 1 part in 10 5

, Transparent, dielectric thin films have absorption coefficients of 1-10^

cm"l. Surface-layer coefficients are estimated as 10"^ - 10 cm~l from

extrapolation of absorption measured versus sample thickness. Comparison of these

values with typical bulk-absorption coefficients of 10"3 cm~l for BK-7 glass and

10~4 cm"l for fused silica indicates the significantly higher concentrations of absorbing particles, impurities and defects in surface layers and coatings.

391
3. Damage to Polished, Uncoated Surfaces

We measured damage thresholds for a series of BK-7 glass substrates, polished by the

standard materials and procedures specified for laser optical components. The surfaces

had measured RMS roughness of 10 A and surface etching revealed minimal subsurface

fracture. Immediately before testing, the surfaces were cleaned by dragging an alcohol -wetted tissue across the surface. Cleaning the surfaces was important because

dust on the surface will burn and induce damage at fluence levels below 14 J/cm?. The

distribution of thresholds measured for 14 samples is shown by the histogram in figure

1. The median 1 ns threshold was 16 J/cm? and 80% of the samples had thresholds above

14 J/cm?. These threshold values are two to three times lower than the bulk damage

threshold of BK-7.

A possible major cause of the surface threshold being significantly less than the

bulk is that the optical grinding and polishing process leaves a physical-disturbed and

chemically-contaminated surface layer called the Beilby layer, with thickness of 10-100

urn. Measurements of absorption as a function of the sample thickness indicate the

absorption coefficient of this surface layer is 10"^ cm

- l to 10 cm- !. Polishing

compounds are an apparent, obvious source for absorbing surface-layer impurities. In fact, chemical analysis at LLL of a variety of common polishing compounds and lap

materials found metallic impurities at concentrations of 10^ ppm. Table 1 lists the

species and concentrations which were measured. Later, a chemical analysis of the

surface layer, removed by acid etch, demonstrated that these impurities did in fact

become imbeded in the surface during polishing. We recently have obtained polishing

compounds with metallic impurities reduced to 1-10 ppm concentrations, and are polishing

glass with them.

Believing that surface-layer absorption limits damage thresholds, we also have

explored removing the surface layer by various methods. Results have been largely

discouraging, probably because the many processes for surface-layer removal

recontami nate or physically damage the surface. Those methods which resulted in no increase and frequently a reduction of the threshold include sputter etching, ion

planing and acid etching. Only one method applicable to optically-figured surfaces

produced significant increase in damage threshold. In that method, a CO2 laser beam

was scanned across the polished surface of fused silica (3). Strong absorption of the

CO2 radiation caused melting and vaporization of material from the surface leaving it very smooth. Damage thresholds of these etched/annealed surfaces of more than two times

the 16 J/cm^ median threshold for polished surfaces has been observed. Possibly the

improvement in threshold results from vaporizing impurities or annealing surface and

subsurface microcracks. Additional samples will be tested to verify the result and

determine the mechanism.

Numerous threshold measurements were made on freshly-fractured surfaces of BK-7 and

fused silica. Suprisingly, the thresholds varied greatly among samples and many values were less than polished-surface thresholds. A possible explanation for the lower

threshold of fractured surfaces is the fracture v accompani ed by micro-shattering which is leaves the surface littered with glass-particle debris. This debris, when irradiated,

may easily melt and induce surface damage.

Although fractured surfaces would appear to be the logical beginning of detailed

surface interaction studies, clearly additional work is required to characterize

fractured surfaces. If, in addition, the cause of these large variations can be

determined and controlled, the fractured surface could then be used to test the effect

of impurities introduced in controlled quantity.

392
4. HR Coatings

4.1 Overcoat

Thin-film HR coatings consist of an odd number of quarter-wave thick layers, typically

15, alternating high-and low-index materials. The first and last layer are the

high-index material. The most widely used materials for visible and near IR wavelengths

are titania (Ti02) and silica ( SiO^) . Multiple reflections of the laser beam by a quarter-wave stack (QWS) creates a standing-wave electric field distribution which has a large maximum outside the coating and then alternating minima and maxima, with rapidly

decreasing amplitude at the successive interf aces(4) . The electric field at the

substrate interface is very weak. This field distribution accounts for the fact that HR coatings normally damage at the first or second layer interface.

We have found that HR damage thresholds are significantly improved by addition of a half wave overcoat. The increase in damage threshold is not related to an alteration of

electric fields. The overcoat layer does not affect the field distribution jn_ the QWS.

A large electric-field maximum falls in the overcoat layer.

Table 1 - Impurity concentrations

emission

Impurity MgO SnO Liquid

Elements Code 920 Av-116 85 b

(ppm) in polishing compounds measured by

spectrometry.

ZOX A1 2 0 3 A1 2 0 3 Reagent

CE-89 Microgrit Microgrit grade

30T 9T ZnO

Al 300 < 5000 nd nd major major <100

Cu 6 1 10 10 4 nd 3 Si 600 1000 nd nd 1 Wt% 1 Wt% <2

Fe 200 600 >1000 1000 1000 1000 nd Ca 3000 100 2000 20 3000 3000 <2

Zn <30 100 nd <1 <30 <30 major

Ni 10 60 nd <1 5 5 <3

Cr <10 30 nd 1000 <10 <10 <10

Ag <1 <1 nd nd <1 3 <1

Sr 10 1 >1000 1000 100 1000 <1

V <10 <10 nd nd 40 20 <30

Pb <10 / <10 600 60 <30 <30 <10

La nd" nd nd >10 Wt% nd nd nd Ba <100 <10 >1 Wt% 5000 <30 <30 <100

Nb <30 <30 >1000 300 <30 <30 <100

Mn 10 <1 30 20 5 5 <10

Mg nd nd >1000 10 500 500 <1 Na <300 nd 100 3000 3000 nd nd

Ti 10 nd nd 800 800 nd <10 Ga nd nd nd 30 30 nd nd Zr <10 nd >1000 nd nd nd nd

d nd means not detected.

b Y and Th level at 100- 1000 according to x -ray fluorescence.

393
The distribution of 1 ns damage thresholds for a series of overcoated (OC) mirrors

is given by the histogram in figure 2. We observed light emission at the OC mirror

surface irradiated at fluences comparable to the nonovercoated (NOC) damage fluence.

However examination by Nomarski microscope at one-hundred times magnification showed no evidence of physical damage to the OC surface. Table 2 compares thresholds for OC and

NOC mirrors.

Table 2. Damage Thresholds (J/cm?) for HR Coatings Measured

with 1 ns Pulses.

OC (12 Samples) NOC (16 Samples)

Maximum 20.3 13.5

Minimum 8.0 5.0

Median 14.4 9.2

We studied the HR damage morphology using a scanning electron microscope (SEM).

Threshold damage sites on NOC mirrors were circular spots, 10-70 ym in diameter, of

material which appeared to have been melted and recrystal lized. In the center of some spots there was a 1.5 ym diameter pit which was at least three or four layers deep.

These damaged spots often occurred at the site of defects which had been visible by

microscopy before irradiation.

Overcoated mirrors had a distinctly different morphology, with two different types

of sites. Irradiation at fluences near threshold, produced small, circular pits with

1-5 ym diameters, at least three or four layers deep, and often located on existing

defects. Irradiation with fluence 1.5-2 times the threshold fluence produced additional

sites of 100-150 ym diameter which were located in apparently defect-free areas. These

sites were not visible by SEM and barely visible by Nomarski microscopy. To enhance the

visibility of these larger sites, a layer of silver was deposited on the coating. The

sites could then be seen as areas where the overcoat had raised slightly, but not ruptured. Because the silica overcoat is under compressive stress, this morphology

suggests the overcoat softened and then raised to relieve the stress.

Possible explanations for the improved damage resistance of overcoated mirrors are

(1) that the absorption of the titania layer may depend on its exposure to the

atmosphere and (2) that the total stress of the coating and its mechanical strength is changed by the presence of an overcoat layer. To study these possibilities, additional

coatings were prepared on which overcoats were deposited after various time delays.

Control parts were included to separate the effect of thermal and vacuum cycling. Those I coatings which received overcoats immediately had the highest damaqe threshold.

However, some improvement was observed for mirrors overcoated after a delav, even nf

several months.

4.2 Substituted High-Index Layers

Titania (TiOj) has the highest refractive index of materials which are normally

used for visible and near I-R mirrors. Thus, although other high-index materials could

be substituted for the titania, they have lower refractive index so, if used in combination with Si02,more layers are required to achieve equal reflectivity. The

basic objection to increased number of layers is simply the complexity and cost of the

coating. However, because the electric field maxima decrease rapidly at interfaces

further from the air interface, the possible advantages of alternate high-index

materials could be realized without increasing the coating's complexity by depositing

additional high/low-index pairs on the basic Ti02/Si02 QWS. To evaluate this

prospect, a set of mirrors was coated by OCLI with one and two additional layer pairs.

The high-index materials in the study were Zr02, Ta205, Al 2O3 and an amorphous

Ti O2 (an OCLI propriety material). The Zr02 and Ta205 mirrors had thresholds

greater than the base Ti02/Si02 mirror, while the amorphous Ti02 had the same threshold and the Al 2O3 mirror had lower threshold than the base stack. However the

base Ti02/Si02 mirrors themselves had anomalously low threshold making these results

394
inconclusive. In a repeat experiment, two Ta205/SiC>2 pairs were added to each of

six, eleven-layer Ti O2 / S i 0^ mirrors. Three of the six also were overcoated. While

the 0C mirror thresholds were greater than the NOC threshold, no improvement resulted

from the added T^Os/SiO? layers.

4.3 Electric-Field Strength

The facts that (1) damage to HR coatings occurs in the outermost layers where the

electric-field strength is greatest and (2) Ti02 is more strongly absorbing than

S i O2 suggested possible improvement from altering layer thicknesses so as to reduce

the average field strength and absorption in the TiO? layers. Three Ti02/Si02

mirrors were coated by OCLI for maximum reflectivity at 1.192 urn, and 1.071 ym, 0.915 vim. When irradiated with 1.06 ym pulses, different standing-wave field intensities were generated. Table 3 gives the peak-and-average-f ield strength in the outermost titania

layer, the field strength at the first interface, the total absorption (measured by

laser calorimetry at OCLI) and the damage threshold for each coating.

Table 3. Field Strength, Absorption and Damage Threshold for Sh if ted-Stack HR Coatings

Wavelength of Standing-Wave Field in Absorption Damage

Max. Reflection ( m) First Ti O2 Layer (ppm) Threshold (J/cm?)

Peak Average Interface

1 193 0 92 0 29 0 92 590 8.0

1 071 0 83 0 40 0 83 450 8.9

0 915 1 .34 1 14 0 92 2300 9.0

Interestingly, although the three mirrors had large differences in peak-and-average-f ield strength and in absorption, their thresholds were nearly equal.

Because the interface field strengths were also nearly equal, we thought that the

interface field might be the important factor. To study that possibility, OCLI coated

two sets of 0C mirrors with different layer thickness. One set was the normal, QWS

design and the other was a nonquarter wave design in which the interface electric field

had a value two-thirds that of the normal, QWS interface field. Several runs of each

design were coated. Different designs were made in alternate runs to reduce systematic

errors. Damage measurements gave the surprising result that those mirrors with lower

interface field strength also had lower thresholds. It is possible that the higher, net tensile stress of the nonquarter wave mirror caused its threshold to be lower. We are presently studying the influence of stress on OC mirror damage thresholds.

4.4 Absorption

To determine the correlation between damage threshold and coating absorption, OCLI prepared a series of HR coatings deposited at different oxygen pressures. Coating

absorptions of 10"5 - 10"^ were measured by 1 aser-calor imetry at OCLI. We found

that coatings with absorption greater than 10" 4 had damage thresholds which generally

decreased with increasing absorption. Those coatings with absorption less than 10"*

had thresholds which were uncorrelated with the absorption. This result however does

not imply that the threshold is independent of coating absorption. The important point

is that damage results from heat deposition in a small volume localized around a defect

or impurity. The absorption coefficient in that small volume can be very much larger

than the average absorption. The laser calorimeter, on the other hand, measures the

average absorption over the the area of the laser beam and the thickness of the

coating. It is this vol umed-averaged absorption which we find is not strongly

correlated with damage threshold. Further understanding of the connection between

absorption and threshold would be greatly aided by the ability to measure absorption in

a spatially resolved volume of 1 ym dimensions.

395
5. AR Coatings

Whereas damage to HR coatings occurs in the outermost layers, AR coating damage

usually initiates at the substrate interface. The damage induced by pulses of durations

up to several nanoseconds is a collection of small pits with diameters of 1-5 ym which

are characteristic of the total thickness of the coating. Viewed collectively by the

unaided eye, these pits appear as a haze. Individual pits can be resolved, and the

evolution of their morphology with increasing fluence studied by optical or electron

microscopy. Figure 3 shows photographs of a surface damage site taken by SEM(5). The

photographs strongly suggest an explosive eruption upward and outward from the substrate

interface. At the edge of the irradiated area, where the beam fluence is low, the

damaged site consists of a raised area with radial cracks in the outer coating layers. At somewhat higher fluence, a 3ym diameter disk is loosened and, at higher fluence, blown

away, exposing the substrate surface. Note the evidence of melted material at the

bottom of the pit and also some material ejected out on top of the surrounding area.

Two additional facts are consistent with the assumption that the interface region

has significantly larger absorption than the surrounding coating and substrate

material. First, although the standing-wave electric-field peak intensity in AR coatings is less than in HR coatings, AR damage thresholds are less than HR thresholds.

This result suggests that the substrate interface, which is irradiated in AR but not in HR coatings, plays an important role. Second, AR coating damage thresholds do not depend strongly on the absorption average over the coating thickness. That result

indicates that absorption is not uniform; rather it is high in some places and low in others. Estimates of the interface absorption coefficient of 10 - 10^ cm

-

* were made at OCLI from the absorption measured for coatings of different thickness.

Damage experiments on AR coatings were designed primarily to study effects of

absorption at the substrate interface and in the coating layers. These experiments

included variations of substrate materials and surface preparation, coating design and

deposition processes and coating materials.

Substrates of BK-7 and fused silica were polished either by conventional or by

bowl-feed process and then coated with identical AR coatings. The damage thresholds did

not depend on the substrate material, but did improve on the bowl-feed polished

surfaces. The distribution of thresholds on conventional and bowl-feed polished

surfaces is shown in figure 4. The increase in median threshold for coatings on bowl-feed polished surfaces is interesting in view of the fact that the damage

thresholds of bare surfaces polished by conventional or bowl-feed process are equal.

Consequently the difference in the coating thresholds is attributable to differences in the chemical state of the surface, which probably changes the coating's absorption.

The effect of undercoat layers of Si02 or AI2O3 deposited between the

substrate and first layer of the AR coating was studied further. The results of this

and earlier studies showed that presence of the undercoat increases the median damage

threshold 30%. However the variation in thresholds among samples of both undercoated

and nonundercoated parts was large. We think the statistical improvement effected by

the undercoat is associated with change in the chemical or impurity environment at the

substrate interface.

6 . Thin Film Materials

A range of oxide and fluoride film materials were deposited in single and multilayer

coatings to determine whether these common materials offer improved damage thresholds.

The oxide films studied were SiO^, TiO? and multilayer combinations of the

low-index SiO? with one of the high-index films Ti02 , A 1 2O3 , Zr02 and

Ta205. The damage thresholds of all these coatings were nearly equal, although some Si02/Ta205 AR coatings were found with thresholds considerably above average.

396
We also made a brief survey of fluoride coatings. Single layers of MgF2, NaF,

Na3AlF5 and multilayers of MgF^/ThF^ MgF2/PbF4 and ThF^ZnS were tested.

The initial results indicated that these materials offer no immediate improvement over

the presently used oxide films.

7. Pulsewidth Dependence of Damage Threshold

Laser-induced damage thresholds generally depend on the laser pulsewidth. Bettis et

al ( 6 ) suggested the empirical scaling law that damage threshold increases as the square

root of the pulsewidth, and presented a heuristic argument for such scaling based on the

intensity dependence of inverse-Bremsstrahlung plasma absorption. We measured damage

thresholds at 0.17 ns, 1.0 ns, 1.6 ns and 3.5 ns. Figure 5 shows the pulsewidth

dependence of damage threshold for polished bare surfaces, HR coatings and AR coatings.

The solid line corresponds to a square root pulsewidth dependence. We find the

square-root increase to be an upper limit which is met only by some bare surfaces.

Coating thresholds increase more slowly and some, particularly AR coatings, are nearly

independent of pulsewidth between 1 ns and 3.5 ns.

8. Summary

Our measurements can be summarized as follows:

1. Conventional polishing compounds and polished surfaces of BK-7 and fused silica

are contaminated with metallic impurities.

2. Pure polishing compounds have been identified, but surfaces fabricated with

those compounds have not yet been tested.

3. Removal of the polished layer by CO2 laser beams increased the damage

threshold; surface removal by acid etching, sputter etching, and ion planning

was ineffective.

4. Breaking holds promise as a technique for generating clean surfaces, but is troubled by microscopic glass particles generated during breaking.

5. Half-wave S i O2 overcoats increased the threshold of Si02/Ti02 high

reflectors, but overcoated reflectors emit light when irradiated at sub-threshold fluences sufficient to damage non-overcoated reflectors.

6. Substitution of AI2O3, Ta205, or Zr02 for Ti O2 in the outer layers

of Si02/Ti02 high reflectors produced mixed results - an advantage was not clearly demonstrated.

7. Alteration of film designs to produce lower standing wave intensities at either

film interface or in the Ti02 layers did not increase damage thresholds.

8. Absorptions measured by laser calorimeter correlate with damage thresholds in high reflectors with absorpti on > 10" 4

, the correlation fails at lower

absorption.

9. Thresholds of 4-layer Si02/Ti02 anti-reflective films deposited on super-polished substrates exceeded the thresholds of similiar films deposited

on conventionally polished substrates, even though thresholds of the bare

substrates were comparable.

10. Fluoride films MgF2, NaF, Na3ALF5, ThF4, and PbF4 offered no immediate

improvement over presently used oxide films.

11. For pulse durations from 0.1 to 3.5 ns. 1064-nm damage thresholds did not increase, in general, proportional to the square-root of pulsewidth. The

square-root scaling was observed on some bare polished surfaces; thresholds of A-R films tested did not vary between 1.0 and 3.5 ns.

397
9. Conclusions

Defects, not yet identified, dominated all surface and film thresholds. Some

techniques for improving thresholds without deliberately attacking the defect problem,

such as re-design of the coating or use of alternate materials, were generally

ineffective. One method of removing impurities and defects from polished surfaces,

laser annealing, was successful; other surface removal techniques failed. Significant

improvements in film or surface thresholds will require identification and elimination

of the defects causing damage. Occasional films and surfaces with high damage

thresholds, and the low value of film and surface thresholds as compared to bulk

thresholds, indicate significant improvements are possible.

Acknowledgments

Many persons participated in the experiments we have described. C. K. Carniglia and

J. H. Apfel of Optical Coating Laboratories designed and fabricated thin films for

measurements reported here. T.A.Tuttle and T. H. Allen, also of OCLI, performed

microscopic analysis of damage and measured the film absorption. P. A. Temple of the

Naval Weapons Center did the CO2 laser annealing of bare surfaces. R.L. Landingham of

the Chemistry and Materials Science Department at LLL analyzed coating compounds and

polished surfaces, andT.M. Quick of the same group developed techniques for breaking

glass. G. E. Murphy and S. E. Peluso assisted in the maintenance and operation of the

lasers used for damage tests, and A. B. Griffith prepared the manuscript.

We are also grateful to other laboratories and individuals for their assistance to the LLL damage program; reports of several experiments are described elsewhere in these

proceedings.

References

1. Milam, D. in Proceedings of the Society of Photo-Optical Instrumentation Engineers,

Vol. 140, Optical Coatings II, De Bell, G. W. and Harrison, D. H. eds. 1978.

2. Decker, D. L. and Temple, P. A. in Laser Induced Damage in Optical Materials: 1977 NBS Special Publ. 509, p. 281; Allen, T. H., Apfel, J. H. and Carniglia, C. K. in Laser Induced Damage in Optical Materials: 1978, NBS Special Publ. 541, p. 33.

3. Temple, P. A., Milam, D. and Lowdermilk, W. H., CO2 Laser Polishing of Fused

Silica Surfaces for Increased Laser Damage Resistance at 1.06 ym, elsewhere in the

proceedings.

4. Apfel, J. H. Applied Optics, 16, 1880 (1977).

5. The damage sites were photographed using SEM by Carrier, G. F. of Corning Glass

Works.

6. Bettis, J.R., House, R. A. and Guenther, A. H. in Laser Induced Damage in Optical

Materials: 1976 NBS Special Publ. 462, p. 338.

398
4

o2

I 2 Z

t—i—i—i—i—i—i—|—i—i—i—

Number tested 14 Median 16 J/cm2 80% exceed 14 J/cm2

10 12 14 16 18 20 n22

1 ns damage threshold, J/cm2 24

Figure 1. Histogram of damage thresholds of

polished, uncoated surfaces of BK-7 and fused

silica measured with 1 ns pulses.

3 - C&

! 2 £

E

3Z

1 1

1 1 1 Total number 12 Median threshold 15 J/cm2 90% exceed 10 J/cm2

D 10 12 14 16 18

1 ns damage threshold, J/cm2

20 22

Figure 2. Histogram of damage thresholds of

overcoated Si0 2 /Ti0 2 mirrors measured with 1 ns pulses.

399
(a)

Figure 3. Damage sites on four-layer Si0 2 /Ti0 2 AR coating observed by SEM(5). The sites shown

in (a), (b), (c) were caused by increasing

fl uence.

400

402
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12 14

FIGURE ka FIGURE kb

Figure 4. Histoqram of damage thresholds of

Si0 2 /Ti0 2 AR coatings on (a) conventionally

polished (b) bowl -feed polished surfaces

measured with 1 ns pulses.

AR (thin film) Graded index

Pulsewidth. NS

Figure 5. Damage thresholds versus pulse width

for bare surface, HR coating and AR coating.

I

In response to a question on the absence of dependence of the observed damage threshold on

the thickness of anti-reflection films, the speaker indicated that film thickness was varied

over a factor of two in the course of the investigation, and no detailed calculations of

field distribution within the film were carried out. He further indicated that in connection

with the studies of phase-separated graded-index surfaces, light scattering was not observed

to be a problem.

403

PULSED DAMAGE THRESHOLDS OF FLOURIDE AND OXIDE THIN FILMS

FROM 0.26 um TO 1 .06 um

T. W. Walker , A. H. Guenther, and C. G. Fry' Air Force Weapons Laboratory

Kirtland AFB, New Mexico 87117

and

P. Nielson

Air Force Institute of Technology

Wright Patterson AFB, Ohio 45433

The pulsed laser-induced damage threshold for CaF2, MgFp. ThF^, MgO,

A^Og, Ti02 > Si O2 , HfOg, and ZrO£ thin films was measured. The measurements

were made at 1.06 um, 0.53 um and 0.26 um with pulse lengths of ~5 and ~15 ns (measured at 1.06 ura). Different film thicknesses of each material were damaged

at each wavelength and pulse length. The film thicknesses were U, 1/2 x, 1/4 x,

1/6 X, and 1/8 X at 1 .06 um.

It was found that the pulse length dependence of breakdown fit with a [pulse length]'2 curve for most of the materials. Exceptions were found in some oxide

coatings at 1.06 um where the breakdown energy density was essentially independent

of the pulse length. The damage threshold of the flouride and Si^ films increased

with a decrease in film thickness. This did not correlate with changes in the E field inside the thin films. (The oxide films did not exhibit as pronounced

a thickness dependence.) The breakdown threshold decreased with decreasing wavelength and correlated inversely with the absorption coefficient, although the correlation was not linear.

There was a marked change in the damage morphonogy of the materials as wavelength was decreased. At 0.26 um all the oxide material damaged in a uniform

region while the flourides damaged as isolated pits. The morphology, pulse length

and film thickness trends are discussed in terms of impurity, avalanche ionization,

and multi photon damage models.

Key words: Laser damage, thin films, pulse duration dependence of damage; wavelength

dependence of damage; impurity damage; avalanche ionization; multi photon induced * damage; film thickness dependence.

The damage thresholds of thin films have previously been studied as a function of laser wavelength (1-6) pulse duration (7,8) and film thickness (9). These experiments have generally been uncorrected since each was performed as a separate study. The interpretation of each uncorrelated

result can vary because of variations of absolute results from separate experiments. We report

the measurement of the damage threshold of several flouride and oxide thin film coatings as a function

of pulse duration, laser wavelength, and film thickness. If a theory of thin film data is to be comprehensive it must explain the correlated variation of the above parameters.

* Presented in partial fulfillment of the Ph.D. degree at the Air Force Institute of Technology,

Wright Patterson AFB, Ohio

|@ Presently at Iowa State University, Ames. Iowa

+ Presently at Air Command and Staff College, Maxwell AFB, AL

1. Introduction

405
2. Experimental Arrangement

Figure (1) is a schematic diagram of the experimental layout. The beam diagnostic path was maintained to be as close a replica of the high energy damaging path as possible. An optical

multichannel analyzer (OMA) was used to assess the beam intensity spatial profile for each laser

pulse. Because of the limited spatial resolution of the OMA (100 ym), the focal plane in the diagnostic path, which is equivalent to the high energy damaging focal plane, had to be magnified

and reimaged onto the vidicon. This was accomplished by the lens L3 shown in Figure (1).

It was necessary to check the equivalence of the two focal planes and the imaging quality

of the lens L3. During damage testing at 2600 A it was found that films damaged uniformly.

These films did not exhibit any isolated pit formation. An obscuration was placed in the near field of the laser beam to produce a distorted far field pattern. The HfO-, thin film sample was damaged in a nonuniform pattern and the resulting morphological record compared to the OMA output.

The result is shown in Figure (2). This procedure was repeated ten times with similar correlation.

Calibration of the total system was complicated and will be detailed in a later report.

All damage shots were taken at the focal plane of the lens at LI. Unfortunately, it was not possible to hold the spot size constant over the four laser wavelengths used in the experiment. In order to minimize the spot size change at shorter wavelengths a 1 meter focal length matched set of lenses were used at 0.35 ym and 0.26 ym. The focal spot sizes at 1.06, 0.53, 0.35, and 0.26 ym

where 360, 280, 220, and 170 ym respectively. These are given as FWHM points.

The pulse durations were also not constant and were a function of laser wavelength. This was

due to the response of the nonlinear frequency connecting crystals. The pulse durations at 1.06,

0.53, 0.35, and 0.26 ym where 15, 14.5, 13.5, and 11.5 ns respectively at the long pulse lengths and

5, 4.8, 4.3, and 3.9 ns at the short pulse lengths. The relative pulse to pulse variation in pulse

length was ±5%.

The experimental error was determined to be ±17%, most of which was attributed to the reproducibility energy meter. The maximum variation in vidicon response across the active

surface was found to be ±7%. The data shown in Tables 1-5 gives the damage thresholds of the

2 tested thin films in Joules/cm . The number following each damage threshold represents

the span over which damage and no damage results overlapped. Each threshold is derived from

approximately thirty laser shots on the sample.

3. Results

The damage thresholds in Tables 1-5 have been corrected for the standing field pattern inside

the thin film by using the equations given by Newnam, et al . (9). The most apparent result is the change in damage threshold with film thickness. This is extraneous to the film thickness dependence

reported by Newnam, et al (9), caused by the change in the standing field pattern with film thickness.

It should be pointed out that much of the film thickness dependence of damage in Ti0„ was removed by

correcting for the standing field inside the film. However, it is clear from the corrected data that

this is not the complete explanation.

The data shows a general trend of decreasing thresholds as the laser wavelength decreases. This

is in general agreement with the results of Newnam, et. al . (1), and shows a general correlation

with the absorptivity measurements. A variation in damage threshold with pulse length is also evident

The weakest overall time dependence is shown by ZrO^. Several of the oxides show a weak dependence

at 1.06 ym, but not at shorter wavelengths. The time dependence also increases for many of the thin

film materials as the film thickness decreases. Finally, Figure (2) shows the damage morphology as

a function of the laser wavelength. It can be seen that there is a consistant decrease in the damage

site size as the wavelength is decreased.

406
4. Conclusions

A theoretical interpretation of this data based on impurity dominated damage will be presented

in a later paper. This theoretical model provides an overall interpretation of the pulse length,

film thickness and wavelength dependence of the damage threshold. The morphology changes can also

be addressed by this model.

In order to understand the damage mechanism in thin films it is extremely beneficial to correlate many of the variables which can occur in the experimental situation. In this

experiment the film thickness, laser wavelength and pulse length were all correlated in an attempt to explain the breakdown process. In addition, the morphology was studied as an auxiliary

source of information. The total integration of these parameters is best explained by an impurity

model of the breakdown process.

The authors would like to acknowledge the outstanding technical assistance of Mr. J. Jasso

and Mr. M. Morris for their contribution in obtaining the experimental data presented in this

paper.

ice

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5. Summary

407
6. References

Newnam, B. E., and Gill, D. H., Special

Dependence of Damage Resistance of

Refractory Oxide Optical Coatings, Laser

Induced Damage in Optical Materials , NBS Special Publication 462, eds. A. J. Glass

and A. H. Guenther, US GPO, Washington D.C. (1976)

Porteus, J. 0., et a!., Multithreshold

Evaluation of 175-nanosecond Pulsed Laser

Damage to Coating Materials at 2.7 and 3.8

m Wavelengths, Laser Induced Damage in Optical Materials , NBS Special Publication 541, eds. A. J. Glass and A. H. Guenther, US GPO Washington D.C, (1978).

Newnam, B.E., and Gill, D. H., Ultraviolet

Damage Resistance of Laser Coatings, NBS Special Publication 541, eds. A. J. Glass

and A. H. Guenther, US GPO Washington

D. C, (1978).

Fradin, D. W. et al.. Comparison of Laser

Induced Bulk Damage in Alkali -Hal ides at

10.6, 1.06, and 0.69 Microns, Laser

Induced Damage in Optical Materials , NBS Special Publication 372, eds. A. J. Glass,

and A. H. Guenther, US GPO, Washington

D.C, (1972).

Smith, W. L., et al . , Picosecond LaserInduced Breakdown at 5321 and 3547 A: Observation ° f Frequency Dependent

Behavior, Phys . Rev . B 15 , No. 8,

4039, (1976TT~

[6] Soileau, M.J., et al., Frequency and SpotSize Dependence of Laser Damage in the Mid-IR

Region, Laser-Induced Damage in Optical

Material

, NBS Special Publ ication 541 , eds.

A. J. Glass and A. H. Guenther, US GPO,

Washington D.C, (1978).

[7] Wang, V., et al., Single and Multilongitudinal

Mode Damage in Multilayer Reflectors at 1.06

m as a Function of Spot Size and Pulse

Duration, Laser Induced Damage in Optical

Materials , NBS Special Publication 435, eds.

A. J. Glass and A. H. Guenther, US GPO,

Washington D.C. (1975)

[8] Bettis, J.R., et al., Spot Size and Pulse

Dependence of Laser Induced Damage, Laser

Induced Damage in Optical Materials , NBS Special Publication 462, US GPO Washington, D.C. (1976)

[9] Newnam, B. E., et al . , Influence of StandingWave Fields on the Laser Damage Resistance

of Dielectric Films, Laser Induced Damage

in Optical Materials , NBS Special

Publication 435, eds. A. J. Glass and A. H. Guenther, US GPO, Washington D.C. (1975).

408
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1.06 m 0.53 m

416
LASER- INDUCED DAMAGE MEASUREMENTS WITH 266-nm PULSESt

T. F. Deaton* and W. L. Smith

University of California

Lawrence Livermore Laboratory

P. 0. Box 5508

Livermore, CA 94550

The results of a survey of 1 a se r- i n d uced damage thresholds

for optical components at 266 nm are reported. The thresholds

were measured at two pulse durati ons--0 . 1 50 ns and 1.0 ns. The

30 samples tested include four commercial dielectric reflectors,

three metallic reflectors, two anti -reflection films, a series

of eight half-wave oxide and fluoride films, and twelve bare

surfaces (fluoride crystals, silica, sapphire, BK-7 glass,

CD*A and KDP). The 266-nm pulses were obtained by frequencyquadrupling a Nd:YAG, glass laser. Equivalent plane imagery

and calorimetry were used to measure the peak fluence of

each of the UV pulses with an accuracy of ± 15%; the uncer- tainty in the threshold determinations is typically ±30%.

Key words: Damage thresholds; laser damage; pulsewidth dependence, picosecond pulses; UV lasers.

1. Introduction

There is currently an increasing interest in the application of visible

and ultraviolet lasers to inertial confinement fusion research. Fusion

j target experiments utilizing the second- and third-harmonic wavelengths of

j the Argus Laser are currently underway or in planning at Lawrence Livermore

Laboratory. Fusion experiments at 266 nm on Argus are probable. A prototype KrF-CH4 Raman-compressed laser system is also being developed which will

involve two UV wavelengths - 268 and 248 nm. There is a clear need for a broad data base of the optical properties of materials that are useful

j near 266 nm.

In response to this need we have performed a survey of damage thresholds

[ at 266 nm at pulse durations of about 100 psec and about 700 psec of some thirty samples comprising a representative set of the materials and coatings

currently available. This subnanosecond data complements the 22 nsec data

j made available last year [1]^. The thin films tested include four total

reflector films, three metallic reflectors, two anti -ref 1 ecti on films, and

a series of eight half-wave fluoride and oxide films. The twelve tested

bare surfaces include many important UV window materials - a 1 k a 1

i - f 1 uor i de

and alkaline earth-fluoride single crystals, hot-forged lithium fluoride,

fused silica, sapphire - and also BK-7 glass, potassium dihydrogen phosphate

(KDP) and cesium dideuterium arsenate (CD*A). It must be noted that the

samples tested in this survey have not been specifically developed to have

high damage thresholds at 266 nm. They are instead representative of

current off-the-shelf components.

j tWork performed under the auspices of the U.S. Department of Energy under

contract W-7405-eng-48.

li *

|| United States Air Force Research Associate.

^Figures in brackets indicate the literature references at the end of

this paper.

i

417
2 . Experi menta

The laser used for the 100 psec pulse duration measurements consisted of

a dye mode-locked Nd:YAG oscillator, two Nd:YAG and two Nd:glass amplifiers.

It has been used in the past for short-pulse 1064-nm damage studies [2],

The 700 psec data were obtained after the installation of an actively modelocked and Q-switched oscillator which provides pulses with durations

ranging from 100 psec to 1.2 nsec [3]. The average pulse duration for the

dye mode-locked oscillator was 150 psec, as determined by streak camera photography. There were of course significant fluctuations in the pulse

duration, on the order of ± 30 psec, due to the statistical nature of the

dye. In this experiment we do not at present have the capability to measure

a s ubnanosecond pulse duration in the UV, so the figure 100 psec is simply

our best estimate of the average fourth harmonic pulse length, which we believe to be accurate to within ± 20 psec. The longer UV pulses were obtained from 1064 nm pulses of 1.02 nsec duration from the actively modelocked oscillator. The accuracy of the 1064 nm pulse duration figure is

better than 5%, but again the UV pulse duration of 700 psec is estimated,

and has an uncertainty of ~ ±50 psec.

The energy of the 1064 nm, linearly polarized laser pulse is determined

by a calorimeter sampling the beam via an uncoated BK-7 beamsplitter (fig.l).

Second harmonic generation is done by an angle-tuned KD*P crystal (Type I)

with conversion efficiency as high as 80% for 100 psec pulses. For some of

the work we used a temperature-tuned CD*A crystal with which we obtained

similarly high conversion efficiency. The remaining 1064 nm light after the

SHG cell is spatially separated from the second harmonic by a prism set

approximately at minimum deviation. The 532 nm beam is similarly sampled

for energy content via a BK-7 beamsplitter; it then enters an angle-tuned

KDP or temperature-tuned ADP crystal for fourth harmonic generation. The

maximum conversion efficiency at this stage was on the order of 30%. The

532 nm light remaining after the cell is spatially separated by a quartz

prism.

The 266 nm optical circuit is shown in figure 2. The beam is focused by

a 2 meter focal length lens; the focal plane occurs at approximately 40 cm beyond the front surface of the sample. After the lens, the beam is split

into two parts by a partially reflecting mirror, with 48% of the energy going

to an absorbing glass calorimeter (GG19 glass). A second bare beam-splitter

directs energy to a vidicon camera and to a multiple-exposure mirror. The

off-set angle of the beamsplitters was 3 degrees. A multiple-exposure photograph of each shot is taken with Plus-X film in a plane optically-equivalent

to the plane of the front surface of the sample. The laser spot size at the

sample plane was approximately 1.0 mm in diameter.

All the samples were irradiated at about 5° from normal incidence, one shot per site. The damage used was the existence of front surface alteration as determined by visual inspection immediately after the shot or

by Nomarski microscopy afterward.

Our standard procedure was to fire enough shots to reduce the threshold

uncertainty to ± 10% in 266 nm pulse energy. When the beam profile data were analyzed, there was found to be a significant variation in the beam

shape, and particularly in the effective area of the beam, from shot to

shot. In some cases this variation produced a narrowing of the uncertainty

of the peak fluence value at threshold, but in others it increased the

uncertainty. Generally the fluence uncertainties are greater for the short

pulses, since these were obtained from a dye mode-locked laser instead of

the very amp! i tude- stabl e AMQ0.

418
The multiple-exposure images of the beam were analyzed by the same techniques used for the 1064 nm laser damage experiments at their laboratory [4].

Experience has shown that the combination of the very stable absorbing glass

calorimeter and our photocal ibration codes yield peak fluence values accurate

to within + 10%.

3. Results and Discussion

The damage thresholds for the bare surfaces tested are shown in table 1. There is variation of about one order of magnitude for the group. The first

eight samples listed are a series prepared in 1976 similarly and for

intended damage testing at 1.06 microns. There is relatively little

variation in thresholds for the series, the average being about 1 J/cm? at

100 psec and 2-3 J/cm? at 700 psec. We hypothesize that the general

clustering of these thresholds may be due more to the surface finishing

techniques than to any intrinsic behavior.

The remaining samples on the table are from various sources. The silica

and BK-7 in particular are substrates intended for coating damage studies

at 1.06 microns. The silica sample was particularly damage resistant, at

least in a relative sense. The polishing techniques used for this sample,

although developed for 1.06 micron, are somewhat more advanced than the

simple hand polishing used for our fluoride samples. It is thus conceivable

that with improved surface preparation the flouride damage thresholds might

be considerably higher.

In light of the widespread application of silica optics in UV laser

systems, it is encouraging to note that the silica threshold is as high as

it is.

An unexpected result is that, at least for the short pulses, the BK-7

threshold was almost as high as the silica and higher than the fluorides, in

spite of its high bulk absorption coefficient of about 6 per cm at 266 nm.

We interpret this result as further evidence that surface properties are to

a large extent masking the bulk properties of these samples.

The KDP sample threshold of about 6.5 J/cm^ at 700 psec is a relatively

high threshold and is encouraging given the application of KDP in second

harmonic generation from 532 nm to 266 nm. The bottom two LiF samples in table 1 were also damage tested at 1.06

microns by Dave Milam e_t. al_. at Lawrence Livermore Laboratory. We found

essentially no difference between the s i ng 1 e- crys ta 1 and hot-forged LiF;

the same was found at 1.Q6 microns. Although 1.Q6 micron damage thresholds for these samples were rather disappointingly low, at 266 nm they

were the highest thresholds measured.

We tested eight half-wave layers of fluorides and oxides on silica

substrates, as shown in table 2. With the exception of the yttrium oxide,

the thresholds are clustered very closely together and are essentially the

same at both pulse durations, and are lower than either the AR films or the

high reflector films on dielectric substrates. Given the clustering and the error bars of the measurements, it is tenuous to assign any systematic

correlation between the thresholds and bulk material properties such as band

gap or index of refraction.

419
The relative magnitudes of the damage thresholds within the set correlate

in a qualitative way with the thresholds for quarter-wave layers of the same materials for 22 nsec pulses reported at this conference last year by

Newnam and Gill [1 ]

We also tested several total reflector and anti -ref 1 ecti ve films. The

thresholds are listed in table 3. We identify the samples only by a letter

because in most cases the coating designs are proprietary, and we consider

it to be ill-advised to try to distinguish between designs based only on this

limited sampling. The dielectric reflectors average about 1.0 J/cm 2 at

100 psec and 1.8 J/cm 2 at 700 psec.

The AR film thresholds were slightly higher than the reflectors at

100 psec, in contrast to the situation at one micron, in which AR films

are typically a factor two weaker than high reflectors. For the 700 psec

duration, the AR's were on the average about as strong as the HR's. The

metallic reflectors exhibited much lower thresholds than the dielectric

reflectors. Finally, we turn to the pulse duration dependence.

The bare surface data are shown plotted versus pulse length in figure 3. The clustering of the fluoride samples is evident. The average pulse length

is about t0 - 5 . The pulse length dependence of the single-layer films is

shown in figure 4. The exponential factor of the pulse length dependence

averages out to nearly zero. The pulse length dependence of the dielectric

reflectors is shown in figure 5. The dependence is between about and t°-5.

4 . Concl us i ons Because only one sample of each specific type was tested, minor distinctions between materials should not be made on the basis of this data. Our

conclusions are that 100 psec the current generally available threshold is

about 1 J/cm 2 with a few important exceptions (silica and KDP). At 700 psec,

2-3 J/cm 2 damage thresholds are to be expected at this time, again with the

exceptions of silica, KDP, and lithium fluoride.

5. Acknowledgements

We gratefully acknowledge the collaboration of B. Flint and L. Stelmac

of Acton Research Corporation, J. Stanford of the Naval Weapons Center,

W. Hargreave of Optovac, and J. Ready of Honeywell, in the supply of samples,

and the capable assistance of M. Taylor in making the measurements.

References

Jj] Newnam, B. E. and Gill, D. H.,

Laser Induced Damage in Optical

Materials: 1978 (NBS Special

Publ ication No. 541 , US GP0,

Washington , D. C. ) , p. 19.0.

[2] Milam, D. Applied Optics 1_6,

1204 (1977).

[3] Kuizenga, D. J., and Martin,

W . E . , Performance of Short s Pulse Oscillators for Argus

and Shiva, presented at ElectroOptics/Laser 1977 Conference in

Anaheim, Ca. (October 25-27,

1 977] .

[4] Milam, D. Measurement and

Identification of Laser Damage

Thresholds in Thin Films , presented at S.P.I.E. Technical

j Symposium East, Washington, D.C

(March 28-30, 1978)

420
2 Table 1. Bare surface damage thresholds (J /cm ) for 266 nm .

o Fl uence (J/cm ) Material 100 psec 700 psec

Li F 0 . 60 + 0 . 39 Ba F

0 . 50 + n 1 3 2 . 06 + nU • 7?/ L

S r F« 0 . 47 n 1 2 1 . 54 u . o I Ca F

2 0 .65 + 0. 1 5 2 . 37 + 0.83

MgF

2

1 . 32 + 0. 32 2 . 69 + 0.41

LaF

3

1 . 80 + 0. 80 3 . 1 0 + 0. 70

A1

2 0

3 0 . 75 + 0. 26

Silica 3 . 00 + 0. 96 1 0 . 00 + 3.00

BK-7 2 . 90 + 0. 84 1 . 94 + 0.71

CD*A 1 . 10 + 0. 27

KDP >0 . 70 + 0. 25) 6 . 48 + 1 . 40

Li F 1 4 . 30 + 2 . 50

Li F (hot forged) 14 .40 + 2 . 50

_ o Table 2. Damage thresholds (J/cm ) for single-layer films

(A/2 at 266 nm) at 266 nm.

100 psec 700 psec

Y

2 0

3 0. 10 + 0.035 0.26 + 0.04

Zr0

2 0.51 + 0.26 0.62 + 0.14

Si0

2 0.72 + 0.26 0.44 +0.13

Al

3 0.65+0.15 0.50+0.07

LaF

3 0.70 + 0.21 0.74 + 0.20

MgF

2 0.80 +0.32 1.24 + 0.33

PI 0.95 + 0.25 0.66 + 0.10

P2 0.85 + 0.23

421
2 Table 3. Film damage thresholds (J/cm ) for 266 nm pulses.

n . . . £ , . 100 psec 700 psec Dielectric reflectors c c

a 1 30 + 0 25 3 85 + 1

b 0 50 + 0 12 0 405 + 0

c 0 92 + 0 20 1 86 + 0

d 0 85 + 0 23 1 54 + 0

e 1 43 + 0 Metal 1 i c ref 1 ectors

a 0 23 + 0 06 0 60 + 0

b 0 10 + 0 04 0 066 + 0

c 0 1 3 + 0 05 0 045 + 0 AR films

a 1 10 + 0 27 1 20 + 0

b 1 60 + 0 54 0 72 + 0

1.06 input BS KDP

Figure 1. Fourth-harmonic generation optics.

422
Lens BS BS

Figure 2. 266 nm damage experimental apparatus.

423
100 700

Pulse length, psec

Figure 4. Pulse-length dependence of damage

thresholds for single-layer films.

0.1

Figure 5.

100 700

Pulse length, psec

Pulse-length dependence of damage

thresholds for dielectric reflectors.

Of all the samples tested, the largest variation in damage threshold at the two different

pulse lengths was observed in sodium fluoride. When questioned about this large variation, [1 the speaker indicated that approximately six months elapsed between those two measurements c\ the same sample, and that the sample was questionable because it had a fogged surface. It was pointed out in discussion that during the six months delay, significant surface con- tamination could have been present.

424
HIGH ENERGY LASER OPTICAL TRAIN PERFORMANCE

A REASSESSMENT

Claude A. Klein

Research Division, Raytheon Company

Waltham, Massachusetts 02154

This paper addresses problems relating to thermally induced phase aberrations

experienced in a power-optics environment. Specifically, it addresses issues of concern in assessing the performance of High-Energy Laser (HEL) systems with

regard to focal intensities and on-target fluences, when thermal lensing phenomena

do occur in the optical train. Following Holmes and Avizonis [Appl. Optics 15,

1075 (1976)] , a model has been developed that should allow HEL-system designers

to complete an evaluation of the degradation in light intensity , which is caused by

irradiance-mapping aberrations in the near-field. Perhaps the most important conclusion that was reached is that theories, calculations, and experiments conducted on a "single-component" basis provide highly unreliable guidelines for passing

judgement on the merits of mirror /window material candidates, or their projected

performance in an optical train.

Key words: Focal intensity; high-energy laser; irradiance mapping; mirror /window materials;

on-target fluence; optical train; phase aberration; power optics; thermal lensing.

1. Introduction

High-energy laser (HEL) systems include an optical train consisting of mirrors and

windows, which may compromise the system's operation because of unavoidable heating resulting from the absorption of some fraction of the incident laser-beam energy. Available experience indicates that catastrophic failure modes associated with thermally induced

stresses or temperature excursions are not a major threat. The performance of the optical

train, as measured in terms of achievable target irradiances , first degrades on account of

"thermal lensing," that is, the process of beam defocusing and beam distorting caused by

thermally generated phase aberrations. Since HEL operation usually requires close-todiffraction-limited characteristics at the far- field focus, this lensing phenomenon must be

carefully assessed and evaluated in relation to specific features of the optical train. In this

paper, we propose to conduct an analytical investigation of relevant processes and discuss

how laser-driven mirror /window distortions may affect the performance of HEL systems with

regard to both focal intensities and on-target fluences. Specifically, it will be our goal to derive an approximate expression for the far-field on-axis irradiance, patterned upon the

model of Holmes and Avizonis [2] , but simple enough to allow HEL system designers to assess the capability of a contemplated optical train, and to evaluate its behavior as a function of key system parameters such as beam-power level and laser run-time.

For this purpose, we have chosen to consider the system illustrated in figure 1, which

should be representative of the type of optical trains encountered in HEL technology [ 3] , and to formulate a treatment that is as general and relevant as possible. The laser device

will be assumed to generate a collimated circular beam of diameter d and power P at the

system's entrance aperture. This input gives rise to a near-field irradiance distribution Igj,

which can be fairly arbitrary, whereas the phase profile is postulated to be uniform. The

beam is transported and processed by means of an optical train , which has all the elements

one might normally encounter in an actual system , including the beam-focusing mechanism

after expansion to diameter D. We note that, in this configuration, the output window re- presents the only transmissive component we will be concerned with. Furthermore, any

propagation effect such as "thermal blooming" that may take place in the beam-relay tube,

will be assumed to generate insignificant aberrations compared to mirror expansion and

window distortion.

Figures in brackets indicate the literature references at the end of this paper.

425
2. Some Key Equations

In the absence of birefringence, the far- field intensity derives from the HuygensFresnel integral [4] and amounts to D/2

I GF = (k/R

Q

) 2

[ / E(p) exp [ i<S cj) CP ,t) ] pd p | 2 (1)

o

at the Gaussian focus. ^ The "phase-aberration function" 6<j> maps the cumulative aberrations

impressed on the beam while propagating through the optical system and relates to the

"wavefront-error function" A simply by means of the propagation constant, i.e., & <$> =

(2 tt/ X ) A . The non-aberrated focal intensity, therefore, is

i2 = (k/R

Q

) 2

[ J E(p) pdpj , which yields [ 5] :

D/2

o

j(o) = K

tt(D/2)

2 GF (AR

o y

(2b)

where k q represents a beam-profile factor (kq

< 1) and accounts for the possibility that

non-uniform beam intensities may cause some reduction in far- field irradiance, even in the

absence of near-field phase distortions.

For weak distortions, we may proceed by expanding exp (i 6 <j>) to second order, in the manner originally suggested by Born and Wolf [4], which leads to a highly compact ex- pression ,

IqP = I^j? { 1-k

2 var [A] } , (3a)

2 2

if the variance is defined in accord with var [A] = <A > - <A> , the brackets symbolizing

amplitude-weighted averages. We note that the reduction in far- field intensity exhibits a

familiar "Strehl-ratio" appearance, which says that the performance of the optical train

should remain nearly diffraction-limited as long as the variance of the wavefront error does

2 not exceed (A/14) . If the square root of the variance becomes larger than one tenth of a wave, Eq. (3a) may no longer be valid, the correlation length of the phase aberration then

becoming an essential parameter [6]. For instance, in the case of random aberrations whose

correlation length is much smaller than the limiting aperture diameter, it can be shown that

the loss of intensity in the far field obeys a generalized Strehl relation such as

I GF = Iq°p • exp {-k2 var [ A] } . (3b)

In this context, Holmes and Avizonis [2] have demonstrated that the degradation factor

2 exp {-k var [A] } provides a convenient tool for optical system analysis irrespective of the

nature or origin of the phase aberrations, so long as the wavelength distortion does not exceed a certain threshold corresponding to, say, / var [ A] = A/5. Throughout the rest of

this paper, we will rely on eq. (3b) for approximating the loss in far-field on-axis bright- ness owing to the emergence of phase aberrations in the near field.

Some drastic simplifications are called for if the calculation is supposed to focus on

those sources of optical distortion directly related to the HEL environment. First, it is postulated that, in a plane located at the entrance of the optical train and perpendicular to the

beam axis, there are no phase aberrations. Second, we take it that beam-induced distortions are coherently additive, whereas all other sources of optical distortion are uncorrelated with

themselves, with each other, or with thermal distortions [1]. Furthermore, it is assumed _

For a listing of the symbols , see table 1

426
that uneorrelated path fluctuations, which normally originate from component imperfections,

are Gaussian and random in amplitude, with zero mean. On this basis, the variance be- comes simply

Var [A] = A

I

2RMS + var [A T

] < 4 > because uneorrelated random wavefront errors enter the far-field irradiance equation in the

form of a sum of the squares of the rms contributions from each individual component

o N

A

2 \

2 2 2

I ,RMS . K1=1 °I,RM.

+ a

i, PM

+ a I,OW *

Table 1. Glossary of symbols.

a

truncation parameter Y

Young's modulus

C : P

volume specific heat z

axial coordinate

d

beam diameter a : expansion coefficient

D : exit aperture diameter QV surface absorptance

E

beam amplitude V volume absorption coefficient

h: heat-transfer coefficient Y : sensitivity parameter

I: intensity /irradiance X A 6 <f> ; phase aberration

I GF : irradiance at Gaussian focus o n

index inhomogeneity

"sr

intensity at system input 6 T

temperature increase

r so

: intensity at system output A : total wavefront error

k: propagation constant

A wavefront error, imperfections

K

thermal conductivity wavefront error, distortions

x.face-plate thickness differential strain L

window thickness 6.:

l angle of incidence

n

index of refraction < : beam-profile parameter

N

number of relay mirrors X: laser wavelength

!VU< iNusseix numuer v : Poisson's ratio P: beam power stress-optics coefficients

r

reduced radial coordinate P = radial coordinate

R : 0

focal length rms wavefront error R mirror reflectance planar stress components

t: dwell time axial stress component

t : c

critical time T

f

: figuring error

t : P

pulse duration %

: mirror distortion coefficient

W

GF = fluence at Gaussian focus X

: window distortion coefficient

Similarly, wavefront errors originating from thermally induced distortions are made up of

contributions from the relay mirrors,^ the primary mirror, and the output window, but since

the profile of the incident beam is not expected to change significantly, they are deterministic in nature and tend to contribute to the variance in accord with

Note that N is defined here as the number of mirrors that handle the unexpanded beam

,

hence includes the secondary telescope mirror in addition to the relay mirrors (see fig. 1).

427
var [AT

] = var N

X AT,RM.

+

1=1 1

A T,PM T ,0W (6)

3. Laser-Driven Aberrations

In this section, two figures of merit are introduced for the purpose of characterizing

the response of mirrors and windows in power optics, from the point of view of thermally

induced wavefront errors.

3.1 Mirror Distortion

Currently contemplated power-optics mirror designs consist of a thin, highly reflecting

face plate, which transfers the deposited radiation energy to a coolant fluid, through the

rear surface, and is stiffly held to a stable, insulated back-up structure that minimizes

mechanical (or thermo-mechanical) modes of deformation. The face plate is typically 1-mm

thick and is subjected to a heat load owing to the small but unavoidable absorption of laser

radiation at the mirror surface. Here, it will be assumed that the face plate reaches a steady-state temperature in a time frame very short compared to the laser-pulse duration.

If bowing and /or related deformations can be effectively suppressed, mirror-induced

optical aberrations originate from displacements of the front surface, which are caused by

thermal bulging of the face plate. The change in thickness is given by

I

6 1 = f e (z) dz , (7a)

' z 0 where the differential strain e (z) must be evaluated in accord with Hooke's law, including

thermal expansion , or

e

z (z) = a6T(z) + (1/Y) [o

z - v (a + aQ

)] (8)

in the case of axisymmetric loadings. With commonly achieved heat-transfer coefficients, say

2

1 W/(cm K) , the face plate should be subjected to minimal axial stresses, which would indi- cate that , in a first-approximation , we have

6£ = (1-R)(£ 2 /2K + i/h) [a (1 + v) I-avI ] (7b)

considering that the axial temperature distribution 6 T(z) consists of two terms: (a) the

linear temperature drop across the face plate, which reflects the thermal conductivity of the

substrate, and (b ) the temperature difference between the back and the coolant, which is controlled by the heat-transfer coefficient.

We now assume, with Sparks [7] , that the support structure is capable of holding the

face-plate rear surface in its original plane. Optical distortions, therefore, are caused by

spatially dependent variations of 6 £ , keeping in mind that a factor of two must be applied

when evaluating optical path differences impressed upon reflection from a mirror face plate

that experiences out-of-plane growth. Since angle-dependent terms always cancel, we conclude that the local wavefront error is best expressed as in (9a), or (9b),

Igj AT,RM " (FoM) RM ' (9a)

428
^so

VPM =

~ (FoM) pM

' (9b)

the minus sign reminding us that mirror expansion tends to shorten the optical path length.

Equation (9) introduces the concept of a figure of merit for cooled, high-average-power laser

mirrors , i.e.

,

F°M

M

= Kgff

2 ' (10)

where

K

eff =1 + (2/Nu)

represents the effective thermal conductivity of the cooled face-plate and , thus , characterizes the heat-removal process, whereas 5 refers to the mirror distortion coefficient, or modified expansion coefficient,

5 = a(l + v) . (12)

We note that this figure of merit is in units of watts per square centimeter per centimeter,

which points to a straightforward interpretation in the sense that 1/FoM^ measures the

"optical growth" of the mirror surface per unit incident power density. In table 2, the

reader will find a listing of relevant properties of key mirror-material candidates (SiC , Mo,

and Cu) with figures of merit for reflectances of 99.8% and heat-transfer coefficients

Table 2. Figures of merit for key mirror-material candidates.

Item Symbol (unit

-) SiC Mo Cu

Expansion coefficient a (10~ 6K

-1

) 3.5 5.0 16.7

Pois son's ratio v (1) 0.3 0.28 0.42

Thermal conductivity K (W cm

_1

K

_1

) 4 1.34 3.99

IR reflectance R(D 0.998 0.998 0.998

* Figure of merit 1 FoMM

(W cm" 3

) 5.49 x 10

7 3.89 x 10

7 1.05 x 107

Figure of merit 2 FoMM

(W cm

-3

) 4.88 x 10

9 2.84 x 10

9 9.37 x 10

8

* -2 -1

Assumes a 1-mm thick face plate and a heat -transfer coefficient of 0.1 W cm K (case 1) or 10 W cm" 2K

_1

(case 2).

ranging from 0.1 to 10 W cm~

2 K

-1

.

3.2 Window Distortion

Optical distortion experienced by laser beams in passing through a solid window results

from the absorption of laser energy at both surfaces as well as within the pane itself. The

phenomena involved are fairly complex and have been the subject of numerous investigations

beginning in 1971 [8]; the reader may consult reference 9 for a comprehensive discussion.

Specifically, if the windows are disk-shaped and subjected to axially symmetric loadings,

429
thermally induced optical path differences (OPD) for unpolarized light can be expressed in

terms of an average OPD for radial and azimuthal polarizations, which yields:

OPD =L {(n-l)[a6T - (v/Y)(o + a

Q

)] + (3n/3T)6T - (n 3 /4) [ n

|( + T^Ka + a

Q

)] } (13)

Here, it is implicity assumed that (a) stress-birefringence plays a relatively minor role,

(b) suitable window material exhibits isotropic features in the plane of the window , and

(c) the stress distribution obeys classical thin-plate theory. In that case, and since wavefront distortions of interest only concern path-length changes stemming from non-uniform

contributions, we may simply write:

A TW =LX 6T , (14)

where x represents the window distortion coefficient, which includes not only the contribution arising from surface bulging through constrained expansion, but also the temperatureinduced isotropic change in index of refraction, and the photo-elastic effect averaged over

the two principal stress directions:

X

= (n-l)ct(l+v) + (3n/3T)o=0 + (n

3aY/4)(7r

H + ir A

) . (15)

As mentioned earlier, birefringence effects are ignored in this model but we may emphasize

that, from the point of view of degradation in light intensity at the Gaussian focus, highly

promising window materials such as CaF^ can be "configured" in a manner that eliminates

stress-birefringence, independently of the polarization of the incident beam [10].

We now consider a pulsed mode of HEL operation for which time scales are such that

lateral heat-conduction and surface-cooling effects can be ignored. In the case of output

windows, this "short-time model" implies that local window temperatures image or map the

beam fluence at the primary mirror, i.e. ,

p

which takes into account both surface and bulk absorption processes. In essence, this

implies that the window generates a local wavefront deformation equal to

=

J so • t n (17 T,W FoMw

' K ' }

where the figure of merit FoMw

regroups all window-related features that affect thermal

lensing:

F°M

W (2 6X»X ' (18>

Conceptually, this figure of merit characterizes the susceptibility to thermal distortion in

the same manner as the mirror FoM but relates the optical path change to a unit amount of

incident fluence instead of intensity. In table 3, the figure of merit of key windowmaterial candidates (CaFg, KC1, and ZnSe) is listed in conjunction with relevant material

properties. Calcium fluoride appears to be, by far, the most promising candidate on the

basis of this figure-of-merit assessment , which disregards the potential impact of other

system components on the overall aberration picture (see sec. 4).

430
Table 3. Figure of merit for key window-material candidates.

Item Symbol (unit) CaF

2

* KCl** ZnSe +

Fracture strength af (kpsi)

i 12.5 4.5 7.5

Heat capacity C (J cm

_3

K

_1

) P

2. 72 1. 35 1. 80

Distortion coefficient X (10

_6

K

_1

) 1 -8.5 73

Bulk absorption 3 v

(l<f

3 cm"

1

) 0.05 0.5 0.5

Coating absorptance 3 s dO" 3

) 0. 15 0.5 1.0

Figure of merit FoMtlI (J cm" 3

) w

77.7 x 10

9 -1.05 x io

8 9.86 x io

6

For < 111 ^oriented , 1-cm thick windows operating at 3.8 ym (DF laser).

For < 111 ^oriented , 1-cm thick windows operating at 10.6 ym (CO laser). t For chemically vapor-deposited , 1-cm thick windows operating at 10.6 pm.

4. Irradiance on Target

According to the model in section 2, the focal irradiance can be approximated as

follows:

! GF

= *GF * 6XP { - k2A i?RMS-k2 var [A T

] } (19)

if the wavefront error resulting from thermal lensing of imperfect components is handled as

in eq. (4). Since the departure from an ideal optical finish is usually specified in terms of rms figuring errors x measured in "waves," and since the window can exhibit spatially

dependent index inhomogeneities , we may rewrite eq. (5) in a more explicit manner, or N

A

I

2,RMS

= 4 £rM I ^^s\ Ql ) + 4x

f

2 )PM

+ 2(n-l) 2 x

f

2 ;OW

+ L

2 6i£MS . (20)

i=l

According to Bennett [1], optical figures of 1/4 wave in the visible can be attained with

large-diameter windows, while 1/8-wave surface figures are readily achievable for metallic

reflectors; it is thus a straightforward task to verify that, in the absence of thermally

induced aberrations, HEL optical trains as described in reference 3 easily meet the requirements of the Marechal criterion at 10.6 ym.

Returning now to our previous discussion of thermal distortion effects, and making use

of the results expressed in eqs. (6, 9, 17), the variance of the beam-induced wavefront

error becomes

var [A,j,] = var W NI

si + 'so

FoMWFoMM

(21a)

where Igj and IgQ refer to the beam intensity and its spatial distribution before and after

expansion. Since the near-field irradiance distribution is expected to preserve its original

profile, and keeping in mind that the variance must be evaluated over the exit-aperture

area, we have:

431
var [A T

] = N(D/d) +1

FoM

W

FoMM

var [I so ] (21b)

At this point, if use is made of a beam-profile factor k-^ for expressing the variance in

terms of the average beam-power density, eq. (21b) can be thrown into a more convenient

form

,

var [A T

] =

t N

I 2

{Til 2) FoMW

(d/2)' FoMM

(21c)

because the primary mirror contribution should be negligible. Three comments are in order:

1) Computer modeling indicates that, for realistic laser beams, the profile factor

will be close to 0.5, even in dealing with the "sugar scoops" of gas-dynamic lasers (GDL)

[11]; in this connection, we emphasize that radial heat flow tends to flatten the temperature distribution and wash out the ripples , which means that thermal diffusion always improves (reduces)the profile factor.

2) The bracket in eq. (21c) regroups the system parameters in two terms: a window

term , which is time dependent , and a mirror term that sums the wavefront error arising

prior to beam expansion; if the figures of merit are both positive, there will be some partial

(or total) cancellation depending upon the pulse history, which may result in a dramatic

reduction of the net wavefront error originating from thermal lensing of optical system components.

3) Thermally induced wavefront errors, Arar [A^,] , are power dependent and in- crease linearly with P; in terms of the far-field performance, this implies that, inevitably,

there will be a point of diminishing returns in pushing the laser output beyond some

sensible limit.

For ease of manipulation, we now define two "sensitivity parameters," first

M

(d/2)

2FoM

(22a)

M

which characterizes the impact of N relay mirrors, and

'W (D/2)"FoM

(22b)

W

which refers to the output window. The light intensity at the Gaussian focus can thus be

expressed in an explicit but still manageable manner:

TrfD/2) P 9 9 9 9 9 'GF

= K

o "T^V • «Pl-(2« <RMS ] •exp[-(2K

1

) 2 (P/X)

2

( Yw t-YM

) 2

] o

(23)

which clearly shows how beam characteristics and system parameters combine to control the

irradiance on target. Upon examining eq. (23), it is immediately seen that, if the window

possesses a positive distortion coefficient , there exists a time t , which may occur during

the pulse, when window lensing exactly cancels the steady-stati mirror distortion and, thus,

suppresses all thermally induced aberrations; this critical time t is given by the ratio

t c = N(D/d) 2FoMw

/FoMM

, (24)

432
and involves the number of relay mirrors , the magnification factor , and most importantly

,

the ratio of the two figures of merit.

We now apply this formalism to the model system specified in reference 3 (d = 6 cm,

D = 36 cm, X = 10.6 ym) on assuming cooled molybdenum mirrors with FoM's as in table 2,

and a CVD-ZnSe output window as described in table 3. Figure 2 depicts how the focal

intensity, at a distance of 1 km> varies as a function of exposure time, for laser beams that

range in power from 1 MW up to a maximum tolerable level of 8 MW. The sequence demonstrates that, indeed, ZnSe windows can have a highly beneficial effect on the performance

of HEL systems operating in a long-pulse regime; the critical time t occurs at 0.73 sec

c

into the pulse, as signaled by a turnover point, which emphasizes the enhancement of the

far-field on-axis intensity throughout the early part of the pulse, at power levels for which

mirror distortion becomes a significant factor.

5. Fluence on Target

Some applications of HEL systems require an evaluation of the energy fluence deposited on target in a single pulse of duration t :

t W

GF

= l GF exp [ " (2T

^2 A

I

2RMS ] * / P exp[-(2K

L

) 2 (P/X)

2 (Yw

t-YM

) 2 ]dt . (25a)

' o

The integration is fairly straightforward and yields

t P

/ "exp [ ]dt = ^

? ( y

P)

\ erf[2^(P/» yM

]+ erf [2 ^(P/ *>(Yw

t p

-

M >] \

, (26) o

which leads to

«L Y W

tt 3/2 (D/2) 4FoMw

w

GF °-

8 2

\ erf [(P/A) V+ erf[(P/X)(VP -

V

] | (25b)

2 XR

o

for a system that verifies the Marechal criterion in the absence of power-driven aberrations and exhibits beam-intensity profiles typical of GDL's. Note that the fluence in- creases very rapidly (fourth power!) with the size of the output-aperture and depends

explicitly on the figure of merit of the window , independently of the bracketed expression, which cannot exceed a maximum value of two.

In discussing eq. (25b), we may consider three distinct possibilities, on assuming positive mirror/window sensitivity parameters:

1) Short- duration pulses (t < y^ / y^) * s seen i n figure 3, which applies to our

model system (sec. 4), that the fluence initially increases linearly with beam power, goes

through an optimum power regime in the 4 to 6 MW range, then falls off as partly compensated mirror aberrations become quite large and dominate the far- field intensity pattern.

2) Critical-duration pulse (t = Ym^W^' ^he Dracket *n ecl- (25b) reduces to the

first error- function term, thus indicating that the fluence grows steadily with beam power

as window aberrations now provide adequate compensation to preserve the quality of the

beam.

3) Long-duration pulses (t > Ym^Y^ - ^s depicted in figure 3 for pulses of 1 and 2 sec, the situation shows strong evidence of saturation in terms of both input power and

pulse length. In effect, the fluence on target tends to reach a plateau of

433
lv (max) _ o GF ~ ^

tt 3/2 (D/2)

4FoMw , _

s * . exp[-(2,/A) A^RMS ] , (27)

2XR

which is independent of the near-field power level but depends on the beam profile in

accord with

(1-e ) 2 / 2~

a / var [exp(-2ar )]

(28)

in the case of truncated Gaussians. Within a factor of two, this is precisely the result of

Klein and Pappis [12] for the energy-deposition capability of HEL output windows, as obtained in the context of a single-component type investigation; this factor thus measures

the "gain" that can be achieved with an optical train that affords compensation, as can be

easily verified by setting y M

0 in eq. (25b).

Finally, and for the sake of comparison, we may consider the hypothetical case of an

optical train whose components are invulnerable to laser-driven aberrations; the fluence on

target then is simply

W'GF * 0.8

tt(D/2) P

(XR ) 2 o

(29)

and increases linearly with beam power and pulse duration, as predicted by elementary

diffraction theory (see fig. 3). More relevant is the case of no window -induced aberrations, for which we have

W£

p * 0.8

7T(D/2) P

(AR

o

) 2

t pexp [-(P/X) 2 ym

2

] (30)

the exponential term reflecting the impact of mirror expansion. The on-target fluence

always peaks at the power level that optimizes the far- field brightness (see sec. 4) and

shows strong non-linear features beyond that point ; this is depicted in figure 3 for shortduration pulses and demonstrates that the presence of the selenide window leads to a substantial enhancement of the energy-delivery capability of our model system.

6. Conclusions

• Equation (23) expresses the focal intensity of thermally aberrated HEL beams in a

simple manner, which clearly shows how beam characteristics and system parameters combine to control the irradiance on target. In this context, an important result emerges:

If the window has a positive distortion coefficient , window lensing suppresses the steady2 state mirror distortion at a time t given by t = N(D/d) FoM^ /FoM^ , where N counts the

number of relay mirrors, D/d is the telescope magnification, and the figures of merit are

as defined in section 3.

• At all other times, the behavior can be strongly non-linear, depending on the

beam-profile factor , but ultimately window distortion dominates and renders the beam unfocusable. In terms of energy fluences deposited on target, this leads to radically differing beam-power dependence patterns for "short" pulses compared to "long" pulses; the

presence of a positively lensing window, however, always results in a substantial enhancement of the energy-delivery capability of HEL systems.

434
• In applying these considerations to a model system representative of the type of

optical trains used for projecting HEL beams at a distant target, we find that CVD-ZnSe

windows can have a highly beneficial impact on the far-field performance of systems operating in a "long-pulse" mode. In the absence of a material window, laser-driven mirror

aberrations always degrade the focusing ability of the optical train, which results in ontarget irradiance levels much below those achievable if there is a selenide window at the

exit aperture.

References

[1] Bennett, H. E. , in Laser-Induced

Damage in Optical Materials: 1976

(NBS special publication 462,

Washington, 1976), p. 11.

[2] Holmes, D.A., and Avizonis, P.V. , Applied Optics 15, 1075 (1976).

[3] Accetta, J. S., and Shea, R.F.,

Proc. SPIE 121, 132 (1977).

[4] Born, M., and Wolf, E., Principles

of Optics (Pergamon Press, New

York, 1975), Chap. 5.

[5] Klein, C. A., in Proc. First Int. Conf. Lasers (STS Press, McLean/

Virginia, 1978) , p. 283 .

[7] Sparks, M., in Proc. High Power Laser

Optical Components and Component Materials Meeting (DARPA, Washington,

1977), p. 66.

[8] Sparks, M. , J. Applied Physics 42, 5029

(1971).

[9] Klein, C. A., Infrared Physics 17. 343

(1977).

[10] Klein, C. A., Applied Phys. Letters 35,

52 (1979).

[11] Petri, F. , Levine, A., and Burns, J., in

Proc. Fifth Conf. Infrared Laser Window

Materials (AFML, Dayton/Ohio, 1975),

p. 488.

[6] Hogge, C, Butts, R., and Burlakoff, [12] Klein, C, and Pappis, J., in Proc. InfraM., Applied Optics 13, 1065 (1974). red Laser Window Materials Meeting

(DARPA, Washington, 1976), p. 39.

435
Figure 1. Schematics of a high-energy laser

optical train.

436
I(T

5x

3x

,o2

5x

3x

io

1

P = I MW

1—i—|i I I i |

1

1—i—| mm.

j I i i i i i i ii l i n i i i i

i 1—i—| i M i

P = 2 MW

PBN -79-405

I llll'.

i

1—I—I

3 5 10 0.1 0.3 0.5

LASER BEAM DWELL TIME (sec)

Figure 2 Time dependence of the light intensity at the Gaussian focus of the model system aescnbed in section 4. The solid curves assume the presence of a CVD-ZnSe window at the system s exit aperture. The broken lines refer to the predicted steady-state irradiance if tnere are no thermally induced window aberrations.

437
PIN-79-341

T

1 1 1 1 1 1

LASER BEAM POWER ( MW)

Figure 3. Beam-power dependence of the energy

fluence at the Gaussian focus of the model

system described in section 4. Laser pulse

durations range from 0.5 to 2 sec.

438
ANALYSIS OF AN IMPERFECTLY COATED CONICAL

ELEMENT FOR HIGH ENERGY LASER RESONATORS

W. P. Latham, Jr. Air Force Weapons Laboratory

Kirtland AFB, NM 87117

The scalar field approximation is normally used in the analysis of eigenmodes and eigenvalues for high energy laser resonators. Recently, Dr. D. Fink

demonstrated the need to account for the vector nature of the electromagnetic

field when conical elements are used in resonators. Dr. G. C. Dente at the AFWL

has developed a theory for the analysis of such polarization effects. When polarization effects are included, an unacceptable mode has been shown to exist in a conical-element, annular-gain laser which is currently being studied at the AFWL.

A special optical coating placed on a conical resonator mirror would make the

resonator mode acceptable by introducing a 90° phase shift between polarization

states which are parallel and perpendicular to the plane of incidence upon reflection at a 45° angle of incidence. The study presented here gives the analysis for

a related resonator within Dente's theory. The results indicate the tolerance

limits of such a coating for a conical element in a high energy device.

Key words: Coatings design; polarization effects -, resonator analysis.

1. Introduction

The solutions for unstable resonator modes are traditionally determined using the Fox

and Li [1]^ iterative approach after assuming a linearly polarized scalar field. For simple

unstable resonators, this approximation is valid because the polarization state is unchanged

on a round trip through the resonator. That is, the linear polarization state is an eigenpolarization state for a simple unstable resonator, so that scalar field theory is valid.

Fink [2] initially showed that for some conical elements, the linear polarization state

is changed on a round trip through the element. One such conical element, which scrambles a linear polarization state, is the retroref lecting cone. The eigen-polarizations of a cone

are radial and azimuthal polarization but not linear polarization. However, a cone would

preserve a linear polarization state upon reflection, if it were coated with a 90° phase

shift coating. If the coating introduces a 90° phase shift between the radial and azimuthal components, there is a 180° phase shift for a round trip through the cone, and it acts

like a plane mirror. That is, for a perfect 90° phase shift, the cone no longer effects the

polarization state of the field, and the traditional scalar field approach can be used.

The polarization effect introduced by conical elements is an important effect in the

annular gain high energy laser resonators currently being studied at the Air Force Weapons

Laboratory (AFWL). The half-symmetric unstable resonator (HSUR) with a coated rear cone is

a simple example of an unstable resonator with a conical element. In the HSUR, the polarization effect is removed by a perfect 90° phase shift. However, for an imperfectly coated

cone, the polarization effect must be included by using both components of the electromagnetic field.

The need to correct the polarization effect defines the requirements on a new high energy laser material. A broad wavelength band 90° phase shift coating which can withstand

high intensities and can be uniformly deposited on a conical mirror is required.

Recently, the general theory of resonators with polarizing elements has been presented

by Dente [3]. The purpose of this paper is to demonstrate that the general form of the

eigen-equations for a HSUR can be solved using standard resonator techniques. Furthermore,

the polarization effects due to variations in the phase shift introduced by imperfectly

^ Figures in brackets indicate the literature references at the end of this

paper.

439
coating a conical element within the HSUR are presented. The imperfect coating analysis for

the HSUR with rear cone defines the tolerance requirements on the coating. Coating designs

have been developed by Southwell [4] and Scott [5], and these are discussed.

2, Resonator Analysis of a Half- Symmetric Unstable Resonator with a Coated Rear Cone.

The general theory for resonators with elements that mix Fourier components of the

field through polarization effects was presented by Dr. G. C. Dente [3]. For the HSUR, only

the mixing of the £=0 and 1=2 states is considered, that is in Dente' s notation L=l. For

L=T, the x- and y-components of the field are

E x

= f

1 (r)e"

1> cos* +

9] (rje^sin* , (1)

and

Ey

= f

1 (r)e

1 *sin^ - g

] (r)e

1(,> cos* . (2)

The general form is obtained by using the sum and difference functions S-j and D, , which are

S-,(r) = f^r) + i 9l (r) , (3)

and

D-,(r) = f^r) - ig^r) . (4)

Substituting eqs (3) and (4) into eqs (1) and (2) gives

E x [S^r) + e'^D^r)] , (5)

and

1-R 1+R

2 2

1+R 1-R

2 2

Ey

= \

[S^r) - e i2\(r)l . (6)

Note that the form of the field is a linear superposition of an 1=0 and 1=2 Fourier component. For a simple unstable resonator, each Fourier component is a mode of the aligned res- onator. Thus Fourier components are sometimes called I modes. The resonator mode for the case studied here is a superposition of two simple resonator i modes.

For a half-symmetric unstable resonator with a coated rear cone, the eigen-equations

are given in matrix form by

(7)

where R is the negative of the ratio of the radial and azimuthal reflectivities. For this case, Kg and 1^ are the 1=0 and 1=2 round trip propagators. The reference plane for a round

trip is taken to be the plane of the rear cone. A round trip is defined by the propagation

from the rear plane to the feedback mirror, the application of the feedback mirror phase

sheet, and the propagation from the feedback mirror to the rear plane. Note that eq (7) can be generalized to a more complicated resonator by including more matrices to describe

the resonator and more % components for fhe field.

The complex reflectivity ratio, R, in eq. (7) determines the strength of the mixing due

to the polarization effect. The 90° phase shift case, which corresponds to the plane mirror, is given by R = -1. For R = -1, the top half and bottom half of the matrix equation in eq. (7) decouple, so that there are two independent equations for S-| and D-j. That is, there

is no mixing. The uncoated rear cone is given by R=l. In his case, the two equations for

S-| and D-| can be solved by substituting one equation into the other. The result is a

440
quadratic ei gen-equation involving x

2 and two propagation operators. For complex values of

R between +1 and -1, the rear element corresponds to an imperfectly coated rear cone. That

is, the rear element is not a plane mirror nor is it an uncoated cone.

The eigen-equations in eq (7) were solved using the power method [1]. For the perfectly coated rear cone (R = -1), the eigenvalues for a normal ha If- symmetric resonator agree to three places with eigenvalues published in the literature [6].

After the solution has converged, the rear cone polarization operator was applied, and

the power in the S-, and D, terms was determined by

where b is the radius of the integration region at the rear plane. The radius b is set to

be sufficiently larger than the geometric beam size to include all of the diffracted light.

The power expression for D-j is identical to eq (8) with replaced by D-| . The power integrals were calculated using a simple numerical integration technique. Results of the power

method calculations for two equivalent Fresnel number resonators for various phase shifts

and wavelengths are presented in the next section.

The results of power method calculations for two equivalent Fresnel numbers for various

phase shifts are listed in Table I. Since D-j is the 1=2 mode, the right-hand column gives

the percentage of power in the 1=2 mode. The rest of the power is in the £=0 mode for this

simple case. Table I indicates that the percentage power in the 1=2 mode scales with the

equivalent Fresnel number. The percentage power in D-| for the Neq=8.5 case is about twice

the percentage power in for the Meq=4.25 case for all values of phase shift.

The results of the power method calculation for two phase shifts for various wavelengths are listed in Table II. As the wavelength is varied without changing the other pa- rameters of the resonator, the equivalent Fresnel number varies. For a wavelength of 3.6ym,

the equivalent Fresnel number of the resonator is nearly Meq s: 9.0, and the power method

does not converge.

Table II indicates that, if the coating can be applied to the rear cone within a ±5°

tolerance of 90°, the mode of the resonator will be very nearly an 1=0 mode. However, the

existing coating design [4] is within this tolerance between wavelengths 3.6vim to 4.0ym

only. If there is appreciable power for smaller or larger wavelengths, there can still be

a substantial percentage of the power in the 1=2 mode.

Southwell [4] initially showed that a 90° phase shift coating could be designed for a single wavelength by using quarterwave pairs of high and low refractive index thin films.

Scott [5] and Southwell [7] have refined the design using 20-layer ZnS and ThF. dielectric

stacks to obtain a near 90° phase shift over a broad wavelength band. For these designs,

the variation in phase shift about 90° is less than ±3°. Thus, the proposed design satisfies the requirement of ±5° about 90° phase shift which was determined by the resonator

analysis. However, there may still be deposition and damage problems with the proposed

coating design. Currently, AFWL is studying the problems of layer thickness deposition on

conical elements and multilayer dielectric stack damage.

(8)

3. Results and Discussion

441
Table I. Percentage power in D, vs. rear cone phase shift.

Phase

Shift

(degrees)

Neq =4.25

90

88.75

87.5

86.25

85

83.75

82.5

Neq =8.5

90

88.75

87.5

86.25

85

83.75

82.5

Table II,

Wavelength

A(j>= 85°

(jjm) Neq

3.55 9.099

3.7 8.730

3.8 8.500

3.9 8.282

4.0 8.075

4.1 7.878

3.55 9.099

3.7 8.730

3.8 8.500

3.9 8.282

4.0 8.075

4.1 7.878

Eigenvalue

Magnitude Phase Percent Power

(degrees) in D-|

0.6484 8.67 0.0

0.6480 7.41 0.137

0.6471 6.10 0.545

0.6455 4.77 1.215

0.6432 3.41 2.136

0.6403 2.00 3.278

0.6367 0.56 4.629

0.6224 -0.40 0.0

0.6221 -1.67 0.255

0.6212 -2.99 1.015

0.6198 -4.35 2.258

0.6178 -5.75 3.957

0.6153 -7.20 6.061

0.6123 -8.70 8.527

Percentage power in D.| vs, wavelength.

Eigenvalue

Magnitude Phase Percent Power

(degrees) in

0.5970 14.72 0.0

0.6099 -7.70 0.0

0.6224 -0.40 0.0

0.6210 6.76 0.0

0.5964 15.44 0.0

0.5900 -14.47 0.0

0.5928 9.37 4.242

0.6044 -13.17 4.933

0.6178 -5.75 3.957

0.6166 1.47 3.353

0.5915 10.17 3.709

0.5839 -20.02 6.554

442
References

[1] Fox, A.G., and Li, T. , Resonator modes

in a maser interferometer, Bell Syst Tech

J. 40, 453 (1961).

[2] Fink, D. , Polarization effects in axicons, Applied Optics J8, 581 (1979).

[3] Dente, G.C., Polarization effects in res- onators, Applied Optics 18, 2911 (1979).

[4] Southwell, W.H., Multilayer coatings pro- ducing 90° phase change, Applied Optics

J8, 1875 (1975).

[5] Scott, M.L., Analysis of LS-14 phase

coatings, Sixth Annual Tri-Service Chemical Laser Symposium, KAFB, 28-30 Aug 79,

p. TH3-3.

[6] Butts, R.R., and Avizonis, P.V., Asymptotic analysis of unstable laser resonators with circular mirrors, JOSA 68,

1072 (1978).

[7] Southwell, U.H., flultilayer coating design achieving broad band 90° phase

shift, Applied Optics, to be published.

443

NONLINEAR ABSORPTION IN DIRECT-GAP CRYSTALS

Vaidyanathan* , T. W. Walker and A. H. Guenther

Air Force Weapons Laboratory

Kirtland AFB, NM 87117

and

S. S. Mitra

Department of Electrical Engineering

University of Rhode Island

Kingston, RI 02881

and

L. M. Narducci

Physics Department

Drexel University

Philadelphia, PA 19104

A critical review of the Keldysh, Braunstein and Basov formulations

of the nonlinear absorption process in direct-gap crystalline solids is presented. The two-photon absorption coefficients in several alkali

halides and semiconductors are calculated by means of the above three models

and compared with available experimental data. It is concluded that the

Keldysh formula offers a sufficiently accurate description of the multiphoton absorption process, even when the photon multiplicity is small.

This is in contrast to the frequently expressed view that the Keldysh

formula is incorrect for small photon multiplicities. It is found that

the Keldysh formula is closely related to Braunstein's formula for

allowed-allowed transitions, while Basov's formula corresponds to the

allowed-forbidden case. It is also shown that the predictive

accuracy of the Braunstein formulas can be increased by the use of

nonparabolic energy bands and excitonic intermediate states. Finally,

the Basov formula, which until now was considered to be the most

accurate description of the two-photon absorption process, is shown to contain several errors.

Key words: Multiphoton absorption; nonlinear absorption; two-photon

absorption coefficients.

1 . Introduction

Nonlinear absorption in crystalline solids has been a subject of great interest during

the past fifteen years [1]. This interest has been stimulated in part by the role of non- linear absorption in laser-induced damage of optical components, as well as by its many

fundamental applications in solid state physics. In spite of an abundance of published

literature, quantitative understanding of nonlinear absorption is far from satisfactory.

Specifically, the two-photon absorption coefficients predicted by different theoretical

methods differ from each other and from the experimental data by as much as three

orders of magnitude [2]. Furthermore, there are important discrepancies regrading the

multiphoton formulas due to Keldysh [3] and Basov[4j. In this contribution we attempt

to resolve some of these controversies and propose two modifications to the Braunstein

model which bring its predictions closer to the experimental data.

*This work was supported in part by an AFSC/NRC Postdoctoral Resident Research Associateship.

ram

445
2. Comparison of the Keldysh , Braunstein and Basov models

The general theory of multi photon absorption in atoms was outlined by M. Goppert-Mayer

[5] in the early days of quantum mechanics. When this early theory is applied to the

electronic transitions from the valence band v to the conduction band c of a direct-gap

crystalline solid by the simultaneous absorption of two photons of frequency w, one obtains the following expression for the transition rate per unit volume:

H H vn nc

vn

6(E

vc

(k)-2Tla)) (1)

In eq. (1) H^'s are the interaction Hamiltonian matrix elements between bands i and

j, whose energy separation at wavelength k is E^., the It integral extends over the entire

first Brillouin zone, the index n extends over all possible intermediate states, and the

delta function expresses the energy conservation requirement.

Braunstein and Ockman [6] simplified eq. (1) by assuming that there was only one intermediate state, which was above the final conduction band, and that all the three

energy bands were parabolic and isotropic (see fig. 1). Depending on the symmetries of the

bands, three types of two-photon transitions were possible: al lowed-al lowed (a-a),

al lowed-forbidden (a-f) and forbidden-forbidden (f-f). Basov et al . [4] simplified eq. (1)

by assuming that the intermediate states were in the initial valence and final conduction

bands themselves. They also assumed the energy bands to be isotropic and quadratic functions of k and they included the two highest valance bands in their calculation. Keldysh

[3] approached the problem of multiphoton transitions differently. He assumed the

electronic wave functions in the presence of the strong radiation to be described not by

the ordinary Bloch functions, but by the Houston functions of the form given below:

** (f, t) = U-> (r) exp (2)

In eq. (2) E and p are the electronic energy and momentum respectively, while U is the

periodic part of its wave function y. The electronic momentum in the presence of the

electromagnetic radiation is given by

(3)

p(t) = p - eftQ sinwt/w

As a result of this, Keldysh's final result contained multiphoton transitions of all order, even though he used only first order perturbation theory to evaluate the transition probabilities. It is also worth pointing out that Keldysh assumed a nonparabolic

dependence of the energy bands on the wave vector:

A

m

vc

E g/

1/2 (4)

E (k) = E , vc

v g\

In eq. (4) E vcis the energy separation between bands v and c whose reduced effective mass

is m*

c and minimum energy gap is E^. All of the above three models assumed the interaction

Hamiltonian to be of the form H= ^ t'P , where t is the vector potential.

446
The two-photon absorption coefficient g is obtained from the two-photon transition

rate per unit volume W with the aid of the equation

2 (5)

3

= 4 foi/r .

We list below the final formulas for the two-photon absorption coefficients obtained

within the context of the Braunstein, Basov and Keldysh models

,a-a

? 5/2 4

£ ire

e c m (

(

m*vc) AE (aE-eJ fvn

fnc (

2^ - E

q )

1/2

u>) FaE - tlto + m* / \1

vn !

: /

(6)

s Br

3/2

3 e cm ("hw) 3 J~AE

- Ti w + m*

vc

^2TTU - E

g jjj' vn

(7)

Basov .2,^ \5 2

e c (liu) m

(8)

Keldysh i

2 ne exp (4 (2 - E/E ))

X exp ( -4 + (-• *3^ /

V 4-2 E /tTo)

v

2

e7 dy

(9)

In eq. (6) - (9) AE is the minimum energy separation between bands v and n whose

reduced effective mass is m* f..'s are the oscillator strengths, e is the high frequency

vn i j

°° dielectric constant of the crystalline material, e is the minimum energy gap between bands ^ 9

v and c, while E is the effective energy gap [3] in the radiation field:

e

2

E

2 E = E +

6 h o

9 9 Am* ...2 ' VC"

where E is the amplitude of the electric . field of the radiation. Eqs. (6) and (7)

1 have taken into account the correction factor -

z pointed out by Mitra et al . [2].

09

447
As one can see there is very little resemblance between the above formulas, and

their interrelationship is unclear. More importantly it has long been stated that the

Keldysh formula is either incorrect or invalid for small photon multiplicity [4, 7-10],

and that the Braunstein and Basov formulas are more accurate. However very little evidence

has been advanced to support these statements. On the contrary the recent calculations

of Mitra et al.[2] reveal that of the three models described above, the Keldysh formulation

gives two-photon absorption coefficients in closest agreement with the experimental results.

Mitra et al. also noted that the predictions of the Basov formula are much larger than the

other two theoretical estimates, as well as the experimental data. This result is also

disturbing since the Basov formulation is a description of the al lowed-forbidden transition,

and one would expect the allowed-forbidden transitions to be much weaker than the allowedallowed transitions. In the following sections we attempt to resolve some of these

contradictions, and propose two modifications to the Braunstein model which result in increased accuracy.

2.1 . Keldysh Formula

We followed the derivation of the Keldysh formula closely and could not find any reason to think this formula is less accurate than Braunstein's and Basov 's formulas

for two-photon absorption. In fact we found that the two-photon absorption edge

resulting from the Keldysh formula agrees very well with that predicted by Braunstein's

formula for al lowed-allowed transitions, as shown below. Close to the two-photon

absorption edge 2f\u - E , the Keldysh formula described by eq. (9) reduces to the

following form. ^

. „ 7re

lt exp(4)(2Tiw - E ) 1/2

B|< " 9 (10)

9e

ro C

2 (2m*

cv

) 1/2 (^) 3

In deriving eq. (10) we have made use of the fact that the effective gap E g is very

close to Eg, which is a necessary condition for the applicability of the Keldysh formula.

The two-photon absorption edge (21fu) ~ E

g

) resulting from the Braunstein's formula

for allowed-al lowed transitions is

ZV2 TreMm* ) 1/2 (m/m -1) (2fico - E.)

1/2

(11) Br g 9e c

2 (Tiw) 3 m

where m is the conduction electron effective mass. Eq. (11) is obtained from eq. (6) if

c one uses the following sum rules [11, 12]

(12)

f = m/(2m* ) vn vc'

f = 5L -1 , (13)

nc m

c

and assumes aE = 2E

g

. This latter assumption is justified since the Braunstein two- photon absorption coefficient is not sensitive [6] to aE as long as AE>E^. From

eqs. (10) and (11), again for 2tia>=E g

448
For semiconductors m << m and m* = m Under these conditions eq. (14) gives

e k

- 1 .7 Bg~

a

; while

for alkali halides m = 0.5m and m* = m , and 8, = 3.4 s^"

a

, c vc c k Br

2.2. Basov Formula

The rather large two-photon absorption coefficients predicted by the Basov formula

in the past evidently arise from several errors in the Basov formula. We will now discuss

these errors and offer corrections.

Basov et al . [4] used the following relations for the interaction Hamiltonian and

vector potential

H = =J A.p (15) mc r >

A

2 _ 8*c I o

o 1/2 2 (16)

0)

2 The proper expression for A consistent with eq. (15) is [13]

A

2

= 2wC 1

• (17)

o e 1/2 2 [U)

Basov et al . also assumed the intra-conduction-band Hamiltonian matrix elements to be

H - M > cc m

c c (18)

with a similar expression for the valence-band matrix elements. The appropriate quantum

mechanical expression for these forbidden transitions is [11, 14]

H vv= H cc= eAJk , {19) mc

where m is the free electron mass. Finally, a correction factor of 1/3 should be

introduced in the Basov formula due to the averaging of the matrix elements, over all directions in l< space. (Lee and Fan [15] earlier stated without explanation that a correction factor of 1/48 should be applied to the Basov formula. Their factor obviously

arises from the averaging of the matrix elements and the correction of the vector potential.)

When all the corrections are made, Basov 's expression for the two-photon absorption

coefficient takes the form

449
3 Basov (») Itllhft ( m*cv)

5/2

E g fvc Uu _ E \ 3/ 2j (20)

with f as given by eq. (12).

This final corrected Basov formula eq.(20) coincides with the result obtained from

Braunstein's formula for allowed-forbidden transitions, eq. (7) if the intermediate

states are taken to be in the initial and final bands themselves, and the two topmost

valence bands are considered for the initial state. Further, when eq. (20) is applied

to several semiconductors and alkali halides, the resulting two-photon absorption

coefficients are found to be much smaller than those predicted by the Keldysh

and Braunstein formulas for allowed-al lowed transitions (see Tables II and III).

2.3. Braunstein Formula

In an attempt to reduce the magnitude of the disagreement between the experimental

nonlinear absorption coefficients and the predictions of the Braunstein formula for

al lowed-al lowed transitions, we introduced two modifications in the Braunstein model.

First, the parabolic energy bands in the Braunstein formula are replaced by nonparabolic

energy bands of the form described by eq. (4). The resulting two-photon absorption

coefficient for al lowed-al lowed transitions is

In the case of alkali halides, the intermediate state is assumed to be an excitonic

level in the forbidden-gap, instead of a higher conduction band. Tables II and III show that these modifications do bring the results of the Braunstein formula for

allowed-allowed transitions closer to the experimental values.

3. Conclusion

The Keldysh formula is shown to be closely related to Braunstein's formula for

allowed-allowed transitions. The Basov formula on the other hand is shown to be a special case of Braunstein's formula for allowed-forbidden transitions. Several errors

in the original Basov formula are indicated and corrected. The predictive accuracy

of the Braunstein formula is increased by including nonparabolic energy bands and

excitonic intermediate states.

On comparing the experimental values of the two-photon absorption coefficients

with those predicted by the above three models, after including the indicated corrections

and modifications, we note the following. The allowed-forbidden formulas grossly underestimate the two-photon absorption coefficient. The Keldysh and modified Braunstein

formulas for allowed-allowed transitions give more reasonable results. However, in many

cases only an order of magnitude agreeement is obtained between theory and experiment.

This points to the need for further refinement of the theoretical models.

450
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453
4. REFERENCES.

[1] J. M. Worlock in Laser Handbook, ed.

F. T. Arecchi and E. 0. SchultzDuBois (North Holland, Amsterdam,

1972), vol 2.

[2] S. S. Mitra, L. M. Narducci , R. A. Shatas, Y. F. Tsay, and A. Vaidyanathan, App. Opt. 14^

3038 (1975).

[3] L. V. Keldysh, Sov. Phys . JETP

20, 1307 (1965).

[4] N. G. Basov, A. Z. Grasyuk, I. G. Zubarev, V. A. Katulin, and O.N. Krokhin, Sov. Phys. JETP 23,

366 (1966).

[5] M. Goppert-Mayer , Ann. Phys.

(Leipzig) 9, 273 (1931).

[6] R. Braunstein, Phys. Rev. ]25_, 475

(1962); R. Braunstein and N. Ockman,

Phys. Rev. 134, A499 (1964).

[7] J. H. Yee, Phys. Rev. 3, 355

(1971).

[8] H. D. Jones, and H. R. Reiss,

Phys. Rev. B 16 , 2466 (1977).

[9] F. Adduci, I. M. Catalano, A. Cingolani, and A. Minafra,

Phys. Rev. B 15 , 926 (1977)

[10] M. H. Weiler, Phys. Rev. B7,

5403 (73).

[11] R. A. Smith, Wave Mechanics of

Crystalline Solids, (Chapman and

Hall, London, 1969), Second ed.,

p. 457.

[12] J. Callaway, Quantum Theory of the

Solid State, (Academic, New York,

1974), part A, p. 248.

[13] L. I. Schiff, Quantum Mechanics,

(McGraw Hill, New York, 1955),

Second ed. , p. 250.

[14] J. Bardeen, F. J. Blatt, and L. H. Hall in Proceedings of the Conference

on Photoconductivity, Atlantic City,

1954, ed. G. H. Breckenridge , B. R. Russell, and E. E. Hahn (Wiley, New

York, 1956), p. 146.

[15] C. C. Lee and H. Y. Fan, Appl

Phys. Lett. 20, 18 (1972).

[16] M. L. Cohen and V. Heine in Solid

State Physics, ed. H. Ehrenreich, F. Seitz and D. Turnbull (Academic, New

York, 1970) vol 24, p. 37.

[17] P. Liu, W. L. Smith, H. Lotem, J. H. Bechtel , and N. Bloembergen, Phys. Rev.

B 17 , 4620 (1978).

[18] R. S. Knox and K. J. Teegarden in

Physics of Color Centers, ed. W. B. Fowler (Academic, New York, 1968).

[19] R. T. Poole, J. G. Jenkin, J. Liesegang,

R. C. G. Leckey, Phys. Rev. }J_, 5179

(1975).

454
Fig. 1.

The energy band model used in Braunstein's formulation, v, c, and n refer to the initial

valence, final conduction, and intermediate conduction bands, respectively. All the three

energy bands are assumed to be parabolic.

The speaker indicated that deposition rates of 50-200 angstroms per minute were used in this

work, but he expressed confidence that these deposition rates could be increased by a factor

of two or more if that were important for commercial production.

455

COMPETING MECHANISMS IN LASER-INDUCED DAMAGE

A. Vaidyanathan*, T. W. Walker, and A. H. Guenther

Air Force Weapons Laboratory

Kirtland AFB, NM 87117

The thresholds for laser-induced intrinsic damage in several

alkali halide crystals are calculated by means of separate avalanche

and multi photon ionization models, as well as by a combination of

18 3 the two. The appearance of 10 electrons/cm in the conduction

band is taken as the criterion for damage. Numerical calculations

are performed for nanosecond and picosecond pulse durations, with

wavelengths ranging from 1.064 to 0.355 um. Upon comparing these

theoretical predictions with available experimental results, it is found that, in general, the avalanche mechanism yields damaging

optoelectric field strengths in good agreement with the experimentally measured values. The field strengths required for

damage solely due to multiphoton ionization are substantially

higher. However, when the photon multiplicity is less than or

equal to four, the damage fields predicted by the multiphoton

model are often smaller than those calculated for catastrophic

avalanche multiplication, and are in better agreement with the

experimentally measured damage thresholds. Furthermore, It is noted that neither the avalanche nor the multiphoton ionization

model, when considered alone, can satisfactorily explain the

experimentally observed frequency and pulse length dependence

of the damage fields over the entire reported range of these

parameters. It is thus suggested that it is more appropriate

to combine these two mechanisms.

Key Words: Avalanche ionization; laser-induced damage; multiphoton

ionization

1 . Introduction

The avalanche multiplication [1] and multiphoton ionization [2] processes are

two competing mechanisms by which a pulse of high power laser can cause irreversible damage to optical materials. In an attempt to understand the relative

importance of these two mechanisms in causing damage to the dielectrics, we have

calculated the damage electric field strengths in several alkali halide crystals

within the frameworks of a classical avalanche model [1] and the Keldysh multiphoton

ionization formula [2], The theoretical predictions are compared with the published

experimental results regarding the optoelectric field strengths at damage as well as their variations with the laser frequency and pulse width. From this comparison it is concluded that in order to better explain all of the experimental observations, the

above two processes must be combined into a single formulation, with due attention

paid to their interdependency.

AFSC/NRC Postdoctoral Resident Research Associate

457
2. Theory

2.1 . Avalanche model

Within the framework of the classical avalanche model [1] the rate of energy

gain of a conduction electron from an alternating electric field t = t cos u>t is given by

where t

k is the electron phonon relaxation time for large angle scattering, and m* is the conduction electron effective mass. The rate of energy loss to the lattice phonons

is

= b> , (2)

where t

l is the relaxation time when both large- and small-angle scatterings are considered and w is the average phonon frequency. If there are n conduction

p CO electrons per unit volume when the laser irradiation is initiated, the conduction

electron density at the end of the pulse t seconds will be

P

(t /t.)

n

c

- n cQ

2 P

, (3)

where t- is the time interval during which a conduction electron starting with zero energy attains an energy equal to the band-gap energy. It is well known that [3] when

18 3 the conduction electron density reaches a value of the order of 10 /cm , the absorption

of the residual laser becomes appreciable, leading to localized heating and subsequent

I o irreversible damage to the optical material. For this reason the appearance of 10

3 conduction electrons/cm is taken as the criterion for laser-induced damage in these

cal cul ations

A more stringent criterion for damage would be the proximity of the plasma

frequency to the laser frequency. However, we did not use this criterion in our calculations, since the multiphoton model (to be described below) becomes invalid due

to the large Stark shifts in the energy bands at the field strengths needed to satisfy

the above condition. We further assume that the ionization of weakly bound sites such

as F-centers , impurities, etc., give rise to an initial conduction electron density of

10 /cm . With these assumptions, we obtain from eqs . (1) - (3) the expression for

the damaging electric field amplitude in mks units:

damage

•(l+w

2^) /A£nl09

0.693t. I

1/2

(4)

where a is the band gap energy of the crystal. Eq. (4) is an expression for the

electric field amplitude in pure dielectric materials at the onset of damage.

458
We will later use this equation to calculate the numerical values of the damage

field strengths in several crystalline dielectrics.

2.2. Multiphoton absorption

Keldysh [2] has theoretically derived an expression for the probability of electronic

transitions from the highest valence band to the lowest conduction band of a direct-gap

crystalline solid when intense electromagnetic radiation is incident on the crystal.

In his analysis the energy of the photon is assumed to be smaller than that of the

band-gap, and transitions are presumed to take place between Stark-shifted energy

bands. Keldysh presented the following expression for the transition rate per unit

volume:

M y w V W

X exp

-it < =- + 1 >

Tlcu

(/T+Y7")

(5)

In eq. (5) y* is the reduced effective mass of the conduction electron and valance hole,

X is the effective band-gap in the radiation field given by

^ = 2A vH+y; E (/iv) 1 (6)

the functions E and K are complete elliptic integrals of the first and second kind, while

Y is the ratio of the radiation frequency to the tunneling frequency given by

efv (7)

The symbol <•*'••> in eq. (5) denotes the integer part of the argument, and the function

Q is given by

Q (y, y) = 2K

(/1+Y

2)_ n=0 \

ti 2 (2 < y + 1 > - 2y + n

)

- 2K (^)

E (^)

1/2,

X exp (8)

459
where $ is the Dawson integral . The laser electric field strengths necessary

to damage the dielectrics solely by multiphoton absorption will be later calculated

with the aid of eqs . (5) - (8), for the condition

no o Wt = 10 electrons/cm 0 * P (9)

2.3. Combination of avalanche and multiphoton models

Vinogradov and Faizullov [4] combined the effects of multiphoton and avalanche

mechanisms by assuming that the two processes were independent of each other. They

obtained the following expression for the conduction electron density:

n

c = (

n co

+ St) exp(u

i t ) . w_

. (10)

p w

i

where W and u. are the multiphoton and avalanche ionization frequencies

respectively.

3. Results and discussion

Eqs. (4) - (10) were used to calculate the damage optoelectirc field strengths

in NaCl , KC1 , KBr, NaF, LiF, and CaF^ for typical laser wavelengths and pulse durations (see Table 1). The following values of the parameters were used in our calculation [1]:

x

k

= 1 .36 x 10" 15 sec, t l

= 8.77 x 10" 16 sec, and -h Wp = JQ eV

In addition the values of the band structure parameters used in these computations

are listed in Table II. The elliptic integrals and Dawson integrals encountered

in the Keldysh formula were evaluated numerically. We included a sufficient

number of terms in the infinite sum in eq. (8) to obtain convergency of

the order of 0.01

3.1. Damage threshold

The damaging field strengths predicted by the avalanche model scatter around the experi

mentally measured values within a factor of approximately four. The electric field strength

needed for dielectric breakdown caused solely by the absorption of four or more photons are much larger, and thus this mechanism plays a secondary role when a > > hw. However,

in many cases (e.g., NaCl, KC1 , KBr, etc.) when A <_ 4 ha>, the damage field in the multiphoton model is smaller than that due to the avalanche mechanism, and in closer agreement

with the experimental results. When the multiphoton and avalanche mechanisms are combined

as in eq. (10), the resulting damage thresholds are closer to the smaller of the above

two estimates. This result apparently arises from treating the two processes as independent

of each other, while in fact they are interdependent.

From Table I we also note that most of the experimental damage thresholds are smaller than the predictions of either theory. The reason for this is probably that

the theoretical calculations are for intrinsic breakdown in pure crystalline materials,

while the experimental values are largely governed by impurities and defects.

460
3.2. Dependence of damage field on frequency and pulse duration

In fig. 1 we have plotted the variation of the optoelectric field strength at damage

threshold in NaCl as a function of the laser frequency. According to the avalanche

model the damage field should increase with increasing frequency approximately

1 12 as (1 + u2t

|, 2

) ( see a ^ so ecl- 4). The multiphoton model on the other hand predicts

a sharp decrease in the damage threshold with increasing frequency. The experimental

data [6] show that the damaging optoelectric field strength increases with increasing

frequency up to that of ruby laser, and decreases on further increase in frequency.

Furthermore, the experimentally observed dependence of the damage field on laser

frequency is much weaker than the predictions of either of the above two theories.

From the comparison of the theoretical predictions with the experimental data we conclude that a combination of the multiphoton and avalanche mechanisms is closer to the real situation than either of the two mechanisms considered alone. We also believe

that including the effect of multiphoton transitions on the avalanche ionization frequency

will bring the theoretical predictions closer to the experimental results.

In fig. 2 we have plotted the pulse width dependence of the damage fields in NaCl at 1.06 pm. It is noted that the avalanche model predicts a weak inverse dependence of the

approximate form F % t , while the multiphoton model predicts a somewhat stronger

0 P -0 155 variation of the form Fq

o, tp '

. The experimental dependence is stronger than either

of these two predictions, being of the approximate form FQ ^ t~

0-25

[14]. This once

again points to the need for further refinement of the theoretical models.

4. Conclusion

It is shown that neither the avalanche nor the multiphoton model alone can explain

all of the experimentally observed features of laser-induced damage. It is suggested that

it is more appropriate to combine these two mechanisms into a single formulation. This

should be done in a more detailed and appropriate manner through treatment of their

i nterdependency . Specifically the effect of multiphoton transitions on the avalanche

growth rate constant should be included. The energy dependence of the collision

frequencies as well as the effects of surface states .impurities , color centers, etc.,

should also be considered. Calculations incorporating the above are currently under active

pursuit.

461
5. References

M. Sparks and C. J. Duthler, Theoretical [8]

Studies of High-Power Ultraviolet and

Infrared Materials, Fifth Technical Report,^

Xonics Inc., Van Nuys , CA, June 1975;

M. Sparks, Current status of electronavalanche breakdown theories,

Laser-Induced Damage in Optical Materials

1975, A. J. Glass and A. H. Guenther,

Eds. NBS Special Publication 435, 1975,

pp. 331-346.

L. V. Keldysh, Ionization in the field

of a strong electromagnetic wave,

Sov. Phys. JETP, Vol 20, pp. 1307-1314,

1965.

N. Bloembergen, Laser-induced electric

breakdown in solids, IEEE J.. Quantum

Electron., Vol QE-10, pp. 375-386, 1974.

An. V. Vinogradov and F. S. Faizullov,

Role of many-photon and impact

ionization in the breakdown of

dielectrics by picosecond laser pulses,

Sov. J. Quantum Electron., Vol 7, pp. 650-652, 1977.

D. W. Fradin, N. Bloembergen, and J. P. Lettelier, Dependence of laser-induced

breakdown field strength on pulse

duration, Appl . Phys. Lett., Vol 22,

pp. 635-637, 1973.

A. A. Manenkov, New results on avalanche

ionization as a laser damage mechanism in transparent dielectrics, Laser-Induced

Damage in Optical Materials: 1977, A. J. Glass and A. H. Guenther, Eds, NBS Special Publication 509, 1977, pp 455-464.

D. W. Fradin, E. Yabl onovi tch , and M. Bass,

Confirmation of an electron avalanche

causing laser-induced bulk damage at 1.06 ym, Appl. Opt., Vol 12, pp. 700-709,

1 973.

[9]

[10]

[11]

[12]

[13]

[14]

W. L. Smith, J. H. Bechtel , and N. Bloembergen , Dielectric-breakdown

threshold and nonlinear-refractiveindex measurements with picosecond

laser pulses, Phys. Rev B, Vol 12,

pp. 706-714, 1975.

D. W. Fradin and M. Bass, Electron

avalanche breakdown induced by ruby

laser light, App. Phys. Lett., Vol 22,

pp. 206-208, 1973.

W. L. Smith, J. H. Bechtel, and N. Bloembergen, Picosecond 1 asgr-i nduced

breakdown at 5321 and 3547 A: Observation of frequency-dependent behavior,

Phys. Rev. B, Vol 15, pp. 4039-4055,

1 977.

R. T. Poole, J.G. Jenkin, J. Liesegang,

and R. C. G. Leckey, Electronic band

structure of the alkali halides. I. experimental parameters, Phys Rev. B, Vol 11 , pp. 5179-5189, 1975.

R. S. Knox and K. J. Teegarden, Alkali

halide crystal excitation, Physics

of Color Centers, W. B. Fowler, Ed. New

York: Academic, 1968, p. 34.

G. W. Ruboff, Far-ultraviolet

reflectance spectra and the electronic

structure of ionic crystals, Phys.

Rev. B, Vol 5, pp. 662-684, 1972.

J. R. Bettis, R. A. House, and A. H. Guenther, Spot size and pulse duration

dependence of laser-induced damage,

Laser-Induced Damage in Optical

Materials: 1976, A. J. Glass and A. H. Guenther, Eds. NBS Special Publication

462, 1976, pp. 338-345.

6. Figures

462
I LASER WAVELENGTH l/tm)

1.064 0.S94 0.532 T

S: 20 -

LASER FREQUENCY (I0

IS rod/sec)

Fig. 1 - Variation of the optoelectric field

strength at the onset of intrinsic damage in crystalline NaCl, with laser frequency.

18 3 (The appearance of 10 electrons/cm

in the conduction band was taken to the

damage criterion.) m - multiphoton model,

a - avalanche model, ma - multiphoton assisted

avalanche, and e - experimental.^ The —

curves represent the extensions of the

theoretical predictions to higher frequencies,

where multiphoton effects should dominate.

UK

u too

io-'° 10

PULSE DURATION Ihc )

Fig. 2 - Variation of the optoelectric field

strength at damage with pulse duration in NaCl

crystal for 1.06um radiation. (The

18 3 appearance of 10 electrons/cm in the

conduction band was taken to be the damage

criterion.) m - multiphoton model, a - avalanche model, and e - experimental.

463
T3

0)Cl

QJ

T3

QJ

5-

roCLEo

ill S3

c >> >> o oa

+-> S- s. o QJ Cl QJ .c C ro c

CL oj ID UJ

O

oa P i/i '0 +j B OJ c

rO (J oj T3 c E

ai

4-> s_ S. ro oj OJ

li- Q.

l/l CU X

o: OJ

Ss: oj

C7> T3 E

i— ro

OJ "O

i— s_ Q- -O

oj cn > c

ro oj

4_) OJ O'er

JZ OJ o

a. +-> c

•r— 1/1 to +J -I- I

i— t/A ro 3 1/1 >

CO

+J i

O OJ CL O

03 i

i— OJ

ro T3 > O

<=C E

<JJ to OA

io

uiininininincocoM rorococorocooors

i— i— i— i— i—i— CM CM CM ooooooooo

is <a- i— ro o

+l +i +l

*3" CM

OA «3- *3- cm i— rs

i— co rs ro • • • • c\j M «j in cm in cm ^3- i— i

=r«3-=d-i— co «3- *3- rs00

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oa o i— era- s <d- in co wnnmsNi— i

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rv ai cn ro co M M id.—

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M N i— ID * cn in ro

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is O

o «d- O O CO

3- =3" «3- CM ID ID ID OI ro o o o id in

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00 Is OA

CM is OA

co o oa o — r— "3"

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oo is in o i— CM

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rs i— i

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id ro in o in ro

oa OA OA 00 O i CM O O O

CO o°

+1 3-

<3- CM ^

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«3" in ' ,— CM CO

is o O

CO is OA

in o 00

O ID OA o m cm CM

OA O ID CO CO i—

CO CM CM i o o o 1 o o o J

*3- cm in id co m

o m co

464
Table II. Values of band structure parameters used

Conduction electron

Crystal Energy Gap (eV) effective mass

(Ref. 11) Free electron mass

(Ref. 12)

NaCl 8.6 0.6

KC1 8.7 0.496

KBr 7.3 0.48

NaF 11.5

Li F 13.6

CaF, 12.1

c (Ref. 13)

The conduction electron effective masses for the flourides are

not available in the literature. 0.5 was assumed as a reasonable

value for these compounds. All the valence electron effective

masses were assumed to be infinitely large.

465

THEORY OF ELECTRON-AVALANCHE BREAKDOWN IN SOLIDS*

M. Sparks, T. Holstein, R. Warren, D. L. Mills,

A. A. Maradudin, L. J. Sham, E. Loh Jr., F. King

Pacific-Sierra Research Corporation

Santa Monica, California 90404

Electron-avalanche breakdown in solids is explained by a theory that is pre- dictive and agrees with experimental results for the magnitude of the breakdown

field and its temperature dependence, pulse-duration dependence, material -to- material variation, and wavelength dependence for A _> 1 urn. The good agreement between experiment and theory with no parameters adjusted is obtained by

using improved magnitudes and energy dependences of the electron-phonon relaxation frequencies. The contributions of both optical and acoustical phonons to electron loss and diffusion must be included. The breakdown field E g is calculated by solving an eigenvalue equation obtained from the diffusion transport

equation. Simple models and interpretations of the diffusion equation afford

physical insight into breakdown and render the breakdown conditions predictable.

Preliminary results indicate that the diffusion approximation fails for wavelengths considerably shorter than 1 ym, but that Eg decreases with decreasing

A as a result of multiphoton absorption before the diffusion approximation

fail s. Keywords: Alkali halides; electron-avalanche breakdown; electron-phonon scattering;

Fokker-Planck equation; laser damage; multiphoton absorption; theory

1. Introduction

In spite of many previous attempts to develop a viable model of laser-induced damage to alkali halides, there has yet to appear a theory capable of simultaneously explaining the

magnitude of the breakdown field as a function of temperature, wavelength, pulse duration,

and its variation from material to material. A re-examination of the model assumptions of

the electron-avalanche theory, with particular attention to the magnitude and energy dependence of the electron scattering rates, shows that the transport-equation approach explains

available data without the need to introduct new mechanisms. In particular, the predictions

of the theory agree well with the new Lebedev data [l] 1 for wavelengths A >_ 1.06 ym. A cardinal reason for the good agreement, contrasted to poor agreement of previous similar

theories, is the use of more realistic electron-phonon scattering rates. Umklapp processes,

which were neglected in previous treatments [2,3], are essential. The resulting electronphonon scattering rates increase with increasing electron energy £ at large £. It is also

necessary to include both optical and acoustical phonons.

2. The Model

In the electron-avalanche picture of dielectric breakdown, a small number of "seed"

electrons drift upward in energy in the conduction band through interaction with the laser

field, retarded by scattering from phonons. The electron undergoes a random walk of

progressively increasing kinetic energy until it achieves a threshold energy £. . The

electron then drops to the bottom to the conduction band after creating an exciton. The

rapid photoionization of the exciton injects a second electron into the bottom of the

18 -3 conduction band. Eventually, in sufficiently high fields, enough electrons (= 10 cm ) are created to disrupt the lattice through Joule heating.

Supported by the Air Force Office of Scientific Research. Background investigations were

^supported by the Defense Advanced Research Projects Agency.

Figures in brackets indicate the literature references at the end of this paper.

1 467
ing to the largest g. Dual-space eigenf unctions must be used in the scalar-product expansion coefficients because the system operator is nonHermitian.

3. Solutions of the Diffusion Equation

An extensive series of studies of the eigenvalue problem outlined above was carried out

for detailed models of acoustical- and optical -phonon contributions to Y

k (&) and y^(£). The

energy variations of these quantities resulting from proper incorporation of Umklapp scat- tering (through a simple model) is an essential feature of the problem. This analysis will

be described in a full-length publication to appear elsewhere. With the detailed results

in hand, it is possible to mimic the results surprisingly well through use of the FokkerPlanck equation with D(£) and V(&) chosen as constants (independent of energy). The solutions to the eigenvalue equation, obtained from eqs. (1) and (7), which are easily attained

analytically, are sufficiently simple to afford insight into the avalanche process and to allow scaling without numerical computation. The solutions are even sufficiently accurate

for use as first-order approximations to the exact numerical results.

From the above breakdown criterion that the avalanche process creates a sufficient

electron density to cause excessive Joule heating, Sparks [4] obtained the result

g tp

6 ^ ~ 17, where tp

6 ^ is an effective pulse length, chosen as 1/3 the actual pulse

duration for Gaussian pulses. This is done because the avalanche rate g has a very strong

variation with E, and only the center of the pulse is effective in triggering the breakdown.

In any case, the breakdown field is insensitive to the value of gt in the nanosecond

P region. From the analytic solution and this criterion, the breakdown field may be written

where

VO

is the value of E at which V vanishes, and

E B =E

d

EV0 ' < 8 >

1/2^, „2_l/2

(9)

Ed

= [1 + 2,ng + £n(l + £ng)]" 1/2

for nanosecond pulses, where

The significance of this decomposition of E g lies first in the weak dependence of Ed (and hence Eg

) on the parameters and t for nanosecond pulses, while E^q depends stronglyon &ph

, y^, ancl w - Tne weak dependence of the breakdown field on the band gap (or, to

be precise, on &.) and on the laser-pulse duration t for nanosecond pulses was at first

surprising. However, the previously used, average-electron criterion for breakdown,

Eg = EyQ, also gave E g independent of and t .

At 1.06 ym, the inequality Y

k << w is valid; while at 10.6 urn, the inequality u « Y

k is valid. Thus, eq. (9) gives

& V0 " ^ J >"'^2

and

E VQ

= uOa^m/eT'* (Y

L /Yk

) '

, at 1.06 ym , (11)

13

468
Although the laser energy is absorbed in finite quanta, a Fokker-Planck equation is used to describe the absorption process and the energy losses suffered from collisions with

phonons. Preliminary results indicate that the Fokker-Planck diffusion equation fails for

wavelengths considerably shorter than 1 ym for sodium chloride. The large-quantum energy

gains then have to be considered as difference processes, rather than being approximated by

differential processes. However, the preliminary results also indicate that multiphoton

processes cause the breakdown field to decrease with decreasing wavelength before the largequantum effects become important. Thus, the Fokker-Planck diffusion equation and multiphoton absorption will be sufficient to explain breakdown if the preliminary results are verified.

The Fokker-Planck equation can be written as a diffusion equation in energy space

3n(£,t)/3t + 3J/3£ 0 , (1)

where n(£,t)d is the number of electrons with energy between £ and £ + d£, and

J(£,t) = V(£) n(£,t) - D(&) 3n(£,t)/3£ , (2)

Here V(£) and D(£) are the effective velocity and diffusion coefficient in energy space. Both depend on energy, temperature, and laser frequency. For fields comparable to the

breakdown field, ignoring small field independent contributions to D, we obtain

V(£) = j.a(a) E

2

- &ph y

l (S) . D(£) = \ a(£)E

2 (3)

Here E is the root-mean-square electric field and the electrical conductivity is defined

as

a(£) =

, (4)

m[u) + Yk (£)]

where "^(S) is the rate at which current carried by electrons of energy £ is relaxed, Y^(£)

is the rate at which the electron loses energy to the phonons, e and m are the electron

charge and effective mass, to is laser frequency, and & ^ is an average phonon energy. The

boundary conditions are

n(£

i ,t) = 0 , (5)

corresponding to no electrons accelerated past the exciton threshold, and

J(0,t) = 2 J(£.,t) , (6)

corresponding to the "flux doubling" feature of exciton creation followed by photoionization.

The partial -differential transport equation (1) is converted to an ordinary-differential eigenvalue equation by using the approximation

n(£,t) = n(£) exp (3t) (7)

and by assuming that the avalanche phenomenon is dominated by the eigenvalue 3 having the

greatest real part. All of the numerical work locates this largest eigenvalue. In fact,

only a single positive real eigenvalue was obtained in simple models for V(£) and D(£)

(for example, V and D constant, or V constant and D ~ £) that render the Fokker-Planck

equation analytically soluble. The approximation in eq. (7) can be justified by general

considerations of the diffusion equation (1) (the time constant x

dl

. f= £

2/D for attaining

the eigenfunction form of eq. (7) is short with respect to the pulse duration), by explicit

solution of the complete partial differential equation (1), or by expanding the initial

electron distribution in eigenfunctions and neglecting all terms except the one correspond469
E (3£ m/e

2

) 1/2

( Y|

_ Yk

) 1/2, at 10.6 ym . (12) V0 v ph

The electron-phonon relaxation frequencies Y

k and y

l are sketched as functions of £ in

fig. 1. From eq. (11) it is seen that at 1.06 ym, the breakdown field is controlled by the

ratio Y|_/Y k

> which is greatest at low electron energies, as illustrated in fig. 1. By contrast, at 10.6 ym, Eg is controlled by the product

y^_ Y^ which is greatest at the highest

electron energies. The values of y^ and Yj_ used in the constant-D-and-V model must be

chosen according—as the low-S values at 1.06 ym and as the high-& values at 10.6 ym. This

necessity to use different y's at different frequencies explains the previous failure of

constant-Y models that was pointed out by Sparks [4].

The result in eq. (8) also explains why the average-electron result Eg = Eyg is too large. The value of E^ in eq. (8) is less than unity. Furthermore, since Ed is nearly con- stant (for nanosecond pulses) the scaling obtained from the average-electron result Eg = E^q

is rather accurate, provided that the appropriate frequency-dependent values of and Y|_ are chosen, as discussed above. This scaling with temperature, wavelength, pulse duration,

and material parameters obtained from eq. (8) or from E R

= E

vf) explains the experimental

scaling wel 1

4. Numerical Results

To obtain quantitative values for the breakdown field, the eigenvalue equation was integrated numerically with a predictor-corrector algorithm using eqs. (2) through (5)

together with the boundary conditions. Figure 2 is the resulting plot of the multiplication

exponent 3 as a function of E for sodium chloride at room temperature and 1.06 ym. For

low fields—which are attained for nanosecond pulses— it can be shown by analytic methods

that

3 = (51/t

p

) g (E

2 /E2 Q

) exp[-E20/E

2

] . (13)

This result in eq. (13) verifies the result above that the breakdown field is sensitive to changes in the value of E un (shifting the 3(E) curve in fig. 2 from left to right according

2 2 to the scaling 3 - exp(E^Q/E ) in eq. (13)) but is insensitive to changes in the value of g (shifting the steep 3(E) curve up and down), the dramatic rise of 3 with increasing E predicted by eq. (13) is seen in fig. 2. By contrast, for high fields the figure shows the

behavior 3 ~ E , which is necessary because the electron energy gain is dominated by the

2 aE term. For picosecond pulses—for which 3 is typically in the high-field region—the

breakdown field is more sensitive to values of £. and t than it is in the nanosecond

i P region. For example, using the breakdown criterion 3t

p

= 51, the breakdown field can be

read off the curve in fig. 2. Changing the value of t and the corresponding breakdown

value of 3g = 51/tp gives small changes in E g in the steep, low-field region appropriate to nanosecond pulses, but gives larger changes in E D in the relatively flatter high-field

2 region of 3 ~ E appropriate to picosecond pulses.

Figure 3 shows the relatively good agreement between theory (solid curve) and experiment [1,5,6,7] points). A major difficulty in comparing theory with experiment is that

there is no way at present to determine whether experimental values are intrinsic or extrinsic. Indeed the recent Lebedev [1] values of E D are greater than previous experiD mental values. Although it is not certain that the intrinsic values have ever been

attained, even in the Lebedev experiments, the good agreement between the theoretical

470
results with no parameters adjusted and the Lebedev experimental results for the magnitude,

temperature dependence, pulse-duration dependence, material -to-material dependence, and

wavelength dependence of Eg suggests that the Lebedev breakdown thresholds are intrinsic

and that the previous lower values of E D were extrinsic.

D Soileau and coworkers [8] suggested that spot-size scaling possibly could reconcile the

results of various workers. They suggested that the value of Eg should be a function of the

focal-volume size because the probability of having a starting seed electron in the volume

decreases as the volume decreases. Experiments with seed electrons supplied by a separate

source, examination of the probabilistic character of breakdown for small focal volumes and

the deterministic character for large focal volumes, or other verification of the spot-size

scaling law is needed because it is currently thought that the seed electrons are easily

generated from impurities, dislocations, or grain boundaries, for example. The breakdown

is thought to be controlled by the avalanche process, rather than by the generation of the

seed electrons. However, there have been no experiments or convincing theories to support

these beliefs, and verification of the suggested mechanism giving the spot-size scaling

would be useful

In order to demonstrate the sensitivity of the value of Eg to values of the parameters

in the theory, a different value of the electron-chlorine-ion scattering cross section

(which appears in and Yj) was chosen. All results were recalculated for this single new value and are shown as dotted curves in figs. 3 through 5. The improved agreement between

theory and experiment for the adjusted value of has no important significance other than

illustrating the sensitivity of Eg to the value of the parameter because adjusting para- meters to obtain agreement between theory and experiment is unwise and almost always misleading. Nevertheless, improved experimental values of a-, would be welcomed because the

unadjusted value used was the value for chlorine gas taken from the atomic-physics

literature.

The agreement in fig. 4 between theory and experiment [1] for the temperature dependence of Eg for sodium chloride at room temperature and 1.06 ym is good. Lower theoretical

values than the experimental values of Eg, as in fig. 4 for 10.6 ym, are quite surprising

because previous theories have universally suffered by predicting values of Eg greater than

the experimental values and because intrinsic processes lower the experimental value of Eg.

The simplicity of the constant-D-and-V model affords the explanation of the low theoretical

10.6 ym values. At 10.6 ym, very-high-frequency (i.e., high-energy) phonons control the

values of y^ and Y|_ because high-energy electrons determine the value of Eg, as discussed

above. Since the reduction in with increasing energy is not known experimentally, the

energy dependence of was neglected. In order to partially compensate for this neg- lected energy dependence, the phonon dispersion relations outside the first Brillouin zone were approximated by constants (equal to the zone-boundary values). This constant highenergy-phonon frequency approximation overestimates the value of Eg. The overestimate does

not occur at 10.6 ym because the low-energy electrons that control Eg at 10.6 ym interact

with low-frequency phonons in the first Brillouin zone. Improved estimates of the 10.6 ym

breakdown field, using more realistic dispersion relations for the high-frequency phonons,

will be made.

The good agreement between theory and experiment [1] for the material -to-material

variation of Eg at room temperature and 1.06 ym is seen in fig. 5.

5. Summary

Numerical studies of the Fokker-Planck form of the kinetic equation using models of

electron-phonon scattering with Umklapp processes included produce good agreement with

471
experiments [1,5] for the magnitude of the breakdown field and its temperature dependence,

pulse-duration dependence, material -to-material variation, and wavelength dependence for

Y > 1.06 ym with no parameters adjusted. Simple results of the constant-D-and-V model are invaluable in understanding the results, scaling to different conditions, relating the

present and previous theoretical results, and in showing that the breakdown at 1.06 urn and 10.6 urn involve different physics.

REFERENCES

[1] Gorshkov, B. G. , Danileiko, Yu. K.,

Epifanov, A. F., Lobachev, V. A.,

Manenkov, A. A., Sidorin, A. V.,

Soviet Physics (JETP) 45, 612 (1977);

Manenkov, A. A., in Laser induced

damage in optical materials: 1977,

Glass, A. J., and Guenther, A. H.,

eds, National Bureau of Standards,

Washington, D.C. (NBS Spec. Publ

509) p. 455.

[2] Holway, L. H. and Fradin, D. W., J. Appl . Phys., 46, 279 (1975).

[3] Frb'hlich, H., Proc. R. Soc. A, 160 , 230 (1937); Frb'hlich, H. , Adv. Phys.

3, 325 (1954).

[4] Sparks, M. , in Laser induced damage

in optical materials: 1975, Glass,

A. J., and Guenther, A. H., eds,

National Bureau of Standards, Washington, D.C. (NBS Spec. Publ. 435),

p. 331; Sparks, M., in Theoretical

studies of high power ultraviolet

and infrared materials, section C and section B, Xonics 5th technical

report, 30 June 1975.

[5] Fradin, D. W. , Bloembergen, N.,

Lettlier, J. P., Appl. Phys. Lett.

22, 635 (1973).

[6] Fradin, D. W., Yablonovitch, E., and

Bass, M. , Appl. Opt., 12_, (700), 1973

[7] Smith, W. L., Bechtel , J. H., and

Bloembergen, N., Phys. Rev., 15_, (4039), 1977.

[8] Soileau, M. J., Bass, M., and van Stryland, E. W., in Laser induced

damage in optical materials: 1978,

Glass, A. J., and Guenther, A. H.,

eds, National Bureau of Standards,

Washington, D.C. (NBS Spec. Publ.

541), p. 309.

472
Figure 1. Energy variation and magnitude of the energy-loss rate y

l and energy-transport

rate Yl for sodium chloride at room temperature.

473
NaCI, 1.06 urn, 293 K

Electric field, E s (MV/crn)

Figure 2. Numerical solution of the Fokker-Planck equation for sodium chloride at temperature and 1.06 urn.

474
10ps 100ps 1ns 10ns

Pulse duration, tp

f!SV\nC °Tr' S °^0 Iu eXPu rimental f 1 ' 5 ' 6 ' 7 ] P^se-duration dependence of the breakdown field EB (points) with the theoretical result (solid curve) for sodium chloride at room temperature and 1.06 ym. The dashed curve illustrates the sensitivity of E

g to the para- meter change discussed in the text.

475
"£&d In^thrJlxt""

8 "

,UStrateS the SenSlt "U^

° f E8

l° *•

476
12

1.06 //m, 293 K

10 -

4

2 -

Theory

Experiment

Theory with Oq

adjusted

LiF NaF NaCI

Material

KCI KBr

Figure 5 Comparison of the experimental [1] material -dependence of the breakdown field

J2M

S nS

atChed

TSar5

) rlh

. the ^fo^tical results (solid-line bars) at room temperature

dfscussedTn the\ext

illustrate the sensitivity of Eg to the parameter change

477
In response to a question concerning the differences between the theory presented here and

the theory presented by Manenkov a year ago, the speaker indicated that the main difference

was in the detail of the treatment of the electron-phonon processes. In this treatment, the

electron-phonon collision frequency was taken to be quite different than in Manenkov's treatment. Furthermore, in this treatment, a number of parameters of the theory were taken to depend on temperature , including band gap, occupation number, etc. , whereas in Manenkov 's treatment the only explicit temperature dependence was in the phonon occupation number. The

speaker was asked why Manenkov' s data could be treated as intrinsic. He indicated that it

might not be intrinsic but that the good agreement with the predictions of the theory re- garding the dependennce of the breakdown -threshold on wavelength, temperature and choice of

material was very encouraging, and that Manenkov' s data appeared to be as close to intrinsic

data as one could obtain. A number of questions came up in the discussion regarding particular approximations in the theory. Finally, the point was raised concerning how intrinsic

avalanche breakdown could exhibit a pulse length dependence when in itself it was a very

rapidly occurring, strongly non-linear process. The point was not satisfactorily resolved in

the course of the discussion.

478
IMPURITY BREAKDOWN MODEL IN THIN FILMS

T. W. Walker0

, A. Vaidyanathan , A. H. Guenther

Air Force Weapons Laboratory

Kirtland AFB, New Mexico 87117

and

P.Nielsen

+ Air Force Institute of Technology

Wright Patterson AFB, Ohio 45433

The impurity model for breakdown given by Hopper and Uhlmann was examined. The case for nonmetallic impurities was derived from the thermal equations and affords the results of Hopper and Uhlmann when the correct limits are taken. These equations were

then compared to an exact solution given by Goldenberg and Tranter, in which it is not assumed that the radial temperature distribution within the particle is a constant.

It was found that the exact solution does not agree with the approximate solutions

when the thermal conductivity of the impurity is small. In this case, the temperature

in the particle is not uniform and the damage threshold can be substantially lower.

The Mie absorption coefficient was then combined with the exact solution. For nonmetallic impurities the results predict a damage threshold which depends on the film thickness. The experimental results for CaF£, ThF^, and MgF£ were found to agree very well with these theoretical predictions. This agreement was found to hold

at both the 5 ns and 15 ns pulse lengths and at 1.06 ym, 0.53 urn, and 0.26 ym wavelengths.

Further theoretical calculations show that different size impurities are the most

damage sensitive at different pulse lengths (and wavelengths). The calculation of

breakdown threshold as a function of pulse length for fused silica surfaces fit very accurately

to a \ft curve, in agreement with the results of D. Milam.

Key Words: Laser damage; impurity-induced damage; pulse duration dependence of damage; Mie

absorption coefficient.

1. Introduction

There has been a great deal of theoretical work on laser-induced damage produced by impurities

in bulk optical material (1-5). At the present it is uncertain whether damage in even very pure

bulk materials is produced by avalanche or multiphoton ionization of by isolated impurity sites (6).

Experimental data (7, 8, 9, 10) indicates a spot size dependence of the breakdown level that is difficult to explain by either multiphoton or avalanche ionization. In thin films the role of

impurities in the damage process is expected to be even greater. For example, the absorption

coefficient of most thin films is between 10 cm

-1 to 1000 cm"

1

, where in bulk form the range is several orders of magnitude smaller. Surfaces fall in between these two extremes of absorption.

This high absorptivity of thin films is generally believed to be due to impurities in the film which

are included during the deposition process. Experimental data also indicates that the breakdown

threshold is more than an order of magnitude lower for thin films that for the bulk form of the

same materials (8)

@ Submitted in partial fulfillment of the Ph.D. degree at the Air Force Institute of Technology,

Wright Patterson AFB, Ohio

* N.R.C. Postdoctoral Resident Research Associate

+ Presently at Air Command Staff College, Maxwell AFB, AL

i 479
2. Theory

The model which will be used in this work for impurity damage is that of spherical absorbing

particles embedded in a uniform host material. The impurity is assumed to absorb the incident

radiation and increase in temperature relative to the surrounding host material. A straightforward solution to the thermal equations can be obtained if it is assumed that the heat flow

out of the particle is determined solely by the thermal conductivity of the host material. This

is true if the thermal conductivity of the impurity is infinite, in which case the temperature

within the impurity is uniform. The solution is straightforward and the result is the same as given by Hopper and Uhlman (2). The temperature in the host is

3QI

2U C D.mr

p h

1 erfc . r~a q-m

1 erfc r-a 2(D

h

t

p

) 2 q+m 2 < D hV

1 exp

q-m

+

q+m- exp

/ w \ j_ (q-m) D,t (q-m)(r-a) +VH ' hp

2a

4a'

erfc r-a + (q-m)(D

h t 2a

(q+m)(r-a) +

{c* +m) Vp

2a

4a'

erfc

L2(D

h t

p )=

r-a + '^»DhV

2 < D hV 2a

Inside the impurity we have

3QI

p 2nCD.ma

P n q-m

1 - exp

1 q+m

1 -exp ^ 4a' 7 V

2a )]

(la)

(lb)

where C is the volume specific heat of the impurity; C, is the volume specific heat' of the host

P \, " material; q = 3C^/C and m = (q(q-4)) 2

; D

h is the thermal diffusivity of the host material.

Equation (1) is the result that is frequently used in impurity damage work (1, 3, 11). It is important to note that equations (la) and (lb) are not valid for q less than or equal to 4. For

q greater than 4 we must have C"h greater than (4/3)C"

p (usually the case for. metallic impurities

in glass). If q is less than 4, the case for dielectric impurities, the correct solution is

3QI

naC

pD

h

(q

2+m2

;

1 + exp

- Cos /

2c' mD

h

t p \ + 2 V 4a

2 / VTT

exp

4a

2 V-

\2a

-y

2 r

sin

(

2yq

Jl_E

2mqD, 2a gcos/ 2^ vP - 2mc' D

h t

p

>

4a' iM -

g_ cos ^

2a

4a'

(2)

dy

It can be shown that under the proper limiting conditioning that both equation (1) and the correct

equation- for q

= 4 are obtained from equation (2). In equation (2) m = (q q-4 ) 2

.

480
Recent research (12, 13) indicates that the impurities in flouride films are oxides. For example, thorium flouride (ThF^) contains Th0

2 and/or ThOF2> Oxides and other dielectric

materials typically have a very small coefficient of thermal diffisivity (14), which violates the basic assumption made in obtaining equations (1) and (2). This means that these equations cannot

be used in treating dielectric impurities in dielectric host materials.

In light of the above, an exact solution is required to the thermal equations of spherical

impurities. The exact thermal equations are given in equations (3) and (4).

d

2 (rT'; dr

2

d

2 (rT)

dr2

- srT = - rOJ_ 0 < r<a

srT = 0

D

h

T = T

K dT' = Ku dT

r > a

r=a

(3)

dr

(4)

The solution to these equations was obtained by Goldenberg and Tranter (13). The result is

T = a^i

p

K

P

3 K

h B)

(5)

(siny-ycosy)(sin(ry/a) dy

2 2 2 2 (csiny-ycosy) + b y sin y

where r] = a /D

p

; c = l-(K

h /K ); b = (K

h /K ) (D /Dh

)"*; K

h and K

p are the host and impurity thermal conductivities, respectfully; D

p is the, impurity thermal diffusivity; t is the laser pulse length.

The exponential in equation (5) involves the constant D t /a, where equations (1) and (2) contain D^t /a. Since the integration in equation (5) is from zero to infinity, it is easy to make a change of variable and cast (5) into the same form. Figure (1) shows a plot of the incident energy per unit area required to heat a particle at its boundry (r = a) to a temperature

of 2000°K as a function of the particle size. Curves (1) and (2) are equations (1) and (5) respectively. The material parameters are for a metallic impurity in a silicate host material.

As expected the agreement is excellent in this case over the range of impurity sizes that were

used. Figure (2) shows another comparison of equation (5) (curve 1) and equation (2) (curve 2). The material parameters are for dielectric impurities and q less than 4. In this case very poor agreement is obtained between the two equations. This is a result of the uniform temperature

assumption made in obtaining equation (2).

The integral in equation (5) converges rapidly and there are no practical reasons for using

equations (1) or (2) instead of (5). It is clear that the Goldenberg and Tranter solution to the

thermal equations is not known to many researchers working in the field of laser-induced

damage. This is pointed out by the fact that in most of the work on impurity dominated damage

(4-6, 9-11, 15-31), there is no reference to this solution. However, in the research cited there

are many references to the solution of Hopper and Uhlmann (1), given by equation (1)- In all fairness to these latter researchers, their prime concern was platinum inclusions in silicate laser glasses, for which case their approach is quite acceptable.

481
The breakdown criteria which will be used in this model will be one of achievement of

a specific temperature. A fixed critical temperature is set, for example the melting temperature

of the host material, and the incident energy density is plotted against the impurity size. The

minimum energy per unit area required to heat an impurity to the critical temperature is taken

as defining the damage threshold. Calculations of impurity damage involving stresses (2), shock waves (16, 23, 28), and even avalanche ionization (11) have been carried out in an attempt to study the final stages of damage. It is assumed in this work that once the critical temperature

is reached the damage proceeds instantaneously and catastrophical ly. This assumption is supported

by the fact that as the temperature approaches the melting point of the host of impurity, the absorption increases dramatically (11, 15, 23, 31). At this point the breakdown then evolves

nonlinearly and rapidly. The slower process by which the critical temperature is reached determines most the physical nature of the breakdown event. Therefore, the critical temperature criteria for breakdown should predict the parametric trends in the experimental data.

There has not been a treatment of impurity dominated breakdown which has used the Mie

absorption cross-section for the impurities. It can be seen from the previous figures that the size of impurity which is easiest to damage is of the same order of magnitude as the wavelength of

the laser pulse. If impurities are of this size then their energy absorption is best described

by Mie absorption.

As equation (5) stands there is no frequency dependence. However, if we introduce an asymptotic

analytic expression for the Mie absorption cross-section in place of Q in equation (5), this is no longer the case. Such an expression can be obtained for the Mie cross-section if the real index (n) of refraction is between 1 and 2, and the imaginary index (n

1 ) is much less than one. We then have

(32, 33)

2 -8n n'a -8 n'a

2TIa

(

1 + e X + £

V 8nn'a/X

(6)

where a is the radius of the impurity and X is the wavelength of radiation. This cross-section is valid for a dielectric impurity and gives a wavelength dependence.

i The range of n and n' over which equation (6) is valid would not include metallic impurities.

The Mie absorption for metallic impurities can be approximated by

Q = ria 2 /Ax + B\

\

n

' a / (7) where A and B are empirical constants which depend on the particular metal.

The greatest difficulty in applying a Mie coefficient in the theory is that the composition

of the impurities is not known for most films. Also, the imaginary index of refraction n' is a function of frequency. As a result of the above points, comparison between experiment and

theory must be relative and not absolute. That is, the impurity model can only predict trends

in the breakdown field and not absolute damage thresholds.

It has been pointed out that resonances can exist in the absorption cross-section (34, 35)

which are not predicted by equations (6) and (7). These resonances can be shown to be of no importance for the cases treated here (36).

3. Comparison of Theory with Experimental Data

The impurity model can be examined in terms of the trends in the experimental data. Figures

(3) to (6) provide a comparison of the ThF. data to theoretical calculations based on equations

(5) and (6). In each figure the damage threshold is plotted as a function of the film thickness.

The solid curves are the theoretical calculations. In the case of ThF^ the impurity was modeled

as ThC^. There is evidence to support the existance of this impurity or ThOT^ in the ThF^ films (13),

Fortunately, thermal constants for ThC^ were available.

482
The most obvious feature of this impurity model is the prediction of an increase in damage

threshold with a decrease in film thickness. This occurs because it is assumed in the model

that the maximum size an impurity can have is limited by the film thickness. That is, as the film thickness increases so do the impurity sizes. This situation has been well supported by

work at Perkin-Elmer (12) in which this phenomenon has been observed for many types of films.

The theoretical curves in the figures are adjusted to the data at each wavelength. This

is done by selecting one data point for either the 5ns or 15ns data and normalizing the appropriate theoretical curve at that point. The other theoretical curve for the remaining

pulse width is then normalized by the same factor so that the relative position between the

two curves is not changed. Therefore, the theoretical curves in each figure have been normalized to a single data point.

It is apparent from the figures that this model gives a good prediction of both the film

thickness and the pulse width dependence of the damage threshold in ThF^. The theoretical curves

also track well with the data as the wavelength is changed and predict the observed decrease in film

thickness dependence of damage at shorter wavelengths. The greatest point of disagreement is at 15ns and 1.06 ym for the 1/8 ym thick film. The measured damage threshold is close to that

of the bare fused silica surface threshold. It is at this point that we would expect the greatest

departure from theory due to the very high energy densities involved. Many more effects can become

important at these high field strengths which could reduce the measured from the predicted threshold.

The theoretical curves show a flattening over more of the film thickness at 0.35 and 0.26 ym as opposed to 1.06 and 0.53 m. It is not evident in the Mie cross-section given in equation (6) and

the thermal equation (5), but this theory predicts that when the laser wavelength is reduced the size

of the impurity that is most sensitive to damage is also reduced. The flattening of the theoretical curves at the shorter wavelengths occurs because the size of the impurity which is easiest to damage

is included by more of the thin films. The particular value of film thickness at which the damage

thresholds begin to rise is also shifted to thinner films at shorter wavelengths for the same reason. Both of these trends are observed in the data.

It can alse be noted that at the shorter wavelengths there is an increase in the damage

threshold of the 4 ym thick films above the theoretical curves. This trend has been discussed

in another work (36)

Although ThF^ theoretical curves indicate that the pulse width dependence is properly described

at all wavelengths the data is only for two pulse widths. It is interesting to compare the laser

pulse width dependence predicted by the impurity model with data taken over a greater pulse width

range. Figure (7) shows how this can be accomplished. The two curves represent 5 and 15ns damage

curves as a function of particle size. It is clear that as the pulse width is decreased the threshold decreases and smaller sized particles are damaged. If a series of these curves are plotted for different laser pulse widths, then the minimum thresholds can be plotted as a function

of pulse width. The resulting graph gives the desired predicted behavior. The procedure was used

to obtain the theoretical curve for fused silica that is shown in Figure (8). The data was taken

from Milam, et al . (37) and is for fused silica surfaces. The agreement between the theory and the data is excellent. Milan (37) made a best fit of a (t

p

) 2 and found that this curve also gave a good

fit to the data. However, this fit is not as good as that shown in Figure (8).

Since the comparison between theory and experiment in Figures (6) through (9) was made by

normalizing the curves at each wavelength, they do not provide a reasonable picture of the predicted

wavelength dependence. Figure (9) compares the theoretical and experimental values of the damage

threshold of M

g

F

2 films as a function of wavelength. The theoretical predictions and experimental

data are given for both 1/8 and lum thick films. The theory and experiment both show a faster drop

in the damage threshold for the 1/8 yra film that for the lym thick film as the wavelength decreases.

The theoretical curve for the 1/8 ym film has been normalized to the 0.26 ym data point. The

theoretical curve for the lym film is normalized so that the relative positions of the two theoretical

curves do not change. The experimental data shows a faster decrease in damage threshold at 0.53 ym

than the theoretical curves. A possible explanation for this can be found in the imaginary index of

refraction used in the Mie absorption cross-section. The index was picked to fit with a transparent

dielectric impurity (n' less than 0.01) at 1.06 ym. For this case the imaginary index of refraction

increases with decreasing wavelength. This feature was not included in the theory because of the

483
lack of knowledge of the impurity composition and concentration. Based on absorption curves for

dielectric materials this increase could be expected to be more pronounced at wavelengths shorter

than 0.53ym. Without a detailed knowledge of the impurities it is not possible to bound the

magnitude of this effect. What can be said is that if the wavelength dependence of the index could

have been included in the theory, the curves would also have exhibited a faster drop in the damage

threshold at shorter wavelengths.

4. Conclusion

The impurity model, with the Mie absorption cross-section, gives excellent agreement with

experimental data. The theory predicts the correct trends of damage threshold with laser pulse

length, wavelength, and film thickness. The complete experimental data is presented elsewhere

(38). In that paper it was pointed out that the morphology of the damage sites changed

with decreasing laser wavelength. In general the site size decreased and the number density of

sites increased. In the impurity model this can be attributed to the increase in the Mie

absorption of the smaller impurities as the wavelength is decreased. That is, smaller impurites

become more likely to damage at shorter wavelengths. Recent research (25, 30) has shown that

the number density of impurities increases as the impurity size decreases. This would explain

the increase in damage site density. Thus, the impurity model also predicts the correct morphology trends.

484
5. References

[1] Hopper, R.W. and Ulhmann, D.R., Mechanism

of Inclusion Damage in Laser Glass, Appl.

Phys. 41, No. 10, 4023,(1970).

[2] Danileiko, Y.K., et al . , The Role of

Absorbing Inclusions in Laser-Induced

Damage of Transparent Dielectrics,

Sov . Phys . JETP 36, No. 3, 541,(1973).

[3] Anisimov, S.I. and Makshantsev, B. I., Role of Absorbing Inclusions in the Optical Breakdown of Transparent Media,

Sov . Phys. -Solid State 1J5 , No . 4

,

743, (1973).

[4] Milam, D., et al., Laser Damage Threshold

for Dielectric Coatings as Determined by

Inclusions, Appl . Phys . Lett . 23, No. 12,

654, (1973).

[5] Danileiko, Y. K., et al . , Role of Absorbing

Defects in the Mechanism of Laser Danage

of Real Transparent Dielectrics, Sov. J_. Quant . Elect . 4, No. 8, 1005, (1975T.

[6] Danileiko, Y. K. , et al . , Surface Damage

of Ruby Crystals by Laser Radiation, Sov . Phys . JETP 31, Mo. 1, 18, (1970).

[7] Wang, V., et al., Single and Multilongitudinal Mode Damage in Multilayer

Reflectors at 10.6 m as a Function of Spot Size Pulse Duration, Laser Induced

Damage in Optical Materials , NBS Special

Publication 435, eds. A. J. Glass and

A. H. Guenther, US GP0 Washington

D.C., (1975).

[8] Bettis, J. R. , et al . , Spot Size and Pulse Dependence of Laser Induced Damage, Laser Induced Damage in Optical Materials , NBS Special Publication 462, US G~P0" Washington D.C. (1976).

[9] Kovalev, A. A., et al . , Dependence of

the Damage Threshold of Polymethylmetacrylate on the Duration of Laser Pulses

and Dimensions of Irradiated Region, Sov . Phys . JETP 43, No. 1, 69, (1976)

[10] Sparks, M. and Duthler, C. J., Theory

of Infrared Absorption and Material

Failure in Crystals Containing

Inclusions, J. Appl . Phys . 44, No. 7, 3038, (1973)7 ~~

[11] Makshantsev, B. I., et al . , Effect of

Absorbing . Development of Avalanche Ionization,

Sov . Phys .Solid State 16., No . 1

, 103, U9T4).

[12] R. Austin and A. H. Ledger, Perkin-Elmer

,

private communications.

[13] Humphreys, T. W., et al., Surface Microanalysis Technicques for Characterization

of Thin Films, to be published in Laser

Induced Damage in Optical Materials , (1979 )

[14] Sparks, M. and Duthler, C. J. , Theoretical

Studies of High-Power Ultraviolet and Infrared Materials , p. 11, Eight Technical

Report, (1976).

[15] Zakharov, S. I., et al., Phenomenology of

Formation of Shock Waves in the Volume of a Dielectric by a Focused Monopulse, Sov . Phys . Dak! . 19 No. 11 , 779, (1975).

[16] Danileiko, Y. K., et al., Prethreshold

Phenomena in Laser Damage of Optical

Materials, Soy_. J_. Quant . Elect . 6. No. 2, 236, (1976).

[17] Aleshin, I.V., et al., Optical Breakdown

of Transparent Media Containing Microinhomogeneities , Sov . Phys . JETP 43, No. 4,

631, (1976).

[18] Libenson, M. N., Plasma-chemical Model for Optical Breakdown of Transparent Dielectrics,

Sov . Tech . Phys Lett . 3, No. 5, 180, (1977).

[19] Artem'ev V.V., et al., Statistics of Microscopic Innomogeneities and Optical Breakdown

of Transparent Media, Sov . Phys . Tech . Phys . 22, No. 1, 106, JT977h~

[20] Picard, R. H., et al., Statistical Analysis

of Defect Caused Laser Damage in Thin Films,

Appl . Opts . 16_, No. 6, 1563, (1977).

[21] Karpov, O.V., et al . , Interaction of Laser Radiation with a Heterogeneous Medium, Zhurnal Prikladnoi Spektroskopi

29, No. 3, 415, (1978).

[22] Lysikov, Y. I., Calculation of the Kinetics of Propagation of a Powerful Light Beam in a Transparent Dielectric with Impurities, Zhurnal Prikladnoi Mekhani ki i Tekhnicheskoi FiziFTj No. 5, 60, (1978).

[23] Kosolobov, S. N. , et al . , Shock Mechanism for

Dissipation of Laser Energy in a Transparent

Dielectric with Microscopic Inclusions and Impurities, Sov. Phys. Tech. Phys. 23, No. 9,

1133, (1978).

[24] Kovalev, A. A. and Makshantsev, B. I., Surface

Damage Caused by Laser Radiation to Solid

Transparent Dielectrics, Sov . J_. Quant . Elect . 8, No. 5, 659, (1978).

485
[25] Artem'ev, V. V., et a!., Correlation

Between the Radiation Breakdown and

Microscopic Inhmogeneities of a Dielectric Coating, Sov . Phys . Tech . Phys. 26, No. 12, 1467, (7978)

[26] Kosolobov, S. N., et al . , An Impact

Mechanism in Diverting Energy From Absorbing Inclusions at Laser Destruction of Transparent Dielectrics,

Opt . Commun . 24, No. 3, 355, (1978).

[27] Morichev, I.E., et al . , Laser Breakdown

and Temperature Dependence of Absorption

in Quartz, Sov . Phys . Tech . Phys . 23,

No. 10, 1254, (197HT".

[28] Ivanov, L. I., et al., Laws Governing the Formation of Shock Waves Under the Influence of Laser Emission of the Absorbing Solids, Problemy Prochnosti , No. 6, 99, (1978).

[29] Sokolovskiy, R. I., and Tyurin, Y. L., Absorption of Intense Radiation in a Transparent Dielectric Which Contains

Microcontaminants , Izvestiya Vysshikh

Uchebynkh Zavedeniy Fizika , No. 6(193),

[30] Leonov, R. K., et al . , Methods of

Invesitgating the Role of Absorbing

Microinclusions in Damage to Transparent

Dielectrics by Laser Radiation; I. Passive Methods, Sov. J. Quant . Elect . 8, No. 6, 729, (197F).

[31] Danilieko, Y. K. , et al . , The Mechanism of

Laser-Induced Damage in Transparent Materials

Caused by Thermal Explosion of Absorping

Inhomogeneities, Sov . J_. Quant . Elect . 8, No. 1, 116, (1978).

[32] Van de Hulst, H. C, Light Scattering by Small Particles , John Wiley & Sons, Inc., Mew York, (1957).

[33] Cook, R. J., and Butts, R. R., Aerosol

Induced Thermo-Optical Degredation of a High

Intensity Laser Beam, Laser Digest , A.FWL-TR-73-273, p. 69,11973^^

[34] Bennett, H. S., and Rosasco, G. u., Resonances

in the Efficiency Factors for Absorption: Mie

Scattering Theory, Appl . Opts . 17., No. 4,

491, (1978).

[35] Rosasco, C. J., and Bennet, H. S., Internal Field Resonance Structure: Implications for

Optical Absorption and Scattering by Microscopic Particles, J_. Opt . Soc . Am 68, No. 9,

1242, (1978).

[36] Walker, T. W., Laser Induced Damage to Optical

Thin Film Coatings, Ph.D. Thesis, Air Force Institute of Technology, Wight Patterson AFB

OH.

[37] Milam, D., 1064-nm Laser Damage Threshold of

Polished Glass Surfaces as a Function of the Pulse Duration and Surface Roughness, Laser

Induced Damage in Optical Materials , NB3 Special Publication 541, US GPO Washington,

D. C. (1978).

[38] Walker, T. W., et al . , Pulsed Damage Thresholds

of Flouride and Oxide Thin Films from 0.26 urn to 1.06 urn, NBS Special Publication 568, US GPO,

Washington D. C. (1979).

486
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494
495
The question was raised about the importance of including the temperature dependence of the

thermal diffusivity. It was pointed out that the thermal diffusivity can change by an order

of magnitude as a function of temperature in going from room temperature to the damage tem- perature for thermal damage in the material and that the change can be of either sign- The

speaker pointed out that this variation has been included, for example, in the work of

Manenkov and his collaborators and that it adds significant complexity to the calculation.

The expected spot size dependence of the damage threshold was discussed at length. The

speaker indicated that on the basis of the model advanced in this paper, the spot size dependence would be expected to be much weaker in thin films than in bulk materials. Be further pointed out that one would not expect to see an absolute correlation between the

absorption in the thin film and the observed propensity for damage, because the absorption

damage threshold corresponds to the number of particles available at that size most easily

damaged, at the pulse duration used in the particular experiment.

496
FREQUENCY AND FOCAL VOLUME DEPENDENCE OF LASER-INDUCED

BREAKDOWN IN WIDE BAND GAP INSULATORS*

M. J. Soileau

Michelson Laboratory, Physics Division

Naval Weapons Center, China Lake, California 93555

M. Bass Center for Laser Studies

University of Southern California, Los Angeles, California 90007

and

P. H. Klein

Naval Research Laboratory

Code 5222, Washington, DC 20375

The frequency and focal volume dependence of the laser-induced breakdown

fields of selected wide band gap insulators were investigated. The breakdown

phenomenon that is reported in this work is the same process that has been

called "intrinsic" damage by other workers. Measurements were conducted with

pulsed (31 to 140 nsec), simple spatial mode lasers at wavelengths of 10.6,

3.8, 2.7, and 1.06 ym. The results indicate that mul ti photon-assisted electron

avalanche breakdown is a likely mechanism for laser-induced failure in these

materials over the wavelength range studied. For small focal volumes, volumes

smaller than the inverse of the free electron density, the frequency dependence

of breakdown is determined by multiphoton ionization of impurity levels.

This initiates an avalanche and leads to the observation that breakdown

fields decrease with increased laser frequency. For large focal volumes, the

free electrons initially present in the crystal initiate the breakdown, and

the frequency dependence is determined by the electron-phonon relaxation

time. Thus, for large focal volumes, the laser-induced breakdown fields

increase with increasing laser frequency. From these results it is clear that

crystal defects and impurities play an important role in what has been called

"intrinsic" damage. As this is the case, breakdown experiments cannot be relied upon for accurate measurements of the nonlinear index of refraction,

n 2

- Key words: Alkali halides; intrinsic damage; IR windows; laser damage; self-focusing;

spot size dependence.

Introduction

The objectives of this work were to examine (a) the frequency dependence of laserinduced breakdown thresholds in alkali halides in a manner that is independent of focalspot radius, (b) the dependence of the breakdown threshold on focal-spot radius in regions

where self-focusing can be discounted, and (c) the potential of improved material processing

techniques for increasing the laser damage resistance of highly transparent materials.

Threshold measurements were made on specimens from three different classes of material

purity at four laser wavelengths (10.6, 3.8, 2.7, and 1.06 ym) and three to seven different

focal-spot radii. The measurements obtained from selected alkali halide crystals at different focal volumes were used to establish empirical scaling laws for the breakdown fields of

these materials. These scaling laws were then applied to determine the damage threshold as

a. function of frequency for a common focal volume and to extrapolate the results of small

focal volume measurements to the limit of large focal volumes. The results indicate that

the dispersion in damage thresholds over the wavelength range of 10.6 to 1.06 ym is a _

Work supported by the Office of Naval Research.

497
function of the focal volume. At small focal volumes (on the order of 3 to 5 x 10 -9 cm 3 ), the laser damage thresholds decrease with decreasing wavelength, while in the limit of

large focal volumes (volumes greater than 10 _lt cm

-3

), the laser damage thresholds increase

with decreasing wavelengths.

The problem of laser-induced breakdown of highly transparent materials has been studied

by many workers [l] 1 and has been the subject of at least three review papers [2-4]. Elec- tron avalanche breakdown has been suggested as the mechanism of failure for the alkali

halides in the 10.6- to 1.06 -urn region [5,6]. However, there has been considerable controversy as to the frequency dependence of the laser-induced damage fields in these materials

[7]. In experiments where the laser light is focused into the material, a pronounced

dependence of the breakdown field on the focal -spot radius has been observed by some workers,

while other workers claim no dependence of breakdown field on focal -spot radius. There

exist discrepancies in the experimentally determined breakdown thresholds and disagreement

as to whether or not the measured breakdown thresholds are intrinsic properties of the

materials. These discrepancies and disagreements are discussed in detail in reference [8].

In the present work, the laser-induced breakdown thresholds of alkali halide crystals were measured. The type of damage studied was that which previously had been called intrinsic damage [2-6]. The single-mode output of the irradiation source was focused into the

bulk to a very small focal spot so that inclusions and other gross defects in the material

could be avoided. Near threshold this type of damage was characterized by (a) a bright

flash and fracture at the beam focus and (b) an abrupt truncation of the beam transmitted

through the material at or near the peak of the pulse. One conclusion of this work is that

this type of failure is dependent upon the defects present in a material. There is some disagreement as to whether or not a property of a material that is related to defects is properly called intrinsic. Therefore, in this work, such failure is referred to as brea k- down or laser-induced breakdown .

Specially prepared high-purity specimens were found to have significantly higher

damage thresholds than commercially available specimens of the same material when irradiated with small focal volumes. However, in the limit of large focal volume, the damage

thresholds for high-purity specimens and the commercially available specimens of a given

material tended to converge to a common value. Within a given group of specimens of a specific material from any one vendor, the variation in damage threshold was greatest at longer wavelengths and smaller focal volumes.

In the discussion that follows, we will show that these properties of the damage

process are consistent with a mechanism in which the electron avalanche breakdown is initiated by multiphoton ionization of defects. The different wavelength dependences are attributed to the requirement that for small focal volumes the "starter" electrons in the ava- lanche process must be produced by multiphoton excitation of defect states; whereas, for

large volumes, there are generally enough conduction band electrons present to initiate the

avalanche process.

Experimental

The basic experimental arrangement used in this work is shown in figure 1. A detailed

description of the experimental procedures and lasers used in this work are given in references [7] and [8]. The occurrence of laser-induced breakdown was determined by (a) viewing

a change in scattering of a coaxial HeNe beam in a 20-power microscope, (b) observing a visible flash and/or fracture in the sample, and (c) monitoring the transmitted waveform

distortion (see fig. 2). In this work it was found that all three of these effects occurred

simultaneously, i.e., when damage occurred, the HeNe laser scatter increased, a visible

flash and fracture was observed, and the transmitted waveform was abruptly truncated. The

threshold for failure was defined as that flux which produced damage in 50% of the sites

when each site was irradiated only once. Each site was irradiated only once, and only bulk

damage thresholds were measured.

Figures in brackets indicate the literature references at the end of this paper.

498
At 10.6, 3.8, and 2.7 ym, the focal-spot radius was varied by focusing with three

different focal length lenses. The lenses used and the resultant measured focal-spot radii

(1/e2 intensity points) are summarized in table 1. The same single-element ZnSe miniscus

lenses (which were designed for minimum spherical aberrations) were used at the three

wavelengths.

Table 1. Measured focal-spot radii for 10. 6-, 3.8-, and 2.7-ym experiments.

Lens

No.

Focal length

at 10.6 ym

(cm)

Lens diameter

(cm)

Focal spot

HF (2.8 ym)

2 radius (1/e in DF (3.8 ym)

intensity ) (ym)

C0 2 (10.6 ym)

1 2.54 2.54 9.55 10.60 20.50

2 6.35 2.54 12.55 13.55 45.00

3 12.70 5.08 26.50 28.50 85.00

The focal-spot radius was varied at 1.06 ym by varying the distance between the focusing lens and the cavity waist (this quantity is designated Z 3

) and by using lenses of

different focal lengths. The 1.06-ym laser was operated in the TEM00 mode (Gaussian intensity distribution), and the focused spot was diffraction limited. As a result, the focalspot radius could be varied in a linear fashion by varying Z 3

. All the alkali halide

specimens were tested at three different spot sizes. Selected specimens were tested at four additional spot sizes. The focal-spot radii used and the relevant beam parameters are summarized in table 2. A detailed description of the techniques used to measure the focal - spot radii is given in reference [8].

Table 2. Focal -spot radii for the 1.06-ym experiments.

Focal length Lens Focal-spot radius

Lens at 1.06 ym diameter (1/e 2 in intensity)

No. (cm) (cm) (ym)

1 5.0 1.0 5.00

5.33

5.64 , 6.15a^

10.29a

2 8.0 1.5 19.29^

22.83d

Indicates focal radius at which all specimens were measured.

Indicates measured values of focal radius. These were found

to be equal to the values calculated from Gaussian optics.

The average pulse widths used in these experiments were 90, 80, 120, and 31 nsec for

the 10. 6-, 3.8-, 2.7-, and 1.06-ym lasers, respectively [8]. Since the pulse widths are not the same for all the data points, one might question whether the data are pulse-width

dependent for this pulse-width region. There is some dispute about the pulse-width dependence for pulse widths of the order of several to hundreds of nanoseconds. Some workers

claim there is a dependence [9], and others claim no dependence on pulse width [10,11]. We tested specimen 78-NC-6 at 10.6 ym with t

p

= 60 nsec and t

p

= 107 nsec. The resulting

thresholds were the same to within ±3.5%, which was well within experimental error. Thus,

we found no pulse-width dependence for these experimental conditions.

499
Three classes of alkali halide crystals were examined in this work: (1) spectroscopicgrade material from the Harshaw Chemical Company 2

, (2) laser-grade material from the Harshaw

Chemical Company, and (3) reactive-atmosphere-processed (RAP) [12] material from the Naval

Research Laboratory 3. The RAP crystals were grown by the Bridgeman method. Gettered argon was the cover gas, and carbon tetrachloride vapor was used for reaction with oxygencontaining anions. The OH" content was approximately 40 to 65 parts per billion, as determined by vacuum ultraviolet spectroscopy 3

. Several single-crystal specimens of NaCl, KC1

,

and KBr from each group were tested.

Manenkov [13] has reported that in testing 100 randomly selected NaCl specimens,

approximately 20% had significantly higher damage thresholds. He suggested that these

exceptional specimens were displaying failure due to an intrinsic process. The specimens

used in this work were carefully screened prior to damage testing in order to maximize the

chances of seeing exceptional specimens of the type measured by Manenkov. Only specimens

satisfying the following criteria were used as specimens for this work: (1) No observable

absorption bands in the 0.5- to 10.6-ym range, (2) residual absorption of the order of 10 -3 cm

-1 or less measured by laser calorimetry [8], and (3) no visible scattering centers when

viewed with a 20-power microscope with HeNe laser illumination (0.6328 ym).

Data

The frequency and focal-spot radius dependence of the laser-induced breakdown intensity

thresholds of RAP-grown NaCl are given in figure 3. The trends in frequency and focal

radius dependence shown in figure 3 are the same for the Harshaw spectroscopic and lasergrade NaCl and for the three classes of KC1 and KBr tested. The peak on-axis intensity

(IB), the rms electric field (Eg), and the peak power at the damage threshold for all the

specimens tested are tabulated in the Appendix. The quantity w0 given in figure 3 and

the Appendix is the radius of the focused beam corresponding to the 1/e 2 points of the

intensity. The values given for the quantities w0 , Ig, and Eg do not include any corrections for the effects of sel f- focusing. Self-focusing does not enter in these experiments

for the reasons discussed in reference [8].

Inspection of figure 3 reveals that there is a distinct dependence of laser damage

threshold on the radius of the focal spot (w0 ) used to irradiate the specimens. As is shown in figure 3, the measured intensity thresholds can be fit by a linear dependence on the effective focal volume (V),

IU, w

Q

) = a[V(x, w

o

)]

_1 + b (1)

In the above equation, the effective focal volume (which is porportional to w^/x) is chosen

to be the volume for which the intensity drops to 0.80 of the peak on-axis intensity. The

"effective" focal volume is then computed using the method derived by Fradin et al . [14].

Note that the focal volume dependence was strongest at longer wavelengths (see fig. 3 and the Appendix). This type of behavior is consistent with the model discussed in the

following section, which assumes that damage is initiated by the most easily ionized defect

within the focal volume. The defect is thought to initiate an avalanche by providing a conduction band electron through multi photon ionization. As the focal volume gets larger,

the probability of finding a defect which can be ionized by a lower order multiphoton

process, and thus lower intensity, increases. At shorter wavelengths, the probability for

ionization of a defect can be much higher for a given order multiphoton process. If one assumes that the density of defects is proportional to the ionization potential, then one would expect a focal volume dependence until a volume is reached that gives high probability

of finding a defect that can be ionized by a low order process. The volume at which this occurs would be smaller at higher photon energies, as is observed in this work.

2 Harshaw Chemical Co., 6801 Cochran Rd., Solon, OH 44139.

3 The RAP-grown materials were supplied by Dr. P. H. Klein, who performed the vacuum ultraviolet characterization of the OH" concentration in the crystals.

500
Discussion of Results

The frequency dependence of the laser-induced breakdown thresholds at constant focal

volume was analyzed in the context of the avalanche breakdown model 4

. In the electron

avalanche model the number of conduction band (or nearly free) electrons increases with

time as [2-4]

t N(t) = N o exp / o(E(t'))dt l , (2)

o where N0 is the density of electrons present at the start of the avalanche, o(E(t')) is the

ionization rate, and E(t') is the rms electric field. The rate of energy absorbed by the

plasma is given by

w(t) = |E(t ) l V) i

. (3) m 1 + tu t c where m and e are the electron effective mass and charge, u> is the laser angular frequency,

and t c is the electron relaxation time.

Yablonovitch [5,6] initially proposed that N0 was the density of free electrons present

in the material prior to laser illumination, and that the frequency dependence was due to the 1/(1 + u

2 t£) term in eq. (3). Sparks [20] pointed out that the value of N 0 estimated

by Yablonovitch was much too large, and that for more realistic values of N 0 the probability

of finding an initially free electron in small focal volumes used in experiments by Yablonovitch [5,6], Fradin [21,11], and other workers was essentially zero. Thus, one must generate the starting electrons which lead to electron avalanche breakdown.

Multiphoton ionization of defects within the gap has been suggested by several workers

as the source of the starting electrons (N0

) in the electron avalanche. It will be demonstrated in the derivation which follows that this mechanism gives a frequency dependence of

the breakdown field consistent with the experimental results of this work.

Consider focal volumes which are small relative to the inverse density of free electrons naturally present in the lattice. Suppose, as was suggested by Milam [22], that for

"intrinsic-like" damage there is a crystal defect of ionization energy e D such that

E[_ < Eq < Eq > (4)

where ej_ is the laser photon energy, and eg is the band gap energy. The defect is assumed

to be uniformly distributed throughout the material with density x\. The number of free

electrons generated by n-photon absorption from these defects is

NQ

= ny

n

E2n

, (5)

where n = (ep/e|_> is the integer part of eq/eL' and Y is a numerical constant proportional

to the matrix elements of the intermediate states. It should be emphasized that eq. (5) is

a simplification, but the key functional dependence on E of the n-photon absorption is correct. The quantity Y

n E2 n is the probability that a single electron will be raised above

the band gap and is, therefore, always s 1, as is /y E.

k The electron avalanche models used to analyze the laser-induced breakdown experiments all have as their origin the dc breakdown experiments of Von Hippie [15] in 1937 and subsequent analysis prior to 1950 of this work by Frb'lich [16], Seitz [17], and Callen [18].

O'Dwyer [19] has compiled a monograph which summarizes the "pre-laser" work on dielectric

breakdown in solids.

501
We define <t>(E) as

t

1

4>(E) =

/ o(E(t))dt . (6)

o

Substitution of eqs. (5) and (6) into eq. (3) gives

£t i i 2 n r 2n d>(E)

m (1 + u) x ) c

If we assume that e|_ << eq, then n = (eQ/e|_> is much larger than one. For large n, the

absorbed energy and thus the laser damage threshold dependence on laser frequency will be

dominated by the quantities containing exponential and power dependence of £|_ and E. This means that for defects with relatively high ionization energy, the frequency dependence of

the breakdown field is just the frequency dependence of the free electron density.

The temporal dependence of the electron density enters through the exponential dependence of the ionization rate. The breakdown process is known to occur very rapidly, within

the 1 nsec response time of the detectors used in this work and within 5 psec for laser

pulse widths of the order of 25 psec [23]. Thus, one can assume E = constant; hence, a(E)

is constant over the breakdown time, t$. This permits <j> to be simplified as

t

4> =

/ a(E(t))dt' = a(E)t

B

. (8)

o

Careful analysis of the pulses transmitted through the specimens indicates that little

attenuation of the laser beam occurs until the rapid breakdown process produces a critical

density, Nc. The critical density occurs when the plasma frequency equals the laser frequency. The frequency dependence of N

c must be included in the analysis of the frequency

dependence of the breakdown process.

The critical density is related to the laser frequency by

us m (hw0 ) m e 0

c e hV L

where B = (m £o)/(^ 2e2 ) ^ s a constant for each material.

Equating eq. (9) with the right-hand side of eq. (2), taking the natural log of both

sides, and rearranging terms gives

*

= 2 £n [e

L ^\ - 2n in(^E) . (10)

Recall that /^E is less than one, and thus the £n(/yE) is negative. As a result, the above

equation can be rewritten as

*

= 2 M [e

L ^ j + 2 ^ |£n(/7E)| . (11)

The exact form of the ionization rate, a(E), is not known. It will be shown that the exact form of a(E) is not important to this analysis as long as 3a/8E > 0. A fit to the

experimental data of Fradin et al. [24] gives

a(E) = A

0

E 3,133

, (12)

where A0

= a constant over the range of laser fields of interest (1 to 10 MV/cm). Fradin's

expression in eq. (11) gives

502
where A = 2/(tgA0 ) = a constant.

Note that eq. (13) contains terms in E and £[_ raised to powers, as well as natural log terms in the same quantities. The ir\{/yE) term varies slowly with E relative to E

3- 1 33

. Similarly, the fcn[e|_(/B/n)] term varies slowly relative to the l/e|_ term. A common approximation in such situations is to set the natural log terms equal to constants [25]. In the

approximation that the natural log terms are constant relative to E

3 - 133 and 1/el, eq.

(13) gives

E 3.133 =a+ JL

j (14)

£ L where a and b are constants.

The important feature of eq. (14) is that the breakdown field decreases as laser

frequency (photon energy) increases , and the frequency dependence is not very strong. This

behavior is consistent with the results of laser breakdown measurements in this work for

small focal volumes. The laser breakdown fields for RAP-grown NaCl are plotted in figure

4, as is the least squares fit of the data to eq. (14). Similar results were obtained for

other materials studied in this work [8].

We have just shown that for small focal volumes the breakdown field decreases with

increasing laser frequency. We now consider the limit of large focal volume. For this case, there are a large number of free electrons present within the focal volume. The

measured breakdown fields are considerably lower than for the case of small focal volume.

The low thresholds at larger volumes mean that the probability of high-order multiphoton

effects is diminished. The rate of energy absorption by the specimen is given by

7 /

ta(E(t))dt'

e |E(t)TN eo

2

x. W(t) =

(1 +

2 2, (15)

If we assume, as before, that the breakdown occurs as N

c is approached and occurs very

rapidly (so that E(t) = E, a constant), then eq. (9) gives

0 0(E)tr

or

N.

a(E) = in

Be N e o

'B

B h

° (16)

The above analysis indicates that the field needed to produce the critical density for this

situation is only weakly dependent on the laser frequency, and the frequency dependence of

the breakdown field will be determined by the w

2 t£ term in eq. (15). This equation can be recast in the form

-B

f M + 22*1/2

E0

(l 0> tc

) (17)

or -(?) (18)

where E 0

= dc breakdown field.

The data from this work for the limit of large focal volumes can be fit to eq. (18) to obtain an estimate of E0 and t c

. The results are given in table 3. The frequency dependences of the breakdown fields in RAP-grown NaCl for the large focal volume limit are plotted

in figure 5.

503
Table 3. Estimates of relaxation time and dc. Breakdown fields for

RAP-grown NaCl , KC1 , and KBr.

Material

Relaxation time

(xlO -15 sec)

Estimated dc

breakdown

field (MV/cm)

Measured dc [15]

breakdown

field (MV/cm)

RAP: NaCl 1.15 1.69 1 .50 RAP:KC1 1.56 1.20 1.01

RAP: KBr 1.22 0.82 0.69

The value of t c given in table 3 for NaCl is in fair agreement with the calculations

by Sparks [20] (t c ~ 1.4 x 10" 15 sec) and with the values suggested by Manenkov [13]

(t c ~ 1.67 x 10" 15 sec). The extrapolated dc breakdown fields are in reasonable agreement

with those measured by Von Hippie [15]. However, one should not put much emphasis on this

agreement since the materials used in the dc measurements were different as were the sources

of systematic error (e.g., space charge effects in the dc measurements).

Concl usions

Measurements were made of the wavelength and focal volume dependence of laser-induced

breakdown thresholds in selected wide band gap materials. The damage irradiations were

performed at 10.6, 3.8, 2.7, and 1.06 ym over a factor of ~\0 k range in focal volumes. The

results indicate that the dispersion in damage thresholds over the 10.6- to 1.06-ym wavelength range is a function of focal volume. At small focal volumes (3 to 5 x 10 -9 cm 3 ), the laser damage thresholds decrease with decreasing laser wavelengths, while in the limit

of large focal volumes (> 10 _1+ cm 3

) the laser damage thresholds increase with decreasing

laser wavelengths. Variations in damage thresholds among specimens of a specific material were found to be greatest at longer wavelengths and smaller focal volumes. High-purity RAP materials were found to have substantially higher breakdown thresholds for small focal

volumes than commercially available specimens of the same materials but had essentially the same breakdown thresholds as the commercially available materials in the limit of large

focal volumes.

This type of behavior is consistent with a multiphoton-assisted electron avalanche

breakdown model. The different wavelength dependences are due to the requirement that for

small focal volumes the "starter" electrons in the avalanche process must be produced by

multiphoton excitation of defect states; whereas, for large focal volumes, conduction band

electrons initially present initiate the avalanche process.

All the results reported in this work are for what has been called "intrinsic" damage,

i.e., damage that occurs at or near the peak of the laser pulse (near threshold), abruptly

truncates the beam transmitted through the specimen, and produces a bright flash and frac- ture at the focal point of the laser. These results indicate that such events are not intrinsic properties of the materials. We found substantial variations in the laser

breakdown thresholds among samples of a given material from a given supplier. Even larger

variations were found among specimens of a given material from different suppliers. Factors

of two variations in breakdown threshold were sometimes found in measurements made in different positions in a given specimen. These results are consistent with what has been

reported by Olness [26,27], Leung et al. [28], Allen et al . [29], Yasojima et al. [30,31],

Manenkov [13], and Soileau et al. [32]. Such variations in breakdown thresholds are inconsistent with the results of Bloembergen [2], Fradin [3], Smith [4], and Yablonovitch [5].

However, careful reading of Yablonovitch' s thesis [5] indicates that he too found variations

of as much as a factor of two in the breakdown threshold in some materials tested at 10.6 ym. Comparison of the results of this work for Harshaw laser-grade NaCl with that reported in references [5] - [7] and [10] for similar focal volumes reveals that breakdown thresholds

agree to within experimental error. Thus, one may conclude that most of the discrepancies

in laser breakdown thresholds found in the literature are due to real variations in laser

damage thresholds among specimens of a given material and the dependence of the breakdown

threshold on focal volume. The laser breakdown thresholds among the RAP-grown alkali

halides tested were systematically higher for the higher band gap materials. However, this

504
correlation of damage thresholds with band gap was not maintained for all specimens tested. For example, the Harshaw laser-grade NaCl laser-induced breakdown threshold at 1.06 ym was

15% lower than that for Harshaw laser-grade KC1.

The technique used by Zverev [33], Fradin [34], and Smith et al . [35] (ZFS) to measure

n 2 and the laser-induced breakdown intensity, I m , is inconsistent with the results of this

work. This inconsistency is due to the focal volume dependence of I m (assumed to be con- stant in the ZFS procedure). The results of this work indicate that self-focusing does not play a role in damage experiments performed with tightly focused beams. A possible explanation for the lack of observed self-focusing effects is that negative changes in refractive

index due to the prebreakdown free electron plasma cancel the positive change in refractive

index due to electrostriction and bound electronic effects (see references [8], [36], and

[37]).

The focal radius dependences determined in this work are believed to be associated

with the densities of defects in the materials having different ionization potentials.

Thus, the dependences are not expected to be universally true. Recent results by Gomelauri

et al. [38] indicate that there is no focal radius dependence for the alkali halides at 2.7

ym for focal radii from 14 to 24 ym, while in this work a factor of two variation of breakdown intensity was found over this focal radius range.

505
APPENDIX

Summary of Damage Measurements

Single-pulse laser damage measurements were made on selected specimens of singlecrystal NaCl, KC1, and KBr. Specimens included two classes of commercially available

material and RAP materials. Damage thresholds were measured as a function of the irradiated volume at 10.6, 3.8, 2.7, and 1.06 ym. The results of these measurements are sum- marized in tables 1 - 7. Listed in these tables are the average values (denoted by < > ) of the peak on-axis intensity breakdown thresholds, <I B

>, the rms electric fields, <Eb>,

corresponding to <Ib>, and the peak power (Pb) of the breakdown thresholds. The quantity

w0

is the radius of the focused beam corresponding to the 1/e 2 points of the intensity.

The quantities w0 , <I B >, and <Eb > listed in tables 1 - 7 do not include any effects of

sel f-focusing.

Table 1. Average breakdown thresholds for RAP-grown NaCl

<lB> <EB> P

B

x(vim) w (MW)

o (ym) (GW/cm2

) (MV/cm)

10.6 20.5 95.70 4.92 1 .26 45.0 10.86 1 .66 0.69

90.0 > 6.67 > 1.30 2.23

3.8 10.6 97.43 4.95 0.34

13.55 65.11 4.04 0.38

28.5 31.03 2.79 0.79

2.7 9.55 66.08 4.07 0.19

12.55 36.58 3.03 0.18

26.5 28.33 2.67 0.63

1.06 6.15 101.0 4.99 0.12

10.29 60.20 3.85 0.20

19.29 54.22 3.69 0.64

22.83 54.79 3.70 0.90

5.0 175.0 6.57 0.14

5.33 126.0 5.57 0.13

5.64 125.0 5.56 0.12

Inspection of tables 1 - 7 reveals that there is a distinct dependence of laser damage

threshold on the radius of the focal spot used to irradiate the specimens. The size depend

ence is seen in all specimens tested, whether they failed by breakdown processes or thermal

processes. Since the object of this work was to study breakdown processes, we will concentrate on the size dependence of breakdown-like events.

There is no question that there exists a size dependence for breakdown-like events. The evidence for such effects is overwhelming. In this work over an order of magnitude of

photon energies for air breakdown, high threshold, and low threshold materials has been

observed. Breakdown-like events are those events characterized by (at threshold) (a)

always occurring at the focal point, (b) truncation of the transmitted pulse within the

detector response time (~ 1 nsec), and (c) a bright visible flash. There are several

plausible physical arguments that could explain observed size dependence, and these are examined in turn in the discussions that follow.

Since much of the data can be fit to a quadratic dependence on focal -spot radius, one concludes that the threshold was peak power dependent, thus indicating that self-focusing

was dominating the results. This is fairly easy to discount by several arguments. First,

the size dependence levels off or disappears entirely as one goes to larger spot sizes.

506
Table 2. Average breakdown thresholds for RAP-•grown KC1

A(ym) w

Q (ym) (GW/cm2

) (MV/cm) P R (MW)

10.6 20. 5 74. 73 4.41 .99

45. 0 6. 36 . 4 1 Qn u

nu > 1.14 1 .45

3.8 l n1 u

D

i

1 mU j . u 5.17 .36

I o . DO A C 43 . nu 3 A? C V 9QCO. cO 9ncX)

1 0 2.29 .51

2.7 9. 55 45. 5 3.44 .13

12. 55 29. 5 2.77 .15

26. 5 15. 83 2.03 .35

1 uu 6. 15 66. 78 AH.I/1 7 n n7Q

10. 29 51

32 3.65 0.17

19. 29 38. 13 3.15 0.45

5. 0 124. 0 5.68 0.097

5. 33 109. 0 5.31 0.1

5. 64 97. 0 5.02 0.097

Table 3. Average breakdown thresholds for RAP -grown KBr.

<IB> A(ym) w

0 (ym) (GW/cm 2

) (MV/cm) P

R (MW)

10.6 20 5 29 9 2.64 0.40

45 .0 4 .12 U . JO nU . ?fi L. U 90 .0 > 2 .82 > 0.81 0.88

3.8 10 .6 69 .6 4.15 0.25

13 .55 35 .8 2 . 96 0.21 28 .5 6 .33 1 .25

2.7 9 .55 41 .5 3.19 0.12

12 .55 15 .7 1 .96 0.78

26 .5 5 .01 1 .11 0.11

1 .06 6 .15 47 .1 3.33 0.056

10 .29 25 .2 2.40 0.084

19 .29 15 .8 1 .91 0.185

Self-focusing would have given the opposite type behavior, i.e., there would have been

little or no spot size dependence for small spots where low total power was required for

damage. For large spots requiring high total power, the spot size dependence would have

saturated at a quadratic dependence. Higher threshold specimens would have shown this

quadratic dependence for the smallest spot sizes. Exactly the opposite type behavior was observed in these experiments. As the focal spot sizes were increased , the dependence of

threshold intensity on the focal-spot radius decreased and in some cases disappeared.

Specimens with the highest threshold at 1.06 ym had the smallest size dependence, e.g., the

RAP NaCl samples. These discussions of the role of self-focusing in these experiments are made more quantitative in reference [8], where self-focusing is discussed in detail.

The influence of thermal diffusion can be ruled out. First, even for the smallest

spots, the thermal diffusion distance (the distance heat deposited in the center of the

beam diffuses during the laser pulse) is 1/10 the focal radius and would contribute, at
Table 4. Average breakdown thresholds for Harshaw spectroscopicgrade NaCl

(I B> (E B

> A(ym) w"q (um) V r"" / (GW/cm2

) (MV/cm) Pn(MW)

R 10.6 20.5 73.4 4. 31 0.97

45.

11 17 1 . 68 0.71 90.0 > 4 11 > 1. 02 > 1 .05

3.8 10.6 94 6 4. 87 0.34

13.55 72. 9 4. 27 0.42

28.5 24 7 2. 49 0.63

2.7 9.55 58 0 3. 81 0.17

12. 55 50 8 3. 57 0.25

26.5 24 7 2. 49 0.55

1 .06 6.15 66 7 4. 05 0.079

10.29 43 5 3. 27 0.15

19.29 33 1 2. 85 0.39

Table 5. Average breakdown thresholds for Harshaw 1 asergrade NaCl.

CI B

> (E B

> P B (um) w (MW)

0 (um) (GW/cm2

) (MV/cm)

10.6 20.5 39. 7 3. 17 0.52

45.0 9 06 1. 51 0. 58 90.0 > 4 11 > 1. 02 1 .68

3.8 10.6 65 59 4. 06 0.23

13.55 45 33 3. 37 0.26

28.5 17 7 2 11 0.45

2.7 9.55 61 03 3. 92 0.18

12.55 35 60 2. 99 U . 1 o 26.5 16 83 2 06 0.37

1.06 6.15 27 65 2. 61 0.033

10.29 14.93 1 . 92 0.050

19.29 12 04 1 . 72 0.151

most, a 20% variation in threshold with spot size. Second, most of the specimens tested

failed at the peak of the pulse near threshold. Thermal failure would have occurred at the

end of the pulse near threshold.

Diffusion of hot electrons (nonthermal electrons) out of the focal volume, sizedependent electron energy loss mechanisms such as self-trapping, and different densities of

defects with various thresholds are other possible mechanisms for spot size dependence.

These three possible mechanisms are not easily dismissed and must be analyzed in detail.

One thing that all of these three size-dependent effects have in common is that they depend

on focal volume. Thus, the experimental data should be analyzed for focal volume dependence, and when making size-independent comparisons, all data should be scaled to a common

focal volume.

Since the 1.06-um focal spots were diffraction limited and the 10.6-um focal spots were nearly diffraction limited, one can compute the focal volumes for these wavelengths.

The focal spots at 2.7 and 3.8 um were not diffraction limited, and thus the focal volumes

at these wavelengths were computed from the measured focal -spot radii and depths of focus.

508
Table 6. Average breakdown thresholds for Harshaw spectroscopicgrade KC1

(] R> <Eb> A(ym) w

o (ym) (GW/cm2

) (MV/cm) P

B (MW)

10.6 20 5 19. 2 2 23 0.25

45 0 5. 97 1 25 0.38

90 0 > 4. 02 > 1 02 1 .27

3.8 10 6 43. 30 3 35 0.15

13 55 22. 23 2 40 0.13

28 5 14. 17 1 92 0.36

2.7 9 55 32. 9 2 92 0.094

12 55 20. 5 2 31 0.102

26 5 14. 7 1 95 0.324

1.06 6 15 52. 1 3 68 0.062

10 29 38. 8 3 18 0.129

19 29 32. 6 2 91 0.381

Table 7. Average thresholds of Harshaw laser-grade KC1

<I B

> (Eb>

P

B

\("ym) W (MW)

Q (ym) (GW/cm2

) (MV/cm)

10.6 20.5 17.65 2.14 0.23

45.0 5.22 1.17 0.33

90.0 > 3.2 > 0.92 1 .07

3.8 10.6 44.53 3.40 0.16

13.55 19.87 2.27 0.12

28.5 15.30 1 .99 0.39

2.7 9.55 35.87 3.05 0.10

12.55 21 .40 2.36 0.16

26.5 13.87 1 .90 0.31

1 .06 6.15 59.50 3.93 0.071

10.29 32.57 2.91 0.11

19.29 39.77 3.21 0.47

i The smallest focal volume used in measuring the breakdown thresholds of all the specimens

was V 5 15 = 0.36 x 10 -9 cm 3

, the focal volume corresponding to w0

= 6.15 ym at 1.06 ym.

V 6 . 15 was chosen as unit of focal volume, and all other focal volumes were normalized using

V 6

' 15 = 1. The data at each wavelength were then fit to a linear dependence on focal

volume,

I(x,w

0

) = a[V(x,w

o )]" 1 + b . (1)

In the above discussions, the focal volume is chosen to be the volume for which the inteni sity drops to 0.80 of the peak intensity.

Tables 8-10 summarize the focal volume dependence of the damage thresholds measured

in this work. Note that in all cases the focal volume dependence was strongest at longer

wavelengths. This type behavior is consistent with the model which assumes that damage is initiated by the most easily ionized defect within the focal volume. The defect is thought

to initiate an avalanche by providing a conduction band electron through multiphoton ionization. As the focal volume gets larger, chances of finding a defect which can be ionized by

509
Table 8. Focal volume dependence of threshold intensity for RAP-grown

material. V is expressed in units of 0.36 x 10" 9 cm 3

, intensity in units of GW/cm2

.

Material x(ym) Best fit(least squares)

Correlation

(r 2

)

NaCl

KC1

KBr

10.6

3.8

2.7

1 .06

10.6

3.8

2.7

1 .06

10.6

3.8

2.7

1.06

a lower order multi photon process and thus

ionization potential of a defect can be much

If one assumes that the density of defects

= 1085 V"

1 + 8.83

= 1099 V"

1 + 7.74

= 1114 V"

1 + 6.63

548 V"

1 + 28.21

= 290 V"

1 + 29.93

= 47.07 V"

1 + 53.97

874 V"

1 + 4 73 879 V"

1 + 4 37 883 v1 + 4 05

675 V"

1 + 11.28

214 V"

1 + 17.20

= 24.72 V"

1 + 2.82

521 V"

1 + 2.74

275 V"

1 + 3.92

29.16 V"

1 + 18.30

,9986

.9997 .9999

,9988

9934

.9999

.9986

.9992

.9995

.9252

.9653

1.000

330 v1 + 3 50 .9986

334 V"

1 + 3 16 .9997

339 v1 + 2 82 1.000

,9920

1 .000

.9647

ower intensity increase. At shorter wavelength

higher for a given order multiphoton process,

s porportional to the ionization potential,

then one would expect a focal volume dependence until a volume is reached that gives high

probability of finding a defect that can be ionized at a low order process. The volume at which this occurs would be smaller at higher photon energies, as is observed in this work.

The focal volume dependence at 10.6 urn was not precisely determined since damage to the surface prevented a determination of the bulk threshold for large spot sizes. The

numbers given for the largest focal volume at this wavelength is that intensity which would

510
Table 9. Focal volume dependence of the breakdown thresholds of

commercially available NaCl . V is in units of 0.36 x 10 -9 cm 3

, Iw in units of GW/cm2

.

Best fit Correlation

Material x(um) (least squares) (r 2

)

NaCl 10.6

(spectroscopic

grade)

3.8

2.7 L. = 219 V"

1 + 31 .11 .7273

1 .06 I = 31 .21 V"

1 + 35.91 .9600

NaCl 10.6 I v

= 392 V"

1 + 8.33 .9986

(laser L = 409 V"

1 + 7.04 .9991 grade) I y

= 426 V"

1 + 5.74 .9917

3.8 396 V"

1 + 16.65 .9986

2.7 r v 322 V"

1 + 18.15 .9788

1 .06 J v

= 15.32 V"

1 + 12.40 .9955

damage only the rear surface when focused in the bulk. Higher intensities would damage the

front surface and thus prevent propagation into the bulk. Thus, the numbers given are a lower limit for the bulk thresholds. These lower limits were used to obtain the top entry

in tables 8-10 for the focal volume dependence. The lower entry assumes that the threshold was the same as that of the previous focal volume. The actual value of the breakdown

threshold for this focal radius was within these two limits.

The pulse widths were not the same for the different wavelengths used in this work.

The pulse widths were approximately 90, 85, 120, and 31 nsec for the 10. 6-, 3.8-, 2.7-, and

1.06-um lasers, respectively. There is some dispute about the pulse width dependence of

breakdown thresholds for pulse widths of the order of several to hundreds of nanoseconds.

Some workers claim there is a dependence, and others claim no dependence. Specimen 78-NC6, a Harshaw laser-grade NaCl crystal, was tested with pulse widths of 70 and 107 nsec at 10.6 um. The resulting intensity thresholds were the same to within ±3.5%, which is within

experimental error. Thus, no pulse-width scaling was applied to the results of this work.

I y = 795 V"' + 9.68 .9986

T y

= 821 V"

1 + 7.76 .9999

I„ = 844 V"

1 + 5.99 .9970

I„ = 583 V"

1 + 25.45 .9729

511
Table 10. Focal volume dependence of the breakdown thresholds of

commercially available KC1 . V is in units of 0.36 x 10 -9 cm 3

, I v in units of GW/cm2

.

Best fit Correlation

Material A(um) (least squares) (r 2

)

(spectroi

1 nu D

! v

=

i v ~

176 V"

1 +

1ft? 1 Ol V +'

O . OD 5.15

. yyoo

.9994

] v 4, UJ

grade)

3. 8 ! v 243 V"

1 + 12.66 .9973

? 7

/ T v

1 J/ V ' 1 A 3R

1

1 uu J v 1QIO.UHOA V +~ OH- 34 . O311

kTI l\L 1 (laser

i

1 nu D *v

=

ICQ 1 0:7 U"^ V +' 166 V"

1 +

t

4.39 .9991

n tt\ Hp \ y r ctue } r v 173 + 3. 87 .9916

3 8 ! v 236 V"

1 + 10.96 .8271

2 7 T v 164 V"

1 + 13.71 .9972

1 06 24.04 V"

1 + 34.83 .8698

512
References

[1] The National Bureau of Standards

series of publications on Laser Induced Damage to Optical Materials,

edited by A. J. Glass and A. H. Guenther, deals with this subject extensively, and the references contained

within these publications are further

guides to the literature on the subject

of laser damage to optical materials.

These reports are available from the

U.S. Government Printing Office,

Washington, D.C. The report numbers

and dates of publications are: NBS Spec.

PublYear

a. 341 1970

b. 356 1971

c. 372 1972

d. 387 1973

e. 414 1974

f435 1975

g- 462 1976

h. 509 1977

i

541 1978

[2] Bloembergen, N. , IEEE J. Quantum

Electron. QE-10 , 375 (1974).

[3] Fradin, D. W. , Laser Focus, 41 (Feb.

1974).

[4] Smith, W. Lee, Opt. Engr. V7, 489

(1978).

[5] Yablonovitch, E. , Ph.D. Thesis, Har- vard University, 1972, unpublished.

[6] Yablonovitch, E. , Appl . Phys. Lett. 19, 495 (1971).

[7] Soileau, M. J., Bass, M., and Van Stryland, E. W. , in reference [1-i], p. 309.

[8] Soileau, M. J., Ph.D. Thesis, University of Southern California, Aug. 1979.

[9] Bettis, J. R., House II, R. A., and

Guenther, A. H. , in reference [1-g],

p. 338.

[10] Manenkov, A. A., private communication,

Sept. 1978.

[11] Fradin, D. W. , Yablonovitch, E. , and

Bass, M. , Appl. Opt. ]2_, 700 (1973).

[12] Pastor, R. C. , and Pastor, A. C, in Crystal Growth in a Reactive Atmosphere , (Pergamon Press, New York, 1975), pp. 117-124. Materials Res. Bull. 10.

[13] Manenkov, A. A., in reference [1-h],

p. 455.

[14] Fradin, D. W., Bass, M. , Bua , D. B., and

Holway, Jr., L. H., Visible and Infrared Laser-Induced Damage to Transparent

Materials , AFCRL-TR-0082 , Air Force Systems Command, Bedford, Mass., 1974.

[15] Von Hippie, A., J. Appl. Phys. 8, 815

(1937); Phys. Rev. 54, 1096 (1938).

[16] Frolich, H., Proc. Roy. Soc. A160, 230

(1937).

[17] Seitz, F., Phys. Rev. 76, 1375 (1949).

[18] Callen, H. B., Phys. Rev. 76, 1394

(1949).

[19] O'Dwyer, J. J., The Theory of Dielectric

Breakdown of Solids (Oxford University

Press, London, 1964).

[20] Sparks, M., in reference [1-f], p. 331.

[21] Fradin, D. W. , Ph.D. Thesis, Harvard

University, 1973.

[22] Milam, D., in reference [1-g], p. 350.

[23] Anther, J. P., and Bass, M., Appl.

Phys. Lett. 31_, 412 (1977).

[24] Fradin, D. W. , Bloembergen, N., and

Letellier, J. P., Appl. Phys. Lett.

22_, 635 (1973).

[25] Powers, Prof. E. A., University of

Southern California, Los Angeles,

private communication, 1979.

[26] Olness, D. , Appl. Phys. Lett. 8, 283

(1966).

[27] Olness, D., J. Appl. Phys. 39, 6 (1968).

[28] Leung, K. M. , Bass, M., and BalbinVillaverde, A.G.J. , in reference [1-f],

p. 107.

[29] Allen, S. D., Braunstein, M. , Giuliano,

C. R., and Wang, V., in reference [1-e],

p. 66.

[30] Yasojima, Y. , Takeda, M. , and Onnishi,

Y., Jpn. J. Appl. Phys. 7, 552 (1968).

[31] Yasojima, Y., Ohmari , Y., Okumura, N.

,

and Inuishi, Y., Jpn. J. Appl. Phys.

14, 815 (1975).

513
[32] Soileau, M. J., Bennett, H. E. , Porteus,

J. 0., Temple, P. A., and Bass, M.

,

in Proc. 5th Annual Conf. on Infrared

Laser Window Materials , 1-4 Dec. 1975,

Las Vegas, Nev. , compiled by C. R. Andrews and C. L. Strecker (Arlington,

Va., DARPA, 1976), p. 391.

[33] Zverev, G. M. , and Poshkov, V. A., Sov. Phys.-JETP 30, 616 (1970).

[34] Fradin, D. W. , IEEE J. Quantum Electron.

QE-9 , 954 (1972).

[35] Smith, W, L. , Bechtel , J. H. , and

Bloembergen, N. , Phys. Rev. B12 , 706

(1975).

[36] Kelley, P., Schmid, A., and Bra'unlich,

Phys. Rev. B 20, 815 (1979).

[37] Schmid, A., Kelly, P., and Bra'unlich,

P. , Phys. Rev. B 16, 1569 (1977).

[38] Gomelauri, G. V., and Manenkov, A. A., Kvant. Elektron. 6, 45 (1979).

Figures

He Ne

ATTENUATOR

ENERGY MONITOR

LASER

(PULSED)

V I \^ 1 / A

/N \ / v

SAMPLE

FOCUSING

LENS

FAST

DETECTOR

REFERENCE DETECTOR

(FAST) TELESCOPE

OBSERVER

Figure 1. Typical setup for laser-induced

breakdown experiments. The linearly polarized, single-mode output of a pulsed laser

operating at 10.6, 3.8, 2.7, and 1.06 ym was focused into the bulk of a highly transparent

specimen. Fast detectors (1-nsec response

time) were used to monitor the shape before

entering and after transmission through the

specimen (see fig. 2). A coaxial HeNe beam

was used for alignment and to monitor the occurrence of damage. Polarizer-analyzer

combinations were used to attenuate the

beam. The analyzer was fixed to insure

constant polarization of the beam reaching

the specimen.

514
Figure 2. Laser waveform related to the occurrence of bulk damage in KC1 at 1.06 ym. The top trace is the reference pulse, and

the bottom trace is the transmitted pulse

showing the abrupt truncation of the pulse

for damage near threshold. The laser burn

profile is also shown (slightly left of

center)

10.6 iim

90.0

= 1085/V + 8.8

3.8 um

13.55 28.5

I - 548A/

n

2 - 0.9988

2.7 iim 9.55 126 EE?

'

26 5

I = 290/V + 26

n

2 - 0.9934

1.06 (jm

10.29

22.83

I - 47/V + 54

n

2 - 0.9999

(GW/cm2

)

100 150

Figure 3. Breakdown intensity for RAP-grown

NaCl. The breakdown thresholds given above

are the average values for four separate

specimens. Each specimen was tested at every spot size and wavelength. The equations at the right of the figure are the

least squares fit of the data to an inverse

volume dependence. Volume, V, is in units

of 0.36 x 10" 9 cm 3

, and the intensity is in units of GW/cm2

.

515
Figure 4. Breakdown fields for RAP-grown

NaCl . In these plots the average breakdown

fields for five NaCl RAP-grown crystals are

given. The thresholds have been reduced to

a common focal volume using the relations

given in the Appendix. The 1.06-, 2.7-, and

3.8-ym measurements were all done at Veff = 7.88 units (one volume unit is 0.36 x 10 -9 cm 3

) and are not scaled in this figure.

Figure 5. Breakdown fields for RAP-grown

NaCl in the limit of large focal volumes, V > 10 _1+ cm 3

. Note that the frequency dependence

for this case is characteristic of electron

avalanche breakdown.

516
A question was raised concerning the test of this theory by damage measurements made in

materials in which an excess of readily ionized electrons was created by one means or another.

The speaker pointed out that work had been done by Yashima and co-workers using color-centered

alkali halides (Japanese J. Appl. Phys. !£, 815, 1975). In material which was free of color

centers, the breakdown field was seen to increase as the wavelength shifted from 1 micrometer to 0.69 micrometers, while in color-centered materials, although the absolute breakdown

field was lower, the frequency dependence was just the opposite, with the breakdown field

decreasing as the wavelength was decreased. This is consistent with a multiphoton process

for the color centers, and would seem to support the predictions of the theory presented in

this paper. The question was raised concerning the extension of this theory to surface

damage. The speaker indicated that he would expect a different spot size dependence because

in surface damage it is only the area of the focal spot rather than the three-dimensional

volume which is of importance. However, it was pointed out that in surface damage it is

often the presence of a uniform contaminant layer which determines the damage threshold, in

which case no spot size dependence would be expected.

517

SBS INFLUENCE ON LASER DAMAGE OF OPTICAL MATERIALS

P. BalkeviSius, E. Kosenko, J. Lukosius, E. Maldutis

Institute of Physics of Academy of

Science of Lithuanian SSR, Vilnius

The influence of the stimulated Brilloin scattering (SBS) on the damage of optical materials has been experimentaly

investigated. It is shown that depending on the experimental

conditions, the SBS changes the threshold and the bulk damage

character. It can decrease or increase the magnitude of the

threshold power of the laser beam, which damages the surface

of the optical systems. The SBS increases (the increase of more than 100 times has been registred) , but not decreases

the threshold power of the bulk damage, as it was thought ' ealier

It is shown, that the reason of the decrease of the bulk

damage threshold and the changes of the damage thresholds of

the surface of the optical system is a redistribution of the

laser beam intensity caused by the SBS, which decreases the

intensity in the focal region of lens.

The influence of the laser beam redistribution on the

damage thresholds and self-focusing is also discussed.

Key words: Damage morphology} damage thresholds; laser bulk

damage; laser surface damage; optical materials; stimulated

Brilloin scattering (SBS); self-focusing.

1. Introduction

The problem of the SBS influence on laser damage of optical materials

has been investigated for 15 years. In most cases the works dealing with

this problem are experimental because a complete solution of the SBS of

focusing laser beam equation can not be obtained even by modern computers.

In the previous work on this problem the generation of intense coherent

acoustical oscillations in the SBS process was regarded as one of the

possible mechanisms of the damage. In later papers the opinion was

expressed that the SBS doesn't play the main role in the damage. A

sufficiently extensive bibliography on this problem and the role of the

SBS in the damage is presented in /1-5/1 ® In the present paper the role of the SBS in optical damage is investigated experimentaly and the changes of the damage thresholds are measured. The main material of the investigation was optical glass K-8

and fused silica.

2. Experimental Methods and Techniques

In the experiment of the SBS influence on the optical damage, it is

necessary to control the SBS power, leaving other conditions unchanged.

Figures in brackets indicate the literature references at the end of

this paper.

519
It is known, that the intensity of the SBS is IgBS ~ exp gljl ». where - laser beam intensity, ^ - length of the active material.

The amplification coefficient S depends on the spectral width of the

laser radiation. If it is sufficiently wide, then g decreases. In

order to monitor the SBS power the spectrum of the laser radiation was

changed.

In the experiments the radiation of the second harmonic of N<3. glass

laser has been used. The intensity distribution over the incident beam

cross section was approximately Gaussian. The beam divergence was close

to the difraction. The main parameters of the radiations are given in the

table.

Table 1. Main parameters of radiations

Laser T7

number i X -V» m t ,ns j D,mm

J ^ .cm"1

characteristics pitching

la 0,53 10 (.0.2 several (up to

five) frequences

passive

lb 0,53 10 4- «0.02 single-frequen- passive

cy

2a 0,53 20 4 2 multi-frequency active

2b 0,53 20 4 <0.02 single-frequen- passive

cy

where T - pulse duration, D - diameter of the beam on a lens.

The use of the second harmonic of the radiation provided the lack

of the feed-back coupling between the laser and the SBS radiation. The

main scheme of the experiment is shown in fig. 1. The time characteristics of the laser and the scattered pulses were registred by the same fast photocell and the oscilloscope. The SBS pulse was detained with

respect to the laser. The spectrum characteristics of the pulses were

measured by Fabry-Perot interferometer. The energetical characteristics

of the pulses were measured by the calorimeters. The damage threshold

for certain focal length of the lens is considered to be the minimal

power of the incident laser pulse upon the sample causing the damage

which is visualy noticed in the beam of the He-Ne laser or in the

appearance of the spark at the moment of the damage. Both methods equaly

determine the damage threshold.

3. Measurements and Results

Investigations of the bulk damage threshold of the glass K-8, placed

in the focal region of spherical lens (f = 33 cm), caused by the radiation

of la or 2a laser, showed that the damage threshold in the case of the

equal radiation power changes several times. It was found, that the SBS

power changes simultaneously. Several conclusions can be made:

1) In the case of sufficiently large transformation of laser

radiation in the SBS, the damage does not take place. This contradicts to

the damage mechanism due to stress, formed by the coherent acoustic

phonons, generated in the process of the SBS.

520
2) The SBS iafluences the damage threshold. A narrow spectrum ra- diation (large SBS) does not cause the damage when the laser power is more then 0.6 MW. On the other hand a wide spectrum radiation (without

SBS) causes the damage when the laser power is 0.3 MS?. For a more through study of the SBS influence on the optical damage

of optical system the damage of the back: surface of sample 1 has been

investigated (fig. lb, the widest line) by two contrary correlating

beams controlled by the spectrum and the intensity. The first was the

laser beam, and the second beam was generated in the process of the SBS

of focused (f = 15 cm) lb laser beam in sample 2, which was placed

1.5*2 mm from the back surface of sample l.'ln this way an eyact

coincidence of opposed beams has been automatically reached.

Having changed the material of the second sample the spectrum of

the second beam also been changed. The energy of the second beam, measured at the front surface of sample 1, reached 0,8 of the first. When the

laser la was used the second beam (SBS) was absent.

It was established:

1) In the absence of the second sample (there is no SBS for both

radiations) the damage threshold does not depend on the width of the

beam spectrum.

2) The damage 1 of the surface of the system, consisting of 2

equal samples (e.g. optical glass K-8) caused by the narrow spectrum

radiation and acomponied by strong SBS, does not take place when the

power of the first bean is 5 times larger than the damage threshold of

a separate sample. When the spectrum is wide, the damage threshold of

the system remains the same as for a separate sample.

3) The damage threshold of the system, consisting of two samples

of different materials (e.g. glass K-8 and fused silica) caused by the narrow spectrum radiation decreases 1.7 times as compared with the

threshold of the separate sample. In the case of the wide spectrum

radiation the threshold remains the same. The increase of the damage threshold can be accaunted by the

interaction of the incident and scattering radiations, which cause a hard exitation of the SBS generation in the first sample. That is why

the intensitv of the incident radiation upon the back surface decreases

considerably.

The decrease of the threshold can be explained by summing up the

actions of the opposed beams. In the first sample the generation of the

SBS in this case is absent due to the violation of synchronism.

In order to understand the mechanism of the SBS influence on damage

deeper, both samples of the previous experiment were "connected", and

the character of the bulk damage of the glass K-8 was investigated. The

radiation of lasers la and lb was focused (f = 15 cm) in the bulk of the

sample. The radiation of laser la (with several frequences) up to 5 MS? was lower than the SBS threshold. In single-frequency regime (laser lb)

the same power was higer than the SBS threshold (0.44 MW). The damages,

obtained without the SBS are ( ~ 20 mm) thin ( < 2\xm ) tracks, beginning

in the focus of the lens (fig. 3).

When the power is increased, the length of the tracks also increases.

The character of the damages caused by a single-frequency laser (with SBS),
is essiantially different: the length of the tracks is not longer than

6 mm, A comparatively large damage in the form of a star (fig. 3),

depending upon the power, in the end of the track directed to the end of

the lens can "be seen. In fig. 4- if? the dependence of the tracks length on the radiation

power of laser given. The experimental results, introdusing the investigations without the SBS, are described well by the formula /6/:

L = 2 \[T (n2/cn

o

) 1/2

(R/a) 2

(PL

- P

t

) 1/2

, where L - the lengths of the track; P^ - the damage threshold (in our case P^ = 0.33 MW); n

Q

and - linear and non-linear refractive indexes;

R and a - curvature radius and beam radius on the surface of the sample.

The length of the track caused by the radiation with the SBS weekly

depends on the laser power.

The obtained results can be explained by the following physical

model. The track is formed as the result of a focus movement opposite

the laser beam. The shortening of the track is due to the fact that the

focal region is covered by the zone in which the process of parametrical

interaction of incident and scattered radiations and transformation of

laser radiation to SBS takes place. The zone moves more rapidly than the

focus of the selffocusing beam. The laser radiation intensity in the

focal region decreases »and the growth of the track stops.

In order to observe directly the redistribution of the laser

radiation and the SBS along the beam the intensity of the luminescence

of rhodamine 6 G solution in heavy water with the angle of 90° towards

the direction of the beam has been investigated.

The radiation of the main harmonic of the two Nd glass lasers 2a

and 2b was focussed into the volume of the solution by spherical lens

(f s 15 cm). The chosen focus length of the lens exhibited strong SBS

when the radiation with the narrow spectrum was used, and the SBS

was absent when the radiation with the wide spectrum was employed. The

zone of luminescence was photographed and photometered.

It has been established, that the zones of the luminescence are strongly different* the radiation of both lasers being equal in power

(fig. 5). In the first case ( £0.02 cm~l, PgBS « 0,8 P

L

) the

increase of the intensity has been observed (1SBS ^1? at tne froat

surface of the cell; it was practically constant up to the focal region

of the lens and it increased slightly in the focal region of the lens.

In the second case ( 4^ s 2 cm-1; without the SBS), the intensity

increases constantly up to the focal region of the lense. It should be

noted, that in the first case the intensity at the first surface of

the cell is almost two times larger, and in the focal region it is

considerably lower than in the second case. In this way it was prowed that the SBS, redistributing the laser

radiation intensity, limits it in the focal region and therefore plays

an important role in the damage process.

Taking into consideration that the SBS threshold is determined not

only by the radiation parameters, but also by the properties of the

522
material, it should be reasonable to expect changes in the relative

damage thresholds for different materials in which the SBS thresholds

(quantity of acoustooptical relation) are different.

The SBS power can be verified by changing not only the laser

radiation spectrum, but also by varying the' length of the interaction

of the laser radiation with the material, i.e, the lens focal length. In

this way the special conditions were chosen, where for the radiation

with the narrow spectrum, the SBS threshold was higer then the damage

threshold (f < 15 cm for K-8 and f ± 3 cm for fused silica) and it was

lower (f > 15 cm for K-8 and f > 7.5 cm for fused silica) than the

damage threshold.

For the radiation with the wide spectrum (laser 2a) the SBS threshold

was higer then the damage threshold in the whole interval of the investigated focal length of the lenses. Fig. 6 shows the dependence of bulk

damage thresholds by laser radiation with narrow ( ; a ) and wide

( o ; o ) spectrum as a function of focal length of lens for glass K-8

( a ; ° ) and fused silica ( a ; O ) . It is clear that in the case of

the radiation with the wide spectrum (without the SBS), the relation of

the bulk damage thresholds of glass K-8 and fused silica is constant in

the investigated interval of the focal length of the lenses. In the same interval of the focal length of the lenses the thresholds differ strongly

when the laser radiation spectrum is narrow: the damage threshold of

fused silica is about 4 times greater than the damage threshold of glass

K-8 for lens f = 3 cm and more than IDO times greater for the lens

f s 10 cm. There is no damage at points marked by arrows. The increase of the observed effect of the damage threshold in

fused silica can be explained by stronger transformation of the main

radiation into the SBS. This is in good agreement with greater acoustooptical relation of fused silica.

4. Discussion

It was proved experimentaly, that SBS, redistributing laser radiation, changed, the damage thresholds. Under the same conditions, when the

narrow spectrum laser radiation generates intensive SBS, the wide

spectrum laser radiation having the same power always produces long thin

tracks. The appearance of long tracks points to the self-focusing. Due

to the fact that there are no absorbtion lines near the laser wave length,

the self-focusing doesn't depend strongly2 on the width of the radiation

spectrum. Consequently, self-compression of laser beam must also

influence the redistribution of the narrow spectrum laser radiation.

It is not dificult to estimate this roughly:

isbs 3 V*? hP- whea xsbs * h

il .pl/s-«i/» d

2

, (i -pl

/p

c2?);

When there are some frequences (modes) of radiation, the time modulation of laser pulse changes nonlinear refractive index and can change

the process of self-focusing. This problem in the theory of self-

-focusing is not discussed. But it is easy to estimate that this

process is not effective and it does not change the damage threshold

sufficiently.

523
where d - the diameter of the beam in the focus of a leas: P „ - crio

1 cr tical power of self-focusing of laser radiation. The comparison of the

intensities of the SBS and the laser radiation, taking into consideration

the self-compression of the laser beam when P,. — P-r i shows that the SBS

intensity increases faster than the laser one. Consequently, the SBS is

much more dependent on the laser intensity change caused by the selffocusing, than the intensity of the laser radiation itself. The decrease

of the beam diameter caused by its self-compression preceeds the selffocusing and forms the conditions for the SBS appearance. But the SBS,

earring out the energy from the sample, limits and even suppresses its

further self-focusing.

The relations between the self-focusing and the SBS can be different

depending on the length of the coherence of radiation. When the length

of the coherence is small, self-focusing suppresses the SBS. When the

coherence length is rather large, self-compression causes the SBS, which

then carries out the radiation, and does not allow the beam to selffocus.

5» Conclusions

1. The SBS increases the bulk damage threshold of optical materials.

It was established that under special conditions the threshold with the

SBS exceeds more than 100 times the damage threshold without the SBS.

2. The SBS changes the damage threshold of the surface of the

optical systems. It was established that depending on the experimental

conditions, the threshold can be increased more than 5 times and

decreased up to 1.7 time.

3. The reason of the increase of the bulk damage threshold and the

changes of the damage thresholds of the surface of the optical system

is considered to be the redistribution of the incident energy caused by

the SBS along the beam.

4. The transformation of the incident energy into the scattered

one occurs in the very front zone of the sample, where the intensity of

both radiations is comparatively low. In the same zone the main part of

the coherent acoustic oscillations energy is localised. These oscillations, due to a large cross-section of the zone, are not sufficiently

intensive to create any damages.

5» The increase of the damage threshold has been observed in optical

glases, in the crystals of quartz and TGS, in the water, heavy water,

benzene and, apparently, takes place 1 in all materials, the damage of

which is accompanied by a strong SBS.

6 • The intensity of the laser radiation in the focal region is

determined not only by the power of the incident radiation, the diameter

of the focal region and self-focusing but also by the power of the

scattering radiation. That is why for the comparisson of optical

resistance of various materials it is necessary to' take into consideration the amount of the SBS on the damage threshold.

7. Strong stimulated scattering for 180° of another type, will,

apparently, influence the damage as in the case of the SBS.

524
fe. References

Laser handbook, ed. by Arecchi

F.T, and Schulz 0 - Du Bois.

North-Holland Publ. Co., 1972.

Shea I.E., Frog. Quant. Electr.,

£, 1 (1975).

Vlasov D.V, , Kasjanov J.S.,

Korobkin V.V» , Fabelinskl I.L.,

Fiz. Tverd. Tela, 17, 3574

(1975).

Balkevi&ius P., Lukosius J.,

Maldutis E, , Zh. Tekh. Fiz.

Fisma, ^, 5^2 (1977).

5. BalkeviSius P., Kosenko E.,

Lukosius J., Maldutis E.

,

Kvantovaja Elektronika, 5,

2032 (1978).

6. Zverev G.M, and Fashkov 7.A.,

Zh. Eksp. Teor. Fiz, 57 . 1128 (1969).

525
12 II 10

i aser beam

o;

5 '

-!--J:i

r l_.

3

/•f o

Pig. 1. Scheme of the experiment.

1 - samples, 2 - SBS generator, 3 - spherical lens,

4 - glass plates, 5 - ?risms, 6 - callorimeter

,

- microvoltmeter , 8 - metallic mirror, 9 - di- versing lens, 10 - interferometer Fabry-Perot,

11, 12 - photocamera with

objective, 13 - photocell,

14 - oscillograph, 15 - cell.

Fig. 2. Typical bulk damaSe

I m^

pko" logy of glass K-8. Lo&S

tricks - without SBS, short

tracks-with SBS. The distance

between tracks -0.5 nun.

fiSER POWER, MW

526
Pig. 4. Distribution of laser axial

intensity along the beam in

rhodamine 6G solution.

30 60 90 120 POSITION, mm

to

7

10'

op-K-8

u,o- fused silica a;±-$h 0,02 cm'

o;o-ii)=2 cm~' m-SBS forK-8

a- SBS for fused

silica

1 10 100

FOCAL LENGTH, cm

Fig. 5. The damage thresholds as a function of the lens focal

length.

527
s

B

t

n
COMPUTER SIMULATION OF DAMAGE MORPHOLOGY

by

P. Kelly and D. Ritchie

Physics Division

National Research Council of Canada

and

A. Schmid and P. Braunlich

Physics Department

Washington State University

Our progress to-date on the computer simulation of the spatio-temporal behavior of a fast, just damaging pulse focused into an optical material is illustrated in the form of a 16 mm movie. The time and space dependent intensity is shown in profile by the assignment

of different colours and allows one to follow beam deformation as the pulse propagates

through the medium.

Our present code PULSE is a major modification of the work we first reported at last

year's damage conference. It is intended to be sufficiently flexible to accommodate any

flux, wavelength, material or temperature dependence of the dielectric function and any

possible mechanism of electronic excitation of the solid (avalanche, multiphoton generation,

etc.), as well as various conceivable mechanisms for converting the energy of the photon

field to excited electron and phonon distributions with an ultimate increase in lattice

temperature.

In particular, we illustrate the simulation of the experimental arrangement of W. L. Smith et al [1] who reported the damage morphology of NaCl using 21 psec pulses at 0.5321 u, where we have included the nonlinear interaction of the polaron model by A. Schmid et al [2]

At last year's symposium [3] we presented an adaption of the computer code RAMSES to the problem of damage in solids. We now consider a significant modification of that work:

namely, the grid is now placed on the solid (over the interaction volume) and the pulse

propagates through it. Our new code PULSE is illustrated in the following 16 mm movie.

Denoting the total energy for intrisic damage, £, as that obtained by W. L. Smith et al. [1] we illustrate the effect of varying the energy content of a clipped gaussian pulse. Our first runs used a coarse grid of 41 y (Ax) by 4y (Ar) so that At ^0.2 psec for 10*C, /LOxC,

2xg and finally £.

In the last "coarse" run there appears to be little non-linear interaction, but by

blowing-up the interaction volume, that is using a grid of 7y (Ax) by 0.9u (Ar) so that

At ^1.04 psec one can see in fine detail the beam break-up close to the optic axis near the

focal point.

And finally we illustrate the history of the ionization fraction, in this case, the

number of polarons created. Close to the center of the interaction volume this fraction

-3 20 3 reaches 5x10 , or 1.2 10 polarons per cm . The computer code and some of the results

will be presented for publication elsewhere.

SUMMARY OF MOVIE

The 16 mm colour film of the above runs was shown twice. In summary, we note that for

high energies, ^ 10 £ very little of the pulse propagates throgh the "focal point" before

there is significant non-linear interaction of the remainder of the pulse is "deflected"

from the hot spot created. As the energy is decreased, more and more of the pulse is required to create the equivalent hot spot and the extent of the spot and the break-up

effects are measurably different. The coarse run with the just damaging energy content £ shows almost no interaction, yet after the pulse has traversed the crystal (we run it

529
through 4000 p of NaCl) the ionization fraction is still high but located in a few microns

around a "focal spot." The last run showed in fine detail the extent of the spot created

and indeed the beam breaks up close to the optic axis. It was pointed out that more work

is required to attach any significance to the present analysis in terms of elucidating on the damage observed by Smith et al. [1] . However, it could be concluded that we have shown

that any theory of intrinsic damage must include the spatial as well as the temporal

behavior of a pulse as it propagates through non-linear medium.

REFERENCES

[1] W. L. Smith et al, Harvard University Technical Report No. 665, 1976 (unpublished).

[2] A Schmid et al, Phys. Rev. B 20, 815, 1979.

[3] NBS Special Publication 541, Glass & Guenther, p. 296, 1978.

[4] H. D. Dudder and D. B. Henderson, Comp. Phys. Comm. 10, 155, 1975.

The discussion centered on the difficulty of observing breakdown at threshold. Since the

highly localized breakdown site is very small when the plasma is first created the amount of

the beam obscured by the plasma is not very great and as a result any attempt to see a sudden

sharp climb in the transmitted beam intensity may prove very difficult. If the pulse is long

enough or if the pulse is well above threshold the initial plasma will grow very quickly,

obscuring a large fraction of the beam, and the well known pulse cut-off will be observed.

530
APPENDIX I — PARTICIPANTS

J. Abate

Laboratory for Laser Energetics

250 East River Road

Rochester, New York 14623

716-275-5181

Wayne Anderson

USAF Academy

ASAFA/DFP

USAF Academy, Colorado 80840

303-472-2394

Joseph H. Apfel

Optical Coating Laboratory, Inc. 2739 Giffen Avenue

Santa Rosa, CA 95402

707-545-6440

Philip C. Archibald

Naval Weapons Center, Code 3818

China Lake, California 93555

714-939-2869

William P. Barnes

ITEK Corporation

10 Maguire Road

Lexington, MA 02173

617-276-2246

Michael Bartowewcz

Lockheed R&D, Dept. 52-54

3251 Hanover Street, Bldg. 201 Palo Alto, CA 94034

415-493-4411, ext. 45051

Michael Bass Center for Laser Studies

University of Southern California

University Park

Los Angeles, CA 90007

213-741-7994

Harold E. Bennett

Naval Weapons Center, Code 38101

Michel son Laboratory

China Lake, California 93555

714-939-2869 or 939-3832

Jean M. Bennett

Naval Weapons Center, Code 38103

Michel son Laboratory

China Lake, California 93555

Arnold Bloom

Coherent, Inc. 3210 Porter Drive

Palo Alto, California 94304

415-493-2111

Norman Boling

Optical Coating Laboratory

2789 Giffen Avenue

Santa Rosa, CA 95403

707-545-6440

E. C. Borsare

W.J. Schafer Associates

10 Lakeside Office Park

Wakefield, MA 01880

617-246-0450

Gordon Boultbee

Optical Coating Lab., Inc. P.O. Box 1599

Santa Rosa, CA 95402

707-545-6440, ext. 202

Professor Rubin Braunstein

Physics Department

University of California

Los Angeles, California 90024

Charles K. Carniglia

Optical Coating Lab., Inc. 2789 Giffen Avenue

Santa Rosa, CA 95402

707-545-6440

Gary E. Carver

Optical Sciences Center

University of Arizona

Tucson, Arizona 85721

602- 626-1493, ext. 4754

George E. Chamberlain

Electromagnetic Technology Division

National Bureau of Standards

Boulder, Colorado 80303

303-499-1000, ext. 3706

Chang H. Chi

Hughes Aircraft Co., M/S 6-D137

Centinella & Teale

Culver City, CA 90230

213-391-0711, ext. 7121

Evan Chicklls

Sanders Associates

MER12-1220

95 Canal Street

Nashua, NH 03061

603-885-3010

Peter P. Chow

Honeywell, Inc. System & Research Center

2600 Ridgway Parkway, MN 17-2329

Minneapolis, MN 55413

612-378-5471

531
Terrance F. Deaton

University of California

Lawrence Livermore Laboratory

L-465

Box 5508

Livermore, CA 94550

415-422-6059

Donald L. Decker

Naval Weapons Center

Code 3816

China Lake, CA 93555

714-939-3247

John A. Detrio

University of Dayton

Research Institute

300 College Park

Dayton, Ohio 45469

513-229-3527

Joseph J. Devaney

Los Alamos Scientific Laboratory

55, MS 529

Box 1663 Los Alamos, NM 87544

505-667-6904

Dr. B. Dischler

Fraunhofer Institut

fur Angewandte Festkorperphysik

Eckerstrasse 4 D-7800 Freiburg

West Germany

T.M. Donovan

Naval Weapons Center, Code 3812

China Lake, CA 93555

714-446-2115

Terry A. Dorschner

Raytheon Company, Research Division

28 Seyon Street

Waltham, MA 02154

Major Glenn R. Doughty

AFIT/ENP

Wright Patterson AFB, Ohio 45433

513-255-2012

Richard W. Dyer

W.J. Schafer Associates

10 Lakeside Office Park

Wakefield, MA 01880

617-246-0450

Johannes Ebert

Universitat Hannover

Angewanote Physik

Welfengarten 1 3000 Hannover

West Germany

GER 0511 7624894

David F. Edwards

Los Alamos Scientific Laboratory

MS-564

Los Alamos, NM 87545

505-667-7102

Ed Enema rk

Optical Coating Laboratory, Inc. P.O. Box 1599

Santa Rosa, CA 95402

707-545-6440, ext. 113

Albert Feldman

Materials Building, A259

National Bureau of Standards

Washington, D.C. 20234

301-921-2840

Nils C. Fernelius

University of Dayton

Research Institute

300 College Park Avenue

Dayton, Ohio 45469

513-254-2641

Dennis Fischer

Thin Film Dept. Manager

Coherent Inc. 3210 Porter Drive

Palo Alto, CA 94043

415-493-2111

Lawrence B. Fogdall, Research Engineer

Boeing Company

Physics Technology

P.O. Box 3707

Seattle, Washington 98124

206-655-6711

Robert P. Freese Institute of Optics

University of Rochester

Rochester, New York 14627

716-275-5632

Wesley D. Gerber

Ford Aerospace & Communications Corp.

Aeronutronic Division

Ford Road

Newport Beach, California 92663

714-759-6545

Joe Giove

ILC Technology

399 Java Drive

Sunnyvale, CA 94086

408-745-7900

532
Ann Tobv Glassman Bill Herrmann, Jr. University of Dayton Optic Electronic Corp.

Research Institute 11477 Pagemill Road

300 College Park Dallas, Texas 75243

Dayton, Ohio 45469 214-349-0190

513-229-3724

Robert Hinebaugh

Ron Goe AGMC/MLLS

Rockwell International Newark AFS

Rocketdyne Division Newark, Ohio 43055

D/588, BA30 614-522-7695

6533 Canoga Avenue

Canoaa Park. CA 91304 R B HemDhill

213-882-3310 Vouaht Corooration

Advanced Technoloav fpntpr

William T. Goosey P.O. Box 226144

Eastman Kodak Co. Dallas Texas 75266

Department 144 20 Avenue E Samuel J. Holmes

Rochester, New York 14650 Northrop Research & Technology

716-726-6442 1 Research Park

Palos Verdes Peninsula, CA 90274

Alex Glass 213-377-4811 Lawrence Livermore Labs Box 808 Alan K. Hopkins

Livermore, CA 94550 Air Force Materials Laboratory

415-422-5354 AFML/LP0

Wright Patterson AFB, Ohio 45433

Art Guenther 513-255-4474/4098

AFWL/CA

Kirtland AFB, NM 87115 Thomas W. Humpherys

505-264-9856 AFWL/ALR

Kirtland AFB, NM 87117

Georae Graves 505-264-1776

University of Dayton

Research Institute Stephen D. Jacobs

300 Colleae Park Laboratory for Laser Energetics

Dayton, Ohio 45305 University of Rochester

513-229-2517 250 East River Road

Rochester. NY 14623

Ma rr von Gu nfpn 716-275-4837 ^ Spectra-Physi cs 1250 W Middlefield Road Carl R. Jacobson

Mountain View, CA 94042 Raytheon Service Co. 415-961-2550, ext 310 Missile Systems Division, M22-2

Hartwell Road

Wilford N. Hansen Bedford, MA 01730

Phvsics DeDartment 617-274-7100, ext. 3156

Utah State University

Loaan. Utah 84322 G.T. Johnston

University of Dayton

Douglas H. Harrison Research Institute

Spectra-Physi cs AFWL/ALR, Bldg. 401 1250 West Middlefield Road Kirtland AFB, NM 87117

Mountain View. CA 94042 505-247-0512

415-961-2550, ext 466

Eric G. Johnson, Jr. James E. Harvey Optical Electronic Metrology Group

University of Dayton Electromagnetic Technology Division

Research Institute National Bureau of Standards

AFWL/ALR, Bldg. 401 Boulder, Colorado 80303

Kirtland AFB, NM 87117 303-499-1000, ext. 3234

505-247-0512

533
Kenneth Jungling

AFWL/ALR

Kirtland AFB, NM 87117

505-264-1776

Thomas Kardos

Broomer Labs, Inc. 23 Sheer Plaza

Plainview, New York 11803

516-249-1544

Paul Kelly

Physics Division

National Research Council

Ottawa, Ontario

Canada

Major Charles D. Kennedy

USAF-AFIT

AFIT-EN Box 4031

Wright-Patterson AFB, Ohio 45433

William E. Kienzle

Science Appl ications, Inc. Suite 310

101 Continental Blvd.

El Segundo, CA 90245

Philipp H. Klein

U.S. Naval Research Laboratory

Code 5222

Washington, D.C. 20390

202-767-3671

Claude A. Klein

Raytheon Company

28 Seyon Street

Waltham, MA 02173

617-899-8400

Robert A. Lawton Electromagnetic Technology Division

National Bureau of Standards

Boulder, Colorado 80303

303-499-1000, ext. 3806

Harald Kuster Institut fur Angewandte Physik

Welfengarten 1 3000 Hannover 1 0511-762-4894

West Germany

David J. Krus Harshaw Chemical Co. Crystal & Electronic Products Dept.

6801 Cochran Road

Solon, Ohio 44139

216-248-7400

Nils Laegreid

Battelle Northwest Labs

Materials Dept.

231-Z-200 West Area Richland, WA 99352

509-942-2417

William P. Latham

AFWL/ALR

Kirtland AFB, NM 87117

505-264-0721

Hubert P. Le Bodo

Chief, Laser Division

Laboratoire National d'Essair

1 Rue Gaston Boissier

Paris France 75015

Kenneth Lei bold Eastman Kodak Co. Research & Engineering Dept. 339

901 Elmgrove Road

Rochester, NY 14650

716-724-5578

Steven R. Lindle

AFWL/ALR

Kirtland AFB, NM 87117

505-264-1776

W. Howard Lowdermilk

Lawrence Livermore Laboratory

P.O. Box 5508, L-465

Livermore, CA 94550

415-422-5498

Mike Lunt Director, Technical Optics, Ltd. Second Avenue, Onchan

Isle of Man Great Britain

0624 4443 (UK)

Ronald L. Lusk

AFWL/ALR

Kirtland AFB, NM 87117

505-264-1776

Fred McClung

Hughes Aircraft, Bldg. 6 M.S. E-125

Culver City, CA 90230

213-391-0711, ext. 4715

Ron W. MacPherson

Electro Optics Division

Centre de Recherches pour

la Defense Valcartier

P.O. Box 880, Courcelette, Quebec

Canada G0A 1R0

418-844-4645

534
Mark Madigan

Optical Coating Lab. , Inc. Technical Products Division

P.O. Box 1599

Santa Rosa, CA 95402

707-545-6440

John R. McNeil New Mexico State University

Box 3-PSL

Las Cruces, NM 88003

505-522-9467

David Milam

Lawrence Livermore Labs

7000 E. Ave. Livermore, CA 94550

415-422-5499

P. A. Miles

Raytheon Missile Systems Division

Hartwell Road

Bedford, MA 01730

617-274-7100, ext. 4515

Charles W. Mitchell

University of Dayton

Research Institute

AFWL/ALR, Bldg. 401 Kirtland AFB, NM 87117

505-247-0512

Dennis Morelli

Optical Coating Lab., Inc. Technical Products Division

P.O. Box 1599

Santa Rosa, CA 95402

707-545-6440

Jacques Mouchart

Laboratoires de Marcoussis, CR-CGE

Route de Nozay

Marcoussis

France 91460

Henry M. Musal , Jr. Lockheed Research Laboratory

D5256, B201

3251 Hanover Drive

Palo Alto, CA 94304

415-493-4411

C. Taylor Myers

Northrop Corporation

Electro-Mechanical Division

500 E. Orangethorpe Avenue

Anaheim, CA 92801

Brian E. Newnam

Los Alamos Scientific Laboratory

P.O. Box 1663, MS 564

Los Alamos, NM 87545

505-667-3519

Davis B. Nichols

Boeing Aerospace Co. M.S. 88-46

P.O. Box 3999

Seattle, WA 98124

206-773-8938

Marc Novaro

Centre d

1 Etude de Limeil

BP 27, Villeneuve

St. Georges

France 94190

J.M. O'Hare

Professor of Physics

University of Dayton

300 College Park

Dayton, Ohio 45469

513-229-2331

Russell Ono TRW Corporation (DSSG), 01/1261

1 Space Park

Redondo Beach, CA 90278

213-535-1792

Roger A. Paquin

Perkin Elmer Corporation

Main Ave., M.S. 278

Norwalk, CT 06856

203-762-4085

Alex Parker

MIT Lincoln Laboratories

P.O. Box 73

Lexington, MA 02173

617-862-5500

Walter T. Pawlewicz

Battelle - Pacific Northwest Lab.

P.O. Box 999

Richland, WA 99352

509-942-2537

Bruce J. Pierce

University of Dayton

Research Institute

UDRI (AFWL/ALR)

Kirtland AFb, NM 87117

505-264-3013

Ben C. Piatt

R. & D. Associates

ATO 9377

Albuquerque, NM 87119

505-264-3013

Robert D. Poirier

Perkin Elmer Corp.

77 Danbury Road, MS 420

Wilton, CT 06897

203-762-4947

535
Thomas M. Pollak

Sanders Associates

MER 12-1214

95 Canal Street

Nashua, NH 03060

603-885-4405

James 0. Porteus

Naval Weapons Center

Code 3817

China Lake, CA 93555

714-939-3827

Frank Rainer

Lawrence Livermore Laboratory

P.O. Box 5508, L-465

Livermore, CA 94550

415-422-4376

John F. Ready

Honeywell, Inc. Material Sciences Center

10701 Lyndale Ave. , S Bloomington, MN 55420

612-887-4430

John Reeves

Hughes, 77 45 24 Centinella & Teale Streets

Culver City, CA 90230

213-391-0711, ext. 3159

Jim Rinefierd

Laboratory for Laser Energetics

250 East River Road

Rochester, NY 14623

716-275-3418

Doug Ritchie

Physics Division

National Research Council

Montreal Road, Bldg. M-36, R-1211

Ottawa, Kiaosi, Ontario

Canada

613- 993-9334

Herbert B. Rosenstock

Code 5504

Naval Research Laboratory

Washington, D.C. 20375

202-767-2225

Werner Ryseck

Siemens AG

8 Munchen 70

Hofmanstrasse 51 Germany

72263297

Ansgar Schmid

Physics Department

Washington State University

Pullman, WA 99164

Marion L. Scott

University of Dayton

Research Institute

AFWL/ALR

Kirtland AFB, NM 87117

505-822-0436

Steven C. Seitel

Naval Weapons Center

China Lake, CA 93555

714-939-2470

Robert R. Shannon

Optical Sciences Center

University of Arizona

Tucson, Arizona 85721

602-626-3345

David Shemwell

Perkin Elmer Corporation

M/S 240, Optical Systems

Main Avenue

Norwalk, CT 06856

203-762-4871

Glenn Sherman

Laser Power Optics Corp.

9062 Willoughby Road

Pittsburgh, PA 15237

412-366-7478

Philip A. Simpson

Electromagnetic Technology Division

National Bureau of Standards

Boulder, Colorado 80303

303-499-1000, ext. 3789

Aaron A. Sanders, Chief

Optical Electronic Metrology Group

Electromagnetic Technology Division

National Bureau of Standards

Boulder, Colorado 80303

303-499-1000, ext. 4341

W. Lee Smith

Lawrence Livermore Labs Box 5508, L-465

Livermore, CA 94550

415-422-8209

Marion F. Soileau, Jr. Naval Weapons Center

Code 3817

China Lake, CA 93555

714-939-3049

Gunnar Sorensen

Institute of Physics

University of Aarhus

8000 Aarhus C Denmark

536
Marshall S. Sparks

Scientific Research Center

1640 Fifth Street, Suite 216

Santa Monica, CA 90401

213-394-0210

Charles L. Strecker

AFML/LPO

Wright-Patterson AFB, Ohio 45433

513-255-4474

Virgil Strahan

Rockwell International

031-278-063

3370 Miraloma Ave. Anaheim, CA 92803

714-632-6547

Paul M. Sutton

Ford Aerospace & Communication Corp.

Ford Road

Newport Beach, CA 92663

714-759-5852

Paul Temple

Naval Weapons Center

China Lake, CA 93555

714-939-3247

Y.S. Touloukian

CINDAS/Purdue University

2595 Yeager Road

W. Lafayette, IN 47906

317-463-1581

Norman L. Thomas

University of California

Lawrence Livermore Laboratory

L-461

P.O. Box 808

Livermore, CA 94550

415-422-0486

James H. Tillotson

Rocketdyne

Advanced Laser Concepts

6633 Canoga Avenue

Canoga Park, CA 91304

213-884-2762

A. Vaidyanathan

AFWL/CA

Kirtland AFB, NM 87117

505-264-3982

Harshadrai Vora

Honeywell Corporate Material

Sciences Center

10701 Lyndale Ave. , S Bloomington, MN 55420

612-887-4412

Thomas Walker

AFWL/ALR

Kirtland AFB, NM 87117

505-264-8212

Nelson W. Wallace

Rockwell International

Rocketdyne Division 055-BA/30

6633 Canoga Avenue

Canoga Park, CA 91304

213-884-3854

David A. Walsh

University of Dayton

Research Institute

300 College Park Avenue

Dayton, Ohio 45469

513-254-2641

Paul E. Werner

Electromagnetic Technology Division

National Bureau of Standards

Boulder, Colorado 80303

303-499-1000, ext. 3439

Kurt Wiedemann

Siemens AG Hofmannstrasse 51 West Germany

08977226892

Thomas Wiggins

AFWL/ALR

Kirtland AFB, NM 87117

505-264-1776

Richard T. Williams

Naval Research Laboratory

Code 5581 Washington, D.C. 20375

202-767-2768

Harry V. Winsor

DARPA/MS0

1400 Wilson Blvd.

Arlington, VA 22209

202-694-3031

G. Richard Wirtenson

University of California

Lawrence Livermore Laboratory

L-465

P.O. Box 5508

Livermore, CA 94550

415-422-1332

Shew M. Wong

Rockwell International

Rocky Flats, B/779

P.O. Box 464

Golden, Colorado 80020

303-497-2080

537
R.M. Wood

General Electric Co., Ltd.

Hirst Research Centre

East Lane, Wembley

Middlesex

United Kingdom

01-904-1262

Matt Young

Electromagnetic Technology Division

National Bureau of Standards

Boulder, Colorado 80303

303-499-1000, ext. 3223

Robert Zimmerer

Scientech, Inc. 5649 Arapahoe Ave. Boulder, Colorado 80303

303-444-1361

538 « U. S. GOVERNMENT PRINTING OFFICE : 1980 311-046/162
NBS-114A CREV. 9-78)

U.S. DEPT. OF COMM. 2. Gov't Accession No J 3. Recipient' $ A'cc*?sion No, BIBLIOGRAPHIC DATA

SHEET

1. PUBLICATION OR REPORT NO. NBS SP 568

4. TITLE AND SUBTITLE

Laser Induced Damage in Optical Materials: 1979

5. Publication Date July 1980

$. Performing Organization Code

(Editors)

Harold Eo Bennett (NWC) , Alexander J. Glass (LLL)

,

Arthur H, Guenther (AFWL) & Brian E. Newnam (LASL)

8. Performing Organ. Report No.

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, DC 20234

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12. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (Street, City, State, ZIP) American Society for Testing and Materials

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Library of Congress Catalog Card Number: 80-600100

| | Document describes a computer program; SF-185, FIPS Software Summary, is attached.

16. ABSTRACT (A 200-word or leas factual summary of most significant infonnation. If document includes a significant bibliography or

literature survey, mention it here.) The Tenth Anniversary Symposium on Optical Materials for High Power Lasers (Boulder

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

30-31 October 1979= The Symposium was held under the auspices of ASTM Committee F-l,

Subcommittee on Laser Standards, with the joint sppnsorship of NBS, the Defense

Advanced Research Project Agency, the Department of Energy, and the Office of Naval

Research. About 150 Scientists attended the Symposium, including representatives

of the United Kingdom, France, Canada, Japan, West Germany, and Denmark. The

j Symposium was divided into sessions concerning Transparent Optical Materials and

the Measurement of Their Properties, Mirrors and Surfaces, Thin Film Characteristics,

Thin Film Damage, Considerations for High Power Systems, and finally Theory and

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

was given to materials for high power apparatus. The wavelength range of prime

interest was from 10.6 pm to the uv region. Highlights included surface characterization, thin film substrate boundaries, and advances in fundamental laser-matter

threshold interactions and mechanisms. The scaling of damage thresholds with pulse

duration, focal area, and wavelength was discussed in detail. Harold E. Bennett of the Naval Weapons Center, Alexander J. Glass of the Lawrence Liver more Laboratory,

Arthur H„ Guenther of the Air Force Weapons Laboratory, and Brian E. Newnam of the Los Alamos Scientific Laboratory were co-chairmen of the Symposium. The Twelfth

Annual Symposium is scheduled for 30 September - 1 October 1980 at the National

Bureau of Standards, Boulder, Colorado.

7. KEY WORDS (six to twelve entries; alphabetical order; capitalize only the first letter of the first key word unless a proper name;

separated by semicolons) Laser damage, laser interaction; optical components; optical

fabrication; optical materials and properties; thin film coatings.

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OFFICIAL BUSINESS

Penalty for Private Use. S300

POSTAGE AND FEES PAID

U.S. DEPARTMENT OF COMMERCE

COM—21S

SPECIAL FOURTH-CLASS RATE

BOOK

1B98