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1/1 O NBS SPECIAL PUBLICATION 414

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

Laser Induced Damage

In Optical Materials:

1974
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APR : :i. 1375

'^57°^ Laser Induced Damage

'no.^\'* In Optical Materials: 1974 iw

Proceedings of a Symposium Sponsored by

Ofl&ce of Naval Research

The American Society for Testing and Materials

And by the National Bureau of Standards

May 22-23, 1974

NBS Boulder, Colorado 80302

Edited by , , ,

Alexander J. Glass

Lawrence Livermore Laboratory

^ Livermore, California 94550

and

Arthur H. Guenther

Air Force Weapons Laboratory

Kirkland AFB, New Mexico 87117

U.S. DEPARTMENT OF COMMERCE, Frederick B. Dent, Secrefary

NATIONAL BUREAU OF STANDARDS, Richard W. Roberts, D/recfor

Issued December 1974
Library of Congress Cataloging in Publication Data

Symposium on Laser Damase in Optical Materials. 6tli. Boulder,

Colo., 1974. Easer induced damage in oiilical materials, 1974.

(NBS Special Publication: 414)

Supt. of Docs. No.: C 13.10:414

1. Optical materials—Defects—Ciongresses. 2. Laser radiation —

Congresses, 'i. La-er materials—Defects—Congresses. J. Glass,

Alexander J., 1933- ed. 11. Guenther. Arthur Henry. 1931- ed. IIL United ."States. Office of Naval Research. IV. American Society

for Testing and Materials. V. United .States. National Bureau of Standards. VI. Title. VII. Series: United .'States. National Bureau

of Standards, .'special Publication: 414. OC100.U57 No. 414 [QC374] 389'.08s 1 621.36] 74-26935

National Bureau of Standards Special Publication 414

Nat. Bur. Stand. (U.S.). Spec. Publ. 414, 2.56 pages (Dec. 1974)

CODEN: XNBSAV

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WASHINGTON: 1974

For sale by the Superintendent of Documents. U.S. (Government Printing Office. Washington. D.C. 20402

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Stock Number 0303-01355
Foreword

These proceedings report in detail the formal papers presented at the 6th Annual Symposium on Laser

Damage in Optical Materials held at the National Bureau of Standards, Boulder, Colorado on May 22 and 23,

1974, This meeting was jointly sponsored by the Office of Naval Research, National Bureau of Standards,

and the American Society of Testing and Materials. The major topics covered were damage at dielectric

surfaces, damage due to self-focusing, damage to dielectric coatings, and (^amage to mirrors and windows

in the infrared, as well as theoretical and laser system studies and the reporting of fundamental properties important in the damage process. Furthermore, several papers dealing with improved diagnostic

techniques were heard.

Because of the growing importance and acceptance of machined components in high power laser systems,

a workshop on the machining of optics was held, under the coordination of Captain T. T. Saito of the Air

Force Weapons Laboratory. Nine papers on various facets of the topic were presented dealing with machining procedures, surface characterization of machined elements, coating of machined components and the

polishing and damage resistance of polished, coated, and bare machined metal reflectors. Abstracts of

these papers are included in the Symposium Proceedings proper.

The co-chairmen. Dr. Alexander J. Glass of Lawrence Livermore Laboratory, Livermore, California,

and Dr, A. H. Guenther of the Air Force Weapons Laboratory, Kirtland AFB, New Mexico, take full responsibility for the summary, conclusions, and recommendations contained in this report, as well as the

summaries of the discussions at the conclusion of each presentation. It is suggested that individuals

interested in the subject of this meeting obtain copies of those publications referenced in the bibliography contained in the summary and conclusions.

It is our intention to convene another symposium next year in Boulder during May to update and

document the state of the art of Laser Damage in Optical Materials at that time. This meeting will

cover the subject historically presented at these symposia with additional emphasis on thin film damage,

machined optics, the problem of the damage of materials and components at 10.6 ym, as well as address

ourselves to the subject of optical system reliability as influenced by laser applications. Hopefully,

reports relating to very short pulses, short wavelengths, and multipulse damage effects will be heard.

We wish to encourage the reader to contact us on matters pertinent to the intent of these conferences.

A. H. Guenther

ill
CONTENTS

Section Page

Foreword ili

A. H. Guenther

Summary and Conclusions vii

A. J. Glass & A. H. Guenther

0.1 Opening Remarks 1 A. J. Glass

DAMAGE FACTORS IN SYSTEM DESIGN

1.1 Laser Beam Divergence and Damage in Glass Amplifiers 2 K. A. Brueckner, B. Guscott, S. Jorna, K. Moncur, & L. Siebert

1.2 Nonlinear Propagation Studies 7 E. S. Bliss

1.3 Suppression of Parasitic Oscillation in Laser Rod Amplifiers 17

J. M. McMahon, R. P. Burns, & T. H. DeRieux

1.4 Optical Requirements for Laser Mirrors 23

H. E. Bennett & P. C. Archibald

1.5 Diffraction Theory of Absorbing Windows 31

M. Flannery & J. Marburger

EXPERIMENTAL DAMAGE RESULTS

2.1 Damage Resistance of Dielectric Reflectors for Picosecond Pulses 39

B. E. Newnam

2.2 Damage Studies at 1.06 pm with 100-200 ps Pulses 48

G. W. Leppelmeier & M. Finkelstein

2.3 Damage Thresholds in ZnSe, A/R Coated NaCl and Micromachined Mirrors by

10.6 ym Multijoule. Nanosecond Pulses 53

E, E. Stark, Jr. & W. H. Reichelt

2.4 Pulsed CO2 Laser Damage in Windows, Reflectors, and Coatings 59

V. Wang, J. E. Rudisill, C. R. Giuliano, M. Braunstein, & A. Braunstein

2.5 Pulsed CO2 Laser Damage Studies of RAP Grown KCl 66

S. D. Allen, M. Braunstein, C. Giuliano, & V. Wang

2.6 A Comparison of 10.6 pm Pulsed Laser Damage in Sputtered vs Electron Beam

Deposited Ge-Coated KCl 76

A. Golubovic, W. Ewing, J. Bruce, J. Comer, & D. Milam

2.7 10.6 ym Pulsed Laser Damage in ZnSe 85

H. Posen, J. Bruce, & D. Milam

2.8 Evaluation of Single Crystal LaCla As CO2 Laser Window Material (Abstract only) ... 93

F. Varsanyi & L. G. DeShazer

2.9 Laser Window Damage from CW 10.6-ym Radiation 94

J. S. Loomis & C. A. Huguley

2.10 10.6 Micrometer CW Laser Damage Studies of Metal Substrate Mirrors 103

T. T. Saito, G. B. Charlton, & J. S. Loomis

2.11 High Absorption Damage in Infrared Filters 113

W. S. Otaguro

SURFACE PREPARATION AND IMPROVEMENT

3.1 Q-Switched Laser Induced Surface Damage at 1.06 Microns 119

N. L. Boling, J. A. Ringlien, & G. Dube

3.2 Damage in Nonlinear Optical Materials at 1.06 ym: Surface Treatment of LiNbOa

in Ar-02 Plasmas and High Pressure O2 Environment 131

J. C. Potosky, C. R. Giuliano, & C. F. Klein

iv
3,3 Surface Characteristics Related to Laser Damage of Lithium Niobate and

Potassium Chloride Surfaces 135

J. 0. Porteus, E. A. Teppo, & J. H. Dancy

MATERIAL CHARACTERIZATION AND INSTRUMENTATION

4.1 Characterization of Infrared Laser Window Materials at the

National Bureau of Standards lAl

A. Feldman, I. Malitson, D. Horowitz, R. M. Waxier, & M. Dodge

4.2 Testing the Surface Quality of Laser Materials * 149

M. J. Soileau & H. E. Bennett

4.3 Statistical Characterization of Mirror and Window Surfaces 157

J. M. Bennett

4.4 Roughness Measurement by Light Scattering 163

J. C. Stover

DAMAGE DIAGNOSTICS

5.1 Laser-Damage-Mechanism Identification by the Measurement of Survival Times 169

D. Milam, R. A. Bradbury, R. H. Picard, & M. Bass

5.2 The Dynamics of Transmitted, Reflected, and Scattered Laser Pulses Above and

Below Damage Threshold: The Search for Precatastrophic Damage 179

C. R. Giuliano

5.3 Rutherford Backscattering Diagnostics of Laser-Irradiated GaAs 190

R. R. Hart, C. R. Giuliano, & H. L. Dunlap

5.4 Surface Defects on Crystals of TiOa and YVOi^ Studied by Laser-Induced

Damage Effects 193

K. M. Leung & L. G. DeShazer

THEORETICAL STUDIES

6.'' Pulsed CO2 Laser Window Damage Processes 200

R. A. Shatas, J. D. Stettler, L. M. Narducci, S. S. Mitra, & H. C. Meyer

6.2 Frequency Dependence of the Nonlinear Optical Susceptibility of Five Glasses .... 207

R. Hellwarth, J. Cherlow, & T. T. Yang

6.3 The Refractive Index Dependence of Pulsed Laser Induced Damage 214

J. R. Bettis, A. H. Guenther, & A. J, Glass

6.4 Extrinsic Absorption in Laser Window Materials 219

C. J. Duthler & M. Sparks

APPENDIX I

Abstracts from "Machining of Optics" Workshop

An Introduction to the Machining of Optics 227

T. T. Saito

The Influence of Tool Feed Rate on the Machinability of CO2 Laser Optics 227

J. B. Arnold, P. J. Steger, & T. T. Saito

Mirror Machining with Conventional Machines 227

W. Ranbauske

Effects of the Residual Surface Roughness of Diamond Turned Optics 228

E. L. Church & J. Zavada

Measurement of Surface Figure and Surface Finish at the Y-12 Plant 228

R. E. Sladky

Application of Electroplating to the Fabrication of Optics 228

F. B. Waldrop, R. C. Waldrop, M. J. Bezik, & W. E. Tewes

Ion Polishing of Metal Surfaces 229

R. A. Hoffman, W. J. Lange, & W. J. Choyke

Ion Milling for Surface Finish Improvement on Machined Metal Mirrors 229

J. Quella

v
Effects of Polishing on Diamond Knife Machined Mirrors

R. L. Gordon

APPENDIX II

Participants

vi
Laser Induced Damage in Optical Materials

6th ASTM Symposium

May 22-23, 1974

The Sixth ASTM-ONR-NBS Symposium on Laser Induced Damage in Optical Materials

was held at the National Bureau of Standards in Boulder, Colorado on May 22-23, of

this year. These Symposia are held as part of the activities in Subcommittee II on Lasers and Laser Materials, of the ASTM. Subcommittee II is charged with the

responsibilities of formulating standards and test procedures for laser materials,

components, and devices. The Chairman of Subcommittee II is Haynes Lee, of

Owens-Illinois, Inc. Co-chairmen for the Damage S)nnposia are Dr. Arthur Guenther,

Chief Scientist of the Air Force Weapons Laboratory, and Dr. Alexander J. Glass,

Head, Theoretical Studies, Y Division, Lawrence Livermore Laboratory.

Over 150 attendees at the Sjrmposium heard 31 papers on topics relating to laser induced damage in crystalline and non-linear optical materials, at dielectric

surfaces, and in thin film coatings as well as discussions of damage problems in

the infrared region due both to cw and pulsed irradiation. In addition, several

reports on the theoretical analysis of laser-materials interaction relative to the

damage process were given, along with tabulations of fundamental materials

properties of Importance in evaluation of optical material response to high power

laser radiation. Attention was given to high power laser system design considerations which relate to improved system performance and reliability when various

damage mechanisms are operable in such systems.

Because of the growing importance and acceptance of machined components in

high power laser systems, a workshop on the machining of optics was held, under

the coordination of Captain T. T. Saito of the Air Force Weapons Laboratory.

Nine papers on various facets of the topic were presented dealing with machining

procedures, surface characterization of machined elements, coating of machined

components, and the polishing and damage resistance of polished, coated, and

bare metal reflectors. Abstracts on these papers are included in the Symposium

Proceedings proper.

The proceedings of these S3rmposia represent the major sources of information

in the field of laser induced damage in optical materials. The Symposia themselves, along with the periodic meetings of Subcommittee II, provide a unique

forum for the exchange of information regarding laser materials specifications

among the manufacturers and users of laser devices, components, and systems.

The Symposia also serve as a mechanism of information gathering to enable the

Subcommittee to write informed and realistic specifications.

Key words: IR windows and mirrors; laser damage; laser materials;

self-focusing; thin films.

1. Principal Conclusions

The sixth annual Symposium on Damage in Laser Materials, sponsored by the American Society of

Testing and Materials (ASTM), in conjunction with the National Bureau of Standards (NBS), and the

Office of Naval Research (ONR) , was held at NBS in Boulder, Colorado on the 22nd and 23rd of May. Over

150 experts on laser materials, components and systems were in attendance. The participants were drawn

from government, industrial, and university laboratories. The attendees included several foreign representatives from organizations such as the Laboratoire Nationale D'Essais, in Paris, France; Commissariat

d'Energie Atomique, in Limeil, France; CRDV, in Quebec, Canada; and ITT Europe, in Harlow, England.

In conjunction with the ASTM Symposium, the National Bureau of Standards played host to three other

meetings on related subjects, a review of ARPA programs on laser materials, a day-long workshop on Optical Surface and Coating Science, sponsored jointly by the ASTM and ARPA, and an evening workshop on Machining of Optics, organized by the Air Force Weapons Laboratory. New techniques of surface preparation, especially bowl feed polishing, ion polishing, and diamond turning, as well as methods for the

quantitative characterization of super-smooth surfaces, were the principal topics of discussion at these

ancillary meetings.

Of the thirty-one papers presented at the symposium, more than one-third gave experimental results

on damage measurements. These included measurements of thresholds in bulk materials, at dielectric

surfaces, at metal surfaces, in nonlinear optics crystals, and in multilayer dielectric films. These

measurements, for the most part, were carried out with well-characterized lasers, on carefully prepared

surfaces, and thus represent information of wide applicability.
The remaining papers dealt with improved methods of surface preparation, characterization of

materials, instrumentation, damage diagnostic techniques, basic damage phenomena, and system design.

The emphasis, throughout the symposium, was on the improvement of damage thresholds, particularly at surfaces and in thin films, due to improved techniques of preparation and fabrication.

In the design of large laser systems, such as those currently under construction, damage

considerations impose severe limits on system design. The improved understanding of index nonlinearity

in transparent dielectrics, its origin, magnitude, and consequences, all confirmed by agreement between

experiment and theory, makes it possible to design large glass systems for optimal operation, avoiding

beam degradation and damage due to self-focusing effects. Correspondingly, in CW systems, the understanding of thermally induced distortion, and availability of reliable material parameters, makes it possible

to design such systems to operate safely within the limits imposed by thermal distortion. The optimal

design is sensitive not only to the material parameters of the system, but to the control of the beam

profile in space and time as well.

Current short pulse systems are limited by surface damage at coated elements to operating fluxes of

a few J/cm^. There still remains considerable room for improvement in thin film damage levels since

the observed thresholds are far below intrinsic limits. This indicates that damage in coatings is still

dominated by defects and impurities. The same coatings, obtained from different manufacturers, exhibit

different damage thresholds, indicating a sensitivity of the surface condition to the fabrication

process. Thin film coatings are very complicated structures, requiring full characterization in terms

of materials, residual stress, adherence to the substrate, morphology (crystalline or amorphous), and,

of course, purity and freedom from defects. Current indications are that sputtered films exhibit better

damage resistance than vacuum-deposited films. The exact cause of this difference, and its susceptibility to improvement by refined techniques, is still uncertain.

As far as surface preparation is concerned, new etching techniques continue to be advanced. The

surface properties of both infrared materials and glass can be improved by use of appropriate chemical

etch, to obtain thresholds close to the intrinsic bulk level. Super-polishing and sputter-cleaning of

surfaces also improve their damage properties, by removal of defects and polishing residue.

The new interest in metal mirrors arises in part from the use of diamond turning in surface

preparation. This technique tends to leave a very clean surface, free of defects, debris, and contamination, which is resistant to both damage and deterioration.

In order to evaluate improved methods of surface preparation, better surface diagnostics have been

developed. In particular, increasing use has been made of surface scattering measurements, to obtain

measures of rms surface roughness down to ten Angstrons. Additionally, selective surface probes,

such as Auger spectroscopy, low energy electron diffraction, and scanning electron microscopy give

reliable information on surface composition, contamination, and structure. A strong correlation between

surface scattering and damage threshold has been established, as has the appearance of increased

scattering after damage.

It is becoming possible to design materials for high damage resistance. The index nonlinearity has

been related to the refractive index value and dispersion, and to the Raman spectrum of the materials

so that a capability exists to predict the value of na from a knowledge of the index dispersion and

Raman scattering. For a broad class of materials, an empirical relation exists correlating higher

refractive index with lower surface damage threshold.

Compositional purity is also a factor in determining damage levels. In alkali halides, for example,

extremely high damage levels have been obtained in materials grown by reactive atmospheric processing

(RAP) . These improved values indicate that earlier work on avalanche breakdown levels in alkali halides

as a function of frequency may not have tested the intrinsic property of the materials, but may have

been influenced by impurities. Specific remedies have evolved for certain materials. Thus, in LiNb03»

oxygen enrichment of the surface leads to a higher damage threshold, since it reduces surface absorption.

In general, purity of materials, care in preparation, and careful removal of chemical residues and

defect structures are all essential to approach the intrinsic damage limit in any material.

Care must be exercised in extrapolating damage values from small area test results to large area applications. Optical damage always finds the weakest spot on the surface, so that the average properties

of the sample may not accurately reflect the operational value of damage resistance.

In conclusion, there have been significant advances in both understanding of damage processes and in

the measurement of relevant parameters in the past year. Intrinsic processes are very well understood.

Extrinsic processes are hard to characterize, and the emphasis is now placed on the use of careful

techniques for material preparation, to reduce the presence of extrinsic influences on material damage.

2. Summary of Papers

The papers presented at the 1974 Symposium have been grouped under six headings: Damage Factors in

System Design; Experimental Damage Results; Surface Preparation and Improvement; Material Characterization

and Instrumentation; Damage Diagnostics, and Theoretical Studies. One-third of the papers presented fall

under the heading of Experimental Damage Results. This group includes information on new materials and on composite elements, over a range of conditions from picosecond pulses to CW. As in previous years, the

purpose of these summaries is to provide the reader with a concise overview of the conference papers,

and to direct his attention to those papers of greatest interest to him.

viii
2.1 Damage Factors in System Design

The degradation of system performance, and the limitations imposed by damage phenomena are overriding

considerations in the design of large laser systems. It is now well established that the two phenomena

which limit the design of large glass systems for laser fusion are small-scale self-focusing, and

parasitic oscillation. Techniques which have been developed to reduce the effect of both phenomena were

described at the S3nnposium.

Kent Moncur of KMSF presented a paper in which he discussed the consequences of the index nonlinearity

in glass lasers, and techniques whereby the beam profile in large glass systems can be controlled. He made

particular reference to the KMS laser system, which operates at 5 to 10 GW/cm^. He concluded that, at this power level, self-phase modulation was inconsequential, but that both large-scale beam distortion

(nonlinear lensing) and small-scale self-focusing (beam break-up) posed serious design problems. To avoid

the development of self-focusing, he advocated the use of apodizing techniques (soft aperatures) , and

described several apodizer designs for high power use. In particular, dielectric coated glass apodizers

withstand power levels of 5J/cm^, but can only be fabricated with relatively small gradients in transmission. Other apodizer techniques were also discussed. Without the use of apodization, as much as 50%

of the beam power can be scattered into a halo, which is of small angular divergence, but outside the

desired beam angle. The development of small scale self-focusing can, of course, lead to physical damage

in the glass itself.

Brian Bliss, of Lawrence Livermore Laboratory, reported on studies of self-focusing effects in both

rod and disk amplifiers. Like Moncur, he analyzed the problems of whole-beam lensing and small-scale,

self-focusing separately. To observe lensing, he passed a high intensity beam through an umpumped glass

rod, and passed a reference beam from the same source along a parallel path, photographing both beam

profiles at a distance of several meters. The lensing effect was quite apparent. A more detailed

observation of self-focusing was provided by use of a Mach-Zehnder interferometer, in which the intensity

dependent refractive index change was observed directly, as a function of time. Analyses of these

observations yielded values of na for laser glass and YAG in good agreement with results obtained from

ellipse rotation measurements.

By use of a shear plate interferometer, a phase and amplitude perturbation of known modulation depth

and spatial frequency can be generated on an intense beam. The growth of this perturbation can then be

observed, by passing the modulated beam through a nonlinear medium, in this case laser glass. By this

technique. Bliss was able to test the predictions of both the linearized theory of small-scale selffocusing, and the full, nonlinear computation . Agreement was excellent between experimental results and

the detailed numerical calculations, and it was found that the linearized theory, discussed by several

speakers at this symposium in 1973, provided an accurate description of the modulation growth, even when

significant deviation of the fringe shape from sinusoidal was seen.

In disk lasers, the presence of intermittent regions of linear material allows diffraction to reduce

the growth of instabilities at high spatial frequencies. For the same power level, the small-scale selffocusing in a disk amplifier and rod amplifier were compared. As expected, the structure which developed

in the disk system exhibited lower spatial frequencies than seen in the same length of glass in a rod

system.

At the Naval Research Laboratory, John McMahon has been developing techniques for the suppression of

parasitics in flash lamp pumped, Nd: glass, rod amplifiers. He identified four kinds of parasitics, as follows. A circumferential mode can build up due to reflection around the interface between the rod and

the cooling water (Mode #1) , or around the water jacket outside the rod, but with gain paths through the

rod (Mode #2). Similarly, longitudinal modes can build up, due to reflections at the end of the rod,

either due to internal reflection at the rod surface (Mode #3) or at the water jacket surface (Mode #4).

Modes 2 and 4 can be suppressed by the introduction of samarium salts, which absorb at 1.06 ym, into the

cooling water. Modes 1 and 3, which develop due to reflection at the rod-interface, are suppressed by

index matching the cooling liquid to the rod. The index matched coolant must be chemically stable, even when exposed to flash lamp irradiation, and transparent to the pump radiation. A solution of Zinc

Chloride (for index matching). Samarium Chloride (for 1.05 ]im absorption) and HCI (for chemical stabilization) was found to be satisfactory for this purpose. The end-reflected mode could be further

suppressed by anti- reflection coating the end of the rod, but this technique may not be directly applicable to long amplifier chains where feedback can occur from other sources. Using parasitic suppression, it looks as if gain-diameter products of aD = 0.6 are achievable.

Without stabilization, a value of aD = 0.2 is marginally unstable. Parasitic suppression allows the use

of higher gains, and therefore reduces the length of glass in the system. Thus higher power densities

can be propagated without small-scale self-focusing. McMahon proposed that stabilized rod amplifiers are more attractive than disk amplifiers up to 100 mm diameter apertures.

For CW or repetitively pulsed systems, thermal distortion and passive distortion must also be

considered. In multi-element systems, the requirements on the surface figure and bulk homogeneity of

each element must be developed in terms of the total system performance.

A most important paper by H. E. Bennett and P. C. Archibald of the Naval Weapons Center considered

the requirements for high power laser mirrors and components, not from an individual component standpoint,

but rather from consideration of total system performance and intended operational functions. These

total system considerations place very severe requirements on individual components to avoid disastrous

ix
cuinmulative effects and minimize the need for active wavefront correction elements. These severe requirements are a) maintenance of excellent figure under thermal load, b) high damage threshold and

absence of damage initiating sites and finally c) low scatter. Marechal's criterion related the cumu- lative phase distortion to the possible peak on-axis energy at the focal point. For a 10 component

system at 3.8 ym, these conditions demand that each component have a figure of A/8 in the visible, and

thermal distortion < 1000 A, with high and uniform reflectivity over the total aperature. The interaction

of various factors contributing to system performance was evaluated so that various system design and

tolerance specifications could be made, e.g., initial and thermally induced distortion. For the case presented above, the sources of wavefront distortion resulting from 1) non-uniformity in the active

laser medium, 2) distortion from figure errors, 3) refractive index variations and 4) atmospheric non- uniformities were made. It was shown that in many cases there was a need for apodization or active

wavefront correction to maintain a high on-axis intensity at the focus of the system. The general con- clusion of this paper, based on achieving high on-axis intensity, quantifies the requirements for

improved state-of-the-art polishing and figuring procedures, and reduction of thermally induced distortion particularly from coating absorption and microroughness. Additionally, the mirror elements should

possess high catastrophic damage resistance with minimum backscatter. It was pointed out that investigation of short wavelength scattering is a powerful non-destructive tool in addressing many important

properties which ultimately influence the total system performance.

In a weakly absorbing window material, especially at 10.6 ym, the effects of thermal lensing and

birefringence will introduce depolarization and aberration into the transmitted beam. M. Flannery and

J. Marburger, of the University of Southern California, have analyzed this problem with an aberration

matrix formulation, and developed a convenient description of the ensuing beam distortion in terms of

two characteristic parameters, which are tabulated for 16 common IR window materials.

The significance of these parameters is as follows. The authors consider a plane-polarized Gaussian

equiphase beam on the window, with the electric field distribution given by = x exp (-r^/w^). The

thermal lensing which such a beam introduces is characterized by a dimensionless time t, which is the

ratio of the Gaussian diffraction length, kw^/4, to the instantaneous effective focal length of the

induced thermal lens at the center of the beam. Clearly, the ratio is proportional to the energy density

deposited in the window material. The spherical aberration introduced in the beam is given simply by

Zt/w"*. Serious aberration is expected to occur when t ~ 7t/4, and at a value of t » 3, the on-axis

Intensity is reduced to one-half its unperturbed value. The authors tabulate the ratio t/£ I^(t')dt',

(where 9 is the window thickness), which is a number characteristic of window materials.

It ranges in value from -1.49 mm/Joule for KBr, to 1.95 x lO^mm/ Joule for Irtran-6. Thus, one would

expect the same thermal lensing effect to occur for Irtran-6 at 10~^ times the transmitted energy, in a beam of the same radius, as in KBr.

The effect of birefringence is characterized by a parameter a, which is also characteristic for a given

material. The effect of birefringence is to Introduce oscillations in the intensity, in the aperture

plane. In the focal plane, this gives rise to an increased beam divergence, and less on-axis intensity.

Significant depolarization effects are seen for values of a ~ 2. For all the tabulated materials, the

authors find a « 1, except for KCl, in which case a = -1.344, and KBr, for which o = -0.598. This paper

reduces a difficult and complex problem to simple form, and provides a useful basis of comparison of

window materials, both for the materials tabulated in the paper, and whatever new materials may be

developed.

2.2 Experimental Damage Results

The main purpose of the ASTM Symposium is to generate understanding of damage phenomena in laser

materials, and to translate this understanding into improved material and system performance. The

measurement and tabulation of relevant material parameters, especially damage thresholds, is an integral

part of this process. In one-third of the papers presented at this Symposium, damage measurement results were reported. In such measurements, the characterization of the laser output, in terms of wavelength,

pulse duration, peak intensity, as well as careful characterization of the material tested, in terms of

purity, homogeneity, defect and impurity levels, are essential. Damage investigators seek corre- lation between observed damage levels and materials properties; especially properties that can be

characterized in passive, non-destructive measurements. The measured damage values must be interpreted

as being valid only under the conditions of test, but provide a basis of comparison of materials, when

comparative measurements are carried out on several materials by the same investigator.

The first paper, on damage resistance of dielectric coatings subjected to picosecond pulses, was presented by B. Newnam of the Los Alamos Scientific Laboratory. Employing a single 30 psec, 1.06 ym

pulse from a TEMqq YAG laser he exposed two types of reflective coatings from several experienced com- mercial manufacturers over a 0.74 mm radius spot size using a 2 m focal length lens. This state-of-theart survey was accomplished by suppling seven coating companies with common, low-scatter superpolished

BK-7 glass substrates fabricated by a separate optical finishing company. All films were e-beam

deposited in a G(HL)^HL^A (H,L = configuration using TiOa and SiOa and, as additional optional coating

materials, ZrOa and SiOa. Single* shot thresholds were determined both by spark threshold using a filtered photo-multiplier tube, and by changes in laser-induced scatter (LIS) observed both visually and

photometrically. Measurements on samples from different manufacturers yielded a range in damage

resistance of 1 to 4 J/cm^ for the TiOa/SiOa coatings and 0.5 to 2 j/cm^ for the Zr02/Si02 coatings.

From all manufacturers where both types of coatings were supplied, this reduced damage resistance for the

Zr02/Si02 samples was indicated. It was concluded that the variance in results for a specific design

X
from each of several suppliers was related to the coating procedures and starting material purity

employed. There was a general but not strong relation between initial scatter and damage resistance.

Reflectors which exhibited a large amount of diffuse weak signal scatter generally exhibited lower thresholds. It is worth mentioning that based upon several intrinsic physical properties, the ZrOa/SiOa films

should have been more damage resistant that the observed result, in which they were only one-half as damage resistant as the Ti02/Si02 coatings. Consideration of energy requirements for melting and calculated linear absorption coefficients suggest the order Zr02 > Ti02 in damage threshold. One must

therefore look for other possible processes such as multiphoton absorption (not very probable) , or film

defects. This latter cause is more likely the culprit as may be suggested from the observed inverse

relation of scattering and damage resistance.

A closely related study by G. W. Leppelmeier and M. Finkelstein of the Lawrence Livermore Laboratory

concerned the investigation of a variety of coated and uncoated elements from different manufacturers.

The damage threshold of multi-layer dielectric films used as polarizing beamsplitters, mirrors and antireflection coating, deuterated KDP , several high-Verdet glasses and micro-machined silver was determined.

The IJ, 125 ± 25 psec laser at 1.06 ym used in this study exhibited less than 4% spatial and temporal

ripple. A main concern in this work was not only assessment of the damage sensitivity of the abovecommercial components, but evaluation of the availability of large area, defect free elements which

yielded high damage thresholds and did not exhibit low level non-linear beam distortion. Specific film

designs were not dictated by the authors, but it was found that the high index layers generally contained

Zr or Ti sometimes with K and Ce added. It was assumed that the lower index material was Si02. The

multi-layer dielectric-coated BK-7 polarizers were normally constructed in 20 to 30 layers and tested at 56.5 ± 0.2°. Samples from seven manufacturers resisted damage from 1.8 to 5.8 J/cm^ and damaged at levels

from 1.5 to 6.7 J/cm^. Seven companies also supplied their best anti -reflection coatings on LLL provided

EY-1 substrates. Damage resistance was from 2 to 5.4 J/cm^ and damage was produced from 2.8 to 6.2 J/cm^.

The damage hierarchy for these two types of coatings was not the same from different suppliers. In the

tests on mirrors, micro-machined silver damaged at levels considerably lower than coated glass reflectors,

by a factor ranging from 1.5 to 3X. Results were also reported on three Faraday rotator glasses Hoya

FR-4, FRN-5 and Owens Illinois EY-1 and several potassium dihydrogen phosphates crystals with varying

degrees of deuteration. All Faraday rotator glasses were quite tough, resisting damage from 4.0 to 7.2

J/cm^. All KDP samples resisted damage from 2.5 to 30 J/cm^. In conclusion, the goal of achieving

reliable optical elements which do not damage at levels below 2.0 j/cm^ for subnanosecond exposures was largely achieved. However, it was stressed that for nanosecond and sub-nanosecond work there is a necessity for beams which are very clean, spatially and temporally, in order to avoid self-focusing and

prevent local maxima in energy density. It was quite clear here, as in Newnam's work, that damage

sensitivity is process-dependent, and that processing techniques to fabricate large aperture elements

free of defects, which will be uniform and damage-resistant, remain to be developed.

With their obvious interest in high power CO2 systems for laser fusion applications, the Los Alamos

Scientific Laboratory is pursuing damage assessments of numerous optical elements subjected to nanosecond

duration pulses. At this meeting, E. Stark, Jr. and W. Reichelt reported recent results on ZnSe, ARcoated NaCl and micromachined mirrors subjected to these short duration exposures. For chemical vapor

deposited (CVD) ZnSe with many voids and inclusions, damage to uncoated samples occurred in the bulk at about 1.5 J/cm^ . For AR-coated ZnSe, from two manufacturers, results indicated damage levels of

2.5 J/cm^ for coatings with reduced defect concentration, while for other samples, containing more

irregular and ragged defects, the damage level was approximately 1.7 j/cm^. NaCl which was AR-coated

with a X/4 layer of NaF damaged at 2.0 J/cm^, while the uncoated NaCl exhibited damage at levels of 3.0

to 4.5 J/cm^ depending on the quality of surface finish. Finally, micro-machined OFHC copper mirror

substrates electroplated with 0.013 cm of gold or copper were fabricated. These mirrors and coatings,

after further micro-machining at the Y-12 facility at Oak Ridge, Tennessee yielded damage levels in excess of 4 J/"cm^<, This was attributed to the removal of defects and polishing debris from the

surface.

A comprehensive study of pulsed CO2 laser damage in infrared windows, reflectors and coatings was

reported by V. Wang, J. E. Rudisill, C. R. Giuliano, M. Braunstein, and A. Braunstein of the Hughes

Research Laboratories. Their results were obtained using a 600 nsec, TEMqo 10.6 ym CO2 laser, described

at previous meetings of this S5nnposium series. An interesting result was that while a reduction in void

and inclusion density in ZnSe led to considerably less measurable scatter, there was only a relatively

smaller increase in damage threshold under these pulsed exposures. Specifically, in the more imperfect

ZnSe window substrates which exhibited an absorption coefficient of 0.0030 cm~^, damage was obtained at

30 to 40 j/cm^ . For the substrates which showed a scattering level only one-tenth as great, and an absorption coefficient of 0.0021 cm~^, damage was observed at 55 J/cm^. However, in the former case, damage was always characterized by inclusion damage, while for the more damage resistant sample damage

was either inclusion-initiated or localized surface breakdown. In tests of thin films deposited on either RAP grown KCl or ZnSe substrates, significant correlation between absorption and damage threshold

was achieved. For example, A/4 wave films of ThFi«, AS2S3 and ZnSe on KCl of absorption of 10 cm"',

<|cm~' and < 0.6 cm"' respectively, damaged at levels of 80 J/cm^, 120 J/cm^ and 140 J/cm^. However, an anti-reflection coating of ZnSe/ThFi, on KCl with reflectivity < 0.2% per surface damaged at a threshold

of 60 J/cm^. In tests of ThFi^/ZnS AR coatings on ZnSe, damage was limited by the substrate to 25 to 40 J/cm^. Similar damage determinations on coated and bare metal reflectors were also obtained.

(ThFi,/ZnSe)^ on Mo, with an absorption coefficient of 0.22 cm"', damaged at 50 J/cm^ while for

(ThFi,/ZnSe) ^ on Mo, with an increased absorption coefficient of 0.32" , the damage threshold was reduced

to 30 J/cm^. By comparison, an Ag/ (ThFit/ZnSe) ^ coating on Al/E-Ni exhibited an absorption coefficient

xi
of 0.2 cm~^ and a damage level of 270 J/cm^. In the first two cases surface conditioning was evident.

The last set of experiments concerned bare metal reflectors. For three samples of conventional polished

Cu, Mo sputtered on Mo, and polished and etched OHFC Cu, damage was noticed at 90-150 J/cm^, 4-8 J/cm^

and 750 J/cm^ respectively. Obviously in the case of the two Cu samples there was a considerably lower

concentration of impurities or surface defects in the etched sample. Here again the results seem to point to inclusions of surface contaminants and surface defects as important factors limiting the damage

resistance of this class of materials.

Laser induced damage studies of KCl , grown by reactive atmosphere processing (RAP), was reported by

S. D. Allen, M. Braunstein, C. Guiliano, and V. Wang all of the Hughes Research Laboratories. The same high power pulsed CO2 laser used in the previous paper was employed. This investigation centered on a comparison of the damage resistivity of variously processed bulk and surface components made of high

grade KCl. Each sample was examined in great detail by Auger, LEED, and SEM techniques, in concert with

evaluation of the absorption characteristics of the individual samples. Samples of both single crystal

and press forged RAP grown KCl were compared with commercially available KCl after both conventional and chemical etch surface processing. For the press forged samples fabricated at 250°C at a rate

of 0.05 inch per minute per inch of remaining length, the resultant grain size averaged 5 to 10 ym in

diameter.

Extending the work of J. W. Davisson of the Naval Research Labs, a chemical treatment of immersion in

concentrated HCl for one to two minutes was rapidly followed by an isopropyl alcohol rinse. Finally, the

residual alcohol and acid were removed by the vapor from a freon TF degreaser. In all cases studied the

absorption of the conventionally polished materials (initial values of 0.00045 to 0.002 cm~^) was reduced considerably (to 0.00013 to 0.0014 cm"'') depending on sample type, source, and fabrication process.

Furthermore, the chemically etched samples were less subject to environmental factors which induce fogging.

Similar improvements in Auger scans and LEED patterns were evident, indicating reduced contamination

density and greater crystalline integrity. It was noted that further surface cleanliness could be

achieved by Argon ion sputtering.

As was mentioned elsewhere in these proceedings, low surface scattering is an important property of

optical elements in high power laser systems, and again the chemical etching process produced measurable

reductions in scattering. Normalizing the scattering from mechanically polished single crystal KCl to 1.0, the chemically etched single crystal sample exhibited a scattering level of 0.45, while for press

forged samples, scattering was reduced from 2.3 to 0.72 when the conventionally polished samples were

further treated by an etch. However, the most important improvement was in surface damage threshold,

where etched single crystal surfaces damaged at from 330 to 2 700 J/cm^ compared to values of 70 to 350 J/cm^ for conventional polished RAP grown KCl. Etched, press-forged samples damaged at levels of

1500 to 1800 J/cm^. For bulk damage in RAP grown samples of both single crystal and press forged fabrication, levels in excess of 4600 J/cm^ was achieved. When these values are compared to other available

data on the laser induced and DC breakdown strength of KCl and NaCl, it was concluded that the electron

avalanche mechanism is primarily impurity mediated and furthermore that future improvements in optical

strength can be made evident through improved process control and higher purity of starting materials.

Many proposals have been made to assess the desirability of one coating process versus another. In

a study to answer this question for the specific case of germanium coated KCl elements subjected to 75

nanosecond pulsed CO2 lasers, A. Golubovic, W. Ewing, J. Bruce, J. Comer, and D. Milam compared both

sputtered and electron beam deposited coatings at the Air Force Cambridge Research Laboratory. This

specific combination of coating material and substrate is important because KCl is hydroscopic, and thus

subject to environmental degradation, as well as being easily marred. Germanium's high refractive index

suggests it suitability as the first layer in a multi-layer AR-coating which can act as a protective

covering as well. The substrates used were reactive atmosphere processed (RAP) and Czochralski grown

KCl of [100] and [111] crystal orientation. All substrates were carefully polished, cleaned, chemically

etched and finally sputter-etched with Ar. The sputtered films were deposited at a rate of l^A/sec at

20 X 10"^ Torr while the substrate was held at 75 °C. The e-beam coating was deposited at 20 A/sec with

the initial pressure of 5 x 10~® Torr increasing to 2 x 10~^ Torr during deposition of the 1 ym thick

coating. Applying the "standard" scotch tape adhesive test both films exhibited weak adherence with the

sputtered film slightly stronger. Auger analysis indicated that Ge X KCl diffusion was more complete on

this sputtered sample. Damage tests indicated a range of thresholds of 0.96 to 1.18 J/cm^ for the

sputtered films, and 0.53 to 0.82 J/cm^ for the e-beam coated samples. There was no correlation with

crystal structure or growth procedures.

In an attempt to study the effects of intergranular boundaries devoid of defect material on the

damage sensitivity of CVD grown ZnSe subjected to pulsed CO2 laser irradiation, H. Posen, J. Bruce and

D. Milam of the Air Force Cambridge Research Labs investigated the morphology and damage threshold of

large grain and highly twinned samples. ZnSe was chosen because of its possible use as a high power

window and coating material. This low bandgap material exhibits low absorption and high yield strength.

All samples tested were sputter-cleaned to remove any carbon introduced in the growth technique. The

polycrystalline ZnSe exhibited an absorption coefficient of 3 = 0.0030 ± 0.0004 cm~^ while the highly

twinned sample had a 6 = 0.0070 ± 0.002 cm"'. Twinning serves to strengthen material since it serves

to form barriers to dislocation propagation. Generally ZnSe is available only as a polycrystalline

aggregate, showing large grain misorientations with highly rectilinear intergranular bands (twins). In addition to classifying various crystal regions as to depth of twinning, refractive index variations and

purity, they were unable to identify any major difference in damage threshold in this II-VI material

correlated with boundary orientation of structure. There was some indication that the morphology of

xii
damage was related to the orientation of the twin relative to the surface. Failure usually occured along

the [110] cleavage plane. The damage sensitivity observations of this study should not be considered

conclusive because of the difficulty of exposing only one type of crystal structure on the surface with

a focal spot size which was large compared to the specific grain boundary locale.

In their continuing program on evaluation of new laser component materials, F. Varsanji and

L. G. Deshazer of the University of Southern California reported their preliminary measurements on LaCla.

This anisotropic crystalline substance has been proposed for use as a 10.6 ym window material, it being

transparent from the visable to 25 ym. Their rather elaborate program of mechanical and optical property

evaluation may be sufficiently complete for a more detailed presentation at a future date.

A paper dealing with CW 10.6 ym damage was given by J. S. Loomis and C. A. Huguley. The

materials studied were coated and uncoated windows of KCl, NaCl and ZnSe. The main thrust of this study

was to correlate temperature rise with intensity (to 70 kW/cm^), focal spot size and irradiation time

(1 to 30 sec) and to measure the distortion interferometrically on coated and uncoated elements. High

strength materials, such as ZnSe and CdTe, had previously been observed to fail, by burning through under

intense illumination, whereas, KCl and NaCl had been observed to fracture, or fail due to the melting of

coatings. Fracture frequently occured after the high power laser was turned off abruptly.

Using a TEMqo CO2 laser of 500 watts output in a 4 mm beam, peak optical pathlength differences, in

632.8 nm fringes, of 3.3, 0.8, and 0.3 were recorded for samples of ZnSe, KCl and NcCl. Distortion was permanent only in one sample of KCl coated with AS2S30 During this series of measurements temperature

gradients stabilized within a few seconds after start of exposure. Thereafter, at each location, the

temperature rise was linear. For samples of ZnSe and NaCl subjected to 70 kW/cm^ loading, no stress

pattern appreared.

At the Air Force Weapons Laboratory, T. T. Saito, C. B. Charlton and J. S. Loomis are continuing their

comprehensive study of metal substrate mirrors for high power CW 10.6 ym applications. The main thrust

of their effort is the damage assessment of mirrors prepared by conventional polishing and diamond machining with noble metal coatings, dielectric coatings and subsequently machined electroplated metal mirrors.

After reporting in detail the output characteristic of their 1 kW cw TEMqq CO2 laser, focusing arrangement and intensity distribution (greater than 200 kW/cm^) at the target surface, they reported extensive

results on a wide variety of samples. In brief, they have found copper surfaces exhibit lower damage

thresholds than molybdenum in contrast to pulsed findings. Silver and gold evaporated on metal substrates

to yield reflectivities greater than 0.99 are found to withstand more than 200 kW/cm^ irradiation over

small areas. Diamond turned optics compete well with conventionally polished optics and, finally

absorption is the major factor limiting the survivability of dielectric coatings. Some coatings, however,

did survive test illuminations greater than 175 kW/cm^. As in other reports of this meeting, precleaning

or conditioning by irradiation at low levels resulted in greater ultimate damage resistance, probably by

gentle removal of dust and other absorbing or scattering particles.

In an engineering study, W. S. Otaguro of McDonnell-Douglas investigated damage to infrared filters by

inadvertant exposure to pulsed ruby laser radiation. Damage morphology, degradation of filter transmission and thresholds for damage from the highly absorbed ruby wavelength were measured. Filter samples were initially cooled to 10°K, for their intended use in infrared radiation sensor systems. The filters,

of Ge/ZnS multilayer construction, transmitted from 8 to 14 ym. With damage defined at that level in

which the passband transmission changed by 4%, Q-switched ruby TEMqq laser pulses of 20 nsecs duration

caused damage at 5MW/ cm^ . This observation was treated theoretically from a heat conduction approach

based on intrinsic absorption of the multilayer materials. Results of experiment and theory were in

general agreement.

2.3 Surface Preparation and Improvement

In actual systems, damage generally first occurs at optical surfaces. Surfaces are characterized by

high concentrations of impurities, defects, and inhomogeneities. Thus, the resistance of an optical

surface to damage can be greatly affected by the techniques of preparation used. Also, since surface

damage is dominated by extrinsic factors, presen^t values can be substantially improved.

It is generally agreed that by improved polishing techniques, smoother optical surfaces (lower rms roughness, and fewer cracks, scratches, and pits) can be obtained, and that smoother surfaces yield

higher damage thresholds. M. Boling, J. A. Ringlien, and G. Dube, all of Owens-Illinois, challenged this

concept in a paper reporting studies of surface damage at 1.06 ym in laser glass. They showed that, if

greater smoothness is obtained by a polishing technique which leaves behind an absorbing residue, a reduced damage threshold will result.

Several other examples were presented which contradicted the hypothesis that smoother surfaces exhibit

higher damage levels. Two quartz samples were polished by conventional methods. One was then subjected

to an acid etch, which left pits of about 100 nm size on the surface. Both samples damaged at 300 to 350 J/cm^. (This might be due to residual defects and impurities still present on the pitted surface,

despite the etching process.)

Boling also showed results on two glass surfaces, one polished conventionally, with barnsite, and the

other bowl-feed polished. The conventionally polished surface showed significant scratch and defect structure, while the bowl-feed polished surface was defect-free on a scale of 10 nm. Both surfaces damaged at

about 150 J/cm^, well below the intrinsic bulk level.

xiii
He then discussed the use of a boiling nitric acid etch, to raise the damage threshold of laser

glass. Unlike the HF etch used earlier by Swain, nitric acid does not attack the silicate structure of

the glass, so it does not alter the optical character of the surface. The etchant is heated to increase

diffusion into the sample. For samples carefully polished with non-absorbing polishing compounds prior

to etching, this technique yielded a threefold increase in surface damage threshold, from the 150 J/cm^

quoted above to values close to 500 J/cm^. This is very nearly the intrinsic limit of the medium.

Boling emphasized the point that surface smoothness alone does not guarantee a high damage threshold,

and that chemical residues on or near the surface are of great importance in the damage process. The hot

acid etch technique is promising as a method which will provide chemically clean surfaces, while not

destroying the optical integrity of the surface.

It has been postulated that surface damage in LiNbOa arises due to a deficiency in oxygen at the

surface, leading to a localized increase in the absorption at 1.06 ym. This hypothesis was tested by

J. Potoskey and C. Giulians^v of the Hughes Research Laboratory, and C. Klein of the Hughes Aircraft

Corporation. They trfed two methods of adding oxygen to the LiNbOa surface, by bombarding the surface

with oxygen in an argon-oxygen rf discharge, and by exposing the sample to oxygen gas at high pressure.

The treated surfaces were tested with a Nd:YAG laser pulse of 17.5 ns duration, focused onto a 64.5 Mm spot. Damage threshold was determined by visual inspection. When the LiNbOa surface was bombarded with 300V oxygen ions in an rf discharge for 30 minutes, a 50% increase in the damage threshold was

observed. The effect was persistent, and the sample retained its improved value up to several weeks

after treatment. However, some regions of the crystal did not show improvement, an effect attributed to the presence of impurities or defects.

Under high pressure oxygen, the crystal showed similar improvement. Samples were exposed from a few

minutes to 24 hours at pressures from 50 to 250 psig. No persistent effect was observed in this case, however. It appears as if surface damage in LiNbOa is partially attributable to oxygen deficiency, and

partially due to defects and impurities. The former cause is amenable to treatment by oxygen enrichment.

J. Porteus, E. Teppo, and J. Dancy, of the Michelson Laboratory reported on studies of surface

damage in LiNbOa and KCl, using both transmission electron microscopy, and Auger electron spectroscopy.

Uncoated samples of LiNbOs polished with AI2O3, diamond dust, SiC/Glycerol, and "Syton" were examined,

along with samples coated with a proprietary coating (probably ThOFa or ThFi, overcoated with AI2O3) and

ThOFa. All samples were in the form usually used for Q-switches.

It has been hypothesized that surface acoustic waves play a role in surface damage in LiNbOa.

Experiments are now underway at the Michelson Laboratory to test this hypothesis in two ways. In one, surface waves will be generated with an interdigital transducer, to see if acoustic waves alone can generate surface pitting. In the other, a surface will be divided by saw cuts, and only one portion

illuminated, to see if pits appear on the isolated regions. Only preliminary results have been obtained

to date.

Auger spectra of damaged coated surfaces indicated the presence of trace impurities. On the

proprietary coating, the Auger spectrometer indicated Nb, S, and CI as being present. The presence of

niobium was attributed to the disruption of the coating during the damage process and exposure of the

underlying niobate. The origin of the other impurities was not determined. The other coating showed

significant amounts of carbon, probably attributable to pump oil. The carbon was concentrated at the

interface between the coating and the substrate. Only preliminary results were presented on KCl.

This paper represents the use of sophisticated surface diagnostics for studying damage phenomena. It is essential in systems as complicated as coated surfaces, that very specific diagnostics be available.

2.4 Material Characterization and Instrumentation

To develop a full understanding of the factors influencing damage processes requires detail

characterization of the materials involved. Intrinsic, bulk physical properties of the materials are

essential, and a measurement program has been established to provide these data. Of the extrinsic factors

influencing damage, surface scattering is one of the most significant. Improved methods for measuring

surface scattering have also been developed.

Partially under the auspices of the Advanced Research Projects Agency, A. Feldman, I. Malitson,

D. Horowitz, R. M. Waxier and M. Dodge have established an infrared laser window material characterization

program at the National Bureau of Standards. The main areas of precision measurement include refractive

index, stress-optical constants (q^j). the temperature dependence of refractive index, the thermal

expansion coefficient and appropriate elastic compliances (S^j). Details of their specific instruments

design, performance and range of applicability were reported. The thrust of this intrinsic material

characterization program is aimed at accurately predicting the distortion produced in infrared windows as

a result of thermal loading. Currently under study are polycrystalline ZnSe, AS2S3 glass, chalcogenide

glass (Ge 33%, As 12%, Se 55%) and KCl. A few of the many important quantities already measured for

KCl are the refractive index at 20°C of 1.6360 at 0.224 ym, 1.4374 at 14.3 ym and 1.4542 at 10.6 ym. The

change of refractive index with temperature over the range of 20°C to 30°C is -2.1 x 10~^/of^ (? 0.27 ym

and -2.5 x 10~^/o|^ at 14.3 ym remaining fairly constant at ~ -3 x 10~^/o^ between 3 ym and 6 ym. The

fact that dn/dT is negative indicates that the variation of n with T is primarily due to a change in

material density. At the spectral extremes where dn/dT is less negative, the temperature dependence

xiv
suggests a shift of U.V. and IR absorption edges to longer wavelengths. Results for stress optical

constants qii, qi2> (112 the elastic compliance S12 and refractive index, were given for AS2S3,

chalcogenide glass and polycrystalline ZnSe at 0.6328 ym, 1.15 ym and 10.6 ym.

M. J. Soileau and H. E. Bennett of the Naval Weapons Center, Michelson Laboratory, described a sensitive instrument for measuring bulk and surface scattering in optical components. The instrument,

called the Optical Functional Tester (OFT) , measures both reflecting and transmitting optics in the

visible and infrared. Light from a He-Ne laser, operating at 0.633 ym, 1.15 ym, or 3.39 ym, illuminates

the sample, and the scattered light from the desired sample volume is imaged on a pyroelectric detector

by a Coblentz sphere, with a focal ratio of 0.5. Bulk and surface contributions to the scattering can be

separately analyzed.

Samples of hot rolled KCl, hot forged KCl doped with divalent europium, "polytran" KCl, and CVD ZnSe

have been tested. The europium-doped KCl and ZnSe samples showed more bulk scattering than the other

samples tested. In both cases, the scattering decreased with increasing wavelength. At 1.15 ym, bulk

scatter in Eu"^: KCl was about one-third that seen at 0.63 ym, while in ZnSe, the reduction was by about

a factor of two.

The standard scratch and dig specifications, MIL-0-13830A, were compared to the OFT. Scratch and dig

standard specimens were examined in the instrument. It was found that the standard specimens correlated

poorly with actual scattering measurements. Thus, a #10 scratch yielded greater scattering than a #20

scratch, and the substrate on which the #10 scratch was scribed yielded higher scattering than the #20

scratch standard.

The OFT offers the advantage of automated, objective evaluation of optical components in terms of both

bulk and surface scattering. Scratches and digs can be identified from a surface scan by the OFT. Socalled scratch and dig standards are seen to be subjective at best, and can be misleading.

Another area of materials classification concerns the characterization of surfaces exposed to high

Intensity laser radiation. Jean Bennett of the Naval Weapons Center reported on several new instruments

developed at the Michelson Laboratory specifically designed to perform these measurements accurately.

The prime driving force for this work is to aid in the determination of surface structure and its re- lation to laser induced damage, and to calculate the magnitude and angular distribution of scattered

light from surfaces prepared by various techniques. To accomplish this task effectively three main

factors must be determined. These are a) the auto-covariance function, b) the rms roughness, and c) the

height distribution function. These basic quantities along with the rms slope and slope distribution

function should sufficiently characterize the geometrical character of normally encountered surfaces of

interest. One should then be able to correlate scattered light with roughness, which can decrease

intensity in the main beam, reduce spurious signals, and assess backscatter which may produce damage by

feedback into the laser. Furthermore, mirror absorption can be increased by a factor of three in overly

rough surfaces. In addition, geometrical roughness can lead to field enhancement and thus lower damage

thresholds in short pulsed laser operations. To perform these assessments reliably and rapidly, an automated FECO (Fringes of Equal Chromatic Order) interferometer with a lateral resolution of ~ 2 ym and

a depth resolution of a few Angstrons has been constructed. This instrument has already been employed

to evaluate the surface character of several glasses, metals and crystals. These include Cervit, KCl,

molybdenum, titanium and copper surfaces prepared by different techniques. On these samples there was a good correlation between rms roughness determined both by the TV FECO interferometer and those from

visual observations of FECO fringes; however, these two methods did not agree absolutely, the visually

determined values generally being larger than those determined by the automated system. The basis of

this disagreement is under further study.

John C. Stover of Dow Chemical has attempted to assess the possible utility of other scattering

concepts and geometries than those in normal use, such as total integrated scattering (TIS) etc., to evaluate surface character more easily, and arrive at a quantification of average surface parameters.

He has attacked this problem from a theoretical standpoint with experimental verification of the scattering from sinusoidal reflection gratings produced by the interference of two coherent laser beams on photographic emulsions. He hopes to extend his analysis in this initial effort to obtain relations

governing the light scattered from arbitrary surfaces (not necessarily Gaussion in roughness disturbation) , resulting in an accurate spatial power spectral density function of these surfaces.

Analyzing the previously described surface theoretically, where scalar theory should apply, he has

achieved qualitative agreement with experiment. This work is now being expanded to arbitrary surfaces

and correlated with standard surface characterization techniques in use at the Michelson Laboratory.

Besides allowing the measurement of rms surface roughness, auto-covariance length, etc., the spatial

power spectral density function should be useful at higher frequencies than presently employed techniques.

The thrust of this research of course is to lead to correlation between geometrical surface character and

damage threshold,

2.5 Damage Diagnostics

In any study of laser damage, it is essential to develop techniques to determine when and if damage

has occurred, and, if it has, to identify the operative mechanism. Furthermore, there is a continuing

search for precursors to damage which might provide a nondestructive test for damage studies. In the

past, increased light scattering, and spark observation, combined with post-mortem observation, have

been used to indicate when damage has occurred. A number of proposals have been put forward in past

Damage Symposia for ways to identify which of several competing mechanisms is responsible for damage

XV
under a given set of circumstances. This year, D. Milam, R. Broadury, R. Picard, all of the Air Force

Cambridge Research Laboratory, and M. Bass of the University of Southern California, discussed the use

of the statistics of sample survival time under continuous illumination as an indicator of the operative

damage mechanism. For a given damage mechanism, the probability of damage per unit time at a given

illumination level is assumed to be a known function, h(t). In an ensemble of experiments, the number

of sample sites surviving, without damage, to a time t is given by

ln[N(t) / N ] = h(t') dt'

.

0 o

If the intensity level is constant, corresponding to an intensity To, then the log - survival function

will be

ln[NCt) / N ] = h(I ,t'] df .

If more than one mechanism is operative the log-survival function will be a sum of similar integrals.

For avalanche ionization, the function h(t) is constant (for constant illumination) and given by

Aq exp (-k/Eo), where Eo is the field amplitude. Thus, the log-survival curve is expected to be a straight line of constant negative slope, with the slope increasing in magnitude with increasing field.

If both defect and intrinsic breakdown are occuring, a curve asymptotic to the two appropriate slopes

will be observed.

For thermal absorption damage, the function h(t) is zero below a critical illumination level, and

goes quickly to a large value as the critical deposited energy is reached. Thus, the log-survival curve approaches a step function in time, with the step being of shorter duration at higher power levels.

More complicated curves arise if several types of absorbing inclusions are present. This hypothesis was

tested using a single mode ruby laser which was shuttered with a fast Pockel's cell. The pulse had a rise time of 0.7 nanoseconds, and a power level of from 50 kW for 50 nanoseconds duration, to 200 kW in

20 nanoseconds. For transparent samples, the transmission of the pulse was monitored, and the decrease

in transmission taken as indicative of damage. The beam was focused to a spot of from 3 to 100 ym in

diameter.

For Ti02-Si02 multilayer coatings, damage was seen to be due to absorption. The log-survival curve showed a sharp cutoff for fixed power, and the cutoff time was inversely proportional to the power. For

ZrOa , single-layer films deposited by e-gun evaporation, damage was seen to be due to absorbing

inclusions. For Suprasil quartz, damage was attributed to defect-enhanced, avalanche ionization in the

bulk material. The data presented showed reasonable agreement with the proposed theory, although in the case of Suprasil, there was considerable ambiguity in assigning the damage mechanism on the basis of the

log-survival curves. Concetto Giuliano of Hughes Research Laboratories, reported on the observation of transmitted,

reflected and back-scattered light during the damage process, as a possible diagnostic test for damage

detection. Samples were sapphire, illuminated with a 20 nanosecond, ruby laser pulse, focused to a 55 ym spot. Damage in this case is accompanied by a significant reduction in both transmitted and

specularly reflected light, undoubtedly due to the formation of a strongly absorbing plasma in the damage

region.

Time resolved measurements indicated that the reflection and transmission follow generally the same temporal behavior. The decrease in specular reflection is seen both for entrance and exit surface damage.

Observations of the diffuse back scattering showed that, above the damage threshold, the back scattering

spreads into a cone of angle 1.5 to 2.0 degrees. As the power is increased, back-scatter structure, in

the form of spots and rings, appear. Temporal monitoring of the diffuse backscatter shows that it

strongly increases at the time that the transmission cuts off, which is the time of first appearance of

the damage plasma. Diffuse back scatter produces a sensitive observable for damage of this kind, since

changes in backscatter are seen even when the absorption is weak. Guiliano was not able to observe a scattering increase reliably before damage was seen, so he could not establish a precursor test based on scattering.

A new diagnostic technique for detecting surface damage was proposed in a paper by R. Hart,

G. Giuliano, and H. Dunlap of the Hughes Research Laboratory. They analyzed a sample of GaAs exposed to radiation at 0.69 \im from a ruby laser. The pulse duration was 20 ns , in a beam of 1.2 mm radius, and

the observed damage levels were from 12 to 15 MW/cm^.

The surface of the crystal was probed by observing the backscattering of 280 KeV alpha particles.

By observing the scattering intensity as a function of energy loss, the scattering could be related to the depth of the scattering layer in the crystal. Backscattered particles were detected at an angle of

150° to the incident beam, with a cooled Si detector.

When the crystal was randomly oriented to the incident particle beam, the scattering was insensitive

to the damage layer. However, when the beam was incident along the <100> crystal direction, ion channeling greatly reduced the backscatter signal. In this case, the signal increase due to surface damage was

clearly discernible. The scattering layer due to damage extends only to about 17.5 nm. The transverse

extent of the scattering layer correlates well with the laser profile.

It was remarkable that the scattering only extended to 17.5 nm, when the surface pits associated

with the damage were from 2 to 4 ym in diameter. Beyond the surface layer, the channeling effect was relatively insensitive to the presence of damage.

xvi
Ti02 and YVO2 are birefringent materials of interest for use in laser applications. K. M. Leung and

L. G. DeShazer of the University of Southern California, reported results on surface damage in uniaxial

single crystals of each of these materials. Damage was induced by irradiation with a TEMqo ruby laser.

The damage mechanism was identified by recording the damage threshold as a function of irradiated spot

size, for spot sizes from 52 pm to 215 ym. Damage was identified by the technique of laser-induced

scattering. The mechanism of damage was confirmed by morphological investigation, using a scanning

electron microscope.

Damage studies were carried out with the optic axis of the crystal both parallel to and perpendicular

to the incident E-field. No difference was observed either in damage threshold, or in morphology.

Damage was attributed to polishing inperfections in the case of TiOa and inclusions of YVOi,.

Extrapolation of the observations to zero spot size yield intrinsic damage levels of 2,5 to 3 times the

defect-dominated values.

2.6 Theoretical Studies

In recent years, there has been significant advance in the theoretical understanding of damage,

processes. Avalanche ionization, for example, has been exhaustively studied, as has self-focusing.

Inclusion damage has been analyzed in several papers. Various theoretical models have been proposed to account for impurity absorption in otherwise transparent materials. As experimental data improves, and

materials of greater purity become available, a corresponding increase in theoretical understanding can be achieved.

R. A. Shatas, J. D. Stettler, M. M. Narducci, S. S. Mitra and H. C. Meyer of the U. S. Army Missile

Command, presented a theoretical discussion of alternatives to the avalanche breakdown process to explain

the observed discrepancies in surface damage thresholds in semiconductors. After reviewing the

phenomenological theory of avalanche breakdown, they discussed the conditions under which the Shockley

theory can be extended from DC to optical frequencies. They show that as long as the optical frequency

is less than the effective collision frequency, the avalanche theory can be expected to apply. Under

this condition, there is a high probability of a "lucky" electron undergoing a momentum- reversing

collision during the optical cycle, and thereby absorbing energy from the applied field. This condition

is shown to occur in NcCl.

In semiconductors like GaAs however, the effective collision rate is too low for the avalanche theory

to apply. The authors' propose that in these materials, collective excitations of the plasma may be

responsible for the observed damage. They examine the threshold for both the two-stream instability and

the parametric instability, and find that in GaAs, the predicted thresholds are in general agreement

with experiment.

The threshold for both plasma instabilities is shown to increase linearly with the ratio of the laser

frequency to the electron mobility. Observed thresholds in GaAs do not increase with increasing optical

frequency, but decrease instead. This effect is attributed to the fact that the band gap in semiconductors is comparable to the frequency, so that at increased optical frequency, electrons can be

excited into conduction states via lower order multiphoton processes.

Recently, there has been great interest in the determination of the third-order nonlinear

susceptibility (x^^'') of transparent dielectrics. R. Hellwarth, J. Cherlow, and T. T. Yang of the

University of Southern California, discussed the determination of the so-called "nuclear" contribution to

)((^) from analysis of the Raman scattering spectrum. This is the contribution to X^^^ from heavy particle

motion in the medium. By use of the Kramers-Kronig relation, both the scalar and tensor components of

the nonlinear dielectric response function can be determined. These values lead directly to the non- electronic contribution to the nonlinear refractive index coefficient, n2.

Samples tested included Homosil quartz, LSO and ED-4 laser glasses, and SF-7 and LaSF-7 optical

glasses. In each case, the electronic nonlinearity was found to account for 70 to 85% of the reported

nonlinear refractive index.

An intriguing relation between observed damage thresholds and material properties was put forward by

J. Bettis and A. H. Guenther of the Air Force Weapons Laboratory, and A. J. Glass of the Lawrence

Llvermore Laboratory. They postulated that the electric field value at damage threshold in transparent

dielectrics might correspond to some critical extension of the electron coordinate, corresponding to a critical distortion of the electron cloud in the polarizable molecule. Consideration of the local field

correction then led to an empirical relation, that the threshold electric field value should vary as

N/ (n^-1) , where N is the number density of molecules, and n the index of refraction of the material.

This hypothesis was compared with experimental results of several investigators. Thin film results

of DeShazer showed generally good agreement. Data reported by Boling et al on surface damage in glasses

showed a 1/ (n^-1) dependence, since the molecular densities were essentially the same for all materials

tested. For vapor phase mixtures of thin film materials, reported by Austin, et al, a l/(n^-l) dependence was found, but the observed damage thresholds were found to be in poor agreement with N/Cn^-l).

No explicit explanation of this anomaly was offered.

The most elaborate comparison was provided by the carefully determined bulk threshold values reported

by Fradin. The threshold field values for halides of sodium, rubidium and potassium were found to be in

excellent agreement with the N/n^-1) relation. The potassium halide curve was displaced from the sodium

xvii

564-378 0 - 74 -2
and rubidium halide data, but was parallel to it. NaF was the only reported material widely variant from

the proposed relation. In all cases, extrapolation of the empirical expression to the index value for air

gave good agreement with reported air breakdown values.

C. J. Duthler and M. Sparks of Xonics, Inc., discussed two aspects of the damage problem, of

particular relevance to laser window materials. In the first part of their talk, they revisited the

problem of surface pit formation due to the heating of subsurface, absorbing inclusions. The failure

mechanism they proposed is based, not on vaporization of the inclusion, but on the adiabatic heating of

the inclusion to create a locally concentrated stress within the material, and then the rupture of the

stressed material along a conical surface by microcrack propagation from the point of maximimi stress.

The energy required In this failure mode corresponds to breaking the bonds of the material only along

the conical surface, rather than throughout the excavated volume. The cone angle of the pit decreases

from 78°, for a deep-lying inclusion, towards 0° as the depth of the inclusion becomes comparable to the

inclusion diameter.

If the heated inclusion lies very close to the surface, failure can occur by microcrack propagation

from the surface into the material, towards the inclusion, resulting in a pinhole- type damage rather than

a conical pit. Subsequent heating will, of course, expand this pinhole. Although the model presented

contains a number of simplifying assumptions, the general agreement with experimental results is satisfactory. The delay time associated with this mechanism is the acoustical transit time to the surface,

of the order of 10 ns for a particle 10 \im below the surface. No such delay has been observed experimentally, probably since the surface fails first at its weakest points, which correspond to inclusions

which are very close to the surface, with correspondingly shorter delay times.

The second part of the paper dealt with the role of molecular impurities in determining the residual

absorption of infrared window materials in regions of nominally high transparency. In KCl and KBr,

absorptions greater than 10~'*cm~'' are seen in spectral regions in which theoretical estimates of intrinsic

absorptions are at least an order of magnitude lower. Atomic impurities can lead to alteration of the

lattice absorption due to the formation of local lattice modes, but these are felt to be at too low a frequency to contribute at 10.6 ym.

Diatomic ionic impurities, in general, exhibit too high a frequency to account for the extrinsic

absorption. The hydroxyl ion, for example, with a resonant frequency of 3640 cm would have to be

present at a concentration of 100 ppm to account for the observed absorption. The authors felt_that the

most likely candidates are polyatomic and multivalent molecular ions, such as N02~, HCO3 , SOu^ and

CrO.,^".

3. Recommendations

As has been mentioned many times, the greatest improvement in damage thresholds of optical elements

yet to be realized is probably in the area of exposed surfaces, particularly if those surfaces are coated.

While considerable advances have been made in this area, resulting in improved components for both CW

and pulsed lasers operating in the visible and the infrared, it has been very difficult to identify the

influence of any one factor and assess its specific role in the damage process. This stems from two main

causes. First, it is generally impossible to isolate and vary a single factor without changing other

influencing parameters. Secondly, due to the presence of synergistic effects, the role played by a given

factor, such as surface roughness, will vary depending on other conditions of the material. From a standpoint purely of engineering and material utility, there is a genuine requirement for large area, defect

free, homogeneous coatings. It is obvious from this year's symposium that the major controlling factors

in coating damage resistance arise in the fabrication process. These include substrate preparation,

ambient pressure, deposition technique, substrate finish, substrate temperature, geometrical arrangement,

purity of starting material, selection of candidate materials, and rate of deposition. It is these

factors which lead to specific film characteristics such as crystal structure, residual stress, defect

concentration, uniformity, impurity concentration, scattering level, adhesion, etc. These are the

observable properties to which the damage process must be related. Thus, there is considerable need for

improved film characterization procedures so that these characteristics can be correlated with specifics

of the ''abrication process. In this way we may learn the influence of particular process factors in

the specific film properties, such as impurity and defect concentration, and be better able to isolate

the individual damage influencing factor such as stress, structure, etc. This program will require more application of advanced diagnostic techniques, such as Auger spectroscopy, LEED and Scanning Electron

Microscopy, as well as the development or application of other more novel characterization techniques.

An area of increasing importance is metal mirror fabrication. Various techniques of fabrication,

involving both coated and bare surfaces, are under investigation. It is to be expected that metal mirrors

will show rapid improvement in damage resistance, because the subject is quite new. The current rate of

improvement is impressive. As in other areas of damage studies however, empirical improvement often

proceeds more rapidly than theoretical understanding. There is clearly a need for improved techniques

of surface characterization, and the development of rapid scan, nondestructive test procedures. The

elegant techniques available to determine surface roughness may form the basis for such instrumentation.

At present, the exact relation between surface roughness and surface absorption remains to be elucidated.

Additionally, there is some theoretical uncertainty in the relation between surface features and

observed surface scattering.

xviii
There exists a large variety of physical probes to examine various features of surface structure.

In order to gain a better understanding of what constitutes a "good" surface for high power laser use, very detailed characterization of surface conditions must be carried out. At a metal surface, for example,

the damage threshold is seen to be sensitive to surface features on a scale of a few nm. This sensitivity is thought to be due to the influence of surface roughness on electron motion within the surface.

A surface layer rich in defects or impurities would be expected to have a similar effect on the damage

propensity of the material. The specific way that fabrication affects the surface layer, the detailed

characterization of the surface layer, on the scale of electron motion as well as on the optical scale,

and the consequent influence of surface properties or damage all need to be examined more carefully. It might be profitable, for example, to prepare a set of surface samples of controlled variation, by polishing with various grit sizes, for different periods of time, and so producing varying deposits of microfracture. These samples could then be polished to produce a smooth surface with varying degrees of subsurface structure. After testing, the smooth polished layer could be removed by etching to exposure

the subsurface making it amendable to microscopic and other types of investigation. Alternately ground

surfaces could be overcoated and a similar investigation carried out. This question of subsurface

structure influences is probably the most significant area remaining to be addressed.

In the case of multilayer films, mechanical and thermal contact between film layers and at the

substrate are also properties of overriding importance. The present preference for sputtered films over evaporated films may be a consequence of the substrate cleaning inherent in the sputtering process,

and the better adhesion of sputtered films. Evaporated film techniques may be amendable to improvement

in this regard.

It was reported at this conference that alkali halides grown by reactive atmosphere processing

exhibit much higher damage thresholds than commercially available material. This improvement is amen- attributed to higher purity of the material. It was also reported that in RAP-grown material, not only were higher damage levels obtained, but that the relative levels of KCZ and NaC£ were reversed. This

strongly suggests that impurities and defects play an Important role mediating the onset of avalanche

breakdown. It might be expected that currently quoted "intrinsic" damage strengths are actually amen- able to improvement. There is still a need for developing a better understanding of damage processes,

especially avalanche breakdown, from a detailed, solid-state theoretic point of view. The use of average

electron properties is questionable, particularly in low-mobility materials, where the mean free path

may be of the order of atomic dimensions.

Materials development and improvement continues to progress. The area of IR window materials is

currently getting the major emphasis. There is substantial room for improvement in this area, as, for

example, in the reduction of void concentration in ZnSe, the reduction of impurity absorption in a variety of materials, and the aforementioned improvement in alkali halide manufacture.

We can look forward to continuing development of materials for the UV and vacuum uv. Here the stateof-the-art is relatively primitive. The special problems of the uv, arising from the high probability of

impurity ionization, the effect of two- and three-photon absorption, and the possibility of color center

formation, remain to be enumerated fully and analyzed. Clearly, all of the problems of multilayer dielectric films and metal surfaces will carry over into the uv spectrum, with added complications

o

As the understanding of various damage-related phenomena improves, and the capability of identifying

relevant material parameters emerges, it becomes possible to formulate figures of merit for optical

materials. An example of this is the analysis of thermal distortion in this proceedings. There is need

for the identification, measurement, and tabulation of these figures of merit, for optical materials

for high power lasers. The availability of this kind of information is, in fact, the ultimate goal of

this symposium.

There is a growing appreciation of the importance of damage processes in designing large laser

systems. Total system considerations must be taken into account in such a design. The interplay of

material properties (as described by the figures of merit mentioned above), structural specifications,

systems operating characteristics (beam quality, pulse length) , and environmental factors comprise what

we call synergistic effects. A material may exhibit a certain damage threshold under single pulse

exposure, but respond quite differently when incorporated into a multi-pulse or CW system. The effect

of flash-lamp irradiation on damage resistance can be important in pulsed solid-state lasers. Failure

of one element may lead to the progressive failure of other, more expensive elements. There is evidence

that when surface damage occurs, material may be splattered onto other parts of the surface, or even re- deposited on adjacent surfaces, leading to damage on subsequent exposure. We must continue to emphasize

that the ultimate use of laser materials is in laser systems, not solely in damage tests, and that the

real environment must be considered in evaluating the material.

It seems clear that MIL-SPEC scratch and dig tests are of limited utility in specification of highly

polished surfaces. The development of new surface specification standards, based on light scattering or FECO interferometry , is highly .recommended, and would be an appropriate undertaking of the ASTM.

Finally, looking ahead to next year's symposium, we must seek to open up the discussion to new topics.

The symposium attracts people interested in the interaction of intense light with optical materials, the

development of new materials, and the design of high power laser systems. We are just beginning to em- phasize system considerations. We have not addressed ourselves to the problems of fiber optics at all,

to date. Our discussions of electro-optic materials have been very limited. These are a few of the directions in which the symposium might grow. The co-chairmen solicit and will welcome suggestions for new

topic areas to be considered in the damage symposium in coming years.

xix
4. Acknowledgement

We would like to acknowledge the invaluable assistance of Dr. Harold S. Boyne,

Mrs. Pauline W. Smith, Mrs. Marjorie L. Wilson and Mrs. Florence M. Indorf of the

National Bureau of Standards in Boulder, Colorado for their interest, support, and

untiring efforts in the operation of this Symposium and in the preparation and publication of the proceedings. Also, we would like to take special note of

Mrs Normal Lear and Mrs. Kay Jentsch, who added pleasurably to the conference operation particularly during the discussion periods following the formal presentations.

The continued success of the Damage Symposia would not have been possible without

their support

.

5. Bibliography

[1]

[2]

[3]

"Damage in Laser Glass", A. J. Glass and A. H. Guenther, Editors, ASTM Special

Technical Publication 469, ASTM, Philadelphia, PA (1969).

"Damage in Laser Materials", A. J. Glass and A. Guenther, Editors, NBS

Special Publication 3A1, U. S. Government Printing Office, Washington, D.C. (1970)

"Fundamentals of Damage in Laser Glass", N. Bloembergen, National Materials

Advisory Board Publication NMAB-271, National Academy of Sciences, Washington,

D.C. (1970)

.

[4] "Damage in Laser Materials: 1971", A. J. Glass and A. H. Guenther, Editors, NBS

Special Publication 356, U. S. Government Printing Office, Washington, D.C. (1971)

[5] "High Power Infrared Laser Windows", N. Bloembergen, National Materials Advisory

Board Publication NMAB-292, National Academy of Sciences, Washington, D.C. (1972).

[6] Proceedings of the Conference on High Power Infrared Laser Window Materials,

(October 1971). C. S. Sahagian and C. A. Pitha, Editors, Special Report No. 127,

Air Force Cambridge Research Laboratories, (1971).

[7] "Laser Induced Damage in Optical Materials

Editors, NBS Special Publication 372, U. S- Washington, D.C. (1972).

1972", A. J. Glass and A. H. Guenther,

Government Printing Office,

[8]

[9]

[10]

"Laser Induced Damage of Optical Elements, A Status Report",

A. H. Guenther, Applied Optics 1_2, pp. 637-649 (19 7 3).

J. Glass and

"Laser Induced Damage in Optical Materials

Editors, NBS Special Publication 387, U. S Washington, D.C. (1973).

1973", A. J. Glass and A. H. Government Printing Office,

"Laser Induced Damage to Optical Materials, 1973-

A. J. Glass and A. H. Guenther, Applied Optics, 1^, pp. 74-88 (1974)

a Conference Report'

Guenther

,

A . J . Glass

A. H. Guenther

XX
0.1 Opening Remarks

Alexander J. Glass

Lawrence Livermore Laboratory

P. 0. Box 808

Livermore, California 94550

Speaking for Art Guenther and myself, I'd like to welcome you all to the Sixth Annual Symposium on Damage in Laser Materials. The National Bureau of Standards has again provided us with these superb

accommodations, and has carried the burden of the preparation of the proceedings for this symposium over

the years, and I want to reiterate the thanks that Haynes Lee has expressed to the Bureau, to the ASTM,

and to ONR for their support in helping us carry out the work of the Symposium.

The organization of the meeting this year is the same as it has been in previous years, in that we

have tried to group papers roughly according to subjects. We have a session this morning on damage and

its consequences in glass systems, then we have a session that is devoted to basic theoretical investigations, and another on damage to crystals, particularly in nonlinear optical materials. Tomorrow,

most of the day will be spent on a variety of papers on coatings, windows, and mirrors.

I think it is clear to everybody there are two fairly well delineated applications areas represented

in these papers. One of them is short pulse, high peak power, essentially single shot lasers, mostly

Neodymium glass for laser fusion. The other is high average power CW lasers for military and industrial

application, which are primarily infrared lasers. In both areas very large systems are on line, or coming on line, depending on where you work. The topics that are being discussed here are not of purely

academic interest. In the design of large systems it turns out that it is damage considerations that

ultimately limit performance and dictate the design, and in many cases, determine the cost of the system.

The cost sometimes becomes infinite, which means the system is infeasible. It is also true that because

of the fact that the damage and the damage phenomena are the limiting factors, that real advances in the

damage area lead to real benefits in terms of cost reduction, as well as in terms of improved system

performance. Because of the fact that these are real problems that we are dealing with and real systems,

you will find, in general that the subject seems to move ahead fairly quickly, and leaves behind it a number of fundamental investigations left undone. I think that it is typical of materials science and

of applied science in general, that you learn how to deal with a problem long before you understand it,

from the basis of first principles. The climate of applied science that exists in most places today is

such that a lot of fascinating physics problems are going to be left behind. It is one of the minor

regrets, but a necessary characteristic of this field, that we have to skip along the surface rather

lightly, with the hope that someday, if people have sufficient motivation and there is a sufficiently

hospitable climate to do so, some of these fascinating problems in materials science and in the interaction of high power light waves with quantum systems can be reviewed and dug into more deeply.

We take a very broad view of damage in the symposium. Many of the things we talk about don't

really have to do with the catastrophic failure of materials but only, like small scale self focusing,

with a reduction in the performance of the system. Along this line, we have a working definition of

damage as being "whatever turns you off".

If you look at the schedule, you will find that there is nothing in it particularly associated with

ultraviolet materials or ultraviolet systems. That is, not to say that damage problems don't exist in

those systems. Even though ultraviolet lasers, excimer lasers, etc. operate at moderate powers in com- parison to the IR and near visible systems, they are already damage limited. You even find people doing

the calorimetry on ultraviolet lasers in terms of the damage to the coated elements. We know something

about the damage mechanisms in the ultraviolet. We know that if one goes to short wavelengths one begins

to get into circumstances in which two-photon absorption can become important in many materials. At the

moment, my impression is that in the ultraviolet systems, extrinsic features such as impurities and imperfections in coatings are still dominant. We can look forward, however, as ultraviolet laser technology advances, to see more investigations in that area. We also have not, in any of the Damage

Symposiums, addressed ourselves to synergistic problems that arise due to the influence of the environment on the elements of the system. This environmental influence alters the damage characteristics.

Certainly, for military or industrial systems, one has a hostile environment consisting of dirt, sea water, or high altitude. A mirror or a coated surface which might bear up very well in the laboratory,

may have an entirely different characteristic when it gets out into the working environment. The same is

true of the high power, short pulse, Neodymium glass lasers for laser fusion. Ask yourself, what do 14

MeV neutrons do to reflecting surfaces, and do they retain their integrity and their reflecting properties at low absorption. I think that everyone working in laser fusion would be delighted to have that

as a serious problem, but it is one that we undoubtedly will have to confront in future meetings of this

kind.

1
1.1 Laser Beam Divergence and Damage in Glass Amplifiers

Keith A. Brueckner, Brian Guscott, Siebe Jorna,

Kent Moncur, and Larry Siebert

KMS Fusion, Inc.

Ann Arbor, Michigan 48106

The power limit in glass amplifiers is determined by the onset of non- acceptable beam distortion of sel f- focusing leading to glass damage, which

results from the power-dependent index in the glass. Self-phase modulation

due to the variation of index with power also can at very high power result

in pulse distortion and gain loss. These effects can be analyzed quantitatively by integration of Maxwell's equations together with the equation

for gain variation of the lasing medium. Under conditions of rapid intensity change, diffraction effects can be ignored and the eikonal equation

used to determine beam propagation.

To study these problems for small amplitude disturbances we have

analyzed the equations using dispersion theory. For larger amplitudes

we have developed several computer codes which allow exact simulation of

the laser system and of selected components which are essential for

control of the beam intensity.

The intensity of the laser beam must be carefully controlled to avoid

both large scale and small scale effects. The large scale effects which

increase beam divergence result from the initial beam radial intensity

variation combined with nonuniform gain in the laser amplifiers. The

small scale effects usually result from diffraction and interference and

can be minimized by reducing interference effects as far as possible. This

can best be done by the use of graduated or "soft" apertures placed at

critical points in the beam. The beam intensity profile can also be

controlled by the use of attenuators with graduated transmission.

These techniques offer highly flexible and effective control of the

laser beam and allow operation well above 5 GW/cm^.

Keywords: Apodizers; glass lasers; self- focusing.

1 . Introduction

The power limit in glass amplifiers is determined by the onset of nonacceptable

beam distortion or s elf - focus ing leading to glass damage, which results from the

power-dependent index in the glass. Self -phase modulation due to the variation of

index with power also can, at very high power, result in pulse distortion and gain

loss. These effects can be analyzed quantitatively by integration of Maxwell's

equation, together with the equation for gain variation of the lasing medium. The

integration of the electromagnetic wave equation can be simplified for small angular

divergence. The relaxation processes in the lasing medium can be limited to transitions from the upper to the lower lasing level, for pulse lengths of a few nanoseconds

or less. Under conditions of rapid intensity change, diffraction effects can be

ignored and the eikonal equation used to determine beam propagation.

To study these problems for small amplitude disturbances, we have analyzed the

equations using dispersion theory. For larger amplitudes, we have developed several

computer codes which allow exact simulation of the laser system and of selected

components which are essential for control of the beam intensity.

2
2. Self-Phase Modulation

The optical path-length in the lasing medium is

L ^ = / n dz opt '

= L n^ + n2 / dz (2.1)

For the present, ignoring the effects of gain saturation, beam nonuniformity , and beam

divergence, the power variation in an amplifier rod is

E2(z) = E^O) exp Gz (2.2)

with G the gain per unit length and E^(0) the field at the entrance to the rod. The • variation of opitcal path-length with time shifts the frequency of the beam, with

6^ ^ 1 ^^opt^

^2.3)

0) c 3t

Equation (2.1) then gives a wavelength shift per amplifier head of

6A(I) =~ -2.81 X 10-4 SP(GW/cm^) ^^^^-l^

^ ^2.4)

3t (nsec)

A typical gain is 0 .064 cm ''" , corresponding in ED-2 glass to energy storage of

0.04 joules/cm\ For a pulse rise time of 50 picoseconds, 5 amplifier heads at high

power, and a peak power of 10 gigawatt s/cm^ , equation (2.4) gives a shift of about

o 10 A, which is considerably less than the lasing line width. Thus, self -modulation

can be avoided by reasonable choices of design parameters.

3. Large Scale Beam Divergence

The optical path-length change causes phase front distortion if the laser power

is nonuniform. For small angular variations, the change in angle of the beam in a single amplifier is given by

«e(r) = — J din

3r 0

L

= n^2-— ' / dilE^(r,z) . (3.1)

3r 0

We assume that the gain may be a function of r but not of z. Equation (3.1) then gives

4TTn„n,,

3r

6e(r) = ^ - P(r,z=0) Cexp G(r)L-l)

j^ ^3^23

The angular variation of the beam can be compensated by a collimating lens provided

that 9(r) varies linearly with r. V/e assume that the beam is collimated so that the

divergence of the beam center is removed. We assume that exp G(r)L >> I and use a typical value of G(0) = 0.032 for the gain at the rod center. We also neglect the

variation of G(r) in the denominator of equation (3.2). The residual divergence then

may be written

6e(r) - 6eQ(r)

2.64 X 10-5 Pp,,(e,z = L) ^ir,z = 0)

TM

Ur P(0,z = 0)

exp[G(r) - G(0)]L

, 2 P(r , z = 0)

3r2 P(0,z=0)

exp[G(r) - G(0)LH^^ J . (3.3)

3
Equation (3.3] shows that the variation of gain with radius must be accurately compensated by the variation of the input beam, if residual divergences less than 10 ^ radians is to be achieved for beam power in the range of 5 to 10 gigawatts.

4. Small Scale Beam Divergence and Damage

A laser beam may also undergo small scale self -focusing with resulting angular

divergence. This effect is excited by small scale nonuniformit ies which are produced

by diffraction effects or by index nonuniformities in the lasing medium.

In the gome tr ic- opt ic approximation, the variation of the power in the beam is

given by —8 P

= -V - V n. (4.1)

3z' J- n ^

2 The power dependence of n gives (for "^^^ << n^]

^ ^ - ^ V 2 P^ (4.2)

3z^

Linearizing equation (4.2) for small variation of power P-^^ gives

:!!^p V

^ P ^4.3)

3z^ " c 0 X

The presence of diffraction adds to equation (4.1) the diffraction correction, valid

for small angles of diffraction,

86P

3z^

diff = —— (V 2) 2 p (4.4)

4Kl^. X

with Kj^ = n^kp and = 2TT/kp the laser vacuum wave number. Combining this with

equation (4.3) gives the dispersion relation

2 4

, -47rn,P„k k

k^ = LiLJi + ^

(4.5)

The growth rate is

The maximum growth is at

giving

P^(z) = P-[_(0) exp Im k^z. (4.6)

. 87rk^n^n^P„

r^i. = " ^ " " (4.7)

^ X max

4
At one micron wavelength, equation (4.7) and equation (4.8) give

(k ) = 101 cm'-'- Pn(GW)'^

Imk^ = 0.053 Pq(GW) cm'-"" (4.9)

At a power o£ 5 gigawatts/cm^ , the e-£olding length is only 3.8 centimeters, allowing

for large growth in a typical path-length of 30-50 centimeters.

The growth of the instability also causes small angle divergence of the beam.

The variation of angle with distance is given approximately by the eikonal equation.

de

dz

= V

4iTn1"

Thus ,

|e(z)

giving at the maximum growth

' ^ ' ^ max

47Tn,

P-^(O) exp |kj z

(4.10)

(4.11)

Sirn,

[cP

0

P^(0) exp 0.053 Pq (GW) z, (4.12)

The linearization of equation (4.2) is valid only if the lateral displacement of the

rays does not exceed the initial lateral structure of the perturbation P, . This

- 1 ^ occurs when 6edz k , at which point the angular deviation is

e (Ax

X

' (4.13)

At the maximum growth point,

6 (Ax = x

' 2TM2Po 1

c

0.52 P, (GW) ^ milliradian, (4.14)

The growth reaches this amplitude when 2 P^^/Pq e^P 0.53 Pq (GW) z is equal to unity.

For an initial perturbation P^/Pg of 1^ ^'^^ ^ power of 5 gigawatts/cm^ , this occurs

in 15 centimeters. The resulting angular deflection of about one milliradian can be

a serious source of transverse energy loss from the beam.

The small scale self -focusing , which leads to angular divergence and hence of

beam energy loss, also can cause glass damage. The self -focusing length is of the

order of k^"*" given in equation (4.5), with P^ the power fluctuation which causes the

self -focusing . Thus, a fluctuation of one gigawatt/cm^ at the wavelength of maximum

growth self-focuses in about 20 centimeters. The large increase of power then can cause breakdown and damage. The growth is less rapid for other wavelengths so that,

in practice, damage appears to occur only for power fluctuations of 5 to 10 gigawatts/cm^

.

The damage may also be prevented by lateral outscatter ing of energy from the selffocusing part of the beam. When this lateral outscattering of energy begins to occur,

the laser beam changes from a sharp edged circular beam to a beam with a diffuse halo

of energy around the outside of the gain energy. When this halo first begins to appear,

there is no visible damage in the glass; however, microscopic damage may have already

occurred. If the power density is increased, the halo becomes larger and the damage

becomes visible in the form of small self -focusing tracks.

5
5. Intensity Control

The intensity of the laser beam must be carefully controlled to avoid both large

scale and small scale effects. The large scale effects which increase beam divergence

result from the initial beam radial intensity variation combined with nonuniform gain

in the laser amplifiers. The small scale effects usually result from diffraction

and interference and can be minimized by reducing interference effects as far as possible. This can best be done by the use of graduated or "soft" apertures placed at

critical points in the beam. The beam intensity profile can also be controlled by

the use of attenuators with graduated transmission.

The KMS Laser Amplifier System uses glass rods from 16 mm to 80 mm in diameter,

then seven disc amplifiers with 100 mm clear aperture are used. Each disc amplifier

has three glass slabs at Brewster's angle. In the design and use of optical components,

we have extensively used our computer codes which solve Maxwell's equations for the

propagation of the beam through the system. The code accurately determines the diffraction effects that are produced by the critical points of the system such as soft

apertures .

A soft aperture with too sharp an attenuating edge will itself produce diffraction rings, while too gradual an attenuating edge wastes a large area of the rod;

therefore, in the design of the soft apertures there is a trade-off between the area

of the rod used and the diffraction pattern generated by the aperture. We have

designed, tested and used several types of graduated apertures.

6. Film Apertures

Film apertures are inexpensive and easy to make, but are suitable only for low

power application. The substrate the emulsion is on should be of good optical

quality, and to avoid any phase front distortion by the emulsion, the aperture may

be contained in an index matched liquid gate.

7. Variable Transmission Metal Layers on Glass

Metal layer apertures are more difficult and expensive to make than film apertures

and can be used only in regions of low power density. The gold coated apertures we tested successfully attenuated the beam, but damaged at a lower power density level

than the film apertures.

8. Dielectric Layers on Glass

Dielectric layers have a high damage threshold and can withstand 5-10 GW/cm^.

These work well as slowly graded attenuators to shape the intensity profile of the

beam, but we have not been able to obtain any edges that are graded sharply and

accurately enough to use as a graduated aperture in front of a rod and still effectively use most of the area of the rod.

9. Piano - spherical Lenses

Piano-spherical glass and quartz lens combinations have been used with index

matched absorbing liquids for attenuation. Both saturable dye and non- saturable

inorganic absorbers have been successfully used. These have worked very well as graduated apertures at all intensity levels. The major problem has been index

matching the absorbing liquid to the glass of the lenses. Once the index match

is obtained, the temperature must be regulated very closely.

We have also designed, but not fabricated, apodizers based on field dependent

rotation on pockels cells and also an optical rotation in quartz.

Using these techniques, we have been able to operate the KMS laser well above

5 GW/cm^ with a minimal amount of glass damage and a reduction in beam distortion.

COMMENTS OM PAPER BY BRUECKMER

Thz. Apzaktn. notzd that the. bzam d^cu e/ig ence jj^om tht iyittm l6 ob^e^ued to be. OjJ

the ofiden. a ^ew tenthi o (, a m-ill'ifia.d'icin at lorn powen. den6-ttij, -Lncnea^-inQ itgniiicantZy mlth pomeK. He commented that M-Lthoat the uie oj) apodtzed ape^tuKei and

cafie^al iyitem deiign, up to 50% the powen. o{, the beam ti, to-dt In the halo a^-iitng

{,fLom imatt i>c.ale ieli {iOc.u.i>-LnQ . WJ^th the uie of, apod-ized ape^tafiei , SO to 90% oj^

the energy can be maintained tn the central beam.

6
1 . 2 Nonlinear Propagation Studies"

E. S. Bliss

Lawrence Livermore Laboratory

Livermore, California 94550

The desire to avoid damaging the ccmponents of high-power solid-state lasersystems being built for laser fusion experiments has led to increased interest in

all aspects of intense pulse propagation. This paper describes a variety of

observations of nonlinear propagation phenomena and the instrumentation used for

measuring them, presents comparisons with theoretical predictions, and outlines

some approaches to minimization of the resulting problems in large systems. The

intent of the paper is to present an up-to-date summary of the progress being made

in characterizing and controlling nonlinear propagation effects.

Key words: Apodization; beam breakup; instability theory; nonlinear propagation;

propagation codes; self-focusing.

1. Introduction

The major problems of intense pulse propagation in a laser system are component damage and

energy loss caused by whole-beam self-focusing, nonlinear enhancement of diffraction patterns, and

beam instability with respect to small-scale intensity or phase irregularities. The development of a laser system producing 150 picosecond pulses with temporal and spatial noise of less than a few

percent at 20 GW/cm^ in a 7 mm diameter beam [1]-'- has made it possible to perform new experiments

in these areas. This paper discusses experimental and theoretical results and the implications for

system optimization.

2. Whole Beam Self-Focusing

The self-focusing of all or part of an intense beam in solid state laser materials has been

extensively studied both experimentally and theoretically [2]. The experiments, with an occasional

exception [3], have consisted of observing the damage resulting from self-focusing, but it is

possible with pulses having a smooth enough intensity profile to observe and measure the selffocusing of fairly large beams nondestruct ively.

One way to do this is to pass a collimated beam through a sample rod and observe the reduction

in beam size that occurs several meters away. For comparison, a small fraction of the same pulse

can be sent to the film over the same path length but without going through the rod. A representative

result of such an experiment is shown in Figure 1. The multiple exposure-level photographs clearly

show both an increased on-axis intensity and a reduction in diameter for the part of the beam vrhich

went through the sample. As expected [4], some distortion of the beam shape also occurs.

The beam photographs of Figure 1 are a time-integrated record of a dynamic process in which the

focusing properties of the sample rod vary continuously during the pulse as the intensity varies

fran zero to its peak value and back to zero again. When the film is replaced by a IG-picosecondresolution streaking-camera, the whole process can be time resolved as illustrated in Figure 2.

'''This work was performed under the auspices of the United States Atomic Energy Commission.

'Figures in brackets indicate the literature references at the end of this paper.

7
In a large solid-state laser-system, dynamic whole-beam self-focusing occurs in every component. The details of the resulting distortion depend on the initial beam profile and virtually

every other system parameter. These facts make meaningful system optimization strongly dependent on the ability to accurately compute the effects of parameter changes, so that only the most promising configurations need actually be tried. Using this kind of experimentally-verified computermodeling combined with a knowledge of state-of-the-art capabilities in the production of such beam

correcting elements as phase plates and apodizers, one can optimize a large system for a particular

well-defined application.

Since self-focusing is a consequence of an intensity-dependent refractive index, the most fundamental observation one can make is an interferometric measurement of the refractive index change as a function of intensity. Figure 3 is a photograph of the time-resolved I^ch-Zehnder interferometer

used to obtain such measurements on sanples of laser glass, Yag, and Faraday rotator glasses. The

interferometer is adjusted to give a pattern of vertical fringes centered on the horizontal slit of

the streaking camera, and index changes occurring in one ann of the interferometer but not in the

other cause the fringes to move horizontally across the slit. The splitter reflectivities in the

interferometer are chosen so that 99% of the pulse energy goes through the sample arm and only 1%

througih the reference arm. This guarantees that index changes in the reference arm are negligible

even though it also contains a rod of the sample material for convenience in alignment. From the

streaking-camera photographs one can plot the fringe shift, and therefore the index change in the

sample, as a function of both time and position. Figure 4 is an example of the results.

The data obtained in any of these self-focusing experiments can be used to determine the nonlinear refractive index coefficient of the sample material. In fact, this was the min motivation

for doing them, and we now know the value of n2 for the materials being used in our large glass

laser systems [5]. Similar measurements on a second group of materials have also been completed [6].

3. Diffraction Pattern Enhancement

Any laser system has limiting apertures which will generate diffraction patterns on the beam if

light is allowed to hit them. Continuing to propagate such a beam through glass at high intensity

leads to the familiar result [7]: the rings self-focus and damage the glass. To control the formation of diffraction patterns, the limiting aperture or apertures must exhibit a smooth transition

from high to low transmission. A variable-thickness dye-cell such as shown in Figure 5 is a particularly simple version of such a graded aperture or apodizer, and we have used this type in one of our systems since late 1972 [8]. Other kinds of apodizers are now in use or under development as discussed elsewhere [9-14].

Of course, "smoothness" is not the only constraint on the transmission function of an apodized

aperture. If the apodized beam has anything but a Gaussian radial-intensity-profile, there will

always be a tendency for rings to form after a sufficient propagation distance; so a careful compromise has to be made between the desire to uniformly fill the system aperture for maximum energy

extraction and the need to avoid damage caused by diffraction ring enhancement.

The importance of choosing the proper apodizing function for optimum propagation is illustrated

in Figijre 6. These results of calculations with a propagation code containing both diffraction and an intensity dependent refractive index [15] show that a slight increase in the steepness of the

profile can cause a dramatic change in the generation and subsequent growth of diffraction features

as an intense beam propagates through a disk amplifier chain [16]. Laboratory experience with

various apodizing functions confirms this prediction.

4. Snail-Scale Beam Instabilities

Everyone who has worked with large solid-state lasers is familiar with the highly structured

beams and finely tracked rods or disks that can result from intense pulse propagation. Figure 7 shows how dramatically the beam profile can change in passing through a single glass rod. The

growth of such structure from an input beam containing various degrees of modulation has been

described by several forms of linearized instability theory and observed experimentally by a number

of workers [17-26].

In an experiment aimed at obtaining a more detailed comparison between experiment and theory, we

have propagated pulses with measured temporal profiles and well-characterized spatial-modulation

through a glass rod at known intensities. The spatial structure is generated by passing the beam

8
through a shear plate composed of the inner surfaces of two closely-spaced quartz wedges. When the

plates are at a small angle to each other, the interference between the main beam and the first

forward reflected beam produces a sinusoidal intensity modulation in one dimension.

The simplest form of instability theory [17,21] predicts that small-scale noise grows

exponentially with a gain coefficient having a maximum as a function of spatial frequency as shown

in Figure 8. For wave numbers near the peaj< of this curve we do experimentally observe pronounced

growth of the modulation as seen in Figure 9, but departures from this theory exceed the experimental error. A form of linearized instability theory which specifically treats the growth of shear

plate type modulation does agree with the experiments, however, and a complete discussion of the

results appears elsewhere [27,28], The conclusion is that the growth of small-scale structure in a

single rod is well understood.

Determining what happens when disks are substituted for the rod is more complicated both

theoretically and experimentally. A qualitative difference between the two cases is observed when a

20 GW/cm beam which has passed through 10 disks with a glass propagation distance of 24 cm is can- pared with the same beam after passing through a 24 an long rod. From the representative beam photos

in Figure 10 it is apparent that the dominant structure out of the rod is of a higher spatial

frequency than that out of the disks.

The reason for this difference is shown schematically in Figure 11. For the beam as a whole,

each disk acts as a very weak self-focusing lens and the effect of a series of disks is to reduce the

beam size. For some smaller-scale beam irregularity of intensity height AI, the focusing effect of

each disk is just cancelled by the tendency for the irregularity to diffract. Finally for very

small-scale structure with the same AI, diffraction overcomes the self-focusing and the irregularities

diffract away. To obtain quantitative information which can more readily be compared to conputer

calculations, a new experiment is being conducted. A single one-dimensional bump is superimposed on an otherwise smooth beam and propagated through a rod or an equivalent number of disks. The growth

rate for the bump can then be measured as a function of its width for both rods and disks.

The glass damage caused by extreme beam breakup is an important concern in the design of large

laser systems, but it is not automatically the over-riding one, since under seme circumstances a frequent glass replacement program may be justified. The more fundamental concern is how the beam

behaves after developing so much small-scale structure.

In an early experiment the ratio of the energy reaching a distant calorimeter to that reaching a nearby calorimeter after propagating through a rod was plotted against the input energy density. The

result, shown in Figure 12, suggests that at high intensities some light is scattered into angles

outside the aperture of the distant calorimeter. We are presently investigating the far field energy

distribution of beams which have suffered various degrees of breakup to determine its effect on

one's ability to focus the beam as a whole, because in the final analysis, optimizing a large system

for laser fusion experiments will require maximizing the energy which can be focused through a targetsize hole.

5. Reference

[1] Speck, D. R. and Bliss, E. S., Lawrence [8]

Livermore Laboratory Laser-Fusion Program

Semiannual Report - January-June 19 73,

UCRL-50021-73-1, pp. 20-22, Available from

the National Technical Information Center.

[2] See, for example, the proceedings of this

symposium for the previous five years,

[3] Newman, B. E. and DeShazer, L. G. , Proc. [10]

3rd ASTM Symp. Damage in Laser Materials,

NBS Spec, Publ, 356, 124 (1971).

[4] Dawes, E. L. and Marburger, J. H. , Phys. [11]

Rev, 179_, 862 (1969),

[5] Bliss, E. S., Speck, D, R, and Simmons ,W.W.

,

to be submitted for publication (1974).

[6] Moran, M. J,, She, C, Y, and Carman, R. L. , ^"""^^ to be submitted for publication (1974).

[7] Fleck, J. A. and Layne, C. , Appl. Phys,

Lett. 22^, 467 (1973). ^^^J

Speck, D. R. and Bliss, E. S., Lawrence

Livermore Laboratory Laser-Fusion Program

Semiannual Report - July-December 1972,

UCRL-50021-72-2, pp. 18-19.

Johnson, B. C. , Lawrence Livermore Laboratory

Laser-Fusion Program Semiannual Report - January-June 1973, UCRL-50021-73-1, pp, 29-31,

Simmons, W. W. , Leppelmeier, G, W. and

Johnson, B. C. , Appl. Optics, to be published

(1974)

o

Johnson, B. C. and Bliss, E. S., Lawrence

Livermore Laboratory Laser-Fusion Program

Semiannual Report - July-December 1973,

UCRL-50021-73-2 (in press),

Brueckner, K. A., Guscott, B., Jorna, S.,

Moncur, K. and Siebert, L,, elsewhere in

these proceedings,

Campillo, A. J, Carpenter, B,, Newman, B, E. and Shapiro, S. L. , Optics Communications, to be published (1974)

,

9
[14] Costich, V. R. and Johnson, B. C, Laser

Focus, to be published (1974).

[15] Fleck, J. A., J. of Computational Physics,

to be published (1974).

[16] Morris, J. R. , unpublished (1974).

[17] Bespalov, V. I. and Talanov, V. I., JETP

Letters 3, 307 (1966)

.

[18] Chiao, R. Y., Kelley, P. L. and Garmire, E.

,

Phys. Rev. Letters l^' 1158 (1966).

[19] Cannan, R. L., Chiao, R. Y. and Kelley, P.L.,

Phys. Rev. Letters _17, 1281 (1966).

[20] Akhmanov, S. A., Khokhlov, R. V. and

Sukhorukov, A. P., in Laser Handbook , ed. F. T. Arecchi and E. 0. Schulz-DuBoi

(North Holland, 1972), p. 1168.

[21] Glass, A. J., Proc. 5th Symp. Laser Induced

Damage in Optical Materials, NBS Spec.

Pub. 387, 36 (1973).

[22] Suydam, B. R. , Proc. 5th Symp. Laser Induced

Damage in Optical Materials, NBS Spec.

Pub. 387, 42 (1973).

6. Figures

m m

Around Through

Figure 1. Photograph showing self-focusing of an approximately 7 mm diameter, 150 picosecond pulse

with a peak Intensity of 2.5 GW/cm^ by a 50 cm long

glass rod at a distance of 7 meters.

[23] Marburger, J., Jokippi, R. , Glass, A. and

Trenholme, J., Proc. 5th Symp. Laser Induced

Damage in Optical Materials, NBS Spec. Pub.

387, 49 (1973).

[24] Camplllo, A. J., Shapiro, S. L. and

Suydam, B. R. , Appl. Phys. Letters 23, 628

(1973) .

[25] Jokippi, J. R. and Marburger, J., Appl. Phys.

Letters 23, 696 (1973)

.

[26] Campillo, A. J., Shapiro, S. L. and

Suydam, B. R. , Appl. Phys. Letters 24, 178

(1974) .

[27] Bliss, E. S., Speck, D. R. , Holzrichter, J.F.,

Erkkila, J. H. and Glass, A. J., submitted for

publication (1974).

[28] Erkkila, J., Bliss, E. S. and Glass, A. J.,

unpublished (1974)

.

10
Figure 2. Isodensity contours of a streaking camera photograph

showing the temporal dependence of the diameter of a selffocused beamo

Figure 3. Time-resolved Mach-Zehnder interferometer for observing the

intensity dependent refractive index change caused by intense laser pulses in

transparent materials. The components positioned off of the main beam lines

are on secondary beams used for streak camera triggering and for monitoring

the temporal variation of the input pulse.

11
EY-1 glass Shot #23, 12 Sept 73

Pulse duration (FWHM) = 165 psec

At = 30 picoseconds

Time

Figure 4. A representative plot of fringe shift (vertical axis) versus

time and position on the slit from data obtained on the time-resolved

Mach-Zehnder interferometer.

EK 9740

Dye flow

Beam

Direction

J

Figure 5. Variable thickness dye apodizer. Some versions use two curved

windows and have slight curvatures on the outer surfaces as well to correct for index mismatch between the dye and window material.

12
E

(A

E

0)

E

u

tn

E

0.5 1.0 1.5

10

I I T I Z = 7.825 m

1 \^ 1 1 1

0 1 2 3 4 5 6

Radius (cm)

Figure 6. Propagation calculations for two intensity profiles of the

form loexp (-r/a) The left-hand curves are the input profiles for

N = 5 and 10. The right-hand curves are the developing beam-shapes

after propagating 7,8 meters at design intensities into the LLL

developmental glass amplifier chain.

Damage

Figure 7. Small scale beam breakup at 20 GW/cm^ in an unpumped 2A cm long glass rod. The photograph at the bottom shows the type of

filamentary damage which results.

564-378 O - 74 - 3

13
Normalized wave number (K/K

Figure 8. The variation of the exponential gain coefficient for the

growth of small-scale intensity modulations as a function of modulation wave number as predicted by the simplest form of linearized instability

theory.

Input Output after propagating 24 cm

Figure 9. Beam photographs and densitometer scans showing the growth

of shear-plate interference fringes spaced 2 ram apart (K = 119 cm~^) on a pulse with a peak intensity of 5 GW/cm^.

14
Rod 10 Disks

Glass propagation distance = 24 cm

Intensity = 20 GW/cm^

Figure 10, Comparison of the beam breakup caused by propagation

through a rod with that caused by propagation through a series of disks.

Figure 11. Schematic drawing of the behavior of various bumps when

propagating through a series of disks.

15
Figure 12. Energy collection at a distant calorimeter as a function of

input energy density. For this plot "relative energy" is defined as that

fraction of the energy exiting the rod which is still within the fixed

1.6 cm aperture of the calorimeter after propagating 6 meters.

COmEMTS ON PAPER EV BLISS

A numbeM. Ojj po-inZ{> nzQoAcLinQ thu zxpMA.tmniaZ izXap oied -in tho. mnk deAcAlbud -in tkii papeA. weAe

cI.aAA,^A.e.d. Tho. qaej,tion mcu, cu>kzd itgcviding tho. ui,Q. icutuAable. dyeA i.n thz dyz apotizn/u, . Thz

iptaksA c.omrmnte,d thcut gmeAatZy iotuxable. dy&6 wejie. uA>zd, but undeA. tka cVLaumitanc&6 -Ln whyich thzy

fiojipondtd In a tinzcui (^cudkion. With fizQcuid to tkz ancLlyi-ii thz zxpeAA-mznt on thz gfiowth o{, thz

dmaUi heAi {^ocu^tng, the, tmpofutancz oi tAzcuting the. inttJjxi condiZiond o{s the problem KeaLidticoJULij

,

that, -its iay, o^ Kepn.oducA.ng in the. anatyi'ii the KeZaZive phcu>e o{, the tntemtty and pha^z moduZatton

IntAoduced at the. enttance {,ace the gla^i, sample by the ihzaxing plate -inteAdeAometeA wa6 e/nphadlzzd.

It wcw pointed out. that thz introduction o{j the appiopntate initial, conditions led to extnzmeJiy good

agKezment between calculation and theory and that the analytic model, mo^ {^u/itheA iubitantiated by

detatted nujneAicat computation.

16
Suppression of Parasitic Oscillation

In Laser Rod Amplifiers

J. M. McMahon, R. P. Burns and T. H. DeRieux

Naval Research Laboratory

Washington, D. C.

The most coinmon glass laser rod amplifier geometry is one in which the laser

rod is centered in a glass water jacket and pumped by Xenon flashlaraps surrounding

the water jacket. By suppressing parasitic modes in the amplifier much higher

gains can be realized than previously achieved with a consequent increase in the

short pulse damage threshold. 30-50% improvements have been attained in the NRL

system and factor of two improvements are possible.

Key words: Disc lasers; laser amplifiers; laser design; parasitic

suppression.

1. Introduction

The generation of intense subnanosecond laser pulses is required for laser fusion and related

applications. Total energies in the kilojoule range and subnanosecond pulse require laser systems

capable of operating routinely at intensities above 10^0 W/cm^. For neodymium glass laser systems

nonlinear effects severely impact the ability to generate such peak powers with high system reliability and efficiency „ The breakup of the laser beam into filamentary regions with consequent damage

to laser components has been studied extensively in recent years as this represents the major limitation for high peak power laser design. Marberger has pointed out that the dominant effect is not

beam collapse (self focusing of the entire beam) but the self focusing of intensity fluctuations on the beam which can have a much lower threshold than whole beam self-focusing. [1]' Suydam has extended

this analysis to laser amplifiers and finds that the gain coefficient for the growth of intensity

fluctuations at the most unstable ripple frequency will be ~.73 of that given by Marburger's analysis

for passive propagation since the intensity varies in the amplifier. [2] More importantly, he found

that the safe operating level will vary directly as the gain coefficient a (for amplifiers where

saturation is not present). Experiments at NRL & LLL are in reasonable agreement with these results.

Apodization techniques have resulted in improved laser performances by minimizing the spatial

frequency "noise" (ripple) on the pulse but these cannot totally eliminate the problem since index

inhomogeneities will always be present to some degree and will constitute a spatial noise source. [3,4,5]

It is more appropriate to attempt to suppress beam breakup by increasing the gain coefficient per unit

length since this will directly tradeoff with safe intensity.

Disc amplifiers offer distinct advantages at large aperture diameters over rod amplifiers, as there is no surface area to volume ratio constraint on the optical pumping. [6] Parasitic oscillation

problem^ have been extensively investigated and adequate solutions appear in hand for practical systems. [7,8,9] On the other hand, the many surfaces in a disc amplifier present a maintenance problem

and we have observed some indications of flashlamp induced damage to disc surfaces.

Rod amplifiers offer a number of significant advantages where they can be used. There is no

surface problem, the pumping efficiency is much higher and it is a much cheaper geometry to construct.

There have been reports in the literature of transient losses under very hard pumping, [10,11] as well

as results showing a very fast roll off in the gain coefficient as the pump energy is increased- [12]

Because of the possibly large benefit in high gain large aperture rods over moderate aperture disc

amplifiers we decided to investigate the reasons for the reported behavior of rod amplifiers.

The CGE rod amplifiers provide a convenient test vehicle for this investigation as all the flashlamp pumping modules are identical and successive amplifiers have twice the beam area while the radiusneodymium doping product is held constant. This allows intercomparison of optical pumping efficiencies

as well as giving different radial and axial gain aspect ratios (a D vs aL)

.

A further experiment was to retrofit the 64 mm laser head with a 44 mm aperture disc laser

structure (shown in figure 1). This allowed a direct measure of the variation of gain with pump

energy in a case where parasitics were stabilized and also allowed a direct intercomparison of the

relative coupling efficiency to disc and rod structures. It was found that there was no fast roll

off in the gain coefficient but rather that it increased as expected from earlier results of the ZAP

1. Figures in brackets indicate the literature references at the end of this paper.

17
optical pumping code. [9,13] It was further found that the pump geometry coupled three times more efficiently to the 64 tmn rod than to the disc structure.

The disc laser results allowed us to predict the relative gain coefficient vs. pump energy for

the 45, 32 and 23 mm amplifier rods. All three amplifiers were found to depart from the predicted

values above a D = 0.19. This factor strongly argued against a flashlamp induced loss coefficient

as the cause of the rollover. The gain dependence argued that it was a parasitic oscillation dependence and the fact that cy D and not cy L was the relevant parameter argued that it was a radial parasitic

rather than a longitudinal parasitic mode.

The pumping geometry for the three CGE amplifiers offers several possible parasitic paths. One

radial mode (henceforth designated mode #1) has a ray path where the ray bounces around the rod at

the critical angle. For the indices of refraction of KD-2 and water this ray can only run in the

outer periphery of the amplifier (K^ > (v'372)R; however scattering of some of the energy off the

roughened sidewall into the center of the rod would deplete the gain. A second radial mode consists

of light reflecting at the critical angle from the interface between the water jacket and air (Mode

#2) . There are two analogous longitudinal modes (Modes #3 and 4)

.

2. Suppression of Lowest Order Parasitic Mode

The approach attempted to suppressing the lowest order parasitic was to dope the water with

Samarium salts to increase the index of refraction. The high 1.06pm absorption of the solutions

employed would suppress modes involving long parasitic ray paths in the water jacket (Modes No. 2 and 4 )

.

We can calculate a threshold condition for Mode No. 1 by noting that the mode will bounce off

the sidewall m times per transit at the critical angle with some loss to scattering each bounce,

hence

Sa^ = m ;e.n R (1)

Where S is the path around the rod ( ~ jtD) and q-^ is the gain value where the mode exceeds threshold (the gain at the periphery is 1.2 the central gain in these amplifiers). We can also calculate

the number of bounces as a function of index mismatch as

1 + 180

90-arcsin(l-e)

(2)

where e = An/no.

For water in the jacket eqs. 1 and 2 give R = 91% which implies Rg = 9%. The Samarium solutions were then used to raise the index to n = 1.46 in several steps. The pumping efficiency was not

affected by the Samarium solution. Figure 2 shows the variation in threshold vs. index mismatch.

The fit to the calculation is quite good and implies that it is indeed mode #1 which first exceeds

threshold. Figure 3 shows the improvement in the 45 mm amplifier. Further it can be inferred that

with a solution with an index of 1.50 the CGE amplifiers would be stabilized (rvD < .37).

3. Results with High Index Solutions

The index of the Samarium solutions could not be increased above n=1.46 before the Samarium began

to precipitate out at 20°C . Solutions of Zinc Chloride and Zinc Bromide were investigated. [14] Figure 4 shows the index of refraction vs. concentration for both the zinc and samarium solutions. ZnCl2

solutions could be stably obtained with an index of 1.53 as determined by a minimum deviation prism

technique. [14,15] While the Zn3r2 solutions had higher indices of refraction, they were highly

absorbing in the laser pump bands. The zinc chloride solutions were measured on a GARY 14 and found

to be quite transparent in the neodymium pump bands. Experiments in the laser heads with zinc

chloride solutions showed that optical pumping produced an undesirable chemical change. Zn(0H)2,

which is a white insoluble compound, was formed in a few shots. This reaction was prevented by adding

3-57o HC;^ to the solution.

Initially a ZnCi2 solution with an index of 1,50 was tried in the 32 mm amplifier. It was found

to perform almost as poorly as the same amplifier with water alone in the jacket. Some samarium

chloride was added to the solution. There was a dramatic Improvement. Figure 5 shows the results

with the 32 mm amplifier with the various solutions.

Factor of two improvements in the gain (23% increase in a) were found with the best solutions

compared to water cladding. However, it was clear that a parasitic involving paths in the water

18
jacket was also happening. The Increase in gain when samarium was added to the water jacket corres- ponded to stabilization of the radial water jacket mode.

The only remaining possible parasitic mode corresponds to a longitudinal oscillation involving

the rod faces, sidewalls and water jacket. To test for this mode, one end of the 23 mm rod was antireflection coated (R — .01). As shown in figure 6, the gain curve followed the predicted curve almost

all the way to peak energy. The roll off was due to the single AR coating being insufficient to avoid

oscillation at the highest gains. With better AR coating(s) this mode would have been completely

suppressed. The fact that the curve exceeds the predicted values is due to improved coupling between

rod and lamps and is also an area where large increases in the gain coefficient may be possible. With

the solution with an index of 1.52 and anti-reflection coatings the gain coefficients of the 23 and

32 mm CGE amplifiers can be increased by kTL and 55% respectively. The gain of the 45 mm amplifier

is pump energy limited but it can be increased by 40%.

4. Pumping Efficiency of CGE Rod Amplifiers

It was found that while the achieved gain coefficient oi varied as R"-*- the pumping efficiency

improved a great deal for the larger amplifiers. This fact was found to have a geometrical interpretation in the coupling of pump light to the water jacket. In this geometry the water jacket

should be 1.33 times the rod diameter to image the rod as the full jacket diameter. This Is in fact

not quite the case with the CGE amplifiers as is shown in table 1. The optimum diameter is listed

as D*o D-^ is the center line diameter of the helical flashlamps.

Table 1.

Amplifier Rod Dia. Jacket Dia. Lamp Dia. (D,

)

V23A2 23 mm 36 mm 30.6 mm 53 mm

V32A4 32 mm 46.5 mm 42.6 mm 62 mm

V45A4 45 mm 58 mm 60 mm 75 mm

A ^omparison of the relative transfer efficiencies of the amplifiers shows that it scales as

(D*/Dj^) (figure 7). With the higher index solutions D* increases and a geometrical improvement in

pumping efficiency of 237c. for the 23 mm head and 197o for the 32 mm head would be expected if there were no additional absorption by the dopants in the water. With the samarium solutions, the efficiency stayed the same as samarium was added, indicating that the blue absorption of the samarium

was sufficient to offset the geometrical improvement. The amount of samarium chloride in the high

index zinc solutions was decreased to 5gm/100 ml (which was still sufficient to stabilize mode #2)

and both the 23 and 32 mm amplifiers showed ~ 127o increases in pumping efficiency. This indicates

that there was an 8-10% loss due to the solution which was more than counter-balanced by improvements

in the geometrical coupling. Table 2 lists the initial and final results for the on axis gain

coefficient for the three CGE amplifiers.

Table 2.

Amplifier (o-jJ)^ (Q'^)p

V23A2 2.9 4.25

V32A4 3.3 5.1

V45A4 2.25 3.10

With helical pumping in the CGE heads and parasitic suppression values of cvD ~ 3.6-3.8 can be

attained. This does not represent the ultimate which can be achieved. With a zinc solution with

an index of 1.53; an q-D = 6 can be attained without parasitic oscillation problems.

19
With close coupled linear flashlamp arrays values of q-D in this range can be attained. Use of a pump geometry similar to the linearly pumped NRL disc amplifier would allow pumping to an .65 o'D<0.7,

The results achieved at Rochester on a 9 cm amplifier are consistent with a pump limited if it is

noted that the water jacket parasitic is evident in their data beginning at cvD ~ .19 (figure 8). With

state of the art pumping geometries energy storage efficiencies of — 2% (at = .6) appear realistic.

5 . Summary

1. The dominant gain limitation in glass rod amplifiers is parasitic oscillation. By use of

available index matching solutions the lowest threshold mode can be stabilized for o'D<0.6.

2. The next lowest threshold mode involves reflection from the rod ends and can be suppressed

in any one of a number of ways

.

3. With helically pumped CGE amplifiers it has proven possible to increase the gain coefficient

by 30-507o using solutions which are stable in the laser head.

4. With optimized linear pumping geometries it should be possible to achieve o'D=0.5. From small

scale self focusing theory and experiments this should result in a safe output power density from

64 mm rod amplifiers 3 times higher than with the standard CGE amplifier.

5. If disc amplifiers and rod amplifiers are equally susceptible to small scale self focusing

the tradeoff point in terms of equivalent gains (9-10/'o/cm) will occur at 6-7 cm aperture diameters

but the ~3 fold efficiency factor in favor of rods may make the economic tradeoff point occur at

10-11 cm aperture.

6. For a system with exponential gain throughout, measurements indicate a safe level of

I < 3 X lO-'--'- a for stable beams with less than X/2 wavefront distortion and no beam breakup. For

an amplifier with cy D = .6, this implies that the safe level is 1.8 x lO''-'- D"-*- W/cm^. For nanosecond

pulses and D < 10 cm, surface damage will be the problem and not small scale self-focusing.

7. No pump induced loss was found in this study. If any such loss was present it could not

have been greater than 17o/cm. The authors of the earlier reports which appeared to show this (present

company included) were probably misled by using the rod fluorescence as an energy storage diagnostic

when the "soft" parasitic was present.

6. Acknowledgements

The authors would like to acknowledge helpful discussions with Dr. J. R. Airey, Dr. J. L. Emmett

and Dr. G. Dub4 which materially assisted the successful completion of this effort.

7. References

[1] J. Marburger, NBS Spec. Publication 356,

51, (1971).

[2] B, R. Suydam, Los Alamos Sci. Lab Report

LA-UR- 73-671 (1973).

[3] J. Marburger, R. Jokipii, A. Glass,

J. Trenholme, Laser Fusion Program Semiannual Report Jan. -June 1973, UCRL-50021-73-1,

145 (1973).

[4] J. F. Holtzrichter , et al., "Laser Target

Interaction Exps. at NRL", APS Plasma Physics

Division Meeting, Monterey, California

(Nov. 1972).

[5] R. Speck, E. Bliss, IEEE Conf. on Laser

Engineering & Applications, Wash., D. C,

May 1973.

[6] J. L. Emmett, et al., "High Peak Power

Lasers", IEEE Int'l. Quantum Electronics

Conf., Montreal, May 1972.

[7] S. W. Mead, R. E. Kidder, J. E. Swain,

F. Rainer & J. Petruzzi, Appl. Optics 11, 345

(1972) .

[8] J. B. Trenholme, NRL Memorandum Report 2480

(July 1972).

[9] J. M. McMahon, J. L. Emmett, J. F. Holtzrichter and J. B. Trenholme, IEEE

J:Quant. Elect. QE-9, 992 (1973).

[10] J. M. McMahon, ASTM Spec. Technical Publication 469, 117 (1969).

[11] C. R. Jones, P. V. Avizones, P. Sivgals, NBS

Special Publication 341, 28 (1970).

[12] J. Soures, S. Kumpan and J. Hoose, Laboratory

for Laser Energetics, U. of Rochester, Report

No. 20, 24 (1973).

[13] J. M. McMahon, 0. C. Barr, and R. P. Burns,

IEEE Proc. Int'l. Electron Devices Meeting

(1973) .

[14] G. Dube and N. L. Boling, Applied Optics, 13,

4, 699 (1974).

[15] J. Strong, Concepts of Classical Optics,

W. H. Freeman &Co., San Fran. , (1958), p. 214

20
8. Figures

Figure 1. 44 mm aperture disc laser structure which was retrofitted in CGE V64AB amplifier head

in place of a 64 mm laser rod.

0 05 010

RELATIVE INDEX MISMATCH Figure 2. Threshold for radial mode #1.

The data points are experimental, the curve is calculated from equations 1 and 2.

05 10 RELATIVE PUMP ENERGY

Figure 3. Gain vs. pump energy for V45A4

amplifier with water (0) and with a samarium

chloride solution with an index of 1.46.

2 3 4 5 CONCENTRATION (gm j/m/HjO)

Figure 4. Index of refraction vs. concentration

for solutions used. X-samarium nitrate;

+-samarium chloride; o-zinc chloride; Q-zinc

bromate.

21
RELATIVE PUMP ENERGY

Figure 5. Gain vs. pump energy for V32A4

amplifier wich various solutions. O-H2O;

O -samarium chloride (n=1.44); X-zinc

chloride (n=1.50); +-z.inc chloride and

samarium chloride (n=1.50).

I \ \ \ I 2 4 6

(cm)

Figure 7. Relative pumping efficiency of 23, 32 and

A5 mm laser heads. Curve is iT)*/'Dj)^ vs. D normalized to the 23 mm data.

/

RELATIVE PUMP ENERGY

Figure 6. Gain vs pump energy for V32A2 amplifiers

with various solutions. O-H2O; •-samarium chloride

(n=1.46); x-zinc chloride (n=1.52); + - zinc chloride + sm chloride (n=1.52); A- same as + but with laser

rod end anti-reflection coated.

y

/

05 I /

/

/ FLASHLAMP RESULT 3.4%

PUMP ENERGY DENSITY (J/cc) Figure 8. U. of Rochester result for a linearly

pumped 90 mm rod amplifier, - Rochester data;

expected result for flashlamp transfer.

COMMEWTS OW PAPER W McUAHON

Thd question moa n.OiU,iid about thz itab-ititij zinc buomldz in i>oliuution undvi aging. The ipojakun.

pointed out that the ioZation only needi to bz itable. in an imnX atmoiphz/Lt, but that long tern

itabititij woi clojinZg a pnoblm (^ok any solution that wa^ oied. He commented that the lamp-induced

OK pump-induced iuK{,ace damage may be a consequence o{, the pump-induced itxeii at the isu/i^^ace due to the itnong absorption by cenium in the lasen. glaa, especialty in highly pumped diioi. Even thii type

0(5 damage Mas stwngly dependent upon the condition o{, the suA^ace. The tendency to damage was re- duced by coJieiuZ cleaning and by stashing the glass discA in an inent atmosphefie when not in uie.

22
1-'^ Optical Requirements for Laser Mirrors

H. E. Bennett and P. C. Archibald

Michelson Laboratory, Naval Weapons Center

China Lake, California 93555

The surface and coating requirements for mirrors used in laser systems are very severe. They may be summarized as follows: (1) excellent figure under

thermal load, (2) high catastrophic damage threshold and an absence of localized

sites where damage is likely to occur, and (3) low scattering levels. Backscattering is particularly important since it can easily damage the laser itself.

The figure requirements for components increase rapidly when multicomponent "real

world" optical trains are used. For example, to satisfy Marechal's criterion

for a 10 component system used at a wavelength of 3.8 ym, each component must

have an initial figure of approximately l/8th wave in the visible region and

thermal distortion over the surface of less than 1000 X. Very high reflectance

coatings with excellent uniformity are thus required. Light scattering at short

wavelengths provides a powerful, nondestructive tool for measuring microroughness

(which reduces the infrared reflectance of metal coatings) , and for identifying

potential sites where damage is likely to occur. The damage threshold of a metal mirror is not necessarily determined by its absorption, and our understanding of the laser mirror problem is still incomplete.

Key words: Infrared absorption; laser damage; laser mirror; laser optical

train; laser window; microroughness; optical figure; scattered light.

1. Introduction

Dr. Glass has defined "laser damage" as "whatever turns you off." One thing which can "turn you

off" is the mirror train in a laser system. The surface and coating requirements for such mirrors are very severe, particularly in multielement systems. They may be summarized as (1) excellent figure

even under thermal load, (2) high catastrophic damage threshold and an absence of localized sites where

damage may occur, and (3) low scattered light levels (particularly low backscattering) . The latter two

requirements are necessary to avoid catastrophic damage to the laser system itself. The first requirement is of a different sort. Failure to meet this requirement will not result in damage to the laser

system but the output beam will not be focussable, "turning you off." In this paper the requirements

for laser mirrors used in an optical train will be discussed, and methods for evaluating and improving

such mirrors will be given.

2. Figure Requirements

If the output beam from a laser is to be focussed, wave front distortion becomes extremely important. The primary sources of wave front distortion are (1) nonuniformities in the active medium, (2)

distortion caused by figure errors in mirrors or windows in the optical train, (3) refractive index

variations in the window, and (4) atmospheric nonunif ormities. Both initial and thermally induced

phenomena contribute to each of these effects. If the resulting wave front distortion is sufficiently

large, it may be necessary to resort to techniques such as apodization or wave front correction using

active optical elements. However the additional inefficiency, complexity and cost involved in such

solutions makes it desirable to minimize wave front distortion rather than to correct for it. The

crucial parameter in most laser applications is the on-axis focussed intensity. This on-axis intensity initially falls off very rapidly as wave front distortion increases, making the optical excellence required for maximum intensity more stringent than that required for high resolution optics.

This situation is in sharp contrast to the relaxed criteria which apply when only a high flux level is

required over an area large compared to the diffraction limit of the focussed beam. The reason for

the stringent requirements on focussed optics is that, as wave front distortion increases, the shape

of a focussed beam near maximum intensity changes very slowly. However the on-axis intensity decreases

quite rapidly, the extra energy going into the diffraction wings. Therefore, although the generally

accepted criterion for optical quality requires that the peak-to-peak wave front distortion not exceed

23
one quarter of the wavelength (the Rayleigh A/4 criterion [1]^) the resolving power of an optical

system for two closely spaced, high contrast objects will remain near the Rayleigh theoretical

resolution limit [2] up to wave front distortions as high as A/2. For low contrast, extended objects

the optical performance of a A/2 system is degraded because of the halo surrounding each image point.

Near theoretical resolving power is, however, only a necessary, not a sufficient condition for good

focus, and higher quality optical systems are required to produce diffraction limited high intensity

focussed spots than to produce diffraction limited optical resolution.

What are the tolerances on multiple elements used in a focussable laser optical train? Even in

the intermediate infrared they are very severe, as can be seen from the following argument. Marechal

has shown [1] that to obtain 80% of the on-axis focussed intensity available from a true Gaussian

beam the rms wave front distortion must not exceed A/14. The peak-to-peak wave front error is then

approximately the Rayleigh quarter-wave criterion. For small wave front distortions the ratio of the

focussed on-axis intensity I to the undistorted wave front intensity Iq (the Strehl intensity) is

given by [3]

f . 1 - iff O

where O is the rms deviation of the wave front compared to that of an undistorted wave, expressed in

units of wavelength A. The Strehl relation is a good approximation [4] for intensity ratios above

60% or rms wave front errors of less than A/10. In using it we also assume a uniform intensity along

the wave front. In practice the intensity will either fall off as a Gaussian or vary in a more com- plicated manner along the wave front, requiring a more detailed analysis than that presented here. In our preliminary calculations, however, let us assume that there are N components in the system, all of

which are allowed equal contributions to the resulting total wave front error o. The contribution

arising from the figuring error in a given component is assumed to be independent both of figuring

errors in other components and of thermally induced errors. From eq. (1), then.

= 0^ + N 0^ (2)

where 0,^, is the total thermally induced error in the wave front and Of is the figure-induced error in

the wave front caused by an individual component. If the thermally induced wave front distortion

in each component is also uncorrelated to that of other components, we obtain

= N . (3)

However, the thermally induced distortion in the different components is probably not uncorrelated to that of other components. For example, if the laser beam heats the center of one component more than

its edge, the beam will probably do the same to the other components. If the thermally induced distortions of the various components are completely correlated, the resulting total thermal distortion

will simply be the sum of the individual contributions, or

= N . (4)

In optical shop practice the figure distortion is measured as a peak-to-peak value rather than an rms value. Approximate peak-to-peak values, represented by p's, can be obtained by multiplying the o's

by 2v^. Then, from eqs. (1) to (4) we obtain for the case when both the initial figure errors of the

individual components and the thermally induced errors are uncorrelated and the Marechal condition is

fulfilled:

and when the thermally induced errors in individual components are correlated:

\2 ¥(7) • <')

Equations (5) and (6) give the maximum peak-to-peak wave front distortion per component which may occur without seriously degrading the focus of the optical train. Even if the Marechal criterion is

relaxed, the allowable distortion increases by only a small amount. For example, if we double the

intensity loss at focus and require only 60% of the theoretical intensity to be achieved instead of

the 80% given by the Marechal criterion, the allowable distortion only increases by a factor of 1.4.

The wave front distortion produced by a mirror is twice the physical distortion of the mirror

surface. The normal optical shop tolerance on a prevision optical surface is A^/4 where A^ is the

visible test wavelength used, typically 5461 A. The most stringent tolerance most optical shops will

1. Figures in brackets indicate the literature references at the end of this paper.

24
accept is A plot of the mirror figure requirements for a 10 component laser system as calculated

from eqs. (5) and (6) is shown in figure 1. The dashed lines indicate optical shop tolerances. The

better the initial figure, the more thermal distortion is allowed, which translates into more relaxed

tolerances on the absorption of mirror coatings or on mirror cooling requirements. In general, a figure is quite adequate for CO2 laser mirrors and even a figure might be acceptable. If the

figure is worse than 0.338 ym, or about 5/8 A^, however, the Marechal condition cannot be met even if no thermal distortion is present. Also, if the thermal distortions in the different components are correlated, the maximum allowable thermal distortion is only 0.107 ym, or less than l/3rd of that for

the uncorrelated case. In practice the allowable distortion should be between these two values if the

mirror is extremely well figured and will decrease as the figure becomes poorer.

Contrast the case for the CO2 laser operating at 10.6 ym with that for the DF laser operating at

3.8 ym. Even a surface is not good enough for a 10 component optical train used with a DF laser;

at least a A^/8 surface is required. Also, the thermal distortion allowed is at best at the lower

limit of that allowed for CO2 , and in most cases will be several times less than that. In the correlated case, assuming a X^/8 initial figuring error, it is only 316 A, which is a very small number

indeed and will require both very highly reflecting coatings and very efficient cooling if it is to be

achieved.

The figuring requirements for windows may also be calculated from eq. (5). If we assume the

figure errors on the two sides of the window are correlated (worst case but entirely possible if both

sides are polished on the same lap) , then the optical path difference per component will be

= 2(n - 1) T (7)

where x is the error in the physical surface and n is the index of refraction of the window (assumed

to be in vacuum or air). For KCl, where at 10.6 ym n = 1.45, tolerances on the surface figure are

seen to be less than half as tight as on mirror surfaces, as shown in figure 2 for a 10 component

system. A half-wave figure would be adequate for CO2 lasers and even a full wave figure error would

not necessarily be disastrous, although the thermal absorption required for such a component would have

to be lower than for one with a better figure. As shown in figure 3 the situation for KCl is in marked

contrast to that for ZnSe (n = 2.405 at 10.6 ym) where the figure and thermal distortion tolerances

are over 3 times more severe than for KCl and are thus worse than for mirror surfaces.

3. Minimizing Thermal Distortion in Mirrors

The static figure of a laser mirror, although often very difficult to achieve, is obtained by more

or less conventional optical shop techniques such as those used for producing astronomical mirrors and

other high quality optics. However, an additional requirement on some laser mirrors is that they

exhibit minimum figure distortion under thermal loading. Much of the heat absorbed by the mirror surfaces can be removed by cooling the mirror, but the greatest gains are made by using mirror coatings

which do not absorb the energy to begin with. Both metal and metal-dielectric coatings are used, and

in both cases the purity of the initial starting materials and the conditions under which the coatings

are deposited are crucial to the resultant absorption the coatings will have. The substrate on which

the coating is deposited must also be exceptionally smooth. Surface microirregularities as small as

30 A rms can significantly affect the infrared absorption of silver and other metals. Optically

polished metal surfaces typically have microroughnesses of the order of 70 A rms, but by careful

polishing, this value can be reduced to less than 30 A rms. [5] By overcoating the metal with a sputtered film of the same material and then polishing that, surfaces having microroughnesses under 15 A

rms can be achieved. [5,6] In general, as the surface becomes smoother the absorption becomes lower.

However, as shown by figure 4, the decrease in absorption is not determined solely by the rms height

of the surface irregularities, and probably depends on the surface autocovariance function as well.

The solid line represents the increase in absorption observed for surfaces roughened by depositing

calcium fluoride on them and then overcoated with silver deposited in ultrahigh vacuum. Finely spaced

irregularities are produced in this way, and the increase in absorption (over a factor of three for

surfaces as smooth as 35 A rms) is higher than for any of the polished metal surfaces tested. On the

other hand, the increase in absorption of the 35 A rms sputtered copper on copper sample was less than

30% and was nearly identical with that of the smoother copper samples. Work is now underway [7] to measure the autocovariance function of such surfaces with the hope that by appropriate modification of

polishing technique and type of substrate the roughness requirement on laser mirror surfaces can be

significantly relaxed.

4. Damage Threshold

In addition to the optical figure requirement, a laser mirror must have a high catastrophic

damage threshold. As with dielectric materials, the damage process in metals is not entirely clear.

Damage may occur at sites where localized defects such as scratches or inclusions exist, and techniques

for detecting such defects are thus important. Scratches may be detected by scattered light meaurements, as described below. Auger spectroscopy, when combined with a sputter attachment for obtaining

25
depth profiles, is a powerful tool for inclusion detection. [8] Figure 5 shows an Auger scan of a copper surface polished with aluminum oxide powder in a water slurry. Even in this unfavorable case

the sensitivity of the Auger system to polishing particles imbedded in the copper surface is about

10~^ gm/cm2 of AI2O3 or 0.17„ surface coverage.

The damage threshold of metal or metal-coated mirrors is not directly proportional to their absorption. It often is significantly higher for uncoated polished bulk metal mirrors than for the same mirrors coated with an evaporated metal film, even though the absorption is lower in the latter case.

It also varies with substrate material. Beryllium copper has one of the highest damage thresholds.

It is significantly better than pure OFHC copper (bulk copper) [9], as shown in figure 6. However,

when the bulk OFHC copper was coated with a dispersion-hardened, sputtered copper film and then polished, the damage threshold increased almost four times [9], and was over 2 1/2 times higher than for

bulk BeCu. When a silver film was evaporated onto the bulk OFHC copper (lower bar graphs) no change in

damage threshold was observed, even though the absorption decreased by over a factor of two. When

silver was evaporated onto the sputtered surface, the damage threshold was nearly twice as high as for

the bulk sample. It was only about half that of the sputtered substrate, however, even though by coating the substrate the absorption was reduced by over a factor of two. When a sputtered silver coating

was deposited on a sputtered substrate, the damage threshold went up almost, but not quite, to the

uncoated sputtered substrate level. [9] The absorption of the sputtered silver film was nearly twice

that of the evaporated film. This example demonstrates that the damage threshold of metal mirrors is

determined not only by the energy the surface absorbs but also by other quantities such as thermal

transfer, surface structure and film adherence. To minimize thermal distortion of the mirror low

absorption coatings are essential, so conventionally sputtered silver coatings [10] are inadequate in

spite of their high damage threshold. One possible solution is to improve the sputtering conditions

with the objective of lowering the absorption of the sputtered silver coating to values comparable to those of evaporated silver films.

5. Scattered Light

If backscattered light from the mirrors in the optical train of a laser is allowed to pass back

through the active laser medium, catastrophic damage to the laser can result. Various attenuation

schemes to avoid this situation can be employed, but one of the best is to minimize scatter from the

mirrors themselves. Scattered light is also a useful nondestructive tool for detecting inclusions,

scratches and other localized defects in the mirror surface. If such defects are larger than the wavelength of light employed, the scattered light will have little wavelength dependence. Total integrated

scatter (TIS) from microirregularities, on the other hand, usually follows a l/A^ law [11], as shown

in figure 7. By scanning the mirror surface at several wavelengths in the visible region it is then

possible, from TIS measurements, to separate and determine both the surface microroughness of the mirror

and the presence of scratches or other localized defects which may lower the damage threshold of the

mirror. Differential scattering measurements at the operating wavelength of the laser are also required, as are reflectance measurements.

An instrument for performing scattering and reflectance measurements on mirrors up to 40-cm

diameter has been described previously. [12] It employs 18 laser lines extending from the ultraviolet

through 10.6 ym in the infrared and is enclosed in a class 100 clean room. Results of scattering

measurements on a gold coated, 25-cm diameter beryllium copper mirror with a polished sputtered surface

are shown in figure 8. The average values of the scatter at different radii are given at the top of

the circles indicating the scan positions, and the points indicate the deviation from these average

values. This mirror is an exceptionally good one polished by the Perkin-Elmer Corporation and sent to

us to evaluate by the Air Force Weapons Laboratory. The rms roughness at the center of the mirror is

only 22 A, as determined by scattered light measurements. [13] There are virtually no significant

scratches on this mirror and the roughness has excellent radial symmetry about the mirror center, as can be seen from the uniform scatter points about the circles indicating average values. The mirror

is thus completely polished out but does show a pronounced radial dependence of roughness. For the

largest circle, which is 25 mm from the outer edge of the mirror, the average roughness is 29 A rms, causing a 60% increase in scatter as compared to the mirror center. The reason for this increase in

roughness is not known, but may be associated with the way in which a large optical surface polishes.

The increase in scattering level with distance from the mirror center is seen more easily in the

radial scan shown at the left in figure 8. There was a small scratch near the edge of the mirror

which also shows up graphically in the radial scan. The still smaller scratch seen halfway out was

almost imperceptible visually, although it is easily located on the trace.

6. Conclusion

The requirements on the elements in the optical train of a laser are very severe, particularly if

the train has several elements in it. If the beam is to be focussed for high energy density and techniques such as apodization are not used, the figure requirements are more stringent than are those for

high resolution optics, and may demand state-of-the-art polishing procedures even for lasers used in

26
the intermediate infrared. Thermally induced distortion is also important, and coatings which absorb

a minimum of the flux incident on the mirror are thus essential. Microroughness contributes significantly to the absorption of metal coatings but can be controlled by polishing sputtered coatings

deposited on the substrate rather than by polishing the substrate itself. Another possibility now

under investigation is the control of the surface autocovariance function by appropriate polishing

techniques. Short wavelength scattered light is a powerful, nondestructive tool for checking mirror

surfaces for microroughness as well as potential damage sites.

In addition to figure requirements, the laser mirror must have a high catastrophic damage threshold, and backscatter must be minimized. The damage threshold of metal mirrors is determined by more

than simply the absorption of the surface. Examples have been reported in which the damage threshold

for silver-coated copper doubled even though the absorption also doubled. The damage threshold per

absorbed photon thus increased by nearly a factor of four without a change in mirror material. Such

results are encouraging but indicate that our understanding of the laser mirror problem is still

incomplete.

7. References

[1] Born, M. and Wolf, E. , Principles of Optics

(Pergamon Press, New York, 1959), p. 467;

Conrady, A. E. , Applied Optics and Optical

Design (Dover Publications, Inc., New York,

1957), p. 136.

[2] Conrady, A. E. , Applied Optics and Optical

Design, Part Two (Dover Publications, Inc.,

New York, 1960), pp. 626-7.

[3] Born, M. and Wolf, E. , op. cit. , p. 463.

[4] Winsor, H. V., Proceedings of the Third

Conference on High Power Infrared Laser

Window Materials, November, 1973 [AFCRLTR- 74-0085 (III)], p. 1069.

[5] Slomba, A., Perkin-Elmer Corp. (private

communication); Soileau, M. J., Bethke, J. and Ashley, E. J. , High Energy Laser Mirrors

and Windows (Semi-Annual Report No. 4, ARPA

Order 2175, Naval Weapons Center, March

1974), pp. 51-58.

[6] Stewart, R. W., Investigate Material Systems

for Mirrors Used in High Power CO and CO

2

Lasers (BNWL-1780, ARPA Order 2175,

December 1972) .

[7] Bennett, J. M. , elsewhere in these

proceedings (1974).

[8] Porteus, J. 0. , High Energy Laser Mirrors

and Windows (Semi-Annual Report No. 3,

ARPA Order 2175, Naval Weapons Center,

September 1973), pp. 88-90.

[9] Soileau, M. J. and Wang, V., Appl. Opt. 13,

1286 (1974).

[10] Stewart, R. W. , Investigate Material Systems

for Mirrors Used in High Power CO and CO2

Lasers (BNWL-1781, ARPA Order 2175, August

1973)

.

[11] Bennett, H. E. , Stanford, J. L. and

Bennett, J. M. , "Scattering from Mirror

Surfaces Used in Space Optics" in Space

Optics, B. J. Thompson and R. R. Shannon,

eds. (National Academy of Sciences, Washington, D. C, 1974), pp. 717-738.

[12] Bennett, H. E. , Archibald, P. C. and

Stanford, J. L. , High Energy Laser Mirrors

and Windows (Semi-Annual Report No. 3,

ARPA Order 2175, Naval Weapons Center,

September 1973), pp.. 5-24.

[13] Bennett, H. E. and Porteus, J. 0., J. Opt.

Soc. Am. 51, 123 (1961).

27
8. Figures

Figure 1. Figure requirements for mirrors used

in the 10 component optical train of a laser

operating at 10.6 ym (CO2) or 3.8 |Jm (DF)

.

Equal contributions to the wave front distortion

by each component are assumed. The subscripts

cor and unc refer to correlated and uncorrelated

thermal distortions. Dashed lines refer to con- ventional figure tolerances used in optical

shops, i.e., an eighth wave surface, quarterwave surface, etc., at a wavelength X = 5461 A.

Figure 2. Figure requirements for KCl windows

used in the 10 component optical train of a CO2

laser. Equal contributions to the wave front

distortion by each component are assumed. The

ordinate gives the fraction of the total wave

front distortion introduced by the window

which can be thermally induced. Dashed lines

refer to conventional figure tolerances used

in optical shops.

FIGURE R6QUIR60 FOR 2/3 THERMAL DISTORTION. 10 COMPONENTS. CO2

K

2

K

4

1 8

REQUIRED FIGURE

0.012 -

0.004

10 20 30 SURFACE ROUGHNESS (X)

Figure 3, Figure requirements for a mirror, a KCl window, and a ZnSe window used in a 10 com- ponent CO2 laser optical train with equal con- tributions to wave front distortion allowed for

each component. Two-thirds of the allowable wave front distortion is assumed to result from

thermal effects. The dashed lines refer to conventional optical shop figure tolerances.

Figure 4. Absorption at 10.6 ym of silver films

evaporated in ultrahigh vacuum (10~^ Torr) onto

surfaces having various roughnesses, as determined by scattered light and FECO interferometric measurements on the silvered surfaces.

The solid line shows the absorption for rough

surfaces formed by depositing CaFa onto quartz

substrates. The OFHC copper surface had a sputtered copper overlayer and is represented

by the squares. There was no sputtered layer

on the BeCu (triangles) , the Mo (diamonds)

and the quartz (circles) . All surfaces were

polished by bowl feed.

28
AUGER ELECTRON ENERGY (eVI 1000 1200

1 1 1

1

Cu: AI2O3 POLISH

\ 1

1 1

1

r—Vlr

Cu

1 1 1 L 1400 1600 AUGER ELECTRON ENERGY (bV)

Figure 5. High-energy portion of the Auger spectrum

from a copper surface polished with AI2O3. The Al

peak from residual AI2O3 particles embedded in the

surface is easily detected. Estimated sensitivity

of this technique is 10^ particles/cm^ or a surface

coverage of 0.1%. From reference 8.

ABSORPTION

0.016

0.020

0.015

0.007

0.007

0.010

DAMAGE THRESHOLD (% OF B«Cu)

BULK B«Cu 100%

SPUTTERED Cu SUBSTRATE

EVAP. ON

BULK 67%

EVAP. ON SPUTTERED 124%

SPUTTERED ON SPUTTERED

SILVER COATING

Figure 6. Damage threshold for copper mirrors at

10.6 ym. Absorption values for the different surfaces are shown on the left and percent improvement

in damage threshold as compared to uncoated BeCu on

the right. Data taken from reference 9.

LIGHT SCATTERED FROM COPPER MIRROR

y 0.

O SCATTEROMETER

OPT. EVAL. FAC.

SCATTER FROM A 25 CM DIAM. MIRROR

AV. SCATTERING

Au COATING

BeCu SUBSTRATE

22X RMS ROUGHNESS WAVELENGTH 4762^

I i lJ WAVELENGTH (tjml RADIUS (CM)

Figure 7. Scattered light from a copper mirror

having a sputtered copper coating which was then

polished. The circles represent measurements

made with the NWC scatterometer , and the squares

are measurements made on the Optical Evaluation

Facility. The solid line is the hemispherical

scattering function predicted from scalar scattering theory for a 31 A rms roughness surface.

Figure 8, Scattered light as a function of

position on a 25 cm dlam. gold-coated beryllium

copper mirror. The circular scans were made at radial increments of 2.5 cm. The average

scattering value for each scan is shown at the

top of the corresponding circle and the points

show the variation about this average. The

radial scan confirms that the scattering level,

although radially symmetric, increases with

radius by about 60% (corresponding to a change in roughness from 22 X at the center to 29 A near the edge of the mirror) . The dashed

line gives the average radial scatter excluding

the scattering from the scratch near the edge

of the mirror.

29

564-378 O - 74 - 4
COMMENTS OW PAPER 8^ BENNETT

30
1.5 Diffraction Theory of Absorbing Windows*

M. Flannery and J. Marburger

University of Southern California

Los Angeles, California 90007

The diffraction-optical theory of coherent polarized beams incident on realistic absorbing windows is developed here with emphasis on tracing the origin of

qualitative features of the diffraction field. The influence of incident polarization, stress induced birefringence, and other parameters, on the transmitted beam

is described, and both analytical and numerical solutions for the transmitted diffraction field are presented. Our development of the theory allows the influence

of the window material to be characterized by two parameters in the transient re- gime rather than the three previously believed to be necessary. Implications for IR window design are summarized.

Key words: Birefringence; laser windows; thermal distortion.

In assessing materials for use in laser systems, one must know how each material parameter influences critical properties of the laser output beam. Unfortunately, the relation between material properties and the transmitted optical field of an absorbing window can be quite obscure. Bendow and

Gianino [1] ' have performed extensive numerical analyses of the diffraction field of a truncated Gaussian

beam focussed through a weakly absorbing window which heats, expands, and produces an inhomogeneous

stress induced birefringence. Because of the large number of parameters involved, the numerical re- sults are difficult to summarize, and it is difficult to attribute specific features of the diffraction

field to specific mechanisms in the window. In this paper, we describe a general diffraction theory of

absorbing windows which is simple enough to allow analytical estimation of many important features of

the transmitted field. These estimates show clearly how such features are related to the material

parameters of the window. The basic formalism of the theory is general enough to allow the analysis of

realistic window systems.

Our starting point is the slowly varying envelope equation for the amplitude E of the total optical

field ? of frequency O): opt ^ ^

^ ^ ikz -ioJt E ^ = Re Ee e opt

Here k = noj/c, and n is the mean refractive index of the medium through which the electromagnetic field

is propagating. The envelope equation in an inhomogeneous medium is (V^ = 3^/3x2 + 3 /Sy^)

c

'X ^ 2 ^

wh^re X is the inhomogeneous part of the linear susceptibility tensor defined through P = [ (n -1)/4it]E +

X'E. We are using unrationalized cgs (Gaussian) units.

In free space (or in air, ignoring inhomogeneities) , eq. (1) becomes

2ik|^ + vJe = 0 (2) oz i

which has the solution

d^ri-^(r-r'

)

2z

E (r',0)

out

(3)

^Research supported by the Defense Advanced Research Projects Agency, Department of Defense, monitored

by Air Force Cambridge Research Laboratories under Contract No. F19628-72-C-0275.

"'"Figures in brackets indicate the literature references at the end of this paper.

31
The exit plane of the window is assumed to be at z = 0, and E is the field at that plane. If a thin out

lens of focal length R is placed at z = 0, then (3) should be replaced by

-ik

2Trz

ik,^ ^, ,2 .kr

'

E(r,z) d r' e e

Eqs. (3) and (4) will be used to find the field beyond the window.

E (r',0)

out (4)

The field f ^ at the exit plane of the window system can be obtained in simple form if the window 3Ut 7 irfraction effects (arising from the V^E term in (1)) can be ignored inside the window.

Then (1) becomes

2ik-^

dz X-E

(5)

This can be integrated easily if x(r,z) is locally diagonal in a coordinate system whose axes do not depend upon z. In this case

^out^'^'"^

r 2

12110) exp

Lkc2 i

0 X(r,z)dz

-W

•E.^(r,-W) . (6)

where z = -W is the entrance plane of the window system. Figure 1 shows the geometry of a typical system.

-v The bracketed quantity in (5) is a two dimensional matrix which we call the aberration matrix X:

A(r)-E.^(r,-W) (7)

This matrix depends upon the, details of the incident field as well as upon material parameters. For a "perfect" window x = 0, A = I and E = E. (except for a constant phase factor). But even in this

out icj case, eq. (4) for the transmitted field is difficult to evaluate except for special cases.

If the incident field has cylindrical sjrmmetry, E. = e. E(r), where e. is a constant polarization unit vector, and if the window response is also cy2indrically symmetric^ then A will be diagonal

in polar coordinates in the window plane:

i$

e '-^ rf + e 66

expi()) cosA + sinA cos(

sinA sin26

(8)

sinA sin26

cosA - sinA cos6

The second form of writing. is appropriate for a Cartesian coordinate system, where f = x cos6 + ysin6,

and

A

The principal phases 0^, $g , are simply the eigenvalues of the exponent in eq. (6). Eq. (8) is important because all 6 dependence is explicit. This means that the angular part of the integration in the

diffraction integral (4) can be performed. The result for the transmitted diffraction field may be

written

E(r,z) E^(r,z) - R(6)-E2(r,z) (9)

where

R(6) (10)

and

32
E (r,z)

o

ikr

-ik 2z

e

z , 'dr' cosA J111 o inj (11)

E2(r,z)

kr

-ik ^ 2z -— e

z r'dr

'

*2iur sinA J„E.

2 m

(12)

Here u k(R

-1

z ), and the argument of the Bessel functions J and is krr'/z.

We can learn much about the transmitted field by meditating upon those equations. and Y.^ both

have the polarization of the incident field, and therefore by (9) all depolarization effects are proportional to Eq. (9) also implies that deg^artures from cylindrical symmetry in the transmitted

field are also proportional to The field £_ itself vanishes on axis (r = 0) because J2 vanishes

there, so the axial field has the same polarization as the incident field, and equals 2^(0,z). When

there is no birefringence, A = 0, and E~ vanishes. The intensity is easily obtained using = 1

:

2 Re E 'RO)

o

•F^2 (13)

If the incident polarization is elliptical with ellipticity e the Cartesian coordinates, then the total intensity is

tan X and with principal axes along

2(Re E E„) cos2x cos2e

o Z (14)

For circularly polarized, or unpolarized light, this implies

i->i 2 I ,2 1 1

2

(unpolarized, circularly polarized)

while for linear polarization

2(Re E E„) cos2e

o z (linearly polarized)

These formulas give the explicit azimutha] dependence of the transmitted intensity for any cylindrically

symmetric incident beam with uniform polarization.

To appreciate the influence of ^ and A upon the diffraction field, we must specify the incident

mode structure and the window response more precisely. It is convenient to adopt as a fiducial case a Gaussian equiphase beam with weak truncation at the window aperture:

E^,(r,0) 2/2e.E. exp -r /w

1 m (15)

The truncation will be specified by the ratio a = d/w, where d is the window radius. If d and the incident power in the whole Gaussian are fixed, then the focal intensity of the transmitted beam is a maximum for a = 1.122. For this value of a, 92% of the incident power is transmitted, and the structure (e.g. Airy rings) introduced in the diffraction field by truncation is weak.

The simplest realistic model for window response is one in which the window is made of an isotropic

material which heats in the beam and develops internal strains as the cooler periphery restricts the

expansion of the warmer interior. These strains generate index changes proportional to the appropriate

elasto-optic tensor elements p... The total phase disturbances $g also contain contributions from

the direct temperature coefficient of refractive index 3n/3T, and from the slight change in path length

as the heated portion of the window expands and bulges outward under the restraining periphery. The

window is assumed to be freely supported at the edges with a medium with the same thermal and elastic

properties as the window. For simplicity in what follows, we shall consider only short heating times

during which the local temperature is not affected by thermal diffusion. Any of these simplifying con- ditions may be relaxed with consequent increases in the complexity of the theory, although analytical

treatments are still feasible in most cases. The reader familiar with the work of Bendow and Gianino

[1] will appreciate that even this restricted window model leads to very complex diffraction fields.

Bendow and Gianino [1] have shown that the principal phases can be written (for the window model

just described) in the form

C5|f(r) + C2g(r) + C, (16)

33
where f(r) is proportional to the local temperature increase and g(r) to the mean integrated temperature

increase from the origin to the radial distance r. For example, during the initial heating regime

before thermal diffusion sets in, a Gaussian incident beam leads to

f(r) = exp(-2r^/w^) (17)

/ N 1 ~ exp(-2r^/w^) , „^ g(r) = ^^-^— (18)

r /w

The coefficients in (16) are

^1 ^ ^ {It ^ + V) (n - 1) + [(1 - v)p^2 - ^11

cj = « {-^ + a(l + v)(n - 1) + %an^Jp^^ - ^-^V^^

C2 = -C® = Han^d + v)(p^^ - p^^)/^ (20)

H = (ekW/C')|'' I^(t')dt'

I = ^E. (0,0)p

o 8tt' in '

Here 9n/3T is the rate of change of index with temperature at zero stress, a is the coefficient of

thermal expansion, V Poisson's ratio, an^ p_j^. the elasto-optic tensor elements. g is the absorption

coefficient, C' the heat capacity per cm , aAd W the thickness of the unstrained window. With this

notation, the aberration functions are

h{.(^\ + Cp f(r)

C2[g(r) - 2f(r)]

Introducing the new parameters

we f ind

T 5 %(C^ + C®), a E + , (21)

(S> = Tf(r) (22)

A = aT[%g(r) - f(r)] . (23)

These are plotted in figure 2 for an incident Gaussian in the pre-dif fusion regime. The importance of

these formulae is that the introduction of birefringence (P-|^2. ^12^ requires only the additional

lumped parameter a for its description in this model. Previous formulations of the theory required two

additional parameters equivalent to £2/0^ and C^/C^.

The significance o^ x can be seen from the role of the mean phase shift (J) in eq. (11). If ^ is

expanded in powers of r , we find

(t)(r) = + (j)2r^ + (f)^r^ + ... .

The term in (j)^ is of no interest here, but that in isf^ is equivalent to a lens of focal length

R = - ^

(24) eff 2(f>2

For a Gaussian beam,

34
f(r) = 1 - 2r^/w^ + 2r^/w^ , (25)

or one finds

Kff = +^ = • (26) err 4t , 2 4a T

2 Thus T is the ratio of the Gaussian diffraction length kw /4 to the instantaneous effective focal

length of the center of the window. Higher orders in the expansion of (j) are also proportional to t. Thus the lowest order spherical aberration term is

(f>^ = 2t/w^ . (27)

Using a modification of a criterion due to Rayleigh [3] , we expect serious degradation of the focal intensity when (j>^w ~ tt/2, or t 77/4. If either the defocusing effect (26) or the aberration effect (27) were present, one would expect a substantial reduction of focal intensity when t ~ 1, but from (25) one

can see that the effects can nearly cancel. In fact the focal intensity drops by half only after the

rather long "time"

This can be inferred either from numerical solutions or from the exact analytical expression for the

foca^ intensity (when birefringence is absent) reported elsewhere [2]. Table 1 gives values of

t/w/ I dt for a variety of materials. This number, which should be small for negligible thermal lensing

effects, is directly proportional to the absorption coefficient 3-

When birefringence is present, a 5^ 0, the field E of eq. (11) is altered by the cos A factor in the

integrand of the diffraction integral. Because there is no birefringence at r = 0 in an isotropic

window with axially syTnmetric strains, A(r) vanishes at r = 0. Thus cos A is unity at the origin and

decreases at finite r values, effectively replacing the smooth incident Gaussian field by one with

radial minima on circles for which A(r) = (m + h)Ti, m = 0,±1,±2.... Figure 3 shows the effect of this

factor on the focal intensity for various values of a. One expects the frequency of the pronounced

oscillations to be roughly proportional to a, and this is indeed the case. The cos A factor reduced

focal intensity by decreasing the effective incident power (which is matched by an increase of power in

the off-axis E„ component) and by sharpening the effective mode structure thereby decreasing the diffraction length. The effect of birefringence on the time required for thermal lensing to cause the

focal intensity to drop to half its original value is shown in figure 4. Of materials interesting for

10.6ym operation, only KCl has a large effect, but birefringence could be a severe problem in the

fluorides.

The off-axis structure in the focal plane is shown in figures 5 through 7 for 0=2. Notice

that the ring structure is contributed entirely by E2 as anticipated in eqs. (12) and (13). The position of the ring in E2 can be estimated by the following argument based upon eq. (12): The magnitude

of E„ at radius r is most strongly influenced by the value of F = r' sinA E. at that value of r' for

which J2(krr'/z) has its first maximum. Since this occurs at J2(3.1) ~ .48"^, and since F(r') has a maximum at some value r' , we expect a maximum in IE2I near r ~ 3.1z/kr^. For the examples shown, this

estimate agrees well with the ring radius in the numerical solutions. TM(The value of r^ is easily determined from a plot of F(r').)

The aberration matrix formalism provides a convenient starting point for the analysis of more com- plicated window systems. Further applications of this formalism are described in the series of progress

reports cited as reference [4].

35
Table 1. Dimensionless Times and Birefringence Parameters

(Based on values tabulated in Ref. 1)

Material a /wj I^(t')dt'

— . Dyo -u . iny

1 1 /. /. O CI

"NT o P 1 — U. ZU/ £ on -O . JU

Is. J. — u. zyt) "7 0

1

LiS J. u

.

— D J . D

LiSDL U

:

on 7

L.a i e

0*^1 7 — iOl

.

H

JS.K.O~ J U

:

job ,

• U / HO

~ • UUOOD /y y u

.

InSb 0? 9790.

Ge .0139 8570,

TI Glass #1173 0? 2890.

TI Glass #20 0? 3780.

Irtran-4 (ZnSe) .0829 4310.

Irtran-6 (CdTe) .0423 19500.

w = window thickness (cm)

^ I = incident axial intensity (Watt/cm ) o . . , : t = txme since turn-on (sec.)

O and T are dimensionless

References

Bendow and Gianino, Applied Physics, 2^, 1, 71 (1973) , "Thermal Lensing of Laser Beams

in Optically Transmitting Materials."

[3] R. V. Luneburg, Mathematical Theory of Optics , University of California Press,

Berkeley, 1966.

Flannery and Marburger, Proceedings of

Conference on High Power IR Window Materials (1971), "I.R. Window Criteria:

'Exact' Analyses of the Thermal Distortion Problem."

[4] USCEE Report 469, I. R. Window Studies,

F. A. Krbger and J. H. Marburger.

Figure 1. Window geometry.

36
Figure 2. Radial dependence of aberration

functions (j) and A,

EFFECT OF B,iRET=RiNG-E)N/Ce"

37
COmEUTS ON PAPER BV FLAWWER/

In fiupome to a qaojitlon on the. intended mpaki,-ii, o^ thJjs Monk, the. ip^akeA. commented that the aJjn

Oj) the woKk deySCAlbed Mca to catt out e^^ecXi o^ blnedHlnQence expticiitly itnce the eidecti Oj) unA.(,ofun the/unaZ lending had aVieady been i/deJLt treated In the eanLiex LiteAotixAe. He ah>o commented that the

model pK<u>ented predicted an oAymmeX/Uc i,ht(,t, on beam iteejiing ei^^ect, mhich had iubiequently been

obienved. ThJj, ihj.{,t -d ien^-Otlve to -the geometry o^ the mount o^ the Mtndoui in mateAlal^ u)tth lange

ptezo eZaitic co edi^tcA-enii, . In f>uch mateMxaJU , ion. example with a 3 potnt mount wtth the M-indoM

tightly 6ecuAed, a veAy lange beam iteeAlng eHect can be obieAved.

38
2.1 Damage Resistance of Dielectric Reflectors for Picosecond Pulses*

Brian E. Newnam

University of California

Los Alamos Scientific Laboratory

Los Alamos, NM 87544

A state-of-the-art survey has been conducted to determine the range of damage

thresholds of electron-gun-produced dielectric reflectors irradiated by 30-ps

laser pulses at 1.06 ym. Seven commercial coating companies, experienced

in producing refractory-oxide coatings, supplied total reflectors of a specified

Ti02/Si02 design deposited on low-scatter glass substrates provided by LASL.

Optional reflectors of ZrOz/SiOa were also evaluated. Single-shot thresholds

were determined by laser-induced scatter and photoelectric detection of spark

radiation. Thresholds of TiOa/SiOa reflectors ranged from 1 to 4 J/cm^; those

for ZrOa/SiOz reflectors ranged from 0.5 to 2 J/cm . Reflectors which exhibited

a large amount of diffuse, weak-signal scatter generally had low thresholds.

Key words: Damage thresholds; dielectric reflector; laser- induced scatter;

picosecond pulses; spark thresholds; thin films; weak-signal

scatter.

1. Introduction

The damage resistance of commercially-produced dielectric coatings is an important constraint in

the design of short-pulse, high-power glass laser systems being built for laser fusion experiments.

Heretofore, very few damage threshold values for subnanosecond pulses at 1.06 \im have been reported.

In the present work, conducted with single 30-ps pulses, we have determined the range of damage thresholds of commercially-produced multilayer reflectors deposited by electron-gun evaporation. To evaluate the effect of only the coating preparation and deposition techniques used by each manufacturer,

the coating design and materials were specified and low-scatter glass substrates were supplied. In addition, each company was invited to submit reflectors of alternate materials and/or designs which

might be more damage resistant than the specified coatings. Results of these damage tests are presented in Section 7. Tests to determine the possible correlation of weak-signal scattering and damage

thresholds are discussed in section 8.

2. Reflector Design and Materials

Seven commercial coating companies, experienced in producing refractory-oxide coatings, supplied

two samples each of 20-layer Ti02/Si02 reflectors according to the design specified in table l. Reflectors of this design had the highest damage resistance in a previous study conducted with 13-ns

ruby laser pulses. [1 ,2]-'- Reflectors composed of ZrOa and SiOz were also submitted by four vendors.

All reflectors were deposited on BK-7 glass substrates two inches in diameter and 0.5 inch thickness.

Since it has been established that coating defects limit the practical damage threshold, it was desired that the surface finish of the substrates not initiate pits and grooves in the coatings. After

examining the surfaces of BK-7 glass substrates submitted by several commercial polishers, the "superpolished" surfaces produced by Itek Corporation were selected for their minimal defects and surface

roughness. The optical quality of the BK-7 glass was grade P which is specially selected from grade A melts. In selecting the best of the surfaces submitted by the several companies, it appeared that the

number of surface defects might be related to the material grade. After polishing, further contact

with the surfaces was prevented by shipping them to the coaters in special aluminum fixtures. Prior

to coating deposition, it was suggested that the surfaces be wiped with methanol and lens tissue

followed by a jet of nitrogen gas.

*This work was performed under the auspices of the U.S. Atomic Energy Commission.

'Figures in brackets indicate the literature references at the end of this paper.

39
Table 1. Damage resistance of dielectric reflectors

(state-of-the-art survey)

A. Reflector parameters

Coaters: 7 commercial vendors

Substrates: BK-7 glass (grade P)

Low-scatter surface polish obtained

with bowl -feed method by Itek Corp.

Specified coating materials & designs: 20 layers

TiOz/SiOa with G(HL)^HL^A layer

configuration for normal incidence

Optical coatings: vendor's considered best*^

B. Laser parameters:

1. Wavelength: 1.064 ym

2. Pulsewidth: 30 ps

3. Beam spot-size radius at sample surface: 0.74 mm

Configuration notation: G is the glass substrate, H and

L are respectively high and low-index films of X/4 thickness, and A is air.

''Four vendors submitted optional reflectors of ZrOz/SiOa.

3. Laser Apparatus

The diagnostic laser consisted of a mode-locked, Nd:YAG oscillator from which a single pulse was switched out by a Pockels cell/spark gap arrangement and augmented by two YAG amplifiers. The net

output was approximately 50 mJ in a 30-ps pulse with a TEMqo spatial distribution. The energy was measured with accuracy of ± 5% with a calorimeter of LASL design [3], and a two-photon fluorescence

camera measured pulsewidth. A fast Si photodiode and Tektronix 519 oscilloscope monitored the modelocked pulse train.

For the present tests, the output was focused with a 2-m lens to a spot-size radius of 0.74 mm at the sample plane. The sample plane was located in front of the lens focal plane to avoid possible

uncertainties in the axial intensity which can result from focusing truncated Gaussian laser beams. [2] A theoretical treatment of this topic has been performed by Farrukh. [4]

The beam profile was first measured at the lens plane by directing a small portion of the beam

onto a Reticon RL 256EC linear scanner array (resolution of 0.05 mm) centered on the beam with a X-Y

translation device. The signal was recorded on a Tektronic 556 oscilloscope and the resultant photograph allowed an instantaneous measure of the beam spot-size. The relative intensification of the

focused beam at the sample surface was determined by measuring the energies required to barely damage

unexposed, freshly developed, Polaroid 410 film at both lens and sample planes. (The damage threshold

for 30-ps pulses of 1.06 radiation was measured to be 20 mJ/cm^.) With such large spot-sizes it is

also possible to measure the focused beam profile at the sample surface directly with the Reticon

array, but a substantial error can result from inaccurate detector placement in a rapidly converging

beam.

The damage test facility was housed in three adjacent rooms. One room contained the laser apparatus, the second was darkened and contained the damage experiment including energy and spark detectors, and the third contained the monitoring electronics. In this way, laser flashlamp emission and room lights did not interfere with visual and photoelectronic monitoring of damage, thereby enabling

accurate detection of damage thresholds.

40
4. Detection of Damage

Damage was detected by two methods simultaneously (see figure l) , Optical breakdown accompanied

by a high-temperature spark was detected with a photomul tipl ier (RCA 6655A) with a purple-transmitting

filter (peaked at 4150 A, 150 A bandwidth) to discriminate against flashlamp energy. The photoelectric

signal caused by the spark emission was measured with a Tektronix 556 oscilloscope. The sensitivity

of the spark detection system was such as to detect a spark 10 ym in diameter with black-body temperature equal to the melting point of TiOa (2093°K).

Evidence of laser-induced disruption of the surface of a coating was manifested by increased

scattering of He-Ne laser light. The beam from a low-power (2 mW) He-Ne laser, traveling collinearly

with the pulsed laser beam, illuminated the test area. By comparing the level of scattering before,

during (with N.D.9.0 safety goggles) and after pulse irradiation, changes in the coating structure

could be detected. We found that visual detection of changes in the scatter level was much more sensitive than photoelectric detection of the same (i.e., with photomultiplier and lock-in amplifier). A

third in situ technique of visual observation with a traveling microscope (30 X) was useful only for

examining damage sites considerably above threshold.

5. Test Procedure

Each reflector was dusted with a jet of Freon gas to remove any possible dust particles. No

other cleaning was considered necessary or desirable. In determining the damage thresholds, a given

area of a sample was irradiated by one shot only. Sites of irradiation were spaced several millimetres apart. The energy of the laser beam was controlled by Schott neutral density filters to lie

as close to the threshold as possible. Thus, easily visible damage rarely occurred. This care was necessary to prevent damage products from one area from influencing the thresholds of adjacent areas.

An adequate sampling of the damage resistance was obtained with 30 to 40 shots.

6. Weak-Signal Scatter

Weak-signal scatter measurements were performed prior to the damage tests. A cw Nd:YAG laser

beam (1 W) was directed at normal incidence onto the center of each reflector. A Si photodiode (EG&G

YAG-44), subtending a solid angle of 3.3 x 10"^ sr, detected scattered radiation at 30° from the

surface normal. It was considered important to use the same wavelength for these scattering tests as

in the damage tests so as to have the same standing-wave electric field patterns in both cases. As

shown in Figure 2, for defect-free films the laser light at the design wavelength, 1.06 ym, drops to

a negligible value after eight layers. Imperfections in the layers (surface roughness and defects)

alter the wave patterns and penetration depth somewhat, and the resultant scattering is dependent on the ratio of wavelength-to-imperfection dimension. This assertion is substantiated by McKenney and

Mott [5] who found a strong wavelength dependence for the diffuse reflectance of dielectric films.

7. Test Results

The damage thresholds for Ti02/Si02 and ZrOa/SiOa reflectors are given in tables 2 and 3 in

terms of peak energy density. Values ranged from 1 to 4 J/cm^ for the former and 0.5 to 2 J/cm^ for

the latter. Thresholds were determined independently by spark detection and laser-induced scatter

(LIS). Generally, thresholds determined by spark were lower than or equal to those by LIS monitoring.

This occurred especially for reflectors with high levels of initial scattering. Occasionally, however,

LIS changes were noted without any measurable spark, probably when an absorbing inclusion was irradiated. Therefore, net thresholds determined with both techniques are also tabulated and are considered

the most useful

.

The thresholds are given as a range of values instead of a single number. The larger value was

the highest energy density which caused no damage; the lower value was the lowest energy density which

caused threshold damage. Occasionally, a laser shot with energy density considerably higher than the

determined threshold produced no damage. Likewise, an energy density significantly lower than the

range indicated sometimes caused damage; such values are given in parentheses. Since the laser spotsize was large (0.74 mm), no statistical nature of damage should be inferred. [6] The range of

threshold values is believed to be a measure of the variable damage resistance of the coatings from

one area to another.

Although only two samples of TiOa/SiOa reflectors were tested from each manufacturer, a relatively

large range of damage thresholds was measured. Since coating design and materials were specified and

equivalent low-scatter glass substrates were used for each coating, only variation in vacuum-deposition

technique and possibly starting material purity remain to explain the differences.

41
Table 2. Damage thresholds of commercial TiOa/SiOa multilayer reflectors^

Reflector . Designation by spark

Peak Energy Density

by LIS net by spark & LIS combined

J/cm^ J/cm^ J/cm^

A-1 2.9-4.0 2 9-4 0 ? 9-4 n (2.4)C (2."4) (2.4)

A-2 2.6-4.1 2.6-4.2 ? 6-4 1 (2.1) (2.2)

B-1 2.9-4.2 3.0-4.2 2 9-3 7 (4.7)

' (4:2)

B-2 3.0-4.3 2 8-4 1 ? 8-4 n (2.5) (2".5) (2.5)

C-1 2.8-4.0 3 1-37

(1.9) (1.9)

C-2 2.3-3.9 2.3-4.0 2 3-3 6 (1.9) (K9)

D-1 2.8-3.4 2 8-3 9 2 8-34

(2.4) (413) (2:4)

D-2 1.2-1.8 1.2-2.5 1 .2-1.8

(2.3) (3 71 (2 31

E-1 1.6-2.3 2.0-2.3 1 .6-2.3

(1 7)

E-2 1 .4-2.2 1 4-? ?

F-1 0.84-1 .7 1.1-1.7 0.84-1.7

F-2 1 .2-3.9 2.0-3.5 1.2-2.2

(1.5) (3.2)

G-1 0.81-1.3 0.96-1 .3 0.81-1 .3

G-2 0.78-1 .0 0.78-1 .0 0.78-1.0

^Design: G(HL)5HL^A.

Identification of coating manufacturers with reflector designation may be

obtained from the author.

Values in parentheses were for single shots considered outside the general

range.

42
Table 3. Damage thresholds of commercial ZrOa/SiOa multilayer reflectors

KcT 1 ec Lor _^ Designation by spark

Peak Energy Density

by LIS net by spark & LIS combined

U/ CHI J/cm^ J/cm'2

A-3 0.71-1.0 , 0.83-1 .2 0.71-1 .0

(1 5)'^ (1.5) (1 5)

A-4 1.5-1.8 1 .4-1 .8 1 .4-1 .8

D-3 0.80-1.2 0.80-1.6 0.80-1 .2

(1 61

D-4 0.87-1.9 1 .4-2.3 0 87-1 .6

(1 91

E-3 0.57-1.6 0.94-1 .6 0.57-1 .6

E-4 0.48-1.6 0.88-1.9 0.48-1 .5 (3.3)

G-4 1.1-2.1 1 .6-2.1 1.1-2 .1

^Identification of coating manufacturer with reflector designation may be

obtained from the author.

'^Values in parentheses were considered outside of the general range.

8. Weak-Signal Scattering

The scatter measurements for the Ti02/Si02 reflectors are plotted in terms of bidirectional

reflectance versus damage threshold in Figure 3. Bidirectional reflectance is defined as the fraction

of incident power Po of unit wavelength, incident from direction (Si, <})i), which is reflected in

direction (62, (i>2) per unit solid angle ^2,

PgOi, 02, ^)-^srbh'kThe factor of tt/cos 62 is required so that the directional reflectance pn (the quantity measured by

integrating spheres) may be directly calculated by integrating pg over the hemisphere according to the

standard definition,

pQ(ei, 4)1, A) = 7^Pb cos e2dn2 . (2)

A general correlation of decreasing damage threshold with increasing bidirectional scatter is seen in Figure 3. The correlation is most apparent for reflectors with largely differing scattering

levels and is weak for reflectors with scattering of the same magnitude. This same type of correlation

was previously reported by Newnam and DeShazer [2] for scattering and damage by 13-ns ruby laser

pulses. Results of that study for Ti02/Si02 reflectors are presented in Table 4. For the eight

Zr02/Si02 reflectors measured, the bidirectional reflectances were all in the range from 10 x 10"^

to 20 X 10-^sr-^

That the damage thresholds were generally lower for coatings with high scatter is a reasonable

result. The effective absorption coefficient of a coating can be enhanced via internal reflections

of the scattered energy. [7] If the contribution of scattering to the net deposition of energy is

great enough, the resultant damage threshold would be lower than the threshold for no scattering. It

is also possible that the scattering process itself does not increase the absorption of energy

significantly. Instead, the level of scatter may indicate the relative presence of other film defects

which do control the threshold by another mechanism. As an example, grooves and pits can cause localized enhancement of the electric field. [8]

43
Table 4. Comparison of weak-signal scatter and spark threshold for

TiOa/SiOz multilayer reflectors designed for 6943

Rp-f 1 pr for

designation

N n pm^ li7pd <^r;=i"H'P>" intensity at 6943 K

Pp ;i U P n o v^n \/ Hpn c "i + \/ threshold (by spark)

J/cm^

0104 1 .0 107-127

0102 2.3 98-110

0101 3.5 83-103

0103 3.7 121-126

SlOl 6.5 44- 56

^Excerted from table 5 of reference 2.

Damage thresholds were obtained with 13-ns pulses from a TEMqo

mode ruby laser and spot-size radius of 0.06 mm at the sample

surface.

9. Discussion

It is interesting that the best ZrOz/SiOa reflectors had less than one-half the damage threshold

(1-2 J/cm^) as that of the best TiOa/SiOa reflectors (3-4 J/cm^). By considering only linear absorption in defect-free films, ZrOa/SiOa reflectors should have been the more damage-resistant. This can

be shown from the following analysis which includes standing-wave electric fields and thermal and

optical properties of defect-free films. It is necessary to consider only the fields in and the

properties of the high-index components, JiOz and ZrOa, since these have much smaller damage thresholds

than the low-index component, Si02.[2] In addition, the first high-index layer would be most likely

to damage first, since the peak standing-wave electric field drops rapidly in subsequent layers (see

Figure 2).

The absorbed energy required to melt a Ti02 film is 6845 J/cm^; that to melt a ZrOa film is

9400 J/cm^, which is -35% larger. [1] But the normalized electric field, |E/Eo|^ at the first H-L interface is also about 35% greater for a ZrOa/SiOa reflector than for a Ti02/Si02 reflector. [1]

So, reflectors with TiOa and Zr02 high-index layers would be expected to have equal damage thresholds

by thermal and electric-field property considerations alone. However, optical properties must also

be included. The UV absorption resonances of ZrOa and Ti02 are at 2480 A [9] and 3200 A [10],

respectively. Considering also that the real part of the refractive index of ZrOa films is less than

that of Ti02 (~2.0 compared to ~2.3 at 1 m) , the linear absorption coefficient of a Zr02 film would

be less than for Ti02. Thus, by linear absorption alone in defect-free films, ZrOa/SiOa reflectors

should be the more damage-resistant. This is opposite to the present experimental results.

If linear absorption was the only operable damage mechanism, unreal i stical ly high values of the

absorption coefficient would be required for both Ti02 and Zr02 films. The corresponding coefficients

for the thresholds measured would range from 1000 to 5000 cm"^. We are, therefore, forced to consider

nonlinear absorption processes, such as multiphoton absorption, and film defects. Two photon absorption

of pure TiOa films was probably not the damaging mechanism since damage at the 4 J/cm^ level would

require a coefficient of ~140 cm"^ which is much larger than the ~1 cm"^ value measured by Waff

and Park. [11] Defects and absorbing impurities, however, could result in both enhanced multiphoton

absorption [12] and localized linear absorption greater than the average over the surface. The observed

correlation between weak-signal scattering and damage threshold supports this latter premise. Previous

investigations of dielectric films designed for 6943 K have uncovered the role of film defects by

measuring thresholds as a function of laser spot-size [13] and pulse duration. [14] It is probable

that similar tests on the present state-of-the-art reflectors will further implicate film defects

as the main limitation on damage resistance.

10, Summary

Several interesting results from this study may be listed:

1. The present electron-gun coating-deposition technology has resulted in higher damage

thresholds for Ti02/Si02 reflectors (1 to 4 J/cm^) than for Zr02/Si02 reflectors (0.5 to

44
2 J/cm^) for 30-ps laser pulses at 1.06 ym.

2. The wide range of thresholds among the seven coaters surveyed appears to be due to the

various coating techniques used.

3. Reflectors with a relatively large amount of scatter have low thresholds. Bidirectional

scatter measurements can be a useful, nondestructive method to segregate low threshold

reflectors.

11. Acknowledgments

The author is grateful to the following individuals and their coating companies who prepared

reflector coatings for this study:

1. C. Broomer of Broomer Research Laboratories

2. V. Costich of Coherent Radiation

3. M. Grindel of Continental Optics

4. Y. Hahn of CVI Laser, Inc.

5. D. Sigler of Laser Energy, Inc.

6. M. Ogne of Optical Coating Laboratory, Inc.

7. G. DeBell of Spectra-Physics Corp.

The cooperation of E. Podbielski of Itek Corp. regarding the glass substrates is also acknowledged.

At Los Alamos Scientific Laboratory, the author was assisted by G. Hoffman in optical microscopy

and K. Winn in preparing the diagnostic laser. Special thanks go to J. Hudson for assistance in data

acquisition and reduction, and to Jo Ann Painter for skillful editing and typing of the manuscript.

12. References

[1] Newnam, B. E., "Laser-Induced Damage Pheno- mena in Dielectric Films, Solids and Inorganic Liquids," dissertation, Univ. of

Southern California (1973). Available from

University Microfilms, Ann Arbor, Michigano

[2] Newnam, B. E., and DeShazer, L. G., in Laser

Induced Damage in Optical Materials: 1972 , edited by A. J. Glass and A. H. Guenther,

(NBS Special Publication No. 372, October,

1972), pp. 123-134.

[3] Watt, B. E., Appl. Opt. 12_, 2372 (1973).

[4] Farrukh, U. 0., "Diffraction and Focusing

of Truncated Gaussian Beams," dissertation,

Univ. of Southern California (1974). Available from University Microfilms, Ann Arbor,

Michigan.

[5] McKenney, D. B., and Mott, L. P., J. Opt.

Soc. Am. 61, 665 (1971).

[6] Bass, M., and Barrett, H. H., in Laser

Induced Damage in Optical Materials: 1972 , opo cito , ppo 58-69.

[7] Winsor, Ho V., in Conference on High-Power

Infrared Laser Window Materials, edited by

C. S. Sahagian and C. A. Pitha, (Air Force

Cambridge Research Laboratories Report

AFCRL-71-0592, 1971), pp„ 351-368.

[8] Bloembergen, N., Appl. Opt. J2_, 661 (1973).

[9] Travina, T. S., and Mukhin, Yu. A., Sov„ PhySo

J. 9, 40 (1966)„

[10] Meehan, J., and Salomon, R. E., J. PhySo Chem„

70, 3642 (1966).

[11] Waff, H. S., and Park, K. , Phys. Lett„ 32A,

109 (1970).

[12] Braunlich, P., in Damage in Laser Materials , edited by A. J. Glass and A. H. Guenther

(NBS Special Publication No. 341, December,

1970), pp. 102-104.

[13] DeShazer, L. G., Newnam, B. E., and Leung,

K. M., in Laser Induced Damage in Optical

Materials: 1973 , edited by A. J. Glass and

A. H. Guenther, (NBS Special Publication

No. 387, December, 1973), pp„ 114-123.

[14] Milam, D., Bradbury, R. A., and Bass, M„, in

Laser Induced Damage in Optical Materials:

1973 , op. cit., ppo 124-132.

45

564-378 O - 74 - 5
13. Figures

Low frequency

passive

filter

Visuol Inspection

Air

.He-Ne laser Measuring

microscope (30X)

Translatoble,

rotatable sonnple mount

Light trap

ght Scatter System

Locl<-in amplifier

Tel^tronix 556 oscilloscope

Figure 1. Methods for detecting laser-induced

damage to optical components: 1) photoelectric

spark detection, 2) laser-induced scatter (LIS)

of a He-Ne beam (electronic and visual) , and

3) visual inspection with a low-power microscope.

L^ H^LHLHLHL H(LHfG

10-22

Relative electric-field intensity distributions for a TiOg/SiOj

reflector.

Figure 2. Relative electric-field intensity

distributions for a Ti02/Si02 multilayer

reflector.

Peak Energy Density, (J/cm )

Figure 3. Initial weak-signal scatter at 1.06 ym

versus damage threshold for Ti02/Si02 multilayer

reflectors.

46
COmEMTS OM PAPER BV WEWNAM

The ipzak^A corm&ntzd that the. n^oaghnsAi oi the iiubiyt/iate Moutd be expected to have an ei^ect on coatings a (Jew degn.eei>, bat ^on. multllayeA coaXlngi, the ei^ect o{, iabitOtute Koaghne^ii would not be

ob^ieAvable, M. J. SoiZeaa OfJ the Uiahetion Labon.aton.y indicated that (^iZmi tested at that labonatoKy

cZeanZy showed Koaghnea c.onjie{>pondA.ng to the iubitttute waghneAi , even w-ith many laye/n.

V. mtam o{, ATCRL commented that oveA IxxAge apeAotuAei , It td the weakest axea mhlch deteAmtnei the

damage thn.ej>hold. The ipeakeA commented that Keg-iom of, hlghei iscattefiing damaged moKe fieadily than loMicattextng negtoni, Ojf the iame dtlm. He al6o indicated that thoie manuiactuAen^ Mho pnodaced the f-iZmi, u)tth the highest damage leveJU> atio produced {^ilmi, with {^ewet weak ipotA [htghJLy icattentng n.egtonA]

.

Tfioi, whether, the low on high vatae o(, damage thnedhold woi taken oi fiepKeientattve o(, a given dilm, the

>iank Oldening oij iamplei wa4 eaentially the 6ame. In all ca^eA nan, iabitnate damage thneAholcUi wexe

much highen than film damage vaZuei. All the f,ilm deiigni tested had a Si02 layen. at the ian{,ace. Even

though thii> givei a highen intennal {,ield value, it wai choien to give a compneAiive itnea, in the outride layen, {,on &tnuctunal itnength.

47
2.2 Damage Studies at 1. 06y m With 100-200 ps PuLses*

G. W. Leppelmeier and M. Finkelstein

Lawrence Livermore Laboratory

Livermore, California 94550

Studies have been conducted of the damage threshold at 1. 06 \im of multi- layer

dielectric films used as polarizing beamsplitters, mirrors, and anti-reflection

coatings, K(H^. . . ^^-^PO^ several high Verdet constant glasses, and micromachined silver. The laser pulse used had less than 4% spatial and temporal

ripple. Pulse energy of up to 1 joule permitted using a coUimated beam. Damage

thresholds in the range 1-5 J/cm^ were obtained.

Key words: Laser damage; mirror damage; thin films.

1. Introduction

Lasers for laser-driven fusion are characterized by subnanosecond pulse lengths,

apertures of hundreds of millimeters, and energy densities approaching bulk material

limits. Thus the optical components in such laser systems are subjected to very high

stresses, in addition to the stringent requirements placed by large apertures and high

optical beam quality. At present, the damage threshold of optical coatings, along with

nonlinear beam distortion, poses a major limitation in the performance of laser-fusion

lasers, particularly for Nd:glass lasers. For this reason we are interested in the

macroscopic damage resistance of films and surfaces, and report here investigation

of the damage threshold at 1. 064 ym, for pulses of 100-200 picoseconds, of multi-layer

dielectric coatings, as used for polarizing beam splitters, mirrors, and anti-reflection

(AR) coatings, as well as several magneto-optic and electro-optic materials.

The laser system is indicated in the schematic of figure 1, and is designated the

"Intermediate Laser System" (ILS). It consists of a Nd:YAG oscillator, single-pulse

selection unit, two Nd:YAG pre-amplifiers (LLL-designed and built) and two CGE glass

amplifiers, one with 16 mm diameter and one with 32 mm diameter. The spatial properties of the beam are determined by a spatial filter between the two Nd:YAG pre-amplifiers

(in early experiments this was simply a pinhole, without lenses) and a soft aperture, or

apodizer, near the input of the first glass amplifier. Temporal smoothness of the pulse

is insured by etalons in the oscillator cavity which provide a time/bandwidth- limited

pulse. Figures 2 and 3 show the spatial and temporal beam profiles, respectively. The

absence of reproduction of the large features in successive traces permits the inference

that the large features are due to film noise or the like, and that the actual ripple is less

than 4%, both in space and time. In early experiments the streaking camera traces indicated that the full width at half maximum intensity was 200 ps -t 25 ps. Most of the

data were obtained more recently with a characteristic FWHM time of 125 ps + 25 ps.

The maximum energy available from the ILS before beam breakup due to nonlinear

effects is about 1 joule, which provides peak energy densities of up to 8-10 J/crr? in a coUimated beam rather than a focused beam. Thus the damage measurements are

obtained for "macroscopic" regions, i.e. , 0.5-2 mm, rather than for small focal regions.

The experiment itself is described in figure 4, which shows the beam emerging from

a 3:1 telescope and passing through an uncoated, wedged beam splitter. One of these

diagnostic beams then goes to a 2. 5 mm aperture in front of a high resolution LLL calorimeter [1]^. The other diagnostic beam is used to produce a multiple reflection

image, either on Polaroid film or Kodak 1-Z high resolution film. The main beam then

passes to the test sample. The 2. 5 mm aperture, the film plane, and the test sample

are all the same distance away from the beamsplitter. For low eneregy densities

(1-2 J/cm^ ) the beam profile was approximately flat across the central 2. 5 mm. As the

energy rises (2-4 J/cm^ ) some whole beam self-focusing occurs, making the peak energy

"This work was performed under the auspices of the U„ S. Atomic Energy Commission.

1. Figures in brackets indicate the literature references at the end of this paper.

48
density somewhat higher than the average over the central 2. 5 mm, which is what the

calorimeter measures. At energy densities higher than 3-4 J/cm^ the edges of the beam

experience a severe self-focusing, causing the peak energy density to occur at the edge of

the beam, rather than the center. When the beam is not extremely smooth, small-scale

self-focusing causes the beam to break up into hot spots with peak energy densities much

higher (2-4 X) than the average.

The results will be presented for average energy density over a 2. 5 mm aperture. In cases where the peak energy density was notably higher than average a note will be made as

to the necessary correction. The ratio of peak energy density to average energy density

(Ep/Eg^J is obtained by tracing the beam profile on the high resolution film with a microdensitometer. Since the ratio of exposures of adjacent spots on the multiple-image film is

equal to the reflection of the partially transmitting mirror, each film is auto-calibrating.

Plotting the exposure versus then allows us to use a polarimeter to obtain the ratio of

peak exposure to average exposure, which is equal toE

2. Experimental Results

Tables 1-4 contain summarized results of the damage tests.

Polarizing beam splitters: table 1 contains the results of damage testing of polarizers.

All the polarizers were tested at an angle of incidence of 56. 5 + . 2 (Brewster's angle for

the substrate, which is BK-7 for all samples). The polarizers"are designed as stop-band

filters, with the transition to transmission occurring for the P-polarization at a shorter

wave length than for the S-polarization, due to the oblique angle of incidence. Generally,

the samples had twenty to thirty layers. Fluorescence analysis indicated that the high-index

layers contained zirconium or titanium, sometimes with potassium or cerium added. We

assume the low index material was Si02 , which would be indistinquishable from the substrate. It was also evident from the angle and wavelength dependence of the transmission of

the two polarizations that decidedly different coating designs are used. We found no correlation of damage threshold with coating materials or polarizer performance.

Table 1. Measured damage thresholds for multilayer dielectric coated polarizers

Substrate Material: BK-7

Unless otherwise noted, pulse times are 125 + ps (FWHM) and energy densities are average over 2. 5 mm circular aperture.

Manufacturer

A

B

C

D

E

F

Resisted Damage

at (J/cm2)

3. 2 2. 8-3. 2 (3 shots)

1.8-2. 5 (3 shots)

3. 6, 5. 3

2. 1, 2. 1

3. 3-4. 8 (10 shots)

3. 3-5. 8(15 shots) on two spots)

2. 9-3. 7 (5 shots)

Damaged at

(J cm2)

3. 8

3. 1-3. 8 (4 shots)

1. 5-2.

0

2. 5-3.0 (4 shots)

4. 3, 6.

7

5. 2-5. 9 (4 shots)

6. 7

3. 1, 3. 5. 4. 4

Remarks

peak energy density, 200 ps

x 1, 3 = peak energy density

peak energy density, 200 ps

X 1. 6 = peak energy density

peak energy density, 200 ps

X 1. 5 = peak energy density

X 2.0 = peak energy density

beam badly broken up

X 1. 3 - peak energy density

Several manufacturers supplied anti-reflected coatings on EY-1 samples furnished by LLL.

The damage tests of these samples are summarized in table 2. The best results are comparable to the best attained for polarizers, but not by the same manufacturers.

49
Table 2. Measured damage thresholds for anti-reflection coating on EY-1

PuLse times are all 125 + 25 ps

Energy densities are average over 2,5 mm circular aperture

Resisted Damage Damaged at Manufacturer at (J cm2) (j cm2) Remarks

A 2. 6-3. 6 (6 shots) 3.6,4.8.5.3 x 1 . 5 = peak energy density

D 4. 0-5. 4 (7 shots) -- x 1 . 3 = peak energy density

F 2. 0-2. 9 (6 shots) 2.8-4.0 x 1 . 3 = peak energy density

G 3. 0-4. 7 (9 shots) 5.0-6.2 x 1 . 2 = peak energy density

Table 3 contains damage testing results for mirrors: micro-machined silver and a BK-7

prism as well as multi-layer dielectrics. The silver mirror was made at Y-12, in a single

cut on 0. 762 mm of electrolytically deposited silver on OFHC copper. While the thresholds

for the dielectric coats are nearly as high as for the glass prism, they are definitely lower

than for the polarizers and AR coats. As would be expected due to superposition of electric

fields, the BK-7 prism damaged at the internal reflectance face.

Table 3. Measured damage thresholds for mirrors

Pulse times are 125 + 25 ps

Resisted Damage Damage at Manufacturer at (J/cm?) (J/cm2) Remarks

B 1,6 1.4, 2.0 0. = 0° X H 2. 0-2. 5 (4 shots) 2.3,2.7,2.7 x 1 . 5 = peak energy density,

0^ = 45°

y-machined Ag 0. 6-1. 0 (15 shots) 1.0, 1.0 •

Earlier tests at 200 + 25 ps. Values are peak energy density.

BK-7 3.0-4.2 4.7, 4.9 90° turning prism

D 2. 6-3. 1 3.3-3. 9 0^ = 4?

B 2. 5-3. 3 (9 shots) 3.5-4.5 0^ =43"

Lastly, table 4 gives damage thresholds for three Faraday rotator glasses and KDP

and KD*P with deuterations of 86% and 97%. FR-4 is a cerium-doped glass made by Hoya

Optical Company. FRN-5 is a terbium-doped glass from the same company, while

EY-1 is a terbium-doped glass made by Owens-Illinois. As can been seen, all three

glasses are very tough. The KDP samples were not subjected to energy densities

any higher than 3. 0 J/cm^. The KDP samples were aligned close enough to k-vector

matching to provide some doubling to 5320 A, but not for maximum second harmonic

generation, as this would have caused a reflection directly back into the glass amplifiers.

50
Table 4. Measured damage thresholds for several magneto-optic and electro-optic materials

Energy densities are peak values

Resisted Damage Damaged at Material at (J/cm^) (J/cn^) Remarks

FR-4 4.0 7 (beam broken 200 + 25 ps

up)

FRN-5 7. 4 -- 125 4_ 25 ps

EY-1 4. 2 200 + 25 ps

KDP 2, 5-3. 0 (7 shots) -- 125 + 25 ps

KD*P (86%, 97%) 2. 5-3. 0 (6-8 shots) -- 125 + 25 ps

3. Summary

The aim of these tests was to evaluate various components for the LLL Nd:glass laser

systems, with the goal of obtaining reliable optical components at energy densities around

2 J/cm^, for pulses in the range of 100-200 ps. In large part, this goal has been obtained,

with the mirors remaining an item for improvement. Our experience underlines the

necessity of very clean beams for nanosecond and subnanosecond work: the slightest modulation in the beam will self-focus and cause a local energy density far in excess of the average. It seems quite clear that damage resistance in dielectric coatings is process-dependent.

Achievement over large apertures of the high damage resistance reported here remains a

difficult and demanding challenge to the coater, as does further increase in the damage thresholds.

References

[1] S. R. Gum, "Volume-Absorbing Calorimeters for High-Power Laser Pulses" , Rev.

Sci. Instr. , July 1974, to be published.

NEAR FIELD DESITOMETER SCAN

SYSTEM DIAGRAM .1^

Mode locked Nd. yag oscillator

Pulse selector

Nd. yag preamplifiers

V

Glass amplifiers

Experiment

Figure 1. Diagram of the Intermediate Laser System.

Generally a spatial filter is inserted between the

ND:YAG pre-amplif iers and a soft aperture is placed

near the input of the first glass amplifier. The

two ND: glass rod amplifiers are made by CGEMarcoussis and are 16 mm and 32 mm in diameter,

respectively. Figure 2. Typical microdensitometer traces of

the time-integrated transverae beam profile.

51
Streak camera photo

At^ 100 picoseconds

Time

Figure 3. Typical microdensitometer traces of streaking camera records

showing temporal character of the beam.

DIAGNOSTICS USED IM DAMAGE THRESHOLD MEASUREMENTS .L3

|— 132 cm^\r-|

Camera

(no lensi

Figure 4. Diagram of the experimental set-up and

diagnostics. See text for discussion.

COMMEWTS ON PAPER BY LEPPELMEIER

It iA}cu> (uke-d X(5 th^ pKeJsmaz ob^cuAlng incluiiom, -in muJUltaLjeji diel-zct/iic itAixctuKU woi

ial{jA^(iA.zwt to cLLiiKLipt the. -itanding wavz patteAn, and thuKzioKz in{)tue.nci the. damage. thAeyi>katd. Vue to icaiteAlng in the matexiaJi, tkLi, wai thought to be antikely. In Incliuion- dominated damage -in muZtitayen.

coatingi, V. UiZam AFCRL nepomted damage at: tnctiLi>ion iiteM, not at: the tnteK^acei o(, the coating

layeAA .

52
2.3 Damage Thresholds in ZnSe, A/R Coated NaCl and Micromachined

Mirrors by 10.6 ym Multijoule, Nanosecond Pulses*

Eugene E. Stark, Jr. and Walter H. Reichelt

Los Alamos Scientific Laboratory

University of California

Los Alamos, NM 87544

Multijoule, nanosecond pulses at 10.6 ym were used to determine the damage

thresholds of uncoated and antireflection (A/R) coated ZnSe, A/R coated NaCl and

micromachined mirrors. The ZnSe results indicate that the A/R coatings have a higher damage threshold than the bulk material. Antireflection coatings on NaCl were found to have a damage threshold slightly lower than uncoated NaCl. Damage

thresholds for micromachined copper and gold are greater than 4 joules/cm^.

Key words: A/R coatings; damage thresholds; machined mirrors; ZnSe.

1. Introduction

The damage thresholds of various optical components are important parameters in the design of large,

short pulse CO2 systems. The threshold may be defined as the incident energy flux level which results

in observable damage to the surface or bulk material of the component. Damage thresholds have been

established for the two prime window material candidates, NaCl and KCl . [1]' Recent emphasis by the Air

Force in developing large ZnSe windows for CO2 laser use has prompted an investigation of both uncoated

and A/R coated ZnSe. The results of this investigation are discussed in this paper.

A/R coatings are necessary in order to reduce beam losses due to Fresnel reflections in the case of

high index of refraction materials. This, of course, is an alternative to the use of Brewster angle

windows. While these losses are of considerable importance in the case of ZnSe because of the high

index of refraction, they also become important in the case of the alkali halides when a number of these

components are involved in a system. In addition, due to the hygroscopic nature of the alkali halides,

it may be prudent to use the A/R coating as a protective coating. Results from a preliminary investigation of A/R coated NaCl are also given in this report.

Gas laser systems require large numbers of metal mirrors for beam handling and focusing. Conventional fabrication techniques for these components are both costly and time consuming. Recent developments in the direct micromachining of optical surfaces [2] implies that this process will reduce mirror

fabrication time and costs. Mirrors fabricated by means of micromachining have been used extensively

in the Los Alamos Scientific Laboratory CO2 laser system. Lower bound damage thresholds have been

established for both copper and gold micromachined surfaces and are reported.

2. Experimental Configuration

The experimental configuration, pulse shapes, etc., have been described in the Damage Symposium

Proceedings of 1973. [3] Consequently, they will be reviewed here briefly for completeness.

The Los Alamos Scientific Laboratory pulsed CO2 laser was used to provide a well characterized

joule level, nanosecond pulse. Joule level pulse generation is accomplished by amplifying a TEM , polarized, millijoule pulse in three electron beam pumped amplifiers as shown in figure 1. The °° energetic pulse is focused into a clean-gas-filled tube by an R = 4m totally reflecting mirror. The

sample is located in flowing clean gas to minimize dust effects. Reflecting surfaces are at a slight

angle from normal in order to prevent reflections from traversing back into the still-activ^ amplifier

system. Average peak energies are determined by means of an aperture and calorimeter.

Damage observations are made visually and with the aid of a telescope. Damage sites are examined

with the aid of optical and scanning electron microscopy.

*Work supported by the U. S. Atomic Energy Commission.

1. Figures in brackets indicate the literature at the end of this paper.

53
3. Results and Discussion

3.1 Dielectric Window Materials

a. Uncoated ZnSe

These samples were prepared by the CVD process at the Raytheon Research Laboratory. Optical

microscopy reveals a high-density of voids and inclusions scattered throughout the volume. The

fabrication process is such that these defects occur predominantly in layers. Sectioned and polished

layers reveal these- bulk defects as well as a highly stressed material as seen in figure 2(a). Some grain

boundary separation is noted at or near the surface in some samples. Electric field enhancement [4] at these defects results in a lower damage threshold and may cause catastrophic damage.

ZnSe damages in the bulk material at an average incident energy of 1.5 joules/cm^. There is a variation in the defect density from sample to sample that would account for variations in the damage

threshold. The damage appears as a progressive clouding of the material with repeated pulses. The

clouds consist of a large number of miniscule dots approximately 3 ym in diameter. These are presumably microcracks radiating from defect centers. The clouds have a defect density of the order of

10^/cm^. Surface damage was not observed when the surface was of optical quality. When surface

damage did occur, it had the appearance of "pull-out", wherein whole grains were lifted from the sur- face as seen in figure 2(b).

A rather spectacular single shot, damage event is illustrated in figure 2(c). The incident laser

beam (1 joule/cm^) direction is indicated by the arrow. A magnified picture of the entrance surface

is shown in figure 2(d). It is speculated that a surface defect such as grain boundary separation at the

surface was responsible for the event. The high electric field resulting from field enhancement at

the defect caused an "explosion" at the surface. Conical fracture occurred in a manner quite

similar to that observed when a steel ball is shot onto a glass surface. [5] Figure 2(e) shows

spallation at the entrance surface. Figure 2(f) is the center spike extending along the beam axis

into the bulk of the material.

Both the observed bulk damage and damage originating at the surface indicate the importance of

material integrity and purity. The electric fields associated with these short pulses require that

the material be defect free. This is particularly true in the case of high index of refraction

materials.

3.2 Antireflection Coated ZnSe

The relatively high index of refraction of ZnSe causes a high Fresnel reflection at each surface.

In order to reduce these beam losses, it is necessary to A/R coat the surfaces. Several ZnSe samples were coated by the Hughes Research Laboratory and the Optical Coating Laboratory, Inc.

a. Optical Coating Laboratory (OCLI) A/R Coated ZnSe

Microscopic observation of these surfaces before irradiation reveal surface defects that might

be in the coating. The defect appearance is a slightly different color cast than the rest of the

surface. The defects were round or oval shaped. The bulk material was relatively defect free.

The irradiated sample surface was refinished by OCLI before coating. Under irradiation the

dielectric surface damaged at 2.5 joule/cm^ Characteristic damage is shown in figure '3 (a) . The shape

and size of the damage spot is similar to the defect spots observed in the coating before irradiation.

These results are indicative of a rather tenacious coating with good damage resistance.

b. Hughes Research Laboratories (HRL) A/R Coated 'ZnSe

As in the case of the OCLI coatings, surface coating defects were noted before irradiation.

These defects were irregular in shape and were rather ragged. Damage was noted in these samples at

an average incident energy of 1.7 joules/cm^ A typical damage site is shown in figure 3(b). The shape

of the site is similar to the surface defects noted in the unirradiated sample. Note the difference in

the substrate finish between the samples shown in figures 3(a) and 3(b). This latter sample had more

bulk defects than in the former sample. This fact may explain some of the difference in damage

thresholds although the substrate surface finish was better in the OCLI sample. Once again this

implies that resistant coatings are possible for ZnSe,

54
3.3 Antireflection Coated NaCl

These samples were prepared by C. Phipps of Lawrence Livermore Laboratory. They were conventional

X/4 NaF coatings on standard finish NaCl flats. The coating had a cloudy cast which apparently did not

affect its efficiency at 10.6 ym. These samples damaged at ^ 2.0 j'/cm^. A typical damage site is

shown in figure 3(c). No special care was taken in surface preparation before coating. Inproved surface

finishing could result in significantly higher damage thresholds.

3.4 Conclusions

The optical quality of CVD ZnSe is presently not sufficient for short pulse CO2 laser applications

at high energy fluxes. It is recognized, however, that this situation is constantly improving. Damage thresholds of the order of 3 joules/cm^ may be expected for both ZnSe and NaCl. The experimental

results again point out the necessity for defect-free bulk material and surfaces. At pulse lengths

shorter than a nanosecond, the electric fields associated with the same energy flux are greater,

consequently material purity and surface quality will become increasingly important.

4. Micromachined Metal Mirrors

4.1 Micromachined Mirrors

Micromachined metal mirrors were prepared by the Y-12 facility at Oak Ridge, Tennessee. Copper

substrates were machined from OFHC copper. The substrates were then electroplated with 0.013 cm of

either gold or copper. The plated surfaces were then micromachined to optical quality. Some of the

machining and plating parameters are:

Surface

Material

Copper

Gold

Plating

Bath

Sel Rex

Temprex HO

Purity

Levels

99.95

99.99

Tool

Material

SC Diamond

SC Diamond

Tool

Radius

1/8 in.

1/8 in.

RPM

of Part

1000

1000

Cutting

Feed

200 pinch/rev

200 y inch/rev

Samples were irradiated as received from Y-12. No attempt was made to reclean the surfaces.

They were dusted with clean N2 although some dust particles were observed on the surfaces in the

clean gas irradiation tube.

4.2 Copper Mirrors

The copper surfaces were irradiated to 4j/cm^ with no permanent surface damage. Figure 4(a) shows

an irradiation site with surface debris from a dust particle which exploded in the laser beam. The

debris washed off in an alcohol bath leaving the surface appearing as it did prior to irradiation.

The primary electron energy was varied in SEM studies of the surface and subsurface and no subsurface

damage was noticed.

4.3 Gold Mirrors

Effects of dust debris were noted in the case of gold surfaces just as in the case of the copper

sample. In one case damage to the surface was observed at an average incident energy of 2 joules/cm^.

This damage site is shown in the optical micrograph in figure 4(b). There is surface debris and some shallow cratering. A series of scanning electron micrographs taken with increasing primary electron

energies is shown in figure 5. These photographs reveal subsurface damage structure as indicated by the

darkening radial zone. A cross section of this damage site is shown in figiire 4(c). It is evident that

there is shallow cratering and subsurface layering - evidence for surface spalling and bulk separation.

The speckled appearance of the gold is an artifact due to imbedded polishing compound. The exact

cause of this damage characteristic is not known although there is a temptation to speculate that it

originated with a dust particle explosion at the surface.

4.4 Conclusions

A reasonable lower bound to micromachined gold and copper surfaces is of the order of 4 joule/cm^.

The experiment reveals the necessity to keep the surface dust free when used in the high energy flux

applications.

55
5. Summary

Table 1 lists the experimental damage thresholds for materials reported in this page. In addition, thresholds for NaCl and KCl are included for completeness.

Table 1. Damage Thresholds or 10.6 ym for Nanosecond Pulses

(joules/cm^)

Material Coating S.C.^^^ S.F.^*^^ S.C.HPLF^^^ Poly/^^S.F. Poly, HPLF Machined

NaCl uncoated 3.0 4.5 3.0

KCl " 1.5

ZnSe " ^^.5

ZnSe A/R, OCLI 2.5

ZnSe A/R, HRL 1.7

NaCl A/R, NaF 2.0

Copper uncoated > 4

Gold " > 4

(a) S.C. refers to single crystal materials

(b) S.F. refers to a surface finish of the order of 40-20 dig, scratch

(c) HPLF to a finish of the order of 10-5 dig, scratch

(d) Poly refers to polycrystall ine material

6. References

[1 ] W. H. Reichelt and E. E. Stark, Jr.,

unpubl ished.

[2] J. B. Arnold, P. J. Steger and T. Saito,

Oak Ridge, Y-12 Plant, Publication Y-DA-5786

[3] W. H. Reichelt and E. E. Stark, Jr., Laser

Induced Damage in Optical Materials, 1973,

NBS Publication 387 (Dec. 1973), p. 175.

[4] N. Bloembergen, IEEE Journal of Quantum

Electronics, QE-10, #3, p. 375 (1974).

[5] I. A. Fersman and L. D. Khazov, Soviet

Physics-Technical Physics, 15, #5, p. 834

(1970).

7, Figures

j\ 1.2 nsec J\\mJ Pulse

Mirror (R=4 m)

Osc

I >-\ Amp I J \ Amp z/ > ^^>-^~\^"'P ^ /

'

Sample

Telescope

Filtered Nj

Experimental Configuration for

NaCl Damage Study

Figure 1. Experimental configurations for damage

studies

.

56
Figure 2. ZnSe samples, uncoated. (a) etched section, 250X; (b) exit damage, 300X; (c) conical

fracture; (d) entrance damage, 40X; (e) entrance damage, lOOX; (f) center damage spike, lOOX.

Figure 3. A/R coated samples. (a) ZnSe lOOOX;

(b) ZnSe lOOOX; (c) NcCl 25X.

57
Figure A. Mlcromachlne surfaces. (a) copper 300X; (b) gold 300X; (c) gold 250X.

Figure 5. Micromachined gold surface. (a) electron

accelerating potential = 25 KV, 300X; (b) electron

accelerating potential = 15 KV, 300X; (c) electron

accelerating potential = 5 KV, 300 X.

COMME.VTS ON PAPER BV STARK

A. EKaimAtiiln the. Haghu RoJidofich Labomton.y comme.ntzd on the. di^icaltLj obtaining a good

limJih, i>cA.citch.-{^H.ee, on InSe., ii the banded itAuctuAe inteA6e.citi the iuA^ace. In pieces in.ee {,wm

banding neoA the iuA{,aae, tkU, diddiaatty wa6 not encoanteAed.

58
2.4 Pulsed COp Laser Damage in Windows, Reflectors, and Coatings

V. Wang, J.E. Rudisill, C.R. Giuliano, M. Braunstein, and A. Braunstein

Hughes Research Laboratories

3011 Malibu Canyon Road

Malibu, California 90265

Recent 10.6 ym damage measurements using a TEM laser of 0.6 ysec pulse

length are reported for a variety of windows, reflectors, and coatings. Reduction of the density of inclusions in ZnSe was found to have a large effect

upon scatter but a smaller effect upon damage resistance. Absorptions and

damage thresholds of thin films of ZnSe, AS2S3 and ThF4 were measured upon

relatively defect-free KCl substrates, and then tested in antireflection and

multilayer reflector configurations. Also reported is a five-fold increase

in damage resistance of simple polished copper reflectors by surface etching.

Key words: AS2S3; laser coatings; laser reflectors; laser windows; pulsed

CO2 laser damage; ThF4; ZnSe.

1. Introdution

Since 1971 we have been involved in the development of damage resistant, low absorption, optical

components at 10.6 ijm.[l,2] Since our prior experiments have indicated that wide bandgap materials

are as damage resistant as simple polished metal reflectors and that semiconductor materials are con- siderably less damage resistant, this paper reports upon recent development of three wide bandgap

materials: ZnSe, !^^2^3' ^"'^ ^^'^4- These results are compared with recent progress in metal

reflectors

.

2. Experimental Conditions

An ultraviolet CO2 TEA laser discharge 50 cm in length, operating at 550 Torr with 1:1:8 mixture

of C02:N2:He was used in this experiment. A two-meter, approximately hemiconfocal cavity operating in

the TEMqo mode with an uncoated germanium resonant output reflector delivers an essentially untruncated

gaussian beam to a folded optical train (illustrated in figure 1). A beam attenuator consisting of two

wire grid polarizers on ZnSe substrates allows better than two orders of magnitude attenuation without

rotation of the output polarization. [3] Wedged NaCl beamsplitters allow monitoring of the energy in

each pulse by a polyvinylchloride (PVF) pyroelectric detector. Absolute calibration of the pyroelectric monitor is provided by a Hadron Model 100 cone thermopile with a 1 cm aperture located in the near field of the laser.

The output pulse of the laser consists of a 200 nsec gain-switched pulse followed by a nitrogen

fed tail of '^3 ysec. Figure 2(a) displays the output detected by a germanium photon drag detector, and

figure 2(b) is an expanded trace showing a typical example of the partially self-mode locked beating in

the output. Power density is measured at the peak of the gain switched spike by using a detector that

averages the rapid pulsations from mode beating, as shown in figure 2[c). An equivalent pulse length

is defined by measuring the area of figure 2(c) and constructing an equivalent rectangular pulse of

equal area and height equal to the height in figure 2(c).

*Work partially supported by Air Force Weapons Laboratory, Kirtland Air Force Base and by Defense Advanced Research Projects Agency under the technical cognizance of Air Force Cambridge Research

Laboratories.

figures in brackets indicate the literature references at the end of this paper.

59
For thermal damage mechanisms, the effect of nanosecond pulses is negligible with respect to the

total pulse energy, so total pulse energy and pulse length closely characterize the pulse. For electrical breakdown mechanisms in inclusion-free dielectrics, simple considerations predict a pulse length

dependence for the electric breakdown field due to the finite time required to form sufficient electrons

through avalanche. Our extrapolation of calculations by Yablonovitch and Bloembergen [4] suggests that

a pulse 2 nsec long would be expected to have a breakdown threshold 4 to 8 times higher in power density

than a 0.2 usee pulse. From figure 2(c), the peak intensity of this pulse is approximately 5 times the

average intensity, so it is the average power in the gain switched pulse that is reported in both this

paper and the paper by S.D. Allen, et al . , [7] on surface and bulk damage in KCl

.

Spot size for a given lens is measured by varying the energy focused on a heat-sensitive plastic

film (3 M, Type 557) in a series of burn spots, and plotting the log of energy versus spot size squared

(figure 5). A pulse of gaussian profile yields a straight line whose slope is a measure of the spot

size. Measurements of spot size using the 12.5 cm germanium lens used for the majority of the tests

indicate an essentially diffraction-limited spot size to beyond the l/e^ intensity points. Measurements of spot size at the focus of the 6.2 cm lens give a diameter of between 0.059 mm and 0.073 mm

(figure 3). The uncertainty in measurement for this lens is due in part to the difficulty of measuring

the low energies used in this calibration. Energy densities quoted for this lens are calculated for the

on-axis peak intensity of a 0.073 mm spot. If a diffraction-limited waist is assumed, energy and power

densities would be 50% higher for this lens at the beam waist.

Both windows and reflectors are tested in a sample chamber evacuated below 1 Torr to eliminate

spurious effects caused by dust and air breakdown induced near the surface of a sample. Damage threshold in this paper is defined as the first visible, permanent change in the appearance of the sample

viewed at high magnification in an optical microscope, unless otherwise stated. During the tests the

sample is observed at ^]0x.

3. Test Results

Zinc selenide has been of major interest as a high power window material. Previous results [2,5,6]

have indicated that inclusions that visibly lie in bands within the material limit the damage threshold,

as shown in figure 4. In an effort to explore the effects of improved materials process control,

regions of low scatter were selected for damage testing. The ZnSe surfaces were etch polished with

potassium ferricyanide on both the samples in tables 1 and 2 to remove a damaged surface layer. Scattering measurements at 10.6 ym show a tenfold reduction in scatter for the selected clear areas. However,

the damage threshold, reported in table 1, shows only a small improvement in damage resistance.

Table 1. Measured Thresholds of Damage in ZnSe Windows

Relative

Scatter,

10.6 pm

Damage Threshold

Material

Absorption

cm Energy Density,

J/cm2

Power Density,

2 MW/cm

Comments

Banded region,

R-26

0.0030 1 30 to 40 50 to 70 Inclusion damage

Unhanded region,

R-26

0.0021 0.1 55 90 Inclusion damage

or localized sur- face absorption

sites

^Based on t = 0.6 ysec

.

Wide bandgap high and low index materials are useful for antireflection coating on windows and

multilayer dielectric reflectors. Zinc selenide (n = 2.41) and AS2S3 (n = 2.38) are promising materials

for high index films and ThF4 (n = 1.35) is the only nonhydroscopic material available of low index and

low absorption at 10 pm. In order to evaluate the performance of individual films, X/4 wave layers have

been deposited on relatively defect-free surfaces of low absorption etch-polished RAP grown KCl. [7]

These substrates allow accurate calorimetric absorption measurements to be made and also provide a damage-resistant substrate.

60
Table 2. Measured Damage Thresholds 1n Th1n Films and AR Coatings

Measured Damage Threshold

Comments Coating and Substrate Absorption Energy Density Power Density^

MW/cm^

X/4 ThF^ on KCl 10 cm""" 80 J/cm^ 130

X/4 AsgSg on KCl <1 cm"^ 120 J/cm 200

X/4 ZnSe on KCl

-1 <0.6 cm

1 /I n 1 / xrt*^ 140 j/cm 230 Poor adhesion of film

ZnSe/ThF^ AR on KCl 0,2% per

surface

60 100

ThF4/ZnSe AR on ZnSe N.A. 25 to 40 J/cm^ 40 to 70 Limited by substrate

ThF4/ZnS AR on ZnSe N.A. 30 to 40 J/cm^ 50 to 70 Limited by substrate

^Based on x ^ 0.6 ysec.

To compare the values given In tabl© 2 for ZnSe films with the bulk values in table 1, we can calculate the electric field distribution in the film as shown in figure 5. Assuming a plane wave entering from the left, figure 5 plots the normalized electric field versus depth into the A/4 film in micrometers. The average electric field in the layer is within a few percent of the field in a bulk ZnSe

window. While bulk ZnSe has an absorption coefficient of 0.002 to 0.003 cm-l (assuming the absorption

1s uniformly distributed), the film has a coefficient of approximately 0.1 cm-t. In this case the

absorbed energy density in the film would be 30 to 50 times larger than that in the bulk samples, and

its damage threshold would be expected to be correspondingly lower. Thus, the higher damage threshold

observed in the film may be a result of the relative lack of absorbing inclusions. In the case of ThF4,

as a result of the relatively high absorption coefficient, linear absorption alone can account for a temperature rise to near the melting point. The performance of ThF4, ZnSe, and ZnS coatings In an antireflection design on ZnSe substrates is shown in the bottom half of table 2. As expected, the incident

flux is limited by damage due to substrate inclusions near the surface.

The performance of some of these films in reflector designs is summarized in table 3. Apparently

because of localized absorbing centers, visible plasmas form near the surface at a low energy density,

but gradual power Increase removes these artifacts without damage to the overall Integrity of the films.

Since visible scattering remains relatively high in these preliminary samples, it might be expected that

considerable improvement is possible. The last sample in table 3j a (ThF4/ZnSe)2 reflector on an electroless nickel substrate, exhibits a reproducible improvement by a factor of more than 4 over the several

sites tested on its surface, without any intentional changes in the film deposition techniques. However, the scatter of this sample was lower than the previous ones. Figure 6 Is an exact computer

calculation of the electric field distribution within this dielectric reflector, including the effects

of absorption. As can be seen, the peak electric field within the reflector occurs at the outermost

Interface between the high and low index films, and is approximately 20°^ higher than the X/4 film on

KCl. The field approaches zero on the outer surface of the stack and is also small at the metal substrate boundary. The substrate absorbs about 0.07°^ of the Incident flux, or i2% of the total absorption of the reflector.

Whereas simple uncoated metal reflectors have proved to be among the most damage-resistant optical

components examined, recent results have Indicated an improvement of 3 to 4 in damage resistance by

superpol Ishing of dispersion-hardened copper mirrors. [6] However, this improvement was not observed

In superpol ished molybdenum. In an effort to isolate the mechanism responsible for the improvement,

samples of OFHC copper were polished and etched by metal lographic techniques designed to remove all

surface contamlnents from abrasives, and to leave an unworked surface highlighting the grain structure.

This surface is not an acceptable mirror in the visible due to high scatter, but exhibits a marked

Increase in damage resistance, similar to that seen in superpol ished materials. Table 4 summarizes

this development, and compares the results with a conventionally polished sample. Etch-polishing

techniques to yield a smoother surface of low scatter are presently under consideration. It is necessary to qualify the applicability of these numbers to large-scale beams. Because of the small beam

sizes incident upon some of the samples (120 ym), three-dimensional heat diffusion can be expected to cool the surface at a greater rate than one-dimensional heat diffusion, as experienced in large-scale

beams. Large-scale beams would thus be expected to exhibit a lower damage threshold.

61

S84.|?B e . 74 . i
Table 3. Measured Thresholds of Damage in ThF„/ZnSe Multilayer Reflectors

Measured Damage Threshold

1

Substrate Coating Absorption

,

%

Energy Density,

J/cm^

Power Density'*

MW/cm^

Comments

Mo (ThF^/Znbe) n 00 U . cd 50 80 Conditions

with gradual

power in- crease

no (

1 nr^/ z.noe j U . 0(1 30 80 Lonai LI ons obvious

scattering

sites

Al/E-Ni Ag/lThF^/ZnSe)^ 0.2 270 450 Scattering

sites visible

^Based on t = 0.6 ysec.

Table 4. Measured Thresholds of Damage in Metal Reflectors

Damage Threshold

Coating and Substrate Energy Density Power Density^ Comments

J/cm^ MW/cm^

Cu, conventional polish 90 150 25.4 cm focusing lens used

150 250 12.7 cm focusing lens used

Mo, sputtered Mo 4 to 8 7 to 13 25.4 cm focusing lens used

coating

OFHC Cu, polish and 750 1200 No imbedded abrasive

etch unworked surface

High visible scatter

12.7 cm focusing lens used

^Based on x = 0.6 ysec.

4. Summary

By careful measurement of spot sizes and temporal profile, we have attempted to precisely define

the on-axis energy and power. Results of tests upon thin films currently indicate that wide bandgap

materials are capable of damage thresholds comparable to metal reflectors, and that simple linear absorption can be the limiting factor rather than inclusion damage.

It has been further concluded that inclusions or surface contamination play an important role in

limiting the damage resistance of copper mirrors. Improvements in surface preparation have improved

damage resistance by better than a factor of four.

5. Acknowledgments

We wish to thank Roger Turk for his invaluable expertise in microscopy and in the metallographic

preparation of the copper reflectors, Bernardo Garcia and John Bowers for the preparation of dielectric

films, and Tomas Home for aid in the construction of the experiments.

62
References

[1]

[2]

Wang, v., Braunstein, A. I., Braunstein, M.

,

and Wada, J. Y. , "Investigation of Pulsed CO2

Laser Damage of Metal and Dielectric-Coated

Mirrors," NBS Special Publication 372, Laser

Induced Damage in Optical Materials (1972)

.

Wang, v., Braunstein, A. I Rudisill, J. E. and Wada,

Laser Damage Studies of Me

Coated Mirrors," NBS Speci

Laser Induced Damage in Op

(1973); and Braunstein, A. Braunstein, M. , Rudisill,

"Pulsed CO2 Laser Damage S and Window Coatings," NBS

387, Laser Induced Damage

Materials (1973)

.

Braunstein, M.

,

J. Y. , "Pulsed CO2

tal and Dielectric

al Publication 387,

tical Materials

I. , Wang, V.

,

J. E. and Wada, J.Y.

tudies of Windows

Special Publication

in Optical

[3] Garvin, H. L. , Kiefer, J. E. and Somekh, S.,

"Wire-Grid Polarizers for 10.6 ym Radiation,"

1973 lEEE/OSA Conference on Laser Engineering

and Applications.

[A]

[5]

[6]

[7]

[8]

Yablonovitch, E. and Bloembergen, N.

,

"Avalanche Ionization and the Limiting

Diameter of Filaments Induced by Light Pulses

in Transparent Media," Phys . Rev. Lett. 29 , 14, 2 October 1972.

Fradin, D. W. and Bua, D. P., "Laser-Induced

Damage in ZnSe," Appl. Phys. Lett. TM^, 11,

1 June 1974.

Braunstein, M. , "Laser Window Surface Finishing

and Coating Technology," Third Conference on High Power Infrared Laser Window Materials,

November 12-14, 19 73.

Allen, S. D. , Braunstein, M. , Giuliano, C. and Wang, V., "Pulsed CO2 Laser Damage Studies

of RAP Grown KCl, this Proceedings.

Soileau, M. J. and Wang, V., "Improved Damage

Thresholds for Metal Mirrors," Appl. Opt. 13 , 6 June 1974.

Figures

GE RESONANT

REFLECTOR

APERTURE TEA LASER

WIRE GRID

POLARIZERS

PYROELECTRIC

ENERGY

MONITOR

4 m RADIUS

REFLECTOR

THERMOPILE

TELESCOPE

VACUUM

FEEDTHRU

DETECTOR

VACUUM

SAMPLE

CHAMBER

Figure 1. Schematic of laser, optical train, and sample chamber.

EXPERIMENTAL LASER PARAMETERS

ENERGY - 200 MJ TEM , DIFFRACTION LIMITED

00

PULSE LENGTH (EQUIVALENT PULSE LENGTH) - 0.6 ySEC

BEAM DIAMETER AT OUTPUT (1/E ELECTRIC FIELD) - 0.5 CM

SPOT SIZES (1/E INTENSITY IN DIAMETER)

25.4 CM f.l. - 0.240 MM

12.7 CM f.l. - 0.120 MM

6.3 CM f.l. - 0.059 - 0.073 MM (EXP. UNCERTAINTY)

ATTENUATOR - WIRE GRID POLARIZERS

SAMPLE CHAMBER - EVACUATED TO <1 TORR

DIAGNOSTICS - VISUAL AT ~10X MAGNIFICATION

DETECTORS - HADRON MODEL 100 THERMOPILE, GE PHOTON DRAG

DETECTOR, PYROELECTRIC DETECTORS

63
—I 1^10 nsec TIME

Flgurt 2. Pulst ttmporal preiilt

(a) Ggminluffl pheten drag dsttetsr plug

500 mz eseilleseaps.

(b) Expanded te 10 nsge/dlvlslen,

(e) Definltien of tqulvalsnt pulss

(pyretltetrle dgtsetor)

.

0 O.e 1.0 1.6 2.0 2.5 3.0 3.g

Flgurg 3. B§am spatial prefils and spot size.

7.3X

NOTE: BANDING (TOP DARK REGION). THIS COINCIDES WITH FACE WHICH POLISHED

POORLY. BOTTOM EDGE COINCIDES WITH FACE GIVING EXCELLENT POLISH.

Flgurg 4, Elnc siltnlde cross section.

64
mmm on paper by ojm

Mthoagh tht s.t&hgd 4uA(5aee napofittd in tPia, papoji e,xhlblt^d iabitantLal iactM?.Hlng, and wa4 not an optitatly ^inuhe.d iuA^acie., the. aathofU izol that tht ttchlng t^ahnlqae u apptiaablz to optically

iiQaAz.d (suAiatui, ctj will. Tku tijpi o^ ttchzd iu/Lf^ace. ihowi, a damage, level cJLoie to the intkin&ic meltinQ limit ol a boAe aoppeA iu^dace. The mateJvial oAed in tku teMt woi otheml&e o^{i-the-6hel(t, oxygeniKQ^t mateAioJL,

65
2.5 Pulsed COp Laser Damage Studies of RAP

^ Grown KCl*

Susan D. Allen, Morris Braunstein, Concetto Giuliano, and Victor Wang

Hughes Research Laboratories

3011 Malibu Canyon Road

Malibu, California 90265

ABSTRACT

Laser damage thresholds in high purity RAP (Reactive Atmosphere Processing)

KCl were measured as a function of surface and bulk processing techniques. Single

crystal and press forged material was prepared in a "conventionally" polished and

HCl etch-polished manner and the bulk and surface damage thresholds measured and

correlated with absorption (cw CO2 laser calorimeter), Auger, LEED and SEM data.

The damage measurements were made with a TEM^q CO2 TEA laser with a 0.6 fjisec pulse

length.

Key words: Etch-polishing; KCl; laser damage; laser windows; 10.6 ym; RAP.

1. Introduction

The present results represent a continuation of work on Hughes Research Laboratories (HRL) Reactive

Atmosphere Processing (RAP) KCl reported by A. Braunstein et al . [1]1 at the 1973 Laser Damage Symposium.

Conventional methods of polishing KCl (Linde A and methanol on a flannel lap) are known to produce

scratched and smeared surfaces, which contribute considerably to the 10.6 lam absorption [2] and scattering. A method of surface finishing which removes the damaged layer has been developed with a resultant

improvement in the 10.6 |j.m calorimetric absorption, scattering, and laser damage measurements. Other

techniques used to characterize the HRL-grown and finished RAP KCl include Auger spectroscopy, LEED, and

SEM. Bulk laser damage threshold measurements on HRL RAP KCl raise interesting theoretical questions as

the predicted trend for the damage threshold of KCl relative to that of NaCl is reversed.

2. Sample Preparation

The procedure for producing HRL RAP KCl has been described elsewhere. [3] The crystals produced

exhibit some low-angle grain boundaries, but can be considered single crystal for most purposes. Press

forged samples were fabricated from boules with minimum visible internal scatter and were essentially

veil free. Press forging was done at 250^0 to a reduction of 50% in height at a rate of 0.05 in. per

minute per in. of remaining length. Grain size in the resulting windows was 5 to 10 jam average diameter.

Windows polished by conventional polishing techniques are extremely environment sensitive. Surfaces fogged over a period of sevpral minutes on damp days. It was therefore difficult to establish

standards of measurement for KCl windows as the data changed with time. Figure 1 illustrates the microscopic difference between fogged and unfogged surfaces. It was also believed that the surface quality

was setting a limit to the absorption and scattering measurements, effectively masking any

improvement in bulk crystal quality that had been achieved.

J.W. Davisson [4] of Naval Research Labs had reported that concentrated HCl was a good etch for

single crystal KCl. We have developed a modification of Davisson's procedure which consistently produces high quality surfaces. Conventionally polished KCl windows are agitated in concentrated HCl for 1 to 2 minutes (depending on the quality of the polished surface and the depth of the scratches), rinsed

quickly in isopropyl alcohol, and then immersed in the vapor of a Freon TF degreaser to remove the alcohol and any remaining HCl.

Work partially supported by Defense Advanced Research Projects Agency under the technical cognizance of

Air Force Cambridge Research Laboratories.

Figures in brackets indicate the literature references at the end of this paper.

66
3. Sample Characterization

Optical micrographs of a conventionally polished and etch-polished KCl window are shown in Figure 2.

In contrast to the scratches and digs observed in the conventionally polished window, the etch-polished

surface shows no discernible features at this magnification. This smoothness is confirmed by SEM photographs at 18,000X (Figure 3).

Absorption coefficients measured calorimetrical ly at 10.6 |jim for HRL RAP KCl, along with similar

data for Optovac KCl for comparison, are given in Table 1. The values quoted for the absorption coefficient are total absorption of the sample, including surface contributions. The range of-values for the

etch-polished HRL RAP KCl is much narrower than that shown by conventionally polished KCl and consistently

lower.* This behavior is an indication of not only improved surface quality, but also improved stability

of the surfaces. It is also now possible for us to distinguish bulk quality differences, which were not

noticeable in conventionally polished windows. As an environmental test, a window cut from an early

boule measured a = 0.00032 cm~^ after etching. When measured after three weeks in a desk drawer, protected only from dust during a period of high humidity, a = 0.00041 cm-1 , and window was not

visibly fogged.

Table 1. 10.6 \xm Absorption of KCl

UD 1 DAD vr 1 HKL KAr KL

1

Absorption, cm ^

Single Crystal

Conventional ly

Pol i shed 0,,0008 to 0.0020 cm'^

Etch

Pol i shed 0,,00013 to 0.00020 cm"^

Press Forged

Conventional ly

Pol i shed 0,,00045 to 0.0021 cm"^

Etch

Pol ished 0,,00022 to 0.00024 cm"^

Optovac KCl

Conventional ly

Pol i shed 0.,001 to 0.002 cm'^

Etch

Pol i shed 0.,0004 to 0.0014 cm"^

*Total absorption of sample - includes bul k and surface.

Preliminary experiments by us and others [5] indicate that the measured absorption coefficients in the

1 to 2 X 10"^ range might actually be instrument limited and represent upper limits to the actual

absorption.

67
Auger analysis and LEED patterns [6] were recorded for four samples representative of different

types of surface finishing: a cleaved C2ochralski (Optovac) window, a similar cleaved window subjected

to a 1 minute HCl etch as previously described, a polished HRL RAP window of unknown crystal lographic

orientation, and a polished blank from the same boule also etched for 1 minute; the results are given in

Table 2, The initial Auger scans show the normal contaminants as all of the surfaces were prepared in

air. Most of these trace contaminants are removed by 6 minutes of Ar"*" sputter cleaning. The residual

oxygen can be ascribed to system contamination. The initial LEED patterns indicate some surface disorder

in the cleaved Optovac sample and the etch-polished HRL RAP sample (B15-7), while the conventionally

polished HRL RAP window (B15-8) displays essentially complete absence of surface order. The etched,

cleaved Optovac window exhibited a great degree of surface order as shown by the appearance of first,

second, and third order diffraction in the LEED pattern. A six minute Ar"*" sputter clean improved all of

the LEED patterns with the exception of the conventionally polished sample (B15-8). An additional hour

of Ar"*" sputter cleaning (removal of 'v 5 |a.m total) was necessary to obtain a diffuse LEED pattern for HRL

81 5-8. The data on the two samples of cleaved Optovac material (etched and unetched) indicate that a relatively thin layer (£0.4 p.m) of surface disorder is generated in the cleaving process, which is re- moved in the etch procedure. The residual diffusivlty of the etch-polished HRL RAP window LEED pattern,

can be at least partially attributed to the fact that the crystals were not cut along a symmetry axis.

The resulting asymmetry of the LEED pattern makes It difficult to characterize. It Is also quite possible

that a longer etch time would have been necessary to remove all of the surface disorder caused by con- ventional polishing.

Table 2. Leed and Auger Data on Various KCl Samples

Sample

Initial

After 6.0 min

Ar+ Sputter-Clean,-

(800 eV, 10 p.A/em^

After 1 Hour

Ar+ Sputter-Clean

(800 eV, 10 H^A/cm'^

Leed

Pattern

Auger

Scan

Leed

Pattern

Auger

Scan

Leed

Pattern

Auger

Scan

1. Cleaved

Optovac KCl' Diffuse S. 0, C Sharp 0, C

0^/0^. - 0.15

0^ < M

Z. Cleaved

Optovac KCl

Etched 1 min

Sharp

(1st, 2nd, 3rd

order)

0,C

(Trace)

Sharp

(Best)

0, I

Of/0^ - 0.25

3. RAP 815-8

Pol i shed

No S, 0, C (Trace)

No 0

(Trace)

Weak,

Diffuse

0

(Trace)

4. RAP B15-7

Etched 1 min

Weak,

Diffuse

S, 0, C (Trace)

Sharper

but still

weak

,

diffuse

0

(Trace)

Relative total (surface and bulk) scattering at 10.6 p-m was measured using a stabilized (0.11)

200 mW cw CO2 laser; these measurements are given in Table 3. The laser beam, focused to a 50 ym spot, was scanned across the sample and the scattering amplitude in the central portion of the scan was averaged

and normalized to a similar measurement on conventionally polished HRL RAP KCl. As is indicated in the

table, the lowest scattering is from etch polished, single crystal HRL RAP KCl. For the press forged

material, etch polished windows show more scatter in the visible than conventionally polished blanks, but

less at 10.6 H-m as shown in Table 3. It is known that grain boundaries are preferentially etched in con- centrated HCl and the enhanced grain boundaries contribute to the visible scatter, but do not affect the

10.6 fj,m scatter appreciably.

68
Tabls 3. Rilativs Scatttrlng 10.6 m

Single Crystal

Mfch Polished KCl 1.0

Etched PoUshid KCl 0.45

Press Forged

Mich PoUshid KCl 2.3

Etched Polished KCl 0.72

4. Laser Damage Thresholds

The laser damage test facility, used to obtain the current results, Is dtseribed In detail In another part of these proceedings. [7] Briefly, the laser Is an ultraviolet prelonlzed TEA laser

operated In the TEMqq mode, with 200 mj maximum output energy. The equivalent rectangular pulse length

1s 0.6 usee, but the actual pulse consists of many 2 nsee longitudinal mode spikes. Damage thresholds

are reported in terms of the average power In the equivalent rectangular pulse. The actual peak power

is approximately 5X the average power, Both a i in. focal length and 2.S in, focal length lens were

used In measuring the damage thresholds with measured beam waist spot sizes of 120 and 70 um, respectively. The measured spot size of 70 ym for the short focal length lens Is probably an upper limit.

The actual spot size lies in a range from 60 ym (the calculated diffraction limited spot size) and the

measured 70 ym. (cf Ref. 7.)

All tests were conducted In an evacuated sample chamber equipped with an x=y translation stage,

Sites to be tested were selected with a 6X microscope to be free of cosmetic defects observable at that

magnification; the onset of damage was monitored with the same microscope. In the samples for which high

damage thresholds were measured, damage was so catastrophic that sophisticated observational techniques

were unnecessary. For those sites where the apparent threshold at 6X magnification could not be reached,

microscopic evaluation at 220X revealed no additional damage morphology. The range of damage thresholds

reported represent the lowest power at which damage occurred to the highest power at which damage could

not be effected.

The surface damage thresholds for polished KCl, reported in Table 4. are given mainly for historical perspective. All of the values reported are for the laser focused on the exit surface using the

5 in. focal length lens. The thresholds vary over quite a range, and do not correlate well with the

absorption data, Since the absorption data were, of necessity, taken before the damage measurements,

changes In the surface quality that occurred between the two measurements cannot be tracked easily.

Table 4. Surface Laser Damage Threshold Polished KCl

Damage Threshold

Sample Absorption Energy Density Power Density

(cm"^) (J/em^) (GW/cm^)

Single Crystal

RAP B24-6 0.00078 70-8S 0.12-0.14

RAP Bll-11 0.00070 > no > 0,18

RAP BS3'4 0.0022 360 0.58

Optovae 0.024 18 0.03

Harshaw "Polytran" 0.0016 15 0.025

61
The character of the exit surface damage morphology for polished KCl is illustrated in Figure 4. The central portion of the damage site has been melted and is surrounded by a ring with a relatively

smooth topography as compared to the undamaged portion of the surface. Radiating from this "cleaned"

area are shallow cleavage planes. This type of shallow damage is observed only on poor surfaces and at relatively low damage thresholds in KCl.

In contrast to polished KCl the etched HRL RAP KCl absorption data do seem to correlate with the

laser damage performance. The highest exit surface damage thresholds for etched HRL RAP KCl (Table 5) are measured on those samples with the lowest absorption and vice versa. Those samples marked by an asterisk were measured with the 2.5 in. focal length lens and the others with the 5 in. focal length

lens. The wide range of values reported for sample RAP B53-1 is an exception to the convention for

reporting thresholds. This particular sample at any one site either damaged, at a relatively low energy

density (600 J/cm^), or did no damage to the limit of the laser power output. The best (and most recent)

etched HRL RAP KCl samples have a measured damage threshold for exit surface damage at least an order of

magnitude greater than the threshold for conventionally polished KCl.

Table 5. Surface Laser Damage Threshold Etch KCl

Damage Threshold

Sample Absorption

1 (cm )

Energy Density

(J/cm^)

Power Density

(GW/cm^)

Single Crystal

RAP B53-2 0.00026 380-520 0.63-0.87

RAP B53-4 0.00043 330-450 0.55-0.75

RAP B53-1 0.00020 600-2500+ 1 .0-4.2+

*RAP B63-2 0.00015 1500-2700 2.4-4.5

Harshaw Cleaved 60-135 0.10-0.22

Press Forged

*RAP B37-17 0.00023 1500-1800 2.4-3.0

*RAP B37-16 0.00024 1650 2.7

Exit surface damage thresholds for press forged HRL RAP KCl are lower than those for single crystalline material, although not enough samples have been tested to date to establish this trend as definitive.

The presence of "V grooves" in the form of preferentially etched grain boundaries in the polycrystal 1 ine

KCl would, according to the theory of Bloembergen, [9] lead to lower damage thresholds due to enhanced

dielectric breakdown.

Exit surface damage morphology for etched HRL RAP KCl differs markedly from that for polished sur- faces (Figure 5). The damage sites are several millimeters in diameter and are composed of radiating

cleavage planes. This type of behavior — minor damage at low thresholds for "dirty" surfaces, catastrophic damage at high thresholds for "clean" surfaces - had been noted by Giuliano [8] in measurements

on ion-polished sapphire at 0.69 p-m. The cleavage planes of exit surface damage sites in etched polycrystalline HRL RAP KCl (Figure 6) are distorted, compared to those in etched single crystal material,

and seem to indicate the presence of grain boundaries. Inclusion damage, both bulk and surface, is

readily distinguishable by damage morphology. A typical inclusion damage site (Optovac KCl, bulk damage)

is illustrated in Figure 7. The damage region extended for a length of 400 ym along the beam path with

many inclusion damage sites observable.

The bulk laser damage thresholds for HRL RAP KCl are given in Table 6, with measurements on one sample of Optovac KCl given for comparison. Because of the instability of the laser at low output

powers, the bulk damage threshold of this particular sample of Optovac material could only be crudely

bracketed. On the other extreme, the maximum available output power of the laser was not sufficient to damage one sample of single crystal and one sample of polycrystal 1 ine HRL RAP KCl. (In all fairness,

it must be mentioned that other workers [10] have noticed a great variability in Optovac samples, and

that we tested windows from only one sample.) For these measurements, the arguments for using the calculated diffraction limited spot size are particularly strong as the samples are thick enough (==1 cm) that a

70
small error in measuring the focal distance and/or placement of the sample would still place the beam

waist in the bulk of the material. Using the diffraction limited spot size would increase the damage

thresholds by a factor of 1.4. Bulk damage in material with a high damage threshold has a very similar

morphology to that exhibited by high damage threshold surface sites. Again the damage sites are massive

and consist of radiating cleavage planes.

Table 6. Laser Damage Threshold Bulk KCl

Damage Threshold

Sample Absorption

(cm-^

Energy Density

(J/cm^)

Power Density

(GW/cm^)

Single Crystal

*RAP B53-1

*RAP B63-2

* Optovac

0.00020

0.00015

2500-3100

> 4600

> 180

< 520

4.2-5.2

> 7.8

> 0.3

< 0.9

Press Forged

*B37-15 (End)

*B37-15 (Center)

Veiled

0.00064'*

3700

> 4600

6.3

> 7.8

Conventionally polished sample, all others etch polished.

5. Dielectric Breakdown

The most generally accepted theory of laser induced breakdown in low absorption solids is that

damage occurs via electron avalanche breakdown, and thus is a function of the peak electric field in the

laser pulse. A recent paper by N. Bloembergen [11] reviews the subject of dielectric breakdown in

solids, and summarizes the experimental evidence to date, p.2,13,14] The data of E. Yablonovitch [12] at 10.6 \im and Von Hippel [14] (dc measurements) are given in Table 7, along with the HRL data. The electric field strength and power densities were calculated and measured slightly differently in the

Yablonovitch and HRL data. Yablonovitch used peak power measurements {- 2X average) [12] and th.e diffraction limited spot size in calculating the breakdown electric fields. The HRL measurements are calculated using the average power and a measured spot size. For comparison, the HRL breakdown thresholds

calculated using the diffraction limited spot size are given in parentheses in Table 7. In order to eliminate the differences in experimental conditions between the Yablonovitch and HRL data, the KCl data was normalized to Harshaw NaCl . According to Yablonovitch, [15] commercial NaCl yields damage thresholds

consistent from sample to sample and thus we used it as a standard. The theoretical prediction from

periodic table trends was that NaCl should have a damage threshold at 10.6 ym greater than that exhibited

by KCl. In data given by Yablonovitch, this trend holds true for samples of comparable purity for both

compounds. The HRL RAP KCl, on the other hand, has a considerably higher damage threshold than Harshaw

NaCl, effectively reversing the trend for KCl/NaCl. This data is seen to lend support to the hypothesis

of Hellwarth [16] that electron avalanche breakdown is impurity mediated, i.e., the observation of dielectric breakdown does not necessarily indicate that the ultimate limit for damage has been reached for

a given material. It is expected that damage thresholds on HRL RAP NaCl (presumed to be comparable to HRL RAP KCl in purity) would be higher than those measured on Harshaw NaCl, and would, perhaps restore

the expected relationship: breakdown (NaCl) > breakdown (KCl).

6. Conclusions

We have developed a technique for generating high quality, relatively undamaged surfaces on HRL

RAP KCl. The improvement in surface quality enables us to better evaluate bulk quality, and various

characterizing measurements have been developed to judge surface and bulk quality. The results of these

measurements seem to correlate qualitatively with the measured damage thresholds. The damage threshold

of the surface has improved by at least an order of magnitude and approaches the bulk damage threshold

in some samples.

The measurements of bulk damage threshold support the theory that sample purity affects the

"intrinsic" electron avalanche breakdown damage threshold.

71
Table 7. Comparison of Laser Damage KCl and NaCl

Yablonovltch (1972) HRL (1974)* Von Hippel (1937)

Power

(GW/cm^)

KCl NaCl KCl NaCl

3.7 7.5 >7.8(10.6) 4.8(6.5)

KCl/NaCL = 0.49 KCl/NaCl = 1.6

KCl NaCl

Electric

Field

(MV/cm)

1,39 1.95 >2.0(2.4) 1.6(1.9) 1.0 1.5

Parenthetical values were calculated using the diffraction limited spot size

(60 fxtn) instead of the measured 70 pm.

7. Acknowledgments

The authors gratefully acknowledge the help and assistance of Roger Turk, Bernie Garcia, Paul

Coker, and Don Williams. The many technical discussions with Art Braunstein were invaluable. The

efforts of the HRL optical polishing shop, George Heussner and Matt Himber, are also appreciated.

8.

[1] Braunsteui, A. I., Wang, V., Braunstein, M.,

Rudisill, J. E., and Wada, J. NBS Special

Publication Laser Induced Damage in Optical

Materials, A. J. Glass and A. H. Guenther,

Bd. , 1973, p. ISl.

[2] Braunstein, M. "Laser Window Surface Finishing and Coating Technology," ARPA Contract

F19628-73-C-0234.

[3] Paster, R. C, and Braunstein, M.,

"Advanced Mode Control and High Power Optics

Technology," Tech. Report, AFWL"TR-72"1S2,

Vol. 11.

[4] Davisson, J. W. , Conference on High Power IR

Laser Windows, Vol. 2, Noveirtaer 1972,

p. S2S-S34.

[B] Haas, M. , Naval Research Labs, private

coirmifflicatlen.

[6] Physical llectronlcs Industries, Inc.,

Idlna, Minn. , Model #10-234G.

[7] Wang, v., it al., these proceedings.

[§] Giullano, C, Appl. Phys. Lett., 21, 39

C1972}.

References

[9] Bloeinbergen, N., Appl. Opt., 12^. 661-664,

1973.

[10] Bemal, G., E., Honeywell, private

communication.

[11] BloeiTbergen, N., IEEE J. of Quantum

Electron., QE-10. 37S-386, 1974.

[12] Yablonovltch, E., Appl. Phys. Lett., 19,

495-497, 1971.

[13] Fradin, D. W. and Bass, M. , Appl. Phys.

Lett., 22, 206-208, 1973.

[14] Von Hippel, A., J. Appl. Phys., 8, 815-832,

1937, Phys. Rev. |4, 1096-1102, 1538.

[IS] Yablonovltch, B., private comnunlcatlen.

[16] Hellwartii, R. W., NBS Special Pi&lication,

Damage in Laser Materials, A. J. Glass and

A. H. Guenther, Ed., 1970, p. 67.

72
9 . Figurgs

ETCH POLISHED HRL RAP KCJt CONVENTIONALLY POLISHED HRL RAP KCl

Figure 2. Optical finishing e@ehniquts.

73
2754-18

3292 - J 7

BEAM 70° TO SURFACE

Figure 3. HCl-etch polished KCl (SEM)

18,000X magnification.

Figure 4, Surface laser damage, conventionally

polished single crystal HRL RAP KCl (B53-4)

.

3292-24

200

I 1

Figure 5. Surface laser damage, etch-polished

single crystal HRL RAP KCl (B63-2)

.

74
Figure 6. Surface laser damage, etch-polished

press forged HRL RAP KCl (B37-16)

.

Figure 7„ Bulk laser damage, single

crystal Optovac KClo

COMMENTS OM PAPER BY ALLEN

The. Ajnpnovmtyvt -in "-LntAlM-ic" damage ZeveJti oveA the, aJLkaU, hoLlde KeAulZi nzpoltzd pn.e-viowi,tLj iMai, ouWiibuuted to the. {,act that RAP-gfioMn mcute/ilat aontciiiU> !s-iQwi(^icaYitty tej>h Ajnpu/uJxeA and de^e.ct{> than

pKevZouAty avaiZable mcutexiat. The pneAenae o{, AjnpuAAXLeJ> ^ thought to inAJxate the avatanahe lom^zatA^on

pfwceM,. M. Ba44 oi USC pointed out that, tn coie o^ the avalanche pH.oceM, the damage thxeJihoZd u, i^n6ttive to local (^ieZd valuers, io the tempofial ht6ton.y oi the laiex pulj>e -c4 much moKe tmpofiXant than -in the caie o{, the/mal damage. Although the absolute valuer o{, damage thxtdhold axe i,omewhat unceAtain, the

fuvtlo o{i the value ion KCl to that don. Had ihould be leM i&n^-OUve to detaJJj, o^ the pulie ihape,

aMwiriing the pulie to be daMily n.epfioducible.

75
2.6 A Comparison of 10.6)jm Pulsed Laser Damage in

Sputtered vs Electron Beam Deposited Ge-Coated KCl

A. Golubovic, W. Ewing, J. Bruce, J. Comer and D. Milam

Air Force Cambridge Research Laboratories

Solid State Sciences Laboratory

L. G. Hanscom Field

Bedford, Mass. 01730

Germanium films deposited on KCl substrates by electron beam and

sputter techniques have been irradiated at 10.6wm. A comparative damage study of germanium films prepared by these techniques under pulsed

irradiation is reported, A novel multiple beam pulsed laser damage

apparatus was used for this study. Well characterized RAP Bridgeman

and Czochralski grown KCl substrates with (100) and (111) orientation

were used.

Key words! Characterization of laser damage; e-beam deposition ef

germanlufflj girmanium coating; laser damage mechanism;

laser induced damage; multiple beam damage apparatus;

potassium chloride; sputtering of germanium.

1. Introduction

The alkali halldes represent a class of materials which are presently under investigation as possible window materials for use In high energy laser (HBLj systems at

10.6ym. In particular, potassium chloride has been shown to have both very low

absorption Cg<2X10°4Gffl-l) and relatively high damage threshold. However, the halides

in general are subject to hygroscopic deterioration and are mechanically poor. Thus,

for practical application these materials must be environmentally protected against

moisture and abrasion by suitable anti-reflective coatings.

Potassium chloride has an index of refraction of n=1.4S4 at 10 . 6ym. Since the

index requirement for a single layer anti-reflective coating (nis\1.4S4) cannot be met

practically, multi-layer films are required for zero reflectance at this wavelength.

Germanium has an index of refraction ef ns4.00 at 10.6ym and has been treated as a candidate for a first layer film for multi-layer anti-reflective coatings. [1]*

Further, this material is ideal for laser damage studies since it is mono-elemental,

well characterized and may be deposited by almost all commonly used deposition

techniques

.

It is the objective of this investigation to compare the 10.6wm pulse laser

damage thresholds of germaniuffl films deposited on well characterized potassium

chloride substrates by electron beam deposition and R. F, sputter techniques. Both

of these techniques have their own characteristics which are reflected not only in

the nucleation and growth processes during deposition, but in the final properties of

the films as well. In this investigation we characterize not only the damage

mechanisms of the germanium films but the substrate, film-substrate interfaces and

films as well

.

As part of this Investigation a novel pulse laser damage experiment is described

for the first time. This novel technique, involving the use of a multiple beam splitting wedge, greatly speeds laser damage studies of materials, and allows an accurate

*Thls research was siflDported jointly by the Air Force and the Advanced Research Projects Agency of the

Department of Defense.

1. Figures in brackets indicate the literature references at the end ef this paper.

76
determination of damage thresholds with few measurements. While the system is used

for pulse damage measurements at AFCRL, the extension of this technique to C. W.

damage studies is obvious.

2. Experimental

2.1 Substrate Preparation

Single crystal substrates of potassium chloride were cut from oriented boules of

both Czochralski and reactive atmosphere process (RAP) Bridgeman material grown at

AFCRL. For this study (100) and (111) orientations were used. Rough grinding was performed by hand on glass plates using 600 and 1000 mesh silicon carbide followed by

ly AI2O3, all using kerosene as a vehicle. Fine polishing was performed by hand on a Politex "supreme" porometric lap (Geoscience Instrument Corp.) using Linde B (0.05m AI2O3) with ethylene glycol as a vehicle. After mechanical polishing, the substrates were heated to 60°C for a period of 2-3 hours prior to chemical polishing.

After cooling to room temperature, the substrates were chemically polished for 10-15

minutes in a 2:1 mixture of glacial acetic acid and concentrated hydrochloric acid,

followed by a thorough rinsing in absolute ethanol and Freon 11. The resulting substrates were clear and unstained by residual contamination. Deposition was performed

as soon as possible after this step, usually within one hour.

2.2 R. F. Sputtering of Germanium Films

To insure identical deposition conditions, a group of four samples, one of each

orientation and growth method, was sputtered simultaneously. Prior to deposition, the

KCl substrates were sputter etched for 15 seconds by argon bombardment at a power

level of 100 watts. Optimal deposition conditions were established by variation of R. F. power levels (and hence both deposition rate and substrate temperature) while

maintaining all other deposition parameters constant. With an R. F. power level of

50 watts the deposition rate was ~lA/second and the substrate temperature was measured

to be 750c. At 100 watts, the deposition rate increased to ~3^/second and the substrate temperature increased to 110-120°C. For all runs the argon pressure was maintained at 20 X 10"3 torr and the source to substrate distance was 40 mm. All sputter

depositions were performed with a Randex model 2400 - 8L turbo-pumped system.

2.3 Electron Beam Deposition

All electron beam depositions were performed in a VEECO VE-400 vacuum system

equipped with a Sloan BXV-6, six kilowatt 180° deflection electron beam gun. The

pressure of this system was 5 X 10"8 torr prior to deposition, rising to ~2 X 10-5

torr during the depositions. For all samples prepared by this technique, the source

to substrate distance was 18 inches (45.7cm) and the substrate temperature was 750C.

Substrate heating was accomplished with quartz iodide lamps directed at the surface

of the substrates to be coated.

Using this arrangement, the depositions were rate limited due to source arcing.

It was found that 20^/second was optimum and this rate was used for all the depositions reported. Deposition control was maintained by an Inficon XMS-3 quartz crystal

deposition controller.

2.3.1 Film Thickness

A film thickness of ly was chosen for these experiments. This thickness represents an arbitrary value chosen less than A/4 to eliminate interference effects, yet

thick enough to be representative for damage measurements. Film thickness was con- trolled by timing the deposition during sputtering at a known rate and by a quartz

crystal thickness monitor during the electron beam evaporation. After each run the

actual thickness was determined by stylus (Dektak) and Tolansky microscope techniques.

2.4 Film Characterization

Several techniques were used to characterize both the KCl substrate surfaces

and the Ge films before and after deposition. The surface quality during the polishing and etching procedures was monitored by Nomarski interference contrast microscopy

Auger spectroscopy was used to analyze the chemistry of the film- substrate interface.

77

564-378 0-74-7
The texture and the crystallinity of the Ge films were examined by transmission electron microscopy and electron diffraction techniques; the method of specimen preparation is described below in detail.

Films as deposited in this study were too thick for transmission electron microscopy. Both undamaged and damaged films were floated off the KCl substrates, thinned

by floating on 1 part "superoxol" to 1 part water for 30-90 seconds, and washed by

floating in deionized water before mounting on 1/8" specimen grids. The final thickness of the films was estimated to be lOOO-ZOOOR. When damaged films were examined

the surfaces were first coated with a thin film of collodion before separation to

prevent breakage. The collodion was removed with amyl acetate before examination.

2,5 Pulsed Laser Damage Experiment

The apparatus for performing pulsed CO2 damage experiments is shown in figure 1.

Provision is made for recording the energy incident at the sample, the spatial profile

of the laser beam in the sample plane, and the waveform of the laser pulse as viewed

by a 500 MHz detection system. These three measurements are discussed individually

below. The characteristics of the TEA CO2 laser are described in table 1.

Table 1, Characteristics of TEA CO^ Laser Pulses

Pulse Energy

Peak Power

Repetition Rate

Spatial Mode

Mode Diameter

Longitudinal Modes

Pulse Profile

::50 Millijoules, Reproducible to 5%

>200 Kilowatts

IPPS

TEM , Gaussian to I/I ^0.01

6mm

Many

Unreproducible mode beating, 100% modulation.

Duration of initial maximum of the pulse

envelope is 75 nsec.

2.5.1 Beam Profile Measurements for Lasers

The most difficult experimental aspect of CO2 damage measurements is that of

properly measuring the spatial profile of the laser beam at the sample plane. This

is usually done by recording burns on some absorbing material during a series of firings in which the laser is successively attenuated. As a result, the laser profile

cannot be monitored for a single firing which is used to actually cause the damage.

Multiple -beam devices [2] are commonly used for beam analysis in the visible.

These devices can be adapted for use in the infrared. If an air-spaced wedge con- sisting of a 50%/100% reflector pair is placed downstream from the lens used to focus

the beam onto the sample, multiple foci are generated. Furthermore, there is a well

defined relative power level incident at successive foci. Ideally, beams are attenuated by 2:1 when generated in a 501/100% reflective wedge. Here, the experimentally measured ratio was 2.1:1. The separation of the multiple foci in the sample

plane can be adjusted by setting the angle of the wedge, and the path length

accumulation in the wedge is minimized by operating with the smallest possible

separation of the dielectric films comprising the wedge.

Beam profile measurements can be obtained from the set of n burns recorded on heavy carbon paper placed either in the weaker beams in the sample plane, or in the

alternate sample plane shown in figure 1. These weaker of the burns presumably result

from the melting of the gel containing the carbon. When illuminated at about 45°

incidence, and viewed at normal incidence in an optical microscope, these burns are highly visible and readily photographed. An example is shown in figure 2. Using

radii measured from such burns, and the relative attenuation factor of the wedge, the

beam profile can be plotted for a single firing. The beam profile at the waist of the

50cm lens is shown in figure 3. The solid curve in that figure is a Gaussian profile

fit to the data. The Gaussian parameter a^, defined by the form.

E^(r) = E_e

2 , 2 r /a

P can be determined to within about 15%.

78
Additional confidence in the profile measurement can be obtained by producing

multiple burns at a range of input powers. For example, if the entire laser output

is incident on an ideal 50^/100% wedge, foci with energy densities proportional to the

series 1, 1/2, 1/4, 1/8, etc are obtained. Attenuating the laser power to 751 of its

total value produces the sequence 3/4, 3/8, 3/16 etc during a second firing. This can

be extended to densely fill the range of possible focal power levels, thereby furnishing many points to document the beam profile.

In practice, if our TEA CO2 laser is left running at Ipps, shot-to-shot variations in the focal size cannot be measured. The focal profile is measured at the

first of each data set, and verified at the end of the set, but not recorded for each

firing

.

Damage experiments are performed by irradiating the sample with multiple foci of

known profile. Typically eight laser pulses are used, with various attenuations, so that 8n sites are irradiated. A damage sample can be processed in about 10 minutes

with most of the time being used to position the sample and return it to its container.

There are some practical limitations in the use of the multiple beam wedge.

Since it must be placed downstream from the focussing lens, damage in the wedge will

occur if lenses of too short focal length are used. This problem is increased due to

the multiple beam interference in the wedge which results in the formation of intense

fringe patterns on the wedge surfaces. There is also a slight astigmatism in the

focal profile due to the existence of the wedge, but this is not a serious problem

with relatively long focal length lenses.

2.5.2 Energy Calibration

By adjusting the wedge angle to collapse the fan of n beams into a single beam,

the total energy passing through the focal plane can be compared with the portion of

the pulse detected at the calorimeter in the side arm. This calibrated calorimeter

response is recorded for each pulse, and the calculated total energy is assumed to be

distributed among the n foci according to the wedge splitting factor, 2.1:1.

2.5.3 Pulse Waveform Measurements

The waveforms recorded by photon drag detector and Tektronix 7904 oscilloscope

show irreproducible mode beating. These waveforms are not recorded in studies of

absorbing materials.

3.0 Results and Discussion

3.1 Character of Undamaged Films

Nomarski photomicrographs of the various surface preparation stages are shown in

figure 4. The effect of the chemical polishing was to remove most of the initial

scratches due to mechanical polishing, although some gross scratches, pits and

occasional grain boundaries may be seen. Bernal [3] et al, have reported non-preferential etching of 0 . 05um/minute using the etching conditions used here. The existence

of etched grain boundaries (found only in the Bridgeman crystals which were slightly

polycrystalline) suggests shorter polishing times should be used for polycrystalline

materials

.

Preliminary substrate preparation using un-annealed crystals was unacceptable as substrate quality was inferior. J. W. Davison [4] has reported similar experience

in his investigations of KCl polishing techniques. All of the Bridgeman KCl used in

this investigation was annealed at 600°C prior to any cutting or polishing.

Film adhesion was measured qualitatively using the standard "scotch tape" test.

Films produced by both sputtering and e-beam deposition were found to be weakly adherent to the chemically polished KCl, although stronger adhesion was observed for the

sputtered films.

Auger analysis of the film- substrate interfaces of films produced by both methods show that for sputtered films the diffusion of Ge^^^Cl is more complete. The

process seems to be describable by the following sequence: During the sputtering of

Ge, the substrate is bombarded by secondary electrons which are produced as part of

the sputter process. The bombardment of the KCl by these secondary electrons causes

79
chlorine depletion at the surface of the KCl, a phenomena observed in Auger and LEED

measurements on alkali halides [5]. As the Ge is deposited, the heat generated locally

at the substrate surface enhances the diffusion of the potassium rich KCl surface layer into the growing Ge film. The Auger profile of a sputtered Ge film on KCl is shown

in figure 5. In contrast, the local heating of the KCl substrate during the conventional deposition process is not sufficient to enhance this diffusion process. However, it was noted that during initial stages of the e-beam deposition stray electrons

from the source would cause color center production in the substrate. The Auger profile of an e-beam deposited Ge film on KCl shows the "leading" of the potassium at the

Ge-KCl interface (figure 6)

.

Appreciable differences in texture were noted between sputtered and evaporated

films as seen by transmission electron microscopy. A typical region of the evaporated

film is seen in figure 7a. The film is essentially structureless at this magnification except for the many holes which result from the topography of the KCl structure.

Thinner regions of the film around protrusions or depressions have been completely

dissolved causing loss of parts of the film. Diffuse rings in the diffraction pattern (figure 7b) appear at the same positions reported by Donovan, Terence, Heineman

and Klause for amorphous germanium [6].

The sputtered films show a spherulite growth seen in figure 8. The spherulites

are composed of radiating fibers which are less than lOOR in width. Diffraction

maxima from these films (seen in figure 9) were broader than those from the evaporated

material

.

3.2 Laser Damage Results

The multiple beam damage experiment produces an array of damage sites on the

specimen, figure 10a. This photograph was taken with transmitted light, the array

being formed by holes in the Ge film. Analysis of such arrays yield threshold values.

The laser damage thresholds for the different films are summarized in table 2.

Table 2. 10.6)jm Laser Damage Threshold

(J/Cm2) of Evaporated vs Sputtered Ge-Coated KCl

Specimen Evaporated Film Sputtered Film

KCl (RAP) (100) 0.53 1.06

KCl (RAP) (111) 0.73 0.98

KCl (CZOCH) (100) 0.82 0.96

KCl (CZOCH) (111) 0.56

2 7 2^--^^

For Laser Beam Parameters used, 1 Joule/cm = 10 watts/cm .

The energy densities quoted represent the lowest energy densities which produce

observable damage and represent an average of multiple irradiations performed at different positions on the sample surface. The threshold values quoted indicate a higher

damage threshold for the sputtered films relative to the e-beam deposited films. However in this investigation no correlation of damage threshold to either the substrate

crystallographic orientation or method of growth could be made.

The damage sites were examined by Nomarski microscopy; a typical damage site is

shown in figure 10b. Here the Ge is completely gone from the KCl surface and several

cleavage cracks were formed in the substrate during damage.

The Ge films were also studied after laser damage by transmission electron

microscopy. Damaged regions were identified by holes in the film. Regions immediately surrounding the holes were crystalline in both the sputtered and evaporated films,

in contrast to the amorphous nature of the film observed prior to damage. One of the

regions in an evaporated film is shown in figure 11.

4. Conclusions

Several conclusions can be drawn from the observations made during this investigation. First, there exists an adhesion problem of the germanium coatings on the

chemically polished KCl which is independent of the deposition technique described

above. The sputtered films tended to show qualitatively better adhesion (see above),

however, none of the films produced in this experiment were able to pass the

80
"scotch tape" adhesion test in a satisfactory manner.

The object of this investigation is to compare the damage thresholds of

germanium films produced by sputtering and e-beam deposition. The observed damage

thresholds for all the films tested were within a factor of two, indicating that

sputtered germanium films can compete or as in this investigation excell in performance against e-beam evaporated films produced in medium high vacuum. However, the

damage threshold measured are low in all cases suggesting that for high power laser

application germanium is a poor first coating.

The damage mechanism observed is consistent with a thermal runaway damage model.

The burn holes in the germanium film suggest a vaporization of a large part of the

film with large amounts of heat being generated at the crater periphery. This is seen

in the crystalline nature of the periphery of the crater edges. Further, the damage

at the substrate surface suggests local heating at the beam site.

The damage profile of both sputtered and e-beam deposited films was essentially

the same. That is, there appeared to be no significant differences in the mechanism

for damage between films. Auger analysis of both kinds of films could show no measurable difference in the contamination, particularly for oxygen which is known to

have an absorption band centered at about ISpm. Further, no correlation could be made

between threshold level and the orientation or growth method of the substrates used

in this investigation.

Finally, the multiple beam laser damage experiment described above has been

shown to be effective in the laser damage studies at 10.6pm.

5. Acknowledgement

The authors thank H. Posen for his helpful suggestions in the course of this

study. Potassium chloride was generously furnished for this investigation by

J. O'Connor of AFCRL, to whom we are greatly indebted. Technical assistance was also

provided by I. Berman and J. Fitzgerald of this laboratory.

6. References

[1] Loomis, J. S., "Evaluation of Coated

Laser Windows". Proceedings of High

Power Infrared Laser Window Materials Conference , Hyannis ,MA. , 30 Oct1 Nov, 1972.

[2] I. M. Weiner, Appl. Opt. 5, 1437

(1966) .

[3] Bernal, et al., "Preparation and

Characterization of Polycrys tal 1 ine

Halides for use in High Power Laser

Windows", ARPA Contract No. DAHC73-C-0227, Final Report, Dec, 1973.

[4] Davisson, J. W. , "Chemical Polishing of NaCl and KCl Laser Windows".

Proceedings of High Power Infrared

Laser Window Materials Conference ," Hyannis, MA., 50 Oct-1 Nov, 1972.

[5] Palmberg, P. W., Todd, C. B. and

Rhodin, T. N., J. Appl. Phys .

, 39,

4650 (1968).

[6] Donovan, T. , et al., "Thin Film

Coatings for Alkali Halide Windows",

Proceedings of High Power Infrared

Laser Window Materials Conference j" Hyannis, MA., 12-14 Nov, 1973.

81
7 . Figures

Figure 2. Burn pattern in carbon paper used

determining beam profile.

60

>-t

z

UJ 40

20

UJ

.2 .4 .6 BEAM RADIUS (mm)

Figure 3. Profile of a focused CO2 laser beam

determined from data obtained during a single

pulse.

Figure 4. Nomarski micrographs of surface of

KCl (100) , Czochralski grown; (A) after

mechanical polish with 0.05 ym AI2O3,

(B) after 2:1 acetic/HCl chemical polish,

(C) after 1 ym thick Ge sputter deposit.

82
1 L_

ke

Figure 5. Auger profile of a sputtered Ge

film on KCl.

+ + +

+ +

-(A

. +

A> A* /.if » »

J L- J L.

ke'. -p' S; T" 'I't - "ST.IES

Figure 6. Auger profile of e-beam deposited

film on KCl.

Figure 7. Structure in thinned evaporated

Ge film.

83
Figure 8. Transmission electron micrographs

of a sputtered Ge film showing spheriiHtic

growth. Figure 9. Electron diffraction pattern

from sputtered Ge film.

• ' *•

.

A 2 mi B

-"0

Figure 10. 10,6 \im pulsed laser damage of

Ge-coated KCl; (A) Macrophotograph showing

an array of damage sites, (B) Nomarski

micrographs of a damage site.

Figure llo Structure of a sputtered Ge

film surrounding the 10.6 ym laser damaged

region.

COmEMTS ON PAPER EV GOLUEOVIC

It MCLi polvvtzd oat that the. {^acX that me.aiu/LmzntJ> can be. made at ieveAnl. poweA leveti, on a g-ivm

ihot, aitng the muZttple Kef^lectxim technique, o{,^eA6 distinct advantages ^on. damage itvudie^. CoJie

muAit be (ixeAcUied hoMeveA, to avoid iyneAg-Utlc e{,{iectA due to ipatteAing {^wm one damage iit^ to an adjacent itte. The exact mzchaniJim o^ the tnteAacJxon betMe.en adjacent damage i-oCes <A itctl undLeoK,

hoMeveA.

84
2.7 lO.Cpm Pulsed Laser Damage in ZnSe

H. Posen, J. Bruce and D. Milam

Air Force Cambridge Research Laboratories

L. G. Hanscom Field

Bedford, Massachusetts 01730

The effect of 10.6ym pulsed laser radiation on large grain ZnSe

is examined with respect to the local crystallography of the material.

In particular, the presence of twin boundaries, a common defect in

II-VI materials, is shown to have very little effect on the damage

threshold. Damage effects were monitored by Nomarski interference

contrast microscopy, electron microscopy and x-ray topography.

Key words: CO2 pulsed laser damage; damage threshold; laser windows; twin

structure; zinc selenide.

1. Introduction

The object of this study was to examine the thesis that the presence of specialized intergranular boundaries, albeit clean of defect material, may adversely affect

the 10.6Mm laser damage threshold in candidate window materials in high power

laser optics systems.

For cubic materials of the alkali halide family, which have the rock salt

crystal structure, polycrystallinity prepared by in situ crystallization techniques

such as the PIR method [1]^ reflect little deterioration in 10.6Mm transmissivity and

appear to exhibit no degradation in 10.6ym damage threshold, directly attributable to

the presence of grain boundaries. These observations have been examined solely under

CW conditions [2] and carefully graded pulsed measurements of damage thresholds as done in this study according to the method of D. Milam of AFCRL have not been made.

Another class of cubic materials, such as CdTe and ZnSe, because of their low

absorption at 10.6Mm and their comparatively larger yield strengths vis-a-vis those of

alkali halides have been also considered as candidate window materials for 10.6ym

laser systems. These II-VI materials are low band gap semiconductors, available only

as polycrystalline aggregates. They exhibit a variety of large grain boundary misorientations, but characteristically, regardless of the crystal growth method exhibit

highly rectilinear intergranular bands which are called twins.

Early experiments on CdTe under CW 10.6ym irradiation exhibited dramatic material failure [3]. There was evidence in these failures that the boundaries acted as local failure initiation sites, although again the width of the CW incident CO2

laser beam did not permit an unambiguous assignment of failure initiation to the twins

or their boundaries.

Some pulsed damage experiments on ZnSe by Fradin and Bua [4] emphasize the relation of 10.6um damage threshold to the possible presence of particular defects rather

than to the local morphology of the crystal. In other experiments on 10.6ijm pulsed

damage in ZnSe [5,6] using extremely short pulse lengths, again no attempt was made to

correlate damage threshold with local crystallography.

For these II-VI materials we will demonstrate in the next section that the presence of twins necessarily implies boundaries of several atomic layers thick of non- cubic material. The presence of such non-cubic layers provide local variations in

the refractive index. Grain boundaries in these materials on the other hand represent boundaries between single crystallites of different orientation, whose crystal

structure remains the same. By their very nature they are large angle boundaries

and represent convenient sites for "stranger" or garbage atoms, which may act as strong local absorptive sites. Early 10.6iim CW studies at AFCRL [3] clearly demonstrated the susceptibility of materials with dirty grain boundaries to low damage

thresholds

.

1. Figures in brackets indicate the literature references at the end of this paper.

85
The thrust of these experiments is confined to evaluating the effect of the presence of twins and their concurrent local variation in refractive index on the damage

thresholds. Thus the boundaries of the twins are wurtzite polymorphs of ZnSe and are approximately 6 . thick.

In view of the coherence of these twins, the specific energy of the twin interface, because of the good atomic fit, is particularly low with respect to the specific

energy of an interface between normal grain boundaries, and defect atoms would prefer

grain boundaries as sites rather than twin boundaries. Thus the major possible

structural contribution to damage threshold variation is the local variation in

refractive index, arising from the different crystalline structure at the twin interface .

Figure 1, illustrates the ZnSe lattice stacked along the [111] polar axis.

The symbols "a" and "c" are repeat distances along [110] and [111] directions. Figure

1(a) shows the normal zinc selenide structure with the usual close packed ABCABC type

stacking along [111] while figure 1(b) -shows the results of a growth error, in which

the structure shown by full lines is a unit cell with hexagonal packing of another

form of ZnSe. Because of the polarity of the structure, the whole lattice is turned

180° about the [111] normal after the twinning.

2. Crystal Twinning in ZnSe

One may regard twinning as a phenomenon that permits individual parts of a crystal to take up a new orientation, usually in response to a growth error, a thermal

transformation or some mechanical impetus. The new crystal is of the same crystal

species as the original crystal, and each is oriented with respect to the other,

according some well defined laws. Only in the immediate vicinity of the boundary can there be a change of crystal structure. Such boundaries may be coherent or incoherent.

Coherent boundaries are plane and parallel to the twin plane and do not give rise to

macroscopic lattice stresses. Twins in ZnSe are classified as growth twins and have

coherent boundaries.

Crystalline twinning was early recognized by mineralogists as a common phenomena

occurring in many minerals. Friedel [7] was the first to systemize twinning according to specific laws. These twinning laws have been refined and expanded and put on

a mathematical basis for all crystal systems by Jaswon [8], and Jaswon and Dove [9].

A predominantly phenomenological exposition of crystalline twinning is extensively reviewed by Cahn [10]. M. V. Klassen-Neklyudova [11] reviews in detail the development of mechanical twins. ZnSe belongs to the cubic zinc blende crystal structure

class and may be regarded as two interpenetrating face centered cubic lattices, one

of zinc and one of selenium shifted with respect to one another 1/4 unit along the

main diagonal of the unit cell. The basic structural unit is a tetrahedron of Zn(Se)

atoms, at the center of which is a Se(Zn) atom, covalently bonded to each of the

Zn(Se) atoms. Thus the structure may be considered as the two-element analog of the

diamond cubic structure, but because not all vertices of the tetrahedra have the same orientation, the structure has no center of symmetry, resulting in the familiar

polarity of ZnSe crystals along the [111] direction.

Aminoff and Broome [12] looked at the geometry of twin structures for zinc blende

(ZnS) , galena (PbS) , diamond, gold and copper, and concluded that if two crystals form

a twin, then the two lattices are directly in contact or the lattices are separated by

a transition region. For monoelemental structures, the transition region corresponds

precisely to the initial structure, whilst for structures like ZnSe, the transition

region has a structure corresponding to one of the polymorphic forms of the initial

structure. Aminoff and Broome showed that there are fourteen possible modes of incorrect deposition when ZnS grows on a (111) face, but the only growth twins known

are ones in which the transition region has the wurtzite structure, the hexagonal polymorph of zinc blende.

3. Experimental Procedure

Selected particular crystals within the ZnSe matrix were to be examined in detail

with respect to their twin structure, exposed to 10.6ym pulsed laser radiation, and

•observed as to when and how damage occurs. However, the grain size of polycry stalline

CVD ZnSe is too small to perform such a study, because of the desirability of indexing

the damage area with other observations. Specimens of large grain multi-twinned ZnSe

manufactured by Eagle Picher Industries, Inc. were selected. Figure 2 shows a macrophotograph of one of these specimens, a disk 17mm in diameter and 4mm thick. Its

86
structure is composed primarily of one large heavily twinned crystal and another

smaller crystal. These specimens were grown in a carbon crucible by the Stockbarger

technique and may contain traces of carbon impurity, as will be discussed later.

Specimens of both CVD and twinned ZnSe were polished using Linde A, and etched in

K3Fe(CN)5 solution according to the method of Braunstein [13]. This etch not only

removes the mechanically damaged surface layer but also reveals the grain structure

and twin structure. The surface grain structure of both types of ZnSe was examined

using Nomarski interference microscopy. Micrographs of typical grain structures

found are shown in figure 3. The linear banding is typical of twin structure where

the light and dark areas represents crystals of different crystallographic orientations, with a definite orientation relationship between the two.

The twinning relationship was established by determining the crystallographic

orientation of each part of the twin and plotting this on a stereographic projection

together with data on the direction of the composition plane. Figure 4 shows the

resulting stereogram with some of the (100), (110) and (111) poles plotted. Solid and

open symbols refer to the two parts of the twin; some poles are common to both twins.

From this stereogram, both the twinning plane and the composition plane are found to

be the (111) plane (dashed line)

.

The finer details of the ZnSe twin structure are shown by transmission electron

micrographs of a two-stage replica of a ZnSe large grain twinned crystal. The crystal

surface had been polished and etched as described previously. Figure 5(a) shows an electron micrograph of a lOym twin band. The difference in texture is due to a difference in the etching characteristics of the twin surfaces with different crystallographic orientations. An area of multiple twinning is shown in figure 5(b). Here

there are at least seven twin bands present in this particular 6ym region. Some twin

bands are as small as O.OZym. Twin bands smaller than this would not be seen by this

replica technique on a surface of this quality.

The surface chemistry of the large grain twinned ZnSe was examined with Auger

spectroscopy. The object here was two-fold: to identify the composition of numerous

black particles seen throughout the bulk of the specimen as well as on the surface ;and

to look for any impurities in the grain boundaries or twin boundaries. Figure 6 shows

spectra taken on a particle before and after sputter cleaning of the surface. The

particle was identified as carbon. No impurities could be found in the boundary

regions after sputter cleaning of the surface.

However consideration of the surface resolution of the Auger beam, and the low

defect chemistry of these materials as established by IR spectroscopy and mass

spectroscopy [14] mitigates against determining accurately the local chemistry of the

twin boundaries.

To insure that the CVD and twinned ZnSe are of comparable optical quality, bulk

absorption coefficients were measured at 10. Gym on both materials by the calorimetric

method. The results are presented in Table 1.

Table 1. 10.6ym Optical Absorption of ZnSe

e(cm"^)

ZnSe (CVD) 0.0030 ± .0004

ZnSe (twin) 0.0070 ± .0002

The large grain twinned material has an optical absorption about double that of

the CVD material. This higher value is not surprising since the twinned material

contains visible carbon inclusions which can act as absorptive and scattering centers.

3.1 Laser Damage Measurements

The experimental arrangement used to perform pulsed CO2 damage experiments at

preselected sites in ZnSe is shown in figure 7. A lens with focal length of 10cm

was used. The profile of the focussed beam in a plane approximately 1cm downstream

from the waist was determined by recording burns on carbon paper while successively

attenuating the beam. The beam profile is shown in figure 8, where the solid curve

is a Gaussian profile fit to the data.

The calorimeter in the side arm was calibrated by comparison with the total

87
energy arriving in the sample plane. Calorimeter readings were taken for each

firing

.

The specimen was placed in the above characterized plane and damage was induced

at an arbitrary place on the. sample. The HeNe laser beam was then focussed through

the sample onto the damage site. Sites could then be preselected for damage experiments by using the HeNe beam as a site indicator.

Damage experiments were performed by successively removing CaF2 attenuators

from the beam until visible evidence of damage was observed on a given site.

Faint yellow light, which may be either white light filtered by the transmission

properties of ZnSe or blackbody radiation from heated sites, was seen on some sites,

while the usual bright white sparks were seen on other sites. Several seconds were

allowed between shots to limit heat accumulation in the sample.

4. Results

Pulsed laser damage was induced on several sites within the individual parts of

the twin crystal as well as on twin boundaries, and in one case on a grain boundary.

Figure 9 is a sketch of a specimen surface with the twin and grain boundaries

indicated. Superimposed on this is the location of several numbered damage sites

and the energy value in J/cm at which damage was observed. Here all of the damage

threshold values fall within the range of 4-11 J/cm^, whether the irradiated site was within a twin or on a twin boundary. No particular trend in threshold energy

with relation to damage site location was observed.

2 In eighteen out of twenty-three runs, damage occured at 10-11 J/cm , indicating

that this value is the damage threshold energy for this material, and suggesting that

at the five sites where lower threshold values were observed, there may have been a flaw in the material, such as a carbon particle, to cause premature damage. Actually

two of these five sites show no visible damage, even when examined at SOX.

The damage sites were examined using optical microscopy at SOX. They appeared

to be located primarily within the bulk of the material, although a few were observed on both front and back surfaces.

Figure 10 shows three typical damage sites, located below the surface. They

appear to have two types of structure associated with them depending on which twin

they are located in. That is. the crystallographic orientation of the twin has a definite effect on the damage morphology. Figure 10(a) is a micrograph of damage site

number 34, with the twin surface orientation ~10o off (110). Damage occurs in the

(110) plane, with numerous damage striations along (Oil) planes. Figure 10(b) shows

damage site number 32, with the surface of this twin 13° off (100) plane. Here

there are no (Oil) striations within the circular damage area . However , outside this

area is another disturbed region where stepwise cleaving has occured. Figure 10(c)

illustrates a damage site located at a twin boundary where both types of damage

morphology were observed.

X-ray reflection topographs were taken of the large grain twinned ZnSe crystal.

Here diffraction contrast can be obtained using either set of twins. X-ray reflections were selected using the computer program TOPOG [15]. Figure 11 shows x-ray topographs of [400] and [440] reflections for one set of twins. The crystal is strained

thus eliminating some of the expected pattern. Figure 12 shows the same reflections

after laser irradiation. Here only two surface damage sites (A § B) are seen, since

this technique samples only about a ISym surface layer. Most of the damage sites are located below the specimen surface.

5. Conclusions

Although no direct correlation between damage threshold at lO.Gpm and site

location was found, there is some indication that the morphology of the damage is

related to the orientation of the twin relative to the surface. When failure did

occur, it occurred by partition along the (110) cleavage planes.

The twinning theory indicates that in cubic ZnSe, the twin boundary has a hexagonal structure, probably a few layers thick. However, when the twin boundary was irradiated, the size of the laser beam used was so large relative to the boundary

that it did not see local variation in index of refraction between these two

structures at the interface. Thus, our original thesis has not yet been disproved,

That the twinning is present and coherent actually serves to strengthen the material.

88
since it forms a barrier to dislocation propagation. However, the question requires

more detailed experimental work.

6. Acknowledgements

The authors wish to thank H. Lipson for the optical absorption data, W. Ewing

for the Auger spectra, J. Comer for the electron micrographs and C. Bergeron for

specimen preparation.

7. References

[1] High Power Infrared Laser Window

Materials (LQ-10 Program) , Quarterly Progress Report No. 6, Air Force

Cambridge Research Laboratories,

AFCRL-72-0559

.

[2] Posen, H. et al. Proceedings of

Conference on High Power Infrared

Laser Window Materials, AFCRL-TR73-0372, (1972) Vol. II p. 383.

[3] High Power Infrared Laser Winsow

Materials (LQ-,0 Program), Quarterly Progress Report No 2, Air Force

Cambridge Research Laboratories,

AFCRL-71-0405.

[4] Fradin, D.W. and Bua, D.P., Appl

Phys Lett 2_4, 555 (1974) .

[5] Wang, v., et al, elsewhere in these

proceedings (1974).

[6] Stark, E.F. and Reichelt, W.H.,

elsewhere in these proceedings

(1974)

.

[7] Fridel "Lecons de crys tallographie"

,

Paris: Berger - Levrault (1926).

[8] Jaswon, M.A., Acta Cryst 9, 621

(1956).

[9] Jaswon, M.A. and Dove, D.B, Acta

Cryst 10, 14 (1957)

.

[10] Cahn, R.W., "Twinned Crystals"

"Advanced in Physis" Vol. 3, (1954)

p. 363.

[11] Klassen-Neklyudova , M.V., "Mechanical Twinning of Crystals", translated from Russian by J.E.S.

Bradley, consultants Bureau, NY

(1964)

.

[12] Aminoff, G. and Broome, B., Z. fur

Krist. 8_0 , 355 , (1931) .

[13] Braunstein, M. , Proceedings of

third Conference of High Power

Infrared Laser Window Materials

,

Vol. Ill, Surfaces and Coatings,

p. 863, (1973).

[14] Hunt, M. , AFCRL, private communication .

[15] Posen, H. - TOPOG, a Computer

Program to compute Penetration

Depth and Setting Angles for

Reflection X-Ray Topography,

to be published.

89
8, Figures

Figure 1. Atomic structure of twin develop- Figure 2, Macrophotograph of a large-grain

ment of ZnSe (from Aminoff and Broome [12]). multi-twinned ZnSe crystal.

TWINNED CRYSTAL POLYCRYSTALLINE CVD

Figure 3. Nomarski micrographs illustrating the grain

structure and twin structure in twinned and polycrystalline CVD ZnSe.

90
Figure 4. Stereogram illustrating twinning

relationship in ZnSe,

Figure 5, Electron micrographs of a replica

of ZnSe surface, illustrating twin structure.

dN|

AFTER

TEA CO2 LASER

CALORIMETER

R- 7%

1/ ,

Figure 6. Auger spectra of a foreign particle in

ZnSe before and after sputter cleaning of the

surface.

CaFj ATTEN.F = 10 cmDAMAGE SAMPLE

HeNe LASER J

Figure 7, Experimental set-up for pulsed CO2

laser damage experiments on preselected sites

in ZnSe. .2 .4 .6 BEAM RADIUS (mm)

.8

Figure 8, Profile of a focused CO2 laser beam.

91
L(I6» Oo.^j (lo.'V

^* • •

•

(ra w (lojj lio.y

• • • %) (ie.ij

(TT)

(1

Figure 9. Energy values of observed pulsed

CO2 laser damage in J/cm^ for ZnSe twinned

crystal.

Figure 10. Micrographs of CO2 laser damage

sites in ZnSe twinned crystal.

\

'lUU

Figure 11. [400] and [440] x-ray reflection

topographs of ZnSe twinned crystal.

Figure 12. [400] and [440] x-ray reflection

topographs of a damaged ZnSe twinned crystal.

WO COmENTS ON PAPER BV FOSEN

92
2.8 Evaluation of Single Crystal LaCla AS CO2

Laser Window Material

F. Varsanyi and L. G. DeShazer

Center for Laser Studies

University of Southern California

Los Angeles, California 90007

We report calorimetric measurements of the LaCls absorption coefficient at 10.6 ym. Single crystal

LaCls of substantial size (one inch diameter, two or more inches long) were grown repeatedly in our laboratories in the past years, mostly for spectroscopic purposes. LaCls is transparent in the visible

extending up to 25 ym and, if pure, is only slightly hygroscopic. Its phonon spectrum has been well

investigated and terminates around 260 cm~^. A rather elaborate program of mechanical and optical

property evaluation was initiated. In the preliminary experiments reported here, we studied a LaCla

single crystal (2 x 11 x 12 mm) with a CO2 laser. We feel LaCla compares favorably with KCl as a laser

window material, but without some of its problems.

Key words: Absorption coefficient; LaCls; infrared materials; thin film material.

COmENTS Ohl PAPER BV VAPSANVJ

H. Poi^n 0(1 AFCRL commzntzd that in on.dzn {^on. a matiAyial to be att/iCLctiv^ oi a 10.6 ym MindoM

mateMMit, it ihould be optically iiot/iopic. Hou3t\JtK, the. u^e o{, LaCl^ oi a coating mateAiat, in thin

iiljm ioKm, ij> itiZl attractive.

564-378 0-74-8

93
2.9 LASER WINDOW DAMAGE FROM CW 10. 6-(im RADIATION

J. S. Loomis and C.A. Huguley

Air Force Weapons Laboratory

Kirtland AFB, New Mexico 87117

Damage studies have been conducted using a CW, one -kilowatt CO2

laser. Some analytic calculation of local temperature rise as a function

of laser power, spot size, and irradiation time will be discussed. Results

of optical distortion and stress -field observations on uncoated and coated

samples of KCl, NaCl, and ZnSe are reported.

Keywords: CO2 laser radiation; CW laser damage; infrared windows ; KCl; NaCl;

optical distortion; ZnSe.

1. Introduction

Our initial investigation into CW laser-induced damage [ 1 /was directed toward determining

the various failure modes which may be encountered. For semiconductor materials such as ZnSe and CdTe, damage was accompanied by very high temperatures at the center of the damage spot which frequently resulted in holes being melted through the window. Alkali halide

windows such as KCl and NaCl were most often damaged through fracture localized near the

irradiated areas. Melting or burning of any coating usually accompanied the fracture; the

latter almost always occured after the laser beam was turned off.

More recently our efforts have been directed toward exploring the temperature dependence

on total power, spot size, and exposure time for windows transmitting CW infrared laser radiation. The desired theory was developed by solving the heat conductive equation, first by

making some simplifying assumptions and then more exactly by numerical methods. We then

attempted to demonstrate various aspects of the thermal problem with measurements of optical

distortion and stress -fields during irradiation by various combinations of power, spot size and

irradiation time.

2. Calculation of Local Temperature Rise

Consider first the case of a homogeneous window which is being irradiated by a uniform

beam. One can make the usual assumptions that the absorption is low so that energy is absorbed evenly throughout the volume of window material, that the thermal properties remain constant during irradiation, and that the irradiation time is less than the typical time constant for

external heat transfer. Under these conditions the temperature of the window will increase

linearly with time and there will be no temperature gradients across the window. The window

will have no thermally induced optical distortion or stress patterns, and damage will occur

only when the temperature of the window reaches the melting point.

Contrast this situation to that of a window which is being irradiated at a small spot at the

middle of the window. The temperature at the center of the spot can be obtained from a simple heat-balance equation:

^R^ d C = -Q + e d P (1)

1. Figures in brackets indicate the literature references at the end of this paper.

94
where ttR d is the volume of material being irradiated (R is the spot radius and d is the

thickness of the window), C is the specific heat of the window, g d P is the absorbed power

( e is the absorption coefficient and P is the total incident power), and Q is the heat flow to the surrounding material. This heat loss term can be approximated as:

Q ^ K T (2)

where K is the thermal conductivity, A = 2 tt Rd is the heat transfer area and R is the

average heat transfer distance. Equations (1) and (2) can be combined to give:

2 d T

ttR C —— = -2TTKT + gP (3)

which has the solution

-t/ T

(4)

where

and

T

B P

2 IT K

R^ C

2 K

This simplified analysis suggests that, in the limit of small beam spot sizes, the temperature at the center of the spot will equilibrate to a temperature which is independent of

spot size with a time constant T which is proportional to the area of the spot. For the case

of potassium chloride irradiated by a 1000 watt beam over a spot of 0. 5 cm radius, there is a temperature rise of about 1. 5°C and a time constant of about 2 seconds.

The actual experimental situation lies between the two extremes. To arrive at an actual

temperature dependence, a numerical solution to the heat conduction equation was made [2]

The solution assumed cylinderical geometry with azimuthal symmetry. Heat transfer was

assumed to be governed by differential boundary conditions with a heat transfer coefficient of

0.002 watt/cm^ - °C, which is appropriate for conduction into still air. The beam profile was

assumed to be gaussian. An implicit finite-difference technique was used to calculate temperature

as a function of time and position.

Figure 1 shows the calculated temperature rise for a potassium chloride window irradiated

for 5 seconds by a gaussian beam with a 1/e spot diameter of 6 mm. The window itself is 3.8 cm in diameter by 1.0 cm thick. Note that the temperature gradient is established within 3-4 seconds. Afterward the temperature at each point in the window rises uniformly.

3, Apparatus

The experimental arrangement used is similar to that reported earlier [1]. That apparatus used a spherical mirror to focus the beam from the CO2 laser (GTE/Sylvania model 971)

onto the sample. Several such mirrors were tried in that system, but in each case the beam

profile aberrated to an elliptical shape. The present experimental apparatus, shown in figure

2, uses a salt lens (25 cm focal length to focus the beam. Graphite apertures 1.3 cm in

diameter are used to stop reflections from the uncoated surfaces of the lens. These components were mounted on a translation stage, allowing different spot sizes on the fixed sample.

Measurements of spot size and transmission of this lens are given in an accompanying paper

[3] _ Window samples were positioned after the focus of the lens so that the intensity was

largest at the front surface of the sample.

Before the laser beam reaches the test apparatus shown in figure 2, it passes through a diagnostic area containing beam switches, a 1 -percent mirror chopper, and instruments to mea95
sure the power, wavelength, intensity profile and polarization of the laser beam. Exposure

times are measured to 1 millisecond resolution and can be set to about 10 millisecond prec is ion.

The interferometer used to measure optical distortion is an Optical Engineering "benchtop" interferometer using a HeNe laser beam. Fringe patterns were recorded either by a motion picture camera (ZO frames/sec, F 5.6) or a 35-mm still camera (1/60 sec, F. 4).

A television camera was used to record visual observations on video tape. Provision was made

for viewing the sample between a polarizer /analyzer combination to observe stress patterns.

4. Optical Distortion

The fringe patterns in figure 3 demonstrate optical distortion in ZnSe, KCl, and NaCl.

The photographs in the lower row were taken after the fringe patterns had stabilized, from 10-

20 seconds after the start of irradiation. The aperture of the interferometer was 2. 5 cm x 2.0

cm and the beam diameter was 4 mm and approximately centered in the interferometer aperture.

Laser power incident on the samples was about 500 watts.

The optical pathlength difference (OPD) at any point over the interferometer aperture is proportional to the change in window temperature integrated along the path of the interferometer

beam. Since the interferometer and the incident laser beam are approximately normal to the

surface of the window, the observed OPD is proportional to the average radial temperature.

The proportionality between pathlength and temperature can be written as:

(j) -

X where (j) is the optical pathlength difference (OPD) in fractions of a wavelength, A , and x

is given approximately by [4 1 .

X - + (n - 1) (1 +v ) a

where n is the index of refraction, a is the linear coefficient of thermal expansion, and v is Poisson's ratio.

The peak OPD measured from figure 3 was 3. 3 fringes for ZnSe, 0. 8 fringe for KCl,

and 0.3 fringe for NaCl. An attempt was made to collect data for several different spot sizes

and materials, calculate peak OPD's and normalize the results by a suitable combination of

material properties and experimental variables. The result Ls shown in figure 4. Heat conduction calculations produced the solid line which shows that a change in spot area of a factor

of 100 only produces a change pf a factor of 2 in peak temperature. The experimental points

were adjusted by making an arbitrary choice for ^ • Neither the value of ^ nor the shape of

the curves which resulted were in agreement with theory. HcM'ever, the uncertainties in power

measurement, spot size, and OPD were large. Further experimental work will be required to improve the quantitative results.

The fringe patterns in figure 5 show optical distortion during irradiation at increasing

laser powers until damage occurred. The peak OPD at each power level was directly proportional to the incident power. As can be seen in figure 5 damage is present in the last fringe

pattern. Prior to that time, however, no permanent distortion of the fringe pattern was

observed.

Interferometric measurements are sensitive to changes in the physical thickness of the

window which may be caused by inelastic flow of material to relieve the compressive stress near the irradiated area. Such areas of flow are visible otherwise only by illumination from

reflected light. Since the large scale spacing of fringes preclude observation of small damage

areas, the interferometric technique is not a sensitive method of detecting microscopic damage

spots. Permanent distortion of the fringe pattern without other physical damage present was

observed only in one sample of KCl, coated with AS2S3.

96
described measurements at powers up to 1000 watts and power densities as high as 70 KW/cm . No more than microscopic damage was observed on any of the uncoated specimens. The damage shown in figure 10 is typical of the damage spots produced on uncoated ZnSe. Sparkles

were seen on the surface at power densities as low as 5 KW/cm , but were not seen at higher

power densities. Even after small damage spots were visible to the naked eye, continued

irradiation of that spot did not produce catastrophic damage. The left side of figure lU does

suggest, however, that multiple damage spots may have been produced by that continued irradiation. Although some of the damage spots seemed to be associated with black inclusions in

the ZnSe, there was no evidence that all irradiated inclusions produced damage or that all

damage was associated with inclusions. The smoky rings about the damage spots suggest vapor

ization of dust or contaminents at the surface.

Damage to uncoated NaCl and KCl was found on one of the two NaCl samples tested and

on all of the four KCl samples tested. Damage appeared to be restricted to the surface in all cases and no fracture was observed on any uncoated sample. Sparkle was observed in the majority of the samples at low power levels. Continued irradiation of damage locations did not

seem to enhance the amount of damage present.

Tests conducted on coated samples generally produced damage to both the coating and substrate and usually destroyed the sample, A description of the coated samples tested recently

and the power levels which produced failure are given in table 1.

TABLE 1

EXPERIMENTAL PARAMTERS FOR COATED WINDOWS TESTED

Sample Description Absorption

(Percent)

Power

(watt)

Time

(sec)

Power Density

(KW/cm^)

ZnSe 20 Raytran substrate with

ZnS/ThF^ AR coating

1. 0 450 3. 5 55

ZnSe 22

(no damage)

Raytran substrate with

ZnS/ThF^ AR coating

1. 0 550 30, 65

KCl 30 Single crystal substrate

AS2S2 half-wave coating

500 4. 0 60

KCl 38 Single crystal substrate

AS2S2 half-wave coating

1. 7 360 3. 5 45

KCl 39 Single crystal substrate

As2S2/ThF^ AR coating

2, 2 I, 000 3. 0 1

KCl 40 Single crystal substrate

As2S2/ThF^ AR coating

1. 6 150 20

97
5, Stress Effects

One of the early calculations for window performance was the dependence on stress in

the window as a function of beam parameters and window properties. [4-5] The radial dependence of the radial and azimuthal stress in a typical window sample irradiated by a Gaussian

beam is given in Figure 6, The conditions are the same as for Figure 1. The stress at the

interior of the crystal is compressive, with the azimuthal and radial stress becoming equal at the center of the window and the radial stress becoming zero at the edge. The edge of the

crystal is subjected to azimuthal tensile stress. Fracture was originally believed to be caused

by the edge tensile stress exceeding the modulus of rupture of the window, which would result

In cracks originating at the edge and propagating into the center of the crystal. It was further

advocated that one way to minimize failure might be to subject the window to a hoop stress

which would counteract the thermally induced tensile stress at the edge of the window.

Our results do not agree with this model of fracture. In most of the samples which

fractured, fracture occurred after the laser beam was turned off and was localized to the vicinity of the irradiated area. To explain this, we first calculated the value of stress at the center of the sample and compared it to the azimuthal stress at the edge of the sample. The

results from thin-plate equations [5] are that:

a ce nter = aE T - -^T 1 L 2 center]

)2T - T , 1

L edge J

edge '

where a is the stress, E is Young's modulus and T is the average temperature of the window.

For a window irradiated over a small area at the center, the temperature at the center will

be larger than either average temperature or the edge temperature. Thus the compressive

stress at the center will be larger than the tensile stress at the edge. Although these compressive stresses are not likely to result in fracture, when the stress exceeds the elastic limit

of the material, slip or flow may result. When the thermally-induced stress is relieved, by

turning off the laser, tensile stress will be created at the boundary between the elastic- strained

material and inelastic-strained material. These tensile stresses may in turn be large enough

to cause fracture near this boundary. This explanation, coupled with the assumption of surface

absorption, also accounts for the observation of fracture localized at the surface of the crystal.

The stress fields produced by a focussed laser beam can be observed by viewing the sample between crossed polarizers. Commercial pciarizer material is available which combines a quarter-wave retardation plate with a half-wave plate. Two such pieces of material may be

arranged so that the stress fields produce dark fringes on bright background instead of the

bright fringes on a dark background produced by the usual half-wave plates. Figure 7 shows

a sample of BaF2 irradiated by a 35 watt CO2 laser beam focused to a 1 mm spot size. A

spectacular stress pattern is observed during irradiation. This pattern then disappears after

the laser is turned off. Figure 8 shows a sample of As2S3/BaF2 coated KCl before and during

irradiation with a focussed beam. This time a residual stress pattern remained after irradiation, as seen in Figure 9- Also, shown in Figure 9 is the small damage spot of melted

coating. In this case irradiation had been cut-off before either substantial burning or fracture

resulted. As with the interferometer technique, viewing the sample does not detect small

damage spots. The damage spot on the right hand side of Figure 9 is not seen in the macro

photograph on the left hand side.

Samples of uncoated ZnSe, NaCl, and KCl were also irradiated at about 500 watts of

power (70KW/cm^). No stress patterns appeared for either ZnSe or NaCl. A faint discoloration, which did not photograph well, appeared for KCl.

6. Damage Morphology

Several samples each of uncoated ZnSe, NaCl and KCl were tested during the previously

98
Two ZnSe samples AR coated at 10.6 ym with ZnS/ThF^ were examined for optical distortion during irradiation. The first sample was subjected to a power of 450 watts into a 1 mm spot size. In less than 3 seconds the window reached its melting temperature and the

burn pit shown in Figure 11 was created. Although it is not obvious from the picture, a hole

was drilled through the window. The damage was accompanied by stress (revealed by crossedpolarizers) but no fracture of the window developed, A sister sample was then tested more

cautiously. Successively higher exposures were made until the sample was subjected to 550

550 watts for 30 seconds, with no damage observed. One explanation for the difference between

the two samples is that the damage may have been caused by a small blemish in either the

coating or the sample. Another explanation is the the conditioning of the second sample may

have improved its survivability.

Our previous work suggested that AS2S3 was an excellent candidate for a damage-resistant

coating on KCl or NaCl. Since that time we have tested three additional samples of AS2S2-

coated KCl, two of which damaged. One sample, viewed during irradiation through a polarizer/

analyzer combination, is shown in Figure 8 and 9. The damage spot covers approximately 10%

of the irradiated area and seems to be caused by melting or vaporiztation of the coating. Residual stress was observed due to laser exposures of 500 watts for 20-40 seconds (60KW/cm ) at different places on the sample. However, the substrate did not fracture at any of these

exposures. Another sample, from a different source, was tested for optical distortion (Figure

5) and failed (after a series of conditioning exposures) at 360 watts in less than 3 seconds (45

KW/cm^). As can be seen in Figure 12, the damage spot consisted of a central region where

the coating had burned away surrounded by a ring of ash. In this case the substrate did fracture (after the laser was turned off).

Two samples of KCl with an AR coating of As2S2/BaF2 were also tested. The first sample failed at 1000 watts in 30 seconds (0.8 KW/cm^). No damage to the coating was observed,

but the substrate fractured at the point of irradiation after the laser was turned off. During

irradiation of the second sample, the coating burned and the substrate fractured (see Figure 13)

in about 1 second at 150 watts (20 KW/cm^). The damage spot in this instance was smaller

than the irradiated area.

7. Conclusion

During this series of measurements we have subjected samples of ZnSe, KCl, and NaCl

both coated and uncoated) to CW radiation up to 1000 watts at power densities up to 70 KW/cm

and for times of 1-30 seconds. It was shown that temperature gradients within the sample

stabilized within several seconds after the start of irradiation, but that the temperature at each

point continues to rise linearly with time thereafter. Measurement of optical distortion can

therefore he made under reasonably steady-state conditions. However, the analysis of interferometer patterns still requires a knowledge of the laser spot size and the thermal properties

of the window. Viewing windows through a polarizer /analyzer combination is a useful technique

for studying the time dependence of the stress distribution within the window in those materials

which are sufficiently birefringent.

Much of the damage which has been observed, especially on uncoated materials, is seen

at or near the surface. Conditionir.i: of the surface at lower power appears to be of some

benefit in increasing the survivability of the window. However, it is not clear what kinds of

surface defects cause the window to damage catastrophically and which merely disturb the cos- metic quality of the window without unduly affecting its performance.

99
8. References

tl] LoomLs, J. S. and Saito, T. T. Third Conference on High Power Infrared Laser Win - dow Materials November 12-14, 1973, Vol - ume III, Air Force Cambridge Research

Labs, AFCRL-TR-74-0085 (111). 1974.

[2] Smith, G. D. , Numerical Solution of Partial

Differential Equations , Oxford University

Press, 1965.

£3] Saito, T. T. , Charlton, G. B. , and

Loomis, J. S. , elsewhere in these proceedings, 1974.

14] Sparks, M. , J. Appl. Phys. 42 , 5029

(1971)

[5] Horrigan, F. A. ,, et. al. , High Power

Gas Laser Research , Raytheon Research

Division, AD 676 226, September 1968.

9. Figures

TIME ( SECONDS) Figure 1, Average radial temperature at the

center and edge of a potassium chloride window

as a function of time. The actual temperature

achieved is proportional to the absorbed power.

SPOT SIZE (nm)

Figure 3. Interferometer patterns of three

materials before and during irradiation by 500 Figure 4. Correlation between peak distortion

watts CO2 laser radiation. The top row is be- and spot size (in normalized form),

fore irradiation and the second row is after

the distortion had stabilized.

100
200 270 360

Figure 5. Interferometer patterns during

irradiation of KCl 38 by the indicated

laser power (watts) into a 1 nm spot size.

0 0.5 1.0

RADIUS

Figure 6. Azimuthal and radial stress as a function of radius (normalized units) , for

a potassium chloride window irradiated by a gaussian beam.

BEFORE

Figure 7, Barium fluoride viewed through a polarizer/analyzer before and during

irradiation by 35 watts into a 1 mm spot

size. The sample is 3.8 cm in diameter.

BEFORE DURING

KCl 30 100 w

Figure 8, Coated KCl viewed through

polarizer/analyzer before and during

irradiation by 500 watts into a 1 mm

spot size.

Figure 9. Same coated KCl after irradiation,

photograph at right is magnified view of the

damage spot.

101
ZnSe17 Mpm

Figure 10. Magnified view of damage spots on surface of uncoated ZnSe sample, which

has been repetitively irradiated.

KCl ^18

Figure 12. Micro-damage area on coated

KCl sample.

ZnSe 20 1 CM

Figure 11. Damage crater in coated ZnSe

sample, shown at the same magnification

under normal illumination (left) and

through crossed-polarizers (right)

.

KCl 38

1 CM

Figure 13. Macro-damage on coated KCl

sample.

MO COmENTS ON PAPER BV LOOMIS

102
2.10 10.6 Micrometer CW Laser Damage Studies

Of Metal Substrate Mirrors

T. T. Saito, G. B. Charlton, and J. S. Loomis

Laser Development Division

Air Force Weapons Laboratory

AFWL-LRE

Kirtland AFB, NM 87117

The CW damage properties of metal substrate mirrors were studied by focusing

a 1 KW CW CO2 laser onto the mirrors. Mirrors studies included polished and

diamond machined bare metals, noble metal evaporated coatings, electroplated

and then machined metals, and dielectric coated. Experimentally determined

damage thresholds and damage statistics are discussed. Extensive efforts to characterize beam area of focused kilowatt beam is included. Evidence of a self-cleaning of mirrors by slowly raising laser power is described. Dust on mirrors reduces the damage threshold below 100 W/cm^.

Key words: CW laser damage; diamond turned optics; dielectric enhanced metal

mirrors; laser beam characterization; metal mirrors.

1 . Introduction

We will describe our initial experiments studying the damage resistance of metal substrate mirrors

illuminated by a focused 1 KW CO^ laser. The experimental apparatus will be described. Theory of the

propagation and focusing of a spherical- gauss ian laser beam will be reviewed. Three techniques for

measuring laser beam radius will be discussed including comparative results. An error analysis of the

damage threshold will be given. Two very interesting results from our experiment are the evidence of

self-cleaning by the laser and drastic effects of dust on reducing damage thresholds. Most samples were characterized by measuring the reflectivity/absorption, total integrated scatter, and examination

by Nomarski microscope.

2. Experimental Apparatus

2 . 1 General

A single-mode Sylvania Model 971 CO-, laser with maximum output power of about 1 KW was used for

the damage experiment. A coincident He-Ne laser was used for alignment. A 25 cm focal length NaCl

lens 330 cm from the laser was used to focus the beam onto the samples which were located 22.5 cm

from the plano-convex lens. The convex side of the lens faced the laser in order to minimize

aberrations. Two graphite blocks with 1.2 cm apertures were symmetrically spaced in front of and

behind the lens in order to block the Fresnel reflections from the lens. The graphite blocks were

1.2 cm thick and spaced by 8.3 cm. A Coherent Radiation Model 213 power meter with a 5% (manufacturer's

specification) accuracy was used to measure the incident power for each shot. The transmission of the

lens aperture system was measured periodically and every time the laser was turned on or the laser was realigned. Eight measurements of the transmission at one given time yielded a result and standard

deviation of 0.68 ± 0.02. The transmission measured throughout the series of experiments yielded an average and standard deviation of 0.68 ± 0.06. Contributions of the uncertainty of the transmission

and power meter yield power readings uncertainty of 61 of the power measured.

2.2 Focusing of a Spherical Gaussian Beam

Let us now consider the propagation of a spherical-gaussian laser beam [1, 2]iwhose spatial

dependence on the radial coordinate, r, of the intensity, I, is expressed in terms of the peak intensity, Iq, and the 1/e^ beam radius, w given in equation 1:

(1) 1=1, e -2-'/-^

The dependence of w on the propagation distance , z measured from the position of minimum waist radius

,

w^, is given by:

1. Figures in brackets Indicate the literature references at the end of this paper.

103
For a lens of focal length f located a distance d from the waist minimum, the square of the beam radius

can be expressed in terms of the distance from the lens, L, and the wavelength of light, X, by

X

TT

, f - L , TT W^ ^

( ) ( o ) +

I d + L (1 - d/f)]

(3)

TT W 2

o'^ |d + L (1 - d/f)j + Cl - L/f) (1 - d/f) (tt w^VX)

In general the smallest spot of a focused/laser beam does not occur at L = f . However in the case of

our experiment equation 3 predicts the smallest spot occurs at 0.1 cm beyond the focal length. We

measured the focal length of the lens using a method suggested by Spawr. [3] A razor blade was located on a translation stage so that it could be moved in the direction of propagation (along L)

and also perpendicular to the direction of propagation. The L position of the blade was changed while

observing the transmitted beam on an x-y detector array made by Laser Precision. The minimum spot

size was defined as that L position where translating the razor blade into the beam attenuated both

the X and y axis of the beam uniformly for the 971 laser operating at several hundred watts but

chopped to transmit 1% of the power. Substracting the 0.1 cm theoretical difference of the distance

to the minimum spot size yielded a focal length of 24.9 cm. This was in good agreement to the calculated focal length of 24.4 cm based on the measured radius of curvature and 10.6 ym index of

refraction of 1.49 from the window materials compendium. [4].

2.3 Measurement of Beam Radius

Three methods were, used to measure CO^ laser beam sizes. One involved scanning an edge of a metal block (or sheet) through the beam ana thereby progressively blocking the beam and reducing the

power incident on a power meter head behind the scanning edge. The second method used a narrow slit

which was scanned across the beam in the same fashion. The third technique scanned a wire across the

beam, while monitoring the change in the wire's resistance caused by heating by the laser. Plotting

resistance as a function of scan position of the wire yields a direct profile of the laser beam. This

method was used to insure that the power profile of the model 941 Sylvania (3 watt) laser was

gaussian. The 941 laser was used to validate the three methods of beam radius measurement. In the

scanning edge measurement technique the edge was moved in discrete steps into the beam. The position

along the scan direction (x) and power transmitted past the edge (P) were recorded. The incident

power (or power transmitted with the block removed) (P ) was also recorded. In order to take into

consideration beam profiles which are not syiranetric, such as those caused by aberration from a focusing mirror, we shall work with a bi-variate gaussian whose intensity distribution is

(4) I = I^ exp -2( w ^ )

The ratio of P/Po is given by

where Z

o

'

2x

C5) P/P. = 1 - ^

/ e dZ

00

h -zV2

E w

o P/Po is determined at each value of x and consequently the left side equation 5 is known. The table

of the normal distribution function provides values for the integral on the right side of equation 5 and thus values o f Zy, are determined. Then since

E

(6) = 2

" ^o^

104
(where x is the position of the intensity maximum)

.

(7) X =

A plot o£ X versus Zg will yield a straight line whose slope is w /2. The procedure for acquiring

data with the scanning slit is basically the same as for the scanning edge except that the incident

power is not measured. For a narrow slit of width Ax the power through the slit Ps is given by

(8) P^ = K e Ax

where K is a constant which depends on the integral in the y direction. The natural log of P is of

the form ^

2 (9) In P = a^ X + sltX + a

s 2 1 o

where a- = —

_

2 w 2 X

Considerations of how narrow the slit width miast be are given in the appendix. A plot of InPs versus

X yields a second order polyiiomial v;here the coefficient of the quadratic term is related to w . Figure 1 presents the results of scanning edge and scanning slit measurements on the Sylvania §41P

laser. The horizontal axis is the distance from the beam waist assumed to be at the inner surface

of the output window. The waist size derived from the measured points is 0.164 cm. The waist size

is produced from the measured points using a parabolic regression assuming the functional form of

equation 2. The solid curve is the theoretical curve for a waist size of 0.16 cm. As can be seen

from the figure there is a considerable discrepancy between the measured and theoretical profiles.

Also, the waist size derived from the measured profile is smaller than the 0.19 cm predicted by laser

cavity theory for this laser. The radius of curvature of the back mirror and cavity length were measured by us and found to be in agreement with the specifications. Consultations with the manufacturer did not resolve the discrepancy. In order to check the edge and slit technique we then utilized

the scanning wire which was in good agreement with the edge and slit. A possible explanation for the

smaller beam is that the inner diameter of the laser tube or some obstruction is aperturing the beam

within the cavity. This would explain the smaller radius and possibly explain why the beam diverges more than predicted by equation 2 at distances greater than 100 cm.

Some comparisons between the techniques are appropriate . A detailed explanation of the techniques

,

and comparisons to burning plexiglass [5,6] will be given in a later publication. None of the techniques require an accurate power sensing device, only one that is linear over a decade of power. The

scanning edge technique is the most flexible and yielded better statistical fits than the slit

technique. The wire technique shows promise for very small beams. Tests of the upper level of power

density for the wire and slit technique have not been completed.

A significant advantage of the scanning edge technique is that in-situ spot measurements can be

made at the power density of this test. We found that a 1 cm thick milled aluminum block would serve well as an edge to be scanned across the beam of intensities greater than 200 KW/cm^ for a 1 KW input.

A razor blade burned in the focus. The beam size for this experiment was measured using the scanning

edge technique at the same location (±0.2 cm) as the damage tests. Taking into consideration the

statistical uncertainty (0.005) in determining w from the slope of x plotted versus Zp and theoretical

contributions of the uncertainty (±.005) of the distance L from the lens yields a spot radius w, of

0.042± .007 cm. The uncertainty in w due to uncertainty in L could have been reduced by performing

the experiment at the minimum w of the focused beam.

2.3 Damage Threshold Uncertainties

Contributions of uncertainty in power (6%) and in area 24% yield an uncertainty of 25% of the

power thresholds. Integrating equation 1 yields a relationship between the total power P.^,, and

peak intensity I

105
Damage threshold values are peak intensities given in KW/cm .

3. Procedure of Experiment

3.1 Illumination

Before each sample was mounted it was dusted using canned freon taking care to prevent the

deposition of liquid freon on the mirror surface. The spot to be illuminated was identified by the

coincident He-Ne laser. The first illumination was for 1 second at 25 KW/cm^ in order to "clean" the

sample. Then 1 second illuminations were made at various levels. As soon as the illumination was completed investigation with a 5X eye-loupe was made while slowly cutting off the He-Ne. This method

is especially powerful for determining in- situ damage when the damage site scatters light. We were

able to detect all damage spots in this way except for the case of copper and KCl. More than one illumination was made on the same mirror. If no damage was observed the last shot was at maximinn

power for 10 seconds. For the case of uncoated mirrors we observed no damage in-situ so we went from

the cleaning shot to maximum power 10 second illuminations. On coated mirrors we tried to increase

the intensity in 50 KW/cm^ increments to obtain an estimate of damage threshold.

3.2 Definition of Damage

After the experiment was completed samples were examined with a Nomarski differential phasecontrast interference microscope. Typical magnification was lOOx. Damage is defined as sites which

are observable by microscope. Laser damage sites exhibit a characteristic chromaticity when viewed

with a Nomarski. One problem with this method is the possibility of overlooking a damage site because

they are so small. We found that some mirrors would have a spark of light during illumination but no damage was detected. [5, 6, 7]

4. Results

4.1 • Characterizations of Samples

Various characterizations of the mirrors were made. The reflectivity and/or absorption at 10.6Mm were measured in the multiple -bounce reflectometer [8] or the absorption calorimeter [9]. Surface

roughness for selected samples was measured using a Fringes of Equal Chromatic Order (FECO) interferometer and a Talystep profile-meter for mirror M-106. [10] Total -Integrated -Scatter (TIS) at

He-Ne wavelength was also measured. The FECO and TIS instrument were built by Perkin-Elmer. [11]

4.2 Bare Metal Mirrors

Table 1 presents the results of our damage study. Identification of the mirror and substrate are given in the first column. In general the mirrors marked with an M are molybdenum substrate mirrors.

The reflectivity or absorption are given in the second column, with the absorption given as 1-A and

so indicated by an asterisk. The % TIS values are given in the third column. In the case where no damage was observed the damage threshold (expressed in KW/cm^) are indicated as being greater than the

maximum intensity of illumination. It is noteworthy that damage was not observed in molybdenum but

was observed in copper. Pulsed (0.8 microsecond) laser studies have indicated that molybdenum has

about a three times lower threshold than copper. [12, 13]

It is inportant to note that our experiments were done in the atmosphere, and it is possible that

oxidation of the copper upon heating increases the absorption and thus results in laser damage. Stark

and Reichelt, who performed their experiment in a nitrogen atmosphere, reported copper to have a higher damage threshold than gold. [7]

In this experiment, damage on the copper was not observable in-situ. Figure 2 presents a Nomarski photograph of a damage site. This damage is different than what is observed under pulse

conditions. SEM studies indicated that near pulse threshold copper fails along grain boundaries. [13]

106
Table 1. Results of CW 10.6ym Damage Study

Mirror R %TIS I^(KW/cm^) Comments

BARE

Mll4-Moly 0.98

M115-Moly 0.981

T-l-Cu 0.992

T-3-Cu 0.991

DIAMOND TIMED

L^-Cu 0.989

Y-12-2(6"]Cu 0.988*

L^-Brass 0.984*

L-^-Au 0.988*

C4 Au on Cu 0.991

CI Ag on Cu 0.993

NOBLE METAL EVAPORATED

C44-BeCu-Cu 0.9898*

T-32 TZM-Mo 0.9933*

GBC-44 BeCu 0.993

T-20FHC Cu 0.98S

T-4 OFHC Cu 0.984

SINGLE QUARTER WAVE DIELECTRIC

M128 Cr/Ag/ThF^

M104 ThF^

M164 KCl

M163 ZnS

M119 ZnS

M61 AsS,

0.87*

0.907*

0.9615*

0.837*

0.963*

0.902*

3

0.2

0.1

0.4

1

0.2

2

0.6

0.07

0.05

0.05

0.9

0.3

0.3

0.9

6

20

10

>200

>170

<210

<210

<220

<210

<200

>220

>220

>210

>210

>210

>190

>200

>210

220

140

>170

200

48

>210

90

7805 Molybdenum

7805 Molybdenum

OfflC; 37A RMS Rough

OFHC; 187A RMS Rough;

Cloth Polished

64A RMS Rough; Oxidized

Electroplated Cu

o 137A RMS Rough; Oxidized

O 74A RMS Rough; Brass

Substrate

46A RMS Rough

UHV+O.C. 21A RMS Rough

Area with Fingerprint

UHV Ag 23A RMS

Badly Scratched

o Au; 5 OA RMS Rough

O Au; 20 OA RMS Rough;

Cloth Polish

Rs=0.991

Rs=0.977

Rs=0.982

Gallard Schlessinger

ZnS; Rs=0.982

Irtran 2; Rs-0.983

DIELECTRIC STACKS

M128

M106

OCLI

5-22-73

0.977*

0.995

0.999*

0.998*

1

7

0.5

0.2

>230

180

200

Rs=0.991; Cr/Ag

(ThF^/ZnSe)

Rs=0.925; 123A Tally;

Cr/Ag (ThF^/ZnSe)

See Text

See Section 4.6 Ag

(ThF^/CdTe)^ on Kanigen

*l-Absorption

107
4.3 Diamond Turned Optics

Diamond turning of optics can be defined as the use of a diamond tool on a precision lathe under

very precisely controlled machine and environmental conditions to fabricate a finished optical

component. [14] Damage values for machined copper are comparable to those of conventionally polished

copper. Future studies may be able to differentiate between the two. Pulsed laser damage studies

have shown diamond turned optics to be inferior to super-polished copper but this difference may be

due to the machined copper being an early product of the diamond turning research program. [12] The

gold and silver for these mirrors were first electroplated on the sample before the final cut was made.

A damage site from machined electroplated copper (on a copper blank) is shown in figure 3.

4.4 Noble Metals - Evaporated

Only one sample in this series failed, the gold on copper. The mirror was made similar to T-3

and it is not surprising that damage occurred since the gold decreased the reflectivity of the bare

copper. It is interesting that mirror T-2 did not fail. This indicates that the threshold is near

220 KW/cm^ for gold coated copper. It is interesting that mirror C-44 did not fail even though there was a finger print in the tested area. GBC-44 was a badly scratched mirror and also survived the

damage test.

4.5 Single Quarter-Wave Dielectric

The reflectivity of the uncoated substrate, R , is also given for this series in the comments

column. The failure of M-128 is attributed to high absorption in the ThF.. Both M104 and M128

exhibited high absorption. The TIS values for M-104 and M-128 indicates that M-128 has a superior

surface finish. One may obtain better damage threshold for KCl in combination with other dielectrics

to protect it from moisture. The test was made several days after the coating was made. Although

efforts were made to minimize water absorption the coatings absorption degraded several percent from

their optimum. The difference in the damage threshold of the ZnS is probably due to the differences

in the absorption of the coating, which in turn may be related to the starting material. All substrates

for this series were molybdenum. Figure 4 presents a Nomarski photograph of the damage site of ThF^

and also the coating peeling off near the damage site. Figures 6 and 7 demonstrate the structure

of KCl damage. Lack of a burned appearance may be a result of failure near threshold.

4.6 Dielectric Stacks

The performance of M-128 is very encouraging because it is the same mirror that failed with just

a ThF. single layer. M-128 was masked and one-half of the mirror had an additional quarter wave stack

of ZnSe which decreased the total absorption by 0.17 and probably explains why the coating survived.

Decreasing the absorption of the ThF^ may yield durable coatings using this combination. A possible

explanation for the failure of M-106 is the exceptionally rough (123 A RMS) surface. The value given

for the reflectivity of the substrate is without the Cr/Ag/ThF^ coatings. Earlier measurements

yielded a reflectivity of 0.991 reflectivity for a Cr/Au coating on the same substrate. OCLI has

developed a high reflectivity coating which is reproducibly greater than 0.998 in reflectivity.

Unfortunately this sample was damaged in an earlier test because of dust on the surface. The damage

site was large enough that we suspect contaminant from the damage may degrade the performance in this

test. However, we found places which did not damage when illuminated with 200 KW/cm^. Further test

of this coating is planned.

4.7 Evidence of Self Cleaning by the Laser

We mentioned earlier that sparks from the surface were observed without any laser damage. We

also noted that if the power was raised in 50 KW/cm^ increments after the cleaning shot that 5-22-73

would survive 200 KW/cm . However, the coating failed when illuminated with 190 KW/cm^ on the first

shot in a new spot. Since it is difficult to differentiate between differences in damage threshold

due to non-uniformities and method of illumination, further study of this phenomenon is warranted.

Indications of self -cleaning by the laser was observed on other coated mirrors.

4.8 Effects of Dust

A Cr/Ag/ThF. coating on a molybdenum mirror was tested earlier to survive at 1 KW/cm^ illumination for 20 seconds. The surface was then covered with dust by shaking a dust rag over it. Half of

the mirror was cleaned and then was illuminated so that the dust line bisected the beam. Damage was observed on the dust half of the mirror at less than 100 W/cm^. This was a drastic demonstration of

the effect of dust in reducing the damage resistance of mirrors.

108
5. Conclusions and Summary

We have described a method to perform in-situ measurement of CW laser beams with intensities

greater than 200 KW/cm^. This test has shown that copper exhibits a lower damage threshold than

molybdenum in the CW case which is opposite the findings for the pulsed case. Silver and gold evaporated on metal substrates to yield a reflectivity greater than 0.99 should survive 200 KW/cm^ illumination of small spots. Diamond turned optics compete well with conventionally polished optics. Not

surprisingly coating absorption is the major factor influencing coating survivability. Dielectric

stacks in this test survived illuminations greater than 175 KW/cm^ . Care must be taken in extrapolating

the damage threshold values of these experiments to cases of the same intensity by larger beam illumination. We hope to perfom theoretical analysis on the effect of beam size on damage threshold.

6 . Acknowledgments

We gratefully acknowledge the advice, assistance and measurements of C. Huguley regarding spot

size, contributions of L. B. Simmons and R. L. Lester, and the contribution of samples from

J. Buckmelter, J. Staal, D. L. Witt, and J. Bryan.

7 . Appendix

Effects of Slit Width on Accuracy of Scanning Slit

Beam Measurement

In order to reduce scanning slit data, equation 8 was obtained by approximating the integral

of the intensity to describe the power transmitted by a slit vertical in the x-y plane

Ax/ 2 2 (11) / -2 0c^ -2Cx-xj2

/

2 w d X 'V Ax e 2 w

0'

Ax/

2

where Ax is the width of the slit. We can estimate how narrow a slit is necessary in order to have

a fraction error of e by breaking the interval of the integral in half and consider an arbitrary x dependence of intensity, f (x)

(12) f

f (X) d X +

Ax/

2

x /

+Ax/2

f (X) dX ^ f(x-^)+f(x+^) Ax

2

The requirement that the fractional error be e is expressed in terms of the difference of equation 12

and the generalized form of equation 11.

(13) Axf (x) f (x-^) + f (X + ) = e f(x) Ax

Equation 13 can be sin5)lified to read

(14) 1 - J

^ £(x - ^) + f(x + ^)

For a gaussian distribution, the maximum rate of change of the slope occurs at x = x^. Therefore,

the most stringent requirement on Ax is established by evaluating equation 14 at x = x . Setting

X = 0 and using a series for the exponentials yields the relationship between the slit width and

0error.

(15) ^

8w'^

For the 3 watt model 971 laser, w > 0.16 cm and our 200ym slit width, the approximation of equation 11

creates an error less than Q.2%.

109

564-378 0-74-9
8 . References

A. E. Siegman, An Introduction to Laser and

Masers , McGraw-Hill, San Francisco, Calif.,

1971.

H. Kogelnik, Bell Tel Syst Tech J 44:455

(1965).

W. J. Spawr, Private Communication, 1974.

C. S. Sahagian, and C. A. Pitha, Compendium

on High Power Infrared Laser Window Materials

(LQ- 10 Program) , AFCRL-72-0170 , Air Force

Cambridge Research Laboratories , Bedford

Mass. , March 1972.

V. Wang, A. Braunstein, M. Braunstein,

J. E. Rudisill, and J. Y. Wada, Laser

Induced Damage in Optical Materials : 1973,

U. S. Department of Commerce 1973.

J. S. Loomis and T. T. Saito, Third Confer - ence on High Power Infrared Laser Window"

Materials November 12-14, 1975 Volume Ifl : Surfaces , Coatings , Air Force Cambridge Research Labs, AFCRL-TR- 74-0085 (III) 1974.

E. E. Stark Jr. and W. H. Reichelt, Laser

Induced Damage in Optical Materials : 1974 , to be published.

[8] Wetzel, M. G., Saito, T. T. , and Patterson,

S. R. , App Opt . 12:1558 (1973).

[9] Saito, T. T., L. B. Simmons, and A. B. Callender, "10.6ijm Absorption Calorimeter for

Metal-Substrate Mirrors," J. Opt. Soc. Am.

64:536 (1974) Presented to 1974 Optical

Society Meeting.

[10] Saito, T. T., "10.6ijm Absorption on Molybdenum Mirrors," to be presented to the AIAA/

ASME Heat Transfer Conference, July 74.

[11] J. R. Kurdock, "Low Absorptance Metallic

Coatings for Metallic Substrates," A. F. Weapons Lab Technical Report AFWL-TR-74-28,

March 1974.

[12] M. J. Soileau, and V. Wang, to be published,

App. Opt. 1974 .

[13] T. T. Saito, W. J. Spawr, V. Wang, to be

published, 1974.

[14] T. T. Saito, Microinch Machining of Optical

Components for Infrared Optics AFWL-TR-73-

290, A. F. Weapons Laboratory, Kirtland AFB,

NM Feb 1974.

"

9. Figures

w Um)

0-58

SPOT SIZE MEASUREMENT

X SCANNING EDGE

r SCANNING SLII

DISTANCE FROM lASER (<m]

Figure 1. Measurement of spot size using the scanning

edge and scanning slit technique versus distance from

the laser. The solid line is the theoretical propagation of a spherical-gaussian laser beam with a minimum w of 0.164,

110
Figure 2. OFHC cloth polished copper with 187 A

roughness damaged at 210 KW/cm^. Nomarski photograph at lOOx.

Figure 3. Electroplated copper then machined:

Damage site of illumination at 210 KW/cm . Nomarski photograph at lOOx.

Figure 4. Molybdenum substrate mirror coated with Figure 5. Molybdenum substrate mirror with A/4 a Cr/Ag/ and X/4 at 10.6 micrometer of ThF^. Notice io.6 micrometer of AS2S3 illuminated at 90 KW/cm^

the coating peeling off near damage site created Nomarski photograph at lOOxby laser. Nomarski photograph at lOOx.

Ill
Figure 6. Molybdenum substrate mirror with X/A

at 10.5 micrometer of KCl damaged at 200 KW/cm^.

Nomarski photograph at lOOXo

Figure 7. Molybdenum substrate mirror with A/4

at 10.6 micrometer of KCl damaged at 200 KW/cm^.

Nomarski photograph at 475x.

Figure 8. Ag and 3 pairs of ThFi,/CdTe on a kanigen substrate, center of damage area where

the coating was not destroyed, probably because

of non-uniformity of damage resistance observed.

Nomarski photograph at lOOxCOUUEMTS ON PAPER BV SAJTO

W. Spawn. o{, Spawn. OpticaZ commtntQ.d on the. dlUlcuJlXy Idtntt^iying damage. In po6t-montejm

e.x.aminatA.on i,u/i(,aceJ> which woAe. ImJuxdily of, poon quatity, I.e., In zxceM of^ 100 kngitAom nm nough

neM. On ^uA/JacM of kigh quoLvty uolth noughneM of thz ondeA oi 30 AngitAomi nm, howeveA, maJUi

icAataheA, digi and damagz-induced locaZ voAlation^ itand out much mone cltanZy, Thui, damage IzveZi

quoted {,ofL nough iuAf^ace^ may actualZy be. kighen. than the Viue thxe^hold.

112
2.11 High Absorption Damage in Infrared Filters

W. S. Otaguro

McDonnell Douglas Astronautics Company

Huntington Beach, California 926^+7

The damage morphology, degradation of filter transmission, and thresholds for

damage of infrared filters subjected to a hostile environment where the radiation

incident on the filters includes high power densities in a wavelength region of high

absorption have been examined. Infrared filters cooled to 10 Kelvin exhibited damage

thresholds at 5 megawatts per square centimeter when illuminated with a Q-switched

ruby laser. A thermodynamic model based on intrinsic absorption of the material com- posing the multilayer coatings is compared to the measured damage thresholds.

Keywords: High absorption damage; Infrared filters; laser- induced damage.

1. Introduction

Optical filters are used to pass radiation in a prescribed wavelength band and reject radiation

outside of this peissband. The optical absorption properties of materials are used to achieve the

desired out of band rejection. Normally, the radiant energy being absorbed by these filters is relatively

low, and the performance of the filters is not seriously degraded by heating due to absorption. However,

in the application of various radiation sensor systems , the amomt of radiant power density outside of the

passband of the infrared filters may be as high as 5 megawatts per square centimeter. Under these

"hostile" conditions, the high absorption of radiation by optical filters outside of their passband may

result in serious degradation in the transmission properties of these filters.

The damage threshold for an infrared filter composed of multilayers of germanium and zinc sulfide was determined by irradiation with a Q-switched ruby laser. A thermodynamic model based on intrinsic

absorption of the material composing the multilayer dielectric coating is compared to the measured damaged

thresholds

.

2. Infrared Filters

The commercial infrared filters obtained for these tests are composed of multilayers of low (zinc

sulfide) and high (germanium) index materials. The index of refraction of zinc sulfide is 2.8U and the

index of refraction of germanium is U.O. The surface layer is zinc sulfide. X-ray fluorescence analysis

indicates traces of aluminum, tungsten, iron and chromium in the multilayer coatings. Also, an ambiguous

peak which could be due to traces of phosphorus , platinum, gold and/or zirconium was detected. The surface

of the infrared filters has small mounds 1 to T microns in diameter. The mxaltilayers ranged in thickness

from 0.65 to 2.h microns. The substrate material is germanium. The passband of the filters is from 8

to ih micrometers

.

3. Experimental Apparatus

The experimental arrangement is shown in figure 1. The Q-switched ruby laser produced a TEMqq mode

with a 20 nanosecond pulse width. The pulse was monitored by a biplanar photodiode. The spatial

uniformity of the laser pulse was recorded on polaroid film. A helium-neon alignment laser was used to

define the beam path of the ruby laser. The infrared filters, cooled to 10 Kelvin in a helium dewar, were irradiated with the vinfocused ruby output. The laser beam entered the dewar through a potassium

chloride window.

The transmission of the infrared filter was measured with a modified Perkin Elmer Model 621

Spectrophotometer. The modification involved the addition of a chopper between the spectrophotometer

source and the infrared filter. This modification prevented the detector from seeing the emitted energy

of the infrsired filter as a signal and allowed the measurement of the infrared transmission spectra of

the filters at cryogenic temperatures

.

113
It. Test Procedure

The damage threshold for the infrared filters was defined as the irradiation level of the ruby laser

where the transmission in the passband of the filter changed by k%. The h% transmission change was selected because the spectrophotometer had a ± i^ measurement precision.

The transmission of the filter was measured in the dewar at ambient conditions and then at 10 Kelvin.

After being irradiated by the ruby laser, the transmission of the filter was measured. The process was repeated until damage occurred.

The infrared filters exhibited damage thresholds due to the Q-switched ruby laser at power densities

of 5 megawatts per square centimeter. The irregular removal of several layers of the coating material

accompanied the reduction of the filter transmission.

A portion of the filter not radiated by the laser is shown in figure 2. The mounds range from 1 to

7 microns in diameter. Debris from the damaged areas in the form of white flaXes was deposited in the

undamaged area.

In figure 3 the filter transmission after an incident pulse of power density 7.2 megawatts per

square centimeter is shown. Although the filter trsinsmission was reduced, the overall bandpass structure

is unchanged. The damage morphology of this filter is shown in figure h. The damage seems fairly uniform with a rather clean demarcation between the regions of material removal and those of no damage. The

color of the damaged area is gray-white in contrast to the black metallic luster of the undamaged areas

.

Even in regions where a high density of pits was produced, small, undamaged portions of the filter surface remained.

A highly magnified view of the damage morphology is shown in figure 5^ The damage is evidenced by

the removal of the top zinc sulfide and germanium layers and the heating of the layers below them. The

breaks are sharp indicating the clean, perhaps explosive, removal of the surface layers. The morphology

of the areas within the pitted regions exhibits a structure similar to the material formation produced

by intense heating to boiling, followed by rapid cooling. The dark spots are areas where the local

heating was intense. Zinc sulfide has recrystallized near the edge of the pits. Flakes of zinc sulfide

are also present. The depth of the damage in figure 5 is 1 to 2 microns. An edge view of the damage area indicates that the surface damage and layer delamination does not extend more than h layers into

the filter coating. Transparent fragments of the filter material, probably the zinc sulfide top layer were found deposited on the front window through which the laser beam entered the dewar.

The infrared transmission of the filter decreased anywhere from 10% to hO% depending on the severity

of the damage.

Results

6. Analysis

According to the Beer Law the intensity of light transmitted through a material, I, is given by:

I = I e

- ad

(1) o

where

I o = incident intensity

a = absorption coefficient, cm

-1

d = path length , cm

° li -1

For germanium at X = 69U3A and 77 Kelvin, the absorption coefficient is equal to 5 x 10 cm .

Therefore

:

0.036

114
Approximately, ninety -six percent of the laser energy incident on the gennanium layer will be

absorbed by this layer. For an extinction coefficient of 0.069, and an index of 4, the amount of

radiation transmitted into the first germanium layer through one layer of zinc sulfide is 32%. The

total energy absorbed by the first germanium layer is approximately 0.T9 Iq r^) where Iq represents

the incident energy density and r is the radius of the laser beam. The surface layer of zinc sulfide

is assumed to affect only the amount of radiant energy incident on the first germanium layer.

A classical thermodynamic formalism is used to calculate the damage thresholds, [l]^ The laser

photons are absorbed by interaction with electrons where the electrons in the valence band are excited

to the conduction band. The excited electrons in the conduction band impart energy to the lattice by

collisions with phonons which result in the heating of the material. During the pulse duration,

2 X 10~S seconds, the excited electrons will collide among themselves and deliver this energy to the

lattice phonons . The energy deposited by the laser pulse in the multilayer coatings may be considered

as being turned instantaneously into heat in the area where the laser pulse is incident. Under these

conditions, the usual equations for heat flow are applicable, [l, 2]

V^T (x, y, z, t) - (l/k) ^^^/^ = -P{..y,.,t) (2

where T = temperature

t = time

k = thermal diffusivity

P = heat production per unit volume per unit time

K = thermal conductivity

The solution to this differential equation is complicated by the temperature and hence, the time

dependence of the thermal diffusivity, k, and the thermal conductivity, K. However, in order to obtain

a solution to eq. (2), k and K will be assumed to be constant. The effect of their variation on the

solution will then be evaluated. The solution to eq. (2) for a = 5 x 10-'* cm~^ and for a laser pulse

flat in time and uniform in the x-y plane is given by: [l, 2] ,

^l/2 1/2-

T (z, t) = [2 F„ (kt)^'"/K] ierfc [z/2 (kt)^''] (3)

For z = 0:

T (0, t) = (2 FJK) (kt/Ti ^1/2

The results of eq. (h) are shown in Table 1.

6 2 _ -9

Table 1. Surface temperature of a germanium layer with F^ = 5 x 10 watts/cm and t - 20 x 10

seconds. The values of k and K are for T .

T (°K) T (°K) o

20 975

100 585

200 TIO

800 1275

1000 1295

The irradiated germanium layer will reach the temperatures given in a time , t = d /l+k where d is equal

to the thickness of the germanium layer. Accounting for the variation in k, the largest t is of the

same magnitude as the pulse duration.

1. Figures in brackets indicate the literature references at the end of this paper.

115
7. Discussion

The damage structure indicates intense heating of the multilayer coatings. The analysis shows

that a 5 megawatt per square centimeter pulse can heat the germanium layer uniformly within the pulse

duration to a temperature range where germanium will melt (1209 Kelvin) and the zinc sulfide layers

will sublime (lUOl Kelvin).

The heating of the first germanium layer and the subsequent sublimation of the zinc sulfide layers

above and below the first germanium layer accounts for the observed damage morphology. The blow off

of the first germanium layer due to pressure produced by the sublimation of the zinc sulfide layer

below it created the surface pitting. The irregular structure within the pits is due to the sublimation

of the zinc sulfide coating during the presence of the laser pulse followed by the rapid cooling due to

the 10 Kelvin environment after the laser pulse had terminated. It is unlikely that the 5 megawatts

per square centimeter laser pulse would be able to vaporize the germanium. Since the damage was con- fined to the top four layers of the coating, the overall filter transmission shape was not altered

except for the reduction of the filter transmission due to scattering and reflection losses resulting

from the surface damage.

Although the damage as seen in figure h appears to be uniform, figure 5 reveals that the damage

structvire is made up of many micro-pits. The exact absorption mechanism in materials of such high

absorption is quite complex. However, it is the creation of a suitable electron density which will

stimulate absorption. [3, ^] The presence of material inhomogeneities in the form of defects, particulate inclusions and impurities will aid the electron growth once the initial electrons are excited

from the valence to the conduction band by the laser photons. The presence of metals detected in the

coatings and the hot spots in figure 5 indicate that the pitted nature of the damage is probably due

to the enhancement of the electron density around these regions of inhomogenieties

.

8. Acknowledgement

The many helpful suggestions of Dr. Isaac Richman during the course of this investigation are gratefully acknowledged.

9. References

[l] Ready, J. F. , "Effects of High Power

Laser Radiation," Academic Press,

New York, 1971.

[3] Hellworth, R. W., Proc.

Damage in Laser Mat ' Is

.

Pub. 3hl, 67 (1970).

2nd ASTM Symp.

, NBC Spec.

[2] Carslaw, H. S. and Jaeger, J. C,

"Conduction of Heat in Solids,"

2nd ed. , Chap 2 Oxford Univ. Press,

London and New York, 1959.

[h] Bliss, E. S., Proc. 2nd ASTM Symp.

Damage in Laser Mat ' Is . , NBC Spec

.

Pub. 3hl, 105 (1970); also

Optoelectronics 3.99 (1971).

10. Figures

FILTER TRANSMISSION

RECORDER

LIQUID HELIUM DEWAR

1CI°K

INfRAREU SPECTROPHOTOMETER

ALIGNMENT USER

Q-SWITCHED RUBY LASER I

OSCILLOSCOPE

FILTER

PHOTOGRAPHIC I SCREEN I

PYROELECTRIC UJ

DETECTOR

Figure 1. Experimental arrangement.

116
Figure 2. Portion of infrared filter not irradiated by ruby laser. The figure on the le

16A microns wide. The figure on the right is 82 microns wide. The mounds range in diame

from 1 to 7 microns. The white flakes are debris from damage areas.

Figure 3. Transmission of infrared filter before and

after damage due to 7.2 megawatts per square centimeter output from a ruby laser.

117
Figure 4. Surface morphology of infrared filter Irradiated with 7.2 megawatts per square

centimeter ruby laser output. The figure on left is 4.3 mm wide. The figure on right is

820 microns wide.

WO COmEMTS ON PAPER BY OTAGUKO.

118
3.1 Q-Switched Laser Induced Surface Damage at I.06 Microns*

N. L. Boling, J. A. Ringlien and G. Dube

Owens-Illinois, Inc. 1700 North Westwood Avenue

Toledo, Ohio 43666

Our studies of the effect of '

' superpolishing' ' on the surface damage

threshold indicate that such polishing does not increase the practical threshold on 'ED-2 laser glass. Impurities introduced in the polishing process often

outweigh any advantages that might be gained in longer polishing to reduce

geometrical defects.

On commercially available, ''conventionally'' polished laser glass, the

surface threshold is about I50 j/cm^ (5 GW/cm^). This is at least an order of

magnitude below the intrinsic bulk threshold. Some problems encountered in

using small beams to measure the surface-to-bulk ratio are discussed.

The effect of treating ED-2 laser glass with boiling nitric acid is also

briefly described. This treatment can permanently raise the threshold by as much as a factor of three, while leaving the surface in good optical condition,

The efficacy of the treatment is highly dependent on the polishing history of

the sample.

Key words: Electron avalanche; laser; laser induced damage.

1. Introduction

In this paper we explore the following three questions concerning I.06 /m Q-switched damage to laser glasses:

1) \4hat is the ratio of the practical damage threshold on ''conventionally'' polished surfaces to

the bulk breakdown threshold?

2) Does ''superpolishing'' of laser glass lead to higher damage thresholds?

5) If the answer to 2 is no, what approach should be taken to achieve higher surface thresholds?

We use the terms ''conventionally polished'' and '

' superpolished' ' very loosely. By ''superpolishing,'' v;e generally refer to a ''bowl-feed'' process that, if done carefully, leads to a surface

with fewer and smaller scratches and a low rms roughness value. However, the effect of the polishing

process on the surface depends on many factors. Among these are the type and size of the grinding ma- terials used, the composition of the polishing compound, the type of lap, the pressvire used on the sample, the polishing time, the amount of water used on the lap, etc. Seemingly insignificant changes in

any one of these can lead to large changes in the modified surface layer created by polishing (see [1]^,

for example). Because of this complexity, we stress that the terms ''super'' and ''conventional'"' are not sul'ficient to describe polishing. We use them only because they have been used to describe the

results of other damage studies.

* This work was supported by the Advanced Research Projects Agency under Contract DAHC 15-72 -C-OI7O.

1. Figures in brackets indicate the literature references at the end of this paper.

119
2. Surface Threshold vs. Bulk Threshold

on Conventional Surfaces

To address the question of b^LLR vs. surface threshold and to provide reference material for question 2 above, we refer to the work of Olness and Swain [2,3]- They investigated the effect of hydrofluoric acid etching on the entrance surface threshold of three commercial laser glasses. A 5 ns, TSMqc

p\ilse from a glass laser was used in the studies. The glasses as they were received from the manufacturer exhibited thresholds of 35 j/cm^ (7 GW/cm^). V/e have made measurements at k ns at Owens-Illinois

that agree well with these values. Upon etching in HF acid and glycerin for approximately two hours,

the threshold increased by a factor of two to three, a significant increase. But when they refinished

the samples using barnesite and a pitch lap, the result was even more dramatic. The threshold increased more than an order of magnitude, going as high as ^QO j/cm^ (76 GV//cm^).

Since these measurements refer to the entrance surface and since the electric vector on the en- trance has the same magnitude as in the bulk, these values can be used to indicate a minimum value of

the bvlk. threshold. That is, since Olness and Swain saw no internal damage [4], we can conclude that

the bulk threshold was at least an order of magnitude higher than the conventional surface threshold.

We have found i\irther evidence for this in other work. Feldman has measured a minimum intrinsic

bulk threshold of 5O-6O GW/cm^ at 25 ns for laser glasses [5]. At Owens-Illinois, we have regularly

been obtaining entrance thresholds of 4 to 6 GW/cm^ (30 ns), again at least an order of magnitude below the minimum bulk threshold.

On the other hand, recent measurements have been made of the ratio of bulk to surface breakdown

levels using the same test setup for both measurements [6]. In these measurements, a small beam (« 20

I'jm) was focused first inside the sample and then on the surface. The probability of breakdown [7] in

these two positions was measured. For conventionally polished BSC glass, a bulk-to-surface ratio of

only 1.7 was obtained. Thus, there is a rather wide discrepancy among reported bulk-to-surface ratios.

To resolve this discrepancy is important, since one would like to know the maximum improvement that can

be expected in svirface thresholds.

One reason for the discrepancy coiild be that in the work of [6] shots on which it was judged inclusions were struck were not counted, nor were the levels at which these inclusions damaged reported.

That is, only shots on which it was judged that intrinsic damage had occurred were counted. This could

lead to the high reported values for surface breakdown {^h GVZ/cm^ or kOO j/cm^ at 12 ns).

Another reason for the discrepancy could lie in the subtleties involved in using very small beams

for damage studies, as was done in . [7] if we accept as a premise the idea that, for small enough

beamB, there is no sharp damage threshold, but rather only a probability for damage at given energy

density, it is profitable to analyze an experiment that compares the probability of bulk breakdown to

surface breakdown. In such an experiment damage is due to at least one free electron gaining enough

energy to begin an avalanche.

Consider figure 1. The laser beam is focused through a microscope lens of focal length f to a spot of diameter d on a sample surface that has no inclusions and is perfectly uniform. Let's suppose

in this ideal experiment that the beam profile is square and not Gaussian as in most real experiments.

That is, it is possible to associate unambiguously an energy density to any shot that causes damage.

In this way we can ascertain the electric field ' ' seen' ' by the electron that starts an avalanche. One

proceeds to hit the sample with a large number of shots at a constant energy level, counts the number

of these that cause damage, and then detenrdnes the probability Ps(e) of damage at that energy level.

Now suppose d is doubled while maintaining the energy density constant, and the experiment is repeated

in a second series of shots. It is obvious that if the beam is small enough to see statistics, the

probability for damage will be greater in the second series. The reason for this, of course, is that

the probability of damage depends on the initial number of free electrons available to initiate damage

in the irradiated volume.

The point of this argument is that, for a beam small enough to see statistics on a uniform surface, the parameter of practical interest is not the probability of damage at a given energy level,

which is system-dependent, but rather the probability of damage per unit area of surface irradiated.

In fact, even this is over simplifying things. The breakdown occurs not at an idealistic two-dimensional surface, but in a surface layer of probably a few hundred angstroms depth. (Vfe consider this

depth to be roughly that of the modified surface layer left by the polishing process. It is in this

layer that one expects higher free electron densities due to stresses, impurities, etc. ) Thus, one should really consider the probability of damage per unit surface volume.

Let us imagine next that the beam is now focused inside the sample to measure the probability of

bulk breakdown Pb(e), as in figure 2. Again, it is obvious that the number of free electrons in the

focal volume Vg is system-dependent, and therefore that Pb(e) is system-dependent.

120
II' we consider the ratio Pb(e)/Ps(E), it might appear that the system-dependence cancels. That

this is not the case can be seen by considering that the ratio Vg/Vs (in figures 1 and 2) changes as the

focal length of the lens changes. That is, Vb is roughly proportional to f^, while Vg a .

Thus, the ideas of electron avalanche and damage probability lead us to conclude that the ratio of

bulK-to -surface breakdown levels cannot be meas\ired with very small beams. However, one might make the

seemingly obvious argument that if the sample surface is placed in the middle of the beam waist, as in

figure 3, and the bulk is seen to damage before the surface, then surely the surface breakdown level is

at least that of the bulk. Al'ter all, the beam has to pass through the surface to get to the bulkl

Again, this argument is wrong if one accepts the idea of damage probability. This can be seen by con- sidering again that Vg/Vs (figure 3) changes with f. If f is doubled, for example, the nimber of free

electrons in the surface volume is quadrupled, while the number of free electrons in the half of the

focal volume in the bulk goes up by about a factor of nine, and therefore PB(E)/Pg(E) is not constant.

V/e have gone on at some length with this analysis of small beam results in an attempt to resolve

the discrepancy in reported ratios of bulk-to-surface breakdown. On the basis of this analysis and re- ported large beam results, we conclude that on conventionally polished laser glass, which exhibits en- trance thresholds of h~6 GW/cm^ at 30 ns, the surface threshold is at least an order of magnitude below

the bulk threshold.

3. The Effect of Scratches on the Practical

Damage Threshold

The observation that the electric field of a laser beam incident on a s\irface is enhanced at

scratches and digs has led Bloembergen [8] to conclude that scratches and digs lead to a lower damage

threshold. This lowering of the threshold by defects would take place whether damage was due to either

avalanche or inclusions. However, because small enough inclusions do not damage, and because free

electrons can dif±\ase out of the high electric field regions around small enough scratches before an avalanche can build up, it was theorized [8] that scratches smaller than about 100 A should not

lower the threshold. Recent experimental work on the comparison of intrinsic bulk breakdown to intrinsic breakdown of superpolished surfaces has seemed to support this theory [6]. In fact, the work of

[6] implies that glass surfaces do not have to be polished very carei\illy to raise the intrinsic

surface threshold to that of the bijlk. TAgain, inclusion damage was excluded in the data.") BSC glass

with scratches as large as 2000 A was found to have an intrinsic surface threshold of about that of the

bulk. (We note that the analysis of Section 2 causes us to question the svirface-to-bulk ratio obtained

in small beam experiments. However, such experiments, if the geometry is maintained, should reflect

changes in the surface-to-bvilk ratio from sample to sample.

)

The work of Olness and Swain [2,3] on HF etched laser glass is pertinent in evaluating the roles of

surface imperfections in damage. After deep etching of the commercial laser glasses they tested, which

could have been expected to exhibit occasional scratches of a few thousand angstroms, the surfaces were

le±^ in extremely rough condition. Defects as large as 3 to 4 microns can be seen in the optical micrographs . [3] In this case, then, the change has been from a fairly smooth s\irface to a very

rough one, and yet the damage threshold has increased by an order of magnitude. This suggests two

points. First, increasing the surface geometrical defects does not necessarily decrease the damage

threshold. Second, surface impurities play the major role in determining the damage threshold.

We have measured the damage threshold of several samples that have been bowl-feed polished. Figures k and 5 juxtapose two surfaces on ED-2 laser glass. Both have been polished in the same manner,

except that the one in figure k was polished longer in an attempt to l\arther remove scratches. This

surface did in fact present a slightly better appearance when viewed in its entirety. Yet the damage

threshold in this longer polished surface was only ^5 j/cm^ (1.5 GW/cm^), while the threshold of the

other surface was 90 j/cm^. These tv;o surfaces, however, were polished with jewelers rouge, which is

known to cause low damage thresholds at I.06 ,bm. Initial damage is always due to inclusions on rougepolished surfaces. But tv/o points again can be made from these data. First, a longer polishing to get

rid of scratches can lead to a much lower threshold. Second, the polishing history, aside from geometrical defects, is of prime importance in damage.

Figures 6 and 7 show two l\ised quartz surfaces polished in the same manner by conventional techniques. The surface in figure 7 has been subjected to an acid treatment, leaving pits of about 1000 A. The thresholds for figures 6 and 7 were respectively 310 and 36O j/cm^, about the same within the accu- racy of the measurements.

Still another set of comparison surfaces is shown in figures 8 and 9- In figure 8 is an ED-2 sur- face conventionally polished with barnesite. Defects as large as 1000 A can be seen. In figure 9 is a very good bowl-feed polished ED-2 surface. No scratches larger than 100 A can be seen. The damage

thresholds of the two surfaces were the same, I50 j/cm^ (5 GW/cm^).

121
In those cases we have described and in others we cannot discuss here, we have been unable to find an example of an increase in threshold by decreasing the geometrical surface defects. In fact, the opposite has sometimes been the case. Vftiat can we inl'er from this? There can be no doubt that the field

is enhanced at these defects, althoiAgh as pointed out by Bloembergen L8], the degree of enhancement can only be estimated. Given that all other conditions are equal , it seems that a badly scratched svirface

must damage before a smooth oneT This might be the case. The key v/ould seem to be in getting all

other things equal. In the examples we have discussed, the surfaces compared were alv/ays different in

ways besides the size of scratches by virtue of having been etched or not, or polished for different

times, etc.

There are, however, also theoretically plausible reasons why some defects are not important. Perhaps electron difiXision out of the highly localized high field region around the scratches is faster

than has been estimated. One can also make the argument that a single 1000 A spherical pit, for ex- ample, would probably not be the first region to damage, simply because the probability of finding a free electron in the approximately lO"-"-^ cnP high field region around it is very small. An extension

of this argument is that the density of defects on a surface should be considered if avalanche is to be

considered the breaJcdown mechanism.

Whatever the case, there are two points that axe clear from these experiments:

1} Surface impurities are in practice oiten more important than scratches and digs in lowering

the threshold.

2) ' ' Superpolishing' ' is not a panacea for raising the surface threshold, and sometimes, because

of the greater introduction of impurities, it can even be detrimental.

h. Raising the S\irface Damage Threshold

by Acid Treatment

Since we have concluded that polishing deposited impurities are of prime importance in the damaging

process, it seems reasonable to investigate ways to remove these impurities. We have been studying

various acid treatments of ED-2 laser glass to this end. Acid treatments to raise the damage threshold

are, of course, nothing new. Several workers have tried various acids. Swain [j] used HF to achieve

large, permanent increases in the threshold, but this acid attacks the silicate structvire of the glass

leaving the siirface in an optically unacceptable condition. Others, including ourselves, have tried

various other acids at room temperature with no effect or only a temporary effect on the threshold.

Recently, we have been using boiling 0.4 N nitric acid on glass surfaces. The damage threshold of

ED-2 laser glass can be changed greatly by this treatment, going from I50 j/cm^ (5 GW/cm^) to as high

as 470 j/cm^. We have subjected samples to this acid treatment, damage tested them almost two months

later, and found the same high threshold. Furthermore, the nitric acid does not attack the silicate

structure of the glass, so the surface is lelt in good optical condition.

The key to the success of this process lies in the high temperature of the acid. We have evidence

that diffusion plays an important role, and diffusion coefficients in glass can change as much as three

or four orders of magnitude as the temperature goes from 25°C to 100°C. This perhaps explains the

negative results obtained by us and others when room-temperature acid treatments were used.

The effectiveness of the boiling nitric acid treatment is highly dependent on the polishing history of the sample. We have worked with several sets of ED-2 samples that have been polished either by

different compounds or in different polishing shops. The increase in threshold after the acid bath has

ranged from 0 to the factor of three discussed above. But the effect is repeatable within a given set

of samples that have all been polished in the same manner. Again, we are seeing the importance of details of the polishing process.

Since we suspect that diffusion plays a dominant role in the acid treatment, an increase in the

bath temperature might render the effectiveness of the treatment less sensitive to polishing details.

Therefore, rather than monitor very closely the polishing process in an attempt to find out what is

responsible for the differences in threshold, our next step will probably be to work at higher temperatures.

5. Conclusions

The surface damage threshold on commercially obtained laser glass is about I50 j/cm^ at 30 ns. This is at least an order of magnitude below the bulk threshold. Resvilts obtained by using very small

beaiiis to measure this bulk-to-surface ratio are questionable.

122
So-called superpolishing of laser glass is not a panacea for raising the threshold, and can even be

detrimental. Impurities introduced by the polishing process are the major cause of damage at JO ns, whether it be fron inclusions or electron avalanche. Hot acid treatment of laser glass can be very effective in increasing the damage threshold by removing these impurities and/or modifying the surface

layer. By paying close attention to polishing and siorface treatment, we have seen the entrance threshold vary between ^5 j/cm^ (on rouge -polished SD-2) and almost 5OO j/cm^ (on acid-treated ED -2).

Vfe wish to express our appreciation to

polished SD-2 sample.

Acknowledgement

IJ. Barnes of Itek Corp. for supplying the bowl-feed Cerox

7. References

[1] Bennett, J. M. and King, R. J., Applied Optics 9, 236 (1970).

[2] Olness, D., J. Applied Physics 39, 6 (I968).

[3] Swain, J., ASTI-1 Damage Symposim, I969,

Boulder, Colo., ASB.I STP 469, Ed. A. Glass

and A. Guenther.

[4] Personal coinmvinication with J. Swain, Lav/rence

Livermore Laboratories, Livermore, Calif.

[5] Personal coramunication with A. Feldman, NBS,

Gaithersburg, Md.

[6] Fradin, D. V/. and Bass, M. , Appl. Phys.

Letters 22, 157 (1973)-

[7] Bass, M. and Barrett, H. H. , IEEE J. Quantum

Electron. QE-6 , 338 (1972).

[8] Bloembergen, N. , Applied Optics 12, 661

(1973).

8. Figures

V

Figure 1. Small beam irradiating a surface volume Vg to measure surface breakdown

probability.

123
Figure 2. Small beam Irradiating a bulk volume Vg to measure bulk breakdown

probabilities.

Figure 3. Sample placed in center of beam waist to determine the surface threshold

relative to the bulk threshold.

124
564-378 O - 74 - 10

125
60DOo

4)

(U

y-iI iHU

0)

Ifl

T3

tuM3M

•H

126
0f

it

1 pm

Figure 6, Conventionally polished fused quartz.

127
1 pm

Figure 7. Conventionally polished fused quartz after acid etch.

128
129
Figure 9. Ed-2 laser glass bowl-feed polished with Cerox.

COmENTS ON PAPER BV EOLING

Tkz ipmktu vjca, cu>kzd horn ika ^u/ijjace loaghmii m<u ckoAxicJizfvizzd and koM MeJLl the p^e6ence OjJ 4u/L^(Xce -impuAyitizA W(X6 mzcauAzd -in thz^z zxpzAlmztvti . He commzntzd that quoLitaXivz mza^uAzd iuAdacz

KoughnzJ)6 inch oi mzoAuJizjmzyvtt, thz wot mzan iqaoAz dzv-taXion o{, thz iuA^^acz wzAz not uAzd, but that

a chaAactz/Uzatio n o^ noaghzA on. imoothzA wa6 obtcUnzd ioZzty on thz bai,ti vtMml impzction o^

zlzctAon mi.QfioQfux.phJi. W-iXh fizgoAd to iuA^acz impuAttizi iomz mlcAopAobz anaZtjiiA , paAtlzala/diij augzA

analyiii Mai uuizd, bat not to a vznij Ioaqz dzQAzz. NzvzAthzlzii , hz zmphajslzzd thz main point that

what ij> cfLuaial in dztzAmining thz damage /ie4'Ciiance o^ a iuAjJace iJ> not iimply thz iuA^acz fioaghnzM

,

no mattzfi how it ii zhoAaztzAtzzd, bat thz klitoAy o(, iafif^acz pAzpoAotion, ThzAz wa6 comidzAablz

diicuiiion o<) thz ctc() jJeAence bztwzzn potiihinQ meJJnoda which bfiing about a fizmoval 0|$ a iuA^acz tayzA

and thoiz mzthodi which mooth a iuA(,acz by mzam o{, ploAtic ^low and thz cxzation o{, a iubiuA{,acz layzA

o(, dzizctA, and impufiUxzA, . It i^ to bz zxpzctzd that in thz {^ofmzA coMz, highzA damagz wilt bz obizAvzd,

Uikz Bom o{, thz UntvzAiity ojj SouthzAn CaZi{^oftnia, o{^{<,zAzd iomz cZaAi(^ying commznti with AzgaAd to hij>

wofik which had been Az{,zAAzd to in Boting'i talk. BaM commzntzd that thzy took Qfizat paiyn to iniuAz

that thziA itudizi wzAz caAAtzd out on imaJit axzoM in ofidzA to avoid thz ambiguity o{, loAgz oazjx

iAAadiation. Thzy (^ound, hz iaid, that a iigni^icant di{i^zAzncz woi obizAvzd in thz moAphotogy

4a/L()ace damagz bztwzzn iuAf^aczi potOshzd by convzntionat mzthodi, which zxhibitzd lowzA thAziholdi , and

thoiz potidhzd by io-cattzd iupzA potvihing tzchntquzA, which zxhibitzd highzA thAZiholdi. Hz

zmphoiizzd that in thziA woAk thz tzAm fioughnzii oA imoothnz&i OjJ thz {iUA{^acz fiz{,ZAAzd to imalZ icalz

z{i(,zct& obizAvzd on imoLt itiuminatzd aAzai. Hz indicated that it woi cAuciaJL in obtaining conAiAtznt

fLZiuZti to thz zxpzAtmznti that thz beam iizz at thz damagz iuA^^acz bz kzpt comtant, that thz ^ocu^ing

gzomztty bz maintainzd. To thii znd, ihoAt {,ocat length Izn^zi wzA-Z zmployzd.

130
3.2 Damage in Nonlinear Optical Materials at 1.06 vm: ^ Surface Treatment of LiNbO, in Ar-0, Plasmas and High Pressure 0, Environment

John C. Potosky and Concetto R. Giuliano

Hughes Research Laboratories

3011 Malibu Canyon Road

Malibu, California 90265

and

Christopher F. Klein

Hughes Aircraft Company

Culver City, California 90230

Surface damage in LiNbOj has been studied at 1.06 ym as a function of different

surface treatments. While the use of ion beam polishing on LiNb03 has resulted in

general degradation of surface finish and decrease in damage resistance, a different

type of plasma treatment shows a distinct improvement. This method of surface treatment is one in which the sample is bombarded in a low energy rf-excited plasma of

argon and oxygen. Under this type of plasma treatment, the damage resistance is substantially improved 50%) over that of the conventionally polished surface of the

same sample. To avoid complications of possible cumulative effects in the damage

experiments, each point on the surface is laser irradiated only once at any given

power. The results are presented in terms of the fraction of surface which resists

damage for a given incident energy.

That this improvement in threshold involves a surface-oxygen effect is supported

by another series of preliminary measurements in which damage tests were performed on LiNb03 at 10 to 15 atm Oo pressure. Under these conditions, the surface was usually

seen to completely resist damage at levels of irradiation at least twice that at which

damage occurred at ambient conditions or in a high pressure nitrogen environment.

Keywords: LiNb03; 1.06 ym; oxygen deficiency; surface damage; surface treatment.

1. Introduction

The relatively low 1 aser -induced surface damage threshold of lithium niobate, LiNb03, poses stringent limitations on its use as a Q-switch or second harmonic generator. However, its large electrooptic tensor elements and its nonhygroscopic nature render it one of the most useful Q-switch crystals.

Thus, a method for increasing its surface damage threshold would certainly be valuable.

It has been reported [1,2]^ that the surface of LiNb03 is apparently deficient in oxygen although

there are various speculations as to its exact role in the damage mechanism. This deficiency is hypothesized to result in greater absorption and thus a lower damage threshold at the surface than in the

bulk material. Experimental verification of this hypothesis is presented herein.

It has been found that surface damage in LiNb03 can be significantly reduced by supplying an excess

of oxygen utilizing either of two methods. One method consisted of exposing the LiNb03 to a low energy

rf excited plasma of argon and oxygen. This appeared to supply an excess of oxygen at the near surface,

resulting in a residual increase in the surface damage threshold. The other method consisted of sur- rounding the sample with pressurized oxygen, also resulting in a significantly increased surface damage

threshold. However, when the sample was removed from the chamber, no residual effects were observed.

This work was supported in part by the Defense Advanced Research Projects Agency through Air Force

Cambridge Research Laboratories.

Figures in brackets indicate the literature references at the end of this paper.

131
2. Surface Damage Threshold Measurement Procedure

The technique employed for the determination of the damage thresholds in these experiments was developed to provide a quantitative threshold value and to detect small changes caused by the surface

conditioning experiments. It was realized that there are several damage mechanisms in simultaneous

operation, some of which are probabilistic in nature. Also, damage in LiNb03 has been reported to be

complicated by multiple shot cumulative effects. [1] For this reason, single pulse damage thresholds

were sought. In addition, previous experience had shown that sample crystals of LiNb03 available had

significantly high densities of inclusions which obscured measurements of the intrinsic surface damage

threshold

.

It was assumed that each point on the surface was characterized by a damage threshold value lying

between a power density below which no point on the surface would ever be damaged, and one above which

every point on the surface would always be damaged. Thus a surface would have a range of damage thresholds which could be measured by firing the laser one time at a particular site, looking for the occur- rence of damage, and moving to another site while keeping the laser output constant. A series of such

measurements taken over the range of damage thresholds yields the probability that the single shot

damage threshold lies below a given power density.

3+

In' order to determine the surface damage threshold for a sample, a single shot Nd :YAG laser

operating in the TEMqq mode at 1.06 ym was focused through a 3.3 cm lens at the entrance surface of the

LiNb03 sample as shown in figure 1. The laser had a pulsewidth of 17.5 nsec (FWHM) and a focused spot

size of 64.5 ym (1/e diameter for intensity). The output was varied by rotating crossed polarizers.

The damage site* was observed with a microscope focused at the LiNb03 entrance surface utilizing an annular mirror oriented at 45° to the laser axis. The laser output was monitored with a beam splitter

and an ITT biplanar photodiode whose output was electrically integrated and displayed on an

osci 1 loscope.

3. Ion Bombardment Treatment

The threshold for laser induced surface damage in optical materials is determined in part by the

physical condition of the surface. [3] Previous work by Giuliano [4] has shown that bombardment of

sapphire crystal surfaces with energetic Ar+ ion beams resulted in a substantial increase in the surface

damage threshold over that of conventionally polished surfaces. Subsequently, attempts were made to similarly condition LiNbO^ surfaces.

A series of experiments was performed to determine whether the surface of LiNb03 could be made more resistant to laser damage. One mechanically polished surface was subjected to various conditioning

plans in which time, energy, and current density of an incident Ar"*" ion beam were varied. The damage

threshold of this surface was then measured and compared with that of another surface of the same crystal which had not been ion bombarded. The results showed that the ion bombarded surfaces were usu- ally slightly more resistant to damage, although the increase in threshold was insignificant.

Considering the surface deficiency of oxygen in LiNb03, an attempt was made to condition the sur- face by oxygen ion bombardment and increase the surface oxygen concentration. A surface was sputtered

in an MRC rf plasma sputter system. The plasma pressure was 4 mTorr argon with 1 mTorr oxygen. The

surface was sputtered for 30 min at 300 V bombardment energy. When the damage threshold of this surface

was compared with that of an untreated surface on the crystal, it was found that a significant ('^^ 50%)

increase resulted. The procedure was repeated on several crystals with similar results as summarized

in figure 2. The time between sputtering and damage testing varied from 30 min to several weeks, indicating a retention of oxygen by the surface. From the data shown in figure 2 it can be seen that the

treated and untreated surfaces have the same low power damage behavior, but differ significantly for

high incident fluxes. Thus for the treated surface, the power density above which damage is observed on every shot is about 50% higher than for the untreated surface. This is interpreted to mean that the

damage resistance is improved in the more resistant areas of the surface, (i.e., inclusion-free regions)

while the treatment has no effect in improving the less resistant areas of the surface (included regions).

It was found that the power level below which no damage was ever observed was essentially a constant. However, the level above which damage was observed on every shot varied significantly from

sample to sample and from conditioned to unconditioned surface on a particular sample.

Damage is defined here as being characterized by the appearance of a small crater or pit generally

accompanied by a plasma spark.

132
4. Pressurized Oxygen Treatment

For the damage tests performed in a pressurized oxygen environment, a windowed chamber was built to house the sample. The surface damage threshold measurement technique is iden*tical with that outlined

above, except for the addition of the chamber (see figure l). With the chamber open to the atmosphere,

damage threshold measurements were made. The chamber was then pressurized with oxygen to 250 psig and

the same procedure repeated. In the 100 sites on 5 different crystals from one vendor, some of

which were uncoated and some ThF4 coated, surface damage was rarely detected with the crystals in

250 psig oxygen. Only internal damage was usually seen at a level of approximately twice that for sur- face damage. However, crystals grown and polished by others did show more surface damage in the oxygen

environment substantiating the need for high quality polishing techniques. [3] In addition, some sur- faces coated with materials other than ThF^ showed significant surface damage. This suggests that ThF^

may be porous enough to allow oxygen to diffuse through it in order to be in contact with the LiNb03

surface.

One important difference was noted between surface damage observed in an oxygen environment and

that observed without. Without oxygen, surface damage rapidly avalanched to the catastrophic level. In pressurized oxygen, a surface damage site may begin to form, but it either becomes no worse with additional shots or worsens only slightly. It was never seen to rapidly avalanche into catastrophic damage,

as such a site would always do without oxygen.

In order to verify that the oxygen was responsible for the changed surface damage characteristics

of LiNb03 rather than some other effect due to the presence of pressurized gas, nitrogen was substituted

for the oxygen. The pressurized nitrogen had no discernable effect upon the surface damage characteristics. As an additional test, another material, yttrium vanadate, YVO^, was substituted for the LiNbO,

in the pressurized oxygen with no perceptible change in the surface damage characteristics.

To determine the minimum pressure and exposure time required to produce an observable effect upon

the damage threshold, these two parameters were individually varied. Minimum pressure appeared to be

50 psig. Below this level, no observable improvement was detected. Above 100 psig, no additional

improvement was seen, but the remaining tests, however, were conducted at 250 psig in order to be

assured of maximum benefit. The time during which the sample was exposed to pressurized oxygen prior to being irradiated was varied from a few minutes to 24 hours with no perceptible difference. When the

sample was removed from the oxygen, no residual damage resistance was observed.

5. Conclusions

The surface damage characteristics of LiNb03 are significantly altered by the presence of oxygen.

An increased surface damage threshold resulted as well as a marked tendency away from the usual rapid

buildup of a damage site with each additional shot.

Two methods for introducing excess oxygen at the LiNb03 surface were used. One method — bombarding

the surface with an argon-oxygen plasma - had the residual effect of increasing the damage threshold

after removal from the plasma. The other — immersing the sample in pressurized oxygen - had no residual

effect. While in the oxygen, however, the surface damage threshold was increased and the tendency for a damage site to avalanche into catastrophic damage with additional shots was significantly reduced.

Certainly, additional work must be performed in order to more quantitatively describe the effects

of oxygen on the surface damage threshold of LiNb03. However, these results clearly indicate that it is

significantly improved by the addition of surface oxygen.

6. Acknowledgments

The authors wish to express their appreciation to Dr. Eric Woodbury for discussions which led to the investigation of pressurized oxygen treatment of LiNb03 and to Dr. H. Garvin for guidance and

assistance in the sputtering processes.

7. References

[1] G. M. Zverev, et al., "Laser-Radiation-Induced

Damage to the Surface of Lithium Niobate and

Tantalate Single Crystals," Soviet J.Q.E. 2[2]

(1972) pp. 167-169.

[2] J. 0. Porteus, E. A. Teppo, and J. H. Dancy,

"Characterization of Polished LiNbOs Surfaces

Using Auger Spectroscopy and Transmission

Electron Microscopy," High Energy Laser

Window Conference, Air Force Cambridge

Research Laboratory, Hyannis, Massachusetts,

12-14 November 1973.

[3] N. Bloembergen, "Role of Cracks, Pores, and

Absorbing Inclusions on Laser Induced Damage

Threshold at Surfaces of Transparent Dielectrics," Applied Optics 12 [4] (1973)

pp. 661-664.

[4] C. R. Giuliano , "Laser-Induced Damage in

Transparent Dielectrics: Ion Beam Polishing

as a Means of Increasing Surface Damage

Thresholds," Appl. Phys. Lett. _21 [1]

pp. 39-41.

133
8. Figures

Si PHOTODIODE

GROUND GLASS

MIRROR

LENS

NdiYAG LASER WITH

ELECTRO-OPTICAL

Q-SWITCH

ALIGNMENT LASER

(HeNe)

INSPECTION

MICROSCOPE

BEAMSPLITTER / ^HR MIRROR

2 mm APERTURE

PRESSURE

CHAMBER

Figure 1, Equipment arrangement for determining laser surface damage

thresholds.

1.0

i 0.9

I-D

CQ- 0.8

W

Q

D

0.7

O

X

CO

LU

DCI

HLLI O

<

0.6

0.5

0.4

< 0.3

Q

LUS3 0.2

DC

00 0.1

o

o

-oo

o

X - UNCONDITIONED

SURFACE

O- CONDITIONED

SURFACE

1000

INCIDENT POWER DENSITY, MW/cm^

3301-2

2000

Figure 2. Laser surface damage threshold distributions for unconditioned

and oxygen ion conditioned surfaces of LiNbOa.

MO COm.iENTS ON PAPER 8/ POTOSKY.

134
3.3 Surface Characteristics Related to Laser Damage of

Lithium Niobate and Potassium Chloride Surfaces*

J. 0. Porteus, E. A. Teppo, and J. H. Dancy

Michelson Laboratory, Naval Weapons Center

China Lake, California 93555

Laser damage of transparent materials is promoted by irregularities in

topography and surface composition. Changes in these characteristics following

laser irradiation can provide insight on damage mechanisms. Transmission electron micrographs of replicated LiNbO^ surfaces which have been chemically etched

show submicroscopic pitting similar to that previously observed following laser

irradiation. Experiments designed to explore a possible role of surface-acoustic

waves in the laser-induced pitting are in progress. Auger analysis of coated

LiNbOj surfaces following visible laser damage have helped to identify redistributed surface material. Profiling of the coating shows evidence of C contamination layers near the substrate surface. Preliminary Auger spectra of KCl surfaces show promise for similar studies on this material.

Key words: Auger electron spectroscopy; ion beam profiling; laser- induced

damage; lithium niobate; potassium chloride; surface acoustic waves; surface characterization; transmission electron microscopy.

1. Introduction

The results reported here represent recent progress in a continuing program on the surf ace-physica

characteristics of laser optical materials. [1,2]' An understanding leading to better control of laser

damage mechanisms is sought via (1) detecting undesirable initial characteristics, such as surface

irregularities or contamination, which are the result of improper preparation and handling, and (2)

analysis of characteristics induced or altered by laser irradiation. Submicroscopic topography is moni

tored by transmission electron microscopy (TEM) . Surface composition and material distribution are determined by Auger electron spectroscopy (AES) , including ion beam profiling. Previous work in this

program has concentrated mainly on uncoated LiNbO surfaces, where a laser-induced submicroscopic pitti

effect was observed. [1,2] The present paper deals with further efforts to understand this effect and

Auger analyses of visibly damaged, coated LiNbO ^ surfaces. Preliminary work on uncoated KCl surfaces

was performed to demonstrate the applicability of AES methods to this material.

2. Experimental Apparatus and Technique

The TEM measurements were performed with a JEOLCO Model 120U electron microscope, using a twostage replication procedure. [2,3] The resolution of the resulting micrographs is limited by the grain

of the replica to approximately 50 A. The AES measurements were performed using conventional cylindrical mirror analyzer systems (Physical Electronics Industries, Model 5000; Varian Associates, Model 981-

2607). [2,4] One of these (PEI) is equipped with a multiplexor and sputter-ion gun for ion beam profiling. [5] To minimize surface charging the electron probe beam was inclined 60° relative to the

sample normal and typically operated at a primary energy of 2 KeV. Lateral resolution, as determined

by the probe beam cross section, is 300 pm or better. Analyses were normally performed in an unbaked

ion-pumped system operating in the low 10~*^ Torr range. Profiling was done at 5 x 10"^ Torr Ar using

a 2 KeV ion beam.

3. Samples

Uncoated LiNbO^ samples were kindly supplied by Isomet Corporation. These are of the form conventionally used for laser Q-switch modulation. Dimensions are 8 x 8 x 24 mm with the long dimension

parallel to the z-axis. The ends have been optically polished using a variety of polishing compounds

*This work was supported by the Defense Advanced Research Projects Agency and by Navy Independent

Research Funding.

1. Figures in brackets indicate the literature references at the end of this paper.

135
including AlgO^, diamond dust, SiC in glycerol and Syton. Two coated LiNbO^ samples showing visible

laser damage were provided through the courtesy of C. F. Klein of Hughes Aircraft Corporation. The

coating on one of these is proprietary; available information suggests ThFi^ or ThOF2 with a protective

overlayer of Al-containing material, possibly AlgO^. The coating on the other sample, which was ana- lyzed by profiling, is nominally ThOF^. Both coated samples have the usual gold electrodes used to apply a modulating field parallel to the x-axis. The KCl samples consist of a freshly cleaved single

crystal and a polished Harshaw Econoflat.

4. Uncoated LiNbOj

A.l Submicroscopic Pitting

It has been demonstrated that exposure of uncoated polished LiNbO^ to laser power densities as low

as 12 MW/cm^ can lead to submicroscopic surface pitting. [1,2] The pits are ~ 100-500 X in diameter with

depths in excess of twice the diameter; they are not confined to the directly irradiated area, but may

extend over the entire polished surface. The pit density and power density required for formation appear

to be strongly influenced by topographical irregularities and quality of surface finish. The significance of this effect for catastrophic damage is that the pits may act as direct absorption sites for

energy from the laser beam. [2]

4.2 Energy Transport Mechanism

An explanation of the nonlocalized pitting effect requires a mechanism for transporting energy

along the surface. One possibility is the scattering of light by dust or inhomogeneities in the surface. [6] Another mechanism is suggested by the excellent propagation characteristics of LiNbO^ for

surface acoustic waves [7], combined with the possibility of exciting these waves by laser irradiation.

[8]

Two experiments designed to test for these mechanisms are now in progress. One of these involves

deliberate generation of surface acoustic waves on a LlNbOj surface and subsequent inspection for pits.

The sample in this case is a YZ-cut acoustic-wave delay line having Al interdigital transducers at either end for sending and receiving acoustic signals. [9] An unexpected problem was immediately

encountered with the procedure used to mount the transducers. An etchant (Electrotec #CAE-600) customarily used to remove the excess Al was found to produce a submicroscopic pitting which is more dense,

but otherwise essentially indistinguishable from that observed following laser irradiation. This may

be seen by comparing figure 1 with figures 8-10 of reference 2. Etching between the transducers can be

avoided by a modification of present procedures, as will be done in future experiments. Nevertheless,

pit formation by an alternative means emphasizes the importance of carefully monitoring sample preparation procedures.

The second experiment is illustrated in figure 2. One end of the uncoated LiNbO^ sample is slotted

with a wire saw as indicated, dividing the surface into four areas designated by letters. The sample is

irradiated through area B using a collimating mask. The surface is then examined by TEM for submicroscopic pitting in each of the four areas. Pitting confined to area B alocie would support the acoustic wave mechanism, since the slots effectively block the progress of acoustic waves along the surface.

Pitting in other areas, on the other hand, would support an alternative mechanism, such as that involving scattered light. Initial studies show slight evidence of pitting in area D, as well as B. However,

in view of the weakness of the effect and the experience with the acoustic delay line it is possible

that the pitting may have been present before irradiation and was overlooked in the control examination.

The experiment must therefore be repeated to establish confidence in the result.

5. Coated LiNbOj

5.1 Sample With Proprietary Coating

The previously described sample with a proprietary coating was analyzed with AES in an effort to identify visible changes in the surface outside the damage site. An optical micrograph of the damaged

portion of the surface taken after analysis is shown in figure 3. A crater and surrounding peripheral

damage area appear as a roundish black spot to the lower left of center. Irregular, lighter-than-average

spots spaced at regular intervals across the field are the result of coating discoloration by the Auger

probe beam. This imaging of the beam permits accurate determination of the analyzed area. A large,

slightly darkened area, made more evident by contrasting surface scratches, covers most of the lower

half of the field. Another large area approximately 2 mm wide, part of which is visible on the extreme

right-hand side of the field is relatively clear (undarkened) . However, the darkening again occurs further to the right, out of the field of view.

Auger spectra were obtained at the damage site and at 1 mm intervals along a line passing through

that position. Normalized peak-to-peak intensities are plotted in figures 4-6. The positional relationship to visible surface features is indicated at the top of these figures. The elements represented

136
in figure 4 show a maximum Auger intensity in the clear sample area, while those represented in figure 5 generally show a minimum here. Figure 6 shows the results for the C and N peaks which do not fall obviously into either category. Strangely, no Th peak was observed, evidently because of the protective Alcontaining overlayer and the shallow sampling depth. The F atom either diffuses to the surface because

of its relatively small atomic size or is a normal constituent of the overlayer. Unfortunately, we did

not have the opportunity to profile the sample in order to clarify this point.

On the basis of these results the clear area of figure 3 represents undisturbed coating, while the

large darkened areas represent overlying material consisting mainly of compounds containing Nb , S, and

CI. The origin of the last two elements is not certain. The association with Nb suggests a relationship of S and CI to the damage event, i.e., as coating impurities ejected from the crater. However,

without either profiling studies or repeating the damage in a controlled environment, the possibility of

preferentially adsorbed atmospheric contaminants cannot be excluded. Judging from the maxima in Nb and

0, and the minima in Al and F signals near the crater, a substantial area of LiNbOj is exposed here.

The failure of the maxima to coincide exactly with the crater position is attributed to surface roughness within the crater area. The roughness suppresses both secondary electron emission and Auger emission. The suppressed secondary emission promotes surface charging which may also influence the observed

Auger intensity distribution via deflection and defocusing effects. Except for a minimum in the crater area due l;,o roughness, the intensity of the C signal is essentially constant over the surface, as might

be expected from superficial contamination occurring after the damage event. The N signal, on the other

hand, shows a single pronounced maximum near the crater, attributed to the formation of nitrogen com- pounds in a spark which accompanied the damage.

5.2 Sample With ThOF2 Coating

The damage on the second coated sample is shown in figure 7. The damage pattern here is much more

complex, and shows a definite periodicity of AO um in the direction of the modulating field. The origin

of the periodicity has not been definitely determined. The facts point to spatial modulation of the

light intensity by interference, possibly a result of misaligned optical surfaces. Intense monochromatic

surface acoustic waves are also excited by optical interference [8] and may have an effect on the plasma

distribution via their associated electric fields. The excitation of surface acoustic waves of the proper frequency (~ 100 MHz) by the Q-switch modulating field is ruled out by its slow rise time (~ 300 nsec)

.

The depthwise composition of the damaged surface was investigated by profiling at selected points

along the surface. It was found that certain areas within the damage pattern, such as the partially

clear circular area near the bottom center of figure 7, are intact except for an overlayer of ejected

substrate material. Cratered areas near the center of the pattern show spectra characteristic of bare

LiNbOj, whereas those near the edge of the pattern are covered with a thin layer of coating material.

This indicates more rapid removal of material from the original coating than from the bottoms of the

craters. Either the craters are more shielded, possibly by plasma, or the original coating is inherently more absorbing, possibly due to impurities.

An attempt was made to detect light-absorbing impurities which may have led to coating damage by

profiling an undamaged area on the opposite end of the sample. The results are shown in figure 8. The

initial turn-on of the electron probe beam in the presence of Ar results in rapid removal of superficial

C and an increase in signal from the coating constituents Th, 0 and F. When the ion profiling beam is

turned on the spectrum shifts due to a change in surface charge, which necessitates retuning the multiplexor. Resumption of the scan with both beams on provides a measurement of the relative concentration

vs depth for each element monitored in the remaining portion of the coating. The profiling rate is

estimated at approximately 35 A/min.

A significant feature of the results for laser damage is the marked rise of the C peak at the

coating-substrate interface, indicating C contamination of the LiNbO^ surface. Another significant feature is contamination of the coating, as indicated by oscillations in the profiles. The prominent maxima in the C profile, which coincide with minima in the Th profile, are produced by inhomogeneities where

C replaces Th to a large degree. These C-rich layers also appear to be abnormally rich in F at the

expense of the 0 content. The marked predominance of C as a contaminant, both in the coating and at the

coating boundaries, strongly suggests its involvement in absorption of energy from the laser beam.

6. KCl Samples

The feasibility of similar Auger studies on KCl laser window samples was investigated by preliminary AES scans on this material. Figure 9 shows the spectra from the Harshaw Econoflat. In addition to the peaks due to K and CI, peaks from 0 and Na were observed. Spectra obtained from a freshly cleaved

control sample were similar, except for the absence of the Na peak. Prolonged exposure to the Auger

probe beam was observed to reduce the intensity of the CI peak relative to that of K, indicating some surface decomposition. [10] A related problem is the tendency toward surface charging, which is promoted by the reduced secondary emission coefficient of the K-rich surface. However, decomposition

proceeds at a sufficiently slow rate that it is not considered to be a serious problem.

137
7. Summary and Conclusions

Submicroscopic pitting, which has been observed on uncoated LiNbO^ surfaces following laser irradiation insufficient to cause visible damage, can also be produced by chemical etching. Experiments

designed to explore the role of surface acoustic waves in the laser-induced pitting have not yet yielded

conclusive results. Auger analysis of a coated LiNbO^ sample with visible laser damage shows large areas covered with Nb, CI and S, which appear to be related to the damage event. Auger studies on a second

coated sample show a thin layer of coating material within craters lying near the edge of the damaged

area, indicating preferential destruction of the original coating. Profiling shows evidence of Ccontaining layers both within the coating and at the substrate interface, suggesting that C plays a major role in surface absorption and damage. Preliminary Auger spectra of KCl surfaces show promise

for laser damage studies on this material.

8. References

[1] Porteus, J. 0., Defense Advanced Research

Projects Agency, ARPA Order 2175, SemiAnnual Report No. 3, Apr. 1973-Sept. 1973,

pp. 87-104; Porteus, J. 0., Teppo, E. A.,

and Dancy, J. H. , Defense Advanced

Research Projects Agency, ARPA Order 2175,

Semi-Annual Report No. 4, Sept. 1973-

Mar. 1974, pp. 84-94.

[2] Porteus, J. 0., Teppo, E. A., and Dancy,

J. H. , Proceedings of the Third Conference on High Power Infrared Laser Window

Materials, Hyannis, Mass., 12-14 Nov.

1973, pp. 947-964.

[3] Murr, L. E., Electron Optical Applications in Materials Science (McGraw-Hill,

New York, 1970), Section 5.3.3.

[4] Bauer, E., Vacuum 22, 539 (1972).

[5] Morabito, J. M. , Thin Solid Films 19, 21

(1973)

.

[6] Emmony, D. C, Howson, R. P., and Willis,

L. J., Appl. Phys. Letters 2^, 598 (1973).

[7] Slobodnik, A. J., Appl. Phys. Letters 14, 94

(1969) .

[8] Cachier, G. , Appl. Phys. Letters 17, 419

(1970) .

[9] White, D. J. and King, R. L. , Defense Advanced

Research Projects Agency, ARPA Order 2175,

Semi-Annual Report No. 3, Apr. 1973-Sept. 1973,

pp. 114-122.

[10] Palmberg, P. W. and Rhodin, T. N., J. Phys.

Chem. Solids 29, 1917 (1968).

9. Figures

Figure 1. Transmission electron micrograph of testing for surface acoustic wave pitting

pitted LiNbOs acoustic delay line after applic- mechanism. Laser irradiation is at

ation of Al etch. Compare with micrographs of normal incidence through a mask which ex- laser-induced pitting in references 1 and 2„ poses a 2-mm diameter circle in area B.

138
Figure 3. Optical micrograph of laser-damaged Figure 4. Auger intensity distribution across portion of a coated LiNbOa surface after Auger a damaged, coated LiNbOs surface for those

analysis. The black spot is the damage crater, elements included in the coating material,

while excessively light spots are images of the Auger peaks used are identified by element and

Auger probe beam. by energy in eV at the right.

EDGE ^CRATER

/^PERIPHERAL DAMAGE

EDGE

CLEAR AREA

Nb(167)

2 4 6 BEAM POSITION (mml

Figure 5. Figure corresponding to figure 4 for elements probably ejected from the damage

crater.

1.0

5 0.8

Vi a z 0.6

uj tu N I- <S 0.4

ocO

z

0.2

T 1 1 1 1 r ^CRATER

EDGE /^PERIPHERAL DAMAGE

CLEAR AREA

EDGE

0(272)

. N(381)

_1 i I L. BEAM POSITION (mm)

Figure 6. Figure corresponding to figure 4 for contaminants originating mainly from

the sample environment.

139
Figure 7. Laser-damaged ThOFa-coated LlNbOs

surface. Note the periodic damage pattern

consisting of vertically elongated damaged

areas which are evenly spaced in the horizontal direction.

a. <

cr

0.4

0.2

I Nb(167)

60 70 ELAPSED TIME (min.)

AUGER ELECTRON ENERGY (eV) 200 400 600

dN(E)/dE

1 1

CI K

I

1 1 1 \ —- POLISHED KChHARSHAW

0 V V—

Na

0

_L 1 1 1 J 1 1 1 1 600 800 1000 1200 AUGER ELECTRON ENERGY (eV) 1400

Figure 8. Ion beam profile of ThOFz-coated

LiNbOa surface using Auger analysis. Curves

on the left show removal of a superficial C layer by the Auger probe beam before ion

profiling has begun.

Figure 9. Auger spectrum from polished

KCl window material.

COMMEMTS OH PAPER BV PORTEUS

Th^ 6pe.akeA. comme.nte.d on tkz idm-itlv^y o{, Augtfi analyi-Oy to iuA^^ace. de.{ie.cX& and -LmpuAAJxeA and

quottd KuaZi^ {^fiom an e,xp&A^me.nt dom w-ith an atiminum -LmpuAAXy on a coppsA. i,uA{,ac2. QmeAato^d by po-tLihInQ the. coppeA maMi aluminum oxldz. Llitng thz AugeA. anaZyiti,, an ImpanAjy tdvdt about: WO nanogfiami,

poji iquanz czntxmeJieA coutd be dztzcttd. We commmtud {^uAtheA that the. AugeA pKobe only penetAoteA the

iuA{,ace to a depth OjJ about 10 A and theAe(tOn.e -it Zi appnopnAote to chanacteAlzz aM, ^emttivtty -in teAm

o{, the {in.actA.on o{, the iuJi^ace tayeA covered by the -mpafvity. In theAe teJim, In the a{,on.ementioned

ex.peAA.ment, a i,en!>'itivtXy o{, about 0^1% iuA^ace coveAage by the aJLumtnum impufvitiy on coppeA coutd be

deZectedo fftitz ZeAnlcke o{ PeAlvin-EtmeA Hjoi^ed the point that the p-ittlng o(^ the 6uA(,ace may not be due

enJuAeZy to the etch uied to n.emo\je the aluminum coaXing {,n.om the i,uA{,ace, but to the onZgtnal atumtnum

coating pwccii. He iaid in expeAAmenti that he had coA/vizd out, no iuch pitting axxi obieAved, when i>un.- j^acei weAe coated uiith manganeAe. The ipeakeA indicated that he had not investigated thZi point.

lAO
4 . 1 Characterization of Infrared Laser Window Materials

at the National Bureau of Standards*

A. Feldman, I. Malitson, D. Horowitz, R. M. Waxier, and M. Dodge

National Bureau of Standards, Washington, D. C. 20234

A program has been established for measuring refractive index, n, stressoptical constants, q. ., change of index with temperature, dn/dT, thermal

expansion coefficient^ a, and elastic compliances, s. ., of infrared laser

window materials. These parameters are necessary for"'determining the optical

distortion that occurs in windows due to heating by the absorption of high

power laser radiation, n and dn/dT are measured over a spectral range 0.2

to 50 ym by the method of minimum deviation on precision spectrometers.

Twyman-Green and Fizeau interferometers, which operate at 0.6328 ym,

1.15 ym, and 10.6 ym, are used for measuring q. ., a, dn/dT and s. .. Materials

currently under study are polycrystalline ZnSet''As2S3 glass, chai^ogenide

glass (Ge 33%, As 12%, Se 55%), and KCl. Results are given for n and dn/dT

in KCl, and q^_. and si2 ir' ZnSe, AS2S3 glass, and chalcogenide glass.

Key words: AS2S3; chalcogenide glass; coefficient of thermal expansion; elastic

constants; infrared laser window materials; KCl; photoelasticity

;

polycrystalline ZnSe; refractive index; stress-optical constants;

thermal coefficient of refractive index.

1 . Introduction

Laser systems of high average power are currently under development. As the output power

increases, the optical components in these systems undergo optical and mechanical distortion due

to absorptive heating. If the distortion is sufficiently severe, the optical elements become

unusable. The parameters necessary for determining this optical distortion [1,2]'^ are absorption

coefficient, refractive index, change of index with temperature, thermal expansion coefficient,

stress-optical coefficients, elastic compliances, specific heat, thermal conductivity, and density.

The temperature change that occurs in a laser window due to absorptive heating will produce

three effects: (1) an expansion, (2) a direct change of refractive index, and (3) a system of

internal stresses due to thermal gradients, which changes the refractive index through the stressoptical effect.

Although some information is available [3,4] on thermal expansion and change of index with

temperature of laser window materials, there is very little data on the stress-optical effect [5]. In the past stress-optical coefficients have been measured in the visible region of the spectrum

using interference methods [5,7]. We have now constructed Twyman-Green and Fizeau interferometers

for making these measurements at 1.15 ym and 10.6 ym as well as at 0.5328 ym. This is the first

time that interferometers have been used for measuring stress-optical coefficients in the infrared.

These interferometers are also useful for measuring thermal expansion, change of index with

temperature, and elastic compliance coefficients.

In this paper we present the methods used for measuring refractive index, n, change of index

with temperature, dn/dT, thermal expansion coefficient, a, stress -optical coefficients, qj_-; , and

elastic compliances, s. ,. We also report on results obtained on AS2S3 glass, a chalcogenide glass

(Ge 33%, As 12%, Se Sskj , polycrystalline ZnSe, and single crystal KCl.

* This research was supported in part by the Advanced Research Projects Agency of the Department

of Defense.

^Figures in brackets indicate the literature references at the end of this paper.

lAl

564-378 0-74-11
We discuss measurements of n and dn/dT by using precision refractometry methods in section 2. The

quantity dn/dT is obtained near room temperature by measuring n at 20°C and at 30°C. Results are reported for KCl.

In section 3 we discuss the measurement of thermal expansion and dn/dT up to a temperature of 800°C.

These measurements involve the use of Fizeau fringes.

In section 4 we discuss the measurement of stress-optical coefficients and elastic compliances.

In isotropic materials there are two independent elastic constants and two independent stress-optical

coefficients. The use of Twyman-Green and Fizeau interferometers in conjunction with samples under

uniaxial and hydrostatic compression supplies us with sufficient data for obtaining all the constants.

In section 5 we present the results of measuring qj_j and si2 on the chalcogenide glass, AS2S3

glass, and polycrystalline ZnSe..

2. Refractive Index and Change of Index with Temperature

In the search for potential window and auxiliary optical component materials for high-power

lasers, KCl appears to be one of the most promising candidates. It has useful transparency from

'V 0.2 to 20 ym (with 90% transmittance between 0.45 and 14 ym) . This covers window utilization in

both the ultraviolet and infrared.

2.1 Index Data

A KCl prism cut from a commercially grown crystal was used in this study. The prism faces are

10 X 10 mm and the refracting angle near 60° . The refractive index was measured by means of the

classical minimum deviation method with two precision spectrometers. The spectrometer schematic

shown in figure la is designed for visible region refractometry and is capable of accuracy of several

parts in 10°. The spectrometer schematic shown in figure lb employs mirror optics and is used in the

non-visible spectral regions. It is capable of accuracy of several parts in 10^. Both instrimients

and procedures for high-precision refractive index measurements have been described in previous

publications [8,9].

Refractive index measurements were made at controlled room temperatures near 20°C at 60 wavelengths from 0.22 pm in the ultraviolet to 14.3 ym in the infrared. The wavelengths were selected

from the spectra of emission sources of Hg, Cd, He, Cs and Zn, and from calibrated absorption bands

of H2O, polystyrene, and methylcyclohexane . A series of calibrated narrow-band filters was also used

in the infrared in the 3.5 to 10.5 ym region.

The experimentally determined index values at 20 °C for KCl are represented by the solid curve in

figure 2. The refractive index decreases from a value of 1.5360 at 0.224 ym to 1.4374 at 14.3 ym, an over-all change of 2 x lO"'^. At 10.6 ym the value of the refractive index is 1.4542 at 20''C.

The refractive index measurement program is being supplemented with a compilation of critically

evaluated refractive indices of key laser materials . A literature search has turned up fourteen

published papers dealing with refractive index of KCl. The data cover the wavelength range from 0.16

to 220 ym and were obtained by a variety of experimental techniques. It is interesting to note that

in the spectral regions of interest to laser designers, the most accurate measurements were made in

1901 [10] , 1908 [11] , and 1927 [5] . The crystals measured were natural salt, and the minimis deviation

method was employed with the sample in prism form. The excellent agreement between the NBS and the

early data is shown in figure 2. The NBS values are lower in index by an average value of 2 x

10"**.

2.2 Thermal Coefficient of Index

Refractive index measurements in KCl were repeated at 22 selected wavelengths at a controlled room temperature near 30°C. The refractive index values at 20°C and 30°C were used to calculate the thermal

coefficient of index, dn/dT. The dependence on wavelength is shown in figure 3. The value of dn/dT

varies from -2.1 x 10"^ K~^ at 0.27 ym to -2.5 x 10"^ K~^ at 14.3 ym; dn/dT remains relatively constant

at about -3.0 x 10"^ k"-"^ in the infrared between 3 ym and 5,0 ym. The fact that dn/dT is negative indicates that the variation in index with temperature is essentially due to the change in density of the

crystal. At the extremes, where dn/dT is less negative, the influence of temperature suggests a shift

of the ultraviolet and infrared absorption edges to longer wavelengths.

142
The dn/dT values of Gyulai [12] and Paschen [13] are shown in figure 3 for comparison. Gyulai's

values for a mean temperature of 33°C cover the wavelength range 0.20 to 0.61 pm. Paschen's data,

for which the mean temperature is unknown, covers the wavelength range 0.59 to 17.7 nm. It can be seen that the absolute values of the earlier work are generally higher than those of NBS. It has

been observed in halide crystals that the absolute value of dn/dT increases with increasing temperature.

3. Thermal Expansion and Change in Refractive Index with Temperature

The thermal expansion and change of refractive index with temperature dn/dT may be measured up

to a temperature of 800°C by an interferoraetric method that uses Fizeau type fringes [14] . In the

visible part of the spectrum these measurements can be made simultaneously.

The interferometer assembly as used in the visible is shown in figure 4. The set of fringes

on the left is obtained between the top and bottom surfaces of the lower fused silica plate, T,

and is calibrated in terms of temperature so that the fringes serve as an optical thermometer. The

center set of fringes is obtained between the top and bottom surfaces of the index sample, R, and

is used to calculate the change in refractive index. Beneath this sample is a frosted area on the

lower plate so that no fringes are formed at the interface. The third set of fringes is formed by

the slight wedge introduced between the upper plate, E, and the lower plate, T, when these fringes

are separated by the expansion sample, S, which is cut from the same material as the index sample, R. These fringes are used to obtain the thermal expansion. The assembly is heated by a vertical ti:ibe furnace and the shifts in the three sets of fringes are recorded simultaneously on a single strip

of 35 mm film as a function of temperature.

The interference pattern generated by the sample of thickness, t, and refractive index, n, is

expressed by the equation,

NX = 2nt, (1)

which relates the optical path, nt, and the fringe number, N, when monochromatic light of wavelength,

X, passes twice through the sample at normal incidence. By differentiating with respect to temperature, T, and rearranging, it is fo\and that

dn ^ X_ AN _ ndt

dT 2t AT dT "

^

'

We calculate the change of refractive index as a function of temperature from the number of fringes

that pass a point of reference and from the change of thickness obtained from the thermal expansion

sample [15]

.

Using a modification of the above method, we intend to extend the measurement of dn/dT to the

infrared.

4. Measurement of Stress-Optical Coefficients and Elastic Constants

The importance of measuring all the stress-optical coefficients in the infrared is not always

well understood. Several laboratories have been measuring stress-induced birefringence at 10.6 ym.

However, these data are not sufficient for determining the optical wavefront distortion due to thermal

stresses, because they measure only the relative stress-optical coefficients [(qji - ^12' ^^'^ ^'-'^ cubic materials]. We know of only one paper which presents measurements of qj j and qj2 separately at

10.6 ym [5], and these measurements, which were done on Ge , did not give the sign of the coefficients.

Both q^j and qj2' combination with the elastic constants, are necessary for calculating the optical

distortion due to stresses from thermal gradients in optical materials [1,2]. Index changes due to

these stresses can be even greater than changes in index due to a rise in temperature. For example,

in a material tailored to have dn/dT = 0, the changes of index will be due solely to the stressoptical effect.

4 . 1 Apparatus

Conventional Twyman-Green interferometers are used [16] . The Twyman-Green interferometer at

0.6328 and 1.15 ym uses helium-neon lasers as sources. The fringes are observed with a silicon matrix

vidicon camera.

The Twyman-Green interferometer at 10.6 ym uses a CO2 laser source, a germanium beam expander,

and a germanium beam splitter. The fringes are viewed either on a liquid crystal detector or on a commercial thermal image plate

.

143
For uniaxial loading, the specimen, in the form of a rectangular parallelepiped is compressed in

a rectangular frame screw clamp equipped with a calibrated load cell and a digital readout voltmeter.

A photograph of the stressing apparatus with the voltmeter is shown in figure 5.

4.2 Experimental Procedure

The sample is placed within one arm of the Twyman-Green interferometer. The fringe shift per unit

of applied stress is measured for stress parallel to the polarization of the radiation, AN^/AP, and

for stress perpendicular to the polarization of the radiation, AN2/AP.

The sample, itself, functions as a Fizeau interferometer if the faces are properly polished;

interference fringes are obtained from reflections from the sample surfaces. The fringe shifts

obtained for stresses parallel and perpendicular to the polarization of the radiation are, respectively,

AN^/AP and AN2/AP.

For an isotropic material we obtain

'12

'12

X

C ANj ANi

2t AP AP~

X ANi

n^t

A AN2

n3t AP B

X

2t

AN2

aF" — (n - 1) Si;

(n - 1) Si;

AN2

AP~ (3)

(4)

(5)

Additional experimental data can be obtained by measuring the stress-induced birefringence.

The experimental arrangement is shown in figure 6. The stress is applied 45° with respect to the

plane of polarization. The analyzer is oriented parallel to the incident polarization. The uniaxial

load applied to the sample is increased until extinction is obtained. The loaded specimen then acts

effectively as a half-waveplate , and considering the double passage of the light through the specimen,

the load is that which creates a single pass quarter wave retardation in the sample. The relative

stress-optic coefficient is

qil - qi2 = X/(2n3pt) . (6)

The value of q^-^ - f^oni the measurement of birefringence is more precise than the value of qj j -

q2^2

obtained from eqs (4) and (5). Thus, we calculate q^. and q^. from the s\im

q^^ ^ + <lj^2' obtained by

adding eqs (4) and (5), and the difference q^ ^ - qi2 ^^om eq (6). This assures us that the birefringence calculated from the numerical values of q-^-^ and equals the measured value.

Equation (3) gives the value for si2- In order to obtain sjj, we use a method similar to the

method for measuring dn/dT. The sample which acts as a Fizeau interferometer is compressed in a pressure vessel. Figure 7 shows the pressure vessel. Hydrostatic pressure is generated by compressing

a fluid. From the shift in fringes per unit pressure, AN/AP, we obtain for an isotropic material

X AN n^ , ^ > ,_. ^11 + 2si2 = 2^-^ - — ^''ll + 2qj2'

Using S12 obtained from eq (3) together with eq (7), we can solve for sjj.

The procedures described in this section are for measuring stress-optical constants and elastic

constants of isotropic materials. These constants can be measured with the same procedures on crystalline materials. Because stresses must be applied along several crystallographic axes, more samples are required for measurements on crystalline materials than for measurements on isotropic

materials

.

1A4
4.3 Results

Measurements of absolute and relative stress-optical coefficients have been made on chalcogenide

glass, AS2S3 glass, and polycrystalline ZnSe . The results are given in Table 1. The data for AS2S3

are presented for three wavelengths, 0.6328 ym, 1.15 ym, and 10.6 ym. Data for the same wavelengths

are presented for polycrystalline ZnSe although absolute values are lacking at 10.6 ym. We have

assiOTied the ZnSe to be an isotropic material . Data for the chalcogenide glass are presented at the

infrared wavelengths.

Table 1. Stress-optical constants, q^j and qj2' ^^'^ elastic compliance, Sj^2' units of 10 m^/N.

a b b b b

n qii '^iz-lu

AS2S3 - 18.6, - 18.8'^

0.6328 ym 2.605 7.63 6.36 - 1.27

1.15 ym 2.452 7.64 6.54 - 1.10

10.6 ym 2.354 8.23 6.95 - 1.28

Chalcogenide Glass

(Ge 33%, As 12%, Se 55%) - 12.0, - 12.2 , - 11.5^

1.15 ym 2.5704 5.32 4.89 - 0.43

10.6 ym 2.4919 5.42 4.94 - 0.48

Polycrystalline ZnSe - 4.0

0.6328 ym 2.578 - 1.34 0.29 1.62

1.15 ym 2.465 - 1.37 0.62 1.98

10.6 ym 2.408 1.96

^These values were not measured in this work, but are given because they were used to calculate the q. .. The values for ZnSe are from single crystal data. """"^

^Estimated accuracy of numbers from this work is ±2%.

F. W. Glaze, D. H. Blackburn, J. S. Osmalov, D. Hiobbard, and Mason H. Black, J. Res. Natl. Bur. Std.

59_, 83-92 (1957) . d

Ray Hilton, private communication — value for Texas Instruments.

eRay Hilton, private communication — value for Bell Laboratories.

The birefringence of the above infrared glasses is of opposite sign to most optical glasses [17]

.

The q^^j is greater than the q^j of most optical glasses by a factor of ten, and qj2 is greater than

the °f most optical glasses by a factor of three [18] . From the theory of photoelasticity and from the data, we expect that the refractive index of

the ZnSe will decrease under hydrostatic pressure. This behavior has been observed previously in

single crystal ZnS [19] . Since dn/dT for ZnS is positive [20] , it would be of interest to observe

whether the same is true for ZnSe. In addition, for ZnSe there is a large dispersion in qi2-

Also included in Table 1 are the values we have obtained for si2. Agreement with the values

of other workers is very good.

5 . Summary

We have outlined the program of optical materials characterization at the National Bureau of

Standards. The properties that we are measuring are refractive index, change of index with temperature,

thermal expansion, stress-optical coefficients, and elastic constants. At the present time, primary

interest is in laser window materials at 10.6 ym. Data have been presented on chalcogenide glass,

AS2S3, polycrystalline ZnSe, and crystalline KCl. Measurements on these materials and on other infrared

145
laser window materials are in progress. In addition, we are developing more sensitive techniques

for measuring the stress-optical coefficients in materials that exhibit a small stress-optical effect.

A similar need exists for optical materials characterization in the ultraviolet. Techniques

similar to those we have used in the infrared can be employed in the ultraviolet should the need

arise

.

^» Acknowledgments

We would like to acknowledge the receipt of samples from Carl Pitha of AFCRL, Bennett Sherman

of the Servo Corporation, Ray Hilton of Texas Instruments, Phillip Klein of NRL, and Colonel William

Goldberg of AFML. We would like to thank W. J. McKean for his technical assistance.

7, References

[1] B. Bendow, P. D. Gianino, A. Hordvik, and

L. H. Skolnik, Optics Commun. 1_, 219 (1973).

[2] B. Bendow and P. D. Gianino, Appl . Phys. 2_, 1 (1973) .

[3] C. S. Sahagian and C. A. Pitha, AFCRL-72-

0170, 9 March 1972, Special Reports No. 135.

[4] C. A. Pitha, AFCRL-TR-73-0372 (1) , 19 June

1973, Special Reports No. 162, Vols. I and

II.

[5] R. L. Abrams and D. A. Pinnow, J. Appl. Phys.

41, 2765 (1970)

.

[6] F. Twyman and J. W. Perry, Proc. Phys. Soc.

London 34, 151 (1922)

.

[7] K. Vedam and S. Ramaseshan, in Progress in

Crystal Physics , Vol. I, edited by R. S. Krishnan (Interscience Publishers, New York,

London, 1958) , pp. 102-138.

[8] C. Weir, S. Spinner, I. H. Malitson, and

W. J. Rodney, J. Res. Natl. Bur. Std. 58_, 189 (April 1957), RP2751.

[9] L. W. Tilton, J. Res. Natl. Bur. Std. 2_, 909

(1929), RP64.

[10] F. F. Martens, Ann. d. Phys. 6_, 603 (1901).

[11] A. Smakula, Office of Technical Services,

U. S. Department of Commerce (October 1952),

p. 81.

[12] Z. Gyulai, Z. Phys. 46, 80 (1927).

[13] Reference 11, page 56.

[14] J. B. Saunders, J. Res. Natl. Std. 35., 157

(1945) , RP1668.

[15] R. S. Krishnan, in Progress in Crystal

Physics , Vol. I (Interscience Publishers,

New York and London, 1958).

[16] N. Born and E. Wolf, Principles of Optics , (Pergamon Press, 1970), p. 303.

[17] R. M. Waxier and A. Napolitano, J. Res. Natl.

Bur. Std. 69A , 325 (1965).

[18] K. Vedam, Proc. Indian Acad. Sci. [A] 31_, 450 (1950) .

[19] R. M. Waxier, IEEE J. Quant. Electron. QE-7 , 166 (1971)

.

[20] Reference 15, page 146.

146
Figure 1. (a) Schematic of spectrometer

used for visible-region refractometry

.

A. source; B. divided circle; C. prism

table; D. collimator; E. telescope.

(b) Schematic of spectrometer used for non- visible-region refractometry. A. source;

B. divided circle; C. prism table;

F. collimating mirror (fixed); G. movable

mirror; H. detector.

POTASSIUM CHLORIDE

TEMPERATURE COEFFICIENT

0

-1

-2

-3

-4 -

0.1

NBS (1974) —

GYULAl (1927) PASCHEN (1908) •

Um *•••• , • • • • ±

1.0 100

WAVELENGTH, jUm

Figure 3. Temperature coefficient of

refractive index of KCl.

Figure 4. Interferometer assembly for the

measurement of thermal expansion and

thermal change in refractive index. E. an interferometer plate; S. thermal expansion

specimen; R. refractive index specimen;

T. interference thermometer; Mg, Mj^, and

Kj, reference lines for the interference

fringes of thermal expansion, refractive

index change and temperature change.

147
TO PRESSURE

(f^"^ GENERATOR

WINDOW EACH

SIDE

Figure 7. Pressure vessel.

WO COMMENTS OW PAPER 8/ FELVMAN.

148
4.2 Testing the Surface Quality of Laser Materials

M. J. Soileau and H. E. Bennett

Michelson Laboratory, Naval Weapons Center

China Lake, California 93555

Research presented at the 1973 Laser Damage Symposium clearly showed that

surface microcracks and pits influence laser damage thresholds for optical

materials. In addition, backscatter from laser amplifiers can result in damage

to oscillator optics in high power oscillator-amplifier systems. The scratch and

dig specifications presently used to specify optical surface quality are inadequate since (1) they address only the problem of surface irregularities larger

than 2 Mm (Bloembergen has shown theoretically that pits or scratches as small as

100 A can significantly reduce damage thresholds) , and (2) compliance is determined in a subjective manner. The relationship between surface quality and light

scattering from transparent optical components is discussed, and an instrument

for measuring bulk scattering and surface quality described. Quantitative measurements on a set of standard scratches and digs are presented, as well as measurements of surface imperfections and bulk scattering in optical materials used in

infrared laser systems.

Key words: Laser damage; light scattering; optical surface quality; scratch

detection.

1. Introduction

Surface quality is extremely important in determining the resistance of an optical element to laser damage. Some of the surface properties which influence laser damage thresholds are: (1) surface defects such as pits and microcracks, as proposed by Bloembergen [I]'', (2) subsurface defects

caused by polishing damage, suggested by Giuliano [2], and (3) surface microroughness which may lead

to resonant absorption, as proposed by Decker. [3] The surface quality specification presently being

used (MIL-0-13830A, the so-called scratch and dig specification) is inadequate since (1) it treats

only the problem of surface irregularities larger than 2 ym (Bloembergen has shown theoretically that

pits or scratches as small as 100 A can significantly reduce damage thresholds) and (2) compliance is

determined in a subjective manner. Other techniques commonly used to determine surface quality are visual inspection, inspection under a microscope, and specifying the type of polish that should be

used to produce the surface, i.e., "super polish" as opposed to standard polish. The various microscopic techniques (Nomarski, dark field, SEM, etc.) serve a very useful purpose in qualitative assessment of surface quality, but give little quantitative information. The term "super polish" is used to describe surfaces prepared by bowl feed, recirculating slurry, or other similar techniques which differ

from classical fresh feed polishing procedures. The use of nonstandard polishing techniques does not ensure superior surface quality; thus, specifying the type of polish does not specify surface quality.

In addition to the surface-related problems, bulk imperfections such as voids and inclusions can cause lower damage thresholds. Scattered light from surface and bulk imperfections can also result in

damage to components in high gain oscillator-amplifier systems. In this paper an instrument for measuring bulk scattering and surface quality is described, and preliminary results on laser window

materials are reported. Quantitative measurements on a set of standards using light scattering techniques can form the basis for surface and bulk quality specifications for laser windows.

2. Instrument Design

The basic concept used in the design of the Optical Functional Tester (OFT) [4] was to make an instrument which could characterize optical components objectively in terms of scattered light from

the optical element and also in terms of present standards. The instrument was to be capable of

measuring both reflecting and transparent optics in the visible and the infrared. An additional

design constraint was that the instrument be capable of measuring scattering from the surface and the

bulk of the component to be evaluated.

1. Figures in brackets indicate the literature references at the end of this paper.

149
The OFT is shown in figures 1 and 2 in its present configuration. A schematic diagram of the

instrument is shown in figure 3. A light beam from a HeNe laser, operating at either 0.6328 ym,

1.15 Mm, or 3.39 Mm, is filtered, chopped at 13 Hz, and transmitted through the sample. (The sample

is mounted on a fixture that allows it to be accurately aligned and rotated so that all of it can be

scanned. It is positioned using a fiber optic microprobe.) The transmitted light passes through the

Coblentz sphere (collecting mirror, M^,) and is measured by pyroelectric detector The scattered

light strikes the Coblentz sphere M^, and is imaged on pyroelectric detector Dj. The signals from both

detectors are fed into lock-in amplifiers, and the outputs are ratioed by a digital voltmeter. This

ratio of the scattered light to the transmitted light (or reflected light if a mirror is being evaluated) is displayed on a chart recorder.

The system can be used to evaluate reflecting optics by placing the laser in the position indicated

as Laser Mirror Mode. In this case the detector D2 would be moved slightly off-axis to allow clear

passage of the illuminating beam, and would be used to collect the specularly reflected beam. The scattered light would be collected by the Coblentz sphere in the same manner as before.

An additional element, not shown in the diagram, is an infrared filter placed between the laser

and the sample that is used whenever the system is operating in the infrared. The purpose of the filter

is to eliminate stray, incoherent visible light from the laser which is always present when the laser

is tuned for an infrared line. Apertures, also not shown, are inserted between the chopper and the

sample to constrain the light beam and eliminate light diffracted from the edges of the chopper blades.

The OFT uses only reflecting optics and broad band pyroelectric detectors. Thus, it can operate

over an extended wavelength range from the ultraviolet through the visible and into the infrared merely

by changing the laser source. Laser sources now commercially available extend from about 0.25 Mm to 10.6 Mm.

In its present configuration, the instrument measures total scattering from both the surfaces and

from the bulk of the component being tested. Because of the small / number of the collecting sphere,

the depth of focus at the detector is small. Contributions from the bulk of the sample and from either

of the two surfaces could thus be separated by restricting the detector aperture and by fine focusing.

3. Evaluation of Surface and Bulk Scattering

The OFT measures scattered light from both surfaces of the sample as well as volume scattering

from the bulk of the material. Since the Coblentz sphere has an / number of about 0.5, the depth of

focus at the detector is quite small. The detector itself has a fairly large sensitive area (5x5 mm),

but by aperturing it to the size of the incident laser beam, a small depth of field at the sample

position is achieved. In this way, the contribution from a particular surface or volume element in the

sample is maximized. Consider a sample of thickness t positioned so that the front surface of the

sample (z = 0) is in focus on the detector aperture. Letting Sjj^ be the total amount of scattered light

passing through the aperture, we have

S. = aS^ + b(t)S^ + Y(z)S,dz (1)

X- o t ^ o b

where S^^ is the light scattered in the forward direction by the front surface, S^. forward scattering

from the back surface, and S^ forward scattering per unit volume in the bulk of the material. The

quantities a, b and y(z) are geometrical acceptance coefficients determined by the aperture and the

angular dependence of the scattering. If the front of the sample is positioned at the focus, a s 1. The back surface will then be out of focus so that the coefficient b(t) « 1. It will vary depending

on sample thickness, refractive index, and the angular dependence of the scattering from the back surface as well as on the aperture used. Bulk scattering occurs between z = 0 and z = t so that Y will

have limits a > Y(t) > b provided the angular dependence of surface and bulk scattering are not too

different.

Approximate values of the acceptance coefficients can be determined experimentally for a "typical"

component by measuring a sample one of whose faces scatters heavily. If we assume that without the

aperture the detector records essentially all of the forward scattering from the sample, then if the

high scatter surface is in the back position, the ratio of signals recorded with and without the aperture gives b. The value of a can be determined by positioning the high scatter surface at the focus,

and the value of Y can be determined approximately by translating the high scatter surface between

the back position and the focus position.

If the fractional scattering per unit volume is the same throughout the specimen, we can remove S^

from under the integral in eq. (1) and obtain

150
S. - aS - bS

h=- '

• (2)

Y(z)dz

In the general case all quantities on the right side of eq. (2) are now known except and S^. The

backscatter from the sample surfaces can be determined by coating the surfaces with an opaque metal

film and measuring the total integrated scatter in reflection using the Optical Evaluation Facility. [5]

If we assume that the forward and backscattering from a surface are approximately equal (there is

experimental evidence to support this assumption as discussed below), we can obtain the bulk scattering

from eq. (2). Thus we have, on a "first cut" basis, separated the surface and volume scattering coefficients and have obtained approximate values for each.

4. Scattering Levels in KCl and ZnSe

The OFT was used to measure surface and bulk scattering parameters of several candidate infrared

laser window materials, including hot rolled KCl [6], hot forged Eu"^ doped KCl [7], laser grade "polytran" KCl [8], and ZnSe grown by chemical vapor deposition. [9] A crucial assumption in the separation

of bulk and surface scattering is that the total integrated scatter in the forward and back directions

from such surfaces be approximately equal. To check this assumption, the forward scattering of two of

the KCl samples and two polished quartz samples was measured at 6328 A. No bulk scattering could be

observed visually from these samples, eliminating the third term of eq. (1) and making it possible to measure forward scattering from the surfaces unambiguously. The samples were then coated with silver

and the backscattering measured using the Optical Evaluation Facility. [5] Care was taken that exactly

the same area of the samples was measured with each instrument. The results are given in table 1 in

terms of derived surface roughness. There is good agreement between the backscattering and forward

scattering results, indicating that for these quite rough samples and also for the relatively smooth

samples the assumption that forward and backscattering are approximately equal is valid. (The roughnesses of the KCl blanks reported in table 1 are typical of many polished KCl surfaces. However,

smoother KCl surfaces can be obtained. Using bowl feed with a triacetin slurry we have produced optically flat surfaces with roughnesses under 40 A rms on both hot rolled KCl and polytran.

I I The Eu doped KCl and the ZnSe specimens exhibited significantly more bulk scattering than did

the other KCl samples. A preliminary analysis of bulk scattering in these materials was performed at 0.6328 and 1.15 Mm using eq. (2); the results are given in table 2. Based on these two wavelengths,

the bulk scattering from the KCl sample seems to decrease with wavelength approximately as l/A^,

whereas the ZnSe goes approximately as 1/A. One should be extremely cautious about extrapolating

these results to longer wavelengths, but if the trend continues, at 10.6 ym the forward scattering

from the bulk would be 2 to 4 x 10~^ cm"-'- for both materials. The total amount of light scattered

per cm of path would be twice these values and would be comparable to that absorbed per cm at 10.6 Um. Such scattering levels would result in increased window absorption for geometries where significant

total internal reflection occurs, as well as increasing the possibility of damage to the oscillatoramplifier part of the laser. The scattering centers may also act to reduce the threshold for laser

damage of the materials.

Table 1. Roughness of polished KCl and quartz surfaces

Specimen Roughness (A rms)

TIS (backscatter) OFT (forward scatter)

Hot rolled KCl

Polytran (KCl)

Quartz No. 1 Quartz No. 2

164

115

13.1

17.8

151

124

12.4

20.9

Table 2. Bulk scattering measurements (forward scattering)

Specimen 0.6328 ym 1.15 m

KCl (Eu"^+) 0.028 cm"?" 0.010 cm"?" ZnSe No. 556 0.026 cm_| 0.016 cm~r ZnSe No. 554 0.069 cm "'" 0.036 cm

151
5. Scratch Detection

The laser damage threshold of laser windows is significantly lowered if scratches are present on the window surface. [1] Scratches also contribute to scattered light, although in a well polished

surface most of the scattered light results from closely spaced microirregularities only a few tens of

angstroms in height, not from the few scratches on the surface. If a scratch is present on a surface

which is scanned by a light beam, when the light strikes the scratch the scattered light level goes up. Thus, by relating the scattered light to the scratch size, we may hope to develop a rapid, simple,

reliable and inexpensive method for checking laser components for scratches.

The currently accepted technique for determining the surface quality of optical elements is described in MIL-0-13830A, the scratch and dig standard specification. This test is difficult, only

semiquantitative, and must be performed visually by a skilled and painstaking inspector if it is to be

meaningful. A comparison is made between the scratches that can be seen on the surface to be tested

and those on a MIL Surface Quality Standard for Optical Elements. The finer scratches on this standard

are extremely difficult to see. However they can be revealed by using Nomarski (Differential Interference Contrast) Microscopy as shown in figure 4. Dig standards are also shown. [10] Both are scribed on 1.5 inch diameter glass substrates that are mounted in a glass case through which measurements are made.

An attempt was made to measure the scattering of the standard scratches through the glass case. However, this was unsuccessful because of the high level of scattering from the encapsulating surfaces.

The standards were then removed from the glass case and evaluated with the OFT. The results of these

measurements are summarized in figures 5 and 6. The scattering levels of each of the standard scratches

are plotted in figure 5 along with 600X Nomarski micrographs of each scratch. Note that the scattering

level of the #10 scratch is higher than that of the #20 scratch. In addition, the average scattering

from the substrate on which the #10 scratch is scribed is higher than the peak scattering for the #20

scratch. This discrepancy on the standard designed to evaluate surface quality as related to scattering illustrates the importance of surface microirregularities as well as scratches and digs in determining the quality of an optical surface.

Figure 6 illustrates the sensitivity of the OFT. The trace on the left is identical to that shown

in figure 5. In the trace to the right the system gain was increased by a factor of 100 to bring out

the #20 scratch. As can be seen in the micrograph of this scratch in figure 5, the substrate for the

#20 scratch is markedly smoother than than for the #10 scratch and the scratch is narrower and better

defined.

The ability of the OFT to identify and make quantitative measurements on scratches and digs on actual components is demonstrated from measurements of the surface quality of an infrared transmitting

dome. In figures 7-9, actual scattered light scans of the clear dome at 0.6328 ym are presented. The

three traces in figure 7 correspond to complete circular scans at various positions off the dome

optical axis (5°, 10.0° and 20.4° from the dome center). A large scratch is indicated in the 5° offaxis scan. The scattering level from this scratch corresponds to that of a #60 scratch.

Figure 8 illustrates the ability of the OFT to identify and track a surface scratch. The dome was displaced 0.5° (approximately 1/3 mm) between each successive scan. The two large "bumps" in the curves in figure 8 correspond to the same scratch. The dome was 50% overscanned to demonstrate the

repeatability of the data. Thus, each scan corresponds to approximately one and one-half times around

the dome at a given position off the dome axis. From this type of measurement one can determine the

number of scratches, their location on the surface, the length of the scratches and the average scattering level of the dome caused by microirregularities.

In figure 9 two complete scans displaced by 1/3 mm from the dome center are shown. The scratch

indicated in this scan corresponds to a #40 standard scratch. Note that the scratch is in both scans, whereas the dig is present in only one. The double peak between the #40 scratch and the dig in the

trace to the left in figure 9 is indicative of a scratch on the order of a #10 standard scratch or smaller.

6. Conclusion

Scratches or pits in optically polished surfaces, and voids, impurity clusters and other volume

defects all act to reduce the damage threshold of laser windows. They also produce scattered light

which can damage components in a laser optical train. The sensitivity of scattering to bulk and surface defects can, however, be used to advantage in testing for the presence of such defects. For

maximum sensitivity the measurements should be made at as short a wavelength as is feasible, since the

scattering cross section of most defects increases with decreasing wavelength.

152
An instrument, the Optical Functional Tester (OFT) has been built for evaluating surface and

volume defects in optical components. By scanning a narrow beam across the sample, both the surface

and the volume may be checked for defects in a fraction of the time required for visual inspection.

In addition, the scattered light test is quantitative, not subjective, and can detect defects which

are virtually impossible to see visually. Scratch and dig standards currently used to specify optical

surfaces can be extended by defining scratch size in terms of light scattering at a given wavelength.

The position and size of even minute scratches on the surface can then be readily determined. It should then be possible to relate scratch size to damage threshold quantitatively. In addition, since

microroughness as well as scratches and digs affects surface scattering, a better measure of true

surface quality can be achieved.

Finally, measurement of scattered light at the operating wavelength of the laser furnishes a functional test of the component performance in use. The system designer can then intelligently

specify what scratch/dig value and what microroughness the surface should have for a given application.

He can also test for volume defects by determining volume scattering levels. For example, scattering

measurements made with the OFT show that some CVD grown ZnSe has high volume scatter as does some KCl

doped with Eu"*"*" for high strength. It will be interesting to see if these defects cause a significant

reduction in damage threshold or result in large scattering levels at 10.6 pm.

7. Acknowledgment

The authors are grateful to Mr. G. J. Hutcheson of the Army MICOM for use of the OFT, which was

built by NWC under contract to MICOM. In addition, we are grateful to Dr. E. Bernal G. of Honeywell

and Mr. John Fenter of AFML for providing such interesting specimens for measurement.

8. References

[1] Bloembergen, N. , Appl. Opt. 12, 661 (1973).

[2] Giuliano, C. R. , in Laser Induced Damage

in Optical Materials: 1972, A. J. Glass

and A. H. Guenther, eds. (NBS Spec. Pub.

No. 372), pp. 55-57.

[3] Decker, D. L. , High Energy Laser Mirrors

and Windows (Semi-Annual Report No. 4,

ARPA Order 2175, Naval Weapons Center,

March 1974), pp. 66-83.

[4] Bennett, H. E., Soileau, M. J., and

Hutcheson, G. J., U.S. Army Missile Command

Technical Report QR-TR-74-1, March 1974.

[5] Bennett, H. E., and Archibald, P. C,

elsewhere in these proceedings (1974).

[6]

[7]

[8]

[9]

[10]

Produced and sent to us by Dr. R. Bernal G. of Honeywell, Inc. for evaluation.

Produced by Honeywell, Inc. and supplied

to us' for evaluation by John Fenter, Air

Force Materials Laboratory.

Produced by Harshaw, Inc.

Produced by Raytheon, Inc. and supplied

to us for evaluation by John Fenter, Air

Force Materials Laboratory.

The scratch standards, Serial No. F-66-1266

and dig standards. Serial No. F-66-1277 were kindly provided by Mr. Nate Scott of

the Frankford Arsenal, U.S. Army, Philadelphia, Pennsylvania.

Figures

Figure 1. Photograph of the Optical Functional

Tester showing overall view and associated

electronics.

153
Figure 2. Close-up of OFT. The fiber optics

sample positioner, Coblentz sphere, transmitted and scattered light detectors, and an

infrared transmitting dome are shown.

OPTICAL COMPONENT FUNCTIONAL TESTER

Figure 3. Schematic diagram of the OFT. Light

from a laser source is used to illuminate the

sample being tested. The transmitted light is

measured by detector and the scattered

light is collected by the Coblentz sphere, M^,

and imaged on detector D^.

SCRATCH AND DIG STANDARDS UNDER DIC (NORMARSKI)

10 20 40 60 80

Figure 4. MIL Surface Quality Scratch and Dig

Standards for Optical Elements. Photographs

are 600X Nomarski micrographs taken through

the sealed glass case in which the standards

are stored.

154
NO. 20 SCRATCH STANDARD

Figure 6. OFT trace of the scattering from the #20

standard scratch at 0.6328 ym. The scale on the

left is the same as in figure 5. The trace on the

right was generated using a factor of 100 increase

in gain in the detector electronics. This figure

illustrates the sensitivity of the OFT to extremely low levels of scattered light.

0.001

0.000

SCATTERED LIGHT MEASUREMENTS ON CLEAR DOME AT 0.6328Mm

0.008

0.006

0.004

O 0.002 z

in 0.004

0.002 0.004

0.002

5° OFF AXIS

2 CM SCAN

10.0° OFF AXIS

4 CM SCAN

20.4° OFF AXIS

8 CM SCAN

2 4 6 SCAN DISTANCE (CM)

Figure 7. Representative scans of the scattering

level of the infrared dome at various distances

off-axis. The scale at the bottom of the figure

corresponds to the circumference of the dome at

the position of the scan.

155
TRACK OF SCRATCH, CLEAR DOME, 0.632;im

5.0° OFF AXIS

0.002

6.5° OFF AXIS

7.0° OFF AXIS

7.5° OFF AXIS

Figure 8. Scans made 5 to 7.5° off axis on the

infrared transmitting dome at 0.6328 ym. The

dome was overscanned by about 50% to demonstrate repeatability. Thus, the two large

peaks in each scan represent the same scratch on the dome surface. This scratch

corresponds to a #60 standard scratch.

Figure 9. Illustration of the ability of

the OFT to detect digs as well as scratches.

The scans were generated by displacing the

dome by 1/3 mm off axis.

SCANS DISPLACED BY 1/3 MM

I NO. 40 SCRATCH NO. 40 SCRATCH

COMMEWrS ON PAPER BV SOJLEAU

Nonjman Boting o OwzvUs-JZJiLYio-it, commtntzd that itnce. it 6zme.d Hxitheji 2.cu>y to obtain iiuJi{^a.ce^ Mtth

RMS ^oughn&i'i o^ 30 A and tkat according to Btombfigzn' i the.on.g, cAacki and cA.e.vtcu OjJ lui than

100 A dhme-mton did not ttad to itgnli-icant (^Ittd tnhancme-nt, that cunAtnt potUsking tzchniquu mold

i,e.m to be zntuxdiy addqwxt^ to wJj,^ iuA{,aaz damage thAeJ>hoZdi to thoJji tnt^mtc livzLi. It woi

potntzd out, howe.v2A, -in d^cuiiton that w-iXh a 30 A RMS voAiation o^ the. iuA^ace. thzKZ mold be a

i>lgni{)-icjxnt nmbeji o^ TOO A cuackA and 6CA.atahu -in the, iuA^ace. Hahold Poizn Oj^ the AiA ToKce CambfUdge.

ReAe-OAch iabonaX.on.y pointed that the mea^uAermnt o^ thz RMS >ioaghneM> o{) the i>uA(iacz. w)a6 not In iX&eld

e.ntlAeJiy Indlcatlvz o{, the damage ne^ldtenze oi the iiuxdace, but Instead that the. conxeZation dlitanae.

aoAOii the iuA^ac^ wa6 atio a ilgnlfflcant peAometeA. In >ieJ>ponAe to a qaeJ>tlon ai to the oHlgln o^ the. icatteAlng that woi obi,eAved, the ipeakeA commented that It uiai piobablij (^lom gAoa icAotche^i on the

ian^ace. latheA than jj/iom the RMS mlcAo AoaghneM. Thli Moi {^uAtheA iuppofuted by fie^uZti obtained a&lng

FECO InteAdeAometAy . A {^uxtheJi te^t o{^ the icatteAlng mechanism Mould be the potatilzatlon dependznae o{^ the. iaatteAlng Mhlch, howevex, had not been meajsuxed at the time the pfiepoAotlon oj$ the papeJt.

156
4.3 Statistical Characterization of Mirror and Window Surfaces

Jean M. Bennett

Michelson Laboratory, Naval Weapons Center

China Lake, California 93555

In order to evaluate the contribution of surface structure to laser damage and

to calculate the magnitude and angular distribution of scattered light from laser

mirrors, windows, and other optical surfaces, the autocovariance function, rms roughness and height distribution functions are required. These and other statistical parameters for optical surfaces can now be determined with a FECO Scanning

Interferometer, which consists of a FECO interferometer, slow scan television

camera, signal averager, minicomputer, and teletype. Wavelengths are measured at 512 equally spaced points along the length of an interference fringe (which contours

irregularities on the optical surface under investigation) . The fringe length

corresponds to a total length of 1 mm on the interferometer surface. The statistics

for the surface are obtained from the wavelength data. The system has the advantages

that it does not depend on visual estimates of fringe width to yield a value of the rms roughness, and additional statistical information such as the autocovariance

function and height distribution function can be obtained which cannot be determined

visually.

Key words: Autocovariance laser damage statistical characterization of surface;

FECO interferometry ; scattered light; surface roughness.

1. Introduction

The surface structure of optical elements, and in particular the surface microroughness , is of

importance in laser applications for three main reasons. First, surface microroughness produces

scattered light. Unwanted scattering not only decreases the intensity of the main beam, but more

importantly it can produce spurious signals and, in the case of backscattering, it can cause damage

to the laser components. Surface absorption can also be caused by surface microroughness. In mirrors,

surface roughness can increase the absorption by as much as a factor of three; the situation for

windows is not so clear, but some experiments indicate that here also the surface absorption is

related to surface microroughness. Finally, surface structure plays a role in laser damage in windows.

Theoretical studies indicate that microcracks of dimensions as small as 100 A may induce dielectric

breakdown in transparent solid materials [N. Bloembergen, Appl. Opt. 12^, 661-66A (1973)].

It is not possible to deduce the surface structure unambiguously from measurements of scattered

light, and at best measurements of this type give indirect evidence about the surface roughness and

whether or not the surface has Gaussian statistics. We have developed a method for directly measuring

the statistical parameters of plane optical surfaces. These parameters include the rms roughness, rms

slope, autocovariance function, height distribution function, and slope distribution function. The

lateral resolution limit of the measurements is about 2 microns, but the depth resolution is of the

order of a few angstroms. In this paper we will describe the FECO Scanning Interferometer with which

we obtain the data, and give examples of typical results we have obtained to date.

Figure 1 shows a photograph of the FECO Scanning Interferometer and a schematic view is shown in

figure 2. White light from a xenon arc Z is collimated by lens L-, and directed onto the sample at

normal incidence by means of beam splitter B. The sample, which has been overcoated with an opaque

layer of silver to increase its reflectance, is placed nearly in contact with a supersmooth optical

flat (roughness less than 10 A rms) that has been coated with an ~ 95% reflectance semitransparent

silver film. Fringes of equal chromatic order (FECO fringes) are formed by interference of multiplyreflected beams between these two surfaces, and contour irregularities on each surface. Lens L2

focuses an image of the interferometer on the slit S of a constant deviation prism spectrograph, and

the FECO fringes can be observed visually in the image plane, photographed, or detected with a slow

scan TV-type camera. By scanning one line at a time with the TV camera, information in digital form

is obtained from the signal averager, which when analyzed by the minicomputer yields the wavelength

of the portion of the fringe contained in the scan line. Thus, wavelengths at 512 equally spaced

discrete points along the length of the fringe are obtained. These are then converted by another

computer program into height differences above and below the mean surface level, and the statistics

of the sample are then obtained. The spacing between the scan lines is set approximately equal to the

resolution of the optical system, so that statistical information is obtained at approximately 2 micron

564-378 0-74-12 157
increments along the fringe. The fringe actually corresponds to a rectangular area on the sample

1 mm in length. The width of the rectangle is determined by the width of slit S, which is approximately 2 microns. Thus, the statistics of the surface roughness are effectively for little squares

on the surface, 2 microns on a side.

We have studied various types of surfaces including very smooth polished optical glasses (Cervit,

fused quartz), polished fluorides (magnesium fluoride, calcium fluoride), polished alkali halides

(potassium chloride, sodium chloride), polished metals (molybdenum, copper, TZM, titanium, stainless

steel), and machined copper. Results for representative samples will be given here. Figure 3 shows

data for Cervit, a polycrystalline quartz with a very low expansion coefficient. This sample had a roughness of 8 A rms, as determined from visual measurements of the fringe width and corroborated by

scattered light measurements. The Polaroid photograph shows the general nature of the fringe, which

in this case is quite straight and structureless. We think that the variations shown in the trace

from the scanning camera in this case are mainly variations in the reproducibility of the system

rather than structural details on the fringe. For the rougher surfaces, there is excellent corres- pondence between structure shown in the Polaroid photographs and in the scanning camera traces. The

autocovariance function in the lower part of the figure shows that the primary correlation on extremely smooth surfaces is short range in nature, and that there is very little long range waviness

on the surface.

The height and slope distribution functions for the same Cervit surface are shown in figure 4. In both cases the histograms are the measured distribution functions and the smooth curves are Gaussians that have the same areas under the curves as do the measured ones. Notice that both the

height and slope distribution functions are very close to Gaussian. This was found to be generally

true for the surfaces we have examined, with a couple of exceptions to be noted later. Surfaces such

as very smooth polished calcium fluoride, magnesium fluoride and the smoothest polished copper have

height distribution functions and autocovariance functions which look quite similar to those we have

just shown for Cervit.

As an example of a polished alkali halide surface, figure 5 shows data for one of the smoothest

polished KCl surfaces which we have seen, having a roughness of 29 K rms. The jagged character of

the fringe is not much different from that of a polished plate glass surface, and the autocovariance

function has the same general form as some polished metals, as will be seen later. However, in

figure 6 we see that there are a few parts of the surface that have very large values of slopes, very

large points and deep scratches. These curves are averages of 8 different scans, so the statistics

should be reasonably good. In addition to the microstructure of scratches which Is measured by the

FECO Scanning Interferometer, polished KCl surfaces have large macroscratches which are clearly visible to the naked eye. Light scattered from these surfaces is affected by the macrostructure as well

as by the microstructure. For this reason, rms roughness values derived from measurements of scattered light are always larger than the FECO values. The rms roughness of this surface deduced from

scattered light measurements was 51 K.

Figure 7 shows a photograph of the FECO fringe, the TV scanning camera trace, and the autocovariance function for a very smooth polished molybdenum surface, having a visually measured roughness of

22 A rms. This material clearly has long range roughness characteristics as shown by the almost

regular oscillations of the autocovariance function around zero. Histograms for the height and slope

distribution functions for the same polished molybdenum surface are shown in figure 8. It is clear

that both height and slope distribution functions are quite good Gaussians, although there are some rather large deviations from the Gaussian height distribution curve.

Figure 9 shows a photograph of the FECO fringe and other information for a 32 A rms polished

titanium surface. The interference fringe is very jagged, indicating scratches and peaks on the surface. This structure causes a very rapid oscillation of the autocovariance function around zero and

an initial extremely precipitous drop from a maximum value at a zero correlation distance. Figure 10

shows that the height distribution function of the titanium surface has a general Gaussian shape, but

there are some rather large excursions from the smooth curve. Part of this structure may be caused

by the small number of scans on different parts of the surface used in the analysis. The slope distribution function is a very well behaved Gaussian with a majority of the facets having small slopes.

As a final example, figure 11 shows a copper surface that was prepared by single point diamond

machining. Under Nomarski illumination one can sometimes see a pattern of grooves of varying periodicity from such a surface. For the better machined surfaces the groove patterns become indistinct

and sometimes disappear entirely. A grooved pattern also shows up on the FECO fringes, and the

extremely sharp character of the fringes clearly indicates that the grooves have smooth sides. In

this figure we see an excellent correspondence between the undulations of the FECO fringe in the

photograph and the undulations of the TV tracing of the fringe center line. This machined copper

surface had a measured roughness of 47 A, but the light scattered was equivalent to that from a much

smoother surface. Measurements on a rougher machined copper sample gave a FECO roughness value of

70 A, and a scattered light roughness value of about 35 A. The structure in the autocovariance

158
function is undoubtedly related to the grooved nature of the surface. In figure 12 we see an anomaly

in the slope distribution function, the only one of its kind we have observed. The first three

slopes are equally probable rather than the smallest slope being the most probable. Also, the height

distribution function is very jagged, undoubtedly because of the semiperiodlc nature of the grooves.

In figure 13 we compare roughness values obtained from visual measurements of the peak-to-peak

roughness to values obtained from the statistical analysis using scanning camera data. There is a good linear relationship between the two techniques which is very encouraging considering the subjective nature of the visual measurements. The main reason why the scanning camera measurements are consistently smaller is that the scanning camera picks out the center line of the fringe and ignores

its width. Since the fringe width is caused by unresolved surface roughness as well as by the finesse

of the Interferometer, there is good theoretical justification why the scanning camera roughness

values are smaller. Because of the linear relation between the two sets of roughness values, even

though the slope of the graph differs from 45°, the scanning camera values can be corrected to give

the visual roughnesses. The minimum visually determined roughness is 8 A, while the minimum roughness

value yielded by the scanning camera is about 3 A. The latter value is determined by the reproducibility of the system. Thus, any structure in the fringe that is larger than about 3 A can be detected

by the scanning camera.

We are in the process of calculating the magnitude and angular dependence of the scattered light

from the surface statistics we have shown here. We will then compare the calculations with direct

measurements of scattered light using the Optical Evaluation Facility described by H. E. Bennett.

Merle Elson is making the theoretical calculations.

In conclusion, I have described a FECO Scanning Interferometer which is capable of yielding

statistical Information on the roughness characteristics of optical surfaces. The system has the

advantages that it does not depend on visual estimates of the fringe width to yield a value of the

rms roughness, and additional statistical Information such as the autocovariance function and height

distribution function can be obtained which cannot be determined visually.

2. Figures

Figure 1. Photograph of the FECO Scanning Interferometer. The

interferometer and light source are located near the upper center,

the scanning camera is at left, signal averager and computer are

at top center, and the teletype, used to communicate with the

computer. Is at right.

159
FECO INTERFEROMETRIC SURFACE SCANNER

SIGNAL MINI- TELE- AVERAGER COMP TYPE

Figure '2. Schematic diagram of the FECO Scanning

Interferometer. The spectrum, containing the

interference fringe that contours irregularities

on the optical surface, is scanned one line at a time by the slow scan TV camera, averaged by the

signal averager, and processed by the minicomputer. Wavelengths at 512 equally spaced

points along the fringe yield statistical information about the surface. The symbols are ex- plained in the text.

0.2 0.3 LENGTH p(mm] Figure 3. Photograph of the FECO fringe, TV

scanning camera trace, and autocovariance

function for an 8 A rms roughness polished

Cervit surface.

POTASSIUM CHLORIDE

HEIGHT

DISTRIBUTION FUNCTION

SLOPE

DISTRIBUTION FUNCTION

0.2 03

LENGTH p(mml

-B 0 B DISTANCE FROM MEAN SURFACE LEVEL (A)

0 2 4 SLOPE (X /micron) Figure 4. Height and slope distribution functions for

the same Cervit surface. The smooth curves are Gaussians that have the same area under the curves as the measured histograms.

Figure 5. Photograph, TV scan, and autocovariance

function for a 29 A rms polished potassium

chloride surface.

160
MOLYBDENUM MOLYBDENUM

HEIGHT

DISTRIBUTION FUNCTION

SLOPE

DISTRIBUTION FUNCTION

0.2 03 LENGTH pImm)

-30 -20 -10 0 10 20 30 0 DISTANCE FROM MEAN SURFACE LEVEL 1^1

2 4 6 SLOPE (S /micron) Figure 7. Photograph, TV scan and autocovariance Figure 8. Height and slope distribution functions

function for a 22 A rms polished molybdenum for the same molybdenum surface,

surfaces

.

AUTOCOVARIANCE FUNCTION

-rA

0.2 0.3 LENGTH p(mm) Figure 9. Photograph, TV scan and autocovariance function for a 32 A rms polished

titanium surface.

HEIGHT

DISTRIBUTION FUNCTION

SLOPE

DISTRIBUTION FUNCTION

-26 0 * o DISTANCE FROM MEAN SURFACE LEVEL (A)

S 10 15 SLOPE iX/mkran)

Figure 10. Height and slope distribution functions

for the same titanium surface.

MACHINED COPPER

LENGTH p(mml Figure 11. Photograph, TV scan and autocovariance function for a A7 X rms machined

copper surface.

MACHINED COPPER

DISTANCE FROM MEAN SURFACE LEVEL (X) SLOPE (X /micron)

Figure 12. Height and slope distribution functions for

the same machined copper surface.

161
VISUAL RIMS ROUGHNESS (Xl

Figure 13. Relation between visual measurements of rms roughness and values obtained from scanning camera data.

COmENTS OM PAPER BV BENNETT

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162
4.4 Roughness Measurement by Light Scattering*

John C. Stover

Dow Chemical U.S.A., Rocky Flats Division

Golden, Colorado 80401

Theoretical and experimental descriptions of how light scatters from sinusoidal

gratings lead to the relationship between light scattered from an arbitrary surface and

the spatial power spectral density function of that surface. With an appropriate measurement system, this relationship lends itself to studying the correlation between damage

threshold and surface topography for surfaces in the 10 K to 500 A (rms) range.

Experimental results are given for various types of sinusoidal gratings (variation in

magnitude, frequency and material) and several arbitrary surfaces. Measurements of

samples submitted by outside laboratories have been taken.

Key words: Light scattering; sinusoidal grating diffraction; spatial spectral

density function; surface roughness.

1. Introduction

Beam damage of laser mirrors is related to the surface roughness and is probably a function of both

the root mean square roughness (o) and the autocorrelation length (T) of the surface (generally not

found by conventional rougness measurement systems). This paper gives theoretical and experimental

results describing the relationship between light scattered by mirrored surfaces (out of the specular

beam) and the spatial spectral density curve of the surface. This curve can be used to compute

both a and T, but contains more information about the surface than just these numbers.

Any arbitrary surface profile can be thought of as being composed of a large number of sinusoidal

surfaces of various magnitudes and frequencies, running in various directions which add to compose the

actual surface. Relatively smooth sinusoidal surfaces (peak to valley less than 2 microinches or

500 angstroms) will reflect most of the light into the zero order (the specular direction) and diffract

small fractions of the light (+1 and -1 orders) on either side of the zero order. The magnitude of

this light is determined by the sinusoidal magnitude and frequency, while the position (angle of

diffraction) is determined by the grating frequency and direction. Any arbitrary surface composed of

many sinusoidal surfaces should then diffract in many directions where each direction and magnitude

defines a sinusoidal component present on the surface. Measurement of the magnitude and direction of

the scattered light then produces the magnitude and frequency of the sinusoidal components present once

it is known exactly how sinusoidal gratings scatter their light. Measurement of these quantities

amounts to measurement of the surface spatial power spectral density function. This curve defines the

surface statistics but not the exact surface profile.

This paper gives first a theoretical explanation (scalar theory) of how light should scatter from

a sinusoidal surface (or grating) and from a surface composed of many sinusoidal gratings. Then the

production and measurement of sinusoidal gratings are described. Scattering measurements are given

from each grating and compared to the theoretical model and the theoretical equation is accordingly

adjusted. Finally, the adjusted theoretical description is applied through a computer to predict how

light should scatter from a known non-sinusoidal surface with favorable results.

*This work was performed under the auspices of the United States Atomic Energy Commission.

163
2. Predictions of Scalar Diffraction Theory

Scalar diffraction theory can be used to develop a relationship between the first-order diffraction

(magnitude and position) and a sinusoidal grating given by the surface:

z = Jsin(2Trfx) (1)

In eq (1), d is the peak-to-valley depth of a sinusoidal wave of frequency, f, imposed on the

x,y plane. Figure 1 shows the orientation of the grating with respect to the incident and reflected

light waves at angles \> and e, respectively.

Application of the Helmholtz Diffraction Integral* results in eq (2) and (3).

sin e = sin \p + qxf (2)

Eq (2) gives the position of any order (q) in terms of i^, fo, and x, the light wavelength. This

relationship is known as the grating equation and is the expected result.

Eq (3) gives the ratio of the power diffracted in the first (Pj) and zero (Pq) order directions as:

Pj_ ^

[" (cos i); + cos e) Ji (h)

"* ^

Po [ 2 cos ij JothJ

In eq (3), Ji(h) and Jo(h) are the first- and zero-order Bessel Functions of the argument h given by:

(3)

A (cos e + cos ii) (4)

Eq (3) is plotted in figure 2 for various spatial wavelengths (A = 1/f) as a function of the

peak- to-val ley grating distance, d.

The Fourier transform of an arbitrary surface will consist of an infinite number of sinusoidal

gratings running in an infinite number of directions at an infinite number of frequencies. However,

the surface can be represented satisfactorily by a finite number of gratings running in discrete

directions at discrete frequencies. Following the notation of the last section, such a composite

surface is given by

z(x,y)

M N

m=L n=i

mn

2 sin ^-^mn^'^^o^^m + y sin(}>^) (5)

where a is the angle measured clockwise from the x-axis to a line perpendicular to the grating lines

and d^n/2 and fmn are the magnitude and frequency of the n_th grating in the mth direction.

If the surface is assumed to be isotropic, then measurements need be made only in one direction

(i.e., for one value of m_) in order to get all the information about the surface. The ratio Pm/Po

(any first order power in the incident plane over the specular reflection) becomes

Pin/Po=

(cos e + cosij;) Ji(hi

2 cos ij; i-] (6)

in which is identical to eq (3). For din/2 f_ly" then hi^<_.25 and Ji (h^ )==^— and Jo(hin)-l. Thus

Pin/Po

IT d in 4 X cos i) (cos e + cos (7)

This equation allows calculation of the magnitude of any spatial frequency present on the surface that

diffracts into the plane of incidence.

*Scalar diffraction theory based on the Helmholtz Diffraction Theory is inadequate to accurately

predict the behavior of the first order as the spatial wavelength A approaches the optical wavelength X. For this reason the derivation of eq (3) is not given.

164
3. Production and Measurement of Sinusoidal Gratings

Sinusoidal gratings were produced by interfering two beams from a He-Ne laser on 10E75 AgfaGeraert high resolution holographic film. The resulting fringes produce a sinusoidal density

variation in the developed film and a proportional surface variation. Grating frequency is adjusted

by changing the angle between the two beams and grating magnitude by changing the exposure time or fringe contrast. The resulting gratings were then coated with a layer of aluminum or copper.

Measurement of the sinusoidal gratings has been accomplished by several interferometric techniques. A Leitz microscope, both with and without a multiple beam attachment, was used for spatial

wavelengths above 500 microinches. A Reichert metallurgical microscope with a polarization interferometer attachment (with a Michel-Levy color chart) was used for most of the gratings with spacing

below 500 microinches. In addition, several gratings have been sent outside to labs for comparison

measurements.*

4. Light Diffraction Measurements

The experimental system is shown in figure 3. It consists of a photomultipl ier tube that measures

the power through a small aperture set at any point on an eighteen-inch circular track. The DVM is a ratio meter adjusted to read Pin/Po- Light scattered at the source is eliminated by an aperture in

front of the sample and by a spatial filter in the detector head. The light source is a He-Ne 5 mW

laser polarized perpendicular to the plane of incidence.

For mirrored surfaces the observation path is divided up into sections. One measurement is taken

inside each section and the total power scattered to that section is calculated. This number is then

used to calculate the magnitude of the sinusoidal surface component that diffracts to the center of

that section. When enough sections are used, the sinusoidal composite surface (composed of M elements

per section) will have the same statistics as the actual surface.

The experimental results have been plotted in figure 2. As expected, they are in disagreement at

short spatial wavelengths. For the purposes of calculating dm from Pm/Po' ^'^ (7) can be modified to

an approximate fit of the experimental data including small values of A.

Pi /Po

TT din (-25 + .00075 A)' (cos 9p + COS i|j)' 4 A COS ijj

A good theoretical explanation of closely spaced sinusoidal gratings is needed.

(8)

5. Roughness of Sinusoidal Composite Gratings

The rms deviation from the mean (o) which normally is thought of as the surface roughness is

defined as

j^z2(x,y) dxdy

.0 0

1/2

(9)

where the surface in question is square of edge When eq (5) for z(x,y) is substituted, a becomes

N

n=l

1/2

(10)

where it has been assumed that the surface is isotropic (i.e., uniformly rough in all directions).

The individual values of dm can be found experimentally by using eq (8) which allows computer

calculation of o.

* NBS in Washington, D.C., c/o Clayton Teague and China Lake Weapons Laboratory in California,

c/o Hal Bennett.

165
The autocorrelation function C(t) defined as

/J

0 0 can be found in a similar manner for isotropic surfaces to be

//z(x,y) z(x-T , y-T ) dxdy

C{t) - ^ 2 1

(11)

z2 (x,y) dxdy

0 0

N

din^ cos(2tt fin x)

C(T)=^i-J-^^ (12)

n=l

"^1"

where t^^ and t are the displacements in the x and y directions respectively and t is the total

displacement. The autocorrelation length T can be defined as the value of t for which

C(t) = e-1 (13)

a and T have been popularly suggested as the two quantities needed to adequately define a surface.

The method introduced in this paper, however, allows measurement of a more valuable quantity -- the

spatial power spectral density function (SDF). This function contains more than just the information

necessary to calculate o and T. The data presented later gives all three quantities. The approximate

SDF for an unknown surface is established from the SDF of the composite surface, S(;(f). This is

taken from the measured values dm as

Sr.(f)

N

i: n=l

din'

6(f - fin) (14)

where 5 is an impulse function. The integrals over f of the SDF for both the composite surface and

the real surface must be equal if both surfaces are to have the same statistics. Thus the values

(din^/8Af) at fn must be points on the actual SDF [S(f)] where Af is the frequency segment on which

the nth measurement is centered.

S(f)

f=f In

din'

8 A f in

(15)

6. Spectral Density Function as a Measure of Roughness

Eq (15) allows calculation of the SDF of the real surface. Examples of what this curve can be

expected to look like are given in figure 4. Note that the zero frequency component is considered

to be zero (because the interest is in measuring deviations from a zero frequency surface) and that

the curve approaches zero as f increases. This should be true as long as A is much greater than

atomic distances on the surface. For various applications, roughness in certain spatial frequency

regions may be more important. The SDF allows discrimination on this basis.

A complete SDF then defines all the roughness present on the surface, allowing comparison of

various measurement systems (stylus, FECO, TIS, scattering vs angle, etc.).

A well -known problem with stylus instruments is that the stylus tip tends to "ride over" surface

valleys that are much smaller than the stylus diameter. Thus the stylus instruments are not capable

of measuring the high frequency roughness. Since the rms roughness is proportional to the area under

the SDF, it is important to know the cut-off point of each instrument if rms results are expected to compare well

.

166
FECO (fringes of equal chromatic order) interferometers with appropriate computer analysis (such

as the system built by J. M. Bennett) [1 ]iare capable of making measurements of the rms roughness, the

autocorrelation function (and length) and the SDF out to about f =0.2 -0.4 (resolution is

about Z\i or f =0.5y"^). Comparison of FECO results to the scattering angle measurements described in

sections D and E is currently being carried out over this part of the SDF. Another instrument

developed at China Lake [2] measures the total integrated scatter (TIS) out of the specular beam.

The percentage of light scattered out of the specular beam is related to the rms roughness by a scalar

scattering theory [2,3] that assumes a Gaussian distribution of the surface roughness. One would

expect TIS devices to have a high frequency cutoff in the neighborhood of 0.6iJ (roughly the spatial

wavelength at which He-Ne laser light is diffracted along the surface for normal incidence). However,

since the analysis is based on the scalar theory, it suffers in the same manner as eq (3) and (7). As can be seen in figure 2, the result is that a TIS measurement does not give enough weight to high

angle scatter. The point at which this effect becomes severe is about 2.5y (lOOy") where the first

order diffraction is almost one order of magnitude below scalar expectations. Thus one would expect

the effective TIS cutoff to be in the 2.5-5y (f = 0.2 - O.Au"^) region, very close to the FECO cutoff.

In fact, good agreement has been reached between FECO and TIS measurements [1]'.

Figure 5 shows the results of using the angle scattering technique on a polished copper sample.

The lower curve is din/2 as calculated by eq (8), which is proportional to the square root of the SDF.

The upper curve is the calculated rms roughness as a function of the highest frequency component

considered. The sample was measured at China Lake on a TIS device at about 18. 2A rms which compares

favorably to 17A at the f =0.3p"^ cutoff obtained by angle scattering.

o Note that angle scattering for this angle of incidence (20°) and this wavelength (6328A) has not

yet reached the point where the SDF starts to approach zero. This is thought to be caused by two

effects. First, eq (8) is prabably inaccurate above f =0.8y"^ (interferometric data goes only

to f =0.4y"i) and second, the system bandwidth may not be large enough yet. Electron microscope

pictures [4] of super-finished metals show relatively large surface features with lateral dimensions

in theO.ly to ly range so the roughness, as indicated, may simply have larger bandwidth than the

measurements taken.

7. Summary

By examining the first order diffraction from sinusoidal gratings as a function of grating depth

and wavelength, a relationship has been developed to measure the approximate spectral density function

of optical surfaces. Use of this curve allows a basis of comparison for different measurement devices

and should be useful as a means of examining different optical production techniques and laser damage

dependence on sur-face geometry. The technique offers a method measurement that does not depend on surfaces having a Gaussian distribution and measures roughness out to higher spatial frequencies than

many devices presently in use.

8. Acknowledgment

The author is grateful to G. Wayne Dutton for making the sinusoidal gratings, to L. William Utley

for making the interferometric measurements on them, and to Dick Skulski (Dow Chemical) and

Jean and Hal Bennett (China Lake Navel Weapons Center) for many helpful discussions.

^Figures in brackets indicate the literature references at the end of this paper.

9.

[1] Bennett, J. M., "Statistical Characterization of Mirror and Window Surfaces,"

Proc. of ASTM-NBS-ONR Symposium on Damage

in Laser Materials, Boulder, Colo.

May 1974.

[2] Bennett, H.E., Bennett, J.M., and

Stanford, J.L., "Surface Irregularities

and Scattering," Proc. of The Infrared

Information Symposium, Huntsville, Ala.

Jan. 16, 1973.

References

[3] Porteus, J.O., "Relation between the Height

Distribution of a Rough Surface and the

Reflectance at Normal Incidence," J. Opt.

Soc. Amer. 53, p. 1394. Dec. 1963.

[4] Discussion with H. E. Bennett at the China

Lake Naval Weapons Center in California

of pictures taken at that facility.

167
OBSERVATION

CIRCLE

GRATING LINES

10. Figures

Figure 1. Position of sinusoidal grating,

incident beam, specular reflection and

first order diffraction spots.

MIRROR 8" RADIUS

APERTURE

SPATIAL

FILTER

LASER

SURFACE

NORMAL

DIFFRACTION FROM A MIRROR

Figure 3. Experimental system to measure scattering as a function of angle from a mirrored surface.

-„ 0.2

qT 0.1

1-<

0.01

Z

a. 0.001 - a

oc9

o

GC

O 0.0001

IIC

liJ Q

DC9

H

^ 0.00001

microtnches

A

Spatial Wavelengths of Experimental Points (microinches) - 100 - 300 A - 500

• - 800

o -1300

T

0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 PEAK-TO-VALLEY DISTANCE (dl (microinches)

Figure 2. Theoretical and experimental

results for sinusoidal gratings at several spatial wavelengths

SPECTRAL 2, DENSITY GRATING

MACHINED SURFACE

NON-PERIODIC SURFACE

I 1 II :

;

ft

\\ — r,

X— ; ^ >• SPATIAL FREQUENCY

Figure 4. Spectral density functions of

several types of surfaces.

-r

.4 .5 .6 .7 (inverse microns) SPATIAL FREQUENCY

Figure 5. RMS roughness and sinusoidal

component magnitude as a function of

spatial frequency.

coMmrrs om paper bv stover

Thz ei$($ect4 ojj polaAA.zation and coheJimce. In the, sccutteAlng iouJicz wejte, cHicuM&d, (U poalblij

InitazncJ-nQ icattaAA.ng to^ixtti. No e.{i(,2.cti daz to zohtfiznce, have, be.e,n obieAved -in icatteAlng mza^uAentdnt^ cioAJvitd out at the. HicheLion labonjoutoiy

.

168
5.1 Laser-Damage-Mechanism Identification by the Measurement of Survival Times

D. Milam, R. A. Bradbury, and R. H. Picard

Air Force Cambridge Research Laboratories

Optical Physics Laboratory

Laser Physics Branch

L, G. Hanscom Field

Bedford, Massachusetts 017 30

and

M. Bass

Center for Laser Studies

University of Southern California

Los Angeles, California 90007

The mechanism responsible for laser-induced damage can be determined

from the statistical fluctuation in the times required to produce damage at many

sites which were sequentially irradiated by equally-intense, square -waveform

pulses. Damage mechanisms are readily separated into three general categories: (1) Homogeneous absorption, (2) damage due to material defects or

inclusions, and (3) intrinsic damage due to electron-avalanche breakdown or

other fast intrinsic mechanisms.

The statistical technique for identifying damage mechanisms will be contrasted with threshold measurement schemes which have been unfruitful in

identifying mechanisms.

Key words: Absorbing inclusions; absorption; bulk dielectrics; dielectric films;

dielectric surfaces; electric field enhancement; electron-avalanche

breakdown; laser-damage statistics.

1. Introduction

It has previously been shown that laser damage in transparent materials at small focal spots is

statistical in nature. Damage may or may not occur at a given incident power level. The

nature of the damage statistics was used [l]^to identify electron-avalanche breakdown as the

intrinsic damage mechanism for nanosecond duration laser pulses. Another statistical

manifestation of breakdown has been observed in damage studies of dielectric films. [2,3] For

most films, the damage threshold is dependent on the size of the damage site, presumably because

the film contains absorbing inclusions or mechanical defects. Such imperfections can be avoided on some firings if the irradiated area is sufficiently small.

Experimental observations of statistical behavior in small-spot damage experiments have been

challenged on a number of significant points. The statistics could result from many effects other

than avalanche breakdown, such as fluctuations in the laser output, improper positioning of the

sample, a lack of material homogeneity or the presence of internal defects, or residual scratches,

dirt, or polishing compound on the sample surface.

1. Figures in brackets indicate the literature references at the end of this paper.

169
A complication in the interpretation of damage statistics observed to date is due to the fact that

single-mode laser pulses with non-constant temporal waveforms have been used. Though the waveform is highly reproducible from pulse to pulse, the fact that the intensity varies during each pulse

makes it difficult to identify the cause of the damage event. For example, the optical field required

to heat a small absorbing inclusion to a damaging temperature is so large [A], and the thermal time

constant of these inclusions is sufficiently short, that the inclusion can cause damage at an intensity

comparable to that which causes avalanche breakdown. One way to try to remove this ambiquity is

to classify the damaging events into two sets [5]: (1) the set of events in which the transmitted laser

pulse is cleanly and rapidly terminated by the damage and (2) events in which termination of the transmitted pulse is not abrupt, or perhaps incomplete, indicating that a large absorbing defects has been

encountered. The statistics of the first set of damage events are consistent with avalanche breakdown. [6]

In order to evaluate the merits of the statistical observations and of the objections to these

observations, an experiment using constant-intensity irradiation of small sites was designed at AFCRL. Our efforts, which began as a test of the hypothesis that avalanche breakdown was the

intrinsic mechanism responsible for damage to transparent materials, have led to a generalized

statistical technique for identifying the cause of laser damage. It is the purpose of this paper to describe that technique in its current state of development.

2. Description of the Technique

Consider an ensemble of identically irradiated sites, N. in number, which are being damaged at an instantaneous rate h(t). If N(t) is the number of sites undamaged at time t, then

. - h(.) NC,

or

The function N(t)/NQ is related to the damage statistics, since it is the probability of a site

surviving for a time t without damaging. We define the plot of In {N(t)/Ng) versus t to be the

survival curve and note that it depends only on the integral of the rate function h(t) which presumably

is determined only by the damage mechanisms in operation and the waveform of the irradiation.

The problem is in every way analogous to fluorescence decay, with irradiated but undamaged

sites being the analogue of excited molecules and h(t) being the analogue of the fluorescence decay

rate. The damage rate may depend on several mechanisms, just as fluorescent de -excitation may

proceed via several different channels. Accordingly, we define h(t) as a sum

n

H(t) = L h. (t) j=l

where one term h. (t) is included for each of the n damage mechanisms under consideration. If the computed funcr'tional forms h. (t) are sufficiently unique, and if one term dominates the sum in a given sample, the limiting dar^^age mechanism can be ascertained simply by comparing the

experimentally determined survival curves with those computed for the various possible damage

mechanisms.

A particularly important advantage of the, procedure outlined above is that damage mechanisms

can be identified without determining the absolute level of irradiation. Mechanism identification

is based solely on a comparison of functional forms. Absolute power density measurements, which

have traditionally been the most difficult aspect of damage experiments can be avoided if one only

wishes to determine the damage mechanism.

170
3. Qualitative Features of Survival Curves

3. 1 Electron- Avalanche Breakdown

The rate of damage by electron-avalanche breakdown has been given in the form

<" = V

where K is a material constant and E(t) is the applied optical electric field. If E(t) has the form

0 (t < 0)

E{t) = Eq (0 < t < t^)

(t > ) ,

the rate h (t) is also a constant, ea

In this case, the survival curve found at a particular power level is a linear function of time, as shown in figure 1.

Following the observation by Crisp and Boling [6] that the local value of the optical electric fie

within the dielectric was the controlling factor in many damage experiments, Bloembergen [7]

described the electric -field enhancement at mechanical defects of various shapes. The enhanced

field will lead to enhanced probabilities for avalanche development. We can use the curves shown in

figure 1 to illustrate the survival curves appropriate to defect-enhanced avalanche breakdown.

Assume that we apply the field Eq to Nq sites, and that some sites contain mechanical defects. If, for simplicity, we consider only two types of defects, some sites will experience enhanced fields

and E^j such that

^2 > ^1 > ^0 •

Then, instead of observing only sites damaging at the intrinsic avalanche breakdown rate r(Eg),

we will also observe subsets of sites which damage at the higher rates F (El) and r(E2). The

observed survival curve will be a nonlinear function of time whose slope is initially large and then

decreases, since the survival probability is the sum of three exponentials with different time

constants. Similar survival curves will be observed if many types of mechanical defects are

considered.

If the field E^ is reduced until only the enhanced rate r(E2) is observable experimentally, the

survival curve, even though resulting from enhanced avalanche breakdown, will be linear in time.

It is only by generating survival curves at a number of optical power levels, beginning with the

lowest that produces a measurable damage rate, that the intrinsic avalanche breakdown of a perfect dielectric can be distinguished from the breakdown at sites containing defects. For a perfect dielectric the survival curve is a linear function whose slope increases with increasing

field amplitude. On the other hand, for a dielectric with mechanical defects, the survival curve

is linear only at lowest field which produces damage, but become nonlinear as the field is increased

171
3, 2 Homogeneous Absorption

If the sample is a homogeneous absorber, all equivalently irradiated sites will survive until

they simultaneously reach a damaging temperature T^j. The damage probability per unit time for

absorption will approach a delta function centered about the time t^ at which the sites damage. The

fractional number of survivors N(t)/NQ will remain at unity until t^, then fall rapidly to zero.

An increase in the applied field will decrease t . The manner in which t decreases with

increasing Eq will reveal whether the absorption is linear or nonlinear. Observation of the proper

relationship between Eq and t^ is possible only if the irradiated area is sufficiently large so that

heat diffusion out of the irradiated area is negligible.

3. 3 Absorbing Inclusions

If metallic or other highly absorbing inclusions are present at some sites, these sites will

normally damage at incident power levels below that required to damage the host material. For

spherical inclusions with diameters greater than'-- X/20, where X is the laser wavelength,

simple heat-transfer arguments show that, under irradiation and with conduction cooling, the

temperature of an inclusion of radius R initially increases as 1/R but reaches a final value proportional to R. This is illustrated in Figure 2. Small inclusions are heated more rapidly, but

reach a lower final temperature.

Damage to a particular inclusion is deterministic. The statistical aspects of inclusion

damage are due to the spatial distribution of the inclusions. If the irradiated spot area is sufficiently small so that some sites irradiated will be free of inclusions, the damage will be a

statistical process.

Exact calculation of the inclusion damage rate h^(t) is difficult, but it is possible to obtain the

qualitative features of the survival curves appropriate to inclusion damage from the heating curves shown in .figure 2. At a given level of irradiation, all absorbers with radius less than some

minimum value Rmin ^^^^ escape damage, their equilibrium temperatures being less than 'the temperature T^j required to produce damage. Slightly larger inclusions will be heated to T^ by

time t-^, when the first damage events will occur. Damage events will cease at time t2 when the

largest inclusion present on any site has reached T^, or when irradiation ceases. The fraction of

survivors N(t)/NQ is unity until Sites containing inclusions that can be heated to damaging

temperatures will damage betwen times t-j^ and t^. Following time t2 the survival curve will again

have constant value because some sites are free of damageable inclusions.

An increase in the irradiation level Eq will reduce both the time to first damage t^^ and the

fraction of sites that remain undamaged. More damage events are expected under increased

irradiation intensity for two reasons: (1) Some smaller and larger inclusions, which previously

escaped damage, can be heated to a damaging temperature at the increased irradiation level, and

(2) the effective area of the site will increase with increasing power due to the Gaussian spatial

profile of the beam.

In practice, it must be realized that the thermal time constant of the submicron-sized

absorbers in question may be less than the temporal resolution of the experiment. The delay until

first damage at time t, may be the time required for the material to respond to the thermally

generated stress instead of the time required to heat an inclusion to T^^,

3. 4 Typical Survival Curves

Typical survival curves for the damage mechanisms discussed above are shown in figure 3. Hopefully, in a given experimental situation, one of the damage mechanisms will predominate,

so that mechanism identification will be possible by comparing the shape of the experimentally

determined survival curves to the characteristic shapes shown in figure 3.

172
4. Experimental Apparatus for Measuring Survival Curves

The constraint of using step-function waveform irradiation, which was assumed for calculation

of the damage rate h(t), is met by using a Pockels cell shutter to extract square -waveform pulses

from the most intense portion of the smoothly varying pulses emitted by a single-mode ruby laser.

The initial laser pulse, with the central switched-out portion missing, and the square pulse re- moved by the shutter are shown in figures 4a and 4b, respectively. The risetime of the pulse is

0. 7 nsec, and its duration can be adjusted from 1. 5 to about 50 nsec. Since the intensity of the

primary laser pulse decreases as its duration increases, square pulses 40 - 50 nsec in duration are

limited in power to approximately 50 kw. A 20 nsec pulse at a power level of approximately 200 kw

is used for most experiments.

The damage experiment is shown in figure 5. A portion of the pulse energy is picked off before

the pulse strikes the sample to serve as an intensity monitor. The remainder of the square pulse is

focussed into the damage sample and then recollimated so that the entire beam is recorded. Both

the transmitted pulse and the monitor pulse are recorded on the same oscilloscope trace by delaying one of them. A hypothetical trace is shown in figure 6. The pulse intensity is obtained by

calibrating the height of the monitor pulse against the energy arriving in the sample plane. The

occurrence of damage and the survival time for each site are indicated by a change in the transmission through the sample when damage occurs.

There are two difficult aspects to these experiments: (1) proper measurement of the spatial

profile at the waist of the tightly focussed beams and (2) proper positioning of the damage samples

when they are thin films or when it is desired to damage a sample surface.

Focal diameters (FWHM) ranging from 3 ^m - 100 /im are used. Beam waists down to about

b jJLra FWHM can be measured with reasonable accuracy from densitometer scans across multiple

images^ of the waist magnified 300 times. Smaller beam waists, obtained by using microscope

objectives as focussing lenses, are more difficult to measure, since they are near the diffraction

limit of our imaging system. As a result, absolute laser irradiation levels determined from focal

diameters 10 /nm (FWHM) or smaller could easily be in error by a factor of 2, even if reasonable

care is taken when measuring the beam. We emphasize again that the damage mechanisms are

identified by the shape of the survival curves and that a knowledge of the absolute damage power

level is not required. However, the absolute power levels are of interest in order to establish

intrinsic damage levels when possible, and an effort is made to measure these levels accurately.

When optical surfaces or thin films are studied, with small diameter spots, great care must

be exercised to ensure that the surface is accurately parallel to the precision x-y translation stage

used to carry the sample; otherwise the sample will be carried in and out of the beam waist as it is translated from site to site. The sample-to-stage parallelism can be achieved using light from

the He-Ne laser alignment beam that is reflected from the surface of the damage sample. As

illustrated in Figure 7, this reflected light, when transmitted back through the focussing lens and

viewed at a distance large relative to the focal length, forms a magnified image of the intensity

distribution in the focal plane of the lens. The distance between the sample and lens is adjusted

until the beam waist is on the sample. The angular orientation of the sample is then adjusted

until the magnified image of the beam waist remains invariant as the stage is translated indicating

that the translation stage and the sample are parallel. The final adjustment is to correct the

sample-to-lens distance for lens dispersion so that the waist of the ruby laser beam will be on the

sample.

A simplified experiment is illustrated in figure 8. Here = 8 sites are irradiated at field

strength Eq. An oscilloscope trace is recorded for each of the eight firings. The data is not

sorted after the experiment in any way except to verify that the applied electric field remains at a value Eq, within acceptable limits, throughout each laser pulse. All data meeting this criterion are

reduced, and a table of experimental results is compiled from which the survival curve is generated.

The curve is then examined for correlation with computed curves.

173

564-378 O - 74 - 13
5. Experimental Results

We have experimentally determined survival curves for a sufficient number of samples to demonstrate the utility of the technique for determining damage mechanisms. We present below

selected data gathered in damage studies of dielectric films, polished dielectric surfaces, and bulk

dielectrics.

5. 1 Damage due to Homogeneous Linear Absorption

Linear absorption in a Ti02 - Si02 multilayer dielectric film is indicated by data shown in

figure 9. At a given power level, all sites survive until a well defined time at which damage occurs. An increase in irradiation level from 1. 5 Gw/cm^ to 3. 9 Gw/cm^ results in a proportional

decrease in t^^ revealing damage due to linear absorption.

An example of the oscilloscope data for an absorbing sample is shown in Figure 10. The

deterministic behavior seen in the survival curves is reflected in the well-defined damage time for

a given irradiation level.

5. 2 Damage due to Absorbing Inclusions

Damage due to absorbing inclusions in a single-layer film of ZrO^ is revealed by the survival curves shown in Figure 11. The delay to first damage, 3. 0 nsec, and the leveling trend as damageable inclusions are exhausted, are both apparent. The data is plotted for Nq ~ 60 sites (0)

and for a smaller subset, Nq = 20 (A), which were the first 20 sites irradiated. Notice that the

basic characteristics of damage due to heating inclusions are already apparent from the survival curve generated by irradiating only a relatively small number of sites. For example, either curve

reveals that damage occured at a greater rate between 4 and 6 nsec after irradiation commenced,

and that about half of the sites survive undamaged.

We commonly observe absorbing inclusions in electron-gun-deposited dielectric films, in

agreement with earlier morphological studies. [4,10]

5. 3 Electron- Avalanche Breakdown

Survival curves indicative of avalanche breakdown in the bulk of samples of Suprasil 1 fused

quartz are shown in Figure 12. Since the focal spot used for these measurements was about

3 /jm in diameter FWHM, the total laser power incident on the sample is recorded instead of

the power density at the focus. [11]

Survival curves at two power levels, 58 kw and 6 3 kw, are linear to within limits reasonably

expected for the small number of samples in each case. However, data at a higher power level,

68 kw, is definitely nonlinear. That curve falls rapidly at first and then falls much more slowly.

Beyond 8 nsec, the remaining sites damage rather abruptly with absorption-like behavior. The

two linear curves and the initial nonlinearity of the third curve are strongly indicative that the

limiting damage mechanism is field-enhanced avalanche breakdown. The abrupt drop for long survival times, characteristic of homogeneous absorption, is thought to be an independent effect and

has been seen at very high power levels in several dielectric films, in the bulk of Owens-Illinois

ED-4 glass, and in Dynasil fused quartz. A number of possible causes for the absorptive behavior at high power levels have been proposed. It could, for example, be due to actual low-level

linear or two-photon absorption, or thermal self-focussing could reduce the spot size and enhance

the avalanche rate.

174
The indication that the limiting damage mechanism in Suprasil 1 fused quartz is defect-enhanced

avalanche breakdown, rather than intrinsic breakdown, is a new result and may appear somewhat

surprising. However, the experiment has been repeated, producing essentially the same survival curves. Moreover, this behavior is consistent with known properties of fused quartz. Bubbles are

generally present in each melt, and even the best quartz, which has been selected from high-purity

melts to contain no bubbles larger than some pre-selected standard size, should still contain very

small voids which enhance the field.

6. Conclusions

We have described the current status of development of a generalized scheme for determining

the cause of laser-induced damage. The technique readily detects absorption with a sensitivity considerably better than a previously described double-pulse [10] method. Absorbing inclusions in

electron-gun-deposited, metal-oxide films, which are known to be present as a result of earlier

studies [4,10] are also readily detected. Electron-avalanche breakdown is seen in the bulk of high

quality dielectrics, also in agreement with prior observations. [5]

There are numerous extensions of the technique yet to be made, and some features, such as the

absorptive behavior of many samples at high field strengths, are poorly understood. However, even

at the present stage of development, the technique described here is the only known way to identify

damage mechanisms within a given single sample without resort to measurements in a series of

materials or at a series of pulse durations.

7. Acknowledgment

The authors thank C. C. Gallagher, of AFCRL, for assistance with the design and assembly

fast electronic circuitry, and Marilyn Sweeney, also of AFCRL, for assistance in preparation of

the manuscript.

8. Figures

[1] Bass, M. and Barrett, H. H. IEEE J. Quant.

Elect. QE-8 , 338 (1972).

[2] DeShazer, L. G. , Newnam, B. E. and Leung, K.M.

Appl. Phys. Lett. 23, 607 (1973).

Newnam, B. E. and DeShazer, L. G. , in Proc- eedings of the 5th ASTM/NBS Symposium on [10]

Damage in Laser Materials , (U.S. GPO,

Washington, 1973).

[3] Bliss, E. S., Milam, D. and Bradbury, R. A.,

Appl. Opt. 12, 602 (1973). [11]

[A] Milam, D. , Bradbury, R. A. and Bass, M.

,

Appl. Phys. Lett. 23, 654 (1973). It has been

demonstrated that intensities as large as

10 watts/cm are required to heat submicron

diameter inclusions.

[5] Bass, M. and Fradin, D. , IEEE J. Quant. Elect.

QE-9 , 890 (1973).

[6] Crisp, M. D., Bollng, N. L. and Dube, G.

,

Appl. Phys. Lett. 21, 364 (1972).

[7] Bloembergen, N. IEEE J. Quant. Elect. QE-10,

375 (1974).

[8] Duthler, C. J., Appl. Phys. Lett. 24, 5 (1974).

[9] Winer, I. M. , Appl. Opt. 5, 1437 (1966).

Milam, D. , Bradbury, R. A. and Bass, M. , in

Proceedings of the 5th ASTM/NBS Symposium on Damage in Laser Materials , (U.S. GPO,

Washington, D. C., 1973).

Subject to statements in Section 4 concerning

the difficulty in accurately measuring small

beam waists, we note that a 1 kw Gaussian-mode

beam focussed to 3 ym (FWHM) has an on-axis

power density of about 10 Gw/cm^ . Many small

sites in Suprasil 1 fused quartz withstand

irradiation at 500 Gw/cm^, give or take a factor of two, for 20 nsec.

175
Figures

ENHANCED ELECTRON

AVALANCHE BREAKDOWN

Figure 1. Survival curves characteristic of

electron-avalanche breakdown. Intrinsic breakdown of the perfect dielectric at field strengths

Eo, Ej, and E2 produce three linear curves. If

field Eo is applied to No sites, and field Ej

and E2 are generated at some of these sites by

field enhancement, all three curves will be

observed simultaneously, resulting in a nonlinear function.

Q.

o

Q.

time

Figure 2. Temperature as a function of time

for spherical absorbing inclusions heated by

an optical field.

ABSORBING

INCLUSIONS

^-<JNTRINSlC AVALANCHE ENHANCED ^~\BREAKDOWN

AVALANCHE 8REAKD0WN^*\^

ABSORPTION

Figure 3. Survival curves characteristic of

four damage mechanisms.

Aiten plates

20 nsec/div

Figure 4. Upper: Pulse emitted by a singlemode ruby laser, with central portion

shuttered out. Lower: Square-wave pulse

selected by the shutter.

Pulse Bjpatsing

Sample

Pulse Through

Sample

Figure 5. Damage experiment. Both the intensity

monitor pulse and the pulse transmitted through

the damage sample are displayed on the same oscilloscope trace.

Figure 6. Idealized oscilloscope trace recording during damage measurements. Intensity is

monitored by the first waveform while the time

at which damage occurred is marked by termination of the pulse transmitted through the

sample.

176
FtLD STRENGTW E, APPLIED TO Nj SITES

Figure 7. Teclinique for aligning the damage

sample with the translation stage on which it is

carried. Light reflected from the sample is used

to form a highly magnified image of the intensity

distribution in the focal plane of the lens. The

sample orientation is adjusted until the image of

the focal plane remains invariant as the sample is

translated.

SITE NLNBER I SUBVIVAL Tine 7

T»C 0 1 2 3 6 7 12 D«MAGeO 0 1 1 1 1 1 1 WO 8 7 6 5 * 3 2

1 m ,75 63 SO 38 a

Figure 8. Conceptual damage experiment and

resultant survival curve.

1.0

1

0 1

01

Somple CR2 -

r °V

I5;jm spot

c r °\

3,9 GW/cm^ P 15 GW/cm* ;

4.3 GW/cm^

10 15 20 25 30

DELAY BEFORE DAMAGE

nsec

35

Figure 9. Survival curve indicating damage due

to linear absorption. The survival time until

damage is proportional to the intensity incident on the sample.

10 nsec/cm

Figure 10. Typical damage data for an absorbing

sample. The time until termination of the pulse

transmitted through the sample (that pulse on

the left of each trace) is proportional to the

pulse intensity. The upper trace has been re- touched on the rise and fall of the pulses.

i .09

SINGLE LAYER ZrOj

9 OW/em*

S 10 IS TIME, nanoseconds

Figure 11. Survival curves indicating damage

due to absorbing inclusions; A total of 60

sites were irradiated (data given by circles),

The first 20 sites irradiated yield the data

shown by triangles

177
BULK DAMAGE

SUPRASIL I (FUSED QUARTZ)

5 10 TIME, nanoseconds

Figure 12. Survival curves Indicating damage

by electron-avalanche breakdown. The initial

concave-upward curvature of the data at

68 kw indicates that limiting damage Is due

to avalanche breakdown at sites where the

electric field is enhanced. The cause for

final downward curvature on that curve is

not understood.

COmENTS ON PAPER BV MILAM

T^e ipmkeA wcl& cu,ke.d If^ Lt woa ponlblt to idzntidy the. noutwio. 0(5 tha inctw&A.on whltk tzd to a

poAticuZoA damage iltz. He Kuponde.d that the. damage moK-phology aj, dttMrnimd by thz pontion o{, thz

lUwuLmting piiJUe. wkZch IntM^tcti thz damage. i-Utz a^tzn. damage, -ci IvujtLated and theA^oKe. -it -u> imeivs-ctivz to the actual tn-UUxitlon pnoceM. He aZ6o commented that -it l^^a^ not poa-ible to ^Ind on. identi(iLj iubmicfLon incluiloyHi lAih-ich oAe deZt to be leApovLdtbte don. the damage, pfilon. to damage,

paxJxcnZaAly ilnce they axe wtdeZy ipnead and theAe U, no way o^ knoMtng which IncZuilon mJLl damage

beionehand. He j^u/i^t/xeA commented, -in geneAoZ, that despite the ^act that -indju^towi, aAe dUtfUbuted

in iize, n.ei>uZti> obtained by thAj> method oAe almoit aJlsMayi anambtguoui negaxdlng identid-ication o{,

the mechanlim o^ damage. Out o^ 50 Auni only 2 on. 3 coieA weAe ^ound -in which a cteoA id&ntt{,ication

could not be made.

178
5.2 The Dynamics of Transmitted, Reflected, and Scattered Laser Pulses

Above and Below Damage Threshold: The Search for Precatastrophic Damage

Concetto R. Giuliano

Hughes Research Laboratories

3011 Malibu Canyon Road

Malibu, California 90265

This paper deals with a detailed study of the temporal shapes on a nanosecond

time scale of laser pulses that are transmitted, back reflected, Brewster reflected,

and backscattered at small angles for different powers below and above surface

damage threshold. The sharp cutoff in transmitted light above damage threshold is

accompanied by a corresponding cutoff in reflected light while the small angle backscattered light shows a sharp increase at the same time. The shapes of the transmitted and reflected pulses are essentially the same over a wide range of powers

from below damage threshold to lOx above threshold. The small angle backscattered

intensity is the most sensitive measure of the very onset of surface damage in that

while the transmitted pulse shows essentially no change in shape for powers slightly

above damage threshold, the backscattered component shows a marked increase in

intensity. These results, along with the intensity dependence of Brewster reflected

pulses, are discussed in terms of possible precatastrophic damage effects.

Key words: Back-scattering; 0.694 \im; precatastrophic damage; pulse dynamics;

reflection; ruby laser; surface damage; temporal pulse monitoring;

transmission,

1. Introduction

The problem of laser-induced damage over the past several years has been attacked by a variety of

approaches, both experimental and theoretical. [1]1 There have been attempts to carefully quantify the

measurements and characterize the phenomena involved. Morphology of damage has been studied in detail

and a great deal of useful interpretation has arisen from these studies. [2-5] Damage mechanisms have

been proposed both as a result of theoretical first principles and interpretation of experimental

results. [2,6-9] Many ingenious experimental schemes have been devised to isolate the effects of different mechanisms and to probe details of the damage process. [10-13] New techniques have been developed for monitoring and characterizing both the sources used for damage studies and the materials being

damaged, and a number of successful attempts tiave been made to increase the damage resistance of optical

materials. [14-17]

The motivation for the work presented here has been first, to examine the dynamics of damaging

pulses in more detail than has been done previously simply to obtain more information for characterizing

details of the damage process and second, to explore the possibility of observing a subtle indication

of the onset of damage at levels of irradiation that are lower than those for generating irreversible

changes. To this end we have studied surface damage by characterizing the behavior of pulses transmitted through, reflected by, and scattered by samples over a range of incident fluxes from below to

many times above damage threshold. Time-integrated and time-resolved data have been obtained for transmitted pulses and for both back-reflected and Brewster-reflected pulses. In addition to examining the

light that is specularly reflected, we also measured the temporal and spatial properties of the light

that is scattered out of the main reflected beam at moderate angles. The amplitude of backscattered

light is very sensitive to the surface damage process and temporal irregularities in the backscattered

light have been observed in the absence of any detectable catastrophic damage. However, this precatastrophic behavior was observed only a small percentage of the time (<5%) when the incident intensity

was just below damage threshold; the remainder of the time a normal linear reflectivity was observed.

*This work was supported by the Defense Advanced Research Projects Agency thi'ough Air Force Cambridge

Research Laboratories.

Vigures in brackets indicate the literature references at the end of this paper.

179
Hence, this monitoring technique, at least within the constraints with which it was explored in this

work, does not appear to represent a practical means for detecting a damage precursor. There have been

attempts by other investigators [18-23] to observe precatastrohpic or precursory damage indications, but none has led to a practical solution to the problem. There is no doubt that a nondestructive technique

for determining the damage threshold of an optical component would be of great value to those working

with high power lasers, and it is the strong conviction of this worker that damage investigations with

this goal in mind should be pursued further.

2. Experimental Conditions

The data presented in this paper were obtained using a single longitudinal mode TEMqq Q-switched

ruby laser that has been described in detail elsewhere. [24] The light was focused onto the surface of

samples using a 19 cm lens; the focused spot radius is 55 ym (1/e for electric field). The material

used for all the data presented in this paper was sapphire, but the behavior was essentially the same for

other materials examined in a less detailed manner and, hence, is believed to be general. Also, whereas

the quantitative results may differ slijhtly for different focusing conditions, the qualitative behavior

is essentially the same.

3, Time-Integrated Transmission and Reflection of Laser Damage Pulses

In this section we report time-integrated percent transmission and reflection as a function of power

for laser pulses. The main motivation for this work has been to determine whether the decrease in transmission of damaging pulses is accompanied by a corresponding increase in reflection as might be expected

if the surface plasma density reaches a sufficiently high value.

The experimental setup for monitoring the pulses is shown in Figure 1. For the transmission experiments, the output from the detector located after the sample (detector No. 2) is integrated electrically

and displayed on one trace of a dual beam oscilloscope (Tektronix 555) while the signal from the detector

that monitors the incident energy (detector No. 1) is displayed on the other trace. Appropriate ratios

are measured over a range of incident power from below damage threshold to many times above threshold.

In a similar manner the percent reflected energy was measured in another set of experiments in which the

reflected light that passes back through the focusing lens is monitored by detector No. 3 and displayed

on the oscilloscope in place of the transmitted signal. The results of these measurements are shown in

Figure 2 where the percent transmission data are presented for comparison with the reflectivity data.

We see from these curves that both transmission and reflection show a monotonic decrease as the power

increases above damage threshold.

4. Temporal Study of Transmitted and Reflected Damaging Pulses

A series of experiments was carried out to simultaneously study the temporal shape of transmitted

and reflected pulses as a function of incident power. The purpose of these experiments was twofold:

first, to determine whether the sharp drop in transmission at the time of damage formation is accompanied by a corresponding change in reflectivity, and second, to explore the possibility that some temporal irregularities might be detectable in the back-reflected pulse in the absence of catastrophic

damage.

As we will see later, examination of the back-reflected light showed no indication of precatastrophic behavior, but there was evidence for such behavior when small-angle backscattering away from the

directly back-reflected beam was studied.

For these experiments the outputs from detectors 2 and 3 in Figure 1 were individually displayed on separate Tektronix 519 oscilloscopes. The use of either line filters or Wratten No. 70 red filters

assured that only laser light was detected. Both oscilloscopes were externally triggered by a pulse

taken from an additional biplanar photodiode that is normally used to monitor the laser output,

This external triggering scheme ensured that the two oscilloscopes are triggered at the same time (within less than 1 nsec). In this way variations in pulse shape for transmitted and incident light can be

temporally related, even though they are separately recorded.

For each of the oscillographs recorded, a double exposure was taken. In the first trace, we re- corded the signals when the laser was fired at the sample with the focusing lens removed from the optical path. For the second trace, the lens was appropriately positioned so that the light was focused on the surface of interest and the laser was fired again. The reproducibility of the ruby laser-amplifier

combination is such that in each case (i.e., with lens and without lens) the total energy incident on

the sample was the same (within 5%). Hence, in the absence of damage, each transmitted and reflected

oscilloscope photograph shows a pair of traces of the same shape and amplitude. When damage occurred

the temporal variations of the damaging pulse can be compared relative to the nondamaging pulse, both

with respect to transmitted light and reflected light.

180
Experiments of this type were carried out in detail for both entrance and exit surface damage on sapphire, and the results of representative shots for different incident laser power are shown in Figures

3 and 4. Because of the difficulty in reproducing high-speed oscilloscope photographs, tracings of these

photographs are shown.

The following features should be pointed out concerning the traces in the figures under discussion:

• The back-reflected pulse cuts off at the same time as the transmitted pulse.

• Most temporal irregularities in the transmitted pulse are duplicated in the reflected

pulse.

• The back-reflected intensity for a damaging pulse is always less than or equal to that of the reference pulse. That is, the specular reflectivity of the interface

during the occurrence of damage is always less than that of the undamaged interface.

Because of the lens diameter (2.54 cm) and distance from the sample surface (20.7 cm) the total

amount of reflected light collected in the experiments described above would have spanned a maximum

half angle of 3.5°.

5. Spatial Properties of Back-Reflected Light From Damaging Pulses

The experiments described in the preceding sections involved measurements of the reflected light collected over a half angle of 3.5°. It was of interest to obtain information concerning the spatial

properties of the light being collected and to compare it with truly specularly reflected light. To this

end an experiment was carried out, as illustrated in Figure 5, where the specularly reflected light was allowed to strike a photographic film (Polaroid Type 47). In addition to photographing the reflected

beam as a function of incident total energy, the spot was photographed for different diameter apertures

placed in the beam between the lens and the sample. In this way angular information could be obtained

for some of the off -axis features.

Qualitative features of these beam profiles are shown in Figures 6 and 7. At low powers a smooth

circular spot is seen that remains essentially unchanged when damage threshold is reached and even substantially above damage threshold ('\^ 2x). At higher powers (above twice threshold) signs of off-axis

features begin to appear and an increasing amount of light is detected in the form of irregular rings

and bright spots.

The angular extent over which the pattern is detected was measured by varing the size of the aperture between the lens and the sample. It was found that an aperture diameter between 5.5 and 7 mm was

the smallest that could be used to give the same pattern as obtained with no aperture in the system

(Figure 7). (The effective limiting aperture caused by the lens would be 12 mm at that position.) Hence

the off-axis features in the photographs extend over a half angle of from 1.5° to 2°.

It should be pointed out here that the measurements of "specularly" reflected light discussed

earlier included the light that is shown in the photographs of Figures 6 and 7. Because of the setup

used in those experiments, the angular acceptance of the detected light was on the order of 3.5° half

angle. Hence, even though a detectable amount of light is scattered out of the specular beam, the total

amount of light collected over the 3.5° half angle shows the general decrease with power above damage

threshold.

6. Measurements of Backscattered Laser Light From the Surface During Damage

Since both the transmitted and reflected pulses show a sharp decrease in intensity following the

creation of damage, one might be inclined to attribute the loss of light to absorption by the damaged

region and/or surface plasma. However, there is also the possibility that an appreciable amount of the

laser light is scattered out of the forward and back direction as well as absorbed.

A measurement of the total amount of scattered light would require the means for collecting and

measuring the light scattered over all directions or a knowledge of the scattering distribution and subsequent sampling of the light over a particular solid angle. Measurements of this type were not carried

out in our experiments but a few experiments were conducted that were directed toward determining the

temporal behavior of scattered light. Again, the purposes were (1) to explore the possibility that an appreciable amount of light might be scattered out of the main beam, (2) to obtain a temporal correlation between the scattered light and transmitted or specularly reflected light, and (3) to explore the

possibility that some indication of precatastrophic damage might be found in the temporal characteristics

of the scattered light.

The experimental setup used for these experiments is shown in Figure 8. It is similar to the previous setup, except that a white card (7 cm sq.) with an aperture was placed approximately halfway between the focusing lens and the sample. Detector No. 3 was placed to monitor the diffuse laser light

181
scattered back from the sample surface onto the white card. The aperture diameter (6.5 mm) was substantially larger than the convergent beam diameter (1.7 mm) at the point where it passed through the hole in

the card.

The experimental data were obtained in a similar way that the previously described data were taken;

the laser was fired once with the lens removed and the second time with the lens placed appropriately in

the beam, both pulses being recorded on the same oscilloscope photograph.

The results of a number of these experiments are illustrated in Figure 9. Here the data are displayed differently from before in that the scattered pulse has been inverted relative to the transmitted

pulse so that the temporal relationship can be seen more easily.

The general qualitative features seen in these traces are

• The scattered light for damaging pulses is characterized by a sharply rising spike

followed by a more slowly decaying tail.

• The peak of the scattered spike occurs at the same time that the transmitted pulse

begins its sharp cutoff.

• The scattered intensity begins to rise from its background level before the transmitted pulse shows any appreciable irregularity.

• In cases where very slight damage occurs so that the transmitted pulse shows little

or no change in shape, there is still an appreciable amount of scattered light

detected. (Relative detector sensitivities are about 50:1.)

The sensitivity of the backscattered amplitude shown in the above experiments indicates that this

type of monitoring is a more sensitive indication of surface damage than examination of either the transmitted or the specularly reflected light. Moreover, the fact that the backscattered light begins to rise

in intensity relative to the background before the transmitted light shows signs of cutoff suggests that

some evidence for precatastrophic damage may be indicated from this kind of measurement.

6.1 Precatastrophic Damage Experiments

Tentative evidence for precatastrohpic damage indications were seen in a few instances in the ab- sence of damage where the scattered light showed a definite irregularity compared with the background

pulse. In these cases the transmitted pulses were essentially identical with or without the focusing

lens in place, whereas the backscattered pulse with the lens in place had a different shape than the

pulse observed with the lens removed.

Examples of this behavior are shown in Figure 10. We see that the scattered pulse with the lens in

place is different in shape from the one in which the lens is removed. In these examples the laser power

incident on the surface was very close to the value where damage occurred but not sufficiently large to produce any detectable damage on that particular shot. All the pulses in the examples shown were followed by damage-producing pulses of slightly higher energy. Because of the detector sensitivity, it is

difficult to obtain large signals at these levels of illumination.

The amplitude of the backscattered signal for the nondamaging reference pulse depends on the size of

the aperture chosen and the alignment of the sample. Sometimes the unfocused beam was not reflected

completely through the hole in the white card and the reference signal would be correspondingly larger

than that obtained for normal incidence. Since the background pulse in these experiments is one in which

the lens is removed from the system, the diameter of the beam in passing through the 6.5 mm aperture is

different in the two cases. (The diameter of the focused beam at the l/e2 intensity points is 1.7 mm and

that for the unfocused beam is 4 mm.) Also the fact that the reference pulse is not focused means that

it strikes a larger surface area (and hence may be scattered differently) raising some question as to the

validity of comparing the pairs of pulses on the double exposure oscilloscope pictures.

In later experiments we changed the procedure for taking data so that the reference pulse could be

employed more meaningfully. For these experiments the focusing lens was left in place for both shots

obtained on the double exposure oscillogram. For the first shot recorded (the low power nondamaging

reference trace) a Wratten neutral density filter ND 0.3 was placed in the laser beam just past the GlanKappa polarizers (Figure 8). The laser intensity in this part of the beam was never high enough at the

levels we used to cause any bleaching or damaging of the filter and the 2x filter attenuation was re- producibly obtained. For the second shot of the double exposure oscillogram (the high power, potentially

damaging shot) the ND 0.3 filter was removed from the main beam and placed in front of detector No. 3 (the one that monitors scattered light) and another filter with the same attenuation as separately

182
measured was placed in front of detector No. 2 (the one that monitors transmitted light). In this way

signals seen by detectors 2 and 3 would be the same for both pulses as long as the sample behaved in a linear fashion. This linearity was confirmed in detail for a series of pairs of low power pulses,

neither of which was intense enough to create damage. The result was a number of double exposure oscillograms in which both pulses were essentially superimposed.

Using the above technique, we carried out a detailed series of experiments for different laser powers very close to but usually below damage threshold. We confirmed the results obtained earlier using

focused and unfocused beams and satisfied ourselves that the reference beam trances as depicted in Fig- ures 9 and 10 are, in fact, valid comparison traces.

Most of the measurements performed using the above described method were directed toward exploring

whether signs of precursory damage could be detected reliably and reproducibly by monitoring the backscattered temporal profile. Occasionally (less than 5% of the time), we would obtain traces similar

to those illustrated in Figure 10 where the transmitted pulse showed no change from the reference pulse

but where the backscattered pulse was distinctly different from its reference pulse and where there

was neither a visible spark of any kind nor any detectable surface damage on subsequent microscopic

examination. Possible explanations for this behavior are local heating causing a slight surface

irregularity that could scatter or deflect the beam temporarily off axis or the formation of a microscopic plasma that could scatter light. Another possibility that cannot be ignored is the chance

encounter of a scattering dust particle that happens to be illuminated somewhere between the screen

and the sample surface.

The practical result of these backscattering measurements is that while definite precursory effects

are observed in the absence of catastrophic damage, they are too subtle to be easily characterized and

at this point do not indicate a practical reproducible means for nondestructive damage threshold

determination.

7. Summary of Pulse Temporal and Spatial Experiments

The results of the above measurements can be summarized as follows:

1. The damaging pulse shows a sharp temporal cutoff both in transmission and reflection

for light collected over a cone of 3.5° half angle.

2. Photographs of the back-reflected light show that a considerable amount of this

light is scattered out of the main beam at small angles (less than 2°) in the form

of bright spots and diffuse rings.

3. In addition to the small angle backscatter seen in the photographs, an easily

detectable amount of light is scattered back at larger angles that is not seen photographical ly.

4. The light scattered back at larger angles (within a 23° half-angle cone) shows a sharp temporal spike at the time the transmitted light and specularly reflected

light cuts off.

5. Some evidence of precatastrcphic effects has been seen through observation of this

large angle backscattered component.

6. Although it is almost certain that the damaged surface site and associated plasmas

absorb a measurable amount of the incident pulse, it is not possible to obtain an accurate measure of the light absorbed during damage without integrating all the

scattered 1 ight.

8. Brewster Angle Reflectivity as a Function of Incident Intensity

During the latter stages of our backscattering measurements in search of precursory damage phenomena

we decided to explore for nonl inearities in the reflectivity at Brewster's angle. The rationale for

this type of experiment was based on the fact that the nonzero value of the reflection coefficient for

real dielectrics at Brewster's angle is attributed to slight residual absorption and that if some subtle

surface disturbance were occurring prior to catastrophic damage, it might be detectable as a power

dependence in this reflectivity. To explore this possibility we set up an experiment shown schematically

in Figure 11 in which the incident energy, the Brewster reflected, and the transmitted light were all

monitored. The signals from detectors 2 and 3 were displayed on Tektronix 519 and 7904 oscilloscopes so that we could look for temporal irregularities in the Brewster reflected light compared with the transmitted light. Reference pulses in the double exposure oscillograms were obtained in a similar manner to that described earlier. Figure 12 shows a series of double exposure oscilloscope traces comparing the

transmitted and Brewster reflected pulses over a range of powers from the onset of damage to about 3.5

times damage threshold.

183
The results of these experiments are summarized below:

• Over a range of incident power from 1/10 damage threshold up to damage threshold the

total energy reflected at Brewster's angle is a linear function of input energy cor- responding to a reflectivity of 10-4.

• Over the range of incident powers below damage threshold the temporal shapes of the

Brewster reflected pulse and the transmitted pulse are identical showing no evidence

for precatastrophic effects.

• The Brewster reflected light shows a cutoff similar to that of back-reflected light

(Figures 3 and 4) but occurring slightly later than the cutoff in transmission.

• While the transmitted and back-reflected pulses remain at a low level from the time

of cutoff throughout the remainder of the pulse, the Brewster reflected pulses return

to a "normal" level before the pulse ends (i.e., the Brewster and reference traces

meet again before the end of the pulse while the transmitted pulse remains cut off).

The fact that the Brewster reflected light cuts off later than the transmitted light suggests that

the point where the initial breakdown occurs is far enough below the surface that it does not affect the

reflected beam initially. Also supporting the idea that the initial point of breakdown is below the sur- face is the fact that there is no increase in Brewster reflectivity as a function of power as would be

expected if there were an increase in surface absorption; the amplitude of the Brewster reflected pulse

is always equal to or less than the reference pulse. The back-reflected beam, however, would be expected

to cut off at the same time as the transmitted beam because the back-reflected light is affected by subsurface effects as well. The situation is further complicated by plasma effects at the surface. Earlier

work on surface plasmas [10,25] has shown that there are two types of plasmas formed during surface damage,

an air plasma that extends along the light direction and a material plasma (explosion plasma) that

extends in the direction of the surface normal. The latter plasma is caused by the deposition of energy

in the material and subsequent explosion. For surfaces tilted away from normal incidence the two plasmas

point in different directions, and the extent of their interactions with reflected light beams will depend on the surface tilt. In particular, the explosion plasma will not interact with the incident light

as strongly for tilted surfaces as for surfaces at normal incidence.

9. Conclusion

The dynamics of damaging pulses have been studied for a number of different conditions of surface

damage. Monitoring of backscattered light at moderate angles (2° to 25°) shows it to be an extremely

sensitive means for detecting subtle aspects of damage initiation. Some signs of precursory damage behavior are noted in monitoring of backscattered light, but within the limits of the experiments performed,

this does not appear to be a practical means for nondestructive testing of laser materials. Areas for

further exploration along these lines are to monitor light scattered out of the main beam at angles other

than the ones reported in this paper. Probing of the irradiated region with other types of optical

probes are also recommended as possible means of investigating the more subtle aspects of surface damage.

The damage process is obviously very nonlinear, and once the critical conditions are met there is a rapid catastrophic runaway. But under suitable experimental conditions it should be possible to observe

the initial stages in a reversible way. It is this author's recoimendation that this type of approach

be applied whenever possible by workers active in this field in order to uncover the most subtle fundamentals of the laser damage phenomenon.

10. References

[1] For detailed references on the bulk of work [5]

done in this area the reader is referred to the contents of National Bureau of Standards j.,. Special Publications No's. 341, 356, 372 and ^

' 387 (U.S. GPO, Washington, D. C, 1970-1973).

[2] Giuliano, C.R. and Marburger, J. H. , Phys.

^"^^

Rev. Lett. 27, 905 (1971).

TBI [3] Boling, N. L. , Dube', G., Crisp, M. D.,

National Bureau of Standards Special

Publication No. 387 (U.S. GPO, [9]

Washington, D.C., 1973) p. 69.

[4] Wang, v., Braunstein, A,, Braunstein, M.

,

Rudisill, J. E. and Wada, J. Y., ibid., [10]

p. 157.

Giuliano, C. R. and Tseng, D. Y. , ibid.,

p. 239.

Hellwarth, R. W. , NBS Special Publication 341

(U.S. GPO Washington, D. C. 1970) p. 67.

Hopper, R. W. and Uhlmann, D. R. , J. Appl.

Phys, 41, 10 (1970).

Bass, M, and Barrett, H. H., lEEE-JQE 8^, 338 (1972).

Zverev, G. M. , Mikhailova, T. N.

,

Pashkov, V. A. and Solov'eva, N. M. , Soviet

Physics-JETP 26, 1053 (1968)

.

Giuliano, C. R. , lEEE-JQE ^, 749 (1972).

184
[11] Milam, D., Bradbury, R. A. and Bass, M.

,

NBS Special Publication No. 387 (U.S. GPO,

Washington, D. C. 1973) p. 12A.

[12] Bliss, E. S., this publication.

[13] Milam, D. , et al. , this publication.

[14] Giuliano, C. R. , Appl. Phys. Lett. 21, 39

(1972).

[15] Swain, J. E., lEEE-JQE 4, 362 (1968).

[16] Allen, S. D. , Braunstein, M.

,

Giuliano, C. R. and Wang, V., this

publication.

[17] Potosky, J. C. , Giuliano, C. R. and

Klein, C. F. , this publication.

'[18] Belikova, T. P., Savchenko, A. N. and

Sviridenkov, E. A., Soviet Physics-JETP 31,

1021 (1970).

[19] Belikova, T. P. and Sviridenkov, E. A.,

JETP lett. _3, 257 (1966).

11. Fi:

[20] Braunlich, P., Carrico, J., Rosenblum, B., and

Schmid, A., NBS Special Publication No. 387

(U.S. GPO, Washington, D.C. 1973) p. 91.

[21] Parks, J. H. and Alyassini, N., NBS Special

Publication No. 372 (U.S. GPO,

Washington, D.C. 1972) p. 104.

[22] Edwards, D. F., Harker, Y. D. , Masso, J. D. and

She, C. Y., ASTM Special Technical Publication

469 (American Society for Testing and Materials,

Philadelphia, Pa., 1969) p. 128.

[23] Edwards, D. F. , She, C. Y. , Masso, J. D.

,

Harker, Y. D. and Schade, H. C. , NBS Special

Publication No. 341 (U.S. GPO,

Washington, D.C. 1970) p. 102.

[24] Giuliano, C. R. and Hess, L. D. , NBS Special

Publication No. 341 (U.S. GPO,

Washington, D.C. 1970) p. 76.

[25] Giuliano, C. R. , NBS Special Publication

No. 372 (U.S. GPO, Washington, D.C. 1972)

p. 46.

res

VARIABLE

ATTENUATOR

INPUT

LASER

BEAM

(GLAN-KAPPA

POLARIZERS)

DETECTOR NO. 3

DETECTOR NO. 2

BEAM SPLITTER

SAPPHIRE

SAMPLE

DETECTOR NO I

Figure 1. Experimental setup used in transmission and surface reflection

measurements

.

185
10

>-"

t= 6

>

I-o ^

lli 4

_i

Ll_ UJ

0

100

T 1 1 1 I M I

I T 1 1

1 I I I I

•REFLECTION BELOW

DAMAGE THRESHOLD

I I I I I J I I I I I

80 -

60

w 40

<

tr

I- 20

0

T—I—I—I I M I

1 r- -TRANSMISSION BELOW

DAMAGE THRESHOLD

2112- n—I—I I I I

I III I I I I

RELATIVE ENTRANCE DAMAGE '» THRESHOLD TRANSMITTED PULSES REFLECTED PULSES

POWER

BELOW

THRESHOLD

-I.Ox

~l,5x

~l.5x

-8.0 X

~l5.0x

—

lOnsec

5

POWER DENSITY, GW/cm'

Figure 2. Integrated transmission and

reflectivity of damaging laser pulses as a function of incident power on sapphire surfaces.

Figure 3. Temporal shapes of transmitted and

reflected pulses for different powers relative

to threshold for entrance damage.

RELATIVE

^""^ THRESHOLD TRANSMITTED PULSES REFLECTED PULSES

POWER

-10

-2.5

-3.0

-4.0

-8.0

— \> .

INPUT LASER

-

BEAM

VARIABLE ATTENUATOR

PHOTOGRAPHIC FILM

DETECTOR NO. 2

/

/ n

(GLAN- KAPPA

POLARIZERS)

I DETECTOR NO. I

> 19cm

\ LENS BEAM

SPLITTER

\ SAMPLE

VARIABLE APERTURE

Figure 5. Experimental setup for photographing

spatial features of specularly reflected light.

t ["— lOnsec

Figure 4. Temporal shapes of transmitted and

reflected pulses for different powers relative

to threshold for exit damage.

186
TIMES ABOVE THRESHOLD APERTURE DIA, mm

1-0

2 0

30

5.0

80

10 0

3.0

3.5

4,0

4.5

5.0

5.5

7.0

9.0

Figure 6. Photographs of back-reflected beam

for increasing laser power above damage threshold (from 1 to lOx)

.

Figure 7, Photographs of back-reflected beam for

different aperture sizes between lens and sample.

Constant laser power for each shot ~10x threshold.

DETECTOR NO 3

VARIABLE r^' DETECTOR NO. 2 ATTENUATOR V 19cm LENS

INPUT LASER- BEAM

(GLAN-KAPPA Ll_l/ADDA '' \ r \ , POLARIZERS! i

1 gEAM SPLITTER . SAMPLE

' 1 ' WHITE CARD

I WITH APERTURE

DETECTOR NO I

Figure 8. Experimental setup used in backscattering

measurements.

187
2S3I - 5RI

TRANSMITTED

BACKSCATTERED

NEAR THRESHOLD

9(a)

/

\—-TRANSMITTED PULSES /

1 ^^-1^ 1 1 1 — 1 1 ^-4, 1

UNFOCUSED

^^^SCATTERED PULSES

^

FOCUSED

1 1 1

FOCUSED

1^-4- 1 1

UNFOCUSED >^-^FOCUSED

^

FOCUSED

SLIGHTLY ABOVE THRESHOLD

9(b)

Figure 10. Backscattered pulses compared with

transmitted pulses below damage threshold

showing temporal irregularities in the backscattered pulse.

^cr^^ol".? DETECTOR NO. 2 SAMPLE

VARIABLE ATTENUATOR

INPUT LASER

-

BEAM m

I9cm LENS

(GLAN-KAPPA

POLARIZERS)

DETECTOR NO

DETECTOR NO 3

Figure 11. Experimental setup used in transmission and Brewster reflection measurements.

Figure 9. Temporal profiles of laser pulses

transmitted through and backscattered by

sapphire samples at different powers

relative to entrance surface damage threshold.

188
THRESHOLD TRANSMITTED

POWER PULSES

-Ix

l.5x

-2x

~2,5x

~3x

~3.5x

BREWSTER

REFLECTED

PULSES

2s:

IT.

n—I—I A I—

-lOnsec »— lOnsec

Figure 12. Temporal shapes of transmitted and

Brewster reflected pulses for different powers

relative to damage threshold.

COMMENTS ON PAPER BV 6IULIAW0

Vav-ld Milam o{j the. kin fon.cz Cambridge. RUdOKck LahoK.aZon.ij comme.nte.d that hz had obioxvdd damage

pmcuMon^ only on puZiU in Mkich damage. occuAJizd ZatM. in the. putiz, that li to iay, the. obieAv&d

change in the mateJilaZ pnopeAtieJ> onty ocuutned on puLiei {>a{f{,lclently enexgetxc to cmate damage. The

ipejxkeA commented that in gene/vxl it wa6 tnae that iLuJjxble obieAvabte^ mmt be choien in oKdeA to obtain

pnecaJxu,tAophic damage in^onjmation. He ne(,eJined to the Monk PankA and AlyaM,ini on the. incnea{>e,d

6catteAing obieAved in thin {^iZmi in the pneience o^ high powex la^eA. puJi(>zi>. In nupome to a qaeAtion

conceAning the penmancnt natuAe o^ the icatt&ni.ng diagnostic, the 6peake.n commented that incneaied

i,catteAi.ng woi onZy obseAvcd duAi^ng high poweA pLLl^e^ and that theAz wo-i no peAmanent e(^{^e.ct. It wa4

pointed out that it ii veAy hoAd to inteApnet dtagnO'^tiC te^ts like icatteAing Mhen the. damaging puZie

iji veAy ^OA above thAeAhold, dae. to the ^act that a lange numbeA o{, independent icatteAing iiXeM oac

cneated and due to the. pneJiZnce. o{, competing pnoceM>eJi . Einalty, it wa6 mentioned that in thii wonk,

damage Ma.!, aMjUyi accompanied by the cneatton 0($ a plasma. This wai, attntbuted to the absence 0(5 abionbing incZusioni) and to the {^act that the SapphiAe mateAiat involved Mas good quoLity and thzAelone veAy

neoA its intAinsic damage Limit.

564-378 O - 74 - 14

189
5.3 Rutherford Backscattering Diagnostics of

Laser-irradiated GaAs

R.R. Hart, C.R. Giuliano, and H.L. Durvlap

Hughes Research Laboratories

3011 Malibu Canyon Road

Malibu, California 90265

The Rutherford backscattering of 280 keV He has been used to study the amount

and depth distribution of lattice disorder introduced in GaAs by intense laser irradiation. The lattice remains largely intact after irradiation at a laser power level

of three times the damage threshold (optical) except within a 175 A surface layer.

In this surface layer the number of scattering centers increases to about 2.5 times

the number of Ga and As atoms present in the surface oxide on undamaged GaAs.

Key words: Ion channeling; laser-damaged GaAs; Rutherford backscattering;

scattering centers.

1. Introduction

The energy spectrum of Rutherford backscattered light ions, which are incident along a major channeling axis of a target crystal, can provide quantitative information of both the amount and depth

distribution of lattice disorder. Such measurements have been especially useful for the study of ionimplantation-produced disorder in Si [l,2].l We have extended this technique to the study of laserdamaged GaAs. See Khan [3] for a general discussion of measurement techniques.

The experimental arrangement is illustrated in figure 1. In the present work, a 280 keV He^^ ion

beam was used for all measurements. The beam was mass analyzed and was restricted to a divergence of

less than 0.02° and to a diameter of about 0.3 mm by a final 0.3 mm diameter aperture. The aperture

could be moved parallel to the target surface to permit measurements across the diameter of the laserdamaged area. Backscattered He particles were detected at an angle of 160° with respect to the incident

beam with a cooled Si detector. The GaAs sample was mounted on a goniometer to permit either <100>

or random alignment of the crystal with tlie incident beam. A biased Faraday cup arrangement was utilized for target current measurements. The GaAs sample used for this work was an n-type wafer which was <100> oriented and polished. The present study was limited to the B face of the wafer.

A local area of the sample was irradiated with the unfocused output of a single transverse and

longitudinal mode Q-switched ruby laser that has been described in detail el sewhere. [4] The pulse duration was 20 nsec and the beam spot size at the sample surface was 1.2 mm (1/e radius for intensity).

Under these conditions the entrance surface damage threshold for the sample for which data is presented

in this paper ranged from 12 to 15 MW/cm2.2 Here, damage is defined as a visual change at the surface

as viewed in the damage setup at 30x magnification. At this low magnification the damage appeared as a change in the surface reflectivity and a fine speckled texture. Under higher magnification, scanning

electron micrographs showed a general micro-molten appearance characterized by damage pits from 2 to

4 H-m in diameter scattered over the entire laser damaged area. The damage morphology is similar to that

shown by Smi th. [5]

Backscattered energy spectra of 280 keV He^^ incident in a random direction and along the <100>

axis of both an undamaged area and near the center of the laser-damaged area are shown in figure 2. Consider first the random spectrum. The edge at a backscattered energy of 226 keV corresponds to the

backscattering of He'''+ from Ga and As atoms at the sample surface. The yield at decreasing backscattered energies corresponds to backscattering from Ga and As atoms at progressively deeper depths below

the surface. The depth scale indicated on Figure 2 (16 A/keV) was estimated by extrapolation of stopping power data of He in Ge. [6]

Figures in brackets indicate the literature references at the end of this paper.

This is defined as the total power divided by ira where a is the 1/e intensity radius giving the power

density on axis.

190
The strong decrease in yield of the<100> spectrum from the undamaged crystal, as compared to the

random yield, is caused by ion channeling effects in a crystalline lattice. [1] The peak at the sample

surface is attributed to backscattering from Ga and As atoms in the surface oxide. The <100> spectrum

obtained by He"*""*" analysis near the center of the laser-damaged area is also shown in figure 2. In com- parison to the<100> spectrum from the undamaged area, a substantial increase in the area of the surface

peak, as well as an increased yield at deeper depths, are noted. The increase in area of the surface

peak is caused by an increased number of scattering centers near the surface of the laser-damaged area. Note that the widths of the surface peaks correspond to the detector resolution, i.e., 175 A for the

GaAs surface. Thus, the actual widths are less than or equal to 175 a. The scattering centers which

contribute to the increased area of the surface peak may be either Ga and As atoms displaced from lattice

sites by greater than 0.1 A or Ga and As atoms present in an increased thickness oxide layer. The

increase in <100> yield at deeper depths is primarily due to dechanneling of the He"*"""" in traversing the

damaged or altered surface region; however, some deep disorder or strain cannot be ruled out. Nevertheless, it seems significant that at depths greater than 175 a a large fraction (greater than 80%) of the

beam remains channeled, indicating that the GaAs lattice is largely intact.

The areas of the surface peaks, the cross-hatched areas of figure 2, were determined on the assumption of linear dechanneling corrections as indicated by the dashed lines. Based on the Rutherford scattering cross section the areas of the surface peaks were converted to the number of scattering centers

per cm2. These results are plotted in figure 3 as a function of distance across the laser-damaged region

of the sample. The profile of energy density of the laser beam is also shown in figure 3, and the damage

threshold (optical) is indicated. It can be seen in Figure 3 that the number of scattering centers per

cm^ begins to increase above the undamaged level near the energy density corresponding to the damage

threshold. Moreover, the number of scattering centers per cm2 increases nonlinearly with the energy density towards the center of the laser irradiated area to a maximum of about 2.5 times the number in the

undamaged regions of the sample.

In conclusion, we have shown that the technique of He^^ backscattering and channeling provides quantitative information regarding the number and depth of scattering centers introduced into a crystalline

surface region by intense laser irradiation. For the case studied here, i.e., GaAs irradiated at a laser

power level approximately three times the damage threshold (optical), the lattice is largely intact

except within 175 A of the surface. In this surface layer the number of scattering centers increases to about 2.5 times the number of Ga and As atoms present in the surface oxide on undamaged GaAs.

2. References

[1] Davies, J. A., Denhartog, J., Eriksson, L. and

Mayer, J. W. , Can. J. Phys. 45, 4053 (1967).

[2] Hart, R. R. , Rad. Effects 6, 51 (1970).

[3] Khan, J. M. , NBS Special Publication No. 372

(U.S. GPO, Washington, D.C. 1972) p. 75.

[4] Giuliano, C. R. and Hess, L. D., NBS Special

Publication No. 341 (U.S. GPO,

Washington, D.C. (1970).

[5] Smith, J. L., J. Appl. Phys. 43, 3399 (1972),

also in NBS Special Publication No. 372, (U.S.

GPO, Washington, D. C. (1972) p. 70.

[6] Chu, W. K. and Powers, D. , Phys. Rev. 187, 478

(1969).

3. Figures

Figure 1, Schematic of backscattering setup.

191
DEPTH, &

1000 800 600 400 200 0

n

\ \ \ I r

BACKSCATTERED ENERGY, keV

Figure 2, Backscattered energy spectra of

280 keV He++ incident in random and <100>

directions on undamaged and near the center

of the laser-damaged area of a GaAs sample.

The cross-hatched areas are proportional to the number of scattering centers within

175 K of the surface.

32B2 - 2

-3-2-1 0 I 2 DISTANCE, mm

Figure 3. The number of scattering centers

per cm^ within 175 A of the surface as a function of distance across a laser-damaged

area of GaAs. The .p rofile of the energy

density of the laser beam together with the

damage threshold (optical) are also

indicated.

COmEMTS ON PAPER EV GIULIANO

HoAold Po-i&n o{i tho. Kin. Tofice. CmbfUdgt RuwAch la.bon.aXaK.Lj indicoittd. thaX he. waA not 06 optanutic

04 the 6pe.akM. about u^tng RuXheJi^ond back-iaatteJiing a6 tvidencz of, cnyitattine. dJjion.deA. He. 6uggz6t&d

-it might be beXttn to uie, {fOn examptt, tnammUiiton oZzcXnon micAOicopy to oAceAtatn that thz mate/Uat

wiX4 -indzzd dtiondzned. The. ipzakeA commenttd that -in thu, mnk the.y weAe not able, to tde.nti{iy the

onX.g-in 0() the dJjsonden, but onty that the back-icatteAlng had -incAea^ed om a n.eMxLt o^ toieJi tltumination.

Thii> lncjiexu>e. -in back-6catteAing might, don example, be due to a chemical pn.oceM> -in which thz gaULim

on a/v{,enic atom -in the lattice. weAe neplaccd by chemtcal. compounds . In any coie, any dAyifiaption th&

latJu.cz mutd be. e.xpe.cted to lead to an -incjiza^e. -in the. back icaXteAlng. John Khan, o{, Lawnejice

LiveAmone. LahoAotonjj , commented that he wa^ iuApnjj>ed that -in the. pne^ence iach pAonounccd damage

monphology that only a imalZ change in the channeZing -in the cAyital Mai obieAved. In the pneAence o^

iach maii-ive changes -in the mateJvial i>uAiace he -indicated that, he mold expect a pnonounced change In

the back-iiCatteAing i-ignal. The 6peakeA -indicated that thli Mai Indeed kii expectation urvtLt the Monk

Mai pen(,onmed and that he too Mai -iuApAlied by the neXatively loM back-icaXteAing ilgnal that conAeiponded w-uth -ilgni^lcant iuA^ace damage. In the ddcui-ilon, the question Mai njjuiied o{, the poi,6lblUMj

that the damage mechaniim con-i-iited of^ melting and necny-italllzatlon. The po-ii>lbltitjy MOi alio nalied

that the damage pnocei6 might Involve the complete nemoval o{, itnongly dliondened matenAXil.

Jean Bennett of, the Ulchetion Labonatony Indicated iuApAiie that -ilgnl^ilcant visible damage Mai ob-ieAved

Mhen the RuutheA{,ond back-icatX.eAing Indicated that the extent o^ the damage wa6 onJiy o(, the ondeA o^ 300

[Angitnomi] Into the itx/ijjace. The -ipeaken commented that, no dlagnoitlcA othen than back-icatteA Mene

employed In the analy-ili theie -iamplei.

192
5.4 Surface Defects on Crystals of Ti02 and YVO4

Studied by Laser-Induced Damage Effects

K. M. Leung and L. G. DeShazer

Center for Laser Studies

University of Southern California

Los Angeles, California 90007

The nature of surface defects on single crystals of rutile (Ti02) and

yttrium orthovanadate (YVO^) was investigated by laser-induced damage

effects using a TEMqq Q-switched ruby laser. It was demonstrated that

damage to these crystal surfaces can be distinguished as defect damage

and intrinsic damage, where the damage thresholds decreased with increasing focal spot-size of the laser beam. The dependence on the directions

of crystallographic axes and electric field was studied. The morphology of

the surface damage sites was examined by the scanning electron microscope

revealing that polishing imperfections were the cause of damage on Ti02

surface and inclusions were the cause of damage in the case of YVO^.

Key words: Damage morphology; polishing compounds; spot-size

dependence; surface defects; rutile crystal damage;

yttrium orthovanadate crystal damage.

1, Introduction

At the Fifth Annual Symposium on Laser Damage, we demonstrated that the distribution

of coating defects can be related to the focal spot-size of the laser beam when measuring the

damage threshold of a dielectric thin film, [l] ^ However, since it is very difficult to identify

what type of defect on thin films enhances laser damage, we transferred our defect investigations

to surfaces of single crystals. We report here that surface defects on transparent dielecti-ics,

either imperfections due to polishing or inclusions due to impurities, can be revealed by using

the same approach as to the dielectric coatings.

The nature of surface defects on two uniaxial single crystals was investigated. By measuring the lowest damage threshold on the samples as a function of laser beam spot-size and by

examining the morphology of the corresponding damage sites, we find that polishing imperfections were the cause of damage on rutile surface and inclusions were the cause of damage in

the case of yttrium orthovanadate surface. The damage dependence on birefringence can be

fully explained by Fresnel reflection losses, not requiring Bloembergen' s model for expIanation[2].

2. Experimental Procedure and Preparation of Samples

The experimental setup and procedure used in measuring the damage threshold of crystal

surfaces is similar to that in the case of dielectric thin films. [1] The laser-induced scatter

(LIS) technique is emphasized for damage diagnosis in our work. The spot-size is the 1/e^

radius of the intensity profile of the near Gaussian TEMqq ruby laser beam, and is measured

by a pinhole scan detector. Focal spot-sizes ranging from 52|Jm to 215fjm were used and areas

of a test sample were irradiated by one shot only. The energy density in the laser beam was

Figures in brackets indicate the literature references at the end of this paper.

193
varied by means of a liquid CUCI2 filter while the output energy of the laser was kept constant.

Defect and damage site morphology were studied using a scanning electron microscope as well

as an optical microscope.

A single crystal of rutile (Ti02) and a synthetic single crystal of yttrium orthovanadate

(YVO4) were used to study surface defects by laser-induced damage effect in this work. Both

crystals are important optical materials since Ti02 is a very common coating compound and

YVO4 has great potential to be a substitute for calcite and is an important solid-state laser

host. They both have a large birefringence (see table 1). The samples used in this work

were cut from single crystal boules and hand-polished to a block. The polishing compounds

used were the usual types of roughing powder, with Linde A and C as the final finishing agents . A detailed spectrochemical analysis of the AI2O2 finishing compounds is displayed in table 2. The dominant trace elements are Si, Fe, and Pb. In general, the quality of the YVO4 surfaces

is better than that of the Ti02 surfaces.

Table 1. Indices of refraction of three uniaxial crystals at 6943A

Material "0

Birefringence

E 0

Calcite 1.486 1. 658 {-) 0. 172

Rutile [3] 2. 88 2. 58 (+) 0. 30

YVO^ [4] 2. 20 1. 98 (+) 0. 22

Table 2. Semiquantitative analysis of the polishing compounds r5]

Element Linde A Linde C

Al 52. % 52.

Si 0. 062 0. 023

Fe 0. 005 0. 0025

Mg 0. 0011 0. 001

1

Ca 0. 00071 0. 0012

Cr 0. 00041 0. 00030

Cu 0. 00031 0. 00013

Ga 0. 0036 0. 0036

Pb 0. 01 0. 031

%

3. Laser-induced Damage Effects on Rutile and YVO^ Surfaces

Damage threshold was measured as a function of the ruby laser beam spot-size.

Figures 1 and 2 show the experimental data along with theoretical curves fitted using the same

model as for the case of dielectric thin films [1,6], It is seen that the damage threshold

depends strongly upon the focal spot-size of the laser beam. For Ti02 , the threshold of defect damage was measured to be about 10 J/cm^ while it was 8 J/cm^ for YVO4. In both cases

a factor of 3 on damage threshold was observed as the spot-size decreased from 190|jm to 52|am. It is found that the LIS method is more sensitive than the spark detection method for

the case of rutile surface, but not for the case of YVO4.

194
Since both crystals have large birefringence, we have attempted to correlate the laserinduced damage effects to the orientation of the incident laser electric field to the crystallographic axes. For the rutile crystal, three different configurations were used in the experiment:

axial, transverse P (optic axis parallel to the incident field) and transverse S (perpendicular to the field). Figure 3 displays these three configurations. For YVO^ , only the transverse S and

P configurations were studied. It was thought that, by variation of spot-size, different defect

damage thresholds could be observed since the enhancement factor depends on the refractive

index of the material which is different for the S and P configurations. The experimental results are shown in figures 4 and 5, For each crystal surface, the S and P data were obtained

and fitted with theoretical curves. However, when the Fresnel reflection [7] was taken into

account to calculate the internal field intensity, the S and P data became identical. For example, table 3 compares the incident and internal field intensities for the intrinsic and defect

damage thresholds of the two crystal surfaces. We conclude that Bloembergen' s type of en- hancement factor does not explain these results and the effect on the crystalline birefringence

Is only present for the incident field. The morphology of damage sites also reveals this aspect.

Table 3. Incident field intensity versus internal field intensity.

All intensities are in units of Joule/cm^.

Crystal Configuration Incident

I.

1

Internal

I.

I

Incident Internal

^d

S 36. 3 29. 3 12. 0 9. 2 Ti02

P 38. 3 29. 3 12. 0 9.6

S 20. 3 18. 1 8. 0 7. 1 YVO4

P 22. 0 18. 9 8. 0 6. 9

4. Damage Morphology

The damage sites were examined by an optical microscope and a scanning electron microscope (SEM). Typical morphologies of the surface damage sites are illustrated in the following

ten SEM pictures (figures 6 to 15). In general, small damage sites induced by a 50 to 80|im

spot-size laser beam are similar for both crystals. Figure 6 shows an example of this small

site for the Ti02 surface. It appears like a 2|jm crater. Large damage sites induced by a laser beam of at least 150(Jm spot-size show different morphology for the two crystals. They

are different in several aspects. For the rutile crystal, a single shot damage site creates

many irregularly distributed small craters within the irradiated area [see figures 7, 8 and 9).

When a plasma was observed on a particular damage site, fracture on the site follows some

crystallographic orientation which may reveal the crystal structure (see figure 10). For YVO^

crystal, no fracture with any pattern was found on large damage sites and occasionally there

were some irregularly distributed small craters within a site but not as numerous as those of

the rutile crystal. However, there is one distinct feature of damage morphology which was not

found on the rutile crystal. Figures 11, 12, and 13 show some of these damage sites. They

appear like "popcorn" where the inclusion is popping out of the surface [8] . In brief, on rutile

the crater morphology is dominant, while on YVO^ surface, the "popcorn" morphology is over- whelming. This indicates that surface defects due to inclusions have enhanced the damage level.

When we compared damage sites corresponding to the S and P configuration tests, no morphology differences could be found. Also it was observed that damage was not always initiated by the structural discontinuity of a scratch, but by some entrapped absorbing compound,

presumably a chemical from the polishing powders. Figure 14 shows a site struck by 150|am

195
beam with five small damage craters along a surface scratch. In addition, figure 15 shows a similar damage site on the YVO4 surface except that the craters do not appear on an obvious

scratch. Thus it seems that an enhancement of the laser electric field was not produced by

the scratch but by absorbing chemicals entrapped in the surface. Recall that traces of iron

oxide were present in the polishing powders.

5. Summary

Two dielectric crystals, rutile and yttrium orthovanadate, were tested for their damage

resistance to the laser irradiation at 0. 69|am. It was demonstrated that the measured damage

threshold of these materials could be distinguished as intrinsic damage and defect damage.

Surface defects are easily studied by this approach. The morphology of the surface damage

sites reveals that entrapped polish particles were the cause of damage on Ti02 surface and inclusions were the cause of damage in the case of YVO^. The fact that the defect damage threshold is independent of the directions of crysta llographic axes with respect to the optical field

supports this conclusion.

6. Acknowledgements

The authors wish to thank Mr. Jack Worrall at the University of Southern California for

his SEM work. This study was supported by Crystal Products Department, Union Carbide Corporation, San Diego, California. Both the rutile and yttrium orthovanadate crystals used in this

study were grown by Union Carbide Corporation.

7. References

[1] L. G. DeShazer, B.E. Newnam and

K. M. Leung, in Laser Induced Damage

in Optical Materials: 1973 , edited by

A.J. Glass and A.H. Guenther (NBS

Special Publication 387, December 1973),

pp. 114-123.

[2] N. Bloembergen, App. Opt. 1_2' (1973).

[3] W. L. Bond, J. AppL Phys. 36, 1674

(1965).

[4] E.A. Maunders and L. G. DeShazer,

J. Opt. Soc. Am. 61_, 684 (1971).

[5] Spectrochemical analysis was performed

for use by Pacific Spectrochemical Laboratory Inc. , Los Angeles, California

90064.

[6] L. G. DeShazer, B. E. Newnam and K. M.

Leung, Appl. Phys. Lett. 23^, 607 (1973).

[7] See for example, M. Born and E. Wolf,

Principles of Optics , 4th ed. (Pergamon

Press, New York, 1970).

[8] C. J. Duthler, Appl. Phys. Lett. 24, 5 (1974).

196
8. Figures

SW6LE CRYSTAL RUTILE (T.O,)

d. r 75 (fitted)

100 ISO SPOT SIZE . W„(,m)200

Figure 1. Spot-size dependence of surface damage

on a single crystal of rutile (TiOa) in the axial

configuration test. The bars on the data points

indicate the range of measured values and are not

error bars. The intrinsic damage threshold is

the upper limit and the defect threshold is

the lower limit.

r

SWOLt CRYSTAL YVO,

^,-275

d.-70 ^"

100 ISO

ttOT 1I7E W,.fdw^ Figure 2. Spot-size dependence of surface damage

on a single crystal of yttrium orthovana-date

(YVOi)) in transverse P configuration.

S P

Figure 3. Schematics of crystal orientations

with respect to incident laser electric field.

Optic axis (OA) of the crystal is indicated

by the parallel lines.

30

20

"t

UiO

HlO

SIHCLt CHYSTHI. YVO4

d. • TO (f,tm) 5 "o

m ISO SPOT SIZE, )V„(^~

Figure 5. Spot-size dependence of surface damage

to YVOi, crystal for transverse S and P configuration tests.

5IN6Lt CSYSTAL Rl/mC (TiQ,)

4. "is /""(f.MaiJ

100 SPOT size w.

Figure A. Spot-size dependence of surface damage

to Ti02 crystal for transverse S and P configuration tests.

Figure 6. SEM photograph of a small damage site

on Ti02 surface; the spot-size is 52 pm. The

crater is 2 ym in size.

197
Figure 7. SEM photograph of a large damage site

on rutile surface; the spot-size is 160 \im. Note

that this is a single shot damage site and the

small craters are irregularly distributed.

iui,

Figure 8. SEM photograph of a large damage site

on rutile surface using transverse S configuration. The spot-size is 150 ym.

Figure 9. SEM photograph of larger damage site

on rutile surface using transverse P configuration. The spot-size is 160 ym and no spark was observed.

Figure 10. A very large damage site with a surrounding white ring on the rutile surface.

A spark was observed in this case and the

spot-size is about 190 ym.

OSfxm

Figure 11(b)

Figure 11. (a) SEM photograph of a large damage site on YVOi, surface; a spark was observed and the spotsize is 160 ym. (b) Close-up view shows that the damage is due to a nonmetallic inclusion, 1.2 ym in

diameter, which was about 0.2 ym below the plane surface. The morphology has crack nucleation similar

to that in reference 8.

198
J

iFlgure 12(a) I^H^^^HHM^HHHHI^HI — Figure 12(b)

Figure 12. SEM photograph of another damage site on YVOi, surface; a huge spark was observed and the spotsize is only 72 ym. The initial LIS level is high. (a) The top view of the site suggests that a cluster

of inclusions was at the center of the site to initiate the damage. (b) Close-up view was taken with the

electron beam at 84° with respect to the surface. No crater is observed on the site.

Figure 13. SEM photograph of another damage site

on YVOi, surface; a spark was observed and the

spot-size is about 150 ym. It can be seen that

material pops out of the site.

Figure lA. A large damage site on YVO^ surface

but no spark was observed. Note that this is a single shot using spot-size of 150 ym and

the distinct small sites distributed along the

surface scratch.

Figure 15. A similar damage site as shown

in figure 14, except one cannot observe

any scratch along the four collinear

damage craters.

WO COmENTS ON PAPER 8^ LEUNG.

199
6-1^ Pulsed CO. Laser Window Damage Processes

R. A. Shatas, J. D. Stettler, L. M, Narducci,*

S. S. Mitra,t and H. C. Meyer

Quantum Physics, Physical Sciences Directorate

U. S. Army Missile Command

Redstone Arsenal, Alabama 35809

We examine the applicability of the extended avalanche breakdown process to describe the semiconductor window surface damage by pulsed high pressure CO2 lasers.

In the extension to optical frequencies of Shockley's avalanche theory construed

originally for a dc electric field, a hard momentum-reversing collision of the free

carrier is required to occur at the instant of optical field reversal. In contrast

to alkali halides, the mobility is very high in semiconductors, and the probability

of a momentum-reversing collision is too small to account for experimental observations of damage thresholds. Alternate models of damage processes are discussed.

They are based on quantum mechanical photon assisted tunneling probability calculations between the bound and the quasi-free states and resonant collective excitations of carriers in the quasi-free state. Theoretical predictions for maximum

pulsed CO2 laser fluxes are made for some typical semiconducting window materials.

Key words: Damage threshold of GaAs; extended Shockley avalanche; pulsed laser

damage; semiconductor infrared windows; solid state plasma; twostream instability threshold.

1 . Introduction

Recent reviews [1,2]^ of laser pulse damage to infrared transparent materials explain the damage

mechanism in terms of the optical electric field-initiated avalanche ionization. The dominant features

of this damage process are ascribed to the ionizing collisions of a few free carriers present initially

which are accelerated above the band gap energies by the alternating field. After several generations

of the free carrier creation, the Drude absorption (absorbance independent of light intensity) is taken

to be of sufficient magnitude to make the material no longer transparent to the incident laser radiation. Although this process is expected to dominate the breakdown in materials such as alkali halides,

in semiconducting window materials such as GaAs, CdTe, and others which exhibit high free carrier mobilities and low band gaps, collective solid state plasma excitations may play a dominant role in interactions leading to damage. In particular, the collective excitations may cause plasma instabilities to occur which lead to an intensity-dependent absorbance.

In section 2, we review the phenomenological theory of avalanche breakdown. In this context, we regard the avalanche as a single-electron initiated process analogous to the gas breakdown. We consider

the two distinct limits of the electron-lattice collisional damping mediated by the free carrier interaction with phonons which determines the carrier behavior in the optical electric field. The strong

damping condition is satisfied whenoujj, the angular laser frequency, is less than the collisional frequency Vg. However, for some semiconductor materials, the reverse is true; because of the relatively

weak damping of a free carrier by collision with the lattice, the free electron-electron interaction

allows for the collective electron excitation of the solid state plasma.

In section 3, we show that the Shockley dc avalanche breakdown can be extended to optical electric

field frequencies if Vg > lu^. However, the extension of the Shockley avalanche breakdown cannot be

justified for the case of Vg < (u^, primarily because of the lack of momentum-reversing collisions. We

demonstrate these differences numerically for two IR window materials, namely NaCl and GaAs.

In section 4, we consider the resonant interaction of carriers with the laser light which occurs when the carrier plasma frequency approaches that of the laser. Above certain amplitudes of the optical

*Permanent address: Department of Physics, Worcester Polytechnic Institute, Worcester, MA.

tPermanent address: Department of Electrical Engineering, University of Rhode Island, Kingston, RI.

1. Figures in brackets indicate the literature references at the end of this paper.

200
electric field, the free carrier plasma is driven unstable and absorbs the laser light anomalously.

Thresholds for the two-stream and the parametric instabilities are deduced; their numerical values agree

within an order of magnitude with experiments.

2. Acceleration of Free Carriers in an Optical Electric Field

The energy distribution function of initially present free carriers is altered by the application

of the optical electric field. Hyperthermal carriers at the high energy tail of the altered distribution undergo collisions when their kinetic energy exceeds the ionization potential and consequently

create a new generation of free carriers. The exact description of the time dependence of the carrier

distribution function requires solving some form of a transport equation. Treatment of this problem

within the framework of the Fokker-Planck equation has been considered by several authors [3,4]. In the

lowest order of approximation, however, the transport problem can be considered in terms of a single

average carrier. The kinetics of this average carrier is described by Euler-Lagrange equations for a nonconservative free carrier-electric field system in which the carrier-lattice interaction is expressed

by a phenomenological dissipation function containing the effective carrier-lattice collision frequency

Vg. Let L = T - V be the Lagrange function of the system and let us express the Rayleigh dissipation

function by

^ = mq

The Euler-Lagrange equations of motion for a nonconservative system formally are written as follows:

^SL_dL

dt ^. dq • dq oq

In our case, they assume the familiar form

mq + e E exp (-itu t) + v mq = 0 , (2)

where m designates the free carrier mass and Eq is the electric field amplitude associated with the

normally incident laser flux S^, For crack and bubble free materials, Eg is given by the expression

J32n S./c

1 +

Cracks and bubbles encompassing domains smaller than the wavelength of the laser can lead to a local

enhancement of Eq as shown by Bloembergen in his consideration of the electrostatic boundary problem for

cracks and inclusions [5]

.

The steady-state solution of eq. 2 for the carrier velocity is given by

q=--E exp (-iu).t)(—: ^TTT ) • W

The free carrier extracts energy from the laser electric field at a rate

V 2„2

W = "l^eCq . e E*) = -2-^^ , (5)

where E is the rms value of the electric field E = E^/^

The case of a relatively weak carrier-lattice interaction au^ > is known as the "high frequency"

limit [6] of the Frohlich model [7], In this case, eq. (5) becomes

V 2^2

• e e E ^1 = - ~ •

201
Note that the lowest otder of the expansion of the carrier distribution function also yields eq. 5 (see

eq. 12 of ref. [3]). The opposite case u)^ < Vg yields the dc limit of the Frohlich model used in his

construction of the dc avalanche breakdown [7] . Here, the average rate of energy extracted from the

electric field by a free carrier is given by

e

A phenomenological dc breakdown model which incorporates the essential features of the electron ava- lanche initiated by the accelerated free carriers has been constructed by Shockley [8], In the next

section, we will consider the extension of his model to optical alternating fields for alkali halide and

semiconductor- type window materials.

3. Shockley Breakdown in an Optical Electric Field

The argument for the avalanche breakdown is essentially built on the extension to an ac field [9]

of Shockley' s phenomenological theory originally conceived for a dc field. This extension is possible

if a few "lucky electrons" experience momentum-reversing collisions which occur just at the time of the

reversal of polarity of the alternating field. Thus, the acceleration of the electron continues throughout the several optical cycles needed to impart to the electron kinetic energy in excess of the ionization potential. This argument predicts that the rms value of the alternating field must be higher than

that of the dc field to obtain an avalanche breakdown. A slight increase in the breakdown field in

alkali halides indeed has been demonstrated by Fradin and Bass for the bulk damage [lO] . A much larger

increase is ascribed for the reactive-atmosphere processed KCl [11]. However, the threshold for sur- face damage of GaAs not only decreases with increasing frequency, but it also takes place at an order of

magnitude smaller field than the dc breakdown [12]. Thus the qualitative behavior of the GaAs surface

damage does not agree with that predicted by the avalanche theory (see table 2 of ref. [13]). A fully

quantitative comparison between theory and experiment requires solving of transport equations of hyperthermal carriers in strong alternating fields as mentioned in the preceding section. Also, the properties of ionizing electrons (such as mobility, effective mass, etc.) are generally significantly different from those that determine the macroscopic mobility and relaxation times at low applied fields.

Nevertheless, some quantitative comparisons between GaAs and NaCl can be made on the basis of macroscopic energy deposition considerations, ionization rates, and the applicability of the dc theory

extended to the ac electric fields associated with the laser radiation.

It is convenient to recast eq. 5 in terms of the time between collisions t = 1/vg and the macroscopic mobility p, which are related by the expression er = iqo.. This yields

W =

,

, (8)

(1 + a),T^)

which we evaluate for GaAs by taking the effective mass of a free carrier to be 0.1 of the electron mass

[14] . In table 1 values of uu^t for both GaAs and NaCl are listed for the laser frequencies of interest.

Table 1. Angular frequency- average carrier scattering time product for different laser frequencies. The

average scattering time t is calculated from room temperature mobility of 3000 cm^ V"-'- s"-'- for our GaAs and of 10 cm2 V"^ for NaCl

Laser cut

X Wavelength (nm) (rad sec )

GaAs

T = 1.9 . lO"''""^ sec

NaCl

T = 5.7 • 10 sec

10.6

14

1.78 . 10^ 3.4 • 10 1.01

1.06 1.78 . lO-*"^ 3.37 • 10^ 10.1

0.694 2.71 . 10^^ 5.14 . 10^ 15.5

202
2 2 This shows that the term ixi^r in the denominator in eq. (8) will dominate. Thus for the same value of

field an order of magnitude less total power will be pumped into the GaAs compared to NaCl. However,

the T used in table 1 is a macroscopic parameter whereas the breakdown is initiated by hot electrons

for which ou^t in NaCl may be nearly constant [l5].

For avalanche formation the ionization rate per centimeter path length T| is the controlling quantity. For E = 10^ V cm~l, experimental data indicate that T] = 3 • 10^ cm~l for NaCl [16] and

T| = 2.2 • lO'* cm"-'- for GaAs [17] . Thus GaAs dc breakdown would be expected at lower fields relative

NaCl as the case probably is. The surface damage we observe occurs at electric fields slightly less

than 10^ V cm~l. For these fields

eEX/E„ ~ 0.5 (9) R

where Ej^ is the energy of the optical phonon and X ^ 15 A is the effective mean free path obtained by

fitting photocurrent near breakdown to the general theory of Baraff [18] . For values of this ratio less

than one the acceleration theory of Shockley [8] holds so that

Tl/E~ exp (-E^/eEX) (10)

where is the effective ionization energy. The number of ionizations per free electron occuring within a laser pulse of length At is

N = exp (liETlAt) - 1 . (11)

For GaAs where E^ 1.8 eV [18], eq. (10) leads to the estimate of T| ~ lO"^^ cm"-*- for E ~ 10^ V cm"^,

and thus for At ~ I0"7 sec, a negligible value is obtained in eq. (11).

Shockley 's theory for the dc ionization rate has been extended to ac laser electric fields by con- sidering the probability of a hard momentum reversing collision occurring during the time interval in

which the field reverses its direction [9] . If ;i is the mean free path between hard collisions and

At = 1/cUj2 Is the time interval during which the laser field of frequency uUjj changes direction, then the

criterion of applicability is that

2v At/£ > 1 (12)

where v is the carrier velocity directed along the field and can be estimated from the low field

mobility v = |i.E, In NaCl the mobility is both low and determined by hard collisions; thus, a reasonable

estimate is that Z vt. The condition given by inequality (12) therefore reduces to that previously

discussed, namely cu^t < 1. However, in the case of GaAs, the situation is less clear. The estimate

X as vT gives, for E = 105 V cm-1, ^ = 5.1 - 10-5 cm which is obviously way too large. We therefore

elect to follow Robson [19] and use the estimate I = (|j,/e) (2< t)m)% where (e) is the average energy per

electron. For (t) given by the room temperature thermal energy, we find the mean free path to be

£ a 6.3 • 10-6 cm and therefore take it to be independent of the field. (The estimates for NaCl for

E = 106 V cm"l are tj,E>r = 5.6 • 10"^ cm and ()j,/e) (2< t )m)% = 6.6 • 10"S cm.) The assumption of a constant mean free path was used by Baraff [18] in his theory of ionization rates in semiconductors which fits

quite well the experimental data [17] on doped GaAs. These estimates may not hold for pure GaAs where

polar optical phonon scattering dominates and leads to a mobility strongly varying with temperature [20].

In GaAs specimens doped to the level of most of our window materials, the mobility is not very temperature sensitive because ionized impurity scattering predominates [14]. As a comparison to the chosen

value ;2 6.3 • 10"^ cm, the effective mean free path for ionization for hot electrons has been found

to be Xi 1.5 • 10"'' cm [17]. Next, the velocity in inequality (12) estimated by use of the mobility

H is so large that it takes at least 10 half-cycles of the CO2 laser field to achieve it. Thus v in

eq. (12) should be an average velocity over those half-cycles with the value v« 0.29 |j,E. With these

two modifications for GaAs and E ~ 10^ V cm"l, we have

2v htii ~ 0.16 (13)

which is below the criterion given in eq. (12). This is sufficient to rule out the applicability of

Shockley' s theory to our results on GaAs for two reasons: First, the sensitive exponential dependence

of the theory on this parameter generates large changes in the predictions. Second, and more importantly, the high mobility used in eq. (12) should not hold for hot electrons in GaAs as the recent work

by Rolsson indicates [19],

203
4. Calculation of Flux Thresholds

for Carrier Plasma Instabilities in Semiconductors

Difficulties associated with the extension of the Shockley dc breakdown model to the multiplication

of free carriers in semiconductors driven by the electric field component of the laser flux have led us to consider alternate processes to explain the damage threshold. Qualitatively speaking, if the carrier

collisions with the lattice are not sufficiently hard to insure the reversal of carrier momentum at the

instant of the reversal of the optical electric field, then perhaps the mutual free carrier interaction

within the carrier plasma may provide an alternate mechanism of an anomalous absorption. Experimentally, the morphology of surface damage in GaAs [12] implies that the absorbance at the damage site by

the end of laser pulse is larger than 10^ cm"-'-. Since the damage sites are not contiguous but consist

of micron-sized depressions, this localization enables us to consider the creation of quasi-free states

in which carriers interact collectively with the laser radiation in distances comparable to a few multiples of the Debye shielding length. For such semilocalized excitations, the effective pseudo-gap between the bound and the semifree state need not assume values ascribed to the crystalline gap. In ref.

[13], we calculated the conditions for attaining a local carrier density needed in a nearly resonant

carrier plasma interaction with the laser flux. A semiclassical treatment of bound to quasi- free transition probability of an electron in a model potential acted upon by a moderately strong monochromatic

Maxwell field has been carried out elsewhere [21] . It also supports the contention that conditions for

a near resonance can be attained in GaAs for experimentally observed surface damage fluxes if the effective pseudo-gap is of the order of 1/3 to 3/4 of the crystalline gap. The higher value corresponds to use of the effective carrier mass in GaAs instead of the electron mass. Because of the smallness of the

excited site, linear absorption is not sufficient to account for the damage. Therefore, the catastrophic damage must involve a nonlinear, intensity dependent absorbance. Since we start with a neutral

solid producing the carrier plasma, charge conservation requires the creation of an equal number of

electrons and holes. The carrier plasma is analogous to a gaseous plasma of electrons and ions. We

therefore should expect the same instabilities that develop in gaseous plasmas and lead to anomalous

heating by lasers to arise in our semiconductor plasma. Since the two-stream instability [22,23] has

one of the lowest thresholds for gaseous plasmas, we calculate its threshold first. In this instability,

the laser radiation excites an electron wave and an ion acoustic wave in the plasma. In deriving the

threshold condition [22,23], we begin with the electron and ion continuity equations:

+ V . (n^Vj) = 0 ; (14)

the electron and ion momentum conservation equations

m.n.v.v.

J J J J (15)

and the Poisson's equation

eV . E = 4tt V e.n. . (16)

The subscript j refers to either electrons or ions, Eq sin co^t is the laser electric field, the v's

are the relevant collision frequencies, and pj denotes pressure. One then proceeds to linearize the

coupled set of eqs. (14) to (15) in the dipole approximation and solve quasi-statically by transforming

into the oscillatory frame of carriers with the smallest effective mass. Inspection of their dispersion

relation shows that a positive imaginary part and hence an instability occurs if

(17)

where Sg is the electron thermal velocity given by s| = Ye(Te/TMe) (^e ^ numerical factor), and u)g is

the plasma Bohra frequency of the smallest-effective mass carriers. Inspection of eq. (17) shows that

for small effective mass highly mobile carriers, the threshold for plasma instability can be quite low.

Recasting of eq, (17) in terms of the macroscopic carrier mobility ^ = e/(mvg) and rewriting the threshold condition in terms of the laser flux S^ [W cm"^] , we obtain

S^SicT (jumv = ceT — (18)

t e jj e e 10.

for the case Tg ~ Tj; = 0.025 eV and ~ (Ug

204
The numerical constant c in eq. (18) is evaluated by comparison with ref. [24] and also takes

into account eq. (3). With GaAs parameters listed with table 1, the threshold flux for the two-stream

instability is calculated to be Sf- ^ 10^ W cm"2. The onset of this instability leads to an enhancement

of the high-frequency resistivity of the solid state plasma in GaAs and hence to the onset of anomalous

absorption and heating.

Another solid state plasma instability of interest in laser induced damage in GaAs is the parametric instability formulated in terms of plasmons in solids by Gersten and Tzoar [25]. However, for this

instability, the plasma is purely electronic, and we have to assume that the holes are immobile. The

instability of interest here is the direct conversion of two identical photons into two plasmons. One

considers that the onset of the instability occurs when the electron distribution function in phase

space f(5,p,t) = fg + F, in the presence of a laser electric field, undergoes a large spatial fluctuation F with respect to the zero-order distribution f^.

One considers the solution of the Vlasov and Poisson equations given by eqs. (19) and (20) for both

components of the distribution function:

df ^ -» ~S f , dcp ~^

dx Bx dp

?(t) .

„2

eV cp 4n n^e|^-l + J' f dpj

(19)

(20)

Kane's nonparabolic Hamiltonian is

H = [(mc*^)^ + (c*p)^] '

, (21)

where c*2 = (gg/2m and Sg denotes the band gap. ^ Using Hamilton's equations of motion SH/3p = q and

9H/9q = -p, we get the free carrier velocity ^= v expressed by

V = c*p[(mc*)^ + p^]"^ . (22)

One then transforms (19) and (20) into the oscillatory coordinates of the electrons and considers the

threshold for plasma instability given by the onset of a net positive growth rate of plasma waves. In

terms of solid state parameters, the parametric instability threshold is exceeded when the following

inequality is satisfied

S > 8 . 10"^ <S (D,/(e|a)

We use again GaAs parameters quoted in table 1 to obtain S(- ~ 7 • 10^ W cm"2 as the threshold flux for

the onset of this instability. Experimental values [12] seen for 40 to 100 nsec CO2 laser pulse damage

of GaAs surfaces agree within an order of magnitude with our calculations. It should be noted that both

the parametric and two-stream instability thresholds are proportional to the ratio cu_g/n.

5, Discussion

Using lowest order approximations, we have attempted in this contribution to demonstrate analytically that the physical damage processes may be quite different for low mobility, large band gap insulators such as the alkali halides and small band gap large mobility infrared transparent materials such

as certain semiconductors. Although further work including higher-order contributions is still needed,

the implications of our considerations encompass more than merely materials for windows. For example,

in large MOPA CO2 laser systems, Faraday rotators comprise about the most significant class of nonreciprocal devices. Currently, InSb is the only material with which successful isolators at 10.6 |im wavelength have been constructed [26], Because of a high mobility and a small effective mass of carriers

in InSb at 77 K, the pulse damage threshold could be primarily determined by the onset of solid state

plasma instabilities discussed in the preceding section.

Despite the difference in the probable damage processes in these two classes of infrared window

materials, the practical prescriptions for increasing the damage threshold are very similar. The threshold in avalanche damage for KCl and NaCl increases with the laser frequency; a similar increase is

predicted for both plasma instabilities discussed in the preceding sections, albeit it is severely

moderated by the lowering of the order of multiphoton transitions in the low band gap semiconductors.

Furthermore, the increase in the energy gap between the bound and the free carriers and decrease in

their mobility contribute to the increase in the threshold of damage initiated by the solid state

205

564-378 O - 74 - 15
plasma instabilities. At the surface, any condition which either increases the local electric field or provides an easy source of free electrons, also contributes to the lowering of damage threshold. In this sense, pulse damage experiments which determine the state of material before and after the illumination are not sufficient to differentiate between various processes of damage under consideration.

However, in understanding the physics of damage, the distinction between various predicted processes

will be quite useful.

6. References

[ 1] Bloembergen, N., IEEE J. Quantum Electron.

QE-10 , 375-386 (1974).

[2] Bloembergen, N., Paper 0-1, 8th Internat.

Quantum Electron. Conference Digest, 74 CHO858-1-QECON.

[3] Holway, Lowell H, , Jr., Phys. Rev. Letters

28, 280-283 (1972).

[4] Epifanov, A. S., The Process of Development

of Avalanche Ionization in Solid Transparent

Dielectrics under High-Power Pulsed Laser

Radiation, Lebedev Physical Institute

preprint, Moscow, 1974

[5] Bloembergen, N., Appl. Opt. _12, 661-664

(1973).

[6] Zverev, G. M. , Mikhailova, T. N., Pashkov,

V. A., and Soloveva, N. M. , Sov. Phys.-JETP

26, 1053 (1968).

[7] Frohlich, H., Proc. Roy. Soc. (London) A160,

230 (1937).

[8] Shockley, W. , Solid State Electron. 2, 35-67

(1961).

[9] Bass, M. and Barrett, H. H., IEEE J. Quantum

Electron. QE-8 , 338-342 (1972).

[10] Fradin, D. W. and Bass, M. , Applo Phys. Letters 22, 206-208 (1973).

[11] Allen, S. D. Braunstein, M. , Giuliano, C,

Wang, Vo, Pulsed CO2 Laser Damage Studies of

RAP Grown KCl, elsewhere in these

proceedings

.

[12] Smith, J. Lynn, NBS-SP-387 Report, p. 103

(Nov. 1973).

[13] Shatas, R. A., Narducci, L, M, , Smith, J,

Lynn, Meyer, H. C., and Mitra, S. S., NBSSP-387 Report, p. 217 (Nov. 1973).

[14] Rowland, M. C, The Preparation and Properties

of Gallium Arsenide in Gallium Arsenide Lasers. Gooch, C. H. , ed. (Wiley- Interscience, New

York, 1969), pp. 133-192.

[15] Yablonovich, E. and Bloembergen, N., Phys.

Rev. Letters 29, 907-910 (1972).

[16] Fradin, D. W., Bloembergen, N., and Letellier,

J. P., Appl. Phys. Letters 22, 635-637 (1973).

[17] Logan, R. A., Chynoweth, A. G., and Cohen, B. G., Phys. Rev. 128, 2518-2523 (1962).

[18] Baraff, G. A., Phys. Rev. 128 . 2507-2517

(1962).

[19] Robson, R. E., Phys. Rev. Letters 31, 825-828

(1973).

[20] Wolfe, C. M. , Stillman, G. E., and Lindley, W.

T., J. Appl. Phys. 41, 3088-3091 (1970).

[21] Shatas, R. A. and Narducci, L. M. , Interband

Transitions in an Intense Infrared Radiation

Field in Trans. 19th Conference U. S. Army

Mathematicians, ARO-D Report 74-1.

[22] Kaw, P. K. and Dawson, J. M. , Phys. Fluids 12,

2586 (1969).

[23] Nishikawa, K. J., Phys. Soc. -Japan 24, 1152

(1968).

[24] Meyer, H. C. and Shatas, R. A., Phys. Fluids

15, 1542 (1972).

[25] Gerstein, I. I. and Tzoar, N., Phys. Rev. Letters 21, 1650 (1971).

[26] Bicknell, W. E., Tomasetta, L. R. , and Bates,

D. H., Paper X.ll, 8th Internat. Quantum

Electron. Conference Digest, 74 CHO-858-1-

QECON.

COMMENTS OW PAPER BV SHATA5

Tho, tiizofiLj pfiopoitd -in thu> papeA wa6 cMoAactzAiztd <X6 a ^tut cut at tht pfiobtm. Thz a6e 0^ thi

conzQ-pti, onz-eJizcXKon e^ijecXtue mcu>i and dt^ {,eAe.ntiaI. mob-itUy may be tnvaJLLd a6 the. eZzcXnoni) bzcome.

meAgetxc, due. to non-panatoticJMj oi^ the. bandU,. In iacZ, the, ipejxkeA commented on the Kole played tn

avaJLancke bKcakdoMn 0^ a typtcal etectfion, not de^cAlbed by the. MameZtian dUstyiibatlon. He pfiopoied

that: a fvLgoKOLU> tAeatment 0^ the piobtem would -involve a detoJJied nmeAyical iolution o{^ the Boltzmann

equations, which would be an expensive undertaking.

206
6.2 Frequency Dependence of the Nonlinear Optical Susceptibility of Five Glasses

Robert Hellwarth, Joel Cherlow, and Tien-Tsai Yang

Electronics Sciences Laboratory, University of Southern California

Los Angeles, California 90007

The frequency dependence of the nonlinear susceptibility throughout the optical region

of fused quartz, ED-4, LSO, SF-7, and LaSF-7 glasses is derived in a novel and useful form

from Raman-scattering and intensity-induced polarization change measurements.

Keywords: ac Kerr effect; glasses; nonlinear optical effects; nonlinear optical

susceptibility; self- focusing; three-wave mixing.

1. Introduction

We have recently determined the frequency dependence of the nonlinear optical susceptibilities of

five glasses, and used these to determine the relative contributions of "electronic" and "nuclear"

mechanisms to their indices for self-focusing. We demonstrated how the nonlinear optical susceptibility

is simply related to the frequency spectrum of polarized and depolarized Raman-scattering cross- sections and the coefficient for intensity-induced polarization changes (IIPC) in glasses. We used our own measurements of Raman cross-sections and Owyoung's measurements of IIPC to determine the

nonlinear optical susceptibilities of fused quartz and the glasses ED-4, LSO, LaSF-7 and SF-7. The

first account of this work is being published elsewhere, but we reproduce it in the Appendix (Sec. 6)

of this paper. In this Appendix the necessary theoretical relations are derived and the experimental

data are summarized, by which the nonlinear susceptibilities and mechanisms are determined. In the

following section 2 we present some transformations of the original results which are of direct use

in predicting a variety of nonlinear optical effects in the glasses.

2. Nonlinear Susceptibilities of Glasses

We have studied the "third order" nonlinear susceptibility tensor in glasses, i.e. the tensor

which gives the term in the expression for the nonlinear polarization of a glass that is third-order

in the propagating macroscopic optical electric fields. (By "optical" we mean fields whose frequencies

are smaller than the bandgap and higher than lattice vibration frequencies.) The third-order term

is the only nonlinear term that is important below damage thresholds. In the Appendix we show that

whereas the "electronic" contribution to the nonlinear polarization can be expressed in terms of

one real parameter o, as in eq. (Al),' the "nuclear" contribution requires two real functions of time,

a(t) and b(t) in eq. (A2), for its determination. Since a(t) and b(t) represent causal response

functions of nuclear motions (a=b=0 for tj<0), the real and imaginary parts of their Fourier integral

transforms, A and B , are related by the usual Hilbert transform of eq. (A7).

In figure 1 of the Appendix are given our measured Raman-scattering cross-sections , to which the

imaginary parts of A and B are related by the simple multiplicative function given in eq. (A6).

Because the self-focWsing indices considered in the Appendix involved only o and the real parts of A^, and B(^ at w=0, the Hilbert transforms at A of the imaginary parts of A(^ and B^, were given in Table 1 only at A=0. However prediction of other nonlinear effects requires a knowledge of Re A^ and Re B^j) at arbitrary w. For example, consider the three-wave mixing observed in glasses by Levenson (ref. [9]

of the Appendix). In these experiments, Levenson superimposed two nearly colinear optical beams

so that the macroscopic electric field at any point equalled

*This work was supported by the U.S. Atomic Energy Commission under U.C. Lawrence Livermore Laboratory

subcontract 2713005.

^The numbered equations 0).(2),etc., in the Appendix will be referred to as eqs. (Al), (A2), etc.

207
5. Figures

The Fourier transforms and B^^ of the time- dependent nuclear response functions a(t) and b(t)

for five glasses, in terms of v/hich the nonlinear susceptibilities are expressed in the Appendix.

The curves represent a) Re +8^) > b) Im (A^^ + B^^) , c) Re and d) Im B^^. The ordinates are

lO'^ times the values in esu. The abscissae give the frequency w in wavenumbers (cm"l).

1 , , ,, , , 1 , 1 r

,

FUSED QUARTZ

b/

^sQ /

/ \ / \ / \

\ -c / 1

\

"

\

\

\

^-"^^

/

/

/

r-'

1

I

- -6-

1 1

—1

1

1

r

1

[ 1r

1

1

. LSO GLASS

\\0

\

\ / \ W — b/ \ j \

\ 1 \

"

^ \ 1 \ \ \ 1 \ ~

'\ \ /' \

"=77:—1I..-- -•

-f ^>i#.. — ^

1 '

1 . , . 1 400 800 1200 400 800 1200

800 1200

90

60

30

-30

1 I 1

I

1

1 LaSF7 GLASS

\ 7 \

\ \

r ^\

~'' \\

\ / -

. . . 1 , . . 1 . , 1 1 400 800 1200

208
E(t)=Re[Ue-^'''ut,ve-i'^vt] (2.1)

where U and V are complex amplitudes, depending on the strength and polarization of the two beams.

Levenson then observed the light radiated at the "mixed" frequency a)=2a)(j-ojy from a nonlinear polarization density which, from eqs. (Al) and (A2), has a complex amplitude

J^JIJ.\/*(a+2A^+B^)+^*U.lJ(a+2B^) (2.2)

where Aeoj-m Here the involvement of the entire functions A and B is evident.

U V (jj 0)

As a second example of the usefulness of the functions A^^ and B^, consider the "ac Kerr effect"

in which colinear waves of the form of eq. (2.1) are again superposed in the medium, and a change in

the optical susceptibility tensor 6xv at frequency is observed because of the presence of the

U-wave. To lowest order in this~change is seen immediately from eqs. (Al) and (A2) to be

expressible in the dyadic form

(2.3)

for calculation of this effect, at

so that A and B are not negligible.

Therefore, we have performed numerical computations of the Hilbert transform eq. (A7), for all

transformed frequency variables below 1300 cm'', and obtained the numerical results for A^, and B^j,

for the five aforementioned glasses, which are displayed in the figure at the end of this section.

3. Summary

We have re-expressed a previous experimental determination (reprinted in the Appendix) of the

entire frequency dependence of the nonlinear optical susceptibility of fused quartz, ED-4, LSO, SF-7,

and LaSF-7 glasses. This re-expression is summarized in curves of the real and imaginary parts of

the Fourier transforms A^^ and B^^ of two scalar lattice response functions defined in the Appendix

(where the electronic nonlinear susceptibility contributions are also tabulated).

4. Acknowledgments

The authors gratefully acknowledge the helpful conversations with A. Owyoung, A. Glass and

M. Levenson.

%i yy (2A^+B^+a)

+1 y*y(B^+ B^+a)

4 1(B +2A +a)U*-U

where vew -w^. Again the entire functions A^ and B^^ are needed

least at frequency shifts v small enough (typically <1000 cm"l)

209
6. Appendix A: reprint of article to be published in Physical Review

in slightly expanded form.

Origin and Frequency Dependence of Nonlinear Optical Susceptibilities of Glasses*

Robert Hellwarth, Joel Cherlow and Tien-Tsai Yang

Department of Physics, University of Southern California

Los Angeles, California 90007

We have determined the frequency dependence of the nonlinear optical sus- ceptibilities of fused quartz and four other glasses, and determined the relative contributions of "electronic" and "nuclear" mechanisms to their nonlinear

optical indices. This is the first such determination in solids. Our method

has been to analyze our own absolute measurement of ordinary differential Raman

scattering cross-sections in conjunction with the coefficients for intensity

induced polarization changes measured by Owyoung in the same glasses.

^(3) The nonlinear optical polarization density P (rt), which is third order in the electric field in

a material, manifests itself in a variety of commonly observed nonlinear optical effects such as beam

self-focusing, self-phase modulation, frequency mixing, and intensity induced polarization changes,

(IIPC). Here we point out a relation between differential Raman scattering cross-sections and P^

which we use to determine both the frequency dependence and physical origins of the nonlinear polarization in five optical glasses. This is the first such determination in solids. We use our own Raman

scattering measurements in conjunction with the IIPC data of Owyoung [ 1 ] to characterize ^ at "optical" frequencies; that is, at frequencies too low to excite electronic transitions and too high for

nuclear motions to follow.

There are, in this case, two distinct physical contributions to P^"^''(rt) (apart from electrostrictlon induced strains which we ignore). First, there is an "electronic" contribution from the non- linear distortion of the electron orbits around the average positions of the nuclei. This polarization

responds rapidly to field changes , within a few electronic cycles ('V10~ sec). In isotropic materials

it contributes a term to p well approximated by the instantaneous form

^ aE(?t)E^(rt) (1)

for frequencies well below the electronic band gap.

ature at fixed density.

The scalar coefficient O is independent of temperA second, "nuclear," contribution arises from an optical field induced change in the motions of

the nuclei; about these changed motions the electronic currents respond linearly to the optical fields.

This contribution can be observed after the sudden impression of a field only following a time lapse of

the order of the time ('^10""'' sec) required f.Qr,a nucleus to execute a vibrational or rotational cycle.

In isotropic materials this contribution to P must be of the form (at some position r)[2]

E(t) dsa(t - s)E (s) + dsE(s)b(t - s)E(t)'E(s) (2)

for frequencies well below the electronic band gap. The nuclear response kernels, a(t) and b(t),

vanish for t < 0 by causality, and generally exhibit a marked temperature dependence at constant density. We determine the electronic coefficient O and the Fourier transforms of a(t) and b(t) for

several glasses using measurements of IIPC and Raman effect in the theoretical relations which we

develop below.

That p^^-^ should cause the elliptical state of polarization of a strong monochromatic beam to change was first predicted and observed by Maker, et al. [3] For a plane wave of frequency w propagating in an isotropic medium, this "intensity induced polarization change" (IIPC) manifests itself by a rotation of the axes of the polarization ellipse through an angle 6(z) which increases with the distance z along the beam. An expression for 9 in terms of a, a(t), and b(t) is easily found by inserting such a plane wave E in (1) and (2), and then verifying that Maxwell's equations are satisfied (to

order E^) with this nonlinear polarization density, provided that [1,2]

*This work was supported by the U. S. Atomic Energy Commission under U.C. Lawrence Livermore Laboratory

subcontract 2713005

''Figures in brackets indicate the literature references at the end of this paper.

210
7rcoz<E (t)>(a + 2B^) cos 2())

8nc (3)

where c is the velocity of light, tan is the ratio of right to left circular polarization amplitudes,

0 (esu used throughout). We

can be determined from differand B is the value of the Fourier transform B, of b(t) evaluated at A

=

o A define = /dtb(t) exp iAt, and A. similarly. We next see how and B^ ential Raman scattering cross sections. Knowing and A^ we can determine a, and thereby the entire

nonlinear polarization (1) and (2), through IIPC measurements which according to (3) give a + 2B^

.

Consider first the well known nonlinear optical effect of ordinary (non-phased-matched) stimulated

Raman scattering in which the presence of a strong plane polarized "pump" wave cos (vt - k z) in a medium causes a second weak wave ReeEexp[i(wt - lc*r) ] to experience exponential gain. If §°in (1) and

(2) is the sum of these two optical fields, then Maxwell's equations are satisfied to order F^E for

real V, k^, and oj, if k is a complex vector s(k' + ig/2) where g, the "stimulated power gain per cm," is

2TTajF Im(A^ + B^) (4)

when e | x, or

TTWF ImB. (5)

when e _|_ X. A = v - oj.

There exists a well known relation between these stimulated gain coefficients and the differential

Raman scattering cross sections d^a^(v,A)/dMA, defined as the fraction of incident photons of frequency V inelastically scattered per unit distance into solid angle dfi and angular frequency range dA

about V - A when the incident and scattered polarizations are either parallel (i = ||) or perpendicular (1 = _[_).[4] Substituting (4) and (5) into eq. (4) of ref. [4] gives the general relations which

we seek:

4 d^a

ImB. ±

Rvoj

3 dfidA

-iiA/kT

(6)

and a similar relation with AaiiA andLiu hOi, -lUji - aj_ substituted for B^ and Oj^ respectively, Because of the

causal nature of a(t) and b(t7, the real parts of their Fourier transforms can be calculated from their

imaginary parts by the usual Kramers-Kronig integrals:

Re B. dv

T ImB

A V

(7)

and similarly for A^

.

We can see therefore from (6) and (7) how the entire transforms of a(t) and b(t) can be deduced

from our measured Raman scattering cross-sections in figure 1 for the five glasses: 1) Homosil fused

quartz (Amersil, Inc.), 2) LSO (American Optical Co.), 3) ED-4 (Owens Illinois, Inc.), 4) SF-7 (Schott)

and 5) LaSF-7 (Schott). The curves in figure 1 were obtained from 90° Raman scattering measurements

using the 514 mn line of an argon ion laser and a double monochromator whose spectral sensitivity was

calibrated with ^ quartz-iodine standard lamp. The absolute cross-sections were obtained by comparison

with the 992 cm line of liquid benzene for which the absolute cross-section has been determined to within 5% by Kato and Takuma[5]. Index corrections of the type described by these authors were performed. Our measurements of the polarized Raman cross-sections of fused quartz yield Raman gains in (4)

about 20% greater than those reported by Stolen and Ippen[6], but within their limits of error.

To obtain the values of B (required to determine the electronic coefficient from IIPC measurements) and A^ (required later to calculate the self focusing index n^) we use (6) in (7) finding

2c dA

3.

CO A

d aL(v,A)

df^dA

-fiA/kT

(8)

and similarly for A . Using the cross-sections of figure 1 in (8) gives the values for A^ and B listed in table 1. Also listed there are Owyoung's IIPC measurements of a + 2B (taken at 694 nm) from

which we deduce the listed values of O. polarization for these glasses.

This completes our characterization of the nonlinear optical

211
Table 1. Nonlinear polarization parameters defined in text as derived from our Raman data and intensity induced polarization change (IIPC) measurements. From these are derived the selffocusing index n2 of eq. (3) and the fraction f^ that is electronic in origin. All quantities

are in esu.

Material n lO^S ^

o lO^^A +B )^

O 0 10-'-^(a+2B^)^ lO^^a 10-'-^n2 10-'-^n2

(eq. 4) (meas.

)

fJZ) e

Fused Quartz 1.46 14+2 57+5 31±2 28±2 11±1 18+3^^ 79±3

LSO Glass 1.52 2&t4 70 ±6 46±3 40±3 15+1

c 26±3

81+3

ED-4 Glass 1.56 26±4 68+7 57+4 52±4 18±1.

3

-1 i-i^d 15±2

15±3^

8 5±3

SF-7 Glass 1.65 210 ±20 315±30 202±20 160±20 58±7 79 ±3

LaSF-7 Glass 1.93 380+30 540+50 252+25 176 ±25 61±8 71±3

^This work [1,12] '^Ref. [9] ^Ref. [8] ^Ref. [7]

Our results may be compared with recent measurements of the self-focusing index n2 for a monochromatic beam. Since the change in refractive index for linear polarization is defined as n^^^^E (t))>, we see from (1) and (2) that, in our terms,

xCT + 2A

2 o + 2B /n (9)

where n is the linear refractive index. Our predictions for n2 in Table 1 (which depend mainly on Owyoung's data[l]) are seen to be consistent with the absolute interf erometric , time resolved pulse

measurements of Bliss et al.[7], and also with the interf erometric comparisons with liquid CS2 made by

Moran et al. [8] Ours are, however, lower than the values obtained by Levenson [9] from 3-wave mixing

experiments near 530 nm. This is most probably due to his less certain calibration. Estimates of

dispersion from phenomenological theories of n2 indicate that n2 should not be more than 5% greater in

the green. [9-12]

The phenomenological theories of n2, developed to aid in the design of low-n2 glasses, have assumed

that n2 is mainly electronic in origin. This is verified by our values in Table 1 for its electronic

fraction f^, which, according to (9), is [1 + 4 (B^ + A^ ) / (3a) ]"•'-

.

The authors gratefully acknowledge helpful conversations with A. Owyoung, A. J. Glass, E. Snitzer,

N. Boling, E. Bliss, M. Moran and R. L. Carman, and thank S. P. S. Porto for the use of his facilities.

[1] A. Owyoung, IEEE Journ. of Quantum Electronics 9, 1064 (1973); A. Owyoung, "Laser

Induced Damage in Optical Materials: 1973,"

ed. A. J. Glass and A. H. Guenther (National

Bureau of Standards Special Publ. 387, Dec.

1973); A. Owyoung, R. W. Hellwarth, and

N. George, Phys. Rev. B5, 628 (1972).

[2] R. W. Hellwarth, A. Owyoung, and N. George,

Phys. Rev. A4, 4 (1971).

[3] P. D, Maker, R. W. Terhune, and C. M. Savage, Phys. Rev. Letters 12_, 507 (1964).

[4] R. W. Hellwarth, Phys. Rev. 130, 1850 (1963),

[5] Y. Kato and H. Takuma, J. Opt. Soc, Am. _61,

347 (1971); J. Chem. Phys. 54, 5398 (1971).

6.1 References for Appendix

[6] R. H. Stolen and E. P. Ippen, Appl. Phys,

Lett. 22, 275 (1973).

[7] E. R. Bliss, D. R. Speck, and W. W. Simons

(to be published)

.

[8] M. Moran, C. She, and R. L. Carman (to be

published)

.

[9] M. D. Levenson, IEEE Journ. of Quantum

Electronics QE-10, 110 (1974).

[10] C. C. Wang, Phys. Rev. B2, 2045 (1970).

[11] J.F.Fournier and E. Snitzer, IEEE Journ. of

Quantum Electronics QE-10, 473 (1974).

[12] N. L. Boling, A. J. Glass and W. Owyoung (to

be published)

.

212
6.2 Figures for Appendix

STOKES RAMAN SHIFT /l(cm"')

Figure 1. Differential Raman scattering cross-sections vs. (Stokes) frequency shift A from 514 nm incident beam for five glasses (A = A/(2Trc) cm"-*-). Solid lines are polarized (||) and dashed lines are depolarized (J_) cross-sections.

COmENTS ON PAPER BV HELLiUAPTH

Tkz pfL&iznaz 0(5 Ntodymlm In thz glcu>i iamplu did not di>ie.cXty a.{,ie.cX the, obieAved Raman ip&cfum.

It dU.d tmd to 'LntAoducz ab^iOKption at the. e-XCyiting MaveZength, and a Q.onM&qae.Yit ^laofLeJ) cent ba.akgn.oand.

When the ah^oiptLon woa itAong, a n.eAuZtant Ion Oj$ ilgnaJi wa^ ieen to occwl.

213
6.3 The Refractive Index Dependence of Pulsed Laser

Induced Damage

Jerry R. Bettis* and Arthur H. Guenther

Air Force Weapons Laboratory

Kirtland Air Force Base, New Mex 87117

and , c

Alexander J. Glass

Lawrence Livermore Laboratory

Livermore, California 94550

The dependence of the laser induced damage threshold on refractive index

has been investigated. A simple theoretical expression, based upon several

factors such as the local electric field and avalanche mechanisms, has been

derived. Comparison is made with several series of previously published

results to verify the proposed dependence. Strikingly good agreement is

obtained between theory and experiment for materials with refractive indices

ranging from 1. 38 to 2. 36. The available experimental data used for comparison

were obtained (1) on the surface of uncoated dielectrics, (2) in the bulk of alkali

halides, (3) with \ / A dielectric coatings and (4) with vapor phase mixture thin

films. Extrapolations of these results to conditions for air at STP give

remarkable agreement with reported values of pulsed laser induced air breakdown.

Key words: Bulk; electro- absorption; electron avalanche, laser damage; local

field corrections; refractive index; surfaces; thin films.

1. Introduction

An early observation of the damage of optical elements by intense pulsed laser radiation showed

that the damage threshold is generally dependent upon the index of refraction of the dielectric.

Notably, in the work of Turner [1]^ a hierarchy of damage sensitivity appeared to exist which

indicated that high index materials were less damage resistant than low index materials. This

paper attempts to elucidate that conclusion.

Since refractive index is a measure of the polarizability (distortion) of electronic distributions

by the local optical electric field in the medium, it is pertinent to aslc how laser induced damage is

related to the optical field. Numerous observations over the past several years have indicated

that the appropriate quantity i^n short-pulse, laser-induced damage is indeed the optical electric

field. Dube 'first showed that this assumption readily explained the asymmetry in the entrance and

exit damage thresholds by recourse to Fresnel considerations [2] . Bloembergen asserted that the

difference in surface and bulk damage thresholds could be explained by the enhancement of the

optical electric field at geometrical or mechanical imperfections and dielectric discontinuities such

as absorbing inclusions [3 ]

. It was further observed that the damage threshold at total internal

^Submitted in partial fulfillment for the degree of Doctor of Philosophy at the Air Force Institute

of Technology, Wright-Patterson AFB, Ohio 45433.

. Figures in brackets indicate the literature references at the end of this

214
reflecting surfaces depended on the polarization of the light wave incident on the reflecting interface.

By calculating the resulting electric fields for p and s polarizations, Fersman and Khazov[4]

demonstrated that whereas the energy and power density at threshold depended on polarization, the

electric field assumed a constant value. Finally, Fradin, Yablonovitch, and Bass |5 ] concluded

that the damage mechanism in alkali halides is the field-dependent process of electron avalanche.

By so doing they were able to explain both the frequency and pulse length dependence of laser-induced

damage. At most operative pulse lengths and wavelengths of interest the breakdown field was nearly

identical to DC breakdown values for the wide gap insulators studied.

2. Local Field Considerations

Granted, then, that the damage process is field dependent. How does this relate to the index

dependence? The refractive index and the local electric field are related through the induced

polarization and the Clausius -Mossotti relation. For a dielectric subjected to a local electric field,

, the induced polarization per unit volume, P , for a linear isotropic medium is

P = Na ( J e E (1) o S.

where N is the density of polarizable species (e.g., molecules), a( lo) is the molecular polarizability,

and is the permittivity of free space. By definition, the polarization is the sum of the displacements from equilibrium of each charge carrier times the charge. Taking <^X^ as the total average

displacement per molecule one obtains

Nq^ <i> = Na(co ) E^ (2)

where q^ is the electronic charge. For dense dielectrics approximating cubic symmetry the

Clausius-Mossotti relation between polarizability and refractive index yields

e"^ - " q^ N <x> (3) o^ '

By applying the Lorentz local field correction to obtain the macroscopic field, E, in the dielectric one obtains the desired relation

q„ N <x>

E = -i^— (4)

eo(" - 1)

The supposition is made that there exists a critical displacement, x^^, not necessarily an average ^.depending undoubtedly on ionization potential, dispersion, band and energy level

structure as'Svell as other related quantities such as effective charges and masses, beyond which an

electronic distribution cannot be displaced without liberating electrons. Since an electron avalanche

is a process by which free electrons are accelerated to ionizing energies by the application of an

electric field, the ease with which electrons can be freed would seem to have a decided effect on a material's damage threshold. Under these assumptions the magnitude of the threshold macroscopic

electric field (E^j^) should vary as

Several other detailed approaches to this relation is given in a paper by Bettis, et al [ 6] . These

include first principle consideration of polarizability and a rate dependent energy gain and loss

calculation for electron avalanches produced by high intensity optical radiation interacting with

dielectric media. Of course, to treat the problem explicitly, one should treat the total assembly of

interacting species individually with careful consideration of their specific environment, density,

locations, and intrinsic polarizability.

215
3. Test of Theory

To test the validity of this approach, one can compare the expression in eq. IS) to data

obtained by various experimentalists. First consider the damage work of DeShazer, et al. , [7]

in which films of various thicknesses and material composition were applied to glass pubstrates.

A tabulation of their results for X/4 films is given in table 1. Included is the function KN/(n^-l)

where the value of K h^s been chosen arbitrarily to facilitate comparison. In the final column is listed values of KN/ (n -1) relative to the thin film damage thresholds for the density and index of

air. The values correspond closely to the reported values for air breakdown of 4. 3 x 10 V/m [8] to 5. 0 X 10^ V/m [9]

.

TflVil p 1 Tabulation of results for A/4 films

.

Material E ^ (10^ V/m)

tn

KN

n^-l

E , Air Breakdown

tn

ZnS 5.71 5.-53 5.0 X 10^ V/m

Ti 0^ 6.59 6.67 4.69 X 10^ V/m

Zr O2 7.58 8.03 4.41 X 108 V/m

Si O2 16.54 18.78 4.13 X 10^ V/m

Mg F2 16.55 30.67 2.53 X 10^ V/M

Comparison of the observed threshold electric field for damage vs. the function

KN —5— • The value of K was arbitrarily chosen to be 9.68 x 10-22 v-M^. The

n -1

values of air breakdown are the ratios of the function for the density and index

of air vs. the material multiplied by the observed threshold field for the

material.

In addition, films of Ti02 of thicknesses X/4, X/2, and 3 V4 were damaged with the damage

being indicated by laser induced scattering (LIS) rather than spark detection as in thp values

quoted in table 1. The appropriate threshold field values were 5.13 x 10 , 3. 78 x 10 , and

3. 69 X 10^ V/m, respectively. These closely grouped values more nearly relate to an intrinsic

property of^Ti02 than do the widely variant incident threshold power densities of 3.45, 0.6, and

1, 8 GW/cm . The difference in threshold power density would vanish if the index of film and substrate were the same. Thus, we note for the case of Si02(n = 1.456) applied to BSC-2 glass

(n = 1. 51) the values of threshold field vs incident power density for a/4 and x/2 films were

1. 65 X 10 , 1. 58 X 10 V/m; and 10. 5, 9. 9 GW/cm^, respectively.

A second set of experimental data, this by Boling, Dube', and Crisp [10] was reduced to figure

1. The data are the results of surface damage studies conducted on seven laser glasses, fused

silica, and sapphire. Approximate values of molecular densities were obtained [11] and although

detailed molecular densities have not been included in expression [5] one can still have confidence

in the comparison since the approximate molecular densities , as supplied by the manufacturers,

were sensibly constant.

Another set of available data is the vapor phase mixture results of Austin, et al [12]. In this

study, vapor phase mixtures of various percentages of ThF^ and ZnS were applied in X/2 films to glass substrates. Perhaps the very nature of these complex mixture films negates exact comparison

with the N/ (n -1) function, for although the threshold field does decrease with increasing index as evidenced in figure 2, the graph of E ^ vs N/(n -1) is reversed in slope. We cannot at present

explain this apparently anomalous benavior. Thus, more careful treatment of binary mixture films

is warranted as well as a detailed look at the surprisingly high optical strength of ThF^.

216
Perhaps the most comprehensive data in which extrinsic influences have been all but eliminated

is that gathered by Fradin and coworkers on wide-gap insulators [5]. This research is different

in character in that threshold conditions for bulk damage were measured in 11 different alkali halides

at 10, 6 |jm and 1. 06 ym and compared to values obtained from D. C. b:^eakdown studies. The results are quite striking when threshold electric field is plotted against N/(n -1). From the previous

discussion we would expect such graphs to be monotonically increasing straight lines and to within

reasonable tolerances such is the case. Similar data at 1. 06 \im is presented in figure 3. The

potassium halides depart systematically from the sodium and rubidium halides. The value taken

for the threshold value in Nal was extrapolated from the D. C. and 10.6 pm data, since the

investigators report that the crystal was extremely hydroscopic and as a result exhibited an anomalously low threshold field at 1. 06 pm. The breakdown field for KF at 1. 06pm was also

extrapolated from the D. C. and 10. 6 ym data since no value was given for 1. 06 pm. The straight

lines which result differ in slope by less than 10% with the only point deviating strongly being the

single point for NaF. In passing, it is worthwhile mentioning that fluorides exhibit a tendency to deviate from expected behavior and therefore should be studied more closely. Extrapolating the

threshold values to the conditions for air yields values of 4. 2 x 10^ V/m to 4. 87 x 10 V/m for the

Q ^ O potassium group and 6. 4 x 10 V/m to 7. 6 x 10 V/m for the Rb and Na group. These extrapolated

values are in close agreement with experimental results for air breakdown at the appropriate

frequency and pulse duration.

One can envision the interaction process as consisting of a catastrophic distortion of electron

distributions. At the very high fields generated by focussed laser beams, second order effects

may become important. Indeed, the second order correction to the refractive index leads to the

well documented onset of self-focussing. Other effects which may become important at elevated

fields are the process of electro-reflectivity and electro-absorption. It has been shown [13] that

due to the quadratic Stark effect an increase in absorption may occur at elevated fields. Any such

increase can lead to an enhanced deposition of energy within a lattice.

While the simple theory presented here gives more than fortuitous agreement with experiment,

it is obvious that a more detailed examination is called for. Obviously for a proper treatment

one should avoid taking average quantities but must instead deal with individual quantities. For

example, the mcflecular polarizability of glasses is a gross average of the polarizabilities of each

species and one would hope that a more detailed look at this facet of the problem would result in

better understanding. Such things as impurities and structures need be considered as well as

the total range of energy states in a solid lattice. Such considerations would naturally lead to investigation of low lying energy traps and the reduced effective mass of electrons lying near

the top of Fermi bands.

References

[1] Turner, A. Francis, Damage In Laser Materials : 1971 , NBS Spec. Pub. 356, 119-123 (1971).

[2] Boling, N. L. and G. Dub^, Laser Induced Damage

in Optical Materials : 1972, NBS Spec. Pub. 372,

40-45 (1972).

[3] Bloembergen, N. , Applied Optics 12 , No. 4,

661-664 (April 1973).

[4] Fersman, I. A. and L. D. Khazov, Sov . J^. of

Quantum Electronics 2, No. 4, 319-323

(Jan-Feb. 73).

[5] Fradin, D. W. , Eli Yablonovitch, and Michael

Bass, Applied Optics 12 , No. 4, 700-709

(April 73).

[6] Bettis, Jerry R. , Arthur H. Guenther and

Alexander J. Glass, Optoelectronics and Laser

Technology , 1974 IEEE Region Six Conference

Record, 43-55 (1974).

[7] DeShazer, L. G. , B. E. Newnam and K. M. Leung,

Laser Induced Damage in Optical Materials : 1973

NBS Spec. Pub. 387, 114-124 (1973).

[8] Nelson, P., P. Veyrie, M. Berry and Y. Durand,

Phys . Letters 13, 222 (1964).

[9] Akmanov, S. A., A. S. Kovrigin , M.M. Strukov and

R.V. Khokhlov, JETP Letters 1, 25 (1965).

[10] Boling, N.L., G. Dube' and M.D. Crisp, Laser

Induced Damage in Optical Materials : 1973

NBS Spec. Pub. 387, 69-79 (1973).

[11] Boling, N. L., Private Communication.

[12] Austin, R. Russel, Raymond C. Michaud,

Arthur H. Guenther, Joseph M. Putman and Richard

Harniman, Laser Induced Damage in Optical

Materials : 1972 , NBS Spec. Pub. 372, 135-165

(1972)

.

[13] Boling, N.L., A.J. Glass and Adelbert Owyoung,

Accepted for Publication in Journal of

Quantum Electronics

.

217
Figures

THRESHOLD ELECTRIC FIELD VS REFRACTIVE INDEX

A SURFACE DAMAGE

AIR BREAKDOWN

Figure 1. Threshold electric field for

damage vs refractive index for glass,

sapphire, and fused silica.

3

2-

THRESHOLD ELECTRIC FIELD

VS REFRACTIVE INDEX

A -J VAPOR PHASE MIXTURES

n> -I

1.6 1.8 2.0 2.4

Figure 2. Threshold electric field for

damage vs refractive index for vapor

phase mixtures of ThFi, and ZnS.

A NoF

THRESHOLD ELECTRIC FIELD

VSN

BULK DAMAGE OF A SODIUM HALIDES O RUBIDIUM HALIDES O POTASSIUM HALIDES

nl 1 1 1 1 1 1 0 10 20 30 40 90 60

( IO"ii.-») ir1

Figure 3. Threshold electric field for

N damage vs —j—r- for 11 alkali halides.

n -1

COMMEWTS OU PAPER BV 8ETTI5 al

Tke. (j-Out quution KeJUattd to wheMieA on. not thz btoxikdoiMn me.chanAj>m pnopoizd -in thJj> papM. usouJLd bz pnobablLUtia. Tkz ipzakzA -indyiccuttd that itncz tku> duoviption only fLe.{jMA to th& A.nitlation o^

thz b>izakdown, the, avaZancke. pnoceMi muZd itltt be pnobabiLUtic. euen though thz ImJMitLon piocui, -U, dttiimtyuMtic. The. LU>e cUa bmakdown vaZati ca a low index limit to the pnopaed icaZing Im WiX6 questioned, poAticuloAJlLj iince the numbeAA given (^on oJji bneakdom values fiz^^eAAed to dust laden oJji

fwXkeA than puAz aOi. The ipexikeA indicated that these points Me/ie included (^on. compa/Uson punpoites

only. He woi {,uAtheA questioned on the use o^ the data ojj TfuidA.n and BaSi, on the bfceakdom iPiengths

0() alkali halides. He replied that in using thoie data wheAe vatues weAe available cut 1.06 micnometeAS,

thoie values weAe used, but wheAe 1 .06 micAomeXeA data weAe. not available, data taken at otheA wave- lengths weAe extAapolated to 1.06 micAometeAS according to the 1 + w^x^ dependence.

218
6.4 Extrinsic Absorption in Laser Window Materials

C. J. Duthler and M. Sparks

Xonics, Incorporated, Van Nuys, California 91406

Two types of extrinsic absorption are discussed. First, our calculations on absorbing inclusions, presented at the 1973 symposium, have been extended to explain

the cone-shaped surface pits observed by Boling. For a spherical inclusion of radius a located a distance d below the surface, the cone half-angle is 6^^^ = cos'l (a/d). The

second type of absorption is that due to polyatomic molecular -ion impurities in alkali_

halides. A literature survey indicates that concentrations of less than 0.1 ppm of NO2 , HCOg, S02-, or Cr02- will result in a bulk absorption coefficient ^(10, 6>im) > 10"4 cm"!.

Key words: Absorbing inclusions; alkali halides; impurity absorption; laser

damage

.

1. Introduction

We have previously reported our calculations of infrared absorption and materials damage in transparent

materials containing inclusions. [ 1 We have extended our studies of extrinsic absorption in transparent laser

materials to include two new effects. First, the mechanism is described for producing cone-shaped surface damage sites that are observed when certain materials are irradiated with a high-power, short-pulse-length laser at

the threshold for damage. Second, certain molecular -anion impurities that substitute for the halide ion in alkalihallde crystals can contribute substantially to the extrinsic 10. 6fim absorption in these materials.

2. Conical Surface-Damage Pits

At the symposium last year, Boling, Dube' and Crisp [2] presented electron micrographs of several materials which suffered pre-plasma surface damage when irradiated with a 30 nsec, 1.06 Jim laser pulse of energy from

2

7 to 25 J/cm . These damage sites are nearly perfectly conical having diameters of a few microns and depths of

the order of one micron.

Due to the statistical distribution of the pits on the sample surface, it was proposed that the damage was initiated by an absorbing inclusion located just below the surface. Differing damage thresholds observed when various

polishing compounds are used indicate that the inclusions may be polishing grits that somehow become embedded

below the surface. Similar damage sites have been observed with thin films by Milam, Bradbury, and Bass. [3]

During the discussion following the talk by Boling, it was proposed that the conical pit was a crater resulting

from the explosion of the inclusion as with conventional explosives. With conventional explosives the crater radius

is related to the 1/3 power of the energy, and it was suggested that this may apply in the case of laser damage.

In a discussion with Dr. Robert Hellwarth after the meeting, we realized that the explosive mechanism is

incorrect since it implies that the material in the conical section is completely broken up. A simple calculation

reveals that there is only enough energy absorbed by the inclusion to disrupt the chemical bonds along the conical

surface and that there is not enough energy available to further disrupt the cone.

This research was supported by the Advanced Research Projects Agency of the Department of Defense and was

monitored by the Defense Supply Service, Washington, D. C.

^Figures in brackets indicate the literature references at the end of this paper.

219
Consider a conical pit of 7 fxm diameter and 2 depth resulting from absorption by a 1 fjxn radius inclusion.

The absorption cross -section of a dielectric inclusion is given by

^abs

= A 77 a3 . (1)

where /S^ is the absorption coefficient of the inclusion material, a is the inclusion radius, and A is a constant of

-1 2 9 order unity. [1] If we use jSj = 10 cm and a laser fluence of 15 J/cm , the energy absorbed is 3x 10 eV. As-

- 16 2 suming that there is one chemical bond of energy 1 eV per 4 x 10 cm of surface, the energy required to re9 move the conical section is 1 x 10 eV.

Therefore a mechanism is needed whereby the conical section is removed intact, or in a small number of

fragments. One is led to consider shock waves which, however, can be quickly ruled out by mechanical equilibrium

considerations. [ 4 ] In order for a compressive shock propagating outward from the inclusion to be able to produce

fracture, the pulse length of the shock must be short compared to the time for sound to travel from the inclusion to

the surface. In sapphire the velocity of sound is 1. 1 x 10^ cm/sec. This yields a round-trip time for sound of less

-9

than 10 sec which is less than even the SOnsec rise time of the compressive pulse. For the sake of completeness, we mention that Fox and Barr [ 5 ] have observed laser-induced shock -wave damage in the back face of aluminum samples.

Hence the inclusion is viewed as acting as a source of quasistatic hydrostatic pressure within the cavity in

the host material. The situation is sketched in figure 1. Neglecting heat flow out of the inclusion, the pressure P

at the inclusion-host interface is found by equating the volume change of the heated and compressed inclusion to the

volume change of compressed but unheated cavity in the host material, which yields

P = 3ajBg^ T . (2)

In eq (2) T is the temperature of the inclusion, is the linear thermal expansion coefficient of the inclusion, and

-1 -1 -1

is an effective bulk modulus given by B^^ ~

'^l

' ^'^^•'^^ ^ ^^'^ ^ denote inclusion and host.

If hydrostatic pressure P is applied within a spherical cavity in an infinite medium, the tensile stress along

the spherical surface is given by a^. = P / 2 . However, due to the proximity of the plane surface in the present

case, the tensile stress is no longer uniform over the spherical cavity.

The exact solution for the 3 -dimensional problem of stresses surrounding a spherical cavity near a plane

boundary is very difficult, and approximate solutions converge very slowly. Fortunately, the exact solutions have

been obtained for the qualitatively similar problem of a long cylindrical cavity aUgned with its axis parallel to the

plane surface. [ 6 ] These solutions can be used to gain insight into the present problem.

It is found that the tensile stress along the cylindrical cavity is anisotropic and is enhanced above the value

that would be obtained In an infinite medium. The stress along the cylindrical surface (j is a maximum at the

angle 6^ = cos (a/d) as is sketched in figure 1. Along the plane surface, maximum stress occurs at

X = 0.

The dependence of the stresses a^(6 = 6^) and a^(x = 0) on the depth of the inclusion is drawn in

figure 2. For an inclusion near the plane surface (d/a -» 1 ), the stresses are very much enhanced over the infinite medium values. As d becomes large compared to a , the stresses approach the infinite medium values of

= P and = 0 , The two curves cross at d = a

.

Due to symmetry, the maximum stress along the surface of a spherical Inclusion is expected to occur at the

same angle 6^^ as is shown in figure 1. However, the exact dependence of the stress on d/a will be slightly different from those shown in figure 2. First, the infinite -medium value for the stress on the spherical surface is

220
CT^ = P/2 . Second, since the stress in the medium surrounding a sphere falls off more rapidly with distance than

those surrounding a cylinder, the a versus d curves are expected to be more strongly peaked near d / a =1 than

in figure 2

.

The mechanism proposed for producing a conical pit is that a crack is nucleated at the point of maximum

stress. For d greater than approximately a , this maximum stress occurs at the angle on the spherical

cavity. The crack then propagates normal to the surface of the spherical cavity until it reaches the plane surface,

directly forming a cone-shaped cavity. This is sketched in figure 3(a). The half-angle of the cone is 6^ , which is

given as a function of d/a in figure 2,

Inclusions very close to the surface (d/a < ) will produce maximum tensile stress at the point x = 0 on the plane surface. As is sketched in figure 3(b), a crack will first propagate from the point x = 0 to the cavity

surface. Further expansion of the inclusion could result in additional failure near 6^ to produce a cone-shaped

damage site.

The following considerations favor fracture first occurring at 6^ as in figure 3(a) rather than at x = 0 as

in figure 3(b): Fracture in bulk materials is strongly dependent on surface cracks and imperfections which have

the same size as the inclusion. Similar, but smaller, defects are expected to affect the fracture in the present

case. If the inclusion is formed by melting surface material over an embedded polishing grit, more defects are

expected on the surface of the inclusion than on the highly polished plane surface. In addition, there is much more

surface area, hence more probability of finding a defect, along the circumference of the inclusion at the angle 6^

than at the point x = 0

,

Calculated laser pulse energies It for failure are very imprecise due to large uncertainties in the failure

strength a- for the material in the vicinity of the inclusion and in the bulk absorption coefficient ^, of the inclu4

sion material. The failure strength a. for sapphire is estimated to be between 6 x 10 psi, observed in bulk

6 1 samples, and the theoretical strength CT^j^gQ.,. = 6 x 10 psi estimated from ^^^^^^q^. = iq^' where E is the Young's

modulus. Assuming = 2 P at the point of failure and using = 6 x 10^ psi, we estimate that failure occurs at

2 approximately It = 400 J/cm accurate to only about two orders of magnitude. Pulse energies for damage observed by Doling et al [2 ] are well within the range of reasonable theoretical values.

For the 30-nsec pulse durations used in many experiments, the assumption of no heat transfer to the host

material is not valid. It is reasonable to expect that heating has little effect on the failure strength and that the

principal effect will be to lower the pressure at the inclusion-host interface, hence increase the required energy

for damage, due to thermal expansion of the cavity in which the inclusion is contained. [ 7 ]

Apart from the time for the crack to propagate to the surface, with very short pulses t » damage

could be delayed until the time after the laser pulse. This will occur if the initially formed isotropic stresses

are insufficient to cause damage but the quasiequilibrium anisotropic stresses, established after the time for

sound to make a round trip to the plane surface, are enhanced sufficiently to cause damage. Delayed surface

damage on dielectric mirrors has been observed by Bliss etal, [8] but it is uncertain whether this is the result

of crack propagation or stress enhancement,

3, Molecular-Ion Impurities

In this section we consider increases in the overall absorption coefficient due to molecular-ion impurities.

This applies mostly to 10. 6 ^wavelength cw systems, in contrast to the previous section wliich was concerned

with fracture due to localized heating in pulsed systems. The increased absorption heats the windows and can result in either undesirable thermal defocusing of the beam or fracture of the window. [ 9 ] Among the most promising materials for use with lasers are the alkali halides, in particular KCl and

BCBr. The absorption coefficient as a function of frequency in a typical material is sketched in figure 4, At low

frequency, the mechanism for absorption is the creation of phonons, with the fundamental Reststrahl peak

221

564-378 O - 74 - 16
occurring in the far infrared. It has been well established experimentally [10] and theoretically [ 11 ] that the

intrinsic absojrption coefficient decreases exponentially with increasing frequency in the multiphonon region.

In the ultraviolet, the mechanism for absorption is the excitation of an electron across the band gap to create an electron-hole pair. In the Urbach-tail region, jS decreases exponentially with decreasing frequency. In between

the two exponential regions, the absorption is thought to be dominated by impurities and imperfections. The absorption coefficient j3 in this region is represented by a wavy line in figure 4.

At the C0„ laser frequency of 943 cm ^ (10. 6\jm wavelength), the absorption for KCl and KBr samples is -4-1 -5 -1 extrinsic, having jS > 10 cm . This is to be compared to the extrapolated intrinsic values of 8 x 10 cm for

KCl and SxlO'^cm'-^ for KBr.

Last year we showed that infrared absorbing j^clusions dispersed throughout the crystals can contribute to

the overall absorption. [ 1 ] In what follows, evidence will be presented showing that another important type of

impurity limiting 10. 6jim wavelength transmission in the alkali halides is molecular-ion impurities that substitute

for the halide ions in these materials.

As an example, the case of substituting for the CI ion in KCl is drawn to scale in figure 5. [ 12 ] This ion fits loosely into the lattice and suffers little distortion. Consequently the internal vibrations of and

other substitutional anions are only slightly affected by the presence of the host crystal. Frequency shifts of the

molecular modes and splittings of degenerate modes are small. Infrared inactive modes may be weakly absorbing

in the crystal.

Because of the small interaction with the host crystal, spectroscopists have used the technique of alkalihalide matrix isolation for a number of years to concentrate and isolate ions for convenient study using conventional absorption spectroscopy. A literature survey has been made of those ions that have been studied in KCl and

KBr crystals. Those that most severely limit 10. 6^m transmission are presented below.

To estimate the absorption at the laser frequency of 1^ = 943 cm due to an impurity mode at Vq, &

Lorentzian line shape

i3(y) = (const) J- (3)

is assumed. In eq (3), A is the full width at half maximum, which is assumed to be frequency independent.

When available, measured line strengths and line widths were used for extrapolation to 943 cm ^. Typically for a

crystal with 0. 1% impurity concentration, the absorption coefficient jS ( i^p, ) at the center of an allowed transition

2 3-1 -1 ranges from 10 to 10 cm , with A v ranging from 4 to 50 cm depending on the particular molecular species

and host.

Impurities that substitute for lattice ions can be grouped into a few general types. First, atomic impurities

can either yield an infrared active local mode of the lattice, or can activate normally infrared inactive phonons.

However, because the frequencies of these modes are generally small compared to 943 cm ^, little absorption

occurs at the C0„ laser frequency. The U center at 500 cm"'^ has the highest frequency of this group and we esti- — -1-4 -1

mate that more than 10 ppm H is needed to produce |3( 1/ = 943 cm ) = 10 cm . [13]

Diatomic ionic impurities that have been studied in KCl and KBr crystals, on the other hand, have absorption frequencies that are too large to result in significant absorption at 943 cm in spite of their large Une widths.

The hydroxyl ion (OH ) with v = 3640 cm"''', is a member of this group. There has been concern regarding the

hydroxyl ion because of the difficulty in removing this ion from alkali-halide crystals. [ 14] However, using

-1 ~ A y = 50 cm in eq. (3), we estimate that greater than 100 ppm of OH at isolated lattice sites is needed to yield

222
)3(943 cm ) = 10 cm , [15] At high impurity concentrations, the OH may combine with other impurities

to absorb more strongly at 943 cm

Polyatomic impurities have several infrared active modes, some of which may occur near 943 cm ^ and produce considerable laser absorption. This group can be further subdivided into small and large ions. Small

polyatomic ions such as fit loosely into the lattice and have large, temperature dependent line widths due to

rotational and translational degrees of freedom. Larger ions such as HCO^ fit tightly into the lattice and have

several narrow, temperature insensitive lines.

2-

In addition to the above groups, polyvalent ions such as SO^ can substitute for the singly charged halide ion.

In order to preserve the charge neutrality of the crystal, these ions must be accompanied by a compensating va- 2+ -1

cancy or by a divalent cation impurity such as Mg . The infrared spectrum and 943 cm absorption is dependent

on the particular compensating species. — — 2- 2-

Our literature survey indicates that 0.03 ppm of either N0„ , HCO„, SO. , or CrO. will result in -1-4-1 z 6 4

4

j3( y = 943 cm ) = 10 cm . Calculated spectra using an assumed Lorentzian line shape along with experi- — — 2-

mental frequencies and line widths are shown in figures 6-8 for NO2 , HCO^, and SO^ . [16]

In each of the cases shown in these figures, the laser frequency, 943 cm \ falls at the edge of a small

number of peaks representing the infrared active internal modes of the impurity. It would seem from looking at

these figures that it would be easy to measure the spectrum of a crystal containing an unknown impurity and hence

determine the impurity. However, there are two difficulties. First, the absorption coefficient at the peaks is only

-2 -1

10 cm so that it would be difficult to make high resolution measurements using conventional spectroscopy.

Second, and more important, there will be more than one impurity present in a real crystal so that the actual

spectrum will look more like that of figure 4 than figures 6-8.

In figure 9 we represent a real crystal by adding the absorption coefficient in the previous three figures.

The individual lines near 1200 cm ^ have merged to form a relatively featureless, broad peak. The only distinctive lines remaining are the two HCO„ lines at 840 cm ^ and 971 cm ^, If in addition we decrease the amount of — ^ 2+ 2-

HCOg and add different compensating divalent cations such as CO along with a small amount of CrO^ , even

the calculated spectrum would appear featureless from 1000 to 1500 cm ^.

The above examples serve to illustrate the importance of molecular-ion impurities on 10. 6 jxm transmission

in alkali halides. A more complete survey of the ions that have been studied in KCl and BCBr crystals will be published elsewhere. [ 17 ]

4. References

[1] C.J. Duthler and M. Sparks, Laser Induced Damage

in Optical Materials, NBS Spec. Pub. 387, 208(197"^

J.AppL Phys. 44, 3038 (1973); Appl. Phys. Lett. 24,

5 (1974).

[2] N. L. Boling, G. Dube', and M. D. Crisp, Laser In- duced Damage in Optical Materials, NBS Spec. Pub.

387, 69 (1973); Appl. Phys. Lett. 23, 658 (1973).

[3] D. Milam, R.A.Bradbury, and M.Bass, Laser In- duced Damage in Optical Materials, NBS Spec. Pub.

387, 124 (1973^

[4] For a review of shock wf^ve induced fracture see

H. Kolsky and D. Rader in Fracture, Vol. 1, edited

by H. Liebowitz (Academic, New York, 1968).

[5] J. A. Fox and D. N. Barr, Appl. Phys. Lett. 22, 594

(1973).

[6] G. B. Jeffery, Phil. Trans. Roy. Soc. Lond. 22J., 265

(1921).

[7] R. W. Hopper and D. R. Uhlmann, J. Appl. Phys. 41,

4023 (1970).

[8] E. S. Bliss, D. Milam, and R. A. Bradbury, Appl.

Opt. 12, 677 (1973).

[9] M. Sparks, J. Appl. Phys. 42 , 5029(1971); M. Sparks

and H.C. Chow, J. Appl. Phys. 45, 1510 (1974).

[10] T. F. Deutsch, J. Phys. Cham. Solids (to be published).

223
[11] M, Sparks and L. J. Sham, Phys, Rev. B8, 3037

(1973) and references therein,

[12] A,R. Evans and D. B, Fitchen, Phys. Rev. B2,

1074 (1970).

[13] A. A, Maradudin in Solid State Physics, edited by

F. Seitz and D. Turnbull (Acadeniic, New York,

1966), Vol. 18, p. 274 and Vol, 19, p, 1,

[14] R.C. Pastor and M. Braunstein, Hughes Research

Laboratories Technical Report No. AFWL-TR72-152, Vol. II (1973),

[15] B, Wedding and M. V. Klein, Phys, Rev. 177, 1274

(1969); M. V. Klein, B. Wedding, and M. XTLevlne,

Phys. Rev, 180, 902 (1969); D, F, Smith, J, Overend,

J. C. Decius, and D.J. Gordon, J. Chem. Phys. 58,

1636 (1973).

[16] The data used for NO2 in KCl are obtained from

V. Narayanamurti, W, D, Seward, and R, O, Pohl,

Phys. Rev. 148, 481 (1966); V.P. Dem'yanenko and

Yu. P. Tsyashchenko, Sov. Phys. - Solid State U,

3043 (1970); R. Kato and J. Rolfe, J, Chem. Phys.

47, 1901(1967); R. Bonn, R. Metselaar, and J. van derElsken, J. Chem. Phys, 46, 1988(1967); A. R. Evans and D, B. Fitchen, Phys. Rev, B2 , 1074

(1970). The HCO3 data from D. L. Burnitt, K. O.

Hartman, and I. C. Hisatsune, J. Chem. Phys. 42,

3553 (1965). The SO|- data from J. C. Decius,

E. H. Coker, and G.L. Brenna, Spectrochim, Acta

19, 1281 (1963); E, H, Coker, J, C. Decius, and

X7B. Scott, J. Chem. Phys. 35, 745 (1961); D. N. Mirlin and I. I. Reshina, Sov. Phys. - Solid State 10, 895 (1968).

[17] C, J. Duthler, J. Appl. Phys. (to be published).

5. Figures

MAX. STRESS

MAX. STRESS

Figure 1. Sketch of an inclusion of radius a at a distance d below the host surface.

1i&

Figure 3. Cone-producing fracture for crack

nucleation at inclusion surface (a) and at

plane surface (b).

DEPTH, a/o

Figure 2. Tensile stresses and cone half angle

as a function of inclusion depth.

Frequency, w

Figure 4. Semi-log plot of absorption in

alkali halides as a function of frequency.

224
t<no>

no; in KCI

Figure 5. Orientation of NO2 impurity in

KCI at low temperature.

T I I I

I

I I I I [

I ! r

I i I

I I I 1 I 1 1 I I L 0 500 1000

Frequency (cm"')

Figure 7. Calculated absorption coefficient

of a KCI crystal containing 0.03 ppm HC03.

T

1 1 1

1—I

1 1 r

I I I I I I I t I \ I I L_

0 500 1000

Frequency (cm"')

Figure 6. Calculated absorption coefficient

of a KCI crystal containing 0.03 ppm NO2.

1—

r

0 500 1000

Frequency (cm"')

Figure 8. Calculated absorption coefficient

of a KCI crystal containing 0.03 ppm

2- 2+

SO^ :Mg^ .

225
T

1 1

1

\

1 1 1

I

I

I T

500 1000

Frequency (cm"')

Figure 9. Calculated absorption coefficient

of a KCl crystal containing a few impurities.

COmENTS ON PAPER BV VUTHLER

HeJtbeAt Lipicomb, dwrn hJji EoKct Q-ombKiAqz Ruz.axch la.bon.aton.y , zommzntzd that hJj, own tdcZYVt

muk had ihom that chloKato, -iom ihoutd be. Indliidud ai poiilbte iouAcu o{j abion.ptlon at 940 and

960 . The. ipe.akeM. commznte.d that thz chZofixute. ton doe^ not znteA the lattice puAeZy In a iabitAXiutional May, bat fiepKeJieni^ inistead, a Zocat chemical change in the compoiitton the. alkaLL

haJU.de. He had two comments to make on the monk o^ HoppeA and Uhlmann, fieponXed at an eanJiieh.

Vamage Sympo^itum. He commented that he ieJit that in thexM. anatyiti o^ incZaitoni deep in the. bulk ojj tAanipoAzvit mate/Liati , that HoppeA. and Uhtman did not aie the connect abionptlon cnoa, iection. He

{^onXheA commented that when the incZuAion tie^ man the 6un.(,ace the anliotnopy in the 6tn.ej>i> mait be

comideAcd, and the iimple iphexlcaJL geomeXny ii> no longeA applicable. It wa6 commented that

cnyitaltine mateAlal ii not neaZty accuAotely deAcnibed by continuum eZoitic theony, but ihouZd be

de^cnibed in tenm o{^ the diicnete lattice and that the poiitxoning o{, an impuAity with nej,pe.ct to

the lattice could be o{, con^idenable impontance in detenmining the e^^ect that impunity in the

neAponAe o{j the matenial. The ipeaken commented that thii wa6 indeed the ca^e, that he w)a6 in totat

agneement, but that to abandon continuum mechanici would intnoduce inondinate complexity in the catculatton which waA atneady only an onden o{, magnitude estimate. Vavid Hvlam, o^ Ain Vonce ReAeoAch

Labonatony, noted in paAiing that in the coAe o{, extnemely ihont pulAci od picoiecond dunation, the

{jiAAt puLie, even though it may exceed the damage thneAhold o^ a neilectlng iun^ace, iA Atilt pnoceAied by that iuA^ace becauie it ii neglected be{,one the damage hoi time to occun.

226
Appendix i

MACHINING OF OPTICS WORKSHOP

AN INTRODUCTION TO THE MACHINING OF OPTICST. T. Salto

Air Force Weapons Lab

AFWL-LRE

Kirtland AFB , NM 87117

ABSTRACT

This paper will introduce machining of optics, presenting some of the history of

machining progress, critical machining variables, material and geometrical capabilities and some new results on optical evaluation of machined surfaces. A most

significant development is electroplating of materials which are otherwise not

presently machineable. For example, molybdenum has been electroplated with silver

or gold and then successfully diamond turned. Research and development ideas for

future machining studies will be briefly discussed.

THE INFLUENCE OF TOOL FEED RATE ON THE

MACHINABILITY OF CO^ LASER OPTICS

J. B. Arnold

P. J. Steger

Oak Ridge Y-12 Plant

Oak Ridge, Tennessee

T. T. Saito

AFWL-LRE

Kirtland AFB, NM 87117

ABSTRACT

The influence of tool feed rate on reflectivity of diamond-machined surfaces was evaluated using materials (gold, silver, and copper) from which CO laser optics are primarily produced. Fifteen specimens were machined by holding all machining parameters constant except tool feed rate, and allowing it to vary only by controlled

amounts. Inspection results of the specimens indicated that tool feed rates significantly higher than 200 microinches per revolution produced detrimental effects

on the machined surfaces. In some cases, at feed rates as high as 500 microinches

per revolution, visible scoring was evident. Those surfaces produced with tool feed

rates less than 200 microinches per revolution had little difference in reflectivity.

Measurements indicated that their reflectivity existed in a range from 96.7 percent

to 99.3 percent

.

MIRROR MACHINING WITH CONVENTIONAL MACHINES

W. Ranbauske

Raytheon (Missile Systems Division)

Hartwell Road

Bedford MA 01730

ABSTRACT

A method is described for fabricating highly aspheric reflective optical elements

using only conventional machines and fabrication processes. Employment of the method

appears to be very practical for the fabrication of mirrors of apertures as large as 1.5 meters diameter for high energy lasers in the 3 to 10 micron wavelength region.

Accuracies in curvature figuring for diffraction limited performance for a current

system design and surface finish to better than one microinch have been achieved.

The mirrors produced have as generatrices conical sections or straight lines, they

are all metallic with a substrate of low thermal expansion, a hard upper surface, and

a top layer of high reflectivity.

227
EFFECTS OF THE RESIDUAL SURFACE ROUGHNESS

OF DIAMOND TURNED OPTICS

E. L. Church

J. Zavada

Frankford Arsenal

Philadelphia PA 19137

ABSTRACT

The residual roughness of diamond turned surfaces is expected to contain grating-like

components. We explore the optical (infrared) properties of such components using

Rice's perturbation theory for a statistically rough surface^ by taking a roughness

autocorrelation function of the form z ^ Co s ( 27Tx/ d ) ; that is, a delta-function power

spectrum corresponding to a surface wavelength d. The Rayleigh wavelengths A±=d

(l+SinS) are critical parameters in describing such properties. For example, when

A>X+, the surface has the reflectivity approaching that of a perfectly smooth surface, and when A<<X-, the reflectivity is that of the smooth surface reduced by the

usual factor: 1 - (AttzCosG / A ) ^ . Here z is the rms vertical roughness, 9 is the

angle of incidence, and X is the illuminating wavelength. When A'V'Xl , additional interesting effects appear as functions of A, 6, the groove direction, the polarization

of the incident radiation, and the optical constants of the surface. The possibilities of observing these effects in re f lectometry , ellipsometry , and scatterometry are considered as means for characterizing and evaluating surfaces of this type.

MEASUREMENT OF SURFACE FIGURE AND

SURFACE FINISH AT THE Y-12 PLANT

R. E. Sladky

Union Carbide Y-12

Box Y Oak Ridge, Tennessee 37830

ABSTRACT

The Y-12 Plant is expanding its optical inspection facilities to provide data for

diamond knife machine tool development programs and for certification of the reflectors that have been produced. Measurements of reflectivity, diffraction pattern, and

surface finish by FECO fringes will be made routinely in the near future. At present,

surface finish is measured with double- and multiple-beam interferometers and

Nomarski polarization. A comparison of results from various materials will be presented. The Foucault knife edge and Ronchi grating are used to evaluate surface

figure

.

APPLICATION OF ELECTROPLATING TO THE FABRICATION

OF OPTICS

F. B. Waldrop

R. C. Waldrop

M. J. Bezik

W. E. Tewes

Oak Ridge Y-12 Plant

Oak Ridge, Tennessee

ABSTRACT

Copper, silver, and gold electroplates are sources of high-purity, flaw-free metal

from which high-quality optical surfaces may be machined. The nature of the electrodeposit can be varied over wide limits, and this fact is demonstrated by using copper

as an example. Super finished metal surfaces can be replicated by electrof orming as an alternate fabrication method.

228
ION POLISHING OF METAL SURFACES

R. A. Hoffman

W . J . Lange

W. J. Choyke

Westlnghouse Electric Corporation

Research Laboratories

Pittsburgh, Pennsylvania 15235

ABSTRACT

An ion polishing apparatus that uses a focused beam of low energy Xe ions incident at

a glancing angle is described. The sample to be polished is rotated and the beam

scanned across the surface to produce uniform removal of material. Starting with

single crystal copper which had been mechanically polished, and by using combinations

of ion polishing and vacuum annealing the absorptivity, determined by calorimetry, was reduced from 0.92% to 0.76%. SEM pictures show that the surface features become

smoother with ion polishing. A similar experiment was performed on a single crystal

copper sample that had been electropolished , and the combination of ion polishing and

annealing lowered the absorptivity from 0.97% to 0.89%.

ION MILLING FOR SURFACE FINISH

IMPROVEMENT ON MACHINED METAL MIRRORS

Rocky Flats

J. Quella

Dow Chemical

- Golden, Colorado 80401

ABSTRACT

An ion beam directed at a surface at a low angle of incidence removes a small amount

of the surface material from the high points of the surface. Results of a preliminary

study indicate that one microinch arithmetic average surface finishes can be improved

25 to 40 percent

.

The process used by Commonwealth Scientific and the results of a preliminary study at

Rocky Flats will be discussed on an informal basis.

229
EFFECTS OF POLISHING ON DIAMOND KNIFE

MACHINED MIRRORS

R. L . Gordon

Ba t t e 1 le-No r thwe s t Laboratories

Richland, WA

ABSTRACT

Work is in progress to determine effects of post machining treatment on diamond-knife

machined copper mirrors prepared by Union Carbide Corporation, Nuclear Division, Y-12.

Machined surfaces are in copper electrodeposited on oxygen-free copper substrates.

Primary emphasis is placed on sample characterization both before and after different

po s tmachining treatments. A relative scatterometer is used to estimate rms surface

roughness and angular distribution of light scattered in the plane of incidence. The

balance of the characterization includes interf erometric observation of mirror figure,

Nomarski microscopy for visualization of surface topography, and calorimetric absorption measurements at X = 10.6)jm.

Results to date indicate that 1.52 in. diameter mirrors can be machined flat to within

two fringes. o At positions of minimum scattering, as-machined surfaces exhibit roughness near 27A rms. Angular distribution of scattered light suggests that the as- machined surfaces approximate saw-tooth gratings with spacings near those predicted by

machining parameters. Observation of diffraction maxima as a function of position on the sample indicate an increasing groove depth with increasing radius, with shallowest

grooves lying near mirror center. These results are verified by SEM stereomicrographs

.

Scattering data further indicate a slight asymmetry in groove shape.

Absorption measurements on as-machined surfaces indicate the presence of non-negligible

scattering at X = 10.6iJm, and an optical absorption at that wavelength of about 0.9%.

It has been demonstrated fhat a 9 minute polis^ in a slurry of colloidal carbon black

reduces roughness from 27A rms to less than 15A rms. Some periodicity in the surface

can still be detected after that duration of polishing. No increase in absorption due

to final polishing of fine-grained deposits was observed.

Future work will determine effects of grinding with Ip and 3y diamond grits before

final polishing on mirror scattering and absorption.

230
Appendix ii

PARTICIPANTS

S. D. Allen

T. H. Allen

L. J. Aplet

D. N. Aspinwall

R. R. Austin

Hughes Research Labs., 3011 Malibu Canyon Rd., Mallbu CA 90265

McDonnell Douglas Corp., P. 0. Box 516, St. Louis MO 63166

Hughes Aircraft Co., Centinela & Teale Sts., Culver City CA 90230

Lockheed Research Labs., 3251 Hanover St., Palo Alto CA 94304

Perkin-Elmer Corp., Main Ave., Norwalk CT 06856

B

F. D. Baird

L. M. Ball

W. P. Barnes, Jr.

M. Bass

J. H. Bechtel

J. M. Bennett

H. E. Bennett

R. B. Bennett

J. R. Bettis

E. Bliss

N. L. Boling

H. S. Boyne

R. A. Bradbury

A. Braunstein

M. Braunstein

N. J. Brown

J. A. Bruce

W. F. B runner

J. Bryan

D. L. Burdick

Lawrence Livermore Lab., P. 0. Box 808, Livermore CA 94550

Optical Industries, Inc., 3006 Enterprise St., Costa Mesa CA 92626

Itek Corp., 10 Magire Rd., Lexington MA 02173

Center for Laser Studies, University of California, Los Angeles CA 90007

Gordon McKay Lab., Harvard University, Cambridge MA 02138

Michelson Lab., Code 6018, Naval Weapons Centers , China Lake CA 93555

Michelson Lab., Naval Weapons Center, China Lake, CA 93555

Battelle-Columbus Labs., 505 King Ave., Columbus OH 43201

Air Force Weapons Lab/DYX, Kirtland AFB NM 87117

Laser Division, Lawrence Livermore Lab., P. 0. Box 808, Livermore CA 94550

Owens-Illinois, 1700 N. Westwood, Toledo OH 43666

Quantum Electronics Division, National Bureau of Standards, Boulder CO 80302

AF Cambridge Research Lab., Hanscom Field, Bedford MA 01730

Hughes Research Labs., 3011 Malibu Canyon Rd, , Malibu CA 90265

Hughes Research Labs., 3011 Malibu Canyon Rd., Malibu CA 90265

Lawrence Livermore Lab., P. 0. Box 808, Livermore CA 94550

Air Force Cambridge Research Lab/LQO, Hanscom Field, Bedford MA 01730

Lawrence Livermore Lab,, P. 0. Box 808, Livermore CA 94550

Lawrence Livermore Lab/L-140, P. 0. Box 808, Livermore CA 94550

Code 6013, Naval Weapons Center, China Lake CA 93555

W. Caithness

J. C. Camm

R. Carbone

G. E, Chamberlain

P.

J.E.

G.C.

J.J.

W.

M.

L.

N.V.

J.N,

Chan

Cherlow

Church

Cobham

Collins

Comer

Cooper

V. Costich

Applied Optics Center, 10 B St., Burlington MA 01803

Avco-Everett Research Lab., 2385 Revere Beach Pkwy., Everett CA 02149

L Division, Los Alamos Scientific Lab.j Los Alamos NM 87544

Quantum Electronics Division, National Bureau of Standards, Boulder CO 80302

Dept. of Elect. Engr., Colorado State University, Ft. Collins CO 80521

Physics Dept., University of Southern California, Los Angeles CA 90007

L4100- 150-2, U. S. Army - Frankford Arsenal, Philadelphia PA 19137

Union Carbide Corp., P. 0. Box Y, Y-12 Plant, Oak Ridge TN 37830

Air Force Cambridge Research Lab/LQO, Hanscom Field, Bedford MA 01730

Air Force Cambridge Research Lab., Hanscom Field, Bedford MA 01730

Dept. of Physics, Naval Postgraduate School, Code 61CX, Monterey CA 93940

Coherent Radiation, 3210 Porter Drive, Palo Alto CA 94304

D

L. G. DeShazer

J. A. Detrio

T. M. Donovan

R. Doussain

C. J. Duthler

Center for Laser Studies, University of Southern California, Los Angeles CA 90007

University of Dayton Research Institute, 300 College Park, Dayton OH 45469

Code 6018, Naval Weapons Center, China Lake CA 93555

Laboratoire National D'Essais, 1 Rue Gaston Noissier, 75 Paris 15°, France

Xonics, Inc., 6837 Hayvenhurst Ave., Van Nuys CA 91406

J. M. Eastman Optics Dept., Spectra Physics, 1250 W. Middlefield Rd., Mountain View CA 94042

S. J, Ekstrom Lawrence Livermore Lab/L-214, P. 0. Box 808, Livermore CA 94550

B. G. Enrique Honeywell Research Center, 10701 Lyndale Ave. So., Bloomington MN 55420

W. S. Ewing Air Force Cambridge Research Labs/LQO, Hanscom Field, Bedford MA 01730

A. Feldman

J. R. Fenter

C. W. Fountain

D, L. Frazen

Materials A-259, National Bureau of Standards, Washington DC 20234

Air Force Materials Lab/LPL, Wright-Patterson AFB OH 45433

Code 5525, Naval Weapons Center, China Lake CA 93555

Quantum Electronics Division, National Bureau of Standards, Boulder CO 80302

231
G

J. F. Giuliani

C, R. Giuliano

A. J. Glass

A. Glassraan

A. Golubovic

R. L. Gordon

A. H. Guenther

H

T. Harris

n. A. Hayes

R. Hellwarth

R, A, Hoffman

S. J. Holmes

R. A. House II

C. A. Huguley

I

J

E. G. Johnson

G. T. Johnston

F. W. Jones

T. Judd

K

J. C. Kershenstein

J. M. Khan

C. Klein

A. Klugman

P. Kraatz

C. S. Krishnan

J. Kurdock

R. G. Kurtz

L

J. Lancelot

H. Lee

G. W. Leppelmeier

K. M. Leung

H. G. Lipson

J. S. Loomis

M

D. MacGrath

R. W. .MacPherson

T. J. Magee

I. H. Malitson

J. Marburger

R. B. McGraw

T. McJilton, Jr.

J. M. McMahon

D. Milam

T. B. Milam

N. K. Moncur

M. Monsler

N

B . E . Newnam

0

D. F. O'Brien

R. W. Ohlwiler

W. Otaguro

A. Owyoung

Code 5551, Naval Research Lab., A555 Overlook Ave., Washington DC 20390

Hughes Research Labs., 3011 Malibu Canyon Rd. , Malibu CA 90265

Lawrence Livermore Lab/L-215, P. 0. Box 808, Livermore CA 94550

Lawrence Livermore Lab/L-140, P. 0. Box 808, Livermore CA 94550

Air Force Cambridge Research Lab., Hanscom Field, Bedford MA 01730

Battelle-Northwest, P. 0. Box 999, Richland WA 99352

Air Force Weapons Lab., Kirtland AFB NM 87117

Raytheon Co./LADC, 4 Standish Circle, Andover MA 01810

Code 5585, Naval Weapons Center, China Lake CA 93555

Physics Dept., University of Southern California, Los Angeles CA 90007

R&D Center, Westinghouse Electric Corp., Beulah Rd. , Pittsburgh PA 15235

Laser Technology Labs., Northrop Corp., 3401 W. Broadway, Hawthorne CA 90250

Air Force Weapons Lab/DYX, Kirtland AFB NM 87117

Air Force Weapons Lab/LRE, Kirtland AFB NM 87117

Quantum Electronics Division, National Bureau of Standards, Boulder CO 80302

University of Dayton Research Institute, 300 College Park, Dayton OH 45469

Union Carbide Corp., P. 0. Box Y, Oak Ridge TN 37830

Laser Optics, Inc., P. 0. Box 3, Danbury, CT 06810

Code 5503, Naval Research Lab., 4555 Overlook Ave., Washington DC 20375

Lawrence Livermore Lab., P. 0. Box 808, Livermore CA 94550

Hughes Aircraft Co., Centinela & Teale Sts., Culver City CA 90230

Dept. 630, Northrop Corp., 3401 W. Broadway, Hawthorne CA 90250

Research & Technical Center, Northrop Corp., 3401 W. Broadway, Hawthorne CA 90250

Materials Technology, TRW Systems, Space Park, Redondo Beach CA 90278

Perkin-Elmer Corp., 77 Danbury Rd. , So. Wilton CT 06897

Dow Chemical Co., P. 0. Box 888, Golden CO 80401

C.E.A. Limeil France, B. P. 27 Villeneuve, St. Georges 94, France

Owens-Illinois, 1035 N. Westwood Ave., Toledo OH 43666

Lawrence Livermore Lab., P. 0. Box 808, Livermore CA 94550

Center for Laser Studies, University of Southern California, Los Angeles CA 90007

Air Force Cambridge Research Lab/LQS, Hanscom Field, Bedford MA 01730

Air Force Weapons Lab/LRE, Kirtland AFB NM 87117

Owens-Illinois, 1020 N. Westwood Ave., Toledo OH 43666

Electro-Optics Division, CRDV/DREW, P. 0. Box 880, Courcelette, Quebec, Canada

Stanford Research Center, 333 Ravenswood, Menlo Park CA 94025

A-251, National Bureau of Standards, Washington DC 20234

University of Southern California, Los Angeles CA 90007

MS 283, Research Dept., Perkin-Elmer Corp., Main Ave., Norwalk CT 06856

General Electric Co., Court Street Plant, Syracuse NY 13201

Naval Research Lab., Washington DC 20390

Air Force Cambridge Research Lab., Hanscom Field, Bedford MA 01730

Pratt & Whitney Aircraft, West Palm Beach FL 33403

KMS Fusion, Inc., P. 0. Box 1567, Ann Arbor MI 48106

Science Applications, Inc., 6 Preston Ct., Bedford MA 01730

Los Alamos Scientific Lab/L-2, P. 0. Box 1663, Los Alamos NM 87544

Air Force Materials Lab., Wright-Patterson AFB OH 45433

U. S. Air Force Systems Command Hdqs/DLSP, Andrews AFB MD 20334

MS 28, McDonnell Douglas, Huntington Beach CA 92 708

Division 5214, Sandia Labs., P. 0. Box 5800, Albuquerque NM 87111

232
p

R. J. Phelan, Jr.

J. 0. Porteus

H. Posen

J. C. Potosky

R. F. Prater

Q

R

W. Rambauske

J. E. Rudlslll

P. M. Rushworth

S

T. T. Saito

R. J. Salmon

S. R. Sheele

R. A. Shatas

C. Y. She

J. L. Siemasz

D. R. Sigler

R. E. Sladky

D. M. Smith

R. L. Smith

T. H. Smith

M. J. Soileau

W. J. Spawr

J. L. Stapp

E. Stark, Jr.

P. J. Steger

C. M. Stickley

D. L. Stierwalt

J. Stover

V. R. Stull

TU

V

K. Vedam

W

F. B. Waldrop

V. Wang

S. W. Warren

R. Webb

E. D. West

R. Wick

C. B. Willingham

R. Windebank

G. I. Wong

S. M. Wong

R. A. Woodruff

D. L. Wright

X

Y

N. F. Yannoni

F. Zernike

Quantum Electronics Division, National Bureau of Standards, Boulder CO 80302

Michelson Lab., Naval Weapons Center, China Lake CA 93555

Air Force Cambridge Research Lab/LQO, Hanscom Field, Bedford MA 01730

Ion Physics Dept., Hughes Aircraft Co., 3011 Malibu Canyon Rd. , Malibu CA 90265

Air Force Weapons Lab/LRE, Kirtland AFB NM 67117

Missile Systems Division, Raytheon, Hartwell Rd. , Bedford MA 01730

Hughes Research Labs., 3011 Malibu Canyon Rd., Malibu CA 90265

MP 276, Martin-Marietta Corp., P. 0. Box 5837, Orlando FL 32808

Air Force Weapons Lab/LRE, Kirtland AFB NM 87117

Dept. 52-21, Lockheed Missile & Space Co., 3251 Hanover St., Palo Alto CA 94304

Hughes Aircraft Co., Centinela & Teale Sts., Culver City CA 90230

Physical Sciences Lab., U. S. Army Missile Command, Redstone Arsenal AL 35809

Physics Dept., Colorado State University, Ft. Collins CO 80521

U. S. Air Force Det 36, Hughes Aircraft Co., Centinela & Teale Sts.,

Culver City CA 90230

Laser Energy, Inc., 320 N. Washington St., Rochester NY 14625

Y-12 Plant, Union Carbide Corp., Oak Ridge TN 37830

RESD, General Electric Co., P. 0. Box 8555, Philadelphia PA 19101

Quantum Electronics Division, National Bureau of Standards, Boulder CO 80302

Chemical Laser Technology, TRW Systems, 1 Space Park, Redondo Beach CA 90278

Michelson Lab., Code 6018, Naval Weapons Center, China Lake CA 93555

Spawr Optical Research, 130 N. Sherman Ave., Corona CA 91720

Air Force Weapons Lab/LRE, Kirtland AFB NM 87117

Los Alamos Scientific Lab., P. 0. Box 1663, Los Alamos NM 87544

Nuclear Division, Union Carbide, Box Y, Oak Ridge TN 37830

Materials Sciences Office, Dept. of Defense - ARPA, 1400 Wilson Blvd.,

Arlington VA 22209

Code 2600, Naval Electronics Lab. Center, 271 Catalina Blvd., San Diego CA 92152

Dow Chemical Co., P. 0. Box 888, Golden CO 80401

Tempo, General Electric Co., P. 0. Box QQ, Santa Barbara CA 93102

Pennsylvania State University, University Park PA 16802

Union Carbide, P. 0. Box Y, Bldg. 9202, Oak Ridge TN 37830

Dept. 30-25, Hughes Research Corp., 3011 Malibu Canyon Rd., Malibu CA 90265

ITT Components Group Europe, Edinburgh Way, Harlow, Essex, England

Materials Research Center, Allied Chemical Corp., P. 0. Box 1021-R,

Morristown NJ 07960

Quantum Electronics Division, National Bureau of Standards, Boulder CO 80302

Air Force Weapons Lab/LRE, Kirtland AFB NM 87117

Research Division, Raytheon Co., 28 Seyon St., Waltham MA 02154

Laser Optics, Mill Plain Road, Danbury CT 06810

Bldg. R-1, TRW Systems Group, 1 Space Park, Redondo Beach CA 90278

Dow Chemical Co., P. 0. Box 888, Golden CO 80401

Ball Brothers Research Corp., P. 0. Box 1062, Boulder, CO 80302

Dept. of Electrical Engineering, Colorado State University, Ft. Collins CO 80521

Air Force Cambridge Research Lab/LQS, Hanscom Field, Bedford MA 01730

Research Dept., Perkin-Elmer Corp., Norwalk CT 06856

233

NBS-n4A iREv. 7.731

U.S. DEPT. OF COMM. 1 . PU B LIC A TION OR R E FORT NO. 2. Gov 't Accession BIBLIOGRAPHIC DATA No. SHEET Spec. Publ. 414

3, Recipient's Accession No.

4. TITLE AND SUBTITLE

LcmeA Indaczd Vamagz In Opticat UateAlati: 1974

5. Publication Date December 1974

6. Performing Organization Code

7. AUTHOR(S) Edited by AltxandeA J. GloM and KfvthuA H. GixQ-WthoA

8* Per f or in i ng Organ . Report No.

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

10. Project/Task/Work Unit No. 2710900/2710569/2712SS5

n. Contract/Grant No.

12. Sponsoring Organization Name and Complete Address (Street, City, State, ZIP)

AmdhA-can Soclzty ^on. Tuting S MatMAMLi , Vhila. , PA J9J03

Vtpt. oi the. Navy, O^jJ-tce 0)5 Naval Rdi^oAch, KnLinQton, Va. 22211

Quucintm Ete.ct/LonA,c6 V'Lv-iiA.on, NaJxanaZ BuM.e.au o{i Standa/idi,

BoaldeA, CO S0502

13, Type of Report & Period Covered

14. Sponsoring Agency Code

15. SUPPLEMENTARY NOTES

Library of Congress Catalog Card Number: 74-26935

16. ABSTRACT (A 200-word or less factual summary ol most si^ilicant information. If document includes a significant bibliography or literature survey, mention it here.) The Sixth ASTM-OMR-NBS Sympoi-Lum on La^eA Induczd Damage, in Optical MateAAjxLi Mai hoZd

at the. National BuAeau StandaAdi in BouldeA, Coloiado on May 22-23, ojj thif, yeoA.

The^e Symposia axe held oi panX. the activi^tie^ in Subcommittee Jl on LoieAi and

LaAeA MateAlalJi , the ASTM. SubcommiXtee II iA choAged with the fLe^iponiibiHtieA o(, {jOfmatating itandoAdt, and teit pn.oceduAeA {^on. louSseA mateAlalii , components , and devices.

The ChaiAman 0($ Subcommittee 11 is Haynes Lee, o{i OMeni-lltinoii, , Inc. Co-choAAmen ion. the damage Sympoiia oAc Va. fiAthuA GuentheA, Chle^ ScientUt of^ the Ma foice dJeapons

Labo/iatoAy, and Vn.. AlexandeA J. Glasi, Head, TheoAetical Studies, V division, LawAence

LiveAJtnofie Labofvatony

.

OveA ISO attendees at the Symposium heoAd 31 papeAS on topics Aelating to laseA induced

damage in cAyitalltne and non-lineoA optical mateAials , at dielectAic iuA^aces, and in

thin {jilm coatingi as weJUi as discuiiiom o;$ damage pAoblemi in the in{,AaAed region due

both to CM and pulsed iAAadiation. In addiJu.on, ieveAol Keponts on the theofieticaZ

analysis o{j lot, eA- mateAials inteAaction Kelative to the damage pKocesi, weAe given,

along Mith tabulattoni o{j {fundamental mateAials pfiopeAtles importance in evaluation

o{i optical mateAial n.esponse to high poMeA la^eA Aadiation. Attention was given to

high poMeA baseA system design consideAations which fielate to improved system peAf^or- mance and AeJUabitiXy Mhen vaAious damage mechanisms oAe opeAable in suck systems.

{continued on AeveAse]

17. KEY WORDS (six to twelve entries; alphabetical order; capitalize only the first letter of the first key word unless a proper name; separated by semicolons

)

IR MindoMS and mifiAons; la^eA damage; laseA mateAials; seZ{,-{,ocusing; thin {ilms.

18. AVAILABILITY [X] Unlimited

1

' For Official Distribution. Do Not Release to NTIS

pC 1 Order From Sup. of Doc, U.S. Government Printitig Office Washineton. D.C. 20402. SD Cat. No. CU. 10:414

1 1 Order From National Technical Information Service (NTIS)

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(THIS REPORT)

UNCL ASSIFIED

21. NO. OF PAGES

256

20. SECURITY CLASS

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24. Ptiee

$3. 15
NBS-114A continued - Laser Induced Damage in Optical Materials : 1974

Becaaic 0)5 the. gfioM^ng AjnponXancz md acczptancz 0|5 mac.kinQ.d compomnti in kigh povoex

IcukOA i>y{,tmi>, a mnkihop on th(i machining o{, optiu wa6 hel.d, undeA thz <ioon.dinojiion

0(5 Captain T. T. Saito 0|) the. kin foKcz \iltapont, LaboKatoiy. Uim papoXi on vaAiom

{iac&t6 0|5 the. topic imqaz pmiMintzd doxULing u)ith machining pn.oce.duAU, iuAi^ace chaAacteAization 0|) machined elements, coating o{j machined components , and the poti&hing and

damage KeAiAtance o{i polished, coated, and bane meXal neilectom,. KbitnactA on theJ>e

papzAA oAe included in the Sympoiium Vnoceedlngi, pnopeA.

The pnoceedingi o{, theAe Symposia nepnesent the majon souAcei o{, in{jOnmation in the

{lield o{, loAeA induced damage in optical mateAiaJb!>. The Symposia tkemseJive^ , along

u)ith the peAiodic meetings o(, Subcommittee II, pnovide a unique {^onum {,on the exchange

o{i in{^onynation negoAding laseA mateAiaZs speci(,ications among the manu^acXwiefU, and

uAQAS 0() loAeA devices, components, and systems. The Symposia atso seAve as a mechanism Oj$ in{,onj!nation gathenlng to enable the Subcommittee to Mfilte in{,onmed and ne.aLLstic

specifications

.
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