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NBS SPECIAL PUBLICATION 669

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

Laser Induced Damage

in Optical Materials:

1982

BOULDER DAMAGE SYMPOSIUM STP 847
NATIONAL BUREAU OF STANDARDS

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Laser Induced Damage

In Optical Materials: 1982

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

Air Force Office of Scientific Research

November 16-17, 1982

NBS, Boulder, Colorado 80303

Edited by:

Harold E. Bennett

Naval Weapons Center

China Lake, California 93555

Arthur H. Guenther

Air Force Weapons Laboratory

Kirtland Air Force Base, New Mexico 87117

David Milam

Lawrence Livermore National Laboratory

Livermore, California 94550

DATE DUE

Brian E. Newnam

Los Alamos National Laboratory

Los Alamos, New Mexico 87545

BOULDER DAMAGE SYMPOSIUM

U.S. DEPARTMENT OF COMMERCE, Malcolm Baldrige, Secretary

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director

Issued January 1984
Library of Congress Catalog Card Number: 83:600625

National Bureau of Standards Special Publication 669

Natl. Bur. Stand. (U.S.), Spec. Publ. 669, 499 pages (Jan. 1984)

CODEN: XNBSAV

U.S. Government Printing Office

Washington: 1984

For sale by the Superintendent of Documents, U.S. Government Printing Office, Washington, DC 20402
Foreword

The Proceedings contain the papers presented at the Fourteenth Symposium on Optical Materials for

High Power Lasers held at the National Bureau of Standards (NBS) in Boulder, Colorado, on November 16-17,

1982. 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, the

Department of Energy, and the Air Force Office of Scientific Research. The Symposium was attended by

approximately 200 scientists from the United States, the United Kingdom, Japan, France, West Germany, and

the USSR. It was divided into sessions devoted to the following topics: Materials and Measurements,

Mirrors and Surfaces, Thin Films, and finally Fundamental Mechanisms. The Symposium Co-Chairmen were Dr. Harold E. Bennett of the Naval Weapons Center, Dr. Arthur H. Guenther of the Air Force Weapons Laboratory,

Dr. David Milam of the Lawrence Livermore National Laboratory, and Dr. Brian E. Newnam of the Los Alamos

National Laboratory. They also served as editors of this report. Dr. Alexander J. Glass of KMS Fusion

acts as Conference Treasurer with Aaron A. Sanders of the National Bureau of Standards as the Conference

Coordinator.

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 Fifteenth Annual Symposium on this topic will be held in Boulder, Colorado, from November 14-16,

1983. 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 1983 do not differ drastically from

those enumerated above. We expect 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 be developed together with an improved understanding of defectdominated damage mechanisms.

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 Fifteenth Annual Symposium.

H. E. Bennett, A. H. Guenther,

D. Milam, and B. E. Newnam

Co-Chai rmen

i i i
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 therein.

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, recom- mendation, 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.

i v
CONTENTS

Page

Foreword iii

H. E. Bennett, A. H. Guenther, D. Milam and B. E. Newnam

Disclaimer iv

Symposium Welcome viii

D. Milam

Welcome on Behalf of the ASTM ix

J. A. Detrio

Summary of Meeting

H. E. Bennett, A. H. Guenther, D. Milam and B. E. Newnam

I. Introduction 1

2. Principal Conclusions 1

3.0 Summary of Papers 4 3.1 Materials and Measurements 4 3.2 Mirrors and Surfaces

3.3 Thin Films 10 3.4 Fundamental Mechanisms 13

4. Recommendations 15

5. Acknowledgments 16

6. References 16

Materials and Measurements

1.3 urn Laser Window Materials Survey Continued 19

R. J. Harris, D. V. Dempsey, D. B. O'Quinn, M. E. Gangl , and W. L. Knecht

Thermal Conductivity of Fluorozirconate Glass and Tellurium Dioxide Single Crystals 28

K. A. McCarthy and H. H. Sample

Optical Characteristics of Germanium at 10 Microns 33

R. M. Wood, S. K. Sharma, and P. Waite

Variation of Laser Induced Damage Threshold with Laser Pulse Repetition Frequency 44

R. M. Wood, S. K. Sharma, and P. Waite

Multiple Pulse Laser-Induced Bulk Damage in Crystalline and Fused Quartz at 1.064

and 0.532 urn 50

L. D. Merkle, M. Bass and R. T. Swimm

Role of Ca and Pb Dopants in Pulsed 10.6 urn Bulk Damage of Superpure NaCl and

KC1 Crystals 60

F. S. Faizullov, V. I. Kovalev, J. Janszky, and R. Voszka

Contaminant and Defect Analysis of Optical Surfaces by Infrared Laser-Stimulated

Desorption 65

S. D. Allen, J. 0. Porteus, W. N. Faith, and J. B. Franck

Low Optical Absorption Measurement of Laser Optical Materials by Interferometric

Laser Calorimetry 75

M. Itoh and I. Ogura

Surface and Free Carrier Absorption Processes in CVD Zinc Selenide 86

K. L. Lewis and G. S. Arthur

v
A Laser Damage Facility in the Ultra-Violet 102

S. S. Wiseall and D. C. Emmony

Use of Hole Gratings in Laser Damage Testing Ill

T. A. Wiggins, T. T. Saito, and J. A. Hosken

Monolithic Cell for Frequency Conversion 118

R. Bossert, S. D. Jacobs, and L. Lund

Mirrors and Surfaces

C0 2 Laser Polishing of Fused Silica: Recent Progress 130

P. A. Temple, S. C. Seitel, and D. L. Cate

Float Polishing and Defects of Fused Silica 138

Y. Namba

Effect of Surface Finishing on the Damage Threshold of Fused Silica at 1.06 urn 146

H. Vora, W. H. Lowdermilk, and J. E. Swain

Scratch Standards Should Not be Used to Predict Damage Threshold 151

M. Young

Bulk and Surface Damage Thresholds of Crystals and Glasses at 248 nm 157

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

Pulsed Laser Induced Optical Surface Damage Studies in Different Gases 164

J. Kardach, A. F. Stewart, and A. H. Guenther

Pulsed Laser- Induced Damage to Metals at Cryogenic Temperatures 178

D. L. Decker, J. B. Franck, W. N. Faith, and J. 0. Porteus

Single and Multiple Pulse Catastrophic Damage in Cu and Ag Diamond Turned

Mirrors at 10.6, 1.06, and 0.532 urn 186

N. Koumvakalis, C. S. Lee, and M. Bass

First Order Analytical Technique for Evaluating the Heat Transfer, Coolant

Channel Thermal and Pressure Ripple, Bending, Burnout, and Coating Damage

on Single and Multilayer Laser Mirror Cooled Heat Exchangers

J. R. Palmer

Adaptive Force Optical Elements Based on Open Pore Structures

V. V. Apollonov, S. A. Chyetkin, V. Yu. Khomich, and A. M. Prokhorov

Optical Figure Requirements for Laser Mirrors Used at Oblique Incidence...

H. E. Bennett

Thin Films

Molecular Beam Epitaxy 234

M. Rona and P. W. Sullivan

on i Ion Beam Sputter Deposition of Optical Coatings HO

J. R. Sites, P. Gilstrap, and R. Rujkorakarn

Optical Characterization of Plasma Deposited Hard Carbon Coatings

B. Dischler, A. Bubenzer, P. Koidl, G. Brandt, and 0. F. Schirmer

Scaling up the Neutral Solution Anti-Reflection Process 255

G. R. Wirtenson, N. J. Brown, and L. M. Cook

Progress in Ultraviolet Damage Testing at Los Alamos 2 ^6

S. R. Foltyn, L. J. Jolin, and B. E. Newnam

vi
Damage Thresholds of Thin Film Materials and High Reflectors at 248 nm.

.

F. Rainer, W. H. Lowdermilk, D. Milam, C. K. Carniglia, T. T. Hart,

and T. L. Lichtenstein

274

Laser Damage Thresholds of Optical Coatings at 351 nm 282

J. A. Abate, R. Roides, S. D. Jacobs, W. Piskorowski, and T. Chipp

Raising the Surface Damage Threshold of Neutral Solution Processed BK-7 by

Pulsed Laser Irradiation 292

J. E. Swain, W. H. Lowdermilk, and D. Milam

Damage Threshold Measurements at 1.06 urn at Centre D' Etudes De Limeil 296

D. Friart, M. Novaro, F. Garaude, B. Geenen, A. Malherbes, J. Guerain,

J. J. Rome, M. T. Buffenoir, P. Perbal , and M. Soulie'

Impurities in ThF 4 Optical Coatings 300

G. J. Hansen, D. Lund, A. T. D'Agostino, and W. N. Hansen

Reflectance Spectrum of Non-Optimum Multilayer Coatings 313

R. Rujkorakarn, R. W. Hannum, and J. R. Sites

Environmental Study of Single and Multilayer Dielectric Films for 2.8 (u)

Subjected to Humidity, Ethanol , HF, and H 2 S 322

J. R. Palmer

Photothermal Deflection Microscopy of Thin Film Optical Coatings 349

W. C. Mundy, R. S. Hughes, and C. K. Carniglia

Fundamental Mechanisms

Interaction Mechanisms of Laser Radiation with Ocular Tissues 355

D. H. Sliney

Spotsize Effects in Laser Damage Testing 368

S. R. Foltyn

Pulsed Laser Induced Damage of an Optical Material with a Spherical Inclusion:

Influence of the Thermal Properties of the Materials 380

M. R. Lange, J. K. Mclver, A. H. Guenther, and T. W. Walker

Temporal Dependence of Laser-Induced Breakdown in NaCl and Si0 2 387

M. J. Soileau, W. E. Williams, E. W. Van Stryland, T. F. Boggess,

and A. L. Smirl

Ripple Structures and Enhanced Absorption Associated with Ordered Surface

Defects 406

M. J. Soileau and E. W. Van Stryland

Charged Particle Exoemission from Silicon During Multi-Pulse Laser Induced

Damage 415

M. F. Becker, Y-K. Jhee, M. Bordelon, and R. M. Walser

A Study of the PRF Dependence of the Accumulation Effect in Multiple Pulse Laser Damage of

Silicon 427

M. Bordelon, R. M. Walser, M. F. Becker, and Y-K. Jhee

On the Nature of Accumulation Effect in Laser-Induced Damage to Optical Materials 436

A. A. Manenkov, G. A. Matyushin, V. S. Nechitailo, A. M. Prokhorov,

and A. S. Tsaprilov

Dispersion of the Refractive Index Near the Absorption Edge in Compound

Semiconductors 448

B. Jensen and A. Torabi

Unipolar Arcing, A Basic Laser Damage Mechanism 458

F. Schwirzke

APPENDIX -- Participants 479

vi i
SYMPOSIUM WELCOME

David Milam

Lawrence Livermore National Laboratory

Livermore, CA 94550

I and my fellow chairmen, Hal Bennett, Art Guenther and Brian Newnam, welcome you to the

fourteenth Boulder Damage Symposium. Our program contains 45 papers. The authors represent 15 universities, 5 government agencies, and 5 companies in the U.S., and government agencies,

universities and companies in 5 foreign countries. In size and composition, this conference is comparable to those of the last few years. Part of this continued interest in laser damage arises

from the fact that the interaction of light and matter is an important physics problem. However,

most damage studies are driven and funded by the need for damage-resistant laser components. Even

if the problems of yesterday had all been solved, design and fabrication of tomorrow's lasers would

require additional damage experiments. These future needs, combined with the unsolved problems at hand, insure that damage studies (and a forum for discussing damage) will be necessary for many

years

.

The conference has six sponsors: the American Society for Testing and Materials, the Air

Force Office of Scientific Research, the Defense Advanced Research Projects Agency, the Department

of Energy, the National Bureau of Standards and the Office of Naval Research. Alex Glass of KMS

Fusion served as treasurer and Aaron Sanders of NBS as coordinator. Ms. Susie Rivera of NBS is the

conference secretary. The contributions of these agencies and individuals are sincerely

appreciated.

We have a continuing problem in preparation of the conference report. It is a majority

opinion of the conference chairmen that each paper presented at the conference should be included

in the report. However, some papers are arriving many months after the conference. This greatly

delays and complicates the completion of editing, so we ask each author to be prompt in supplying a manuscri pt.

Having completed these administrative announcements, I wish to comment briefly on a general

problem within the damage community. The Q-switched and mode-locked lasers with which I am familiar emit pulses with durations that are typically less than 50 ns, at wavelengths between 250 nm and 1000 nm. The safe operating level for these lasers is less than 20 J/cm2, and usually

less than 10 J/cm2. The safe fluence is determined by laser damage except for lasers emitting

pulses with durations well under 1 ns, for which self-focusing is the limiting mechanism. By

contrast, damage studies conducted with these same laser pulses sometime result in threshold values

which are more than 10 times greater than the operating levels in current lasers.

In some instances these large thresholds are observed in scientific studies of materials which

do not damage in operating lasers. For example, we are not surprised to find that the bulk

threshold for fused silica is much greater than safe operating fluences.

It is, however, more difficult to understand the significance of oversize thresholds measured

in direct support of the development of a particular laser. I can think of only two explanations.

Either we are studying a component which is not limiting laser performance, or we are using a measurement that does not provide data directly applicable to laser design. Valid data on nonlimiting problems may be of value once present limitations are removed. Inappropriate

measurements may hinder component improvement by indicating incorrect paths for the development.

The importance of performing experiments that provide absolute tnresholds (independent of

local relative standards) has been discussed by many speakers at this conference. The measurement

capability within the damage community has steadily improved. We are now able to provide

experimental answers for many questions. I believe our results would have increased significance

if we would compare measured threhsolds with current operating levels to insure that our questions

are relevant.

vi i i
Boulder Damage Symposium 1982

ASTM Welcome

John A. Detrio

University of Dayton

Dayton, Ohio 45469

Welcome to the Fourteenth Annual Boulder Damage Symposium. Sustaining excellence over 14 years

is a challenging task. It is a credit to the organizers of this meeting, Hal Bennett, Art Guenther,

David Milam, Brian E. Newnam, Aaron A. Sanders and to the authors and participants of the Boulder Damage

Symposium that this meeting continues to attract international attention, and to be the forum for

presenting the state-of-the-art of optical materials for high power lasers.

Each year the contents of the Symposium's program gives some insight into trends in the science

and technology of high power laser optical components. This year's program contains work which continues trends begun at earlier meetings toward measurements of repetitively pulsed damage mechanisms,

and the influence of preparation and environmental factors on damage mechanisms with emphasis on

surface and coating preparation methods. These trends are indications of the maturation of laser

technology and the increased practical application of pulsed lasers. The relatively rapid transition

from laser science to laser technology is just one more example of the accelerated pace of technology

and its effect on our lives.

The demand for reliable, long lived optical components for high power lasers is a certain sign

that these components are expected to perform over an extended useful life in practical situations.

These applications include chemical and materials processing, manufacturing, energy research and

production, and medical applications. The practical application of a technology does not put an

end to scientific or theoretical research. In fact, the development of more carefully controlled

materials properties and more precise measurements permit more sensitive tests of theories now than

ever before.

The attempts to understand and control the failure mechanisms in optical components has

required a reexamination of the materials themselves, their preparation and fabrication, and the

careful testing and characterization of these materials. It is in this role of testing and evaluating materials that ASTM, the American Society for Testing and Materials, supports and endorses

the Boulder Damage Symposium. ASTM is a non-profit, volunteer organization dedicated to the

development of standards for materials and test methods.

Being a non-profit agency, ASTM provides primarily moral support to the symposium. For example,

the proceedings of earlier damage symposia are available from ASTM. This activity fulfills ASTM's

ix
secondary role of helping in the spread of scientific knowledge. We are pleased to participate

in the damage symposium.

Once again, let me say welcome to the Boulder Damage Symposium with a special warm welcome to

our many guests from overseas. We are very happy that you could come and share your work with us.

Your attendance confirms my sentiments about the value and importance of the Boulder Damage

Symposium.

x
Laser- Induced Damage in Optical Materials

Fourteenth ASTM Symposium

November 16-17, 1982

The Fourteenth Annual Symposium on Optical Materials for High Power Lasers

(Boulder Damage Symposium) was held at the National Bureau of Standards in Boulder,

Colorado, November 16-17, 1982. 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, the Office of

Naval Research, and the Air Force Office of Scientific Research. Approximately 200

scientists attended the Symposium, including representatives of the United Kingdom,

France, Japan, West Germany, and the USSR. The Symposium was divided into sessions

concerning Materials and Measurements, Mirrors and Surfaces, Thin Films and finally

Fundamental Mechanisms. As in previous years, the emphasis of the papers presented at

the Symposium was directed toward new frontiers and new developments. Particular

emphasis was given to materials for high power apparatus. The wavelength range of

prime interest was from 10.6 urn to the uv region. Highlights included surface characterization, thin film-substrate boundaries, and advances in fundamental laser-matter

threshold interactions and damage mechanisms. Harold E. Bennett of the Naval Weapons

Center, Arthur H. Guenther of the Air Force Weapons Laboratory, David Milam of the

Lawrence Livermore National Laboratory, and Brian E. Newnam of the Los Alamos National

Laboratory were co-chairmen of the Symposium. The Fifteenth Annual Symposium is

scheduled for November 14-16, 1983 at the National Bureau of Standards, Boulder,

Col orado.

Keywords: laser damage; laser interaction; optical fabrication; optical materials

and properties; thin film coatings.

1. Introduction

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

was held, as in previous years, at the National Bureau of Standards in Boulder, Colorado, November 16-17,

1982. 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, the Office of Naval Research, and the Air Force Office of Scientific Research. Working sessions

of the Committee F-l Subcommittee on Lasers were held on Monday, November 15. Approximately 200 scientists attended the Symposium, including representatives of the United Kingdom, France, Japan, West

Germany, and the USSR. The Symposium was divided into sessions concerning Materials and Measurements,

Mirrors and Surfaces, Thin Films and finally Fundamental Mechanisms. In all, approximately 45 technical

presentations were made. Harold E. Bennett of the Naval Weapons Center, Arthur H. Guenther of the Air

Force Weapons Laboratory, David Milam of the Lawrence Livermore National Laboratory, and Brian E. Newnam

of the Los Alamos National Laboratory were co-chairmen of the Symposium. Alexander J. Glass of KMS Fusion, is Conference Treasurer and Aaron A. Sanders of the National Bureau of Standards acts as Conference

Coordi nator. 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 Fifteenth Annual Symposium, scheduled for November

14-16, 1983, at the National Bureau of Standards, Boulder, Colorado.

2. Principal Conclusions

Various surface effects received considerable attention this year. It was shown that water, hydrogen, oxygen and OH contribute in a multitude of ways to the laser-damage process. For example, surface

absorption in germanium is believed to be caused primarily by the presence of a hygroscopic oxide layer

l
which results from surface treatment. However, polishing with diamond on a tin lap is one way to reduce

the effect. Many other materials also exhibit surface effects. Spatially localized variations, of orders

of magnitude, in water-related contaminant levels have been found and correlated with low damage thresholds.

Laser cleaning (or laser-induced desorption) appears to passivate the surface, reducing its affinity

for water. More vigorous laser treatment, i.e. raising the surface to near melting, increases the damage

threshold even further, as is understandable from the laser absorption studies, reported last year, which

showed that some water-related contaminants are so tightly bound that very high surface temperatures are required to drive them off. It is interesting to note that improvement in damage threshold is not re- stricted to regions of strong water-band absorption. Beneficial gains at 1.06 urn are significant, for

example, even though no appreciable water band exists at that wavelength.

Polishing procedures as well as laser annealing affect the surface and presumably the laser damage

threshold. New finishing techniques such as float polishing and mechanochemical polishing offer various

degrees of promise in reducing the subsurface damage resulting from conventional polishing techniques

while still producing surfaces with excellent optical figures and low surface micro-roughness levels.

Scratch standards for specifying optical surfaces are not the answer.

As regards measurements, the keys continue to be absorption and the determination of other thermally

related properties. As such, materials measurements are being reported at cryogenic temperatures. One

anticipated conclusion is that the thermal properties of crystalline materials will tend to improve as the

temperature is reduced. There is expected to be little improvement for glassy materials, however. Metals, on the other hand, will improve their optical performance at low temperatures, particularly in the

i nf rared.

In the area of damage testing, new facilities are still being reported, and damage testing is being

carried out at wavelengths as short as 193 nm. The importance of multiple-pulse testing is finally being

realized and, although the reason for the degradation in materials after being exposed to repeated laser

pulses is often obscure, some progress was reported. For example, the fact that the 1.06 urn bulk threshold of germanium at 100 Hz was only 15% of that measured at 1 Hz was attributed to the additional free

carriers present at the higher pulse rate, while laser-induced slip is suggested as the dominant mechanism

for metal mirrors used in multiple-pulse applications.

Application is the final test and the system is the final application. Of necessity, then, systems

analyses continue to be developed. A technique for obtaining a first cut of a cooled mirror design for a proposed laser train, without recourse to a large computer, has been developed, an adaptive optics mirror

design which does not require actuators was presented, and the adequacy of normal incidence tolerances for

turning flats used at large angles of incidence was verified.

As the Symposium turned to the vital area of thin film research and development, recent progress with

non-standard coating deposition techniques was emphasized. The primary goal is to produce coatings of

higher quality than available by use of the commercially-standard techniques involving electron guns and

resistive heating. In contrast to the polycrystall ine films usually produced by these standard techniques, ion-beam-deposited coatings of Ti^, Ta 2®5' as we ^ as were ^ ounc' to ^ e near ^Y amorphous and

in a compressive state of stress. At the other structural extreme, molecular-beam epitaxy of certain

classes of compounds produces single-crystal films in high vacuum. It offers the possibility of reduced

absorption of energy by electrons in low-energy states and reduced probability of electron avalanche

breakdown. However, deposition of wide-bandgap dielectrics of interest for high-power laser optics has

yet to be demonstrated by this method.
In pursuit of coatings having the very desirable diamond-like properties, hydrogenated amorphous

carbon films were produced in an rf-excited glow discharge of benzene vapors. Especially low absorption

was attained at 2.8 and 3.8 urn (15 and 25 cm , respectively) which is only a few times greater than that

of crystalline diamond. Finally, as an economical alternative to physical vapor deposition processes,

gradient-index antiref lection (AR) coatings produced with the Neutral Solution Process were shown to be

scalable to very large-diameter glass optics without any reduction in damage threshold.

Coating damage studies were concentrated in the ultraviolet this year. The average multiple-shot

2 damage thresholds for state-of-the-art multilayer reflectors are 3, 5, and 8 J/cm at 248, 308, and

351 nm, respectively, for 10-ns pulses. Using non-quarterwave thickness designs, 25% to 50% higher

thresholds are obtained. As a valuable diagnostic tool in multiple-shot damage testing, real-time video

monitoring has proven itself in observing and recording the onset and evolution of coating damage.

From a survey of the damage thresholds of single-layer and multilayer reflectors for 248 nm, several

interesting trends were observed. High-index films had lower thresholds than low-index films, thresholds

correlated inversely with extinction coefficient and uv bandedge, but no correlation with film stress was

apparent. For reflectors using a given low-index material, higher thresholds were obtained with highindex layers with minimum absorption. For a fixed high-index component, higher thresholds resulted from

use of the low-index component with minimum refractive index. The physical basis for this last result is not understood.

Coating designs using hal fwave-thick overcoats on reflectors and undercoats with AR coatings consistently have had higher damage resistance on the order of 50%. However, one study this year found these

designs to be ineffective for subnanosecond-pul se laser radiation at 351 nm. Pre-irradiation of optical components with subthreshold laser intensities was shown again to increase

the damage threshold of wide-bandgap dielectrics. For the case of AR coatings produced by the Neutral

Solution Process, the surface threshold was increased by a factor of two by this treatment.

Under the subheading of thin film diagnostics, the capability of two techniques to detect localized

absorbing film contaminants or impurity sites was demonstrated. Internal reflectance spectroscopy applied

to the problem of water contamination in ThF^ films lead to the conclusion that the origin of the water

may well be in the starting material rather than entering during or after deposition. The application of

photothermal deflection microscopy likewise identified many coating defects not revealed by Nomarski

microscopy.

Passing to fundamental damage mechanisms, this year's contributions included aspects pertinent to each of bulk, surface, and thin-film damage phenomena. Of special interest to all laser users was a review of ocular damage mechanisms which are based on either thermal, photochemical, or high-field

strength interactions. Eye damage is not necessarily a prompt reaction to laser radiation. In particular, photochemical effects may manifest long after exposure, and lenticular cataracts may be delayed by

years. The most dangerous spectrum for retinal injury is 400-500 nm. The spot-size dependence of laser damage has been a controversial issue at these symposia for many

years. This year a physical model was presented which, at least for thin-film coatings, demonstrated that

the conventional definition of damage threshold (average of the lowest laser fluence causing damage and

the highest fluence producing no damage) is inherently spot-size dependent. A new threshold definition

was proposed: the maximum fluence for which no damage was observed over a sufficient number of test sites.

That this conservative definition is independent of laser spot size was supported by abundant test results

involving optical coatings.

Although a number of experimental studies have concluded that absorbing defects are the initial sites

of damage in optical coatings, a relative ordering of damage resistance of the various coating materials

3
has been observed in numerous studies. A significant analytical result presented this year demonstrated

that the threshold scales with the thermal properties of the host coating material with insignificant

dependence on those of the impurity.

A study dealing with geometric surface defects demonstrated experimentally that linear defects oriented perpendicular to the laser electric field have increased absorption and reduced damage threshold.

This observation explains the unequal damage resistance of diffraction gratings when alternatly irradiated

with S- or P-orientations of the electric field.

A number of multiple-shot studies at this and at previous symposia have revealed the accumulation

nature of damage for some materials. Among the list of materials which have exhibited this phenomenon are

* silicon, KD P, NaCl , fused silica, Cu mirrors, and optical polymers. In each case, the number of laser

shots required to produce observable damage monotonical ly increased with decreasing laser fluence.

Finally, a new damage mechanism, unipolar arcing, was introduced to the laser-damage community.

Occurring only after generation of a plasma at a surface, pits and craters result from electrical current

arcs between the optical element and the adjacent plasma.

3.0 Summary of Papers

The subject matter of the Fourteenth Annual Boulder Damage Symposium consisted broadly of four topics: (1) Materials and Measurements, (2) Mirrors and Surfaces, (3) Thin Films and (4) Fundamental Mechanisms for Laser Damage. In this section a concise summary of each paper is given. Our intention here is only to provide the reader with an overview and critique of the Symposium and to identify the authors and

institutions which contributed to it. The interested reader is referred to the papers themselves for

further details. Each topical area is introduced with a brief commentary on the status of the underlying

problems addressed, and comments are often made about the import of the paper in our view.

3.1 Materials and Measurements

Materials are a primary concern of the Symposium, and an assessment of the status of materials applicable to a given type of laser or spectral range is a valuable contribution to both the Symposium and the

field.

R. Harris , D. Dempsey , D. O'Quinn , and M. Gangl of the University of Dayton Research Center and

W. Knecht of the Materials Laboratory at Wright-Patterson Air Force Base presented measurements of the

summed bulk and surface absorption at 1.3 urn and the thermo-optic coefficient, dn/dT, in a variety of

crystals and glasses. Their stated goal was the assessment of window materials for the 1.315-pm iodine

laser. However, their technique for measuring absorption did not appear to give an unambiguous ranking of

the various candidate materials. In their words, "The use of larger, non-standard size samples for several of these materials resulted in larger measured absorption coefficients than obtained for the same materials measured earlier with the standard 38-mm diameter samples." Further, their reported absorptions

-4 -3 -1

(generally 10 to 10 cm ) were comparable to, or slightly greater than, the expected surface absorptions which were included in the measurements, so the data may not be truly representative of the various

materials. Some values of dn/dT given by these authors were, within experimental uncertainty, equal to values quoted by the manufacturers, but a general comparison of their results with previous measurements

was not provided.

Other thermal properties of materials are also of great interest in connection with laser effects.

The origin of these properties can often be best understood by low temperature determinations.

K. McCarthy and H. Sample of Tufts University measured the thermal conductivity of a fluorozirconate glass

(ZBLAN, 57.7 percent ZrF., 1.14 percent BaF„, 5.8 percent LaF,, 4.0 percent A1F V and 20.1 percent NaF)

4
and of TeCL crystals over the temperature range from 1.5 to 100 K. The thermal conductivity of the glass

~3 1 "1

(10 W dm °K at 4°K) increased with increasing temperature, a behavior that is typical of other

glasses. The conductivity of the Te02 crystals reached a maximum, 10 W cm

lo

K

^ at 5°K, and then decreased with increasing temperature. For both materials, the estimated conductivity at 300°K was approximately 10 W cm K . The authors pointed out that low- temperature operation may be more useful in

crystals, where thermal conductivities tend to increase with decreasing temperature, than in glasses where

conductivities usually decrease with decreasing temperature.

One important crystalline infrared material is germanium and R. Wood , S. Sharma , and P . Wa i te of the

GEC Research Laboratories in Wembley, England reviewed its optical characteristics. At 10.6 um, absorption in the bulk of germanium is due to free-carrier absorption. Simple theories were presented which

adequately describe the variation in free-carrier concentration with temperature and the absorption coefficient as a function of both temperature and impurity level. Minimum absorption at 10.6 um was obtained

by using n-type doping to produce a resistivity of 9 Q-cm. The authors measured the absorption in crystals of varied length. The data indicated that surface absorption contained a contribution whose magnitude (0.05 - 0.7 percent) varied with sample preparation, and a major contribution (up to 0.7 percent) was believed to be caused by the presence of hygroscopic germanium oxide surface layers. Surfaces polished

with diamond on a tin lap, and tested in vacuum, had the least absorption.

In a companion paper, the same authors reported laser-damage thresholds of a germanium crystal irradiated with pulses from a TEA CO2 laser operating at pulse repetition rates of 1, 10, 50, and 100 Hz. Thresholds decreased monotonical ly with pulse rate; at 100 Hz the threshold was only 15 percent of that

measured at 1 Hz. The relative decrease of threshold with repetition rate was correctly modeled by as- suming that damage resulted from free-carrier absorption which increased as the sample temperature increased. Therefore, thresholds were lower at high repetition rates at which the sample did not cool

sufficiently during the interpulse interval.

Laser damage thresholds in silicates are of continuing interest and L. Merkle , M. Bass , and R. Swimm

of the University of Southern California focused 21-ns, 1064-nm pulses and 15-ns, 532-nm pulses into the

bulk of crystalline quartz and fused silica and measured the number of shots required to produce damage as

a function of pulse intensity. The diameters of the focal spots were about 14 urn at 1064 nm and 7 um at 532 nm. For each shot they measured the absorption of the laser pulse and the relative scattering by the

focal volume. For the pulse which caused a detectable change in scattering, they recorded the waveforms

of the incident and transmitted laser pulse. For all samples, damage could be produced by irradiating the

sample with many pulses at intensities ranging from 55 to 90 percent of the single-shot threshold suggesting that multiple-shot damage was cumulative. However, they were unable to detect any change in scattering or absorption prior to damage. Further, the fluence levels in the tiny focal volumes, up to

3 2 10 J/cm , were so large relative to operating levels in lasers that it is difficult to use their results

to predict lifetimes for optical elements.

Another class of compounds of continuing interest are the alkali halides. F. Faizullov and

V. Kovalev of the Lebedev Physical Institute and J. Janszky and R. Voszka from the Laboratory on Crystal

Physics in Budapest used 150-ns, 10.6-um pulses to test ultrapure crystals of KC1 and NaCl that had been

doped with either calcium or lead. The most surprising result was that the bulk damage threshold of KC1

2 2 "8 - 3 increased from 0.3 GW/cm to 1.3 GW/cm as the calcium dopant level was increased from 10 to 10 mol/

mol. The authors believe that the insertion of divalent calcium into a potassium site forms a cation

vacancy which tends to trap excess potassium and reduce the probability of formation of colloidal clusters of potassium.

5
For many years, a group at the Center for Laser Studies at the University of Southern California

under the direction of S. Allen has been studying laser-assisted coating deposition. Additionally, sur face effects have been under study for a comparable length of time at Michelson Laboratory, Naval Weapons

Center. It is thus not suprising that these combined efforts would lead to the related field of laserinduced desorption (LID). This year S. Allen, together with J. Porteus , W. Faith , and J. Franck of the

Michelson Laboratory, evaluated this technique for its use in the analysis of optical surfaces for contaminants and defects. Principal emphasis was given to a study of water and other contaminants on normally

transparent substrates. Detection and identification was made by use of an ionizer and a standard quadrupole mass analyzer. Spot sizes ranged from 120 to 380 urn. A variation in contaminant concentration of

several orders of magnitude in some cases was noted when various regions of a sample were scanned. This

localized variation was attributed to defects. It is gratifying to note that there was a strong correlation between sites affording higher levels of desorbed material and sites having a low damage threshold.

One last comment worth mentioning is that laser-cleaned surfaces evidenced little affinity for water

indicating a potential passivation of the surface.

Absorption is often (but not always) related to reduced laser damage thresholds, and this year

M. Itoh of the Institute of Chemical and Physical Research in Saitama and I. Ogura of the University of

Tokyo described a somewhat novel i nterferometric technique for measuring absorption. The samples were

placed in one arm of an interferometer and heated by an intense beam propagating through the sample along

a path at right angles to the i nterferometric probe beam. Fringe patterns recorded at intervals during

heating indicated the local temperature distribution in the sample. The surface absorption and bulk

absorption coefficient were computed by comparing solutions to the heat flow equation with the observed

fringe shifts. For KC1 , the measured surface absorption and bulk absorption coefficients at 10.6 urn were,

-3 -3 -1

respectively, 1.1 x 10 and 1.0 x 10 cm , values that are in reasonable agreement with data reported

in previous symposia. However, Itoh and Ogura reported a very large bulk absorption at 488 nm in quartz,

-2 -1

9.5 x 10 cm . The magnitude of this absorption could be peculiar to their quartz sample (the exact

type of quartz was unspecified), but it might also indicate a systematic difficulty in the absolute calibration of their instrument.

Extrinsic absorption of CVD ZnSe at 10.6 urn was the subject of research by K. Lewis of the Royal

Signals and Radar Establishment, Malver, United Kingdom, and G. Arthur of the Atomic Weapons Research

Establishment, Aldermaston, United Kingdom. Surface absorption, measured by attenuated total reflectivity

(ATR), was dominated by the presence of water and hydrocarbons. The water thickness was estimated to be

100 A\ Cleaning of surfaces with trichloroethane and acetone reduced surface absorption by an order of

magnitude. In stoichiometric ZnSe, the absorption coefficient at frequencies near 10.6 urn was about

0.003 cm a value not corrected for surface absorption. In ZnSe grown in a Zn-rich environment, the

absorption coefficients ranged from 0.007 to 0.107 cm

1

. This increased absorption in Zn-rich material

was attributed to free-carrier absorption thought to be caused by the presence of impurities which form

acceptors (Cu or Na substituted in Zn sites) and donors (possibly In in Zn sites). Annealing in an inert

gas environment effectively reduced this absorption by several orders of magnitude, possibly by impurity

diffusion from grain boundary sites.

A new laser damage facility assembled at the Loughborough University of Technology was described by

S. Wiseall and D. Emmony . Some preliminary results at 193 nm and 249 nm were also reported. Unfortunately, in these initial measurements the authors varied the intensity by moving the sample beyond the

focus. As B. Newnam pointed out in the question period, possible spurious effects of air breakdown at focus and the possible influence of diffraction effects caused by beam truncation earlier in the optical

train make the beam profile at the damage site quite uncertain. The spot size also varies, but as the

6
authors correctly pointed out, the spot-size dimensions (2-5 mm) were large enough so that little spotsize dependence would be anticipated. Measurements were reported at 193 nm on several alkali and alkaline-earth fluorides, and a decrease in damage level with increasing refractive index was observed as expected. Multilayers of unknown design or composition were tested at 249 nm on one pyrex and several

fused quartz substrates. Those films not designed for exposure to a fluorine atmosphere exhibited when

exposed, low damage thresholds. No comparison was made to other results in this wavelength range reported

in the literature, possibly because so little was known about the present samples. One should not draw

firm conclusions from this work about the damage thresholds of uv multilayers until more carefully con- trolled and documented experiments are performed. The facility at Loughborough presents the opportunity

for performing further research and is particularly welcome for tests at 193 nm. In a display of the validity of the principles of physical optics, T. Wiggins , T. Saito , and

J. Hosken of the Air Force Academy placed hole gratings ahead of the focusing lens in their damage experiment to obtain an array of small focal spots whose relative amplitudes and sizes were predictable by

Fourier analysis. In examples of multiple-site damage induced on diamond-turned metal surfaces by pulses

from a Nd: glass laser, the severity of damage was well correlated with the relative amplitudes in the

array of foci

It has become increasingly evident that as far as laser inertial confinement fusion is concerned,

short wavelengths are the way to go. There are two approaches, direct production through short wavelength

lasers or by frequency multiplication of longer wavelength systems. The latter approach allows one to employ presently-available, well-engineered, high-intensity, solid-state glass laser systems. The principle engineering problem is the production of large-aperture, efficient, frequency conversion assemblies

which are damage resistant. To this end, R. Bossert , 5. Jacobs , and L. Lund , of the Laboratory for Laser

Energetics at the University of Rochester have reported on the design, assembly, and operation of a 6-cm

aperture monolithic cell for up-converting 1.06 urn radiation to a 0.351 urn output. The cell employs an efficient polarization mismatch scheme of KDP Type II crystals useful at high energies. Specifics of the

tonstruction are given together which details on the utility of various damage-resistant, constant index

matching fluids. (Koolase was the final choice).

3.2 Mirrors and Surfaces

Surfaces are a key concern in any large, multi-element laser optical train. Techniques that improve

surface damage threshold, by reducing surface absorption, scattered light, or wavefront distortion (making

it possible to focus the laser beam to a high intensity spot), are of interest to the entire laser community. For many years now, we have been waiting in anticipation concerning the potential of laser polishing

or laser annealing of optical surfaces as a means of raising their damage threshold. In the continuing

saga of this developing story, P. Temple , S. Seitel , and D. Cate of the Michelson Laboratory, Naval Weapons Center, discussed the recent progress in the CO,, laser polishing of fused silica. While previous

emphasis was on assessing the pulsed 1.06 urn damage resistance of such heated surfaces, recent efforts

concentrated on determining the rate of sublimation, surface removal vs. surface modification, desorption

and, most importantly, surface figure modification. This work, together with that reported on LID, suggests that the most important problems may be surface preparation for coating and protection against

environmental degradation. The authors stated that very little material was removed during the surface

treatment process.

Another technique for producing very high quality surfaces with minimum surface distortion may be

float polishing. Y. Namba , recently at the University of New Mexico, discussed a new float polishing

7
machine developed in Japan to produce very flat optical components with ultra-low surface microroughness.

Optical figures as good as a twentieth of a visible wavelength peak-to-peak on small samples were re- ported. The surface microroughness was measured to be 2 A rms and subsurface damage was minimal. Unfortunately, the polishing mechanism is not well understood.

Another paper dealing with fused silica surfaces and the effect of various surface polishing techniques on laser-damage sensitivity was given by H. Vora of the Honeywell Corporate Technology Center with

W. Lowdermilk and J . Swa

n of the Lawrence Livermore National Laboratory. Samples were polished mechanochemically using SrCO^, a MgO and NiO slurry, colloidal silica, as well as sapphire. The latter was thought to introduce scratches and subsurface microfractures. All such polished samples were tested using

2 1-nsec, 1.06 urn laser pulses. Damage levels ranged between 9 and 17 J/cm . Unfortunately, no measurements of surface roughness were given or laser spot size mentioned. It was concluded that subsurface

microfracture is not by itself an important mechanism in lowering the pulsed laser-induced damage threshold, an observation previously well documented.

Conventional polishing techniques remain the most widely-used surface finishing technique. Such

surfaces often contain observable scratches which may reduce the laser damage threshold. M. Young of the

National Bureau of Standards at Boulder indicated that current scratch standards are not appropriate for

predicting the relative laser damage threshold of optical components. Scratch standards are not specified

in terms of width, but in terms of visual appearance, a qualitative cosmetic parameter not easily quantified. There was some correlation in measurements performed at the Bureau of Standards between near-angle

scattered intensity and scratch number, but close to half of the scratches could just as easily have been

given the next higher or lower scratch classification. Also, currently available scratch standards do not have a reliable relation to scratch widths, although the widths of most of the standard scratches, denoted

as between 10 and 80, are between 1 urn and 10 urn. The width of a #10 scratch is often expected to be

1 urn, a #60 scratch 6 urn and so on. The author concluded that, "If I were buying parts for a high power

laser, I would hate to rely on such a loose standard for avoiding catastrophic damage to my components."

Unanswered was the question, directed from the audience, as to what more pertinent laser surface specification should be used.

The single-shot ultraviolet damage thresholds at 248 nm of uv-transmitting silicate glasses, fluorides, halides, alkaline earths and KDP and its isomorphs were reported by F. Rainer , W. Lowdermilk , and

D. Milam of Lawrence Livermore National Laboratory. Pulses 20 ns in length were used. For the silicate

2 glasses, damage thresholds were 5 to 9 J/cm first appearing primarily at the rear surface. Color centers

2 formed in UVFK54 glass at less than 2 J/cm . Improvements in rear-surface damage threshold for these

glasses are probably possible but were not realized. In LiF, the bulk and rear-surface damage thresholds

2 were nearly the same, about 10 J/cm or more. Thresholds for CaF 9 were similar, while those for MgF~ and

2 *

^ NaF were in the 17 to 18 J/cm region. KDP, KD P, and ADP had entrance-surface thresholds in the 3 to

2

9 J/cm region. Nonlinear absorption produced transmission losses as great as 30 to 50 percent in these

latter materials, so rear-surface damage did not occur. These results suggest that the 248 nm, two-photon

absorption coefficients are larger than published values at 266 nm by a factor of 2 to 10. The effect of ambient electronegative gases on damage threshold was investigated by J. Kardach of the

Air Force Institute of Technology and A. Stewart and A. Guenther of the Air Force Weapons Laboratory. A

2 1.06 urn laser having a pulse length of 5 ns and a focused spot size of 176 urn (1/e radius) was used for

- 6 these experiments. Gas compositions of nitrogen, Freon 14 (CF^), and SFg at pressures from 5 x 10 torr

to 2 atmospheres were used. Half-wave thick films of Z^, various sol gel coatings, diamond-turned

copper, uncoated fused silica, ARG-2 glass, and porous surface layers of ARG-2 glass were tested. No detectable variations in threshold damage with ambient gas type or its pressure were found. These results

8
suggest that damage in the cases tested occurred just beneath the surface of the sample where damage initiation is not affected by the sample surroundings. Eron'ka, et. al., previously observed a variation in damage threshold, defined as spark generation, with changes in ambient gas. This result is not in disagreement with present results since "damage threshold" is defined differently. Spark thresholds are generally higher than "first detectable permanent damage" and would be sensitive to electron rate- controlling factors present in avalanches.

Laser damage to metals at cryogenic temperatures was reported by D. Decker , J. Franck , W. Faith , and

J. Porteus of the Naval Weapons Center. Measurements were made at 139°K and 276°K on diamond-turned gold

samples and at 133°K and 300°K on diamond-turned copper samples. The increase in observed damage threshold at cryogenic temperature was about 1.3 to 1.5X and is in good agreement with that calculated from the

Drude theory at a wavelength of 2.7 pm. The onset of selective damage scaled in the same ratio, suggesting that localized thermal defects such as those caused by a crack, void or grain boundary, rather

than absorbing particles, may be the origin of localized defect damage.

N. Koumvakalis , C. Lee , and M. Bass of the Center for Laser Studies, University of Southern California investigated the relationship of single- and multiple-pulse damage thresholds in diamond-turned

copper and silver-coated mirrors as a function of wavelength and spot size. They concluded that the

single-pulse damage threshold can be explained as defect initiated and will be independent of spot size

above a critical value which depends on defect density. Multiple pulse damage was found to occur at lower

irradiance levels than the single-shot threshold but approached the single-shot threshold as the spot size

increased. The authors suggested that the multiple-shot threshold is an intrinsic process related to laser-induced slip and is defect dependent only in the upper limit. This paper provides a mechanism for

multiple-shot damage which suggests that beam uniformity may be very important in establishing the damage

thresholds for the optics in multipulse systems.

J. Palmer of TRW presented a comprehensive analysis of a first-cut technique for designing a heat

exchanger mirror which does not require the use of a large computer program. First, the optical, structural and thermal properties of the mirror performance are estimated as an output. Then the reflectance

requirements on the mirror surface, the decision as to whether to use single, double, or triple coolant

channels for the mirror design, bending of the mirror surface under thermal and structural loading and

maximum defect size on the mirror surface which can be tolerated are estimated. Once this process is completed the boundaries of the mirror design are known and detailed design can begin.

In a paper presented by title only, V. Apollonov , S. Chyetkin , V. Khomich , and A. Prokhorov of the

Lebedev Physical Institute continued their discussion of porous heat exchanger mirror structures. They

suggested extending their use to adaptive optics. In their proposed design the force correcting the

surface figure is not applied through mechanical actuators as is normally the case, but rather through

control of pressure on segments of the heat exchanger. While this technique may not have use in high

band-pass situations, it could be very useful in optimizing or controlling high energy cw, or high average

power laser systems.

The objective of many laser systems is to focus radiation on a target. In addition to the decrease

in focused intensity caused by catastrophic laser-induced damage of any of the optical components, wavefront distortion will decrease the peak focal intensity. At these annual symposia, H. Bennett of the

Naval Weapons Center has frequently presented various analyses of the effects of imperfect surfaces on beam focusabi 1 ity. This year he compared the influences of random surface figure irregularities and

sphericity of otherwise flat reflecting surfaces when used at nonnormal incidence. Not surprisingly, he

found that wavefront distortion due to random irregularities is a maximum at normal incidence and de- creases monotonical ly as the cosine of the angle of incidence. However, sphericity, which introduces

9
astigmatism, produces zero wavefront distortion at normal incidence but grows with angle as sin 6 up to

55°, after which there is a diminishing effect. Additionally, sphericity causes a small focal shift

which, in most applications, can be either compensated or tolerated. Comparing the relative magnitudes of

the two figure imperfections at any angle of incidence, it was determined that the distortion introduced

by sphericity, which meets a prescribed peak-to-vall ey figure specification, will be less than that from

random figure errors.

3.3 Thin Films

As in past years more papers were presented in the general area of thin film technology than in any

other subject area, presumably because there is room for considerable improvement. There are two principal schools of thought regarding the most expeditious scientific approach to realize more damage-resistant

optical coatings. One group suggests investigation of new coating deposition techniques while the other

prefers to understand the damage mechanisms and relate them to film characteristics and the deposition

process variables— obviously, both should be pursued.

A strong candidate for a "new" deposition technique, proposed because of our present understanding of

the film structure/ damage interplay, is Molecular Beam Epitaxy. This film growth technique was the

subject of a paper by M. Rona and P. Sullivan of the Arthur D. Little Co. Emphasis was on the deposition

of single-crystal films produced in ultra high vacuum (10 torr). For the semiconductor industry, films

with rms surface roughness of <5 A (determined by crystal structure, not substrate) with <1 percent variation in thickness over a 5-cm diameter have been grown. It is expected that films grown by this technique

will exhibit reduced absorption by electrons in low energy states and reduced probability of electron

avalanche breakdown as a result of longer electron mean free paths. All that is needed is someone with

enough courage ($) to contaminate an MBE apparatus for developing hard, wide-band gap, laser coatings.

Another new deposition process which is being pursued vigorously is ion-beam sputtering. J. Sites ,

P. Gil strap , and R. Rujkorakarn of Colorado State University have been depositing layers of Si^, Ta^O^

and Ti02 using beams of 500-1200 eV argon and oxygen ions which are neutralized by an equal number of

electrons. A second beam at lower energy is used for stress modification and pre-deposition sputter

cleaning of the substrate. Films deposited in this manner are mechanically stable, nearly amorphous,

relatively dense, and extremely adherent, with a beneficial compressive internal stress. Values of absorption coefficients ranged from 2.5 to 18 cm for half-wave thick films of Si^ and Ti^, respectively.

Damage tests performed at the Air Force Weapons Laboratory gave thresholds of 15 to 40 J/cm^ for 114-|jm

radius (1/e intensity), 5-ns FWHM pulses at 1.06 pm for single-layer films and only si ightly less for a three-layer structure. This latter result is significant in that it may indicate low interlayer absorption. The only impurity that could be identified was carbon (<1%).

Hard carbon coatings are a very promising thin film development and B. Dischler , A. Bubenzer ,

P. Koidl , G. Brandt , and 0. Schirmer from the Fraunhofer-Institute for Applied Solid State Physics in West

Germany have now attained absorption coefficients as low as 15 cm ^ at 2.8 pm for hydrogenated material by

exciting an rf discharge in benzene vapors. This value is only a few times higher than that of crystalline diamond. The films were hard, chemically inert, amorphous and had a relatively high index of refraction, 1.93. The substrates used were Ge, Si, and glass. The possible dependence of short wavelength

absorption on dangling-bond density was also discussed.

The preceding three papers involved physical vapor deposition of optical coatings. However, coatings

produced by chemical processes at moderately- low temperatures are also being evaluated for large optical

components. Motivated by the high damage resistance and economy of the Neutral-Solution Process (NSP) for

borosilicate laser optics, G. Wirtenson and N. Brown of Lawrence Livermore National Laboratory and L. Cook

LO
of Schott Optical Glass, Inc., developed procedures and equipment to obtain antireflective surfaces on large-diameter glass optics up to 90 cm. Typically, the peak transmittance realized was 99.2 to 99.5

percent through two surfaces after a processing time of two to four days, depending on the wavelength of

interest. The surfaces were found to be extremely fragile so that cleaning by drag wiping should be

avoided. Further, contamination by hydrocarbon particulates, e.g., dioctyl phthalate, could increase the

surface reflectance over a period of time to the initial 4 percent level. Fortunately, simple flushing

with ethyl alcohol restored the surfaces to their initial post-process state. The high damage resistance

2 of 12 J/cm measured for small NSP-process witness pieces was also obtained on a 46-cm lens.

Extending from 248 nm to 1064 nm, the next five papers reported results of damage tests on a wide

variety of coating materials. The ultraviolet laser damage test facility now in operation at Los Alamos

National Laboratory was described in detail by S. Foltyn , L. Jolin , and B. Newnam , and a summary of damage

data for uv reflectors and crystalline materials was presented. The system employs in-line beam diagnostics and a real-time video monitoring system. Three excimer wavelengths are available, and testing is

performed at 35 pps using a nominal 10-ns pulsewidth and a spot-size diameter of 600 pm. The best

2 multiple-shot thresholds consistently obtained for multilayer reflectors have been 3 J/cm at 248 nm,

2 2

5 J/cm at 308 nm and 8 J/cm at 351 nm. Nonquarterwave designs have resulted in significantly higher

(25-50%) thresholds, particularly at the shorter wavelengths. At 248 nm, thresholds for single-crystal

2 and press-forged LiF windows were 6 and 3.5 J/cm , respectively. Of special interest was the dependence

2 2 of the threshold of KD*P crystals on the number of laser pulses: 3 J/cm after 7000 shots or 8 J/cm after

500 shots.

The objective of a coating damage survey at 248 nm by F. Rainer , W. Lowdermi 1 k , and D. Milam of

Lawrence Livermore National Laboratory and C. Carniglia , T. Hart and T. Lichtenstein of Optical Coating

Laboratory, Inc. was to determine the relationships between damage thresholds and the physical properties

of various film materials which might serve as a guide to selecting optimum coating combinations. Half- wave thick layers of 15 oxide and fluoride films were deposited by electron-gun evaporation at OCLI onto

fused silica substrates polished by the bowl-feed process. Damage thresholds for these coatings, using

2 2 20-ns pulses, ranged from 0.3 J/cm for Zr0

2 to 25 J/cm for ThF^. Not unexpectedly, high-index films had

lower thresholds than low-index films in agreement with previous studies reported at these symposia.

Further, thresholds correlated inversely with extinction coefficient k and the uv band edge, and no general relationship with film stress was apparent. Thirteen different high reflectors composed of 15 layers

plus a half-wave overcoat of the low-index component of the above materials on BK-7 substrates also were

2 tested and ranked in order of threshold. Values ranged from 0.5 J/cm for the IrO^/YF^ combination up to

8 J/cm^ for MgO/LiF. For reflectors made of a given high-index material, pairing with a film having the

lowest refractive-index realized the highest threshold. Using a given low-index film, thresholds cor- related with use of high-index film materials having the lowest absorption.

The use of ultraviolet laser drivers for inertial confinement fusion experiments has made the wavelength 351 nm especially interesting since it is produced by both the XeF excimer and frequency-tripled

Nd:glass lasers. In their damage experiments of coated optics for this wavelength, J. Abate , R. Roides ,

S. Jacobs , W. Piskorowski , and T. Chi pp of the Laboratory of Laser Energetics surveyed the influences of

substrate type, polish, and post treatment on over- and under-coated damage thresholds of optical thin

films. Materials studied were Ta^/SiO,,, Sc^/SiO,,, Al^/MgF,,, and Y^/SiO,, pairs. Films intended

for use as dichroic elements (1064 and 351 nm) were tested at the two wavelengths with pulses of 0.4 and

1 ns over large areas (5 mm in diameter). To quote the authors, "What is striking about the data ....is

2 that thresholds are consistently between 2 and 3 J/cm and that the methods that seem to markedly affect

the damage thresholds of optical coatings at 1.06 pm seem to have little or no effect at 351 nm."

11
Multiple-shot irradiation just below the single-shot damage threshold has been reported by several

laboratories to significantly raise the damage resistance of optical coatings, surfaces and the bulk

interior of crystals. As further evidence of the benefit of subthreshold conditioning, J . Swa j

, W. Lowdermilk , and D. Milam of Lawrence Livermore National Laboratory reported an impressive factor of two increase in the single-shot threshold of Schott's Neutral-Solution Processed BK-7 surfaces. Without

pre-irradiation, large-diameter optics with this gradient- i ndex anti ref 1 ecti ng surface have a mean thresh2 old of 12 J/cm for single 1-ns pulses at 1064 nm. Irradiated by 6 to 8 shots below threshold until

? laser- l nduced light emission ceased, a 25 J/cm threshold was measured on both surfaces. Further, this

factor of two improvement was obtained over the entire range of 1 to 20 ns for which the threshold increased as the square root of the pulsewidth.

In support of the upgraded eight-beam Octal glass laser fusion facility at the Centre d

1 Etudes de

Limeil, France, the damage resistance of antiref lection coatings and reflectors produced by three French

coating firms (MATRA, MTO, and CILAS) was measured. The results of these tests involving nearly 500

samples and 200 components were summarized by D. Friart , M. Novari , and F. Garaude of the Commissariat a l'Energie Atomique, B. Geenen , A. Malherbes , and J. Guerain of the Societe MATRA, J. Rome , M. Buffenoir , and P. Perbal of M. T. 0., S. A. and M. Soulie of CILAS. Using 3-ns pulses focused to a 1.8-mm diameter

2 spot size, damage thresholds ranged between 3 to 6 J/cm for coatings of ZrC^/SiG^, TiO^/S"^ and singlelayer MgF£. Coatings with ZrO^ as the high-index component damaged earlier than those using Ti^, and

large-diameter components had slightly lower thresholds than the smaller test samples. These thresholds

and trends are in essential agreement with those determined over the past decade at the several American

laser fusion laboratories.

The remaining four papers concerned with thin films involved measurements of coating imperfections,

contaminants and stability in hostile gaseous environments.

The problem of water in thorium fluoride films was addressed by G. Hansen , D. Lund , A. D'Agostino , and W. Hansen of Utah State University using internal reflectance spectroscopy. Using this nondestructive

technique, monolayer amounts of water can be identified quantitatively and their position at the film

surface or in the bulk of the film can be determined by modifying the position of the standing wave produced when the critical angle is exceeded. The ThF^ films were deposited on ZnSe or Ge substrates at the

Air Force Weapons Laboratory and some had overlayers of CeF^ or ZnSe and underlayers of Si 0 . The observed

water level varied greatly from film to film but was unaffected by the presence of a capping layer. The

lability of the water was studied by heating the films and by immersing them in water and in D2O, where

the isotope shift observed indicated how easily ion exchange occurred. The authors concluded that (1) the

water in many cases is very tightly bound (in agreement with the conclusions of J. Porteus, et. al. at last year's conference), (2) overlayers did not reduce the H^O presence in the films examined but that

some films were more porous than others, and (3) films which had little water initially or after being

heated did not reabsorb water. For the ThF^ films studied, they suggested that water was present in the

starting material before evaporation rather than entering them during or after film preparation. The

internal reflectance technique was again demonstrated to be a very powerful tool, and it is to be hoped

that these authors will use it on films in which the starting material and deposition conditions are more carefully controlled.

The effect on multilayer high-reflectance coatings of errors in thickness or index of refraction of

individual layers was revisited by R. Ruykorakarn , R. Hannum , and J. Sites of Colorado State University.

They pointed out that thickness errors affect the secondary minima more than the reflectance maximum. The reverse is true for increases in absorption coefficient. The reflectance maximum is also more sensitive

to changes in index of refraction than it is to thickness errors.

12
The environmental stability of thin, single- and multilayer-dielectric coatings was investigated by

J. Palmer of TRW. Thin film coatings of A^G^, Si, ZnS, MgF,,, CeO, IrO^, and Y^Q^ were deposited on substrates of silver-coated float glass, molybdenum, gold, copper, and aluminum. The test samples were

exposed to hostile environments of 95-percent relative humidity at 35°C, 95-percent saturated ethanol

vapor at 35°C, and 10 percent by volume HF and H,,S. Considerable effort clearly went into this survey,

but unfortunately it is difficult to extract much fundamental information from the results.

We continue to look for meaningful yet facile indicators of damage sensitivity. Nowhere is this more desirable than in the thin-film optics area. With this objective, W. Mundy and R. Hughes of Pacific Union

College and C. Carniglia of Optical Coating Laboratory, Inc., investigated the sensitivity of photothermal deflection microscopy. This microprocessor-controlled technique, although simple, provides high

spatial resolution and identification of low-level absorption areas. The system employs a chopped Ar-ion

laser (514.5 nm) as the heat source while a HeNe laser is used for the probe. The sample was scanned in

4-um steps. Interestingly, many coating defects not identified by Normaski microscopy were discovered.

The most obvious area to be investigated is a correlation of the sites so covered with their damage sensitivity. If this proves fruitful, then many more research applications will arise such as elemental analysis, calibration with absorption data, and time-dependent absorption studies.

3.4 Fundamental Mechanisms

While practical measurement of damage thresholds of materials, surfaces, and thin films is necessary

to define the present state-of-the-art, many of the advances in producing better optical components have

been the fruit of basic studies relating to fundamental damage mechanisms. This year, a wide range of

topics was addressed, ranging from ocular damage susceptibility, scaling with laser and materials properties, damage precursors and accumulation with multiple-shot irradiation, first-principles calculation of

the refractive index, and an electro-mechanical surface mechanism called unipolar arcing.

In the first laser biological effects paper to be presented at these symposia, D. Sliney of the U. S. Army Environmental Hygiene Agency, Aberdeen Proving Ground divided the various mechanisms for eye damage

from laser radiation into three categories: (1) normal -thermal , (2) photochemical, and (3) high-field

strength. Photochemical effects, in particular, may produce effects long after the event (hours or days)

and lenticular cataracts may occur years after irradiation occurs. The most dangerous wavelength region

for retinal injury is from 0.4 to 0.5 urn, but other wavelengths are also hazardous at higher fluences.

High-field damage from subnanosecond exposure, particularly multiple-pulse exposure, is not yet well

understood. Much remains to be done in all three areas of research.

The next three papers were concerned with scaling of damage susceptibility with laser parameters

(spot-size and pulsewidth) and materials properties. In a very important paper by S. Foltyn of Los Alamos

National Laboratory, a statistical theory was presented which predicts that single-shot damage thresholds

can often be interpreted to make the resultant damage threshold independent of spot size. The conventional damage threshold determination, by contrast, leads to a spot-size dependence whose interpretation

can be very confusing. The statistical procedure proposed should, in principle, make it possible to utilize well-controlled small lasers to determine damage thresholds which are equally valid when large

areas of a component are irradiated using a large device. It was assumed that the damage process is defect related, only one type of defect was considered in the theory, and cumulative effects were ignored. Data for optical coatings was presented which showed that in many cases the assumptions are justified.

The bottom line is that the intercept for threshold damage, i.e. maximum fluence at which no damage occurs

for all irradiated sites, does not change as the spot size changes. Only the slope of the survivability

curve (damage probability vs laser fluence) is affected.

13
Scaling the damage thresholds of optical coatings as a function of various material properties has

been addressed several times at these symposia. This year, M. Lange and J. Mclver of the University of

New Mexico and A. Guenther and T. Walker of the Air Force Weapons Laboratory considered the relative roles

of absorbing inclusions and the host material in determining the damage threshold. Beginning with the

exact solution of the heat diffusion equation obtained thirty years ago by Goldenberg and Tranter, an approximate analytical solution was obtained. This solution clearly reveals that the damage threshold

scales with the thermal properties of the host material as T^pCpKt, with an insignificant dependence on those of the impurity. (The factors in this formula are the melting temperature, density, specific heat,

thermal conductivity and laser pulse width.) Comparison of their analytical result with an exact numerical solution was excellent.

M. Soileau , W. Williams , E. Van Stryland , T. Boggess , and A. Smirl of North Texas State University

measured bulk thresholds in fused silica and NaCl crystals with both 0.53-pm and 1.05-pm pulses having

durations from 4 to 200 ps focused to spot-size radii ranging from 3.9 to 14.2 urn. They observed that the

threshold breakdown field increased with decreasing pulse duration in functional agreement with M. Sparks' theory for avalanche breakdown. However, measured values of E^ were smaller at 0.53 pm than at 1.05 urn, and they increased with decreasing spot size, observations which are in conflict with avalanche

theory. The authors believe the wavelength dependence could be explained by a theory invoking multiphoton-initiated avalanche breakdown, and that the spot-size scaling should be attributed to extrinsic

effects. They reported data which indicated that self- focusing was not a relevant factor.

The ripple structure often associated with laser damage of surfaces was revisited by M. Soileau and

E. Van Stryland of North Texas State University.

A model had been proposed by Soileau and Temple at the 1976 Boulder Damage Conference which related ripple

formation to the presence of defects and correctly accounted for their spacing and polarization dependence. In the present paper, the dependence of absorption on orientation of the defects was verified

experimentally both by determining the damage threshold of a grooved NaCl sample oriented in and perpendicular to the plane of polarization and by monitoring the absorption photoacoustically of a grating with

spacing comparable to the wavelength, 10.6 pm. Reduced damage threshold and increased absorption occurred

when the linear defects were aligned orthogonal to the laser field. A still unexplained anomaly is why

the ripple spacing for metals and semiconductors is the free space wavelength A whereas for wide-band gap

dielectrics it is A/n where n is the index of refraction.

The shifting emphasis from single-shot-per-site studies to multiple-shot, repetitively-pulsed laser

damage has directed attention to precursor phenomena and accumulation of microdamage to detectable levels.

In such a study, M. Becker , Y. Jhee , M. Bordelon , and R. Walser of the University of Texas discussed

irradiation of p-type silicon with 1064-nm pulses having durations of 60-80 ps. The pulses were delivered

at a rate of 13 Hz, and focused to a spot approximately 100 pm in diameter. The silicon samples were

-5

tested in vacuum (10 torr) which allowed collection of ions or electrons ejected from the surface and

comparison of charge collection with observations of damage. Damage was observed after an "incubation"

period whose duration varied with laser intensity. They believe the incubation period indicates accumu- lation of a "reversible excitation" other than temperature. Exoemission of charge, the primary topic of

the paper, was not observed unless damage (detectable by scanning electron microscopy) was induced. The

authors made no comment regarding how the proposed incubation period preceding damage might be related to exoemission of charge.

In a companion paper by the same authors, they reported that the multipulse damage threshold of

single-crystal silicon for 1.06-pm pulses had no dependence on pulse repetition frequency. A

14
support of this observation.

Multiple-shot irradiation was also the test regime of the damage studies of organic and inorganic

glasses reported by A. Manenkov , G. Matyushin , V. Nechitailo , A. Prokhorov , and A. Tsaprilov of the

Lebedev Physical Institute. They reviewed literature which indicates that multiple-shot thresholds are

less than single-shot thresholds, proposed a statistical framework for analysis of multiple-shot data, and

presented results of recent experiments in which polymers were irradiated with either 1064-nm or 694-nm

pulses. The experimental data showed that scattering from the focal volume increased steadily with re- peated shots prior to damage, but that the intensity of laser-induced uv light emission remained at a constant low level until a few shots prior to damage. For spot sizes between 0.1 and 8 mm, the threshold

measured with 200 shots was at least a factor of 10 below the single-shot threshold, but spot-size scaling

was weaker in multiple-shot than in single-shot thresholds. The authors concluded that reduced multipleshot thresholds resulted from accumulation of microdamages.

The index of refraction of a material is closely related to both its single and multiphoton damage

threshold. B. Jensen and A. Torabi of Boston University presented a derivation which makes it possible to calculate the magnitude and spectral dependence of the index of refraction from first principles even near

the band edge. The derivation was developed for compound semiconductors and depends on the Kane model of

the band structure. Its applicability to wider bandgap materials has yet to be determined.

Different mechanisms of damage are manifest at various irradiation levels. While this series of

meetings has principally been concerned with that regime around the threshold for the first observable,

non-reversible damage, one can sometimes learn about threshold damage mechanisms by studying pre-catastrophic indicators, e.g., photoconductivity. A presentation by F. Schwirzke of the Naval Postgraduate

School on unipolar arcing in metal mirrors fits into the above-threshold category. It was shown that,

after the production of a plasma resulting from the interaction of a laser and a metallic surface, there

are three principal interrelated damage mechanisms: thermal evaporation, ion sputtering, and unipolar

arcing. The first two, being principally thermo-mechanical , are well known to the laser damage community.

The third, unipolar arcing, being primarily an electrical interaction with thermal-mechanical aspects, is

not. Results of such high-level, laser-induced interactions are pits and craters resulting from microarcs between the optical element and the adjacent laser heated plasma. When the plasma is not present,

unipolar arc craters are not observed.

4. Recommendations

One of the clearest trends at the Fourteenth Annual Boulder Damage Symposium was the general realization that laser radiation in an actual system is going to strike a component surface more than once and

that we should therefore understand the effects of multiple irradiations on component performance. Unexpected problems arise when a component is subjected to hundreds, thousands or millions of laser pulses and

the physical processes behind these phenomena, although much clearer than previously, are still not well

enough understood. More work on multipulse or multiexposure damage testing is therefore clearly desirable. Coupled with and closely related to the multipulse requirement is the move to study shorter wavelength laser effects. Many visible and ultraviolet lasers must be used in a pulsed mode. They bring in other new problems as well. In the infrared one is concerned with multiphonon effects and thermal heating

of surfaces, coatings and coating defects. At shorter wavelengths an entirely different set of concerns

becomes important. Electronic transitions suddenly become of primary importance and multiphoton effects

must be understood. In addition, the light quantum now has enough energy that a range of effects in defects, impurities, etc., now becomes possible and photochemical as well as photothermal effects must be

15
considered. Surface desorption or adsorption of contaminants must be considered, surface states must be

determined in much finer detail and surface preparation processes need to be developed and perfected. The

entire question of disturbed surface layers and surface microroughness takes on added importance at shorter optical wavelengths and more interactions with solid state and semiconductor physicists, who have been

worrying the surface-interaction-with-electron problem for years, is recommended.

There is a question as to whether we can continue to tolerate the inadequate surface standards with

which optics has been plagued for years. As we go to visible and ultraviolet wavelengths the absorption

and scattering from surfaces is cursed by factors which are entirely unrelated to the "scratch-dig" sur- face standards by which optical surfaces are traditionally specified. The problem clearly is becoming

acute and the ASTM, which was instrumental in initially starting this conference, together with the Optical Society of America and other technical organizations, such as the Quantum Electronics and Applications

Section of the IEEE, are the logical organizations to resolve this situation.

To progress toward more perfect optical coatings, continued, thorough evaluation and optimization of

novel deposition techniques is encouraged. Especially those capable of producing totally amorphous or single-crystal films should be compared with those resulting in polycrystal 1 ine structure. Molecular-beam

epitaxy of wide-bandgap materials should be demonstrated, and ion beams should be exploited to determine

if dense, water-free coatings are feasible.

To facilitate optimization of the coating deposition parameters in the pursuit of ultra-low loss

multilayer reflectors, high-precision measurement techniques with four and five-decimal place resolution

need to be utilized on a routine basis. Finally, finely-resolved spatial maps of coating absorption

should be compared with similar maps of damage susceptibility.

A welcome trend is the realization that lasers and laser optical trains may not always operate at room temperature, and low temperature or cyclic performance in particular must be investigated. The

sometimes slow but consistent progress in better understanding the engineering details of real systems

should be encouraged. This Symposium is not the place to discuss detailed hardware designs but it is very

appropriate to consider the general principles on which good hardware designs are based. Future papers in

such areas are encouraged.

5. Acknowledgments

The editors would like to acknowledge the invaluable assistance of Mr. Aaron A. Sanders 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. Particular thanks to Ms. Susie Rivera for her part in the preparation and publication of the Proceedings and to Ms. Pat Whited of

the Air Force Weapons Laboratory for conference coordination. We are also grateful to Commander Theodore

Saito of the Air Force Academy and Dr. Kenneth Jungling of the University of New Mexico for serving as rapporteurs for the poster sessions.

6. References

[1] Glass, A. J.; Guenther, A. H. , eds. , Damage in laser glass, ASTM Spec. Tech. Publ. 469 (ASTM, Philadelphia, PA, 1969).

[2] Glass, A. J; Guenther, A. H. , eds., Damage in laser materials, Nat. Bur. Stand. (U.S.) Spec. Publ.

341 (1970).

16
[3] Bloembergen, N. , Fundamentals of damage in laser glass, National Materials Advisory Board Publ.

NMAB-271 (National Academy of Sciences, Washington, D.C., 1970).

[4) Bloembergen, N. , High power infrared laser windows, National Materials Advisory Board Publ. NMAB-356

(National Academy of Sciences, Washington, D.C., 1971).

[5] Glass, A. J.; Guenther, A. H. , eds. , Laser induced damage of optical materials: 1972, Nat. Bur. Stand. (U.S.) Spec. Publ. 372 (1972).

[6] Glass, A. J.; Guenther, A. H. , eds., Laser induced damage in optical materials: 1973, Nat. Bur. Stand. (U.S.) Spec. Publ. 387 (1973).

[7] Glass, A. J.; Guenther, A. H. , eds., Laser induced damage in optical materials, 1973: a conference

report, Appl. Opt. 13, 74 (1974).

[8] Glass, A. J.; Guenther, A. H. , eds., Laser induced damage in optical materials: 1974, Nat. Bur. Stand. (U.S.) Spec. Publ. 414 (1974).

[9] Glass, A. J.; Guenther, A. H. , eds., Laser induced damage in optical materials: 6th ASTM symposium,

Appl. Opt. 14, 698 (1975).

[10] Glass, A. J.; Guenther, A. H. , eds., Laser induced damage in optical materials: 1975, Nat. Bur. Stand. (U.S.) Spec. Publ. 435 (1975).

[11] Glass, A. J.; Guenther, A. H. , eds., Laser induced damage in optical materials: 7th ASTM symposium,

Appl. Opt. 15, 1510 (1976).

[12] Glass, A. J.; Guenther, A. H. , eds., Laser induced damage in optical materials: 1976, Nat. Bur. Stand. (U.S.) Spec. Publ. 462 (1976).

[13] Glass, A. J.; Guenther, A. H. , eds., Laser induced damage in optical materials: 8th ASTM symposium,

Appl. Opt. 16, 1214 (1977).

[14] Glass, A. J.; Guenther, A. H. , eds., Laser induced damage in optical materials: 1977, Nat. Bur. Stand. (U.S.) Spec. Publ. 509 (1977).

[15] Glass, A. J.; Guenther, A. H. , eds., Laser induced damage in optical materials: 9th ASTM symposium,

Appl. Opt. 17, 2386 (1978).

[16] Glass, A. J.; Guenther, A. H. , eds., Laser induced damage in optical materials: 1978, Nat. Bur. Stand. (U.S.) Spec. Publ. 541 (1978).

[17] Glass, A. J.; Guenther, A. H. , eds., Laser induced damage in optical materials; 10th ASTM symposium,

Appl. Opt. 18, 2112 (1979).
[18] Bennett, H. E. ; Glass, A. J.; Guenther, A. H. ; Newnam, B. E. , eds. , Laser induced damage in optical

materials: 1979, Nat. Bur. Stand. (U.S.) Spec. Publ. 568 (1979).

[19] Bennett, H. E. ; Glass, A. J.; Guenther, A. H. ; Newnam, B. E. , eds., Laser induced damage in optical

materials: 11th ASTM symposium, Appl. Opt. 19, 2375 (1980).

[20] Bennett, H. E. ; Glass, A. J.; Guenther, A. H. ; Newnam, B. E. , eds., Laser induced damage in optical

materials: 1980, Nat. Bur. Stand. (U.S.) Spec. Publ. 620 (1981).

[21] Bennett, H. E. ; Glass, A. J.; Guenther, A. H. ; Newnam, B. E. , eds., Laser induced damage in optical

materials: 12th ASTM symposium, Appl. Opt. 29, 3003 (1981).

[22] Bennett, H. E. ; Guenther, A. H. ; Milam, D. ; Newnam, B. E. , eds., Laser induced damage in optical

materials: 1981, Nat. Bur. Stand. (U.S.) Spec. Publ. 638 (1983).

[23] Bennett, H. E. ; Guenther, A. H. ; Milam, D. ; Newnam, B. E. , eds., Laser induced damage in optical

materials: 13th ASTM symposium, Appl. Opt. 22,3276 (1983).

18
1.3 ym Laser Window Materials Survey Continued

R. J. Harris, D. V. Dempsey, D. B. O'Quinn,

and M. E. Gangl

University of Dayton

Research Institute

Dayton, Ohio 45469

and

W. L. Knecht

Air Force Wright Aeronautical Laboratories,

Materials Laboratory

Wright-Patterson Air Force Base, Ohio 45433

This work represents a continuation of efforts to assess candidate window materials

for use in the iodine laser operating at 1.315 ym. The results presented here are effective optical absorption coefficient, 3 as measured using laser rate calorimetry

at 1.319 ym, spectral transmission from the ultraviolet through the infrared region, and

thermo-optic coefficient, 8n/8T, at four wavelengths in the visible and near infrared.

The materials reported in this paper include fused silica, sapphire, water clear ZnS,

LiF, MgF£, stabilized cubic zirconia, undoped Q-98 glass from Kigre, and Schott IRG-3,

LG-660, LG-670, LG-680, LG-750, LG-760, and LG-810 glasses.

Key Words: A^O^; infrared materials; laser calorimetry; laser glass; LiF; MgF£;

optical absorption; Si^; thermo-optic coefficient; ZnS; ZK^

1 . Introduction

'

The measurements reported here were performed as a part of the continuing effort to identify

and assess candidate materials for use in the development of the iodine (I*) laser operating at 1.315 ym. This effort has been ongoing for the last three years and several materials properties were reported earlier [1,2,3]. Here we report properties of additional materials studied.

The optical absorption coefficients given here were obtained using laser rate calorimetry

[4,5] using a Quantronix 114 Nd:YAG laser modified to operate at 1.319 ym. These values are the

effective absorption coefficients, 6 calculated using the equation

(1 " R

b

} p !a ^Saijl m

eff " TT+R^T PT

" 1+n 2 I At P T u;

where P^ is the power absorbed in the sample, Py is the power transmitted, R^ is the reflection

coefficient at the exit face, n is the refractive index, m is the mass of the sample, Cp is the

heat capacity, £ is the sample thickness, and (AT/At) is the sum of the heating and cooling slopes.

The value reported is the sum of the bulk and surface absorption, whence the term "effective":

6 eff

E B

B +

( 1+x ) e

s /£ ( 2 )

where (3g is the bulk absorption coefficient and 3

g is the fractional absorption per surface. The

x is a factor between 1 and n depending on the coherence conditions of the irradiating light [6].

1 9
Since the surface absorption is included in the measurement, any remarkable condition of the surface

such as cloudiness or roughness is included in the sample descriptions given below.

Spectral transmission measurements were made using a Beckman 5270 and Perkin-Elmer 180

spectrophotometer with a combined spectral range of 0.25 to 50 urn. Transmission traces of some

of the more uncommon materials will be presented below also.

Thermo-optic coefficient measurements were made using an interferometric technique described in detail in Reference 7. The method involves observing the temperature change necessary for a one fringe change in optical path for internal interference in the sample and then calculating the

thermo-optic coefficient using the equation

8n/9T = X/22.AT - not

where X is the wavelength of the laser used for the measurement, AT is the temperature change per

fringe, a is the thermal expansion coefficient and the other quantities are as defined above. The

measurements were made in the temperature range from 25 to 85 degrees C at 0.6328, 1.15, 1.319,

and 3.39 ym.

2. Experimental Results

The results presented here are those obtained on materials studied since references 2 and 3. Spectral transmission traces are presented only for the glasses since the other materials have

essentially featureless spectra in the high transmission region. For the 3n/3T data the values of

the constants used in the calculation are presented in a separate table.

Two varieties of fused silica (SiOg) were obtained for characterization, Suprasil W-l from

Heraeus-Amersil and Vitreosil IR-1 from Thermal American Fused Quartz Company. The samples were

70 mm in diameter by 13 mm thick. These dimensions are approximately twice the standard size

sample we have been characterizing, so the calorimetric absorption cannot be directly compared

with the materials of the standard size with confidence. Their greater thermal mass made the

measurement difficult since it strained the capability of our equipment.

-3 -1 The Suprasil W-l absorption coefficient was 0.364x10 cm which is nearly twice as high as

-3 -1 that obtained in earlier measurements on smaller samples of 0.190x10 cm , but it is still lower

than other types of fused silica (see Ref. 2). It is unclear whether this difference is due to the difference in sample size or difference in batch. The spectral transmission is not significantly different from that of the smaller samples. The transmission curve is shown in Figure 1.

20
Wavelength (urn)

Figure 1. Spectral transmission of Suprasil W-l . Sample thickness is 13 mm.

The Vitreosil IR-1 showed considerable variation sample to sample for the absorption coefficient and one of the three showed very large variation as a function of location on the sample.

There was also considerable difference in the strength of an absorption band near 2.7 urn, presumably the OH band, from sample to sample and in the one odd sample in the 1.3 ym absorption.

The other two showed homogeneity to ±0.2% in transmission at 2.7 ym. The transmission spectra

for the 2.5 to 5 ym spectral region are shown in Figure 2, and Figure 3 shows a map of transmission for the odd sample depicting the severe inhomogeneity. The absorption coefficient ranged

-3 -1 from 0.4 to 0.73x10 cm . Interferometry and Schlieren photography did not show conclusive

evidence of inhomogeneity in refractive index for the odd sample.

10C

l 5 3 0 3.5 •4 0 4.5 5.0 Wavelength turn)

Figure 2. Spectral transmission of three different samples of Vitreosil

IR-1 showing strong differences in the intensity of the absorption

band at 2.7 ym. All samples are 13 mm thick.

21
Figure 3. Transmission map of Vitreosil IR-1 sample #3949 showing

approximate lines of constant transmittance. The sample was measured at 2.7 urn at various locations and showed the above

extreme inhomogeneity.

Sapphire (A^Og) samples were obtained from Crystal Systems and Union Carbide for measurements. The Crystal Systems samples were 70 mm diameter by 13 mm thick and as noted above may not

be directly comparable to other sample measurements. The absorption coefficients for the three

-3 -1 samples showed variability from 0.3 to 0.65x10 cm from sample to sample but little inhomogeneity

on a particular sample. These values are somewhat higher than earlier measurements on Crystal

Systems sapphire of 0. 211 xl 0~^cm~^ of the standard 38 mm diameter size (see Ref. 2).

The Union Carbide sapphire were standard size samples. The measured absorption coefficient

-3 -1 for them was 0.359x10 cm which is approximately 50% higher than the best Crystal Systems

material. All samples were oriented with the c-axis perpendicular to the face (0001).

Five 70 mm diameter by 7 mm thick LiF samples were received from Harshaw. The samples showed

little sample to sample variation or inhomogeneity in individual samples. The absorption

-3 -1 coefficient measured for these samples was 0.534x10 cm which is significantly higher than the

-3 -1 earlier measurement of 0.133x10 cm on standard size samples. Again, this disparity may be due

to sample size.

Harshaw also sent five 70 mm diameter by 7 mm thick MgF2 samples. The samples were randomly

oriented with only one of the five having the c-axis nearly perpendicular to the face as desired.

The orientation of this sample was about five degrees off. The measured absorption coefficient

-3 -1 for these samples was 0.710x10 cm with again little variation between samples or within a -3 -1 sample. This is again somewhat higher than earlier measurements of 0.307x10 cm on standard

size samples.

(R) A. sample of RAYTRAN ^ water clear ZnS from a recent batch was sent from Raytheon for comparison with an older sample. The spectral transmission was not significantly different from the

older material; however, the absorption coefficient was lower. The newer material had a measured

3 1 3 1 absorption coefficient of 0.608x10 cm as compared with 1.23x10 cm for the older material.

22
A sample of stabilized cubic zirconia, ZrOg with 12.0 mol% ^2®3 was received from Ceres

Corporation and polished by Adolph Meller. It was polished only to window shine specification so the surface was too uneven for transmission measurements. The measured absorption coefficient

was 0. 848x1

0" 3 cm

_1

.

Results from measurements on Kigre, Inc. Q-98 phosphate glass lightly doped with Nd were

reported in Ref. 2 and Kigre made a special melt of the glass with Al substituted for the Nd

dopant. Virtually all the absorption band features in the visible and near IR were removed, as can be seen in the comparison of the transmission spectra in Figures 4 and 5. There are three

small residual bands in the visible which appear to be due to some residual Nd contamination in the

-3 -1 melt. The measured absorption coefficient increased, however, from 0.360x10 cm for the Nd

-3 -1 doped glass to 1.36x10 cm for the Al doped glass.

Wavelength lumi

Figure 4. Spectral transmission of Nd doped Q-98 glass from

Kigre. Sample thickness is 10 mm.

LOO

1 2 3 4 5 Wavelength (uml

Figure 5. Spectral transmission of Al doped Q-98 glass from Kigre. The

small structure in the visible corresponds to the strong bands in the

Nd doped material and is probably due to residual Nd contamination in the melt. Sample thickness is 10 mm.

Several glasses were received from Schott Optical for characterization. These included IRG-3

dense lanthanum flint glass; three silicate glasses, LG-660, LG-670, and LG-680; two phosphate

glasses, LG-750 and LG-760; and one fluorophosphate glass, LG-810. The LG series glasses are laser glasses and are doped with Nd. The transmission spectrum of IRG-3 is presented in Figure 6.

23
The laser glasses show several absorption bands in the visible and near IR, but are featureless

in the neighborhood of 1.3 um. The transmission spectrum of LG-670 is shown in Figure 7 as representative of the general nature of all the LG series. The absorption coefficients of these

-3 -1 glasses fall in the range of 0.58 to 4.12x10 cm and are given in Table 1, which summarizes the

results of all the materials measured. Two of the glasses, LG-670 and LG-680, were returned to Schott after their polish by Unertle Optical for hardening. This process involves an ion exchange

of sodium for lithium in a hot salt bath and produces a clouded surface. Post hardening measurements were done on these samples and showed increased scattering at the surface and also

increased absorption. The increased absorption may be due to the surface effects. The samples

will be repolished and remeasured to more fully investigate the increase.

ioo

~ ~"

; /'

1 l 4 5 Wavelength turn!

Figure 6. Spectral transmission of Schott IRG-3 glass.

Sample thickness is 5 mm.

too

0

1 2 3 4 5 Wavelength (um)

Figure 7. Spectral transmission of Schott LG-670 laser

glass. The structure is due to Nd doping. Sample

thickness is 7 mm.

24
Table 1. Summary of Calorimetric Absorption Results at 1.3 ym

Material (Source) $ ~ 10" 3 cm

1 Results

Li F (Harshaw)

MgF

2 (Harshaw)

RAYTRAN® ZnS (Raytheon)

Zr0

2 :Y2 03 (Ceres)

A1

2 03

:(Union Carbide)

(Crystal Systems)

Fused Silica:

Suprasil W-l (Heraeus Amersil)

Vitreosil IR-1 (Thermal American Fused Quartz Co.

LG-660 (Schott)

LG-670 (Schott)

LG-680 (Schott)

LG-750 (Schott)

LG-760 (Schott)

LG-810 (Schott)

Q-98, undoped (Kigre)

IRG-3 (Schott)

0.534

0.710

0.608

0.848

0.359

0.293

0.647

0.501

0.364

0.406

0.727

0.548

0.605

0.575

2.20

0.898

2.99

0.798

0.848

0.844

1 .36

4.12

Best

Worst

Average

Best

Worst

Average

After hardening

After hardening

Measurements of 3n/3T were also made on several of the materials in the survey. The values

obtained in the past year are presented in Table 2. The value for LiF is a correction of a value

presented in Ref. 3. The values of the constants used in calculating the 3n/3T results are given in Table 3. The MgO was produced by Adolph Meller. The LiF was supplied by Harshaw. The

C0RTRAN samples are glass made by Corning and designated 9753 and 9754 by the manufacturers. The

Fused Silica is Suprasil II made by Heraeus-Amersil . The Q-98 glass is a laser glass produced

by Kigre, Inc. and was doped with aluminum instead of neodymium. Its 3n/3T value is slightly

lower than its neodymium doped analog. The IRG- and LG- series glasses are from Schott Optical,

IRG-3 is a lanthanum dense flint glass, IRG-N6 is a Ca Al silicate glass, IRG-7 is a lead silicate

glass, and the LG-660 and LG-670 are neodymium doped silicate glasses.

25
Table 2. Summary of 3n/8T Results

3n/3T t 10" 5 /°C

Material 0.6328 ym @l . 1 5 ym (91 .32 ym 03.39 ym

MgO 1 .13±0.46 1

.

10±0.35 1 .15±0 .38 1 .11±0.24

Li F -2.00±0.37 -2. 02±0.20 -2.10±0 .19

Cortran 9753 0,41±0.25 0. 48±0.25 0.39±0 .15 0.60±0.15

Cortran 9754 0.88+0.43 0. 75±0.32 0.74±0 .20 1 .17±0.44

Fused Silica 1 .00±0.24 1

.

03±0.23 —-

IRG-3 0.02±0.23 -0. 2H0.12 -0.20±0 .04 IRG-N6 0. 39±0. 14 0. 34±0. 13 0.39±0 .25

IRG-7 -0.22±0.20 -0. 29±0.12 -0.26±0 .05

Undoped Q-98 Glass 0.05±0.15 -0. 22±0.09 -0.27±0 .10 Opaque

LG-660 -0.08±0.27 -0. 16±0.34 -0.06±0 .29 LG-670 -0.08±0.16 0, 07±0.12

Table 3. Constants Used in 3n/3T Calculations

Material

r c^io'Vc n@.6328 ym n@l .15 ym n@l .312 ym n@3.39 ym

MgO 10.5 1

.

74 1 .74 1

.

74 1 .74

Li F 35.0 1 . 39 1 .39 1

.

39 1 .39

Cortran 9753 6.2 1 . 61 1 .61 1. 61 1 .61 Cortran 9754 6.2 1 . 66 1 .66 1

.

66 1 .66

Fused Silica 0.49 ) . 45 1 .45 1. 45 1.41

IRG-3 8.1 1

.

84 1 .81 1

.

81 1 .78

IRG-N6 6.3 1 , 59 1 .58 1

.

57 1 .56

IRG-7 9.6 1

.

56 1 .55 1

.

54 1 .52 Undoped Q-98 Glass 8.2 1

.

55 1 .59 1. 59 Opaque

LG-660 10.7 1

.

52 1.51 1

.

51 1 .51 LG-670 9.26 1

.

57 1 .56 1. 56 1 .56

3. Conclusions

Optical absorption at 1.3 ym, : spectral transmission, and the thermo-optic coefficient have

been measured for a variety of glass and crystalline materials for possible use with the iodine

laser. The use of larger, non-standard size samples for several of these materials resulted

in larger measured absorption coefficient than obtained for the same materials measured earlier

with the standard 38 mm diameter samples. This increase might be associated with difficulties

in fabricating larger samples, but is more likely due to difficulties in making the calorimetric

measurement on samples of large thermal mass. The most noteworthy of the large samples was the

Vitreosil IR-1 which showed considerable inhomogeneity in absorption and also in transmission.

26
The Schott LG series glasses were consistently low in absorption, but none were as good as the

Nd doped Q-98 laser glass from Kigre.

4. References

[1] Fernelius, Nils C, Dempsey, David V., Walsh, David A., O'Quinn, David B., and Knecht,

Walter L., "Survey of 1.3 ym Window Materials," Proceedings of the Twelfth Symposium on Optical Materials for High Power Lasers, Boulder, Colorado (1980).

[2] Fernelius, Nils C, Dempsey, David V., O'Quinn, David B., Gangl , Michael E., and

Knecht, Walter L., "Survey of 1.3 ym Window Materials Continued," Proceedings of the

Thirteenth Symposium on Optical Materials for High Power Lasers, Boulder, Colorado (1981).

[3] Harris, Richard J. and Gangl, Michael E., "Thermo-Optic Coefficient of 1.3 ym Laser

Window Materials," Proceedings of the Thirteenth Symposium on Optical Materials for High

Power Lasers, Boulder, Colorado (1981).

[4] Pinnow, D. A. and Rich, T. C, Appl . Opt. T_2, 984 (1973).

[5] Hass, M., Davisson, J. W., Klein, P. H., and Boyer, L. C, J. Appl. Phys. 45, 3959 (1974).

[6] Fernelius, Nils C. and Johnston, George T., "Discussion of a Theory of Analysis of Rate

Calorimetry which Includes Coating Absorption," Proceedings of the Tenth Symposium on Optical Materials for High Power Lasers, Boulder, Colorado (1978).

[7] Harris, R. J., Johnston, George T., Kepple, G. A., Krok, P. C, and Mukai, H.,

Appl. Opt. 16, 436 (1977).

27
Thermal Conductivity of a Fluorozirconate Glass

and Tellurium Dioxide Single Crystals

Kathryn A. McCarthy and H.H. Sample

Physics Department, Tufts University, Medford, MA 02155

Measurements of the thermal conductivity of a fluorozirconate glass and two tellurium dioxide crystals have been made in the 1.5 - 100 K temperature range.

The thermal conductivity of the glass increases linearly with temperature in the

lowest temperature region, "plateaus" from 8 K to 20 K, and then increases again.

These measurements are part of a continuing program to measure thermal conductivities

of materials suitable for electro-optic devices and as transmitting materials in the

visible and in the infrared, in the 1.5 to 400 K temperature range.

Keywords: Cryogenic; fluorozirconate glass; tellurium dioxide; thermal conductivity; thermo

measurements

.

Knowledge of the heat conduction rate in materials used with high power lasers is essential in

the design of such systems. Here we report measurements of the thermal conductivities of a fluorozirconate glass (ZBLAN glass, see below) and of single crystals of tellurium dioxide (Te02), in the

1.5 - 100 K temperature range. The present data are part of a continuing program to measure the

thermal properties of materials useful as infrared (IR) transmitting windows and/or for IR acoustooptic devices. The thermal conductivity measurements for these and other materials are currently

being extended to T = 400 K.

Thermal conductivity was measured via the steady state method, utilizing a cryostat described

previously [1]. Except as noted below, the absolute experimental accuracy of the measurements is

better than 5% at all temperatures.

The ZBLAN glass specimen [2] had a composition (inmol%) as follows: ZrF^ = 55.7, BaF2 = 14.4,

LaF3 = 5.8, AIF3 = 4.0, NaF =20.1. It had a density p = 4.26 g/cm3

, index of refraction nD

= 1.504,

and coefficient of linear expansion a = 174 x 10"^ °C -1

. The specimen had dimensions 9.3 mm x 4.8 mm

x 27.5 mm long and an etched surface.

The tellurium dioxide single crystals were obtained from two sources. Sample No. 1 [3] had

dimensions 3.8 mm x 3.8 mm x 26 mm, with the long dimension (heat-flow direction) parallel to the

c-axis of the crystal; the surface was optically polished and the crystal was of optical quality.

Unfortunately, this sample cracked before a complete set of measurements was accomplished. Sample

No. 2 [4] had dimensions 4.3 x 4.7 x 25 mm, cut so the heat-flow direction was along a (110) crystal

direction (i.e. perpendicular to the c-axis). Its surface was not polished.

Figure 1 shows the thermal conductivity results for the ZBLAN specimen. The error bars at the

highest temperatures are due to uncertainties in correcting for radiation heat losses [5]; otherwise

measurement errors are smaller than the size of the data points. As can be seen, the thermal con- ductivity is in the 10~3 Watt cm- l K~l range, increases with increasing temperature, and shows a "plateau" in the 8 -20 K region. These features are similar to those observed in other amorphous

systems [6-8]; for comparison we have included typical behavior for silica and borosilicate glasses

[9]. At our lowest measurement temperatures, K = Tl-4, also similar to the temperature dependence

for other glasses, at comparable temperatures.

While it is generally agreed that the existence of the plateau region in all glasses is due to an abrupt decrease in phonon mean free paths with increasing frequency, the exact cause of the

decrease has not been completely resolved [6]. The similarity of the ZBLAN glass data to borosilicate glass data above 20 K leads us to believe that the conductivities of the two materials will

also be similar at still higher temperatures. So, for example, a conductivity on the order of

10-2 W cm-1 K- 1 is to be expected for ZBLAN glass at T = 300 K.

Figure 2 shows the thermal conductivity results for the two tellurium dioxide crystals. For

these high conductivity specimens, it was necessary to use very small temperature gradients below

T = 4 K (0.1% % AT/T < 1%), resulting in rather large random measurement errors, shown by the error

bars. Above 8 K, the errors are less than the size of the data points. As mentioned above,

28
10

-2

£ 10

u

-3

Silica

glass

O O O O

o '

Borosi licate

glass

ZBLAN Glass

ZrF4

: 55.7

BaF2

: 14.4

LaF3

= 5.8

AIF3

= 4.0

NaF 20. 1

10"

10

T(K)

20 50 100

Figure 1. Thermal conductivity of a f luorozirconate (ZBLAN) glass.

29
Figure 2. Thermal conductivities of tellurium dioxide single crystals: o, Sample No. 1 (Q parallel

to c-axis); •, Sample No. 2 (Q perpendicular to c-axis). The solid line illustrates the T^ dependence of K at low temperatures.

30
Sample No. 1 cracked before a complete set of measurements could be made, which explains the paucity

of data for this specimen. The general features to be noted for the tellurium dioxide data are:

1) the T

3 dependence at low temperatures, shown by the solid line in Figure 2; 2) the high peak

values of the conductivity at about 5 K (10-15 Watt cm- l K

-

-^); 3) the rapid decrease of K with

temperature for temperatures above the peak (typical of high conductivity crystalline^insulators)

;

and 4) the lower conductivity of Sample No. 2 (Q i c-axis) compared to Sample No. 1 (Q ^ c-axis),

at higher temperatures.

3 The T dependence of K at low temperatures can be used to calculate [10] a Debye temperature

for Te02 of 6 = 250 K, higher than the value (160 K) calculated from room-temperature elastic constant

data [11]. The high peak value of K is due to the relatively low Debye temperatures (high specific

heat) coupled with low specimen impurity and defect concentrations which would reduce (and smear out)

the conductivity peak. We believe that the lower conductivity (by about 20%) of Sample No. 2 relative

to Sample No. 1 is due to the intrinsic orientation dependence of K in this anisotropic material.

However, the two crystals were from different suppliers, and the possibility that some of this difference is due to a higher impurity concentration in Specimen No. 2 cannot be ruled out. Finally, extrapolation of the Te02 data to higher temperatures leads us to estimate a conductivity of (3±1) x 10 -2 W cm~l K

- -'- at T = 300 K, in good agreement with the value quoted by the manufacturer [12], for

Sample No. 1.

It is interesting to note that while these two types of IR materials have quite different con- ductivities at low temperatures, their room temperature conductivities are nearly the same. This

fact may be important for the design of high power laser systems. If getting heat out of a window

material or optical component is a problem, then operating the component at reduced temperatures can greatly improve the situation, for components constructed of high purity crystalline materials. This

will not be the case for amorphous solids (glasses), whose conductivities generally decrease as the

temperature is lowered.

The authors wish to thank M. Drexhage for supplying the ZBLAN glass sample, and both Crystal

Technology Inc. and Itek Corporation for the loan of the TeO_ specimens.

References

[I] R.A. Kashnow and K.A. McCarthy, J. Phys. Chem. Solids 30, 813 (1969); M. Jirmanus, K.A. McCarthy,

H.H. Sample and J. Bulman (unpublished).

[2] Sample obtained from M. Drexhage, RADC/Hanscom Field, Bedford, MA.

[3] Crystal Technology Inc., 1035 East Meadow Circle, Palo Alto, CA.

[4] Sample prepared by S. Daigneault, Itek Corporation, 10 Maguire Rd. , Lexington, MA.

[5] Our measurement technique automatically corrects for radiation losses to first order, so no "correction" has been explicitly applied to the data in figure 1. The error bars represent

±25% of the calculated effect of the radiated heat on the conductivity.

[6] M.P. Zaitlin and A.C. Anderson, Phys. Rev. BL2, 4475 (1975).

[7] H. Bottger, Phys. Stat. Sol. B6_2, 9 (1974).

[8] R. Berman, Thermal Conduction in Solids (Clarendon Press, Oxford, 1976).

[9] Data tabulated in Thermophysical Properties of Matter, The Thermophysical Properties Research

Center Data Series , edited by Y.S. Touloukian, R.W. Powell, C.Y. Ho and P.G. Klemens (IFI/Plenum,

New York, 1970), Vol. 2. Silica: p. 922, curve 55; borosil icate : p. 922, curve 39.

[10] The relationship K = l/3cv£ = 2tt\^t\/15 fi^v^ (see Ref. 6) is used in conjunction with v =

(V/6Tr 2

) 1 / 3 (k9/-h) to extract the Debye temperature (9), given the T

3 dependence of the conductivity (K), the volume per atom (V) of TeO£ of 1.47 x 10~23 cm3

, and taking the phonon mean

free path (&) to be on the order of the cross-sectional dimensions of the crystal or [4.3 x 4 . 7

] 2 = 4.5 mm.

[II] N. Uchida and Y. Ohmachi, J. Appl. Phys. 40, 4692 (1969).

31
. , of 30 u« cm"

1 K"

1 onoted In the manufacturer's [31 product information

121 r^r.^rsno^n aLaU, ne 30

^ K •

32
Optical characteristics of germanium at 10 microns

R M Wood, S K Sharma and P Waite

GEC Reserach Laboratories, Hirst Research Centre

Wembley, England, HA9 7PP

A review of the parameter values and an in-depth interpretation of the relevant properties

of germanium has been made in order to produce a self-consistent theory of the factors

determining the absorption properties of germanium windows in CO2 laser systems.

Theoretically generated absorption coefficient versus temperature curves have been computed

and compared with experimental results showing that within the wavelength and temperature

range considered the simple free carrier absorption theory is quite adequate in predicting

the behaviour of the bulk absorption in doped and intrinsic germanium. Resulting from the

unequal absorption cross section of electrons and holes to 10.6 u.m radiation it can be shown

that the bulk absorption coefficient can be 'fine tuned' to provide different

characteristics, in particular increased laser induced damage threshold. It will be shown

that many of the beam handling problems found come from the surface component of absorption

rather than the bulk. The source of surface absorption often lies in the polishing

procedure with trapped polishing material and sub-surface damage. It has also been shown

that the main source of residual surface absorption is in the growth of a hygroscopic oxide

layer. The thickness of this layer has been studied by calorimetric, c.w. laser damage and

gas drying techniques and good agreement has been gained. Water absorbed in this layer may

be drawn off by placing the sample in a vacuum or by cleaning with IPA. The growth of the

oxide layer has been studied and attempts made to passivate the surface.

Keywords: C0 7 lasers, Germanium, Absorption, Calorimetry

1 Introduction

Germanium is widely used in infra-red laser systems as a window material due to its low

dispersion and its low absorption at room temperature in the wavelength range 2-12 u.m. The lower of

these two limits is defined by the bandgap of the material (figure 1). Decreasing the wavelength of

radiation incident on a germanium window from 2 u.m results in an absorption coefficient which

increases exponentially [1]. There is usually additional structure superimposed on this exponential

rise, brought about by exciton formation or the presence of defects and impurities in the crystal. At the long wavelength limit multiphonon infra-red absorption becomes the dominant process. This is a characteristic of the germanium lattice and the limit of 12-13 u.m is defined by the fundamental L0

phonon frequency associated with it. As the wavelength of incident radiation is increased beyond 12

u-m the absorption coefficient increases as before. However, due to the selection rules governing the

formation of phonons by the lattice, there is a great deal of structure associated with it [2,3].

This is shown in figure 2. Within the transparent wavelength region the residual absorption is a compound of the weak tails of the fundamental lattice and electronic absorptions, and the intrinsic

carrier absorption (figure 3). In practice one would expect to see some contribution from impurities

and defects to arise as well. In general, the absorption coefficient behaves according to the

approximate relationship shown in equation (1) below [4,5,6].

p = Aw" 2 ...(1)

p = absorption coefficient in cm -1

A = constant of proportionality

w = frequency of incident radiation

33
This expression describes the excitation of free carriers into higher states of their host band. The

constant, A, applies at a given temperature only.

2 Theory

2.1 Band structure of germanium

The first few conduction bands and the upper valence bands of germanium are reproduced in figure

1 (from reference 7)

.

The bands close to the intrinsic bandgap of germanium are shown semi -quantitatively in this

figure. The valence bands are essentially the same as those in GaAs and Si, with a 'heavy hole' band

maximising in energy at about (000). The conduction band does not reach its lowest energy at the zone centre as in the case of GaAs, but instead does so at eight locations on the zone boundary, giving the

material an indirect bandgap of 0.67 eV at 300°K [8] and a direct bandgap of 0.803 eV at 300°K [9].

At lower temperatures the indirect path involving phonon co-operation will dominate, whereas at higher temperatures the direct path will contribute. An average activation energy for excitation of

free carriers is therefore found to be 0.37 eV in the temperature range 300-1000°K [10]. The

temperature coefficient is the same for both the direct and the indirect gap and has been found to be

-4 x 10" 4 eV °K _1

.

2.2 The effect of doping germanium

Doping can be either p or n type, depending on the dopant used. For resistivities below about 50

Q-cm, n-type material has a lower bulk absorption coefficient than p-type at 10.6 ^m. This arises

because of the higher absorption cross-section of holes to this wavelength of light. Capron and Brill

[11] studied thp absorption coefficient of n and p type germanium as a function of the doping level

and their findings are presented in figure 4.

These results were examined by Bishop and Gibson [12], who treated the problem of interpretation

as follows:

The total absorption of a germanium sample can be expressed as:

AT0T = pL(T,W)+a e Ne(T,W)+a h N h (T,W)+p II (T,W)+p s (T,W)

...(2)

where

Atot = total absorption

a e

= electron absorption cross-section

«h = hole absorption cross-section

PII = ionised impurity absorption

PL = lattice absorption

Ne = concentration of electrons

Nh = concentration of holes

Ps = surface absorption

For a crystal in equilibrium we can write:

NeNh

= Ni 2 ...(3)

where parameter Ni is the intrinsic carrier concentration.

Al so

Ni = 2 _2ji£

3/2

.(Ne

N h )

3/lt .exp{-E

g /2kT} ...(4)

h

2 = Ne ) in the intrinsic case

= Nn ) and a = e(

N

eii e+N n u n ) -..(5)

where |j. e , p. n are the mobilities of holes and electrons; a is the conductivity in cm"* and e is the

value of the fundamental electronic charge in coulombs.

34
From equation (3)

:

N h

- Ni2/Ne ...(6)

substituting equation (6) into equation (2):

2 AtOT = PL+«eNet + a ^ +P II+Ps ...(7)

N e

It has been shown that for low concentrations of dopant impurity, the contribution of the

component pjj to the overall absorption coefficient, is completely independent of carrier

concentration [2], as is the contribution deriving from the sample surfaces.

Therefore differentiating equation (7) with respect to Ne

: dAT0T = ae+(ahNi 2/Ne

2 ).(-l)

dNe

dAT0T/dNe

= ae -ahNi 2/Ne

2 ...(8)

For minimum Ajqj, this differential must equate to zero.

«e

=

cthNi 2/Ne

2

Ne

= <^h 1/2

.Ni ...(9)

a e

Substituting in values for electron and hole mobilities at 300°K [10,13] and values for a e and ah

the absorption cross-sections [5,3,14], the minimum resistivity at room temperature can be calculated

and compared to the results of Capron and Brill [11].

The values chosen to fit these data were as follows:

Hh = 1850 cn^V-is" 1

He = 4000 cm2 V- 1 s" 1

<x e

= 0.15.10- 16 crrr 2

ah = 6.50»10" 16cm- 2

Ni = 2.7. 10 13 cm" 3

These give a value for the room-temperature minimum-absorption resistivity of 9 Q-cm, and a theoretical fit to the data of Capron and Brill as shown in figure 4.

The effect of doping germanium p-type is such that the absorption coefficient increases with

increasing concentration of holes in the valence band. For resistivities lower than 30 Q-cm at room-temperature, heavy absorption occurs as shown in figure 4. Germanium doped n-type exhibits a minimum in the absorption coefficient as explained mathematically above. This arises because of the

different absorption cross-sections of electrons and holes to 10.6 |j.m radiation. The electron

cross-section derives from free carrier absorption and the hole cross-section from transitions within

the valence band [15], The minimum occurs when the concentrations of electrons and holes are such

that each population has the same absorption; a eNe - afiNh.

The value of the resistivity when this criterion is fulfilled is governed by the respective

absorption cross section values and by the intrinsic carrier concentration of the material.

The effect of doping on the thermal conductivity of germanium has been investigated [16]. For doping concentrations of <10^ cm

-3 the effect on this transport property is negligible. However,

heavily doping the material with impurities such as tin can give decreases of up to 50%.

2.3 Temperature dependences

2.4 Specific heat

Studies have shown that in germanium, the specific heat rises steadily in the region of interest

(300-450°K) [17]. From these results, however, it will be noted that the specific heat value changes

by only 8% in this temperature range.

35
2.5 Absorption and transmission

At 10.6 urn, a wavelength at which germanium appears transparent, the absorption tails due to the

lattice and the bandgap, are negligible. In this region the absorption can be attributed to the free

carriers present in the crystal. As a result, the absorption characteristics of germanium are very

strongly dependent on the population density and nature of any impurities present, and the temperature

dependence of the absorption coefficient is thus linked to the temperature dependence of the free

carrier concentrations.

By using ideas discussed in Section 2.2 and expressions found in the literature [10,12],

theoretical curves for the carrier concentration variation as a function of temperature (figure 5) and

absorption coefficient variation as a function of both temperature and impurity level (figures 6,7,8) can be produced.

Figure 5 shows how carrier concentrations increase with increasing temperature in the intrinsic

regime according to equation (10):

Nj = Ne

= Nh= 1.76*10 16 *T

3 /

2 ex p{-4550.7/kT} ...(10)

where

Ni = intrinsic carrier concentration

Ne

= electron concentration

N n

= hole concentration

T = temperature in °K

k = 1.38*10- 23 J°K- 1

When germanium is doped n-type or p-type, the simple exponential behaviour in equation (10) is destroyed. The additional curves in figure 5 show the behaviour of a series of germanium samples

containing differing donor concentrations. There is a range of temperature within which the

conduction band is swamped by carriers provided by the shallow donors in the forbidden gap. This

region is called the 'exhaustion range' and usually corresponds to the range of temperature inside

which most semiconductor devices are designed to operate. Devices operating under such conditions

have the advantage of being largely insensitive to fluctuations in temperature, due to the majority

carrier concentration remaining the same. With regard to figure 6 and referring to the work of Young

[6], undoped (50 Q-cm) germanium begins to absorb appreciably at temperatures in excess of about 325°K

when thermal runaway is initiated. Very approximately, this corresponds to an absorption of 0.1 cm"*

(see figure 6). The intrinsic sample of germanium, as can be seen from figure 5 will never have a region of temperature-independent carrier concentration and as is provided by doping. If the sections

of electrons and holes were equal in germanium, one could never achieve an absorption coefficient

lower than that found for intrinsic germanium at any given temperature by doping the material. The

two absorption cross-sections are, however, unequal as stated earlier in 2.2. This state of affairs

should allow a certain amount of 'fine tuning' to be undertaken as far as absorption characteristics

are i concerned. From the paper by Capron and Brill [11] and Bishop and Gibson [12] as well as the curves in figures 6 and 7, it can be seen that by doping germanium n-type, the absorption coefficient

can be artifically lowered by depression of the hole population, with its correspondingly larger

absorption per carrier. This gives an 'ideal' sample as being that possessing a room temperature

resistivity of 9 Q-cm. However, it can be shown that for a sample doped in such a way, the onset of

thermal runaway will be close to that of the intrinsic material, ie. «325°K. If a larger margin for error is required, the material should be made more strongly n-type to increase the high temperature

limit of the exhaustion range of the sample. A sample, for instance, of 3.1 Q-cm (figure 6) has an absorption which stays below the value of 0.1 cm" 1 until ~350°K; this is 20-25°K later than the point

at which the corresponding absorption occurs in 9 Q-cm material.

3 Experimental measurements

3.1 Apparatus

A differential calorimeter system has been developed capable of being used to make measurements

of absorptions down to 10"^ with ±1% repeatability. A feature of this system is the provision of a vacuum cell in which the sample (and sample standard) can be inserted (see figure 9). Use of this

system allows measurements to be made under laboratory ambient conditions, under vacuum and under

controlled atmospheric conditions.

36
3.2 Measurements

Results of transmittance measurements are shown in figure 10 where the transmittance of an undoped germanium sample was monitored as the sample was warmed over the range 290-360°K, which agrees

with the value of 309°K taken from the computer-generated plot in figure 8. This figure was shown to be reproducible with no hysteresis.

Measurements of absorption have been made on a series of well characterised germanium substrates

under ordinary laboratory ambient conditions. These values agreed well with those previously measured

[11] (see figure 11). However, it was realised that there was an appreciable surface contribution to

be allowed for (of the order of 0.3% surface).

This was proved because of measurements made on thin (0.4 cm) and thick (1 and 2.2 cm) samples.

It should be noted that the Capron and Brill [11] measurements were made on 1 cm thick samples with no allowance for surface absorption. For this reason a correction should be made to the absorption

versus resistivity graph and this is shown in figure 11.

Further measurements made on the germanium substrates in the vacuum calorimeter proved that the

surface absorption can be modified by the surface state and the ambient atmosphere. Figure 12 shows

that for any given substrate there is an appreciable removable absorption which can be replaced by

exposure to air. The removable surface absorption was proposed as being due to both hydrocarbon and

water layers, depending on the previous handling and cleaning procedures and the ambient conditions.

The measured removable surface contributions varied from 0.05% to 0.7% per surface. There was also an apparent variable non-removable absorption of up to 0.7%. The maximum estimated surface

absorption was 0.94%.

Subsequent measurements made on the same samples [18] showed that the non-removable absorption

was directly due to the polishing processes. The original polishing was done with alumina. When the

surfaces were treated to a Syton polish the non-removable absorption was reduced to about 0.1% and

after final polishing with diamond powder on a tin lap to 0.05% per surface. The different polishing

processes altered the removable surface contributions and the best surfaces showed average removable

surface contributions of 0.15% per surface.

Estimates of the surface absorption were made by two other techniques. The first of these gave

support to the view that the absorption was due to water by the accurate weighing of a molecular seive

which had been arranged to catch water vapour removed from the surface of a heated germanium

substrate. In all, 16.9 u.m3 of water was found to have evaporated from a 25 mm dia 4 mm thick sample.

This corresponded to a Iyer of water 1.3 x 10"6 cm thick yielding an absorption contribution of 0.13%

surface based on the known absorption of water at 10.6 p.m.

The total surface absorption was also measured by irradiating a pair (2 and 4 mm thick) of

germanium substrates with a 400 W COg laser beam. The samples cracked after 14.9 s and 27.8 s exposures respectively. If it is assumed that the samples were approximately identical then it follows that the absorption that led to damage must be largely associated with the surface and a simple thermal analysis yields the result that the surface absorption was approximately 0.65% per

surface.

3.3 Discussion

In the theory developed a surface absorption, p s

, was defined but subsequently assumed to be

negl i gi bl

e

AT0T » MT > W )

This is only satisfactory in the cases where the total absorption is high compared to the total

absorption,

(a) in the case of a thick sample,

(b) in the case of a high absorbing sample (i.e. p-type or n-type with 3<p<50.

It is also possible to circumvent it by making comparative measurements on thick and thin

samples

.

In the case of most laser windows the surface contribution may not be negligible and can lead to anomolous results.

37
It can be shown that badly prepared or damaged surfaces totally negate a low bulk absorption

coefficient and greatly increase the risk of thermal runaway. It has been shown that the surface

absorptance varies both with the surface state and with the ambient atmosphere. Even apparently well

polished surfaces have been shown, by X-ray topography (see figure 13) to contain several micron thick

polycrystal 1 i ne layers [19], the legacy of the original cutting process. Most of this absorption is attributable to water vapour which is absorbed on to the sample surfaces when it is left standing in air.

The amount of the surface absorption is conditional on the surface finish. For example, an A1^0J polished surface can have a high non-easily removable absorption. This polycrystal 1 i ne layer can be

substantially removed by Syton polishing without removing the absorption. When, however, that surface

is dried (with IPA or under vacuum) a much lower absorption may be measured. The IPA treated surface

may take two or three days to recover its absorption but a vacuum dried surface will recover virtually

instantaneously on the re-admittance of the atmosphere or of water vapour. Repeated Syton polishing

gradually leads to both a lower total absorption and a lower removable water absorption. Other

surface finishing techniques, e.g. ion-beam etching, HF etching, diamond turning and diamond polishing

have been shown to give temporarily lower absorption. It is thought that the source of the absorption

is a hygroscopic germanium oxide layer. This would explain th difference i n behaviour as cleaning with

IPA or HF would remove most of the oxide layer. Work is at present being undertaken to verify this.

4 Conclusions

A review of some of the relevant properties of germanium was provided in Section 2. The various

components which go to make up the total absorption coefficient of a germanium sample were discussed,

with emphasis upon the importance of the concentration of free carriers in the transparent wavelength

range. A review, together with an in-depth interpretation was provided, of the work done by Capron

and Brill [11] and Bishop and Gibson [12] on doped germanium. From the results presented in these

publications, it can be shown that the bulk absorption coefficient can be 'fine tuned' to provide

different characteristics. This arises from the unequal absorption cross-sections of electrons and

holes to 10.6 ym radiation; holes absorb 43 times as much as electrons at this wavelength.

A germanium sample which exhibits a room temperature resistivity of about 9 ft cm has a bulk

absorption coefficient corresponding to the minimum. However, considering thermal runaway

probabilities, a greater margin of safety can be allowed by doping the material such that at room temperature the resistivity is about 3 n cm, involving a relatively small trade-off in an increased

bulk absorption coefficient.

The work of Morin and Maita [10] was used together with that of the authors mentioned earlier to provide a means of theoretically generating absorption coefficient versus temperature curves. Comparison of the resulting expressions with experimental results (figure 8) were very favourable,

showing that, within the wavelength and temperature ranges considered, the simple free-carrier

absorption theory is quite adequate in predicting the behaviour of the bulk absorption in germanium.

It has been shown in Section 3 that the surface component of absorption is more problematic, and

is consequently less tractable in solution. Work is currently being done to discover how the surface

of a germanium sample degrades with time and how to protect it from doing so. It has been shown that

the source of the surface absorption is due to water adsorbed on the sample surface and that this may

be drawn off by placing the sample in a vacuum or by cleaning with IPA or HF.

5 Acknowledgement

This work has been carried out with the support of Procurement Executive, Ministry of Defence,

sponsored by DCVD.

6 References

[1] F Urbach

The long wavelength edge of photographic sensitivity and of the electronic absorption of solids

Phys Rev 92, 1324, (1953)

[2] R J Collins and H Y Fan Infrared lattice absorption bands in germanium silicon an diamond

Phys Rev _93, (4) , 674, (1953)

38
[3] H Y Fan, W Spitzer and R J Collins

Infrared absorption in n-type germanium

Phys Rev _101, (2), 566, (1956)

[4] Semiconductors and Semimetals

Vol 3, p 409. Acad. P. N.Y. (1967)

[5] W Kaiser, R J Collins and H Y Fan Infra-red absorption in p-type germanium

Phys Rev 91, (6), 1380, (1954)

[6] P A Young

Thermal runaway in germanium laser windows

Appl Opt JLO, (3), 638, (1971)

[7] F Herman et al Quantum theory of atoms, molecules, and the solid state

Ed P 0 Lowdin (Acad. P., 1966) p 381

[8] S J Fray, F A Johnson, J E Quarrington and N Williams

Lattice bands in germanium

Proc Phys Soc _85, 153, (1965)

[9] S Zwerdling and B Lax Oscillatory magneto-absorption of the direct transition in germanium

Phys Rev J06, 51, (1957)

[10] F J Morin and J P Maita

Conductivity and Hall effect in the intrinsic range of germanium

Phys Rev 94, (6) , 1525, (1954)

[11] E D Capron and 0 L Brill

Absorption coefficient as a function of resistance for optical germanium at 10.6 u.m Appl Opt., 12, (3), 569, (1973)

[12] J Bishop and A F Gibson

Absorption coefficient of germanium at 10.6

Appl Opt 12, (11) , 2549, (1973)

[13] P P Debye and E M Conwel

1

Electrical properties of n-type germanium

Phys Rev 93, 693, (1954)

[14] A H Kahn

Theory of the infrared absorption of carriers in germanium and silicon

Phys Rev 97, (6), 1647, (1955)

[15] M Hass and B Bendow

Residual absorption in infreared materials

Appl Opt 16, (11) , 2882, (1977)

[16] A V Ioffe

Effect of various impurities on the thermal conductivity of the germanium lattice

Sov Phys Sol State, 9, (8), 1914, (1968)

[17] D Gerlich, B Abeles and R E Miller

High temperature specific heats of Ge, Si, and Ge-Si alloys

J Appl Phys, 36, (1) , 76, (1965)

[18] C J Hutchinson, C Lewis, J A Savage and A Pitt

Surface and bulk absorption in germanium at 10.6 u.m App Opt 21, (8), 1490, (1982)

[19] J Foley, S K Sharma and R M Wood

Absorption calorimetry and laser induced damage threshold measurements on germanium and zinc

sel endi e Boulder Damage Symposium, NBS Special Publication 620, 248 (1980)

39
Figure 1 : The band structure of germanium Figure 2 : Variation of the absorption coefficient

of n-type (60 n cm) germanium with

temperature and wave number

10

Absorption (cm-<)0

I. LATTICE ABSORPTION |H- TRANSPARENT REGION

Structure caused by j multiphonon dynamics:] K-Sclection rules in Photon - Phonon

interaction.

I m. FREE CARRIER

ABSORPTION

In this region free carrier absorption is dominant

obeying Urbachv rule.

Magnified Section

For Germanium

this region shows BaO

. dependance. Impurities can

I provide a basal plateau absorption!

I component. Weak tail absorptions

Structure due to exciton

formation

impurities,*"' etc

Of

0a< Q'*

dependance

Frtquency.U (5"1

)

B at (J"*

dependance B«exp(A(t)-Oa>/kT)

dependance

The three basic regions of absorption in Germanium. I- Lattice dominated, n. Transparent region in which free carrier concentration has a large part

in the determination of the absorption coefficient and IS. strong exponential

rise in absorption due to inter-band transitions. 0}g determined by the activation energy.

05

0 2

g 0 1

005

002

0 01

P-Type Germanium

20 30 40

Resistivity (a-cm)

50 60

(26) Fitted line (after Bishop and Gibson. 1973)

Measured values (after Capron and Brill, 1973

)

(25)

Figure 3 : Absorption in germanium Figure 4 : Absorption coefficient of germanium

at 10.6 pm

40
Room Temperature

Resistivity o. em"'

Electron Concentration Corresponding to

Various Doping

Levels. 10 n/cc

588 1 f 38*4 \ 2903 \ 4693 1364 2364

Holt Concentration

for Equilibrium

Conditions.

1-37 r

3-933

3-4%

3-059

2622

2-185

1-748

1-311

874

•437

0

1 /Temperature 100 134-9 169-9 204 0 230 0 274-9 305 0 344 0 3799 414-5 Temperature Deg.K.

445 5

Figure 5 : Germanium carrier concentration

versus temperature

Figure 6 : Absorption coefficient of n-type

germanium versus temperature and

dopant levels

11-956

10-758

5-598

8-398

7-198

5-995

• 4-755

3-595

2399

1-195

0

Room Temperature

Resistivity Q cm-'

100 134 0 169 0 204 5 239 0 274-5 309 5 344 5 379 5 414-5 449-5 Temperature Deg. K.

15

105

3 6

'e 315

2-7 o

t_> c

2-25 o

Q. 1-8 o

si 1-35

9

•45

0

A Measured values (after Young PA. 1971)

100 135 170 205 240 275 310 345

Temperature Deg. K.

380 415 4S0

Figure 7 : Absorption coefficient of p-type

germanium versus temperature and

dopant levels

Figure 8 : Absorption coefficient of intrinsic

germanium versus temperature

Dump

Vacuum Calorimeter Air Calorimeter

To Vacuum Pump

Figure 9 : Schematic of differential calorimeter system

41
100

r*

90

80 -

70 -

60

SO

A%

40 h

30

20

10

R = 1 5%

x

*

Room Temperature Values

Absorption 2-7%

Reflectance 15%

Transmission 95 8K>

• Measured Transmission T

x Calculated Absorption A

AK> = 100- T- 1-5

-J i i

Absorpton coefficient in cm" 1

006

005

0-04

003

002

0 01

000

Published results (After Capron & Brill. 197 J)

• • • Values of absorption coefficient as

in Fig. 8. Normalised to sample

thickness of 1cm as used by Capron

& Brill. 197 J (Surface component

included

)

Theoretical phonon absorption limit

Capron a Brill results, minus the surface absorption of 0 661b

( NB. Values normalised assuming a total surface Absorption of 0 66% per sample]*

« W163 had a surface absorption of 0-94% (Est.)

20 30 40 50

Resistivity in (fl-cm)

60 70

0 10 20 30 40 50 60 70 80 90 100 °C

Temperature

Figure 10 : Measured transmission and calculated Figure 11 : Measured absorption coefficient of

absorption versus temperature n-type germanium as a function of

resistivity

Absorption coefficient in cm-1

0030

0 025 -

0020

0015

0 010 -

0005

Atmos. 2x10"2t Atmos. 2x10"2 t Atmos.

Press. Prees. Prees.

2x10

"2t 2x10" 5t Atmos.

Press

DRV

Cos

Conditions under which

absorption was measured

Figure 12 : Behaviour of the absorption coefficient of a 4 mm thick germanium sample under various

conditions in a vacuum calorimeter

42
43
Variation of Laser Induced Damage Threshold with

Laser Pulse Repetition Frequency

R M Wood, S K Sharma and P Waite

GEC Research Laboratories, Hirst Research Centre

Wembley, England, HA9 7PP

Laser induced damage threshold studies have been made on single crystal germanium window

substrates at 10.6 u,m using a pulsed TEA laser at prf's from single shot to 100 Hz. These

measurements have been made using a system employing real time transmi ttance and surface

inspection. The reflectance and transmittance of the component under test is monitored via an energy integration network and minicomputer and the real time transmittance (using

storage scopes) and the surface morphology are monitored simultaneously using a split video

system.

These studies have shown that the LIDT of germanium drops markedly with prf, the 100 Hz value being about l/10th that of the single shot value. It has been shown that this

behaviour is consistent with the analysis of free carrier absorption in intrinsic and

n-type doped germanium which leads to thermal runaway.

Keywords: Laser damage, CO2, Lasers, prf, Germanium, Thermal runaway, TEA laser, Non-linear

absorption

1 Introduction

The laser induced damage threshold of single crystal germanium is dominted by the electrical and

thermal properties of the crystal rather than by dielectric breakdown. Since the absorption

coefficient at 10.6 u.m in germanium has been shown to be non-linear, thermal runaway is expected to occur and the LIDT should be dependent on resistivty.

Experiments have been performed using a new repetition-rate TEA laser system and the LIDT results

have been found to be prf dependent, the 100 Hz value being 15% of the single-shot value. Preliminary

theoretical calculations, using an exponential model of sample cooling with time, have shown a reasonable agreement between the normalised calculated and measured prf LIDT values.

2 Apparatus

A new pulsed laser induced damage threshold measurement apparatus based around a 100 Hz pulsed

TEA laser has been set up and both prf dependent and cumulative damage measurements have been made.

The laser is a pulsed hybrid TEA laser. This laser can be operated at upto 100 Hz. A schematicof the system is shown in figure 1. The CW section smoothes the TEA pulse and allows a clean

TEM00 pulse to be extracted. The pulse length can be tuned from 50 ns to at least 1 u,s by changing

and controlling the gas mix.

The measurement system consists of three simultaneous measurements. The surface morphology of

the sample under test is viewed directly using a sensitive video camera with a suitable zoom lens (x 12 ). This allows the spot at which the samples is placed to be both viewed and recorded. The

reflectance and transmittance of the component under test is monitored via an energy meter,

integration network and subsequently recorded in a micro-computer. The real time transmittance or reflectance can also be measured, using the same energy meters and high speed storage oscilloscopes

(the outputs of these can also be stored via a separate video recorder or even on the same screen as the surface morphology using a split screen video technique). The video system, the computer and the

oscilloscopes are all slaved to the laser and the data obtained from each laser pulse can be recorded.

After laser damage has been induced either the laser is switched off or the sample moved to a new site

and the experiment repeated.

4 U
After the experiments have been completed the records are played back and the frames containing the

laser damage is abstracted and analysed.

The use of these three measurment techniques is extremely useful when prf or cumulative studies

are being made. The reflectance and transmittance measurements are extremely sensitive ways of

detecting the onset of laser induced damage. The video recording technique allows any changes that occur in the sample surface to be recorded, and it also records the formation of the laser damage

plasma spark.

3 Measurements

Multiple pulse irradiation experiments are being made on a range of CO2 laser components and

materials. This paper summarises the results which have been made on uncoated germanium substrates

using a 100 ns pulse duration. These substrates were first cleaned and characterised and then set at the focus of a 50 mm lens. First of all the single-shot damage threshold was found by irradiating a clear area at successively higher power levels until damage occurred. This measurement was made on a number of adjacent areas. The laser was then run at 10 Hz and the power level gradually increased

until damage occurred. The power was varied about this level on adjacent areas and the number of

pulses required to cause damage was recorded. This procedure was repeated at 50 Hz and 100 Hz prf

settings. A plot of the LIDT values against the number of pulses required to cause damage is presented in figure 2.

4 Observation of damage

It will be seen from figure 2 that each repetition rate has its own multi-pulse LIDT. With

repeated irradiation at levels 10% below these levels no damage was observed to at least 10^ pulses.

These measurements were repeated on several different samples of germanium and although the specific

LIDTs varied the fractional drop with respect to the single-shot value remained approximately constant

(see table 1)

.

Inspection of the video film of the sample surface and the laser energy meter records showed that

the laser damage at the prf threshold took place over about three shots. At these levels the plasma

spark was initiated, grew and finally diminished. The transmitted beam gradually fell to a minimum.

Under continued irradiation no further damage took place. At irradiation levels above the 'prf

thresholds' the damage was more catastrophic and generally drilled through the sample. At the 'prf

threshold 1 levels the damage occurred at the peak of the power pulse whilst at higher power levels the

damage was initiated earlier in the pulse.

5 Discussion

Laser induced damage to germanium is mainly due to the limitation and the continuation of thermal

runaway. Taking the simplified case of an unfocussed beam and ignoring the contribution due to dielectric breakdown at scratches and similar surface damage sites, we find that for a normal, thin

germanium window, the energy aE0 is absorbed in a cylinder of radius r and length h (i.e. in a volume

-rcr^h), where a is the absorption coefficient (cm~l) and E0 is the total amount of incident energy.

Unless beam size is comparable to the diffusion length L, (L=2(Dt) ), there is negligible lateral

spread of the energy in the pulse time x. The peak temperature rise at the centre of this cylinder is approximately given by the expression:

DT = T - T0

= 4« Eo ... (1)

pC-nr 2 h

assuming a Gaussian beam profile.

Taking normal parameter values:

h = 4 mm, p

= 5.46 gm cm" 3

, C = 0.376 Jgrn-^K" 1

, a = 0.01 cm" 1

, E 0

= 50 mJ

then the temperature rise is 37.6°C for r = 50 \im and

0.15°C for r = 2.5 mm

This calculation has assumed a constant absorption a. Once the temperature rise is sufficient

however, non-linear absorption takes place (figure 3) leading to thermal runaway. As a preliminary

45
calculation to obtain the temporal temperature profiles as a function of incident power for a typical TEA pulse, the centre-weighted absorption coefficient was taken from figure 3 yielding the curves

shown in figure 4. This figure illustrates the dominance of thermal runaway as a mechanism for laser

induced damage in a non-linear absorber.

Extending the problem to the case of a window sample which is irradiated with a train of

identical laser pulses, the situation becomes more complex. Suppose a second pulse, P2, is indicent

before the effect of an initial pulse PI is completely dissipated by the transport mechanisms

operating in the sample. The initial conditions of the heat equation for P2 would then have to contain the 3-dimensional temperature profile created by PI. Each laser pulse therefore generates a step-like temperature-rise with a subsequent thermal -conductivity dependent decay (figure 5). For a specific prf there will be a range of peak powers and pulse energies where no damage will occur

(except from probabilistic avalanche or cumulative laser degradation effects). The lowest pulse

power/energy at which damage just occurs is termed the prf damage threshold.

An attempt has been made to quantify the prf damage thresholds as a function of prf using a simple model for the situation outlined above. Table 1 includes the calculated values for the prf

damage threshold, normalised to the single shot level. These figures were calculated using a simple

exponential cooling model with a characteristic time-constant of % = 0.07 seconds (figure 6). It can be shown that for a given prf the damage threshold should be approximately constant if all the pulses

are consistent in peak power and duration. There is only a slight variation which is dependent on specific sample parameters such as thermal diffusivity.

It is expected that all semiconductors should show approximately the same behaviour as long as thermal runaway dominates the LIDT. Materials with large band-gaps may be found to exhibit avalanche

breakdown effects before non-linear thermal effects become operative.

Table 1 LIDT versus prf

prf

Normalised prf

(Hz)

Experimental Theoretically calc.

(t = 0.07)

Single Shot 1.0 1.0

10 0.75 0.76

50 0.50 0.51

100 0.15 0.13

Acknowl edgement

This work has been carried out with the support of Procurement Executive, Ministry of Defence,

sponsored by DCVD.

6 References

[1] R M Wood, S K Sharma and P Waite

Optical characteristics of germanium at 10.6 microns

Laser Damage Symposium NBS Boulder, Col USA, 1982

46
Brewster

attenuators \

CjT^ x 2 Zoom

>

i Zoom

CW

TEA

laser

Video

recorder

Fi gure 1 : Schematic of prf laser and LIDT monitoring apparatus

4000 r

850 1000

Temperature, °K

Figure 3 : Absorption coefficient of n-type

germanium, 9 n cm, N-type

10

9

8

7

»

5

-•-> a

<v L

Single shot L.I.D.T.

o_ _ao 10 Hz limit o

_o_oo_o o- -50 Hz limit

_o_o _ _ m 100 Hz limit

1000 2000

No. of shots

3000

Figure 2 : Plot of prf LIDT vs number of shots

for germanium

10,000

8,000

x 1-3 Pulse

power

x 1-0 Pulse

power

x 0-5 Pulse

power

100 200

Time, ns Figure 4 : Temperature rise of germanium during

a TEA laser pulse

47
Limiting temperature

Number of shots

Figure 5 : Peak temperature at the centre of a sample under repeated irradiation

Single shot level

1-0,

Figure 6 : Normalised LIDT values as a function of 1/PRF for prf values of

10, 50 and 100 Hz in germanium

48
Several questions and comments were stimulated concerning the "physical processes leading to thermal

runaway in germanium. The speaker cited previous studies that determined that free-carrier concentration,

and thereby absorption, increase with temperature. Absorption leading to runaway was attributed to extrinsic defects or impurities, such as dopants, but the speaker asserted that even intrinsic absorption

can produce thermal runaway. R. Walser, however, stated that intrinsic silicon cannot exhibit runaway.

M. Soileau added that multiphoton absorption is dominant for semiconductors irradiated by short, "100 ps, laser pulses. A. Manenkov suggested that repetitively pulsed laser calorimetry would prove conclusively

whether or not temperature increases from pulse to pulse.

49
Multiple Pulse Laser-Induced Bulk Damage

in Crystalline and Fused Quartz at 1.064 and 0.532 ym

Larry D. Merkle, Michael Bass and Randall T. Swimm

University of Southern California

University Park, DRB 17 Los Angeles, CA 90089-1112

We report bulk damage measurements on single crystal quartz and two types of fused

silica under single and multiple pulse laser irradiation at 1.064 ym and 6.532 ym. Whereas

the single pulse damage threshold varies little among the materials or with wavelength, the

multiple pulse induced damage behavior depends strongly upon material and wavelength, with

damage rarely occurring within a few thousand pulses except very near threshold in some samples, but always occurring within a few hundred pulses even below 60% of threshold in others. Laser calorimetry and measurements of light scattered during irradiation give

no evidence of change prior to the pulse on which macroscopic damage occurs. However,

the distribution of the number of pulses needed to produce macroscopic damage as a function

of intensity is indicative of an accumulation process in which some minimum intensity is required to initiate the accumulation of microscopic damage.

Key words: cumulative; damage; fused silica; multiple pulse; single pulse; quartz.

1. Introduction

The subject of multiple pulse laser-induced damage to transparent optical materials at intensities too low to cause observable damage in one pulse has been less thoroughly studied than single

pulse damage, and is certainly far from being well understood, though several interesting studies

have been reported [1-10]. Not only is the subject of obvious practical importance for the prediction of the longterm survivability of laser optics, but the comparison of multiple pulse with

single pulse damage may offer significant insights into the physical processes which cause laserinduced damage.

Multiple pulse induced damage may be of at least two distinct types. In some cases, the damage

behavior indicates that the material has no "memory" of preceding laser pulses, but damages after

many pulses simply because the probability of damage on any one pulse is less than unity [1,2].

In other cases, evidence is seen for changes in the sample properties prior to macroscopic damage

indicative of the accumulation of microscopic damage leading to the catastrophic failure event [3-91.

We report here the results of a study of multiple pulse laser-induced bulk damage in singlecrystal quartz and fused silica aider 1.064 ym and 0.532 ym wavelength irradiation. Several techniques have been used to monitor parameters which can give information regarding the damage process

and the presence or absence of microscopic changes prior to the macroscopic damage event. These

include calorimetry, light scattering, observation of incident and transmitted damaging pulse waveforms, microscopy of damage sites and analysis of the variation with intensity of the number of

pulses needed to obtain macroscopic damage. The experiments will be discussed more fully in Section

2.

The results of the experiments will be reported in Section 3 and discussed in Section 4. It will be shown that the observed macroscopic failure after many pulses must arise from the accumulation of microscopic changes in the material brought about by the earlier pulses, but that these

microscopic changes are very subtle, eluding direct observation in these measurements. It will also

be shown that the accumulation mechanism appears to require some minimum incident intensity to initiate or support the growth of the microscopic changes or damages. These and other conclusions will

be discussed in Section 5.

2. Experiments

The experimental apparatus employed in this study was designed to allow the monitoring of several

parameters during each experiment and is diagrammed in figure 1. The laser is Molectron MY-34 YAG

laser with single axial mode configuration giving pulses of near-Gaussian spatial and temporal

profile at a pulse repetition frequency of 10 pps [11]. The fundamental output at 1.064 ym has a

50
pulse duration of 21 nsec FWHM, and was focused into the sample to give a focal spot radius (e ) of 13.8 pm in fused silica and 14.5 pm in crystalline quartz as measured in air and corrected for

refraction at the sample surface. With second harmonic generation 0.532 pm output was produced

having a pulse duration of 15 nsec FWHM and focal spot radius (e~ ) of 6.6 pm in fused silica and

7.0 ym in crystalline quartz. A beam splitter allowed monitoring of each pulse by a Laser Precision energy meter which was calibrated to give the incident energy in single-pulse experiments and

the average incident energy in multiple-pulse experiments. Using other beam splitters, the incident and transmitted waveforms were monitored by Hewlett-Packard 5082-4207 PIN photodiodes and a Tektronix 7844 dual -trace oscilloscope with camera.

In multiple-pulse experiments only the damaging-pulse waveforms were recorded. In this work

"damage" is defined by the appearance of a flash of light, the attenuation of the transmitted

pulse, and the production of macroscopic zone of melted and cracked material, as observed by

scatter of a coaxial He-Ne laser beam. This is the same definition of macroscopic damage widely

used in the study of single pulse damage. These changes were always found to occur on the same pulse in these experiments, and it was convenient to use the flash of light, produced by plasma

generated in the damage event and detected by a properly filtered RCA C31059 photomul tipl ier, to trigger the oscilloscope. The output from this PMT was also used to close a Uniblitz shutter to terminate the experiment after damage occurs.

Scattered pulsed laser light from a 2.5 mm diameter region of the sample centered on the focal

spot was imaged by a lens and aperture onto a Hamamatsu R632 photomultiplier filtered to receive

only the laser wavelength. The output of this PMT and of another PIN plntodiode monitoring the

incident energy were connected a LeCroy 2249SG analog to digital converter, which sent signals proportional to the incident and scattered energies to a Hewlett-Pakcard 9825A calculator. This

allowed the counting of pulses and the detection of any change in scattering efficiency of the

sample prior to macroscopic damage. It also provided a means to monitor laser stability, so that

any experiment in which the pulse to pulse variability was significantly greater than the usual

5% could be rejected.

In some experiments, a thermocouple was attached to the sample, which was suspended by threads

to minimize thermal contact with the sample chamber. In this way, a laser calorimetric measurement could be made to detect any increase in the absorption coefficient of the sample prior to macroscopic damage.

The single-crystal quartz samples used in this work were obtained from Sawyer Research Products.

The fused silica samples were of two types: Suprasil-1 and UV grade Corning 7940. The crystalline

quartz samples were oriented such that the beam propagated along the z-axis for the 1.064 pm experiments. Due to the ease of obtaining surface damage at 0.532 pm, it was necessary to turn

the samples to obtain a longer path length at this wavelength, corresponding to beam propagation

along the y-axis in quartz.

3. Experimental Results

The number of pulses needed to obtain macroscopic damage is plotted versus the fraction of

single-pulse damage threshold intensity in figures 2 and 3 for crystalline quartz and fused silica.

Figure 2 displays the results of 1.064 pm wavelength experiments, figure 3 the 0532 ym results.

The single-pulse damage thresholds in this work were defined as the average of the intensity below

which single-pulse damage was never obtained and that above which single-pulse damage was always

observed. The single-pulse damage threshold intensities for the different materials and wavelengths

are listed in table 1. Due to the width of the intensity range within which single-pulse damage

only sometimes occurred (typically 10-15% of the threshold intensity), the damage thresholds of the

three materials may be considered to be roughly equal at each wavelength. Note, however, that the

multiple-pulse damage behavior of the crystalline quartz varies systematically from that of the

fused silicas. At a given fraction of threshold intensity, the crystalline material tends to withstand more pulses before suffering macroscopic damage than do the fused silicas. Indeed, at 1.064 pm quartz requires more than 1000 pulses to damage below 90% of threshold, whereas the 1000

pulse intensity is about 80% of threshold for Suprasil-1, and about 75% for Corning 7940. Multiplepulse damage is observed at smaller fractions of threshold at 0.532 pm, but with the exception of

the solid circles in the crystalline quartz data, to be discussed below, the trend is similar. The

intensity at which 1000 pulses are required to cause macroscopic damage is about 70% of threshold

in crystalline quartz, about 60% in Suprasil-1 and about 55% in Corning 7940.

The solid circles in figure 3A represent experiments performed on sites in one portion of one crystalline quartz sample. Evidently this region damaged more easily than the majority of the

material. On the other hand, microscopy and light scattering measurements have been unable to detect any significant differences between this region and the remainder of the sample. The material

51
from which the quartz samples were cut did not include any seed material and the cause of the

weakening is uncertain. Since these data came from only one contiguous region within one sample,

they will not be considered further in this paper.

A recent study of multiple-pulse laser induced damage in al kal i-hal ides found evidence of an increase in the absorption coefficient on the several Dulses just preceding macroscopic damage, indicative of the growth of absorbing microscopic damage sites [6]. On some of the exoeriments at 1.064 ym in the present study laser calorimetric measurements were performed to search for changes

in absorption prior to macroscooic failure. No such change has been detected. This indicates that

if microdamage is accumulating on Dulses Drior to the observed damage flash, its size must be limited

as follows. Based upon the observed noise in the calorimetry measurements, it is estimated that an increased absorption on the order of 0.4 yj per pulse could be detected. If this energy were absorbed in one small spot, such as at an absorbing inclusion or microdamage from an earlier Dulse, a region of material about five to six microns in diameter would be melted. This large size only

weakly limits the size of microdamage sites which could escape detection by calorimetry. Also, the

time required for diffusion of heat from the beam path to the thermocouple , especially in fused silica, may prevent detection of changes in the absorDtion coefficient in the last few pulses prior to macroscopic damage when irradiating at a 10 Hz repetition frequency.

The monitoring of scattered laser light represents a more sensitive means of detecting microdamage accumulation and was used in nearly all of the multiple-pulse experiments. Figure 4 shows the

output of the incident and scattered light detectors, described in the previous section, for one particular experiment. This plot is representative of the data observed in all of the experiments,

in that no increase in the scattered light signal relative to the incident energy could be detected

prior to the pulse on which the macroscopic damage occurred. We estimate that a 10% increase in

scattering could have been reliably detected. Based upon the capacity of the analog to digital

converter, the approximate sensitivity of the photomultiplier tube and the losses in the optical

train, the minimum detectable amount of energy scattered into the solid angle of the scattered light

collecting lens is estimated to be about 1.6 picojoules per pulse in typical 1.064 ym experiments

and 2.2 picojoules per pulse in typical 0.532 ym. From the Rayleigh law of scattering by one small

object his would correspond, for the collection geometry and typical incident energies used, to detection limits of about

VAn = 7.7 x 10" 4 (ym)

3 (1)

in the 1.064 ym experiments and

VAn = 3.5 x 10" 4 (nm) (2)

in the 0. 532 ym experiments. Here V is the volume of the scatterer and An is the absolute difference

in index of refraction between the scatterer and the surrounding material.

The photography of the incident and scattered waveforms of damage-producing pulses provides additional information regarding the damage process. Figure 5 shows tracings of two oscillographs of

damaging pulses, both from experiments on Corning 7940 with multiple-pulse 0.532 ym irradiation.

Due to slight variations in detector alignment from day to day, the relative amplitudes are of less

significance than the pulse shapes. The rather sudden truncation of the transmitted waveform (lower

trace) in figure 5A is typical of all damage event waveform data, both single-pulse and multiplepulse induced, at both wavelengths, except for 0.532 ym multiple-pulse damage events below about 70%

of threshold intensity in fused silica. This sudden truncation generally occurs at or before the

peak intensity point of the waveform, though sometimes it occurs within a few nanoseconds after the

peak at intensities only slightly below maximum. This behavior is similar to that observed by Bass and Barrett [2], who interpreted it as evidence for electron avalanche initiated by a probabilistic

mechanism. In the fused silica materials at intensities of 0.532 ym light below 70% of threshold,

transmitted damaging-pulse waveforms like that shown in figure 5B were sometimes observed, becoming

the predominant type of event in Corning 7940 below about 60% of threshold. In these damage events

attenuation of the transmitted waveform begins in the trailing edge of the pulses and takes several

nanoseconds to decrease to minimum transmission. Such distinctly different waveform truncation

behaviors may indicate the operation of different damage mechanisms in the different intensity

regions.

Micrographs of two damaged sites in Suprasil-1 are shown in figure 6. Both were caused by

0.532 ym irradiation, the damage in figure 6a by a single pulse just at threshold intensity, that in

figure 6b by many pulses at 55 - 60% of threshold. The latter is a damage site caused by a slowattenuation damage event of the type just discussed. Its far smaller size is not surprising in view

of the fact that damage occurred late in the pulse so that little energy was available in the trailing edge to be absorbed by the damage site. Other than the size difference, there is no systematic

difference in morphology between the slow-attenuation multiple-pulse damage sites and the fast52
attenuation single-or multiple-pulse damage sites. Nearly all observed damage sites exhibit a spindle-shaped melted region which begins at or very near the focal plane and extends upstream

a distance of about 300 ym for 1.064 ym -induced damage and up to about 200 ym for 0.532 ym - induced damage. This suggests that, with the exception of a very few anomalous sites, the melted

region is controlled by the intensity distribution of the irradiating beam, not by the distribtuion of damageable defects.

4. Discussion of Results

The lack of an observable change in the absorption coefficient or the scattering efficiency pri- or to multiple-pulse induced macroscopic damage may indicate that no change in the material precedes

the observed damage event or that such material changes do occur but are too subtle to be detected

by the techniques used in this study. The statistics of the distribution of damage events with re- spect to the number of pulses needed to cause macroscopic damage can be used to distinguish between

these types of mechanisms [1,2]. If no change in the material is caused by the pulses preceding the

observed damage, that is if the material has no "memory" of preceding pulses, and if the laser is

sufficiently stable (as could be verified in these experiments using the ADC to record pulse energy

and the oscilloscope to monitor pulse shape,) then damage after many pulses must indicate that there

is a constant nonzero probability of damage on any pulse. In such a case, the distribution f(NJ of

many experiments at constant intensity as a function of the number of pulses N needed to produce

damage is expected to follow a binomial distribution [1,2]

f(N) = P

1 (1-P1

) N_1

(3)

where P, is the probability of damage on any one pulse. At any given intensity more damage events

would be expected to occur in one pulse than two, more in two pulses than in three, and so on.

The data of figures 2 and 3 do not fit this description, excluding the data points from the

"weak" region of crystalline quartz. Rather, at any given intensity below the single-pulse

threshold the probability of damage appears to be quite small for the first few pulses, then be- comes significant for larger numbers of pulses before dropping off again after many pulses. This

behavior may be seen more clearly figure 7, a plot of the ditribution of damage events in Corning

7940 irradiated by 0.532 ym pulses in one narrow intensity range. The smooth curves compare

eq. (3) with P, equal to the observed mean value of N and a Poisson ditribution of the same mean

value. The data are too sparse to allow identification with any particular distribution function,

and the Poisson ditribution is chosen only as an example of a peaked function. However, it is clear that a peaked distribution can describe the data far better than eq. (3).

Such a distribution, in which the probability of macroscopic damage is higher after several

pulses than after one pulse, strongly indicates that the early pulses modify the material in some way. Thus the distribution of observed multiple-pulse damage events suggests an accumulation

damage process even though no material changes prior to macroscopic failure have been detected.

Numerous types of accumulative changes in the material may be envisioned which could increase

the probability of damage on subsequent laser pulses. Eron'ko, et.al., have proposed that multiplepulse damage in insulators occurs by the accumulative breaking of bonds in the material to gradually

create microscopic damage sites [4,5], In their model, the average number of pulses needed to induce macroscopic damage varies as

N « exp(-Kc) (4)

where e is the RMS electric field strength of the electromagnetic wave. The N versus intensity

data for crystalline quartz and the 1.064 y data for fused silica vary too strongly with intensity to allow meaningful comparison witheq. (4), but the data for fused silica at 0.532 ym do permit

comparison. Figure 8 is a plot of log N versus the square root of the intensity for the Corning

7940 data. This should be linear if eq. (4) correctly describes the damage process. The data

clearly do not follow a straight line, nor do the Suprasil-1 data at 0.532 ym when plotted similarly.

Thus other mechanisms for the damage must be considered.

The behavior of the data in figure 3 for both types of fused silica could be fit by two straight

lines. This would be indicative of two damage mechanisms operative in different intensity ranges

which require a number of pulses to produce damage varying exponentially with intensity. However,

such an interpretation would be unphysical, in that two damage mechanisms whose probability in- creases exponentially with intensity would add in such a way that the more strongly-varying function

would dominate at higher intensities. This is exactly opposite to the behavior of the data.

The curvature of the data plots in figures 3B and 3C can more plausibly be interpreted as evidence for a damage mechanism driven by accumulative material changes which cannot occur below some minimum irradiation intensity or pulse energy. In such a case irradiation at intensities

53
approaching the minimum intensity from above would require increasing numbers of pulse to achieve

macroscopic damage. Below the minimum intensity, an infinite number of pulses would be needed.

One possible functional form for the variation of N with intensity which gives this behavior is an hyperbola. This functional form arises in the case that the rate of accumulation of microscopic

precursors to macroscopic damage is zero below some critical intensity and increases linearly

with intensity above the critical intensity. The solid curves in figures 3B and 3C represent

fits to the data by the hyperbolic forms

N ^Suprasil

= —— " 2 ' 266 <*? (I/I

t

) - 0.625

N(I) r v• = 1.867 - 3.340 (6) 'Corning v ' (I/I

t

) - 0.570

where Lis the single pulse damage threshold intensity. The critical intensities 0.6251, for

SuprasiT-1 and 0.571. for Corning-7940, are not intended to be precise, but the overall good fit

supports the suggested existence of a critical intensity for the damage mechanism operative in fused silica under 0.532 ym irradiation in this intensity regime.

Whereas the exponential intensity dependence of the bond breaking model of Eron'ko et. al.,

[4,5] arises rather generally in physical processes based upon excitation or tune! ling between

quantized states of individual atoms or small groups of atoms, the hyperbolic dependence noted

above is suggestive of cooperative phenomena. Examples of such phenomena include melting in which

a minimum temperature must be reached to initiate the phase transition, and mechanical failure in which a minimum stress must be applied to induce plastic deformation. The lack of observable

changes in absorption or scattering prior to macroscopic damage limits the size of any such melted

or otherwise damaged regions but the intensity dependence, taken together with the distribution

of damage events at any one intensity discussed earlier, gives evidence that such microscopic

damage does occur.

The data presented here show that multiple pulse induced macroscopic damage occurs after fewer

pulses at a given fraction of single pulse damage threshold intensity in fused silica than in single crystal quartz. It also occurs after fewer pulses ata given fraction of threshold intensity

under irradiation at 0.532 ym than at 1.064 ym. The difference between the materials may suggest

interaction of the light with already-existing microscopic structural defects or the generation

of absorbing point defects such as color centers, as both are more prevalent in fused silica than

in the crystalline material. The wavelength dependence is consistent with either mechanism, since

shorter wavelengths interact more strongly with particles smaller than the wavelength and various

point defects may exist with absorption bands in the visible but not in the near infrared.

5. Conclusions

We have presented a study of single and multiple pulse laser-induced bulk damage in crystalline

and fused quartz at two wavelengths of irradiation. Parameters such as the absorption coefficient,

the relative light scattering efficiency, the shapes of incident and transmitted laser pulse waveforms, and damage morphologies were studied as well as the variation with intensity of the number

of pulses needed to cause macroscopic damage.

It has been found that, whereas the single pulse damage threshold intensities for the materials

tested are very similar, fused silica suffers macroscopic damage after fewer subthreshold pulses

than are typically required to damage crystalline quartz. Also, macroscopic damage occurs in fewer

pulses under irradiation at a given fraction of threshold intensity at 0.532 ym than at 1.064 ym. These trends are in agreement with earlier studies [3,10,12].

At a given intensity of irradiation below the single pulse threshold, the probability of the occurrence of macroscopic damage is peaked about a number of pulses greater than one. This strongly

suggests that macroscopic damage occurs by means of the accumulation of laser-produced microscopic

changes in the material. In view of the above observations, possible mechanisms for this accumulation may involve the growth of small structural defects initially much smaller than the light

wavelength, or the generation of point defects which absorb green light more strongly than near

infrared.

The data presented here indicate the subtlety of the accumulation process. The lack of observable changes in the absorption coefficient or the efficiency of light scattering prior to macroscopic damage limits the size and nature of the microscopic changes which precede such damage.

54
In particular the light scattering experiment, which is the more sensitive of the two techniques,

limits the volume V of any region in which the index of refraction varies^from that of the unirradiated material by An to a value such at VAn is less than a few times 10" (ym).^ The similarity

of the damage sites created by multiple pulse irradiation to those created by a single pulse, and

the conformity of the region of melted material to the beam path and focal plane suggest that the

damage depends upon intrinsic material properties or upon rather common and homogeneously distributed defects.

The generation of macroscopic damage in these materials may be a rather complex phenomenon.

The observation of distinct types of waveform truncation at damage and the different material

dependences of single pulse and multiple pulse damage behavior suggest that two or more damage

mechanisms may be operative in different intensity regions.

The variation of the number of pulses needed to produce macroscopic failure with intensity at 0.532 urn gives evidence that a minimum intensity or pulse energy may exist below which the observed

damage processes are not operative. Of course, these data do not exclude the possibility that other

mechanisms may cause damage at lower intensities after irradiation with larger numbers of pulses than were used in this study. However, it is interesting to note that the transparent optical elements

in pulsed lasers typically suffer only coating failure, not bulk damage, even after 10^- 10^ pulses.

For example, the components of the laser used in this study have survived 10^ pulses without bulk

damage at intensities of about 100 MW/cmS This may itself suggest that a minimum intensity for the

initiation of multiple pulse damage does indeed exist.

We are presently undertaking experiments using 0.355 ym wave length irradiation to extend

the wavelength dependent aspect of the present study. The ability of the shorter wavelength

photons to interact more strongly with very small defects and to access different resonant absorption transitions will provide further information on the damage process or processes operative

in Si0o .

Acknowledgement: This work was supported by Department of Energy project agreement number

DE-AT03-81NE33080.

References

[1]. M.

[2]. M.

[3]. C.

[4]. s.

[5]. s.

[6]. s.

[7]. E.on

Y. She and D. F. Edwards, NBS Spec. Publ . 372, 11 (1972).

B. Eron'ko, S. N. Zhurkov and A. ChmeT , Sov. Phys. Solid State 20(12), 2064 (1978).

N. Zhurkov, S. B. Eron'ko and A. ChmeT, Sov. Phys. Solid State 22 (10), 1775 ( 1980).

-T. Wu and M. Bass, Appl. Phys. Lett. 39 (12), 948 (1981).

W. Van Stryland, W. E. Williams, M. J. Soileau and A. L. Smirl, Proc of 1982 Symposium

Laser Induced Damage, to be published.

[8]. M. F. Becker, Y. -K. Jhee, M. Bordelon and R. M. Walser, Proc. of 1982 Symposium on Laser

Induced Damage, to be published.

[9]. A. A. Manenkov, G. A. Matyushin, B. S. Nechitailo, A. M. Prokhorov and A. A. Tsaprilov, Proc. of 1982 Symposium on Laser Induced Damage, to be published.

[10]. S. K. Balitskas and E. K. Maldutis, Sov. J. Quant. Elect. 11 (4), 541 (1981).

[11]. W. Fountain and M. Bass, IEEE J. Quant. Elect. QE-18, 432 (1982).

[12]. M. J. Soileau and M. Bass, IEEE J. Quant, Elect. QE-16 (8), 814 (1980).

55
Table 1. Single-pulse bulk damage threshold intensities

for two wavelengths of irradiation

Material ^threshold

at 1.064 urn

^threshold

at 0.532 urn

Crystal 1 ine

quartz

Suprasil-1

Corning 7940

132 GW/crn

113 GW/ciri

113 GW/cm2

102 GW/cm

94 GW/cm'

111 GW/cm 2

THERMOCOUPLE

ADC AND

HP 9825A

PMT FOR SCATTERED

LIGHT

- TO OSCILLOSCOPE TRIGGER AND SHUTTER

Figure 1. Experimental Appartus

10,000

1000

100

10

"'A

' i i i x

" ' i

CRYSTAL!. INF OlMPTZ XOO CTD 'SAWYER

l

1.064 prn

X

X

X

"i i

10,000

05 10

I / I THRESHOLD

1,000

u. '00

10

FUSED 0 SILICA

«*> (SurRA51l - 1:

1.064 urn

0.5 1.0

I / I THRESHOLD

10,000

1,000

100

10

'c —1 1

~-

FUSFTJ x X SILICA °§ 1 CORN 1 ltd 79101 X 1064

X

X *

-.

X\

XXW.X* XX- 0.5 10

I / ^threshold

Figure 2. Number of pulses versus intensity for damage experiments using 1.064 ym wavelength

irradiation. X's represent experiments in which macroscopic damage was observed after

the number of pulses indicated. Open circles represent experiments which were terminated after the indicated number of pulses without macroscopic damage. The peak

intensity is normalized to the corresponding single pulse threshold intensity in Table 1.

56
10,000

1000

100

10

i 1 i—i 1 r 'A


 o —i 1 r

CRYSTALIIKI X mm

(SMffER) X x \

0532pm

PULSES

u.o

X

•

NUMBER

x wxx *

05 10

I /I THRESHOLD

10,000

ipoou. 100

10

-I 1 1 1 1 FUSED SIUCA

(SUPMSU - I I

0.532 fjm

J I I I 1 L 0.5 IP I/Ithreshold

ipoo

100

10

•V

' 0 QB>T 1 -I 1 1 1 r

FUSED SILICA

<continG 79«0) > X

X X

x

XJ

< XN

X \ X

X N.

"' ' 1 1 x r 05 10 I/I thresholo

Number of pulses versus intensity for damage experiments using 0.532 urn wavelength

irradiation. X's represent experiments on "normal" samples in which macroscopic

damage was observed after the number of pulses indicated. Open circles represent ex- periments which were terminated after the indicated number of pulses without macroscopic damage. Filled circles in A represent damage-producing experiments on the

"weak" sample discussed in the text. The peak intensity is normalized to the corres- ponding single pulse threshold intensity in Table 1. The curves in B and C represent

equations (5) and (6).

1,000

o 500

o»V

15

•6o

oa

0

1

1 Corning 7940

r i > = 0.532 urn somple LA

. site 103

E '0.93 m j N * 75 Sc»m«ED

I

1

1 36 72 PULSE NUMBER

ADC output for a representative multiple pulse damage experiment. Upper trace: signal

proportional to the energy of the scattered light on each pulse. Lower trace: signal

proportional to the incident energy of each pulse. For this experiment 1,1. ^65

GW/cm^ and macroscopic damage occurred on the 75th pulse.

50mV 10nS 20 mV 10 nS

i 4

\

Damaging-pul se waveforms for 0.532 ym experiments on Corning 7940 fused silica. A: Typical waveforms for a "fast-attentuation" damage event. B: Typical waveforms for

a "slow-attentuation" damage event. In each case the upper trace is the incident

waveform and the lower trace is the transmitted waveform.

57
Figure 6. Damage sites in Suprasil-1 fused silica irradiated at 0.532 urn. A: Typical single

pulse damage site. B: Typical mutiple pulse damage site for which the damaging

pulse attenuation was "slow."

Figure 7. Distribution of damage events in one intensity range from the Corning 7940 data of

Figure 3C. Curve A is binomial distribution, Equation (3), with 1/P i= 19 . 25

.

Curve B is a Poisson distribution with a mean of 19.25.

to.o

1,000-

FUSED

SILICA (CORNING 7940) 0.532

0.8 < 1.0

Figure 8. The data of figure 3C plotted versus the square root of the normalized intensity.

33
The paper attracted extensive questioning, during which the speaker stated (1) that the repetition

rate during multiple-shot tests was 10 pps, (2) the spot sizes (e~l in intensity ) were about

14ym in 1064-nm tests and 6.6-7]m in 532-nm tests, and (3) the beam propagated along the Z axis

in the crystalline quartz sample. A member of the audience suggested that "aluminum'' platelets

were present in some early samples of 7940 silica, and that the crystalline quartz probably

contained sodium iron silicate . The speaker believed such platelets might influence damage, but

thought sodium compounds were likely to be present as individual ions. He further stated that he

believed it would be unwise to extrapolate his multiple-shot data to predict an absolutely safe

fluence, and commented on preliminary data that suggested that thresholds at 1 pps might be

slightly greater than those at 10 pps.

59
The Role of Ca and Pb Dopants in Pulsed 10,6 jwm Bulk

Damage of Superpure NaCl and KC1 Crystals

P.S. Paizullov, V.I. Kovalev

P.N. Lebedev Physical Institute, USSR Academy of Sciences, Moscow, USSR

and

J. Janszky, R. Voszka

Research Laboratory on Crystal Physics, Hungarian Academy of Sciences,

Budapesht, Hungary

The study has shown considerable increase (about 3 times from 0,4 GW/cm ) of the TEA C0

?-laser bulk damage threshold in superpure RAP grown NaCl

and KC1 crystals with dopping by divalent metals Ca and Pb. The dopants

were added into the melt with concentrations 10-8-10""3 mol/mol for KC1

and 2*1 0""7-1 0""4 mol/mol for NaCl. The concentration of contaminants

(including OH) in samples under the test is less thafi 10"' mol/mol.

It is known, that bulk damage threshold in optical materials is difined by

absorbing microinclusions. These could be the metal colloid particles

in superpure alkali halides. Physical explanation of the found phenomenon

is based on the analogy of the colloid part icles-P-centres system in

crystals with the liquid-vapour system.

Keywords: NaCl and KC1; TEA C02-laser; Bulk damage threshold; Divalent

metals; Dopping; Colloid particles; P-centres.

1 Introduction

It is well known that absorbing microinclusions play the main role in the

bulk optical damage of real optical materials. The ascertion of this fact, the investigation of microinclusion bulk damage principles, and the improvement of the

optical materials technology made it possible to increase considerably the bulk optical damage thresholds /1,2/. The thresholds, nevertheless, remain two or three

orders lower than a theoretical limit determined either by an avalanche or a multiphoton ionization /3,4/* As a result, one may conclude that the absorbing microinclusions play the main role in bulk optical damage of real optical materials. The

dimensions of inclusions in modern laser optical materials are, however, too small

to be discovered by traditional methods. Of course, this makes it difficult to

identify the nature and composition of these inclusions. Respectively this impedes

the interpretation of experimental results and the improvement of optical materials technology for highpower applications. The controlled insertion of any dopant

into the test material would allow one to check the influence of a dopant chemical

composition on the optical damage threshold. Obviously, the role of a dopant composition in any material would become apparent if the concentration of other contaminants appears to be considerably less that of the dopant under test.

In this work we investigated the effect of Ca and Pb dopant taken in a wide

range of concentrations (10~s to 10-3 mol /mol ) in superpure NaCl and KC1 crystals on the bulk optical damage under C0

2 laser pulsed irradiation.

2 Experimental procedure and test specimens

The optical damage threshold was measured by the method described earlier /5/.

Laser pulses of 150 ns (PWHM) at P(20) line were emitted from a TEA C0

p

laser. The

laser provided a TEMqo mo&e beam. The beam was focused by ITaCl lens 0^55 mm P.L.

into the bulk of samples under test. The effective cross-section of the laser beam

in the focal plane of the lens was 2:10"*4Cm

2 , The accuracy of relative measurements

was 10% and that of the absolute one - 20-25%.

60
Test specimens of NaCl and KC1 were RAP grown in hermetic quartz crucibles

with- multiple zone-refining after growing. The concentration of monovalent metal

contaminants was not more than 10"° rnol /mol , and that of the anion contaminants

(including OH) - 10~7 mol /mol . Divalent metal dopants were added in melt with

concentrations of 10~8 to 10-3 mol /mol in KC1 and 2«10~' to 10~4 mol /mol in

NaCl. The details of the growing process and contamination control methods are given in /6/.

3 Experimental results

Figures 1 and 2 represent the results of bulk damage threshold measurements

(the average of 4-6 shots) in the samples under test. Vertical lines in Figures

are the average dispersion of thresholds at different points of the sample. From

Fig. 1 it is seen that the bulk damage threshold decreases markedly with the increasing of Pb concentration (C ) in NaCl from 10*"5 mol /mol which agree with /!/%

and can be accounted for the precipitation of Pb and its salts for C > 10~5moi/mol.

The doping of NaCl by Ca decreases the bulk threshold as well, but not so strongly, as with doping by Pb at C > 10-5 mol /mol . Moreover, the threshold decreases with decreasing of Ca concentration in NaCl. This effect is much more evident in KCl:Ca (Fig. 2). The threshold drops three times with decreasing of Ca concentration in KC1 from 10~5 to 10-8 mo]_ /mol . Besides, unlike NaCl, the threshold

in KCl:Ca does not decrease at high concentration (more than 10"5 mol /mol ).

4 Discussion

This result strange, as it may seem, can be explained in the following way.

Alkali halide crystals always contain an overbalance of alkali metals or halides

even after most Improved methods of crystal growing and purification. As is known,

the excess of an alkali metal is solved much easier than that of a halide. The

excess of the alkali metal provokes two kinds of structure defects in crystals:

F-centres, like point defects of the lattice, and macroscopic metal inclusions,

the so called colloid particles. F-centre is the anion vacancy, which traps an overplus electron (Fig. 3a). The diffusion-like migration of anion vacances in the

crystal lattice can lead to the case, when many anion vacances cluster together at one point of the lattice, and form a macroscopic defect-metal colloid particle. As a matter of fact, the F-centre is the minimal metal colloid particle of 1 nm size,

which consists of 6 metal atoms (Fig. 3a). But in practice, the term colloid is

extended to much larger sizes, in fact, more than ~5 ran /8/.

The system of metallic colloids and F-centres is an example of a condensed

phase and dispersed solute in thermodynamic equilibrium in the crystal /8,9/. In

other words, at crystal temperature T, a part of an overplus metal is in the Fcentre form, and the remaining metal is in the colloid form. In this case the colloid particles would be "evaporated" by the increasing crystal temperature T. That

is the effect /10/, which can explain the results of the increasing optical bulk

damage by a factor of 5-10 of the alkali halide crystals with the help of thermal

treatment (annealing) /11/.

From the analogy of colloid particles-F-centres and the liquid-vapour systems it follows that the colloid particles can be "evaporated" (bulk damage threshold of the crystal can be increased) without high-temperature annealing, if one succeeds in decreasing the concentration of the excess alkali metal (in other

words, pumping out of the vapour at constant temperature). Such effects may take

place, for example, in alkali halides by the divalent metal doping. In this case the cathion vacancy is formed near the divalent ion (Fig. 3b). The electrical neutrality of a crystal is provided by a compensation of the cathion vacancy with the

divalent ion. The cathion vacancy can trap excess alkali metal and decrease the

probability of the F-centre formation at constant concentration of the excess alkali metal. This phenomenon should decrease the dimensions of colloid particles in

the crystal (and increase the damage threshold).

Obviously, such effects can appear when the dopant is totally soluable in the

crystal. Otherwise, the doping metal would precipitate and decrease the bulk damage threshold (NaCl with Ca and Pb concentrations > 10"5 mol /mol ). The absence of

the threshold decreasing in KCl:Ca is probably due to a better soluability of Ca

in KC1.

61
At the same time, it is necessary that the concentration of alkali metal was

small enough for the compensation of alkali metal excess by the divalent metal doping without precipitation of the doping metal. The samples under test in our work

were taken to fit these requirements. The increasing of point defects and colloids

concentration in crystal can increase the yield stress and hardness by impending

the dislocation motion /8,9/. However, the optical damage threshold would not be

heightened as well, in accordance with a nonlinear model of microinclusions heating by pulsed laser radiation /1 3 , 1 4/ (which is more preferrable for pulsed damage than the linear model of Hopper and Uhlmann /15/). In accordance with the nonlinear model, the damage threshold of optical material is independent of mechanical properties, and is fully defined by absorbing defects characteristics. That is

why the investigation of the initial cause of optical damage is the most important

question.

Among the reasons why the observed dependence of the optical damage treshold

had not been earlier discovered, one should note the important role of admixture

of the hydroxil OH. The typical OH concentration in air-grown crystals is

10"3 mol /mol • The measurements in KC1 with a special doping by 10-3 mol /mol

concentration of OH show that the threshold intensity is 0.4 GW/cm2 (Pig. 2).

This value is lower than the damage thresholds of OH free samples under test

(Pig. 2).

5 Summary

The study of the bulk damage thresholds in superpure NaCl and KC1 crystals

with the gain switched TEA C02~laser pulses has yielded a very important result.

The effect of considerable damage threshold increasing with doping by divalent metals has been discovered in superpure NaCl and KC1 crystals.

We have suggested a physical explanation based on the analogy of colloid particle s-P-centres with the liquid-vapour systems.

Although this work deals with colloids in ionic matrices only, our results

may help to interpret any result on colloids in other superpure optical materials.

The authors are grateful to Drs A.V. Vinogradov from Lebedev Physical Institute and V 0 V. Nabatov from Shubnikov Crystallography Institute for their valuable

discussions and assistance.

6 References

I Danileiko Yu.K., Manenkov A. A., Prokhorov A.M., Khaimov-Mal »kov V.Y., JETP, 58,

31 (1970).

2. Aleshin I.V., Imas Y.A., Komolov V.L. Optical strength of low-absorbing materials (in Russian), Leningrad, 1974.

3 Epifanov A.S., Manenkov A. A., Prokhorov A.M. JETP, 70, 728 (1976).

4 Vinogradov A.V., Paisullov P.S. Sov. J. of QE, 7, 6!>Q (1977).

5 Kovalev V.I., Morosov V.V., Paisullov P.S. Sov. J. of QE, 4, 1208 (1975).

6 Voszka R., Tarjan I., Berkes L., Krajsovsky J. Kristall und Technik, 1_, 423

(1966); Raksanyi K., Voszka R. Kristall und Technik, 4, 227 (1969).

7 Kabatov V.V., Beliaev L.M., Dimenko N.N., Voszka R. Acta Phys. Acad. Sci.Hung.,

47, 107 (1979).

8 Hughes A. P., Jain S.C. Adv. in Phys., 28, 717 (1979).

9 Seits P. Rev. Mod. Phys., 26, 7 (1954).

10 Vinogradov An.V., Tribel'sky M.I. JTP Letters, 5, 595 (1979).

II Gorshkov B.G., Danileiko Yu.K., Epifanov A.S., Lobachev V.A., Manenkov A. A.,

Sidorin A.V. JETP, 72, 1171(1977).

12 Kittel Ch. Introduction to solid state physics. Pourth Edition, New York,

London, Sidnay, Toronto.

13 Danileiko Yu.K,, Manenkov A. A.. Nechitailo V.S., Prokhorov A.M., Khaimov-Mal •

kov B.Y. JETP, jO, 1030 (1972).

14 Anisimov S.I., lakshantsev B.I. PhTT (Phys. Sol. Stat.). 1_5, 1090 (1973).

15 Hopper R.W., Uhlmann D.R. J. Appl. Phys., 41, 4023 (1970).

62
10' itr w

Fb COHCEJURATIOM , MOL/ROL

Pig. 1. Bulk damage threshold (relative units) in NaCl

samples versus Pb concentration.

W8

iQ'6

i0'

k iO'

2

DOPAST COKCWTMTIOH, MOL/XOL Pig. 2. Bulk damage threshold in NaCl and KC1 samples

versus Ca concentration. Triangle is the threshold

for KC1-0H.

63
0©0©0

© v ©0©

«) 0©0©0

Y F-CPTTRB

0©0oo© ©0Q0©0

8j 0©0©0©

\ CATHIOT

\ vacancy

Pig. 3.a.Schematic model of an F-centre.

b. Cathion vacancy in alkali-halide lattice with

a divalent metal dopant.

When asked why increased Ca doping reduced thresholds in NaCl, but increased thresholds in KCl , the

speaker said Ca was less soluble in NaCl than in KCl, and that precipitates of Ca or of calcium

salts occurred in NaCl crystals doped with > 10~5 mol/mol of Ca. in answer to additional

questions , the speaker stated (1) that the threshold of the undoped crystals was 0.4 GW/cm2 for

test beams 100-150\m in diameter , a value believed to be relatively low by several in the

audience , and (2) that he had been unable to detect F centers in the crystals.
Contaminant and Defect Analysis of Optical Surfaces by

Infrared Laser-Stimulated Desorption

S. D. Allen

Center for Laser Studies

University of Southern California, Los Angeles, California 90089-1112

J. 0. Porteus, W. N. Faith, J. B. Franck

Michelson Laboratory, Physics Division

Naval Weapons Center, China Lake, California 93555

Infrared laser induced desorption (LID) has been demonstrated tor water and other

contaminants on nominally transparent substrates. The apparatus consists of a UHV chamber

interfaced with several pulsed IR lasers. Detection of the desorbed fluence is via an ionizer and quadrupole mass analyzer. For the samples measured to date, the LID is defect dominated for both small (120 ym) and large (380 pm) spot sizes. For the larger

spot sizes, the data for multiple irradiations of the same site (N/l) can be explained

using a simple defect model. The LID fluence of water and other contaminants such as hydrocarbons can vary by orders of magnitude from site to site on the same sample. Similar variations were observed for average values of the desorption signal as a function

of sample and substrate finish. The surface damage threshold of these materials cor- relates with the measured LID fluence of contaminants, i.e., samples and sites with

high contaminant fluences damage at low thresholds. Samples investigated include:

polished CaF2 (two vendors), BaF2, NaCl and etched CaF2 and NaCl . A comparison of

desorbed water and hydrocarbons at 2.8, 3.8 and 10.6 pm suggests that water may not provide the dominant mechanism for localized optical absorption and damage. Laser

cleaned surfaces which were dosed with water at - -100°C showed no evidence of

readsorbed water on subsequent LID testing, indicating a passivation of the surface.

Key words: surface contamination, laser induced damage, surface defect analysis,

laser induced desorption.

Introduction

As materials for high power laser optics have improved, it has become increasingly true that

failure occurs at surfaces and interfaces in the optical' component where contamination is most

probable. Last year at this conference we reported initial results from an experiment designed

to quantitavely measure such contamination [1]. The experiment consists of an UHV chamber interfaced

with a pulsed IR laser. The laser wavelength is chosen such that adsorbed molecules couple strongly

to the laser field and absorb sufficient energy to cause desorption. The desorbed molecules are detected with a quadrupole mass analyzer. In most cases, the substrate is highly transparent to the

laser wavelength. The advantage of this choice of experimental parameters is that large amounts of

energy can be transferred to the adsorbed molecules at intensities significantly below the damage

threshold of the substrate. Most other laser induced desorption (LID) experiments are conducted

under conditions in which the substrate is highly absorbing [2].

The HK/DF and C0„ TEA lasers with nominal wavelengths at 2.7, 3.8 and 10.6 pm respectively were

used to irradiate samples of CaF 2 , BaF 2

, NaCl, KC1 , Cu, Mo and several coatings. Most measurements

were made with the HF laser as it is strongly absorbed by H

2 0, a pervasive contaminant. Detected

desorbed species include H

2 0, OH and hydrocarbon fragments. Detection sensitivity for adsorbed H 2 0 scales with the laser spot size and is estimated at several percent of a monolayer using the present

experimental geometry. (HF laser, D-, , 2

= 380 pm). Initial desorption experiments were carried out

on polished and etched samples which had been subjected to an 8 hour bakeout at 250°C, as the

tenacity of surface water and hydroxide ions at elevated temperatures is well known [3]. Measurements of desorption fluence as a function of laser energy density showed considerable data scatter

which increased for smaller spot sizes D-j /e2

= 120 pm) . As each data point was taken on a new site

(1/1), there appeared to be a large site to site variation on approximately a 100 pm scale. This

effect could be due to selective adsorption of contaminants at defect sites such as scratches,

cracks, pits and pores.

65
LID Passivation

In order to overcome this experimental limitation, we used laser induced desorption to clean

several "large" areas using multiple overlapping laser irradiations. These areas were then dosed

with many Langmuirs of both at room temperature and -100°C in an attempt to obtain a uniform

adsorbed layer. The sample was subsequently re-irradiated and rj£ desorbed H2O was detected. It was clear from the thermal desorption that occurred as the samples warmed, that water was deposited

on the substrate at approximately -100°C, but none was detectable once the substrate had warmed

up to slightly below room temperature. Unfortunately, we were unable with the current experimental

apparatus to maintain the sample at reduced temperatures during the laser induced desorption. We can conclude from these results for CaFz (100) and (111), BaF2 (111) and NaCl(lOO), that laser

induced desorption causes changes in the surface which inhibit F^O adsorption at room temperature.

Similar surface passivation effects have been observed for chemically etched KC1 surfaces [4].

SEM examination of such single crystal etched surfaces shows an essentially featureless surface

with an occasional low angle grain boundary. A reasonable explanation, therefore, is that adsorption ot H2O takes place predominantly at defect sites and that LID somehow removes these sites.

These results have obvious applications to surface preparation of high power laser optics . Fur- ther experiments such as SEM and TEM examination of LID cleaned surfaces should be conducted to determine what changes take place in the laser irradiated surface.

Simple Detect Model

In order to quantitatively analyze the defect mediated adosrption, a simple model was developed. As shown in Figure 1, we assume that the defect density and contaminant adsorption on the surface is uniform and that there is a single threshold laser intensity necessary for desorption. The assumption of a threshold intensity is a good approximation for a process which is undoubtedly governed by an activation energy whether it is strictly thermal or has some component

of bond selective excitation. In addition, comparison of the desorption energy for H2O on CaF2 , for example, with the available laser energy shows that two photons of 2.8 ym are necessary for

each H2O molecule desorbed. The LID is thus highly nonlinear in the laser intensity and can be

reasonably approximated by an intensity threshold. Above the threshold, all adsorbed molecules

are assumed to be desorbed. Using this model, the desorption fluence, i>, is proportional to the

irradiated area above the threshold intensity. For a gaussian beam, this becomes

¥ = m7rrT2H

¥=m77p2 [ln3>0 -ln$TH] If the desorbed tluence is plotted as a function of the natural logarithm of the axial energy

density, $0

, the slope ot the resulting straight line yields information on the defect density

and size. The zero desorption intercept is the threshold intensity for the LID process.

An example of LID data for H2O on etched CaF2 using the HF laser (D-|/e

2 = 380 urn) in a 1/1

experiment is given in Figure 2. The large data scatter indicates that tnis model is not applicable on the scale of several mm (the spacing between irradiated sites is 1 mm). If we similarly plot the data for a N/1 experiment for the same system, but use the cumulative desorbed

H2O tluence, we obtain a much improved signal to noise ratio as shown in Figure 3. Similar

graphs have been plotted for many sites on CaF2, both (100) and (111), BaF2 and NaCl as shown in Figures 4, 5, 6 and 7. Although there is evidence of the site to site data scatter shown in Figure 2, the curves tend to group naturally into several types. As indicated by the site coordinates, given in mm, similar sites occur in areas separated by up to a centimeter. In some curves,

e.g., Figure 3, two distinct straight lines are evident, implying an initial desorption from

defects with a relatively low threshold intensity and subsequent appearance of a second type of

defect with a higher threshold. Occasionally one site will show an unusually high LID signal.

For example, sites 5,15 and 7,0 in Figures 5 and 6 respectively lie signficantly above the

remaining data. In each case the anomalous curves are approximately parallel to but offset

vertically from the other curves. A possible explanation is the occurrence of a particularly

large, lower threshold desorption site within the initial irradiated area. If such defects are uncommon, e.g. foreign particles, they are not accounted for using the proposed model. A large

signal will result on the first but not on subsequent shots if a large desorption site falls

within the initially irradiated area.

In the absence of measurements at intensities near the indicated threshold and other gaps

the data, we have not attempted to draw any quantitative conclusions from either the measured

thresholds or the density factors. What is remarkable is that the measured curves are so simple, implying in most cases, one or more types of defect sites uniformly distributed over an area several

hundred microns in diameter.

66
A comparison with the previously reported data [1] shows that the 1/1 LID signal measured on CaF2 (IOC) was more uniform from site to site and that the signals were more than an order of magnitude larger. The initially measured CaFp samples were polished and supplied by a different

vendor. Independent damage measurements [5] of the two types of samples showed consistently lower

damage thresholds for the material reported in reference 1. Poor quality, low damage threshold, high

LID fluence surfaces are reproducibly bad from site to site, whereas good quality, high damage

threshold surfaces exhibit low average LID fluences but large site to site variations. These observations are consistent with the proposed defect model.

Wavelength Dependence of LID

As shown in the previous paper [1] , the desorption efficiency of the DF laser for H2O was

less than that observed for the HF laser, but the disparity was not as large as would be expected

on the basis of the absorption coefficients of HoO at 2.7 and 3.8 ym. As shown in Table 1, HgO

absorbs strongly in the 2.7 ym region and virtually not at all at 3.8 ym [6]. This seeming

contradiction is also seen in the few points in Figures 3, 5, 6 and 7 where N/1 data for the DF and CO2 lasers does not differ significantly from the HF LID data. There are several explanations

which should be explored. The simplest is that the particular HF laser wavelengths used in this

experiment do not effectively couple to the adsorbed OH bonds. The dominant spectral components

in the HF laser are 2.795 ym (34%); 2.71, 2.835, and 2.635 ym (20% each). The spectrum of H

2 0 adsorbed on CaF2 powders [3] shows a broad spectral feature centered at 2.12 ym and extending

from approximately 2.7 to 3.3 ym attributed to hydrogen bonded H2O and a sharp absorption line at 2.72 ym ascribed to strongly held free HgO molecules. In no case was there any absorption observed near 3.8 ym. An alternative explanation is that several contaminant species are co-adsorbed.

As shown in Table 1, C-H bonds absorb in the DF laser region and C-C bonds in the CO2 laser

region. As alcohol is frequently used as a polishing vehicle and in the etching process for the

surface preparation of the tested materials, it is reasonable to assume that some fraction of the

adsorbed and surface occluded material is alcohol.

Sequential desorption measurements of CH3 and H2O were made at both HF and DF wavelengths.

It was observed that on no site was one contaminant observed in the absence of the other. The ratios

of desorbed fluence for first irradiation/second irradiation for the two components are given in Table 2. At 3.8 ym, the ratio of desorbed fluence for HpO or CH3 remains approximately the same whether H2O or CH3 is measured first. In contrast, similar measurements at the HF wavelengths yield

a large change in ratio with change in observation order. A comparison of N/1 LID of the various

suspect contaminants at closely spaced sites could quantify these apparent differences.

There is some evidence of cumulative heating effects for N/1 LID measurements in CaF2 at 10.6 ym where the substrate is a bulk absorber. Water continues to be desorbed on sequential shots even when the energy density and therefore the irradiated area above threshold is fixed. An estimated temperature rise of at least several hundred °C due to the bulk absorption would lower the

effective LID threshold in the heated area. As the area heated by bulk absorption and the substrate

temperature increases, the LID also increases even for the same effective laser spot size. What is surprising in these experiments is the relatively high damage threshold observed in spite of the

high bulk absorption. We were unable to damage the etched CaF2 (111) in N/1 experiments up to 150 J/cm2 measured for etched (111) NaCl (Fig. 7). This result emphasizes the importance of surface

defect absorption versus bulk absorption in determining the laser damage theshold.

Cone! usions

We have shown that for the surfaces considered, i.e., CaF

2

, BaF2

, and NaCl, LID cleaning provides passivation for subsequent H

2 0 adsorption. These materials are representative of the ionic

crystals used for high power infrared optics. The LID signal initially observed is, therefore,

probably associated with adsorption of H2O and other contaminants at native defects in the surface

such as scratches, cracks, grain boundaries, etch pits, pores, etc. A simple defect model can explain the laser pulse energy dependence of the LID fluence if measurements are made at a single

site (N/1) Site to site LID measurements can differ by orders or magnitude, but some areas on the

order of mm^ show similar behavior, suggesting the possibility of using LID for nondestructive

mapping of contaminant concentrations. A series of experiments involving the measurement of LID

as a function of wavelength and multiple species at a single site imply that contaminants other than

H2O may significantly influence laser damage of these surfaces.

Acknowledgments

The authors are pleased to acknowledge the support of the Office of Naval Research Special

Research Opportunities Program and Navy Independent Research Funding.

67
References

[1] J. 0. Porteus, W. N. Faith, and S. D. Allen, this conference, 1981;

S. D. Allen, J. 0. Porteus and W. N. Faith, Appl . Phys. Lett. 41_, 416 (1982).

[2a] Absorbing Substrates:

L. P. Levine, J. F. Ready and E. Bernal G. , J. Appl. Phys. 38, 331 (1967).

H. F. Winters and E. J. Kay, Appl. Phys. 4_3, 789 (1972).

P. A. Temple, D. K. Burge, and J. M. Bennett, NBS Special Pub! . 462, 195 (1976).

R. Gauthier and C. Guittard, Phys. Status Sol i di A 38 , 477 (1976T7~

J. P. Cowin, D. J. Auerbach, C. Becker, and L. Wharton Surf. Sci. 78, 545 (1978).

[2b] Low Absorption Substrates:

J. Heidberg, H. Stein, E. Riehl, and A. Nestmann, Z. Phys. Chem. Neue. Folge. 121 , 145 (1980).

T. J. Chuang, J. Chem. Phys. 76, 3828 (1982).

[3] P. B. Barraclough and P. G. Hall, Surf. Sci. 46, 393 (1974); J. Chem. Soc. Faraday Trans. 171_,

2266 (1975).

[4] S. D. Allen, M. Braunstein, C. Giuliano and V. Wang, NBS Special Publication 414_, 66 (1974).

[5] J. 0. Porteus, unpublished results.

[6] J. D. Roberts and M. C. Caserio, "Basic Principles of Organic Chemistry", W. A. Benjamin, NY (1965).

68
Table 1

COMPARISON OF H

2 0 AND HYDROCARBON SEQUENTIAL LID

A (ym)

3. 8

2.8

ON CaF

2 (100)

CH

3 /H

2 0

0.4

0. 3

H

2 0/CH

3

2. 4

14

Table 1. Ratio of desorbed H

2 0 and CH-^ fluence for first irradiation/

second irradiation on the same site at both HF and DF

wavelengths.

Table 2

CHROMOPHORE

0-H

O-H

(hydrogen bonded)

C-H

C-0

C-C

ABSORPTION BANDS

2.7 - 2.8

2.9 - 3.1

3.0 - 3.7

7.7 - 11.

0

8.0 - 12.

0

LASER

HF

HF

DFco

2 co„

EXAMPLES

water, alcohols

water, alcohols

hydrocarbon, alcohols

alcohols

hydrocarbons

Table 2. Absorption bands of suspected contaminants on optically polished

and/or etched surfaces.

69
Fig. 1 Schematic diagram of the defect density model used.

2

I

2 = 2I 1' p is the 1//e radius of the gaussian beam,

and I,, , is the minimum intensity at which desorption

threshold

takes place from the surface defects.

0.4

e 0.3 c

UJg0.2

LU

-Jo

0.1

CaF2 <100>

etched

HF

F= 8.5

10

$>(J/cm^)

Fig. 2 LID fluence of mass 18 (^0) as a function of axial

energy density for a 1/1 experiment on etched

(100) CaF~. D , 9 = 380 nm at HF.

2

70
$ (J /cm*)

100

Fig. 3 Cumulative LID fluence of mass 18 (H.,0) as a function

of axial energy density for a N/1 experiment using the

same system as Fig. 2. Each irradiated site is designated

by its coordinates in mm.

0.4-

.0.3-

LU

_Jo

UJ

0.2-

3-

0.1

5,6

o 15, 1

• 15, 3

X 15, 5 v — 15, 17 — 17, 1 * —

17, 3

A 17, 11

11, 1 DF

© — 11, 7 DF

o — 13, 1 DF

etched CaF2 <100>

10

<t> (J/cm2

) 100

Fig. 4 Cumulative LID fluence of mass 18 (H.,0) as a function

of axial energy density for an N/1 experiment on (100)

CaF2* D, , 2 = 380 um at HF and DF. 1/e K Irradiated sites are designated by their coordinates in mm,

71
0.4

0.5

£c

_j

oLU

_Jo

-0.1

s- w

CaF2 <111>

HF

9,7 (DF)

9, 21 (DF)

0

Fig . 5

*(J/crrO

Cumulative LID fluence of mass 18 (H

2 0) as a function of axial energy density for an N/1

experiment on (111) BaF

2

. D 1/e 2 = 380 um at HF

and DF. Irradiated sites are designated by their

coordinates in mm. BqF2 <m>

HF,

10 o * (J/cm2

) 100

Fig. Cumulative LID fluence of mass 18 (H

2 0) as a function

of axial energy density for an N/1 experiment on (111)

BaF

2

. = 380 ym at HF and C0

2

. Irradiated sites are designated by their coordinates in mm.

72
Fig. 7 Cumulative LID fluence of mass 18 (H^O) as a function

of axial energy density for an N/1 experiment on

(100) NaCl. D i/e

2 = 380 urn at HF and C0

2

. Irradiated sites are designated by their coordinates

in mm. (Back surface damage at the CC^ wavelength

is indicated by the CO- notation on two graphs.)

73
Fig. 8 Cumulative LID fluence of mass 18 (H

2 0) as a function

of axial energy density for an N/1 experiment on an SiO

film on CaF.. D 2 = 380 ym at HF . Irradiated sites

2- 1/e are designated by their coordinates in mm.

in answer to a comment that the input fluences in the experiment were high relative to the input at 2.7\m that should be needed to evaporate water, the speaker stated that the laser operated at both 2.7 and 2.8\m, and that there probably were spectral shifts in the absorption spectra of

some surface bonded water, in response to further questions , the speaker cited two observations

that suggested that most of the desorbed water had initially been trapped in the polished layer:

(1) the amount of desorbed water varied for samples polished by different vendors, and (2)

laser-cleaned areas that were exposed to water and retested did not show evidence of water

contamination in subsequent desorption tests.

74
Low Optical Absorption Measurement of

by Interf erometr ic Laser

Laser Optical Materials

Calor imetry

Masahide ITOH

The Institute of Physical and Chemical Research,

Hirosawa, Vako-shi , Saitama, 351, Japan

and

Iwao OGURA

Institute of Industrial Science, University of Tokyo

Roppongi , Minato-ku, Tokyo, 106, Japan

A calorimetr ic method for measuring the low absorption coefficients

of laser optical materials is proposed. It uses the interferometer at 633

nm to measure the bulk and the surface absorption coefficients by the

change of the optical path length due to the temperature rise caused by

the absorption of CO2 laser beam. Non-contact measurement as well as linear and rapid response of the absorption makes possible multi-point

measurement with high time-resolution. The surface absorption can be

separated from the bulk absorption. With the aid of digital technique,

the bulk and the surface absorption coefficients of infra-red materials

such as ZnSe , KC1 , NaCl , etc. , are obtained with the range of 10"3cm

_1

. Key Words: inter f erometry ; laser calorimetry; highly transparent

materials; laser window; surface absorption; bulk absorption; KC1 ; ZnSe.

1 . Introduction

The widespread use of high power lasers requires further studies of the very

low loss optical materials. The progress in the technique of measuring small

absorption coefficients has contributed to the development of such materials. Laser

calorimetric method is one of the most powerful techniques of measuring the low

optical absorption of the infrared laser window materials [1~5]'.

Especial ly , inter f erometr ic laser calorimetry has a merit of obtaining the inner

temperature distribution of the transparent sample in multi-point in parallel

because of non-contact measurements [6,7] . We will also propose some applications

of this method.

2. Thermal Equation

The coordinate system of the interf erometr ic laser calorimetry is shown in

fig. 1. The CO2 laser beam passes through the sample at right angle with He-Ne

laser probe beam. The sample absorbs the CO? laser light proportional to its

absorption coefficient. The heat gives rise to the change in optical path length.

This change is observed as the movement of the fringe by the interferometers.

Thermo-optic coefficient 7 that is, the change rate of the optical path length

versus the temperature, is written as:

dL , < n dd dn

y = — = (n-1 ) + d , (1 ) dT dT dT

Figures in brackets indicate the literature reference at the end of this paper.

75
where L is the optical path length, T the temperature, d the thickness and n the

refractive index.

The temperature distribution in the sample obeys the partial differential

equation of heat conduction given by [8]

1 du o = V'-u + F . (2)

k dt

Here u is the temperature difference between the sample and the ambient, and k is

the thermal diffusivity. The heat source term F is

P 1 f (x-L r /2) 2 + (y-L„/2) F = t exp ' k (kw)2 V u)2

xj (l-a)0e^2+a(5(z)+(l-a)e_/?2 S(z-Lz )]

J

, (3)

where P is the power of the laser beam of the heating radiation source, w is the

radius of the spot size, Lx , L y

, and Lz are the sample size, S(z) is the delta

function, and a and # are the bulk and surface absorption coefficients,

respectively. We calculated the temperature u of x,y,z,t, under the boundary

condition of the third kind with the Fourier method. The solution of the eq . (2) is

" - E E E A ijk (t)-gUx)-gUy)-gi(z) , (4)

i=0 ;=0 k=0

where

A ijk = ^(1- e-^) , Mi;k

Mi;k = K( (/l?) 2 +(^)2 +(/lf) 2

) ,

OO oo oo F = E E E /wk-gf (x) -g«(y) -gf (2) . i=0 ;=0 k=0

The change in optical path length that we observed by the interferometer is

proportional to the integral of the temperature between the sample thickness, as

U = 7 t'u dy = 7 L, E E —(l- e"f ** ) g? (x )

• (z ) . (5)

"'O i=0 k=0 MiOk

In the case of low optical absorption, change in the optical path length is the

linear combination of the surface and the bulk absorption terms that we write Us and

Ub y respectively

:

where

U = a Us + (i Ub , (6)

v. - 1 r Wi- —) + e - «2,.o,f.-i

g

ff- ,) i

.

ic=o L L, V A J ~\ i V 2 (2.x ) 2K

2 +2kLVl

Ub = E R00k - k=0

R.Ok = -^(1- e^f ) -gf -gf -a, , MiOk where aj is the i-th expansion coefficient of the cross sectional (x-) component of

the heat source F. Us and Ub can be calculated with the constants of the sample,

such as specific heat, density, thermal diffusion constant, heat transfer

76
coefficient, etc. Figure 2 shows a distribution of the the optical path length that

is proportional to the inner temperature of the transparent sample.

3. Experiments

We used the two types of interferometers. If the opposite surfaces of the

sample are both optically flat, the interferometer of the Twiman-Green type is used

(fig. 3(a)). Otherwise we have to use the holographic interferometer (fig. 3(b)).

As probe laser beam, He-Ne laser is used, where the wavelength is 633 nm , and the

power is 5 mW. The light source that induces temperature rise is continuous wave

CO2 laser. The the wavelength is 10.6 nm , the power 10 W and the spot-size is 1 mm. The cubic KC1 samples are used, where its sides are 20 mm. Sample is set on

the edges of the bakelite holder.

Figure 4 shows an example of the fringe movements using holographic

inter ferometry . CO2 laser beam passes through the center of the sample from right

to left. We can see the temperature distribution inside the sample from this

patterns. About 30 seconds after CO2 laser turns off, the temperature in the sample

becomes uniform. Using the change of optical path length after 30 seconds, we can obtain the thermo-optic coefficient 7. The samples are two pieces of KC1 where measuring wavelength is 10.6 nm. And

we also measured the quartz and CaFz at 488 nm of the Ar laser. We read out the

fringe movement at the center of the sample. For the calibration of the movement

due to the interferometer, we subtract the outer movement of the fringe from the

center. Intensity profile of the interference pattern is assumed to be sinusoidal.

We first measure the peak to peak intensity of the fringe to move the one arm of the

interferometer by piezo electric transducer, and obtain the change in the optical

path length by interpolating the peak-to-peak intensity.

4. Results and Discussion

After we measure the optical path length change U of x,y,z,t, we can determine

the surface absorption coefficient a, and the bulk absorption coefficient (i by the

least square method (fig. 5). Figure 6 is an example of the experimental data and

the fitting line of two different of KCl's. We may say that experimental data are

in good agreement with our method.

The surface and the bulk absorption coefficients are obtained (table 1). The errors of each value are about 10 %. We consider that this error is caused by the

change of environmental condition during experiments, and the thermal stresses. On

the other hand, internal stress effect may be obtained by measuring the stress

birefringence with interferometers.

Table 1. Determined bulk and surface absorption coefficients.

Specimen Wavelength Bulk Absorption

($ xl03 (cmH

) Sur f ace

a Absorption

x 103

Quartz 488 nm 95 + 7 5.3 ± 0.3

CaF2 488 nm 6.6 + 0.5 4.0 ± 0.6

KC1 (1 ) 10.6 nm 2.1 ± 0.2 1 . 1 + 0.3

KC1 (2) 10.6 nm 1.0 + 0.2 1 . 1 + 0. 1

We also propose another application of the interf erometr ic laser calorimetry

that is to obtain the distribution of the bulk absorption. We can obtain the change

in optical path length in multi-point in parallel. In this case we used the 1024

point linear image sensor as the detector, and processed the data with mini-computer

(fig. 7).

77
The partial differential equation of heat conduction (eq. (2)) is rewritten

into the difference equation:

J_ Ui, j+ \-U

Ly At (Az)2 pc

(7)

Subscript i and j denote the spatial and temporal parameter respectively. If we measure the spatial distribution of the optical path length change with the short

time repetition, we can obtain the distribution of the bulk absorption coefficient

The test experiments of measuring the bulk absorption distribution of ZnSe is

shown in fig. 8. In this case, we assumed the only bulk absorption, and

consequently, the increase of the absorption near the sample surface was observed.

Connecting the interferometer with the micro-computer, we can not only get the

surface and the bulk absorption coefficients separately, but also get the

distribution of the absorption in the sample.

Laser inter f erometr ic calorimetry has the merit of high speed response to the

temperature field. With the pulsed CO2 laser, damage occurs within 100 ns . We have

observed the stress and the thermal wave inside a ZnSe sample under the TEA CO2

laser irradiation where the pulse duration is 100 ns and total power is 1 J. The

surface and the bulk linear or nonlinear absorption will be obtained by analysing

the wave form.

In the case that the sample does not have optical parallel, such as lenses,

prisms or rough surface, we may use the Schlieren method. If we use the 2-nd

harmonic wave of the Nd-YAG laser as the probe light we can observe few nano-second

thermal or stress effect. With the use of double pulsed YAG laser, we may take a double exposed holography, and after development the hologram, the deformation less

A/100, that is, about few nano-meter can be measured by analysing the reconstructed

holographic 3-dimentional image using digital fringe scanning technique.

We have developed a technique of measuring the surface and the bulk absorption

coefficients separately using an i nterf erometr ic laser calorimetry. It uses the

interferometer at 633 nm to measure the bulk and the surface absorption coefficients

separately by the change of the optical path length due to the temperature rise

caused by the absorption of 10.6 um CO2 laser beam. We measured the bulk and the

surface absorption coefficients of KCl*s, and obtain l.OxiCT3 cm

-1 and l.lxl 0"3

, respectively. We also measured the distribution of the absorption inside the sample

and observed the thermal and stress wave induced by the pulsed laser light. We

believe that all optical measurements have some merits of high sensitivity and high

time response to study the low optical absorption materials.

The authors acknowledge stimulating discussions with Dr. K. Kuroda. The

authors also thank Dr. H. Saito and Prof. T. Ose for their encouragements.

[1] Temple, P. A., Examination of laser damage sites of transparent surface and

films using total internal reflection microscopy; Laser Induced Damage in

Optical materials; 1979, October 30-31 1979, NBS Boulder Co., eds . by H. E. Bennett, A. J. Glass, A. H. Guenther and B. E. Newnam, NBS Special Publication

568, U. S. Government Printing Office, Washington, D. C. 1980, p333

.

[2] Wu , S.T.; Bass, M., Intensity dependent absorption of Alkal i-hal ides at 10.6

urn ; Laser Induced Damage in Optical materials', 1980, September 30-0ctober 1 1980, NBS Boulder Co., eds. by H. E. Bennett, A. J. Glass, A. H. Guenther and

B. E. Newnam, NBS Special Publication 620, U . S. Government Printing Office,

Washington, D. C. 1981, p75

.

[3] Braunstein, R.; Kim, R. R.; Braunstein, M., Wavelength modulation spectroscopy

of laser window materials; Laser Induced Damage in Optical materials ; 1980,

Pi

5. Conclusion

6. Acknowledgement

7. References

78
September 30-0ctober 1 1980, NBS Boulder Co., eds . by H. E. Bennett, A. J. Glass, A. H. Guenther and B. E. Newnam , NBS Special Publication 620, U. S. Government Printing Office, Washington, D. C. 1981, p29

.

[4] Rosenstock, H. B., Absorption Measurements by Laser Calorimetry, J. Appl . Phys

.

50: 102-110 1970.

[5] Itoh, M.; Saito, H., Pulsed Photoacoustic Technique to Measure the Absorption

Coefficients of the Highly Transparent Materials. Opt. Commun. 44: 229-231

1983.

[6] Itoh, M.; Ogura, I., Absorption Measurement of the Optical Materials by Real

Time Holographic Interf erometry . Opt. Commun. 33: 183-187 1980.

[7] Itoh, M.; Ogura, I., Absorption measurements of laser optical materials by

interf erometr ic calorimetry, J. Appl. Phys. 53: 5140-5145 1982.

[8] Carslaw, H. S.; Jaeger, J. C. , Conduction of Heat in Solids. London; Oxford

University Press; 1959.

8. Figures

Sample

Probe He-Ne Laser

C02 Laser

Interferometric

Fringe

Figure 1 . Coordinate system.

79
Figure 2 Distribution of the optical path length. The right side of the figure is due to the

only surface absorption, and the left is due to the bulk absorption.

80
Laser Beam

(a)

Li

He-Ne

Laser Beam

to

Detector

CO2 Laser Beam

(dial

He-Ne Laser Beam ^

(b)

6% Li

Figure 3. Schematic of Interferometers, (a) Twiman-Green type which is used if the opposite

surface of the sample are both optically flat, (b) Holographic type which is used,

otherwise

81
Laser on 10 sec 30 sec

240 sec

Laser off 30 sec 60 sec

Figure 4. Example of the fringe movement observed by the holographic interferometer.

t = 20 sec Ub

Figure 5. Procedure of measurements.

82
(1)

Us/Ub

Figure 6. Plot of the ratio of U/Ut, vs. Us AJb • U is the measuring optical path length (OPL), Ub is the calculating OPL due to the bulk absorption, and Us is the calculating OPL due to the surface absorption.

83
Sample

Probe He-Ne Laser

y C02 Laser

Linear Image Sensor

1024 elements

Micro-Computer

LSI-1 1/02

Figure 7. Experimental set up to obtain the distribution of the bulk absorpti on.

E

u

CD

X

O

03O

c

o

o

<

0

i • :

Sample : ZnSe

20X20X20 mm

• :

- i

• •

i

-

L L

10

Position (mm)

Figure 8. Distribution of the bulk absorption.

84
The accuracy of the reported absorption coefficients for alkali halide crystals was questioned.

Reference was made to previous measurements on such crystals at 10 microns performed by B. Bendow and

coworkers in the 1970 's. In these, stress-induced changes in the optical path length (via thermal

lensing), amounting to ~20-30 percent, had complicated the determination of the absorption coefficients.

85
Surface and Free Carrier Absorption Processes in CVD Zinc Selenide

K L Lewis

Royal Signals and Radar Establishment

St Andrews Road, Malvern, Worcs. WR14 3PS UK

and

G S Arthur

Atomic Weapons Research Establishment

Aldermaston, Berks. RG7 4 PR UK

An investigation is reported of the effects of extrinsic absorption processes on the

infra-red properties of ZnSe prepared by chemical vapour deposition. Surface absorption

processes are discussed with reference to the absorption of water and hydrocarbon impurities

from the atmosphere. Particular attention is paid in the case of bulk absorption to the

effects of free carrier scattering processes which produce a wavelength dependence of the

absorption coefficient of the type 3 = X

p . In material free from second-phase inclusions,

the intensity of infra-red absorption at 10.6 ym measured calorimetrically , has been found

to be correlated with a red photoluminescence process produced under near band-gap

irradiation. Detailed studies of the photoluminescence show that the electronic structure

of the material is dominated by the presence of sodium and copper acceptors, together with

a donor tentatively ascribed to indium. In well -compensated material, deeper centres

appear to predominate.

The behaviour on annealing of material exhibiting face carrier absorption can be

described by a model involving the diffusion of impurities such as Fe, Ni or Cr from grain

boundary sites.

Keywords: Calorimetrically; free carrier absorption; photoluminescence;

surface absorption; zinc selenide.

1. Introduction

The 1

1 —V I semiconductor compound ZnSe is of current interest as an infra-red transmitting

material. A combination of wide bandgap (2.7eV) and low lattice vibrational frequencies allows

transmission over a wide wavelength range (0.5-22ym), covering most of the visible spectrum, two thermal imaging bands (3-5 and 8-12pm) and the operating frequencies of several infra-red lasers

(HF, DF and CO2). Production of such material by chemical vapour deposition (CVD) techniques avoids

the difficulties that are likely to be encountered in the scale-up of melt growth or hot-pressing

processes, and results in a polycrystal 1 ine product with a grain size of 50-100pm. The material is

n-type with a DC resistivity of lO^^-lO^ncm. However, the electrical properties are set by the

presence of potential barriers at the grain boundaries, preventing the application of conventional

techniques of electrical assessment. Russell et al [1] have examined the material using Electron

Beam Induced Current techniques and suggest that the electrical properties at the grain boundaries

are a consequence of the segregation of hydride species or transition metal impurities.

The infra-red properties of ZnSe are a direct consequence of the impurity content and defect

structure of the material. For example, Dutt, Kim and Spitzer [2] , who studied aluminium-doped

single crystal ZnSe, found that the intensity of infra-red absorption was dependent on the aluminium

content and the conditions of annealing in zinc-rich atmospheres. It is believed that a selfcompensation of charge carriers occurs in Al -doped ZnSe such that the shallow donor level of

Al (E/\ = 0.026eV) is compensated by the formation of a complex species (Al

z

n -V^p ) which acts as a deep acceptor. Annealing of ZnSe:Al in zinc-rich atmospheres results in a suppression of the zincvacancy concentration and therefore reduces the self -compensation . This results in an increase in the free-carrier absorption effect. However, the local vibrational mode spectrum of such material

86
contains two additional absorption bands which cannot be explained on the basis of this defect

model, and the electronic structure may be complicated by the presence of other defects or impurities. Recently, Dean [3] has highlighted the role of impurities in providing shallow or deep

centres which determine the electrical properties of 1 1 -V I materials, and emphasises the need for a greater degree of impurity control.

Kulakov and Grinev [4] have also examined melt-grown ZnSe:Al, and have found that it is necessary to reduce the free carrier concentration to less than lO^cm"^ j n order to achieve absorption coefficients at 10ym of less than 0.001cm"'.

Some impurities give rise to specific absorption bands in the infra-red. For example, SiO

species give rise to a narrow band near 9.1ym [5] , whilst Cu and Fe give rise to extremely broad

features, centred at 2ym and 3.3ym respectively [6] . The Fe band has little effect on 10.6ym

absorption, but the long wavelength end of the Cu band produces a significant absorption at 10.6ym.

On the basis of the data of Kulakov et al [6] , it is necessary to reduce the Cu content to less than

1 ppm in order to achieve absorption coefficients at 10.6ym of less than 0.001cm"'.

In a study of absorption processes in optical materials, it is also essential to ensure that

differences in the properties of the bulk material are not affected by random differences in surface

absorption effects, which may vary from sample to sample depending on surface preparation and cleaning conditions. This is of particular importance when the contribution of bulk absorption is small,

and laser calorimetry is required for its accurate measurement. This work examines surface absorption and infra-red free carrier absorption (IRFCA) processes in CVD ZnSe using a variety of

techniques

.

IRFCA is produced in CVD ZnSe grown under Zn-rich conditions and results in an increase in 10.6ym absorption coefficient by several orders of magnitude, despite low levels of electrically

active impurities. The origin of this effect is discussed in relation to the defect structure of

the material

.

2. Material Growth and Assessment

The ZnSe examined in this study was produced by chemical vapour deposition from mixtures of zinc

vapour and HoSe gas as previously described [7] . The zinc and h^Se were of nominal 99.999% purity

(Koch Light ttd and British Oxygen Co. respectively), and were transported to the deposition zone within the furnace using an argon carrier stream of greater than 99.999% in purity, derived from a liquefied source. Impurity levels in the product were determined by atomic absorption spectroscopy

and neutron activation analysis. Material was selected for study from batches of similar purity.

Type 1 material was grown near stoichiometric conditions, whilst type 2 was grown under zinc-rich

conditions. Table (1) shows that there are no significant differences in purity of the two materials. The inequality signs represent detection limits.

Samples for optical assessment were cut from the deposit and polished using diamond abrasive

compounds. Infra-red absorption spectra were measured using a Perkin Elmer model 597 spectrophotometer, whilst absorption coefficients near 10.6ym were determined using a laser calorimeter [8]

.

The calorimetric system was used as previously described with the exception of the replacement of

the laser by a 10W step-tu nable CO2 laser (GTE Sylvannia model 950). Photoluminescence (PL) spectra were determined under Hg lamp excitation using a 1 metre monochrome tor (Bentham Instruments) and

lock-in signal processing. Photoluminescence Excitation (PLE) spectra were recorded using conventional lamp/monochromator techniques but with a double-beam sampling system which automatically

corrected for the spectral dependance of intensity produced by the lamp/monochromator source. Samples for PLE measurements were etched lightly with an aqueous solution of potassium ferricyanide/

potassium hydroxide to remove the surface work-damaged layer. For low temperature measurements,

samples were fixed onto the cold finger of a vacuum cryostat.

3. Surface Absorption Processes

The infra-red absorption properties of the ZnSe surfaces were examined using Attenuated Total

Reflectivity (ATR) sampling. In this technique, the infra-red beam is propagated along a thin

polished plate of material by a series of total internal reflections. At each reflection, the

evanescent wave 'samples' a portion of the surface, and on passage along the plate, 25 to 50 such

samplings occur. The ATR spectrum of a polished CVD ZnSe plate, which has been standing in the

laboratory air for a few days is shown in figure (1). The spectrum is dominated by features due to 0-H and -CFL vibrational modes. The OH species presumably arise from atmospheric moisture whilst the CH

n derive from hydrocarbon pollutants. The origin of the sharp band at 1540cm"l is open to debate.

It is not due to the bending vibrations of the -CHn groups since these give rise to the two weaker

bands near 1390 and 1450cm"'. The sharpness of the band suggests that it may be due to -C=0 species,

with the shift in frequency from the usual 1650 - 1750cm"' being a consequence of the electronic

87
interaction with the bulk ZnSe, rather as in formates and organic acid salts of the type

(R-CO2)" M

+ [9]. Indeed, Kirovskaya and Pimenova [10] have studied the adsorption of carbon dioxide

on the related compound ZnTe, and suggest that the surface complex formed has the carboxylate ion

structure

.

Notably absent from figure (1) are any absorption features in the spectral range 900-1000cm"^

.

However, the presence of the 0-H band near 3300cm"l shows that H2O is present which would also give

rise to a flat featureless absorption plateau in the range 900-lOOOcm-' . In addition, the liquid

water spectrum contains a broad absorption feature near 1550cm"^ which may be superimposed on the

band attributed above to -CC^ species. Laser calorimetric measurements on a sample of ZnSe (table 2)

clearly demonstrate the effect of a surface layer of water, and using a value of bulk absorption

coefficient for H2O at 10.6ym of l(rcm~ , we estimate a water film thickness of 100A.

Cleaning of samples using trichloroethane , followed by acetone, was found to reduce the intensity

of the absorption features in figure (1) by an order of magnitude. Samples used for the absorption

studies discussed in the following section were each subjected to this cleaning procedure.

4. Absorption Due to Free Charge Carriers

Samples of ZnSe produced under Zn-rich conditions have been found to exhibit IRFCA. Values of

absorption coefficient 3 for several examples, including material grown near stoichmetric conditions,

are listed in table (3). Despite the similarity in purity of the material (type 1 cf. type 2),

variations in the magnitude of e extend over several orders of magnitude. The calorimetric measurements were carried out using several laser frequencies near 10.6ym in order to check whether the

results were being influenced by optical resonance phenomena. Despite deliberate fabrication of the

samples to be slightly wedge shaped, some resonance was detected in certain cases, and the values of

absorption coefficient used for the purposes of the following discussion are mean values of the

measurements at the different spectral lines of the CO2 laser. No correction is applied for the

effects of residual surface absorption.

The spectral dependence of absorption (measured using the spectrophotometer, figure (2)) reveals

the free-carrier nature of the absorption processes, since 6 is related to wave length A by an expression of the form

On the basis of the discussions of Dutt et al [2] and Jensen, [15] the value of the exponent p of 4.3 suggests that the absorption is dominated by contributions from an impurity scattering

mechanism. Jensen [11] derives an expression for the absorption coefficient of the form

8 = kx p

(1)

(2)

x = (3)

88
and H and D are complex functions of x and r. Other symbols are defined in table (4). In the limit

of r -» °°, equation (2) simplifies to

x

for the case where the final electron state lies in the parabolic region of the conduction band,

oro = \ (6)

when the final electron state lies in the hyperbolic region of the conduction band.

Equations (5) and (6) therefore predict values of p = 3.5 for the parabolic limit, and p = 5 for the hyperbolic limit, since

1.240

A = "hv~

Assuming that the density of charged impurities Ni is equal to the free carrier concentration

np , values of n e may be estimated from equation (1). Usinq the parameters listed in table (4), we obtained carrier concentrations of 0.9 x 10 lb and 6.6 x 10lc, cm"J for the two limits discussed above,

This corresponds to a concentration of electrically active impurity of between 0.4 and 3 ppma. A comparison with the actual values listed for type 2 material in table (1) shows that several impurities are present at concentrations in this range, but it is not possible to state which is

dominating the free-carrier absorption process.

5. Photoluminescent Properties and Electronic Structure

Samples exhibiting the IRFCA effect described above are characterised by a red photoluminescence

centred at 1.97eV. Indeed, figure (3) shows that for several samples, the experimentally determined

absorption coefficient at lQ.6ym is correlated with the luminescence intensity at 1.97eV. The

spectral distribution of the emission when excited by Hg light at 2.84eV is shown in figure (4). A similar emission spectrum is obtained under 3.40eV excitation, although the emission intensity is

reduced by an order of magnitude.

The photoluminescence excitation PLE spectrum of the red emission is shown in figure (5). Two

features are found; the first, at about 2,38eV is the characteristic excitation band for the 1.97eV

process, whilst the second at 2.62eV is much narrower and is due to excitation processes occurring

via shallow impurity states. These spectra are similar to those published by Jones and Woods [12]

for copper-doped ZnSe, demonstrating the importance of Cu impurities in determining the photoluminescent (PL) properties of the CVD ZnSe. It is clear that the red emission is not due to the selfactivated luminescence process since the PLE spectrum contains two maxima, and not just one peak near 2.7eV [ 12] .

As the temperature of measurement is reduced, the narrower PLE band shifts to higher energy,

also revealing some structure in the form of a sharp dip at 2.783eV. Such sharp features are characteristic of exciton processes, and in this case is probably the I]" 66 ? acceptor [3] . The PL spectra exhibit considerable structure at near band-gap energy when examined at liquid helium

temperatures (figure 6). In samples exhibiting IRFCA, the bound-exciton spectral region contains

lines due to 1^ (sodium), I

-j dee p ancj an unidentified donor (l2 Z

) line [13] . On the basis of a comparison with single crystal ZnSe containing chlorine, the In 1 line is about 0.55meV lower in energy suggesting that the donor may be Inv n or F se according to the data of Merz et al [14] . Donoracceptor pair (DAP) features are also found, especially the P(Na) series [ 3] . In material produced

near stoichiometry , the I-^ line is absent, to be replaced by an I-j x (Lithium) line of reduced

89
intensity. However, the most significant difference is the presence of a broad DAP-like feature at just below 2.60eV. It is thought, [21] that this may be the DAP band associated with the l-| dee P exciton.

Measurements of thermally-stimulated current and luminescence (TSC and TSL) carried out on a sample exhibiting strong IRFCA reveal the presence of two traps [15] . The first, at O.leV, is

ascribed to the sodium acceptor, identified by the 1-)^ line above. The second at 0.28eV is too deep

to be associated with In Zn or (Eq ~ 0.029eV) and is probably due to a lattice vacancy or vacancyimpurity complex. Merz et al [ Tb] suggest that the binding energy associated with the I

^ deep

acceptor is about 0.2eV on the basis of Halsted and Avens [17] empirical rule. TSL measurements on a sample exhibiting weak IRFCA reveal two different trapping centres, with binding energies 0.38eV and

0.72eV. It is likely that these are the two centres associated with green and red PL in ZnSe : Cu

[ 12,18] .

The exact nature of the electronic structure of the material is dependent on the identification

of the i ^deep acceptor. Dean and Merz [19] suggested that the i deep

cen ^ re involves a Zn vacancy

following a study of the effect of annealing of ZnSe in vacuum and in zinc vapour. The I

-j deep ii ne

is enhanced following annealing in vacuum, but reduced by the zinc vapour anneal, suggesting that

the vacuum anneal favours the formation of zinc vacancies. We have examined the effect of annealing

a CVD ZnSe sample exhibiting strong IRFCA in an atmosphere of high purity argon, and found that

dramatic reductions in the magnitude of the IRFCA (figure 7) and 1 .97eV PL occur. This evidence,

together with the fact that IRFCA is only found in material grown under Zn-rich conditions, demonstrate the possible importance of the zinc vacancy. On the other hand, recent evidence discussed by

Dean [3], suggests that Ijdeep is associated with Cu^ from a study of the Zeeman behaviour of the

emission. Much of the effects previously ascribed to the formation of vacancies by annealing in vacuo may merely be a result of impurity diffusion processes, from sites such as grain boundaries or inclusions, whilst growth under Zn-rich conditions merely reduces the incidence of impurities on the

zinc sublattice. Such a model has to be reconciled with the results of irradiation experiments on nominally undoped ZnSe. For example, Taguchi and Palmer [20] found that an I] emission line near

2.780eV was produced by lOOkeV proton irradiation at room temperature, and was ascribed, on the basis

of a comparison with 5 MeV electron irradiation and EPR evidence, to a zinc vacancy. However, it is

known that the dominant impurities in the CVD material are the transition metals (Fe , Ni , Cr) and a hydride species [1], and cathodoluminescent studies [1] suggest that these are segregated at grain

boundary sites. The transition metals are well-known luminescence 'killers' [22] - a consequence of

their ability to provide efficient shunt pathways for electron-hole recombination. Thus the impurity

diffusion model described above is a likely explanation for the behaviour of CVD ZnSe on annealing.

The electronic structure of material exhibiting IRFCA which evolves from the above discussion is

shown in schematic form in figure (8). The only donor species identified as being present is Z, and

on the basis of a consideration of the residual impurity content of the feedstock used is tentatively

ascribed to In^

n (Eq = 0.029eV). The red emission of 1.97eV is due to a transition from this donor

to the deep Cu Zn acceptor (E^ = 0.72eV). The PLE maximum at 2.38eV occurs via the transition A to B whilst the narrow maximum of 2.62eV is probably due to a transition from the sodium acceptor to the

donor level. The P(Na) emission series of low temperatures arise from transitions from the indium to

the Na^. Following Dean [3] , the 0.38eV trap is attributed to a complex such as (Cu;7 n -C£se ) . The

origin of the 0.28eV trap is unclear; if it associated with the O.leV trap (Nazn ), then it may involve

a complex centre such as (Na Zn -V se ).

The position of the Cuzn acceptor in figure (8) implies that the activation energy of quenching

of the Cu-red emission at 1 .97eV should be about 0.7eV. However, we determine a value (figure 9) nearer O.leV. In a study of many ZnSe:Cu samples, Jones and Woods [12] also report similar

discrepancies. This suggests that a charge transfer process may be occurring between acceptor

centres in the CVD ZnSe, possibly as a consequency of the presence of transition metal impurities.

The evidence provided by the results of the PL, TSC and TSL stud/ies suggest that the free-carrier

absorption is a consequence of the presence of shallow electronic states in the material . In the case where the dominating electronic states are deep (eg the 0.38 and 0.72eV traps), the magnitude of

free-carrier absorption effects are very much reduced. From the point of view of material growth,

this sets a requirement on the control of levels of impurities such as Al , Na , Li, CI, In, Ga etc in

order to achieve low values of photon absorption at 10.6pm.

6. Conclusions

A study has been carried out of optical absorption due to surface processes and free charge

carriers in CVD ZnSe. Surface absorption is dominated by the presence of water and hydrocarbon

impurities, but can be reduced by careful cleaning of the surface. Infra-red absorption due to freecarrier scattering processes hras been found in ZnSe produced under zinc-rich conditions. The intensity of the absorption process has been found to be correlated with a photoluminescence at 1 .97eV

90
produced in the material under near band-gap irradiation. The electronic structure of the material

is complex, and involves acceptors due to Cu-^ and Na^

n as well as a donor tentatively ascribed to In^p. Some of the PL and PLE phenomena can be explained on the basis of these impurities. The free

carrier absorption is reduced by several orders of magnitude following annealing under inert gas con ditions. This may be a result of the diffusion of impurities such as transition metals and hydride

species from grain boundary sites.

The authors are indebted to the support of the Chemical Analysis Group at AWRE, and to Drs P J Dean and M Skolnick at RSRE, Malvern for the low-temperature photoluminescence measurements.

7. References

[ 1] Russell, G.J., Waite, P., Woods, J. and Lewis, K.L. : Proc. Conf. on Microscopy of

Semiconducting Materials, Oxford, April 1981. Inst Phys Conf Ser 60 (7) 371 (1981)

[2] Dutt, B.V., Kim, O.K. and Spitzer, W.G. : J Appl . Phys 48 2110 (1977).

[3] Dean, P.J. : Czech J Phys B30 272 (1980).

[4] Kulakov, M.P. and Grinev, V.I. : Inorg. Mat. 16 139 (1980) (from Izv. Akad. Nauk SSSR, Neorg

.

Mat. 16. 223 (1980)). ~~

[5] Lewis, K.L. and Hill, J.; Proc. 7th Inf. Conf. on CVD 629 (1979).

[6] Kulakov, M.P., Savtchenko, I.B. and Fadeev, A.V. : J Cryst. Growth 52_ 609 (1981 ).

[7] Lewis, K.L., Cook, D.J. and Roscoe, P.B. : J Cryst. Growth 56 614 (1982)

[8] Gibb, R. and Lewis, K.L. : J Phys E: Sci Inst. TJ_ 304 (1978).

[9] Lee Smith, A. : 'Applied Infra-Red Spectroscopy 1 54_ "Chemical Analysis", Wiley.

[10] Kirovskaya, I. A. and Pimenova, L.N. : Inorg. Mat. 12 191 (1976) (from Izv. Akad. Nauk SSSR,

Neorg Mat. 12_ 221 (1976)). ~~

[11] Jensen, B. : J Phys Chem Solids 34 2235 (1973).

[12] Jones, G. and Woods, J. : J Lumin 9 389 (1974).

[13] Dean, P.J. : Private Communication.

[14] Merz, J.L., Kukimoto, H., Nassau, K. and Shiever, J.W. : Phys Rev. B6_ 545 (1972).

[15] Waite, P. : PhD Thesis, Durham University (to be published).

[16] Merz, J.L., Nassau, K. and Shiever, J.W. : Phys Rev B8 1444 (1973).

[17] Halstead, R.E. and Aven, M. : Phys Rev Lett H 64 (1965).

[18] Stringfellow, G.B. and Bube, R.H. : Phys. Rev T_7]_ 903 (1968).

[19] Dean, P.J. and Merz, J.L. : Phys Rev. 178 1310 (1969).

[20] Taguchi, T. and Palmer, D.W. : Paper presented at 2nd BACG meeting on the Crystal Growth and

Characterisation of 1 1 -V I Compounds. (1980)

[21] Dean, P.J. : Inst. Phys. Cont. Ser 46 100 (1979).

[22] Aven, M. and Prener, J.S. : 'Physics and Chemistry of 1

1 -V I Compounds'. North Holland (1967 ).

91
Table 1. Purity of Material Examined

Impur i ty

Type 1 ZnSe

Type 2 ZnSe

Cu 0.34 0.45

Fe 0.9 0.9

Mg < 3 < 3

Ni 0.25 0.25

Cr 0.8 0.8

Co < 0.6 < 0.6

Al < 2.7 < 2.7 Mn < 0.05 < 0.05

CI 0.12 < 0.12

Br < 0.05 < 0.05

I < 0.03 < 0.03

Values are given in units of ppma.

In addition, traces of Ca were detected using semi -quantitative emission spectrographs techniques.

Elements other than those listed above were not detected.

Table 2. Effect of Moisture on Absorption of ZnSe at 10.6ym

Sample State Absorbance of 10.6ym

Sample diamond polished,

rinsed in tichloroethane

,

acetone and finally water.

Allowed to dry for 1 hour.

Sample re-immersed in acetone, allowed to dry.

0.00356

0.00149

(Sample thickness = 0.255cm)

92
Table 3. Absorption Coefficients of ZnSe Samples Near 10.6ym

(values are uncorrected for the effects of residual surface absorption)

oaliip 1 c iype

Aym

3

-l cm

1 1 10.19 0.0032

10.25 0.0031

10.33 0.0025

10.49 0.0026

10.55 0.0026

10.60 0.0025

10.65 0.0029

10.69 0.0026

10.73 0.0034

2 2 10.30 0.011

10.60 0.0080

10.69 0.0073

3 2 10.49 0.026

10.60 0.024

10.76 0.031

4 2 10.19 0.025

10.30 0.027

10.49 0.029

10.60 0.031

5 2 10.30 0.071

10.49 0.060

10.60 0.044

10.76 0.054

6 2 10.19 0.107

10.30 0.092

10.49 0.106

10.60 0.105

93
Table 4. Parameters Used for Estimation of Carrier Concentration

Symbol Pa rameter Value

Frequency

Absorption Coefficient

: g Bandgap

n Refractive Index

Electronic Charge

e0 Static Dielectric Constant

m_ Effective Electron Mass

Planck Constant

Velocity of Light

Electron rest mass Spin-orbit splitting

,-12

0.124 eV 0.11 cm"

1

2.67 eV (= 4.277 x l(f " erg)

2.4

4.8029 x 10"10 esu 9.12

0.17 m 6.624 x 10"27

erg s (rt 2.998 x 10

10cm S"

1

9.109 x 10"28

g

h/ 2 TT

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96
97
90K

2-2 2U 2-6 2-8 3-0

Photon Energy / eV

Fig 5. Photoluminescence Excitation Spectrum for Red Emission

Process.

98
deep

deep LO

deep 2L0

1

ZnSe type 2

2K

2-8 27 2-6

Photon Energy / eV

2-5

Fig 6. Photoluminescence Spectrum of Type 2 ZnSe Recorded at 2K

Under UV Laser (3.44eV) Excitation. (Courtsey of Dr P J Dean,

RSRE, Malvern).

99
200 400

Anneal time / min (700 C)

Fig 7. Variation of 10.6pm Absorption Coefficient of Type 2 ZnSe

on Annealing in High Purity Argon Atmosphere.

2 7eV 2-62

2-38 1-97

Cu

Zn

InZn

P(Na)

-Na

•(Cu7

- CI ) Zn Be

01 Zn 0-38

Fig 8. Proposed Electronic Structure of Type 2 ZnSe.

(the P(Na) transition is observed at liquid helium temperatures

100

A Laser Damage Facility in the Ultra-Violet

S.S. Wiseall and D.C. Emmony

Department of Physics

Loughborough University of Technology

Loughborough, Leicestershire LEll 3TU

U.K.

A laser damage facility has been set up based on a one Joule KrF excimer laser.

Damage threshold measurements at 193nm have been made on polished samples of LiF, MgF2>

and SrF2 and cleaved samples of LiF, NaF, CaFp, MgF2» SrF2 and BaF2- Observations on the thresholds are discussed and some interesting aspects of the damage morphology

described.

Damage threshold measurements have also been made at 249nm on a range of multilayer

dielectric reflectors. The results indicate that Pyrex may be used as a substrate instead

of fused silica without a reduction in the damage threshold and that the damage threshold

of excimer mirrors exposed to a fluorine environment is substantially reduced over that of

unused mi rrors

.

Key Words: excimer laser; fluoride crystals; laser damage; ultra-violet reflectors; thin

f i lms

.

1. Introduction

One of the major limitations in the development of high energy excimer lasers is the relatively

low damage threshold of the laser mirrors themselves. The purpose of this work was to provide the

KrF laser development programme at the Rutherford Laboratories with a laser damage and diagnostic

f aci-1 ity.

Preliminary damage threshold measurements at 193nm have been made on a range of fluoride crystals. The major excimer wavelength of interest is 249nm. Damage threshold measurements at this

wavelength have been made on a number of multilayer dielectric reflectors.

2. Experimental Details

A schematic diagram of the experimental arrangement is shown in figure 1. The philosophy behind

the experimental approach was to use a large beam area incident in the damage plane to simulate real

tests on lasers with large beam diameters. This also enabled a large percentage of the area of the

sample to be tested making identification of weak spots more probable.

The experimental arrangement is based on a Lambda Physik EMG 200 excimer laser that gives a maximum output of about one Joule at 249nm. The output energy at this wavelength was measured with a Laser Instrumentation Calorimeter and was stable, shot to shot, to about 5%. The temporal pulse

profile illustrated in figure 1 was measured with a vacuum photodiode (EMI Gen-Corn, PD1912 S5) and a Tektronix 466 oscilloscope. The temporal profile remained constant from shot to shot and had a half

width of about 30ns. The energy density in the damage plane was varied by moving the sample away

from the focus. Thus, a variable spot size was used in these damage threshold measurements. In the case of threshold damage caused by defects a spot size dependence _has been observed at 10.6ym, 1.06ym

and 248nm on metals, transparent dielectrics and thin films p] , [2], [3]. The spot size dependence

occurs for the case where the beam dimensions are comparable to the spacing between the defects which

are initiating the damage. In the tests reported here no spot size dependence is expected because

effective spot diameters of 2-5mm were used. The spot size dependence of the damage threshold observed at 10.6pm and 1.06ym on copper mirrors was only observed for spot sizes less than 150ym [2] . Two methods were used to measure the beam size as a function of distance from the lens. The first involved burn patterns on the grey back side of instant polaroid film (Type 667, Coaterless Land Pack film).

The burn area plotted as a function of the lens-sample distance is shown in figure 2. The threshold

values were determined by measuring the pulse energy and the distance beyond the lens where damage

occurred. The two main sources of error in the fluence value calculated using this approach were the

accuracy of the measurement of the lens-sample plane distance and the measurement of the burn area, especially near the lens focus. The fluence values calculated in this way represent some average

fluence and not necessarily the peak fluence. The second method investigated for measuring the

beam size as a function of distance beyond the lens was to use Dylux 608 film [4]. This

102
material turns blue on exposure to UV light near 250nm. The optical density of the blue image was measured by scanning with a helium neon laser. The reproducibility of the beam profile was checked

on a shot to shot basis using the Dylux film and to within experimental error no differences were

observed. The Dylux film was found to suffer from a number of disadvantages. The first is that the

Dylux is not sensitive at 193nm. The peak optical density of the exposed film was found to be some- what time-dependent, even though the film was stored in the dark. A decrease in optical density

between 5-10% was observed in the first week. The main problem with the Dylux film was its limited

dynamic range. A graph of the peak optical density of the Dylux exposure plotted as a function of

the lens-sample distance is shown in figure 3. The horizontal section indicates saturation is occurring as the lens focus is approached. For these reasons the threshold values quoted in this

paper were calculated using the burn area method.

Laser induced damage was defined as the observation of any physical change in the sample when

observed through a X10 microscope. The formation of damage was observed as the shot was fired, the

sample being illuminated with white light. The damage specimen was also observed when removed from

the damage plane at a higher magnification. Threshold damage was often observed as the formation of

several micropits ^10ym in diameter; it was found that these were quite difficult to identify once

the sample had been moved out of the damage plane.

Two damage thresholds were defined. The first definition was the minimum fluence to guarantee

damage in a single shot. The second definition was the maximum fluence not to cause damage after 600

laser shots. This test was performed at a repetition rate of 2Hz. These two definitions seemed to be useful from the practical viewpoint.

3- Damage Thresholds at 193nm

Table 1 summarises the single shot and multishot thresholds of the fluoride crystals. Three

polished samples were tested, Li F, MgFg and SrFp, the rest were cleaved specimens. The Li F and CaF^

wgre supplied in two purity grades, listed as IR grade and UV grade. The tests were performed at

0 incidence. A maximum of three crystals of each type were tested, listed as No. 1, No. 2 and No. 3.

Threshold damage was observed on the entrance surface. All of the cleaved specimens had single

shot damage thresholds in the range 1.5-2.5Jcm~2 except BaF^. Polished samples of Li F and had

single shot damage thresholds somewhat lower than their equivalent cleaved counterparts. This

suggests that the polishing compound remaining in the surface is absorbing at 193nm and initiating

damage. In support of this the polished sample of Li F fluoresced green and the cleaved samples

fluoresced blue. The green fluorescence was particularly strong from surface scratches.

The cleaved surfaces of all of the crystals were 1-2 days old before damage measurements were

made. The only exceptions are those marked with an asterisk in table 1. The single shot damage

thresholds of the two NaF crystals and one Li F crystal were measured within several minutes of them

being cleaved. The high damage threshold of these measurements can be accounted for by the high

water solubility of these two crystals compared to the others.

The single shot damage threshold is about 2-4 times greater than the multishot level. Damage

was generally observed within the first 10-30 shots of a multishot test. This probably means that

damage is initiated in one shot at a lower level than we can detect with our present method. After

10-30 shots the damage has grown sufficiently to be observed.

Figure 4 shows a graph of these damage data plotted against refractive index. Apart from NaF,

there appears to be a trend of higher index materials having lower damage thresholds. This trend has

been observed previously at other wavelengths [5].

4. Damage Morphology at 193nm

4.1 Fluoride Crystals

Some interesting aspects of the damage morphology have been observed on the fluoride crystals.

These are illustrated in figures 5-9.

Figure 5 shows the triangular surface pattern on a cleaved sample of S^. A similar surface

cleaved pattern, but rectangular, is shown in figure 6. Damage with the triangular form was only

observed in the alkali-earth fluorides, the rectangular cleaving only being observed in NaF and Li F. The origin of the cleaving is probably thermal shock the surface experiences from the surface

generated plasma. This type of phenomena has been observed in previous damage work at 1.06ym and

10.6ym on several crystalline material s [5] , [6]. The ring structure shown in figure 7 was observed

on Li F.

103
Rings were observed in experiments involving more than one laser shot. The number of concentric

rings increases with shot number. Ring formation due to a plasma has been observed previously by

Boling et. al at 1.06ym [5]. Bulk damage in Li F is shown in figure 8 and illustrated pictorially

in figure 9. This type of damage was observed at only one site in the polished sample of Li F and the

cleaved sample of SrF2< The tail of the damage is not necessarily perpendicular to the entrance

surface, but in all cases tapered away from it. In a few cases the tail was observed to be curved.

The cause of this type of bulk damage is uncertain.

4.2. SiO Overcoated Aluminium Mirrors

Whilst setting up the damage facility the need arose for some relatively inexpensive aluminium

beam steering mirrors. When the first of these was placed in the beam damage was immediately

observed. The aluminium did not have a MgF2 overcoat as expected but a 1/4^ SiO overcoat. The

silicon monoxide absorbs strongly at 193nm. The interesting aspect of the damage was the diffraction colours observed from the damage region when illuminated with white light. The damage region

was also illuminated with a helium neon laser and characteristic diffraction rings were observed.

Analysis of the ring diameters gave a scattering centre dimension of 1.4pm.

An SEM replica picture of the surface is shown in figure 10. The surface consists of a distribution of uniform ripples or globules of diameter 1.5ym. The globule formation was observed to occur

just above threshold at 0.2Jcm~2. The globule size was not dependent upon the angle of incidence of

the laser light in the range 0-45°. The globule size was also independent of the shot number in the

range 1-20 shots. The picture contrast though did improve somewhat with shot number.

In various parts of the damage area parallel and circular lines appear. These are illustrated

more clearly in figures 11 and 12. The parallel ridges follow the scratch, the scratch width was typically in the range 0.1-0.3ym. The ridge spacing for the parallel lines and circular lines was

the same as that of the background pattern.

_2

Simple calculations show that at the incident energy density of 0.2Jcm there is sufficient

energy to melt the SiO. Ripple patterns on metals and semiconductors have been observed previously

DG ' 00' Various model s have been proposed and are often described by interference between the

incident (and reflected) wave and a surface scattered evanescent wave |T| . These models predict a ripple periodicity dependent upon the angle of incidence with a periodicity equal to the laser wavelength at normal incidence. The parallel and circular patterns observed here also suggest an interference or wave phenomena. The issue is confused though because the diameter of the surface

undulations does not appear to be angle dependent and the ripple periodicity is a factor of seven

times the free space wavelength of the laser light.

5. Damage Thresholds at 249nm

Table 2 summarises the single shot damage thresholds of some multilayer dielectric reflectors

listed as A, B and C. Their nominal reflectivity before testing was 99%. They all had fused silica

substrates unless specified. The description 'used' means they had been operated in a fluorine

environment as an excimer mirror before the tests were performed. The multishot threshold of these

mirrors were also tested and found to be a consistent factor of 3-4 times less than the single shot

threshold. The damage threshold of mirror B was previously determined by the Lawrence Livermore

Laboratory. Their figure of 6Jcm"2 compares favourably with ours of 7.9Jcm~2. Three main points of

interest arise from table 2. _2

(i) The first is the large difference between the threshold of mirror B at 7.9Jcm and the

unused samples of type C at about 2Jcm~2. Mirror B was designed for operation in a non- fluorine environment, whereas mirrors in C were all fluoride coatings made to improve

their fluorine resistance.

(ii) The next point is the factor of six difference between the threshold of the unused

reflectors in C and the Dynasil 4000 substrate used to produce them. This substrate

had a single shot threshold of 12Jcm~2. This large difference illustrates the damage

threshold limitation arising from thin film overcoatings.

(iii) The final point is the large reduction in the damage threshold of used mirrors compared

to unused mirrors of the same kind. The used mirrors had a large number of milky

blotches visible when illuminated in white light. This is presumably caused by fluorine

attack on the multilayers and/or substrate. The used high reflector in table 2 with a Pyrex substrate suggests that as far as damage thresholds are concerned, there is no great advantage in using a fused silica substrate. Newnam et. al [91 havenoticeda 10%

improvement in multilayer dielectric reflector damage thresholds at 249nm when produced

on a fused silica substrate compared to a BK-7 glass substrate.

104
6.. Conclusions

In conclusion, we have set up a laser damage and diagnostic facility in the ultra-violet and

have measured damage thresholds at 193nm and 249nm.

_2

The damage thresholds of the fluoride crystals at 193nm were in the range l-3Jcm . A trend

of lower damage thresholds for high index materials has been observed.

The damage threshold of the multilayer dielectric reflectors at 249nm suggests that Pyrex may be

used instead of fused silica as a substrate material. The damage threshold of an excimer mirror

exposed to a fluorine environment was found to be much less than that of an unexposed mirror.

The authors would like to thank the Science and Engineering Research Council for a research grant to carry out the work. We would also like to thank B.D.H. Chemicals Limited and Technical Optics

Limited for some of the test samples and Dr. F. O'Neill of the Rutherford and Appleton Laboratory for

his help and encouragement.

7. References

[1J C.S. Lee, N. Koumvakalis, M. Bass, Appl. Phys. Lett. 41(7), 1 October 1982.

[2] L.G. DeShazer, B.E. Newnam, K.M. Leung, Appl. Phys. Lett. 23, 11, 1 Dec. 1973.

[3] Foltyn, S.R.; Newnam, B.E.; Jolin, L.J. Laser damage results and analyses for UV reflectors

under multiple-shot irradiation. Nat. Bur. Stand. (U.S.) Spec. Publ . 1981 Nov. Awaiting

publication.

[4] M.W. Taylor, J. Goldhar, J.R. Murray, Appl. Opt. 21, 1, 1 Jan. 1982.

[5] Boling, N.L.; Dube, G.; Crisp, M.D. Laser surface damage studies on several glasses. Nat. Bur. Stand. (U.S.) Spec. Publ. 387, 1973.

[6] Reichelt, W.H.; Stark, E.E. Radiation induced damage to NaCl by 10.6ym fractional joule,

nanosecond pulses. Nat. Bur. Stand. (U.S.) Spec. Publ. 387, 1973.

[7] D.C. Emmony, R.P. Howson, L.J. Willis, Appl. Phys. Lett. 23, 11, 598, 1 Dec. 1973.

[8] F. Keilmann, Y.H. Bai : J. Appl. Phys. A, 29, 9-18 (1982).

[9] Newnam, B.E.; Foltyn, S.R.; Jolin, L.J. Multiple-shot ultra-violet laser damage resistance of

nonguarter wave reflector designs for 248nm. Nat. Bur. Stand. (U.S.) Spec. Publ. 1981,

awaiting publication.

105
damage

sample

calorimeter

vacuum photodiode

dylux film

o

5mv/div

^ 10 ns/dtv

temporal pulse profile

of KrF excimer

Figure 1. Experimental Arrangement.

200

AREA / cm-2

, X / mm

100 180 260 340

Figure 2. Burn area of polaroid film plotted against distance beyond the lens,

.5"

.4-

3

2

1

PEAK OPTICAL

DENSITY

/ X mm

-40 0 40 <60 120

Figure 3. Peak optical density of Dylux film plotted against distance beyond the lens.

106
Table 1. Damage thresholds of fluoride crystals at 193nm.

crystal

type

single shot

threshold ±30%

/ Jcm-2

no.1 no.2 no. 3

multi-shot

threshold ±30%

/ Jcm-2

no.1 no.2 no.

3

LiF I.R grade

UV grade

nnl K hpri

3.9*

2.4

1.8

2.2 2.6

3.3

1.0

NaF cleaved 1.3 2.9 3.0

CaF^ I.R grade

UV grade

2.1 2.51.9

2.6

0.5

0.65

fl^ polished 2.6 2.6

SrF£ cleaved

p olished

1.6

1.3

2.2

0.5

BaF? cleaved 1.2 1.0 0.6

Table 2. Damage thresholds of multilayer dielectric reflectors at 249nm.

TYPE DESCRIPTION SINGLE SHOT

THRESHOLD /Jem"2

A USED 0.6

B UNUSED 7.9

C UNUSED 2.6 UNUSED 2.0

USED 0.3

USED 0.3

USED duran

substrate

0.75

fused silica substrate

dynasil 4000

12.3

107
FLUENCE/Jcm"2

NaF

°

0 MgF

2 °g

o0L1F

OO

CaF

2

°

o °SrF,

"?8aF

1.4 1.5 1.6 REFRACTIVE INDEX

Figure 4. Single shot damage level plotted against refractive index.

Figure 5. Surface damage on SrF

?

,l shot,

fluence 4.2Jcm-2

. Figure 6. Surface damage on LiF, 100 shots

fluence 1.7Jcm-2

.

Figure 7. Surface damage on LiF, 25 shots,

fluence 1.6Jcm~2 .

108
1

1

entrance surface

100-500 urn 20-100 urn

Figure 8. Bulk damage in Li F 3 shots,

fluence 1 . 9Jcm~2.

Figure 9. Geometry of bulk damage,

mm

Figure 10. SiO overcoated Al mirror,

1 shot, 25° incidence, fluence

.18Jcm' 2

, Mg X2000.

Figure 11. SiO overcoated Al mirror,

1 shot, 25° incidence, fluence

J8Jcm"2, Mg X4000.

Figure 12. SiO overcoated Al mirror,

3 shots, 45° incidence, fluence

.18Jcm- 2

, Mg X400O.

109
when asked about the coating materials used in the reflectors that were tested, the speaker replied

that reflectors with "thresholds of about 2 J/cm2 " were made of unidentified fluorides . Materials

in the remaining reflectors were unknown. The speaker commented that attempts were being made to use a model incorporating a surface acoustic disturbance to explain the circular fringe patterns

observed on damage sites. A strong criticism from the audience was made of the placement of damage

samples downstream of the beam waist in the damage experiment

.

110
Use of Hole Gratings in Laser Damage Testing

T. A. Wiggins,* T. T. Saito, and J. A. Hosken

United States Air Force Academy

Colorado 80840

Placing a metal plate with an array of round holes of radius a and separation d in front of a lens which focuses laser radiation for small-spot damage testing produces a set of intense spots of different intensities. The number of spots and their relative

intensities depend upon the ratio d/a and the symmetry of the array. The l/e^ size of

each spot is negligibly larger than that due to the lens alone if a large number of

holes is illuminated. The advantage of using a hole grating is that, if the intensity

loss which depends on d/a is acceptable, damage data can be acquired more rapidly since

identical spots having a wide range of intensities are produced from a single shot.

Further, the role of defects or surface quality can be assessed since several spots

receive equal intensities. Beam quality and sharpness of focus can also be more easily

observed. Examples of damage to copper and nickel surfaces will be presented.

Key words: defects; hole grating; laser damage; small-spot damage

1. Introduction

In small-spot laser damage testing, it would be convenient to present a range of intensities

to a sample in a single shot so that the threshold can be determined more easily than if a series

of shots of different intensities are used. A method for doing this has been suggested [1], How- ever it has the disadvantage of providing an intensity variation in only one direction. A two- dimensional method is presented here.

If an array of round, equally-spaced apertures, a hole grating, is placed in front of the

focussing element used in damage testing, the element will form a Fourier transform of the array,

which is itself an array of regions of high intensity. This grating obviously produces a decrease

in the intensity available for testing, due both to the transmission of the array, and because the

transmitted energy is divided among several intense regions. There are, however, advantages in the

use of a hole grating if the intensity loss does not preclude its use. They are: 1) several

different intensities are available from a single shot; 2) except for the central one, more than

one spot receives a given intensity; 3) the spacing of the spots can be adjusted by selection of

the aperture spacing and focal length of the focussing element; 4) the locations of the spots are known so that they can be easily located to assess the extent of the damage produced; and 5) the

effect of two or more adjacent simul ataneous intense spots can be investigated.

2. Intensity Calculation

Designating the hole radii as a, their spacing d, and the number of holes in a single straight

line that are uniformly illuminated as N, the intensity of a spot for a linear array can be written

where J] is a first order Bessel function and x = ir sine/A. An equation of similar form applies

to a two-dimensional array. The half-intensity angle of each spot will be approximately 1/N of the

spacing between spots.

**Supported in part by AFOSR through the SCEEF. Summer Fellow Program.

*Permanent address, Department of Physics, The Pennsylvania State University, 104 Davey Laboratory,

University Park, PA 16802

Jl(2xa)

-r2

I = I 0 xa

si n(Nxd)

N sin(xd)

111
3. Experimental Results

Two hole gratings have been used to demonstrate their feasibility. They were selected area from screens produced by Buckbee-Mears , St. Paul, MN by photo-etching of stainless steel sheets.

Enlargements of the hole gratings are shown in figure 1. The dimension of the gratings were

a = 1.1 x TO" 2 and d = 5.6 x 10" 2 cm for the 60° array and a = 1.1 x 10" 2 and d = 4.0 x 10" 2 cm for the 90° array. Zero intensity occurs at an angle e * 0.61 A/a, the angular radius of the Airy

disc. The diffraction patterns produced by these gratings are shown in figure 2 made using a beamexpanded He-Ne laser. The relative intensity of the central spot compared with that produced

without the hole grating was measured using a pin-hole apertured photodiode and the He-Ne source. The results were 1:65 for the 60° and 1:31 for the 90° arrays. The values can be compared to the

results 1:65 and 1:18 estimated from the calculated transmissions, (2tt//3) (a/d) 2 and (a/d) 2

, respectively and the ratios of the intensities of the central spots compared with the summations of

the intensities of all the spots within the Airy discs. The agreement for the 60° array is very

good. The discrepancy for the 90° array is probably due to variations in the hole radii resulting

in a blurring of the Airy disc.

Relative intensities have been predicted for these cases and are shown in figure 3. They

have been calculated from the values of J-|(2xa)/xa for values of x = mA/d, where m is an integer.

It is noted that the largest order within the Airy disc is given by m < 0.61 d/a. This is 3 for

the 60° and 2 for the 90° array.

Damage testing on diamond turned metals was performed with a Nd:glass laser system. Figure

4 shows the result using the 60° array on nickel and the 90° on copper. The exact replication of

damage for spots predicted to have the same intensities is reinc rkabl e. It is noted, however, that near threshold, differences can appear indicating the utility of this method for a study of defectinitiated damage.

Figure 5 which shows damage on a copper surface illustrates another use of the hole grating,

that is in the observation of the alignment of a laser cavity. In a high gain medium such as Nd:glass, some misalignment will not prevent lasing. However, such a misaligned cavity will

produce a highly divergent beam as evidenced by the damage shown in this figure. This information

could of course be obtained without the use of a hole grating. However in this case, a series of

shots of different intensities might be required to distinguish between the effects of the system

and those due to scratches, dirt and defects on a surface.

It is noted that if the holes have different radii, in particular following a Poisson

distribution, the Airy disc can be removed and a wider, smooth range of intensities obtained.

4. References

[1] Wiggins, T. A.; Walker, T. W. ; Guenther, A. H. A novel technique for investigating impurity

initiated short pulse laser damage in thin films. Bennett, H. E.; Glass, A. J.; Guenther,

A. H. ; Newman, B. E. , ed. Laser Induced Damage in Optical Materials: 1980. Nat. Bur. Stand.

Spec. Publ. 620; 1981 October. 277-281

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Figure 5. Damage produced on diamond turned copper with the 90° array

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117
MONOLITHIC CELL FOR FREQUENCY CONVERSION

R. Bossert, S. D. Jacobs, and L. Lund

Laboratory for Laser Energetics

University of Rochester

250 East River Road

Rochester, New York 14623

We report the successful design, assembly, and operation of a single cell for frequency conversion

of Nd:glass laser radiation from X = 1054 nm to X = 351 nm. Our approach combines the highly energyefficient "polarization mismatch" scheme previously conceived (1)1 and developed (2) with the

simplicity of the "tandem" crystal approach recently demonstrated (3). The resultant "monolithic"

conversion cell consists of two KDP Type II crystals, assembled with tuning axes orthogonal and

contained between a single pair of windows. An index-matching liquid is used to eliminate reflections

from all internal surfaces.

In this paper we describe a simple birefringence-sensitive method for marking and orienting

circular doubler and mixer crystals with tuning axes orthogonal to better than 60 seconds of arc. The

monolithic cell design is described, with emphasis on the chemical compatibility of component

materials (crystals, spacers, seals, and metal surfaces) with index-matching liquids. A comparison

between Halocarbon (4,5) and Koolase (6) index-matching fluids, after several months of operation

(greater than 400 shots), shows the latter to exhibit better long-term, photochemical stability.

The performance of the monolithic cell is superior to that of separate doubler and mixer cells. It exhibits better long-term pointing stability and produces a cleaner 3u beam.

Key words: frequency conversion; Halocarbon; index-matching fluid; Koolase; KDP; Ndrglass; photochemical degradation; polarization mismatch; third harmonic; Type II crystal.

1. Introduction

The invention (1) of a scheme for highly-efficient energy conversion of Ndrglass laser radiation to its third

harmonic (3co = 351 nm), and the subsequent demonstration of the process using two bare crystals of Type II KDP

(2), have provided the inertial fusion program with an attractive approach toward improving the performance of

laser fusion targets. An overall energy-conversion efficiency, from lu> (X = 1054 nm) to 3u) (X = 351 nm), of 80%

has been predicted and demonstrated for Ndrglass systems using real beam temporal and spatial energy profiles,

The actual deliverable 3 a) energy, however, has fallen below 80% because of optical materials problems. Nonlinear

loss processes at 2u) (X = 527 nm) (4) and photochemically-induced degradation at 3oi (7) have impeded the use of

index-matching liquids for the recovery of energy losses from Fresnal reflections at the conversion crystal

surfaces.

In this paper we describe a frequency-conversion cell which successfully solves the 3 to photochemical

degradation problem at moderate intensity levels and offers additional advantages over previous cell designs. In what follows, we briefly review (a) the "polarization mismatch" scheme for frequency conversion, (b) describe a monolithic conversion cell which combines both doubling and tripling crystals between a common pair of cell windows, (c) give fabrication and alignment guidelines for optimum cell assembly, and (d) discuss long-term cell performance, with special emphasis on the photochemical stability of "Koolase" — the index-matching liquid of

choice for this design and our laser systems.

2. Concept

The "polarization mismatch" concept for frequency conversion is shown schematically in figure la. Both the

KDP second-harmonic generator (SHG) and the KDP third-harmonic generator (THG) are Type II cut, such that the

z-crystallographic axis (the crystal optic axis) makes an angle of approximately 59° with the polished optical

surface normal of each crystal, lw laser radiation, incident on the SHG, is linearly polarized at 35° to the

1 Numbers within parentheses indicate the literature references at the end of this paper.

118
o-direction of the doubler. Provided that the intensity of the incident laser radiation and the thickness of the SHG

are appropriately matched (1,2), equal numbers of 1 wand 2 u> photons emerge from the SHG, which is angle-tuned

for phase matching. These photons are subsequently mixed in the THG to produce 3 w radiation. As indicated in figure la, the THG is rotated with respect to the SHG by 90° about the system optical axis, and must therefore be

angle-tuned in a plane orthogonal to that used to tune the SHG. The thickness of both crystals is chosen to be the same.

Figure lb gives the overall third-harmonic, energy-conversion efficiency versus average 1 co beam input

intensity for crystals whose thickness is 20 mm. A maximum internal conversion efficiency approaching 80% is possible for real-beam (gaussian temporal and flattened gaussian spatial) input intensities of 0.7 GW/cm 2

. The

"polarization mismatch" scheme simultaneously provides high-conversion efficiencies and minimizes sensitivity to beam divergence and phase matching angle (1).

3. Monolithic Cell Design

The orthogonality of SHG and THG crystals in the "polarization mismatch" scheme described above, allows one to design a single frequency-conversion cell which contains both crystals. Angle tuning for proper phase

matching can then be accomplished in a standard gimbal mount. This simplification was first demonstrated by

Summers et al. (3).

The design shown in figure 2a incorporates the SHG and THG crystals between a common pair of fused silica windows. These four optical elements are gently, but firmly, held together by spring loaded plungers and swivel

pads. Three sets of glass spacers separate the crystals from each other and the end windows, and enable indexmatching liquid to coat all six internal optical surfaces. The two external surfaces of the cell possess dual

wavelength 1 co - 3 in AR coatings. The characteristics of materials chosen for the construction and testing of a 60 mm clear aperture device are indicated by the code in figure 2a. The gimbal mounted device is shown in figure

2b. A thermistor temperature sensor, mounted within the cell body in close proximity to the crystals, controls current to an infrared heat lamp, which temperature stabilizies the cell to ±0.05° C. Temperature stabilization is required to decouple the cell from temperature fluctuations that occur in the GDL (Glass Development Laser) laser bay.

One obvious advantage to the monolithic cell design, when compared with separate SHG and THG designs, is that it requires fewer optical elements and AR coated surfaces. In addition, we have chosen to make the input and

output windows identical to simplify spare parts inventory. The use of a single liquid for index matching of all internal surfaces is a mixed blessing (see section 5.0).

4. Assembly and Testing

It is fundemantal to the monolithic cell configuration that the SHG and THG crystals be installed with their o and e directions orthogonal, and that the o and e directions be distinguished from each other (see fig. la).

Satisfying the former condition ensures that angle tuning one crystal in the gimbal mount is decoupled from angle

tuning the second crystal. Satisfying the latter condition ensures that the SHG may be properly oriented with

respect to the incoming 1 go laser radiation.

By utilizing the uniaxial birefringence of KDP, it is possible to devise a simple, inexpensive method for orienting and marking SHG and THG crystals prior to loading and filling the monolithic cell. Figure 3 demonstrates

the two procedures required. Rotation of a crystal through 360° around its optical surface normal, while between

crossed polarizers, produces four intensity minimas in the lock-in detected HeNe signal. These minimas define the

ordinary ray (o) and extraordinary ray (e) directions projected onto the polished crystal surface. These directions

may be marked onto the crystal barrel. By orienting one of these directions at 45° to the incident HeNe

polarization direction, the intensity between crossed polarizers versus angle of incidence can then be measured. If the axis of tilt and the o-direction coincide, a rapidly varying series of intensity maxima and minima , separated by

about 0.7 mrad (depending on laser beam divergence), can be observed. If the axis of tilt and the e-direction

coincide, very little intensity change over tens of mrad can be observed. Both conditions are depicted in figure 3. This orientation technique can define the o- and e-directions of an SHG or THG crystal to within 60 seconds of arc, and is presently more sensitive than our ability to mark and mechanically orient circular crystals in a celL

The 60 mm clear-aperture, monolithic conversion cell, shown in figure 2b, has been operating in our UV thinfilm, damage-testing facility (8,9) since July, 1982. Compared to the separate SHG/THG, two-cell method used

prior to this date, the monolithic cell exhibits greatly improved, long-term pointing stability and has generated a cleaner 3co beam. Figure 4 shows the improved spatial energy profile as measured at a CID camera in our thin119
film, damage-testing facility. Also shown in this figure is the small-signal, phase-matched, angle-tuning curve for

3 a) laser radiation. The monolithic cell has remained at the peak of the 3 to tuning curve since July 1982, whereas

the separate cell configuration required monthly pointing corrections on the order of 1 mrad.

5. Index-Matching Liquid

"Koolase" is the refractive index-matching liquid of choice for our monolithic conversion cell. This

proprietary azeotrope is composed of esters, heterocyclic, and primary hydroxyl compounds (6). It has a viscosity

of 2 est. at 21 °C, a refractive index of 1.44 at X = 337 nm (7), and negligible optical absorption at X = 527 nm and

X = 1054 nm. The linear absorption coefficient for Koolase at X = 351 nm can be as good as 0.37 cm-1

, but this may vary depending on the batch. This fluid was developed as a non-toxic, non-hydroscopic, laser flash tube

coolant, and is therefore photochemically stable under intense UV irradiation. We have determined that Koolase is compatible with KDP and a number of other materials. Table 1 gives the results of a thirteen-month, passive soak

test, conducted at room temperature. Particulates may be removed from the 2.0 um filtered, as purchased liquid,

by additional filtering through a Nalge 0.20 um nylon membrane filter using a glass syringe.

Table 1. Materials compatibility with Koolase

COMPATIBLE NOT COMPATIBLE WHY

KDP • Transparent mylar Swelling

» Fused silica • Dyed mylar Bleeding

Indium • Vinyl tubing Increased UV absorption

• Gold • Tygon tubing Increased UV absorption 6061 Aluminum • Gum rubber tubing Increased UV absorption

• Black anodlzed aluminum • Potyflo tubing Increased UV absorption

• 316 stainless steel • Silicon tubing Increased UV absorption

• Polypropylene • Buna N O-rlng increased UV absorption

• Nylon membrane filter • Neoprene O-rlng Bleeding

• Viton • Polyurthane O-ring Increased UV absorption

• Red silicon orlng • Black delrin Increased UV absorption

• White silicon orlng • Lexan Increased UV absorption

• White delrin • Teflon TFE Increased UV absorption

Teflon PFA • Copper Chemical reaction

• Teflon FEP

• RTV 108

In both our 60 mm monolithic cell and a separate 140 mm THG cell, over the past three to six months,

Koolase has shown no long-term, photochemically-induced degradation to itself (see fig. 5) or to the KDP crystal

surface. The number of laser shots taken and average 3u)fluences, during this time period, were 350 @ 0.2 J/cm 2 and 100 @ 0.75 J/cm 2

, respectively, at a pulsewidth of 1 nsec. The passive absorption losses for loo, 2o>, 3 to laser radiation in the three 150 um-thick, fluid gaps of the monolithic cell have remained below 1% total during this time

period.

Halocarbon oil (4,5), another index-matching fluid that has been found effective and stable in a 140 mm SHG

cell at X = 527 nm, has not exhibited adequate photochemical stability in a 140 mm THG cell. Figures 6a and 6b

indicate that, after two months and 200 shots @ 0.75 J/cm 2

, Halocarbon oil showed an increase in UV absorption,

accompanied by chemical attack to the KDP crystal surface.

120
6. Conclusion

In this paper we have described the design, construction, and performance of a monolithic cell for frequency

conversion of moderate intensity, Nd:glass, laser radiation from X = 1054 nm to X = 351 nm. This design takes

advantage of the highly energy-efficient "polarization mismatch" conversion scheme, wherein two Type II KDP

crystals of equal thickness are orthogonally oriented between a common pair of cell windows. Long-term pointing

stability and 3 w beam quality are enhanced with the monolithic cell design. Long-term photochemical stability

problems, associated with the use of a single index-matching liquid for laser radiation at 1 (4 2 oj , 3 w> have been

solved.

The authors would like to thank Dr. J. A. Abate for providing the 3 co CID photographs, Dr. S. Craxton for valuable suggestions concerning portions of the manuscript, and Eugene Kowaluk for typing the manuscript.

This work was supported in part by the Exxon Research and Engineering Company, General Electric Company,

Northeast Utilities, New York State Energy Research and Development Authority, the Standard Oil Company

(Ohio), the University of Rochester, and the Empire State Electric Research Corporation. This work was also supported by the U. S. Department of Energy Inertial Fusion Project under Contract DE-AC08-80DP40124.

7. References

(1) Craxton, R. S. Theory of high efficiency third harmonic generation of high power Ndtglass laser radiation.

Opt. Comm. 34: 474-478; 1980 September.

(2) Seka, W.; Jacobs, S. D.; Rizzo, J. E.; Boni, R.; Craxton, R. S. Demonstration of high efficiency third

harmonic conversion of high power Nd:glass laser radiation. Opt. Comm. 34: 469-473; 1980 September.

(3) Summers, M. A.; Boyd, R. D.; Eimerl, D.; Booth, E. M. Two-color frequency conversion system for high

power lasers. Technical Digest - CLEO'81, OSA, 1981 June. p.30.

(4) Smith, W. Lee. High-intensity transmission properties of index-matching fluids for laser fusion applications.

Technical Digest - CLEO'81, OSA, 1981 June. p.32.

(5) Available from Halocarbon Products Corp., 82 Burlews Court, Hackensack, NJ 07601.

!6) Available from Kimball Associates, 846-B Rummell Road, St. Cloud, FL 32769.

!7) Jacobs, S. D.; Abate, J. A. Compatibility of transmissive optical materials with high intensity 0.351 ym laser radiation. Technical Digest - CLEO'81, OSA, 1981 June. p.86.

8) Abate, J. A. Laser damage thresholds of optical coatings at 0.351 nm. Technical Digest - CLEO'81, OSA,

1981 June. p.86.

9) Abate, J. A.; Roides, R.; Jacobs, S. D.; Piskorowski, W.; Chipp, T. Laser damage thresholds of optical

coatings at 351 nm. Presented at the 14th Annual Sympoisum on Optical Materials for High Power Lasers,

Boulder, Colorado, 1982 November.

121
122
100

>- o

0.0 0.20 0.40 0.60 0.80 1.00

INPUT INTENSITY GW/cm2

Figure lb. 3 go conversion efficiency as a function of lco input intensity

for two 20 mm thick KDP Type II crystals. Theoretical curve assumes gaussian temporal and flattened gaussian

spatial laser beam profiles.

123
INPUT

1.054 /urn OUTPUT

0.351 urn

CODE ITEM

A Swivel Pad

B Input Window

C Index Matching Liquid

D Spacer

E Type II KDP SHG

F Type II KDP THG

G Output Window

H Cell Housing

I Liquid Seal

J Spring Plunger

DESCRIPTION

Solid Stop for Window

Fused Silica with 1co- 3a> AR

Koolase 0.20 ptm Filtered Glass, t = 150 jum

0 = 58.90° , t = 20 mm

0 = 59.03°, t 20 mm

Biloca with Icj~3co AR

Electroless Nickel

Plated Aluminum 6061

Viton O-ring 75 Durometer

Stainless Steel

Figure 2a. Monolithic frequency conversion cell.

124
Figure 2b. 60 mm, clear-aperture, monolithic cell in a gimbsl mount. Heat

lamp in foreground is for temperature stabilization.

125
• This technique is accurate to 60 seconds ol arc, and is presently more sensitive than our ability to mark and mechanically orient the crystals in the cell

TILT ABOUT o-DIRECTION

100 r

30 20 10 0 10 20 30 ANGLE OF INCIDENCE (mrad)

TILT ABOUT e-DIRECTION

100

> t 3 u

111 H

DC Z

30 20 10 0 10 20 30 ANGLE OF INCIDENCE (mrad)

Figure 3. Method for identifying and marking ordinary ray (o) and extraordinary ray (e) directions

projected onto the polished crystal surface.

126
BEAM PROFILES USED FOR THE

LLE 3 CO DAMAGE TESTER

MONOLITHIC CELL SEPARATE SHG AND THG

Figure 4. Tuning curve and CID camera processed, spatial-energy profiles of

3w beam. Monolithic cell imparts better pointing stability and

beam quality to the UV laser damage-testing facility.

127
0-0 1 ... I ... I ... I ... I 0.24 0.28 0.32 0.36 0.40

X(Atm)

Figure 5. UV stability of Koolase after 3 months and

100 shots.

128
J I I I I I I I : 0.26 0.30 0.34 0.38

Figure 6. Compatibility of Halocarbon oil and UV radiation.

a) Degraded UV transmittance after 2 months and 200 shots.

b) Chemically attacked KDP crystal surface resulting from a).

129
CO2 Laser Polishing of Fused Silica: Recent Progress

P. A. Temple, S. C. Seitel, and D. L. Cate*

Michelson Laboratory, Physics Division

Naval Weapons Center, China Lake, California 93555

A review of CO2 laser polishing of fused silica for increased surface damage

resistance to pulsed 1.064-ym radiation is presented. Discussed will be recent efforts

which have concentrated on (1) determining the rate of sublimation of the surface during

CO2 laser treatment to address the question of surface removal vs surface modification,

(2) Auger line shape and water desorption studies of mechanically polished vs CO2 laser

polished surfaces, and (3) efforts at figure retention of CO2 laser polished surfaces

using a new treatment apparatus which allows simultaneous CO2 laser treatment of both

sides of a sample.

Key words: fused silica; laser damage; laser desorption; laser polishing.

Introduction

In previous work we have reported improvement in the 1.064-um pulsed laser damage threshold for

surfaces of fused silica which have been polished by several minutes of surface exposure to an intense

CO2 (10.59 lim) CW laser beam [1-3]. That work did not answer the fundamental question of why the

pulsed laser damage threshold increased with CO2 laser treatment. In this paper we give evidence

that large amounts of surface are not removed by the treatment process, indicating that the

improvement is most likely due to either the removal of small amounts of impurity or the modification

of the surface material itself, rather than the gross removal of large amounts of material to reveal

undisturbed bulk material.

ESCA spectra of a silicon peak were taken of the fused silica surface before and after CO2

laser treatment. These data indicated that the CO2 treated surface exhibited a slightly higher

binding energy, indicating that the silicon was more highly oxidized in the CO2 treated surface.

Laser desorption spectra were also taken, using a pulsed 10.59-Mm laser as stimulus. These data

showed that surfaces which had been CO2 polished evolved less water when illuminated atsubdamage

threshold intensities than did the mechanically polished fused surface. When the 10.59-pm pulse

strength was increased to cause damage, it was observed that water, which is normally present in bulk

fused silica, was uniformly distributed around the damage site out to several millimeters from the

damage site.

Lastly, a new apparatus is described which allows the simultaneous treatment of both sides of a fused silica substrate. This is an advantage when parts with minimum strain are desired.

Previous Work

Figures 1 through 3 show damage results presented previously [1-3]. Figure 1 is a bar graph

showing the number of sites which survived 100 exposures at each of two intensities on four CO2

laser polished samples and on a typical mechanically polished sample. The "treatment schedules" are

of increasing CO2 laser power. Figure 1 shows that a surface which only has been mechanically

polished will damage on approximately .one-half the exposures at Is and will damage on essentially all

exposures at 1.81 Is. However, after CO2 treatment using schedule 3, there was no damage after 100

exposures at Is and 100 exposures at 1.81 Is. Reference 3 describes in detail the schedules used to polish these parts.

Figure 2 shows the beam energy required to cause damage as a function of beam waist position

in the vicinity of the sample surface. In these measurements, 9-nsec, 1064-nm pulses were focused

D. L. Cate is a cadet at the U.S. Air Force Academy.

Numbers in brackets indicate the literature references at the end of the paper.

130
to a 16.5-um-radius spot size. The sample was positioned so the beam waist was well inside the

sample. Several sites were tested to determine the input energy required to damage 50% of the bulk

sites tested. The sample was then moved 0.25 mm further from the lens so the beam waist was closer to the surface, and the threshold was remeasured. This process was repeated to determine the damage

threshold when the beam waist was (1) in the bulk material, (2) on the surface, and (3) in the air in

front of the surface. The data shown in figure 2 are for a mechanically polished silica sample and

for a silica part CO2 polished by schedule 3. The position 0.0 corresponds to the sample surface

being positioned in the beam waist. For the mechanically polished part, the input energy required to cause damage remained constant as the sample was moved to probe several bulk sites. The input energy

reached a minimum when the waist was on the surface and then increased. The data for the CO2

polished surface are similar, except that no dip in threshold was observed when the beam waist was on

the surface. These data suggest that the schedule 3 CO2 polished surface is as damage resistant as the bulk.

Figure 3 shows data from large-spot testing. Samples prepared by treatment schedule 3 had the

highest thresholds: one survived repeated irradiation at approximately 60 J/cm . This is to be

contrasted with a value of 16 to 23 J/cm for the mechanically polished part which is identical except

for not having been CO2 laser polished as a final finishing step.

Mass Loss Measurements

Figure 4 shows the mass of a single fused silica sample as a function of repeated CO2 laser

irradiation. The 175-, 200-, and 225-watt laser powers correspond to schedules 1, 2, and 3,

respectively, of figure 1. The balance used was a Mettler electronic balance with a manufacturer's

repeatability specification of ±2.0 Mg. This corresponds to a surface removal of ±135 A over the 1.0

square-inch area which was irradiated with the CO2 laser beam. By zeroing the instrument before

each measurement, it was possible to stay well within this specification. Figure 4 shows that the

sample mass changed by less than the ±135 A uncertainty imposed by the manufacturer's specification.

This indicates that the surface is not being removed in massive amounts, revealing "fresh" bulk-like

fused silica with its anticipated bulk-like higher damage threshold. We are left with the possible

removal of selected impurities by CO2 heating or by modification of the silica surface as the reason

for incre'ased damage threshold.

ESCA Spectrum of Silicon in Fused Silica

Figure 5 shows ESCA spectra of a mechanically polished fused silica surface and a schedule 3 CO2 polished surface. The data show a slight shift to higher binding energy in the CO2 polished

surface, indicating a possible increase in silicon oxidation state after CO2 polishing.

Laser Desorption Studies

A mechanically polished part and a CO2 polished part were examined by laser desorption using a

10.59-lJm, 100-nsec pulsed laser as the activation source. The parts were placed in a UHV chamber

which contained a residual gas analyzer, as described by Porteus et_ al . [4]. Masses 15, 16, and 18 were monitored for relative signal intensity. Masses 15 and 16 are hydrocarbon peaks and 18 is

H2O. The results are shown in figure 6. It can be seen that all three species are less prevalent

in the laser polished parts than on the mechanically polished parts. In all cases, the same laser

energy was used for all shots. The intensity was well below the 10.6-Mm pulsed damage threshold for

the silica surface. The reduced hydrocarbon concentration of the CO2 polished parts was in

agreement with Auger data, not shown.

When a sample was damaged by the 10.59-pm beam by intentionally increasing the laser energy, it was found that water was removed from the damage pit and distributed in a circular pattern about the

damage pit. The pattern was several millimeters in radius. Figure 7 shows this effect. In figure 7 are shown mass 18 data for a region of the sample prior to damage. The site at 0.0 was then damaged

with a single pulse. Water desorption data were then taken again along the same row and along an adjacent row displaced 0.5 mm from the first row. It is clear that the mass 18 signal has increased

after damage. It is not clear if the water has been deposited after being ejected from the damage pit

or if the sample surface has been modified, possibly by fracturing, near the site. In any case, these

data indicate that damage data taken too near to existing damage sites may be modified by the previous

damage. It should be noted that fused silica contains substantial amounts of water in the bulk

material. The data also indicate that the amount of water present on a CO2 polished surface is less

than on a mechanically polished surface. Whether this plays any role in the 1064-nm damage threshold

improvement is certainly not obvious.

New CO2 Polishing Apparatus

Figure 8 is a schematic diagram of the CO2 laser polishing apparatus. The device in the lower

left, labeled mirror assembly, is designed to allow treatment of both sides of a sample

131
simultaneously. Figure 9 shows in detail the mirror assembly [5]. Shown is the laser path of the

rastered beam during one-half of its travel. During the other half, it strikes the two mirrors on the

right side of the diagram and the right side of the sample. As the beam is raised and lowered as well

as moved left and right by the "moving mirror" (fig. 8), the entire sample is swept by the laser

beam. This device is small enough to be inserted into a vacuum chamber and will allow the

simultaneous treatment of both faces of a sample in order to minimize the distortion described

elsewhere [1-3].

Summary

Results of earlier measurements were presented which demonstrate the increased laser damage

resistance of CO2 polished fused silica surfaces. The possibility of surface sublimation resulting

in gross removal of material was investigated by tracking the weight loss of a sample as it was treated with successively increasing CO2 laser powers. No detectable weight loss was seen, indicating that the increase in pulsed damage threshold was due to some mechanism other than bulk

removal of "disturbed" material. The ESCA spectra of mechanically polished and CO2 polished

surfaces indicate that there is some shift in peak position of a silicon line. This is an indication

that silicon in CO2 polished surfaces is more highly oxidized than in mechanically polished

surfaces

.

Laser desorption measurements were made on CO2 polished and on mechanically polished surfaces.

These measurements indicated that CO2 polished surfaces release less water and masses 15 and 16 hydrocarbon than the mechanically polished surfaces. These results are consistent with Auger data.

When a CO2 polished surface was damaged, it was observed that a region of several millimeters radius now was capable of giving up large amounts of water when pulsed with subdamage 10.59-ym pulses. The

origin of this effect was not determined, but it may have been splattered water-rich ejectorate, or the surface may have been altered to allow the release of water trapped just below the surface.

Finally, an apparatus designed to allow the polishing of both sides of a fused silica substrate

was described. This apparatus makes use of four mirrors to redirect the rastered beam in a pattern

which strikes first the left side and then the right side of the sample.

References

[1] Temple, P. A.; Milam, D. ; Lowdermilk, W. H. "C02- laser polishing of fused silica surfaces for

increased laser damage resistance at 1.06 micrometers," in Laser Induced Damage in Optical

Materials: 1979 , Bennett, H. E.; Glass, A. J.; Guenther, A. H. ; Newnam, B. E., ed. 1979 October

30-31; Boulder, CO. Nat. Bur. Stand. (U.S.) Spec. Publ. 568; 1980 July. Pp. 229-236.

[2] Temple, P. A.; Soileau, M. J. "1.06 micrometer laser-induced breakdown of C02-laser-polished

fused S102," in Laser Induced Damage in Optical Materials: 1980 , Bennett, H. E. ; Glass, A. J.; Guenther, A. H. ; Newnam, B. E. , ed. 1980 September 30-0ctober 1; Boulder, CO. Nat. Bur. Stand.

(U.S.) Spec. Publ. 620; 1981 October. Pp. 180-188.

[3] Temple, P. A.; Lowdermilk, W. H. ; Milam, D. "CO2 laser polishing of fused silica surfaces for

increased laser damage resistance at 1064 rim," Appl. Opt. _21_; 3249-3255; 1982.

[4] Porteus, J. 0.; Faith, W. N. ; Allen, S. D. "Laser desorption analysis of H2O and other

contaminants from optical surfaces," presented at the 13th Annual Symposium on Optical Materials

for High Power Lasers; Boulder, CO; 1981 November.

[5] Cate, D. L. A New Mirror Apparatus for Symmetric CO2 Laser Polishing of Fused Silica , Naval

Weapons Center, China Lake, CA; NWC Technical Memorandum 4896; 1982 September.

132
3.28 mJ

34.3GW/cm2

307 J/cm2

MECHANICAL (1) POLISH

TREATMENT SCHEDULE

Figure 1. Bar graph showing the number of sites which survived 100

shots at each of two intensities on three multipass CO2 polished

samples and on a typical mechanically polished sample. The sites were illuminated on a 0.5-mm grid with only one exposure per site.

6.0

5.0

4.0

| 3.0

ui

LU

2.0

1.0 -

1.5 1.0 0.5 0 0.5 — -> + POSITION mm

1.0 1.5

/ — +

1.06/im

9.0 nsec 16.5Mm SPOTSIZE

O MECHANICAL POLISH

A C02 LASER POLISH

2.0

Figure 2. Beam energy required to cause damage as a function of beam waist position.

The solid lines are not mathematical fits but are hand-sketched to aid in interpretation.

133
60 r

(m 50

u

K 40

CO

UJQ

>O

cc

LiJ 2

30

E

UJ

20

10

[

_L _L MECHANICAL

POLISH

1 SCHEDULE

SAMPLE TREATMENT SCHEDULE

Figure 3. The 1064-nm, 1-nsec damage threshold on fused

silica surfaces as a function of treatment schedule used

in multipass CO2 laser polish.

15.94794

92

m

v.<

<

to

90

88 -

86 -

15.94784

MFGR. REPEATABILITY SPEC.

(±135A/sq.in.)

_L INIT. 100 150 175 200

POLISHING LASER POWER (w)

225

Figure 4. The mass of a sample of fused silica which was laser polished

over a 1-inch-square area at successively higher laser intensities.

134
112 110 108 106 104 102

VOLTS

Figure 5. ESC spectrum of a silicon transition in a mechanically

polished and a CO2 laser polished spectrum.

M=18 LASER

MECHANICAL

M=16 LASER

MECHANICAL

M=15 LASER

MECHANICAL

0 10 20 30 40 50

RELATIVE DESORPTION SIGNAL

Figure 6. Relative desorption signals for masses 15, 16, and 18

for a mechanically and a CO2 laser polished surface. The molecular

desorption was stimulated by a 10.6-um, 100-nsec laser pulse of

subdamage threshold intensity.

135
q

t/i

DC

LU

UJ

< 2

UJ

T T I BEFORE DAMAGE

AFTER, SAME ROW.

AFTER, AJACENT ROW

A

a

-6 -4 -2

DISTANCE [mm)

0

FROM DAMAGE SITE

Figure 7. Desorption signal for mass 18 as a function of position.

The signal which resulted from the single damage pulse at 0.0 mm

is not shown.

IMAGE

PLATE BEAM SPLITTER AND HeNe LASER

\ MIRROR

ASSEMBLY

C02 LASER

M (MIRROR)

3.5 m

Figure 8. Diagram of the CO2 laser

polishing apparatus.

136
BASE PLATE

V

SAMPLE HOLDER

Figure 9. Top view of the two-sided polishing apparatus.

The laser path shown sweeps one side of the sample. During

actual use, the beam sweeps the entire front mirror, thus

striking both sides of the sample.

137
Float Polishing and Defects of Fused Silica*

Yoshiharu Mamba

Institute for Modern Optics

Department of Physics and Astronomy

The University of New Mexico

Albuquerque, New Mexico 87131

A float polishing machine has been newly developed to finish optical components. The

machine has a rail head and a precise main spindle of both high rigidity and rotational accuracy,

and can cut a tin lap of 460 mm in diameter to the degree of 1 ym in flatness. Very good

flatness of 0.03 urn (A/20) on BK-7 glass of 100 mm in diameter and extremely small surface

roughness of 2.2 angstroms rms on fused silica were obtained by the machine. Defects of fused

silica were examined by etching and Nomarski microscopy. Volume defects such as bubbles or inclusions in ym size seem to exist in fused silica.

Key words: Defect; flatness; float polishing; fused silica; laser damage; surface roughness.

1. Introduction

Both machining accuracy and productivity are the most important factors in the manufacturing of

optical components. Moreover, higher surface damage thresholds are required on the optical components for high power lasers. Previous works reported that the threshold was a function of the sur- face conditions, such as surface roughness [1]J pits, microcracks [2], entrapped polishing particles

[3,4] or contamination [2] from polishing compounds and laps. Therefore, the surface of supersmooth, damage-free and no contamination is required for the optical components for high power lasers

[5] in order to improve the laser-induced damage threshold on the polished surface and interface

between the substrate and coating film. The properties of the interface and coating film seem to be

affected by qualities of polished surface of the substrate.

The float polishing technology [6,7] was invented for polishing electronic materials ultrafinely

without deformed layer. Sapphire and Mn-Zn ferrite single crystals were polished with various fine

powders in distilled water on a tin lap, and super smooth surface finishes better than 10 angstroms

Rz with crystal lographic perfection were obtained [6,8]. Therefore, this technology seems to be

useful for making high energy laser components.

In this paper, a newly developed float polishing machine for making optical components, some results of surface topography on the float-polished glasses, and examination of defects on fused

silica samples polished by the state-of-the-art are described.

2. Development of Float Polishing Machine

The float polishing technology has been widely used in manufacturing magnetic heads for video

cassette recorders in Japan and has improved the performance of the heads remarkably [9]. Very good

flatness A/20 = 0.03 ym on the area of 7x7 mm^ excellent retention of edge geometry less than 100

angstroms in radius and extremely small surface roughness better than 10 angstroms Rz were obtained

on single crystal surfaces by the float polishing [6-10].

A float polishing machine was newly designed as shown in Fig. 1 in order to use the float

polishing technology for making optical components. This machine of double housing type looks like a diamond turning machine [11], and has a high accuracy vertical main spindle with hydrostatic bearing

of both high rigidity and rotational accuracy, a cross rail with a rail head and work spindle, and a filter unit for ultra-clean air. The machine can cut a tin lap of 460 mm in diameter to the degree

of 1 ym in flatness with a diamond tool as shown in Fig. 2. The key of making workpieces flat by

polishing is the flatness of a lap, because the flatness is transferred on the work surfaces during

*Work supported by the Air Force Weapons Laboratory, Kirtland AFB, New Mexico, under contract

F29601-82-K0017.

figures in brankets indicate the literature references at the end of this paper.

138
polishing process. The continuously dressed flat annular machines are widely used for polishing

ultra-flat optical surfaces [12]. In this case it is the most important that the pitch surface wears

uniformly, the flatness of which is indirectly monitored by measuring the flatness of the standard

sample on the pitch surface periodically.

A spiral groove of 2 mm in pitch is cut on the commercially-pure tin lap withacut-off tool of

high speed steel as shown in Fig. 3. Chips are removed by vacuum during the cutting process. Then

the tin lap surface is cut with a diamond tool as shown in Fig. 4, and the feed rate is 0.3 mm/rev.

After cleaning up the lap with distilled water and putting a stainless steel ring on the base plate

of stainless steel under the tin lap for making a vessel, the polishing fluid mixed with distilled

water and 2 wt% Si02 powder (Aerosil 300, Degussa Corp.) of 70 angstroms in nominal diameter is poured into the vessel. Workpieces to be polished are put on the work holder, then the latter is mounted at the edge of the work spindle in order to make a universal joint.

Figure 5 shows the actual polishing condition. During the polishing process, the work spindle

rotates the work holder of 180 mm in diameter, and the polishing pressure is applied on the workpieces only by the weights of the work holder and workpieces so as to maintain the stable movement

of workpieces. The workpieces are floating in a stable state, or are suspended in the polishing fluid

by the hydrodynamic behaviour of fluid between the workpieces and grooved tin lap. In this figure

the tin lap and work holder are rotating at the same revolution of 80 rpm in the same direction.

3. Polishing of Glasses

The conventionally pitch-polished BK-7 glass of 100 mm in diameter and 30 mm in thickness was polished with the new float polishing machine as shown in Fig. 6. In this case, the diameter of the

work holder was the same as the workpiece which was mounted under the work holder with a doublesticky tape in order to prevent the workpiece from the thermal deformation due to waxing process.

Figure 7 shows the flatness of the float-polished BK-7 glass sample, measured with Zygo Mark III, Phase Measuring Interferometer System. Before the measurement, the sample was left on the Zygo

system in a temperature-controlled measuring room for more than 24 hours in order to eliminate

thermal errors. The result shows that the peak-to-valley flatness is 0.049 x (wavelength) = A/20 = 0.031 vim and rms flatness is 0.006 x (wavelength) = A/167 = 0.0038 ym = 38 angstroms on the sample.

In this connection, Zygo Mark III, Zapp System detected that the peak-to-valley flatness was 0.029 x (wavelength) = A/34 = 0.018 ym and rms flatness was 0.006 x (wavelength) = A/167 = 0.0038 ym = 38

angstroms on the same sample. The float-polished sample seems to be very flat beyond the measuring

limit of the measuring machine.

Fumed silica named Aerosil 300 of Degussa Corporation has the very low stock removal power for

polishing fused silica, however, it is too small in size to make any scratch on the fused silica

surface and is very pure as same as silicon single crystals which are widely used in the semiconductor industry. Pure fumed silica composed of the same composition as fused silica seems to be useful

for preventing the contamination during polishing [2,6]. J. M. Bennett measured the surface profile

of float-polished fused silica samples of 38 mm in diameter with Talystep [13] as shown in Fig. 8. The measuring conditions were the stylus radius of 1 ym and loading of 2 mg. The figure shows 2.2

angstroms rms roughness in the measured length of 800 ym on polished fused silica, 1.9 angstroms rms

in 60 ym length, 1.6 angstroms rms in 15 ym length, and the instrumental noise corresponds to 1.1 angstroms rms. From the previous experience of polishing single crystals [8], this surface roughness

number of 2.2 angstroms rms may be considered to reduce to some extent by the successive experiment.

Colloidal silica of alkalinity has higher removal rate than the fumed silica powder which shows

acidic in water, however, the rougher surface of fused silica is obtained by the float polishing with

the colloidal silica than that with the fumed silica.

4. Defects of Fused Silica Surface

Polishing technology for fused silica has been developed to reduce the surface roughness and

mechanically-damaged layer [1,14]. Sawed and ground glasses have numerous reticulate cracks under

the surface [2]. Controlled grinding was performed to remove the surface damage, assuming if proper

amounts of the surface were removed during each stage of grinding and polishing, the damage would be

disappeared [14]. However, various defects were detected visually under the Nomarski interference

microscope of Nikon Optiphot on all of the tested samples [1,14,15] by etching with hydrofluoric

acid, and such defects have remarkably reduced with the times. Figure 9 shows typical defects on well-polished fused silica surface [1] after 5 minutes etching with 5 % HF. These defects are classified as follows; (1) a series of semicircular microcracks, (2) a hollow due to a semicircular

microcracks, (3) a faint scratch, (4) a tiny pit due to bubble, inclusion or residual damage of premachining, (5) depression due to different phase. Most of the defects above-mentioned can be

removed by careful polishing processes [14,15], but tiny pits of few micrometers in diameter were

139
remained on all samples tested in this study.

Figure 10 is a Nomarski micrograph on the etched surface of fused silica after polishing by VTI,

Inc. and laser irradiation by AFWL, and shows two black spots representing two separate pits and a laser damaged site. This sample was remarkably well-polished and very smooth. Only defects on the

polished surface after etching were tiny pits and small number of depressions which were detected

only on this sample. These small pits of few micrometers and sub-micrometers can be considered to be

caused by bubbles and inclusions contained in the bulk material or by small cracks due to pre-machining. As the results of observing the birth and death process of tiny pits on the fused silica surface during the float polishing, it was detected that new pits appeared and pits previously existed were removed during polishing. Therefore, these pits seem to be caused by volume defects such as bubbles and inclusions contained in the bulk material. It is necessary to study such a small volume

defects in fused silica in the future, and the total internal reflection microscopy [16] is one of

the best method to detect the defect.

In order to study the effects of bulk material, surface and coated film on the laser damage

threshold, fused silica has been used as bulk material because it is optically uniform and can be

used for optical components in the range of visible and near infrared. However, the fused silica

samples used in this experiment may be considered to contain volume defects which are not known in detail, so that in order to perform the basic research work on laser damage in optical materials it

is recommended to replace fused silica with single crystal materials which still contain point and

line defects, but rarely volume defects. Sapphire seems to be one of the best materials as such

single crystals, and it is easy to polish without damage layer [6], It is relatively easy both to detect various defects on the sapphire surface of good quality and to know the origin of laser damage

of bulk surface by using various modern methods because of crystal.

Figure 11 shows a series of a laser damage site on the fused silica sample, taken with the

Nomarski microscope of Nikon Optiphot. 7.6 angstroms rms surface roughness was measured by TIS

measurement on the test surface before laser irradiation [14]. Most of the patterns caused by

laser irradiation of 1.06 urn wavelength did not change the status even after etching with 5 % HF for

5 minutes and 25 % HF for 5 minutes. In the case of crystallographic materials, plastically-deformed

region or area having residual stress can be revealed by etching because these areas are more active

and have anisotropy of etching rate. Figure 11 shows that the slight laser damage on the front

surface may be only changed topographically but not metallurgically, and the damage site did not change the condition after etching. This phenomenon seems to be unique existing only on the

amorphous material, therefore the laser polishing can be considered to be effective for eliminating

defects on the mechanically-polished fused silica [17,18].

5. Conclusions

From this paper the following conclusions may be drawn:

1) The flatness of A/20 = 0.03 urn and rms flatness of A/167 = 0.0038 ym on BK-7 glass sample of 100 mm in diameter and the surface roughness of 2.2 angstroms rms on fused silica sample of 38 mm in diameter were obtained by using the newly-developed float polishing machine.

2) Pits of few micrometers or sub-micrometers in size were appeared on all fused silica samples

tested in this study after carefully-polishing and etching with HF. These pits seem to derive from

volume defects such as bubbles and inclusions in the bulk.

3) The difference of metallurgical subsurface disorder between the polished surface and 1.06 urn laser damaged site on fused silica can not be observed by etching with 5 % and 25 % HF. This is the

feature of amorphous material and the laser polishing seems to be available to get the higher damage

threshold on conventionally-polished surfaces.

6. Acknowledgments

The author would like to thank Dr. A. H. Guenther of Air Force Weapons Laboratory and Prof. M.

0. Scully of University of New Mexico for encouraging this work, Prof. K. C. Jungling of University

of New Mexico and Dr. A. F. Stewart of Air Force Weapons Laboratory for helpful discussion, Dr. J. M. Bennett of Naval Weapons Center and Mr. W. E. Kunzler of Air Force Weapons Laboratory for measuring

surface roughness, Mr. J. W. Bender of International Laser Systems, Inc. for various help to this

experimental work, and Mr. S. F. Soares of University of New Mexico for the experimental work.

A part of the work to make the float polishing machine was supported by Grant-in-Aid for

Developmental Scientific Research No. 485063 of the Japanese Ministry of Education.

140
7. References

[I] House, R. A., Bettis, J. R. , Guenther, A. H. and Austin, R. , Correlation of Laser-Induced Damage

with Surface Structure and Preparation Techniques of Several Optical Glasses at 1.06 urn, Laser

Induced Damage in Optical Materials: 1975, NBS Spec. Publ. 435, 305 (1976).

[2] Landingham, R. L., Casey, A. W. and Lindahl, R. 0., Effects of Various Polishing Media and

Techniques on the Surface Finish and Behavior of Laser Glasses, The Science of Ceramic Machining

and Surface Finishing II, NBS Spec. Publ. 562, 231 (1979).

[3] Boling, N. L., Dube, G. and Crisp, M. D. , Laser Surface Damage Studies on Several Glasses, Laser

Induced Damage in Optical Materials: 1973, NBS Spec. Publ. 387, 69 (1973).

[4] Leung, K. M. and DeShazer, L. G. , Surface Defects on Crystals of Ti0 2 and YVO^ Studies by LaserInduced Damage Effects, Laser Induced Damage in Optical Materials: 1974, NBS Spec. Publ. 414,

193 (1974).

[5] Bass, M., Leung, K. M. and Tang, C, Pulsed Laser Induced Damage at the DF Laser Wavelength,

Laser Induced Damage in Optical Materials: 1976, NBS Spec. Publ. 462, 136 (1976).

[6] Namba, Y. and Tsuwa, H., Ultra-Fine Finishing of Sapphire Single Crystal, Annals of the CIRP,

26/1, 325 (1977).

[7] Namba, Y. and Tsuwa, H., Float Polishing of Mn-Zn Ferrites and Characteristics of Finished

Surfaces, Proceedings of 4th International Conference on Production Engineering, 1017 (1980).

[8] Namba, Y. and Tsuwa, H., Mechanism and Some Applications of Ultra-Fine Finishing, Annals of

CIRP, 27/1, 511 (1978).

[9] U. S. Patent 4,358,295 November 9, 1982. "Polishing Method".

[10] Namba, Y. and Tsuwa, H., Ultrafine Finishing of Ceramics and Metals by Float Polishing, Laser

Induced Damage in Optical Materials: 1980, NBS Spec. Publ. 620, 171 (1981).

[II] Donaldson, R. R., Large Optics Diamond Turning Machine, LLL, UCRL-52812 (1979).

[12] Cooke, F., Brown, N. and Prochnow, E., Annular Lapping of Precision Optical Flatware, Optical

Engineering, 15, 407 (1976).

[13] Bennett, J. M. and Dancy, J. H., Stylus Profiling Instrument for Measuring Statistical Properties of Smooth Optical Surfaces, Applied Optics, 20, 1785 (1981).

[14] Bordzol , L. and Kurdock, J. R., Dielectric Samples Program, Final Report, Report No. 14955,

Contract No. F29650-80-C-009 (1980).

[15] Fused silica sample polished by VTI, Inc. (1982).

[16] Temple, P. A., Total Internal Reflection Microscopy: A Surface Inspection Technique, Applied

Optics, 20, 2656 (1981).

[17] Temple, P. A., Milam, D. and Lowdermilk, W. H., C0 2 -Laser Polishing of Fused Silica Surfaces for

Increased Laser Damage Resistance at 1.06 ym, Laser Induced Damage in Optical Materials: 1979,

NBS Spec. Publ. 568, 229 (1980).

[18] Temple, P. A. and Soileau, M. J., 1.06 ym Laser-Induced Breakdown of C0 2 -Laser-Pol ished Fused

Si0 2 , Laser Induced Damage in Optical Materials: 1980, NBS Spec. Publ. 620, 180 (1981).

141
Figure 1. Newly developed float polishing machine

in a clean room.

M

Figure 3. Cutting a spiral groove on a tin lap

with a high speed steel tool.

Figure 5. Actual condition of float polishing of

3 samples of 76 mm in diameter at a time.

Figure 2. Diamond-turning of a tin plate

like a mirror.

Figure 4. Final cut of the tin lap with a diamond tool

.

Figure 6. Actual condition of float polishing

of a BK-7 glass sample of 100 mm in diameter.

142
Figure 7. Flatness of a float-polished BK-7 glass sample of 100 mm in diameter, measured with Zygo

Mark III, Phase Measuring Interferometer System.
? I I

Figure 9. Nomarski micrograph of well-polished fused silica [1] after 5 minutes etching with 5 % HF„

50 yrn

Figure 10. Nomarski micrograph of laser damaged site on the fused silica sample polished by VTI

,

Inc., taken with Nikon Optiphot (Two arrows indicate pits after 5 minutes etching with 5 % HF).

144
(a) After laser irradiation

(b) After first etching using 5 % HF for 5 minutes

10 um

(c) After second etching using 25 % HF for 5 minutes

Figure 11. Nomarski micrograph of a laser damaged site on the polished fused silica [14],

145
Effect of Surface Finishing on the Damage

Threshold of Fused Silica at 1.06ym*

H. Vora

Honeywell Corporate Technology Center

10701 Lyndale Avenue South

Bloomington, Minnesota 55420

and

W.H. Lowdermilk and J.E. Swain

Lawrence Livermore National Laboratory

University of California

P.O. Box 5508, L-470

Livermore, California 94550

In an effort to determine the effect of surface finishing on the damage threshold

of fused silica at 1.06 ym, several fused silica samples were polished mechanochemically

using SrC03, MgO and NiO slurry, and colloidal silica as polishing media as well as mechanically using 0.3pm sapphire abrasive. Employment of a hard abrasive, such as sapphire, in polishing of fused silica introduces scratches and subsurface microfractures,

which are not observed in mechanochemically polished samples of fused silica because the

material removal in mechanochemical polishing occurs through chemical reactions which are activated by application of pressures during polishing. This paper describes various

mechanochemical polishing techniques used to prepare fused silica surfaces as well as the results of damage threshold measurements made on mechanochemically and mechanically

polished samples of fused silica using 1-ns, 1.06-ym laser pulses.

Keywords: Fused silica; mechanochemical polishing; 1-ns, 1.06-ym damage.

1. Introduction

Earlier measurements of the surface damage thresholds for 1-ns, 1.06-ym laser pulses for a series

of BK-7 and fused silica samples whose surfaces were prepared principally by polishing on a pitch lap

using a slurry of cerium oxide in water have yielded the median value of 16 J/cm^, which is two to three times lower than the bulk damage thresholds of these materials [1]!.

Recently, it has been shown by Temple, etal. [2] that the damage threshold of mechanically

polished fused silica surfaces can be increased to a value comparable to the bulk value by carbon

dioxide laser polishing. Two possible explanations were given for the observed increase in the

surface damage threshold of fused silica by carbon dioxide laser polishing: vaporization of the

absorbing impurities (polishing compounds, etc.) from the mechanically polished surface and healing

of surface and subsurface microfractures introduced by conventional grinding and polishing processes.

The objective of this work is to determine if the surface damage threshold of fused silica can

be improved by mechanochemical polishing. The material removal in mechanochemical polishing occurs through chemical reactions between the workpiece and the polishing medium and as a result of this,

fused silica samples free of scratches and subsurface microfractures are relatively easy to obtain

using this technique. If the measured damage threshold values of conventionally polished fused

silica surfaces are lower than the bulk due to the presence of surface and subsurface microfractures,

then significant improvement in the surface damage threshold of fused silica can be realized by

mechanochemical polishing. However, if the damage threshold of bare fused silica surfaces is limited by the presence of absorbing impurities in the polished surface, then mechanochemical

polishing may not have a significant effect on the surface damage threshold of fused silica. The reason for this is that, except for the employment of higher pressures in mechanochemical polishing,

* Work supported by the Lawrence Livermore National Laboratory under Contract #7298101.

Numbers in brackets indicate the literature references at the end of the paper.

146
the mechanochemical polishing is performed in a manner very similar to the conventional polishing.

In both techniques, the workpiece surface is in intimate contact with the lap surface and the

polishing medium during polishing. Because of this, the absorbing impurities (which can be polishing

compound itself or some impurities present in the polishing compound and lap) are as likely to imbed

in the mechanochemically polished samples as in conventionally polished samples.

2. Results and Discussion

Damage threshold measurements for 1-ns, 1.06-ym laser pulses were made on four sets of fused

silica samples; each set consisting of three samples which were polished simultaneously. One set was polished mechanochemically with colloidal silica to completely remove scratches and subsurface

microfractures introduced by previous grinding and lapping operations. This was verified by

Nomarski microscopic examination of the polished surfaces after etching in a 5% HF acid solution.

The remaining three sets were similarly polished with colloidal silica initially. Subsequently, one set was polished with a slurry of NiO and MgO in water, another was dry mechanochemically polished

with SrCC^, and the third set was polished with 0.3ym sapphire. Scratches and subsurface microfractures were observed in fused silica samples polished with sapphire, but not in those polished

with SrC03 or the slurry of NiO and MgO in water. The hardnesses on Mohs scale of SrC03, NiO, MgO

and AI2O3 are respectively 3.5, 5.5, 5.5 and 9 as compared to Mohs hardness in the range of 5.5-6.5

for fused silica. Nomarski micrographs taken before and after HF etching of mechanochemically

polished surfaces of fused silica are shown in figure 1.

All fused silica samples used for damage threshold measurements were polished on a polyurethane

impregnated polyester lap, which restricts flatness to 1/2-1 light band per centimeter of the

polished surface. Measurements were made of the removal rates obtained by mechanochemically polishing

fused silica on this lap using SrC03, MgO and NiO slurry, and colloidal silica as polishing media

and the following parameters: pressure, 1.4x10^ N/m2; lap diameter, 25cm; lap rpm, 50; and stroke

frequency, 30cpm. The data on material removal rates are shown in figure 2 and figure 3, where it

is seen that the measured removal rates are 0.3ym/hr for SrC03, 6pm/hr for colloidal silica and

8ym/hr for NiO and MgO slurry.

The results of damage threshold measurements are summarized in table 1, where it is noted that

the exit surface damage thresholds for 1-ns, 1.06-vim laser pulses for all mechanochemically polished

fused silica samples lie in the range of 10^17 J/cm^, which is only marginally higher than the range

of 9-11 J/cm^ measured for fused silica samples polished with 0.3pm sapphire. These results are comparable to those for surfaces polished with cerium oxide. Thus, the process of simply eliminating

surface and subsurface microfractures through mechanochemical polishing is not by itself effective in significantly raising the damage threshold of fused silica.

Table 1. 1-ns, 1.06-ym Exit Surface Damage Thresholds

of Various Fused Silica Samples

Polishing Medium Damage Threshold

Set Initial Final ( J/cm?)

1 col loidal si 1 i ca 16.,8 + 1.,8 11. 5

+

1,,2

9. 7 +1,,0

2 colloidal si 1 i ca SrC03 16,,8 +1 ,9 14. 4

+

1,,5 11. 8

+ 1,,4

3 col loidal silica MgO & NiO 13. 8

+

1,,8 10,,4 + 1,,0 10. 2 +

1,,0

4 colloidal si 1 ica 0.3pm sapphire 11,,0 42,,6

9.,7 +

1,,0

9.,3 + 1,.0

147
The authors would like to thank Drs. K.D. McHenry and B.G. Koepke of the Honeywell Ceramic

Center for suggesting SrC03 compound for mechanochemical polishing of fused silica.

3. References

[1H Lowdermilk, W.H., Milam, D. and Rainer, F. , in Laser Induced Damage in Optical Materials: 1979,

(NBS Special Publication 568), Bennett, H.E., Glass, A.J., Guenther, A.H. and Newnam, B.E., Eds.,

p. 391 (1980).

[2] Temple, P. A., Lowdermilk, W.H. and Milam, D. , Appl . Opt. n, 3249 (1982).

(a)

i 1 SO (J in

(b)

Figure 1. Normarski micrographs of a mechanochemi cally polished sample of fused silica

taken (a) before and (b) after etching.

148
0 1 2 3 4 5 6

Polishing Time, hr

Figure 2. Weight loss of fused silica during dry mechanochemi cal polishing with SrC03

.

Polishing Time, hr

Figure 3. Weight loss of fused silica during mechanochemi cal polishing with slurries of

colloidal silica and NiO and MgO.

I

149
The speaker was asked if surface roughness had been measured , and whether the surfaces had been

damage tested at X = 555 nm. He replied that neither of these measurements was made, in further

comments , both by the speaker and from the audience , the beam used in the damage measurements was reported to be 2-3 mm in diameter , and previous tests of CC>2-polished surfaces were cited as evidence that surface damage thresholds could feasibly be as large as 30-50 J/cm2 for 1-ns,

1064-nm pulses.

150
SCRATCH STANDARDS SHOULD NOT BE USED TO PREDICT DAMAGE THRESHOLD

Matt Young

Electromagnetic Technology Division, 724.02

National Bureau of Standards

Boulder, Colorado 80303

The scratch and dig standards are the most widely used surface quality standards in the industry. In the proceedings of the 1980 symposium, Harold Bennett showed theoretically that damage ought to be initiated near a defect and related damage threshold to defect size. Evidently because of one or both of these facts, some purchasers may use the

scratch standards to specify the surface quality of components intended for high power

laser systems. Although damage is often associated with the presence of a defect, this is an inappropriate use of these purely cosmetic standards; the classification of a particular

scratch correlates only very loosely with its width or depth. Even if a component is made

of glass, little or nothing pertinent to damage threshold may be determined by classifying

a scratch according to the cosmetic standards.

Keywords: Laser optics; scratch-dig standards; surface specification.

Laser induced damage to optical surfaces is at least sometimes associated with the presence of a defect on the surface, and Bennett has shown that the threshold for such damage ought to be related to the defect's size and absorption coefficient. 1 In part for this reason, designers of high power laser

systems might be tempted to use the scratch and dig standards for specifying optical surface quality.

This would be an improper use of those standards.

The classification of a dig is based on the diameter of the dig; however, the classification of a scratch is based on its visual appearance compared with that of a master standard (Fig. 1). Because

of the complexity of the cross section of most scratches, classification, or scratch number, cor- relates only very loosely with easily measurable quantities such as depth or width. Inasmuch as width

is the parameter singled out by Bennett for estimating damage threshold, I anticipate that scratch

number will correlate equally loosely, if at all, with damage threshold.

Objective Measurements on Scratch Standards

Because of the difficulty of specifying and procuring standards based on their visual appearance,

the National Bureau of Standards has taken on a project to perform objective measurements on sets of

standard scratches. The purpose of the project is to enable the contractor to specify the scratches

in terms of a more objective criterion than visual appearance. Nevertheless, for the forseeable

future, the standard itself will remain a cosmetic one in that optical surfaces in the field will

still be compared visually with the standards. Only the standards will be evaluated photoelectrical ly.

Figure 2 shows the polar scanning apparatus used to examine the scattering patterns of the

scratches. The unique aspect of this system (which is described more fully in Ref. 2) is the presence

of a pupil, or virtual stop, at the location of the scratch; the pupil is the image of the aperture

stop and guarantees uniform illumination in the plane of the scratch.

I also examined the scratches with an optical processor set up for high pass filtering, or edge

enhancement. Although the purpose of this experiment was to examine ways of visualizing cosmetic

defects, I also placed a fine slit in the image plane and measured the intensity scattered by each of

151
several sets of scratches. This measurement is akin to total integrated scatter (TIS) , except that

these measurements include only angles between about 0.03° and 4° (an effective F number of about 7).

Finally, the geometrical cross section of the scratch provides depth and width information with

which to compare the results of scattering measurements.

Experimental Results

This paper is a by-product of the investigations of the scratch standards; I will report only on experimental measurements germane to the subject at hand.

After examining the polar scans of five sets of certified scratches, my colleague, theorist Eric

Johnson, and I restricted our consideration to the range between 5 and 10°. Figures 3a and 3b show on an arbitrary scale the relative intensity scattered by the scratches as a function of scratch number

(10, 20, 40, 60, and 80). The open circles represent the average values, and the closed circles

represent the highest and lowest values in each set of measurements. Data shown are distilled from

angular scans that covered the range from -30° to 30°.

The figures show considerable correlation between scratch number and scattered intensity, but

also much overlap, especially at the angle of 10°.

The optical processing experiments show roughly similar results — rough correlations with a good

deal of scatter. For example, Fig. 4 plots relative intensity as a function of scratch number, width

and depth.

Depth and width plotted as functions of scratch number (Fig. 5) show more scatter for the numbers

10 and 80 scratches than for the other designations; this is in part because some of the other

scratches appear to have been made with the same stylus and are nearly indistinguishable. The two anomalous number 80 scratches also are nearly indistinguishable.

Figure 5b, by the way, shows that the width of a given scratch is not equal to one tenth the

scratch number, although you will often hear that claimed.

Cone! usi ons

It would be easy to say that the scratch number has nothing whatsoever to do with any observable

property of the scratch and thereby get a good laugh. But the statement has the disadvantage of being

untrue. What is true, however, is that all the data show a great deal of scatter, so much that close

to half the scratches with a given classification could just as easily have been given the next lower

or higher classification. In addition, scratches are made of glass and viewed in transmission;

scratches in some other material (which usually has a greater index of refraction) and scratches

viewed in reflection usually look far worse. If I were buying parts for a high power laser, I would

hate to rely on such a loose standard for avoiding catastrophic damage to my components.

The scratch standards are cosmetic standards and do not pretend to be anything else. Let us leave them to those people who are legitimately concerned with cosmetic defects and find proper performance standards for opticists concerned with damage threshold.

152
Acknowl edgements

Mrs. Shelley Etzel carried out the tedious prof i lometer measurements admirably. Dr. Eric Johnson

was a great help and sometimes a great comfort during most phases of this project. The work was supported in part by Calibration Coordination Group, Department of Defense, and by the U.S. Army Armament

Research and Development Command.

References

1. H. E. Bennett, Insensi ti vi ty of the Catastrophic Damage Threshold of Laser Optics to Dust and

Other Surface Defects, in Laser-Induced Damage in Optical Materials: 1980, National Bureau of

Standards (U.S.) Spec. Pub. 620 (1980), 256-263.

2. M. Young, Objective Measurement and Characterization of Scratch Standards, Proc. SPIE, Vol. 362,

in press.

/

/ /

/

/ /

/

i

Figure 1. The scratch standards are purely cosmetic.

/Field

1 ens

Figure 2. Apparatus used to determine scattering as a function of angle (after Ref. 2).

153
Figure 3a. Intensity scattered at 5° off axis as Figure 3b. As Fig. 3a, but data taken at 10° off

a function of scratch number. The axis,

open circles show the average of five

sets of scratches; the closed circles

are highest and lowest recorded

values.

inc

+J

4->

or. a1

0.25

Scratch Number Scratch Width (ym)

Figure 4a. Intensity scattered into F/7 cone Figure 4b. Data of Fig. 4a as function of meas-

(~4°) as a function of scratch ured scratch width. The arrow indinumber for three sets of scratches. cates one extreme point that is not shown on the graph.

154
>1

•-> c

CD

ce.

/

|||H\ o

To 4.0, 4.2

Depth o ooo

Scratch

o ? i

oO

i l 10 20 40 60 80

Scratch Depth (Mm) Scratch Number

Figure 4c. Data of Fig. 4a as function of meas- Figure 5a. ured scratch depth.

Measured scratch depth as function of

scratch number for five sets of

scratches. The arrow indicates two extreme points that are not shown on the graph.

<-> I/O

Scratch Number

Figure 5b. Measured scratch width as function

of scratch number for five sets of scratches.

155
Those commenting on the paper agreed that scratch-dig specifications were imprecise measures of

surface quality, and the speaker suggested that an objective surface roughness standard was needed.

156
Bulk and Surface Damage Thresholds of Crystals

and Glasses at 248 nm*

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

Lawrence Livermore National Laboratory

Livermore, CA 94550

We used 20-ns, 248-nm pulses to study surface and bulk damage in bare, polished

samples of UV-transmitting glasses, fluoride crystals and isomorphs of potassium

dihydrogen phosphate (KDP). The intent was to test enough samples to establish the

safe operating limits for currently available windows and crystals finished by

routine techniques for optical fabrication. In fluoride crystals, thresholds for

bulk inclusion damage and for rear-surface damage were comparable. The median

rear-surface threshold was 16 J/cm^. The fluence limit in fused silica windows

was determined by the threshold for rear-surface damage, which ranged from 5 to 9 J/cm2 . For the isomorphs of KDP, the limiting fluence was the threshold for

front-surface damage, which ranged from 3 to 9 J/cm 2.

Key words: damage; fluoride crystals; KDP isomorphs; laser-induced damage; UV glasses.

1. Introduction

Continued interest in the use of short-wavelength lasers as drivers for inertia! confinement

fusion experiments stimulated us to study the laser damage thresholds of many UV transmitting

materials. In this paper we present the results of damage tests conducted on bare, polished

surfaces and bulk materials at 248 nm using 20-nsec, p-polarized laser pulses.

2. Experimental Procedure

We used a discharge-pumped KrF laser to irradiate the samples. Typically, the beam diameter

(e~2 in intensity) at the sample was 1.5 mm, but the beam contained intensity spikes which were

as small as 0.1 mm in diameter (at 90% of peak fluence). For each shot, the absolute fluences in the sample plane were computed through analysis of a photographic image of the intensity

distribution in the beam.

From 7 to 20 sites were tested on each sample with each site being irradiated only once. The

samples were mounted on a two-position rotating stage which allowed the test site to be moved from

the irradiation station in the beam line to the field of view of a Nomarski microscope. Test

sites were photographed, at magnifications of 100 to 420, before and after each shot. The samples

were also visually inspected with a microscope, and with the unaided eye using intense white light

il lumination

.

Laser damage was defined to be a permanent alteration on the surface, or in the bulk, that

could be detected by these examinations. Threshold was defined to be the average of the highest

nondamaging fluence and the lowest damaging fluence. Details of these techniques are described in detail elsewhere [1], [2].

3. Experimental Results

3.1 UV Glasses

Fused silica (polished with abrasives that consist largely of CeO^) is routinely used in fabrication of UV-transmitting windows and lenses. Figure 1 gives the thresholds for six samples

of fused silica and one sample of a UV-transmitting glass, UVFK54. Sample 4 was tested three

times, once as received from the vendor (data set 4A) and twice after it was repolished with

high-purity ZrO? (data sets 4B and 4C, which are discussed below). Arrows plotted in figure 1 indicate that thresholds were only determined to be greater or less than the plotted value. Bulk

damage was a limiting problem only in UVFK54, in which color centers were formed in single shots

*Work performed under the auspices of the U.S. Department of Energy by the Lawrence Livermore

National Laboratory under contract W-7405-ENG-48.

157
at fluences as low as 2 J/cm^. Bulk damage, consisting of self-focusing tracks that emanated

from the rear surface, occurred in the UV7940 sample, but only at fluences greater than the

threshold for rear surface damage. For all Suprasil samples, the bulk thresholds were greater

than either front-surface or rear-surface thresholds.

For conventionally polished samples of fused silica (data sets 1,2,3,4A and 6; Fig. 1), the

maximum operating levels were set by the threshold for rear-surface damage, which ranged from 5 to

9 J/cm2. Since we do not know how 248-nm surface-damage thresholds are influenced either by

polishing compounds and procedures or by surface cleanliness, two additional experiments were

performed. One surface of sample 4 was repolished with high-purity ZrO?. This surface was retested twice (data set 4B), once as a front surface and once as a rear surface. Within

experimental uncertainty, thresholds were unchanged (or perhaps slightly reduced). However,

during these repeat tests, low thresholds (data set 4C) were observed for the surface of sample 4 which had not been repolished. We can only speculate that this resulted from surface

contamination. We also tested one surface that had been repolished using a bowl-feed process

(front surface of sample 5); its threshold was below the front-surface thresholds of

conventionally polished surfaces included in these tests. Although positive results were not obtained in these two attempts to increase surface damage thresholds, we believe thresholds might

be improved by additional study because the bulk threshold of fused silica is not a limiting

problem.

<3~ Heraeus - Amersil

20

16

Suprasil II

Corning

UV7940

Schott

UVFK54

>

3 12

'Oo

J.'

ca

r.9c T

R±

T t

F B

1

T

R

1

4A

T

Pi

i

4B

Tt

F B i

T

R1

4C

-r t F B

t 4 F B

T

1 R

1

F —

Figure 1. Laser damage thresholds of UV glasses for 248-nm, 20-ns pulses. The letter designations

are F for front surfaces, B for bulk material, and R for rear surfaces.

3.2 Fluoride Crystals

Because they have good UV transmission, fluoride crystals are candidate materials for use in 248-nm lasers. We tested 6 single-crystal samples of Li F, and 13 fine-grained polycrystal 1 ine Li

F

samples made by press forging. Both types of samples were cut from 4 different boules to avoid

biasing the data to the characteristics of an individual boule. Figure 2 shows damage thresholds

for each sample, and the median threshold for each group of samples. We were unable to determine

the threshold for rear-surface damage in the single-crystal samples because the surfaces had a high density of residual polishing scratches. The experimental arrangement allows preshot and

postshot microscopy of the entrance surface, and minor damage can be detected on even a structured

surface. Damage on exit surfaces was detected by independent microscopic examination of the

158
surface following completion of the experiment, and minor damage could not always be distinguished

from pre-existent surface structure.

The data in figure 2 indicate that bulk damage is a limiting problem in Li F. The median

threshold for bulk damage was only slightly greater than the threshold for rear-surface damage.

The bulk damage consisted of starburst-type fractures with 4 orthogonal rays. The number and

sizes of these fractures did not always scale with fluence, and in some cases the spacing of the

fractures was comparable to the diameter of the laser beam. This suggests that the damages were

caused by isolated defects or inclusions, and that use of a larger beam, or testing of more sites,

might have located volumes with lower thresholds. However, no one boule yielded crystals with

predominantly high or low thresholds, although thresholds of the single-crystal samples were 40%

greater than thresholds of forged crystals. Therefore, the data set taken as a whole suggests

that the bulk material in randomly selected Li F crystals will be resistant to damage at fluences

up to 16 J/cm2 , and that forged crystals will survive 10 J/cm2 . Median bulk thresholds should

be higher, 26 J/cm2 for single crystals and 18 J/cm2 for forged samples.

40

CME

o

rV) so

CD c+» 20

COO)

COE

CO Q 10

Front surface - • 9

Med. 33 o

o

Med. 8 24

o

o

o

Bulk material

oo

Med.• 26

8

Rear surface

o

o

oo

Med. ~ 16 ^ooo—

0%

o

0

L J 1 —

• Single crystal o Press forged crystal

Figure 2. Damage thresholds of 6 single LiF crystals and 13 press-forged LIF crystals for 248-nm, 20-ns pulses.

The surfaces of the single crystals and the forged samples exhibited a common damage

morphology. At threshold, damage produced a spatially uniform alteration of the surface. In microscopic examinations with bright field illumination, these damages were visible as areas with

different surface luminosity, which suggests that the surface had been locally smoothed or melted. When examined with Nomarski microscopy, the damaged areas were also visible, indicating

that surface height as well as surface texture was altered. As the incident fluence was increased, the shape and size of the damage increased proportionally. At fluences well above

threshold, surface damage in the forged crystals consisted of a surface array of orthogonal cracks

and of isolated surface pits. It is possible that some surface pits resulted from fracture of

material around inclusions located near the surface. Further, our studies of surface damage in LiF proved to be somewhat irrelevant because bulk thresholds for these materials were low.

We also tested 3 CaFj crystals and one sample each of MgF^, BaF2 and NaF. Thresholds

for these samples are given in figure 3. To allow easy comparison of thresholds of all fluorides

tested, we also give in figure 3 the median threshold and spread of thresholds of the LiF

crystals. Only for MgF2 and NaF did the bulk threshold for damage exceed the threshold for

front-surface damage. Except for one CaF2 crystals, which had an unusually low threshold, and

159
the BaF2 crystal, rear-surface thresholds of these additional fluorides are comparable to those

of Li F crystals.

The threshold surface-damage morphology varied for each sample, but in general consisted of a highlighting of existing surface defects and polishing scratches. A subjective assessment of

laser-induced fluorescence, as judged by the naked eye, was made for all samples. No fluorescence

was observed in the MgF2 sample and only a slight amount in the others. This is perhaps

relevant since the MgF2 crystal had good bulk damage resistance. In magnitude of fluorescence

the materials, ranked in the order (low to high): MgF2, Li F2 , NaF, BaF2 and CaF2-

40

E— 30

Q

Ur. B 20

s

10

CaF,

F

/ B

CaF.

T

F T

I R

T

l

B

CaF.

T T FB

i i

MgF,

1 T

R

I

BaF. NaF

FB

LiF

Yv1

1

1

1

B

AA

LiF

press

forged.

v

V

A

1

1

1

1

1 -

1

1

1

1 B

Rj

1 AA—

Medians

and ranges

Threshold designations: F = front surface, B = bulk material, R = rear surface

Figure 3. Laser damage thresholds of fluoride crystals for 248-nm, 20-ns pulses.

Three of the samples, one each of MgF2, BaF2 and CaF2, had been tested earlier using

266-nm pulses with durations of 100 and 700 psec [3]. A plot of those thresholds, together with

the current 20-ns, 248-nm thresholds is shown in figure 4 as a function of pulsewidth. The slopes

of the straight-line fits to these few data points indicate the front-surface thresholds scale as the square root of the pulsewidth.

3.3 Isomorphs of KDP

Damage thresholds of KDP and its isomorphs are of general interest since these crystals are routinely used to produce fourth-harmonic 266-nm radiation from input 1064-nm pulses. Since

thresholds at 248 nm and at 266 nm should be approximately equal, we tested four potassium

dideuterium phosphate (KD*P) crystals, and one crystal each of KDP and of aluminum dihydrogen

phosphate (ADP). Front surface thresholds of these crystals, shown in figure 5, ranged generally

from 3 to 5 J/cm? although the threshold of the KDP sample was greater than 6 J/cm?. This can

be compared with the surface threshold of a KDP crystal that was measured earlier with 0.7-nm,

266-nm pulses, 6.5 + 1.4 J/cm2 [3]. Therefore, the available data suggest that UV thresholds of

KDP are rather consTant over the range of pulse durations from 0.7 ns to 20 ns. If this result is generally true, it will severely limit use of KDP in frequency quadrupling of long duration

1064-nm pulses.

These crystals are mechanically soft and difficult to polish, and all surfaces tested had a high density of polishing scratches that could be observed with a Nomarski microscope. With minor

exceptions, all crystals had similar damage morphologies. At fluences slightly above threshold,

160
damage consisted of diffuse alterations of surface texture and depth, somewhat like the uniform

surface damage described above for LiF. The surface alteration accentuated the appearance of

scratches on KDP and KD*P, but actually masked the scratches on ADP. Damage in KDP and ADP was always accompanied by the faint emission of white light. The KD*P samples did not emit light

except at fluences 2-3 times greater than threshold. At approximately 20 J/cm2, in all crystals, front surface damage sites contained linear cracks, and at higher fluences, jagged

pieces of crystal were broken off the surface. _ 30

CME

in •a

0)

iff E

CO o

8

CO3

to

*-> C

o

10

£ 1 -

0.3

i i i i -i—i i i 1

1

i i i 1

1

TT

248 nm

266 nm

266 nm

'iii j i i i i

• MgF2

o BaF,

X CaF,

j i i i 1

1

J_I_L 0.1 0.3 1 3

Pulsewidth (nsec)

10 30

Figure 4. Pulsewidth dependence of front-surface laser damage thresholds of fluoride

crystals in the UV.

We presented only front-surface thresholds in figure 5, because we were unable to produce

bulk damage or rear-surface damage in these crystals, except at fluences well above the

front-surface threshold. This is unusual, since intensity at the rear surface of a transparent

window is greater than intensity at the front surface by the ratio Irear/Ifront = 4n2/(n+l)2, where n is the refractive index [4]. KDP is mildly birefringent; its 248-nm

indices of refraction are approximately 1.52 and 1.56 [5]. Therefore, the intensity ratio was Irear/Ifront = 1.47. Assuming that the rear surfaces were, on the average, as resistant to damage as were the front surfaces, the lack of rear-surface damage implies that the transmission

across the sample was <_ 1/1 .47 = 68%.

To test this conclusion, we measured transmission in these six crystals as a function of

intensity. At very low fluences, the net transmission of KDP and KD*P was approximately 90%, and

the loss was primarily due to surface reflectivity. At fluences approximately eaual to the damage

thresholds (4-10 J/cm^ delivered in 20 ns), additional intensity dependent losses as large as 20% (averaged across the spatial distribution of the beam) were observed. Since the intensity

distribution was highly irregular, this implies that the intensity dependent loss for the intense

spikes in the beam was greater than 20%. Transmission of the ADP sample was only 50% even for

very low fluences. Nonlinear absorption was not observed in LiF.

The published value of the two-photon absorption coefficients at 266 nm in KDP and ADP are, respectively, (2.7+2.0)xl0" 10 cm/W and (2.4+1 .8)xlO- 10 cm/W [6]. Using these values, the

predicted nonlinear losses in 1-cm thick samples irradiated at intensities ranging from 0.25 to 0.50 GW/cm? is 2-20%. Our transmission measurements, supported by the failure to observe

rear-surface damage, imply that the 248-nm two-photon absorption coefficients were larger than

published 266-nm by a factor of 2-10.

161
_ 12

CME

3 10

33

o

jC

03

CO =©m

o

S3

c

8

6 -

co 4 —

4-

2 -

—

—

i —

—

T

-

\

- :

i i

KD*P KDP ADP

Figure 5. Front surface .laser damage thresholds of KDP isomorphs for 248-nm, 20-nm pulses.

4. Conclusions

We used 20-ns 248-nm pulses to measure laser-damage thresholds of several samples each of

UV-transmitting glasses, fluoride crystals and isomorphs of KDP. The safe operating fluence for

all fused silica windows that we tested was set by the threshold for rear-surface damage, which

ranged from 5 to 9 J/cm?. These thresholds are comparable to those of the most damage resistant

reflective and antiref lective 248-nm thin-film coatings [1],[2]. Bulk damage was not a limiting

problem in fused silica, but color centers were formed in UVFK54 by single irradiations at less

than 2 J/cm?. Because the bulk of fused silica is damage resistant, improvements in surface

damage thresholds are possible, but positive results were not obtained in brief attempts to improve thresholds, either by use of nonabsorbing abrasives or by alteration of the polishing

procedure. Further, when one surface of a window was repolished, the threshold of the opposite

surface was decreased, presumably due to surface contamination.

In Li F crystals, the threshold for bulk inclusion damage was only slightly greater than the

rear-surface threshold. The median rear-surface threshold of forged Li F crystals was 16 J/cm?;

all Li F crystals survived at 10 J/cm?. Two CaF2 crystals damaged at comparable levels; the

third damaged in the bulk at 6 J/cm. Because the surface thresholds are comparable to bulk

thresholds, performance of these crystals cannot be improved by improving the surfaces. In the

only samples of MgF£ and NaF tested, bulk thresholds were greater than surface-damage thresholds

which were 17-18 J/cm?. If these thresholds are representative of the materials, improvements

in surface thresholds are possible.

Isomorphs of KDP had entrance-surface thresholds between 3 and 9 J/cm?, and exhibited

nonlinear absorption which prevented damage of the rear surface.

162
5. References

[1] Rainer, F.; Milam, D.; Lowdermilk, W. H., Laser damage thresholds of thin film optical

coatings at 248 nm. Nat. Bur. Stand. (U.S.) Spec. Publ.; (Laser Induced Damage in Optical

Materials: 1981) - to be published.

[2] Rainer, F.; et.al,, Scandium oxide coatings for high-power UV laser applications. Appl . Opt.

21 (20): 3685-3688; 1982 October 15.

[3] Deaton, T. F.; Smith, W. L., Laser-induced damage measurements with 266 nm pulses. Nat. Bur. Stand. (U.S.) Spec. Publ. 568: 417-424; 1980 July.

[4] Crisp, M.D.; Boling, N. L. ; Dube, G. Importance of fresnel reflections in laser surface

damage of transparent dielectrics. Appl. Phys. Lett. 21(8): 364-367; 1972 15 October.

[5] Milek, J. T.; Welles, S. J. Linear electrooptic modulator materials. U.S. Dept. of Comm,

Nat. Tech. Info. Service Doc. AD704 556; 1978 January. 177p.

[6] Liu, P.; Smith, W. L.; Lotem, H. ; Bechtel, J. H. ; Bloembergen, N. Absolute two-photon

absorption coefficients at 355 and 266 nm. Phys. Rev. B 17(12): 4620-4632; 1978 June.

163
PULSED LASER INDUCED OPTICAL SURFACE

DAMAGE STUDIES IN DIFFERENT GASES

John Kardach*

Air Force Institute of Technology

Wright Patterson Air Force Base, Ohio 45433

and

Alan F. Stewart and Arthur H. Guenther

Air Force Weapons Laboratory

Kirtland Air Force Base, New Mexico 87117

The pulsed laser induced damage threshold of several optical surfaces

has been measured as a function of the pressure and type of ambient gas.

Measurements were made in vacuum and various pressures of nitrogen, Freon 14

(CF^) and SFg in an attempt to decrease the growth rate of the avalanche

which accompanies pulsed laser induced damage. Test samples included

conventional dielectric coatings of Zr02 , diamond turned copper, bare

surfaces of fused silica and ARG-2 glass, and "porous" surface layers of

both leached ARG-2 glass and some sol-gel coatings.

From this preliminary study, it is apparent that the ambient gas has no observable effect on 1-on-l surface damage thresholds (although some improvement in N-on-1 damage levels had been reported). The present finding

is in agreement with previous observations that damage sites appear to be

initiated on or just below the surface of optical elements. For uncoated

surfaces, both conventional and porous, initiation sites seem to be isolated

impurities embedded near the surface resulting in the formation of pits. In

either case, such damage initiation levels are unmodified by the presence of

electronegative gases, and no change in threshold is observed.

Key words: Laser damage; surface; gases; leached surfaces.

I ntroduction

The present understanding of avalanche breakdown in solids, or more

specifically at their surface, has two major deficiencies [1]. In the first

place, the source for the "seed" or initiating electron is usually not known or specified. Explanations range from cosmic ray induced ionization to easily ionized

low-lying impurity states. Secondly, the rate of generation of electrons in a solid phase avalanche process is open to much speculation. Without a deeper

understanding of these two facets of the breakdown process, it is impossible to

adequately predict when the breakdown process will ensue under a given set of

conditions. However, if the concentration of "seed" electrons or the

multiplication rate during the avalanche process can be altered by some method, a greater understanding of the total damage process may be revealed or at least

parameterized. To this end, we resorted to the potential use of efficient electron

scavengers to either modify the seed electron concentration or more probably slow

the avalanche growth rate, which leads to the observation of a visible spark. This

damage associated spark can efficiently absorb the remaining laser energy, and the

ensuing heated plasma reradiate onto the optical surface, producing the

catastrophic, irreversible morphologically observed changes in the sample surface.

We have investigated the effect of various electronegative gases on laser

induced surface damage thresholds. It is well known that gases such as sulfur

hexaflouride can strongly inhibit electrical breakdown in sparkgaps and other

discharge devices. Such electronegative gases also exhibit high laser induced

breakdown thresholds. If surface damage occurs because of an avalanche process,

the close proximity of such an electronegative gas could modify the damage

1. Figures in brackets indicate the literature references at the end of this

paper

.

^Submitted in partial fulfillment of the Master of Science degree in Physics at the

Air Force Institute of Technology, Wright Patterson Air Force Base, Ohio 45433.

164
process. Although independently suggested, it was discovered during the course of

this investigation that an increase in N-on-1 laser damage threshold for porous

surfaces immersed in SFg had been previously reported in the literature ]2] . The

objectives of this research effort were to study the effect of electronegative

gases on various classes of optical surfaces a.nd carefully parameterize the effect.

Data for this study was obtained using a 1.06um laser having a 5 nsec FWHM

pulse duration. The testing matrix included measurements in vacuum and four or five pressures of three gasses—nitrogen, Freon 14 (CF^), and SFg —along with a selection of samples, A /2 (at 1.06um) thin films of ZrO^ , diamond turned copper,

bare surfaces of fused silica and ARG-2 glass, and "porous" surface layers on leached ARG-2 glass as well as both leached and unleached sol-gel coatings ]3],

Experimental

All experiments were conducted with an Nd:YAG laser, beam diagnostics and

testing chamber as depicted in fig. 1. The laser and diagnostics are as described

in refs. 4 and 5. With good beam quality in a TEMQ mode, the laser produces a few hundred millijoules in a pulse duration of 5 nsec FWHM. With a magnification

of 10X, the spatially resolved beam profile of 175ym (1/e radius in intensity)

could be reimaged onto a vidicon target for real time measurement. In addition,

beam scans were taken throughout the focal volume using a 36pm wide slit and a sensitive pyroelectric detector. The intensity distribution at the sample is

obtained from the beam energy, the spatial profile obtained from the vidicon system

or beam scans and the temporal profile from a fast biplanar photodiode.

The testing chamber is detailed in fig. 2. The system is cryopumped for

operation to 10 Torr. A gas manifold and accurate capacitance pressure gauges

allow for filling the chamber with , CF^ or SFg to pressures up to 2

atmospheres. The sample mount, shown in fig. 2, is designed to provide a full

range of motion adjustments by remotely controlled stepper motor drives.

Damage testing was conducted on a variety of sample types. Fused silica

surfaces produced by controlled grinding were tested uncoated and with X/2 thin

films of ZrOg . Commercially available diamond turned copper mirrors and ARG-2

glass were also employed as test specimens. The ARG-2 glass is phase-separable,

and half of the samples were acid leached to produce a "porous" antiref lective

surface layer [6]. The "porous" layer in leached ARG-2 extends several microns or

so inside the surface. This is contrasted with a "porous" layer depth of less than

a half micron in samples tested with sol-gel coatings. In addition, the sol-gel

coatings could be etched to approximately double the average pore diameter, up

to 50 A [7, 8]. These coatings had a measured projected surface area of 10m /gm

and a composition of 66 Si02

• 18 B 2 0 3• 7 A1 2 0 3• 6 Na 2 0 • 3 BaO [9]. Pore

diameters in leached ARG-2 range up to an order of magnitude larger.

Each sample was dedicated to a particular gas. Experiments began with testing

in vacuum at the base pressure of the chamber, typically 5 x 10~ Torr. Gas

pressures used for successive tests were ~ 1, 50, 200, and 600 Torr. The sample

surface was allowed to stabilize for about an hour at each new pressure before

testing began. Samples were initially cleaned by a standard alcohol-laden lens

tissue pull.

Each irradiated site was examined under a Nomarski microscope to determine if

damage had occurred. Spark formation was found to be a positive indicator only at

fluences well above threshold. At or near threshold, the damage morphology always

consisted of a field of one or more small pits in every sample tested.

Because the samples were sealed in an environmentally controlled chamber, it was not efficient to check for damage after each 1-on-l irradiation. Thus, a set

of 30 to 40 ranging exposures were first made, the sample was removed and an initial estimate of the damage threshold was determined. The sample was reinserted, and the remaining exposures were made in the vicinity of the estimated

threshold; in all, between 120 and 160 testing events were accomplished for each

gas and pressure. Representative examples of the test data are shown in figs. 3 and 4, exhibiting the damage threshold analysis in both J/cm and GW/cm .

165
The definition of damage threshold [5] used in this experiment is given by eq.

(1).

E _ _ I j -, E no damage + E . damage _threshold = 1 max mm (1)

Area 2 Area

This damage threshold represents an average value midway between the defect

dominated minimum damage level and the highest nondamaging fluence (presumably

approaching the intrinsic value). From figs. 3 and 4, it is very clear that the

damage threshold defined in this fashion is an average value. Another figure of

merit listed in table 1 is the spread defined by

S =

Emax

no damage "Emin damage

E

* threshold

The spread combined with the damage threshold completely describe data sets such as

in figs. 3 and 4.

Results

The data from these experiments representing over 10,000 shots is summarized

in table 1. As indicated, the ambient gas had no noticeable effect on surface

damage thresholds for the conventional samples tested. From previous observations,

it was apparent that damage in single layer thin film coatings seems to be

initiated primarily at the substrate-coating interface [5]. Unless the coating is

extremely thin or permeable to gases, the surrounding atmosphere would have little

effect on threshold pit formation. Similarly, pit formation observed in damage

tests on fused silica indicates damage is initiated at the defect or inclusion on

or just below the surface. Damage in diamond turned copper also showed pit

formation near threshold, even though damage was expected to be purely thermal in

origin. Representative near threshold damage sites for each of the seven surfaces

tested appear in figs. 5 and 6.

From the data in table 1 , it is clear that the ambient gas has no observable

effect on surface laser damage thresholds as defined herein. At least this is true

for the samples studied and with beam spot sizes of a few hundred microns. The

spread in each experiment is larger than any observed charge in threshold. With a larger spot size (>lmm) , it is entirely possible that a sharp threshold with near- zero spread would be observed [10]. Under those circumstances, it might be

possible then to measure with confidence a 20 percent increase in damage

threshold. However, in these experiments the spread ranges from 0.19 to 1.51

essentially masking any potentially observable effect.

The minimum damaging fluence for a sample is expected to be completely

dependent on resident defects. It is important to note, however, that additional

defects can be introduced by the measurement technique. Large damage sites can splatter molten material over a wide area. Also, plasma formation in the ambient

gas could lead to deposition of contaminants. It was for this reason that SFg and

CF^ were chosen as the electronegative gases. The major fraction of SFg decomposition is SF^ , a gaseous product. Furthermore, fully flourinated CF^ leaves no carbon deposits or any observable pressure increase after the passage of an electric arc, while other mixed chlorine or flourine compounds decompose, forming

CI2 or Fg [11]. These are important considerations since there is evidence that

for certain laser-ambient gas-sample composition (particularly thin films)

combinations, there may be chemical reactions, including some in the solid phase,

taking place in the presence of optical breakdown [12].

The spacing between sites was 1 mm or larger, which appeared to be adequate to

preclude associated damage or other modification of adjacent sites. The sol-gel

coatings were not very uniform, and testing at higher pressures involved sites

nearest the edge of the substrate (the most troublesome area). The spreads

measured in these particular data sets are very large and overlap, precluding any

definitive conclusions.

The largest spreads listed in table 1 were measured for the leached surface

ARG-2 samples. These surfaces were extremely nonuniform but exhibited definite

166
zones with higher damage thresholds. We can only conclude from this observation

that the acid leaching process is not yet sufficiently well controlled. With a large diameter beam of a millimeter or more, this leached surface nonuniformity

might average out and become unobservable

.

Discussion

The data summarized in table 1 shows that the ambient gas has no observable

effect on the surface damage threshold as defined in these experiments. This

appears to be in contradiction with data reported by Eron'ko et al. in ref. 2.

However, the Russian data dealt with an experiment as contrasted to ours in table

2.

Let's examine table 2 in some detail and discuss the differences between the

two experiments and how these differences might affect the results obtained.

First, Eron'ko' s pulse duration was 70 nsec , while in this work it was 5 nsec FWHM;

we would expect that the longer pulse would produce a larger spread in the damage

threshold determination as one moves from site to site on the sample. The same increase in spread would be seen from differences in the beam diameter, where the

Soviet work was 60ym, while in this work it was 330ym at the 1/e points. Again,

the smaller spot size should lead to a large variability in the damage threshold;

however, no comments relative to the spread were given in the cited Soviet

reference. The wavelengths were the same in the two cases, but test procedures

were different. In the Soviet work the experiment was performed in an N-on-1 mode,

while in this work it was 1-on-l . It is certain that preconditioning can play a major role in N-on-1 testing. However, Eron'ko reported that a positive effect was

seen from the first irradiation, which means that preconditioning was not a concern

and that their results should be similar to this 1-on-l investigation. The damage

threshold definition was also different, but here we feel that the difference is

significant. In the Soviet work damage was assessed by the production of a spark

and related plasma, while in this work damage was the identification of the

formation of pits from Nomarski microscopy. In the Soviet work an improvement in

the threshold was noted when surfaces were immersed in an electronegative gas,

particularly when those surfaces were porous. No improvement was noticed in this

work. These conclusions are not necessarily in disagreement. Furthermore, the

affects of variation of the pressure on the damage threshold over the range of 10~5

to 600 Torr were also studied. No change in threshold was noticed. The gases were sufficiently similar in the two experiments, with SFq being common to both investigations. And finally, samples studied were also sufficiently similar in that in

both the controlled grind fused silica, porous coatings, and dielectric thin films were investigated.

The significant difference was in the definition of damage threshold. As was previously stated in this paper, sparks or plasma threshold were only observed at

higher radiation levels, well above the threshold for pit formation. One might

expect that the sparks produced by an avalanche process to be controlled to some degree by an electronegative gas in which electron generation and loss rates would

be affected, and thus it is possible that the Soviets would see a "beneficial"

effect. And it might be added that since the catastrophic failure of optical

surfaces are primarily produced by the heated plasma accompanying the laser

irradiation, one might ameliorate this problem by the use of an electronegative

gas. However, on the other hand, if a more meaningful definition of damage is the

first observations of an irreversible structural change, then the addition of

electronegative gas will not play any major role in modifying a threshold, so defined

.

It is unlikely that an N-on-1 experiment can be designed with sensitivity to

the formation of individual submicron pits. If we had defined damage threshold in

another manner—the occurrence of "uniform" damage, for instance—our data would

have overlooked the defect dominated sites with the lowest damaging fluence. This

points to an intrinsic problem with N-on-1 testing since most diagnostics (spark

formation, probe beam scattering, etc.) are sensitive only to large scale "uniform"

or catastrophic damage.

Conclusions

The data from this experiment show that short pulse laser induced damage

thresholds of various optical surfaces are defect dominated. Even with porous

167
surfaces such as leached ARG-2 and sol-gel coatings, the low damage threshold of

resident defects predominate over any effect which is due to the ambient gas. The

large spread observed in our data indicates that further studies in this area require more homogeneous porous surfaces. Better control and uniformity have been

reported for neutral solution coatings, and they should be investigated in future

work. In addition, it may be necessary to use larger beam diameters to reduce the

spread in the data before the effects (if any) of immersion in an electronegative

gas can be seen. Spectroscopic observations of laser induced damage to optical

surfaces immersed in index matching liquids or other environments only show spectra

characteristic of the optical element and not of the ambient media [13], further

supporting our contention that the damage initiation process is not appreciably

affected by the environment in a gross sense (however, absorbing species adsorbed

on the surface can play a role).

The absence of any noticeable effect is further supported by the result that

ambient gas type or pressure does not play an observable role in laser induced

damage of optical surfaces when that damage is defined as the first permanent

observable morphological change.

References

[1] There is an extensive literature

on avalanche breakdown. See, for

example, the section "Electron

Avalanches" in "Index of Papers

Laser Induced Damage in Optical

Materials Symposia: 1969-1978"

published by Air Force Weapons

Laboratory, Kirtland AFB , New

Mexico 87117.

[2] S. B. Eron'ko, G. T. Petrovskii,

A. V. Shatilov, A. K. Yakhkind

,

and L. V. Aleksandrova , Sov . J. Opt. Technol., 43_, 29 (1976).

[3] W. H. Lowdermilk and D. Milam,

Appl. Phys. Lett. 36 (11), 891

(1980). L. M. Cook, W. H. Lowdermilk, D. Milam, and J. E. Swain, Appl. Opt., 21^, 1482

(1982)

.

[4] T. W. Walker, A. H. Guenther, and

C. G. Fry, NBS Spec. Pub. 568, 405

(1980) . T. W. Walker, A. H. Guenther, and P. E. Nielsen, IEEE

J. Quant. Elect., QE-17 , 2041-2065

(1981) .

f5] A. F. Stewart and A. H. Guenther,

NBS Spec. Pub., "Proceedings of

the Thirteenth Symposium on Optical Materials for High Power

Lasers (1981)," to be published.

\6] Y. Asahara and T. Izumitani, J. of

Non-Crystalline Solids, j42_, 269-

280 (1980).

[7] S. P. Mukherjee and W. H. Lowdermilk, Appl. Opt., J21_, 293

(1982)

.

[8] M. J. Minor, J. Opt. Soc . Am.,

66 , 515 (1976).

[9] Sol-gel coatings were provided by

C. J. Brinker, Division 5845,

Sandia National Laboratories,

Albuquerque, New Mexico 87185.

r 10 ] D. Milam, "Optical Coatings - Applications and Utilization II,"

SPIE, 140 , 52-61 (1978).

[11] J. P. Manion, J. A. Philosophos,

and M. B. Roberson , IEEE

Transactions Electrical

Insulation, El-2 , No. 1, 1-10

(1967)

.

[12] V. A. Artem'ev, A. M. BonchBruesch, and Y. I. Chernaza,

Ivestiya Akademii Nauk SSSR,

Seriza Fizidieskaya 44 , 2108

(1980) .

[13] C. Yamanaka, T. Sasaki, M. Honggo, and Y. Nagao, NBS Spec.

Pub. 356, 104 (1971).

168
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173
Sol-Gel Coating in 50 Torr CF4 (50x).

b. Leached Sol-Gel Coating in 1 Torr CF4 (100*)

Fig. 6: Damage Morphology Near Threshold

174
Ik

c. ARG-2 in 50 Torr CF4 (lOOx)

d. Leached ARG-2 in 600 Torr N

2 (lOOx).

Fig. 6: Damage Morphology Near Threshold

175
Table 1 Damage Thresholds and Spreads

VACUUM PRESSURES

SAMPLE GAS (10~ 5 TORR) 1 TORR 50 TORR 200 TORR 600 TORR

Zr02 N2 7

2

.6 J/cnr 8 .4 J/cm27 .5 J/cm2 7. 8 J/cm2

7

9

.3 J/cmz (0 .46) (0 .339) (0 .633) (0. 503) CO .669)

CF4

8

9

.90 J/cmz 10 .0 J/cm2 11 .9 J/cm2 10 . 3 J/cm2

8

9

.7 J/cnT

C P . 566

)

(0 .628) (0 .414) (0. 293) CO • 474 1)

SF6

9

9

.7 J/cnr 7 .9 J/cm28 .7 J/cm2 9. 2 J/cm2

9

9

.5 J/cmz CO . 548) (0 .611) (0 .608) (0. 560) CO .474')

Cu Mirrors N2

2 9.7 J/cnT 2 .8 J/cm23 .2 J/cm2 3. 1 J/cm2

2 9.9 J/cnT

(0 .639) (0 .554) (0 .614) (0. 589) (0 .647)

CF a 2 .5 J/cm2

2 ,7 J/cm2

2 8 J/rm2

3 . 2 J/rm2

2 .9 J/cm2

(0 .469) (0 .730) (0 .933) (0. 589) (0 . 647

)

SF«b

1 .9 J/cm21 , .9 J/cm2

1 .7 J/cm2

2 . 1 J/cm2

1 .9 J/cm2

(0 .818) (0,.998) (0 .794) (0. 823) (0 .955)

Fused Silica N2 57 .9 J/cm2 49 9.6 J/cm 47 .2 J/cm2

Substrates (0 . 665 ) (0 .430) (1 .170)

(Si02

) SF-b ?8

48

(0

.3 J/cm2

.527)

58

(0V

r, t 1 9

.7 J/cmz .972)

Unleached 15 .6 J/cmz 17,.9 J/cm2 17 .4 J/cm2 19

9

.7 J/cmz Sol-Gel (0 .922) (0..057) (0 .573) (0 .437)

De rived

Coat ings CF,, 17 „ _ , 9 .7 J/cmz 20 .8 J/cm2 16 .2 J/cm2 12

_ , 9

.3 J/crrT

( 0 . 531

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CO . 269

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CO 395 ^ CO .630)

SF6 18

9

.5 J/cm-4 16,.2 J/cm2 17 .0 J/cm2 14 9.4 J/cmz

C 0 . 552 ) (0,.614) (0,.518) CO . 801 ) Leached N

2 14 .7 J/crn^ 15 .4 J/cm2 18 .7 J/cm2 14

9

.5 J/cm-4 Sol-Gel 799 ^ (0,.526) (0 .443) CD 1 87

DfZ* T* i y prl UKZ X J- V C; U Coatings CF4 17 •0 J/cmz 20 .2 J/cm2 20 .4 J/cm2

^ U ^4^ (0,.711) (0,.508)

SF6 16 .3 J/cm^ 17,.4 J/cm2

19,.7 J/cm2 18

o .4 J/cnT

f( 0u (0,.739) (0..608) CD 74^ ^ Unleached N2 65 .6 J/crn^ 64 .2 J/cm2 59 .7 J/cnT

ARG-2 f 0\

1 ' (0,.504) c

o

• DUO ) CF4 44

o .0 J/cnT 38 .4 J/cm2 49

o

.1 J/cnT

(0 .939) (0,.646) (1 .005)

SF^6 45 .9 J/cm2 43 , .0 J/cm2 54 .7 J/cm2

(0 .369) (0,.341) (0 .976)

Leached 38 .0 J/cm2 39,,8 J/cm2 50 .3 J/cm2

ARG-2 (1 .446) (1.,426) (1 .508)

SF6 21 .1 J/cm2 25 , .7 J/cm2 23 .8 J/cm2

O . 143) (0,,661

)

(1 .454)

176
Table 2 Environmental Effects Comparison

EXPERIMENTAL CONDITION ERON'KO ET AL. THIS WORK REMARKS

Pulse Duration 70 nsec 5 nsec FWHM Spread Variability

Beam Diameter 60pm (?) 330pm (1/e 2

) Spread Variability

Wavelength 1 .06pm 1 .06pm Same

Test Procedure N on 1 1 on 1 Soviet effect seen from first irradiation ~ 1-on-l

Damage Threshold

Definition

Spark/Plasma Pit Formation

from Noma r ski

Microscopy

Different

!

Improvement Noted Yes No Not necessarily in

disagreement

Pressure Effect Didn't do

_5

10 * 600 Torr No change

Gases SFg , air SF6 , CF4 , N2

, vacuum Sufficiently

simi lar

Samples Controlled ground

fused silica,

Si02 coating,

porous surfaces

A/2 Zr02 films

metal mirrors,

control led

ground fused

silica, ARG-2 and

sol-gel porous

surfaces

Suf f icient ly

simi lar

177
Pulsed Laser- Induced Damage to Metals at Cryogenic Temperatures

D. L. Decker, J. B. Franck, W. N. Faith, and J. 0. Porteus

Michelson Laboratory, Physics Division

Naval Weapons Center, China Lake, California 93555

Experimental pulsed laser-induced damage thresholds have been determined on diamond-machined metal surfaces at cryogenic temperatures. These multithreshold

measurements are in substantial agreement with a simple one-dimensional thermal model

previously developed. Differences in damage character do exist, primarily a decreased

frequency of localized, imperfection-related damage on cooled surfaces. A detailed

intercomparison of experimental and theoretical results is presented with discussion of the

practical and fundamental significance of laser mirror operation at cryogenic temperatures.

Key words: cryogenic temperatures; laser damage; optical absorption in metals; surface

imperfection.

Introduction

For laser pulse lengths longer than the electron relaxation time ( ~ 10

14 sec), laser-induced

damage in pure metals is thermal in origin. As energy is absorbed from the pulse, the sample surface

temperature rises. As the surface temperature rises, the gradient also becomes larger and the

thermally induced stress will reach the yield stress of the material; further increase will ultimately

result in mechanical slip. As the temperature rises further, melt will be eventually reached. On a well-prepared metal surface, slip will occur at a small fraction of the pulse fluence required to melt

the surface [l]. 1 For a uniform surface, both slip and melt thresholds are constant at different

points on the surface. However, in actual observation, variation does occur due to surface defects,

the origin of which has been the subject of much concern and conjecture [1,2]. On metal surfaces,

these defects are most often supposed to be absorbing particles of some sort. A statistical

description of the defect-related damage can be derived from "small spot" testing a large number of

sites, providing a particularly useful way of describing surface uniformity [3].

Thus, although the uniform effects of slip and melt are assumed to be understood, the spatially

selective effects are not. It is ordinarily assumed that the intrinsic IR optical absorption can be

described by the Drude model with the necessary temperature dependence computed from the temperature

dependence of the dc conductivity [4]. If the optical absorption and resulting heat flow can be

accurately modeled, a theory of laser-induced melting results. Such a theory has been applied to previous laser damage data, and, for samples initially at room temperature, reasonable agreement has

been obtained between theory and experiment [1]. To further examine the validity of these earlier

theories and also to explore the consequences of the anomalously low absorption at 2.7 Urn wavelength

previously reported for silver and copper surfaces at low temperature [4], a series of laser-induced

damage studies has been conducted with the sample initially at cryogenic temperature. As will be

shown, two primary effects occur which combine to produce a higher threshold fluence for melting when

a metal sample is initially at a cryogenic temperature. The optical consequence is to decrease the

initial absorption, and the thermal consequence is to increase the total heat required to reach

melting.

Theory

The Drude free-electron model provides a convenient theory to describe the IR absorption of a metal. The absorption A is simply related to the plasma frequency cop and the relaxation time t,

Work supported by the Office of Naval Research and Naval Weapons Center Independent Research

funding.

Numbers in brackets indicate the literature references at the end of the paper.

178
A - 2/(jo t . (1)

P

2 2 This expression is valid in the limit (<^pX) » (ojt) » 1, which applies accurately to silver,

copper, and gold at near- and mid-IR wavelengths [4]. Furthermore, x can be related to the dc

conductivity a by the Lorentz-Sommerfeld' relationship which follows from simple transport theory:

x = 4ifo/u)p (cgs units) . (2)

For temperatures higher than one-third of the Debye temperature (9n) for the metal (see Table 1),

l/o = p0 + Pl T , (3)

where Pq and P\ are constants, independent of temperature [5]. Hence,

A - 2? (P 0 + PlT) = A0 + AiT . (4)

This explicit temperature dependence has been verified for silver, copper, and gold in the infrared,

although complicated hysteresis and time-dependent "annealing" effects were also seen [4,6],

Laser-induced melting can be simply modeled if a square pulse and one-dimensional heat flow are assumed. The latter approximation is valid for the case in which the irradiated spot size is large

compared to the thermal diffusion depth [7]. The one-dimensional heat flow equation and appropriate

boundary conditions are

K3

2 T/3x2 = C3T/3t , T(x,0) =T 0 , -K(9T/dx)

x=Q

= IA , (5)

where K is the thermal conductivity, C is the heat capacity of the material, and I is the laser

fluence. If A is temperature independent, the surface temperature is

T(0,t) = T0 + [2IA/(ttKC) 1/2

]t

1/2

. (6)

If the pulse length is tp, then setting the final surface temperature to the melt temperature T^,

T(0,tp) = Tm, and solving for Itp, we have the melt threshold fluence 9^:

B - Ol^i" ( T - T0 )t 1/2

. (7) M 2A M p

The product KC is only weakly temperature dependent. The temperature dependence of A can be

approximately taken into account by replacing A with its average value [A(To) + A(T )]/2.

„ - (*KC) 1/2

1/2

9 M = A(T 0

) + A(TM) M

T « ) C p (8)

,1/2 T„ - T 0 Ai TM + T7

<»KC> t 1/2

, A S AjT (9)

179
The ratio of the melt threshold fluence for a sample at an initial temperature to that at Tq is

then

VT l) TM

+ T0

9 M(T 0 ) = TM

+ Ti (10)

The first term in this expression accounts for the decrease in absorption for lower initial

temperature, and the second for the increase in heat required to reach melting. It is instructive to examine a specific case. For silver,

9 M

(88°K)

efeoino -1.16*1.23 = 1.43

Notice that the two contributions to the increase in melt threshold are nearly equal in magnitude.

If we assume ji priori that A is temperature dependent, A = Aq + A^T, and again adopting a one-dimensional heat flow model, the surface temperature is given by the approximate expression [8]

r 1/2

T(0,t) = 2(T0 + A0 /Ai)e - A0 /A! , where 5 = IAxCt/KC) (11)

valid for £ » 1. Setting T(0,t

p

) = TM and solving for IT

p

,

TM+ Aq/Aj

i r tm

+

A0 /Ai)

1/2

(t KC) 1/2

P

Ai (12)

The ratio of the melt threshold fluence for a sample at an initial temperature T^ to that at To is

then

eM (TO I *n[A(TM

)/2A(Ti)]

jCTq)

\ £n[A(TM

)/2A(T 0 )]

1/2 £n(TM

/2T!)

£n(TM

/2T0 )

1/2

(13)

Although this expression is based upon a more realistic treatment of the temperature dependence of the

absorptance, it does not permit a ready examination of the physical effects as does the earlier

expression [eq (10)]. In the analysis section, it will be shown that eqs (10) and (13) yield results

of similar magnitude.

Experiment

The experimental laser-induced damage reported in this paper was obtained in a manner similar to that previously reported [1,7], The laser produced a reproducible Gaussian spatial profile in pulses

180
of 100 nsec duration_at 2.7 urn wavelength. Samples were mounted in an ultrahigh vacuum chamber with a base pressure of 10 Torr. Cooling was accomplished by bringing the samples into contact with a liquid nitrogen reservoir. The samples were then rotated and aligned for damage testing. Once in

position for laser irradiation, the samples would begin a long (many-hour) climb in temperature back

to ambient. During damage testing, the temperature increase would be no larger than 5°K. The

temperatures reported are the average temperatures during a damage sequence. Temperatures were monitored with calibrated thermocouples mounted directly on the samples. Two sample types are reported here: diamond single-point machined, electroplated, high purity gold and OFHC copper. The

samples were machined off-center with an interrupted cut and with a round-nosed tool [9]. Figures 1 and 2 show the typical appearance of crater and slip damage to OFHC copper both for an initial

temperature at 133°K (a) and at 300°K (b). Figures 3(a) and (b) show the results at cryogenic and

ambient temperature for diamond-machined electroplated gold. No differences in damage morphology with

initial sample temperature were evident in examining hundreds of damage sites on both copper and

gold. Table 2 presents a summary of multithreshold results for both sample types, with fluence values

given as well as the ratio of cryogenic and ambient threshold fluences.

In addition to the determination of threshold values for melt, slip, and other damage effects,

data were also obtained to determine the onset fluence for pitting. Pitting is a localized melting

occurring as a consequence of spatially selective absorption or thermal effects [1-3,7]. Using a power law fit to the pitting frequency data with P = 0.1, the following onset fluences were obtained

using the statistical model of Porteus [3] for the OFHC copper sample previously examined,

I0 (133°K) = 176.1 J/ cm

2

; I 0 (300°K) = 134.9 J/cm2

I 0 (133°K)

I 0 (300°K)

= 1,31

Analysis

The advantage in computing ratios of melt thresholds, as in eqs. (10) and (13), is that only a minimum of material constants are required for an analysis. Using values of the melting temperatures

for copper and gold as given in Table 1, the following ratios are obtained using eq. (13) for copper

and gold, respectively:

3 M(133°K)

3 (300°K) M

and

6 M (139°K)

6 M

(276°K)

= 1.41

= 1.33

These values agree well with the measured values given in the previous section for copper and gold,

1.32 and 1.25, respectively. The discrepancy in both cases is about 6% and is well inside that

permitted by experimental uncertainty. For a given sample type, except for the flash threshold ratio

on electroplated gold, the ratios are remarkably consistent with the melt threshold ratios for the

several damage effects reported. This is not unexpected, since in all cases the effects are thermal

in origin. It might be argued that slip is a more complicated phenomenon, involving the mechanical

properties of the material. However, actual slip occurs at the same temperature (gradient) regardless

of the initial conditions. The cryogenic absorption and heat capacity effects determine the details

in the rise to this threshold condition, at which temperature (gradient) the mechanical properties are

the same in all cases.

181
Not expected, however, is the discovery of the same threshold ratio for the onset of selective

damage, an effect assumed to be defect related. It has been long assumed that the localized

laser-induced damage seen on bare metal surfaces is due to small, highly absorbing particles, perhaps

submicron in size [1-3]. The temperature and wavelength dependence of the absorption associated with

such a particle is obviously unknown, but would not be expected to be the same as the metal itself.

The simplest explanation for the effects observed here is that the defect is not an absorbing particle

but is rather thermal in origin. A small localized crack could thereby isolate a piece of the

surface. In absorption, this particle behaves just as the parent metal, but as laser heat is

deposited in it, it increases in temperature very rapidly, reaching melt temperature before the bulk.

There is no evidence for the anomalously low absorption previously observed for diamond-machined

copper and silver. The very low absorption reported earlier would result in a further increase in

melt threshold values. In fact, the measured values are lower than those computed for both copper and

gold [4]. The simple Drude model with a temperature dependence introduced from experimental values of

the dc conductivity provides an absorption model in excellent agreement with the measured thresholds

for diamond-machined OFHC copper and gold.

Conclusions

No unusual laser-damage morphology is associated with a sample at a cryogenic initial

temperature. This statement follows from microscopic examination of hundreds of damage sites on diamond-machined gold and copper. The experimentally determined melt threshold values for copper and

gold vary with initial sample temperature in good agreement with simple theory. In this theory, it is

assumed that the optical absorption varies linearly with temperature. There is then no evidence for

the anomalous near-IR absorption previously reported for diamond-machined and evaporated metal films.

The diamond-machined optical surfaces examined in this work behaved in accordance with the simple

Drude model.

In a separate series of measurements, the onset fluence for selective damage on diamond-machined

copper was determined using the statistical model developed by Porteus. Surprisingly, the selective

damage onset fluence varies with initial sample temperature in the same fashion as the melt threshold

fluence. The simplest explanation is that the absorption mechanism for selective damage has the same temperature dependence as uniform damage. The lower threshold values and selective nature of this

damage must then be due to localized thermal defects. Such a defect results from the inability of a small surface region to conduct heat deposited by the pulse into the interior of the sample. The

surface temperature thus reaches a damaging value more readily than it would over uniform material.

Such a spatially nonuniform defect could be caused by a crack or void near the surface, and in OFHC

copper is often observed in association with grain boundaries.

Defect-related damage, of course, has been detected on a wide variety of sample types including

dielectric-coated multilayer mirrors. Much work has been accomplished in an attempt to detect and

identify the "absorbing" defects responsible. It seems very likely that in many cases the defects are actually thermal in origin and that no impurity particle is involved. This simple idea accounts for

the inability, in most cases, to discover a_ priori a defect by optical means which upon subsequent

damage testing will produce local damage.

Although cryogenic laser damage measurements are difficult to perform, unique information about

both intrinsic and extrinsic selective damage mechanisms can be obtained. No doubt it would be

worthwhile to repeat the experiment described in this paper on dielectric-coated samples. As a practical matter, operating a bare metal diamond-machined silver, copper, or gold mirror at 80°K will

result in a decrease in absorption by a factor of about 5 in the IR and an increase in the thermal

conductivity of most metals by a factor of about 1.5. As demonstrated in this paper, both intrinsic

and extrinsic damage thresholds on bare copper and gold can be increased by a factor of 1.3 to 1.5 over the same temperature range. No one would consider cryogenic laser mirror operation and its

additional complexity for ordinary use. However, in some critical applications, the advantages

summarized above may well argue for cryogenic operation.

182
References

[1] Porteus, J. 0.; Decker, D. L. ; Seitel, S. C. ; Soileau, M. J. "Dependence of metal mirror damage

thresholds on wavelength, material, pulse length, and preparation method," in Laser Induced

Damage in Optical Materials: 1980 , Bennett, H. E. ; Glass, A. J.; Guenther, A. H. ; Newnam, B. E.

,

ed. 1980 September 30-0ctober 1; Boulder, CO. Nat. Bur. Stand. (U.S.) Spec. Publ. 620; 1981

October. Pp. 201-209.

[2] Seitel, S. C.; Porteus, J. 0.; Decker, D. L. ; Faith, W. N. ; Grandjean, D. J. "Effect of surface

preparation methods on laser-induced pitting of Cu mirrors at 10.6 Mm wavelength," IEEE J.

Quantum Electron. QE-17 ; 2072-2077; 1981.

[3] Porteus, J. 0. "Determination of the onset of defect-driven pulsed laser damage in 2.7 um

optical coatings," (U) presented at the High Power Laser Optical Components Topical Meeting,

Boulder, CO, 1982 November.

[4] Decker, D. L. ; Hodgkin, V. A. "Laser mirror operation at cryogenic temperatures," presented at

the 13th Annual Symposium on Optical Materials, Boulder, CO, 1981 November. (Proceedings in

[5] Kittel, C. Introduction to Solid State Physics , 5th ed.; chapter 6. New York; John Wiley &

[6] Decker, D. L.; Hodgkin, V. A. "Wavelength and temperature dependence of the absolute reflectance

of metals at visible and infrared wavelengths," in Laser Induced Damage in Optical Materials:

1980 , Bennett, H. E.; Glass, A. J.; Guenther, A. H. ; Newnam, B. E. , ed. 1980 September 30-

October 1; Boulder, CO. Nat. Bur. Stand. (U.S.) Spec. Publ. 620; 1981 October. Pp. 190-200.

[7] Porteus, J. 0.; Decker, D. L. ; Faith, W. N. ; Grandjean, D. J.; Seitel, S. C; Soileau, M. J.

"Pulsed laser-induced melting of precision diamond-machined Cu, Ag, and Au at IR wavelengths,"

IEEE J. Quantum Electron. QE-17 ; 2078-2085; 1981.

[8] Carslaw, H. S.; Jaeger, J. C. Conduction of Heat in Solids . London; Oxford University Press;

[9] Saito, T. T., ed. Advances in the Precision Machining of Optics , Proceedings of the Society of

Photo-Optical Instrumentation Enginers ^3_; 1976.

process.

)

Sons; 1976.

1948.

Table 1. Values of the melting temperature T^ and the

Debye temperature Oq for silver, copper, and gold

Metal TM (°K)

Ag

Cu

Au

1259

1356

1337

220

315

165

183
Table 2. Measured threshold fluence values for diamond-machined electroplated gold and

OFHC copper. Values shown are in J/ cm and are for a pulse of 100 nsec duration at 2.7 ym. Additional details given in the text.

Diamond-machined electroplated gold L' J. <XV11\J LLU tnflrhi II Id l^HX npH LiCVX OFHP \JL uu r r T = 139°K T = 276°K Ratio T = 133°K T = 300°K rvcx Liu

Slip 124 ± 28 105 ± 4 1.18 90 ± 2 Melt 174 ± 11 139 ± 6 1.25 251 ± 13 190 ± 25 1.32

Pit 174 ± 11 139 ± 6 1.25 251 ± 13 190 ± 25 1.32

Crater 174 ± 11 140 ± 6 1.25 274 ± 15 218 ± 23 1.26

Flash 174 ± 11 199 ± 23 0.87 274 ± 15 218 ± 23 1.26

Figure 1. Nomarski micrographs of damage sites showing cratering on a diamond-machined OFHC copper

surface with the initial sample temperature at 133°K in (a) and 300°K in (b). No differences in

morphology are evident.

184
100 vim

' T = 133°K T = 300°K

Figure 2. Nomarski micrographs of damage sites showing slip on a diamond-machined OFHC copper surface

with the initial sample temperature at 133°K in (a) and 300°K in (b). No differences in morphology

are evident.

T - 139°K 1 100 urn ' T - 276°K

Figure 3. Nomarski micrographs of damage sites showing cratering on a diamond-machined electroplated

gold surface with the initial sample temperature at 139°K in (a) and 276°K in (b). No differences

in morphology are evident.

185
Single and Multiple Pulse Catastrophic Damage in Cu and Ag Diamond

Turned Mirrors at 10.6, 1.06 and 0.532 urn

N. Koumvakalis, C. S. Lee, and M. Bass

Center for Laser Studies, University of Southern California

University Park, DRB 17, Los Angeles, CA 90089-1112

The results of single and multiple pulse damage studies at 10.6, 1.06 and 0.532

Vim in diamond turned bulk Cu and diamond-turned electrodeposits of Ag on Cu are presented. The single pulse damage threshold decreases or remains constant with

increasing spot size as previously reported. On the other hand, the multiple pulse

threshold increases with increasing spot size. The difference in their behavior

suggests that the two phenomena are the result of different mechanisms. Localized

surface defects and impurities appear to be responsible for single pulse damage,

while for multiple pulse damage a model is proposed based on the cumulative effect

of plastic deformation induced by thermal stresses.

Key words: Copper mirror; damage; defect; diamond-turned; multiple pulse; pulsed

calorimetry; silver mirror; single pulse; thermal gradient stress.

1. Introduction

The continuous development of high power pulsed lasers has made the question of radiation induced resistance of the optical components employed an important problem. Copper and silver mirrors

because of their high reflectivity and good thermal conductivity are used extensively in laser

applications. These mirrors must withstand high-intensity, short-pulse irradiation in multiple as well as in single pulse operation. Last year [1] we presented data for Cu mirrors, where, at 1.06

urn the multiple pulse damage threshold increased with increasing beam size. The effect was attributed to stress developed during the pulse with no further analysis.

The scope of the present work is to analyze the observed behavior of Cu and Ag mirrors under

10.6 urn, 1.06 ym and 0.532 ym multiple and single pulse irradiation and present a model to explain

the damage processes. In the case of multiple pulses, we examine the thermomechanical stress degradation of the metal surface under laser irradiation. As a result, we can describe multiple

pulse damage in terms of a cumulative process where the damaged medium has a memory of the preceding

pulses if the power is more than a certain threshold value. The single pulse damage process is described in terms of a defect triggered breakdown.

2. Experimental

The samples used were 0FHC bulk Cu and electrodeposited Ag on Cu substrates diamond turned in the facilities of the NWC in China Lake. We also used 0FHC bulk Cu diamond turned by Intop Corp.

The damage test facilities are depicted in figure 1 and are in principle the same as those used

in previous damage and calorimetry studies [1,2]. An important addition was the use of a SHG crystal in the Molectron NdrYAG laser which allowed irradiation with 0.532 ym light.

The repetition rate of both lasers - 10 Hz for the Nd:YAG at both 1.06 and 0.532 ym and 5/8 Hz for the CO^ laser - set a limit to the maximum number of shots used for the experiment, the limit

being 10

4 shots for the NdrYAG and 1,500 shots for the C0

2 laser.

The quality and reproducibility of the laser beams is of crucial importance in damage experiments. Therefore their? temporal as well as spatial profiles were periodically monitored. For the

Nd:YAG laser at 1.06 ym and 0.532 ym, we used a fast photodiode to measure the temporal profile and

a Reticon diode array to monitor the spatial distribution of the laser beam. For the CO2 laser, we used a photon drag detector to monitor the temporal waveform while the spatial profile was determined by beam scans.

The sample was positioned in a calorimetric chamber evacuated to -100 mTorr (see figure 1). A site was then selected on the sample's surface and irradiated with many pulses of constant

energy. Damage would occur instantly or after a number of pulses. For the 10.6 ym and 1.06 ym

186
irradiation damage was monitored by using laser calorimetry. As described previously [1,2] upon

damage the behavior of the heating curve displayed in real time would change dramatically while at the same time one could observe a flash or spark. Due to the increased absorption of Cu and Ag

at 0.532 urn detection of damage with laser calorimetry presented serious technical problems.

Therefore, a photomultipl ier with appropriate filtering was used which detected the increased

scattering which accompanied damage as well as the broad spectrum of wavelengths produced by the

intense plasma formed during the catastrophic damage process.

After damage, the number of pulses required to cause failure N, and the corresponding intensity 1^, were recorded. Then a new site was selected and irradiated at a lower intensity, where

more pulses were required to produce damage until an intensity was reached showing no evidence of

causing failure after 1,500 pulses at 10.6 ym and 10,000 pulses at 1.06 and 0.532 urn.

The same experiment was then repeated for several beam sizes in order to determine the multiple as well as the single pulse damage intensity as a function of the beam size. (In this work

beam size is given as the radius of the beam where the intensity is 1/e^ of the axis intensity).

3. Results

Figure 2 shows a plot of the log of the number of shots N vs intensity Tor diamond turned Cu at 10.6 ym, 1.06 ym and 0.532 ym, while figure 3 shows the same plot for diamond turned Ag. One

notices that as the intensity is reduced more pulses are required for damage.

Figure 4 shows a plot of the multiple pulse damage intensity for a certain number of shots

(open circles) as well as the single pulse damage (solid circles) as a function of beam radius for

diamond turned Cu at 10.6, 1.06 and 0.532 ym. Figure 5 shows the same plot for diamond turned Ag.

The single pulse damage threshold decreases or remains constant as the beam size increases. In contrast the multiple pulse damage threshold increases with increasing beam size in a linear

fashion.

An examination of the damage morphology with a Nomarski microscope reveals that single pulse

damage generally occurs at localized defect sizes. On the other hand, in multiple pulse damage,

prior to catastrophic failure (i.e. before a flash or spark were observed), the irradiated sites

become roughened and the degree of roughness increased with increasing number of pulses. The re- sults for Cu are shown in reference 2. Figure 6 shows the results for Ag at 0.532 ym. In Figure 6a, the site was irradiated for 1,500 pulses, while in Figure 6b, the site was irradiated

tor 5,000 pulses. Catastrophic failure was not observed in either case. We were able to detect

the increased scattering due to surface roughening at 0.532 ym with the photomul ti pi ier before

catastrophic failure occurred.

4. Discussion

The observed beam-size dependence for single pulse damage is similar to the one observed for

damage to thin films and surfaces of transparent media [3,4,5]. The model suggested in that work,

which seems also to be valid for metal mirrors, assumes that the nature and distribution of surface defects determine the beam size dependence. As a result, the probability of the laser beam

striking a defect will be greater for larger beam sizes. If the beam size is larger than the mean distance between defects so that the beam area striking the samples encompasses several ot them,

the damage threshold will be invariant with spot size increase. Apparently the range of beam sizes

used in the present experiment ( r

-j/ e

2 between 50 and 250 ym) satisfies the condition. There is recent evidence [6] that multiple pulse damage in dielectric reflectors follows the same trend in its beam size dependence as the single pulse damage, namely the laser damage intensity decreases

or remains constant for increasing beam size. Damage following the above trend can be characterized as extrinsic.

An entirely different description applies to the multiple shot damage we observed in Cu and Ag.

The sample morphology before catastrophic damage occurred was indicative of an accumulation process

suggested before [7J. To explain the accumulation of deformation leading to catastrophic damage as well as the unexpected spot size dependence of the multiple pulse damage threshold we considered the

generation of stress by a local temperature gradient associated with a non-uniform temperature field

The temperature distribution produced by a Gaussian beam at the end ot a square pulse of duration t , assuming that the thermal diffusion length is negligible in comparison to the beam

dimension, is given by:

187
T(r) = ^Ifi_ exp (-r

2 /w2

) PC ^ (1)

for w » /4Kip

where

:

A = absorptance

*0 = peak intensity

p = material density

C = specific heat

k = thermal diffusivity

2 w = spot radius at i/e

t = pulse duration

P

The shear stress of a plane metal surface subjected to heating is proportional to the temperature

gradient introduced, so that the maximum shear stress (occurring where the temperature gradient is maximum) is given by:

AI n aEx -i amax = _0 P 1 {2)

s /2e(l-v)pC v

where the additional parameters are:

E = Young's modulus

a = coefficient of thermal expansion

v = Poi sson ' s ratio

When the shear stress exceeds the yield stress of the material, the metal deforms plastically.

The onset of plastic deformation is defined when:

a > a

s y

Substituting a from (2) and solving for the critical now intensity I ^ y

/2e~ (1-v) pCo

I y(w) = aTT

1 w

According to Porteus et. al . [3], the most dominant damage mechanism on clean and carefully

prepared surfaces, as is our case, is slip bands resulting from thermal stresses and having a threshold significantly lower than the one required for catastrophic damage. The slip bands and

the other plastic deformations occurring when the yield strength of the surface is exceeded during

each pulse, will manifest themselves as an increase in surface roughness. Repetitive pulse illumination will cause an accumulation of plastic deformation and a continuous increase in surface rougn- ness until catastrophic damage occurs.

5. Conclusion

The single as well as the multiple pulse damage behavior of Cu and Ag diamond turned mirrors

at 0.532 urn, 1.06 ym and 10.6 ym as a function of ihe laser beam size have been examined. For the

single shot damage, a probabilistic model can be applied in which the nature and distribution of

defects are responsible for the observed behavior. Such a damage process can be characterized as intrinsic. In the case of multiple pulse damage, a cumulative process triggered by thermal

stresses leads to the observed spot size dependence and demonstrates that multiple pulse damage is dominated by an intrinsic process.

188
The authors express their thanks to Dr. D. Decker from the NWC in China Lake and P. Engstrom

from INTOP Corporation for kindly supplying the crystals. They also want to thank Dr. L. Merkle

for helpful discussions. This work was sponsored by ONR-SRO Research Grant No. N00014-70-C-0896.

6. References

[1] Koumvakalis, N.; Lee, C. S. ; and Bass, M. , Proceedings of the 13th Symposium on Laser Induced

Damage in Optical Materials, Boulder, Colorado, Nov. 1981 and to be published by the National

Bureau of Standards.

[2] Lee, C. S. ; Koumvakalis, N. ; and Bass, M. , Appl . Phys. Lett. 41 (7), 1922.

[3] DeShazer, L.G.; Newnam, B. E. ; and Leung, K. M., Appl. Phys. Lett. 23, 707 (1973).

[4] Wang, V.; Giuliano, C. R. ; and Garcia, B., NBS Special Publication No. 435, 1973, p. 216.

[51 Bettis, J. R. ; House II, R. A.; and Guenther, A. H., NBS Special Publication No. 462, 1976,

p. 338.

[6] Foltyn, S. R. ; Jo! in, L. J.; and Newnam, B. E., Proceedings of the 14th Symposium on Laser

Induced damage in Optical Materials, Boulder, Colorado, 1982, November and to be published

by National Bureau of Standards.

[7] Musal Jr., H. M. NBS Special Publication No. 568, 1979, p. 159.

[8] Porteus, J. 0.; Decker, D. L.; Jernigan, J. L.; Faith, W. N.; and Bass, M. , IEEE J. Quantum

Electr. QE-14, 776 (1978).

TO PUMP

4±

VARIABLE

B.S. ATTENUATOR

1.06^m 4*.0.53/um

Ndi YAG LASER

and SHG

\ ? MOVABLE

MIRRORS

10.6/un

C02 TEA LASER

ond CW DISCHARGE SECTION

CHART RECORDER

Figure 1. Schematic of experimental apparatus.

189
NWC DT Cu (OFHC) • 42 WAVE L E NGT H = 0. 5 32 ( fi m

SPOT-SIZE = 72 ( M ml

10 0 100 200 300 tOO 500

N-PULSE DAMAGE THRESHOLD (MW/cm!

) to)

10°

T

NWC DT (OFHC) Cu « 39 WAVELENGTH = I.06(mTM) SPOT-SIZE = 50 (,irr.)

,0"! J 1— —l_^-W—

0 200 100 600 800 1000 N-PULSE DAMAGE THRESHOLD (MW/fcm2

) (b)

10"

10

NWC DT Cu » 48

WAVELENGTH = (0.6 (^m) SPOT-SIZE = 110 (fjm)

O 200 400 600 800 1000

N-PULSE DAMAGE THRESHOLD (MW/cm'l

(c)

Figure 2. Measured log N vs 1^ for diamond-turned OFHC Cu irradiated at a) 0.532 ym

b) 1.06 ym c) 10.6 ym.

NWC DT EP-Ai) a 14 WAVELENGTH = 0.532 (j,n SPOT-SIZE = 52 Urn)

J_

0 100 200 300 400 500

N-PULSE DAMAGE THRESHOLD (Mw/cm ?) (ol

IO

l

NWC DT EP-Aq #54

WAVELENGTH = 1.06(^1 SPOT-SIZE = 100 ( M m)

1 ' O 200 400 600 800 1000 N-PULSE DAMAGE THRESHOLD IMW/tm')

(b)

10'

1 1 1

NWC DT EP-Aq »53

WAVELENGTH: 10.6 (^m) SPOT-SIZE =50 (urn)

L O 200 400 600 800 100O N-PULSE DAMAGE THRESHOLD (MW/tm*)

(c 1

igure 3. Measured log N vs 1^ for diamond-turned electroplated Ag irradiated at a) 0.532 ym

b) 1.06 ym c) 10.6 ym.

190
9 500r 2_

g E400- g5 300- < J_ o </> < uj uj iooh

NWC DT (OFHC) Cu 42 WAVELENGTH . 0.532 (um)

100 150 SPOT SIZE

(o)

800-

600-

20CNWC Cu (OFHC) OT • 39 WAVELENGTH 1.06 (urn)

-*

50 100 150 200 SPOT SIZE (um)

(bl

250 300

1000-

[ 800-

400-

NWC Cu (OFHC) DT #48

WAVELENGTH = 10.6 (um)

50

_

L

100 150 200 SPOT SIZE lum)

(c)

250 300

Figure 4. 1^ vs W for diamond-turned OFHC Cu irradiated at a) 0.532 urn b) 1.06 ym

c) 10.6 vim, N - 10

3 shots.

J; ^-4oo- U p

II

2

UJ j"J 13 5 looH M-

-4-

NWC DT ELECTROPLATED Ag #14"

WAVELENGTH = 0.532 liiinl 50 100 150 SPOT SIZE Ifim) (o)

200

800-

600- 400-

NWC ELECTROPLATED Ag DT * 54 WAVELENGTH *1.06 (,<m)

i

100 150 SPOT SIZE

(b)

200

(um)

S^-'OOOf > 800-

S 200-

5"

NWC ELECTROPLATED Ag DT * 53 WAVELENGTH = 10.6 Ifiml

50 100 150 200

SPOT SIZE (um)

(c)

250 300

Figure 5. 1^ vs w for diamond turned electroplated Ag irradiated at a) 0.532 um

b) 1.06 ym c) 10.6 um. N = 10

3 shots.

191
210 um

a)

210 um

b)

Figure 6. Nomarski micrograph of laser-induced surface roughening in diamond-turned electroplated

Ag at constant intensity for two different numbers of pulses, a) N = 1,500

b) N = 5,000.

In response to questions , the speaker emphasized that thresholds increased when spot size increased

only during multiple-shot tests on metal surfaces . For single-shot tests, thresholds decreased

when spot size increased.

192
"First Order Analytical Technique For Evaluating The Heat Transfer, Coolant Channel

Thermal and Pressure Ripple, Bending, Burnout, and Coating Damage on Single and

Multilayer Laser Mirror Cooled Heat Exchangers"

James R. Palmer

TRW

One Space Park, Redondo Beach, California 90278

Laser mirror cooled heat exchangers become somewhat complicated to evaluate when the

analyst must consider elements other than the heat transfer. This analytical technique is organized to assist in the first order evaluation of a cooled heat exchanger

mirror, withoutusing large, complex codes, in order to determine if the mirror heat

exchanger is within the boundaries of the problem to be investigated.

The process goes step by step, first evaluating the requirements for single or multiple pass, determining the pressure drop through the mirror and the gradient through

the face plate. The temperature gradient for the channel web is determined and the

relationship of pressure and thermal ripple are integrated with the channel web

distortion to achieve a RMS ripple value. The overall distortion of the mirror,

i.e., a focus term, is evaluated as a function of total gradient through the total

thickness. The relationship of coating defects, i.e., damage, with respect to location over a web and over a cooling channel, acting in concert with the cooling fluid,

is determined. The cooling fluid properties and film coefficients are then related

to the coating defect, and defect zone, wherein burnout, nucleate boiling and film

boiling may be established as an additional constraint on the design. As a result,

the investigator will have a reasonably well rounded - upper bound, view of how the

mirror will respond to the particular conditions imposed by the particular laser

system.

Key words: coating defects; cooled laser mirrors; defect damage; hot face design;

limiting flux density; mirror damage, vapor barrier-induced burnthrough.

Introduction

A technique is suggested for providing a comprehensive analytical technique for evaluating,

to the first order, the thermal, structural, and optical properties of an optical element. Primarily, the technique addresses cooled Laser Mirror Heat Exchangers. However, as an aside, many of

the structural expressions will apply to uncooled mirrors when combined with expressions presented

by the author, last year.

Essentially, the technique describes the process for evaluating the fluid mechanics within

the mirror coolant channels. The heat transfer and steady-state gradients are developed in concert

with the analytical and graphical solution of burnout, incipient and pool boiling, and saturation

temperature of the coolant fluid.

Based on the heat transfer and fluid mechanics, the analytical expression for static deflection are developed for the channels, webs, and overall bending of the optical elements. The last

element of the technique is the development of the analytical solutions for determining the maximum

defect size that may be tolerated based on limiting temperature condition of coating or sub-cooled

boiling at the coolant channel wall. The last element of the technique is the development of

analytical solutions for evaluating temperature gradients and bending of transmissive optics.

The objective of this paper is to develop the various analytical expressions in a step wise

fashion in order to lead the reader through each process and, then, to show how the various elements

fit together in a comprehensive fashion. At the very outset, of course, the flux density that will

irradiate the optic must be known. Secondly, one must know the reflectivity of the optical surface.

From this information one will find the absorbed flux density. So that, simply:

Numbers in brackets indicate the literature references at the end of the paper.

193
Fo = I 0

(l - R) (1)

where

:

I0

= Incident Flux Density, BTU/KR-FT 2 (W/CM2

) R = Reflectivity

Fo = Absorbed Flux Density, BTU/HR-FT 2 (W/CM2

)

Assuming parallel flow and uniform absorption over the surface of the heat exchanger, determine

the amount of flux density that will be absorbed over each top channel, see Figure 1.0. The absorbed

flux density for each channel will be partioned between the free span over the flowing fluid and the

webb that separates one channel from the other. Typically, the webb thickness will be 5.4 times the

channel width and will receive 16% of the absorbed energy. The distribution of absorbed energy be- tween the channel span and the channel webb will be:

Q(.84) = Absorbed energy through span, BTU/HR (Watts) (2)

Q(.16) = Absorbed energy through webb, BTU/HR (Watts) (3)

The fluid mechanics of the system are reasonably straight forward. At the outset, the amount of

temperature increase in the flowing fluid should be determined, based on system requirements. So that, temperature increase is found from:

Ay = Q/CpG 3600 (4)

where

:

Q = Total absorbed energy/channel, BTU/HR (Watts)

C = Specific Heat, BTU/#-°F (kCal/kgm-°C)

P = Density, #/FT. 3

( kg/M3

) G = Mass Flow Rate, i/Sec, (M 3 /sec.)

Having determined the appropriate temperature increase of the cooling fluid for the allowable

mass flow rate, the additional values may be determined so that:

Gp VDM p Re

= —or^iL Ll] (5 )

where

:

R

g

= Reynolds Number, dimensionless

G = Mass Flow Rate, FT. 3 /sec (M 3 /sec)

V = Velocity, FT. /sec (M/sec)

D u = Hydraulic Diameter, FT. (Meters)

n

n = 4 AREA [91

H DIAMETER L J

y = Viscosity, #/FT.Sec. (kg/m-sec)

A polynomial expression for (y) has been derived, for english units, to provide a curve fit for

water between 32°F and 200°F and atmospheric pressure. This polynomial expression was derived usinp

the Forward Doolittle Technique:

194
Ft _ Sec

= 1.61861 (10~ 3

) - 1.661365 (10~ 5

) (T°F) (6)

+ 4.8557 (10~ 8

) (T°F) 2 [3]

The pressure drop through the channel is found from:

L 2 A

p #/in

2

= [f

DH

+ EK

L

] V

P [4] [5] (7)

144 (2g)

where

:

L = Length of channel flow, Ft. (m)

D

H

= Hudraulic Diameter, Ft. (m)

EkL = 1.5 for entrance and exit losses, non dimensional

V = Velocity, Ft. /Sec. (m/Sec.)

3 3 p

= Density, #/Ft. (kgm/m )

R

e

> 2000 f = [1.84 log 1Q (R

e

)-l .64]" 2 - 0 Idel 'check [6]

R

g

> 2000 f = .184(R

e

)" 2 - 0 Blausis [7]

R > 2000 f = .0056+. 50(R ) "

32 Koo [8]

e e

R > 2000 f = |i Moody ' [9]

e k

e

For water, the expression, ( Eq. 7), reduces to:

A

p #/in.

2

=

j [(fjj- + 1 .5] V

2 (6.729)1 0" 3

|

The steady-state heat transfer and subsequent steady state temperature gradient through the top

face plate may be found from the following expressions. For the portion of the abosrbed flux to be removed from the channel span:

The heat transfer coefficient for Laminar flow, i.e., R

e

< 2000, is found from the Graetz expression:

Nu =

h c D

H = 3.65+

°- 065 (¥L

> R

e

P R ?n

* a

k 1+0.04[(D

H /L) R

e

P R

] 7 (9)

Sieder and Tate have developed an empirical equation for Laminar flow conditions, such that:

Nu/c DH = 1.86 [ R

e

P R W/3 ("B)

°' U

[n] (10)

k L m s Where:

h c

= Film heat transfer coefficient, BTU/HR-Ft.-°F (w/cm

2

- C)

k = Thermal Conductivity of cooling fluid, BTU/HR-Ft.-°F(W/Cm-°C)

y

g

= Bulk fluid Viscosity, #/Ft. Sec. (gm/cm-sec.)

= Viscosity of liquid at surface temperature, #/Ft. Sec. (gm/cm-sec.)

P R

= PRANDTL NUMBER = -^(3600)

195
For water between 32°F and 200°F at atmospheric pressure, the following polynominal may be used for

determining the thermal conductivity of water, in english units: [12]

k BTU/HR-FT.-°F=. 290402+9. 563962(10" 4 )(T°f)-2.1 96228(10' 6 )(T°F)2 (11

)

Prandtl number for water between 32°F and 200°F at atmospheric pressure may be found from the

following polynominal:

P R =18.8104-.208228(T°F) + 6.29383(1

O

-4

) (T°F)

2

For fully developed turbulent flow, i.e., R >2000, the film coefficient may be found from any of

the following expressions:

Dittus Boelter Expression

Nu T =J^H_ = .023 [R ] 0,8

[P D ] (1/3)

[13] (13)

1 k

Petuhkov Expression

NuAD (f/8 > R

e

P R [14] (14)

1 k 1 .07 + 12.7(f/8).i>(P

R -bb-l .0)

Seider and Tate Expression

h D N "T

=-Mi = .023 [R

J°- 8

[PD ] 1,3

[y

B]

0 * 14

[15] [16] (15)

From the information generated from the above expressions, the termperature gradient across the

stagnation boundary and the temperature gradient across the face-plate may be determined. So that;

Inside wall temperature will be found from:

t T„an-W W <' 6 >

where

:

2 2 A = Channel Surface Area. Ft. (cm )

A = Channel width x channel length

The steady state surface temperature is found from

d

d 0.84Q = -kA^T (17)

The variables are separated, so that;

0.84Q /

_' COld

' Kdt

m r r

A Jm = -Jk

hot

with (k) independent of (T)

f Hot " T Cold 0-84Q =iA

(T H„ t - Trnl ,) [18] (19)

196
where:

L

= thickness of face-plate, Ft.(M)

T Hot

= Re ^ ectlve Surface Temperature

T Cold

=

* ns1(-' e wa ^ temperature = T -|-|

So, then, from (19).

t

T

_ 0.84QU) j (20) Hot k'A + Twall

1 '

The Webb temperature may be determined, for both the reflective surface and the bottom of the

Webb that attaches to the face-plate of the second channel, or substrate, if a second set of

channels is required. Basically, the Webb may be treated as a rectangular fin with heat transfer

at the end. The generalized solution for this case is:

^|-m2 0=O (21)

dx

9=C iemx

+ C

2 e"mx (22)

For the case of heat flow from the edge of the fin, the boundary conditions are: .t(xH,.g.-Vee

(23)

at (x)=0, 0=9

Q

(24)

Differentiating (Eq. 22) and substituting (Eq. 23)

mb „ -mb h r „ mb r -mbn (25)

C-,me -C 9me = - e|_C-.e - C

? e J k

1 c

Substituting (Eq. 24) into (Eq. 22) provides:

G = C,+C 9 (26)

0 I c Solving (Eq. 25 and Eq. 26) simultaneously gives:

e

=ejr emb(x/b- 2) +ae-mb(x/b)

] [ (27)

-2mb a-e

where:

_m+h /k

m-h /k

e

h

c

= Eq. 10, 13, 14, or 15

2h r m=[T-| ] S = Webb thickness, Ft.

k S o o

0 =T -T.. . b = channel + face-plate depth, Ft, o o Hot

9=T- T Bulk

h e =k/b

197
EQ. 27 May be differentiated for the ideal case, so that;

de = d_

dx dx

) o(emb(x/b-2) + a -mb(m/b)

, „ -2mb

ct + e

at(x) = 0 de 0 m(e" 2mb

- a] dx =

(28)

(29)

a + e So, then, heat flow through the base of the fin:

J6Q"kS

°

L<[ - ".W"'

- 2mb

» (30)

A U - - -

, -2mb

a + e

move conveniently,

.16Q = kSo

L o M0

o [{h&/mk) + TANH mb l

( 31)

1 +(h

e /mk) TANH mb

Since .16Q is a known value, Eq. 31 may be written to solve for the temperature at the

reflective surface of the fin and the tip of the fin. Re-arranging Eq. 31.

T j ,16Q [1 + (he/mk) TANH mb] (32)

Vwall k S L m L(he/mk) + TANH mbj oo

where:

L Q

= Length of the channel, Ft. and from Eq. 27, for x = b

T - T

= ( VTWan)[e mb0 " 2)+a e

"mb

l H9], [20], [21] (33)

e Bulk

a - e ^

Knowing the various temperatures, as determined by the foregoing expressions, it is possible to determine the deflection caused by the various thermal gradients.

For deflection due to thermal gradient across the channel span, which is a fully clamped geometry, is found from the following:

Bending Moment:

M = a(THot ' T Cold )(1 + v )D (34)

L

where:

T = Eq. 20 y = Poisson's Ratio

Hot

TCold

= E «- 16

L = Thickness of face-plate

a = Coefficient of expansion of face-plate

material , in/in-°F

D = Modulus of Rigidity

D = Et

3

, T2TT72]

2 Modulus of Elasticity, #/in

198
The apparent load is found from:

<ft - M(16) (35)

where:

R = Half-width of channel span

Deflection at the center of a Rectangular Element fully clamped and width of channel to length of

channel ratio > 3.0, which is the case for most mirrors:

A] = f^<2R) 4 1 2.6(10" 3

) [22], [23], [24], [25], (36)

D

In order to be most conservative, the webb will be allowed to grow without restraint, i.e.,

without Poisson effect, so that:

A

2 = a(T

o

" T

e )b [26] (37)

The channel span deflection resulting from hyrdraulic pressure is found from:

% = ^ + V2

" 1 (38)

where:

2

P-| = Entrance pressure, #/in

2 A D = Pressure drop through the heat exchanger, #/in

P = Eq. 7

For clamped rectangular geometry as indicated by (Eq. 36)

,4 -, A

3 =Pl (2R)

D (2.6)10

3 [27] (39)

The ripple for the face-plate as a result of the thermal gradient across the face-plate, the

axial gradient along the webb and the deflection from the hydraulic pressure will be found

from

:

RMS

(A)=

A 1+A 2+ A3

2 vr J

(40)

The next area of concern is the overall bending of the optic. The radius of curvature due to temperature difference between the coolant channels and the back surface of the substrate is found for a free geometry:

R-JL [28] (41)

0 aAT

where

:

t = Substrate thickness, inches

a = Coefficient of Expansion

Ay = T

g

- Ambient temperature

T

g

= Temperature at the end of the webb

3

n Et R = Radius of the optical element inch

199
The apparent load due to the bending is found from:

a _ s pn + u)

Q

o

" R (R)2

o [29] (42)

For a plate simply supported

A = 20 . o

0 64D 16D( 1+v

)

[30], [31]

For a circular plate that is supported on the edge at three (3) points, 120° apart, with the

load uniformly distributed, the deflection at the center would be:

A

o

=

/\ 4

. 0362ttQ

o

R [32]

(43)

The next element of the analysis is to evaluate the possibility of subcooled boiling, i.e.,

burnout. It should be pointed out that a defect that becomes absorbing will allow the spot

(defect) site to increase in temperature in as much as the heat transfer across the stagnation

boundary of the fluid is bv conduction onlv. In this problem, only the localized spot is affected. Little heat is carried to the surrounding by diffussion since the differences in thermal

diffusivity of the face-plate materials and the cooling fluids are large. Essentially, then,

the fluid will be a virtual insulator to the channel wall and will allow the channel wall, at

the localized spot, to increase in temperature without producing a significant change in the bulk

fluid flowing in the channel. The localized spot (defect) at the wall, then will experience

local izec' boiling. [33].

There are techniques for evaluating the points of boiling as a function of the referenced film

coefficient, temperature of the bulk fluid, velocity of the bulk fluid, and the pressure

in the system. A graphing technique for evaluating burnout and incipient boiling has been

explained by the author in a previous paper [34]. However, the technique is essential to this

first order analysis system and may be evaluated using a plot of the following expressions as shown in the graph of figure 6.0.

The heat transfer before boiling if found from:

: [(T

W

- V + { T

S T»)] [35] (44)

where

:

T.j = Inside channel wall temperature

T<- = Saturation temperature of the cooling fluid at average

pressure of the mirror

Tg = Bulk average temperature of the cooling fluid

Incipient boiling is found from:

= 15.66(F)

] - 156

[Tw

- T

s

] 2 - 30/P [36] (45)

where:

P = Average absolute pressure, PSIA

Fully developed boiling is found from:

TQl l" (Tu - Isj EXP (P/900)

'

1 LaJb "

I 60 (10

c [37] (46)

200
Burnout, based on velocity, is found from the McAdams correlation:

= 4(10

5 )V 1/3

+ 4.8(10

3 )(TC

- T D )V

1/3

[38] (47)

B.O

S B

By plotting equations 45 through 47, as shown in figure 6.0, the maximum temperature increase at

a defect site on the channel wall may be determined. As pointed out previously by the author,

the maximum temperature for reflective coating should not exceed 200°C(392°F) . This value is some what conservative; however, transcending this temperature invites catastrophic damage, based

on experience. This temperature may be achieved either on the coating surface or through

burn-back from incipient boiling at the channel surface, resulting from an absorbing defect of

the appropriate size. [39]

The maximum defect size for 100% absorbing defect may be evaluated from the Palmer-Bennett model.

[40]

A = F0

sTa t

1 .1 284 - 2ierfc R . j K

' 2 v

/~aT j For Reflective Surface (48)

A T

where:

=

Fo ^~aT hierfc Z - 2i erfc (

Z

2 + R

2

)

5

j (49)

K

' 2 ^aT ~\?or channel wall

Ay = Maximum temperature increase over steady-state. This

value will depend on which element is controlling -- viz., maximum coating temperature or incipient boiling

at channel wal 1

.

Z = Thickness of face-plate, FT.

FQ

= Absorbed Flux Density, BTU/HR-Ft.

2

K = Thermal Conductivity of face plate material,

BTU/HR-FT. -°F

2 a = Thermal diffusivity of face-plate material, FT. /HR

T

= Run time, HRS.

CO ierfc (x) = f [1-erfU)] d?

x erf (x) = 2 * _ ^2

it fe d£

o

a° = a(6/aT)/Z For 6 VaT > Z

R = Defect radius, FT.

Knowing the value of Ay, the maximum defect radius may be found from:

Zierfcf^—)= 1.1284 - K(A

T

} *Z£t j-j^ (50)

A value of anti -[2ierfc(x)] has been developed through a set of polynominals derived from using

the forward Doolittle Matrix Technique; so that:

For (x) = j2ierfc ) - Ml \ OR jl.1284 - "^jO-!

(x) = o to 1(10" 4

) R = 2^t(2.6)

201
(x) = .0001 to .002 R = 2^t[2.576486 - 656.947(x) + 1 .87357(10

5 )(x )2]

(x) = .002 to , 0253 R = 2^1 [2. 062964 - 52.9737(x) + 1 . 0761 5(1

0

3 )(x ) 2

]

(x) = .0254 to .1364 R = 2^t[1 .58956 - 8.663(x) + 26.882(x)

2

]

(x) = .1365 to .3119 R = 2.J^x[l .2648 - 3.121 (x) + 3.1847(x)

2

]

(x) = .3120 to .6284 R = 2^t[1 .04463 - 1 .671 (x) + .7755(x)

2

]

(x) = .5285 to 1 .1284 R = 2^t[.8424 - 1 .0125(x) + .23617(x)

2

]

Conversly

The polynominal for 2ierfc (x) for intervals of (x) will be: [41]

x Interval 2ierfc (x)

0 - 0.299 1.1284 - 1.998(x) + l.ll(x)

2

0.30-0.5499 1.1150743 - 1 .90863(x) + 0.9543(x)

2

0.55 - 0.9999 1.012 - 1.5524(x) + .64346(x)

2

1.0 - 1.599 .6271 - 0.76914(x) + 0.241964(x)

2

1.6 - 2.0 .21 573 - 0. 21 1 65(x ) + 0. 0525(x)

2

2.1 - 2.5 .05124 - .041425(x) + .0083939(x)

2

In order to illustrate the techniques, a working example will be given to take the reader through

the use of each expression. Additionally, an output of the Doc Jim Code, which calculates the

values, will be shown at the end of this portion.

Probl em:

3 A cooled laser mirror, with three channel passages, as shown in figure 3.0, will absorb 10

BTU/HR-Channel . The face-plates of the heat exchanger will be 0.020" thick. The channel

geometry will be 0.081" wide, 0.020" high with a webb thickness of 0.015". The channel will

be 6.0" long. The cooling medium will be water, which will enter the mirror at 70°F and 150 PSIG

entrance pressure. The face-plates will be made of Moly. The velocity of the water will be

35 FT. /Sec. at fully developed profile. Run time is 15 seconds.

From Eq. (2), (3) we know that the portion of energy for the channel span and webb will be:

Span = .84(1

0

3

) = 840 BTU/HR

Webb = .1 6(1

0

J

) = 160 BTU/HR

The hyrdraul ic diameter of the cooling passages w'll be:

n - U (-081 x .020) ] 12" 1 U

H f (2 x .081 + 2 x .020)

J

D

H

= 2.67(10" 3 )FT.

The temperature rise of the water for each channel will be, from Eq. 4:

Ay = 10

3 BTU/H R

i n BTU « # « FT. , .081 x .020 , ?finn Sec^

1 .0 j-^- 62.35 35 ( m ) 3600

A

T

= 11 .315°F

202
The first cut viscosity will be based on the end point temperature of the water, i.e., 70 +

11 .315 = 81 .385°F.

V

= 1 .61861 (10" 3 )-1 . 661 365(1 0" 5

) (81 .31 5)+4 .8557(1 0" 8) (81 .315)

2

y

= 5.887(1 0- 4 )|L^-

From Eq. 5

R = 35 FT. /Sec. (2.67)1

Q

" 3 FT. 62.35 #/FT.

3

R

g

= 9896.8

e 5.887(1

o

-4 )TM*^

Friction from Eq. 7

f = [1.84 log 10

(R

e )-1.64]" 2 - 0

f

3 .065(1 0~ 2

)

Pressure Drop from Eq. 8

2

p

=

| [.03065 -2 ;67( 1 0 -3) ] +1 - 5

j 35

2 (6. 729)1 0"

A

p = 59.677 #/in.

Since R

e >2000, the film coefficient will be found from Eq. 13. The Prandtl and thermal

conductivity will be found from Eq. 12 and Eq. 11 respectively:

P R

- 18.8104 - .208228(81 .31 5)+6. 29383(1

0

-4

) (81 .31 5)

2

P R

- 6.04

k = .290402 + 9. 56396(1 0" 4)81 .31 5-2 .1 96228(1

0~ 6 )81 .31

5

2

k = .3536 BTU/HR-FT.-°F

The heat transfer film coefficient is found from Eq. 13:

h c =

2

3 67( 10 -3 )(-023)(9896.8)-

8 (6.04)

1/3

hc

= 8719 BTU/HR-FT.

2 -°F

The heat transfer area over the span is:

A = (.081 x 6)144

_1

A = 3.375( 10" 3 )FT.

2

Recheck Viscosity, Prandtl and Conductivity based on

T = (104.2 + 7fl) ?

-1

T= 87.1°F

u = 5.399(1

0" 4

) PR = 5.448 k = .3908

/ new/ R

e

= 1 0792 f .0299 A = 58.55 #/in.

2

h Q= 10002 BTU/HR-FT. 2 -°F

TUall

= 104 ° F 840 BIU (^20)

The reflective surface of the mirror will be from Eq. 1 9- Tu = HR 1? —

Hot

84 hr_ft -of (3.375)l0'

3 Ft2 +104°F

203
T Hot = 109°F

Temperature balance along the webb will be evaluated from Eq. 32 and Eq. 33

M = 10002 x 2

M = 436.5/FT.

b = 3 .33(1

0" 3

) FT.

h £

= 84/3 .33(1

0" 3

)

h g

= 25202 BTU/HR-FT,

m + he/k

a m - he/k

LQ

= 6" OR 0.5 FT.

a = 5.3956

TANH(mb) = .8966

T

o

=

160

BTU

HR

1+( J874 x 8966)

. FT. 2 J + 104 F

84 BTU HR-FT -°F ^ 12 yFT.

(.5 FT.)

.6874 . once

py 2 + -8966

4 36.5 ) FT.

T =

o

BTU

258.605 HR-FT. 2 BTU 34.6695 HR-FT. 2 -°F

+ 104°F

T

o=111.5°F Reflective Surface temperature over the webb

From Eq. 33

T = [112-87. 1][EXP(-1 ,45485)+5. 3956 x EXP(-1 .45485)] +

o 71 o e F 5.3956 - EXP (-2 x 1 .45485) ' r

T = 36.43 + 87.1 °F

e 5.341

T„ = 93.92°F

The span bending moment will be found from Eq. 34.

M 2.77(1

0~ 6 )/°F(1 09-1 04) (1 ,3)D

.02"

D = 46(1

0

6

) ( .02)

3 12(1-. 3^)

D = 33.7

M = .03033 #-in./in.

_ 03033(1 6)

g " TTT~y)(.08112) 2 227.58 #/in.

227 . 58( .081 ) 33.7

A -j = 1. 9198(10"^ )y

2.6(1

0" 3

)

( .0254)1

0

6

204
A

2

= 2.77(10" 6 )(111 .5 - 93.92) 3 .33(1

0" 3

) x 12 x .0254 x 1

0

6

A

2 = 4.947(1 0" 2

)v

A

3 104(.081) 4 33.7 2.6(10 -3

) x .0254 x 106

A

3 = 3.4766(1

0" 3 )y

RMS (A) = [1 . 91 98(1 0" 2

) + 4 . 947 (1 0" 2) + 3 .4766(1 O"

3 )] ( 2x^/2)

_1

RMS (a) = 2.5507(10" 2 )y

The overall bending of the optic which is 12.0" in diameter with a 2.0" thick substrate is from Eq. 41 through Eq. 44.

P 2.0

0 (93.92 - 70) 2.77 (10" 6

) R = 30185 inches

0 r r> 46(10°) I

6

D TT(T-.32r

D = 33 .6996(1

0

6

)

= 8(33 .6996)1

0

6

(1 .3)

0 30185 (6)2

= 322.5 #/in.

2 v

o

Bending for a simply supported plate:

= 322. 5(6)

4 + 322.5(6)

4

0 64(33. 6996) (1

0

6

) 1 6(33 .6996)1

0

5

(1 .3) A

Q

= 7.9(1

0" 4

) inches

A

o

= 20. ly if there were only one (1) flow channel

Bending for an optic support at three places 120° apart:

From Eq. 44

= .0362U) 322. 16)1 ( Q254)1 Q

6 A

o 33.6996(1 0 ) I-U^jiu

A

Q

= 35.826p if there was only one (1) flow channel

The boiling and burnout characteristics of this particular problem can now be evaluated for

graphical analysis.

From Eq. 45

[l]= hc " TU

" V +

< T

S

" T B»

h c

= 10004 BTU/HR-FT.

2 -°F

P = 118.7 PSIA so that T

$

= 340°F [42]

Tn = 87.1 °F

T

s = 340°F

205
For (Tw -T

s

) = 10, 20, 30

£| = 2.63 (1 0

6

), 2 .73(1 0

6

)

, 2 .83(1 0

6

] A For incipient boiling:

g| = 15.60(F)

1 - 156

[Tw-T

s

] 2 - 30/P

For (Tw

- T

s

) = 10, 20, 30

0.234

LAJi

= 4 .46(1 0

5

)

, 1 .85(1 0

6

)

, 4 .26(1 0

6

;

For fully developed boiling

jfl (Tw-T

s )EXP(P/900)

60

(10

6

)

For (T

w

- T

$

) = 10, 20, 30

= 1308, 20923, 1 .06(10*

Burnout is found from:

(4(10

5 )(35)

1/3

+ 4.8(10

3 )(340-87.1)35 1/3

L

hJb.o

B.0

5.28(10°)

From figure 6.0, which is a graphical plot of the previous expression, it becomes apparent that

the wall temperature may be substantially above 391 °F, which is the limit of the coating.

Consequently, the maximum temperature rise for a defect will be:

Allowabl e A j

= 391 - 111.5

Ay = 279. 5°F

FQ

= 1(10

3

) BTU/HR/(.081 x 6 x 144" 1

)

Fq

= 2 . 963(1

0

5

) BTU/HR-FT.

2

So that, from Eq. 49:

R 279. 5°F 84

2ierfc2 \Jut

R_ 2/cn

= 1.1284

( H

BTU

)

\

m HR-FT.-°F/

2.963(10* BTU

HR-FT.

.2604

2 ~- (4. 16) 10" 3 HR

The polynominal for solving the anti-2 ierfc(.2604)

R 2 (4. 16) 10" 3HR

0.5

1.2648 - 3.121(.2604) + 3. 1847( .2604)'

R = .122 FT. DIAMETER = 2.9 inches which is much larger than a single channel

The following pages show the results of the Doc Jim Code which solves each of the elements which

have been shown in the example.

206
The last area to be visited with analytical evaluation is the problem one encounters with transmissive optics. The major mechanism for damage in a CW Laser System is the surface absorptions,

vis-a-vis, bulk absorption, self-focusing, photon absorption, et al

.

In most cases, the beam foot-print covers only a central portion of the transmissive element.

Consequentially, there are both axial and radial gradients which set up bi -axial stresses. Additionally, bubbles and striae become notch points or stress risers in the material. In the case

of thermal loading, bubbles and striae become absorbing defects, much like what has been discussed

in the foregoing. The following, then, is a technique for evaluating the temperature rise on the

first and second surface of transmissive optics and the resulting stress and deflection that will

result. The temperature rise on the front surface of a transmissive optic with a substantial

dimension is found from: r , N o.5

I- at

°i A = — (1.1284) (51)

°F K

When: R/2(ctT) 0,5

> 3.0

The temperature rise on the back surface of a transmissive optic for this set of conditions is found from:

A TI op lax)

0,5

1.1284 [42] (52)

Where

:

Fo = I (1-R), BTU/HR-Ft.

2

, Absorbed flux density

I - Incident Flux density, BTU/HR-Ft.

2

3 = Attentuation coefficient, cm7*

t = Thickness of the optic, cm.

For a transmissive optic that is irradiated by a spot, i.e.,

Where: R/2(ax)

0-5

< 3.0

The temperature rise on the surface is found from:

3T

-F

"

~ K j

1 ' 1284 " 21erfc 7^T5 }<">

and for the back surface, for the same conditions:

F + IJl-e

°2 ° JUt)°- 5

T op - K \— 2(ax)°-

5 1.1284 - 2ierfc ^-fr-<r > (54)

Thermal shock becomes the major problem with these damage mechanisms. Once the coating becomes

debilitated, the thermal stresses will begin to build quite rapidly.

Kreidl and Wood propose a model that suggest that an axial gradient sufficient to produce a tensile stress of 1000 PSI between the two surfaces should be a limit of the design. So that,

207
,

) = 2(10

3 )(1-m)

lA 4 T

; 1000 Ey L J

Where

:

\i = Poisson's Ratio

E = Young's Modulus, (#/in. )

y = Coefficient of thermal expansion, (in./in-°F. )

Fundamentally, then, the damage due to thermal gradient for a transmissive optic

When: R/2(aT) 0,5 > 3.0

(55)

2000 >

F°l

(aT

0.5

K (1.1284)

F + I n (l-e" 3t

) 02 0

K (1.1284)

,-1.0

,0.5

x Ey (1-m!

and for: R/2(ax) u-J

< 3.0

,0.5

2000 >

Fn (ax)

1 °1

K

F„

2

+

'o'

1

- 6 "")

(1.1284 - 2ierfc

(1.1284 - 2ierfc

2( ,0.5'

2(« ,0.5'

x Ey( 1-u!

•1.0

(56)

(57)

As the equations indicate, the value on the right hand side will be a negative value. The

convention only indicates the tensile or compressive condition. The front surface will be

compression and the second surface will be in tension.

sign

in

The bending of the transmissive optics may be found, as previously, from EQ. 41. For most conditions, the optic may be treated as simply supported around the peripheral edge.

Then, for a round window:

R0 =4,aAT

\

= 8D(l+y)

0

R0(R)

2

(58)

(59)

and

A o 64D 16D( 1+y) [44]

For a rectangular window that is simply supported:

(60)

0 aAT ^ = 8p ( t +v

)

where: R=.5 x short side dimension, inches

0

R (Rr o

v

'

(61)

(62)

208
QJ2R)'

A

o

= ^T (X) [45] (63)

For a ratio of the sides, i.e., the long side divide by the short side:

S (long) = 7

S (short)

L

For interval (Z)

:

1.0 to 2.0 -6.9953(10~ 3 )+1.34996(10~ 2 )(Z)-2.4674(10~ 3 )(Z)

2

2.0 to 5.0 3.6464(10" 3 )+1.20792(10" 2 )(1 Z) -3.9140( 10" 3 )(LnZ]2 [46]

5.0 to - .01302

There are additional conditions where transmissi ve optical elements are used, e.g., a second

surface reflecting beam splitter, or a simple second surface reflector.

For: R/2(aT) 0 - 5 >3.0

A

2 \

+ I 0

< 1

- e

' 2et

) 1.1284 (ax)

0.5

(64)

'F K

The second surface temperature rise is found from:

Fo

2

+ V1 -^') 1.1284 (ax)

0.5

K (65)

0 5 For: R/2(ax) ' < 3.0 The front surface temperature rise:

A

2 + I 0 (l-e" etz

) (ax) 0,5

| 1.1284 - 2ierfc

>F K

For the second surface temperature rise:

2(ax)

0.5 (66)

o^ o

v

K

et.

(ax)

0.5

1.1284 - 2ierfc

2(ax)

0.5 (67)

209
Typical Thermal -Mechanical Properties

of Commonly Used Optical Materials

Table 1.0 (46)

Optical Material

Young's

Modulus (E)

Thermal

Dlffusivity (a)

2 Ft /HR

Thermal

Conductivity (K)

BTU/HR-FT-°F

Linear Coefficient

of Expansion

1n/1n- F

^oly 46(10

6

) 2.05 84 2.77(10'6

)

Copper 17(10

6

) 4.42 224 9.28(10"6

)

Silicon Carbide 55(10

6

) .3525 40 1.94(10' 6)

3erylco-10 17(10

6

) ^ 2.356 121 9.80(10"6

)

Ilectroless Nickel 21(10

6

) 7.4(10' 4) 2.9 7.64(10"6

)

Fused Silica 10.6(10

6

) 0.033 0.8 3.11(10' 7)

JLE 9.8(10

6

) 0.03 0.76 1.67(10" 8)

ferodur 13.3(10

6

) 0.03 0.948 6.67(10" 8

)

210
REFERENCES

1. Schl ichting, Hermann, Boundary - Layer Theory , 6th ed., New York; McGraw-Hill; 1968; pp 744.

2. Denn, Morton, Process Fluid Mechanics . Englewcod, New Jersey; Prentice-Hall; 1980; pp 383.

3. Stanby, L.T. Practical Statistics For Petroleum Engineers. Tulsa, Okla.; Petroleum Publishing; 1973; pp 139.

4. Kreith, Frank, Principles of Heat Transfer , 2nd ed., Scranton, PA; International Textbook

Co.; 1966; pp 620.

5. Olson, R.M., Engineering Fluid Mecahnics , 2nd ed., Scranton, PA; International Textbook

Co.; 1967; pp 392.

6. Idel'Chik, I.E., Handbook of Hydraulic Resistance , Jerusalem, Israel; Israeli Program for

Scientific Translations; 1960; pp 560.

7. Denn, 0p_. Cit.

8. Olson, Ojd. Cit .

9. Ibid .

10. Edwards, D.K.; Denny, J.E.; Mills, A.F., Transfer Process . 2nd ed. , New York, N.Y.;

McGraw-Hill Book Co.; 1973; pp 421.

11. Sieder, E.N.; Tate, C.E., Heat Transfer and Pressure Drop in Tubes . Ind. Eng. Chem. 28;

1429; 1936.

12. Stanley; 0p_ Cit .

13. Dittus, F.W. ; Boelter, L.M. , Heat Transfer in Automobile Radiators of the Tubular Type , Vol 2. Berkley, CA; University of California Berkley Publ . Eng; 1930. pp 443-461.

14. Petukhov, B.S.," Heat Transfer and Friction in Turbulent Pipe Flow with Variable Properties " in Advances in Heat Transfer , Vol. 6, James P. Hartnett and Thomas F. Irvine, ed.

New York, N.Y.; Academic Press; 1970; pp 503-564.

15. Sieder and Tate; 0p_. Cit .

16. Kreith; 0p_. Cvt.

17. Ibid .

18. Ibid .

19. Kern, D.Q.; Kraus, A.D., Extended Surface Heat T ransfer . New York, N.Y.; McGraw Hill Book

Co. ; 1972. pp 805.

20. Harper, D.R.; Brown, W.B., Mathematical Equations for Heat Conduction in the Fins of Air

Cooled Engines ; N.A.C.A. Rept. 158, 1922.

21 Berger, Arthur," Principl es of Heat Transfer Augmentation 1: Single-Phase Heat Transfer " in Heat Exchangers , S. Kakac; A. E . Bergles; F. May^uger, ed. New York, N.Y.; McGraw-Hill

Book Co.; 1959; pp 580.

22. Timoshenko, S.; Woinowsky-Kreiger , S., Theory of Plates and Shells . New York, N.Y.; McGraw-Hill Book Co.; 1959; pp 580.

23. Roark, Raymond, Formulas For Stress and Strain. 4th ed., New York, N.Y.; McGraww-Hill Book

Co.; 1965; pp 432.

211
24. Harvey, John, Pressure Component Construction . New York, N.Y.; Van Mostrand Relnhold Co.; 1980; pp 479.

25. Sneddon, I.N.; Berry, D.S.," The Classical Theory of Elasticity ," in Elasticity and

Plasticity , Vol. VI. S. Flugge, ed. Berlin, Germany; Springer-Verlag; 1958; V-126.

26. Timoshenko; 0p_. Cit .

27. Ibid .

28. Harvey; 0p_. Cit.

29. Ibid .

30. Ibid .

31 . Roark; 0p_. Cit .

32. Timoshenko; 0p_. Cit .

33. Palmer, J.R.; Bennett, H.E., "Relationship Between Coating Defects and The Limiting Flux

Density a Cooled Laser Mirror Can Withstand," in Proceedings of 13th Annual Symposium

on Optical Materials for High Power Lasers , Bennett, H.E.; Glass, A.J.; Guenther, A.H. Neuman, B.E., ed. 1981 Nov. 17-18.

34. Ibid .

35. Rohsenow, W.M. ," Boil ing" Chapter 13 in Handbook of Heat Transfer , W.M. Rohsenow;

J. P. Hartnett, ed. New York, N.Y.; McGraw-Hill Book Co.; 1973; 13.1-13.75.

36. Long, L.S., Boiling Heat Transfer and Two Phase Flow . Huntington, N.Y.; Robert E. Kreiger

Publishing Co.; 1 975; pp 242.

37. Van Stralen, S.; Cole, R., Boiling Phenomena , Vol. I. New York, N.Y.; McGraw-Hill Book

Co.; 1 979; pp 443.

38. McAdams, W.H., Heat Transmission , 3rd ed . New York, N.Y.; McGraw-Hill Book Co.; 1954;

pp 392.

39. Bennett, H.E., " Insensitivity of the Catastrophic Damage Threshold of Laser Optics to Dust

and Other Surface Defects," in Proceedings of 12th Annual Symposium on Optical

Materials for High Power Lasers , Bennett, H.E.; Glass, A.J.; Guenther, A.H.;

Newman, B.E., ed. 1980 September 30 - October 1; Nat. Bur. Stand. (U.S.) Spec.

Publ. 620; 1981 October; 256-263.

40. Palmer and Bennett; 0p_. Cit .

41. Ibid.

41. Schneider, P.J., "Conduction," Chaper 3 in Handbook of Heat Transfe r, W.M. Rohsenow, ed.,

New York, N.Y.; McGraw-Hill Book Co.; 1973; 3.1 - 3.134.

43. Kreidl, N.J.; Rood, J.L., "Optical Materials," Chapter 5, Vol. I, in Applied Optics and

Optical Engineering , R. Kingslake, ed., New York, N.Y., Academic Press; 1965 ;Pg . 74.

44. Harvey, op. cit .

45. Timoshenko and Woinowsky-Kreiger, op. cit .

46. Barnes, W.P., "Optical Materials-Reflective," Chapter 4, Vol. VII , in Applied Optics and

Optical Engineering , R. Shannon and J. Wyant, ed., New York, N.Y.;Academic Press;

1979; Pg. 102.

212
*******************************

DOC J I fa LASER' fa I PROP PROGRAM

THERMAL D I SRI BUT I OH FOR THE FIR

ST AND SECOND MIRROR CHANNEL

*: * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * EVALUATION OF, MOLY

WITH THE FOLLOWING PROPERTIES

CONDUCTIVITY^ 84 BTU.-HR-FT-AF

COEFF OF EXPANSION, <cc> =

2 . 77777777778E-6 IN-'IN-AF

MODULUS OF ELASTICITY, <E>=

46000688 # I N " 2

INLET PRESSURE OF- 158 #.--IN-"-2 OPTICAL ELEMENT RADIUS OF 6

INCHES

OPTICAL ELEMENT THICKNES OF 2

INCHES ****** * * * * * * * * * * * * * * * * * * * * * * * * * R E Y N 0 L D S NO = V 0 %

-" v Re= 18853 SI 85 192

* * * * * * * * * * * * * * ** * * * * * * * * * * * * * * * FRICTION FACTOR

f =< 1 . 84*LGT<Re>-l . 64 >

-N -2

t"= 2 . 98761 58 1345E-2

* * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * PRESSURE DROP

ip=< <F*L/D >+2K > *M ^2*«* < 1 44*2*9 >

&p= 58 .4737826147 PS I ** * * * * * * * * * * * * ** * * * * * * * * * * * * * * * 0 I TTUS-BOELTER FILM

h= 023*<Re> A . 8*<Pr) - . 333* < fo-'CO h= 1888^ .8377948 BTU-'HR-FT ' 2-AF ********************** * * ** * * * * * PETIJKHOV FILM COEFFICIENT

h=':*.-e>*Re*Pr. 1 . 67+ 12 . 7< * ''8 > * . 5*

t P . 66-1 > > *K -'0 h= 11967.4755972 BTU-' HR-FT " 2-iF *******************************

VELOCITY OF FLU 10=

35 FEET •-•'SECOND * * * * * * ** ********* * * * * * ******** * VISCOSITV-l.=

5.3683547931 1E-4 # -'FT-SEC ******************* * * * ** * * * * * * * DIAMETER OF DUCT=

88267 FEET

*******************************

LENGTH OF DUCT=

.5 FEET

* * * * * * *************** * * ******* * PRANDTL NUMBER=

5 .41398744428 D I MENS I ONLESS

* * * * * * * * * * * ************* * * * * * *

DENSITY OF FLUID=

62 35 #/FT-'-3 *******************************

CONDUCTIVITY OF FLUID

398834918999 BTU-'HR-FT-iF ******* * * * ** **************** * * * TEMPERATURE AT INSIDE SURFACE

OVER THE CHANNEL

SURFACE TEMP= 184.981215827

DEG F ***** * * * * * * * * * * * * * * * * * * * * * * * * * * TEMPERATURE AT END OF THE WEBB

SURFACE TEMP= 94 4548664832

DEG F

* * ******** ******* * * ****** * * * * * * TEMP. OF THE RECEIVING SURFACE 0

VER THE CHANNEL

SURFACE TEMP= 189 839486632

DEG F

* * * * * * * * * * * * * * * * * ****** * * * * * * * * TEMP OF THE RECEIVING SURFACE 0

VER THE WEBB

SURFACE TEMP= 112.518977152

DEG F ************************** * * * * * THE A V G WATER TEMPERATURE IS

87 4586875135 DEG F *******************************

TEMPERATURE INCREASE OF THE FLOW

IMG WATER i NECESSARY TO UNLOAD

1889 BTU'HR- CHANNEL

AT= 11.3146338735 DEG F *******************************

THERMAL BEND 1 G FOR THE CHANNEL

AS= 1 .58748721696E-2 <>> METERS *******************************

THERMAL BENDING FOR THE WEBB

AS= 5 . 89686 14788 1E-2 METERS *******************************

PRESSURE BEND ' G FOR THE CHANNEL

AS= 8 . 79143779499E-3 <!v>METERS

THE AVERAGE PRESSURE=

184.216851388 #^INA2.

213
***** ********

RMS RIPPLE OF THE CHANNEL

RMS dS= 2.64553129512E-2

CiO METERS

* * * * * * * * * * * * * ** * * * * * * * * * * * * * * * * THE BENDING MOMENT PER UNIT OF C

LAMPED EDGE < M >=cc*£T*<l + a '? * D

' t M= 1487.94583935 #-IHIH

RPPflRENT LORD <<i>=8#D*l + v>/'R A 2*F

o

1 = 336.654630968 #- IH"-2 *************************** * * * * THE CHANGE IN RADIUS OF CURVATUR

E Ro = t.' <:o:*AT)

A= 29442.9559537 IN ******************* * * * * * * * * * * * * THE CHANGE IN CURVATURE FOR A FP

EE EDGE i=q*R'-4.--<' 16*D*U + p> >+<i*R

A 4/ < 64*0)

&= 15.5285388554 C*0 METERS

SUM<2&>- 15 5285398554

< m :• METERS FOR A FREE EDGE GEOMET

RY*********** * * * ****** * * * * * * * * * * * THE CHANGE IN CURVATURE FOR A TH

REE POINT SUPPORT-A= . 0362*11*<1*R'-

4'D

£= 36 7321 704267 < v > METERS

* * * * * * * * * * * * * * * * * * * * * * * * * * * * * * *

* * * * * * * * * * * * * * * * * * * * * * * * * * * * * * *

D DC JI M ROHSEH0 W-BERGLES

PROGRAM FOR BURNOUT AND BOILING

BURNOUT- I NC I P I ENT-POOL BOILING **************************** * *

FORCED CONVECTION

<Q/-fi>=Hc*< <Tw-Ts> + <Ts-Tbul k> ? Tw-Ts=10 DEG iFTwall* 351 DEG F Q/A>= 2640855.6 BTUVHR—FTA 2

Tw-Ts=20 DEG AFTiA«all= 361 DEG F

Q/A> = 2748895.6 BTLL'HR-FT'^2

Tw-Ts=38 DEG eFTwall* 371 DEG F

Q/A>= 2848135.6 BTU^HR—FT"2

j w -Ts=40 DEG AFTwall* 381 DEG F

Q/fl>= 2948175.6 BTUVHR-FTA 2

******** * * * ***** * * * * * * * * * * * * * * * INCIPIENT NUCLEATE BOILING

(Q.^fl> = 15 . 6*<P>"-i . 156*<Tw-Ts>^2 .3

3/P" .8234

Tw-Ts=l£i DEG iFTwall* 351 DEG F Q/A>= 444539.744656 BTU.'HR-FT'V 2

Ti,i-Ts=20 DEG iFTwalJ* 361 DEG F Q •' A '< = 1849732 16233 BTU-- HR-FT-^2

Tw-Te=38 DEG iFTwall* 371 DEG F Q/A')= 4258807 72915 BTU "HR-FTA 2

Tw-Ts=48 DEG iFTwall= 381 DEG F

Qvfl>= 7695879.43378 BTU'HR-FTA 2 *******************************

POOL BOILING

0 •' A > = < <Tw-Ts>*EXP<P '900;> ;"-4*lE6

Tw-Ts=10 DEG £.FTwall= 351 DEG F

Q/A>= 1387 70403911 BTI_L'HR-FT ^2

Tw-Ts=28 DEG iFTwall* 361 DEG F

Q/A>= 20923.2646256 BTU'-'HR-FT'^2

Tw-Ts=30 DEG AFTwall* 371 DEG F Q/A>= 105924.827167 6TU-HR-FT^2

Tw-Te=48 DEG iFTwall* 381 DEG F

Q/A>= 334772.234811 BTU''HR-FTA 2

* * * * * * *********** * * * * * * * * * * * * * * BURNOUT AS A FUNCTION OF VELOCIT

<Q.-A)=4E5*VM 1.^3) + 4 8E3* < Ts-Tbu!

k>*V'M 1/3)

VELOCITY* 35 FT -'SEC

''Q/A>= 5294948 .45761 BTU -'HR-FT "•2 *******************************

PROPERTIES OF EVALUATION

SATURATION TEMP= 341 DEG F ABSOLUTE SATURATION PRESSURE*

113.7 PSIA

FILM COEFFICIENT* 18904

BTU.-'HR-FT-^-AF

AVERAGE BULK TEMP= 87 . 1 DEG F VELOCITY OF COOLANT* 35 FT.•'SEC COOLANT MATERIAL*WATER *******************************

214
1 * 1 * * % * % * * * * * * * * * * * * * * * * * * * * * * * DOC J IN MAXIMUM DEFECT ANALYSIS

****** * * * * * * * * * * * * * * * * * * * * * * * * * THE MAXIMUM DEFECT SIZE FOR fl T EMP RISE OF- 279.5 -WILL BE-

.243929127634 -FT. IN D I A

.

D= 74357.3Q13364 (. o > METERS

* * * * * * * * * * * * * * ** * * * * * * ** ****** *

2 i er f c < Z-'"2* < or*T >

"'

. 5 ':•

. 2ierfc= 1 1284

* * * * * * * * * * * * * * ** * * * * * * * * * * * * * * *

2 i e r + c ( < z " 2 + r * 2 )

•'-

. 5 < 2 * < o: * f > . 5

l|

2iertc= . 2683993 1 1 1 £5

RECIPROCAL FUNCTION OF THE Z'iert

RECIPROCAL* .668027426545

* * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * THE ANALYSIS HAS BEEN BASED ONMOLY

-WITH THE FOLLOWING PROPERTIES

o:= 2 F

T -"' 2 'HP

t'= 4 . 1666S66S667E-3 -HOURS., i.e..

15 -SECONDS

K = 84 ETU/-HR-FT-F

T= 1 6666 6666t-£7E — 3

FEE T OF THICKNESS- .02 -INCHES

AT= 279.5

ALLOWABLE INCREASE IN TEMPERATUR

E-DEG F

Q = 296300

MAX ABSORBED L 0 A0 - B T U

'

' H R - F T

•'

• 2 ********************** *3;S:******

************ *******************

* DOC JIM COOL LASER MIR GRAD .

* FOR-MOLY-HOT FACE MAT * * * * *. * *. * * * * * * * * * * * * * * * * * * * * * * * * * * SURFACE TEMP INCREASE ************** * * * * * ******** * * * * AT= 278.96907438 DEG F &T= 154.9828191 DEG C SURFACE TEMPERATURE 111 5 + 273 96907433 = 390 46907438

DEG F

***************** **************^ *******************************

RADIUS OF DEFECT

R= 122 FEET

R= 37189 4528273 C*»> METERS *******************************

TEMPERATURE INCREASE *******************************

£T= 278.96967433 DEG F AT= 154.9828191 DEG C

BACK SURFACE TEMPERATURE 184 + 273.96907438 = 332.96907438

DEG F *******************************

FOR IRRADIANCE TIME

r= 4 16666666667E-3 HOURS

* * * * ****************** * * * * * * * * * * THE THERMAL LOAD IS

Fo= 296380 BTU- HR-FT 2 * * * * * ********* * * * * * * * * ******** ** 2ier t c-X •' < 2* < oc*-r > A . 5 > 2ier+c= 1.12834 FRONT SURFACE ********************************

2ierf c-<T^2+RA 2> cc*Y> A . 5

2ier+"c= .261944953747

BACK SURFACE

** * * * ************ * * * * ***** * * * * * * MATERIAL PROPERTIES OFMOLY

THERMAL CONDUCT I W I TY= 84

BTU-'HR-FT-iF

THERMAL D IFFUS I V I TY= 2 FT " 2 - HR

TOTAL THICKNESS OF THE ELEMENT*

1 . 66666666667E—3 FT

T= .82 INCHES

* * * * * * * ********** * * * * * * * * * * * * * *

215
Figure 1.0

PARALLEL FLOW CONFIGURATION

BRAZE

INTERFACE

SURFACES

*D IS HEAT EXCHANGER DIAMETER

Figure 2.0

216
PARALLEL PLOW CONFIGURATION

STAGGERED CHANNEL GEOMETRY

I HOT WALL

THICKNESS

SUBSTRATE MIN

1

*0 IS HEAT EXCHANGER DIAMETER

Figure 3.0

THREE-POINT

COVER ATTACHMENT

THREE-POINT MOUNT / MIRROR SURFACi

STAINLESS STEEL

MOUNT RING

HEAT EXCHANGER

PLATE ALIGNMENT

PIN (2 TYPICAL)

COOLANT PORT

AND TRANSFER

TUBE

STRUCTURAL

SUBSTRATE

THERMAL

HEAT EXCHANGER

(2 PLATES)

Figure 4.0

"LONG"

CYLINDER

HEAT SHRINK AND

PINNED ATTACHMENT

217
VERTICAL FLOW CONCEPT

Firju!-e 5.0

l id i ) « ) t aoo»»j»s«J i io

Figuro i.O

218
The speaker was asked whether he thought it feasible to generate a family of curves that would

allow a typical user of cooled mirrors to determine whether he would be operating at a safe power

level: the speaker believed such curves were feasible . He said, however, that it was not possibl

to specify an optimum pressure drop across an arbitrary set of channels , or to predict the effect

of nonuniform, non reproducible beam shapes.

219
ADAPTIVE FORCE OPTICAL ELEMENTS BASED ON OPEN PORE STRUCTURES

V.V.Apollonov, S. A.Chyetkin, V.Yu.Khomich, A.M.Prokhorov

P.N.Lebedev Physical Institute of the USSR Academy of

Sciences, Leninsky prospect, 53, Moscow, USSR

Formation and transmission of high evarage power laser radiation flows with

a divergence close to diffraction limit is one of the actual problem in high intensity laser optics.

This paper presents the results of investigations of the phase correctors

for laser system optical tracts basing on cooled force optics elements /I/. A reflecting surface shape is controlled by deformation of the porous heat exchanger.

The phase correctors* application, made on the base of force optics elements

is not restricted by the adaptive optics. For example, the use of adaptive reflectors in the optical location laser systems will allow to extend the possibilities

of latters due to beam formation with the required optical properties of the wave

front (WP). A possibility to control the shape of reflecting surface of the cooled force optics elements, executed on the base of open porosity structures is considered within the following model assumptions: porous heatexchanger with the radius R and thickness H( H «R) is tightly fixed over the base and lateral surface

into an absolutely rigid material (as compared to porous structure (PS))body over

the whole surface except those parts, which are connected with the actuator system ( as the absolutely rigid material cylinders with radius Ct«R), capable to

produce given transfers or amplifications. The layer deviding radiation and heatcarrier prepared on free PS heatexchanger surface is a base for reflecting coating.

An assumption of the absolute rigidity of the force optics elements* and ac- tuators* body allows not to consider the deformation of latters while forming a given shape of the reflecting surface W f

( Y , V ). W . (t,^) is determined by

the following way: for the incident beafiTwith WP q>

{ (%, \p f

e and initial reflecting surface shape ^("Z,^) , conjugated Wrefs is determined by connection:

where N - is the laser radiation wavelength, N - is the integer.

To the main types of Cp^ we refered the wave front tilt, defocusing, spherical abberation /2-3/, and as a function of the given reflecting beam WP form - surface, i.e. (P2 (l,^)=0 . For given 0t and

^

it iQ impossible an accurate formation of the reflecting force optics element surface shape W - by the control system with a finite

number of actuators. Therefore, one should find the optimal way to design the moving structure of actuators, namely, their number and mutual location in order to

provide a required accuracy of conjugation of the reflecting adaptive force optics

element surface shape with the main types of WF laser radiation abberations.

Before solving the above problem let us consider the deformation transmission

characteristics of the separate actuators of the adaptive force optics element.

Within the approximate linear theory of elastic porous heatexchanger deformation of the cooled force optics elements the problem of definition of the reflecting surface shape W

f (resulted from the action of actuator, placing on the unreflected side of PS at distance o out of mirror axis) reduces to the equation

A/:

W| ZrO =Uoe(a 2 -(^x)2 -^) ; |^| 2 = H

^0; W| t = R = 0

where V - is the Poisson coefficient, 0 - is the Kheviside function, U0 -

is the value of actuator shifting, whose solution for W - - W (z H) is the

following: rer '

220
CO oo

where - is the Beasel function

of the order of K, >(/

K) - is the j-th equation root: \ (y\

K)

) - 0 ; <£; K = n

(k)

I ijjd- W/q

and the effort to be applied to actuator is defined as; J 'J Aj 2.11-vO

where E - is the Junge module of PS heatexchanger.

Prom the analysis of above dependences it is seen, that a transmission deformation function (i.e. response function of reflecting surface to actuator action)

is of smooth Gaussian, which width is measurable with the actuator radius

(Pig. 1 A). Dependence of the maximum response function on parameters HR and AR

is represented in Pigs. 1 B and 1 C, and dependence of the response function width

A » defined over level 1/2 - in Pigs. 1 D and 1 E. Dependence of dimensionless

specific pressure _ I i-2v?

' P( 1+0) R An_ Q MP-H. where G - is the module pf PS

\W^) nu 0 , a

2-'TM

R rtK-R

shifting out of HR and AR parameters is represented in Pigs. 1 P and 1 G.

Prom the above dependences it follows, that in case of wide response function with A > 1 » its maximum value V</o / Uo ^ 1 whereas in case of narrow

response function (A = I) W0 /(J 0 reaches its maximum value equal to I, and value &

tends to minimum. According to relationships, represented in Pigs. 1 B and 1 C

the maximum of Wo/Wo reaches at AR, such that AR :> I5HR.

In KOAT systems, operating in real time, the reflecting mirror surface should

change according to a change of phase distortions along laser beam propagation

trasse, whose times of "cooled" existence is only O.I - I waec. Therefore a frequancy range width of actuator system effects should be I - 10 KHz and in deformated PS no resonance effects should be exhibited.

We define a spectrum of the relative PS vibrations using the approximate

dynamical elasticity equation:

1 2!W _ A w . 3V

C

2 31 2 ~ fli w +

3 Z

z

on the base of which the frequency spectrum of the adaptive force optics element

is defined as:

("0 2 *

2 ,, „ . v2/rw..i2/„ /. w,. - V%

c

where C - is the sound velocity in PS material.

The "nodes' ' position over unreflecting PS side is defined by the following

expressions

:

Dm (>rw >R) = 0 • m^ie) =*(i + 2e)/2

The crests' position over unreflecting PS side is defined as:

^m(^j t, mjt/R)=o ; m^ct.

221
Thus, if the actuators' frequency spectrum do not reaches 5TC/H , effects, affected upon the stability of operation of the adaptive f

ment control system, should be observed. If the operation frequenci

then one of the method to avoid the resonance effects is to choise

sition in the bundles of porous layer vibrations.

The analysis of the PS stressed state shows, that the reflect

shape is the Fourier series over a full (on circular R radius apert

functions of { Ok. (^j

CK) t/R ) C05 K Y } . That is any phase distortion re function F(ti^) such, that one exists an integral ^ F

^ , ^ ^

can be corrected with any accuracy £ , having used accordingly a tem of actuators; £ is defined here by a number of used actuators

al position.

In the process of correction one manufactures the reflection

no resonance

orce optics elees are over ft C/H

the actuator poing surface

ure) system of

presented by

projected sysand their mutusurface shape

ref

.

A/

n = i where are the center coordinates of n-th actuator over the unreflecting porous layer side, (t, 1?) - are the coordinates of the reflecting surface

point, \\ n - is the response function of reflecting surface to n-th actuator

action. A problem of conjugation of the reflecting force optics element surface

shape and laser radiation flow with the disturbed wave front F

(

t , ^ ) reduces

to definition of

; [ Gn ^n] at wnicl1 an eri*or of conjugation £ is

determined as:

A/ 8 =

S

s where N - is the actuator number, - is the aperture function, giving

a range to the reflecting mirror surface for which the adaptive equation is carried out should be minimum.

According to a method of the most little squares, mimimum of £ achieves

over a large number of values { U n 1 ^

tne condition 1> £ / y — q

is satisfied for any i^n^/Y

Vector

{ Un] » minimizing the functional £ is solution of the linear equations* system

where the matrix elements A K a of dimensions are defined as

A Kn ~

s i.e. are the characteristic of number and mutual positions^of the force optics

element actuators' system. The elements BQ

of the vector B of dimension N:

B„ = 4SJW1 (t.v)F(t.«P)^ n ds

are defined not only by a force optics elements construction, but the function

type, describing WP disturbance of laser beam.

A compensation error £ is defined as?

A/

n= i where | F I ^ = 4" ^ W, ( X ,t ) F

*

( X ,V ) d S

^ s

222
We determined a type of elements A|<n » &n » I F I for the Dasic abberation

In the case, if the aperture function Wj. ( t , ^ ) — 9(Ri - t)

where - is the radius of "active" subaperture and R-j <C R , the elements of

the transmission matrix of mirror are the following:

A * _ y sr AKj(G = 8m)AKi(g=en)0OSK(^n-^m)

A nm — L 4- (k) (k) i qn , / jy

./!

K> a. nibu

w:

)r) --"i (<'3k jK

- t (^

KV)

+ iv v f

4JK (g-u

eh

2

SDjR (1 + Sk,o)

where

If # —> j , i.e. control is carried out along the whole aperture of adaptive

force optics element: - V f A ft 9 A Jk(^ 1W

Vr)(Sk.O + COS KCPn-ym))

With correction of WP tilt, described by expression

F(t,v) = tlgdL sinV 0(R,-i)

where ck - is the tilt angle of WP surface, the vector element Bn

is the following:

Bn =

- I ,RRi V * IC Q \ hSA. ^ Sin %

and expression for | F I

IFI

2 =|19

2o(KV

With correction of cylindrical bend of flat WP , described by function at the same subaperture R.^ < R

FM) = ± (x

2

sin

2

1» -R 1

2 )G(Ri-t)/2R 2

where Rj - is the cylindrical surface radius, the components BQ are defined

as follows:

Bn - X 32R2 a X°

)3 ^

(®jo)

Aio(M„) f UiM"*) .ovMl) 2* 10)

tfcosMn V Aj2 (g = gn)

f

223
and 6

5R,

If

7

! ~ 32 R

2

Rl

At correction of defocusing F(t,v)± (R

2 -t*)/2R 2 where R2 - is the spherical

surface radius, the components BQ

are equal:

r ima y :UX°VrU(^C0'°/r)

where Z m«» = R V( 2 R a ) and | Fl

2 = R Vtt 2 ft£ ) . Spherical abberation is determined as p(t | vp) - ± 0.25 B( tVR ~ i ) so component Bjj is equal

and 2 max = B> / 4 and I F I = ^ 2 mo<* / ^ • In case of email-scale distortion for approximation of which we use the gaussian

function: = U i exb(-K 0 t

a

) where Ko = 2. / O j 2 , Q L -is the cross- section size, determined over level 1/ Q

z

, such that Q £ ^ Q 7 |F| 2 - U L Q?/4R

and the corrected vector Bn

is defined as:

2°°A/tf/?\ ( (o)22

Q _ UiQ t r A jo C 5 ^ 6n ) / Ai Oi \ -

1G a R jrj^MJT ^""Tr1

"'

Thus, we have design a model of the adaptive force optics elements executed

on the base of structure with open porosity. A problem of conjugation of the reflecting surface shape with the basic abberation types is solved. The main expressions, describing an algorithm of optimal phase-conjugation are obtained.

References.

1. V.V. Apollonov, P.I.Bystrov, V. P. Goncharov , A.M.Prokhorov , V.Yu.Khomich,

Kvantovaya electronika, 6, No 12, 1979, p. 2533.

2. Avisonis P.V. Overview of adaptive optics technology .-Physics of Quantum

Electronics, 1978, v. 6, p. 1-54.

3. Hogge C.B. Adaptive optics in high energy laser system. - Physics of Quantum

Electronics, 1978, v. 6, p. 55-120.

4. V.V. Apollonov, A.M.Prokhorov, V.Yu.Khomich, S. A.Chyetkin. Pis'ma v ZhTP, 7,

22, 1982, p. 1388.

224
25

20

15

10

Pig. I A. Response function

of APOE reflecting surface

made on the base of porous

structure due to parameters AR and HR.

5

W =

I w_

2 AR U 0

l AR = o.oi

i HR=o.o2

HR=0.0H

\ \

\

\

\

\ I

HR = 0.06

HR = 0.0&

S n \ x \ \

"~* - ^ Is \

HR = 0.i

" ^ ^ w

~~ ^ — c

\

S\

AR 2flR 34R

225
0.5

Wo

U 0 HR

-= ~ 0.02

S

' / ' /

7 /

i ' 1 it/' U 1 I ,/ / //

'////V/V/// ////

</f

0.2

AR

0.05

Pig. I B.

0.1

asan

Wo

Uo

N

\ \ \ \

\ A \ ^

\ \

AR

0.05

\ X 0.04

\

^

0.03

0.02 - 0.01 HR

0.02 0.1 Pig. I C.

Dependence of maximum response function of APOE reflecting

surface on parameters AR and HR.

HR

0.1b

A

\

o.i\\\\\ N

HR - 0.2

0.02- 0.02

0.01

Pig. I D.

0.05

AR —>- HR

O.OZ

Pig. I E.

OA

Dependence of APOE response function width 0n parameters AR and HR

226
26-

20

10-

2-

S

26-

20-

10

O.Oi 0.05

AR —

AR = o.oi

AR = 0.02

Pig. I P.

0.02 0.04

Pig. I G.

0.0

HR —>•

Dependence of specific pressure value ^>-^\-i^ t>yu 0 £a 2 in the

place of actuator and porous heat exchanger connection on parameters

AR and HR.

227
Optical Figure Requirements for Laser Mirrors Used at Oblique Incidence

H. E. Bennett

Michelson Laboratory, Physics Division

Naval Weapons Center, China Lake, California 93555

In order to achieve near diffraction-limited power densities in a focused laser

beam, the wave front distortion introduced by the relay optics must be closely con- trolled. The figure tolerances for the individual components required to achieve this

control are usually derived from normal incidence arguments. However, turning flats

are usually used at nonnormal incidence, typically 45 degrees. It is shown that the

rms figure irregularities are less important at nonnormal incidence than at normal

incidence, but that sphericity, which introduces astigmatism, is much more important at nonnormal incidence. If a focal shift can be tolerated, however, the wave front error

at nonnormal incidence caused by a turning flat containing sphericity will be less than

that at normal incidence from a flat having an equal peak-to-valley random figure error.

Keywords: laser mirrors; optical error budget; optical figure; optical flats; spherical power; turning mirrors.

The ability of a laser system to focus energy is greatly influenced by the optical train through

which the light is focused. The Marechal criterion is often used for superior imaging optics [l]. 1 To meet this criterion, the cumulative rms wave front distortion for wavelength A must not exceed

A/14, which is approximately equivalent to the well-known Rayleigh quarter-wave criterion. The

optical resolution achievable using an optical train meeting the Mare'chal or Rayleigh criterion

is equal to that of a perfect optical system. The on-axis focused power density, however, is only 80%

of that for a perfect system [1], leading to the conclusion that it is more difficult to achieve the

highest quality power handling optics than it is to achieve diffraction-limited imaging optics.

The ratio of the axial intensity I (for an aberrated signal) to I (for an aberration-free

system) is given by the Strehl relation [2]:

0

where A is the wavelength and a is the rms deviation of the wave front from its ideal value. Equation (1) has been shown [3] to be a valid indicator of optical image quality in the range 0.6 < 1/

I

Q<

1.0. In designing an optical train, figure tolerances must be assigned to the various components

[4] so that the resultant wave front error, after passing through the system, does not exceed the

desired value of a.

Usually lenses and mirrors containing power are used at nearly normal incidence. However,

plane mirrors are frequently used at nonnormal incidence. Only a slight sphericity is then required

to produce significant astigmatism. The sphericity tolerance is the subject of this paper.

Wave Front Analysis

When mirrors are used at normal incidence, the optical path difference caused by a surface

irregularity of height h-j exceeds it by a factor of 2. The rms wave front error a] is thus twice

the rms figure error 6-j of the mirror surface. If the mirror is used at nonnormal incidence, the

contribution of an irregularity of height h. to the wave front error will be 2h.cose, where e is the angle of incidence. It follows that

Introduction

2ttq v 2

A

1 (1)

* Work supported in part by Navy Independent Research Funding,

figures in brackets indicate the literature references at the end of the paper.

228
a. = 26 .cose (2)

and surface irregularities relative to the best-fit sphere are less important at oblique incidence

than they are at normal incidence. At nonnormal incidence, however, a spherical surface having a sagittal depth h gives an astigmatic contribution to a. If a quasi-plane mirror containing a slight

amount of spherical power is used at 45 degrees, for example, the path difference between the beams

reflected from the edge of the mirror and from its center is 2hcos45° = /2h , as shown in figure 1.

A circular beam incident on a nearly plane mirror will produce an elliptical footprint with

semimajor axis ba and semiminor axis b, where 2b is the diameter of the incident beam. This footprint is shown on a circular mirror in figure 2(a). The semimajor axis of the ellipse is b , where

b cose = b . (3)

The sagittal depth of a plane wave front after reflection from a spherical mirror of radius of

curvature R at angle e is (b|/2R)2cose along the p-direction; along the s-direction, (b 2 /2R)2cose.

Here p and s are directions parallel and perpendicular, respectively, to the plane of incidence.

After reflection, the beam is still circular in cross section but the wave front is toroidal, as shown in figure 2(b), and astigmatism caused by the difference in sagittal depth of the wave front

in the p- and s-directions will be present at focus, as illustrated in figure 2(b). The maximum

difference is

1 k2,

o (ba

- b )cose

Substituting from eq (3), it becomes

2 2

b sin e R cose

The best-fit sphere will bisect the astigmatic wave front, as illustrated in figure 2(c). The

maximum deviation of the wave front from a best-fit sphere will then be half the deviation of the wave fronts from each other.

At 45 degrees to the s- or p-directions, the aberrated wave front coincides with the best-fit

spherical wave. The difference between the aberrated and best-fit spherical wave fronts as a function of azimuthal angle $ and distance from the optic axis r is then

2 • 2

where 0 < r < b and <j> = 0 at 45 degrees to the s-direction. The rms deviation of the astigmatic

wP

2

~drd(\>

(4)

wave front from a best-fit spherical wave front is then a - where

a

2 b / 2 2

2 o.o

a

irb^

so that

a

a

b 2

4/3R

tanesine . (5)

Spherical Wave Front Error

In addition to the astigmatic wave front error, there is also a spherical wave front error (the

best-sphere). Since the two errors are correlated, the resultant wave front error is their sum. In many applications, however, the spherical wave front error can be neglected since it only displaces

the focal point of the system, as illustrated in figure 3. In the original focal plane, the on-axis

intensity is decreased from I to I , where [5]

229
and &i is the maximum difference in sagittal depth, as illustrated in the figure. A focal shift by

an amount Af, where f is the original focal length and

Af = 2 ai (f/b)

2

, (7)

restores the on-axis focal intensity to its initial value.

Comparison of Normal and Oblique Incidence Wave Front Errors

The decrease in on-axis intensity caused by astigmatism introduced by using a slightly spherical mirror at nonnormal incidence, cannot be corrected by a focal plane shift. If we neglect the

spherical wave front contribution, a comparison of the effect of random and spherical figure error

in mirrors can be summarized as follows. From eq (1) the decrease in on-axis intensity caused by

figure error [2] is 1 - (2tto/a) 2

. At normal incidence, a = 26 where 6 is the random rms deviation

of the mirror from a best-fit sphere. At a nonzero angle of incidence e, astigmatism is also present;

thus, from eqs (2) and (5),

a

2

= (26cose)

2 + (b2 /4/3R) 2 tan2 0sin 2

6 , (8)

where the sum of the squares is taken because the contributions of random figure errors and astigmatism are assumed to be uncorrected.

If the flat is filled by the beam at angle e, the sagittal depth of the sphericity h is

Substituting eq (9) into eq (8), the rms wave front error a is given by

a = (46

2 + sin 4

e) 1/2cose . (10)

Usually, optical figure specifications are made in terms of the peak-to-valley deviations from

a reference plane. Since random figure errors on a well-polished part are vaguely sinusoidal, they

may be related approximately to the peak-to-valley random figure error p

v by

P

v

= 2/26 . (11)

An optician can readily perform the measurement of peak-to-valley figure error by placing a refer- ence flat on the part being made and observing the interference fringes formed. Thus a quarter-wave

surface, assuming no sphericity were present, would be one for which p v

= A/4, where A, the wavelength of light, is typically either 6328 A (if a HeNe laser is used to make the measurement) or

5461 A (if a mercury light is used). If sphericity were present but no random irregularities, a quarter-wave surface would be one for which h = A/4. Figure 4 gives a plot of peak-to-valley figure error and sagittal depth versus wave front error.

Conclusion

It is clear from eq (10) that the tolerance on sagittal depth can be significantly looser than

that on random figure irregularities if a slight focal shift can be tolerated. If the figure error

of the mirror is specified in terms of the magnitude of peak random irregularities allowed at normal

incidence (i.e., p y

) and only spherical power is present, its sagittal depth can be 3.4 times the pv value at an angle of incidence of 45 degrees and 3.3 times it at 60 degrees. At grazing incidence,

the wave front error caused by either random irregularities or spherical power approaches zero. We

230
may conclude that an optical flat which meets a given peak-to-valley figure specification for random

surface figure errors will always introduce less wave front error when used at nonnormal incidence

than it will at normal incidence. In addition, if a small focal shift can be tolerated, the wave front error introduced by sphericity which meets the peak-to-valley figure specification will be

less than that introduced by random figure errors.

With modern computerized interferometric equipment, it is posssible to separate random errors about a best-fit sphere from the sagittal depth of the sphere and determine both 6 and h. With such

equipment, a good idea of the actual performance of a turning flat may be obtained from eq (10).

References

[1] Born, M. ; Wolf, E. Principles of Optics , 3rd ed. New York; Pergamon Press; 1965. Pp. 468-469.

[2] Strehl , K. "Atmospheric irregularities and zonal aberration of objectives," Z. Instrum. 22^ 213-217; 1902.

[3] King, W. B. "Dependence of the Strehl ratio on the magnitude of the variance of the wave aberration," J. Opt. Soc. Am. 58; 655-661; 1968.

[4] Bennett, H. E. "Thermal distortion thresholds for optical trains handling high pulse powers,"

in Laser Induced Damage in Optical Materials: 1976 , Glass, A. J.; Guenther, A. H., ed. 1976

July 13-15; Boulder, CO. Nat. Bur. Stand. (U.S.) Spec. Publ . 462; 1976 December. Pp. 11-24.

[5] Lowenthal , D. D. "Mar£chal intensity criteria modified for Gaussian beams," Appl . Opt. 1_3; 2126-2133, Errata, 2774; 1974.

PLANE WAVEFRONT

\

Figure 1. Astigmatism produced by oblique reflection

from a spherical mirror.

231
BEAM FOOTPRINT TOROIDAL WAVEFRONT

BEST FIT SPHERE

(a) (b)

Figure 2. Origin of toroidal wave front caused by oblique

reflection from a nearly flat spherical mirror. The ellipse

in (a) shows the area of the circular mirror illuminated at

a 45-degree angle of incidence by a round beam. After

reflection, the wave front in the s-direction has a different curvature from that in the p-direction as seen in (b).

The best-fit sphere averages between them.

I/I 0 = 1 - 7^2/6

FOCAL SHIFT

Af = 2a., (f /a) 2

Figure 3. Drop in on-axis intensity I caused by spherical wave front error restored to I by shifting focal plane to right.

232
ANGLE OF INCIDENCE IDEG)

Figure 4. Root mean square wave front

caused by random peak-to-valley and

sagittal quarter-wave figure errors.

The speaker was asked about the validity of having treated lack of sphericity and random height

fluctuations as independent quantities , and replied that while he did not believe there was necessarily any correlation between the two types of error, the basic effect of dropping the

assumption would be the requirement of slightly tighter surface specifications.

233
Molecular Beam Epitaxy

Mehmet Rona and Philip W. Sullivan

Arthur D. Little, Inc. Cambridge, MA 02140

MBE is a technique for the deposition of single crystal films in ultra high vacuum.

The technique has been developed within the last five to ten years predominantly by the

semiconductor industry.

In vacua of approximately 10 "^Torr, materials are thermally evaporated from boron

nitride crucibles onto a substrate, the temperature of which is precisely regulated. Single

crystal films up to 5 cm OD have been grown Q that demonstrate surface roughness no worse than

one atomic layer locally (approximately 3-5A) and global film thickness variations less than

one percent. Multiple films of different chemical compositions but similar lattice structures

have been deposited on one another with precisely defined chemical boundaries and no interlayer diffusion.

Multilayer dielectric coatings for high energy laser systems have consistently failed to provide performances expected on the basis of bulk materials properties. Instead, the observed

effects support the following simplified model of the behavior of dielectric materials in the

presence of intense electric fields.

The substitution of polycrystalline films by single crystal films should offer improvement on two counts: (a) reduced absorption of energy by electrons in low energy surface

states, and (b) longer mean free paths of electrons in the conduction band, with reduced

possibility of electron avalanche breakdown and massive energy transfer to the lattice.

Keywords: Laser coating; molecular beam epitary; optical thin film; single crystal films.

1. Introduction

The rapid development of semiconductor technology over the last thirty years has imposed increasingly stringent design requirements on the materials technologist. The late 1970's and early 1980's

have seen a huge increase in interest in devices which require highly precise control of physical

dimensions and semiconductor composition. Although silicon still dominates the semiconductor device

market there is much interest in very high speed and optical devices fabricated mainly from compound

semiconductors whose constituents can be found in groups III and V of the periodic table, e.g. ; GaAs,

InP and alloys GaAlAs, GalnP, GalnAsP, etc. Such materials have the same crystal structure as Si but

have electronic properties which make them more attractive for use at high frequencies and in active

and passive optical components. Such components are, by nature, smaller and characteristically more

dependent on surface and interface roughness. Moreover dopant homogeneity and incorporation level

must, of necessity, be accurately controlled. For these reasons, many modern devices are fabricated

in thin, single crystal (or epitaxial) layers which are grown onto a bulk crystal slice. Several

techniques are used to grow epitaxial layers; these will be discussed in the next section. All of

the techniques, however, are characterized by the following:

A) Crystal growth rates are slow compared to bulk crystal growth rates and thus

allow precise control of layer thicknesses and therefore device size, at least in the dimension parallel to the growth direction.

234
B) Dopant incorporation and alloy composition is well controlled. N- and Ptype levels may be set with some precision and resolution.

2. Epitaxial Growth Techniques: VPE, LPE and MBE

The three major growth techniques used for growing epitaxial layers are vapor phase epitaxy

(VPE), liquid phase epitaxy (LPE) and molecular beam epitaxy (MBE). A comparison is given in Table

1 of the three techniques for the growth of GaAs.

Table 1. A Comparison Of Growth And Material Parameters For GaAs Grown By LPE, VPE and MBE

LPE VPE MBE

CVD MO-CVD

Growth Rate (ym/min) <\,1 M).l -vO.l ^o.oi 4

Growth Temp. (°C) 850 750 750 550

o Thickness Control (A) 500 250 ioo(so)

1

5

0 Interface Width (A) >50 ^65 <20(16)

3 <15(9)

2

Dopant Range (cc ) io13-io

19

10 13410

19

io

14

-io

19

io

14-io

19

Mobility, 77°K (cm2 /Vs) 150,000 - 200,000

150,000 - 200,000

120,000 105,000

(1) Reported but unpublished to date (R.D. Dupuis , Appl. Phys

,

Lett. , 35 (1979), 331)

(2) Minimum sputter auger measured width with broadening correction (CM. Garner, et al., Appl.

Phys. Lett., ^34 (1979, 610)

(3) Minimum sputter auger measured width with broadening correction (R.D. Dupuis et al., in Gallium

Arsenide and Related Compounds , Inst, of Physics, London (1970), p.l)

(4) This growth rate has recently been increased by little more than one factor of ten.

SOURCE: Reference 1 (4th footnote has been added).

In VPE, a heated stream of compounds or elements is passed over the substrate slice. Chemical

interactions of the gases with the substrate cause the layer to form and grow. The apparatus normally

consists of a quartz furnace in which the substrate is held and through which the reactants pass, at

or near atmospheric pressure.

LPE involves cooling a solution which is saturated with the components required to grow the crystalline layer. The solution is maintained in intimate contact with the substrate.

[1 21 Molecular Beam Epitaxy ' can be defined as the growth, under ultra high vacuum conditions,

of single crystal layers of electronic materials, from thermally generated beams of the constituent

elements. For example, to grow gallium arsenide (GaAs) separate crucibles containing the elements

gallium and arsenic are heated in an evacuated chamber. The resulting vapor products Ga and As

4 are directed towards the surface of a suitable single crystal slice, often called the substrate.

If the substrate is clean and presents a plane crystal surface to the arriving molecules and the

lattice size is similar or identical to that of GaAs then the substrate crystal structure seeds the

235
growth of a single crystal GaAs layer. Such a layer will then be called epitaxial (Greek roots,

epi meaning "close to" taxis meaning "arrangement") because it has nearly the same crystal structure

as the substrate. If the substrate is GaAs then the layer is said to be homo-epitaxial, otherwise,

the layer is heteroepitaxial.

Although the MBE technique is basically vacuum evaporation, it differs in critical features from

conventional vacuum deposition methods. Indeed interest in MBE technology evolved from the difficulties

encountered by workers, in the late 1950' s and 1960's, in the preparation of single crystal layers

r 3 1 ii by vacuum evaporation. In 1958 Gunther reported the use of the so-called "three temperature method"

to grow compound semiconductors. To grow InSb separate crucibles containing the elements were heated

to different temperatures T and in order to regulate the stoichiometry of the layer grown onto

the substrate (held at temperature T ) . All of the films were polycrystalline. In the following

decade many groups grew compound semiconductors of poor crystalline quality using vacuum evaporation.

In 1968 Davey and Pankey^ succeeded in growing GaAs onto a heated GaAs substrate by using an impinging beam of gallium atoms along with a sufficiently high background pressure of arsenic. In the same year, Arthur ^ reported the growth of GaAs from thermally generated molecular beams in an all

metal vacuum system. Arthur used liquid nitrogen panels to collimate the beam sources and studied

[ 6 ] the interaction of As molecules with the GaAs substrate surface. Soon after, Joyce and coworkers

performed a definitive set of experiments which determined the surface reaction kinetics of Ga,As^

and As species on a clean GaAs surface. These experiments led to what is now referred to as Mole-

[2 7] cular Beam Epitaxy (MBE) . Cho and coworkers ' developed the technique to its present level by

studying the growth and characteristics of GaAs and AlGaAs layers. Their pioneering work on the fabrication by MBE of a large number of important devices kindled an interest which has spread to more

than 100 groups throughout the world.

An important distinction that must be drawn between vacuum evaporation and MBE, is found in the

ultimate pressures and relative cleanliness of the techniques. Vacuum evaporation is normally as- sociated with metal contact preparation in diffusion and ion pumped glass bell-jar vacuum systems

with ultimate background vacuum pressures in the range 10 ^ to 10 ^ Torr, while the ultimate background pressures in MBE are usually in the range 10 to 10 Torr, an improvement by four orders

of magnitude. To put this comparison into perspective it is instructive to convert pressures into

number densities of molecules. As the background pressure drops in the vacuum system, that is, as

3 the number of gas molecules per cm decreases, it takes longer for background or contaminant gases

to completely cover a freshly clean substrate with a single atomic layer. At background pressures

of 10 Torr such a layer forms in one second. The much lower pressures available in the MBE vacuum

system guarantee that once the crystal surface is clean it remains so for a period which is very

long compared to a typical layer growth run. This is crucially important since the quality of the

epitaxial layer is sensitive to impurities, both on the substrate surface and incorporated during

the growth.

In summary, the properties of thin metal, semiconductor and dielectric films grown by conventional vacuum deposition are limited by cleanliness. The layers are invariably of extremely poor crystalline

quality. At best the films are composed of polycrystals or randomly oriented microcrystalline areas whose lateral dimensions are similar to the film thickness. In addition, impurities, which migrate

to the polycrystal grain boundaries, often have a deleterious effect on the electronic properties of

the layer. Polycrystalline layers are often rough on a microscopic scale, and difficulties encountered

236
in accurately defining sharp geometrical device shapes can be a severe hindrance in optical com- ponents. On the other hand, layers grown by MBE are epitaxial, that is, they are single crystals.

Edges which have been etched in device fabrication steps are characteristically exceptionally smooth

and film surfaces are smooth on an atomic scale. Consequently the interface roughness between two layers grown consecutively in an MBE system can be less than a few atomic steps. Such characteristics

make MBE layers eminently suitable for use in optical devices. Furthermore, the growth of dielectric

multilayers by molecular beam epitaxy does not replicate surface topography from layer to layer.

3- Equipment for Molecular Beam Epitaxy

Figure 1 shows a commercially available MBE machine of the type installed in the Arthur D. Little, Inc., MBE laboratory. A schematic drawing is shown in Figure 2. In the design of MBE

Figure 1. Photograph of Arthur D. Little MBE Installation

equipment close attention is paid to the elimination of sources of impurities. Essentially an MBE

system system consists of a stainless steel ultra high vacuum chamber with thermal effusion sources containing suitable elements to grow the compound crystal of interest. The cells are constructed of

refractory materials such as pyrolitic boron nitride. Additional ovens may be used to generate beams

of impurity elements to dope the material, n- and p- type. Each oven has an aperture positioned so that the vaporization products are directed towards the substrate which is mounted on a heated block

of molybdenum. A small molybdenum plate or shutter can be interposed between each cell aperture

and the substrate to facilitate abrupt changes in doping density and/or structure. For example,

by opening and closing the aluminum oven shutter during simultaneous gallium, arsenic and aluminum

evaporation the growth of superlattices of AlGaAs/GaAs can be achieved. Similarly, the repeated

actuation of shutters in front of cells containing n- and p- type doping materials during gallium

arsenide growth has led to the development of a new type of doping superlattice. Most MBE systems

have extensive internal cryopanels which are double skinned stainless steel containers through which

liquid nitrogen is passed, thereby reducing the temperature to 77K. The cryopanels reduce background

237
LOAOLOCK

\

Components

8 Thermal sources

2 e gun sources

Microcomputer controls

Cryoshields

Rotating and heated

substrate holder

Cryopump

400 1/sec ion pump

Ti sublimation pump

2 Nude ion gauges

(1 flux meter)

Diagnostics

Reflection Electron

Diffraction (RED)

Residual Gas Analyzer

(QMA)

Components

Ar ion gun (etch profiling)

850°C Sample heating capability

Substrate holder

Nude ion gauge

Cryoshield

200 1/sec ion pump

Ti sublimation pump

Components

Ar ion gun (cleaning)

250°C substrate holder

Nude ion guage

Cryopump

Diagnostics

Scanning Auger Microprobe (SAM)

Secondary Electron Emission Microscope (SEEM)

Figure 2. Schematic of Arthur D. Little MBE System

238
pressures by removing condensible gas species and inhibiting the scatter of effusing species

from the stainless steel walls of the vacuum chamber.

4. Diagnostic Facilities

One of the advantages of MBE over other growth techniques is that the ultra high vacuum environment permits the use of in-situ diagnostic techniques. The technique which is routinely used in MBE

is reflection electron diffraction (RED) in which an electron beam of energy 5 to 15 keV impinges on

the substrate surface at a glancing angle. The electron beam is diffracted by the ordered array of

surface and near surface atoms in the crystal in much the same way as x-rays are diffracted by a crystal, or light is diffracted by a periodic grating. The diffraction pattern generated by the

crystal surface/electron beam interaction is conveniently displayed on a fluorescent screen situated

within a vacuum viewing port diametrically opposite the electron beam gun. Surface roughness on a microscopic scale gives rise to a spotty pattern due to transmission of the electron beam through the

3 dimensional surface projections while an atomically smooth surface is characterized by a streaked

pattern. For example, the pattern obtained from a clean polished (100) surface of a cubic semiconductor crystal slice is usually "spotty". The spots signify that the surface, although mirror smooth to the naked eye, is microscopically rough. On exposing such a surface to epitaxial growth in an MBE

system, it has been shown in many cases, that after only 150 Angstroms of growth the spots in the

diffraction pattern become elongated indicating a significant improvement in surface smoothness.

The reason for this is that the surface morphology of the layer is determined by the crystal structure

and not by the topography of the underlying substrate . Thus single crystal layers grown by MBE are characteristically smooth and are well suited to devices in which surface or interface morphology

is a crucial factor, for example, integrated optical components and devices which make use of the

2-dimensional effects associated with semiconductor interfaces.

Other important diagnostic techniques are Auger electron spectroscopy (AES)* and quadrupole mass

spectrometry. The former is used to study the chemical species present on the surface of material.

As shown in Figure 2, the SAM unit is situated in a chamber which can be isolated from the growth

environment. Substrates may be introduced into the analysis chamber without disturbing the vacuum

conditions in the growth chamber. The substrates may then be cleaned and inspected by SAM to insure

the complete removal of contamination from the crystal surface. Quadrupole mass spectrometry is used

to identify and estimate the partial pressure of background species in the vacuum system. It is

often used on a routine basis to decide whether the vacuum environment is suitable for growth of an epitaxial layer.

5. Materials, Devices and Structures Grown by MBE

Although GaAs has been used as an example in much of the preceding description it should be realized that many materials have been grown by MBE. The list of materials given in Table 2 is not intended to be complete but serves to illustrate the point.

*Sometimes referred to as a Scanning Auger Microprobe (SAM)

239
Table 2. Crystalline Films Grown With Molecular Beam Epitaxy

IV: Ge IV-VI: (PbGe)Te Metals: Al

GeSi PbS Ag

Si PbSe Au

PbTe Fe

I-VII: KC1 (PbSe)Te Sn

(PbSn)Se

II-VI: CdS (PbSn)Te

CdSe SnTe

CdTe PbEu

ZnS PbYbTe

ZnSe PbMnTe

ZnTe

Zn(SeTe) III-V: AlAs

HgTe (AlGa)As

HgCdTe A1P

GaAs

II-VII: BaF Ga(AsP)

2 Ga(AsSb)

2 (Gain) As (BaCa)F

2 (Gain) (AsP)

SrF

2

(Gain) P

GaSb

GaN

(AlGa)N

InAs (InAl)As

InP InGaAlP

InSb

Progress in devices and structures fabricated using MBE material is also proceeding apace. A

very successful program aimed at developing solid state double heterostructure lasers has been undertaken by Tsang "'at Bell Labs.

Similarly the growth of superlattices has received considerable attention along with the development of the modulation doped hetero junction. In this device an n-doped layer of AlGaAs is grown onto an MBE-grown undoped GaAs layer. Since there is a higher electron affinity in GaAs than in AlGaAs

excess electrons spill over from the AlGaAs into a thin surface region of the GaAs close to the interface. The electrons which find themselves in this region are spatially separate from their parent

donor ions and thus are free to move without impedance from ionized impurity scattering centers.

The electron mobility is therefore high. This is the basis of the high electron mobility transistor

(HEMT) - a device made possible by MBE technology.

Many other new applications are possible. For example, the possibility arises of vertically

stacking devices; of "writing" devices into epitaxial layers by steering ionized dopant beams; of

growing consecutive layers of metals, insulators, semimetals and semiconductors. Applications in new areas of physics are also currently under investigation, e.g., ultrasmall electronic devices,

quantum interference effects, metallic superlattices and metastable phases.

240
MBE technology is being developed in a number of ways. Increases in substrate size have already been accommodated from dimens ions. less than 1" diameter up to 3" diameter. It is expected that

MBE systems can be scaled up further. Furthermore, the development of "beam processing systems" has

already been predicted. In these systems, thermal effusion sources would be only one of several techniques to transport energetic elements and molecules to the substrate. Other ways may involve low

energy ionization and electron beam evaporation. Substrates could also be processed in a number of

ways within the vacuum system, e.g., by laser, electron beam or thermal annealing.

6. Characteristics of Multilayer Dielectric Optical Coatings

Simple multilayer dielectric (MLD) coatings consist of thin films of alternating regions of high

and low indices of refraction, each having very low optical absorption. Although the design of such

film stacks are relatively straightforward, in practical cases, serious problems are encountered.

They fall into two somewhat, but not completely, independent areas; problems related to the absorption

characteristics of the films and their damage thresholds.

A vexing aspect of optical performance of thin films is that although the optical absorption can be made small, it is still, in general, orders of magnitude higher than would be predicted from the

absorption coefficients of the bulk material used in making the film. Furthermore, the damage thresholds

of material surfaces are always somewhat lower than that of the bulk.

While the excess optical absorption of films can be attributed to impurities and high surfaceto-volume ratio of the polycrystals , the relationship between the film's structural properties and

its resistance tc damage manifests itself in a complicated manner. For example, the effects of

surface topography may enter the problem both in defining thermal paths and in the formation of high

field regions. Furthermore, the existence of interface electronic states contributes to the absorption characteristics of the interface and the increased scattering caused by surface roughness

plays a role in determining the damage threshold. However, in all of these matters, as well as the

gross surface properties of a film, the grain size of microcrystals forming the film plays a central

role.

In addition to these two general problem areas, a third and equally important problem area concerns the optical requirement that the optical phase-shifts should be constant within one percent

and independent of position across the area of the film.

7. Application of Molecular Beam Epitaxy to the Fabrication of Optical Coatings

The majority of current processes for manufacturing MLD mirrors involve conventional vacuum

deposition techniques such as evaporation and sputtering. These methods produce amorphous or microcrystalline films; we argue that they cannot improve upon the performance limitations already set by

optical absorption and damage thresholds, and described in the preceding paragraphs.

As described earlier, molecular beam epitaxy (MBE) is an ultra high vacuum technique which has

recently reached a degree of near-commercial maturity in the semiconductor device fabrication industry.

Layered structures called "Superlattices" containing extremely well defined and almost atomically

smooth interfaces can be grown with an unequalled morphology and high or precisely controlled purity

single crystal films. The technique has a demonstrated performance in semiconductor physics and technology

241
which makes it a plausible process candidate to satisfy the materials, morphological and thickness uniformity requirements of MLD mirror manufacture. We justify this conclusion with the following observations .

Single crystal films or films with large crystal grain sizes and thus small surface-to-volume

ratios are expected to have fewer numbers of localized states in the energy gap and less optical absorption. Further, an electron in the conduction band is less likely to be the instigator of laser-induced

breakdown in a crystal where the momentum relaxation time is longer. Single crystal films with low

density of defects and smooth interfaces would directly contribute to a longer momentum relaxation time.

These constitute the microscopic basis for the expectation that epitaxial films will have superior performance as MLD components in comparison with polycrystal films.

An equally important additional microscopic advantage of epitaxial growth is that grown layers do

not replicate the surface roughness of the substrate, but instead tend to be atomically smooth. The

reason for this phenomenon is that the surface morphology is determined by the crystal structure and not by that of the underlying surface topography. Long-range order of the crystal insures that the free

energy of an epitaxially placed atom in a valley on a rough surface is less than that of one which is

placed epitaxially on a peak. For amorphous films, the free energy is independent of position on the

(rough) surface since there is no long-range order.

As a production process, the existence of a load lock and facilities to clean by heating and ion

bombardment of substrates in-situ in UHV environment make JIBE a unique method to minimize the existence

of "extrinsic" damage centers in the finished product.

From a materials point of view MBE has already been demonstrated on many very important optical

materials such as ZnS, ZnSe, CaF^, etc. Table 2 contains a list of materials which have been successfully

deposited epitaxially with the MBE process. Although the list is not exhaustive, it suggests that it

is now possible to examine experimentally, the behavior of MBE grown MLD reflector stacks in intense

laser beams.

9. References

* [1 ] M.B. Panish, Science, Vol 208, pp 916-922, (May 1980).

[2 ] A.Y. Cho and J.R. Arthur, Prog. Solid State Chem., 10, 157 (1975).

[3] K.G. Gunther, Naturwissenschaf te, 45, 415 (1958).

[4] J.E. Davey and T. Pankey, J. Appl. Phys., 39, 1941 (1968).

t5] J.R. Arthur, J. Appl. Phys., 39, 4032 (1968).

[6] C.T. Foxon, and B. A. Joyce, Surf Sci. 64, 293, 1977.

[7] A.Y. Cho, J. Vac. Sci Technol, 16, 275 (1979).

[8] P.W. Sullivan, RFC Farrow and G.R.Jones, submitted to J. Cryst. Growth Nov

.

(1981)

[9] P.W. Sullivan, T.I. Cox, RFC Farrow, G.R. Jones, D.B.Gasson, C.S.Smith, 28th American Vacuum

Society Meeting, USA Nov. (1981).

[10] M. Faurie, Presented at MBE Workshop, Santa Barbara, Sept. 1981.

[11] W.T. Tsang, Appl. Phys. Lett. 34, 473 (1979).

*Authoritative review articles

242
Ion Beam Sputter Deposition of Optical Coatings

J. R. Sites, P. Gilstrap, and R. Rujkorakarn

Department of Physics, Colorado State University

Fort Collins, Colorado 80523

A beam of 500-1200 eV argon and oxygen ions, neutralized with an equal number of

electrons, strikes a target, and the sputtered particles from a deposited layer of S^,

T^Og, or Ti O2 - A second beam at lower energy is aimed at the substrate and used for

stress modification and pre-deposition sputter cleaning. Deposited films are mechanically stable, nearly amorphous, and extremely adherent; internal stress is compressive.

The primary contaminant is carbon, which is less than one percent. Refractive indices near 1 urn are 1.47 for Si0?

, 2.03 for Ta-0r, and 2.27 for TiOp. Optical absorptance of

half wavelength films at 1.06 pm is near 10 for S^ and Ta^O^ and near 4 * 10 for

Ti0 9

. Damage tests with a 114 ym radius, 1.06 urn wavelength laser beam show an onset

2 energy flux for visible damage in the 15 to 40 J/cm range for single layer films,

slightly less for a three layer structure.

Key words: optical coatings; ion beam; Sn^; Ta20^; Ti^; absorptance; laser damage.

1. Introduction

Ion beam sputter deposition has proven to be a useful technique for producing thin films for

applications including solar cells [1], optical recording media [2], insulators on semiconductors

[3], silicides [4], and diamondlike films [5]. The broad beam Kaufman-type ion source developed for

space propulsion [6] is nearly ideal for this purpose. A discharge chamber with magnetic confinement of electrons produces a relatively uniform plasma from essentially any gas or mixture of

gasses. A grid system extracts positive ions and a neutralizing filament adds an equal number of

electrons to the beam. The neutral beam does not diverge appreciably and can be used with insulating targets and substrates.

In the work described here a Kaufman-type ion source is used to fabricate oxide films of both

high and low refractive indices for reflectance enhancement. Results to date are very encouraging in that both the mechanical and optical properties of the films are approaching those desired for

coating mirrors subjected to large optical power densities.

2. Experimental Procedure

The ion beam sputter deposition apparatus is shown in figure 1. The primary ion source produces a beam of monoenergetic positive ions, in the present case either argon or an argon/oxygen

mixture. The ion energy is controlled by the potential difference between the source and ground

and is typically 500 to 1200 eV. The ion current density is controlled by the gas input to the source and the discharge potential; it is typically 1.0 to 1.5 ma/cm^.

The primary ion beam strikes a target that is either metallic or the oxide being deposited.

In either case some additional oxygen is necessary and is mixed with the argon in the source. Oxide targets require about 10% oxygen in the beam; metallic targets, 30 or 40%. The system con- figuration allows a change of targets in situ , a change which can be made either quickly or gradually.

A second ion beam is aimed directly at the substrate. It is generally run at a lower energy

(^300 eV) and is used to sputter clean the substrate prior to deposition. This step seems to be

important in producing films with very high adherence. The second beam is also used during deposition in an effort to modify stress in the deposited layers.

243
Cryogenic not snown vacuum

system _ 8 P<3x 10 Torr

Figure 1. Ion beam sputter deposition apparatus.

3. Results

Three oxides have been deposited by the ion beam technique: SiC^, ^a 2^5' anc' ^iC^-

quartz and silicon targets were used to produce SiO^, with similar results either way. Only

metallic targets have been used for the other two oxides.

The refractive indices of the three oxides were deduced from the reflectivity spectrum of

single layer films and are illustrated in figure 2. The SiCL index is essentially that expected;

Ti0 2

2.27

Ta 20 5

2.03

Si0 2

1.47

Figure 2. Refractive indices of deposited films.

The Ti O2 value is fairly high for a nearly amorphous film, implying that it is quite dense; the

Ta20,- refractive index is slightly lower than anticipated, but there is relatively little reported

work for comparison.

In addition to the single layer films, three layer coatings of high-low-high index films on quartz have been fabricated. Both Ta ? CL and TiCL have been used for the high index material, SiCL

244
for the low index. Figure 3 shows the reflectivity spectrum of quarter wave Ta20g-Si02"Ta20g

100

>

>

HO

LU

I Li. Ill

50

Three layer Coating —— measured

calculated

1.0 2.0

WAVELENGTH [jicm]

Figure 3. Comparison of calculated and measured reflectivities for quarter wave TagOg-SiOg-l^Oj.

coating.

layers coating quartz with the thicknesses selected for a peak reflectivity at 1.06 urn. The agreement with the spectrum anticipated from calculation using the matrix technique [7] is quite good.

Compositional analysis of the ion beam sputter deposited films utilized ESCA techniques. In all the samples there was carbon impurity on the surface, but it almost completely disappeared under

a light sputter etch. The residual carbon was just at the limit of experimental resolution, about

one percent. All other impurities were below the resolution of the instrument (see fig. 4). In

63%

Si

32% C

5%

1

0

66%

Si

34%

0

o

69% Ta

30% C

1%

SURFACE OF Si0 2 75A INTO Si0 2 75A INTO Ta 20 5

Figure 4. Composition of nominal SiC^ and Ta^O^ films.

general the ESCA measurements suggested that the films contained slightly less oxygen than the

nominal Si and 7&2®$ labels. (TiC^ films have not yet been analyzed.) At low oxygen concentrations in the sputter beam, the oxygen content of the films not surprisingly dropped to lower values.

245
The adherence of the oxide coatings has been very good as is typical of ion beam sputter

deposited films in general. Apparently the relatively high energies of the sputtered particles

(^20 eV) create both dense films and ones that adhere well to a substrate. Both the single layer

7 2 and three layer coatings exceeded the limit of our adherence tester (10,000 psi or 6 x 10 N/m ). In this technique a metal piece is epoxied to the coating then pulled until the coating comes off

or until 10,000 psi is reached. When we forcibly removed the metal piece by twisting with a pair of

pliers, the substrate generally cracked before the coating failed.

Stress in our single layer films was always compressive. It was determined by measuring the

radius of curvature of a thin substrate before and after deposition [3]. We found the SiC^ films

to have a larger internal stress than either the Ta

2 0g or the TiO^. Attempts to modify the stress

using the second ion beam were only partially successful (see fig. 5). The effect of the second ion

o

o

o

L-1 1 1—

O 250 500

SECONDARY BEAM ENERGY [eV]

Figure 5. Stress in single layer films showing the effect of substrate bombardment with a second

ion beam.

beam was to lower the Si^ stress somewhat and leave that of the Ta^O^ essentially unchanged. The

difference in stress between the two materials, however, is decreased considerably. Measurements

of Ti O2 stress modification have not been completed.

Optical absorptance of half-wave, single layer films was measured at 0CLI using absorptance

calorimetry with 1.06 urn radiation. As seen in figure 6, the absorptance of both Si0 ? layers and

-4 TagOg layers was near 10 . That of TiC^ was somewhat higher. Also shown in figure 6 is a half

wavelength Si 0 film deposited by sputtering Si09 without any oxygen in the beam. The much larger

absorptance clearly illustrates the need to add extra oxygen.

Laser damage tests at AFWL showed that both single layer and three layer films have a reason- able ability to withstand damage from high intensity radiation. When damage did occur, it was most

pronounced at small, micron sized scattering centers in the films. We believe these defects are caused by residual particles not removed by our cleaning processes. Generally the damage threshold

was higher when the laser failed to hit any defects, lower when it struck one or more, as illustrated by the dashed boxes in figure 7. Less intensive tests were made on the three layer coating

and as with some of the other measurements, we have not yet tested the Ti02 films.

246
TiQ 2

SiC>2

Ta 20 5

8x10

-5 1.1x10-4

3.8x10

-4

HALF -WAVE ABSORPTANCE ABSORPTION COEFFICIENT

Figure 6. Optical absorption of single layer films.

CM

E

o

<

UJCO

z

40 -

3 30

lli O

<

20

10

Si0 2

Ta 20 5

Wavelength : 1 .06

Radius of Beam: 114/urn

Ta 20 5

-Si0 2 -Ta 2 0 5

/ A. A .A x \ 4 4 4 /

0

1 1 1 1 1 '

Figure 7. Laser energy density necessary to visibly damage coatings. Dashed boxes enclose region

where damage sometimes occurred and sometimes did not.

247
4. Conclusions

We find that ion beam sputtering is an attractive technique for the deposition of optical

coating layers. The films are very sound mechanically; they have excellent adhesion, they are relatively dense, and as far as we can tell, they are stable. Internal stress is compressive and

can be modified somewhat with a second beam aimed at the substrate. Optical absorption in single

layers is relatively low, and the energy density necessary for laser damage is relatively high. For multilayer films, it is straightforward to change from one composition to the other, and we see

no major difficulties in scaling the technique to more layers and larger sized surfaces.

We would like to thank Kent Geib of our University for the ESCA measurements, Tom Allen and

Jerry Dodds of Optical Coatings Labs for the absorption calorimetry, and Alan Stewart and Ed Miesak

of AFWL for the laser damage tests. The work reported here is supported by the Air Force Weapons

Laboratory under Contract F29601-82-K-0009.

5. References

[1] Burk, D. E.; DuBow, J. B.; Sites, J. R. Fabrication of OSOS cells by neutral ion beam sputtering. Proc. 12th IEEE Photovol taics Specialists Conf. 971-974; 1976.

[2] Rossnagel , S. M.; Robinson, R. S.; Kaufman, H. R. Ion beam sputter deposition of high density

optical recording media. Unpublished report, CSU Physics Department, 1982.

[3] Meiners, L. G.; Pan, R. ; Sites, J. R. Oxide barriers on GaAs by neutralized ion-beam sputtering. J. Vac. Sci. Techno!. 14(4): 961-963; 1977 July/Aug.

[4] Weinmeister, R. E.; Mahan, J. E. Electronic transport and microstructure in molybdenum

silicide thin films. Appl . Phys. Lett. 39(12): 347-351; 1981 December.

[5] Banks, B. A.; Rutledge, S. K. Ion beam sputter-deposited diamondlike films. J. Vac. Sci.

Techno!. 21(3): 807-814; 1982 Sept. /Oct.

[6] Kaufman, H. R. Technology of electron bombardment ion thrusters, in Advances in electronics

and electron physics, Vol. 36, L. Merton, ed. New York: Academic Press; 1974. 265-373.

[7] Abeles, F. Sur la propagation des ondes electromagnetiques dans les milieux stratifies.

Ann. Phys. 5: 504-520; 1950.

[8] Rossnagel, S. M.; Gilstrap, P.; Rujkorakarn, R. Stress measurement in thin films by geometrical optics. J. Vac. Sci. Technol . in press.

In response to questions, the speaker stated (1) that the packing fraction for the films had not been measured , and (2) that oxygen had only been introduced into the chamber through the ion beam

itself since it was thought that molecular oxygen wasn't sufficiently reactive . When asked to clarify his assertions that the films were noncrystalline and very adherent, the author conceded

that his data indicated absence of crystallites greater than 100 A in size, and that sputter

deposited oxide films on glass usually had good adherence.

248
Optical Characterization of Plasma Deposited Hard Carbon Coatings

B. Dischler, A. Bubenzer, P. Koidl , G. Brandt, O.F. Schirmer

Fraunhofer-Institute for Applied Solid State Physics (IAF)

Eckerstr. 4, D-7800 Freiburg i. Br., West-Germany

Hydrogenated amorphous carbon films (a-C:H) have been deposited on Ge, Si and glass

substrates in a RF excited glow discharge from benzene vapour. These films are hard,

chemically inert and IR-transparent and therefore very promising for protective and antireflective coatings for IR-optical components. We report on the optical properties of

a-C:H and their quantitative dependence on deposition parameters. Absorption spectra of

a-C:H coatings in the wavelength range 0.2 - 20 urn have been extracted from transmission

and reflection measurements. In addition CO2 laser damage thresholds are reported. For

a-C:H with refractive index n = 1.93 the following absorption coefficients have been measured: 250, 25, 15, 200 and 780 cm~1 at wavelengths of 10.6, 3.8, 2.8, 1.3 and 1.06 urn respectively. The increased short wavelength absorption and its possible dependence on dangling bond density - as determined by electron spin resonance - is discussed.

Key words: absorption; amorphous hydrogenated carbon; hard coating; infrared; laser damage;

optical properties; plasma deposition; thin film.

1. Introduction

Amorphous hydrogenated carbon (a-C:H) offers the potential of extreme hardness, resistance

against chemical attack and optical transparency over a wide spectral range. Consequently this material has found increasing interest as a protective coating for optical components. Although hard carbon coatings are offered commercially, this material is physically not well understood. In particular

there is insufficient quantitative information on the optical properties and their dependence on the

conditions of preparation.

We shall report on the optical absorption of a-C:H showing that at HF and DF laser wavelengths

the absorption of our films is an order of magnitude below the best values quoted previously. In addition CO2 laser damage data will be reported.

2. Preparation and Film Properties

Hard carbon coatings have been prepared by rf [1-41 and dc [5,61 plasma deposition, ion beam

[7-101 and sputtering techniques [11-12]. We used the first method since it offers the possibility

to grow homogeneous large area coatings on insulating substrates with relatively high deposition

rates

.

a-C:H has been deposited onto negatively biased Ge, Si or glass substrates in a 2.3 MHz-excited RF glow discharge sustained by benzene vapour. The apparatus as described previously [13] has

been implemented by an optical pyrometer for the measurement of the film temperature during growth.

The deposition process and the film properties can be controlled precisely and reproducibly by

the two parameters: negative bias voltage, Vb, and hydrocarbon pressure, P. As an example, figure 1 shows the dependence of the refractive index on Vg and P. The inner rectangle defines the parameter

range used for the deposition of a-C:H. The solid lines are curves of equal refractive index. As shown by the dotted lines there is a wide parameter range within which n is determined by a single

parameter, namely Vg/P.

The tunability of the refractive index is important for the design of optical coatings. By adjusting the refractive index exactly to n = 2.0, single layer AR coatings on Ge have been realized

with a residual reflection below 0.2% [13].

Table 1 summarizes important deposition parameters, thin film properties and their dependence

on Vg and P. A detailed study and a discussion of the range of validity of the given relationships

will be published separately [14].

3. Optical Characterization

Transmission and reflection measurements in the wavelength range 0.2 - 20 urn have been performed

on two double beam spectrometers, each equipped with an on-line computer (Perkin Elmer 330 + 3500

data station and Perkin Elmer 580B + 3600 data station). All transmission (T) and reflection (R)

spectra are stored electronically and the loss spectrum (A) is computed according A = 1-R-T. Surface

losses can be ruled out because A scales exactly with film thickness (in units of half wave layers).

Comparison with calorimetrical ly determined absorption shows that scattering can be neglected. Thus

249
Table 1: Deposition parameters, a-C:H thin film properties and their interdependence

Property Val ue/Range Linear Function of

Deposition parameters

bias voltage, Vg gas pressure, P growth rate

film temperature

0.4 - 1.8 kV

10 - 60 ubar

100 - 2500 A/min

40 - 140° C

VVP

v 2 pl/2

a-C:H Properties

density

refractive index

hydrogen content

optical gap

Knoop hardness

electr. resistivity

1.5 - 1.8 g/cm 3

1.8 - 2.2

- 30 at%

0.8 - 1 .8 eV

1250 - 1650 kp/mm2

1 ?

~ 10 ficm

V

V

-1/2

1

Vg, weak P-dependence

Vg, weak P-dependence

to be determined

to be determined

the loss spectrum represents essentially the absorption of the a-C:H thin film. After correcting for

standing wave effects [15], the absorption coefficient is calculated. Figure 2 shows the absorption

coefficient between 2.5 and 14 um. The strong absorption at 3.4 urn results from C-H stretching vibrations. The absorption beyond 6 um is due to C-H deformation and C-C bond stretching vibrations. At

10.6 um the method described yields an absorption coefficient between 250 and 300 cm~1. This value

has been confirmed by laser calorimetry.

Figure 3 shows the absorption from the CH-stretch band towards higher energy. Spectra of three

different carbon films with refractive indices of 1.85, 1.93 and '2.16 are shown. The absorption minimum occurs around 2.8 um. The spectral dependence is quite different from an Urbach type absorption

edge as observed e.g. in diamond (fig. 3a). There is a subexponential increase of the absorption to- wards higher energy. Replotting the curves in figure 3b shows that - except for the lowest tail - the

absorption in a-C:H follows the relation

(aE)

1/2 =G(E-Eopt ).

This relation also holds for the absorption edge of a-Si:H [16,17]. Extrapolation of the straight

lines to the abscissa yields the optical gap, E 0 pt- The gradient G was found to be constant. Thus,

the differences in optical absorption for E > E op t are fully characterized by Ep

p

^. The optical gap can be influenced by means of V B and P (see table 1). It is important to note that the sub gap absorption tails shift with the optical gap itself. Thus Eop t also determines the near infrared absorption of a-C:H.

In table 2 we list the measured absorption of different a-C:H coatings at important laser wavelengths. Very low absorption is found at 2.8 and 3.8 um especially for the low index films. These

absorption coefficients are an order of magnitude below the best values reported before [18]. The near IR absorption of the n = 2.0 sample has been confirmed by low temperature calorimetric spectroscopy [19,20]

.

4. CO2 Laser Damage

Most applications of hard carbon coatings concentrated so far in the 8 - 12 um region. Therefore

CO2 laser damage measurements are of importance.

Germanium windows coated on either side with a quarter wave AR film of a-C:H (n = 2, thickness

=1.3 um) have been used for damage measurements.

AT GEC, Hurst Research Centre, single shot CO2 laser damage thresholds have been measured using

the experimental setup as described in this volume [23]. 1/1-damage thresholds ranging between 110

and 280 MW/cm2 have been observed.

Multiple pulse measurements have been performed at Eltro GmbH., Heidelberg. A C0£-TE laser emit250
ting pulses of 50 n.s width with a TEM00 transverse mode profile has been focussed to a 1.5 mm diameter spot. At a continuous irradiation at 1 Hz prf a N/1 -damage threshold of 50 MW/cm2 corresponding to 5J/cm2 has been measured. This lower value as compared to the single shot damage threshold

indicates an accumulation or memory effect.

Table 2: Optical absorption coefficients (cm" ) in a-C:H

Eopt

Refr. Wavelength (urn) (eV) Index 10.6 3.8 2.8 1.3 1 .06

1.76 1 .85 250 15 < 10 52 150

1 .28 1.93 250 25 15 200 780

1.10 2.00 250 40 26 550 1900

0.96 2.16 250 110 60 2800 6000

a) a) a)

a) from ref. [18]

121 429 446

5. Discussion

It has been shown that besides the already established usefulness of a-C:H in the 10 um region,

this material has a window of very low absorption at HF and DF laser frequencies. The residual absorption as well as the increase towards higher energy reflects the onset of electronic absorption

and is characterized by the optical gap.

It has been found that E 0 pj- and the content of bonded hydrogen depend in much the same way on the deposition parameters (see table 1 and [14] ), suggesting a correlation between these two quantities. In fact there is a linear dependence of E 0 pt on the hydrogen content, as shown in figure 4. A similar behaviour has been observed in a-Si:H and a model by Cody et al . [17] suggests that hydrogen relaxes structural disorder by saturating dangling bonds thus leading to an increased optical gap. Although the microstructure of a-C:H is more complex than in tetrahedral ly coordinated a-Si:H

[15] , the similarity of the absorption edge and its dependence on hydrogen content suggest that the same physical model holds for both materials. In fact, electron spin resonance experiments, as described below, indicate that dangling bonds are present in high concentration in our a-C:H thin films.

ESR experiments have been performed at room temperature on a 9.7 GHz (X-band) spectrometer. A signal with a g-factor of 2.0027 and a typical linewidth of 2 Gauss , previously attributed to dangling

bonds in a-C:H [21,22], has been observed. Using an a-C:H sample prepared with Vb = 1.5 kV and

P = 33 ubar, a spin density of 9 x 10^ cm

-3 has been determined. Work to establish a correlation

between dangling bond density and hydrogen content is under way.

6~. Summary

The optical properties of plasma deposited hard a-C:H coatings were investigated. For the first

time complete optical absorption spectra between 0.2 and 20 urn have been measured. Laser damage

thresholds at 10.6 pm are reported, showing that a-C:H is suited for medium power CO2 laser applications. The absorption in the visible and near infrared is determined by the optical gap, which depends linearly on hydrogen content and can be varied between 0.8 and 1.8 eV. Very low absorption is achievable in the 2 to 6 urn region. This result together with the chemical resistance makes a-C:H

very interesting as a coating material for HF and DF laser components.

We are indebted to R.M. Wood (GEC, Wembley), J. Ruger and F. Prein (Eltro, Heidelberg) for

providing the laser damage data and to S. Hunklinger (MPI, Stuttgart) for communicating the low

temperature calorimetry results.

251
7. References

[1] L. Holland and S.M. Ojha, Thin Solid Films 38, L17 (1976)

[2] D. A. Anderson, Phil. Mag. 35, 17 (1977)

[3] S. Berg and L.P. Anderson, Thin Solid Films 58, 117 (1979)

[4] K. Enke, H. Dimigen and H. Hubsch, Appl . Phys. Letters 35, 291 (1980)

[5] D.S. Whitmell and R. Williamson, Thin Solid Films 35, 255 (1976)

[6] B. Meyerson and F.W. Smith, J. Non-Cryst. Solids 35-36 , 435 (1980)

[7] S. Aisenberg and R. Chabot, J. Appl. Phys. 42, 2953 (1971)

[8] C. Weissmantel, Thin Solid Films 92, 55 (1982), and references therein

[9] E.G. Spencer, P.H. Schmidt, D.C. Joy and F.J. Sansalone, Appl. Phys. Letters 29, 118 (1976)

[10] J.C. Angus, M.J. Mirtich and E.G. Wintucky, in Metastable Materials Formation by Ion Implantation , S.T. Picraux and W.J. Choyke, Eds., p. 433, Elsevier, N.Y. (1982)

[11] B.A. Banks and S.K. Rudledge, J. Vac. Sci. Technol . 2j_, 807 (1982)

[12] D.R. McKenzie, R.C. McPhedran, L.C. Botten, N. Savvides, and R.P. Netterfield,

Appl . Optics 21_, 3615 (1982)

[13] A. Bubenzer, B. Dischler, A. Nyaiesh in Laser Induced Damage in Optical Materials: 1981 , eds. H.E. Bennett, A.H. Guenther, D. Milam, B.E. Newnam (NBS Spec. Publ .

)

[14] A. Bubenzer, B. Dischler, G. Brandt and P. Koidl , submitted to J. Appl. Phys.

[15] B. Dischler, A. Bubenzer and P. Koidl, Appl. Phys. Lett, (in press)

[16] E.C. Freeman and W. Paul, Phys. Rev. 20, 716 (1979)

[17] G.D. Cody, T. Tiedje, B. Abeles, B. Brooks, and Y. Goldstein, Phys. Rev. Letters 47, 1480 (1981)

[18] T.J. Moravec, in Summary of the Workshop on Diamond-Like Carbon Coatings , Albuquerque, N.M.,

April 1982, p. 7 (B. Bendow, Ed.)

[19] A. Bubenzer, S. Hunklinger and K. Dransfeld, J. Non-Cryst. Solids 40, 605 (1980)

[20] S. Hunklinger, private communication

[21] R.J. Gambino and J. A. Thompson, Solid State Commun. 34, 15 (1980)

[22] T. Shimizu, M. Kumeda and Y. Kiriyama, Solid State Commun. 37, 699 (1981)

[23] R.M. Wood, S.K. Sharma and P. Waite, "Variation of Laser Induced Damage Threshold of Germanium

with Laser Pulse Repetition Frequency" in Laser Induced Damage in Optical Materials: 1982 , eds. H.E. Bennett, A.H. Guenther, D. Milam, B.E. Newnam; (NBS Spec. Publ., this volume)

252
50

40

°- 30

UJ

or

en

UJ

or

x

u

20

10

n = REFR ACTI V E INDEX AT X=1

_i I i

i i i I i j i i I i_ 0.5 1.0 1.5 2.0

BIAS VOLTAGE \fe(kV) »

Figure 1: Curves of equal refractive index (solid lines) as a function of bias voltage and benzene

vapour pressure.

600

400

'Eu

» 200

4000

1

;

i

1— "i i i i i i

\ a - C : H _ f\

i i 1 1 1 3000 2000 1500 1000 700

V (cm

Figure 2: Infrared absorption coefficient of a-C:H.

253
Figure 3: (a) Absorption edge of three a-C:H

films with different refractive index,

(b) Results for the same three samples,

replotted to demonstrate optical gap

behaviour.

254
Scaling Up the Neutral Solution Anti-Reflection Process

G. Richard Wirtenson and Norman J. Brown

Lawrence Livermore National Laboratory

University of California

Livermore, California 94550

and

Lee M. Cook

Schott Optical Glass Inc. 400 York Ave. Duryea, Pennsylvania 18642

ABSTRACT

The Neutral Solution Process^>2 f or chemically producing anti-reflection coatings

has been scaled up and used successfully to treat components as large as 90-cm diameter.

Since the process is primarily determined by diffusion through the surface, it has proven

to be relatively insensitive to fluid flow effects which influence the chemical

concentration build-up at the surface. Large parts visually appear more uniform than

small parts processed in model tanks. The major problems encountered have been

associated with using plastic support components beyond their normal temperature limits

and in handling parts to the required cleanliness outside of a clean room environment.

Key words: Anti-Reflection; Coating; Laser Damage Resistant Neutral Solution; Surface

Leaching; Thin Film.

1. Introduction

About a year ago, the Schott Glass Co., developed a chemical back-etch process for producinq

low reflectance surfaces on certain glasses^ >2,3. This process produced such promisingly useful

properties as regards reflectance and laser damage, that the Lawrence Livermore National

Laboratory purchased the right to use it for the Laser Fusion Program and immediately began

scale-up operations for the Novette Laser. Pending patent clarification, chemical details cannot

be disclosed at this time. However, this restriction does not impede discussion of the process in sufficient detail for the laser damage community to have meaningful discussions on practicality

and economic feasibility.

The process is not completely understood at this time, but usable surfaces to 90-cm diameter

have been produced that show excellent low reflectance characteristics, good damage threshold, and

considerable promise for improvement in the reflectance levels. Reflectance minima have been

produced at 530 nm and 1060 nm both singly and almost simultaneously, and the process can be

controlled to yield minima anywhere from about 400 nm to at least 2500 nm. Figure 1 shows a two surface transmission scan obtained from a 5-cm diameter BK-7 witness piece processed with an 80-cm

Novette focusing lens. An unprocessed piece would transmit about 92%. This scan was made on a Cary 17 spectro- photometer and verified on a Perkin-Elmer 330 which yielded identical results.

At the same time, it is a rather extreme result, more typical results showing 0.5 to 0.8% losses

through two surfaces in the peak transmission regions.

2. Operational Process Sequence

The process sequence begins in the clean-room where pneumatically expanding rinqs grip the

part on its edge to lift it into the ring seats of plastic holders. From this point on, points

within the clear aperture are never contacted by solids.

Work performed under the auspices of the U.S. Department of Energy by LLNL under Contract No. W-7405-Eng-48.

255
Just before being immersed in the clean deionized water in an electropol ished stainless steel

tank, the optics are given a final rinse with alcohol to remove a layer of hydrocarbons that

quickly forms on glass surfaces in almost any environment. Failure to do this can result in a blotchy surface. Immersed in the covered tank, the part and fixture are moved from the clean room

to the processing area. Here they are quickly loaded into polypropylene plastic tanks (fig. ?)

filled with deionized water. The water is filtered to 1 micron when it enters the processing area

and again as it comes from area holding tanks; however, airborne particulates most likely

contaminate the processing tanks.

The deionized bath is brought to temperature on a programmed linear ramp. Chemicals are then

introduced for a minimum one hour etch. This solution can be flushed with four throughputs of

deionized water from tanks at the same temperature or the etch can be allowed to remain in the

tank since it is compatible with the process. Chemicals are then added to the tank and the

processing is allowed to progress for two to four days depending on the wavelength of interest.

40 to 70 hours of processing time, allows an hour or two latitude in timing. By means of witness

pieces, progress is monitored at about two thirds of the estimated period and at least twice more

towards the end of the period using spectrophotometer measured transmission maxima and a linear

extrapolation. Processing is quenched by a flush with deionized water at processing temperature

using the four times throughput. Again a linear programmed ramp is used to cool the tanks to room

temperature. Slow cooldown is precluded by a slight erasure of the process in protracted exposure

to hot deionized water. Throughout the process, the tanks have been enclosed in portable clean

hoods. These are now removed and the part in its holder is quickly removed to the stainless

steel, water filled transport tank. Surfaces are manually sprayed with water during this transfer

to prevent drying. Some particulates are undoubtably transferred and others acquired during this

transfer, but immersed in a bipolar fluid, they do not appear to fix onto the surface. Their

concentration in the clean tank fluid is decreased a hundredfold during the sloshing transport to the clean room. Because the processed surfaces are extremely sensitive to water spotting, Freon,

immiscible with water, is introduced into the tank and water is skimmed from the surface. The

liquid Freon is continuously cycled through a filter and water trap. The chilling effect of Freon

evaporation precludes removal of the part into the direct atmosphere since this could result in severe thermal stress or condensation of water vapor. A second stage with plastic windows is

attached to the portable tank and brought to saturation with Freon vapor. The optic is raised

into this vapor for drying. It is first scanned visually for traces of water droplets which are readily distinguishable on the surface as the Freon sheets off. If any are present, the lens is repeatedly dunked until none appear. It is then allowed to dry in the freon atmosphere with

little chilling and is subsequently removed (fig. 3) into the clean room atmosphere. This process

is shown schematically in figure 4.

A number of elements are processed on one side only. These are protected on one side by a double Viton "0"-ring and a cover. When the part is immersed in the processing tank, the cavity

fills with deionized water. The cavity is vented to the atmosphere through tubes rising above the

maximum fluid level to permit outgassing and accommodate expansion.

This equalizes pressure and prevents serious ingress of chemical into the back cavity in the

event of "0"-ring failure of which there have been several. This failure mode has been eliminated

by the use of heat-sealed vulcanized "0"-rings. This cavity is wedged open prior to post

processing immersion in Freon.

3. Damage Threshold

The Neutral Solution processed parts show damage thresholds (fig. 5) at least two to three

times that of vapor deposited layers producing the same optical results. We can postulate a number of possible reasons for this.

a. The process involves chemical etchinq. We may be etching away chemical contamination and

traces of physical disruption produced by the polishing process.

b. The interface between bulk and leached layers is not only less contaminated but both

structurally and chemically continuous.

c. The altered layer has a density gradient which could possibly absorb dynamic shock or impede electromagnetic effects. Electron microscopy indicates the pores are on the order

of 40-60 angstroms as shown in figure 6.

4. Surface Characteristics

The Neutral Solution processed surfaces show slightly nonlinear effects when compared to the

linear progression of a vapor deposited coating. This is because there exist in the former both

density and chemical gradients in the leached layers.

256
The surfaces are extremely fragile and cannot stand much physical contact. We have

successfully drag wiped several 5-cm specimens without noticeable effect on damage threshold, but

drag wiping on some others has shown visible scratching so this has been avoided with all parts.

We have also seen surface damage when spraying these parts with Freon at 80 psig.

The surfaces are prone to hydrocarbon adsorption which can increase surface reflectivity to non-processed levels. This adsorption is obviously a function of the environment. Some specimens

at Schott have been exposed to the atmosphere for almost a year with changes of only a couple of

tenths of a percent in reflectivity. Others at both Schott and LLNL have shown almost complete

but reversible regression in two months. The effects have been erratic and appear as a reflective

film growing inward from the edges. FTIR (Fourier Transform Infrared Spectrscopy) and GCMS (Gas

Chromatography/Mass Spectrometry) have identified this film as predominately the hydrocarbon,

dioctyl phthalate. It should be noted that dioctyl phthalate molecules are of a size that will

fit within the pores and have an index near 1.5, virtually identical to BK-7. Simple flushing

with ethyl alcohol quickly restores the surfaces to their immediately post process state as far as reflectivity is concerned. Figure 7 shows the reduced transmittance due to contamination of the

processed surface by dioctyl phthalate and the restored transmittance after ethyl alcohol

washing. Some surfaces exposed to vacuum on one side only have shown almost total regression to preprocessed reflectance levels in a few weeks while the side exposed to atmosphere showed little

effect. Other surfaces have regressed at a slower rate. Similar recovery effects were observed

with hydrocarbon solvents. Deliberate exposure of surfaces to high molecular weight hydrocarbons

such as index-matching oils produce significantly less changes in the reflectance again in agreement with the electon microscopy data on surface porosity.

Since the process is in part an etching process, it tends to uncover subsurface damaqe and

traces of incomplete polishing. Several pieces have been covered with fine scratches and sleeks

after processing. The evidence that these are due to polishing is that they may be restricted to

a single surface when both have received identical processing. These surfaces had all been

carefully inspected prior to processing and in a number of cases, carefully documented, these

surfaces had not contacted anything solid from the time of their arrival at LLNL. It is not clear

whether these sleeks, etc., contribute to lowering the damage threshold since the etching tends to open up and round the crack apex.

5. Design Considerations

The chemistry is quite delicate and very sensitive to contamination, particularly metallic

contamination. Common plasticizers also have an effect. A small rubber "0"-ring was sufficient

to disturb a reaction in about one cubic foot of solution. To-date we have limited contact of the

processing fluids to the glass, Viton, Teflon, and natural polypropylene.

The process takes place at a temperature near the boiling point of water. This is considerably above a practical use limit of the polypropylene selected for lens holding fixtures,

etc. As a result we have been plagued with plastic shrinkage. This tends to be a function of

plastic thickness with thicker parts, presumably less completely cured, showing a greater

shrinkage. The shrinkage is also non-isotropic. Pieces shrink differently in length and width

and from side to side. This amounts to 2-3%, a significant amount in 48 inch holders. The

problem has been ameliorated but not eliminated by a precook or preshrinking operation between

rough and finish machining operations, but slow shrinkage is still being observed through over a dozen cycles of several days duration each.

During processing the fluid temperature is maintained within a fraction of a degree. The size

of the parts and the strong thermal coupling between part and fluid make thermal stress a dominant

consideration during heating and cooldown cycles. These are linearly programmed and are of 2 to 6 hours duration each depending on part thickness. These long-times preclude visual observations of

the part during processing, so small witness samples are periodically removed (fig. 8) for this

purpose.

If the witness samples are of nearly identical composition and processing, their processing

rate may be dominated by the processing rate of the large part. However, the rate is strongly

dependent on annealing rate and glass composition, and virtually identical glasses from different

melts have been found to vary somewhat in processing rates.

We are working on a laser ref 1 ectometer to detect changes in reflectivity through the fluid,

but it is not yet clear how effective this will be.

For work at a specific laser frequency, the uncertainty in location of the part minimum is not

as intolerable as it sounds. The minima are quite broad and the differences between the part and

the witness samples have been on the order of 10% or less in peak wavelength position.

257
Bubbles are almost always present in flowing solutions of these chemicals at these

temperatures. The possibility that these may be trapped and collect on an underside dictated

processing in a vertical position.

Considerable effort was expended on achieving uniform flow. This has not been a problem. One

advantage of the process is that in retrospect rates seem to be determined more by diffusion

processes inside the surface than by chemical concentration at the surface. Thus moderate

attention to flow uniformity is sufficient. We have been pleasantly surprised to find that larger

pieces process more uniformly than smaller pieces because it is easier to maintain uniform flow in

a larger setting.

To summarize to this point, the process produces surfaces with some features desirable and

others less than desirable. We trying to determine these and how or whether we can live with

them. We have found the process is scalable and relatively inexpensive, demonstrating this on parts as large as 90 -cm diameter. Figure 9 shows the largest aperture processed in both the

development (24-cm diameter) and scale-up (90-cm diameter) phases. Figure 10 shows processed

80 -cm diameter f/20 spatial filter lens. The diameter and quantity of parts scaled-up during

FY-82 is shown in figure 11. We are ignorant of many details and the crash nature of the program

has precluded optimization, but there is considerable promise for future growth.

6. Surface/Subsurface Chemistry

While working on this process, the authors, two chemists and an engineer with diverse

backgrounds in glass chemistry, vacuum coating, and optical polishing have been forced to share

their ignorance. The process has raised questions for which we have, as yet, no answers, but

questions that may be of interest to the laser damage community.

These questions have to do with the contamination of surfaces and the depth of contamination

as influenced by pre-leaching processing parameters. Before beginning this process we knew that

there was a surface layer a fraction of a micron deep that somehow differed from the substrate

bulk. The outer quarter to half micron of fused silica etches much more rapidly than the bulk in HF for example. Underlying this layer are a pattern of abrasive particle tracks and a roughness

greater than that of the polished surface. Some material has obviously been transported over the

surface at least to a depth of a few tens of angstroms, the typical track depth on finely polished

surfaces. The remaining thousand or more angstroms may be similarly regeled material or material

changed by diffusion of species into or out of the layer, or just structurally disturbed.

Early in the process development, we needed some large pieces of glass for dummy runs. We had some acceptable stock of vacuum coated elements unusable for other purposes due to laser damage.

We etched off the coatings with hydrofluoric acid. In this process, we have seen rubbing streaks

etch into the substrates below the coatings, streaks representing stroking of a type we had not

used. We are certain these are residuals from a cleaning process occurring between polishing and

coating. Yet these pieces had received near state-of-the-art cleaning just prior to coating.

Next we found that a brief polish was insufficient to remove chemical traces introduced in the

stripping process resulting in a different processing rate. A second lengthy polish still did not result in a normal processing rate. Similar effects have been seen on witness samples. Desiring

a quick turnaround we simply repolished some witness pieces removing slightly over one micron of

surface. They processed differently. Next we polished off about five microns. These came closer

to a normal processing but there were perceptible differences.

We do have witness samples successfully reprocessed to virgin material response by passing

through a grind sequence. However, it is not clear whether it is the quantity of material removed

or the suddeness of the removal preventing recontamination that produces the result. Figure 12 shows this effect.

We have seen differences in processing rates for parts polished in different shops,

differences in parts from the same shop, and differences in rates between two surfaces of the same

part. These have been small with little effect on the result, but they have been perceptible. At the same time we have not observed rate differences on differing parts of a single surface even in highly figured aspheric surfaces of large diameters. Thus surface figure is not perceptibly

changed during processing^. Thus, for this type of leaching process, the chemical

state-of -the-surf ace has an important rate-determining effect. A relatively consistent surface

may be prepared, especially for plano-plano parts, using state-of-the-art finishing techniques.

However, the grinding and polishing process does not necessarily guarantee a well defined surface

chemistry. It appears that one cannot simply give mechanical guidelines for the finishing

process. At the same time one cannot logically hope to chemically characterize each glass surface

before treatment.

258
It seems difficult to conceive of a diffusion process permitting significant contamination

into or leaching out of microns of material, thousands of atom layers, but we are seeing results

of contamination persisting through at least a few microns of polishing removal, and we are not the only ones. A recent Soviet paper^ investigating the effects of prepolishing treatment on polishing rates inferred for at least one glass a perceptible effect persisting through 25 microns

of polishing removal. One possibility is that prepolishing contamination persists through

appreciable polishing because some contaminants reinfect the fresh surface as layers are removed.

There is a growing body of evidence about ion absorption and re-sorption on glass surfaces during

the leaching process. Tait and JensenS have reported effects of solution and glass additives on leaching characteristics of nuclear waste glasses. Zinc, in particular, produced markedly lower

leaching rates and the effect is the same whether the ions are introduced into the glass or into

the leaching solution. More importantly surface analysis shows strong enrichment of this ion

within the leached surface layer. Similar surface enrichment effects and changes in leach rates

have been recently reported by Buckwalter and Pederson^ for leaching in lead and aluminum metal

containers. One of the authors has observed environment effects when leaching flat glass

samples. In this case, tin found before treatment on a single glass surface as a contaminant from

manufacturing is observed on both surfaces after leaching in apparent high localized

concentrations. What is important about these findings is that the absorbing ions remain on the

leached layer for very long leaching times, i.e., they are continually adsorbing and resorbing

onto the moving leaching boundary as the corrosion process works it way into the glass. As many

of these elements are commonly encountered as structural materials, as additives to many plastics,

and because the chemical nature of the polishing process is largely the same as aqueous corrosion

of glass, it is suspected that surface enrichment of polished glass surfaces by metal ions may be more wide spread than has been thought.

7. Summary

The neutral solution process adds an important class of high laser damage threshold surfaces

to the anti-reflection treatment repertoir. The ease with which the process can be scaled to large size optics is also attractive. Both glass chemistry and the present delicacy of the

surfaces require special design consideration. Observations made during the processing raise

questions on the control of near surface chemistry during glass polishing operations, questions

that may be important for laser damage work.

259
References

1 Schroeder H., "Thin Film Formation on Glass Surfaces in Chemical Coating Processes", Proc.

LJ Tenth Int. Cong, on Glass, Kyoto 8-118 (1974).

[21 Cook, Lee M., and Mader, Karl-Heinz, "Intergral AR Coated Glasses for High Energy Laser

LJ Applications", Proceedings Electro-Optic/Laser 1981, Anaheim, CA, Nov. 17-19, 1981.

I3j Cook, Lee M., Ciolek, Sally, and Mader, Karl-Heinz, "Intergral Antiref lective Surface

Production on Optical Glass", Journal of the American Ceramic Society, Vol. 65, No. 9,

September 1982.

4 Kudryavtseva, N.L., Yakovleva, T.P., and Khodakov., G.S., "The Effect of Polisher Suspension

on the Micro-Hardness of Optical Glass", Soviet Journal of Optical Technology", Vol. 48, No.

12, December 1981.

5[ Tait, J.C., and Jensen, CD., "The Effect of Zn (II) Ion Adsorption on the Durability of

Sodium Borosilicate Glasses", Journal of Non-Crystalline Solids, Vol. 49, 1982 Special Issue.

6| Buckwalter, C.Q., and Pederson, L.R., "Inhibition of Nuclear Waste Glass Leaching by

Chemisorption", Journal of the American Ceramic Society, Vol. 65, No. 9, September 1982,

260
c

o

c

CO

c

o

0) Q.

400 600 800 1000 1200

Wavelength (cm)

Figure 1. Spectrophotometer transmission through two surfaces of 1 cm thick

BK-7 witness piece.

Figure 2. Loading an 80-cm diameter

crystal array apodizer

Figure 3. Removal of an 80-cm diameter

f/20 spatial filter lens

from the freon cleaning stage.

261
Witness pieces processed in developmental phase

Clean room

Process

area

Clean Mount

Immerse

Transfer Heat

Etch

Fiush Process

Freon rinse Vapor dry

Demount

Flush Cool

fransfe;

NSP surface (average for 15 samples) 12.5 Laser annealed NSP surface 23.3

Witness pieces processed in operational phase

NSP surface 12.2

Laser annealed NSP surface 25.0

46-cm aperture lens NSP surface 15.0

Note: 532 nm damage threshold values are similar to 1064 nm thresholds

Figure 4. Operating sequence of scaled up Neutrol Solution Process.

Figure 5. Damage threshold at 1064 nm,

1 nsec (J/cm^)

fit.-' *V3

Figure 6. Transmission electron micrograph of replica of neutral solution processed surface.

c

+4

400 600 800 1000 1200

Wavelength (nm)

Figure 7. Two surface transmission of 1-cm thick primary green part contaminated

with doctylphthal ate during 2 months storage

(B) and after ethyl alcohol rinse (A).

262
Figure 8. Removing a witness during

processing of a large optic.

Figure 9. Largest aperture processed

during development (24-cm diameter)

and operation (90-cm diameter).
25 50

Diameter (cm)

75 100

Figure 11. Diameter and quantity of parts treated to-date in neutral solution

scale-up operation .

400 600 800 1000 1200

Wavelength (nm)

Figure 12. Two surface transmission through 1-cm virqin material (A) material repolished without regrind (B) and material repolished and etched (C).

264
When asked whether aging of neutral-solution-processed surfaces would be a serious system problem,

the speaker replied that rinsing aged surfaces with ethanol had been effective in restoring high

transmission . The speaker was asked whether the rate of processing depended on the preparation of

the substrate surface, and replied that sample-to-sample variations in processing rate were observed , but site-to-site variations in processing rate of the surface of a particular large

optical element had not been observed.

265
Progress in Ultraviolet Damage Testing at Los Alamos

S. R. Foltyn, L. J. Jolin, and B. E. Newnam

University of California, Los Alamos National Laboratory,

Chemistry Division, Los Alamos, New Mexico 87545

A laser damage test facility is described which employs in-line beam diagnostics and

a real-time video monitoring system. Testing at three excimer laser wavelengths is accom- plished at 35 pps with a nominal 10-ns pulsewidth. The test spotsize (1/e2 diameter) is typically 0.6 mm. Samples have consisted primarily of multilayer dielectric reflectors

composed of a variety of materials. In particular, the best results - =3 J/cm2 at 248 nm, =5 J/cm2 at 308 nm, and =8 J/cm2 at 351 nm - have been obtained consistently for coatings

utilizing ThF 4 , AI2O3, or SC2O3 as the high-index component. Significant improvement has

resulted from the use of non-quarterwave designs at 248 and 308 nm. Also presented are preliminary 248 nm test results for LiF -- both single crystal and press-forged ~ and

KD*P.

Key words: dielectric reflectors; excimer laser; KD*P; LiF; non-quarterwave designs;

video monitoring.

1. Introduction

Although the basic principles behind, and operation of the Los Alamos uv damage test facility

[1] have remained unchanged over the past three years, we describe here some experimental improvements which provide capabilities for (1) remote video monitoring; (2) high throughput testing; and

(3) real-time beam diagnostics. A summary of test results for multilayer dielectric reflectors at

248, 308, and 351 nm is presented along with test results for LiF (press-forged and single crystal)

and KD*P.

2. Apparatus

The experimental layout is shown in figure 1. Propagating at about 300X the diffraction limit,

the beam is relayed from the laser room with a 2-m lens and is reflected twice at small angles

before being focused into the test plane at left center. A few parts per thousand are transmitted

through the dielectric turning mirrors and are used to power a fast vacuum photodiode and a silicon

photodiode array for temporal and spatial profiling, respectively. A 5° uncoated Suprasil wedge

provides somewhat larger beam fractions to a calorimeter which monitors average power and to a pinhole/pyroelectric detector combination used to monitor peak fluence. Hardware descriptions are given in [2]. While these diagnostics provide the capablity to observe and correct changes in beam

quality during testing, the documented beam characterization is always performed in the actual test

plane. For alignment purposes, a He-Ne beam is split into two paths, one being collinear with the

uv beam and the other off-axis. The location of the ultraviolet focal volume is pinpointed in space

by the intersection of these two beams.

Damage is observed in real-time with a long working-distance microscope [3] at about 100X.

Magnifications from 45X to 1000X are available. Because of the long working distance and the use of

transparent substrates, the microscope can be placed behind the sample and can thus be used at

normal incidence. An alternative scheme is being devised for use with opaque substrates. A high

resolution video system [4] attached to the microscope provides not only for operator safety but

allows the use of a video cassette recorder for preserving visual images of damage onset and growth

during' irradiation. One application of this system is described elsewhere in these proceedings [5].

In the future, it may be desirable to sync the video framing rate (30 Hz) to the laser so that an individual video frame can be recorded for each laser shot. Figure 2 is a photograph of the test

section showing, right to left, sample holder mounted on an X-Y stepper assembly, microscope modified for horizontal optical axis, and video camera.

266
3. Test Results

3.1. LiF

Test results for six LiF samples (three press forged, three single crystal) are shown in fig- ure 3. Probability of damage is determined experimentally by testing ten sites at each of several

fluence levels and plotting the number of damaging sites versus peak test fluence. The zero-percent

intercept of a linear regression fit is defined as the laser damage threshold. This is in contrast

to conventional notation in which threshold is approximately the fluence at which damage probability is 50%. The slope of a probability curve is determined by the relationship between beam size

and the density of defects which are responsible for damage. A steep slope correlates with high

defect density and vice versa. A more detailed discussion of this relationship and the threshold

definition is contained in [5].

The bands in figure 3 represent the observed variations in each type of sample. The cross- hatched area is for single crystals while the clear area was obtained for press-forged samples.

From the analysis above, it can be concluded that press forging does not alter the defects themselves since thresholds are no different than for single crystal samples, but that the average

density of defects has been increased by forging.

Damage in all samples was observed primarily in the bulk material. In that the test beam was focused on the front surface, beam divergence in the sample resulted in a fluence uncertainty of up to 25%. However, a change in the opposite direction results if self focusing is present. An initial analysis indicates that the two effects cancel one another and that the beam is effectively

collimated in the volume of interest.

3.2. Multilayer Dielectric Reflectors

Table 1 is a summary of test results for a variety of reflector designs at three wavelengths.

Peak fluence thresholds are as defined in the previous subsection and represent average values over

no fewer than three samples. Wavelength scaling is evident; however, the data base is not sufficient to allow quantification of trends.

Table 1. Test Results for 99% Reflectors

HR Coating

Threshold at 248 nm (J/cm2

) Threshold at 308 nm (J/cm2) Threshold at 351 nm (J/cm2)

ThF

4/Na3AlFg 3.3 2.8

Al

2 0

3 /Si0

2 3.0 5.2 8.5

Sc20

3 /Si02, MgF

2 2.9 5.4

Same as above, non-qw 4.2 6.8

Figure 4 illustrates the ability to improve 308-nm damage resistance by shifting the standing- wave field peak out of the high-index/low-index interface region [6,7]. This is accomplished by the

use of outermost layers of nonquarterwave optical thickness. The results shown are for three layerthickness variations plus a standard quarterwave design for comparison: The use of in-chamber shut- ter masking allowed all four designs to be deposited [8] in a single run. Thresholds are summarized

in table 2.

267
Table 2. 308 nm Thresholds for non-QW Reflector Designs

Design Threshold

1

- Field suppressed in MgF

2 5.2 J/cm2

2

- All quarterwave 5.4 J/cm2

3

- Field suppressed in SC2O2 (one pair) 6.6 J/cm2

4

- Field suppressed in SC2O.3 (two pair) 6.8 J/cm2

For a detailed analysis of a similar test at 248 nm, see [9].

3.3. KD*P

A somewhat different damage behavior was observed during testing of four KD*P crystals

(fig. 5). Depending upon fluence level, damage was delayed by a variable amount: Several thousand

shots were required at a few J/cm2 while, at 8-9 J/cm2

, damage was produced in only a few hundred

shots. From a physical point of view, threshold is the asymptotic value of the solid curve; how- ever, in a practical sense, a system operating at a very low pulse rate could run at much higher

levels for an extended period without experiencing damage problems.

4. Conclusions

We have described some recent improvements to an existing ultraviolet laser damage test facility which center around a video monitoring system and in-line beam diagnostics. Test results for

state-of-the-art reflectors are presented as well as preliminary results for Li F and KD*P.

5. References

[1] Foltyn, S. R. ; Newnam, B. E. ; Jolin, L. J. Laser damage results and analyses for uv reflectors

under multiple-shot irradiation. Proceedings of the 13th Boulder Damage Symposium, November,

1981 (to be published).

[2] ITT 4014 vacuum photodiode; Reticon 256G silicon photodiode array (25 urn resolution); Scientech

36-0001 surface absorbing calorimeter; Molectron J3-05 pyroelectric detector behind a 50 pm

diameter pinhole.

[3] Bausch & Lomb "MicroZoom" microscope.

[4] RCA TC1005/01 camera; Conrac SNA-14C monitor.

[5] Foltyn, S. R. Spotsize effects in laser damage testing. Elsewhere in these proceedings.

[6] Gill, D. H. ; Newnam, B. E. ; McLeod, J. Use of nonquarterwave designs to increase the damage

resistance of reflectors at 532 and 1064 nanometers. Nat. Bur. Stand. (U.S.) Spec. Publ. 509;

1977 December, p. 260.

[7] Apfel , J. H. Optical coating design with reduced electric field intensity. App. Opt. 16(7)

1880-1885; 1977.

[8] These reflectors were designed and deposited by Optical Coating Laboratories, Inc., Santa Rosa,

CA.

[9] Newnam, B. E. ; Foltyn, S. R. ; Jolin, L. J.; Carniglia, C. K. Multiple-shot ultraviolet laser

damage resistance of nonquarterwave reflector designs for 248 nm. Proceedings of the 13th

Boulder Damage Symposium, November, 1981 (to be published).

268
MULTIPLE- SHOT LASER

DAMAGE TEST FACILITY

REAL-TIME VIDEO MONITORING

• HIGH RESOLUTION CAMERA

AND MONITOR

• VIDEO CASSETTE RECORDER

• 1/100 th SECOND CLOCK

= «=OELECTR:c DETEC"=

VIDEO CAMERA

MICROSCOPE Jul

TEST PLANE -

VACUUM

\ PHOTOOIOOE i irPELLICLE |

' SHUTTER

IN-LINE DIAGNOSTICS

• PEAK FLUENCE

• SPATIAL PROFILE

.• AVERAGE POWER

• PULSEWIDTH

Figure 1. Line drawing of the Los Alamos laser damage test facility.

269
Photograph of test section. Sample holder on X-Y steppers is located

a few cm from objective of microscope (center). Beam enters from

right.

270
PEAK TEST FLUENCE (J/cm 2

)

Figure 3. Test results for Li F . The cross-hatched region is for single crystal

samples while press-forged material gave results represented by the

clear area. Slopes of probability curves indicate a higher average

defect density for the press-forged samples.

271
Figure 4. Probability curves for 380 nm reflectors which are identical except

for the thickness of the outer four layers. A 25% improvement was realized by suppressing the peak SW field in the high index material.

272
rOO

uj 6

QO

QC

CLg 4

Q

UJ

QC

3

QJ °

CSL CO

I-O

X

co 2

LL. o

UJm

LASER DAMAGE TEST

RESULTS FOR FOUR

KD*P CRYSTALS

X= 248 nm, t = 5 ns SURFACE DAMAGE

CRYSTAL NUMBER

- 1315

1316

1317

1318

2 3 4 5 6 7 8

PEAK TEST FLUENCE (J/cm2

)

10

Figure 5. Delayed damage in KD*P crystals.

273
Damage Thresholds of Thin Film Materials and High Reflectors at 243 nm*

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

Lawrence Livermore National Laboratory

Livermore, CA 94550

C. K. Carniglia, T. Tuttle Hart, and T. L. Lichtenstein

Optical Coating Laboratory, Inc. Santa Rosa, CA 95403

Twenty-ns, 248-nm KrF laser pulses were used to measure laser damage thresholds for

halfwave-thick layers of 15 oxide and fluoride coating materials, and for high

reflectance coatings made with 13 combinations of these materials. The damage

thresholds of the reflectors and single-layer films were compared to measurements of

several properties of the halfwave-thick films to determine whether measurements of

these properties of single-layer films were useful for identifying materials for

fabrication of damage resistant coatings.

Keywords: absorption; environmental stability; index of refraction; halfwave-thick

layers; laser-induced damage; reflectors; stress; UV coating material.

1. Introduction

Because UV lasers are potentially useful for the investigation of inertial confinement

fusion, we have continued development of thin-film coatings for KrF lasers. In previous

experiments we tested a large number of coatings made from a limited set of thin-film coating

materials, principally Si 0^, MgF2 and SC2O3, which were selected primarily because they

are highly transmissive at 248 nm [1], [2]. Lengthy studies of a few materials allow optimization

of the deposition parameters for these materials, and provide a sensibly large data base for

evaluation of coatings made from the materials. However, since time and expense limit the number

of such studies that can be performed, there is the possibility that potentially superior

materials will remain unidentified. It would, therefore, be advantageous to discover

relationships between damage thresholds of coatings and the physical properties of the coating

materials which could be used to guide material selection. In this paper we describe a material

survey whose aim was to establish such relationships.

There are two general formats for survey studies of thin-film materials. An evaluation of a large number thin-film materials can be made by studying either single-layer films of the

materials or by studying properties of multilayer coatings made from the materials. Testing of

halfwave-thick single layers is attractive because, in principle, the materials can be

individually evaluated and because internal electric fields experienced by halfwave films during

laser damage testing do not depend strongly on the refractive index of the material. The physical

properties of single-layer films may, however, be affected by contact of the film with both the

substrate and the atmosphere. Testing highly reflective multilayer coatings made of the materials

to be evaluated eliminates the influence of the substrate, and provides direct information about a useful coating design. Also, environmental degradation or contamination of a given film layer

could be prevented by overcoating the reflector with a film of a more stable material. The

disadvantages of using multilayer reflectors are that the materials are tested in combinations,

rather than individually, and that given a number of materials, there are many potentially

interesting material combinations.

It is also important to note that a survey study of thin film materials cannot be expected to provide an unambiguous ranking of the relative worth of many coating materials. A survey, by

necessity, includes only a few films of each material, but characteristics of thin films are reproducible only for those materials for which deposition has been optimized, which is the task one seeks to avoid by conducting a survey. A survey may reveal general relationships between

laser damage thresholds and measured physical properties of films, and these relationships would,

in turn, indicate which materials should be studied more carefully.

*Work performed under the auspices of the U.S. Department of Energy by the Lawrence Livermore

National Laboratory under contract W-7405-ENG-48.

274
We decided to evaluate the effectiveness of material surveys Dy testing both halfwave-thick

layers of 15 oxide and fluoride materials, and highly reflective quarterwave stack reflectors made

from 13 combinations of the same materials. The coatings were made at Optical Coating Laboratory,

Inc. (OCLI) by electron-beam evaporation of the materials onto fused silica substrates that had

been bowl-feed polished at OCLI. For single-layer films, measurements of the refractive index,

absorption, stress, environmental stability and the position of the UV band edge were made. For both films and high reflector (HR) coatings, the threshold for damage by 20-ns, 248-nm KrF laser

pulses was measured at Lawrence Livermore National Laboratories (LLNL). Results of these

measurements and a discussion of possible correlations between the thresholds of the reflective

coatings and physical parameters of the materials are presented.

2. Experimental Procedure

Damage thresholds were measured at LLNL with 20-ns, 248-nm pulses generated by a discharge-pumped KrF laser. At the surface of the sample, the beam was 1.5 mm in diameter at the

e"2 intensity level, but the intensity distribution was nonuniform. The highest fluences in the

beam occurred at isolated maxima, where fluence was uniform to within + 5% over areas not less

than 0.1 mm in diameter. For each shot, the beam was photographed with Kodak 1-Z spectroscopic

plates, and the intensity distribution was determined by densitometry. Peak fluence for each shot

was computed by numerically integrating the intensity distribution and normalizing the integral to agree with the measured pulse energy.

The samples were mounted on a rotating stage which allowed the irradiated test site to be

moved into the field of view of a Nomarski microscope. Test sites were photographed before and

after irradiation at a magnification of 420. The test sites were also inspected visually using

either Nomarski or bright-field microscopy, and under intense white light illumination using the

unaided eye. Damage was defined to be a permanent surface alteration that was detectable by any

of these inspection techniques.

Each test site was irradiated once. The average number of sites tested per sample was

seven. Damage was defined as the average of the lowest fluence that caused damage and the highest

fluence which did not cause damage. A detailed description of the laser system used in these

tests and of our experimental procedure has been previously presented [1].

Other parameters of the coatings were measured at OCLI. Transmittance and reflectance were measured with a Cary 17-DX spectrophotometer. Index of refraction for the individual coating

materials was calculated from the measured reflectance and transmittance of thick single-layer

films. The absorption coefficient of a film material was measured by coating a halfwave-thick

layer of the material on a high reflector and measuring the decrease in reflectance. Film stress

was determined by interferometric measurements of the stress-induced flexure of coated substrates

0.38 mm in thickness. The absorption edge was defined to be the wavelength at which the

transmission of a coating with a 1.5-um optical thickness was 50%. Since tranmission

measurements could not be made at wavelengths below 200 nm, the band edge could not be measured in some films. These materials were separated into two groups: those with some measurable

absorption at 200 nm (absorption edge less than 200 nm) and those with no detectable absorption at 200 nm (band edge much less than 200 nm).

3. Single-Layer Films

Halfwave-thick layers of 15 oxide and fluoride films were fabricated by OCLI using

electron-beam evaporation of the materials onto fused silica substrates that had been bowl-feed

polished by OCLI. The films of most of these materials had generally good cosmetic appearance,

and contained only isolated defects with typical dimensions of 1 urn. There were, however, some exceptions. Films of YF3 were mosaic arrays of 10-um-sized areas presumably caused by local

stress fracture. Some areas on Na3AlF5 films were clean, but other areas had a streaked

appearance. Films of both ZrO^ and NaF had a hazy, fogged appearance.

We measured laser damage threshold for two halfwave-thick layers of each material. The

damage thresholds are given in figure 1. The morphology of damage in each film was also

recorded. Three general types of damage were observed, and the nature of the damage correlated

with the measured damage threshold. In low-threshold films, damage appeared as micropits which

were aligned in either straight or curved rows that resembled polishing sleeks. In medium-threshold films, damage usually was seen as a spatially uniform area (shaped like the

incident laser beam) which was visible by either bright-field or Nomarski microscopy. This

implies alteration of both surface texture, which effects white-light visibility, and surface

height, the parameter observed in Nomarski microscopy. The altered areas also contained a few

pits, which were typically 1 urn in diameter. High-threshold films, such as SiO? and ThF4,

had damage morphologies much like the damage that is observed on front surfaces of bare polished

275
fused silica: pits 1 urn in diameter which are surrounded by halos at high fluences.

The damage thresholds ranged from less than 1 J/cm? in ZrO?, which is comparable to the

damage fluence for some metallic films, to 25 J/cm2 in ThF4, which is about twice the

front-surface threshold of bare polished fused silica [3]. The latter is an unusual result for

which we have no satisfactory explanation; thresholds of coated surfaces rarely exceed those of

bare surfaces. However, the experimental observation we are reporting is straightforward. When

we use the test procedure described above, we find 1) thresholds ranging from 9 to 15 J/cm^ for

front surfaces of bare polished silica substrates [3], 2) thresholds exceeding 20 J/cm? for some silica surfaces coated with low-index films, and 3) the same morphology of damage for both types

of surfaces. It is also interesting to note that substrate structure appeared to influence damage

morphology on low-threshold films, but not on moderate- or high-threshold films. Absorption by

impurities trapped in polishing scratches could have caused this damage, but all substrates were similarly polished, so scratches on substrates irradiated at higher fluences should also have

damaged. The implication is, therefore, that in low-threshold films, the film material itself had

been influenced by the presence of the polishing sleek.

30

25

20

-oof

£ 15

10

II

Zr0

2 Na

3 AIF6 Th02 Y

2 03 Hf0

2 Sc

2 03 MgO Al,02~3 YF, NaF LiF MgF, LaF, SiO„ ThF.

Figure 1. Laser damage thresholds (20 ns, 248 nm) of halfwave-thick films.

Next, consider the comparison of the film thresholds to physical properties of the films. In figure 2, four properties of the films are plotted as a function of the average threshold for each

material. Thresholds were largest in the films with low refractive index, which (excepting

S i 0 2 ) were all fluorides. The high-index films (which were all oxides), and the films of

Na3AlF6 (which had poor physical properties), had low thresholds. Our results are generally

similar to those of Newnam and Gill who used 22-ns, 266-nm pulses to test single layers of six

oxide materials and three fluoride materials [4], and those of Walker, et.al., who tested films of

six oxides and three fluorides with 15-ns, 266-nm pulses [5], Among these three studies, both the

threshold ranking of individual film materials, and the absolute values of thresholds reported for

a given film material, vary considerably, suggesting that it is difficult to establish true

comparisons of materials in thin-film form. However, the general correlation between high

thresholds and low refractive indices is present in all three studies. This correlation was first

observed by Turner [6], and later developed as a threshold scaling law by Bettis, et. al . [7],

The scaling law predicts some of the results of the three UV damage studies. However, the scaling

rule is based on the assumptions that damage is caused by electron avalanche and that thresholds

should scale according to the strength of the local electric field. The law would not be

applicable if linear absorption were the dominant mechanism for damage in UV film materials.

276
Na3AIF6 ZrOx

C

E

-o

03C

o

SI<

a

(A

2.4

2.2

2.0

1.8

1.6

1.4

0.012

0.010

0.008

0.006

0.004

0.002

240

230

220

210

200

+20

0

-20

-40

-60

-80

Y2°3

ThO,

r Hf02 Sc2°3

MgO

J_

r AI 2 03

,-YF

i

MgF?n

3

IMaF

i I

i

r LaF 3

r Si02 £TMF

4

i i i i i

-- r

--

Low n materials 9

High n materials

• T «

©

j i'ii

o

• • ••

i i i i i i i

i i i i i i 1

1

i i i i i i i *i 0.3 0.4 0.6 0.8 1 2 3 4 6 8 10 20 30

Damage threshold (J/cm 2

)

Figure 2. Index of refraction, absorption, position of the UV band edge and film stress for 15 UV coating materials plotted as a function of the average of the damage thresholds measured

on two halfwave-thick samples of each material.

277
We also found that thresholds varied systematically with film absorption and with the

spectral position of the UV band edge. Highly absorbing films had low thresholds. All low-absorption films had UV band edges below 200 nm, so absorption and position of the band edge were closely related. In fact, since the oxides were the materials which had band edges in the near UV, high absorption, and also high refractive index, it is possible that the apparent

correlation between UV damage thresholds and index is accidental. Again, there were exceptions.

NaF was very absorptive, but had a moderately large threshold, and scattering losses in both

Na3AlF5 and NaF were so large that we could not measure the positions of the band edge.

The only previous study of the correlation between UV thresholds and film absorption

concluded that linear absorption was responsible for damage induced by 266-nm 22-ns pulses in Zr02 [4].

Finally, we found no correlation between film stress and damage threshold, and a possible

correlation between cosmetic appearance and threshold. The Zr0 2 and NaF films had a foggy

appearance, and the film of Na3AlFs exhibited streaked areas. The threshold of the Zr02 film was less than that of any other film tested, and these two fluoride films had thresholds less

those of other materials with comparable properties.

Although the material characteristic responsible for damage in a given material is not known,

trends in the entire set of data identify the characteristics that are associated with good

resistance to damage by 248-nm irradiation. The film material should have low refractive index,

low absorption, band edge located at a wavelength well below 248-nm, good cosmetic quality, and

good environmental stability when exposed to high humidity. For single-layer films stress did not influence thresholds.

4. High Reflectors

Reflectors for use at 248 nm were fabricated by OCLI from 13 high-index/low-index

combinations of the materials that were studied as single-layer films. Four reflectors of each

type were made, two in each of two coating runs. Each reflector had a minimum of 15 quarterwave-thick layers and was overcoated with a halfwave-thick layer of the low-index material

used in the reflector stack. The coatings were deposited onto conventionally polished substrates

of BK-7.

10

8 -

2 -

Zr0

2 YF,

Zr02 MgF2

Th02 MgF2MgO

MgF2

Al

2 03 NaF Y2°3

MgFHf0

2 ThF.

Sc2 03 LaF, Hf02 SiO,

Al2°3

Na3 AIF

6

Sc2 03 YF,Sc2°3

MgFMgO

LiF

Figure 3. Laser damage thresholds (20 ns, 248 nm) of quarterwave-stack multilayer highly

reflecting coatings made from 13 combinations of high-index and low-index materials,

278
The cosmetic appearance of most reflectors was usually like that of halfwave-thick films of

its constituent materials, but the surface density of defects was slightly higher in reflectors.

All reflectors with Zr02 and NaF had a foggy granular appearance, and reflectors with YF3 had

stress fractures. Other unusual reflectors included the following: those with Hf02 had many

cosmetic defects and sections of missing coating; those with Na3AlF6 were densely fogged, and

had large circular voids; the ThC^/MgF^ reflectors appeared scuffed and nonuniform.

Damage thresholds of these reflectors, shown in figure 3, fell into three groups: below 3 J/cm2, between 3 and 5 J/cm2, and greater than 5 J/cm2 . Reflectors with the lowest

thresholds were fabricated with high-index materials that had low thresholds when tested as single

layers, ZrC>2 and ThC>2. In contrast, the low-index material Na3AlF5 had a low threshold

when tested as a single layer, but when combined with Al 2O3 , produced a reflector with

moderate threshold. Reflectors of Sc203/MgF2 and MgO/LiF had the highest thresholds, the

median values being 5.6 and 7.0 J/cm^, respectively. It is interesting that these largest

reflector thresholds were considerably greater than the thresholds of single layers of the

high-index materials, and considerably less than the highest thresholds observed in single layers

of the low-index materials.

Rank

High-n

material Low-n

material Index

n Abs.k Abs. edge Stress Threshold Cosmetic Environ- mental

12 MgO LiF MgF 2 • O • • O 9 O

123

Sc2 03 MgF 2 YF 3 LaF3 • O O X X 9

12

Al 2 03 Na3 AIF6 NaF 9 9 • X • 9

12

Hf02 Si0

2 ThF 4 • O 0 • 0 9 9

12

Zr0 2 MgF 2 YF3 • 9 9 O • . • 9

12

34

5

Sc2°3 Y 2 03 MgO

Th02 Zr02

MgF 2 X • 0 X 0 • X

12Sc2 03 Zr0 2

YF3 G • 0 0 0 • 9

Figure 4. Comparison of threshold ranking (high to low) of reflectors to ranking of coating

materials with respect to increasing refractive index, absorption, position of band

edge, and stress, and with respect to decreasing threshold cf halfwave-thick films,

cosmetic appearance and environmental stability. Reflectors are grouped into subsets

containing a common material, and within each subset, are ranked by decreasing

threshold. For each subset, we indicate whether each possible ranking of the varied

materials agreed (•) or disagreed (X) with the threshold ranking. In some instances

there were either minor disagreements (0) or the material ranking could not be

established (?).

A major purpose of this study was to determine whether the threshold ranking of reflectors

could have been predicted from either the threshold ranking of single layers of the constituent

materials, or the ranking of the constituent materials in terms of their physical properties. A comparison between these rankings is shown in figure 4. The lefthand columns of Fig. 4 list the

pairs of materials from which reflectors were made. The list is organized so that all reflectors

containing a given material appear as a group. Groups of reflectors with a common high-index

material are at the top of the figure; groups with a common low-index material are at the bottom.

Inside a given group, the reflectors are ranked in order of decreasing threshold. In the

279
righthand part of the figure, we use four symbols to indicate whether the ranking of thresholds in

a group of reflectors agreed (•) or disagreed (X) with a particular ranking for the materials that were varied in that group. The open circle indicates an insignificant inconsistency between

threshold rankings of reflectors and a material ranking based on only a slight difference in physical parameters or reflector thresholds. The symbol (?) indicates no material ranking was possible because material parameters were equal or unknown.

As an example, consider the set of three reflectors that contained SC2O3 as the

high-index material and either MgF?, YF3 or LaF3 as the low-index material. Threshold

ranking (high to low) for these reflectors agreed with material rankings based on increasing

refractive index (n = 1.43 for MgF2, n = 1.54 for YF3, n = 1.59 for LaF3) and increasing

absorption (k < .001 for MgF£, k < .001 for YF3, k = .001 for LaF3); disagreed only

slightly with the ranking based on increasing magnitude of film stress (-50 Kpsi for MgF2, -49

Kpsi for YF3, -91 Kpsi for LaFi); and strongly disagreed with the rankings based on decreasing

damage thresholds of the film (14.4 J/crn? for MgF2, 4.6 J/cm? for YF3, 17.7 J/cm2 for

LaF3) and decreasing cosmetic quality (the YF3 films had stress fractures). Films of MgF2,

YF3 and LaF3 had comparable environmental stability and comparable positions for their LIV band

edges, so rankings could not be established on the basis of these properties.

The entire set of data in figure 4 shows two general trends. First, when various low-index

films were used with a given high-index film (top half of figure), the highest reflector

thresholds were correlated with use of the low-index material with lowest refractive index.

Absorption, position of the UV band edge, and stress were reasonably good indicators of the merit

of the low-index materials, whereas thresholds of single-layer films and cosmetic appearance were sometimes poor selection criteria. Attempts to correlate environmental stability with reflector

thresholds were inconclusive. Second, when various high-index materials were used with a given

low-index material, optimum reflector thresholds were correlated with minimal absorption and best

cosmetic appearance in the high-index film. Of these, we suspect that absorption is the strongest

parameter. There was also reasonable correlation between reflector thresholds and either position

of the UV band edge or threshold of single-layer films of high-index materials. The refractive

index, stress and environmental stability in high-index materials did not correlate with reflector

thresholds

.

5. Summary

We measured laser damage thresholds for halfwave-thick layers of 15 oxide and fluoride

materials, and for HR coatings made from 13 combinations of these materials. Several physical

properties of the single-layer films were also measured.

Of the HR coatings tested, those made of Sc203/MgF2 and of MgO/LiF had the highest

median thresholds, which were, respectively, 5.6 and 7.0 J/cm2. The thresholds of single-layer

films ranged from less than 1 J/cm? for Zr02 to more than 20 J/cm? for Si O2 and ThF4«

Thresholds were greatest in films of materials for which refractive index and absorption were

small, and the position of the UV edge was well below 248 nm. Thresholds of single-layers did not correlate with film stress.

Measurements of thresholds for halfwave-thick single layer films did not identify the

materials from which the reflectors with greatest threshold were fabricated. The survey was moderately successful in identifying material characteristics which effect thresholds of

reflectors, and identified a promising material combination, MgO/LiF, which we had not previously

studied.

In reflectors made of pairs of materials having a common high-index material, and various low

index materials, the largest thresholds for the reflectors correlated with use of the low-index

material with lowest refractive index. In reflectors with a common low-index material, thresholds

correlated with use of the high-index material with the lowest absorption.

6. Acknowledgements

We appreciate the assistance of Steven Brown, Julius Goldhar, John Lutz and Ross Rapoport in maintaining the damage facility, and of Diane Kelly in preparation of the manuscript.

280
References

Rainer, F; Lowdermi 1 k , W. H. ; Mil arn , D. ; Hart, T. T. ; Lichtenstein, T. L. ; and Carniglia, C. K., Scandium oxide coatings for high-power UV laser applications. Applied Optics 21(20):

3685-3688; 1982 October 15.

Hart, T. T. ; Lichtenstein, T. L. ; Carniglia, C. K.; and Rainer, F., Effects of undercoats and

overcoats on damage thresholds of 248 nm coatings, to be published in the report of the 1981

Boulder Conference.

Rainer, F.; Lowdermi lk, W. H. ; and Milam, D., Bulk and surface damage thresholds of crystals

and glasses at 248 nm. Elsewhere in these proceedings.

Newnam, B. E.; and Gill, D. H., Ultraviolet damage resistance of laser coatings. Nat. Bur. Stand. (U.S.) Spec. Publ. 541: 190-201; 1978 December.

Walker, T. W.; Guenther, A. H. ; Fray, C. G. ; and Nielson, P., Pulsed damage thresholds of

fluoride and oxide thin films from 0.26 um to 1.06 urn. Nat. Bur. Stand. (U.S.) Spec.

Publ. 568: 405-416; 1980 July.

Turner, A. F., Ruby laser damage thresholds in evaporated thin films and multilayer

coatings. Nat. Bur. Stand. (U.S.) Spec. Publ. 356: 119-123; 1971 November.

Bettis, J. R.; Guenther, A. H. ; and Glass, A. J., The refractive index dependence of pulsed

laser induced damage. Nat. Bur. Stand. (U.S.) Spec. Publ. 414: 214-218; 1974 December.

281
LASER DAMAGE THRESHOLDS OF OPTICAL COATINGS AT 351 nm

J. A. Abate and R. Roides

B.P. Oil, Inc. Laboratory for Laser Energetics

University of Rochester

250 East River Road

Rochester, New York 14623

S. D. Jacobs, W. Piskorowski, and T. Chipp

Laboratory for Laser Energetics

University of Rochester

250 East River Road

Rochester, New York 14623

The damage thresholds of several optical coatings to 351 nm laser radiation will be presented

along with a discussion of the optical sytem used to make these measurements.

We have investigated the influence of substrate type, substrate polish - standard commercial vs. superpolished, post coating baking, and the effect of overcoats and undercoats on the observed damage

threshold. The coating materials investigated include Ta205/Si02, Sc203/Si02, Al203/MgF2> and

Y 2°3/Si02. We report measurements on both AR and HR dichroic coatings designed to operate at 1.054 ym and 351 nm.

Keywords: Coatings; laser damage; multilayer dielectric coating.

As the laser fusion community proceeds to frequency convert their Ndrglass lasers to shorter wavelengths,

the importance of the performance of optical coatings at these wavelengths becomes of great interest. Of

particular interest to the University of Rochester's Laboratory for Laser Energetics (LLE) is the performance of these coatings at the tripled frequency of Nd:glass, 351 nm. The fluence level at which the optical coatings can

transport the UV beam will have a major impact on the size, cost, and energy-on-target in any UV upgrade to the

24-beam laser at LLE, known as OMEGA.

Over the last several years a major effort has gone into measuring and attempting to improve the damage

thresholds of optical coatings at 1.06 um. Until recently, very little has been done at shorter wavelengths. What

has been done (1) has shown that the damage thresholds are considerably lower than those at 1.06 um. State-ofthe-art coatings for 1.06 ym have damage thresholds (2) for 1 nsec pulses of 6 to 10 J/cm 2

, for high reflectors

(HR) and 4 to 7 J/cm 2 for anti-reflectors (AR). The results reported here and elsewhere (3), show damage

thresholds with 351 nm light, for 1 nsec and 400 psec pulses rarely over 2.5 J/cm 2 for standard optical thin films.

A diagram of the apparatus used to measure the damage thresholds is shown in figure 1. Damage testing is done by irradiating a given sample and characterizing the incident light pulse's energy, spatial intensity

distribution, and pulse width. These parameters change from shot to shot, so it is imperative that they be

accurately determined for each and every shot.

The LLE UV damage tester uses the output of the Glass Development Laser (GDL) after the 40 mm rod

amplifier. This gives typically an output of 1.6 J of 1.06 um light and the laser system can be fired every 10 minutes. This output is directed to a set of KDP cyrstals, operated in a similar manner to Seka, et al. (4), to produce 351 nm light. The efficiency of conversion is 50%. The residual 1.06 ym and 0.53 ym light from the

tripling process is removed by a dichroic mirror and the 0.35 ym light is then focused onto the coating sample. A

half-wave plate and a set of two dielectric polarizers are used to throttle the amount of energy that is delivered onto the sample. This keeps the loading on the KDP crystals the same for all intensities on the sample and, as a

result, keeps the beam profile and pulse width relatively constant during the tests. The beam is 5 mm in diameter

Numbers within parentheses indicate the literature references at the end of this paper.

282
when it strikes the sample.

An uncoated wedge picks off a portion of the beam and directs it to a set of diagnostics to measure the

energy, pulse width, and intensity profile of the beam for each shot. The intensity profile is recorded both on film

and solid state TV camera. The TV image is recorded in a mini-computer with the aid of a high-speed video

digitizer. Figure 2 shows a block diagram of a data collection system. The maxiumum energy density is determined from this spatial distribution and the total energy on the sample. This analysis takes approximately 5 minutes. The determination of coating damage is done by taking photo-micrographs of the portion of the sample

that is irradiated, both before and after the shot. We have investigated the effect of a number of different parameters on the performance of several optical

coatings to 351 nm laser irradiation. In figures 3 through 7, the results of tests on some 82 samples are presented.

In each of these figures we have indicated the coating vendor; the pulse width the tests were performed at; the

coating type and operative angle of incidence; the substrate type (P - Pyrex, F - fused silica, or BK - BK-7) and any

substrate vendor information, if available; the method of polishing used on the substrate whether standard

commercial polish or in several samples in figure 6, a super-polish performed by OCLI; the post-coating baking

conditions, if any; and the coating materials and design, (the coating design is indicated schematically in terms of quarter-wave layers of the individual materials. For example, in figure 3,] 9 <TS)T2S indicates that on the

substrate, "1 ", there are 9 alternating quarter waves layers of Ta^^, "T", and SiC>2, "S" followed by a quarter wave of Ta205 and a half wave layer of Si02 at the surface). The accuracy to which the damage threshold was

determined for each sample is indicated by the width of the box representing that sample. The mean threshold for similar groups of samples is both plotted as a dashed line and indicated at the right-hand side of the figure.

Several trends become apparent on analysis of the data. The highest damage threshold that we have

measured has been an AR-treatment to BK-10 glass developed by Schott Glass. In this process, known as the

"Neutral Solution Process," (5) the surface of the glass is etched to a quarter wave depth by a weak acid, leaving a surface with a reflectivity of less than 0.5%. Previous surface treatments (6) to produce AR properties have not been very durable and could not be cleaned without damage to the surface. The Neutral Solution Process is cleanable and appears fairly durable. This plus a damage threshold of 9 J/cm 2 would make this coating look very

promising for future UV systems, except that all the glasses it can be applied to presently have been shown to be

solarized at 351 nm (7). It can be seen from figures 4 and 6 that thicker AR coatings tend to have a lower

threshold than thinner AR's. There is also evidence in figure 6 that the addition of a barrier layer between the

substrate and the coating shows some improvement in the damage threshold, even though the number of samples

tested is small. In terms of the materials used in the AR coatings tested, SC2O3 consistently showed higher

thresholds than those containing Y2O3. Also the SC2O3 film produced by OCLI showed a 30% improvement when

deposited on "super-polished" substrates rather than on substrates with a standard commercial polish. No

significant improvement was seen in those coatings that underwent a post coating bake.

The results in figures 4 and 5 show that coatings deposited on fused silica substrates tend to have lower

thresholds than coatings deposted on BK-7 or Pyrex. We did not see any noticable improvement in the HR coatings

tested when an half-wave overcoat was added to the coating design. We did, however, see an overall improvement

when the order of the layers was reversed in the design, for a group of dichroic HR coatings shown in figure 5 (ID

827 and 829).

In figure 8 we show phase contrast photo-micrographs at both 50X and 1280X representatives of the damage

morphology seen in the coatings we have tested. In general, the size of the characteristic HR damage is somewhat

smaller and usually more dense in nature than the AR damage at threshold. For both types of coatings near

threshold the damage takes the form of small individual pits or craters in the coatings.

What is striking about the data presented in figure 3-7 and data presented by other authors is that thresholds are consistently between 2-3 J/cm 2 and that the methods that seem to markedly effect the damage thresholds of optical coatings at 1.06 um seen to have little or no effect at 351 nm.

References

(1) Douton, T. F.; Smith, W. L. 1979 Symposium on materials for high power lasers. Nat. Bur. Stand. (U.S.) Spec.

Publ. 568; 1979.

Newnam, B. E.; Gill, D. H. 1978 Symposium on materials for high power lasers. Nat. Bur. Stand. (U.S.) Spec.

Publ. 541; 1978.

283
(2) Walker, T. W.; Guenther, A. H.; Nielsen, P. IEEE 3. Q. E. QE-17 10: 2041-2065, 1981 October.

Loudermilk, W. H.; Milam, D.; Rainer, F. 1979 Symposium on materials for high power lasers. Nat. Bur. Stand. (U.S.) Spec. Publ. 568; 1979.

(3) Rainer, F.; Deaton, T. F. Tech. Digest CLEO '81 - WH-2, Washington, DC, June 10-12, 1981.

Abate, 3. A. Tech. Digest CLEO'81, THB-10, Washington, DC, June 10-12, 1981.

(4) Seka, W.; Jacobs, S. D.; Rizzo, J. E.j Boni, R.; Craxton, S. Opt. Comm. 34: 469; 1980.

(5) Cook, L. M.;Lowdermilk, W. H.; Milam, D.; Surain, J. E. App. Opts. 21(8): 1482; 1982 April 15.

(6) Minot, M. J. 3. Opt. Soc. Am. 66: 515; 1975.

(7) Jacobs, S. D.; Abate, 3. A. Tech. Digest CLEO'81, THB-11, Washington, DC, June 10-12, 1981.

Acknowledgement

This work was partially supported by the U.S. Department of Energy Inertial Fusion Project under contract DE-AC08-80DP401 24 and by the Laser Fusion Feasibility Project at the Laboratory for Laser Energetics which has

the following sponsors: Empire State Electric Energy Research Corporation, General Electric Company, New York

State Energy Research and Development Authority, Northeast Utilities, The Standard Oil Company (Ohio), and the

University of Rochester. Such support does not imply endorsement of the content by any of the above parties.

284
Streak Camera

(AR @ 0.53 jim) (AR @ 1.06/jm,0.53/jm)

Figure 1. UV Damage Tester.

285
Data

Figure 2. Block diagram of data collection system.

286
Pulsewidth: 0.4 nsec

Type: 3wHR/45°

Substrate: P - Pyrex, FS - Fused Silica, BK - BK-7 (INRAD)

Polish: I I Commercial

Baking: 343° C for 2 hours

VENDOR

Materials Design

UR7LLE

TajOg, SI02

]9(TS)T2S

ID

731

730

PP

CZIFS

OFSl

i BK BK |

| IZZIP J i I i_

MEAN DAMAGE

THRESHOLD

1.7 J/cm2±0.4

12 J

3 4 DAMAGE THRESHOLD (J/cm2

)

Figure 3. 351 nm damage tests on highly reflecting coatings at 0.4 nsec.

VENDOR ID

Materials Design

Pulsewidth: 0.4 nsec

Type: 3cj AR/0°

Substrates: Fused Silica (Virgo), BK-10 (Schott)

Polish: I I Commercial

Baking: 343° C for 2 hours

or No Bake (NB) MEAN DAMAGE

THRESHOLD

Schott

Neutral Solution

UR/LLE

Y203, Si02 ]YS

892 B

B92 C

757

682

-ryA—

BK-10 C

BK-10 9.0J/cm2±0.4

FS

;qfs

^=3FS

CZDFS

some UV/ozone

cleaning

NB

C±)FS

; i

2.7J/cm2 ±0.8 J

DAMAGE THRESHOLD (J/cm2

)

Figure 4. 351 nm damage tests on antireflecting coatings at 0.4 nsec.

287
Ptiltewldth: 1.0 nwc

Type: 1u - 3w HR/45° and 57°

Substrate: P - Pyrex(ESCO), FS - Fused Silica, BK - BK-7(Virgo)

Polish: CZJ Commercial

Baking: 343° C for 2 hours

VENDOR

Materials Design

ID

UR/LLE

TajCVSIC-a

J4(3T3S)3T6(ST)

]4(3T3S)3T6(ST)2S

gni'ii

812

828

J4(3T3S)3T6(ST) 327

]6(TS)4(3T3S)3T2S 829

57°

45°

45°

45°

1 '

1

1

1 1 dp! p

of

IIP DP

4fs CZIFS, NB FSFS(iziFs

FS, NB

MEAN DAMAGE

THRESHOLD

2.5 J/crrT ± 0.3

2.0 J/cm ± 0.2

°5CZDFS

| DFS

i CFS

i OP, NB

BK, NB

CZIFS

[±jfs DFS

I OBK, NB

J i I i i 0 12 3 DAMAGE THRESHOLD (J/cm2

)

2.0 J/cm2 ± 0.3

2.6 J/cm2 ± 0.3

Figure 5. 351 nm damage tests on highly reflecting coatings

at 1.0 nsec.

288
PulaowklUc 1J) mac

Type: 1u - 3oj AR/0°

Substrate: Fusad SUtes (Vendor : V - Virgo , I - Inrad)

Polish: I I Commercial aVOCLl super polish Baking: 343° C for 2 hours

or No Bake (NB)

VENDOR

Material* Design

ID

UR/LLE

YjOj/SIOj 13*

]3Y 3S

•23

A 1 2O3 Barrier Layer | t4*

J2S3Y3S 853

)2S 3Y 3S S53

OCLI Inc

8cj03/SI03

Baking Unknown

All with 3X/2 Barrier Layer

icbs

IB07 as MM

an

IBM

CM,

MlcrocoaUng*. Inc AljOj/MgFj

Baking Unknown

Redone civ

S-23 bv

i O|V

ojv ov!

1 01

CTV

01

S

SI 3

*200

•201 •202 •203

CZJV ojy NB

Tdv

O v

r-.» av

, gv

' Bv

V

V

0 12 3 4 OAMAOK THRESHOLD (J/cm2

)

MEAN DAMAGE

THRESHOLD

1.6 J/cm* ±0.3

1J) J/cm2 ± 0.2

1.7 J/cm'± 0.2

1.9 J/cm2 ± 0.3

2.5 J/cm2 ± 0.3

2.1 J/cm2 ± 0.3

Figure 6. 351 nm damage tests on antireflecting

coatings at 1.0 nsec.

289
Pulsewidth: 1.0 nsec

Type: 1 co - 3cj AR/45"

Substrate: Fused Silica (Virgo)

Polish: (ZD Commercial

Baking: 343° C for 2 hours

VENDOR 9D

Materials Design

UR/LLE

AIO2, Y203 , SI02 830

J2A13Y3S

—

n

r

1 D

0

MEAN DAMAGE

THRESHOLD

2.1 J/cm* ± OA

i

' J

DAMAGE THRESHOLD (J/cm2

)

Figure 7. 351 nm damage tests on antirefleeting coatings at 1.0 nsee.

290
291
Raising the Surface Damage Threshold of

Neutral Solution Processed BK-7 by Pulse Laser Irradiation*

J. E. Swain, W. H. Lowdermilk and D. Milam

Lawrence Livermore National Laboratory

Livermore, CA 94550

Gradient-index antiref lecting surfaces can be produced by neutral solution

processing of bare polished surfaces on borosilicate glasses. These processed surfaces

have a median surface damage threshold of 12 J/cm? for 1-ns, 1064-nm pulses. The

surface damage thresholds can be increased to fluences as large as 25 J/cm2 by

irradiating the surface with 1064-nm wavelength pulses at fluences below the initial

threshold of 12 J/cm2 . For surfaces in their initial state, or surfaces, subjected to repetitive laser irrauiation, surface damage thresholds increased as the square root of

pulse duration over the range of pulse durations from 1-20 ns.

Key words: neutral solution processing, laser damage, laser conditioning, antiref lection

f i lms.

Schott Optical Glass Inc. recently developed a process for producing gradient-index

anti-ref lecting (AR) surfaces on borosilicate glass by leaching the glass in a nearly neutral

solution of Na2HAs04 in water [1]. This "neutral solution process" (NSP) was used to produce

AR surfaces on BK-7 borosilicate crown glass, wnich is widely used for optical components of

visible and near-infrared lasers. These NSP AR surfaces exhibited a median aamage threshold of 12 J/cm2 for 1-ns, 1064-nm pulses, which is more than twice the 5 J/cm2 median threshold of

si 1 ica-titania multilayer AR coatings [2]. Figure 1 compares these data. Recently we found that

the damage threshold of NSP AR surfaces could be increased by as mucn as a factor of two by

irradiating the surface with laser pulses at fluence levels slightly below the initial 12 JVcm?

threshold.

Our experimental procedure for measuring laser damage thresholds has been described [3]. The

procedure consists of irradiating a site on the surface of a sample with a pulsed Nd-glass laser

beam approxiinately 2.5 mm in diameter. We examine the irradiated site, both before and after the

laser shot, using a Nomarski microscope with magnification of 100. The sample is moved after each

shot so that each site is irradiated only once. The fluence of the laser pulse is increased with

each shot until damage occurs. The occurrence of damage is identified by a permanent change in the surface morphology observed using the Nomarski microscope. This measurement technique, in which each site on the surface is irradiated only once, is commonly called a "1-on-l" measurement.

The occurrence of damage in 1-on-l tests of NSP AR surfaces was always accompanied by

laser-induced emission of light. This light is probably recombination radiation from the plasma

generated at the surface by the intense laser irradiation. Light emission from NSP AR surfaces

was also frequently observed in the 1-on-l tests at fluence levels slightly below those required

to produce damage visible by microscopic examination. If, rather than moving the sample after

each shot, we continued to irradiate the same surface area at fluence levels below the 1-on-l

threshold, light emission ceased after a small number of shots. The required number of shots was typically 1-8 and depended on the laser pulse duration. We founo that after such treatment, the

laser fluence could be raised to a level well aoove the 1-on-l threshold before light emission or damage occurred. The threshold for damage determined by this procedure is called the "n-on-1"

damage threshold.

We have measured 1-on-l and n-on-1 thresholds for NSP AR surfaces using 1064-nm pulses at five pulse durations: 1,3,6,9 and 20 ns. Tnresholds for a particular sample are shown in figure

2. Tnresholds, either n-on-1 or 1-on-l were defined to be the average of the least fluence that

caused damage ano the greatest fluence that caused no damage. The uncertainty depended on the

separation of the two fluences which defined threshold, and on the uncertainty in fluence

measurements themselves, + 5%. In 1-on-l measurements, front and rear surface damage thresholds

were the same, as expected for a substrate with AR films on both surfaces. In the n-on-I measurements with 6,9 and 20-ns pulses, rear-surfc.ce thresholds were less than front-surface

thresholds, an ooservation for which we currently have no satifactory explanation. The

*Work performed under the auspices of the U.S. Department of Energy by the Lawrence Livermore

National Laboratory under contract W-7405-ENG-48

.

292
two straight lines drawn through the data have slope one-half. Therefore, over the range of pulse

durations from 1 to 20 ns, tnresholds of NSP surfaces scale in proportion to tne square-root of

pulse duration.

E

CDto

0)

_Q

4 6 8 10 12 14 16

Damage threshold — J/cm2

(1 ns, 1064 nm pulses)

Figure 1. The median one-on-ont surface damage threshold of neutral solution processed BK-7 is a factor of two higher than that of electron beam deposited dielectric AR coatings.

A possible mechanism for the increase in damage threshold measured in the n-on-1 tests is the

removal of absorbing impurities from the surface by subthreshold irradiation. We believe

absorption of energy from the laser pulse by particulate impurities is a common cause of damage to optical surfaces of transparent dielectric materials irradiated oy laser pulses with near-ir or shorter wavelengths [4]j When irradiated initially at high fluence, as in the 1 -on- 1 tests, these

particulates absorb laser energy and can reach temperatures above 1000°K, which is sufficient to induce surface damage by stress fracture or melting. When irradiated at lower fluence, the

particles reach lower temperatures, which may be sufficient to promote desorption, but

insufficient to produce damage. In some photographs of NSP AR surfaces taken with a scanning

electron microscope, particles are visible in areas that were not subjected to n-on-1 irradiation,

but absent from irradiated areas.

Another possible mechanism for removing particulates by low fluence irradiation is the

emission of electrons or ions from the surface [5]. The quantity of charge emitted may be

sufficient to substantially alter the static electric forces which normally bind particulate

impurities to the surface.

The heating and desorption mechanism is, however, more consistent with our observation that

"laser cleaning" is more easily accomplished using pulses of longer duration. When surfaces were irradiated with 20-ns pulses at subthreshold fluence, light emission usually ceased after one

shot. Using pulses of snorter duration, more shots were required to "clean" the surface

sufficiently that light emission ceased. At the shortest pulse duration used (1-ns), 6-8 shots were usually required. The maximum fluence at which the "cleaning" can be performed is that just

below the 1 -on- 1 damage threshold, which, as shown in figure 1, increases as the square root of

the pulse duration. Consequently, these results indicate that the total fluence accumulated over

all the shots required to "clean" the surface and affect an increase in threshold is roughly

constant.

293
In conclusion, we have found that the damage threshold of gradient-index AR surfaces of

neutral-solution-processea BK-7 glass can be increased by as much as a factor of two by pulsed

laser irradiation at fluence levels just below the 1-on-l damage threshold. Damage thresholds of

these AR surfaces increase with the square root of the laser pulse for pulse durations in the

range from 1 to 20 ns, and the improvement in threshold due to "laser cleaning" occurs over this

entire range of pulse duration.

We are grateful for technical assistance by S. E. Peluso and for assistance by Treva Janssen

and Diane Kelly in preparing the manuscript.

References

[1] Cook, L. M.; Mader, K. H.; and Schnabel, R. "Integral antiref lecting surfaces on silicate

glass", U.S. Patent Application, Ser. No. 309,149, October 6, 1982.

[2] Cook, L. M. ; Lowdermilk, W. H.; Milam, D. and Swain, J. E.; Applied Optics 2J_, 1482 (1982).

[3] Milam, D. SPIE, Vol. 140, 52-61 (1978).

[4] Lowdermilk, W. H. ano Milam, D. IEEE J. Quant. Elect. QE-17, 1881 (1981).

[5] Logothetis, E. M. and Hartman, P. L. Phys. Rev. J87, 460~Ti"969).

1 10 100

Laser pulse duration (nsec)

Figure 2. Pulse duration dependence of damage thresholds measured with 1064-nm laser pulses on the front surface and rear surface of a window with NSP AR surfaces. The straight

lines are square-root functions. "Laser cleaning" produced n-on-1 thresholds that were nearly twice as large as 1-on-l thresholds.

294
in response to several questions, the speaker stated: (1) the minimum reflectance obtained at a tuned frequency is about 0.1%, but reflectance less than 0.5% at both 532 nm and 1064 nm can be

obtained by tuning the minimum to a frequency between these wavelengths , (2) the surface

reflectance is highly uniform even on 90-cm parts, (3) the ratio of front surface to rear surface

laser-conditioned thresholds was greatest at 20-ns, where the ratio was about equal to the factor

of 1.4 expected for the bare glass, (4) the term "laser cleaning" was being used as a generic term,

and the threshold increase could be due to a variety of effects such as burning away of sharp

points, evaporation of absorbing residue from the pores, or melting and collapsing the pore

structure, (5) stress-related damage mechanisms have not been considered , (6) the surfaces made by

this process are stronger than those etched into phase-separated glass, but weaker than hard e-beam

deposited oxide films, (7) the process works on any borosilicate glass but no such glasses usable

in intense 355-nm beams are presently known, and (8) Livermore is producing the films for their own

use under license from Schott

.

295
DAMAGE THRESHOLD MEASUREMENTS AT 1.06 ym AT CENTRE D' ETUDES DE LIMEIL

D. Friart, M. Novaro, F, Garaude

Commissariat a l'Energie Atomique, Centre d'Etudes de Limeil

B.P. n° 27, 94190 Vi 1 leneuve-Saint-Georges , France

B. Geenen, A. Malherbes, J. Guerain

Societe MATRA, 93 avenue Victor Hugo

92502, Rueil-Malmaison, France

J.J. Rome, M.T. Buffenoir, P. Perbal

M.T.O. S.A., 11 rue Ampere

B.P. n° 6, Massy Cedex, France

M. Sou "Me

CILAS, Route de Nozay

91460 Marcoussis, France

We have measured the damage threshold of a serie of different optical components at 1,06 um, 3 ns. These optics were coated with anti-reflective, high reflectivity and

polarizing films. Damage thresholds for A.R. and H.R. were around 5 J/cm 2

.

Key words : laser induced damage, damage threshold, anti-reflective coatings, high reflectivity

coatings

.

1. Introduction

The Octal laser facility is an eight beam neodymium glass laser recently reconditionned and upgraded.

It is now in operation at the Centre d'Etudes de Limeil, France. Its realization required the development of new mul tidiel ectric coatings with high level damage thresholds.

Consequently during 1981 we developped a damage test facility at 1.06 gm wavelength. During

1981 and early 1982 we have tested almost 500 samples and 200 components manufactured by french

companies (MATRA, MT0 and CILAS).

The damage threshold of A.R. and H.R. coatings were of the order of 5 to 6 J/cm 2 on these samples.

2. Damage threshold determination

The damage tests at 1.06 ym were carried out with the laser facility described below and shown

in figure 1. First, a QUANTEL oscillator generated a 3 ns,'-3 mJ laser pulse. A 20 ps time resolution

streak camera was used to record 50 shots ; its appeared that there was no modulation in the pulse.

Second, the pulse was amplified trough a 6 mm followed by a 9 mm diameter road amplifiers with

small signal gain around 100.

Attenuation of the beam was realized with a half wave plate inserted between two polarizers.

At this level the beam fluence was around 0.5 J/cm 2

. We increased the fluence with a 2.5 m focal

length lens. Sample were positionned at 2 m from the lens in order to obtain a sufficiently large

296
irradiated area. At this point the beam diameter was 1.8 mm.

DAMAGE THRESHOLD TEST FACILITY AT 1.06 yM

PIN 10D

n

250 mJ % 1 f-

_ -7 Polarizers

^

lens f = 25 m

< 2 m

telescope

Amplifier 0 3 mm Amplifier 0 6 inm

3 ns , 3 TM^\^_

XA 1003

Monomode oscillator A

oscilloscope

MEASUREMENT OF THE PEAK FLUENCE

SEIN Recorder

FIGURE 1

297
Energy was measured with a PIN 10 D detector calibrated with a SCIENlECH calorimeter inserted

in the test beam. We used a combination of pinhole and detectors described schematically in figure 1 to measure

the peak fluence (J/cm 2 ). The pinhole had a 300 ym diameter and we assumed a constant fluence over

this surface which is small compared to the spot size of the beam (1.8 mm).

The samples were controled by using a one hundred times magnification telescope.

3. Results

Zr0

2 and Ti0

2 were used as high index materials for A.R. and H.R. coatings. Results obtained

on testing samples are presented in table 2, results on Octal components are presented in table 1.

Better testing reproductible ity and high damage thresholds were obtained on samples coated

with Ti0

2 /Si0

2

. A.R. coatings with a monolayer of MgF

2 showed a strong dependance of damage threshold with

substrate material and polishing on both samples and components.

For H.R. coatings we noticed lower results for large components than for small samples

(3.7 J/cm 2 compared to 6 J/cm 2 ). No satisfactory explanation has been found yet, but very few large

components have been coated with Ti0

2 /Si0

2

-

Table 1 - Damage Threshold of Components Table 2 - Damage Threshold of Samples

Coating Supplier Dimension of rompanents (mm| and substrate

material

Number of coatings

tested

Median

Damage Threshold

J/cm2

A R Monolayer

R - 1 %

MTO

i.io" FR4

18 5,3

172 EK7

24 5,8

90 Silicate Rod

24 5,1

130 BK7

16 4

MATRA

200 BK7

9 4 200

BK7 9 5,4

A R Zr02 /Si0

2 Multilayers

R = 0,2%

MTO

130 BK7

27 3,1

MATRA

200

BK7

9 3,7

120 Silicate Rod

24 5

CI LAS

64 Silicate Rod

30 3,8

Polarizers

ZrOj/SiOj MATRA

130 x 286

« BK7

30 4,9

70 x 150 BK7

5 4,5

H R

TiO /SiO

2 2 MATRA

200 15 3,7

200 7 2,6

200 7 3,8

200 9 3,7

200 9 4

Supplier Coating

Number

of Sample

Median

Damage Threshold

J/cm2

Standard Deviation

1 H R

6 4,9 2,5

MATRA

Zr0

2_/Si02 6 3,3 1.2

H R TiO /SiO

2 2

4 3,2 1,2

16 6,7 1,9

5 6 2

A RZi0

2 /Si0

2

18 4,2 1,2

26 3 1

A RTi0

2 /Si02

4 4,6 0,6

10 5 1,1

MTO

A R

2 Layers 20 3,2 0,6 A R MgF

2 10 4 0,7

A R MgF2 13 5,4 0,6

A R MgF2 20 5,3 0,6

298
In response to a question regarding the computation of fluence by measuring the energy transmitted

through an aperture , the speaker replied that measurements had been made with apertures of various

sizes, and the results were consistent , so he believed that his test beam was uniform over the

aperture used in fluence measurements. The speaker also stated that visual inspections by

microscopy were made before and after each shot to determine whether damage had occurred , and that

the damage threshold was defined to be the average of the lowest fluence causing damage and the

greatest fluence that caused no damage.

299
Impurities in ThF^ Optical Coatings*

Galen J. Hansen, Daines Lund, Alfred T. D'Agostino,

and Wi 1 ford N. Hansen

Utah State University

Logan, UT 84322

Impurities in T h F 4 films simulating HEL optical coatings were

studied using mult i -internal reflection IR spectroscopy. Water was the

main impurity found. as vapor and liquid was used to examine the

lability of the water impurity. Films were also heated in vacuum while

simultaneously observing their IR spectra to see at what temperature the

water was driven off, and whether the water would then be reabsorbed.

Results indicate the following: 1) the water was tightly bound within the

films, and was not free to come and go; 2) some films were more porous

than others; 3) CeF3 and other caps on top of films in this study did not

help eliminate moisture contaminaton; 4 ) films with no water or which

lost their water did not absorb any additional water; 5) films showed no moisture degradation over time. The general conclusion was that water

impurity in T h F 4 films originated with the T h F 4 starting material and did

not enter the films during or after film preparation.

Key words: deuterium oxide; HEL optical coatings; impurities; optical

coatings; reflection spectroscopy in thin films; thorium fluoride; water

contami nati on

.

1. Introducti on Impurities can be a significant problem in high energy laser optical

codtings. They can directly absorb the radiation to promote laser damage, and they

can cause degredation of the coating otherwise. In the case of thorium fluoride

films, moisture is especially troublesome because it absorbs at desired laser

operational wavelengths. To date it shows a stubborn tendency to be there,

sometimes in large amounts.

In this work, a number of thorium fluoride films were examined for impurity

content. Water was the main impurity found. The amount and behavior of the

unwanted moisture was studied in varous films. It was hoped to determine the

origin and nature of the impurities, and whether pure durable T h F 4 films can be

made.

Multiple internal reflection IR spectroscopy is a very sensitive method for

in-situ non-destructive observation of individual molecular species. Using this

technique, monolayer amounts of impurity molecules can be identified and

quantified [1]. This spectroscopy was the main technique used in this

investigation. A typical set-up is that shown in figure 1. Sensitivity is

directly proportional to the number of reflections (N) at the sample, i.e.

absorbance K N, and is also proportional to the electromagnetic field squared, i.e.

<E^>, at the site of the impurity. By arranging to have both N and <E^> large,

sensitivity is maximized. (A geometric factor of l/cos9 is also important, but

assumed constant here.) The large fields obtainable by using internal reflection

is illustrated by figure 2. By adjusting polarization and angle, the large <E 2

> values can be placed to maximize the impurity spectra [1,2].

While figure 2 is for a two-phase system, the <E^> values at the interface

are changed little by the presence of a small amount of impurity, and the picture

is still valid. For three or more phases, the situation is more complicated but

still tractable. Thus, the large <E^> values can be placed where desired in a system of thin films. Palik and Holm have also made impurity studies of thick

* This work was sponsored by DARPA, and monitored by AFWL, Air Force Systems

Command, United States Air Force, Kirtland Air Force Base, New Mexico.

300
films using internal reflection spectroscopy [3]. They tended to emphasize

absorptance calculations rather than <E> field manipulation.

In this work we used to help determine the nature of the water in the

T h F 4 films. The infrared spectra of D^O are shifted with respect to H 2 0 , so that

any exchange was readily detected.

The purpose of this study was to determine in a general way the nature and

long term behavior of impurities in various T h F 4 films. For this purpose T h F 4 films were deposited on infrared transparent internal reflection plates. The

films were prepared in various ways. Variations included starting material

purity, heating of substrate, capping with cerium fluoride and other materials,

and detailed deposition procedures. All films were furnished by the Air Force

Weapons Lab. A table of representative films is found below. Results showed that

the nature of optical films depends critically on preparation procedures.

Table 1. Thorium Fluoride Film Description

Film

#1(1)

Substrate,

thickness

i nternal

incidence

angle

Zn Se-4mm

= 30°

#2(1) ZnSe-2mm

= 45°

#3(2)

#4(2)

#5(3)

Zn Se - 2m m

= 60°

Zn Se - 2mm

= 45°

Ge-2mm

= 30°

# of ThF Film Preparation Additional Comments

internal Thickness Conditions Overlayer

reflections or underlayer

22

25

14

25

22

5000 A

5000 A

4000 A

3000 A

2. 5y

Preheated

to 150°C

before and

during

deposition

1 0 0 A S i 0 Overl ayer

1000 A CeF

3 Ove rl ayer sides

Coated

both

sides

Coated

both

sides

Coated

both

sides

Coated

both

Coated

one side

only

(1) . Group 1 - Some samples originally contained large amounts

of water. Such films are assigned to group 1, and Films #1 and

#2 are representative of group 1 samples.

(2) . Group 2 - Some samples originally contained moderate

amounts of water. Films #3 and #4 are representative of the

group 2 sampl es.

(3) . Group 3 - Some samples originally contained very little

water. Film #5 is one such film.

301
2. Experi mental

2.1 Samples

The samples examined in this study consisted of thin T h F 4 films varying in

thickness from 3 0 0 0 & to 2 . 5 y , deposited on ZnSe or Ge substrates. Some of the

films had an additional overlayer of C e F 3 or underl ayer of SiO. The substrates

were 50mm x 20mm on the side, with ends cut at 30°, 45°, or 60° for internal

reflection. All substrates were 2mm thick, except for the 30° ZnSe substrates

which were 4mm thick. The number of reflections within a sample was determined

by the length and thickness of the substrate, and by the internal angle-ofincidence, which was usually the same as the angle of the substrate end. Samples

were used for both internal reflection and transmission IR spectroscopy.

The samples examined could be divided into three groups based on water

content and the date of preparation. Five samples (samples #l-#5) are herein

described, along with the results of their experimental treatments. However, they are only representative of a larger number of samples which were studied. Group

one was represented by samples #1 and #2, while group 2 included samples #3 and

#4, and sample #5 represented group 3.

o Sample #1 consisted of 5000A T h F 4 evaporated on a 30° ZnSe substrate v^hich was preheated to 150°C before and during deposition. For sample #2, a 100A SiO

layer was evaporated on a 45° ZnSe substrate, with 5000A ThF

4 coated on top. Al sg

included in group 1 are T h F 4 films capped with ZnSe. Sample #3 consisted of 4 0 0 0 A ThF 4 deposited on a 60° ZnSe substrate, while for sample #4, a 3000A T h

£ 4 film

was deposited on a 60° ZnSe substrate, with a final overlayer of 1000A CeF^.

Sample #5 comprised 2.5u ThF A on a 30° Ge substrate. Sample #5 was coated only

on one side, while samples #1 through #4 were coated on both sides equally.

Internal IR spectra were taken of all samples upon being received and again at the

start of this study.

2.2 Treatments

The following is a chronological breakdown of the various treatments to which

the samples were subjected (also see table 2).

Samples of group 1 and 2 were first soaked in D2O vapor for 7 days; then

heated in D

2 0 vapor to over 90°C for 2 hours; then heated in H

2 0 vapor at 70°C for

24 hours; then finally heated in vacuum at 70°C for 24 hours. Each of these

treatments was performed by placing the samples in a vacuum desiccator and the air

evacuated. When subjected to H

2 0 or D2O vapor, a small amount of the liquid was placed in a beaker in the desiccator, and heated by placing the desiccator in an oven. Internal reflection IR spectra of each sample were taken before and after

each of these treatments.

Later, samples from group 1 were individually placed in a 1 0

~ ^ torr vacuum

and heated at intervals up to 400°C while simultaneously taking their IR transmission spectra. The vacuum chamber was specially designed to fit into the

sample compartment of a Cary White 90 IR Spectrophotometer. It consisted of a glass cylinder with KBr windows on the ends and a standard taper cap on the top to

which was connected the sample holder. The sample holder comprised a rectangular

shaped aluminum tube which was wrapped with stainless steel sheathed heating

coils. Samples were fitted upright inside the tube. The temperature was

controlled automatically and monitored per a thermo-couple placed near the sample.

Heating the sample did not interfer with the IR spectrum because the sample and

reference beams were chopped before the sample compartment. After heating, D£0

vapor was allowed into the vacuum chamber. Then the samples were removed and

placed in D

2 0 vapor for two weeks at room temperature. We were then so bold as to

submerge sample #2 in liquid D

2

0 for 6 days. Some samples from group 3 were also

placed in liquid D

2 0 for 18 hours. However, these samples contained very little

water originally, and were not heated or treated in any other way beforehand.

302
3. Results

A. The broad IR absorption bands of the moisture found in the T h F 4 films

were nearly identical to those of pure water [4].

B. The amount of water impurity originally found in the T h F 4 films as received varied greatly from one group of films to the next. The films of group 1,

represented by samples #1 and #2, originally contained large amounts of water,

which can be seen as the large F^O absorption bands in figures 3 and 4. Some of

the films in this group had ZnSe caps. Those samples of group 1 whose substrates

were heated during film preparation, and those capped with ZnSe did not vary

significantly in water content from the other samples in the group. The films of

group 2, including samples #3 and #4, contained moderate amounts of water as seen

by the F^O absorption bands in figures 5 and 6, which are much smaller than the

bands in figures 3 and 4. Note that the spectra of sample #3 were taken

internally at a 60° angle-of- incidence which allowed only 14 reflections and made

the sensitivity smaller than for 45° samples. The films of group 3 represented by

sample #5 contained almost no water (fig. 7). In figure 7, the HgO bands are indeed very small. The hydrocarbon seen in figure 7 is due to the uncoated side

of sample #5.

C. When left in vapor at room temperature, what was at first thought to

be D2O-H2O exchange took place readily in samples #1 and #2 (figs. 8,9) and not at

all in samples #3 and #4. However, there is also present a rather large band at

1440 cm~l due to HDO (figs. 8,9). There must have been some D-H exchange taking

place. The samples were then heated in D2O to see what percentage of the H 2 0 absorption bands could be displaced by D2O and HDO bands. Figures 8 through 12

show that the H2O absorption bands were largely replaced by D2O and H^O bands in

samples #1 and #2, somewhat in sample #4, and not at all in samples #3, even at

temperatures greater than 90°C. Note also that the HDO band in sample #2 was rather small after heating. This is because the amount of HDO is related to the

remaining amount of H2O. Part of the remaining band at 1440 c m " * was probably

due to hydrated ZnSe.

D. When heated in H2O vapor, the D2O and HDO bands were replaced by H2O

bands once again (figs. 13-15). However, two additional bands could be seen at

1430 cm

-

* and 1540 cm"*. These bands were originally present in sample #1 which

was preheated to 150°C when first made (fig. 3), and in sample #4 which was capped

with CeF^ (fig. 6). These two bands seem to be hydrated ZnSe. After being heated

at 70°C in a vacuum, the water bands remained unchanged. Very little water was

driven off any of the samples at this temperature.

E. Sample #2 was placed in vacuum (fig. 16) and heated, while simultaneously

observing its IR spectrum. The water did not begin to leave appreciably until the

sample was heated above 200°C. Most of the water was driven off when heated at

300°C for several hours (fig. 17). The same was true for other samples which

contained water, even some which had not been subjected to the previously

mentioned treatments of samples # 1 - # 4 . However, in all cases there remained a small amount of residual moisture, even when heated to 400°C.

F. Once gone, water impurity did not re-enter the films. This seems to

disagree with the results of Palik, et.al. [ 5

n

. This result was true both for

films which originally contained very little water, and for films whose water was driven off at elevated temperatures. After being submerged in liquid D 2^ for

almost a week, sample #2 (water was drived off) showed no evidence of D20 impurity

(fig. 18). Sample #5 and other films which originally contained very little

water, but were treated in no other way, showed no D2O contamination, even after

being soaked in liquid D2O for a day.

A summary of sample treatment and results is found below in table 2.

303
Table 2

Fi lm Treatment Original

niii l n 2 u Amt of D-Hc XL fid iiy c room temp

Amt of D-H

c X L II d 11 y c heated >90°

Amt of Water

after h o 3 t c± r\ in H

2 0 70°C

Amou nt

n f U 1 Porosi ty

#1 A La rge La rge Large Returned to

original amt

La rge

n A,B,C Large Large Large Returned to

original amt

La rge

#3 A Moderate None Very

little

Returned to

original amt

None

#4 A Moderate Very

Little

Moderate Returned to

original amt

None

#5 D Very Little

A -Soaked in DoO vapor for 7 days; then heated in D2O vapor to >90°C

for 2 hours; then heated in H

2 0 vapor at 70°C for 24 hrs; then heated

in vacuum to 70°C for 24 hrs.

B - Placed in 1 0

~ ^ torr vacuum, and heated in intervals up to 400°C.

C - Removed from vacuum, and placed in DoO vapor at room temp for 2 weeks; then submerged in D2O liquid for b days.

D Placed in liquid D2O for 18 hours,

4. Di scussi on

A number of important conclusions regarding the preparation and integrity of

good pure durable thorium fluoride films can be drawn from the preceeding results.

Some conclusions are certain while others are probable.

A. To begin with, some thorium fluoride films contained a great deal of

moisture, perhaps five percent by volume, within the film [6], The broad,

intensely absorbing spectral regions of this moisture, at 2.6u - 3.8u, 5.8y - 8y,

and >10u, would surely affect the optical performance of the films. The fact that

this moisture impurity had the same spectral signature as liquid water indicated

that it was present mainly as bound water and not as something else such as hydroxide or hydrolyzed fluoride, although some of these may have also been present.

B. None of the films dealt with in this study showed signs of degradation

under moderate conditions over a period of 1-4 years. A small increase in the

moisture content of some of the films over time or in humid atmosphere was noted,

but this never amounted to more than a one or two percent increase. Cracks in the

film or spots of moisture degradation were seen only when heated in steam. (We

made no attempt to study film damage due to heating in water vapor.) Even when

submerged, none of the films were altered. A study which dealt more with T h F 4 film degradation is found in reference 7.

C. Evidently the water was tightly bound within the T h F 4 film. The water did

not leave freely under vacuum, but was driven off only at temperatures greater

than 200°C. So, what was thought to be H

2 0 - D

2 0 exchange in result B must

actually have been H-D exchange. D

2

0 absorption bands due to H-D exchange are indistinguishable from those from D2O - H

2 0 exchange. However, the HDO band at

1440 cm"* in figures 8, 9, and 12 is also evidence of H-D exchange. Also, the

water was evidently bound within the ThF* structure by the 0, leaving the H's

304
free. This is evidenced by, 1) The vibrational bands of the tightly-held water

which are the same as those of liquid H^O, and 2) the relatively free H-D exchange

i n sampl es #1 and #2.

D. The H-D exchange also indicated that some films were more porous than

others. In order for the D of D2O to exchange with the H of HoO freely, (a good

deal of H-D exchange was evident in samples #1 and #2 after sifting in D 2 0 vapor

at room temperature for less than 12 hours), the D-O evidently was able to

penetrate the film and approach the H2O. Sample #3 did not allow any D-H

exchange, most likely because it was not porous and did not allow D2O penetration.

The H-D exchange also suggested that the water is not trapped within impenetrable

little cells inside the T h F 4 films. Heating the samples up to 4 0 f) 0C would surely

have ruined the films if the water had been trapped inside. Heating the substrate

during film deposition did not seem to improve film porosity, at least up to

150°C. Palik et. al. claimed that heating the substrate to 200°C was "adequate"

for making good films [

5

~} . However, he did not mention how this applies to Th F4 films containing water.

E. Evidently the CeFo cap and other caps do not reduce the moisture content

of samples. In this study, all capped samples contained as much moisture as similar samples without caps. In fact, the C e F 3 itself may contain moisture and

be more porous than the underlying ThF*, as evidenced by comparing the H-D

exchange of samples #3 and #4 (fig. 5 and 6, 11 and 12, 14 and 15). In a study

done by Donovan and Temple, it was found that a ZnSe cap on the ThF^ film did

reduce the H content of the ThF^ film by an order of magnitude [8].

F. It is an important point that for those films prepared with very little

water impurity, and for the dehydrated films, there was no additional water

absorption. Even in humid atmosphere, and even submerged in liquid for days the

films did not acquire moisture, but remained anhydrous. At first it was assumed

by nearly everyone that the wet films acquired their moisture after being taken

out of the preparation chambers. This is now quite doubtful. Also, there was far

too much moisture in most of the films to have come from the residual gas of a 10"° vacuum chamber. Heating the substrate to 150°C during deposition or adding a CeF3 or ZnSe cap did not help eliminate moisture by more than a few percent.

Therefore, the moisture in the T h F 4 films most likely originated with the T h F 4 source material.

G. The evanescent wave of the totally reflected (internal) IR radiation is

strongest at the film-substrate interface. Its penetration depth at 45° is on the

order of 0.2y. So the <E^> field will not be very big at the air-film interface

of sample #2, and virtually zero for sample #5. Transmitted IR radiation "sees"

the air-film interface. Therefore, if the hydrocarbon bands of sample #2 at 2930

cm"-'- and 2850 cm"-'- are larger for transmission than for internal reflection (figs.

13 and 16), most of the hydrocarbon must be located on the outer surface of the

film. The same reasoning can be used to determine the location of the moisture

within the films. This has been done in reference 9. Profiling the water content

within some ThF^ films has also been done by Donovan and Temple using nuclear

resonance [8].

5. Concl usi on The key to preparing pure durable thorium fluoride films is evidently to

start with pure dry feed stock. Surely this is possible if careful inorganic

preparation techniques are used. Some salts are impossible to dry if you start

with the hydrated form. (CrClg*6H20 is an example.) However, if they are made

anhydrous to begin with, they have no affinity for water. Perhaps this is true of

thorium fluoride films.

The fact that the results of this study do not agree with some results of

other studies merely points out the necessity of cooperation between the various

research groups, and the need for more detailed information concerning method of

film preparation, origin and nature of starting materials, as well as treatment of

films immediately after preparation.

305
References

[1] Hansen, Wilford N. Reflection Spectroscopy of Optical Coatings. J. Opt. Soc.

Am. 69(2): 264-272; 1979 February.

[ 2

1 Hansen, Wilford N. J. Opt. Soc. Am. 58(3): 380-390; 1968 March.

[3] Palik, E.D.; Holm, R.T. Internal R e f 1 e c t i o n - S

p e c t r o s c o py Studies of Thin

Films and Surfaces. Opt. Eng. 1 7 ( 5 ): 51 2- 524 ; 1 978 Se pt em be r- 0c t o be r

.

[4] Rusk, Alvin N; Williams, Dudley; Ouerry, Marvin R. Optical Constants of

Water in the Infrared. J. Opt. Soc. Am. 61(7): 895-903 ; 1971 July.

[5] Palik, E.D.; Gibson, J. W. ; Holm, R.T.; Hass, M.; Braunstein, M.; and Garcia,

B. Infrared Characterization of Surfaces and Coatings by Internal-Reflection

Spectroscopy. Appl. Opt. 17(11): 1776-1785; 1978 June.

[6] Anderson, Wayne J.; Hansen, Wilford N. Reflection Spectroscopy Analysis of

Surfaces and Thin Films. Proceedings of SPIE Conference, April 1981.

[7] Humpherys, T.W.; Lusk, R.L.; Jungling, K.C. Charactarization of Thin Films

for Laser Systems Using Surface Analytical Techniques. J. Vac. Technol. 18(2)

296-300, 1981.

[8] Donovan, T.M.; Temple, P. A. The Relative Importance of Interface and Volume

Absorption by Water in Evaporated Films. NBS Spec. Publ. 568:237-246, 1979.

[9] Hansen, Wilford N. Spectroscopic Study of Small Absorptions in Optical

Coatings. AFWL-TR-81-139; 1982 April. 57 p.

306
aucm

rH •H *H o

4-1 in

CD 1 cu

crj 10-1 M O 00

jj 1 a

CO 0J 1—1

COC CO cd

CO • cj

CO

14-1 * u

iJ o i-M

flj

C >-i

a \ o

,£ o CN|| H C

O

4-1 CD a

cu (3

J_l

a

O ••

4_( '->

—i

•H

CO 4-1 4-1 cd O -H

x) 0) >

rH CO iH -H

M-l 4J *H CU -H k to

cu c J H CU

id a)w

rf c y s-j o

cu te 4JNI

N

Th in 5<>

cuMM

•H iiIIIl we

!8:g

•I <-Buozu

i-s 1L<

UJUJ

a:

m

-< ,r,

X

ma. Si

uio

ccDo

.<—-«>

a: 0

cu -a Cfl •H cu 1) w -a 4J c 0) •H cd H et

CO tr ta E -c CO 3 c H -c

4JH c

0) c 41 CO

CD

r& (3 4-1 c o O c c S-i 01 •H 4J — u

4J CJ cd u cu J- CX a H X

rH CO c o cd

cu pd c_u

u

M de

4-11 H O C2

ni co en >H C rH •H n

CU

w E -

4-1

H

(3 •H !+J H cd o

cu

Lj3

307
Figure 3. Sample #1 Internal reflection

5000A ThF on ZnSe

6=30° 22 reflections

As received

Figure 4. Sample #2 Internal reflection

100A SiO, 5000A ThF on ZnSe

0=45° 25 reflections

As received

Figure 5 . Sample #3

4000& ThF on ZnSe

0=60° 14 reflections

As received

Internal reflection

Figure 6, Sample #4 Internal reflection

3000A ThF 1000A CeF on ZnSe

6=45° 25 reflections

As received

308
40 HI MtfJONS 60 '0 »0 M O 0

Figure 7. Sample #5 Internal reflection

2.5y ThF on Ge (one side)

9=30° 22 reflections

As received

Figure 8. Sample //l Internal reflection

5000A ThF on ZnSe

0=30° 22 reflections

Soaked in DO vapor 7 days rm. temp.

Figure 9. Sample #2 Internal reflection

100A SiO, 5000A ThF on ZnSe

0=45° 25 reflections

Soaked in DO vapor 7 days rm. temp.

309
Figure 10. Sample #2 Internal reflection

100& SiO, 5000& ThF on ZnSe

6=45° 25 reflections

Heated >90°C 2 hrs. in DO vapor

> M 10 50 MJCKM 6U /o 10 ' ~ fO tt O it b II JO i

Figure 11. Sample #3 Internal reflection

4000A ThF^ on ZnSe

6=60° 14 reflections

Heated >90°C in D.O 2 hrs.

310
Figure 13. ' Sample #2 Internal reflection

100A SiO, 5000A ThF on ZnSe

9=45° 25 reflections

Heated in HO vapor 24 hrs. 70°C, then

in vacuum at 70°C 24 hrs.

Figure 14. Sample #3 Internal reflection

4000A ThF

4 on ZnSe

9=60° 14 reflections

Heated in HO vapor at 70°C 24hrs., then

in vacuum at 70°C 24 hrs.

WAVMMB tOS( 1 WAVTNUMCI IOM 1

Figure 15. Sample #4 Internal reflection

3000A ThF., 1000A CeF„ on ZnSe

6=45° 25 reflections

Heated in HO vapor at 70°C 24 hrs., then

in vacuum at 70°C 25 hrs.

311
1 K

i:-h

tffttr

iiiili,1

i r

:

r —— -

U .: H ::it-

' i:

'. ii!" ;rrr—

•

:

'

i

:•' ,1; M : I.

[11 !. :i i

; '0 K'Up

: III

| ;!;' |!

!

i :nj hl

: i

il'll 1

i ! "TV ii : i '$7

mipDpi

11; hi pi

: il;!i!'!< [111 ..'I

iliiijiii Ii! iilPm] 1

1 ii |p

'i ; i

.

i-iUi T I'-

ll

3000

cm"

1 Sample #2 Transmission through both sides

100A SiO, 5000A ThF

4 on ZnSe

Same as fig. 13

100A SiO, 5000A ThF^ on ZnSe

Heated at 300°C for several hrs. in vacuum, then

Sample #2 Transmission through both sides

)A E itec at 400°C for %hr.

I

"'

Sample #2 Transmission through both sides

100A SiO, 5000A ThF on ZnSe

Soaked in liquid T> 20 for 6 days (submersed)

312
Reflectance Spectrum of Non-Optimum Multilayer Coatings

R. Rujkorakarn, R. W. Hannum, and J. R. Sites

Physics Department, Colorado State University

Fort Collins, Colorado 80523

The reflectance spectrum of light normally incident on arbitrary multilayer coatings

is calculated by the matrix technique. The purpose is to use the details of the measured

spectrum from a reflection enhancing coating to identify an individual layer and the

specific parameter (thickness, refractive index, or absorptance) that are responsible for

less than optimum peak reflectance. As previously discussed by Bennett and co-workers,

the secondary features of the spectrum, particularly the adjacent minima, are more sensitive than the peak to thickness errors; the peak is more sensitive to problems with the

index or with absorption. For concreteness, we have defined optimum conditions as those

producing the maximum possible reflection at 1.06' ym. We display results for variations

in a three layer coating, which is a useful starting point for exploratory experimental

work, and for a fifteen layer coating which is nearing the infinite limit for practical

coatings.

Key words: absorption; equivalent substrate; extinction coefficient; multilayer coatings;

peak reflectance; quarter-wave thickness; refractive index deviation; secondary features;

thickness errors; transparent substrate.

1. Introduction

The reflectance spectrum of a multilayer optical coating can be accurately calculated, and

often is, by multiplying the 2x2 matrices associated with the layers of the coating [1]. Less common, however, is the calculation of reflectance spectra variations with systematic deviations

from optimum conditions in one or more layers of a multilayer coating.

In the current work we have mathematically varied the thickness, refractive index or absorption

of one of the outer layers of an otherwise ideal reflection enhancing multilayer coating. We have

then examined the primary maxima and minima of the resulting spectrum to determine which are most

sensitive to these deviations from ideality. For concreteness, we considered coatings with low

index of refraction layers at or near 1.48 and with high index at 2.03. Thicknesses were chosen to yield maximum reflection at a wavelength of 1.06 ym, and the substrate was taken to be transparent.

We show here results for three layers (high-low-high indices), which is the simplest effective

multilayer case, and for fifteen layers. Clearly, however, the general trends shown in the graphs

to follow can be applied to other coating parameters.

2. Reflectance Calculation

Following Abeles [1], the reflectance of a system of layers between an incidence medium (index

n 0 ) and transparent substrate (index n ), shown in figure 1, can be calculated from the characteristic (M) of the coating.

The fundamental period matrix (Cj) is related to the Fresnel reflection coefficient rj and the

phase retardation <5j of each layer such that

1L

J r

j+l

e

(2)

i6

j

- i6

j eJ eJ

313
For normal incidence, the Fresnel reflection coefficient is defined as

n i-l~

n

i r. = -J—1—1 (3)

3 n.. 1+n . ^

The outermost layer is j = 1 , that next to the substrate, j = m, and the substrate j = s. 6j = 2irn jdj/ x where nj is the refractive index, and x is the wavelength of radiation.

Several closed form expressions can be calculated by approximating the matrix calculation [2,

3, 4]. The reflectance of a coated substrate is given by [5]

MM* _ 21 M MM*21 m^' "n"n

The reflectance can be maximized through the constructive interference of alternating high-low

refractive index layers, each one-quarter wavelength thick [6]. That of a glass or quartz substrate

(^4%) can be enhanced to something in excess of 99% for a specific wavelength.

3. Thickness Errors

The calculated reflectance of a three-layer coating is shown in figure 2. The solid line in each case is for three layers, each at exactly one-quarter wavelength for 1.06 pm radiation. The

dashed line indicates a layer that is too thick, and the dotted line, one that is too thin. The

maximum reflection does not change much (<1%) even with the artificially high 20% deviations shown.

As previously noted [7], the minima on either side of the major peak (Rmax) snow large changes in

amplitude, especially when one of the high index layers is altered. When the outer layer thickness

is too large (fig. 2a), the adjacent low wavelength minimum (Lm -j n ) decreases nearly to zero, and the

high wavelength minimum (Hm -j n

) increases. With the inner layer (fig. 2c) the opposite is true. For thickness deviations in the middle layer (fig. 2b) the minimum reflectance is nearly unchanged,

but the adjacent low wavelength maximum (Lmax ) varies strongly with thickness.

The trends of figure 2 are perhaps better illustrated in figure 3 where the secondary features

of the reflectance are plotted as a function of thickness deviation. Since the features are qualitatively different in each case, it should be possible to use the calculated spectrum to identify which layer differs from its ideal thickness.

Fifteen layers (fig. 4) present a more difficult situation for identifying thickness problems,

but the general trends persist. The reflectance variations when the outermost layer is varied (fig.

4a) are qualitatively similar to the three layer case. The largest reflectance peak (^99%) is much

narrower. In figure 4b, we see a secondary peak reflectance near 0.5 urn. This peak occurs when the

thickness of alternate layers is increased or decreased relative to one another by a total of 20%.

This peak is analogous to the enhanced feature of Lmax at 0.85 ym in figure 4a and becomes more

pronounced as the number of thickness altered layers is increased. Moreover, the high envelope at the left-hand side of the secondary peak indicates that all high index layers are too thick. On the

contrary, when all the low index layers are too thick, the high envelope appears on the right.

4. Variations in Index

Returning to three layer coatings, variations of the calculated reflectance spectra with

refractive index are shown in figure 5. Here the large variations shown are even less likely than

20% thickness variations, but are again depicted to show the trends in the spectra. Rmax is lowered

or raised as the ratio of high to low indices is decreased or increased.- Again, however, the

behavior of the secondary features differs considerably depending on which layer is affected. For

the middle, low index layer (fig. 5b) neither Lm in nor Hm -j n changes significantly with index changes,

but Lmax is enhanced with a smaller high to low index ratio. The coating yields a larger Lmax with

a refractive index increased for any high index layer, but the effect on the minima depends on which

layer is altered. When the outer layer index is increased both minima increase, with the inner

layer both decrease.

For fifteen layers, the trends of the secondary features are basically the same as for the

three layer coating when the outermost layer index is varied. There is relatively little variation

in Rmax and the minima despite the same large variation in refractive index. However, index errors

show up significantly at Lmax . The increase of Lmax due to index change is about three times of

that in the three layer case.

314
5. Absorption

Absorption in a multilayer coating consists of the effects due to film volume, layer interface,

scattering and surface plasmons. For simplicity we assume an ideally smooth surface, which allows

us to neglect contributions for the scattering and surface plasmons. Overall absorption can be

characterized by the extinction coefficient k which is the imaginary part of the complex refractive

index n = n-ik. k can be defined in terms of the absorption coefficient £ by the equation k = BA/4tt. Here the calculated reflectance spectra for a three layer coating with and without absorption are displayed in figure 6. As k increases in figure 6, Rmax and Lmax decrease while the

adjacent minima become more shallow. Such trends also persist for the case of a fifteen layer

coating, with larger effects due to the higher number of layers. In contrast to the results of

multilayer coatings on metallic substrates [7], we see that Rma x is more sensitive to absorption

variations than Lm -j n and Hm -j n. The discrepancy can be explained in terms of an equivalent substrate

of refractive index n£ [4]. n^ is a dielectric equivalent of the metal substrate and its innermost

film layer. This layer differs from quarter-wave thickness to achieve phase compensation between

the metallic substrate and other layers of the coating. In general ns<<n s where n s is the refractive index of a normal substrate. While the absorption remains comparable at the minima for

coatings on both kinds of substrate, n| gives rise to a smaller amount of absorption at Rmax than

n s . Eventually, Rmax becomes less sensitive than the minima to the change in k as appeared in the

results of Bennett and Burge [7].

6. Conclusion

In the calculated reflectance spectrum of light normally incident on a multilayer coating, we find that in addition to the primary peak Rmax , secondary features such as Lm -j n , Hm -j n and Lmax can

be very informative for determining the ideality of the coating.

For a three layer coating consisting of alternating high and low index dielectrics on a transparent substrate, the reflectances and wavelength locations of the secondary features are good

indicators of thickness errors in any individual layer. Uniform refractive index variations in the

outer or inner layer gives rise to indistinguishable Rmax and such cases can only be differentiated

on the basis of their secondary features. Furthermore, Rmax is considerably more sensitive to the

index change than to the thickness change. If absorption takes place equally in all layers, and

interfacial scattering is neglected, the influence it has on the minima is less than that on Rmax - Thus, the increase (decrease) of reflectances at the minima (maxima) can indicate the amount of

absorption.

Calculated reflectance spectra for fifteen layer coatings reveal trends qualitatively similar

to a three layer case when only coating errors in the outermost layers are considered. Due to the

large number of layers, little difference appears in the characteristics of the secondary features

for errors among the three outer layers. Rmax is narrow and its magnitude is close to one. Absorption raises the minima and lowers the maxima. The secondary maximum is enhanced to form a distinctive peak, if the high and low index layers are alternately too thick and too thin.

Acknowledgement

This work was supported under the grant from Air Force Weapons Laboratory Contract F-29601-82-

K-0009.

References

[1 ] Abeles, F. Sur la propagation des ondes electromagnetiques dans les milieux stratifies. Ann.

Phys. 5, 505 (1950).

[2] Giacomo, P. Propriety chromatiques des couches re"fle"chissantes multi-die'lectriques. J. Phys.

Rad. 19, 307 (1958).

[3] Sparks, M. A simple method for calculating the optical properties of multilayer-dielectric

reflectors. J. Opt. Soc. Am. 67, 1590 (1977).

[4] Bennett, H. E.; Burge, D. K. Simple expressions for predicting the effect of volume and interface absorption of scattering in high-reflectance or antireflectance multilayer coatings. J. Opt. Soc. Am. 70, 268 (1980).

[5] Heavens, 0. S. Optical Properties of Thin Solid Films . London; Butterworth; 1950. 72 p.

[6] Macleod, H. A. Thin Film Optical Filters . New York; Elsevier; 1969. 4 p.

315
Bennett, H. E.; Burge, D. K. Limits to the validity of evaluating a quarter-wave, high

reflectance multilayer through analysis of its secondary structure. 13th symp. on optical

materials for high power lasers; Boulder, 1981.

Pulker, H. K. Optical losses in dielectric films. Thin Solid Films 34, 343 (1976).

Loomis, J. S. Absorption in coated laser windows. Appl . Opt. j_2, 877 (1973).

0,

Figure 1. Structure of a multilayer coating.

316
WAVELENGTH (pm) WAVELENGTH c^m)

40 -

(C) 3 LAYERS —

d

H dL(i.2dH ) —

d

H dL dH dH dL(o.8dH)

i f r /

,'

' ~J\ I

: /''

V

i 4 . 8 1. 2 1.6 2. 0 2. WAVELENGTH (jum)

40

30

20

10

(d) Rmax

3 LAYERS dH dL d H

Lmax /

Lmin

x

, 8 1.2 1.6 WAVELENGTH (^m)2. 3 2. 4

Figure 2. Calculated reflectance spectra for thickness variations of: (a) the outer layer,

(b) the middle layer, (c) the inner layer, and (d) with no variations, d^ = 1.06

pm/4nH> d L

= 1-06 ym/4nL where n^ = 2.03, n|_ = 1.48.

317
Figure 3. Reflectances of the secondary features vs. thickness deviation of: (a) the outer

layer, (b) the middle layer, and (c) the inner layer.

318
>_ 60

(a) j: \ 15 LAYERS

I ! .... (i.2dH)[dL dHl 7 — dH

[dLdH]7

, (Q8dH

)[dL dH] 7

1 6 .9 1.2 1.5 1.8 2.1

WAVELENGTH <^j m)

60

50

(b) 15 LAYERS

. (i.idH)[(o.9dLXi.idH )]7

- dH

[dL dH]

7

- (o.9dH

)[(i.id^(o3dH )]

7

.6 .7

WAVELENGTH (^m)

. 9

Figure 4. Calculated reflectance spectra of a fifteen layer coating when (a) only the outer

layer thickness is altered (by 20%), and (b) all the layer thicknesses are altered

(each by 10%).

319
3

. 4 .8 L2 1.6 2. 0 2. 4 WAVELENGTH (^rrO

Figure 5. Calculated reflectance spectra for refractive index variations of: (a) the outer

layer, (b) the middle layer, and (c) the inner layer, n^ = 2.03 and n|_ = 1.48.

320
321
Environmental Study of Single and Multilayer Dielectric Films for 2.8

(m) Subjected to Humidity, Ethanol , HF, and H,,S

James R. Palmer

TRW

One Space Park, Redondo Beach, California 90277

This study was conducted in order to evaluate the response of various coatings on various first layer metals when subjected to 95% relative humidity at 35°C, 95% saturated

ethanol vapor at 35°C, 10% by volume of HF and H^S.

The study is conducted in two phases. In the first phase, four (4) sample single

layer dielectric coatings on a silver base layer, deposited on a molybdenum substrate, were evaluated The coatings cgnsisted of SiO with a thickness < 283 A and ZnS with

coating thickness values of 314A and 5244A, and 5473A. The samples were subjected

to Humidity, liquid water, HF, and H 2 S in series. Reflectance and scatter measurements were made on the samples before and after each environment.

In the second phase, forty-eight combinations of single layer films on metals and

six multilayer dielectrics on a silver alloy was evaluated using a double blind

test. One matrix of the twenty four configurations were subjected to the environments

in series. One matrix was subjected to each environment independently.

Each coating sample was examined under the microscope and a photograph made at

5x and 400x. After each run, the sample was again examined and photographed.

Additionally, another matrix was subjected to mechanical tests of eraser rub, cheese

cloth, tape, and a spectrophotometri c scan to determine any H 2 0 or OH absorption.

Photographs from the first phase of the study and optical changes as a function of

environment are shown graphically in the paper. The details of the coating chemistries

and test plan for evaluation are discussed and the results and conclusions are presented.

Key Words: coating chemistry; coating damage; defect damage; HF, NH^, H^S environments;

mirror damage; single layer , multilayer dielectric coatings.

Introduction

A large number of single and multilayer dielectric coatings were produced on one inch diameter

float glass substrates. A very small selection of samples were produced on one and a half inch

diameter molybdenum substrates. The coatings and mechanical testing were provided by OCLI. The

Molybdenum substrates were tested and measured at Naval Weapons Center China Lake. The float glass

coated samples were tested and evaluated at TRW. Additional testing was performed on samples including NH^, air conditioned air , and outside air environments. The results of the tests performed on the

float glass are shown in figures 1.0, 2.0, and 3.0, in tabular form. The results of the tests and

measurements performed at the Naval Weapons Center are shown in figures 4.0 and 5.0.

The results of the environmental tests at the Naval Weapons Center show that reflectivity

and scatter change in moderate fashion for all the tests through HF. However, there is a demonstrable change in both reflectivity and scatter when the samples were subjected to wet H^S. There has

been some criticism of the wet H^S environment as being too severe. However, the test does provide

a substantial bound to the types of conditions that the coatings and substrate metals may be able to withstand.

Essentially, the single layer dielectric coatings on the float glass did not withstand the

various environments very well. Except for Al^O^ and SiO all of the oxide coatings failed the OCLI spectrophotometry absorption measurement, i.e., the oxide coatings manifested an absorption greater

than 5%. Single layer A1„0

3 and ZnS on Au , Cu, and Al proved superior in their resistance to chemical response to the various environments. With the exception of Ag/(ThF^/Zn Se)5, the multilayer

dielectric coatings demonstrated substantial resistance to the effects of the various environments.

For the most part, coatings that were subjected to the outside air indicated substantial

chemical activity after one week of exposure. Some of the effects were very dramatic. Because of

the chemical makeup of the air, however, this environment is much too stringent and is not regarded

322
as a real application environment that one would encounter in most normal operating conditions.

The NH

3 environment had a substantial effect on MgF^/Cu, bare Cu, Si/Ag alloy and ZrO^YpO^/Ag

alloy. The remainder of the samples indicated no changes after two hours in the environment.

Continued experiments with samples that are maintained in an air conditioned environment

indicate that there will be chemical activity early on; or, there will be no activity at all. Each

week, the samples are cleaned, i.e., ethanol is allowed to stay on the coated surface for two minutes.

The surfaces are cleaned using a cotton swab. The surfaces are blown clean with dry nitrogen. After

the first two weeks, minor changes were noticed at pre-existing defect areas on Al^CU/Au and ZnS/Au.

No other areas of the coating appeared to be affected. MgF

2 /Cu, bare Cu, and most all of the Ag

alloy based single layer dielectric coatings manifested substantial changes in the A1

? 03 and ZnS. Additionally, there have been no changes in any of the other coatings which were unaffected from the outset.

There were two phases to this study. The first phase involved ZnS/Ag/Mo, SiO/Ag/Mo and Ag/Mo.

The SiO/Ag coating was on the order of 283°A of thickness. There were three (3) coating thickness

values for the ZnS/Ag/Mo: 314 A, 5244 A° , and 5473 A°.

This first set of samples were placed in storage for one month after first undergoing characterization for reflectivity and total integrated scatter. After the samples were removed from storage

they were again characterized for reflectivity and scatter. Each sample was subjected to each of the

environments in series. The first environment was humidity. The test required 95% saturated vapor

at 35°C. The samples were allowed to stay in the environment for half an hour. At the end of the

half hour, the samples were measured for reflectivity and scatter. If there was a change of less

than 0.1% the sample was re-introduced for one hour. If there was a change > 0.1% < 0.15% the

samples were re-introduced for one-half hour. If there was a change > 0.15% the sample was no longer

subjected to this test and was prepared for the next environment.

The second environment consisted of placing de-mineralized water on the surface for one-half

hour. After the half-hour was complete, the surface was blown dry with dry nitrogen and evaluated

for reflectivity and scatter. According to the criteria indicated above, the samples were, or were

not re-introduced to the environment. The thin ZnS demonstrated substantial staining at the end of

the first half hour.

The third environment was a mixture of 10% by volume of HF in nitrogen. Ten seconds were al 1 owed

for the mixture to be introduced and completely diffuse in the bomb. The samples were allowed to continue in the environment for twenty seconds of residence time. Ten seconds were al lowed for

purging the HF-nitrogen mixture from the bomb. This environment was continued for two cycles.

Reflectivity and scatter measurements were made at sixteen hour and forty hour intervals.

The last environment, and perhaps the most severe, was a 10% by volume of H„S in nitrogen with a relative humidity of 95% at 35°C. The samples were subjected to the same cycle auration as the HF. The same criteria for evaluation was employed. The samples were only introduced into this environment for two cycles. The results of this phase of the experiment are shown in figures 4.0 and 5.0.

The second phase of this experiment used float glass samples with a multiplicity of coating and

metal chemistries. The material combinations that were selected for this experiment are:_

SLDC

Au/AL

2 03

, Au/Si, Au/ZnS, Au/MgF

2

, Au/Ce0

2

, Au/Zr0

2

, Au/Y

2 0

3 Cu/AL

2 0

3

, Cu/MgF

2 AL/AL

2 03

, AL/MgF

2

, AL/Zr0

2 Ag/Si, Ag/MgF

2 Ag ALL0Y/MgF

2

, Si, Ce0

2

, Zr0

2

, ZnS, Zr0

2

Y

2 0

3

, AL

2 03

323
MLDC

Au/(AL

2 0

3 /ZnS)

5

, Au/(SiO/Si) 5

Cu/(AL

2 0

3 /ZnS)

5

, Cu/(SiO/Si)

5

AL/( SiO/Si ) 5

, AL/(AL

2 03 /ZnS)

5

Ag/(ThF

4 /ZnSe)

5

, Ag ALLOY(SiO/Si ) 5

, Ag ALLOY(AL

2 03 /ZnS)

5

The selection criteria used by OCLI for determining which coating and metal combinations would

be most suitable, were:_

DIELECTRICS METALS

INDEX OF REFRACTION CHEMICAL REACTIVITY

ABSORPTION (OH) REFLECTIVITY AT 2.8 ym - R > 95%

PACKING DENSITY ADHERENCE

STRESS HARDNESS

CHEMICAL REACTIVITY

Unlike the first phase of this experiment, the second phase was undertaken to evaluate a large

selection of metals and coating combinations on the basis of chemical activity, rather than optical

changes. There were a total of twenty-four different metal -coating combinations. There were twentyfour sets of these samples produced for this experiment.

This set of experiments was set up as a double blind test. One set of samples was introduced to each set of environments in series. Another set of samples were introduced to each environment

independently. In this manner, any chemical activity would be distinguished as being a function of

the individual environment or a cumulative effect of one environment to another. The result of this

technique produced an interesting outcome in that those chemistries that were deemed to have failed

did so on both the serial and independent sample simultaneously. None of the failed chemistries

did so independent of the other like chemistry for any given environment. A schematic of the double

blind test is shown in figure 6.0.

The test set-up is shown in figures 7.0 and 8.0. The samples were placed on stacked sheets of

wire mesh on a central rod. The forty-eight specimens were placed on the screens and the complete

set was inserted into special bell jar with a stopper at the mouth. The gas lines were inserted

through the stopper. The bell jar was then suspended in a water bath to maintain a constant temperature commensurate with the gas temperature to preclude condensation on the samples or bell jar

walls. The respective gas environment was passed through a heat transfer coil which was immersed

in the water bath to insure the proper temperature of 35°C.

At the outset, all of the samples were examined with a microscope and photographed at 5x and

400x. The surfaces were scanned to find any outstanding defects in the coating. The position of

such defects were marked on an X-Y coordinate system so that any chemistry that resulted in these

areas would be disregarded after the samples had been submitted to the environments. All 576 samples were characterized and photographed in this manner and the photographs were placed in lab books.

The serial and independent samples were subjected to the humidity test, first Four (4) one- half hour tests were conducted. After each half hour test, the samples were examined under the

microscope and scanned at 5x. Any areas that became suspect were then examined at 400x. Following

the test, the independent samples were vacuum bagged with desiccantand stored.

The ethanol was introduced with nitrogen to a 95% saturation level at 35°C. This technique is less severe than placing liquid water on the surface; however, it was felt that subjection of the

samples to two hours of saturated vapor at 35°C would be more consonant wi th actual operational

orocedures. The same orocedure for evaluation was used to determine oass or fail.

For the HF test, pre-mixed bottled gas was used. The HF was 10% by volume in nitrogen. The

samples were subjected to four twenty second cycles. Again, the temperature was maintained at 35°C.

This test was utilized to reproduce an upset condition in a device which would spray HF gas onto

324
operating optics. The same procedure for evaluation of the coatings was used.

The l-LS test also used 10% by volume in nitrogen pre-mixed bottle gas. The samples were subjected to four cycles of twenty seconds each. The temperature was maintained at 35°C. Unlike

the previous phase of this experiment, the l-LS in nitrogen was maintained in the dry condition. The

dry condition was regarded as more representative of the actual conditions that may be expected.

Again, the same conditions for evaluation and discriminating were used.

The results of the OCLI mechanical, and spectrophotometri c tests, along with the TRW environmental results, are shown in tabulated form in figures 1.0, 2.0, and 3.0.

Additional tests were performed on the sample sets. One set of twenty four samples were placed

in a cage, on screens, and placed on the roof of a TRW building in El Segundo, Ca . The samples were protected from sunlight so that a photochemical process would not be introduced into the problem. A startling result confronted the investigators after the first week. Almost all of the samples

manifested substantial deleterious effects. The experiment continued for two additional weeks and

the deterioation of the coatings continued for this period of time. After reviewing the average

chemistry of the air at the locality, which contained substantial PPM of ozone, this test was determined to be much too stringent for the actual conditions that the coatings would be expected to experience.

One set of samples was subjected to 99% saturated NH

3 in 1% wet nitrogen for thirty minutes, at

35°C. MgF

2 /Cu, bare Cu, Si/Ag ALLOY and Zr02 Y

2 03 /Ag ALLOY indicated substantial change after the

first half-hour run. Three additional half-hour runs were made and none of the other samples

indicated any changes. One may conclude from these results that, for the most part, the coatings are resistant to slightly moist NH3-

The last portion of this phase of the experiment, which is still ongoing and may ultimately be

the most important, is the air conditioned environment. One set of samples were placed on screens in an isolated air conditioned room. The room temperature is maintained at 68°F + 5°. The humidity is maintained at 40% + 5%. No one goes into the room except once a week when the samples are removed

for examination and photographing and returned to the room. The samples are microscopically

examined at 5x and 400x.

After the first two weeks, minor changes were noticed at pre-existing defect areas on AL^O^/Au

and ZnS/Au. No other areas of the coating appeared to be affected. MgF~/Cu, bare Cu, and most all

of the Ag ALLOY based single layer dielectric coatings indicated substantial changes. However, in

the eight weeks since the first examination, there has been no additional deterioation of the

Al^O^/Au or ZnS/Au. Additionally, outside of the coatings that initially failed, there has been no change in the remaining samples.

In recent weeks, the experiment has been expanded in that each sample is subjected to reagent

grade ethanol placed on the coating surface for two minutes. After the two minute internal, the

coating surface is cleaned with a cotton swab. The ethanol is blown from the surface with dry

nitrogen and the surface is examined under the microscope. The only sample that has suffered

deleterious effects is the evaporated bare gold. After the first series of ethanol rub, the gold

coating manifested severe scratch lines.

At the end of this paper there are a number of photographs of the various coatings that have

failed under various environments that have been discussed. The result of these experiments has been

to narrow the field of suitable metal and coating candidates that may be used in this particular

application. A very important result of these various situations relates to the care that must be

taken with optics, that have similar coatings, when used in laser optical systems.

Acknowledgements

This work was supported by DARPA. The author would like to give special recognition to Michael Kenney for his efforts in preparing and photogranhing the test specimens.

325
mi c 693-1 Cs/Au/ZnS THIN F F P PF F IX P P P P F

693-2 Cr/Au/ZnS THICK P F P PP P PP 1Z F F F P F

693-3 Cr/Au/A1203 THIN P P P PP P PF 1Z P P P P F

693-1 Cr/Au/A12

0

3 THICK P P P PP P PP 1Z P P P P F

694-1 Cr/Au/Ce02 THIN F F P PF P P 5Z P P P P F

691-2 Cr/Au/Ce02 THICK P F P PF P P »5Z P P P P P

694-3 Cr/Au/Si THICK P F P PP P PP 1Z P P P P F

694-4 Cr/Au/Si THIN P F P PP P PP 1Z P P P P F

695-1 Cr/Au/MgF2 THIN P F P PP P PP 5Z P P P P F

695-3 Cr/Cu/MgF2 THIN F F P PP F 5Z P P P F F

695-4 Cr/Cu/MgF

2 THICK P P PP F 5Z P P P

696-1 Cr/Au/Zr02 THIN F F F »5Z P P P

696-2 Cr/Au/Zr02 THICK P P PP P PP »5Z P P P

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2 thick P P PP P PP 5Z P P P P

697-3 AI/ZRO2 THIN P P PF P P P P P P

697-4 A1/Zr02 THICK P P PP P PP P P P P

Figure 1.0

326
CHEMISTRY DIELECTRIC

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s /Si .05 F p P 1Z Tl P P P P

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Figure 2.0

327
CHEMISTRY

DIELECTRIC

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+ Ag/Alloy

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328
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329
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331
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332
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333
HUMIDITY

693-2-10

Cr/Au/ZnS Thick

693-2-6

Cr/Au/ZnS Thick

2126-9

Ti/AgAlloy+MgF,

2126-6

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334
HUMIDITY CONT'D.

693-2-9 2126-10

Cr/Au/ZnS Thick Ti/Ag Alloy + MgF

2

335
ETHANOL ONLY CONT'D.

2127-10 712-1-10

Ag/Ag Alloy + Si Thin Ag/(ThF4/ZnS)

ETHENOL + HUMIDITY

336
693-2-11

Cr/Au/Zns Thick

2126-11

Ti/Ag Alloy + MgF,

337
700-3-12 2126-12

Cr/Cu/MgF

2 Thin Ti /Ag AT| 0y + MgF

2 Thick

338
H

9 S CONT'D.

2127-12

Ti/Ag Alloy + Si Thin

2130-12

Ti/Ag Alloy + Ce09 Thick

2129-12

Ti/Ag Alloy + CeO ? Thin

2138-12

Ti/Ag Alloy+Al

2 0

3 Thick

339
H

9

S + HF + ETHANOL + HUMIDITY

Ti/Ag Alloy + Ce0

2

340
H

?

S + HF + ETHANOL + HUMIDITY CONT'D.

341
A. C. ENVIRONMENT PASSED

342
A.C. ENVIRONMENT FAILURES

60x Before 400x After

Ag Alloy/Si Thin

60x Before 400x After

Cr/Cu 5 QW MgF

2 @ 725nm

343
A.C. ENVIRONMENT FAILURES

2128-7

60x Before

Ag Alloy/Si Thick

2128-7

400x After

344
A.C. ENVIRONMENT FAILURES

60x Before »$-3-13

Cr/Cu 1 Qw «gF

2 * 725rm

400x After

345
NH

3 REJECTS

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695-4-13

Cr/Cu §Q* MjF

2 9725nm

60x Before 400x After

1138-12

As Alloy/ZrDJT.O, Thick

346
60 x Before

2127-8

Ag kWvf/SI Thin

ON ROOF FOR 1 MEEK

400x After

700-2-14 Cr/Cu/(SiO/Si) 700-3-14 Cr/Cu

347
ON ROOF FOR 1 WEEK

2131-14 2130-14

Ti/Ag Alloy + ZrO? Thin Ti/Ag Alloy + Ce0

2 Thick

348
Photothermal Deflection Microscopy of Thin Film Optical Coatings

W.C. Mundy, R.S. Hughes

Department of Physics and Computer Science

Pacific Union College

Angwin, CA 94508

C.K. Carniliga

Optical Coating Laboratory, Inc. 2789 North Point Parkway

Santa Rosa, CA 95401

A system has been developed which employs the photothermal deflection effect to map

the absorption characteristics of thin film optical coatings used in high energy laser

applications. The high spatial resolution, low-level absorption data provided by this

system may reveal flaws which could become damage sites when exposed to high energy laser

radiation.

The system, in which an argon ion laser (514. 5nm) is used for the pump source and a He-Ne laser is used as the probe source, has been used to study a halfwave optical thickness Zr0 2 coating on a super-polished fused silica substrate. The sample was mounted on

an x-y translation stage which was driven by stepper motors in increments of 4pm in either

direction. Qualitative data are presented of mappings of areas of the coating in which

defects were found which were not observed with Nomarski microscopy. Measurements are in

progress to ascertain correlations between the absorption and laser induced damage sites.

Key words: dielectric thin films; microscopy, optical absorption; photothermal deflection

microscopy.

When electromagnetic radiation is absorbed by a medium, some of the radiant energy is converted

to thermal energy, which causes local changes in the index of refraction of the absorbing medium and

of a thin layer of the atmosphere adjacent to the sample surface [1]. This variation in the index

of refraction, and hence the amount of absorption of the incident pump beam by the sample, is measured by the deflection of a probe light beam which intersects the pump beam near the sample surface.

If the wavelength of the pump beam is varied, an absorption spectrum of the surface at the location

of the intersecting beams can be generated. This phenomenon has been utilized in the recent development of photothermal deflection spectroscopy [2-5]. If, for a given pump wavelength, the sample is

translated in both directions parallel to the sample surface, a map of the absorption characteristics

of the surface can be obtained. This technique has been referred to as photothermal deflection microscopy [6].

This paper reports preliminary work demonstrating the feasibility of using photothermal deflection microscopy (PDM) to provide high spatial resolution maps of the absorption characteristics of

thin film optical coatings. A small area of a coating has been mapped as part of the development of

the technique, but the limiting characteristics and parameters of this technique are yet to be ex- plored.

The experimental arrangement is shown schematically in figure 1. The 514. 5nm line (approximately 350mW cw) from an argon ion laser was used as a pump source. This beam was directed so that

it was normal to the sample surface and it was focused on the surface with a 120mm focal length lens.

The probe beam from a two milliwatt He-Ne laser was focused with a 75mm focal length lens at the sam- ple surface adjacent to the pump beam. Its angle of incidence was approximately 35 degrees with re- spect to the normal. The relative positions of the pump and probe beams were adjusted to maximize

the deflection of the probe beam. The sample was mounted on an x-y translation stage which was driven

by stepper motors in increments of 4pm in either direction. An EG & G FND-100 photodiode was positioned to receive the probe beam that was reflected from the front surface of the sample. A 632. 8nm

pass filter with lOnm half-band-width was placed in front of the detector. When the detector was translated across this reflected beam, a Gaussian curve was obtained for the intensity distribution of

the beam in a plane perpendicular to the beam at the detector. The detector was then located on the

edge of the beam where the derivative of the Gaussian distribution was a maximum so that small angular

deflections of the beam would result in a maximum variation of the detected beam intensity. The sig349
nal from the detector was amplified with a lock-in amplifier synchronized with a chopper, which modulated the pump laser beam at a frequency of 37Hz.

The signal from the amplifier was routed to a voltage-to-resistance converter and then to one of

the eight paddle ports of an Atari 800 computer. (An Atari computer was selected because it has ana- logue inputs and programmable I/O lines.) The value of resistance was digitized by the computer and

the signal was then available for processing and display. The response of the computer was linear

with the output of the lock-in amplifier to within 0.5%.

The output register of the Atari can be loaded with desired bit patterns which can be made

available at any of the joy stick ports. This capability was used to generate a signal which was decoded by a "one of sixteen" decoder chip to drive operational amplifiers that controlled the stepper motors on the translation stages on which the sample was mounted. Figure 2 shows a block diagram

of the signal electronics.

The computer was programmed to repeatedly translate the sample a predetermined distance and measure the absorption level until the desired horizontal scan was obtained. Then predetermined vertical displacements were executed, each followed by another horizontal scan, until the desired area

was covered. The color of each pixel of a color monitor was determined by the magnitude of the absorption signal. This generated a four-color (a limitation of the Atari) contour map of the absorption characteristics of the sample. Absorption magnitude detail for a given contour can be enhanced

by manipulating the stored data. For example, enhancement was obtained by varying the absorption

signal levels at which the colors were changed and by stepping through the same four colors more than

once. All absorption data were relative as the PDM system has not been calibrated.

This system was used to map the absorption characteristics of single-layer thin film optical

coatings made from materials used in high energy laser applications. Preliminary scans demonstrated

that the high spatial resolution, low-level absorption data provided by this system may reveal flaws

which could become damage sites when exposed to high energy laser radiation. Figure 3 shows an absorption contour map of a 250ym x 5S0ym area of an evaporated Zr0 2 coating on a super-polished fused

silica substrate. The Zr0 2 film had an optical thickness equal to half of the 514. 5nm pump wavelength. Each square pixel provides information about the absorption for a given sample position.

The sample was translated 4pm between each data point or pixel. The four shades represent 4 different ranges of absorption levels. These are 0 to 15, representing the background level of absorption,

15 to 40, 40 to 70, and 70 and greater representing the highest level observed. These data were

repeatable as observed in several scans made over a period of a week.

Figure 3 shows several small absorption sites and one fairly large one. An enlargement of this

major absorbing site was obtained by rescanning the sample and displaying each element using a four

times larger pixel (an option of the Atari graphics). Figure 4 shows a 75ym by 150i_tm enlargement of

the primary absorbing site made in this manner. This does not correspond to an increase in reso- lution, since adjacent pixels still correspond to sample sites separated by 4ym, but it allows the

details of the absorbing site to be more easily observed.

The area of the sample shown in Figures 3 and 4 was inspected using Nomarski microscopy. No features were observed corresponding to the major absorption site detected by photothermal deflection

microscopy. This indicates that photothermal deflection microscopy can detect sites which might not otherwise be seen and which might be precursors to laser damage.

Areas that should be pursued using PDM include studying the correlation between absorption and

laser induced damage sites, performing high resolution elemental analysis of absorption sites, calibrating the system so that absolute absorption data are available, developing submicron spatial reso- lution, and carrying out wavelength-dependent absorption studies.

References

CI] Smith, D.C. "Thermal defocusing of C0 2 laser radiation in gases, " IEEE J. Quantum Electron.

QE-5 , 600 (1969).

[2] Boccara, A.C.; Fournier, D. ; Badoz, J. "Thermo-optical spectroscopy: Detection by the mirage

effect," Appl. Phys. Lett. 36, 130 (1980).

350
Boccara, A.C.; Fournier, D. ; Jackson, W. ; Amer, N.M. "Sensitive photothermal deflection technique for measuring absorption in optically thin media," Opt. Lett. 5^ 377 (1980).

Murphy, J.C.; Aamodt, L.C. "Photothermal spectroscopy using optical beam probing: Mirage

effect," J. Appl. Phys. 51, 4580 (1980).

Jackson, W.B.; Amer, N.M.; Boccara, A.C.; Fournier, D. "Photothermal deflection spectroscopy

and detection," Appl. Opt. 20, 1333 (1981).

Murphy, J.C.; Aamodt, L.C. "Optically detected photothermal imaging," Appl. Phys. Lett. 38,

196 (1981).

X-V TRANSLATOR

POSITION SENSING

DETECTOR

FOCUSING LENS

(f :120mm)

PROBE BEAM FOCUSING LENS (f =75 mm)

PROBE BEAM 632. 8nm

HeNe

LASER

PROBE BEAM

DIRECTING MIRROR

RATIO DETECTOR

PUMP BEAM 514. 5nm

I O

ARGON-ION

LASER

PUMP BEAM DIRECTING MIRROR

CHOPPER

Figure 1. Photothermal deflection microscopy experimental arrangement.

351
VOLTAGE TO RESISTANCE PREAMP

V/R CONVERSION

ZERO

STEPPER MOTOR INTERFACE

DECODER

ATARI JOYSTICK ^

f

1 "

2-

PADOLE IN

13

24

S7

S

a

}

£>=}

MOTOR 1 PHASE 1

MOTOR 1 PHASE 2

MOTOR 2 PHASE 1

MOTOR 2 PHASE 2

Figure 2. Block diagram of the signal electronics

*

I

^^J^n^T^ of/

DM

n COntOUr video display of A/2 optical thickness of Zr0 2 on a superpolished fused silica substrate. Contour covers a 250ym x 550pm area.

352
Figure 4. 75ym x 150ym contour of primary absorbing site in figure 3.

353
Several in the audience questioned whether a study of thin films of materials gave a true measure of the relative merit of the materials . The speaker replied that the data had good internal

consistency ; with few exceptions , films of a given material had similar thresholds even though they were made in two or more coating runs, but conceded that the results might be biased by the

relative experience of the coater in depositing various materials . In response to other questions,

the authors clarified that (1) coating stress was inferred from interferometric measurements of

stress-induced flexure of thin substrates that were coated simultaneously with damage-test samples,

(2) that he had no explanation why substrate polishing seemed to effect low-threshold films but not

high-threshold films since all substrates were similarly polished , and (3) that AI2O3 had not

been included in the list of most promising materials because the half-wave layers of AI2O3

were highly stressed . A strong criticism was made of the fact that the data were presented as

linear plots of thresholds as a function of variables such as refractive index and stress, instead

of through functional forms developed during previous studies of the scaling of damage thresholds

with physical parameters of the coating.

354
Interaction Mechanisms of Laser Radiation With Ocular Tissues*

David H. Sliney

US Army Environmental Hygiene Agency

Aberdeen Proving Ground, Maryland 21010, USA

Many investigations of laser injury to the eye have been conducted over the past 20

years for the purpose of setting standards for safe exposure of the human eye. Such

studies have also played an important role in the developments of laser applications in

medicine and surgery. A review of this extensive body of experimental data suggests that

there are at least three dominant mechanisms of injury which are well accepted. However,

other effects cannot be ruled out, and several studies suggest that the time-course of

injury and repair as well as the physical aspects of the interaction of biological tissue

with optical radiation must be considered. The scaling relations of spot-size, and

pulse-duration dependence of threshold differ from those applicable to material damage.

KEY WORDS: Eye; laser damage; optical breakdown repetitive pulses; retinal injury; spot-size

dependence; thermal injury ; thermoacoustic injury.

1. Introduction

In the course of development of laser biomedical applications and safety standards for human

exposure, it is necessary to critically examine not only the thresholds of injury in biological

tissue, but also the mechanisms of laser interaction with the tissue and the mechanisms of

biological damage and repair. It would be unwise to simply determine acute injury thresholds and

then to set exposure limits below these thresholds. Knowledge of the mechanisms of interaction and

damage are important because of our limited ability to predict the delayed onset of adverse

biological effects. The delayed appearance of lenticular cataract only years after ocular exposure

to a radiation insult is but one example of such delayed effects. Through knowledge of the

mechanisms of interaction of laser radiation with biological tissue and from the extensive

experience with the effects of heat and conventional optical radiation exposure on man, it is possible to provide the best judgement of what levels should not be hazardous to health. Depending

upon the nature of the injury mechanism, one is better able to predict the likelihood for delayed

effects at levels below acute injury thresholds. The presently accepted ocular exposure limits

attempt to take these considerations into full account [1].

Unfortunately, there are at least three types of exposure where the cycle of injury and repair

are not well understood: ultrafast (sub-nanosecond) pulsed exposure, the spot-sized dependence of

retinal injury for q-switched exposure durations, and the additive effects of multiple-pulse

exposures. We shall examine each of these problem areas and one more, and suggest some possible

explanations for some of the biological effects that have been reported. It is, of course, impossible in this brief paper to review all of the many studies of laser biological effects;

however, several representative references will be provided on each major point. The absence of

references to many specific reports that were considered in this presentation should not be

interpreted as suggesting that the referenced papers are the only relevant ones. Many others have

been reviewed [2]. Furthermore, a major purpose for presenting this review at the Boulder Damage

Symposium is to encourage physical scientists and engineers to take a greater interest in this

subject. Hopefully this can lead to a wider critique of biomedical studies and to suggestions for

further, meaningful investigations.

* The opinions or assertions contained herein are the private views of the author and are not to be

construed as official or as reflecting the views of the Department of the Army or Department of

Defense.

355
2. Principal Damage Mechanisms

It is widely accepted today in the biomedical community that there are at least three dominant

mechanisms of injury involved in laser damage to the cornea, lens and retina of the eye. These

three dominant mechanisms of injury are initiated by: a photochemical event, a thermal event, or

an interaction dependent upon the electric field strength of the incident optical wave. The latter

mechanism (or mechanisms) apply to ultra-fast exposures. The thermal effects are clearly dominant

in the infrared and for almost all pulsed exposures between 1 ms and 1-5 seconds. The

photochemical events are generally limited to wavelengths less than 550-650 nm, and then are clearly evident only for lengthy exposures of 10 s to several hours. There are clearly cases where

two or more injury mechanisms jointly play a role. Sometimes it has been necessary to choose

extreme parameters of exposure to study the mechanism, e.g., very large retinal images or very long

exposure durations that may be unrealistic in our normal environment. Varying the wavelength has

also led to interesting findings.

A common error that is often made when interpreting the biological effects of laser radiation

is to consider only the physical interaction of the optical radiation with matter and the

dissipation of the absorbed energy away from the site of exposure (e.g., by heat conduction). We must remember that a biological system is dynamic, and that immediately following an insult to tissue some biological actions can be expected. Several repair processes can start to work almost

immediately. Also, several adverse reactions can be initiated by the initial insult and become

apparent only long after exposure. Delayed onset of the appearance of tissue injury is particularly common with photochemi cal ly initiated events. Examples of photochemical effects are: retinal injury from lengthy exposures (10-1000 s) to short-wavelength light or skin or corneal

injury following UV exposure. Retinal lesions from 441.6-nm (He-Cd) laser radiation at injury

threshold typically do not appear for 48 hours [3] and erythema and photokeratitis typically do not appear for several hours following exposure to 300-nm UV radiation [1].

Although erythema (sunburn of the skin) and keratoconjuctivitis (welder's flash) following UV irradiation of the skin and eye have been studied extensively from the beginning of the century,

the exact chain of events following exposure, and even the site of the initial photochemical event

and the chromophores are not really known. Several therories exist, but no theory attempts to

explai/i all aspects of the delayed reaction. Since reciprocity of UV irradiance and exposure

duration exists for exposures of microseconds to 1000 s or more, and since the action spectrum for

each of these effects has a sharp cutoff in the 305-315-nm band (where individual photons have

insufficient energy to cause the reaction), no one doubts that the injury mechanism is initiated by

a photochemical event. Photosensiti zers permit less energetic near-UV (UV-A) photons to elicit the

same result as 300-nm photons.

Photochemical retinal injury from laser exposures lasting greater than 10 s has only recently

been studied extensively [3-6] and is today still not well understood. Figure 1 shows the

approximate action spectrum measured outside of the eye for this effect. Thresholds for UV retinal

damage, if measured at the retina, are lower than at 440 nm, but fortunately the intact crystalline

lens filters out the UV and protects the retina. The thresholds at the retina for this effect vary

from about 20-30 J/cm 2 at 442 nm to 3000 J/cm 2 at 632 nm as shown in Figure 2 [2,3]. Ham and his

colleagues have studied many histological cross-sections of photochemical ly damaged retinae of

rhesus monkeys and argue that damage is always seen first in the retinal pigmented epithelium

(RPE), [3] but others claim that photochemical injuries show damage throughout all layers of the

retina [5-6]. Figure 3 shows the retinal layers. There are probably several chromophores in the

retina that are sensitive to blue light and ultraviolet (UV-A) radiation. In fact, the appearance

of UV-A retinal lesions and 441.6-nm lesions are different [3,4]. The time course of appearance

also varies with wavelength, just as does the time course of appearance of UV erythema varies

between 254-nm and 300-nm radiation. Clearly the melanin pigment in the RPE plays a major role,

but as Ham has pointed out, melanin could act as both a source and sink for free radicals, because

of its unusual characteristic of containing stable free radicals, possessing semiconductor

properties, surface metal ions, and its characteristic of both scavenging for free oxygen and

producing free radicals, depending upon the light environment [3]. Others report that the melanin

in the RPE changes with age, [7] that it loses its photoprotecti ve ability with increased light

exposure, that acute retinal lesions from blue-light exposures contain melanin granules resembling

senile melanin, [3] and that senile macular degeneration (a major cause of blindness in old age) is caused to some extent by short-wavelength light [8]. Since melanin is the major concentrated

absorbtion site of laser radiation in the retina, it plays the key role in retinal thermal injury,

photochemical injury, and probably also in ultra-fast injury events. As more basic research is performed on melanin, more understanding of laser injury mechanisms will follow. Possible

blue-light chromophores for retinal injury include flavoproteins , glutathione peroxidase and other

light-activated enzymes, and cytochrome-C [1,2].

356
300 400 500 600 700

Wavelength nm

Figure 1. Spectral Weighting Functions. The spectral weighting function now used for the analysis

of retinal injury from blue-light [2](BA

) is compared with the night vision, or scotopic (V^),

and the photopic (V.) daylight spectral response of the human eye. The peak of the "blue-light

hazard" function is at 435-440 nm.

3. Determining Injury Thresholds

Thresholds of biological injury may be determined by direct observation, by microscopic study

(histology) or by measuring a functional change. In any case, a probit plot is frequently made

(Figure 4) to provide some statistical insight into the quality of the data and the injury

mechanism. As shown in Figure 4, the true-statistical variation in injury threshold from different

exposure sites or different individual animals may be distorted by poor focusing at the retina or

by other experimental errors. While some errors will shift the probit line to the right, other

errors (such as focusing) will reduce the slope of the line.

4. Retinal Thermal Injury

Retinal thermal injury has been studied extensively, [9-14] and many mathematical models of

heat flow and temperature rise in the retina have been developed [9-13]. If the average

temperature rise in the RPE is calculated, it is shown that the temperature rise required for an experimentally determined retinal lesion increases for shorter and shorter exposure times from 10 s down to about 7 ms. But then, unexpectedly this temperature rise decreases for pulse durations

less than approximately one ms [1,2,11]. Figure 5 illustrates that the peak temperature rise

calculated for the average density RPE is greatest for 1-ms exposures; there is little difference

in this trend other than peak values, whether one calculates this temperature dependence at the

center of a minimal, 16-^m diameter image or at a point just outside the image. The same threshold

injury dependence applies to large image size lesions [11]. Hayes and Wolbarsht argued that for

pulse durations less than 1 ms , one must consider the melanin granule temperature rise and its

cool-down [14]. At threshold irradiances for 20-ns exposures the melanin granules are raised to incandescence. Marshall notes that vacuoles in mitochondria are seen following q-switched

exposures, but not in retinal injury from 1-s exposures [15]. Several differences in the

onset-time of the visible lesion and in the histological appearance of the lesion resulting from

sub-microsecond exposures have led most of us to argue that injury mechanisms for q-switched

exposures are not limited to the simple thermal coagulation of proteins.

357
10 I I I I I I iiil i

i I I I iiil I

I I 1 mil | I I I I I III I I I I II I I i I 10

1 10° I0

1 I02 I03 I0

4 EXPOSURE DURATION (sec.)

Figure 2. Photochemical and Thermal Retinal Injury Thresholds. Threshold retinal injury from a photochemical retinal injury mechanism appear only for exposure durations exceeding 10 s; the

appearance of injury is delayed; the effect is limited to short wavelengths (higher photon

energies) and reciprocity of irradiance and exposure duration is observed. The thresholds for

the 1064-nm wavelength are clearly those of a thermal damage mechanism. Data points are from

reference 3 (exposures less than 10^ s) and 6 (longer durations) [2].

Figure 3. Retinal Structure. This drawing shows the different layers of the retina and adjoining

choroid. The neural retina layers are nearly transparent. Most light is absorbed in the 3 to 10

jjm thick RPE and the choriod capilaries. Left panel shows variation of layers in central retina

at the fovea where neural layers are pulled back. Adapted from Polyak [in 2J

.

358
mW INTO EYE

Figure 4. Probit Plots. The use of probit analysis to determine a slatistical "threshold" (the

ED-50) is particularly important when dealing with the structural and pigment variation common in

biological systems. The steepness of the curve gives a clue to injury mechanism, but also shows

the care of the experimental procedure. When attempting to achieve a minimal (nearly

diffraction- limited) focal spot on the retina, a slight refractive error or corneal scattering

will lead to a higher "threshold" (ED-50) and a less steep slope of the probit curve. C2,25J

.

£C0

the m

Figure 5. Time-Temperature Histories for Injury Thresholds as Calculated by a Thermal Model [2].

The upper curves are calculated for 0.01-ms to 10-s exposure for minimal image size lesions in

the rhesus monkey. Temperature rises are at the center of a 10-um thick RPE at the center of a 16-um image. The upper bold curve is an envelope of the peak temperatures reached for each

time-temperature history. The lower curves are for the same minimal threshold irradiance data in the rhesus monkey, but this time the point of temperature calculation was moved to a point at the

center of the 10-um thick RPE, but 20 urn radially from the center of the image. The latter

points would indicate the time-temperature history of cells just adjacent to the imaged retinal

area. Note that the peak of the temperature elevation for the 10-us pulse does not occur until

0.1 ms , and the boiling point of water is never reached by any of the latter curves.

359
There are two curious results that are not really explained by any of the mathematical models

for thermal injury. All of the thermal models would predict that for exposures less than

approximately 100 jjs the temperature rise would be so great prior to any significant heat flow that

there should be no difference in injury thresholds for exposure durations less than 0.1-1.0 ms, and

there should also be no variation of threshold with image size. The melanin-granule models suggest

some decrease in damage threshold for the RPE cells themselves for the shorter exposure durations,

since the incandescent granules could destroy adjacent structures in the cell, but certainly no image size dependence of injury could be explained. Heat flow from large images during 1-s

exposures is insufficient to cool the center of the image for the same irradiance that causes no injury in a smaller image. This leads to the image-size dependence predicted for exposure

durations exceeding 1 ms. The rate-constants for thermal coagulation of proteins are such that

thermal models that include a rate-process dependent term predict that injury from a 20-ns exposure

should occur before heat flow becomes significant (i.e., in less than 100 jus ) [16]. The

temperature rise in the outer receptor layer where irreversible damage occurs is 10 times less for

a 20-ns pulse than for a 1 ms pulse, but the time-temperature history in this layer should be the

same for either pulse, since heating occurs in this layer only from conduction of heat from the

adjacent RPE. The rate-process models do predict one scaling relation: the decrease in threshold

radiant exposure (J/cm?) as a function of the pulse duration t raised to the 3/4 power for exposure

durations ranging form 20 jus to 10 s (Figure 6) [9]. Another scaling relation is seen in Figure 7: the retinal injury threshold for aJJ_ exposure durations, from 30 ps to 10 s varies approximately as the reciprocal of the image diameter for image sizes of 20 urn to at least 1 mm.

Another anomaly not explained by the thermal models is the fact that the thresholds for 10-20

ns exposures are greater than the thresholds for 500-ns to 10-us exposures for the same image size

and same wavelength [2]. This curious phenomenon is shown in Figure 6, and many more data points

between 10 ns and 1 jjs that are not shown offer solid evidence that this temporal dependence of

threshold for such short exposures is real and not an experimental artifact. What is the

explanation? Two possibilities appear reasonable to me. As the pulse duration decreases, more

energy could go into a noninjurious dissipative mechanism such as an accoustic transient at threshold. Of course this very mechanism is known to be the principal cause of severe, almost

explosive damage in the retina for suprathreshol d exposure levels. An alternate, and perhaps more appealing explanation is that melanin may be a saturable absorber like some other organic

molecules, and as the irradiance increases for shorter exposure durations, the amount of energy

absorbed in the granules becomes less. I have tried to detect such behavior by irradiating a vial

of melanin in a water suspension with a q-switched pulsed laser beam, but was unable to achieve a clear result because I could not prepare a sufficiently dense and uniform suspension of melanin

granul es

.

Ips Ins I/js Ims Is 1000s EXPOSURE DURATION (S)

Figure 6. The Dependence of Retinal Injury Threshold Upon Pulse Duration. The injury thresholds

in the rhesus monkey are plotted as a function of total laser energy entering the eye vs exposure

duration. The dashed line shows the laser safety limits (ANSI Z-136.1). Note the anomalous

increase in thresholds for both 1064-nm [•] and visible light [] at 10 and 20 ns when compared

to thresholds at 10-20 /js. Safety limits do not exist for mode-locked pulse durations. Adapted

from refs 2 and 18.

360
1000

100

— 10 3

o

— I ms 514 5

* 10

<

z

0.1

0.01 10 100 1000

RETINAL IMAGE DIAMETER ( »m

10000

Figure 7. Scaling Relation for Retinal Injury Threshold with Spot Size [2]. While the reduced

exposure threshold for large spot sizes ( d

f

) is expected for longer exposure durations (due to

heat conduction), the same dependence of approximately l/d r is surprising for 15-ns and 30-ps

pul ses.

Perhaps the most curious contradiction of the thermal model is the collection of experimental

threshold data that clearly show that pulsed exposures separated by durations adequate for almost

complete cooling of small image sizes (e.g., 100 ms) show an additivity of exposure. Minimal image

sizes should cool within 1-10 ms and the thermal model predicts that no reduction in injury

threshold should result from exposure to a train of pulses separated by that duration, even if the

pulsed exposures are superimposed over the same retinal area. A large collection of data now suggests that one can empirically predict that the threshold exposure of a single-pulse in a train

is decreased proportionally to N" 1/4 where N is the number of pulses in the train [17]. This

relation holds for 10-ns pulses, 10-jjs pulses, and 10-ms pulses over a wide range of pulse

separations and exposure durations for small image sizes. According to one study it appears not to occur for large image sizes [18]. But then another study with large image sizes does tend to show

the additive effect [20]. One phenomenon — biological repair -- may explain these and the other

anomal ies.

361
5. A Rapid Repair Mechanism

Mathematical models of heat flow suggest that the elevated temperature in a 20-um diameter

irradiated area will return to normal within a few milliseconds [13]. However, all of the

repetitive-pulse injury thresholds show an additivity for pulses separated by several milliseconds

and even 100 ms. One possible explanation for this apparent contradiction would be the presence of

a rapid repair mechanism for retinal thermal injury following short-pulse exposure. I would

suggest one hypothesis: the existance of a repair mechanism that precludes the appearance of a thermally induced lesion for small spot sizes at levels near a threshold for immediate injury. It would be consistent with present threshold data to hypothesize that if exposures are delivered

during the period of primary activity of this repair mechanism (l ms to 10 s), any further

exposures may inhibit the repair process. In fact, one could even expect that the area already

exposed could be more vunerable to further injury if exposed during this repair period.

Furthermore, if we hypothesize that this repair mechanism is most effective for smal

1

image sizes,

we can explain most of the experimental data. It certainly would be plausible that closure of a damaged hole in a ruptured membrane would be more rapid and more effective for the smaller lesion.

If the repair mechanism involves the activity of surrounding, unexposed (or uninjured) cells, or the movement of enzymes, then the rapid repair of a few cells would be quickest and most effective

for the smaller image areas. It is true that I am being very speculative, since the evidence for

such a repair mechanism at present is still quite limited. In my judgement, the study of retinal

pigment epithelium (RPE) cells in vitro (i.e., in a dish) by King and Geeraets [19] lends support

to this hypothesis. In their study the thresholds of injury for 20-um and 135-um diameter spots in their cell culture exposed to 30-ns ruby (694.3-nm) laser radiation were the same; i.e., there was

no variation of threshold with spot size.

It was once argued that the spot-size dependence for short-pulse exposures was due to the

inability to see very small retinal lesions through the slightly scattering ocular media with the

ophthalmoscope, but later studies showed that this contrast problem played only a minor role and

then only for image diameters below 50 jum. Since there would be no external limiting membrane and

no underlying Bruch's membrane and choriocapi 1 1 ari s (structures on either side of the RPE), a repair process from adjacent cells above and below (or damage of the absent structures) would not be possible in the cell culture.

Since the retinal irradiance threshold reduction for a train of very many pulses and for very

large spot sizes (<^1 mm) are similar, these lowered thresholds may be the lower asymptote for

retinal injury thresholds. The higher thresholds for the appearance of a single, small-image

exposure or intermediate exposures may be the result of a rapid repair process. Further threshold

studies of very large retinal image sizes and very long trains of pulses appears to be justified.

A study by Greiss, et al . , shows that thresholds for repeated exposures of the retina at 1064 nm (nonphotochemical process of injury) for 0.9-mm image diameters in the rhesus monkey retina were

not reduced from a single-pulse threshold as shown in Figure 8 [18]. We would therefore expect

that lesions from different pulse conditions would appear somewhat like those shown in Figure 9,

which to some extent agrees with the descriptions of experimental thresholds. Regrettably, there

is another earlier study which contradicts this. Walkenbach found a reduction in the threshold

with additional pulsed exposures for image sizes of 200 urn to 800 um diameter [20].

The large volume of studies of laser damage to nonbiological materials is useful to consult

when considering biological injury thresholds from pulsed lasers. A spot-size dependence and

repetitive-pulse additivity of damage thresholds in coatings and bulk solids have sometimes been

reported, but the direct comparison to retinal tissue is often not possible. For example, the

spot-size dependence for damage in "transparent" materials is argued to be due to the probablistic

character of exposing a small area of the material which also contains a microscopic imperfection

or absorption site. The same argument has been made for optical coatings [21]. Such a spot-size

dependence is for focal diameters of less than 400 ;um in these low absorption materials and damage

threshold exposures are at least 100 times greater than for the RPE. A more realistic comparison

can be made with damage thresholds of absorptive layers, such as photographic print films or carbon

paper [22,23]. Bergqvist, et al . , [22] did not find significant additivity for trains of 1064-nm

pulses focused to a 20-um spot on a dark x-ray film. Studies of 694.3-nm damage thresholds for

20-ns exposures in my laboratory some years ago did not reveal a spot-size dependence for focal

spot diameters greater than 100 um [2]. From all of these observations, I favor a working

hypothesis that a retinal repair process is influencing the observed thresholds for an opthalmoscopi cal ly visible lesion for q-switched laser exposures [1].

362
10 09

08

07

06

0 5

0.4

0 3

02

A

7b

- \ 8 -

. -

1

I

i i i mill i

I I 1 1 ml l LjUUujul

0

Himbtr of pulses

IO0

Fiqure 8. Ratio of Retinal Injury Thresholds for Different Pulse Trains. The empirical relation

N-l/ 4 is plotted as a solid line. Relative threshold exposure per pulse is shown for 0.9-mm

image size h ; for 30-um spot size macular exposurej); and for 30-um paramacular exposures _g_;

open symbols~are 1064 nm; dark symbols, 532 nm. From Greiss, et al . , (1980) [18].

SINGLE PULSE

LARGE SPOT

MINI SPOT

LINE OOO

O o O o

1 M s 1ms 1s

MULTIPLE PULSE

LARGE SPOT

MINI SPOT

LINE

1ms 1s TIME

10s 000

10s

Figure 9. The Hypothesized time Course of the Appearance of a Threshold Retinal Lesion. Note that

the repetitive pulsed exposure during the time depicted here would prevent repair of the

irradiated area.

363
An alternative hypothesis for this spot-size dependence could also be related to the method of

detecting a visible lesion. When the investigator observes a damage site during and after the

pulsed exposure, he often reports the appearance of a brief enhancement of retinal reflection at the exposure site [25]. This could be the result of a rapidly formed gas bubble at the junction

between the RPE and the retina. The gas bubble is reabsorbed into the adjacent tissue more readily

for the smaller lesion. The longer the bubble lasts could be the greater the adverse effect upon

adjacent tissue.

Still another (but related) alternative hypothesis would be the release of a biochemically

toxic agent following membrane rupture [26]. When released in a sufficiently great quantity (as

with a large image), the toxic dose is sufficient to result in permanent injury. Whatever factors

play a significant role await further research.

6. Retinal Effects From Semiconductor Diode Laser Exposures

A puzzling change in the appearance of the retina has been reported by Beatrice, et al .

,

following exposure of the rhesus monkey retina to trains of Ga-As (904-nm) laser pulses with

average power levels entering the eye of only 22 uW to 260 uW [27]. This effect, which sometimes

fades, and has not been detectable by histology, is not always seen and it is difficult to relate

to pulse repetition frequency. Attempts to duplicate the finding have not always been successful,

although a retinal change appearing as a darkening was observed at 300 uW by Ham and Mueller for a high-frequency pulse train. Since some emissions of blue light at 452 nm were noted as a result of

frequency doubling in those specific diodes that caused some of the "changes," it appeared

reasonable to speculate that the alteration at such low levels (well below thermal injury

thresholds) could have been due to the self-oscillation found in most semiconductor diode lasers.

The self-oscillation typically occurs at frequencies of 250-500 MHz, and individual pulses are less

than 100 ps [28]. This behavior of diode lasers and self-mode-locking of most He-Ne lasers may

point to an explanation of two curious experimental findings for lowered thresholds for trains of

exposures lasting several seconds. Besides the GaAs laser thresholds for 10-1000 s, the threshold

reported for 1000-s He-Ne exposures may be slightly lower due to this effect. Since most future

human exposure to laser radiation may occur through the use of Ga-As lasers employed in fiber-optic

communication systems and from He-Ne lasers, the study of these new phenomena as they affect

retinal injury is clearly important. I should like to emphasize, however, that the presence of a visible change does not necessarily mean damage, any more than the detectable bleaching of visual

pigments upon exposure to intense light. It may well be that these changes at 850-905 nm may be

due to the photoactivation of enzymes [29,30]. Biscar reports that bovine a-chymotrypsin was activated at a very narrow band of wavelengths between 853 nm and 857 nm. This demonstrated that

such effects could be produced in the near-infrared, and at a very select wavelength band [30].

Such observations force one to consider selective laser chemistry as a potential mechanism for

laser injury in biological systems at irradiance levels believed only recently not to be of concern

[31-32], To my knowledge, such phenomena have yet to be shown to be plausible as injury

mechanisms, but such possiblities cannot be overlooked. The effects of resonant interaction of a mode-locked train of pulses with a crystal lattice, although improbable in the retina, cannot be

completely ruled out either, when one considers the regular organization of some retinal structures

[33]. Lund has recently reported an apparent wavelength dependence of retinal injury thresholds

between 700 nm and 950 nm that does not follow the absorption spectrum of melanin [34]. However,

there were pulse width variations with the dye laser used. These could have lead to some of the

differences. We must await the results of further studies to find the answer to this new puzzle.

7. Conclusions

Although much of the biological threshold data for laser injury of the eye can be understood as following the prediction of either a photochemical or thermal model, some remaining data require more complex models -- such as the incorporation of biological repair or biological amplification

of the initial effect. Work continues in this area. Other areas of needed research include the

further understanding of picosecond-pulsed injury of the retina, although most evidence suggests

that it depends on the peak power in the pulse for pulse durations between 5 ps and 100 ps. Competing damage processes resulting from multi-photon absorption and electron avalanche effects as occur in solids, [35] probably explain the higher threshold for 532 nm when compared to 1064 nm at the same pulse duration (30 ps) as the data of Goldman show [36].

Interest in Nd:YAG mode-locked and q-switched laser injury of the retina has recently been

elevated by the introduction of these types of lasers to cut membranes by producing microplasmas in the clear media of the eye [37,38].

364
Still further research is also necessary to understand the effects of speckle upon retinal

function at very low, chronic exposure durations as reported by Zwick [39]. It may well be that if we force ourselves to stare at a large area diffuse laser reflection, the speckle which produces a myraid of areas of very high contrast borders at the retina may affect those neural mechanisms that

detect borders in our normal world. Fortunately, most of us find it uncomfortable to stare at speckle for long periods, and this effect is hopefullly just a laboratory curiosity. Clearly there

is still much to learn vis-a-vis laser irradiation of the eye.

I would like to thank Dr. M. L. Wolbarsht of Duke University for helpful criticism during the

development of this paper, and Dr. J. Taboada of the USAF School of Aerospace Medicine who pointed

out the existence of references 30 and 33 (Biscar, and Ratjska & Piekara).

8. References

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for human exposure limits, Kvant Elektron . , 7(10) : 2271-2281 (1980) and Soviet J. Quant.

Electr . , 10(10) : 1278-1284 in translation.

D. H. Sliney and M. L. Wolbarsht, Safety with Lasers and Other Optical Sources , New York,

Plenum Publishing Corp. (1980).

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radiation damage: dependence on wavelength, power level, and exposure time, Vision Res .

,

20(12): 1105-1111, (1980).

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spectrum for retinal injury from near-ultraviolet radiation in the aphakic monkey, Am. J. Ophthal

.

, 93(3) :299-306, 1982.

T. B. Williams and B. N» Baker, (Eds.), The Effects of Constant Light on the Visual Process , Plenum Publishing Corp., New York (1980).

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Doc. Opthal ., 44(2):379-402 (1977).

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R easearch , Vol. 2, Academic Press, New York, pp. 119-179, (1980).

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R esearch (M. L. Sears, Ed.), Yale University Press, New Haven, (1981).

R. G. Allen, Retinal thermal injury, j_n (M. L. Walbarsht and D. H. Sliney, eds.) Ocular

effects of non-ionizing radiation, Proc. Soc. Photo-Opt. Eng ., 229:80-86 (1980).

J. J. Vos, A theory of retinal burns, Bull . Math. Biophys . , 24:115-128 (1962).

A. M. Clarke, W. Geeraets, and W. T. Ham, Jr., An equilibrium thermal model for retinal

injury, Appl . Opt ., 8:1051-1054 (1969).

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Applications in Medicine and Biology , Vol. 1, Plenum Press, New York (1971), pp. 125-162.

M. A. Mainster, T. J. White, J. H. Tips, and P. W. Wilson, Retinal-temperature increases

produced by intense light sources, J. Opt, Soc. Am ., 60:264-270 (1970).

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Aerospace Med ., 39:474-480 (1968).

J. Marshall, A. M. Hamiliton, and A. C. Bird, Hi stopathol ogy of ruby and argon laser lesions

in monkey and human retina, a comparative study, Br. J. Opthal . , 59(1) : 610-630 (1975).

C. Hu and F. S. Barnes, The thermal -chemical damage in biological material under laser

irradiation, IEE Trans Biomed Eng, BME-17:220-229, 1970.

365
B. E. Stuck, D. J. Lund, and E. S. Beatrice, "Repetitive Pulse Laser Data and Permissible

Exposure Limits," Report No. 58, Letterman Army Institute of Research, Presdio of San Francisco, San Francisco, CA, (1978).

G. A. Greiss, M. F. Blankenstein and G. G. Williford, Ocular damage from mutiple-pulse laser

exposures, Health Physics , 39(6) : 921-927 (1980).

R. G. King and W. J. Geeraets, The effect of Q-switched ruby laser on retinal pigment

epithelium in vitro, Acta Opthalmol ogica , 46:617-632 (1968).

. Walkenbach, J. E., Determination of retinal lesion threshold energies of pulse repetition

Nd+ 3

: YAG laser in the rhesus monkey, Masters Thesis, Medical College of Virginia, Richmond,

VA, June 1972.

R. H. Picard, D. Milam, and R. A. Bradbury, Statistical analysis of defect-caused laser

damage in thin films, Appl . Opt ., 16:1563-1571 (1977).

T. Bergqvist, B. Hartmann, and B. Kleman, Imaging properties of the eye and interaction of

laser radiation with matter, vn (Tengroth and Epstein, eds.) Current Concepts in

Ergrophthalmology , pp. 55-71, Ergophthalmologica International i s , Department of

Opthalmology, Karolinska Institute, Stockholm (1978).

J. E. Sol lid, S. J. Thomas, and C. R. Phipps, Jr., Damage threshold variation with spot size

at 10.6 urn for Kalvar and Polaroid films, Appl . Opt . , 18(4) : 424 (1979).

D. J. Lund, E. S. Beatrice, and S. T. Schuschereba , Bioeffects concerning the safe use of GaAs laser training devices, pp. 15-29 in (Beatrice, E. S., ed.), "Combat Ocular Problems,"

Letterman Army Inst. Res., 1982.

E. S. Beatrice, Letterman Army Institute of Research, private communication.

M. L. Wolbarsht, Duke University, private communication.

E. S. Beatrice, D. J. Lund, M. Carter, and D. M. Talsma, "Retinal Alterations Produced by Low

Level Gallium Arsenide Laser Exposure," Report No. 38, Letterman Army Institute of Research,

Presidio of San Franscisco, San Franscisco, CA (1977).

R. W. Dixon, Current directions in GaAs laser device developmemt, Bel 1 System Tech. J.

,

59(5) :669-722 (1980).

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Photobiol .

,

32:841-848 (1980).

J. P. Biscar, Photon enzyme activation, Bull . Math. Biol .

,

38:29-38 (1976).

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selectively, Optical Engineering , 19(1): 10-20 (1980).

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action of laser radiation, Sov. J. Quantum electron ., 8(10) : 1263-1268 (1978), originally

Kvantovaya Electron ., 5:2243-2251 (1978).

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with matter, Appl. Opt ., 17(23) : 3815-3817 (1978).

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Letterman Army Inst. Res., (1982).

| H. E. Bennet, A. J. Glass, A. H. Guenther, and B. E. Newman (Eds.), Laser Induced Damage in Optical Materials, "NBS Special Publication 568, US Department of Comerce, National Bureau of

Standards, Boulder, CO, (1980).

| A. I. Goldman, W. T. Ham, Jr., and H. A. Mueller, Ocular damage thresholds and mechanisms for

ultrashort pulses of both visible and infrared laser radiation in the rhesus monkey, Exp. Eye

Res. , 24(1) :45-56 (1977).

366
[37] D. H. Sliney, YAG Laser Safety, Chap 5, in (S. L. Trokel , ed.) YAG Laser Ophthalmic

Microsurgery , Appl eton-Centry-Crofts , East Norwalk (1983).

[38] M. A. Mainster, D. H. Sliney, C. Davis Belcher, III, and S. M. Buzney, Laser photodi srupters

:

damage mechanisms, instrument design and safety, scheduled for publication in Ophthalmol ogy , 1983.

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367
Spotsize Effects in Laser Damage Testing

S. R. Foltyn

University of California, Los Alamos National Laboratory

P. 0. Box 1663, Los Alamos, NM 87545

For the past three years a program of testing and evaluation of KR coatings at excimer

laser wavelengths has been in progress. During this period, an unconventional method has

been employed to plot damage data: Ten sites are irradiated at each test fluence and the

fraction of damaging sites is plotted as a function of peak fluence. The resulting curves slope from a fluence at which no damage was produced to a higher fluence at which all ten

sites damaged. Attempts to explain wide slope variations among a variety of tested samples

have led to a simple model which correlates slope, defect density, and Gaussian spotsize of

the test beam. Specifically, the model predicts steep slopes if the beam is large compared

to the mean defect spacing. In contrast to conventional notation, threshold is defined as the highest fluence at which the sample could not be damaged. Implicit in the model is

that threshold thus defined is independent of spotsize while conventional thresholds have a calculable dependence. In the limit of large spotsize, the two thresholds are identical.

Test results for various spotsizes are reported and analyses of previously published

experimental data are presented in support of this model.

Key words: coating defects; damage probability; damage statistics; defect density; laser damage;

size effects; spotsize; spotsize effects.

1 . Introduction

One of the more interesting and certainly one of the most frustrating aspects of measuring laser

damage thresholds is that the results of such measurements are influenced to varying degrees by an enormous range of parameters. Variables are introduced throughout the life cycle of a test sample;

during fabrication (materials, polishing, cleaning, coating design or deposition parameters), during

testing (wavelength, pulselength, spotsize, statistical variations), and in-between (storage, handling, cleaning) [1]. As a consequence, individual test results become meaningless without extensive

characterization and documentation; valid intercomparisons between test facilities become virtually

imposs ible

.

In this work, the effect of one of these variable parameters - spotsize - is analyzed. It is

shown that spotsize effects can arise from an improper, although traditional, definition of threshold

and that a more realistic method of defining threshold leads to test results which are independent of

spotsize

.

2. Spotsize Effects

The first reported observations of spotsize effects - the apparent increase in damage threshold

with decreasing test spotsize - appeard in the literature nearly ten years ago [2,3]. A physical

model was described in which the samples under test contained a distribution of damage-prone defects

that were more likely to be irradiated (hence damaged) by large laser spots than small ones. Similar

observations have been described and analyzed in succeeding years [4-11], but despite considerable

attention, there remains an aura of skepticism surrounding thresholds measured with small spots and a corresponding drive to do very expensive large-spot (>1 cm) testing with high-energy lasers. Interestingly, the beastly nature of such lasers (i.e., poor beam quality and repeatability) can itself

introduce large uncertainties in the test results.

3. Threshold Definition

The Los Alamos excimer laser damage test facility has been in operation for the past three years

[12,13], during which time an unconventional method has been employed for plotting test results and

defining damage threshold. This is illustrated in figure 1. Here, a 308 nm reflector of Sc 2 0 3 /MgF 2 design was irradiated at nine different peak fluence levels. Ten discreet sites on the coated

surface were tested at each fluence and the fraction of sites that damaged (i.e., the probability of

368
damage) was plotted as a function of peak test fluence. Damage threshold is defined as the zero- percent crossing of a linear regression fit - 6.2 J/cm 2 in this case. (Note that in figure 1, a logarithmic fit is used; the difference in threshold is insignificant).

Figure 2, in contrast, shows the same data replotted in the conventional manner. For each site

tested, a closed circle was placed at the "damage" level, or an open circle was used to indicate no

damage. Traditionally, threshold is defined as the average of the highest nondamaging and the lowest

damaging levels, corresponding roughly to the fluence at which half of the test sites damaged - 8.6 J/cm2

.

The zero-percent, or onset, definition is more satisfying for several reasons:

* Damage probability curves demonstrate threshold-saturation behavior - the zero-percent

definition of threshold is consistent with accepted notation (e.g., laser gain curves).

* The onset fluence constitutes a maximum "safe" operating level. The reflector characterized in figure 2 will damage at levels well below the conventionally defined threshold

fluence

.

* As will be shown, the slope of a probability curve, and accordingly, the 507o damage probability, depends upon the test spotsize. The conventional threshold definition is, therefore, a convolution of the actual defect threshold and the focusing geometry used for

testing

.

4. Influence of Spotsize on Probability Curves

4.1. A simple Model

In order to demonstrate the effect of spotsize on the slope of a damage probability curve, a simple model is presented which begins with the familiar Gaussian intensity distribution

4> (r)

= <t> Q exp (-2r

2

/u)Q

2

) (1)

where the temporal integral has been performed and §, *. is the fluence at radial distance r, (J> is

the peak fluence at the beam center, and to is the beam radius at the 1/e 2 amplitude (spotsize).

Consider a test surface that is populated wit-h a random distribution of identical defects, each of

which has the property that, when immersed in a fluence exceeding some defect threshold fluence

damage occurs. Referring to figure 3, eq (1) can be rewritten to express the area A^ within which

damage can occur in terms of the peak test fluence and spotsize:

2 . , « n id /4> \

From Poisson statistics, the probability P^ that A^ contains one or more defects (i.e., the probability of damage is

PD

= 1 - exp (-AAD

) (3)

where K is the surface density of defects. Combining eqs (2) and (3) yields

369
which, as required, is the probability of damage as a function of peak test fluence with spotsize and

defect density as parameters.

Thus far, the model is two-dimensional but easily converted to the 3-D case that is applicable

to damage in bulk materials. However, for a multilayer dielectric reflector - particularly with a low index ratio as is typical for uv materials - it is useful to sum 2-D damage probabilities over

each of the high-index/low- index coating interfaces where the standing-wave fields are peaked. At

the ith interface, the peak fluence is reduced by a factor i") . given by

'nL \2(i-l)

(5)

where n^ and n^ are the low- and high-indices, respectively. Summing over all interfaces for r].0 >

<pth (precludes negative contributions to the damage probability) gives the total damage probability

for a multilayer reflector:

2. n u) A. o

A numerical evaluation yields the curves of figure 4. At a fixed beam size and index ratio (the

chosen values are similar to those encountered experimentally) , the only adjustable parameter is

defect density. In figure 4, where a relatively low density of 2.5 mm

-2 was used, contributions from

the first and successive interfaces are plotted separately. It should be noted that for a n^ of 2.0 or larger and a similar defect density, the first-interface probability rises steeply ana little

damage occurs in deeper layers.

Examination of eq (4) reveals that as the exponent of the fraction ($ ,/<)> ) is increased, the

damage probability increases, giving rise to a steeper slope. It is this relationship that leads to

a decrease in conventional threshold for large spots or high defect density.

4.2. Experimental Support

In that the previous model was developed assuming a spatially Gaussian intensity distribution,

measurements were conducted to demonstrate the applicability of the 248 rim spot used in these tests. While a TEM^ beam is not necessary, it is required that A^ increase logarithmically with peak

fluence. As shown in figure 5, this condition is met experimentally. Other pertinent test conditions include pulse repetition rate (35 pps) and pulselength (15 ns).

Mean spot diameters of 0.31, 0.58, and 1.46 nm were used to irradiate eight samples from a single run of Al 2 0 3 /Si02 reflectors. A complete data set for the three samples tested with 2u)q = 0.58 mm is shown in figure 6. The solid line is the result of a least-squares fit and serves as a reasonable approximation to the hypothetical probability curve of figure 4. The dashed lines form a band which is ± one standard deviation from the best fit. Similar bands in figure 7 illustrate the

observed variations in slope with spotsize. Note that, in the conventional sense, threshold

increases from about 2 J/cm2 for the large spot to almost 7 J/cm2 for the 0.31 mm spot, while the

zero-probability intercepts remain unchanged. It should be pointed out that even though the best fit

for each spotsize gave an onset fluence of 1.8 J/cm2

, the onset uncertainty, as indicated by the

width of the bands in figure 7 , increases for smaller spots

.

An attempt was made to correlate the results of figure 7 with those predicted by eq (6) . Plots were generated for the appropriate spotsizes using the defect density as an adjustable parameter.

Solid lines within the shaded bands of figure 8 illustrate that a quite reasonable fit is obtained

for A = 3.5 mm

-2

.

To check this result, an independent measurement of defect density was carried out by damaging

and counting the defects . in a relatively large coated area. Fluence was adjusted to produce damage

in the outer HL interface only. Defects in each of fifty-five 1 mm 2 samples areas were counted

370
resulting in the histogram of figure 9, which is a fair approximation of the Poisson distribution

function. The average measured defect density was 3.2 mm

-2 which is in agreement with the fitted

value of 3.5.

5. Quantitative Spotsize Dependence of Conventional Thresholds

In order to quantify the spotsize dependence inherent in the conventional threshold definition,

Eq. (4) can be rewritten in terms of d

Q

, the mean defect spacing:

This expression is valid for damage in a single layer coating, on a surface, or, as indicated

earlier, for a multilayer reflector with a high index ratio. Now, considering that the conventionally defined threshold <J> is the peak fluence for which P = 50%, eq (7) can be manipulated to express the functional reTOionship between conventional threshold, defect threshold, spotsize, and mean defect spacing:

, f+conv^ 2.4 f"o\

The significance of this expression becomes apparent in figure 10. Here, the conventional

threshold (normalized to the defect threshold) is plotted versus spotsize (normalized to the mean

defect spacing). Conventional threshold exceeds the defect threshold by 10% for spotsizes as large

as four times the distance between defects. In figure 11, 694 nm thresholds obtained by DeShazer, et

al. in 1973 are shown to correlate with this predicted behavior.

6. Other Models

The statistical model of Section 4.1 is a very simple one. More sophisticated models exist

[14,15] which propose that, instead of being singular, defect thresholds are distributed. This

feature is perhaps more physically satisfying, but is not necessary to explain the results presented

in section 4.2. In addition, it is not intended that the work here preclude the applicability of

thermal models or the possibility of a spotsize effect resulting from self-focusing or other processes .

7. Conclusions

It is shown that spotsize effects can arise as a consequence of dependence of the conventionally

defined threshold upon the slope of a damage probability curve. A model is developed and experimental evidence presented suggesting that the zero-probability intercept, or damage onset, is spotsize independent. It is proposed for this reason, and others, that the laser damage threshold be

redefined accordingly.

The author wishes to acknowledge both the work of L. J. Jolin, who performed the damage testing,

and the help and support of Brian Newnam.

371
8. References

[I] The most comprehensive source of information regarding parametric studies of laser damage can be found in the proceedings of the first thirteen Boulder Damage Symposia.

[2] DeShazer, L. G. ; Newnam, B. E.; Leung, K. M. The Role of Coating Defects in Laser-Induced

Damage to Thin Films. Nat. Bur. Stand. (US) Spec. Publ. 387; 1973, ll4p.

[3] Aleshin, I. V.; Bonch-Bruevich, A. M. ; Zinchenko, V. I.; Imas, Ya. A.; Komolov, V. L. Effect

of Absorbing Inhomogeneities on Optical Breakdown of Transparent Dielectrics. Sov. Phys

.

Tech. Phys. 18: 1648; 1974.

[4] Wang, V.; Guiliano, C. R. ; Garcia, B. Single and Multilongitudinal Mode Damage in Multilayer

Reflectors at 10.6 |Jm as a Function of Spot Size and Pulse Duration. Nat. Bur. Stand. (US)

Spec. Publ. 435; 1975, 216 p.

[5] Bettis, J. R. ; House, R. A.; Guenther, A. H. Spot Size and Pulse Duration Dependence of LaserInduced Damage. Nat. Bur. Stand. (US) Spec. Publ. 462; 1976, 338 p.

[6] Tang, C. C.; Leung, K. M. ; Bass, M. Re-examination of Laser Induced Breakdown in the Alkali

Halides at 10.6 |jm. Nat. Bur. Stand. (US) Spec. Publ. 462; 1976. 346 p.

[7] Van Stryland, E. W. ; Soileau, M. J.; Swirl, A. L. ; Williams, W. E. Pulse-Width and Focal-Volume

Dependence of Laser-Induced Breakdown. Nat. Bur. Stand. (US) Spec. Publ. 620; 1980. 375 p.

[8] Sioleau, M. J.; Franck, J. B. ; Veatch, T. C. On Self-Focusing and Spot-Size Dependence of

Laser-Induced Breakdown. Nat. Bur. Stand. (US) Spec. Pub). 620; 1980. 385 p.

[9] Gorshkov, B. G. ; Danieleiko, Yu. K. ; Manenkov, A. A.; Prokhorov, A. M. ; Sidorin, A. V. Size

Effect and Statistics of Laser Damage to Alkali-Halide Crystals at the 10.6 |jm Wavelength.

Sov. J. Quantum Electron. 11(1): 81-84; 1981 January.

[10] Bessonova, T. S.; Danileiko, Yu. K. ; Nikolaev, V. N. ; Sidorin, A. V. Optical Strength of LiF

Crystals. Sov. J. Quantum Electron. 11(10): 1383-1384; 1981 October.

[II] - Stewart, A. F. ; Guenther, A. H. Preliminary Experimental Results of Spot Size Scaling in

Laser-Induced Damage to Optical Coatings. Presented at Thirteenth Annual Symposium on Optical

Materials for High Power Lasers, November, 1981 (to be published).

[12] Foltyn, S. R. ; Newnam, B. E. Multiple-Shot Laser Damage Thresholds of Ultraviolet Reflectors

at 248 and 308 nanometers. Nat. Bur. Stand. (US) Spec. Publ. 620; 1980. 265 p.

[13] Foltyn, S. R. ; Newnam, B. E.; Jolin, L. J. Laser Damage Results and Analyses for uv Reflectors

Under Multiple-Shot Irradiation. Presented at the Thirteenth Annual Symposium on Optical

Materials for High Power Lasers, November, 1981 (to be published).

[14] Danileiko, Yu. K. ; Minaev, Yu. P.; Nikolaev, V. N. ; Sidorin, A. V. Determination of the

Characteristics of Microdefects from Statistical Relationships Governing Laser Damage to Solid

Transparent Materials. Sov. J. Quantum Electron. 11 (11): 1445-1449; 1981 November.

[15] Porteus, J. 0. Determination of the Onset of Defect-Driven Pulsed Laser Damage in 2.7 |Jm

Optical Coatings. Presented at Topical Meeting for High Power Laser Optical Components,

November, 1982.

372
Sc2 03 /MgF2 HR AT 308 nm

PEAK TEST FLUENCE (J/cm2

)

Figure 1. Results of a standard test at 308 nm of a Sc 2 0 3 /MgF 2 reflector. Each triangle represents

the fraction of ten test sites which damaged at a particular test fluence. Damage threshold is the

zero-probability intercept - 6.2 J/cm2

.

Sc2 03 /MgF2 HR AT 308nm

100

C 80

60

40

V)

Idt 20

r—t *• WW —

i

W»—

THRESHOLD 0% INTERCEPT

6.2 J/cm8

DAMAGE

NO

DAMAGE

/ CONVENTIONAL NOTATION £ THRESHOLDS MIDPOINT BETWEEN / HIGHEST NON-DAMAGING /k A AND LOWEST DAMAGING

FLUENCE.

3BB00OCOCBD lO O

" 8.6 J/cm 2

_0 Q_

7 8 9 10

PEAK TEST FLUENCE (J/cm2

)

Figure 2. The data of figure 1 are replotted in

the conventional manner (open and closed circles).

As defined conventionally, threshold is 8.6 J/cm2

.

373
Figure 3. For a given defect threshold

fluence, the area within which damage

can occur increases logarithmically

with peak fluence.

wo RADIAL DISTANCE FROM BEAM CENTER wo

Figure 4. Calculated damage probabilities for a multilayer

reflector with a low index ratio. For the case shown,

damage can occur in the five outermost HL interfaces.

The net damage probability is, to a good approximation,

a straight line.

374
0.4 -

o

-e- 0.6 -

0.8

• MEASURED BEAM AREA

2o>0 (measured) = 0.60 mm

-GAUSSIAN FIT TO POINTS

2w0

(fit) = 0.596 mm

OF FIT = 0.999

0 0.05 0.10 ' AREA OF SECTION (mm2

)

0.15

Figure 5. Spatial comparison of test beam to an ideal

Gaussian. A good fit assures the validity of a com- parison between theory and experiment.

100

UiO

<

<

O

>-

80

60

q 40

CO<

CDO

ccCL 20

AVERAGED RESULTS FOR 3 REFLECTORS PRODUCED IN A SINGLE COATING RUN

MEAN SPOT DIAMETER; 0.58 mm

DASHED LINES REPRESENT

± ONE STANDARD DEVIATION

2 3 4 5 PEAK TEST FLUENCE (J/cm2

)

Figure 6. Results of testing three reflectors with a spot

diameter of 0.58 mm. The dashed lines extend ± one standard deviation from the best fit. The error bars

indicate a ± 5% uncertainty in measurement of both

fluence and probability.

375
OA

yj

o

<r

1;< 60

1

1

5

>

4U

<

o 20

cc

a.

0

1 1 M 1 1 \ /

/ \ /

2w0

= .46 mm 2w0= 0.58 mm / 2w0= 0.31 mm /

-

• TEST RESULTS FOR HR

COATINGS AT 3 SPOT

DIAMETERS

• SHADED AREA REPRESENTS

V BEST FIT + STD. DEV. 1/ 1 1 1 1 0 4 6 8 10

PEAK TEST FLUENCE (J/cm 2

)

14

Figure 7. Results of testing identical reflectors at three different spotsizes.

PEAK TEST FLUENCE (J/cm 2

)

Figure 8. The solid lines within the shaded areas are calculated curves obtained from eq (6) by using the

appropriate spotsize values and treating defect density

as a fitted parameter. The best fit is obtained for

a defect density of 3.5 mm" 2

.

376
20

0 2 4 6 8 10 12 14 16 18 20 22

NUMBER OF DEFECTS/SAMPLE

Figure 9. A defect counting experiment was performed to verify

the fitted value of defect density. Fifty-five 1 mm2 samples were damaged yielding an average defect density of 3.2 mm"2 . The histogram is a fair approximation of the Poisson distribution

function.

377
1 .0

0.81 i I i J i I i I i I 0 2 4 6 8 10 12

cu0 /d0

Figure 10. Quantitative behavior of the conventional

threshold as a function of spotsize.

Figure 11. Test results from reference 2 plotted

on the curve of figure 10. Good agreement demonstrates the applicability of a simple model to the

explanation of spotsize effects.

378
The speaker was asked if he had considered more than a single type of defect in his analysis, and replied that the fit of his data to the 'single-defect- model was adequate, in response to a second question, he said the 'effective density of defects" was determined by counting defects in large-area damage sites irradiated at a few percent over threshold fluence.

379
PULSED LASER INDUCED DAMAGE OF AN OPTICAL MATERIAL WITH A

SPHERICAL INCLUSION: INFLUENCE OF THE THERMAL

PROPERTIES OF THE MATERIALS

M. R. Lange and J. K. Mclver

Institute for Modern Optics

University of New Mexico

Albuquerque, New Mexico 87131

and

A. H. Guenther and T. W. Walker

Air Force Weapons Laboratory

Kirtland Air Force Base, New Mexico 87117

In a recent set of papers, a model was developed to describe the role

of spherical inclusions (impurities) in the pulsed laser induced damage of

optical thin films (dielectrics). The numerically generated relation of

the damage threshold in J/cm as a function of the radius of the inclusion

had a minimum, whose value varied as the square root of the incident laser

pulse length. This prediction agrees with the frequently observed trend

that longer laser pulses yield higher damage thresholds.

In this paper it is shown analytically that the damage threshold as a function of radius of the spherical inclusion does indeed have a minimum

that varies as the square root of the laser pulse length. Furthermore,

this minimum is shown to be relatively insensitive to the thermal

properties of the impurity for a wide range of typical optical materials.

However, the effect of the thermal properties of the host on this minimum

is more complicated. One is able to show that for many host/impurity

combinations, the minimum damage threshold varies as the square root of the

thermal conductivity of the host, thus indicating that the thermal

properties of the host material may be a useful indicator of pulsed laser

induced damage sensitivity.

Key words: Laser damage; damage threshold; spherical inclusion; thermal

properties; impurity.

Introduction

One possible reason for the low damage threshold of optical thin films is the

increased absorption, relative to the material in bulk- form, of the incident

radiation by the ever present impurities within the film. For the particular case

of dielectric films, a previous study [1,2] concentrated on developing an understanding of the variation of the damage threshold of a thin film as a function

of the radius of a spherical inclusion embedded in the material. That study

predicted that the damage threshold should vary as the square root of the laser

pulse length t. Numerous experimental investigations have provided evidence for

this assertion (see, for example, the work of Milam [3,4]).

In the work herein, the results of a further study of laser damage due to a spherical inclusion embedded in a host are presented. In particular, the

sensitivity of the damage threshold to the thermal properties of the host and

inclusion are investigated. One finds that for many cases of interest, the damage

threshold is relatively insensitive to the thermal properties of the impurity but

varies measurably with the thermal properties of the host. From this study, the

point of damage is found to scale linearly with a material dependent temperature

and as the square root of the products of the thermal conductivity and specific

1. Figures in brackets indicate the literature references at the end of this

paper

.

380
heat of the host material. This variation of the damage threshold with the thermal

properties is in addition to the /tdependence mentioned previously.

Model

As is the case with previous investigations [1,2,5] of impurity initiated

damage, the present model has a spherical absorbing particle with radius "a"

embedded in a host material. The inclusion absorbs radiation from the incident

laser field and rises in temperature. Once the temperature of the impurity or host

reaches some critical temperature (e.g., melting point), the system is assumed to

damage. From this simple description, it is patently clear that the rate at which

the heat is conducted away from the inclusion can play a crucial role in

determining the damage threshold

.

As has been discussed in previous work [2] , the well known approximate solution

of Hopper and Uhlmann [6] for this problem is not valid (nor was it meant to be)

when the coefficient of thermal diffusivity in the impurity is small. This

restriction excludes the possibility of using their solution when the impurity is a dielectric. For this, the case of prime interest, it is necessary to turn to an exact solution of the heat diffusion equation for the temperature, such as given by

Goldenberg and Tranter [7]

.

i h o

F( , = (sin y - y cos y) sin y

^ (1)

y

2

[ (c sin y - y cos y) 2 + b

2 y2 sin2 y]

K. ,D7" K,

o /t\ i- i / n , n

Y = a

2 /D h = — —, c = i - _ , hi l

K is the thermal conductivity in J/s-cm-°K, and D is the thermal diffusivity in cm /s. The subscripts refer to the host h or the impurity i. It should be noted

that the above equation is not in its most general form. It has been rearranged to

give the temperature at the surface of an impurity of radius a in cm. The

quantities Q and I that occur in eq. (1) are the absorption cross section in cm and the intensity of the incident laser pulse in watts/cm .

A rigorous treatment of the absorption of radiation by the impurity requires

that the Mie cross section be used for Q. In that case, the absorption would be

frequency dependent. This topic will be the subject of a later paper. Since in

this study one is interested in the effects of the thermal properties, the small

wavelength limit of the Mie cross section will be used; i.e., Q = ia2 .

In keeping with common practice, the temperature at which damage occurs is

assumed to be known. The dependent variable in eq. (1) is then the intensity for a given pulse length necessary to achieve this temperature. To conform to the damage

threshold usually measured in experiments, the energy per unit area (J/cm ) is

defined as

E = It (2)

Results

In a previous study [2] employing eq. (1), the damage threshold E was investigated as a function of the radius of the impurity. This work showed that

the function E(a) had a minimum (see fig. 1), and that this minimum damage

threshold varied as Vt. This scaling has been verified experimentally [1,3,4],

381
Since a sample that contains inclusions will most likely have a distribution of

sizes [8], the damage threshold of the system will be determined by the minimum of

E(a), if it occurs at a radius a that is within the thickness of the film. As can

be seen from fig. 1, minimums do exist within the film for common pulse lengths and

thin film material and geometrical parameters. For this reason, the term damage

threshold will for the remainder of this discussion refer to the minimum of E(a).

Because the determination of the thermal properties of the inclusions and host

is a difficult experimental problem, a theoretical study of the sensitivity of the

damage threshold to these quantities is a worthwhile endeavor. It is, of course, a relatively simple procedure to vary the thermal constants and determine the damage

threshold numerically. In fact, this has been done, and the results are presented

for the case of platinum in glass (fig. 2) and Th02 in ThF 4 (figs. 3-4). These two

examples cover the most interesting regions of parameter space; i.e., impurities

with both good and poor thermal conductivity embedded in a poor thermal conductor

(see table 1). It is important to note that if the heat capacity C

p

in J/g-°K and

the density p in g/cm3 of the material is assumed to be held fixed, then D and K

vary at the same rate since K = pC D. P

Unfortunately, numerical treatments, besides being time consuming, do not

readily lend themselves to determining functional dependencies. For this reason,

an approximate method of evaluating the integral in eq. (1) has been developed.

The key to this method is the realization that as long as D.t/a 2 >l, the integral

is dominated by the exponential. This means that the remaining portion of the

integrand can be replaced by a power series in y . The zeroth order approximation

consists of retaining terms of order one in the power series. The radius at which

damage first occurs is then given by

a 0 = —2— ' ( 3 )

while the damage threshold is given by

/p. C , K.t

E 0 = 1ST h Ph h

.

Equation (4) is particularly interesting because it not only verifies

the /t dependence previously suggested but also predicts that the damage threshold

scales linearly as the temperature at which damage occurs and as the square root of

the product of the specific heat and conductivity of the host. In this reasonable

approximation, the damage threshold is independent of the properties of the

impurity.

In order to visualize the effect of the properties of the impurity on the

damage threshold, it is necessary to keep terms of order y in the expansion in the

integral. In this case, the damage threshold is given by

/p,C .K.t

E = 16T

h Ph h

1 (5)

1 TT 1 + TT p , C . [15 p,C - 10 - tl

i pi 1 h ph h

J 120p, C , p . C . FT

h ph l pi l

Note that this expression also scales as T/t~. , however, the dependence on the

thermal properties is more complicated. For dielectric materials, the second term

in the denominator is <0.1, thus making it a relatively unimportant contribution to

the final answer. The~ first order correction, therefore, does not substantially

influence the scaling predictions of eq. (4).

A comparison of eq. (5) with the numerical integration of the exact solution

shows suprisingly good agreement (see figs. 2-4). This reinforces the conclusion

382
that for impurity-dominated damage, the threshold scales as T^Pj

1 Cph Kh

t and ^ s relatively insensitive to the thermal properties of the inclusion.

Discussion

Although this model provides some new scaling laws for damage thresholds, it is

not capable of making quantitative predictions. It also ignores several

potentially important phenomena. One of these is the frequency dependence of the

absorption cross section. Another is the finite thickness of the film and the

effect of the substrate. The list of neglected factors and phenomena can become

lengthy. On the other hand, if absorption of radiation occurs primarily in the

inclusion, then at what rate the temperature rises in any part of the impurity

still depends on how fast the surrounding material can remove the heat from the source. It is, therefore, not unreasonable to assume that further refinements of

this basic model will not substantially change the conclusions presented in this

paper

.

[1] T. W. Walker, A. H. Guenther, and

P. Nielsen, Pulsed Laser-Induced

Damage to Thin-Film Optical

Coatings - Part I: Experimental,"

IEEE J. Quantum Electron., QE-17 , 2041 (1981).

[2] T. W. Walker, A. H. Guenther, and

P. Nielsen, "Pulsed Laser-Induced

Damage to Thin-Film Optical

Coatings - Part II: Theory," IEEE

J. Quantum Electron., QE-17 , 2053

( 1981) .

[3] D. Milam and R. A. Bradbury,

"Laser Damage Threshold for

Dielectric Coatings as Determined

by Inclusions," Appl. Phys. Lett.,

23 , 654 (1973).

[4] D. Milam, "1064NM Laser Damage

Threshold of Polished Glass

Surfaces as a Function of the

Pulse Duration and Surface

Roughness," Laser Induced Damage

in Optical Materials: 1978 , A. J.

Glass and A. H. Guenther Eds., NBS

Spec. Pub. 541, 164, Washington

DC: U.S. Gov't Printing Office

(1978) .

[5] C. J. Duthler and M. J. Sparks,

"Extrinsic Absorption in Laser

Window Materials," Laser Induced

Damage in Optical Materials , A . J

.

Glass and A. H. Guenther Eds., NBS

Spec. Pub. 414, 219, Washington

DC: U.S. Gov't Printing Office

(1974) .

nces

[6] R. W. Hopper and D. R. Uhlmann,

"Mechanism of Inclusion Damage in

Laser Glass," J. Appl. Phys., 41 , 4023 (1970).

[7] H. Goldenberg and J. C. Tranter,

"Heat Flow in an Infinite Medium

Heated by a Sphere," Brit. J.

Appl. Phys., 3_, 296 (1952).

[8] V. V. Artem'ev, A. M. BonchBruevich, I. E. Morichev, I. E.

Ivanov, and A. G. Vinogradskii

,

"Statistics of Microscopic

Inhomogeneities and Optical

Breakdown of Transparent Media,"

Sov. Phys. Tech. Phys., _22, 106

(1977)

.

[9] M. Sparks and C. J. Duthler,

Theoretical Studies of HighPower Ultra-violet and Infrared

Materials , Eighth Tech. Rep., 11

( 1976) .

[ 10 ] Handbook of Thermophysical

Properties of Solid Materials , Rev. Ed., A. Goldsmith, T. E. Watemman, and H. J. Hirschhorn,

New York: MacMillan Company

(1961) .

[11] Thermophysical Properties of

Matter , Y. S. Touloukian, Dir.,

New York: Plenum Press (1970).

383
Table 1 Quantities Employed in the Analysis of an Impurity Embedded in a Host

Thermal Properties for ThF^ (Host) and Th02 (Impurity)

K

h

= 0.1 (J/cm-s-°K) (9)

K

i = 0.142 (J/cm-s-°K) (11)

D

h

= 0.05 (cm 2/s) (9)

D

i = 0.0594 (cm2/s) (11)

Thermal Properties for Glass (Host) and Platinum (Impurity)

K

h

= 0.013 (J/cm-s-°K) (6)

K

± = 0.67 (J/cm-s-°K) (6)

D

h

= 0.003 (cm 2 /s) (6)

D

L = 0.24 (cm 2 /s) (6)

Damage Condition

T

c

= 2000 °K

384
0.8

0.7

0.6

0.5

0.4

2 0.3

0.2

0.1

-i i 1.0 2.0 3.0 4.0 5.0 6.0 RADIUS (X 10 -5 cm)

Fig. 1 Damage Threshold as a Function of Impurity Radius for Various

Pulse Lengths, the Host is Glass While the Inclusion is Platinum

o.i

o.oi o.oooi

NUMERICAL

X APPROXIMATE

SLOPE - .50

t* 1 X 10'°sec

0.001

K

h (»/cm-°K)

0.01

Fig. 2 Damage Threshold as a Function of Host Conductivity: Platinum Impurity

385
1 0

SLOPE = .55

NUMERICAL APPROXIMATE

O.Ol 0.1

K

h (W/cm-°K)

I .0

Fig. 3 Damage Threshold as a Function of Host Conductivity:

ThO- - Impurity

10.0 r

NUMERICAL

x APPROXIMATE

t = 1 X 10"° sec

0.1

0 01 0. 1

Kj (W/cm-°K)

I .0

Fig. 4 Damage Threshold as a Function of Impurity Conductivity: ThF 4 - Host

386
Temporal Dependence of Laser-Induced Breakdown in NaCl and SiC^

M. J. Soileau, William E. Williams, Eric W. Van Stryland,

Thomas F. Boggess, and Arthur L. Smirl

Department of Physics

Center for Applied Quantum Electronics

North Texas State University

P. 0. Box 5368

Denton, Texas 76203

The laser-induced damage (LID) thresholds of fused silica and single crystal NaCl were studied at wavelengths of 0.5 and 1 ym for pulses as short as 4 psec for a variety of

focal spot sizes. The problem of sample-to-sample variation was minimized by performing

parametric studies on a single sample at a time. Beam distortion measurements and polarization dependence studies of the LID thresholds demonstrate that the contribution of selffocusing to the LID measurements in this work was negligible. The damage threshold field,

Eg, was found to increase as the pulsewidth was decreased in both materials at both

wavelengths. The strongest pulsewidth dependence observed was approximately an inverse

square root proportionality observed in NaCl at 1 ym for pulses shorter than 10 psec. For conditions of equal pulsewidth and the same focal spot size, Eg was less at 0.5 ym than at

1 ym for both materials.

Key Words: Laser Damage, Picosecond Pulses, Fused Silica, NaCl, Avalanche Breakdown, 1.06 ym, 0.53 ym, 1 .05 y m.

1. Introduction

The problem of bulk laser-induced damage (LID) in nominally transparent materials has been the

subject of extensive investigation [1]. However, after more than 15 years of study, bulk laserinduced damage is still not well understood. A major obstacle to developing models for such damage

in highly transparent materials is the lack of a consistent data base for the dependence of LID on such basic parameters as laser wavelength and pulsewidth. The problems of sample to sample variations and the complex interdependence of the damage thresholds on laser frequency, pulsewidth, and

focal conditions [2] make the interpretation of isolated data points difficult.

We have tried to minimize these problems by studying LID as a function of wavelength and

pulsewidth for a variety of focal conditions on a given sample. In Ref. 2 the LID-threshol ds of a sample of fused S i O2 and a single cyrstal NaCl sample were studied at 1.06 ym as a function of

pulsewidth for pulses ranging from 40 psec to 31 nsec using a variety of focal conditions. In that

work we found that the laser-induced breakdown threshold field, Eg, (i.e., the r.m.s. field corresponding to the LID threshold peak-on-axis irradiance) was only weakly dependent on the laser pulsewidth. We found that, for the range of parameters used, we could fit all the data in Ref. 2 to the

fol lowing empirical relationship

E B

= Wp/4 # + B (D

387
where A and B are constants for a given sample, t

p is the laser pulsewidth and co 0

is the laser focal

spot radius. In the present work, we find that this relationship breaks down for pulses shorter

than 10 psec.

Here we extend the pulsewidth dependence studies at a wavelength of 1 ym to pulses as short as

4 psec, and we study the LID characteristics of fused Si O2 and single crystal NaCl at 0.53 ym for

pulsewidths in the 20 to 200 psec range. We find that the empirical relationship given by eq. 1 no longer holds for laser pulses shorter than 10 psec. For the shortest pulses (4 - 10 psec at 1.05 ym), Eg varied as approximately the inverse fourth root of the pulsewidth in Si0£ and as approximately the inverse square root in NaCl for all the focal conditions studied.

For conditions of equal pulsewidth and the same focal spot size, Eg was less at 0.53 ym than at 1.06 ym for both materials for pulses in the 40 to 200 psec range. This resul t is contrary to the

predictions of a simple avalanche breakdown model . Al so, the observed dependence, while in the

right direction, is much too weak for a strictly multiphoton process. A multiphoton initiated

avalanche breakdown process [2] could possibly account for the observed wavelength dependence.

In addition, by careful ly studying the polarization dependence of LID and by measuring the

distortion of the transmitted beams, we show that the contribution of self-focusing to LID in our experiments is much less than has been assumed by other workers. In fact, for certain focal conditions we demonstrate that the effects of self-focusing are negligible.

2. Experiment

The lasers used in this study were a mode-locked Nd:YAG oscillator-amplifier system and a modelocked Nd:Glass oscillator system which have been described el sewhere [2,3], The YAG 1 aser was operated at 1.06 ym and the glass at 1.05 ym. In each case a single pulse of measured Gaussian

spatial profile was switched out of the mode-locked train and amplified. The pulsewidth of the Nd:YAG laserwas varied from 40-200 psec by selecting various etalons as the output

coupler of the oscillator. The glass laser pulsewidth varied from 4 to 10 psec. The pulsewidth and

energy of each pulse were monitored. The width of each pulse was determined by monitoring the ratio

of the energy in the second harmonic produced in a nonlinear crystal to the square of the energy in the fundamental . A more detai 1 ed description of energy and pul sewidth measurement are gi ven in

Ref. 2.

A temperature tuned CD*A crystal was used with the Nd:YAG laser to produce pulses at 0.53 ym. Care was taken to filter any residual 1.06 ym radiation from the 0.53ym beam. The energy in the

fundamental pulses at 1.06 ym was kept below a value which would produce saturation effects in the

spatial profile of the second harmonic. Two-dimensional scans of the 0.53 ym beam with an optical

multichannel analyzer (0MA) verified the absence of saturation effects and the Gaussian spatial

profile of the 0.53 ym beam. Such vidicon scans of the spatial beam profile showed that the shotto-shot variation in the beam width was less than our detection resolution limit of approximately

1%. Light-by-light scattering measurements in a Li IO3 crystal [4] indicated that the 0.53 ym pulses

had a Gaussian temporal shape. The results of these measurements confirmed that the 0.53 ym pulsewidth scaled as the 1.06 ym pulsewidth divided by /2~. This scale factor was used to compute the

0.53 ym pulsewidths from the measured 1.06 ym pulsewidths.

The laser beam was focused into the bulk of the sample using single element, "best form",

lenses designed for minimum spherical aberrations. Three lenses of focal lengths 37 mm, 75 mm, and

150 mm were used at various distances from the beam waist to produce the focal spot radii for these

experiments. The lowest f -number condition used in these experiments was f/10.3. In each case, the

388
beam diameter was kept below maximum values necessary to ensure diffraction limited performance.

Aberrations caused by focusing through the planar surfaces of the samples were calculated to cause

an error in the field of less than 0.1% for the worst case. The output energy of the CD*A crystal

was monitored continually using a sensitive photodiode peak-and-hol d detector, absolutely calibrated

with respect to a pyroelectric energy detector. Transmission through the sample was monitored by

another peak-and-hol d detectors.

In some cases, the transmission detector was replaced by a vidicon tube used in conjunction

with an optical multichannel analyzer to scan the transmitted beam profile. Such scans were made at the damage threshold irradiance and for irradiance levels ten times below the damage threshold. In this way, a total time integrated beam distortion at the beam waist of the order of A/5 could be

detected.

The short pulsewidth data (4-9 psec) were taken on the same samples using a microprocessorcontrolled NdrGlass oscillator system operating at a laser wavelength of 1.05 ym [3]. This system

produced single pjlses of measured Gaussian spatial profile. Shot-to-shot energy fluctuations were determined to be ~20%. The same focusing lenses were used as with the 0.53 ym measurements. Beam scans as a function of distance from the 1 aser were empl oyed (as was al so done using the 1.06 ym

laser) to determine the beam divergence (0.4 mrad.) and the position and size of the output beam

waist. This information and the lens focal length were then used to calculated the beam waist at the lens focus. Energy on target was varied using a calibrated Glan polarizer. The procedures for

measuring and continuously monitoring both pulsewidth and energy are identical to those described

previously with the NdrYAG laser system [2].

The breakdown threshold for a given pulsewidth was taken to be that irradiance which produced

damage 50% of the time. Each site was irradiated only once. Damage was defined as the appearance

of a visible flash in the bulk of the sample or by the observation of forward scattered light from a coaxial HeNe laser as viewed through a ten-power long-working distance microscope. The microscope

also was used to verify that damage had occured at the beam focus and was not due to inclusions.

For 0.53 ym damage, the appearance of 0.53 ym scattered radiation and HeNe scattered light occurred

simultaneously for both Si0 2 and NaCl. In the cases of the 4-9 psec 1 ym data, for both Si0 2 and

NaCl , there was a small range of incident energies where damage had occured as determined from

scattered HeNe radiation, but no flash was observed. N-on-1 experiments (multiple shot irradiation

at the same sample site) conducted at both wavelengths, indicate no change in the breakdown threshold when compared to the 1-on-l experiments (only one irradiation per sample site).

3. Sel f-Focusing Considerations

In order to produce laser-induced damage in the bulk of a highly transparent material one must

tightly focus the light into the material. Since the beam propagates through the material, one must

consider the effects of self-focusing on the results of bulk damage measurements, particularly when

the power required to induce damage is of the order of the predicted critical power for selffocusing. Marburger [5] has derived an approximate solution to the nonlinear wave equation for a focused Gaussian beam in a nonlinear medium. This solution (called the constant shape approximation) assumes that, in the presence of self-focusing, the focused beam waist is given by

a = (1 - P/Pi)

1/2

a

0

(2)

389
where P^ is the critical power for self-focusing

a 0

is the focal radius in the absence of self-focusing, and

P is the input power.

Some workers have assumed that the damage threshold irradiance for highly transparent materials

(such as those used in this study) is an intrinsic property [6] of the material and that any

apparent dependence of the breakdown irradiance on a Q is due to self-focusing [7,8,9] The breakdown

power then was measured for various values of a 0 and the results fit to the equation

1/P

B

= 2/I

B

Traj; + 1/P

1 (3)

where Pg is the power required for breakdown and

Ig is the intrinsic damage threshold.

A number of authors have used this technique to extract values for Ig and P-j for nanosecond [7,8]

and picosecond pulses [10,11] In some cases P^ was calculated [11] in accordance with the relation [5]

Pj = c A

2 /32tt n 2 (4)

where A is the wavelength, and n

2 is the nonlinear refractive index. A new focal radius was calculated using eq. (1). The n2 values used in these calculations were taken from nanosecond measurements .

There are serious problems associated with using the above mentioned procedure. Subsequent

work has shown that Ig is not an intrinsic property of a transparent material as evidenced by large

sample to sample variations observed in a given material [2,12]. A second problem is that other

self-action effects such as a saturation of the self-focusing or free carrier defocusing may become

important at the extremely high irradiance levels required for damage. The presence of these "free"

electrons creates a negative change in the index of refraction which can limit the self-focus

collapse, [5,13,15] defeat self-focusing entirely, [15] and in some cases lead to sel f-defocusing of

the beam. [16,17] Therefore, before any analysis of damage data can begin, one needs to devise

tests which will verify the presence, or absence of self-focusing effects.

One such test arises directly from self-focusing theory which predicts that the critical power

for self-focusing is 1 ower for linearly polarized light compared to that for circularly polarized

light [18-21]. Experimental measurements in liquids, such as CS 2

, conducted by ourselves [22] and

others [20,23] have confirmed that this is indeed the case. In solids, Feldman et_ a_l_. [19] measured

ratios of circular polarization thresholds to linear polarization thresholds on the order of 1.1 to

1.3 for fused quartz. These measurements were conducted using nanosecond pul ses at 1.06 pm where

both el ectrostricti ve and electronic contributions to n

2 are believed to be important. For picosecond pulses the dominant contribution to n 2 is believed to be electronic self-focusing, which is polarization dependent. Thus, if self-focusing effects are important in our measurements, then the

breakdown thresholds should be different for different polarization configurations.

In figure 1 we present the results of polarization dependent studies for a sample of fused

quartz at a laser wavelength of 0.53 ym and 1.06 \xm. The breakdown thresholds are given in terms of

the r.m.s. electric field, in MV/cm, corresponding to the peak-on-axis irradiance producing damage.

In both cases we used the shortest pulses available to us, which corresponded to the highest input

390
power for the gi ven focal spot radius of 5 vim (HW 1 /e^ M). As can be seen the breakdown fiel d for

linear polarization equals that for circular polarization. In both materials no polarization dependence was observed for pulses between 4-9 psec (FWHM) at 1.05 ym even for the largest spot size used

in this study. Similar results were obtained in measurements on a NaCl sample at a 0.53 ym laser

wavelength for 30 psec pulses. Thus, for the focal conditions used, we observed no polarization

dependence of the breakdown thresholds for either fused quartz or NaCl.

Another, independent test for the presence of self-focusing is to examine the beam spatial

profile after transmission through the sample for irradiance levels far below and near the damage

threshold. In figure 2 we show two vidicon traces taken in the far field through the center of the

0.53 ym beam after transmission through the fused quartz sample. The focal spot radius (HW 1 /e^ M)

at the beam waist for this configuration, in the absence of self-focusing, in approximately 7 urn. The solid trace was taken near the damage threshold. The dashed trace was taken at an irradiance

level approximately 10 times below the damage threshold. Filters were placed in front of the

vidicon to adjust the irradiance at the vidicon surface to the same value in both traces in order to minimize any problems in detector nonl inearity. Inspection of figure 2 shows no detectable beam

distortion for input powers approximately equal to the damage threshold power. Scans conducted for

a focal spot size of 3.4 ym at the 0.53 ym laser wavelength also show no distortions.

The authors are aware that the technique just described monitors the time-integrated beam

spatial profile; however, this technique is sensitive to a A/5 distortion in the total time

integrated profile and has proven to be a sensitive technique for monitoring the onset of selfdefocusing in solids [16]. These measurements, when taken together with the polarization experiments described above, indicate that self-focusing effects were not important for the geometries

used in these experiments. However, the reader should be aware that in other experimental

geometries (i.e., less tightly focused beams) se 1 f -f ocusi ng effects can be important. In fact,

preliminary measurements in Si O2 at 0.53 ym indicate that for a 14 ym focal spot radius the far

field beam radii for irradiance at the damage threshold differs from that at low irradiance levels

by approximately 28% (indicating that self-focusing has taken place). For that same focal geometry

the ratio of the LIB threshold for circularly polarized light to that for linearly polarized light

is approximately 1.4. These self-foe using studies are not yet compl ete and wi 1 1 be reported in

detail at a later date. We emphasize that no data are included in this paper for which either the

polarization test or the beam distortion test indicate the presence of self-focusing.

4. Experimental Results and Discussions

Tables I - V 1 1 1 summarize the results of measurements of the laser-induced breakdown thresholds for two different samples of single crystal NaCl and fused silica [24]. The NaCl sample

labeled 78-NC-6 is the same sample used in previous picosecond damage studies [2] and in studies at longer pulsewidths and wavelengths [15,25]. Both Si 0g samples have been used in nanosecond studies

at 1.06 ym [26] and the sample labeled 79-FQ-125-1 was used in previous picosecond studies [2]. The

uncertainties listed in the tables of data are the relative errors obtained by the method of Porteus

et a 1 . [27], The absolute errors, which include the relative errors and absolute errors in energy,

pulsewidth and focal spot radius, are estimated to be ±20% in the breakdown electric fields.

Tables I - 1 V summarize the results of our measurements at 0.53 ym for laser pulsewidths in the

25 to 200 psec (FWHM) range. These results will be compared here with the results of our earlier

studies at 1.06 ym for these same material s over a similar range of pul sewidths. Tabl es V- VI 1

1

contain the results of measurements on these same samples at 1.05 ym for pulses in the 4-10 psec

391
range. In addition, Tables VI and VIII contain LID thresholds at 1 ym in the pulsewidth range of 40

to 200 psec for a 7.2 ym focal spot radius. These thresholds are taken from our previous work

(Ref. 2) and were interpolated from measurements made at focal spot size of 6.1 and 10.3 ym.

In the paragraphs that follow, we examine the pulsewidth dependence of the threshold breakdown

field, Eg, for a given sample at a given wavelength and focal soot radius. We then examine the

wavelength dependence of Eg for a given pulsewidth and spot size.

The dependence of Eg on pulsewidth (tp) is more clearly seen by plotting Eg versus the inverse

pulsewidth on a log-log plot. Figures 3 and 4 are such plots for a NaCl sample (82-NC-l) and a Si

0

2 sample ( 79-FQ- 7940- 1 ) at 1.05 ym. Note that for a given spot size and for pulses shorter than 10

psec the data for each sample can be fit with a straight line. Over this limited pulsewidth range

E 8 * *i

X

< 5 >

where x - 0.3 for the S i 0g sampl e and x - 0.5 for the NaCl sampl e. The displ acement of each set of

data points corresponding to different focal spot sizes indicates a relatively strong spot size

dependence in these samples at this wavelength.

Figures 5 and 6 are similar plots for the same two samples ( 79-FQ-7940- 1 and 82-NC-l) at 0.53 ym for pulsewidths in the 25-200 psec range. Again one can fit the data with a linear dependence. However, in this case x <. 0.1 for the S i O2 sample and x *0.3 for the NaCl sample. Similar

trends are seen in the data for the other Si O2 anc' samples given in tables II, IV, VI, and

VIII.

At 0.53pm and 1.06 ym the pulsewidth dependence of Eg observed for both materials is in good

agreement with the predictions of various avalanche breakdown models [28,29,30]. For example the

model proposed by Sparks et^ aj . [28] predicts very little pulsewidth dependence for relatively long

pulses (nsec) and a dependence of Eg on pulsewidth which approaches an inverse square root dependence for relatively short pulses, i.e., ten's of picoseconds. The inverse square root of pulsewidth dependence implies that the breakdown fluence is constant (as can be seen in tables VII and

VIII for the NaCl samples). We find that for NaCl in the long pulsewidth limit (tp > 1 nsec) the

breakdown field is nearly constant, whereas in the short pulsewidth limit (t p

< 10 psec) the breakdown f 1 uence is nearly constant. The trend in the t p dependence of Eg for Si O2 is similar to that

no

seen in NaCl, however, the strongest dependence of Eg on t p observed was Eg a tp" * for pulses

shorter than 10 psec at 1.05 ym. It is important to note that the strongest pulsewidth dependence

of Eg observed in these measurements was the approximate inverse square root of pulsewidth dependence observed for NaCl for t p

< 10 psec at 1.05 ym. In on a /al anche breakdown model this dependence implies an ionization rate (g) which is proportional to the input irradiance. Then the

buildup of carriers is given by [28]

2

N = N e 6t

= N e AE t 00 (6) v '

where N is the carrier density

N

Q is the initial carrier density or carrier density produced by multiphoton ionization.

A is constant.

392
It is commonly assumed that damage occurs when the carrier density reaches a critical value, N

c

. Thus eq. 2 gives the following relationship for Eg

E B

= ^[(N

c /NQ

)/A] 1/2

(7)

and the [] term is constant for a given sample and laser frequency. The breakdown fluence.eg, is

o proportional to Eg times the laser pulsewidth. Therefore eq. 6 implies that eg is a constant

for 3 proportional to E 2.

In the Sparks avalanche breakdown model the ionization rate 3 is proportional to E 2 in the high

electric field limit. This limit corresponds to the situation in which the increase in energy of

the electrons in the conduction band is simply proportional to the input irradiance and that all

losses are negligible. This simply says that the ionization rate is limited by the rate at which

the input light beam can supply energy to the conduction band electrons. For the low field limit,

i.e. longer pulses, 3 is exponentially dependent on E and the resulting pulsewidth dependence is relatively weak. In figure 7, we have reproduced the theoretical curves derived by Sparks et a 1

.

[28] showing the predicted dependence of the breakdown electric field, Eg, on pulsewidth and have

extended the pulsewidth scale to longer and shorter pulsewidths. These curves were obtained by

numerical integration of the Fokker Planck diffusion equation with appropriate boundary conditions

reflecting energy loss by electrons to phonons. The solid curve was obtained using a value for a

Q

-i , the electron-chlorine ion scattering cross section, taken from atomic physics literature. The

dotted curve labled "a

c i adjusted" was obtained using a value for this parameter adjusted so as to give a better fit to experimental LID data available at the time. This cross section appears in the

equations used to calculate the el ectron-phonon relaxation frequencies. The experimental points

shown were obtained in this work at 1.05 urn for the NaCl sample labeled 78-NC-6 (Table VIII) and in our previous picosecond study at 1.06 ym [2]. Note the excellent agreement between the Sparks

avalanche model and the data obtained here and in Ref. 2 for the 5 ym spot size. Data obtained in the same sample for a larger spot size (7.2 ym) show a similar trend in the functional dependence of

Eg on pulsewidth but the values of Eg are lower than those given by this model. The agreement

between the absolute values predicted by theory and experiment for 5 ym spot size may be fortuitous

since the Sparks model describes intrinsic avalanche breakdown, i.e., the starter electrons

necessary to create the avalanche are assumed to be already present. Such a model predicts no spot

size dependence of the breakdown field. The relatively strong spot size dependence observed in NaCl

at 1.06 ym indicates that damage in this material is probably initiated by some extrensic process;

for example, multiphoton ionization of material defects.

Other workers have used an approximate inverse pulsewidth dependence (i.e., x = 1 in eq. 3) to scale breakdown data for various materials [11]. This is a much stronger dependence than is

observed in this work. Their strong pulsewidth dependence was determined by combining the 1.06 ym,

30 psec data in Ref. 9 and 1.06 ym, 15 psec data in Ref. 31. These two data points were taken with

different focal spot sizes. That is, the 30 psec data point in Ref. 9 was taken with relatively

small focal radii (4.7 to 5.9 ym l/e 2 radius) and was reduced with the assumption that self-focusing

was dominating the observed damage. The 15 psec data point in Ref. 31 was taken with a relatively

large focal radius (12.4 ym HW 1/e 2 M) and thus higher input power. In the latter work the authors

assumed that self-focusing was not present in their experiment. The relatively large pulsewidth

393
dependence deduced from these two isolated data points is probably due to differences in experimental conditions used in these two measurements and the different methods of data reduction.

We now examine the wavelength dependence of the breakdown thresholds for laser pulsewidths in the 25 - 200 psec range. In figure 8 we have plotted, in bar graph form, the breakdown electric

fields for Si

0

2 (sample 79-FQ-125-1) and NaCl (sample 78-NC-6) for 1.06 ym and 0.53 ym for various

pulsewidths at a fixed focal spot radius (7.2 ym). The 1.06 ym LID thresholds at the 7.2 ym focal

radius are taken from Ref. 2 and are interpolated from measurements at spot sizes of 6.1 and

10.3 ym. In two cases where the pulsewidths did not exactly overlap for the two wavelengths we interpolated the 0.53 ym data between two pulsewidths for which data was available. This procedure was made necessary by the fact that only a limited number of pulsewidths were available at each

wavelength studied. The errors due to interpolation are estimated to be within the error bars shown

in f i gure 8.

Note that for each pulsewidth range plotted in figure 8 the breakdown field is less at 0.53 ym than at 1.06 ym for both the Si

0

2 and the NaCl sample. Avalanche breakdown theory predicts an increase in Eg with decreasing wavelength, which is clearly inconsistent with the results shown in figure 8. The observed decrease in breakdown field with wavelength, while in the right direction,

is much too weak for a strictly multiphoton process. One could possibly account for these results

by a multiphoton initiated avalanche breakdown model which has been previously suggested.

Smith et a_l_. [11] observed an increase in the breakdown threshold with decreasing wavelength

for picosecond pulses. However, the data presented in that work was scaled for the presumed effects

of se 1 f -f ocusi ng using eq. (2) for 1.06 ym and eq. (1) for 0.53 ym over a range of pulsewidths and

spot sizes in which we observe no such effects. In addition it is unclear whether the same samples

were used in the wavelength comparison. Manenkov [12] observed an initial increase in the damage

threshold for a sample of NaCl from 1.06 ym, to 0.69 ym, then a decrease in the damage threshold at 0.53 ym for pulses ranging from 15 to 8 nsec. The data presented in that work was not scaled for

sel f-focusing.

5. Summary

Laser-induced breakdown was studied as a function of pulsewidth and wavelength for a variety of

focal conditions in fused Si

0

2 and single crystal NaCl. Beam quality measurements and polarization

dependence studies indicated the absence of self-focusing effects in these experiments such that no self-focusing corrections need be used.

For the two materials studied the breakdown field (Eg) increases with decreasing pulsewidth.

The observed pulsewidth dependence for a given spot size and wavelength is consistent with the

pulsewidth dependence predicted by various electron avalanche breakdown models. However, the spot

size dependence observed in this study is not predicted by any avalanche theory and is probably due

to the extrinsic nature of the observed damage. The strongest pulsewidth dependence observed was in NaCl at 1.05 ym for pulses shorter than 10 psec. For these short pulses Eg increases as the inverse

square root of the pulsewidth, indicating that the breakdown fluence is constant.

For conditions of equal pulsewidth and the same focal spot size, i.e., pulsewidths from 45 to

1 75 psec and a focal spot size of 7.2 ym, Eg is less at 0.53 ym th<in at 1.06 ym for both material s. Avalanche breakdown theories predict an i n crease in Eg for shorter wavelengths. The observed

decrease in breakdown field with wavelength, while in the right direction, is much too weak for a strictly multiphoton process. A mul tiphoton-initiated avalanche breakdown process might account for

these results. In such a process electrons are excited to the conduction band by multiphoton

394
excitation of impurities or defect states within the material bandgap (i.e., extrinsic effects).

After a few electrons are present in the conduction band, avalanche ionization takes over and

dominates the damage process. Thus, the frequency dependence observed may be partly due to the

multiphoton initiation process and the pulsewidth dependence indicative of an avalanche ionization

process.

The authors acknowledge the support of the Office of Naval Research, the National Science

Foundation, The Robert A. Welch Foundation, and the North Texas State University Faculty Research

Fund.

References

[I] A central depository of work in this area are the proceedings of the symposium on "LaserInduced Damage in Optical Materials", National Bureau of Standards Special Publications, #341

(1970), #356 (1971), #372 (1972), #387 (1973), #414 (1974), #435 (1975), #462 (1976), #509

(1977), #541 (1978), #568 (1979), #620 (1980). These may be obtained from the Superintendent

of Documents, U.S. Government Printing Office Washington, D.C. 20402. Papers in these proceedings contain further reference to materials in this field.

[2] E. W. Van Stryl and, M. J. Soi 1 eau, Arthur L. Smi rl , and Wi 1 1 iam E. Wi 1 1 iams, Phys. Rev. B 23,

2144 (1981).

[3] A. L. Smirl, T. F. Boggess, B. S. Wherrett, G. P. Perryman, and A. Miller, IEEE J. Quantum

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[4] E. W. Van Stryland, W. E. Williams, M. J. Soileau, and A. L. Smirl, to be published in the

Proceedings of the 1981 Conference on Laser- Induced Damage in Optical Materials, Boulder,

Colorado.

[5] J. Marburger, in Progress in Quantum Electronics edited by J. H. Sanders and S. Stenhold

(Pergamon, Oxford, 1975), Vol. 4, Part 1, and references therein.

[6] Nicolass Bloembergen, IEEE J. Quantum. Electron. QE- 10 , 375 (1974).

[7] G. M. Zverev and V. A. Pashkov, Sov. Phys. JETP, Vol. 30, 616 (1970).

[8] D. W. Fradin, IEEE J. Quantum El ectron. QE-9 , 954 ( 1972).

[9] W. Lee Smith, J. H. Bechtel, and N. Bloembergen, Phys. Rev. B 12, 706 (1975).

[10] W. Lee Smith and J. H. Bechtel , Appl . Phys. Lett. 28, 606 ( 1976).

[II] W. L. Smith, J. H. Bechtel , and N. Bloembergen, Phys. Rev. B j_5, 4039 ( 1977).

[12] A. A. Manenkov, Natl. Bur. Stand. (U.S.) Spec. Publ . j)09, 455 ( 1977).

[13] R. W. Hellworth, Natl. Bur. Stand. (U.S.) Spec. Publ. 34_1» 67 ( 1970).

[14] E. Yablonovitch and N. Bloembergen, Phys. Rev. Lett. 29, 907 (1972).

[15] M. J. Soileau, Ph.D. Thesis, University of Southern California, 1979, unpublished.

[16] E. W. Van Stryland, A. L. Smirl, T. F. Boggess, and F. A. Hopf, in Picosecond Phenomena III (Springer-Verl ag, New York, 1982), p. 368.

[17] P. Kelly, A. Schmid, and P. Braunlich, Phys. Rev. B 20, 815 ( 1979).

[18] Y. R. Shen, Phys. Rev. Lett. 20, 378 (1966).

[19] A. Feldman, D. Horowitz, and R. Waxier, Natl. Bur. Stand. Spec. Publ . 3_72, 92 (1972).

[20] C. C. Wang, Phys. Rev. 1_52, 149 (1966).

395
[21] R. W. Hell worth, Progress in Quantum Electronics, Vol. 5, pp. 1-68, Pergamon Press, N.Y., 1977.

[22] M. J. Soileau, W. E. Williams, E. W. Van Stryland, and S. F. Brown, Proceedings of the Conference on Laser Induced Damage to Optical Materials, 1981, to be published by Natl. Bur. Stand. Spec.

[23] D. H. Close, C. R. Giul iano, R. W. Hel 1 worth, L. D. Hess, F. J. McCl ung, and W. G. Wagner, IEEE

J. Quant. Electron. QE-2 , 553 (1966).

[24] The fused silica samples were acquired from Mark Optics, 1510 East St. Gertrude Road, Santa

Ana, CA 92705. The Si

0

2 designated 78-FQ-125-1 was a General Electric #125, high purity water

free sample and the sample designated 78-FQ-7940-1 was a Corning #7940 sample which contained

substantial water, but is otherwise a high purity material. Both NaCl samples were single

crystal "laser grade" material from the Harshaw Chemical Co., 6801 Cochran Road, Solon, Oh o 44139.

[25] M. J. Soileau, M. Bass, and P. H. Klein, NBS Special Publ. 568, p. 497 (1979).

[26] M. J. Soileau and M. Bass, IEEE Journal of Quantum Electronics QE-16 , 814 (1980).

[27] J. 0. Porteus, J. L. Jernigan, W. N. Faith, NBS Spec. Pub. 509_, 507 (1977).

[28] M. Sparks, T. Hoi stein, R. Warren, D. L. Mills, A. A. Maradudin, L. J. Shan, E. Loh Jr., and F. King, NBS Spec. Publ. 568, 467 (1979).

[29] S. Brawer, Phys. Rev. B 20, 3422 (1979).

[30] A. S. Epifanov, IEEE Journal of Quantum Electron. QE-17 , 2018 (1981).

[31] D. W. Fradin, N. Bloembergen, and J. P. Lettlier, Appl. Phys. Lett. 22, 635 (1973).

396
Table I. LID data for sample 79-FQ-7940-1 (Si0 2

) at 0.53 ym. The error values listed are the

relative uncertainties in the threshold for damage determined by the method used in Ref. 27. The

absolute accuracy of this data and the data presented in the other tables is estimated to be ±20% in the breakdown field. In this table and those that fol low W = focal spot radius (HW 1/e 2 M) in

microns, t„ = laser pulsewidth (FWHM) in psec, Ig = breakdown irradiance, Eg = breakdown field, Pg = breakdown power and eg = breakdown fluence.

W t p Ig Eg Pg eg

(ym) (psec) (TW/cm 2

) (MV/cm) (MW) (J/cm 2)

25 ± 4 1.32 + 0.13 18.3 + 0.9 1.07 + 0.11 35.0 ± 3.5

7.2 53 ± 5 0.78 + 0.08 14.1 + 0.7 0.64 + 0.06 44.0 ± 4.4

110 ±10 0.78 + 0.08 14.1 + 0.7 0.64 f 0.06 91 ± 9

37 ± 5 1.40 + 0.14 18.9 + 0.9 0.55 + 0.06 55 ± 6 5.0 105 ±15 1.20 + 0.12 17.5 + 0.8 0.47 + 0.05 134 ± 13 140 ±15 1.14 + 0.11 17.0 + 0.8 0.45 + 0.05 169 ± 17 180 ±20 1.10 + 0.11 16.7 + 0.7 0.43 + 0.04 210 ± 21

30 ± 5 1.68 + 0.17 20.7 + 1.0 0.31 + 0.03 53 ± 5 3.4 72 ±10 1.34 + 0.13 18.5 + 0.8 0.24 + 0.02 102 ± 10 105 ±15 1.07 + 0.11 16.5 t 0.8 0.19 + 0.02 83 ± 8

Table II. LID data for sample 79-FQ-125-1 ( S i 0 2) at 0.53 ym. See Table 1 for explanation of

symbols and error bars.

w tp *B E BP B

e B

(ym) (psec) (TW/cm 2

) (MV/cm) (MW) (J/cm 2

)

25 + 4 0.96 + 0.10 15.7 + 0.8 0.79 + 0.08 25.5 + 3.0

7.2 105 + 15 0.48 + 0.05 11.0 ± 0.5 0.39 + 0.04 53 + 5 150 + 15 0.39 + 0.06 9.9 + 0.8 0.32 + 0.04 62 ± 6

34 + 4 1.02 + 0.10 16.1 + 0.8 0.40 + 0.04 37 4 5.0 120 + 15 0.86 + 0.09 14.8 + 0.7 0.34 + 0.03 109 + 11 180 + 20 0.80 + 0.15 14.3 1.0 0.32 + 0.03 153 + 25

32 + 4 1.16 + 0.15 17.2 + 1.0 0.21 + 0.02 39 + 4 3.4 120 + 15 0.91 + 0.09 15.2 + 0.5 0.16 + 0.02 115 + 12 180 + 20 0.75 + 0.12 13.8 + 1.0 0.14 + 0.01 143 + 14

397
Table III. LID data for sample 82-NC-l (NaCl) at 0.53 ym.

W

(ym)

"P (psec)

B (GW/cnO (MV/cm) (KW) (J/crn 2

)

35 ± 4 57 + 6 3.80 ± 0.25 174 + 17 2. 10 + 0.20

14.0 80 ± 8 35 + 4 2.98 ± 0.15 107 + 10 2.96 + 0.30

110 ± 15 32 + 3 2.87 ± 0. 15 100 + 10 3. 77 + 0. 38

41 ± 9 91 + 9 4.80 ± 0.35 120 + 12 3.95 + 0.40

7.2 82 ± 7 64 + 6 4.03 ± 0.19 40 + 4 5.50 + 0.60

120 ± 15 47 + 5 3.45 ± 0.19 29 + 3 6.00 + 0.60

26 ± 4 139 + 14 5.95 ± 0.48 26.0 + 0.3 3.83 + 0.38

37 ± 3 115 + 12 5.41 ± 0.33 21.0 + 0.2 4.50 + 0.50

3.4 57 ± 3 85 + 9 4.64 ± 0.30 15.0 + 0.2 5.10 + 0.50

73 ± 4 71 7 4.24 ± 0.52 12.8 + 0.12 5.50 + 0.60

90 ± 10 64 + 6 4.04 ± 0.18 12.0 + 0.12 6.10 + 0.60

115 ±15 35 + 4 3.00 ± 0.31 6.4 + 0.64 4.30 + 0.40

Table IV. LID data for sample 78-NC-6 (NaCl) at 0.53 ym.

w t

p

I BE B

P B

(ym) (psec) (GW/cm 2

) (MV/cm) (KW) (J/cm 2)

25 ± 5 120 + 12 5*53 + 0.33 370 + 40 3.19 + 0.32

14.0 105 ±15 48 +5 3.50 + 0.20 148 + 15 5.37 + 0.54

170 ±20 34 + 6 2.95 + 0.30 105 + 10 6.18 + 0.62

30 ± 5 148 + 15 6.14 + 0.30 120 + 12 4.72 + 0.47

7.2 105 ±15 50 +

5 3.55 + 0.20 40 + 4 5.53 + 0.56

150 ±15 36 + 4 3.03 + 0.16 29 + 3 5.74 + 0.57

28 ± 4 184 18 6.86 + 0.31 33.0 + 3.3 5.48 + 0.55

85 ± 5 82 +8 4.56 + 0.23 15.0 + 1.5 7.39 + 0.74

3.4 100 ±10 66 + 7 4.10 + 0.21 12.0 + 1.2 7.02 + 0.70

120 ±10 55 + 8 3.75 + 0.30 10.0 + 1.0 7.05 + 0.71

140 ±10 44 + 7 3.35 + 0.30 8.0 + 0.8 6.57 ± 0.66

170 ±20 34 +

5 2.95 + 0.21 6.0 + 0.6 6.17 ± 0.62

398
Table V. LID data for sample 79-FQ-7940-1 (Si02

) at 1.05 pm.

tp l B

E Bp8

e

3

(psec) (TW/cm 2

) (MV/cm) (MW) (J/cm 2)

7.9 + 0.6 1.12 + 0.11 16.8 + 0.8 3.44 + 0.34 9.4 + 0.9

D • J + U • J 1 . c J + 0 1

"3 1 7 7 + 1 n J • OH + U • JO ft A + n ft U . O 5.2 + 0.3 1.42 + 0.14 18.9 + 1.0 4.37 + 0.44 6.9 + 0.6

3.6 + 0.3 1.75 + 0.18 21.0 + 1.1 5.39 + 0.54 6.7 + 0.7

8.5 + 0.5 2.27 + 0.23 24.1 + 1.1 1.85 + 0.19 20.5 + 2.0

7.5 + 0.5 2.20 + 0.22 23.7 + 1.1 1.79 + 0.18 17.5 + 1.8

6.5 + 0.5 2.60 + 0.26 25.7 + 1.3 2.12 + 0.21 18.0 + 1.8

5.5 + 0.5 2.75 + 0.28 26.5 + 1.2 2.24 + 0.22 16.1 + 1.6

4.5 + 0.5 3.14 + 0.31 28.3 + 1.3 2.56 + 0.26 15.0 + 1.5

3.6 + 0.4 3.65 + 0.37 30.5 + 1.5 2.97 + 0.30 14.0 + 1.4

8.5 + 0.5 3.04 + 0.30 27.8 + 1.4 1.19 + 0.12 27.6 + 2.8

7.5 + 0.5 3.24 + 0.32 28.7 + 1.3 1.27 + 0.13 25.9 + 2.6

6.5 + 0.5 3.56 + 0.37 30.1 + 1.5 1.40 + 0.14 24.6 + 2.5

5.5 + 0.5 3.94 + 0.39 31.7 + 1.5 1.55 + 0.16 23.1 + 2.3

3.6 + 0.3 5.00 + 0.60 35.7 + 2.0 1.96 + 0.20 19.2 + 1.9

Table VI. LID data for sample 79-FQ-125-1 (SiOo) at 1.05 pm. The data for 7.2 pm size and tp

= 47

to 175 psec is taken from Ref. 2 and is interpolated from measurements made at spot sizes of 6.1 and

10.3 ym and a laser wavelength of 1.06 urn.

W t

p

I B

E B

P B

e B

(ym) (psec) (TW/cm 2

) (MV/cm) (MW) (J/cm<

9.5 ± 0.5 0.88 + 0.09 15.0 + 0.7 2.71 + 0.27 5.01 + 0.50

8.5 ± 0.5 0.90 + 0.09 15.1 + 0.7 2.77 + 0.28 8.14 + 0.82

14.0 6.5 ± 0.5 1.06 ± 0.11 16.4 + 0.9 3.26 + 0.33 7.32 + 0.73

4.0 ± 0.5 1.8 + 0.20 21.4 + 1.2 5.54 + 0.56 7.66 + 0.77

175 ±15 0.88 + 0.12 15.0 + 1.2 0.72 + 0.09 163 +

20 92 ±15 0.92 + 0.13 15.3 + 1.1 0.75 + 0.10 90 + 12 47 ± 6 0.96 + 0.14 15.6 ± 1.0 0.78 + 0.10 48 +

7 9.5 ± 0.5 1.93 + 0.19 22.1 ± 1.1 1.57 + 0.16 19.5 + 2.0

7.2 7.5 ± 0.5 2.05 + 0.21 22.8 ± 1.2 1.67 + 0.17 16.3 + 1.6

6.5 ± 0.5 2.15 + 0.22 23.4 ± 1.2 1.75 + 0.18 14.9 + 1.5

5.5 ± 0.5 2.30 + 0.23 24.2 ± 1.2 1.87 + 0.19 13.5 + 1.4

4.5 ± 0.5 2.89 + 0.29 27.1 ± 1.4 2.35 + 0.24 13.8 + 1.4

3.5 ± 0.5 3.31 + 0.33 29.0 + 1.4 2.69 + 0.27 12.3 + 1.2

9.4 ± 0.5 2.60 + 0.26 25.7 + 1.3 1.02 + 0.10 26.0 + 0.3

7.5 ± 0.5 3.26 + 0.33 28.8 + 1.4 1.28 + 0.13 26.0 + 0.3

5.0 6.5 ± 0.5 3.77 + 0.38 31.0 + 1.5 1.45 + 0.15 26.1 + 0.3

5.5 ± 0.5 3.98 + 0.40 31.8 + 1.6 1.56 + 0.16 23.2 + 0.2

4.5 ± 0.5 4.49 + 0.45 33.8 + 1.6 1.76 + 0.18 21.5 + 0.2

399
Table VII. LID data for sample 82-NC-l (NaCl) at 1.05pm.

w tp l B

E BP B

e B

(urn) (psec) (TW/cm 2

) (MV/cm) (MW) (J/cm2

)

9.5 + 0.5 0.25 + 0.03 8.00 ± 0.44 0.77 + 0.08 2.55 + 0.26

8.5 + 0.5 0.26 + 0.03 8.18 ± 0.41 0.81 + 0.08 2.38 + 0.24

7 ^ + U • 3 n 9Q U. Uj q en + n a'x O. JU x U.*tj U . 03 n no C . J

I

+ 6.5 + 0.5 0.37 + 0.04 9.64 ± 0.57 1.12 + 0.11 2.55 + 0.26

5.5 + 0.5 0.40 + 0.04 10.10 ± 0.48 1.23 + 0.12 2.38 + 0.24

4.5 + 0.5 0.45 + 0.05 10.70 ± 0.58 1.37 + 0.14 2.12 + 0.21

7.5 + 0.5 0.66 + 0.07 12.9 ± 0.73 0.53 + 0.05 5.22 + 0.52

6.5 + 0.5 0.75 + 0.08 13*8 ± 0.73 0.61 + 0.06 5.15 + 0.52

5.5 + 0.5 0.88 + 0.09 14.9 ± 0.81 0.71 + 0.07 5.12 ± 0.52

4.5 + 0.5 0.97 ± 0.10 15.7 ± 0.80 0.79 + 0.08 4.65 + 0.47

3.6 + 0.3 1.38 + 0.14 18.8 ± 0.87 1.12 + 0.09 5.28 + 0.53

7.5 + 0.5 0.74 + 0.07 13.7 ± 0.65 0.29 + 0.03 5.86 + 0.59

6.5 + 0.5 1.00 + 0.10 15.9 ± 0.83 0.39 + 0.04 6.91 + 0.69

5.5 + 0.5 1.14 + 0.11 17.0 ± 0.83 0.45 + 0.05 6.66 + 0.67

4.5 + 0.5 1.29 + 0.13 18.1 ± 0.91 0.51 + 0.05 6.20 + 0.62

Tab! e VIII. LID data for sample 78-NC-l (NaCl ) at 1.05 urn. The data for the 7.2 ym spot size and

tp = 45 to 167 psec is taken from Ref. 2 and is interpulated from measurements made at spot size of

6.1 and 10.3pm and a laser wavelength of 1.06 pm.

W t

p

I B Eg P B

e B

(pm) (psec) (TW/cm 2

) (MV/cm) (MW) (J/cm 2)

4.0

7.2

7.5 + 0.5 0.26 ± 0.03 8.06± 0.54 0.79 ± 0.08 2.03± 0.20

6.5 + 0.5 0.28 ± 0.03 8.44± 0.44 0.86 ± 0.09 1.95± 0.20

5.5 + 0.5 0.32 ± 0.03 9.03± 0.44 0.99 ± 0.10 1 .87+ 0.19

4.5 + 0.5 0.38 ± 0.04 9.80+ 0.53 1.17 ± 0.12 1 .82+ 0.18

167 + 16 0.083± 0.011 4.6 ± 3.3 0.068± 0.008 14 + 1.5

100 ± 10 0.110± 0.008 5.1 ± 0.4 0.090± 0.006 11.7 ± 1.2

45 +

3 0.118± 0.018 5.5 ± 0.4 0.096± 0.010 5.56± 0.60

6.5 + 0.5 0.58 ± 0.06 12.2 ± 0.6 0.43 ± 0.04 4.00+ 0.40

5.5 + 0.5 0.65 ± 0.07 12.9 ± 0.73 0.53 ± 0.05 3.83± 0.38

4.5 + 0.5 0.80 ± 0.03 14.3 ± 0.63 0.65 ± 0.07 3.83± 0.38

3.5 + 0.5 0.95 ± 0.10 15. f> ± 0.64 0.77 ± 0.08 3.56± 0.36

7.5 + 0.5 0.92 + 0.09 15.3 + 0.73 0.36 + 0.04 7.30± 0.73

6.5 + 0.5 0.94 + 0.09 15.5 ± 0.69 0.37 + 0.04 6.53± 0.65

5.5 + 0.5 1.13 + 0.11 17.0 + 0.75 0.44 + 0.04 6.62± 0.66

4.5 + 0.5 1 .20 + 0.12 17.5 i 0.83 0.47 + 0.05 5.78± 0.58

3.4 + 0.1 1 .60 + 0.16 20.2 + 0.94 0.63 + 0.06 5.78± 0.58

400
\=0.53^m

P B =0.55MW

t p

=37±5psec

LINEAR POLARIZATION

I

CIRCULAR POLARIZATION

0 10 20

E B [MV/cm)

30

K =1.06/um

PB=1.4MW

t p=6.5±0.5psec

LINEAR POLARIZATION

CIRCULAR POLARIZATION 1

10 20 30

EelMV/cm)

Figure 1. Polarization dependence of Eg for SiC^. The horizontal axis is Eg, the r.m.s. breakdown field corresponding to the peak on-axis irradiance at damage. Pg is the breakdown threshold

power. Note the lack of polarization dependence for both wavelengths. The cross-hatched portions

shown above are the relative uncertainties of the thresholds using the procedures given in Ref. 16.

Figure 2. Beam distortion measurements. This is a plot of the far field beam profile of the 0.53 ym beam after propagation through the Si O2 sample. The quantity u

Q

= 7 pm is the focal spot radius

inside the sample calculated using linear Gaussian optics. The curve labeled high power was taken

with the input power at the damage threshold level and the one labeled low power was for input power

approximately equal to one-tenth the damage threshold power.

401
(

E B(MV/cm)

Figure 3. Pulsewidth dependence of Ed for SiOo at 1.05 pm. The three sets of points correspond

to different focal spot radii and the relatively large displacement of the three sets of points is indicative of a large spot size dependence in the damage threshold. The slope of the least square

fit for each set of points is approximately 0.3 for this pulsewidth range (t D

< 10 psec).

1*10

15 20

EB(MV/cm)

30 35 40 45 50

Figure 4. Pulsewidth dependence of Eg in NaCl at 1.05 Mm. The slope of the least square linear

fit to each of the three sets of points is approximately 0.5. This indicates an inverse square root of pulsewidth dependence of Eg for this pulsewidth range (t p

< 10 psec).

402
1 10 100

EB(MV/cm]

Figure 5. Pulsewidth dependence of Eg in SiC^ at 0.53 pm. Note Eg is nearly independent of tp in this pulsewidth range (20 to 300 psec) and that there is little spot size dependence in Eg for the

two spot sizes shown (3.4 and 5.0 pm).

i i l i i i i I I I 1 l

i i i i i 10 2

103

EB(MV/cm)

Figure 6. Pulsewidth dependence of Eg in NaCl at 0.53 ym. Note that there is little spot size

dependence in Eg for the two spot sizes shown (3.4 and 7.2 ym). The 1 east squares 1 inear fit of

these data give a slope of 0.3, or nearly an inverse fourth root dependence of Eg on t p for pulsewidths in this range (20 to 200 psec).

403
£

m

22

20

18 E 16

14

12

10

6

6

4

2

65

*-» \

\ \ \

\ \ \ \

\ \ 5.0/xm

- Nj \

* 7.2^im

T \ \

- * xK

No Adjusted Parameters

It

1 1 1

i

i i 1ps lOps lOOps Ins PULSE DURATION, t p

10ns 100ns

Figure 7. The RMS breakdown field data for NaCl (78-NC-6) at 1.06 ym are plotted as a function of

pul se duration, tp. The sol id 1 ine and dotted 1 ine were obtained from the theory devel oped by

Sparks et^ aj_. [28Jfor NaCl at room temperature. The dotted line uses a different value for the

absorption cross section for CI ions in the theory than the solid line.

tp(psec)

150+15

104+15

45+3

EB

(rVIV/cm)

E B(MV/cm]

Figure 8. Wavelength dependence of the breakdown field Eg for NaCl and Si 0£ for a variety of

laser pulsewidths. All the above data was taken on the same sample of NaCl and the same sample of

Si The 1.06 ym thresholds are taken from Ref. 2 and are interpolated from measurements made at spot size 6.1 and 10.3 ym.

404
There were several questions regarding the damage mechanism. The author stated that he did not

believe electron avalanche was the only mechanism observed in his experiments , because thresholds

at 532 nm were less than those at 1064 nm. He speculated that the low 532-nm thresholds might be

attributed to avalanche breakdown assisted by multiphoton ionization , and also agreed with the

audience that the volume density of damageable defects probably increased as wavelength decreased.

The author believed surface thresholds should also scale with laser parameters, but declined to speculate on the exact parametric dependences. The fused silica used in the experiment was water-free material from General Electric Corporation.

405
Ripple Structures and Enhanced Absorption

Associated With Ordered Surface Defects

M. J. Soileau and Eric W. Van Stryland

Center for Applied Quantum Electronics

Department of Physics

North Texas State University

Denton, Texas 76203

Laser-induced ripple structures on material surfaces have been observed by a number

of workers for various material s incl uding metal s, semiconductors and die 1 ec tries. A model has been proposed [1] which correctly accounts for the spacing and polarization

dependence of the ripples, and the association of the ripples with material defects. In this model the ripples are associated with the interference of the incident field with the

nonradiative field induced by the interaction of the incident field with the material

defects. The association of these features with defects is often difficult to see since

real optical surfaces have many defects having a range of sizes and orientations. In this

paper experimental results which unambiguously show the association of these features with

defects are presented. Quantitative measurements of laser-induced damage thresholds and

photoacoustic absorption measurements show a reduction of damage thresholds and increased

optical absorption for surfaces with controlled, linear defects aligned orthogonal to the

incident laser field (as predicted in ref. 1).

Key Words: Laser-generated ripples; surface damage; surface defects; NaCl ; ZnS; ZnSe;

CdTe; 1.06 ym; 10.6 ym.

1. Introduction

Laser-generated ripple patterns (LGRP) have been observed in laser-induced damage (LID) to surfaces of various wide bandgaps dielectrics [1-14], semiconductors [14-26], and metals

[ 1 ,2,1 4,25,26,28-36]. These features have al so been observed in diel ectric fi 1 ms used as anti

ref 1 ection coatings [6-9]. Careful examination of micrographs in a number of earl ier LID papers

reveal LGRP on surfaces which were initially unnoticed or were wrongly interpreted as features which

were caused by multiple reflections within the sample [37-41]. Recent work has shown these features

to be common artifacts of laser annealing experiments, while other experiments have revealed LGRP

associated with laser-assisted thin film deposition [36]. The observation of LGRP over such a large

range of wavelengths (0.17 to 10.6 ym) and such a wide range of materials has prompted Van Driel to speculate that LGRP are a universal phenomenon common to all laser-surface interactions [42].

Most of the LID experiments referenced above were conducted using optically polished surfaces.

Such surfaces, particularly those on exotic materials such as the al kal i-hal ides, contain a high

density of defects, e.g., scratches, digs and residual polishing material. LGRP are found to be

directly associated with these pre-existing surface defects. Figure 1 shows LGRP running parallel

to scratches which are orthogonal to the impressed laser electric field, whereas no LGRP are

produced by the same type defect parallel to the field. Similar features have been observed for

point defects and grain boundaries in polycrystal 1 ine materials.

The observations above lead to a model by Temple and Soileau [1,2] for the formation of LGRP

that assumes that these features are initiated by the interaction of the impressed field with preexisting surface defects. A similar, much more rigorous, model has been proposed by Sipe et al

[43]. In that work, the authors show that ripple formation may be initiated by the components "57 the surface sub-microscopic roughness which are oriented for optimum coupling with the impressed

field. Other models [12,36,44] have been recently proposed and these all have the common feature

that LGRP result from some sort of scattered wave launched by surface roughness (i.e., submicroscopic surface defects) which then constructively interferes with the impressed field to produce

ripples.

406
The recent models have the common feature that they predict that LGRP are initiated by surface

roughness [12,36,43,44]. The model proposed by Temple and Soileau [1,2] predicts that defects of

specific sizes and orientation with respect to the incident field should have maximum efficiency for

creating LGRP (the model by Sipe et al [43] has a similar provision which they refer to as the

"efficacy factor"). Reference 1 prelTicTs that LGRP are easiest to form for scratches of width equal

to the laser wavelength and oriented orthogonal to the electric field of a linearly polarized beam.

The effects of scratch orientation on LGRP generation on dielectric surfaces have been reported in measurements of the LID thresholds of diamond-turned optical surfaces [10] and more recently in

specially prepared Si surfaces [23].

In this paper we report the results of measurements of the thresholds for LGRP formation on

NaCl surfaces. We attempted to make the defects nearly "resonant" with the impressed laser field by

grinding the NaCl surface with particles of average diameter approximately equal to the laser

wavelength (10.6 ym). In addition, we made the observation of LGRP in ZnS, ZnSe, CdTe and other

material s.

2. Experimental

A single crystal NaCl specimen was first etched to remove most residual , randomly oriented

scratches, due to polishing by the crystal vendor. The sample was then ground on the edge of a polishing wheel using 10 ym alumina grit so as to produce a high density of parallel scratches. A linearly polarized CO2 TEA laser operated in the TEMoo spatial mode at 10.6 ym was used to irradiate

this specially prepared sample. The laser was gain-switched with a partially mode locked temporal

profile. The laser was operated without and the temporal width of the gain-switched envelope

was approximately 100 nsec (FWHM). The laser was focused onto the exit surface of the sample. The

f 1 uence was varied using a set of Brewster angle polarizers arranged so as to keep the orientation

of the laser electric field vector constant at the sample surface. The threshold fl uence for the

onset of LGRP formation was then measured for the cases where the surface scratches were oriented

parallel to and orthogonal to the laser electric field.

This same laser system was used to produce LGRP on the exit surfaces of optically polished ZnSe

and ZnS. In addition, a Nd: YAG laser was used to produce LGRP on various materials. The Nd: YAG

laser was linearly polarized and operated at 1.06 ym in the TEMoo spatial mode (this laser is more completely described in ref. 45).

The CO2 laser system described above was also used as the source for orientation dependent

absorption measurement for an aluminum diffraction grating. The grating spacing was 9 ym. The

absorbed energy was monitored using a photoacoustic technique. The laser-induced acoustic signal was detected by a piezoelectric tranducer pressure contacted with vacuum grease to the rear surface

of the metal grating. The absorption, as determined by the piezoelectric peak voltage output, was measured for the cases where the grating grooves were parallel to and orthogonal to the laser

electric field.

3. Results and Discussion

Figure 2 is a Nomarski micrograph of LGRP on the specially prepared NaCl surface. The micrograph on the right corresponds to the situation for which the parallel scratches are orthogonal to the incident field vector. Note that as a scratch enters into the high field region it seems to "grow" until its width is approximately equal to the wavel ength of the 1 ight in the material . A well coordinated ripple pattern results since adjacent ripples are optimumly spaced for maximizing

the induced surface field. The micrograph on the right is a picture of a damage sight for the

situation where the scratches are parallel to the incident field. The only ripples seen in this

micrograph are associated with a scratch (probably left over from the original polishing) which is

nearly normal to the impressed field.

We measured the threshold fl uence for the onset of LGRP with the laser field parallel to and

orthogonal to the parallel scratches. The results of these measurements are shown in figure 3. Note that the threshold for ripple formation is 40% higher for the case where the scratches are

oriented parallel to the impressed electric field.

The above result is consistent with the model presented in reference 1 and can be understood in terms of the well known phenomenon of the orientation dependence of absorption of a diffraction

grating [46]. Figure 3 is a plot of the 10.6 ym absorption of an aluminum grating with 9 ym grating

spacing. The absorption was measured for the cases where the grooves are parallel to and normal to the laser electric field for linearly polarized light. As can be seen in figure 4, the absorption

is 4.6 times larger for the case where the grating lines are ortogonal to the impressed field

compared to the case where the grating lines are parallel to the incident laser field. So, for this

407
specimen the surface absorption is 4.6 times greater for the case where the defects (i.e., the

grating lines) are normal to the incident field. By comparing the results shown in figure 4 with

those shown in figure 3 we see that the threshold for initiating LGRP is lowest for the same orientation of the surface defects (scratches) which lead to maximum surface absorption. As the

ripples form, coupling of light into the surface increases, and this in turn enhances the formation

of more ripples. In fact, an exponential growth of LGRP has been observed [12].

LGRP observed in diel ectrics, semiconductors, and metal s are very simi 1 ar in that they run

normal to the impressed field and are associated with surface defects. They differ in one important

way: the ripple spacing is the free space wavelength ( * o) for semiconductors and metals, whereas

their spacing is ^o/n, (n is the index of refraction), in the wide bandgap dielectrics. Table 1 summarizes the materials for which LGRP have been observed in our lab and the observed ripple

pattern spacings. One may speculate about the reason for the difference. One possibility is that

the semiconductors undergo a semiconductor to metal phase transition prior to ripple formation. The

model given in reference 43 predicts that a transition from ^o is X o/n spacing occurs for index of

refraction between 4 and 1.5. We attempted to observe the occurance of LGRP with both spacings by

examining materials of intermediate index. CdTe and ZnSe were examined at 1.06 ym, and ZnSe and ZnS were examined at 10.6 ym. In addition ZnSe has been studied at 3.8 ym. In all cases the LGRP

observed had spacings of ^ o for these materials.

4. Summary

We have studied LGRP on surfaces with oriented defects (parallel scratches). The threshold

for initiation of LGRP was 40% lower for the case where the defects are normal to the impressed

laser electric field. Scratches normal to the field produce well coordinated ripple patterns which

are clearly associated with the scratches. Photoacoustic measurements of absorption at 10.6 ym of

an aluminum grating with 9 ym spacing show that surface absorption is 4.6 times larger for the

grating lines normal to the field vector. Thus, we see that LGRP formation is easiest for defect

orientation for which surface absorption is maximum.

Ripple patterns were produced on surfaces of CdTe and ZnSe at 1.06 ym and on surfaces of ZnSe

and ZnS at 10.6 ym. In all cases the ripple spacing was ^o, the free space wavelength. Ripples

observed on metal s and semiconductors al 1 have a spacing of ^ o, whereas those observed on wide

bandgap dielectrics are spaced at ^o/n.

This work was supported by the Office of Naval Research, the National Science Foundation, The

Robert E. Welch Foundation and North Texas State University Faculty Research funds.

5. References

[1] P. A. Temple and M. J. Soileau, "Resonant Defect Enhancement of the Laser Electric Field," NBS

#462, pp. 371 -378 (1976).

[2] P. A. Temple and M. J. Soileau, "Polarization Charge Model for Laser-Induced Ripple Pattern in Dielectric Materials," IEEE Journal of Quantum Electronics, QE-17, 2067-2072 (1981).

[3] M. J. Soileau, H. E. Bennett, J. Porteur, P. Temple, and M. Bass, "Laser Damage to HEL Window

Materials as Related to Surface Damage," in Proceedings of the Fifth Annual Conference on Infrared Laser Window Materials, published by the Defense Advanced Research Projects Agency,

Arlington, Virginia, pp. 391 -417 (1976).

[4] M. J. Soileau, J. M. Bennett, J. 0. Porteur, W. Faith, J. Jernigan, and T. T. Saito, "10.6 ym

Pulses Laser Damage Studies of Diamond Turned KC1 Window Surfaces," NBS #462, pp. 149-157

(1976).

[5] M. Bass, K. M. Leung, C. Tang, and M. J. Soileau, "Pul ses Laser Induced Damage at the DF Laser

Wavelength," NBS #462, pp. 1 36-144 (1976).

[6] M. J. Soileau, A. D. Baer, J. 0. Porteur, J. L. Jernigan, W. N. Faith, T. M. Donovan, and A.

K. Green, "Laser Damage Studies of Low Index Coating Materials at 10.6 ym," NBS # 462, pp. 264-270 (1976).

408
[7] C. C. Tang, M. Bass, M. J. Soileau, E. W. Van Stryland, "10.6 ym Laser Damage in Coatings

Containing As 2

S

3 and As 2 Se 3

," NBS #509, pp. 316-324 (1977).

[8] T. M. Donovan, A. D. Baer, J. H. Dancy, and J. 0. Porteur, "Defects and Impurities in As 2

S

3 , As 2 Se 3 and NaF Coatings," NBS #509, pp. 342-351 (1977).

[9] J. 0. Porteur, J. L. Jernigan, and W. W. Faith, "Mul ti threshol d Measurements and Analysis of

Pulsed Laser Damage on Optical Surfaces," NBS #509, pp. 507-516 (1977).

[10] M. J. Soileau, J. 0. Porteur, and D. L. Decker, "Laser-Induced Breakdown of Diamond-Machined

Window Surfaces," Appl. Opt. ]_9, PP« 3043-3044 (1980).

[11] M. J. Soileau and Eric W. Van Stryland, "Laser-Generated Ripple Patterns Associated with

Surface Defects," JOSA, 72, p. 1 756 (1982).

[12] F. Keilmann and Y. H. Bai, "Periodic Surface Structures Frozen into C02 Laser-Melted Quartz,"

Appl. Phys. A 28 , pp. 1 -10, (1982).

[13] C. T. Walters, "Surface Scattering at LSD-Wave Initiation Sites on Nonmetallic Materials,"

Appl . Phys. Lett. 2_5, pp. 696-698 (1 974).

[14] M. Siegrist, G. Kaech, and F. K. Kneubuhl , "Formation of a Periodic Wave Structure on the Dry

Surface of a Solid by TEA-C02 -Laser Pulses," App. Phys. 2, 45-45, 1973.

[15] M. Birnbaum, "Semiconductor Surface Damage Produced by Ruby Lasers," J. Appl. Phys. 36_, 3688-

3689 ( 1 965).

[16] D. C. Emmony, R. P. Hawson, and L. J. Willis, "Laser Mirror Damage in Germanium at 10.6 ym,"

Appl. Phy. Lett. 23, 598-600 (1973).

[17] G. N. Maracus, G. L. Harris, C. A. Lo, and R. A. McFarlane, "On the Origin of Periodic Surface

Structures of Laser-Annealed Semiconductors," Appl. Phy. Lett. 33_, 453-455 (1 978).

[18] H. J. Leamy, G. A. Rozzonyi, T. T. Sheng, and G. K. Celler, "Periodic Regrowth Phenomena

produced by Laser Annealing of Ion-Implanted Silicon," Appl. Phys. Lett. .32, 535-537 (1 978).

[19] M. Oron and G. Sorensen, "New Experimental Evidence of the Periodic Surface Structure in Laser

Annealing," App. Phy. Lett. .35, 782-784 (1 979).

[20] R. M. Walser, M. F. Becker, J. G. Ambrose, and D. Y. Sheng, "Heterogeneous Nucleation of

Spatially Damaged Structures in Crystalline Silicon with Picosecond 1.06 ym and 0.53 ym Laser

Pulses," in Laser and Electron-Beam Sol id Interaction and Materials Processing , edited by

Gibbons, Hess and Sigman (Elsevier, North Holland, 1 98 1 ) , pp. 177-184.

[21] M. F. Becker, R. M. Walser, Y. K. Thee, and D. Y. Sheng, "Picosecond Laser Damage Mechanisms

at Semiconductor Surfaces," in Picosecond Lasers and Applications , SPIE vol. 322 (SPIE,

Bellingham, Washington, 1982), pp. 93-98.

[22] P. M. Fauchet and A. E. Siegman, "Surface Ripples on Silicon and Gallium Arsenide Under Picosecond Laser Illumination," Appl. Phy. Lett. 40, 824-826 (1982).

[23] J. F. Young, J. E. Sipe, J. S. Preston, and H. M. van Driel , "Laser-Induced Periodic Surface

Damage and Radiation Remnants," Appl. Phy. Lett. 41_, 261-264 (1981 ).

[24] P. M. Fouchet and A. E. Siegman, "Surface Ripples on Silicon and Gallium Arsenide Under GaAs

Picosecond Laser Illumination," Appl. Phy. Lett 40, 824-826 (1982).

[25] J. F. Figueira and S. J. Thomas, "Generation of Surface Microstructure in Metals and Semiconductors by Short Pulse Co2 Lasers," Appl. Phys. B 28, 267 (1982).

[26] J. F. Young, J. S. Preston, H. M. van Driel , and J. E. Sipe, "Laser Induced Periodic Surface

Structure: II. Experiments on Ge, Si, Al and Brass," Phy. Rev. B 27, 1 155-1 1 72 (1983).

[27] Noriaki Tsukada, Sumio Sugata, and Yoh Mita, "New Experimental Evidence of Surface Ripples on Gallium Arsenide in Laser Annealing," Appl. Phys. Lett. 42, 424-426 (1983).

[28] M. A. Cutter, P. Y. Key, and V. I. Little, "Temporal Development of Optically Etched Gratings:

A New Method of Investigating Laser-Induced Damage," Appl. Optics 13, 1399-1404 (1974).

409
[29] J. C. Koo and R. E. Dusker, "Diffraction From Laser-Induced Deformation on Reflective

Surfaces," Appl. Phys. Lett. 28, 614-616 (1976).

[30] D. L. Decker, M. J. Soileau, J. 0. Porteur, and J. M. Bennett, "Surface, Optical and Laser

Damage Characteristics of Diamond- turned Metal Mirrors," NBS #462, pp. 158-164 (1976).

[31] J. 0. Porteur, C. W. Fountain, J. L. Jernigan, W. N. Faith, and H. E. Bennett, "Pul sed-Laser

Stress Phenomena on Highly Reflecting Metal and Alloy Surfaces," NBS #509, pp. 204-214 (1977).

[32] N. R. Isenor, "CO? Laser-Produced Rippl e Patterns in Ni x Pi Surfaces," Appl . Phy. Lett. 31 , 148-1 50 (1977). —

[33] Clifton W. Draper, "Regrowth in Laser Initiated Elemental Metals and Alloys: Surface Nonuniformities and Coupling Phenomena," NBS #620, pp. 210-221 (1980).

[34] A. K. Jain, V. N. Kulkarmi, D. K. Lood, and J. S. Uppal , "Periodic Surface Ripples in LaserTreated Aluminum and Their Use to Determine Absorbed Power," J. Appl. Phys. 52, 4882-4884

(1981). ~~

[35] S. J. Thomas, R. F. Harrison, and J. F. Figueira, "Observation of the Morphology of LaserInduced Damage in Copper Mirrors," Appl. Phys. Lett. 40, 200-202 (1982).

[36] S. R. J. Brueck and D. J. Ehrl ich, "Stimulated Surface-Plasma-Wave Scattering and Growth of a Periodic Structure in Laser-Photodeposi ted Metal Films," Phy. Rev. Lett. 48, 1678-1681 (1982).

[37] A. I. Braunstein, V. Wang, M. Braunstein, J. E. Rudisill, and J. Wada, "Pulsed C0£ Laser

Damage Studies of Windows and Window Coatings," NBS #387, pp. 151-158 (1973).

[38] V. Wang, A. Braunstein, M. Braunstein, J. E. Rudisill, and J. Y. Wada, "Pulsed CO2 Laser

Damage Studies of Metal and Dielectric Coated Mirrors," NBS #387, pp. 157-169 (1973).

[39] T. T. Saito, G. B. Charlton, and J. S. Loomis, "10.6 Micrometer CW Laser Damage Studies of

Metal Substrate Mirrors," Boulder, NBS #414, pp. 103-1 12 (1974).

[40] Susan D. Allen, Morris Braunstein, Concetto Ginl iano, and Victor Wang, "Pul sed CO? Laser

Damage Studies of RAP Grown KC1 ," Boulder, NBS #414, pp. 66-75 (1 974).

[41] J. 0. Porteur, M. J. Soileau, and C. W. Fountain, "Character of Pulsed Laser Damage in Al at

10.6 m Inferred From Single-Crystal Targets in Vacuum," NBS #462, pp. 165-172 (1976).

[42] H. M. van Driel , "'Universal' Laser-Surface Interaction," Laser Focus 21. 12 (1983).

[43] J. E. Sipe, J. F. Young, J. S. Preston, and H. M. van Driel, "Laser-Induced Periodic Surface

Structure: I. Theory," Phy. Rev. B 27_, 1 141-1 154 (1983).

[44] Zhou Guosheng, P. M. Fauchet, and A. E. Siegman, "Growth of Spontaneous Periodic Surface

Structures on Solids During Laser Illumination," Phy. Rev. B 26 , 5366-5381 (1982).

[45] E. W. Van Stryl and, Arthur L. Smi rl , Wi 1 1 i am E. Willi ams, and M. J. Soi 1 eau , "Pul sewidth and

Focal Volume Dependence of Laser-Induced Breakdown," Phys. Rev. Bl , 23^, 2144-2151 (1981 ).

[46] R. Ritchie, "Surface Plasmas in Solids," Surface Sci. 34, 1-19 (1973).

410
(NaClj

Figure 1. Laser Generated Ripple Patterns (LGRP) in NaCl. The above patterns were produced by

normally incident 10.6 um laser lighten the exit surface of NaCl. The laser light was linearly

polarized in the direction shown by E. The higher power photo on the right illustrates that the

ripples are associ a ted wi th defects normal to E whereas scratches para 1 1 el to E have no ripples

associated with them.

Figure 2. LGRP's on the exit surface of a NaCl specimen. The laser wavelength was 10.6 um. The

laser was linearally polarized in the direction indicated by the arrow (E). Each site was

irradiated only once.

411
E 1 Scratches

E II Scratches

0.0 0.5 1.0

DAMAGE FLUENCE (Relative Units)

1.5

Figure 3. Threshold for Formation of LGRP's. The above is a plot (in relative units) of the

fluence required to produce the LGRP for the cases where the linearly polarized electric field is

paral lei to and normal to the surface scratches. Measurements were made on a NaCl sample (exit

surface) using 10.6 urn radiation.

Em

Ei

1.0 2.0 3.0 4.0

Relative PAS Signal

\0/um

(550XJ

71

J

Figure 4. Grating Absorption. The bar graph on the right is the PAS signal (in relative sites) for

normally incident, linearly polarized, 10.6 ym laser radiation. The micrograph on the right shows

the surface of the Al grating.

412
Material 1 .06y 2.7y 3.8y 10. 6y

KC£ X o /n A o /n

NaC£ A Q

/n X Q

/n A Q

/n

KBr A Q

/n A Q

/n

Si0

2 (fused) Vn

BaF

2

A 0/n X o/n

SrF

2 Vn

MgF

2 A Q

/n

Si X o

CdTe X o

ZnSe X

o

A 0

ZnS X

o

CdS X

o

Cd(S

.25

Se

.75 } X o

ZnTe X o

A£ Xo

X

o

Cu X o

Ag X o

Table 1. The above is a listing of the materials, wavelength, and LGRP spacing

observed in this work. Additional materials have been studied by other workers

(see the references listed in the introduction).

413
In response to questions, the speaker stated (1) that he had not yet performed experiments at 532 nm because of a resolution limitation in the available microscope, and (2) that he did not believe the

spacings in ripple patterns seen in damages could be explained by mechanisms which invoke surface

plasmas or surface polaritons . A comment from the audience noted that damage to absorptive

materials occurred at the entrance face, and involved superpositions of fields in air, whereas

damage in transparent or thin samples occurred at the exit face and involved superposition of

scattered and incident fields in the sample, and suggested this might account for some systematic

differences in observed ripple spacings

.

414
Charged Particle Exoemi ssion From Silicon During

Multi-Pulse Laser Induced Damage

M. F. Becker, Y-K. Jhee, M. Bordelon, and R. M. Walser

Electronics Research Center and Electrical Engineering Department

University of Texas, Austin, Texas 78712

The damage induced in silicon by multiple, picosecond Nd.-YAG laser pulses at 1.06ym

has been studied over the past few years. Observations of damage nucleation and growth,

and the evolution of a spatially coherent damage structure led to an electronic model

for the damage mechanism. To further understand the damage nucleation, we studied

electron and ion emission by recording exoemission and laser intensity simultaneously.

We found an incubation period for which we could not detect any laser damage. Initiation of damage was observed coincident with the initiation of exoemission. Furthermore,

the length of the incubation period varied inversely with the laser intensity and

diverged at a threshold intensity. The change in exoemission versus time and laser

intensity was related to the damage morphology, as seen by SEM. For exoemission versus intensity, the emitted positive and negative charges are equal, and emission follows the

fifth power of intensity. Excellent correlations are observed between all the experimental observations.

Key words: charged particle emission; exoemission; incubation; laser induced damage;

multi-pulse laser damage.

1. Introduction and Background

The damage induced in silicon by multiple, picosecond Nd:YAG laser pulses at 1.06ym has been

studied over the past few years. Observations of damage nucleation and growth, and the evolution of

a spatially coherent damage structure led to an electronic model for the damage mechanism [1-2].

We characterize the damage process as a first order, non-equilibrium phase transition in which the

nucleation and resulting multi-pulse damage is heterogeneous. The damage is not the result of

uniform melting and resolidification; as would be observed at higher intensities or shorter wavelengths [3]. The damage has been shown to follow classical nucleation and growth behavior [4-5],

including having an incubation period for which no detectable changes are observed. In phase tran- sition theory, the incubation period in which the nucleation rate is not steady state is characterized strictly by reversible processes and not by microscopic irreversible events. Some evidence of

reversibility in the incubation period has been reported as pulse repetition frequency (PRF) dependent multi-pulse damage [6-7].

In the experiments reported here, we use the exoemission of charged particles into vacuum as a probe of the energetics of the damage process. We have found charge emission to be a very sensitive

indicator of the first appearance of surface damage on silicon, and is thus useful for measuring the

duration of the incubation period. We have also correlated SEM observations with exoemission mea- surements to arrive at a sequence of events during the growth of multi-pulse laser damage.

The use of semiconductor grade silicon gives several advantages in matgrial characterization

and perfection. There is about one impurity atom for a cube of silicon 100A on a side. On the

scale of the laser wavelength the material is homogeneous and well characterized. In addition, the

1.06ym wavelength is just beyond the band gap of silicon and the material is still somewhat transparent with an absorption length of 1mm. In this case, silicon has more in common with other semiconductor optical materials irradiated at or below their band gap. Their electron density can rise

due to single or low order multi-quantum absorption. In silicon at 1.06pm, there is no problem

obtaining a reasonable density of free electrons during the first instant of irradiation. A final

point to consider is that regardless of the mechanism of damage, we have observed similar coherent

multi-picosecond pulse damage morphology on widely dissimilar materials; such as amorphous silicon

and single crystal aluminum.

2. Experimental Configuration

A block diagram of the experimental set-up is shown in Figure 1. A passively mode-locked

Nd:YAG laser with a single pulse selecting switch produced 60-80ps, 250yJ pulses at PRFs up to

415
20Hz. The beam was directed to an ion pumped vacuum chamber (pressure < 10~ 5 torr) containing the

silicon sample. The sample was placed at or somewhat in front of the focus of a lens which could be

translated transversely to illuminate new regions of the target. The fluence was varied by using a multi-plate reflective attenuator. Using a micro-processor controlled data acquisition system, the

energy and quantity of charge emitted could be collected for every laser shot. The size of the

capacitor, C, in the charge collecting circuit was changed to vary the charge detection sensitivity.

At its highest sensitivity, the detection limit was lCH3c. The laser spot size at the sample was about lOOum and the PRF was kept at 13Hz.

The silicon samples were p-type <100> silicon wafers of high resistivity (35ohm-cm). They were prepared using standard semiconductor processing cleaning techniques before being placed in the vacuum chamber. The samples were examined optically and with a scanning electron microscope both

before and after damage. The surfaces were always perfectly smooth and featureless before damage;

and after damage, structures as smaller than .Turn could be easily seen.

3. Experimental Results

3.1 Incubation Period

It was first established that the initial exoemission event coincided with the first observable

damage structures as seen by SEM. Numerous negative experiments were performed in which many near multi-pulse threshold laser pulses were incident on a sample location without either exoemission or observable damage. When damage occurred, it was always accompanied by exoemission and vice versa. Generally, when a damage event occurred, the amount of emitted charge correlated with the area of

the damage pits. The smallest observed exoemission event was an order of magnitude larger than the

instrumental detection sensitivity.

Using the first observed exoemission as an indicator of the end of the incubation period, we measured the number of pulses to the first emission event versus the laser fluence. The data is

plotted in figure 2. The inverse of the number of pulses, N, is used in order to clearly show the

linear dependence on 1/N and to show the intercept point where 1/N = 0 and N diverges. Sites which

had pulses that were well above the averaae energy were discarded. Note that for N < 4 there are quantization steps in figure 2. No damage was observed below the intercept fluence, and no damage

was observed for anomolously low numbers of pulses. This clearly indicates that the probability of

damage is an increasing function of pulse number; and for these samples, damage does not proceed as the growth of "micro-damage."

3.2 Growth of Damage and Exoemission

After damage was initiated, the exoemission versus fluence was measured. The exoemission

process after first damage is very nonlinear and noisy. The data in figure 3 shows averaqe exoemission for the first 50 exoemission events at a given average fluence. As with all the observations,

the emitted positive and negative charqe was equal and gave equivalent information. The straight

line drawn through the data in figure 3 represents a fifth power dependence. The good fit indicates

that charge emission occurs through a five quantum process. Just below the lowest fluence in figure 3, was the infinite number of pulse threshold, and just above the highest fluence is the

single pulse damage threshold.

The development of exoemission as a function of the number of laser pulses at a 13Hz PRF is shown in figure 4 for a relatively high fluence (upper curve) and for a low fluence (lower curve).

Note that the vertical scale on the lower curve has been magnified 10 times. Both cases reach a stationary situation after a long enough time. These curves are correlated with SEM observations at the points on the curves marked with numbers. A sampling of these observations are shown in figures 5 and 6. Figure 5a shows an example of one of the smallest damage events observed after one exoemission event at a low fluence. A characteristic of lower fluence is that the pits are formed

into rows of more than two pits each. The round spheres are ejected material and were not present

before damage. Figure 5b shows the asymptotic form, and it is similar for all fluences where

multi-pulse damage occurred. There is also evidence of material removal, transport, and redeposition in this region. As the density of pits decreases in the center of the spot, the exoemission

also decreases. Figure 6a shows the result of one exoemission event at higher fluence. There are numerous pit pairs over the beam area. Figure 6b shows the higher fluence damage spot after 4 exoemission events. Already the coherent damage structure has filled in the beam spot. It is formed by the linking of the pits in chains and the subsequent generation of adjacent chains [1-2].

Note that in figure 6b there were some zero emission pulses after the damage began to initiate, and

although damage initiated on the third pulse and there were four exoemission events, there were a total of 20 laser pulses incident on the site.

416
4. Conclusions

We observe that exoemission is coincident with the first observable damage morphology, which

follows an incubation period. The length of the incubation period is inversely proportional to the

laser fluence. These observations along with the findings in a related experiment [8] on the PRF dependence of the incubation period, strongly suggest that incubation represents a period of

increasing damage probability. The accumulated quantity is not micro-damage or temperature in this

case [8]. It appears that the excitation is reversable and not a morphological change.

When damage does nucleate, the accompanying exoemission appears as a five quantum process. It

is tempting to correlate this observation with the photoelectric threshold of silicon which is between four and five photons in energy for both direct and indirect processes. This might indicate

that the excitations in the exoemission process are from the valence band and not an excited state. The problem with this interpretation is that the positive charge emission is equal to the negative

emission and can not be directly related to photo-electron emission.

The growth of multi-pulse damage and the exoemission rate correlate and show stationary behavior. The emission process correlates most closely with the pit structure on the damage surface.

SEM reveals that material removed from the pits is ejected and some of it returns to the surface as small spheres. In addition, there seems to be a vapor phase growth of areas immediately adjacent to the pits indicating a complex vapor-liquid-solid material transport process.

This research was supported by the DoD Joint Services Electronics Proqram through AFOSR

Contract F49620-77-C-0101

.

5. References

[1] R. M. Walser, M. F. Becker, and D. Y. Sheng, "Laser Damage of Crystalline Silicon by Multiple

1.06um Picosecond Pulses," in Laser I nduced Damage in Optical Materials , November 1981, Nat.

Bur. Stand. Spec. Publ . 621 (not yet in print)

.

[2] M. F. Becker, R. M. Walser, Y. K. Jhee, and D. Y. Sheng, "Picosecond Laser Damage Mechanisms

at Semiconductor Surfaces," in Picosecond Lasers and Applications , January 1982, Los Angeles,

CA, SPIE Vol. 322; pp. 93-98.

[3] J. M. Liu, R. Yen, H. Kurz, and N. Bloembergen, "Phase Transformation on and Charged Particle

Emission from a Silicon Crystal Surface, Induced by Picosecond Laser Pulses," Apol . Phys. Lett. 39(9); 755-757, November 1, 1981.

[4] D. Y. Sheng, R. M. Walser, M. F. Becker, and J. G. Ambrose, "Heterogeneous Nucleation of

Damage in Crystalling Silicon with Picosecond 1.06ym Laser Pulses," Appl. Phys. Lett. 39(1);

99-101 , July 1 , 1981

.

[5] R. M. Walser, M. F. Becker, D. Y. Sheng, and J. G. Ambrose, "Heterogeneous Nucleation of

Spatially Coherent Damage Structures in Crystalline Silicon with Picosecond 1.06um and 0.533ym

Laser Pulses," in Laser and Electron-Beam Solid Interactions and Materials Processing , T. J. Gibbons, W. Hess, and T. Sigmon eds., New York, Elsevier, 1981, pp. 177-184.

[5] S.-T. Wu and M. Bass, "Laser Induced Irreversible Absorption Changes in Alkali Halides at 10.6pm," Appl. Phys. Lett. 39(12) ; 948-950, December 15, 1981.

[7] S. K. Balitskas and E.K. Maldutis, "Bulk Damage to Optical Glasses by Repeated Laser Irradiation," Sov. J. Quantum Electron. Vl_(4) ; 541-543, April 1981.

[8] M. Bordelon, R. M. Walser, M. F. Becker, and Y-K Jhee, "Observations of Long-lived Metastable

Excitations in Multi-pulse Laser Damage of Silicon," in these proceedings.

417
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419
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425
In response to a comment that damage craters discussed here resembled the craters caused by

unipolar arcing which was always associated with the presence of visible plasma, the speaker stated

that visible plasmas were not observed during production of the least catastrophic of their damage

sites The speaker further commented that they had been unable to ascertain a relationship between

the work function of silicon and the coefficient in the power law dependence of ion emission or intensity.

426
A Study of the PRF Dependence of the Accumulation Effect

in Multiple Pulse Laser Damage of Silicon

M. Bordelon, R. M. Walser, M. F. Becker and Y-K. Jhee

Electronics Research Center

Electrical Engineering Department

The University of Texas at Austin

Austin, Texas 78712

We report on the effect of varying the laser pulse repetition frequency PRF (from

0.03 Hz to 15 Hz) on the accumulation effect in the multiple pulse damage of silicon

by picosecond 1.06 ym pulses. When the laser intensity fluctuations are controlled on

a pulse-to-pulse basis, no PRF dependence is observed. This implies that the precursors to damage are either characterized by very long reversible decays or are the

result of irreversible processes. Our calculations show that thermal accumulation is negligible in these experiments for even the largest PRF.

Key words: picosecond, damage, silicon, nucleation, nonequil ibrium phase change, multiple pulse, pulse repetition frequency.

1 . Introduction

The objective of the experiments reported here was to study the thermodynamic reversibility of

the subtle precursors to catastrophic damage of crystalline silicon irradiated by multiple, picosecond pul ses of 1.06 ym radiation. The existence of difficult to detect precursors in multiple

pulse laser damage is evidenced by an accumulation effect [1-5], the salient feature of which is an inverse relationship between the laser intensity and the number of pulses required to produce observable damage. Conventionally, a shift in this relationship with a change in the laser PRF is interpreted as evidence of reversibility on a time scale ^(PRF)"^.

Previous studies have, for example, revealed a PRF dependent accumulation effect in optical

glasses for 7 ns pulses of both 1.06 ym and 0.533 ym radiation [4] and reversible, predamage, absorption changes in alkali halides [5] induced by 100 ns pulses of 10.6 ym radiation. Fauchet [7], however, in experiments nearly identical to ours, failed to observe a PRF dependence in the damage of

crystalline silicon. Fauchet's data was, however, primarily taken for laser intensities for which

N » 100 pulses were required to observe damage. It is also not clear that the laser intensity

fluctuations, which are critical in this region, were monitored on a pulse-to-pulse basis. Furthermore, the methods used to detect damage in these experiments increase the possible ambiguity of the

resul ts

.

The time scale in which reversible precursors might occur in multiple pulse laser damage is an issue of crucial importance in understanding the accumulation effect. While irreversible precursors

to multiple pulse laser damage will certainly occur in most cases, it is not clear that observable

reversible processes may also occur, perhaps coincidentally. The latter is expected, for example,

in a new perspective of this laser damage process as a nonequil ibrium phase transition [6,13]. In this picture, the accumulation effect is associated with an incubation period, or induction time,

characterized by the nonsteady growth of critically-sized embryos. Thus, nucleation and growth

will occur simultaneously, and it is not clear that the prenucl eation reversible fluctuations can be

independently observed.

To answer this question and to attempt to verify the results of Fauchet [7], we conducted the

experiments described in Section 2. If the laser-silicon energy transfer is reversible on time

427
scales on the order of yg sec - 10 sec during the incubation period, it may be possible to observe a change in either the microscopic extent of damage, or in the damage threshold, by varying the PRF of

a fixed number of laser pulses.

To reduce the ambiguity of these experiments, it is crucial to control or select the laser intensity fluctuation statistics and monitor these on a pulse-to-pulse basis. It is also important to use a high resolution technique to estimate the change in damage produced by varying only the PRF.

In our experiments, this was done by inspecting the irradiated beam spot with high resolution electron microscopy.

2. Experimental

The laser used in these experiments was a passively mode-locked Nd:YAG operating at 1.06 ym

with FWHM pulses of 60 to 80 picoseconds. The spatial and temporal distribution of intensity were

both Gaussian and typical intensities used were between 2.0 and 4.0 GW/cm (0.16 to 0.32 J/cm )

.

The samples used were intrinsic <111> silicon wafers 15 mils thick. The samples were ul trasonically

cleaned and boiled in a series of baths containing high purity trichloroethyl ene, acetone, and

methanol. No defects or foreign particles were observed by optical microscopy with magnifications

up to 1600X.

2 Single pulse damage experiments were conducted first, and a damage threshold of 6.0 ± 0.5 GW/cm

-4 2 was determined for an irradiated area of 3.21 x 10 cm . As noted by others [8], we have observed

that the single pulse damage thresholds are inversely proportional to the area irradiated. Next,

the laser beam was attenuated so that the average intensity was between 3.0 and 4.5 GW/cm . A mechanical shutter in the beam line was used to select 20 pulses for each damage spot.

Damage produced by PRFs of 15 Hz, 10 Hz, 1 Hz, 0.1 Hz and 0.03 Hz was studied. The PRFs of 15 Hz and 10 Hz were produced by varying the frequency of the flash lamps. The lower frequencies were

produced by switching out individual pulses with the shutter while the laser operated at 15 Hz. The

energy of each pulse was recorded by a calibrated PIN photodiode interfaced with a microprocessor

which calculated the average intensity and the standard deviation of the pulse train. The extent of

damage and the damage threshold were determined by high resolution optical and electron microscopy.

When comparing the extent of damage for the different PRFs, the data was intentionally statistically biased by selecting damage spots such that the lower PRFs had a higher average intensity

than the higher PRFs. This was done to ensure that the effect of positive laser intensity fluctuations, which are assumed to increase metastable lifetimes (if such states exist), would have a greater effect on the experiments with a lower PRF. Individual damage runs were rejected from the

data when the standard deviation, a, of the pulse train was greater than 20% of the average intensity, and whenever the absolute difference between the average intensity and any individual pulse

intensity was greater than 2a.

3. Experimental Results

When comparing the extent of the damage for any two PRFs, the result of the statistical bias was that slightly more damage occurred at the lower PRFs. Thus, for the same average intensity and

pulse number, there was no difference in the extent of damage. The PRF independence of damage was further supported by the deterimination of the damage threshold, which was in the range of 3.3 to 3.6 GW/cm 2 for all PRFs. The damage morphology observed was the same for all PRFs. As first reported by Sheng [3], the

damage process induced by picosecond pulses of 1.06 ym radiation is a heterogeneous nucleation involving a first-order phase change to the liquid-like state. For each PRF, the first sign of

428
damage was characterized by discrete damage sites that were shallow and elongated in shape with one

or two small nipples within each depression. The depressions and resulting pits always formed with

their long axis perpendicular to the electric field of the linearly polarized laser radiation. Both

of these structures are evident in the SEM picture of figure 1 of damage initiated by 23 pulses at

2 an average intensity of 3.74 GW/cm . Growth of the damage then occurs in a direction paralle 1 to the electric field resulting from the interference between the wave scattered from the pits and the

incident wave. This results in the formation of pit chains separated by the wavelength of the laser.

This "ripple" structure is illustrated in Fig. 2.

4. Discussion

In the experiment presented here, the damage of silicon was not PRF dependent, which suggests

that the mechanism of accumulation is not reversible on time scales less than 30 seconds. However,

it can not be ruled out that over long periods, the mechanism is reversible. Reports of the optical properties of some amorphous semiconductors has shown reversibility on the order of hours and

days [11].

To determine the effects of heating, calculations were made of the initial temperature profile

(t=0) and for several t > 0. In previous work [6], it was shown that for intensities up to single

pulse damage threshold, both one and two photon absorption occur in silicon with corresponding

absorption coefficients of a = 10 cm~^ and 6 = 52 cm/GW. If uniform illumination is assumed, the

intensity relative to the surface is given by the differential equation

^-=-(cd+3I 2

) (1)

Rearranging and integrating explicitly gives

T

/ x _ a(l-R)I

9 UX; " exp(-gx) m

a+e(l-R)I 0 [l-exp(-ax)] Uj

where I is the incident intensity and R is the reflectivity. Since silicon is not highly absorb0

-1 ing at 1.06 ym, the thermal diffusion length, ^Ktp, during the time of the pulse is « [ot+BI(x)]

This allows the initial temperature distribution to be approximated by

where R is 0.3, the pulse length, t , is 80 psec (worst case), and the heat capacity, Co is 0.56

2 Joules/cm °K [9]. The above gives a maximum surface temperature change, AT, of 62.2°C for an inci2 dent pulse of 4.0 GW/cm . This suggests that if only homogenous absorption were occurring, the

sample would never damage unless thermal accumulation is significant. To check that possibility,

the temperature decay was calculated assuming a one-dimensional semi-infinite sink with no radiative

losses of heat at the surface [10].

A numerical solution of eq. (3) was obtained by a computer. Since the thermal diffusivity, k,

is a function of temperature (decreasing with increasing temperature), a worst case approximation

was obtained by assuming that k remains constant at its 100°C value, which is 0.536 cm /sec. The

results of these calculations are shown in figure 3. These results show that thermal accumulation

cannot be responsible for damage at any of the PRFs used, which is further supported by the experimental results. In the above calculations, free carrier absorption was assumed to be negligible

18 19-3

since a charge carrier density of 10 to 10 cm" is produced for the intensities used [12].

429
Proposed mechanisms on the origin of damage of silicon surfaces by picosecond 1.06 ym radiation

must explain (1) the inhomogenous absorption required to melt localized regions, (2) the symmetry

breaking involved in the observation that damage is initially oriented perpendicular to the electric

field of the laser, and (3) the mechanism of accumulation of microscopic change that allows many

pulses with sub-single pulse threshold intensity to damage. A theory involving the resonant absorption of radiation by surface "particle" plasmas [6,13] gives an explanation for the inhomogeneity of

damage and has predicted the orientation of the pit chains and the distance separating individual

pits in each chain.

What has not been answered in this experiment is the mechanism of accumulation. An accumulated

change is required to allow the localized region to have different absorption characteristics with

respect to the bulk silicon. Experiments which would be beneficial to such a study would look for

structural or electronic changes occurring during the incubation period before the onset of damage.

5. Conclusion

Multiple phase damage of silicon by picosecond 1.06 ym radiation has been shown to be independent of the pulse repetition frequency (PRF). Damage as a result of thermal runaway was incompatible with our calculated heating and cooling transients and with the observation of heterogeneous

damage. For similar reasons, the presence of defects and impurities are not believed to be responsible for the accumulation of damage since localized heating would have a higher rate of cooling and

could not begin to explain the consistently observed symmetry of the damage. The possibility of an avalanche ionization mechanism is neglected because of the low electron density assumed, and since

the product of the excitation frequency and the el ectron-phonon collision time, gjt, is greater than

100. Although the mechanism of accumulation could not be determined, this experiment clearly supports the perspective of the observed laser damage as a nonequil ibrium phase transition involving

precursor formation of long-lived excitation.

This research was supported by the DoD Joint Services Electronic Program through AFOSR Contract

No. F49620-77-C-101

.

References

[1] M. F. Becker, Y-K. Jhee, M. Bordelon and R. M. Walser, "Charged Particle Exoemission from Silicon During Multi -pulse Laser Induced Damage," these proceedings.

[2] R. M. Walser, M. F. Becker, D. Y. Sheng and J. G. .Ambrose, "Heterogeneous Nucleation of Spatial'

ly Coherent Damage Structures in Crystalline Silicon with Picosecond 1.06 ym and 0.533 ym Laser Pulses," Laser and Electron Beam Solid Interactions and Material Processing , J. F. Gibbons, W. Hess and T. Sigmon, eds., Elsevier, New York, 1981, pp. 171-184.

[3] D. Y. Sheng, R. M. Walser, M. F. Becker and J. G. Ambrose, "Heterogeneous Nucleation of Damage

in Crystalline Silicon with Picosecond 1.06 ym Laser Pulses," Appl . Phys. Lett. , 39 (1), 19,

1981

.

[4] S. T. Wu and M. Bass, "Laser Induced Irreversible Absorption Changes in Alkali Halides at 10.6 ym Laser Pulses," Appl. Phys. Lett. , 39 (12), 948, 1981.

[5] S. K. Balitskas and E. K. Maldutis, "Bulk Damage to Optical Glasses by Repeated Laser Irradiation," Sov. J. Quantum Electronics , 11 (4), 541, 1981.

[6] M. F. Becker, R. M. Walser, Y-K. Jhee and D. Y. Sheng, "Picosecond Laser Damage Mechanism at Semiconductor Surfaces," in Picosecond Lasers and Ap plication, Jan. 1982, Los Angeles, CA,

SPIE, Vol. 322, pp. 93-98.

430
[7] D. M. Fauchet, Phys . Lett. 93A , 155 (1983).

[8] M. J. Soileau, M. Bass, P. H. Klein, "Frequency and Focal Volume Dependence of Laser-Induced

Breakdown in Wide Bandgap Insulators," in Laser Induced Damage in Optical Materials: 1979 , NBS Special Publication 568, p. 497 (1980).

[9] This value is reported by N. Bloembergen, Fundamen tals of Laser Solid Interactions and Laser Processing , -1 978 , S. D. Ferris, et al., ecL AIP Conference Proceedings #50 (1979), p. 1. Other papers in the same proceedings use Cpp = 1.6 J/cm3 °K. See M. von Allmen, et al .

,

"Annealing of Silicon with 1.06 ym Laser Pulse," p. 43. The former should give a worst case approximation.

[10] H. S. Carslaw and J. C. Jaeger, Conduction of Heat in Solids , 2nd edition, p. 276, Oxford at

the Clarendon Press, 1957.

[11] M. I. Korsunskii, Anomalous Photoconductivity (New York: Wiley, 1973).

[12] D. Y. Sheng, Ph.D. Dissertation, The University of Texas, 1981.

[13] R. M. Walser, M. F. Becker and D. Y. Sheng, "Laser Damage of Crystalline Silicon by Multiple

1.06 ym Picosecond Pulses," in Laser Induced Damage in Optical Materials: 1981 , NBS publication of 1981 Boulder Damage Symposium.

431
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In response to questions , the speaker said he was unable to specify whether the proposed long-lived

excitation was an atomic state or a macroscopic effect. He further emphasized that the density of

excited states ( <1019 cm~2 ) and material temperatures (<100°C) reached in these experiments

were much lower than those observed in annealing experiments which might produce lo^l to 1022

excited states/cm^ and temperatures greater than one half the melting temperature .

435
On the Nature of Accumulation Effect in the LaserInduced Damage to Optical Materials

A.A.Manenkov, G.A.Matyushin, V.S.Nechitailo,

A.M.Prokhorov and A.S.Tsaprilov

Lebedev Physical Institute of the USSR Academy of Sciences

Moscow 117924, USSR

Features of multiple-shot laser-induced damage to optical materials

are analyzed. The methodology of studying the accumulation effect responsible for catastrophic damage, based on the statistical approach, is suggested and the basic characteristics of multiple-shot damage are introduced. Possible mechanisms of microdamage in materials under multiple laser irradiation are discussed. The experimental data on the laser-induced

damage to optical polymer materials are presented, which demonstrate some basic properties of the accumulation effect. The ways of suppressing

the accumulation effect in transparent polymers are proposed and realized

which result in a considerable increase of laser damage resistance.

Key wordsj accumulation effect in laser-induced damage; laser-induced

damage; laser-induced me chanochemical reactions; microdamage; macrodamage; optical polymers; single-shot and multiple-shot laser induced damage;

statistics of damage.

1 Introduction

The resistance of optical materials to a high-power laser radiation in the

multiple irradiation regime is known to be in many cases essentially lower than

that at a single-shot. This fact has been established rather long ago 1,2, but

systematic studying of multiple-shot damage has not been so far undertaken and the

main attention in the problem of laser-induced damage was concentrated on the investigation of damage characteristics in the single-shot regime. However, taking

into account the practical importance of the question of laser damage resistance

of optical materials in the multiple-shot regime, we consider investigation of the

causes responsible for the effect of multiple laser irradiation to be exclusively

important

.

In principle, the multiple irradiation effect may be connected with two essentially different processes; 1) the appearance of visible macrodamage at the N-th

shot without any irreversible changes in all the previous shots (this process is

associated with a random appearance of the so called "happy electron", resulting

in an avalanche break-down 2 ; 2) accumulation of microdamages from shot to shot

up to the formation of macrodamages at the N-th shot 3-7 . Note that the realization of the happy electron model in laser damage has not so far been experimentally confirmed. As to the model of gradual growth of microdamages up to macrosizes,

the possibility of its realization has been demonstrated, probably for the first

time, in 4, 5 , when investigating nonlinear light scattering by absorbing defects

initiating laser-induced damage.

The dynamics of accumulation of irreversible changes in material under multiple laser irradiation can be studied using different techniques, i.e. by observation of the incident radiation scattering and the visible and near UV glow kinetics accompanying the damage process 4-17 , by the methods of electron spectroscopy 18 , IR-spectroscopy 19 , supersonic spectroscopy 20 and pulsed laser ca- lorimetry 21 . As the experiments show, among these methods those of light scattering and visible and UV glow detection are the most simple and informative for

studying the accumulation effect.

The accumulation effect has been so far revealed and investigated in the silicate glasses 4-7, 18-20 , alkali-halide crystals 11, 21 , ferroelectric crystals 17 , and polymers 8, 10, 12-16 . It was established that in the crystals

and glasses the accumulation effect is revealed at the laser radiation intensities

only several times below the single-shot damage threshold, whereas in the polymers

it is observed in a much greater (up to 100 times) intensity range.

436
To account for the accumulation effect, different mechanisms have been suggested. For example, in silicate glasses it was interpreted as "the radiation-stimulated accumulation process of thermof luctuation decomposition of the intermolecular

bonds" l"22, 23.1 > or ^

was associated with the accumulation of gases formed as a result of thermode struction [ 24 j . In the transparent polymer materials the accumulation effect was accounted for by a multi-quantum photolysis of macromolecule

s

L25, 261 , by the formation of absorbing products (like soot), due to high-temperature thermode struction of the polymer {727-29J , or by the mechano-chemical reactions initiated hy thermoelastic stresses around the absorbing defects in a samole

fH, 30, 31].

The analysis of the pointed works shows that despite a series of interesting

experimental observations and suggested explanations, the accumulation effect hasnV.

yet been enough studied. Purposeful experimental and theoretical investigations

are needed to elucidate its nature. In this connection we have made in the present

paper an analysis of the basic regularities of the accumulation effect, their con- nection with the single-shot laser-induced damage characteristics and possible mechanisms of this effect. We present also some typical experimental results on the ac- cumulation effect in organic glasses, where it is more pronounced as compared to

the other optical materials.

2. The Basic Gharacteristics of the Accumulation Effect

The accumulation effect in the laser-induced damage is usually investigated as follows. The single-shot damage threshold for the surface or bulk of a sample is

determined_, the incident radiation intensity reduces to a definite level below the

damage threshold, and the critical number of laser pulses N resulting in visible

macro damage is determined. The experiments show L10J, that a considerable variation of the N values is observed (more than by a factor of 10) from site to site

in the sample. This fact indicates a statistical nature of the accumulation effect

and to describe it, it is apparently needed to introduce the main conceptions and

characteristics similar to those used at statistical analysis of the single-shot

damage 0 We suppose that such adequate characteristics are: an averaged over a great

number of irradiated sites critical number of laser pulses 1\T (I), at which catastrophic macrodamages jDecur in the sample at fixed intensity livel I; multiple-shot

damage threshold I«. L16J defined as the intensity level at which a visible failure

in the sample occurs at a fixed number of pulses H with a pregiven probability

Wjt(I), the latter being determined by the ratio of the number of damaged sites in

tne sample to the total number of the irradiated sites at a fixed intensity I and

number of laser pulses N; multiple-shot damage threshold distribution function

f(Ijr) defined by the ratio of concentration An of defects in the sample having

the N-shot damage thresholds in the interval (Ijt» Ijt+^Ijt) to the value of interval Z\L, The function f(Iw ) makes it possible to calculate W,

T (I) by the formula:

W Cl)= l-e«p[-Jc,A J/ CI»yctt«] CD

/V A ° where A is the irradiated volume of surface of the sample.

In general, function f(Ivr) depends on the incident radiation parameters (wavelength, pulse duration, and so on) and on the sample characteristics (absorption

coefficient and size of defects, thermoelastic properties of the material etc).

Due to the statistical character of the damage described by the distribution function f(I„), a spot-size dependence of thresholds I„ should be observed, which is

confirmed by the experimental results T15, 16J. It should be expected that at the

multiple-shot damage, like at the single-shot damage, an unambiguous correspondence will be observed between the spot-size dependence of I„ and that of W„ upon I.

More over, because of the same nature of the processes leading to damage at single

and multiple irradiation, one can suggest that a difinite connection exists between the damage characteristics in these two cases. This connection can be expressed by the evolution of the single-shot damage threshold distribution function

f (I. j usually introduced at statistical analysisof laser-induced damage in the

single-shot irradiation regime (see review paper [33j and references threin ), in

the process of successive laser shots:

437
or

ta^Aicr,), . . .J^j^JV ,

(2)

J. t 1,) - J J ( I,) where

j"^' ATT_g —XA. is the evolution operator, whose form is determined by a concrete mechanism of the accumulation effect in different materials. Operator A, generally being nonlinear, depends on the incident radiation parameters (wavelength,

width and shape of the pulse and so on) and can be found solving an inverse problem, i.e. from the experimentally obtained dependences of the multiple-shot damage

characteristics on the radiation parameters and sample characteristics.

Evolution of the distribution function may, in principle, lead to both the

decreasing of the material laser damage resistance in the multiple-shot irradiation regime, compared to the single-shot damage resistance, and to its increasing

("laser polishing and conditioning). In the experiments an effect of damage threshold decreasing is as a rule observed at multiple irradiation (I,-

r <I 1 ), though

there is some opposite evidence (1^ > I) available in literature U4» 35j

.

3. Possible Mechanisms of the Accumulation Effect

The accumulation effect in laser-induced damage physically implies accumulation from shot to shot of irreversible changes in the interaction region of the

sample up to the formation of visible macrodamage. Irreversible changes may occur

due to photochemical, thermochemical and mechanochemical reactions and different

phase transitions. The first process may be initiated both by direct laser radiation (due to multiphoton absorption), and by radiation of highly heated absorbing

defects. Thermo- and mechanochemical reactions are most probably initiated in the

vicinity of the absorbing defects because of the matrix heating and local thermoelastic stresses occuring in this case. Responsible for the accumulation effect in

laser induced damage at multiple irradiation may naturally be only the reaction

products, which absorb the incident radiation due to linear or nonlinear in intensity process.

The pointed mechanisms of irreversible changes at multiple laser irradiation

were used in literature to account for the experimental results on the accumulation effect. Thus, in ["25, 26J a model was discussed of gaseous product accumulation

in microcavities (about 10~4cm in size) as a result of multiquantum photochemical

reactions, leading to destruction of polymer molecules. The formation of macrodamages in this model was considered as a result of a wedging effect of the gases.

This model is not however in agreement with the available experimental data

["15 » 1 6J on laser-induced damage to polymers, indicating to the absence of any

gaseous products in noticeable amount in macrodamages.

A similar "gas wedging model" of the accumulation effect was discussed to

analyse the experimental data on laser-induced damage in silicate glasses . According to this analysis the accumulation effect must not be observed at the

laser pulse repetition rates < 10* Hz, which however contradict to many experimental observations f4-7, 18, 19, 36].

To account for the accumulation effect in silicate glasses a mechanism was

also suggested in [22, 23j of thermofluctuational decomposition of intermolecular

bonds, stimulated by: laser radiation. But it remains unclear in this model how

the electric field of the electromagnetic wave directly affects the intermolecular bonds leading to the decreasing of bond breaking activation energy.

In |72 7—29.7 a thermochemical mechanism was discussed of accumulation in polymer materials of strongly absorbing products (like soot) formed around the initial

absorbing inclusions. Since the rate constants of thermochemical reactions exponentially depend on temperature T :K^

c

jj?*exp(-U o /kT) (where U

Q

is the activation energy

usually being 2t-3 eV), and become noticeable only at very high temperatures, such

an accumulation mechanism may be observed only in a very narrow interval of incident radiation intensities L28}. But in many materials (including silicate glas438
ses £4-7, 18-20, 36j, alkali-halide crystals [l 1 , 2lJ , ferroelectric crystals

[17, 37] , transparent polymers f8, 10, 12-16, 25-27, 29j ) the microdamages occur

at the incident radiation intensity several times (in the case of polymers up to

100 times) below the single-shot damage threshold, which cannot be explained on the

basis of a "pure" thermochemical mechanism.

We assume that the most probable mechanism of accumulation of irreversible

changes in materials at a multiple laser irradiation is associated with mechanochemical reactions ("14, 30, 31] , whose rate constants K^^exp(r(Uo-0 (^/^essentially

increase compared to K^uat the expense of activation energy decreasing by elastic stresses § ( "ft" is the structure-sensitive factor). The latter occur in the

vicinity of the absorbing defects due to their heating by laser radiation. In the

process of mechanochemical reactions the excited active particles (radicals, ionradicals, surface and solvated electrons, solitons, etc.) may appear, which absorb

the laser radiation and lead to the chain reaction development and microdamage formation. As a result of such irreversible changes in the material, nonlinear absorption increases in successive laser shots, leading to catastrophic damage. Analysis

of the mechanochemical mechanism of the accumulation effect [J4, 30, 31J shows that

it must be considerably different for optical materials with different thermoelastic properties (breaking strength, Young's modulus, expansion coefficient, etc)

and rate constants of mechanochemical reactions. In particular, one may expect an essential difference of the intensity range A I below the single-shot damage threshold, in which the accumulation effect may be observed. Indeed, the experiments

confirm this expectation: to inorganic glasses this effect occur in a narrow range

of A I C4-7, 18-20, 36]], whereas in organic glasses (polymers) it is observed in

a wide intensity range ["8, 10, 12-16, 25-27, 23J. Such a peculiarity of the polymer materials, compared to the other transparent

dielectrics allows to study on this class of materials the main characteristics of

the accumulation effect. In the next section we shall present some experimental data on the accumulation effect in the laser-induced damage of polymethylmethacrylate - a typical representative of organic glasses.

4. The Accumulation Effect in Transparent Polymers

The accumulation effect in laser-induced damage to polymer materials has been

revealed and studied in many works (j , 2, 8, 10, 12-16, 25, 27-31J. Here we present some new results obtained for polymethylmethacrylate (PIMA) and modified polymethacrylate (MPMMA), widely used in conventional and laser dptics, in particular

for creation of solid-state dye lasers T381, bleachable filters [33], and optical

elements for laser ophtalmology devices £"40J.

In the investigated specially purified samples (~12j, a pronounced accumulation effect has been observed in a wide intensity range: the 1-shot to N-shot damage

thresholds ratio I-i/ITM- is equal to 3 and 30 for MPMMA and PMMA, respectively (see

fig. 1). The intensity dependence of N has an asymptotic character: IT ~^ ©° , at the intensity aporoaching some minimum value I . .'which is as much as 0.03I-,

for EMMA and 0.3I-, for MPMMA .

'

We have studied the dynamics of the accumulation process by observing the incident laser radiation scattering and the glow in visible and near UV spectrum

range, accompanying the damage. Figo 2 shows the dependence of the light scattering intensity at >» =0„69/w m on the number of irradiation pulses I of a fixed

site ii* the MEMMA sample at I = 0.51*. It is clearly seen that the scattering intensity monotonously increases from shot to shot up to IT = 20, at which a sharp

increasing of the scattering intensity is observed and a catastrophic damage occurs. A similar behaviour was observed for the dependence of glow intensity in the

visible and UV spectrum range on the number of laser pulses ( > = 1»06/im, Z -

= 20 ns) with intensity I = 0o5I* irradiating a fixed site in the bulk of the EMMA

sample (see fig. 3 ). It was observed in these experiments that the amplitude and

the temporal shape of the glow pulses undergo essential fluctuations. This indicates a random character of the laser-induced damage process.

We have also investigated statistical properties of the laser-induced damage

to polymers by studying the spot-size dependences of the damagethresholds I* and

Iw

at a fixed number of shots (IT = 200) and probabilities W.. and W^ (both equal to

0.5) for 1-shot and IT- shot damage as well as the dependences VL and W^ on the incident radiation intensity. Pig. 4 shows these dependences for the EMMA sample irra439
diated by YAG.Nd laser pulses at A = l»06Am» It is seen that the 1-shot damage

threshold I

1 has a stronger dependence on the focusing lens focal spot diameter d, , compared to that for N-shot damage threshold Iw

» Such a behaviour of

spot-size dependences of I

T and 1^ correlates with the intensity dependences of damage probabilities and W,, »

* * Indeed, a more gently sloping dependence

MAI) corresponds to a steeper dependence I-CcLJ and, vice versa, a weak spot-size

dependence I^Cd,) corresponds to a steeper dependence Wj,(I). These qualitative re- lationships were confirmed by a detailed analysis of the obtained experimental data on the basis of a statistical model of laser-induced damage, described by formula

(I). In particular, it follows from this analysis that at successive laser irradiations the initial distribution function f (I..) of defects is transformed into the

functions fjr -(I-) having a more narrow distribution compared to f (I-) and shifted

to smaller Values of damage threshold I-, Determination of a concrete form of these

functions on the basis of experimental data will make it possible to clear up the

form of the evolution operator A, which, as it follows from section 2, is the basic

characteristic of the accumulation effect.

Besides the statistical properties described above of the accumulation effect,

we have also studied the dependence of the U-shot damage thresholds l„ on the laser

radiation parameters (pulse width f and wavelength A ) and physico-mechanical

characteristics of the samples (temperature, visco-elastic parameters of the material, content of low-molecular-weight additives).

These studies have shown that, similar to the case of single-shot damage [ A^J , the following relationship takes place in the multiple-shot irradiation regime:

I N= BA/'C for Z- = 3* 5cTns and A = 1 .06* 0.33Jnm with B = const.

The temperature rise of the EMMA and MFMMA samples up to the glass-transition

temperature and addition of plasticizers into polymers leads to a considerable (more than by a factor of 10) increase of their laser damage resistance in the multiple-irradiation regime, whereas both these factors relatively weakly affect the

single-shot damage threshold [10, 14, 16, 42]. It was observed that the effect of

the laser damage resistance increasing in multiple-shot irradiation regime essentially depends on the type of plasticizer El6.] and its mole concentration. These re- sults indicate that properly chosen additives in the polymer are able to suppress

the mechnochemical reactions due to quenching of the excited active particles ari-.

sing in the vicinity of the absorbing defects from thermoelastic stresses C"14,30,31]

A similar suppression effect in the laser-induced microdamage accumulation can probably be attained in other optical materials by means of their structural modification resulting in the change of thermoelastic properties and decreasing of the rate

constants of mechanochemical reactions.

5. Conclusion

Analysis of the laser-induced damage to optical materials in the multiple-irradiation regime shows that decreasing of their laser damage resistance in a series

of successive shots is associated with accumulation of irreversible changes in the

material. In other words, a catastrophic damage at the N-th shot is a result of ac- cumulation of microdamage s appearing at the previous N-1 shots.

In the present work v/e have developed the methodology of studying of the accumulation effect in the laser-induced damage, based on the statistical approach, and

introduced the basic characteristics of multiple-irradiation damage: critical num- ber of pulses II (I), the N-shot damage threshold ITM, N—shot probability for damage

V/^d), W-shot damage threshold distribution function f(I,.,), evolution operator A

of the single-shot damage threshold distribution function in a series of successive

laser pulses. This methodology and the introduced characteristics allow to investigate purposefully the regularities of the accumulation effect.

The experiments conducted on laser-induced damage to optical polymer materials

made it possible to establish specific properties of multipie-irradiation damage

(weakening of spot-size dependence of I„ compared to that of I.,, narrowing of intensity dependence of W^compared to that of W^(I)), indicating a statistical character of the accumulation effect. Among the mechanisms of accumulation effect considered, the most probable is that associated with the irreversible changes in the

material due to mechanochemical reactions under multiple laser irradiation.

The investigation of laser-induced damage to polymers in the multiple-irradi440
ation regime permitted to find efficient methods of suppressing the accumulation

effect (by variation of molecular-mass distribution of macromolecules and by introduction of additives quenching the excited active particles and preventing the development of chain mechanochemical reactions). One can expect that such methods

can be efficient for suppression of the accumulation effect in the laser-induced

damage to other optical materials.

6. References

1 Ashkinadze, B.M. ; Vladimirov, V.I.; Likhachev, V.A.; Ryvkin, S.M„; Salmanov,

V.M. ; Yaroshetsky, I.D. JETP, 50, 1187, 1966.

2 Bass, M. ; Barrett, H. JEEE J. Quant .Electr. QE-8, 338, 1972.

3 Likhachev, V. A. ; Ryvkin S.M. ; Salmanov, V.M. ; Yaroshetsky, I.D. Solid State

Physics, 8, 3432, 1966.

4 Nechitailo, V.S. Ph. degree dissertation, PIAN USSR, Moscow, 1974.

5 Danileiko, Yu.K.; Manenkov, A. A.; 'Nechitailo, V.S. Soviet Quantum Electr. 3,

538, 1976.

6 Danileiko, Yu.K.; Manenkov, A. A. ; Nechitailo, V.S. FIAN Proceedings, 101, 31,

1978.

7« Danileiko, Yu.K.; Manenkov, A. A.; Nechitailo, V.S. Laser Induced Damage in

Optical Materials. NBS Spec.Publ. 620, 369, 1980.

8 Aldoshin, M.I.; Gerasimov, B.G.; Mizin, V.M. ; Nechitailo, V.S. Abstracts of

the YIII All-Union Conference on Coherent and Nonlinear Ootics, Tbilisi, Metsnieraba,I, 136, 1976.

9. Danileiko, Yu.K.; Manenkov, A. A.; Nechitailo, V.S. Soviet Quantum Electr. 5,

194, 1978.

10 Aldoshin, M.I.; Gerasimov, B.G a ; Manenkov, A.A.; Nechitailo, V.S. Soviet Quantum Electr. 6, 1866, 1979°

11 Krutyakov, V.P.; Smirnov, V.N. JTP 49, 2647, 1979.

12 Manenkov, A.A.; Nechitailo, V.S. Soviet Quantum Electr, 7, 616, 1980.

13 Manenkov, A. A.; Nechitailo, V.S.; Tsaprilov, A.S. Izvestiya AN SSSR, Ser. physicheskaya, 44, 1771, 1980.

14 Dyumaev, K.M., Manenkov, A. A., Maslyukov, A. P.; Matyushin, G.A. ; Nechitailo,

V.S.; Prokhorov, A.M. Laser Induced Damage in Optical Materials. NBS Spec.

Publ., 1981.

Dyumaev, K.M.; Manenkov, A. A. ; Maslyukov, A. P.; Matyushin, G.A. ; Nechitailo,

V.S. Soviet Quantum Electr. 1_0, 3, 1983.

15 Manenkov, A. A. ; Nechitailo, V.S.; Tsaprilov, A.S. Izvestiya AN SSSR, Ser.physicheskaya, 46, 1200, 1982.

16 Dyumaev, K.M.; Manenkov, A. A.; Maslyukov, A. P.; Matyushin, G.A.; Nechitailo,

V.S.; Tsaprilov, A.S. Soviet Quantum Sleetri, 9, 1318, 1982.

17 Belyaev, N.N. ; Bredichin, V.I.; Rubakha, V.I.; Preidman, G.I. JETP 83, 1065,

1982.

18 Tikhomirov, G.P.; Turovskaya, T.S. OMP, No5, 65, 1977.

19 Eron'ko', S.B. ; Zhurkov,S.N. ; Chmel, A.B. Solid State Physics 20, 3570, 1978.

20 Kogtina, I.G.; Kinber, B.E. Soviet Quantum Electr. 7, 2427, 1980.

21 Wu, S.T.; Bass, M. Appl.Phys.Lett . , 39, 948, 1981.

441
22 Zhurkov, S.N. ; Eron'ko, S.B.; Chmel, A.E. Solid State Phys. 22, 3040, 1980.

23 Zhurkov, S.N.; Eron'ko, S.B.; Chmel, A.E. Solid State Phys. 24, 733, 1932.

24 Novikov, N.P. ; Novikova, Noil.; UPJ 27, 516, 1902.

25 Agranat, M.B.; Krasyuk, I.K.; Novikov, II. P.; Perminov, V.P.; Yudin, Yu.I.;

Yampol'sky, P. A. JETP 60, 1748, 1971.

26. Novikov, N.P. UPJ 26, 1676, 1981.

27 Butenin, A.B.; Kogan, B.Ya. Soviet Quantum Electr. 3, 1136, 1976.

28 Liberman, M.A. ; Tribelsky, I.I. JETP 74, 194, 1978.

29 Kovalev, A. A. ; Makshantsev, B.I.; Pilioetsky, N.P. ; Sidorin, Yu.V. ; Stonik, O.C

Soviet Quantum Electr. 7, 1287, 1980.

30 Manenkov, A. A, ; Nechitailo, V.3.; Tsaorilov, A.S. Soviet Quantum Electr. 8,

838, 1981.

31 Nechtailo, V.S. Izvestiya AN SS3R, Ser. physicheskaya 46, 1194, 1982.

32 Manenkov, A. A.; Matyushin, G.Ao ; Nechitailo, V.3.; Prokhorov, A.M.; Tsaprilov,

A.S. Soviet Quantum Electr. 10, No6, 1983.

33 Danileiko, Yu.K. Izvestiya AN SSSR, Ser. physicheskaya, 46, 1119 (1982).

34 Wang, V., Giuliano, C.R.; Garsia, B. Laser Induced Damage in Optical Materials.

NBS Spec. Publ. 435, 216, 1975.

35 Bass, M. ; Leung, K.M. JEBE. J. Quant .Electr. QE-12, 82, 1976.

36 Baliskas, S.K., Maldutis, E.K. Soviet Quantum Electr. 8, 902, 1981.

37 Zverev, G.M. , Naumov, V.S.; Pashkov V.A. ; Sidoryuk, O.E.; Skvortsov, L„A.

Izvestiya AN SSSR, Ser. physicheskaya, 46, 1135, 1982.

38 Gromov, D.A.; Dyumaev, K.M.; Manenkov, A.A. ; Maslyukov, A. P. Matyushin, G.A.;

Nechitailo, V.S.; Prokhorov, A.M. Izvestiya AN SSSR, Ser. physicheskaya 46,

1956, 1982.

39 Gromov, D.A. ; Dyumaev, K.M. ; Manenkov, A. A.; Maslyukov, A. P.; Matyushin, G.A.;

Nechitailo, V.S.; Prokhorov, A.M. Izvestiya AN SSSR, Ser. physicheskaya 46,

1959, 1982.

40 Akopian, V.3.; Berlin, A. A. ; Vinogradov, A. P.; Gromov, D.A. ; Danileiko, Yu.K.;

Dyumaev, K.M.; Krasnov, M.M. ; Krupnov, G.V.; Maslyukov, A. P.; Matyushin, G.A.

;

Naumidi, A. P.; Nechitailo, V.S.; Prokhorov, A.M. Izvestiya AN SSSR, ser. physicheskaya, 46, 1996, 1982.

41 Aldoshin, M.I.; Manenkov, A 0 A„ ; Nechitailo, V.3.; Pogonin, V.I. JTP 49, 2498,

1979. ~~

42 Aldoshin, M.I.; Gerasimov, V.G.; Menenkov, A.A«; Maslyukov, A. P.; Nechitailo,

V.S.; Ponomarenko, E,P, JTP 49, 2496, 1979.

442
Intensity ratio, I/I

Figure 1. Mean critical number of pulses U

cr

at

which the damage in organic glasses PMMA and MPMMA

occurs, versus the intensity I of incident radiation at the wavelength ,\ = O.GSUm. I. is the singleshot damage threshold. '

443
Figure 2. Scattering intensity of ruby-laser radiation versus the number of pulses at the multiple-shot

damage in MEMMA at the incident intensity- I = 0.51-,.

444
Number of Pulses

Figure 3* Intensity of the visible-UV glow accomoa

nying the laser-induced damage in PMMA versus the

number of pulses at 1.06/nm irradiation at the inci

dent intensity I = 0.5I 1

.

445
Spot Size (mm)

Figure 4. Spot-size dependence of the 1-shot and 200-shot

damage thresholds (I. and I

2 oo respectively) of PIMA at

1.06*^ m irradiation.

446
(J

Intensity (Rel. Units)

Figure 5. Probabilities for single-and multipleshot damage (7/

1 and W200> respectively) in PMMA

versus the incident radiation intensity at the

laser wavelength 1 .O^m and the beam diameter

0.5 mm.

The speaker was asked if he had laser conditioned a site with repetitive low fluence shots and then

tested to see if damage accumulated at that site during a set of repeated shots, as it did no nonconditioned sites. The speaker had not done this experiment. He further stated that plastics

impregnated with dyes behave basically as undoped plastics provided the dyes were stabilize by

adding an unspecified chemical. In comments from the audience, similar accumulation of damage was reported in plastics and in lithium niobate , and laser conditioning of KDP was mentioned. The

speaker was asked whether the reports of damage accumulation arose from the inability to detect the

first small damage induced, and agreed that this was the case and said that the lowest fluence to cause such micro damage was the real threshold for physical damage.

447
DISPERSION OF THE REFRACTIVE INDEX NEAR THE ABSORPTION EDGE

IN COMPOUND SEMICONDUCTORS

B. Jensen, A. Torabi

Boston University

Boston, Massachusetts 02215

An analytical expression for the real part of the refractive index n near the fundamental absorption edge has been calculated for a semiconductor in terms of experimentally

known quantities. It is derived from a quantum mechanical calculation of the complex

dielectric constant which assumes the band structure of the Kane Theory. The expression

obtained is a function of band gap energy, effective electron and heavy hole masses,

the spin orbit splitting energy, the lattice constant, and the carrier concentration

for n-type or p-type materials. The refractive index is given as a function of frequency

and the basic material parameters listed above with no adjustable constants. Dispersion

near the fundamental absorption edge, which has been observed experimentally for a number

of III -V and I

I —V I compounds, is predicted and its calculation enabled. Comparison of

theory with available experimental data is given.

Key words: Optical constants; optical materials; refractive index; semiconductors; wavelength

dependence.

1. Introduction

An understanding of the spectral dependence of the refractive index of a semiconductor in the

region near the fundamental absorption edge is a subject of considerable importance in integrated

optics, as the refractive index plays a fundamental role in the guiding, coupling, and modulation

of radiationP^ Experimental data on the spectral dependence of refractive indices are available

for a number of compounds. However, there have been few theoretical analyses of the subject.

The spectral dependence of n can be calculated using the general theory of dispersion and the

Krarners-Kronig relationship, but this requires a knowledge of the complete spectrum of the imaginary

part of the dielectric constant e„. In practice, application of this method leads ultimately to the

introduction of parameters whose relation to the experimentally available quantities, such as band

gap energy, effective masses, and free carrier concentration, is more or less vague.

^ ^The

spectral dependence of the refractive index is usually described by formal empirical relationships,

involving several adjustable parameters . ^ 1) ^ s (<nown experimentally that at photon enerqies

small compared with the band gap energy, the complex dielectric constant, from which the complex re- fractive index is derived, is given by the quasiclassical result of the Drude theory. C2)-(15) ^

frequencies close to the fundamental absorption edge however, the Drude theory result for the

imaginary part of the complex refractive index is know to fail . ^^"^^ The experimentally observed

result can be accounted for by a quantum calculation which reduces to the classical result at low

(19)-(21 ) frequencies. '

v '

An analytical rexpression for the real part of the refractive index near the fundamental absorption edge has been calculated for a compound semiconductor in terms of experimentally known quan-

(22 )-(24) tities. 'It is derived from a quantum mechanical calculation of the complex dielectric

constant^^ which assumes the band structure of the Kane theory. We present theoretical

448
results and give a comparison with experimental results for a number of compounds in Section III. A

(22) (24) detailed derivation is given elsewhere. /s v '

2. Results

Near the fundamental absorption edge, the real part of the refractive index n is found as a function of the experimental parameters for a given semiconductor which are listed in Table I. The

independent variable z is defined in terms of the photon energy fiw in units of the band gap enerqy

G as

z = ( 1 - (Tfw/G) ) h

(1) We define

n(0,0) = ( 1 + 2c Yg)* = index of refraction at the absorption edge (z = 0) for (2)

a pure material with zero carrier concentration.

In (2), the constant c q

is given in terms of experimental quantities, specifically band gap

energy, spin orbit splitting energy, and effective electron and heavy hole masses, in Table I. The quantity Yg is a function of the lattice constant a (8) of the form

Y B

= m(a

Q

- a) = n

2 (0,0) - 1 (3)

2c

o

where

a o

0

- 1 (3.04 ± 0.08) A

1 , 1 1

1 -V compounds

0

- 1 .346 ± .020) A

1 , II-VI compounds

o (7.49 ± 0.04) A , 1 1

1 -V compounds

o (17.37 ± 0.88) A , II-VI compounds

(4)

(5)

The real part of the refractive index is given as a function of frequency and carrier concen-

(22) (24) tration, or z and y F

, near the fundamental absorption edge by v " v

n

2 (z,y

p

) = 1 + 2c

Q

[(Y

g

- y

p

) - z( tan

_1

(Y

B /z) - tan

_1 (y

F /z) )] (6)

where

]/3

yp = 2 (n g/N ) , degenerate n-type materials (7)

yF = jc , degenerate p-type materials (8)

where n g

is the free electron concentration for n-type materials and p is the free hole

concentration for p-type materials and * = (m /m ) h

(< 12) (9)

r

v n r

v c

k h (3,2 p)

1/3 O0)

In (7) through (10), N

y and m^ are functions of band gap energy and effective masses defined

in Table I and m

n is the effective electron mass at the band edge. The concentration dependence

449
of n occurs through the term y^. Equation (6) applies to degenerate materials with non-zero carrier concentration where the Fermi level lies within the conduction band (for n-type materials)

or the Valence band (for p-type materials). The expression for a pure material with negligable free

carrier concentration is obtained from (6) by setting y^ = 0, and non-degenerate materials are discussed in Reference (22). Equation (6) contains no adjustable constants. If carrier concentration, band gap energy, lattice constant, effective electron and effective heavy hole mass, and spin

orbit splitting energy are known, exact numerical results can be calculated from (6). Table II gives the experimental values of these material parameters and values of theoretical quantities which

are functions of experimental parameters for GaAs, InP, InSb, CdTe, and ZnTe. We use the small spin

orbit splitting limit if the spin orbit splitting energy is less than or equal to the band gap energy,

and the large spin orbit splitting limit if the spin orbit splitting energy is greater than the band

gap energy, as the calculation has been performed in these two limits only.

An attempt has been made to compare theory with experimental data from various sources. In Table III, theoretical results calculated as described above and experimental results are given for

ZnTe. In Table IV, theoretical results calculated using the approximation discussed in Ref. (22),

which involves replacing tan~^(Yg/z) by tt/2, are given along with experimental results from sources listed in the References. Since Yg ranges from around 4 to 6, while z << 1 , the approximation is

permissible for z sufficiently small. Theoretical results (solid lines) are plotted along with

experimental results in Figures 1 and 5. Comparison for each of the five compounds considered is

discussed in detail in the Figure Captions. It can be seen that overall, agreement between theory

and experiment is quite reasonable, considering that no adjustable parameters are involved if Y DD is determined from the lattice constant. These results should be useful in technological applications, as they give a method of calculating the refractive index as a function of frequency in terms

of measurable parameters for a class of important semiconductors.

This research, was supported by the department of Energy under contract # DE-AC02-79ER10444 .AO00

.

450
[I] E. Garmire in "Integrated Optics,"

T. Tamir, Ed., Springer-Verlag (1979).

pp 243.

[2] D.F.T. Marple, J. Appl . Phys. 35_, 1241

(1964).

[31 H.C. Casey, D.D. Sell, M.B. Panish,

Appl. Phys. Lett. 24, 63 (1974).

[4] D.D. Sell, H.C. Casey, K.W. Wecht,

J. Appl. Phys. 4J5, 2650 (1974)'.

[5] D.F.T. Marple, J. Appl. Phys. 35, 539

(1964).

[6] E.G 0 Valyashko, Karin Gerrmann, Sov. Phys. -Semiconductors 1_, 904 (1968).

[7] G.D. Pettit, W.J. Turner, J. Appl.

Phys. 36, 2081 (1965).

[8] A.N. Pikhtin, A.D. Yas'kov, Sov. Phys.-

Semi conductors 12_, 622 (1978).

[9] F. Stern, Phys. Rev. T33> A1653 (1964).

[10] T. Tagaki, Japan. J. Appl. Phys. 21,

L167 (1982).

[II] V. Gopal, Infrared Phys. 22, 255

(1982).

[12] J.K. Kung, W.G. Spitzer, J. Electrochemical Soc. 121, 1482 (1974).

[13] R.N. Zitter, K. As'Saadi, J. Phys.

Chem. Solids 35, 1593 (1974).

[14] R.T. Holm, J.W. Gibson, E.D. Palik,

J. Appl. Phys. 48, 212 (1977).

[15] S. Perkowitz, J. Phys. Chem. Solids

32, 2267 (1971).

[16] W.G. Spitzer, J.M. Whelan, Phys. Rev. 114, 59 (1959).

[17] W.P. Dumke, M.R. Lorenz, G.D. Pettit,

Phys. Rev. B 1, 4668 (1970).

[18] R.M. Culpepper, J.R. Dixon, J. Optical

Soc. Amer. 58, 96 (1968).

References

[19] E. Haga, H. Kimura, J. Phys. Soc. Japan, 18, 777 (1963); 19, 471 (1964);

19, 658T1964).

[20] B. Jensen, Ann. Phys. 80, 284 (1973);

J. Phys. Chem. Solids M, 2235 (1973).

[21] B. Jensen, Aon. Phys. 95_, 229 (1975).

[22] B. Jensen, A. Torabi, IEEE J. Quantum

Electron. March (1983). To be published.

[23] B. Jensen, A. Torabi, J. Appl. Phys.

April (1983). To be published.

[24] B. Jensen, A. Torabi, IEEE J. Quantum.

Electron. May (1983). To be published.

[25] B. Jensen, IEEE J. Ouantum Electron

Q-18 , 1361 (1982).

[26] E.O. Kane, J. Phys. Chem. Solids-1, 249

(1957).

[27] J.I. Pankove, in "Optical Processes in Semiconductors," Prentice-Hall, Englewood

Cliffs, 1971. pp 412-413.

[28] D. Hennig, B. Strehlow, Phys. Status

Sol idi 107, 283 (1981).

451
Table 1

Experimental parameters and theoretical quantities which are functions

of experimental parameters.

•r

r 1

V

o n(0,0)

band gap energy

electron effective mass at the band edge

heavy hole effective mass at the band edge

spin orbit splitting energy

o lattice constant (A)

free electron concentration for n-type materials

free hole concentration for p-type materials

m(a

o

- a,

3.04(7.49 - a)

.346(17.37 - a;

2/3

3/2

A/G «1

A/G «1

III-V compounds

II-VI compounds

small spin orbit splitting unit

large spin orbit splitting unit

frequency at fundamental absorption edge

m m /(m +

n p n (G/2m

n

c

2 )^2

"h7m a c

n o

reduced mass

(m /m )

: v n r' c

(4/3) 1/V

c

N (m /m ) 3/2

v r n

4 Tre 2 N*/m

v n /- 2 J/3

(3 TT p) '

(3/2) n (w

2 /w

2

)

\, (1 + 2c Y R

) 2 = index of refraction at absorption edge for

0 pure material (n = p = 0)

452
Table 2.

Values of experimental parameters and theoretical quantities which are functions of experimental parameters for various semiconducting compounds.

T = 300°K GaAs InP InSb CdTe ZnTe

G(eV') 1 .43 1.35 0.18 1 .50 2.28

m /m

n 0.07 0.07 0.0133 0.11 0.15

m /m

'V

0.50 0.40 0.18 0.35 0.663

5.653 5.869 6.479 6.477 6.101

Y 5.554 4.966 3.063 3.779 3.889

n

0 UA) c

2/3 2/3 3/2 2/3 2/3

12.4 12.7 80.0 9.63 6.68

N*(cm~

3

) V 20

1.18x10

u

1 .03x10 4.76x1

0

20

2.01x10

20

6.66x10

w

g (sec

_1

)

1

5

2.17x10'° 1.95x10

15 14 2.73x10

4 2.28xl0 15 3.47xl0

15

w

v (sec

_1

) 2.31xl0

15

2.16x10

15 14 '3.37x10 2.41xl015 3.76xl0

15

c o 1.133 1.114 3.422 1.118 1 .175

Experimental parameters are taken from Reference (27) with the exception

of m /m = .663 for ZnTe. A value of m /m = .6 for ZnTe is reported in Reference (28). There is a considerable amount of variation in reported

values of m for many compounds, as discussed in Reference (23).

Table 3. Refractive Index of ZnTe

x(um) n(experiment) n(theory)* n(theory)

0.589 3.05 3.0247 3.042

0.620 3.00 2.9852 3.000

0.830 2.84 2.8614 2.863

1.240 2.78 2.7799 2.770

2.060 2.71 2.7260 2.710

* Calculated using Y g

_ 4

= 3.982 ± 0.022 with m = 0.6m fixed. The

variance is 3.99x10 . p

++ Calculated using the approximation discussed in Reference (22), with

m , treated as an adjustable parameter, found to be m = 0,663 ± 0.1414

compared with a reported experimental value of m = 0?6m.(28) The

index of refraction at the absorption edge is n(8) = 3.2016 ± 0.0187

and the variance is 2.38x10 .

453
Sample Wavelength

Table 4.

(urn) Energy (eV) n(exp) n(theory)

fi - f\ ~ GaAs i on n cca U. bb4 o o a a 3. 34b o o o /i 3. 334

1 . bb n onn U. oUU O OIC 3. 3/b o o 7 n

3. 3/U

1 0 7

1 .3/ n nnn u. yuu o /inn

J . 4UU o one o . jyo

1 O/l

1 . ^4 i nnn 1 . UUU O /l OA 6 . 4Z4 O /IOC O . 4 1 O

1 10 i in 1 . 1

U

O A CO j . 4b oO /ICO j . 4bo

1 .03 1 .20 3.485 3.496

0.95 1.30 3.540 3.543

0.91 1.35 3.570 3.573

T ws D o nc L . Ub n en U . bU 0 1 oc o i nc -5 . 1 Ub

1 77 n 7n U. /U O 1 A C 6 . 1 4b O 101

1 C C

1 . bb n on U.oU O 1 cc d . 1 bb o i c n

0. i by

1 0 7 1.0/ n on u . yu 0 1 on 0 . 1 oU o i on o . i yu

1 0 A

1 . £4 i nn 1 .UU o oon o . ££U O OO/l o. ££4

1.12 1.10 3.253 3.264

1.03 1 .20 3.300 3.312

0.99 1.25 3.333 3.342

InSb o nn o. UU n ic U. 1 b

a nnn 4 . UUU a n/i 7 4.U4/

9.00 n io U. 1 O

o non 3. y3u o nc i o . yb 1

i n nn n to U. 1 d. O O 0 0 o . ooj o on7 o . yy/

i i nn

1 1 . OU n in U. 1 1 O QCC J. bob O OA C. 6 . o4b

12.00 0.10 3.816 3.806

13,00 0.09 3.800 3.773

14.00 0.08 3.766 3.746

CdTe 0.903 1 .37 2.91 2.916

1.000 1.24 2.84 2.845

1.10 1.12 2.81 2.798

Zn I e n con (J. boy o in L . 1 U

o nc j . Ub o n/i o

n con (J. D^U o nn £ . UU o nn o . UU o nnn o . UUU

0.830 1 .49 2.84 2.863

1 .24 1 .00 2.78 2.770

2.06 0.60 2.71 2.710

** Theoretical values are calculated using the approximation discussed

in Reference (22), which involves replacing tan" (y R /z) by it/2 in Eq. (6)

454
1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 II 1 1 i 1 1 1 1 1

Ca As

300 K

1 1 1 1 1 1 1 1 1 1 HI 1 II

Energy ( ev

)

J g Figure 1. Theoretical results for n for GaAs with a carrier concentration of n = 6x10

(solid line) plotted together with experimental results for a sample of the same carrier

concentration obtained from Reference (2) (+++++).

1 1 1 1 1 1 1 1

1

In P

1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1

300 K G = 1.35 eV mp = .4 m

+ /

1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 Ml

Energy ( eV

)

Figure 2. Theoretical results for InP for a sample with carrier concentration n = 5x10

(solid line) plotted along with experimental results on a sample of the same carrier

concentration obtained from Reference (7) (+++++).

16

455
1 1 1 1 1 1 1 1! 1 1 1 1 1 1 11 1 1 1 Mill

In Sb

295 K

1 1 1 1 I I 1 11 1 l 1 l l 1 ll 1

0 10.0 100.0 Wavelength ( ^.m

)

2.8xl0 16 cm"

3 are Figure 3. Theoretical results for InSb (solid line) for a sample with n compared with experimental results, £++++*+) on samples 2n of Reference (fe), which has an impurity concentration N . = 2.8x10 cm . In makinq the comparison, we have assumed n

=

Wavelength (/im

)

Figure 4. Theoretical results for CdTe (solid line) are compared with experimental results

(+++) from Reference (5) An instrinsic sample, with n &

= p = 0, has been assumed,

456
1—I—I I I MIT 1—I I I I I I IT r

Zn Te

300 K m p = ( .663 ± .1153 ) m

J L I I I I I I I J I L

Wavelength ( mm

)

Figure 5. Theoretical results for ZnTe (solid line) are compared with experimental results

(+++++) from Reference (5). An instrinsic material, with n = p = 0, is assumed.

457
UNIPOLAR ARCING, A BASIC LASER DAMAGE MECHANISM

F. Schwirzke

Neval Postgraduate School

Monterey, California 93940

Laser beams interact with surfaces by a variety of thermal, impulse

and electrical effects. Energy coupling is considerably enhanced once surface electrical breakdown occurs. The laser heated plasma interacts

then with the surface via three major interrelated damage mechanisms:

thermal evaporation, ion sputtering, and unipolar arcing. While the first

two are purely thermal and mechanical effects, unipolar arcing is an electrical plasma-surface interaction process which leads to crater

formation, usually called laser-pitting, a process which was often observed

but not well understood. Without any external voltage applied, many

electrical micro-arcs burn between the surface and the laser heated plasma,

driven by local variations of the sheath potential with the surface acting

as Vioth the cathode and anode. Unipolar arcing represents the most damaging

and non-uniform laser-plasma-surface interaction process since the energy

available in the plasma concentrates towards the cathode spots. This causes cratering of the surface. Unipolar arcing is the primary plasma-surface

interaction process once breakdown occurs. The onset of arc damage is

coincident with the onset of plasma formation. Never was there a plasma

evident without attendant unipolar arc craters. At low irradiance there was no other laser damage (like melting) observed, all damage was in the form of

unipolar arc damage.

Key Words: Laser damage mechanism; Laser- induced unipolar arcing;

Unipolar arc model.

1. INTRODUCTION

Progress in pulsed power technology has been hampered by many plasma

physics and technical problems not the least of which has been the problem

area of plasma surface interactions. Surface breakdown by high power laser

radiation leads to the formation of a plasma layer in front of the laser

illuminated surface. Unipolar arcing then occurs at a sufficiently high

plasma electron temperature.

The term unipolar arc was first used by Robson and Thoneman [1] in

1958. In their article they described the existence of an arc which

"required only one electrode and is maintained by the thermal energy of the

plasma electrons." The basis of their model is that at equilibrium there

must be zero net current between an exposed conducting plate and a plasma.

A retarding potential is established between plate and plasma. This

floating sheath potential Vf prevents all but the higher energy electrons

in the maxwellian distribution to reach the surface. Thus at equilibrium

the net current becomes zero. The sheath potential depends on the electron

temperature. If the electron temperature, T e

, is sufficiently large such

that Vf exceeds the potential to initiate and sustain an arc, the authors

argued that there will be strong local emission of electrons from a cathode

spot on the plate into the plasma. Figure 1 represents this process. The

plasma potential then lowers from V f to the cathode fall potential V c of

the arc. With the plasma potential now lowered more electrons can return to

the plate, thus closing the current loop and maintaining the plasma's

quasi-neutrality . The circulating current is given by the increased

458
© ION # ELECTRON

^^^^^^^

mLmM \

c<Vf

RETURN CURRENT

Figure lb. Equilibrium flow

of electrons and ions to

isolated plate with electron

emission from a cathode spot.

electron return current to the plate of area A due to the reduction of the

sheath potential from Vf to Vc ,

I c = Aene

n e is the electron density, k the Boltzmann constant and e the elementarycharge. The arc was sustained, they argued, until I c fell below a minimum

current I a

. The value of I a depends on the material of the plate. The

Robson-Thoneman mechanism implies a constant plasma electron density n e contributing to the return current over a large area A of the wall.

The existence of unipolar arcs has been verified with extensive

experimental evidence. Arc tracks have been observed on many tokamak

surfaces [ 2-6 ] .

2. UNIPOLAR ARC MODEL

While the Robson-Thoneman mechanism describes essentially the return

current flow by eq (1), our model elaborates upon the electric fields which

are set up in the plasma and drive the arc. The condition of plasma

quasi-neutrality , n e = n-^ , leads to the formation of a sheath wherever

the plasma is in contact with a wall. Since the electrons have a higher

thermal velocity than the ions, an ion rich boundary layer is formed and the

plasma potential is positive with respect to the wall. The magnitude of

this sheath- or f loating- potential is proportional to T e , depends weakly

on the ion/electron mass ratio Mj_/m e , and is independent of the plasma

density,

V f = (kT e /2e)An(Mi /2irme ) (2)

The sheath width is proportional to the Debye shielding length

A D

= (kT e /4wn e

2

) 1/2

(3)

The plasma loss rate to the wall is controlled by 1»he sheath potential.

It accelerates the ions and electrostatically reflects most of the electrons

WW

PLASMA \\\\\

SHEATH

PLATE

Figure la. Equilibrium flow of

electrons and ions to isolated

plate

.

1/2,

(kT ) e

( 2inO

/ -eV

exp

'-eV,

kT£ -exp

kT, (1)

459
such that electrons and ions reach the jsurface in equal numbers. Since the

ions are much more effective than electrons in the desorption and sputtering

of surface particles and the rates increase steeply with energy in the range

of interest, sputtering and desorption of loosely bound particles [7]

become worse if the sheath potential increases due to an increase of the

electron temperature near the wall [8]. Electron and ion recombination on the surface contribute to surface heating.

Unipolar arcing, where the wall acts as both the anode and the cathode,

occurs if the sheath potential increases sufficiently to ignite and sustain

an arc. The Robson-Thoneman unipolar arc model implies a constant plasma

density. However, for an arc to develop it is also necessary that the ion

density increases above the cathode spot, in order to enable a larger

electron current to flow into the plasma. Figure 2 shows schematically our unipolar arc model which takes into account the increase of plasma density

due to ionization of neutrals released from the cathode spot.

The initial experiments were conducted on type 304 stainless steel

targets [9] which had been highly polished. Surface protrusions,

metallurgical inhomogenities , inclusions or whiskers from processing

operations such as milling and grinding can serve as initial cathode spots.

Furthermore, plasma waves and associated potential fluctuations in the

sheath [10] can enhance electron emission and initiate cathode spots. The

increased electric field strength on surface protrusions will also increase

the ion flux from the plasma to these spots. Increased ion bombardment and

recombination rates lead to a locally increased surface temperature. This

results in desorption of neutral gas molecules and evaporation of metal

atoms from the surface spot into the plasma. The ionization of a small

fraction of the released neutral atoms results in a local increase in

density of the plasma above the cathode spot.

The equation of motion for the electron fluid is given by

3V n e

Where p e = n eme is the electron mass density, Ve the fluid velocity

of the electrons, P

e the pressure, j the current density, and o the

electrical conductivity. The electron inertial term is insignificant for

the characteristic time for arc evolution. Thus, setting eq (4) equal to zero and furthermore for B = 0, solving for E, one obtains,

E = i - VP

o n e e

For j = 0, the ambipolar electric field becomes EQ

= -(1/e n

g

) VP^

For a weakly ionized plasma, the ratio j/ a = AE becomes independent of

n e and small in comparison to E Q

:

. n eV (m v) m V v VP kT m y

2 AE = 1 = — -

e = e e < E

e s e _e _ ° (n e

2

X ) eA

e ° n

e

e ed 2 ed

e e

460
Figure 2. Unipolar arc model

Figure 3. Unipolar arc craters. The outer rim diameter is 5.5 ym. The

dark cathode spot at the center has a diameter of 0.7 ym, smaller than the

laser wavelength of A =1.06 ym.

461
Our model implies that the mean free path length X e is larger than the

dimensions of the neutral cloud above the cathode spot. Since the build-up

of the plasma pressure gradient is caused by ionizing a fraction of the

neutral particles, we assume X e>d, where d is the characteristic pressure

gradient length. Even if X e $d,E< E Q since the current associated drift

velocity of the electrons Ve <<u, where y is the electron thermal velocity.

The probability that a neutral atom is ionized in or near the sheath is

of the order of the Debye length divided by the mean free path length. This

ratio is much less than unity. For values of ne = 5 x 10 cm -J and

kT = lOeV, which may be representative for an initial laser produced low

density plasma layer, X D

= 10~4 cm. The electron mean free path for

ionizing a neutral atom is X e = l/nQ

aj_. Under vacuum conditions the neutral

particle density nQ

in the sheath depends on the desorption and

evaporation rates from the surface. Iron atoms emitted at the melting

temperature of 1526°C leave the surface with a thermal velocity of about

10^ cm s ~ Their time of flight to pass through the sheath would be

10~9 s. If, for example only one monolayer, i.e., 2 x 10^5 particles

cm

- 2 are released from a surface spot during this time, the neutral

particle density within a Debye length will increase to n Q

= 2 x 10^-9 cm

-

^.

(For comparison, atmospheric density at 0°C corresponds to 2.7 x lO-^ cm"^.

)

If we assume that the ionization cross section is 10~l'7crn 2

f \ Q becomes

0.005 cm. Thus a small fraction, Xj-,/

X

e = 2 x 10~2, of the neutral atoms

would be ionized within a Debye length from the surface. This, however,

increases the plasma density from 5 x 10 cm

- to ni = (2 x 10~2) no ='

4 x 10-^ Cm"3 ,i.e., by a factor of 800 within 1 ns. Correspondingly, the

local X £ would rapidly decrease to 3.7 x 10~6 cm. An ion produced in the sheath is driven back to the surface by the

electric field which is of the order

E ~ Vf/Xrj (5)

The time of flight to return to the surface is proportional to the Debye

length and the acceleration in the electric field a = (eVf ) /(M^Xp )

.

Thus

t = (2XD /a) 1/2 = A D (2Mi/eVf ) 1/ 2

For the initially assumed Debye length of 10- 4 cm an iron ion needs

about lOps to fall back to the surface. Since the Debye length decreases

with build-up of the local plasma density above the cathode spot, later

released neutrals which become ionized in the sheath need only picoseconds

to return to the surface. The increased ion bombardment and release of

recombination energy lead to a further increase in surface temperature.

This in turn represents a feedback mechanism, leading to a further release

of absorbed gases and material evaporation at a sufficiently high power

deposition. Since the mean free path for electrons is much larger than the

sheath width, the additional ionization of neutral atoms in (or near) the

sheath and the acceleration of these ions in the sheath implies a concen- tration of available plasma energy towards the arc spot. Of course, most of

the neutral atoms released from the surface become ionized within the

plasma. The locally increased plasma density reduces X D and thus the

sheath width. Correspondingly, the locally increased plasma pressure above

the cathode spot leads to an increasing electric field Earc

(V/cm) to drive

the arc current:

|E I z V A < V-/X,, = 6.6 x 10" 3 (nQ kT ) 1/2

(6)

' arc ' c D ~ f D e e'

where kT e is measured in electronvolts and ne in reciprocal cubic

centimeters. The high density plasma above the cathode spot is biased with

respect to the surface via the cathode fall potential of the arc. The high

density plasma dominates the surrounding lower' density background plasma.

The increased plasma pressure above the cathode spot thus also leads to an

462
electric field E r in the radial direction, tangential to the surface:

E = - kT dn

_e e + 2 (7) r en dr a

Neglecting the j/a term, this radial field reduces the plasma potential in a ring-like area surrounding the higher plasma pressure above the arc spot by

kT n (r)

Av(r) = —* In e (8) e neo

The ratio of the maximum plasma density above the cathode spot to the

unperturbed plasma density can easily be of the order (ne /n eo ) - 10 3 or larger. The reduced sheath potential in this ring area allows more electrons from the high energy tail in the maxwellian distribution to reach

the surface, this closing the current loop of the unipolar arc, figure 2.

In fact, equating AV with the sheaths floating potential, eq (2)

AV = V

f —

kT n kT

® ln = __e . Mi

e neQ 2e

^ 2^

we find that independent of the electron temperature the sheath potential

approaches zero when

e Mi

1/2

eo \ e

In this case, the electron return current to the surface is determined by

n e and the electron thermal velocity. This electron saturation current

is

en v (9)

i = -r— A

s 4

Equation (9) can be used to estimate the order of magnitude of the surface

area A needed for the electron return current. If the arc current is 10 Amp

and n e

= {M^/2^me ) neo = 130n eo for the mass Mj^ of iron ions

and if we consider a laser produced plasma near the surface with initially

n eo

= 5 x 10-1-4 cm-3 and kT e = 10 eV, the return current area

is A = irr^ = 3xl0~9 m^ . Actually, ne(r) will rapidly increase

above the assumed value, reducing A further. Thus the estimated value of

the radius of r < 30 ym is of the same order as the radius of the outer

crater rim. The location of the outer crater rim thus may indicate how far

the return current area has expanded during the limited burntime of the

arc.

463
3. EXPERIMENT

A plasma generated by a neodymium laser pulse was used to study

unipolar arcing on solid material surfaces. A Korad K-1500 laser was operated in the Q-switched mode. Nominal beam pulse half width of the laser

was 25 ns . Incident laser energy was varied by inserting neutral density

filters of varying transmittance in the beam path. For low energy shots the

laser amplifier was not fired. Laser energies on target between 0.0075 and

10 joules were obtained by these techniques. The beam was focused to

various spot sizes on the target to provide further variation in power

density at the target. Laser energy at each shot was measured using a Laser

Precision RK-3200 Pyroelectric Energy Meter, with a RE 549 detector reading

via an 8% splitter in the main beam.

The laser pulse was directed onto targets placed in a vacuum chamber.

The vacuum system using an oil diffusion pump and a liquid nitrogen cooled

baffle, was capable of providing chamber pressures of the order 10"^

Torr. To obtain a smooth surface the target disks were metallographically

polished with a final polishing slurry of 0.05 ym AI2O3. Before being

mounted in the test chamber, each specimen was cleaned with acetone. After

one laser shot the damage on the polished surface was observed with an optical and a scanning electron microscope, Cambridge Stereoscan S4-10.

4. LASER-INDUCED UNIPOLAR ARCING

wide range between two

dynamic conditions on

1. High Irradiance

Laser induced unipolar arcing was studied over a extreme, basically different laser-power and plasmavarious metals and nonconducting surfaces.

A high power laser pulse is focused onto the target. The laser

produced plasma expands from the small focal spot over the target surface

into areas which were not illuminated by the laser. The area damaged by the

plasma is much larger than the laser focus area. The existence of unipolar

arc craters in these areas proves that unipolar arcing is a plasma-surface

interaction process, independent of the laser-plasma interaction processes,

like self-focusing, filamentation, instability, etc. The laser just heats

the electrons to a sufficiently high temperature.

2. Low Irradiance

A low power, defocused laser pulse is used to determine the laser power

density threshold for onset of unipolar arcing. Laser intensity variations

over the cross section (hot spots) cause breakdown and arcing only in areas where the power density was sufficiently high. The area damaged by the

plasma is smaller than the laser illuminated area. The high and the low irradiance experiments will be discussed in the

following paragraphs and the results on conducting and non-conducting

surfaces will be compared.

4.1 High Irradiance Laser Induced Arcing

In the Q-switched mode the range of energies on the target was selected

as 3-5J, giving a power range of 100-200 MW. The laser focal spot diameter

is approximately 0.5 mm. For a 200 MW pulse this gives a typical power

density of 10-*-! W cm"^. The temperature of the laser-produced

plasma was measured spectroscopically , kT e

~ lOOeV. Time of flight

measurements showed that the plasma expands rapidly with a velocity of about

10^ cm/s from the focal spot on the target surface in the normal and

radial directions.

The laser impact crater of about 0.75 mm diameter is surrounded by a plasma damaged surface area of about 6 mm diameter. Although no external

voltage is applied, about 300,000 to 600,000 unipolar arc craters/cm2 are observable on the stainless steel surface which was exposed to the

radially expanding plasma but not to the laser radiation. Figures 4-6 show

464
a sequence of SEM photos of the plasma-damaged surface for increasing distance from the laser focal spot. Close in, figure 4 shows a superposition

of arc craters. The oldest one, burning for a longer time during the

existence of the laser-heated plasma, have larger outer rim diameters of

30-50 y m. Smaller craters within larger ones are probably initiated towards

the end of the laser pulse. They have smaller outer rim diameters. If not

refilled by the liquid metal, craters show a dark spot of about 1 Pm size at

the center representing the cathode spot. The size of the outer rim craters

also decreases with increasing distance from the focal spot (figures 5 and

6). Figure 6 shows the onset of crater formation near the edge of the

expanding plasma.

Figure 4. Overlapping craters near laser focal spot. Outer rim crater size

depends on arc burn time.

Figure 5. Crater formation at 1.5mm from laser focal spot which is located

beyond the left side.

465
Figure 6. Onset of unipolar arc crater formation on highly polished

stainless steel.

Deep cathode craters of 0.5 to 1 ym diameter are clearly visible with outer

crater rim formation just barely beginning. The depth of these fully

developed cathode craters is 3-6 ym.

The short lifetime of the laser-produced plasma allows the study of the

temporal development of the arcs. The initial breakdown phase of unipolar

arcing is documented by the small but deep cathode craters formed furthest

away from the focal spot, figure 6, where the plasma conditions favorably

for arc formations lasted only for a short time because after laser shut off

T e decreases rapidly in the expanding plasma. Further in, if allowed to

burn long enough, the surface metal area A needed for the electron return

current melts. The liquid is pushed outwards in the radial direction by the

expanding dense plasma and an outer crater rim of 5-50 ym diameter is formed

depending on the arc duration.

The depth of the initial arc crater of 3-6 y m, is larger than the dia466
meter of 1 ym. This indicates that the pinch magnetic field of 40,000 Gauss

for a current of 10A and r = 0. 5 ym contributes to the concentration of the arc energy onto the small cathode spot, see figures 2 and 3. If kTe = lOeV,

this field can confine a plasma with a density up to 4 x 10^-^ cm . At this

density the Debye length A D

= 10~^<<1 ym and it is justified to assume that a high density plasma exists in the arc hole. This represents a hollow

cathode configuration and the ionization rate should be high. The depth

of the cathode spot craters corresponds to a removal of 10^ to 10^

monolayers

.

Arcing continues only as long as the laser pulse is heating the

electrons for 50-60 ns. The natural life time of a micro-arc seems to be of

the order of 10 ns . Unipolar arcing on different materials has been

studied. All metals show a very similar kind of arc craters. Evidence of

unipolar arcing has been found on all targets irradiated at atmospheric

pressure, that also arced in vacuum indicating that the metal vapor pressure

near the surface is of the order or even larger than the atmospheric

pressure

.

4.2 Determination of Low Irradiance Threshold for Onset of Arcing

The incident laser power density for the onset of arcing was determined

for type 304 stainless steel and type 2024 aluminum [11]. Since the energy

output is more easily controlled by the introduction of filters than by

adjusting laser power supply voltage, the incident power on the target was controlled in this manner. A polaroid camera was positioned above the

target to note plasma formation by recording the attendant light. The beam

spot size was measured by placing exposed polaroid film at the target

position within the vacuum chamber and conducting a series of laser shots to

find the average spot size area. Using this experimental arrangement a series of laser shots were made

to determine the power density threshold for breakdown and plasma formation

and the power density threshold for unipolar arc formation. Surface

breakdown and the formation of a visible plasma layer occur at a laser power

density of about 10 MW/crn^. Unipolar arcing has been shown to be then the

primary plasma-surface interaction process once plasma formation occurs. Figures 7 and 8 provide a comparison of the extent of damage between

25.5 MW/cm^ and 5.4 MW/cm^. Figure 9, an enlargement of figure 8, shows

the minimal damage observed at 5.4 MW/cm^ . The characteristic shape of

unipolar arcs becomes clearly visible in the enlarged photo, a small dark

cathode spot at the crater center surrounded by a pronounced rim.

A review of the target damage reveals that the damage was not evenly

distributed across the laser illuminated area but rather was concentrated in one quadrant, due to non-uniformity in the laser beam, figures 7 and 8. The

locations of the beam's hot spots coincide exactly in shape and location

with the observed target damage. Since the area used in calculating power

density assumed a uniform distribution of energy, the threshold density

reported above can only be accurate as an order of magnitude power density

for this particular laser.

The data thus obtained suggest that the power density threshold for the

onset of breakdown and for the onset of unipolar arcing are the same. The

onset of arc damage is coincident with onset of breakdown and plasma

formation. Never was there a plasma evident without attendant unipolar

arcs. At low power density there was no other direct laser damage (like

melting) observed on the target. In fact, all damage was in the form of arc damage

.

Figures 10 and 11 show patterns of micro-arc craters produced on polished stainless steel by an unfocused medium power Q-switched laser

pulse. Arcing seems to follow the interference fringes of the laser output

as clearly visible near the edge of the laser illuminated area. Breakdown

and arcing occur in the regions of constructive interference. The expanding

plasma pushes the liquid surface layers to form a rim in the region of

destructive interference.

467
Figure 7. Arc damage on stainless steel,

defocused laser pulse of 25.5 MW/cm^ . Figure 8. Onset of arcing on stainless steel at 5.4 MW/crn^ . Same magnification as figure 7.

Figure 9. Further magnified view of figure 8. Onset of breakdown and arc formation at 5.4 MW/cm^

.

468
4.3 Comparison of Conducting and Non-Conducting Surfaces

Laser induced arcing on metal surfaces has been studied extensively.

All metals investigated, stainless steel, copper, titanium, aluminum,

molybdenum show similar arc damage, a cathode crater of about 1 ym diameter,

3-6 ym deep and an outer crater rim of 5-50 ym diameter. Evaporation of

material from the cathode crater and the return current flow through the

material are essential for the formation of a unipolar arc. A consequence

of our unipolar arc model is that reduced surface conductivity and high

melting point materials should influence the arc formation. The effect of

unipolar arcing for various types of materials has been studied to determine

whether material characteristics have any effect in reducing the arcing

damage. Titanium carbide coatings were deposited by an activated reactive

evaporation process on stainless steel surfaces. At 20 °C the electrical

conductivity of TiC is 1/15 the one of SS304, 1.4 x 10 5 (ohm-cm) _1 . The

melting temperature of TiC is 3410 K, roughly twice the value of SS304 at

1770 K. The large size characteristic unipolar arc craters for metals were not observed on laser illuminated TiC coatings [12]. Also, thermal energy

and momentum coupling to various target materials was investigated to determine what effect unipolar arcing had on the coupling process [13.]. Even

though the heat conduction coefficient of TiC is about the same as the one of SS304, a 10 ym TiC coating on SS304 reduced the thermal coupling to about

1/3 of the SS304 value. This leads to the conclusion that large scale unipolar arcing on metal surfaces increases the coupling of energy from the

laser heated plasma into the target.

A series of TiC-coated targets was irradiated at power densities

ranging from 300 to 1000 MW/cm^ and the targets were subsequently examined

for surface damage. Figure 12 pictures the edge of the damage area. There

are regions of apparent melting and resolidification in droplet form,

cracking and arcing. Individual arc craters are clearly visible on the

right hand side of the figure, in the transition zone to the undamaged area. This indicates again that unipolar arcing is the primary damage mechanism.

However, crater sizes are considerably smaller. They range from 0.1 to 0.3

micron diameter, with crater densities of approximately 4.5 x 10^ cm

- ^ in regions close to the melted material. Cracking occurs along a narrow

band of arc craters, figure 13, indicating that large stress formation is

associated with the cratering.

Silicon coatings of 1-2 micron thickness on SS304 and 2024 aluminum

were also studied [14]. Figure 14 is a SEM photograph of a 1 micron silicon

coating on polished aluminum prior to laser irradiation. This coating shows

a bumpy structure. After irradiation at the power density for onset of

plasma formation tiny spheres of various degrees of irregularity appear, but

the coating does not appear to have melted through. Figure 15 shows the

coating near the edge of the laser exposed region. Arc formation starts on the top of the bumps. The crater density is approximately 5 x 10^ cm

-

, and the crater diameter is about 0.1 micron, again, about 1/10 of the crater

size observed on metals.

5. DISCUSSION

The experimental results presented in this paper show that unipolar

arcing is the primary laser-plasma-surface interaction mechanism which leads

to a very non-uniform energy deposition and crater damage on the surface.

Since the scale lengths are very small, typically 1 ym for the cathode spot on metals and the electrons provide the current flow, unipolar arcing should

be considered as a fast evolving process with arcs forming within a nanosecond or less.

The initial absorption of laser radiation and plasma heating occur in

the underdense plasma close to the surface. The release of material from a cathode spot leads to locally increased absorption of laser radiation and

build-up of the plasma pressure until the critical density is reached. Heat

conduction by electrons and short wave length plasma radiation provide the

energy transport through the overdense plasma to the target surface. Many

laser-plasma instabilities have been discussed which may cause non- uniformities near the critical density region. No attention has been

469
Figure 10. Laser-produced arc pattern near edge of laser illuminated area. Arrows show onset of arcing in ring pattern.

Figure 11. Laser-produced micro-arc pattern on SS304, defocused laser.

470
Figure 12. Unipolar arc craters on TiC coated SS304 after irradiation at

300 MW/cm2 .

Figure 13. Cracked TiC coating on SS304 after irradiation with 1096

MW/ cm2

. The bottom part shows removal of a surface layer. Along the edges

of the removed TiC area, cracking follows a narrow band of arc craters.

471
Figure 14. One

irradiation.

micron silicon coating over polisher1 2024 aluminum before
given to arcing which leads to non-uniform ablation of the material and thus

also to a modulation of the critical density contour. The onset of arcing

on conducting and non-conducting surfaces will h>e discussed in the following

paragraphs

.

5.1 Formation of Low Density Breakdown Plasma

The initial breakdown phase may be characterized by the build up of a low density plasma layer in front of the surface with a density

neo <<n ec where n ec is the critical density. This initial plasma

layer may be formed by photo electrons and ionization of neutral particles

at a distance A /4 from the surface where for normal incidence the first

maximum in the electric field occurs due to the superposition of the

incident wave and the wave reflected on a metal surface. A sheath will form

immediately since the sheath potential Vf is independent of the plasma

density. The electron temperature might be of the order of 5 to 20eV.

Spatial variations of the laser intensity will produce density variations in

this initial plasma layer. The electron density can be estimated from the

laser produced arc pattern of figure 11. The ring width of about 30 micron

is much larger than the laser wave length. The existence of the pattern of

this size implies that the local sheath width should be smaller than 3 0 ym,

otherwise the pattern would be "washed out." This assumption X £><<30 yrn allows to estimate a lower limit of the plasma density. Assuming again an initial A D

= 10 -4 cm and TQ

= 10eV, we estimate neo > 5 x 10 14 cm-3 .

In accordance with eq (6) the sheath electric field increases with the

local build up of the plasma pressure as E a pl/2 . The dielectric

strength of air at atmospheric pressure is 30 kV/cm. If we take this as a typical value for onset of sparking we find from eq (6) that a plasma layer

of only n eo >_ 2 x 10^cm-3 and e = lOeV provides already a sheath electric field of E >^ 30 kV/cm to start spark formation at

atmospheric pressure. As discussed previously, any release of neutral

particles -From the surface leads to a rapid build up of the local plasma

pressure and this further increases the sheath electric field. Thermally

enhanced field emission might begin at about 10^ V/cm, that is at a plasma

density of about n e = 1.25 x lO-'-'cm-3 . Finally, thermionic electron emission from a cathode spot will lead to

the formation of an arc. The local build up of the neutral and plasma

density then also leads to non-uniform absorption of laser radiation.

5.2 Heating of a Surface Plasma Layer by Laser Radiation

Electrons change momentum through collisions with positive ions and

neutral particles. The frictional force acting on the electron fluid is

equal to the rate of change of momentum, and the equation of motion for a cold unmagnetized plasma becomes now

8V m

e = -eE - m V (v . + v ) e 3t e e ei eo

where v e i and v eo are the collision frequencies for momentum

transfer due to collisions with ions and neutral particles respectively. The

collision frequency in a fully ionized plasma depends on n-j_ and T e as

v . = 0.29 x 10~ 6 (n.Z 2£w A)/(T ) 3/2

ei l e where T e is measured in eV and n^ in cm-3

, Z is the ionic charge and

the Coulomb logarithm is of the order In A ~ 10 . Electron-neutral

collisions dominate for a weakly ionized plasma, v ei <<v eo ; anc^ veo depends on the neutral particle density nQ the collision cross section o Q

(a is of the order 5 x 10~"16 CP1 2 ) an(j t^e electron

thermal velocity

veo = n o

ao

y

473
Thus, assuming that v ej_ and v eo are independent of the fluid

velocity Ve , we find for an electromagnetic wave of frequency oj for Ve the solution

2 n e . 1 = en V = -S-7 ; t—;—— E = aE J e e m (v . + v + iw

e ei eo

where the conductivity

a = n e

2 f(v . + v ) - iw

e L ei eo J

e I ei

, 2 21

eo' J

The dispersion relation for transverse electromagnetic waves in a plasma can be written as c^ = £0j2/k2 wj_th the dielectric constant given by

e = 1 +

0

idle,

Thus the dispersion relation becomes

.22 2 k c = w

_ V _ He (Vei

+ V eo

}

(vei + V eo

)2 + »

2 u

[(vei + Veo

)

2

+ ^

where the plasma frequency is defined as ojp e

= (n e e2/me

e 0

) 1/2

For the assumed initially weakly ionized plasma layer of ne

= 5 x 10 14 cm J (which is many orders of magnitude below the critical density of

nec = 1021 cm"^ for Nd laser radiation) and a neutral density layer of

no

= 2.7 x 10l3 cm-3 (which corresponds to atmospheric density at 0°C and

760 Torr) , we find oj2 e << oj2 ( an<3 \> Q ^ << ~ 2 x 10^2 << to and eq (10)

simplifies to

2 2 o = L _ _pe _ Pe eo = ^2 = _ 2 =

2 _ 2 _ -2—2 = l - 3 = n = ^n ? R

- - nR

- n

z - .in^ (11)

/k i 0) / eo

The refractive index n2 = e is complex. Because of (v eo/co) << 1 the real

part of the refractive index becomes the familiar expression for an electromagnetic wave in a plasma

, / M 2/ M/2 nR

a ck/aj = (1 - wpe

/w )

The imaginary part of the complex wave vector k describes the damping of

wave amplitude due to collisions. Since the intensity I is proportional

to the square of the amplitude, we find that the absorption coefficient

in the equation

1 = Z o

e

becomes

a = -2 lm(k) = -2(a>/c) rij

474
From eq (11) also follows, -2 n R

nj = Im(e). Thus a can be written

a = (d)/cnR) Im(e)

The absorption length t = l/a becomes

ceo 2

CO V pe eo

3.3 x 10 n A v e eo

31 cm

(12)

where n e is measured in cm 3 and X in micron.

Since z/£<<1 the intensity absorbed by the plasma becomes

I -I s I (z/!)

o o

(13)

It is noteworthy from eq (12) and (13) that the absorption of laser

energy per electron in a weakly ionized surface layer of depth z is

independent of n e

2 2 Iz I ai v I X v o _ o pe eo o eo rTzl ~ 2 e n ceo e

(14)

3.3 x 10 31

Consequently, electron heating will occur for any initial electron density

present near the surface and it will increase with the desorption of surface

layers of absorbed gases (CC>2# H2O, etc.) because v

e o increases with

the build up of the neutral particle density. For atmospheric pressure

initially, the laser damage threshold is of the order 15j/cm2 for short

pulse tld radiation. From eq (14) we estimate then that an electron absorbs

5.6eV. Ionization will occur if this energy becomes thermalized between the

electrons. The existence of the sheath will enhance the electron-electron

collision rate since most of the electrons will be reflected in the sheath

and their velocity component V z goes through zero, thus increasing the

Coulomb collision cross section.

For a perfectly reflecting surface the time averaged value of a standing wave shows spatial intensity variations as

B

2 iE 2 sin 2 jap)

with maxima of 4 times increased intensity located at z = A/4,3A/4,... This

higher intensity and the release of neutral particles from the surface

should considerablv increase the electron heating in the A/4 region. Figure

16 shows schematically this influence of the neutral density gradient on the

preferential rapid electron heating, ionization and breakdown in a surface

layer, near z = A/4. The average speed of water molecules released from the

surface at 25°C is 592m/s . For A = 1.06 ym, the time for desorbed molecules

to travel a distance of A/4 is thus 0.4 ns. This time will be even shorter

as rapid surface heating will occur. This discussion indicates that

electrons can gain sufficient energy to ionize neutrals, thus further

increasing the electron density and consequently the absorption coefficient.

This feedback mechanism will lead to rapid ionization and build up of the

local plasma pressure until the arc ignites. Furthermore, laser energy will

be preferentially absorbed and less reflected where the arc burns since

relative large amounts of material will be ejected from the cathode crater.

475
atmospheric

Reflec ring

Surface

Laser

Distance Z

Figure 16. Absorption of short-pulse laser radiation near a reflecting

surface. The neutral density gradient in front of the surface is caused by

blow-off of surface material.

This contributes to a non-uniform laser energy deposition and plasma build

up above the cathode spot until the critical density is reached.

5.3 Unipolar Arcing on "Non-Conducting" Materials

It is apparent that laser-target interaction with its attendant

plasma- induced arcing is a dynamic process which must be described in terms

of constantly changing variables. Such variables include, but are not

limited to, nlasma temperature and density, laser power and pulse duration,

neutral particle density, absorbed gases, target material and surface

preparation, physical properties of the target, like thermal and electrical

conductivity and vapor pressure as function of surface temperature. The

observed small scale cratering on non-metallic materials indicates that

their surfaces exposed to a plasma require sufficient conductivity to provide the electron return current flow to the cathode spot. The modification

of the sheath potential due to the radial pressure gradient, eq (7) and (8)

provides an r-dependent electron flow from the plasma to the surface ring area 2TTrAr

i = 2-rrrArn (r)e(kT /2 7rm)-L/z exp -e[V (r)-V (r)]/kT (15) e e p w e

where Vp(r) is the plasma potential Vw

(r) the potential of the wall and

the sheath potential is V s (r) = Vp(r)-Vw

(r). The ion flow to the

surface is determined bv the number of ions entering the sheath, which

corresponds to the ion saturation current [15] and it is independent of the

476
.sheath potential

ig = 0.4 enj_(r) (2kT e

/Mj[ )2TTrAr

Differences between i~ and i + Since nj_(r) = n e

(r) it is the exponential term in eq (15) which controls

the net surface charge density. Electron and ion flows are equal when the

sheath potential has the value of the floating potential, Vs = Vf . For

Vs (r) < Vf we find i~>i+ . Since in accordance with eq (8) the plasma

potential decreases with increasing distance r from the the high plasma

pressure region above the cathode spot the surface areas 2TrrAr will be

charged more negative with increasing r. The r-dependent net surface charge

density causes then a surface radial electric field with the electron

emitting cathode spot being more positive than the negatively charged sur- rounding ring areas. Electron surface mobility and diffusion will determine

the electron-ion recombination rate on the surface and the electron return

current flow to the cathode spot.

The small size observed for craters on silicon and TiC in comparison to

the larger size cathode craters for metals may be qualitatively explained

by the higher resistivity encountered by the surface electron return current

which converges radially inward to the cathode spot. If ohmic heating is

assumed to be the primary source for vaporization of material from the

cathode spot, a smaller cathode spot diameter is required to achieve

sufficient current density. The power dissipated per crater volume V~m

is

P/Vm

= j 2 (r)/a

s where a s is the surface conductivity of the material. At the cathode spot we must have j(r) ~

j arc - Neglecting differences in arc current densities and binding energy, dimensional analysis indicates that the crater

size r D should scale with conductivity roughly as as

01 r Q

2 which is

in qualitative agreement with the experimental observation that lower con- ductivity material show smaller crater size. In the case of TiC the resistivity is almost two orders of magnitude larger than the one for aluminum.

The size of craters observed on TiC is 0.1 to 0.3 ym diameter while for

metals the cathode crater is typically 0.7 to 1.5 ym. The actual size would

depend also on the burn time of the arc. It is recommended that unipolar

arcing on various non-conducting materials be further investigated.

SUMMARY

Unipolar arcing has been shown to be the primary plasma- sur face

interaction process when a laser produced plasma is in contact with a surface. Evidence of unipolar arcing was found on all targets irradiated at

atmospheric pressure that also arced in vacuum, stainless steel, titanium,

molybdenum, copper, and aluminum.

Cratering was observed even for a defocused and low-power laser pulse.

The minimum laser power density required for the onset of breakdown on the

surface is also sufficient to cause arc damage. Never was there a plasma

evident without attendant unipolar arc craters. About 500,000 arc craters

per cm 2 have been observed on laser illuminated metal surfaces although

no external voltage is applied. Smaller size craters with a density of

about lO^/cm^ have been found on higher resistivity materials. The

higher resistivity requires the radially inward surface return current to

converge to a smal ler cathode spot size to achieve sufficient power density

to vaporize and ionize the material required for running the unipolar arc. The local increase of the plasma pressure above the cathode spot leads

to an electric field configuration which drives the arc current and also

facilitates the return current flow to the surface and cathode spot.

Unipolar arcing concentrates the available laser-plasma energy towards

the cathode spot. Large scale unipolar arcing on metal surfaces increases

the coupling of energy from the laser heated plasma into the target. The

ejection of a plasma jet from the cathode crater also causes highly

477
localized shock waves to propagate into the target, softening it in the

process. Thus, material erosion is much more severe than it would be the case for uniform energy deposition over a larger area. This research has wide spread applications. Any situation in which a sufficiently hot surface plasma exists there will be unipolar micro-arcing.

The physics relates to other forms of electrical breakdown on surfaces and

electrodes

.

ACKNOWLEDGEMENTS

This work was supported by the Naval Postgraduate School Foundation

Research Program and in part by the Naval Research Laboratory.

REFERENCES

[I] Robson, A. E. and Thonemann, P. C, Proc. Phys . Soc. 13_, 508(1959)

[2) McCracken, G. M., J. Nucl. Mater. 93/94 , 3(1980)

[3] Clausing, R. E., Emerson L. C. and Heatherly, L. E. , J. Nucl. Mater.

93/94 , 150(1980)

[4] McCracken, G. M. and Goodall, D. H. J., Nucl. Fusion 1_8, 537(1978)

[5] Cohen, S. A., Dylla, H. F., Rossnagel, S. M., Picraux, S. T. and

Magee, C. W. , J. Nucl. Mater. 76/77 , 459(1978)

[6] Staib, P. and Staudemaier, G. , J. Nucl. Mater. 76/77 , 78(1978)

[7] Schwirzke, F. , Oren, L., Talmadage, S., and Taylor, R. J., Phys. Rev.

Letters 40, 1181(1978)

[8] Oren, L . , Taylor, R. J., and Schwirzke, F. , J. Nucl. Mater. 7 6/77 , 412(1978)

[9] Schwirzke, F. and Taylor, R. J., J. Nucl. Mater. 9 3/94 , 780(1980)

[10] Gierke, G. V., Ott , W., and Schwirzke, F., Proc. Fifth International

Conference on Ionization Phenomena in Gases, Munich 1961, p. 1412

[II] Schwirzke, F., Beelby, M. H., and Ulrich, H. G. f "Basic Mechanisms that

Lead to Laser Target Damage", Naval Postgraduate School Report

NPS 61-82-002(1981)

[12] Schwirzke, F., Bunshah, R. F., and Taylor, R. J., Thin Solid Films

83(1981)117

[13] Methenv, P. M., An investigation of energy coupling in various arc susceptible and resistant conductors, M.S. Thesis, Naval Postgraduate

School, Monterey, California, June 1982

[14] Jenkins, W. F., and Schmidt, W. R., Thermal coupling and damage

mechanisms of 1.06 micron laser radiation and laser produced plasma

on selected materials, M.S. Thesis, Naval Postgraduate School,

December 1982

[15] Schwirzke, F., Phys. Fluids, 9, 2244(1966)

in response to questions, the author stated that his proposed damage mechanism did require

existence of a continuous low-density plasma over the surface prior to initiation of a unipolar

arc, but he believed that boil off of surface gases would provide the necessary plasma. He

believed the predamage plasma density might be so low that the plasma was not apparent by simple

visual inspection, and that his mechanism was consistent with the data of Kardach et. al. which was

also presented at this meeting.

478
APPENDIX - - PARTICIPANTS

Dr. Joseph A. Abate

Laboratory for Laser Energetics

University of Rochester

250 East River Road

Rochester, NY 14623

(716) 275-5181

Dr. M. A. Archarekar

ILS 3404 N. Orange Blossom Tr. Orlando, FL 32804

(305) 295-4010

N. Allen

Rutherford Appleton Laboratory

Chilton

Didcot

Oxon. 0X11 OQX

England

235-21900

Susan D. Allen

Center for Laser Studies

Univ. of Southern California

Los Angeles, CA 90089-1112

(213) 743-6705

Joseph Apfel

Optical Coating Laboratory

2789 Griffen Ave. Santa Rosa, CA 95402

(707) 545-6440

Tsuyoshi Asaeda

Canon U.S.A.

3191 Red Hill Ave. Costa Mesa, CA 92626

(714) 556-1036

John F. Asmus

Maxwell Laboratories

Univ. of California

8239 Sugarman Dr. La Jolla, CA 92037

(714) 279-5100

David S. Atlas

Eastman Kodak Company

D/177 Special Fabrication

901 Elmgrove Rd.

Rochester, NY 14650

(716) 588-5261

E. Douglas Baird (L-792)

Lawrence Livermore National

Laboratory

P.O. Box 808

Livermore, CA 94550

(415) 422-7633

Robert John Barton

Ministry of Defence

A.W.R.E.

Al dermaston

Reading, United Kingdom

William J. (Pat) Beauchamp

Optical Coating Lab

2789 North Point Parkway

Santa Rosa, CA 95401

(707) 525-7649

Michael F. Becker

Dept. of Electrical Engineering

The University of Texas at Austin

Engineering Science Bldg, Room 133

Austin, TX 78712

(512) 471-3628

Bernard Bendow

BDM Corp.

1801 Randolph

Albuquerque, NM 87106

(505) 848-5000

H. E. Bennett

Michel son Laboratory

Naval Weapons Center

Code 38101

China Lake, CA 93555

(714) 939-2869 or 939-2970

Jean Bennett

Michel son Laboratory

Naval Weapons Center

Code 38101

China Lake, CA 93555

Ralph R. Berggren

Avco Everett Research Lab., Inc. 2385 Revere Beach Parkway

Everett, MA 02149

(617) 381-4158

Lt. Col . Jerry R. Bettis

European Office of Aerospace R&D

(EOARD) Box 14 FPO New York, 09510

London, England (01) 409-4505

Autovon 235-4505

E. C. Borsare

W. J . Schafer Assoc.

10 Lakeside Office Park

Wakefield, MA 01880

(617) 246-0450

479
Richard P. Bossert

Laboratory for Laser Energetics

University of Rochester

250 E. River Rd.

Rochester, NY 14623

(716) 275-3418

Theodore W. Broberg

Honeywell, Inc. 2600 Ridgway Parkway MN17-1622

P.O. Box 312

Minneapolis, MN 55413

(612) 378-5974

Richard R. Brown

The Boeing Company

M.S. 8C-23

P.O. Box 3999

Seattle, WA 98124

Charles K. Carniglia

Optical Coating Laboratory

2789 Griffen Ave. Santa Rosa, CA 95402

(707) 545-6440

Michael Clancy

Rocketdyne Div. of

Rockwell International

6633 Canoga Ave., MS-FA40

Canoga Park, CA 91304

(213) 700-4908

Spencer D. Clapp

W. J. Schafer Assoc., Inc. 22222 Sherman Way, Suite 205

Canoga Park, CA 91303

(213) 704-1455

Garth Clarke

Westinghouse R&D

5180 Manont Road

Murrysville, PA 15668

(412) 256-7762

Joseph F. Connolly

CVD Incorporated

35 Industrial Parkway

Woburn, MA 01801

(617) 933-9243

Robert Culver

Aerojet Electro Systems

P.O. Box 296

MS Bldg. 160, Dept. 4344

Azuza, CA 91702

Frank Daly

Sundstrand Optical Technologies Inc. 2697 Lavery Court #8

Newbury Park, CA 91320

(805) 499-2636

Donald A. Dalton

UTRC/OATL

P.O. Drawer 4181

West Palm Beach, FL 33402

(305) 840-1052

Edward Daniel ewicz

Laser Power Optics Corp.

1211-V Sorrento Valley Rd.

San Diego, CA 92121

(619) 455-0751

Terry Deaton

Frank J. Seiler Research Lab

U.S. Air Force Academy

Colorado Springs, CO 80840

(303) 472-3502

Don Decker

Naval Weapons Center

China Lake, CA

(619) 939-3247

John A. Detrio

University of Dayton

Research Institute

300 College Park

Dayton, OH 45469

(513) 229-3527

T. M. Donovan

Naval Weapons Center

Code 3818 NWL China Lake, CA 93555

(619) 446-2115

John Dugan

ITEK

10 Mcquire Road

Lexington, MA 02178

Richard Dyer

W. J. Schafer Associates

10 Lakeside Office Park

Wakefield, MA 01880

617-246-0450

Dr. Osama H. El-Bayoumi

U.S. Air Force

RADC/ESM

Hanscom AFB, MA 01737

617-861-5832

Ahmet Elci

University of New Mexico

Albuquerque, NM 87131

David Emmony

Dept. of Physics

University of Technology

Loughborough Leicestershire LE11 3TU

United Kingdom

480
Albert Feldman

National Bureau of Standards

A257 Materials Bldg.

Washington, DC 20234

(301) 921-2817

Dennis G. Fischer

Coherent, Inc. 2301 Lindbergh St. Auburn, CA 95603

(916) 823-9550

Lawrence Fogdall

Boeing Aerospace

MS-2R-00, P. 0. Box 3999

Seattle, WA 98124

(206) 773-2455

Stephen R. Foltyn

Los Alamos National Laboratory

MS-J 566, CHM-5

Los Alamos, NM 87545

(515) 667-1123

Neil Forbes

Ferranti PLC

Dunsinane Ave. Iundee DD2 3PN

Scotland, United Kingdom

Venelsoa Foster

Optical Coating Laboratories, Inc. 2789 Northpoint Parkway

Santa Rosa, CA 95401

(707) 525-7652

Dougl as L . Franzen

724.02

National Bureau of Standards

325 Broadway

Boulder, CO 80303

(303) 497-3346

Daniel Friart

CEA Limeil

BP 27 94190 Villeneuve - St. Georges

France

E. Joseph Friebele

DARPA/DS0

1400 Wilson Blvd.

Arlington, VA 22209

(202) 694-1346

David Gallant

Rocketdyne Div.

Rockwell International

P.O. Box 5670 KAFB Albuquerque, NM 87185

(505) 846-0196

Robert L. Gallawa

724.02

National Bureau of Standards

325 Broadway

Boulder, CO 80303

(303) 497-3761

Victor Garni

z

Rocketdyne Div.

Rockwell International

P.O. Box 5670 KAFB Albuquerque, NM 87185

(505) 844-4691

Paul Gil strap

Physics Dept.

Colorado State Univ.

1799 S. College St. #17

Fort Collins, CO 80525

(303) 493-1425

Ann T. Glassman

Avionics Laboratory

AFWAL/AARI-2

Wright-Patterson AFB, OH 45433

(513) 255-5922/2698

Ms. Alison C.J. Glover

British Aerospace P.L.C.

Laser Systems Dept. F.P.C. 67 P.O. Box 5, Filton

Bristol BS12 79W

United Kingdom

(0272) 693831 xl012

William T. Goosey, Jr. D/177 Special Fabrication

Eastman Kodak Company

901 Elmgrove Rd.

Rochester, NY 14650

(716) 588-5261

George Graves

University of Dayton

300 College Park

Dayton, OH 45469

(513) 229-2517

David L. Gri scorn DARPA

Defense Sciences Office

1400 Wilson Blvd.

Arlington, VA 22209

(202) 694-1346

William Grossman

Mathematical Science M.W. 2755 Northup Way

Bellevue, WA

(206) 827-0460

Dr. Arthur H. Guenther

AFWL/ALO

Kirtland AFB, NM 87117

(505) 844-9856

481
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BIBLIOGRAPHIC DATA

SHEET (See instructions)

1. PUBLICATION OR

REPORT NO.

NBS SP 669

2. Performing Organ. Report No 3. Publication Date Januarv 1984

4. TITLE AND SUBTITLE

Laser Induced Damage in Optical Materials 1982

5. author(S) Harold E. Bennett (NWC), Arthur M. Guenther (AFWL)

David Milam (LLNL) and Brian E. Newnam (LANL)

6. PERFORMING ORGANIZATION (If joint or other than NBS. see instructions)

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

7. Contract/Grant No.

8. Type of Report & Period Covered

Final

9. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (Street. City, State, ZIP)

National Bureau of Standards (NBS) Defense Advanced Research Project

American Society for Testing and Materials (ASTM) Agency (DARPA)

Office of Naval Research (ONR) Air Force Office of Scientific

Department of Energy (DOE) Research (AFOSR)

10. SUPPLEMENTARY NOTES

Library of Congress Catalog Card Number: 83-600625

I | Document describes a computer program; SF-185, FlPS Software Summary, is attached.

11. ABSTRACT (A 200-word or less factual summary of most significant information. If document includes a si gnificant

bi bliography or literature survey, mention it here) The Fourteenth Annual Symposium on Optical Materials for High Power Lasers (Boulder

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

November 16-17, 1982. 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, the Office of Naval

Research, and the Air Force Office of Scientific Research. Approximately 200

scientists attended the Symposium, including representatives of the United Kingdom,

France, Japan, West Germany, and the USSR. The Symposium was divided into sessions

concerning Materials and Measurements, Mirrors and Surfaces. Thin Films and finally

Fundamental Mechanisms. As in previous years, the emphasis of the papers presented at

the Symposium was directed toward new frontiers and new developments, Particular

emphasis was given to materials for high power apparatus. The wavelength range of

prime interest was from 10.6 urn to the uv region. Highlights included surface characterization, thin film-substrate boundaries, and advances in fundamental laser-matter

threshold interactions and damage mechanisms. Harold E. Bennett of the Naval Weapons

Center, Arthur H. Guenther of the Air Force Weapons Laboratory, David Milam of the

Lawrence Livermore National Laboratory, and Brian E. Newnam of the Los Alamos National

Laboratory were co-chairmen of the Symposium. The Fifteenth Annual Symposium is

scheduled for November 14-16, 1983 at the National Bureau of Standards, Boulder, CO.

12. KEY WORDS (Six to twelve entries; alphabetical order; capitalize only proper names; and separate key words by semicolons)

Laser damage; laser interaction; optical fabrication; optical materials and properties;

thin film coatings.

13. AVAILABILITY

| x 1 Unl imited

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